Patent Publication Number: US-10759730-B2

Title: Method for producing acetic acid

Description:
TECHNICAL FIELD 
     The present invention relates to methods for producing acetic acid. 
     BACKGROUND ART 
     A methanol carbonylation process (methanol-acetic acid process) is known as a method for industrially producing acetic acid. In this process, for example, methanol is reacted with carbon monoxide in the presence of a catalyst in a reactor to form acetic acid, the resulting reaction mixture is subjected to evaporation in an evaporator to give a vapor phase, the vapor phase is purified through a light ends column and subsequently through a dehydration column to give an acetic acid product, or further purified through a heavy ends column subsequent to the dehydration column, and, further through a product column to give an acetic acid product. 
     In the acetic acid production process as above, an offgas from a process typically using a reaction system or a purification system includes useful components (such as methyl iodide, water, methyl acetate, and acetic acid). Before discarding of the offgas, the useful components are recovered from the offgas typically by absorption treatment with an absorbing solvent in a scrubbing system. However, the offgas also includes hydrogen iodide. Hydrogen iodide probably corrodes the interior of equipment and facilities in the acetic acid production process and/or causes environmental issues when released into the atmosphere. Removal of hydrogen iodide from such offgas is therefore also required. 
     Patent Literature (PTL) 1 discloses an acetic acid production method in which a specific process stream is brought into contact with a first absorbent selected from the group consisting of acetic acid, methanol, and methyl acetate, or further brought into contact with a second absorbent including at least one of methanol and methyl acetate, to thereby remove hydrogen iodide from the process stream. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2016-121126 
     SUMMARY OF INVENTION 
     Technical Problem 
     The resulting solution after absorbing such useful components is then fed to a distillation column and is separated, by distillation, into the useful components and the absorbing solvent (absorbent). By the distillation, the useful components such as methyl iodide are generally concentrated in an overhead stream from the distillation column, and the concentrated methyl iodide can be recycled to the reactor and reused in the reaction step. 
     Disadvantageously, however, the method disclosed in PTL 1, when employing methanol or methyl acetate as the absorbing solvent, is susceptible to improvements in separation efficiency in separation, by distillation, of the solution after absorbing useful components, because the absorbing solvent and methyl iodide have a relatively small difference in boiling point from each other. The method disclosed in PTL 1, when employing acetic acid as the absorbing solvent, tends to cause acetic acid to corrode the interior of a distillation column during separation, by distillation using the distillation column, of methyl iodide from the solution after absorption. In addition and disadvantageously, the method disclosed in PTL 1 is insufficient in recovery of hydrogen iodide, and can hardly recover methyl iodide, which is a useful component, and hydrogen iodide, which is to be removed, separately from each other, because these two components are recovered together. 
     Accordingly, the present invention has an object to provide an acetic acid production method that enables, in a scrubbing system, at least one of efficient separation and recovery of methyl iodide from a solution containing methyl iodide; restrainment of corrosion of the interior of a distillation column during distillation of the solution; efficient separation and recovery of hydrogen iodide and methyl iodide from an offgas from the acetic acid production process; and sufficient recovery of hydrogen iodide. 
     Solution to Problem 
     To achieve the object, the inventor of the present invention made intensive investigations with focusing the solubility of hydrogen iodide and methyl iodide, the difference in boiling point between methyl iodide and an absorbing solvent, and the sequence of absorptions of hydrogen iodide and methyl iodide. As a result, the inventor found that the use of, as the absorbing solvent, a solvent having a large difference in boiling point from methyl iodide enables efficient separation of methyl iodide by distillation. The inventor also found that the use of an absorbing solvent that is less corrosive can restrain the corrosion of the interior of the distillation column. The inventor also found the use of two different absorbing solvents differing in composition, as an absorbing solvent to absorb hydrogen iodide and another absorbing solvent to absorb methyl iodide: and found that the use enables efficient separation between, and recovery of, hydrogen iodide and methyl iodide form an offgas from the acetic acid production process. In addition, the inventor found that the use of a specific absorbing solvent enables sufficient absorption of hydrogen iodide. The present invention has been made on the basis of these findings and further investigations. 
     Specifically, the present invention provides, in one aspect, an acetic acid production method that includes a first absorption step and a second absorption step. 
     In the first absorption step, at least a portion of offgases formed in an acetic acid production process is fed to an absorption column and is brought into contact with a first absorbent in the absorption column to allow the first absorbent to absorb an iodine compound from the offgas, where the first absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. 
     In the second absorption step, the first gas is brought into contact with a second absorbent in an absorption column to allow the second absorbent to absorb an iodine compound from the first gas, where the second absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, basic aqueous solutions, and acetic acid, and where the second absorbent differs in composition from the first absorbent. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. 
     The present invention also provides, in another aspect, an acetic acid production method that includes a carbonylation step (reaction step) and a separation step and may further include an acetaldehyde-removing system. This acetic acid production method includes a first absorption step and a second absorption step. 
     In the carbonylation step, methanol is reacted, in a reactor, with carbon monoxide in the presence of a catalytic system, acetic acid, methyl acetate, and water to form acetic acid, where the catalytic system includes a metal catalyst and methyl iodide. 
     In the separation step, a reaction mixture from the carbonylation step is separated, using at least one selected from evaporators and distillation columns, into a stream including the metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in light ends than the acetic acid stream. 
     In the acetaldehyde-removing system, acetaldehyde is removed, using a distillation column or columns, from at least a portion of a condensate resulting from condensation of the stream richer in light ends. 
     In the first absorption step, at least one offgas is fed to an absorption column, where the at least one offgas is selected from the group consisting of an offgas from the reactor, an offgas from the evaporator or evaporators, an offgas from the distillation column or columns in the separation step, and an offgas from the distillation column or columns in the acetaldehyde-removing system. The fed offgas is brought into contact with a first absorbent in the absorption column to allow the first absorbent to absorb an iodine compound from the offgas, where the first absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. 
     In the second absorption step, the first gas is brought into contact with a second absorbent in an absorption column to allow the second absorbent to absorb an iodine compound from the first gas, where the second absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, basic aqueous solutions, and acetic acid, and where the second absorbent differs in composition from the first absorbent. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. 
     The first absorbent preferably includes water. 
     The second absorbent preferably includes acetic acid. 
     One of the first absorbent and the second absorbent preferably has a water concentration of 10 ppm by mass or more. 
     Preferably, of the first absorbent and the second absorbent, one has a water concentration of 10 ppm by mass or more, and the other has an acetic acid concentration of 10 ppm by mass or more. 
     The acetic acid production methods preferably include a stripping step by which the second solution is separated, by distillation, into an overhead stream rich in methyl iodide, and a bottom stream rich in acetic acid. The methyl iodide-rich overhead stream may be recycled to a reaction step. 
     A charge liquid to a distillation column in the stripping step preferably has a methyl iodide concentration of 100 ppm by mass or more. 
     A charge liquid to the distillation column in the stripping step preferably has a hydrogen iodide concentration of less than 100 ppm by mass. 
     The present invention also provides, in yet another aspect, an acetic acid production method that includes a carbonylation step (reaction step), an evaporation step, a light ends-removing step, and a dehydration step. The method may further include at least one of a heavy ends-removing step and an acetaldehyde-removing system. This acetic acid production method includes a first absorption step and a second absorption step. 
     In the carbonylation step, methanol is reacted, in a reactor, with carbon monoxide in the presence of a catalytic system, acetic acid, methyl acetate, and water to form acetic acid, where the catalytic system includes a metal catalyst and methyl iodide. 
     In the evaporation step, a reaction mixture from the carbonylation step is separated, using an evaporator, into a vapor stream and a residue stream. 
     In the light ends-removing step, the vapor stream is separated, by distillation, into an overhead stream rich in light ends, and a first acetic acid stream rich in acetic acid. 
     In the dehydration step, the first acetic acid stream is separated, by distillation, into an overhead stream rich in water, and a second acetic acid stream richer in acetic acid than the first acetic acid stream. 
     In the heavy ends-removing step, the second acetic acid stream is separated, by distillation, into a bottom stream rich in heavy ends, and a third acetic acid stream richer in acetic acid than the acetic acid stream before being subjected to the distillation. 
     In the acetaldehyde-removing system, acetaldehyde is separated, using a distillation column or columns, from at least a portion of a condensate resulting from condensation of the overhead stream rich in light ends. 
     In the first absorption step, at least one offgas is fed to an absorption column, where the at least one offgas is selected from the group consisting of an offgas from the reactor, an offgas from the evaporator or evaporators, an offgas from a distillation column in the light ends-removing step, an offgas from a distillation column in the dehydration step, an offgas from a distillation column in the heavy ends-removing step, and an offgas from the distillation column or columns in the acetaldehyde-removing system. The fed offgas is brought into contact with a first absorbent in the absorption column to allow the first absorbent to absorb an iodine compound from the offgas, where the first absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. 
     In the second absorption step, the first gas is brought into contact with a second absorbent in an absorption column to allow the second absorbent to absorb an iodine compound from the first gas, where the second absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, basic aqueous solutions, and acetic acid and where the second absorbent differs in composition from the first absorbent. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. 
     One of the first absorbent and the second absorbent preferably has a water concentration of 10 ppm by mass or more. 
     Preferably, of the first absorbent and the second absorbent, one has a water concentration of 10 ppm by mass or more, and the other has an acetic acid concentration of 10 ppm by mass or more. 
     The first solution may be recycled to at least one apparatus selected from the group consisting of the reactor, a decanter for reserving the condensate, a distillation column in the dehydration step, and a distillation column in the heavy ends-removing step. 
     The acetic acid production method may include a stripping step by which the second solution is separated, by distillation, into an overhead stream rich in methyl iodide, and a bottom stream rich in acetic acid. 
     A charge liquid to a distillation column in the stripping step preferably has a methyl iodide concentration of 100 ppm by mass or more. 
     A charge liquid to the distillation column in the stripping step preferably has a hydrogen iodide concentration of less than 100 ppm by mass. 
     The overhead stream rich in methyl iodide may be recycled to at least one step selected from the group consisting of the reaction step, the evaporation step, and distillation steps. 
     The bottom stream rich in acetic acid may be recycled to at least one step selected from the group consisting of the reaction step, the evaporation step, the light ends-removing step, the dehydration step, and the heavy ends-removing step. 
     The first absorption step and the second absorption step may be performed individually in different absorption columns. 
     Advantageous Effects of Invention 
     The present invention, when employing as an absorbent a solvent including at least one of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions, enables efficient separation of methyl iodide by distillation. This is because the absorbent used herein has a larger difference in boiling point from methyl iodide, as compared with the case where methanol or methyl acetate is used as the absorbent. In addition to or instead of this, the configuration enables restrainment of corrosion of the interior of a distillation column as compared with the case where acetic acid is used as the absorbent. The present invention, when employing a solvent including water or a basic aqueous solution as the absorbent, enables sufficient absorption of hydrogen iodide. In addition, the present invention employs two absorption steps using two different absorbents differing in composition. This configuration enables efficient separation between, and recovery of, hydrogen iodide and methyl iodide. This is because the two different absorbents have different solving powers to hydrogen iodide and methyl iodide, and, one absorption step using one of the two absorbents gives a solution enriched with one of hydrogen iodide and methyl iodide; and, the other absorption step using the other absorbent gives a solution enriched with the other of hydrogen iodide and methyl iodide, as compared with the former solution. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts a schematic flow chart illustrating a scrubbing system according to an embodiment; 
         FIG. 2  depicts a schematic flow chart illustrating a scrubbing system according to another embodiment; 
         FIG. 3  depicts a schematic flow chart illustrating a scrubbing system according to yet another embodiment; 
         FIG. 4  is a production flow chart illustrating an acetic acid production system according to an embodiment; 
         FIG. 5  depicts a schematic flow chart illustrating an acetaldehyde-removing system according to an embodiment; 
         FIG. 6  depicts a schematic flow chart illustrating an acetaldehyde-removing system according to another embodiment; 
         FIG. 7  depicts a schematic flow chart illustrating an acetaldehyde-removing system according to yet another embodiment; 
         FIG. 8  depicts a schematic flow chart illustrating an acetaldehyde-removing system according to still another embodiment; and 
         FIGS. 9( a ) and 9( b )  depict schematic flow charts illustrating the structures of scrubbing systems used in examples and comparative examples. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The acetic acid production method according to an embodiment of the present invention includes a first absorption step and a second absorption step. In the first absorption step, at least a portion of offgases formed in an acetic acid production process is fed to an absorption column and is brought into contact with a first absorbent in the absorption column to allow the first absorbent to absorb an iodine compound from the offgas, where the first absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. In the second absorption step, the first gas is brought into contact with a second absorbent in an absorption column to allow the second absorbent to absorb an iodine compound from the first gas, where the second absorbent includes at least one liquid selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, basic aqueous solutions, and acetic acid, and where the second absorbent differs in composition from the first absorbent. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. Each of the first absorbent and the second absorbent may include each of different solvents (liquids) as above alone or in combination. The categories C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, and ketones may overlap one another. 
     In the acetic acid production method according to the present invention, at least a portion of offgases formed in the acetic acid production process is fed to an absorption column or columns and is subjected to the first and second absorption steps. Non-limiting examples of the offgas to be fed to the first and second absorption steps include an offgas from a reactor in a reaction step, an offgas from an evaporator in an evaporation step, an offgas from a distillation column in a separation step, and an offgas from a distillation column in an acetaldehyde-removing system, where the steps and apparatuses will be described later. 
     Each of the first and second absorption steps may be independently performed in one absorption column or two or more absorption columns. For example, assume that offgases from two or more processes are subjected to the first absorption step. In this case, the first absorption step may be performed by an adsorption technique using two or more absorption columns (e.g., a high-pressure absorption column in combination with a low-pressure absorption column), because the offgases differ from each other in composition and/or in pressure. The first and second absorption steps may be performed in a single absorption column. 
     Non-limiting examples of the C2 or higher alcohols for use in the first and second absorbents include monohydric alcohols such as ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, t-butyl alcohol, cyclohexanol, ethylene glycol monoethyl ether, and ethylene glycol monobutyl ether; and polyhydric alcohols such as ethylene glycol, propylene glycol, and diethylene glycol. 
     Non-limiting examples of the esters of C3 or higher carboxylic acids include monocarboxylic esters exemplified typically by propionic acid esters such as methyl propionate and ethyl propionate, lactic acid esters such as methyl lactate and ethyl lactate, and butyric acid esters such as methyl butyrate and ethyl butyrate; and dicarboxylic esters such as adipic acid esters. 
     Non-limiting examples of the esters between carboxylic acids and C2 or higher alcohols include carboxylic acid ethyl esters such as ethyl acetate, ethyl propionate, ethyl lactate, and ethyl butyrate; carboxylic acid propyl esters such as propyl acetate; and alkylene glycol acetates such as ethylene glycol monoethyl ether acetate. 
     Non-limiting examples of the ethers include aliphatic ethers such as dimethyl ether, methyl ethyl ether, diethyl ether, ethylene glycol monoethyl ether, and ethylene glycol monobutyl ether; aromatic ethers such as diphenyl ether; cyclic ethers such as tetrahydrofuran; and alkylene glycol ether acetates such as ethylene glycol monoethyl ether acetate. 
     Non-limiting examples of the ketones include aliphatic ketones such as acetone, methyl ethyl ketone, dimethyl ketone, ethyl methyl ketone, and diethyl ketone; aromatic ketones such as diphenyl ketone; and alicyclic ketones such as cyclohexanone. 
     Non-limiting examples of the basic aqueous solutions include aqueous solutions of alkali metal salts such as lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium acetate, sodium carbonate, and sodium hydrogencarbonate; aqueous solutions of alkaline earth metal salts such as magnesium hydroxide, calcium hydroxide, and barium hydroxide; and aqueous solutions of carboxylic acid salts such as sodium acetate. 
     The second absorbent is an absorbent that differs in composition from the first absorbent. Non-limiting examples of this case include the case where a component that is not contained in one absorbent is contained in the other absorbent; and the case where the two absorbents are identical in component or components but are different from each other in content (proportion) of at least one component. 
     Assume that at least one selected from C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water, and basic aqueous solutions is used as one of the first absorbent and the second absorbent. This configuration enables efficient separation of methyl iodide in a stripping step described later, where the stripping step is the step of separating, by distillation, methyl iodide from the first solution (solution containing methyl iodide and the first absorbent) resulting from the first absorption step, or from the second solution (solution containing methyl iodide and the second absorbent) resulting from the second absorption step. This is because the solvent herein has a larger difference in boiling point from methyl iodide, as compared with the case where methanol or methyl acetate is used as an absorbent. In addition to or instead of this, the configuration enables restrainment of the corrosion of the interior of a distillation column as compared with the case where acetic acid is used as an absorbent, because the absorbent used herein less causes corrosion of metals, as compared with acetic acid. In general, when a mixture of acetic acid and methyl iodide is subjected to distillation, acetic acid is concentrated at the column bottom and may corrode the column bottom, because the temperature of the column bottom is raised by heating. Assume that water is used as one of the first absorbent and the second absorbent. This absorbent can sufficiently absorb hydrogen iodide, because water has high solving power to hydrogen iodide. Also assume that a basic aqueous solution is used as one of the first absorbent and the second absorbent. This absorbent can also sufficiently absorb hydrogen iodide, because the basic aqueous solution not only has, due to water therein, high solving power to hydrogen iodide, but also neutralizes hydrogen iodide. In addition, the absorption step herein is performed as two steps using two different absorbents differing in composition, i.e., the first and second absorbents. This configuration enables efficient separation between, and recovery of, hydrogen iodide and methyl iodide. This is because the two different absorbents have different solving powers to hydrogen iodide and methyl iodide, and, one absorption step using one of the two absorbents gives a solution enriched with one of hydrogen iodide and methyl iodide; and, the other absorption step using the other absorbent gives a solution enriched with the other of hydrogen iodide and methyl iodide, as compared with the former solution. 
     The first absorption step is the step of bringing an offgas formed in the process into contact (in particular, countercurrent contact) with the first absorbent to allow the first absorbent to absorb an iodine compound from the offgas, and whereby a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. Specifically, the offgas is continuously introduced into an absorption column with which the first absorption step is performed; whereas the first absorbent is continuously introduced through a line into the absorption column, where the line is coupled to the absorption column at a height higher than the offgas feeding height. In the column, the ascending offgas and the descending first absorbent are brought into countercurrent contact with each other, to allow the first absorbent to absorb an iodine compound from the offgas. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. The first absorption step may be performed using one absorption column or two or more absorption columns. 
     The second absorption step is the step of bringing the first gas, which has an iodine compound concentration lowered by the first absorption step, into contact (in particular, countercurrent contact) with the second absorbent and thereby a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. Specifically, the second absorbent is continuously introduced through a line into an absorption column with which the second absorption step is performed, where the line is coupled to the absorption column at a height higher than the first gas feeding height. In the column, the ascending first gas and the descending second absorbent are brought into countercurrent contact with each other, to allow the second absorbent to absorb an iodine compound from the first gas. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. The second absorption step may be performed using one absorption column or two or more absorption columns. Assume that the first absorption step is performed using two or more absorption columns. In this case, first gases from the two or more absorption columns may be merged and then subjected to the second absorption step, or may be fed individually to one or more absorption columns and be subjected to the second absorption step. The first and second absorption steps may be performed using a single absorption column or using two or more different absorption columns. 
     Non-limiting examples of the iodine compound(s) to be absorbed by the absorbents in the first and second absorption steps include iodine compounds present in the process, including hydrogen iodide; and alkyl iodides such as methyl iodide, ethyl iodide, and hexyl iodide. Among them, the iodine compounds to be absorbed by the absorbents are preferably hydrogen iodide and methyl iodide, where hydrogen iodide may corrode the interior of a distillation column, and methyl iodide is a useful component usable in the reaction step. In particular, it is preferred that the iodine compound to be absorbed in the first absorption step is hydrogen iodide, and the iodine compound to be absorbed in the second absorption step is methyl iodide. Assume that the first and second absorption steps are performed using different absorption columns. In this case, when hydrogen iodide is sufficiently absorbed in the first absorption step, the resulting first gas has a very low hydrogen iodide concentration. This less causes corrosion in an absorption column used in the second absorption step and allows the absorption column to be made of a material having low corrosion resistance (low-grade material). In addition, such significantly lowered hydrogen iodide concentration in the first gas to be fed to the second absorption step allows the resulting second solution to have a lower hydrogen iodide concentration, because hydrogen iodide is little absorbed by the second absorbent in the second absorption step. This less causes corrosion in the distillation column during distillation of the second solution in the stripping step, and allows the distillation column to be made of a low-grade material. Each of different iodine compounds may be absorbed alone or in combination in each of the first and second absorption steps. 
     Accordingly, the first absorbent preferably includes water or a basic aqueous solution, from the viewpoint of high hydrogen iodide absorptivity; and the second absorbent preferably includes at least one solvent selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, and acetic acid, from the viewpoint of high methyl iodide absorptivity. In this case, hydrogen iodide and methyl iodide can be efficiently separated from each other and recovered. This is because hydrogen iodide can be sufficiently recovered from the offgas by the action of the first absorbent in the first absorption step; whereas methyl iodide is little absorbed by the first absorbent, but sufficiently recovered by the action of the second absorbent in the second absorption step. Among them, the second absorbent preferably includes acetic acid, from the viewpoint of eliminating or minimizing contamination of the reaction system or the purification system with unnecessary components when methyl iodide is recycled to the system. The use of the first and second absorbents as above enables selective recovery and removal of hydrogen iodide and methyl iodide respectively in the first absorption step and in the second absorption step. This enables recovery of methyl iodide containing approximately no unnecessary hydrogen iodide and allows such methyl iodide to be easily reused in the reactor. In addition, the configuration can restrain the corrosion of a distillation column during purification, by distillation using the distillation column, of acetic acid used as the absorbent in the second absorption step. 
     Each of the first and second absorbents may independently include one or more components other than the liquid(s). Non-limiting examples of the other components include solvents other than the liquids, exemplified typically by methanol; esters of C1 or C2 carboxylic acids, such as methyl acetate; methyl esters of C3 or higher carboxylic acids; carboxylic acids such as acetic acid (in the case of the first absorbent) and formic acid. Non-limiting examples of the other components also include impurities used or formed in the acetic acid production process. 
     The concentration of the at least one solvent (liquid) in the first absorbent is typically 10 ppm by mass or more, preferably 20 ppm by mass or more, more preferably 50 ppm by mass or more, furthermore preferably 100 ppm by mass or more, and particularly preferably 200 ppm by mass or more, or may be 300 ppm by mass or more, 400 ppm by mass or more, 500 ppm by mass or more, 1000 ppm by mass or more, 1 mass percent or more, 5 mass percent or more, 10 mass percent or more, 20 mass percent or more, 30 mass percent or more, 40 mass percent or more, 50 mass percent or more, 60 mass percent or more, 70 mass percent or more, 80 mass percent or more, or 90 mass percent or more, where the at least one solvent is selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, and water (including the case where a basic aqueous solution is used). The upper limit of the concentration is 100 mass percent, but may be 99.999 mass percent, 99.99 mass percent, 99.9 mass percent, 99.5 mass percent, 99 mass percent, or 98 mass percent. The water concentration in the first absorbent may fall within the ranges. 
     The concentration of the at least one solvent in the second absorbent is typically 10 ppm by mass or more, preferably 20 ppm by mass or more, more preferably 50 ppm by mass or more, furthermore preferably 100 ppm by mass or more, and particularly preferably 200 ppm by mass or more, or may be 300 ppm by mass or more, 400 ppm by mass or more, 500 ppm by mass or more, 1000 ppm by mass or more, 1 mass percent or more, 5 mass percent or more, 10 mass percent or more, 20 mass percent or more, 30 mass percent or more, 40 mass percent or more, 50 mass percent or more, 60 mass percent or more, 70 mass percent or more, 80 mass percent or more, or 90 mass percent or more, where the at least one solvent is selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, water (including the case where a basic aqueous solution is used), and acetic acid. The upper limit of the concentration is 100 mass percent, but may be 99.999 mass percent, 99.99 mass percent, 99.9 mass percent, 99.5 mass percent, 99 mass percent, or 98 mass percent. In particular, in the second absorbent, the concentration of at least one solvent selected from the group consisting of C2 or higher alcohols, esters of C3 or higher carboxylic acids, esters between carboxylic acids and C2 or higher alcohols, ethers, ketones, and acetic acid (in particular, acetic acid concentration) preferably falls within the ranges. The acetic acid concentration in the second absorbent may fall within the ranges. 
     Of the first absorbent and the second absorbent, the water concentration in one absorbent is typically 10 ppm by mass or more, preferably 20 ppm by mass or more, more preferably 50 ppm by mass or more, furthermore preferably 100 ppm by mass or more, and particularly preferably 200 ppm by mass or more, or may be 300 ppm by mass or more, 400 ppm by mass or more, 500 ppm by mass or more, 1000 ppm by mass or more, 1 mass percent or more, 1000 ppm by mass or more, 1 mass percent or more, 5 mass percent or more, 10 mass percent or more, 20 mass percent or more, 30 mass percent or more, 40 mass percent or more, 50 mass percent or more, 60 mass percent or more, 70 mass percent or more, 80 mass percent or more, or 90 mass percent or more. The upper limit of the water concentration is 100 mass percent, but may be 99.999 mass percent, 99.99 mass percent, 99.9 mass percent, 99.5 mass percent, 99 mass percent, or 98 mass percent. 
     Hydrogen iodide includes molecular hydrogen iodide. When at least a portion of hydrogen iodide is ionized in a polar medium (generally a medium containing water), hydrogen iodide includes both molecular hydrogen iodide and dissociated hydroiodic acid. The two forms are convertible to each other. The hydrogen iodide concentration herein can be determined by potentiometric titration, or by subtraction technique in which the concentration is determined by subtracting other ionic iodides from the totality of ionic iodides. 
     By the potentiometric titration, the hydrogen iodide concentration is determined through acid-base titration using a potentiometric titration end-point. In particular, the hydrogen iodide concentration may be determined by performing titration to the potentiometric titration end-point typically with a standard lithium acetate solution. The subtraction technique is the technique of determining the hydrogen iodide concentration by subtracting the concentrations of iodides that are assumed to be involved in measurement of corrosion metals or non-hydrogen cations from the total concentration of all ionic iodides present in the sample. 
     The hydrogen iodide concentration in the first solution, as determined by the subtraction technique, is typically 0.01 ppm by mass or more, or may be 0.1 ppm by mass or more, 1 ppm by mass or more, 10 ppm by mass or more, 50 ppm by mass or more, 100 ppm by mass or more, 200 ppm by mass or more, 300 ppm by mass or more, 400 ppm by mass or more, 500 ppm by mass or more, 600 ppm by mass or more, 700 ppm by mass or more, 800 ppm by mass or more, 900 ppm by mass or more, 1000 ppm by mass or more, 2000 ppm by mass or more, 3000 ppm by mass or more, 4000 ppm by mass or more, 5000 ppm by mass or more, 6000 ppm by mass or more, 7000 ppm by mass or more, 8000 ppm by mass or more, 9000 ppm by mass or more, or 1 mass percent or more. The hydrogen iodide concentration is typically 10 mass percent or less, or may be 5 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, or 3000 ppm by mass or less. 
     The hydrogen iodide concentration in the first solution, as determined by potentiometric titration, is typically 0.01 ppm by mass or more, or may be 0.1 ppm by mass or more, 1 ppm by mass or more, 10 ppm by mass or more, 50 ppm by mass or more, 100 ppm by mass or more, 200 ppm by mass or more, 300 ppm by mass or more, 400 ppm by mass or more, 500 ppm by mass or more, 600 ppm by mass or more, 700 ppm by mass or more, 800 ppm by mass or more, 900 ppm by mass or more, 1000 ppm by mass or more, 2000 ppm by mass or more, 3000 ppm by mass or more, 4000 ppm by mass or more, 5000 ppm by mass or more, 6000 ppm by mass or more, 7000 ppm by mass or more, 8000 ppm by mass or more, 9000 ppm by mass or more, or 1 mass percent or more. The hydrogen iodide concentration is typically 5 mass percent or less, and preferably 2 mass percent or less. The hydrogen iodide concentration is typically 10 mass percent or less, or may be 5 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, or 3000 ppm by mass or less. 
     The methyl iodide concentration in the first solution is typically 30 mass percent or less, or may be 25 mass percent or less, 20 mass percent or less, 15 mass percent or less, 10 mass percent or less, 7 mass percent or less, 5 mass percent or less, 4 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, 2000 ppm by mass or less, or 1000 ppm by mass or less. The methyl iodide concentration is typically 10 ppm by mass or more (e.g., 50 ppm by mass or more), preferably 100 ppm by mass or more (e.g., 500 ppm by mass or more), and more preferably 1000 ppm by mass or more (e.g., 2000 ppm by mass or more). 
     When the method includes two or more first absorption steps, the concentrations of the individual components in the first solution correspond to the concentrations of the individual components in all the first solutions separated and obtained in the two or more first absorption steps. 
     The hydrogen iodide concentration in the second solution, as determined by the subtraction technique, is preferably lower than the hydrogen iodide concentration in the first solution, is typically 5 mass percent or less, or may be 4 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, 3000 ppm by mass or less, 2000 ppm by mass or less, 1000 ppm by mass or less, 700 ppm by mass or less, 500 ppm by mass or less, 300 ppm by mass or less, 200 ppm by mass or less, 100 ppm by mass or less, less than 100 ppm by mass, 50 ppm by mass or less, 30 ppm by mass or less, 20 ppm by mass or less, 10 ppm by mass or less, 5 ppm by mass or less, 3 ppm by mass or less, 2 ppm by mass or less, or 1 ppm by mass or less. The hydrogen iodide concentration is typically 0.0001 ppm by mass or more, or may be 0.001 ppm by mass or more, 0.01 ppm by mass or more, 0.1 ppm by mass or more, or 0.5 ppm by mass or more. 
     The hydrogen iodide concentration in the second solution, as determined by potentiometric titration, is preferably lower than the hydrogen iodide concentration in the first solution, is typically 5 mass percent or less, or may be 4 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, 3000 ppm by mass or less, 2000 ppm by mass or less, 1000 ppm by mass or less, 700 ppm by mass or less, 500 ppm by mass or less, 300 ppm by mass or less, 200 ppm by mass or less, 100 ppm by mass or less, less than 100 ppm by mass, 50 ppm by mass or less, 30 ppm by mass or less, 20 ppm by mass or less, 10 ppm by mass or less, 5 ppm by mass or less, 3 ppm by mass or less, 2 ppm by mass or less, or 1 ppm by mass or less. The hydrogen iodide concentration is typically 0.0001 ppm by mass or more, or may be 0.001 ppm by mass or more, 0.01 ppm by mass or more, 0.1 ppm by mass or more, or 0.5 ppm by mass or more. 
     The methyl iodide concentration in the second solution is preferably higher than the methyl iodide concentration in the first solution, is typically 1 ppm by mass or more, or may be 10 ppm by mass or more, 100 ppm by mass or more, 1000 ppm by mass or more, 5000 ppm by mass or more, or 1 mass percent or more. The methyl iodide concentration is typically 20 mass percent or less (e.g., 15 mass percent or less), and preferably 10 mass percent or less (e.g., 8 mass percent or less). 
     When the method includes two or more second absorption steps, the concentrations of the individual components in the second solution correspond to the concentrations of the components in all the second solutions separated and obtained in the two or more second absorption steps. 
     The acetic acid production method according to the present invention may include a distillation step (stripping step) of subjecting, to distillation, the first solution from the first absorption step and/or the second solution from the second absorption step. By the stripping step, the first and/or second solution is subjected to distillation and is separated into an overhead stream rich in components having lower boiling points than the first and/or second absorbent, and a bottom stream rich in the first and/or second absorbent. The method, when including the stripping step, is excellent in economic efficiency, because this configuration enables separation between useful components and the bottom stream rich in the first and/or second absorbent, allows the useful components to be recycled typically to at least one of the reactor, the evaporator, and the distillation columns, and allows the separated absorbent to be reused as an absorbent in the first and/or second absorption step. 
     The methyl iodide concentration in a charge liquid to a distillation column in the stripping step is typically 1 ppm by mass or more, or may be 10 ppm by mass or more, 100 ppm by mass or more, 1000 ppm by mass or more, 5000 ppm by mass or more, or 1 mass percent or more. The methyl iodide concentration is typically 20 mass percent or less (e.g., 15 mass percent or less), and preferably 10 mass percent or less (e.g., 8 mass percent or less). 
     The hydrogen iodide concentration in the charge liquid to the distillation column in the stripping step is typically 5 mass percent or less, or may be 4 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, 3000 ppm by mass or less, 2000 ppm by mass or less, 1000 ppm by mass or less, 700 ppm by mass or less, 500 ppm by mass or less, 300 ppm by mass or less, 200 ppm by mass or less, 100 ppm by mass or less, less than 100 ppm by mass, 50 ppm by mass or less, 30 ppm by mass or less, 20 ppm by mass or less, 10 ppm by mass or less, 5 ppm by mass or less, 3 ppm by mass or less, 2 ppm by mass or less, or 1 ppm by mass or less. The hydrogen iodide concentration is typically 0.0001 ppm by mass or more, or may be 0.001 ppm by mass or more, 0.01 ppm by mass or more, 0.1 ppm by mass or more, 0.5 ppm by mass or more, or 1 ppm by mass or more. 
     The hydrogen iodide concentration in the charge liquid to the distillation column in the stripping step, as determined by potentiometric titration, is typically 5 mass percent or less, or may be 4 mass percent or less, 3 mass percent or less, 2 mass percent or less, 1 mass percent or less, 5000 ppm by mass or less, 3000 ppm by mass or less, 2000 ppm by mass or less, 1000 ppm by mass or less, 700 ppm by mass or less, 500 ppm by mass or less, 300 ppm by mass or less, 200 ppm by mass or less, 100 ppm by mass or less, less than 100 ppm by mass, 50 ppm by mass or less, 30 ppm by mass or less, 20 ppm by mass or less, 10 ppm by mass or less, 5 ppm by mass or less, 3 ppm by mass or less, 2 ppm by mass or less, or 1 ppm by mass or less. The hydrogen iodide concentration is typically 0.0001 ppm by mass or more, or may be 0.001 ppm by mass or more, 0.01 ppm by mass or more, 0.1 ppm by mass or more, 0.5 ppm by mass or more, or 1 ppm by mass or more. 
     Non-limiting examples of the light ends to be concentrated in the overhead stream resulting from separation in the stripping step include iodine compounds (such as methyl iodide and hydrogen iodide), water, methyl acetate, dimethyl ether, methanol, acetaldehyde, and formic acid. At least a portion of the overhead stream, when including useful components such as methyl iodide, may be recycled to the reactor (reaction step). The recycling to the reactor allows the useful components to be reused in the reaction step, and this offers excellent economic efficiency. 
     Assume that the offgas to be subjected to the first absorption step is an offgas from a decanter for reserving a condensate resulting from condensation of an overhead stream from the after-mentioned light ends-removing step. In this case, the overhead stream from the stripping step may include a large amount of acetaldehyde. Accordingly, it is acceptable that, before being recycled through the decanter to the reactor, the overhead stream from the stripping step is fed to the acetaldehyde-removing system, and acetaldehyde is removed therefrom by the working of the acetaldehyde-removing system. The overhead stream may also be recycled to a distillation step or steps (such as a light ends-removing step, a dehydration step, and/or a heavy ends-removing step) downstream from the evaporation step. 
     The methyl iodide concentration in the overhead stream from the stripping step is typically 5 mass percent or more, or may be 10 mass percent or more, 20 mass percent or more, 30 mass percent or more, 40 mass percent or more, 50 mass percent or more, 60 mass percent or more, 70 mass percent or more, or 80 mass percent or more. The upper limit of the methyl iodide concentration is typically 99.9 mass percent (e.g., 99 mass percent), preferably 98 mass percent (e.g., 95 mass percent), and more preferably 93 mass percent (90 mass percent), or may be 80 mass percent, 70 mass percent, 60 mass percent, 50 mass percent, or 45 mass percent. 
     Of the bottom stream, which is separated and obtained from the column bottom of the distillation column in the stripping step, at least a portion may be continuously discharged out of the system; and at least a portion may be cycled to at least one of the first and second absorption columns. At least a portion of the bottom stream may be recycled typically to at least one of the reaction step, the evaporation step, and purification steps (distillation steps) downstream from the evaporation step. 
     Hereinafter scrubbing systems including the first and second absorption steps according to embodiments of the present invention will be illustrated.  FIG. 1  depicts an exemplary schematic flow chart illustrating a scrubbing system for use in the present invention, according to an embodiment. This scrubbing system  8  includes an absorption column  81 , an absorption column  82 , an absorption column  83 , and a distillation column  84 . The acetic acid production method according to the embodiment performs first absorption steps in the absorption columns  81  and  82 ; a second absorption step in the absorption column  83 ; and a stripping step in the distillation column  84 . 
     The absorption column  81  is a unit (high-pressure absorption column) to perform a first absorption step of absorbing and recovering an iodine compound from, of offgases, a high-pressure gas. This first absorption step is the step of bringing the high-pressure gas into contact with a first absorbent to allow the first absorbent to absorb an iodine compound from the high-pressure gas, and thereby into a first gas having a lower iodine compound concentration as compared with the high-pressure gas, and a first solution containing the iodine compound and the first absorbent are to be separated. 
     Specifically, the high-pressure gas is continuously introduced through a line  58  (high-pressure gas feed line) to the absorption column  81 ; whereas the first absorbent is continuously introduced through a line  59  (first absorbent feed line) to the absorption column  81 , where the line  59  is coupled to the absorption column  81  at a height higher than the high-pressure gas feeding height. In the column, the ascending high-pressure gas and the descending first absorbent are brought into countercurrent contact with each other, to allow the first absorbent to absorb an iodine compound from the high-pressure gas. Thus, the first gas having a lower iodine compound concentration as compared with the high-pressure gas, and the first solution containing the iodine compound and the first absorbent are to be separated. At the absorption column  81 , the first gas is obtained from the column top through a line  60 ; and the first solution is obtained from the column bottom through a line  61 . 
     A non-limiting example of the high-pressure gas is an offgas from the reactor (or reaction step). In this case, the offgas discharged from the reactor may be fed through the line  58  to the absorption column  81  directly, or as a non-condensable gas resulting from separation, using a condenser, of the offgas into a condensate and the non-condensable gas. The temperature of the first absorbent before being fed to the absorption column  81  is typically 1° C. to 120° C. and is a temperature within such a range that the first absorbent is neither frozen nor boiled. 
     The absorption column  81  is selected from rectification columns such as a plate column and a packed column. The packing in the packed column may be any of structured packings and dumped packings. The absorption column  81 , when being a plate column, typically has 1 to 100 theoretical plates. The column internal pressure is typically from atmospheric pressure to 5 MPaG (gauge pressure) and is generally equal to or lower than the reactor internal pressure. The column internal temperature is typically about 1° C. to about 120° C. 
     The absorption column  82  is a unit (low-pressure absorption column) to perform a first absorption step of absorbing and recovering an iodine compound from, of offgases, a low-pressure gas. This first absorption step is the step of bringing the low-pressure gas into contact with the first absorbent to allow the first absorbent to absorb an iodine compound from the low-pressure gas, and thereby into a first gas having a lower iodine compound concentration as compared with the low-pressure gas, and a first solution containing the iodine compound and the first absorbent are to be separated. 
     Specifically, the low-pressure gas is continuously introduced through a line  62  (low-pressure gas feed line) into the absorption column  82 ; whereas the first absorbent is continuously introduced through a line  63  (first absorbent feed line) into the absorption column  82 , where the line  63  is coupled to the absorption column  82  at a height higher than the low-pressure gas feeding height. In the column, the ascending low-pressure gas and the descending first absorbent are brought into countercurrent contact with each other, to allow the first absorbent to absorb an iodine compound from the low-pressure gas. Thus, the first gas having a lower iodine compound concentration as compared with the low-pressure gas, and the first solution containing the iodine compound and the first absorbent are to be separated. At the absorption column  82 , the first gas is obtained from the column top through a line  64 , and the first solution is obtained from the column bottom through a line  65 . 
     Non-limiting examples of the low-pressure gas include an offgas from the evaporator (or evaporation step); an offgas from the light ends column (or light ends-removing step); an offgas from a decanter for reserving a condensate resulting from condensation of the overhead stream rich in light ends from the light ends column; an offgas from the dehydration column (or dehydration step); and an offgas from the heavy ends column (or heavy ends-removing step). Each of these offgases may be fed through the line  62  to the absorption column  82  directly, or as a non-condensable gas resulting from separation, using a condenser, of the offgas into a condensate and the non-condensable gas. The temperature of the first absorbent before being fed to the absorption column  82  is as with the temperature of the first absorbent before being fed to the absorption column  81 . 
     The absorption column  82  is selected from rectification columns such as a plate column and a packed column. The packing in the packed column may be any of structured packings and dumped packings. The absorption column  82 , when being a plate column, typically has 1 to 100 theoretical plates. The column internal pressure is typically from atmospheric pressure to 5 MPaG and is generally equal to or lower than the reactor internal pressure. The column internal temperature is typically about 1° C. to about 120° C. 
     The first gas (line  60 ) from the column top of the absorption column  81  and the first gas (line  64 ) from the column top of the absorption column  82  are merged with each other and fed through the line  69  to the absorption column  83  with which the second absorption step is performed. On the other hand, the first solution (line  61 ) from the column bottom of the absorption column  81  and the first solution (line  65 ) from the column bottom of the absorption column  82  are merged with each other, a portion of which is discharged through a line  66  out of the system continuously or batchwise; whereas the remainder is combined with a fresh first absorbent fed through a line  67  continuously or batchwise. The resulting mixture is fed through a line  68 , divided into the lines  59  and  63 , and cycled respectively to the absorption columns  81  and  82 , and reused as the first absorbent in the first absorption steps. It is also acceptable that all the first solutions are discharged out of the system without cycling, and a fresh first absorbent is fed to the absorption columns  81  and  82 . At least a portion (e.g., the first solution discharged out of the system through the line  66 ) of the first solutions may be recycled to at least one of the reactor, the evaporator, and a distillation column with which a distillation step is performed. For example, assume that the first absorbent includes water. In this case, the first absorbent efficiently absorbs hydrogen iodide, and almost all the first solution is cycled to the first absorption columns and reused as the first absorbent, and when hydrogen iodide is concentrated, a portion of the first solution is recycled to the reactor. This is because water in the reactor is consumed by the shift reaction with carbon monoxide (H 2 O+CO→H 2 +CO 2 ). Water, when contained in a large amount in the first solutions, may not be sufficiently consumed in the reactor, and the first solutions may be recycled to at least one of the aqueous phase in the decanter, the dehydration column, and the heavy ends column, instead of, or in addition to, being recycled to the reactor. In this case, the component in the first solution is concentrated typically in the aqueous phase in the decanter, at the column top of the dehydration column, and at the column top of the heavy ends column; and, of the resulting substance, a portion is recycled to the reactor, and another portion is discharged out of the system. The embodiment illustrated in  FIG. 1  employs the same first absorbent in both the absorption columns  81  and  82 . However, in another embodiment, the method may employ different first absorbents and subject the different first absorbents typically to cycling, discharging out of the system, and/or recycling. 
     The absorption column  83  is a unit (generally, a low-pressure absorption column) to perform a second absorption step of absorbing and recovering an iodine compound from the first gas from the first absorption step. This second absorption step is the step of bringing the first gas into contact with the second absorbent to allow the second absorbent to absorb an iodine compound from the first gas, and thereby a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. 
     Specifically, the first gas is continuously introduced through the line  69  into the absorption column  83 , whereas the second absorbent is continuously introduced through a line  70  (second absorbent feed line) into the absorption column  83 , where the line  70  is coupled to the absorption column  83  at a height higher than the first gas feeding height. In the column, the ascending first gas and the descending second absorbent are brought into countercurrent contact with each other, to allow the second absorbent to absorb an iodine compound from the first gas. Thus, the second gas having a lower iodine compound concentration as compared with the first gas, and the second solution containing the iodine compound and the second absorbent are to be separated. At the absorption column  83 , the second gas is obtained from the column top through a line  71 , and the second solution is obtained from the column bottom through a line  72 . The temperature of the second absorbent before being fed to the absorption column  83  is typically 1° C. to 120° C. and is a temperature within such a range that the second absorbent is neither frozen nor boiled. 
     The second gas (line  71 ) from the column top of the absorption column  83  is discarded, because the second gas is a gas from which useful components have been collected and removed. The gas discharged from the line  71  is usable as a carbon monoxide source to be introduced into the bottom portion of the evaporator  2 , or into the residue stream recycle lines  18  and  19 . The second solution (line  72 ) from the column bottom of the absorption column  83  is fed to the distillation column  84 . 
     The absorption column  83  is selected from rectification columns such as a plate column and a packed column. The packing in the packed column may be any of structured packings and dumped packings. The absorption column  83 , when being a plate column, typically has 1 to 100 theoretical plates. The column internal pressure is typically from atmospheric pressure to 5 MPaG and is generally equal to or lower than the reactor internal pressure. The column internal temperature is typically about 1° C. to about 120° C. 
     The distillation column  84  is a unit with which a stripping step is performed. The stripping step in the embodiment is the step of separating, by distillation, the second solution into an overhead stream rich in useful components (in particular, methyl iodide), and a bottom stream rich in the second absorbent. At the distillation column  84 , vapors as the overhead stream are continuously drawn from the column top portion to a line  73 ; and bottoms are continuously drawn from the column bottom portion to a line  74 . There is disposed a reboiler  84   b.    
     More specifically, the second solution continuously introduced into the distillation column  84  is subjected to distillation treatment and separated into the overhead stream rich in useful components (in particular, methyl iodide), and the bottoms rich in the second absorbent. The distillation column  84  is selected typically from rectification columns such as a plate column and a packed column. The overhead stream from the distillation column  84  is introduced through the line  73  into a condenser  84   a . The condenser  84   a  cools and partially condenses the overhead stream from the distillation column  84  into a condensate and a gas. Of the condensate, a portion is refluxed to the distillation column  84 , and another portion (or the remainder) is distilled through a line  73   b . In the embodiment illustrated in  FIG. 1 , the condensate is refluxed to the distillation column  84 , but the whole quantity of the condensate may be distilled out through the line  73   b . The non-condensable gas, which has not been condensed by the working of the condenser  84   a , may be fed through the line  73   a , merged into the line  62 , and cycled to the absorption column  82 , or may be discarded, or may be recycled to a site immediately upstream from a condenser for condensation of an overhead stream from the column top of a distillation column such as the light ends column, the dehydration column, or the heavy ends column. It is also acceptable that the recycled non-condensable gas is then subjected to separation from a condensate by condensation using a condenser, and cycled again through the line  62  to the absorption column  82 . 
     The overhead stream (line  73 ), which is rich in useful components (in particular, methyl iodide), from the column top of the distillation column  84  may be recycled typically to at least one of the reactor, the evaporator, and distillation columns disposed downstream from the evaporator. The recycling of the overhead stream to the reactor allows the useful components (in particular, methyl iodide) to be reused in the reaction step and gives excellent economic efficiency. Assume that the offgas to be subjected to the first absorption step is the offgas from the decanter. In this case, the overhead stream rich in useful components may include a large amount of acetaldehyde, because components having lower boiling points as compared with the second absorbent are concentrated in the overhead stream. Accordingly, it is acceptable that, before being recycled through the decanter to the reactor, the overhead stream rich in useful components is fed to the acetaldehyde-removing system, and acetaldehyde is removed therefrom by the working of the acetaldehyde-removing system. Of the bottom stream (line  74 ) from the column bottom of the distillation column  84 , a portion is discharged though a line  75  out of the system continuously or batchwise; and the remainder is combined with a fresh second absorbent fed through a line  76  continuously or batchwise, is cycled through the line  70  to the absorption column  83 , and is reused as the second absorbent in the second absorption step. It is also acceptable that the whole quantity of the second solution is discharged out of the system, and a fresh second absorbent is fed to the absorption column  83 . At least a portion (e.g., the second solution discharged through the line  75  out of the system) of the second solution may be recycled typically to at least one of the reactor, the evaporator, and distillation columns downstream from the evaporator (such as an evaporator, the light ends column, the dehydration column, and the heavy ends column). 
     The distillation column  84 , when being a plate column, typically has 1 to 50 theoretical plates when acetic acid is subjected to distillation therein, where the number of theoretical plates may be determined depending on the composition of the second solution to be subjected to distillation. The distillation column  84  in this case is operated at a reflux ratio of typically 0 to 3000, where the reflux ratio may be determined according to the number of theoretical plates. In the distillation column  84 , the column top pressure is set typically to 1 to 500 kPaG; and the column bottom pressure is set to a pressure which is higher than the column top pressure and is typically 10 to 700 kPaG. In the distillation column  84 , the column top temperature is set typically to a temperature which is lower than the boiling point of the second absorbent at the set column top pressure and is typically 40° C. to 130° C.; and the column bottom temperature is set typically to a temperature which is equal to or higher than the boiling point of acetic acid at the set column bottom pressure and is 118° C. to 165° C. (and preferably 120° C. to 160° C.) 
       FIG. 2  depicts an exemplary schematic flow chart illustrating a scrubbing system according to another embodiment, where the scrubbing system includes the first and second absorption steps. In this embodiment, the first absorption step is performed only with the absorption column  81 . At the absorption column  81 , the first gas from the column top is fed through the line  60  to the absorption column  83  with which the second absorption step is performed; and the first solution is drawn from the column bottom through the line  61 . A portion of the first solution is discharged (line  66 ) out of the system; and the remainder is combined with a fresh first absorbent (line  67 ), and is recycled, as the first absorbent, through the line  59  again to the absorption column  81 . The other configurations than these are as with the embodiment illustrated in  FIG. 1 . 
       FIG. 3  depicts an exemplary schematic flow chart illustrating a scrubbing system according to yet another embodiment, where the scrubbing system includes the first and second absorption steps. In this embodiment, the first and second absorption steps are performed with a single absorption column  85 . Specifically, in the first absorption step, an offgas from the process is continuously introduced through a line  77  (offgas feed line) into the absorption column  85 ; whereas the first absorbent is continuously introduced through a line  78  (first absorbent feed line) into the absorption column  85 , where the line  78  is coupled to the absorption column  85  at a height higher than the offgas feeding height. In the column, the ascending offgas and the descending first absorbent are brought into countercurrent contact with each other, to allow the first absorbent to absorb an iodine compound from the offgas. Thus, a first gas having a lower iodine compound concentration as compared with the offgas, and a first solution containing the iodine compound and the first absorbent are to be separated. At the absorption column  85 , the first gas further ascends in the column, whereas the first solution is discharged from the column bottom through a line  79 . In the second absorption step, at the absorption column  85 , the first gas further goes up in the column higher than the first absorbent feeding height, whereas the second absorbent is continuously introduced through a line  86 , which is coupled to a portion adjacent to the column top of the absorption column  85 . In the column, the ascending first gas and the descending second absorbent are brought into countercurrent contact with each other, to allow the second absorbent to absorb an iodine compound from the first gas. Thus, a second gas having a lower iodine compound concentration as compared with the first gas, and a second solution containing the iodine compound and the second absorbent are to be separated. At the absorption column  85 , the second gas is drawn from the column top through a line  89 , and the second solution is collected on a unit  87  and drawn through a line  88 , where the unit  87  is capable of receiving a liquid falling down from the second absorbent feeding height and is exemplified typically by a chimney tray. From the absorption column  85 , the second solution is fed through a line  88  to a distillation column  84  with which a stripping step is performed; and the first solution is drawn from the column bottom through the line  79 . Of the first solution, a portion (line  90 ) is discharged out of the system; and the remainder is combined with a fresh first absorbent (line  90 ′) and recycled as the first absorbent through a line  78  to the absorption column  85 . Of the bottom stream (line  74 ) from the column bottom of the distillation column  84 , a portion is discharged through the line  75  out of the system continuously or batchwise; and the remainder is combined with a fresh second absorbent fed through the line  76  continuously or batchwise, is cycled through the line  86  to the absorption column  85 , and is reused as the second absorbent in the second absorption step. The other configurations than these are as with the embodiment illustrated in  FIG. 1 . Specifically, the second solution from the absorption column  85  is fed to the stripping step, but at least one of the first and second solutions may be recycled to at least one site in the process. 
     The acetic acid production method according to the present invention may include, in the acetic acid production process, a carbonylation step (reaction step) and a separation step. In the carbonylation step, methanol is reacted with carbon monoxide to form acetic acid. In the separation step, a reaction mixture from the carbonylation step is separated, using at least one selected from evaporators and distillation columns, into a stream including a metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in light ends than the acetic acid stream. The separation step preferably includes, for example, an evaporation step and a light ends-removing step. In the evaporation step, the reaction mixture from the carbonylation step is separated, using an evaporator, into a vapor stream and a residue stream. In the light ends-removing step, the vapor stream is separated, by distillation, into an overhead stream rich in light ends, and a first acetic acid stream rich in acetic acid. The separation step may include a dehydration step. In the dehydration step, the first acetic acid stream is separated, by distillation, into an overhead stream rich in water, and a second acetic acid stream richer in acetic acid than the first acetic acid stream. 
     Instead of the evaporation step and the light ends-removing step, the separation step may include an evaporation-light ends-removing step. This step is the step of separating the reaction mixture from the carbonylation step into a stream including the catalyst, an overhead stream rich in light ends, and a first acetic acid stream rich in acetic acid. Instead of the light ends-removing step and the dehydration step, the separation step may include a so-called light ends-removing-dehydration step which is a light ends-removing step that also functions as a dehydration step. Namely, this step is the step of separating, by distillation, the vapor stream into an overhead stream rich in light ends, and an acetic acid stream which has been dehydrated to a water concentration at a similar level to that in the second acetic acid stream. Accordingly, the evaporation-light ends-removing step may be one that functions also as the dehydration step (an evaporation-light ends-removing-dehydration step). Acetic acid streams rich in acetic acid from the light ends-removing-dehydration step and the evaporation-light ends-removing-dehydration step correspond to the second acetic acid stream. 
     The acetic acid production method according to the present invention may further include at least one step selected from steps (a), (b), and (c) as follows. 
     The step (a) is a heavy ends-removing step of separating, by distillation, the first or second acetic acid stream into a bottom stream rich in heavy ends, and a third acetic acid stream richer in acetic acid than the acetic acid stream before being subjected to the distillation. 
     The step (b) is an adsorptive removing step of treating the first or second or third acetic acid stream with an ion exchange resin to give a fourth acetic acid stream. 
     The step (c) is a product step of subjecting the first or second or third or fourth acetic acid stream to distillation to give a fifth acetic acid stream richer in acetic acid than the acetic acid stream before being subjected to the distillation. 
     The acetic acid production method according to the present invention may include an acetaldehyde-removing system (acetaldehyde-removing step). In the acetaldehyde-removing system, acetaldehyde is separated, using a distillation column or columns, from at least a portion of a condensate resulting from condensation of the overhead stream rich in light ends. 
     In the acetic acid production method according to the present invention as above, at least one offgas is preferably fed to an absorption column to be subjected to the first absorption step in the absorption column, where the at least one offgas is selected from the group consisting of an offgas from the reactor, an offgas from the evaporator, an offgas from the distillation column(s) in the separation step, and an offgas from the distillation column(s) in the acetaldehyde-removing system. 
     In the acetic acid production method according to the present invention as above, the overhead stream rich in methyl iodide separated and obtained in the stripping step may be recycled to at least one of the reaction step, the evaporation step, and distillation steps, for allowing methyl iodide to be advantageously reused in the reaction step in the reactor. 
     Assume that a second absorbent containing acetic acid is used in the acetic acid production method according to the present invention as above. In this case, the bottom stream rich in acetic acid, which is separated and obtained in the stripping step, is preferably recycled to at least one step selected from the group consisting of the reaction step, the evaporation step, the light ends-removing step, the dehydration step, and the heavy ends-removing step. This is because acetic acid, when included in the final product without discarding, can be effectively utilized, and heavy ends other than acetic acid, which may be included in the bottom stream, can be separated and removed in the heavy ends-removing step. Assume that the concentrations of components having higher boiling points as compared with acetic acid excessively increase, where the components are exemplified typically by lithium iodide, the catalyst such as a rhodium catalyst, high-boiling hydrocarbons, and high-boiling carboxylic acids, each of which is contained in a trace amount in the offgas. In this case, the bottoms (bottom stream) are desirably recycled out of the system. This is because the temperature of the distillation column in the stripping step in this case is elevated to accelerate corrosion of the interior of the distillation column; the bottoms are contaminated with the material or materials of the distillation column due to the corrosion and thereby change in composition; and this leads to lower absorption efficiency in the absorption step. 
     Assume that the acetic acid production method according to the present invention employs a water-containing first absorbent. In this case, the resulting water-containing first solution (aqueous solution) from the first absorption step is preferably recycled to at least one apparatus selected from the group consisting of the reactor, the decanter for reserving the condensate, the dehydration column, and the heavy ends column. This is because water in the reactor is consumed by the shift reaction with carbon monoxide (H 2 O+CO→H 2 +CO 2 ). The water-containing first solution, when recycled to the aqueous phase in the decanter, is to be treated together with the aqueous phase. The water in the solution, when recycled to the dehydration column, is concentrated at the column top of the dehydration column; and when recycled to the heavy ends column, is concentrated at the column top of the heavy ends column. Such concentrated water is then recycled, or discarded. 
     Hereinafter, one embodiment of the acetic acid production method according to the present invention will be illustrated.  FIG. 4  depicts an exemplary production flow chart (methanol carbonylation process) of an acetic acid production system according to an embodiment. Acetic acid production equipment according to the acetic acid production flow includes a reactor  1 , an evaporator  2 , a distillation column  3 , a decanter  4 , a distillation column  5 , a distillation column  6 , an ion exchange resin column  7 , a scrubbing system  8 , an acetaldehyde-removing system  9 , condensers  1   a ,  2   a ,  3   a ,  5   a , and  6   a , a heat exchanger  2   b , reboilers  3   b ,  5   b , and  6   b , lines  11  to  56 , and a pump  57 . The equipment is configured so as to be capable of producing acetic acid continuously. 
     In the acetic acid production method according to the embodiment, a reaction step, an evaporation step (flash step), a first distillation step, a second distillation step, a third distillation step, and an adsorptive removing step are performed respectively in the reactor  1 , the evaporator  2 , the distillation column  3 , the distillation column  5 , the distillation column  6 , and the ion exchange resin column  7 . The first distillation step, the second distillation step, and the third distillation step are also referred to respectively as a light ends-removing step, a dehydration step, and a heavy ends-removing step. Steps in the embodiment are not limited to those mentioned above. In particular, the equipment may not include one or more facilities such as the distillation column  5 , the distillation column (heavy ends column)  6 , the ion exchange resin column  7 , and the acetaldehyde-removing system  9  (such as an acetaldehyde-removing column). The equipment may further include a product column disposed downstream from the ion exchange resin column  7 , as described later. 
     The reactor  1  is a unit with which the reaction step is performed. The reaction step (carbonylation step) is the step of continuously forming acetic acid through a reaction (methanol-carbonylation reaction) represented by Chemical Formula (1) below. During steady operation of the acetic acid production equipment, the reactor  1  contains or houses a reaction mixture, which is stirred typically with a stirrer. The reaction mixture includes starting materials methanol and carbon monoxide, a metal catalyst, a promoter, water, production target acetic acid, and various by-products. In the reaction mixture, a liquid phase and a gas phase are in an equilibrium state. Chemical Formula (1) is expressed as follows:
 
CH 3 OH+CO→CH 3 COOH  (1)
 
     The starting materials in the reaction mixture are liquid methanol and gaseous carbon monoxide. Methanol is fed from a methanol storage unit (not shown) through the line  11  to the reactor  1  continuously at a predetermined flow rate. Carbon monoxide is fed from a carbon monoxide storage unit (not shown) through the line  12  to the reactor  1  continuously at a predetermined flow rate. The carbon monoxide does not always have to be pure carbon monoxide and may include a small amount (typically 5 mass percent or less, and preferably 1 mass percent or less) of one or more other gases such as nitrogen, hydrogen, carbon dioxide, and oxygen gases. 
     The metal catalyst in the reaction mixture is used to promote or accelerate the methanol-carbonylation reaction and may be selected typically from rhodium catalysts and iridium catalysts. A non-limiting example of the rhodium catalysts is a rhodium complex represented by the chemical formula: [Rh(CO) 2 I 2 ] − . A non-limiting example of the iridium catalysts is an iridium complex represented by the chemical formula: [Ir(CO) 2 I 2 ] − . The metal catalyst is preferably selected from metal complex catalysts. The catalyst may be present in the reaction mixture in a concentration (in terms of metal) of typically 200 to 10000 ppm by mass, preferably 300 to 5000 ppm by mass, and furthermore preferably 400 to 2500 ppm by mass, relative to the totality of the liquid phase of the reaction mixture (liquid reaction mixture). 
     The promoter is an iodide to assist the action of the catalyst and may be selected typically from methyl iodide and ionic iodides. Methyl iodide can offer the action of promoting the catalysis of the catalyst. Methyl iodide may be present in a concentration of typically 1 to 20 mass percent, and preferably 5 to 15 mass percent, relative to the totality of the liquid phase of the reaction mixture. The ionic iodides are iodides that form iodide ions in the liquid reaction mixture, of which ionic metal iodides are typified. The ionic iodides can offer the action of stabilizing the catalyst and/or the action of restraining side reactions. Non-limiting examples of the ionic iodides include alkali metal iodides such as lithium iodide, sodium iodide, and potassium iodide. The ionic iodide(s) may be present in the reaction mixture in a concentration of typically 1 to 25 mass percent, and preferably 5 to 20 mass percent, relative to the totality of the liquid phase of the reaction mixture. For example, when an iridium catalyst is used, a ruthenium compound and/or an osmium compound may be used as the promoter. These compounds may be used in a total amount of typically 0.1 to 30 moles (in terms of metal), and preferably 0.5 to 15 moles (in terms of metal), per mole (in terms of metal) of iridium. 
     Water in the reaction mixture is a component necessary for the formation of acetic acid, due to the reaction mechanism of the methanol-carbonylation reaction, and is a component necessary for dissolving water-soluble components in the reaction system. Water may be present in the reaction mixture in a concentration of typically 0.1 to 15 mass percent, preferably 0.5 to 10 mass percent, more preferably 1 to 6 mass percent, and furthermore preferably 1.5 to 4 mass percent, relative to the totality of the liquid phase of the reaction mixture. The water concentration is preferably 15 mass percent or less, so as to minimize energy necessary for removing water in the acetic acid purification process and for performing the acetic acid production more efficiently. To control the water concentration, water may be fed to the reactor  1  continuously at a predetermined flow rate. 
     Acetic acid in the reaction mixture includes acetic acid that has been charged into the reactor  1  before operation of the acetic acid production equipment; and acetic acid that is formed as a main product of the methanol-carbonylation reaction. Acetic acid as above can function as a solvent in the reaction system. Acetic acid may be present in the reaction mixture in a concentration of typically 50 to 90 mass percent, and preferably 60 to 80 mass percent, relative to the totality of the liquid phase of the reaction mixture. 
     A non-limiting example of major by-products contained in the reaction mixture is methyl acetate. Methyl acetate can be formed through reaction between acetic acid and methanol. Methyl acetate may be present in the reaction mixture in a concentration of typically 0.1 to 30 mass percent, and preferably 1 to 10 mass percent, relative to the totality of the liquid phase of the reaction mixture. 
     Another non-limiting example of the by-products contained in the reaction mixture is hydrogen iodide. When the catalyst with or without the promoter as above is used, hydrogen iodide is unavoidably formed due to the reaction mechanism of the methanol-carbonylation reaction. Hydrogen iodide may be present in the reaction mixture in a concentration of typically 0.01 to 2 mass percent, relative to the totality of the liquid phase of the reaction mixture. 
     Non-limiting examples of the by-products also include hydrogen, methane, carbon dioxide, acetaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, dimethyl ether, alkanes, formic acid, and propionic acid, as well as alkyl iodides such as hexyl iodide and decyl iodide. The reaction mixture may also include metals resulting from corrosion of the equipment, such as iron, nickel, chromium, manganese, and molybdenum (such metals are hereinafter also referred to as “corrosion metals”); and other metals such as cobalt, zinc, and copper. The corrosion metals and other metals are collectively also referred to as “metals such as corrosion metals”. 
     In the reactor  1  housing the reaction mixture as above, the reaction temperature is set typically to 150° C. to 250° C., the reaction pressure as a total pressure is set typically to 1.5 to 3.5 MPa (absolute pressure), and the carbon monoxide partial pressure is set typically to 0.4 to 1.8 MPa (absolute pressure), preferably 0.6 to 1.6 MPa (absolute pressure), and furthermore preferably 0.9 to 1.4 MPa (absolute pressure). 
     Vapors in the gas phase in the reactor  1  during operation of the equipment typically include carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. The vapors can be drawn from the reactor  1  through the line  13 . The inside pressure of the reactor  1  can be controlled by regulating the amount of the vapors to be drawn out, and is typically held constant. The vapors drawn from the reactor  1  are introduced into the condenser  1   a.    
     The condenser  1   a  cools and partially condenses the vapors from the reactor  1  to separate the vapors into a condensate and a gas. The condensate typically includes methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. The condensate is introduced and recycled from the condenser  1   a  through the line  14  to the reactor  1 . The gas typically includes carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The gas is fed from the condenser  1   a  through the line  15  to the scrubbing system  8 . 
     In the embodiment illustrated in  FIG. 4 , all of a gas (line  20 ) from the condenser  2   a , a gas (line  32 ) from the condenser  3   a , a gas (line  37 ) from the condenser  5   a , and a gas (line  45 ) from the condenser  6   a  are merged into the line  15  and fed to the scrubbing system  8  (pattern A). However, it is also acceptable that the gas from the condenser  1   a  alone is fed through the line  15  to the scrubbing system  8 ; and all the gases (lines  32 ,  37 , and  45 ) from the condensers  3   a ,  5   a , and  6   a  are merged into the line  20  and fed to the scrubbing system  8  (pattern B). In the pattern A, for example, the gas from the condenser  1   a  is fed through the line  15  and through the line  58  or line  77  to the absorption column  81  or to the absorption column  85  in the scrubbing system  8  illustrated in  FIG. 2  or  FIG. 3 . In the pattern B, for example, the gas from the condenser  1   a  is fed through the line  15  and through the line  58  to the absorption column  81  in the scrubbing system  8  illustrated in  FIG. 1 ; whereas the merged gas through the line  20  is fed through the line  62  to the absorption column  82  in the scrubbing system  8  illustrated in  FIG. 1 . 
     By the working of the scrubbing system  8 , useful components (such as methyl iodide, methanol, dimethyl ether, water, methyl acetate, and acetic acid) are separated and recovered from the gas from the condenser  1   a  (line  15 ), through the first and second absorption steps, and, as needed, further through the stripping step, as described above. In the embodiment, the separation and recovery employs a wet process which is performed using an absorbing liquid (absorbent) for collecting useful components from a gas. For example, a condensate derived from vapors from the after-mentioned distillation column  6  is usable as the absorbing liquid or liquids. The separated and recovered useful components (such as methyl iodide) are introduced and recycled from the scrubbing system  8  (in particular, through the line  73  from the column top of the distillation column  84  in the stripping step) through the recycle line  48  to the reactor  1 . Though not shown in the figure, it is also acceptable that the line  48  is introduced (coupled) to charge lines of the condensers  1   a ,  2   a ,  3   a , and  5   a , and whereby the useful components are cooled, condensed, and recovered. Gases (e.g., the line  71  in  FIG. 1  and  FIG. 2 ) from which useful components have been collected are discarded through the line  49 . Gases (e.g., the lines  73   a  and  89  in  FIG. 3 ) from which useful components have been collected may be subjected to separation of condensable components (condensates) therefrom using a condenser, and/or be recycled to the absorption column. The gas discharged from the line  49  is usable as a carbon monoxide source to be introduced into the bottom portion of the evaporator  2  or into the residue stream recycle lines  18  and  19 . The treatment in the scrubbing system  8 , and the subsequent recycling and discarding are applicable to the after-mentioned gases (lines  20 ,  32 ,  37 , and  45 ) to be fed to the scrubbing system  8  from other condensers. 
     In the reactor  1  during operation of the equipment, acetic acid is continuously formed as described above. A reaction mixture containing such acetic acid is continuously drawn from the reactor  1  at a predetermined flow rate, and introduced through the line  16  into the subsequent (downstream) evaporator  2 . 
     The evaporator  2  is a unit with which the evaporation step (flash step) is performed. The evaporation step is the step of partially evaporating the reaction mixture to separate the reaction mixture into a vapor stream (volatile phase) and a residue stream (low volatile phase; residual liquid stream), where the reaction mixture is continuously introduced through the line  16  (reaction mixture feed line) into the evaporator  2 . 
     The evaporation may be performed by reducing the pressure with or without heating. In the evaporation step, the vapor stream temperature is typically 100° C. to 260° C., and preferably 120° C. to 200° C.; the residue stream temperature is typically 80° C. to 200° C., and preferably 100° C. to 180° C.; and the evaporator internal pressure is typically 50 to 1000 kPa (absolute pressure). 
     The ratio of the vapor stream to the residue stream, which are separated from each other in the evaporation step, is typically from 10:90 to 50:50 in terms of mass ratio. The vapors formed in the step typically include methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, as well as alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, and decyl iodide. The vapors are continuously drawn from within the evaporator  2  to the line  17  (vapor stream discharge line). 
     Of the vapor stream drawn from within the evaporator  2 , a portion is continuously introduced into the condenser  2   a ; and another portion is continuously introduced through the line  21  into the subsequent (downstream) distillation column  3 . The vapor stream has an acetic acid concentration of typically 40 to 85 mass percent (preferably 50 to 85 mass percent), and more preferably 50 to 75 mass percent (e.g., 55 to 75 mass percent); a methyl iodide concentration of typically 2 to 50 mass percent (and preferably 5 to 30 mass percent); a water concentration of typically 0.2 to 20 mass percent (and preferably 1 to 15 mass percent); and a methyl acetate concentration of typically 0.2 to 50 mass percent (and preferably 2 to 30 mass percent). The vapor stream has a hexyl iodide concentration of typically 0.1 to 10000 ppb by mass, generally 0.5 to 1000 ppb by mass, and frequently 1 to 100 ppb by mass (e.g., 2 to 50 ppb by mass). 
     The residue stream formed in the step includes the catalyst and the promoter (such as methyl iodide and/or lithium iodide), which have been contained in the reaction mixture; and water, methyl acetate, acetic acid, formic acid, propionic acid, and other substances that remain without volatilization in the step. The residue stream is continuously introduced from the evaporator  2  through the line  18  into the heat exchanger  2   b , using the pump  57 . The heat exchanger  2   b  cools the residue stream from the evaporator  2 . The cooled residue stream is continuously introduced and recycled from the heat exchanger  2   b  through the line  19  to the reactor  1 . The line  18  and the line  19  are also collectively referred to as a “residue stream recycle line(s)”. The residue stream has an acetic acid concentration of typically 55 to 90 mass percent, and preferably 60 to 85 mass percent. 
     The condenser  2   a  cools and partially condenses the vapor stream from the evaporator  2  to separate the vapor stream into a condensate and a gas. The condensate typically includes methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. The condensate is introduced and recycled from the condenser  2   a  through the lines  22  and  23  to the reactor  1 . The gas typically includes carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The gas is fed from the condenser  2   a  through the lines  20  and  15  to the scrubbing system  8 . The acetic acid formation reaction in the reaction step is an exothermic reaction. A portion of heat accumulated in the reaction mixture is, in the evaporation step (flash step), transferred to the vapors derived from the reaction mixture. The vapors are cooled in the condenser  2   a  to give a condensate, and the condensate is recycled to the reactor  1 . Specifically, this acetic acid production equipment enables efficient removal of heat by the working of the condenser  2   a , where the heat is generated in the methanol-carbonylation reaction. 
     The distillation column  3  is a unit with which the first distillation step is performed. The distillation column  3  in the embodiment is characterized as a so-called light ends column. The first distillation step is the step of subjecting the vapor stream, which is continuously introduced into the distillation column  3 , to distillation treatment to separate and remove one or more light ends therefrom. More specifically, the first distillation step is the step of separating, by distillation, the vapor stream into an overhead stream rich in at least one light end selected from methyl iodide and acetaldehyde, and an acetic acid stream rich in acetic acid. 
     The distillation column  3  may be selected typically from rectification columns such as a plate column and a packed column. The distillation column  3 , when being a plate column, typically has 5 to 50 theoretical plates and is operated at a reflux ratio of typically 0.5 to 3000, where the reflux ratio may be determined according to the number of theoretical plates. In the distillation column  3 , the column top pressure is set typically to 80 to 160 kPaG, and the column bottom pressure is set to a pressure higher than the column top pressure and is typically 85 to 180 kPaG. In the distillation column  3 , the column top temperature is set typically to a temperature which is lower than the boiling point of acetic acid at the set column top pressure and is from 90° C. to 130° C.; and the column bottom temperature is set typically to a temperature equal to or higher than the boiling point of acetic acid at the set column bottom pressure and is from 120° C. to 165° C. (preferably from 125° C. to 160° C.) 
     At the distillation column  3 , the vapor stream from the evaporator  2  is continuously introduced through the line  21 ; vapors as an overhead stream are continuously drawn from a column top portion to the line  24 ; and bottoms are continuously drawn from a column bottom portion to the line  25 . There is disposed the reboiler  3   b . An acetic acid stream (first acetic acid stream; liquid) as a side stream is continuously drawn, through the line  27 , from a portion at a height position between the column top and the column bottom in the distillation column  3 . 
     The vapors drawn from the column top portion of the distillation column  3  include light ends in larger amounts as compared with the bottoms and the side stream from the distillation column  3 , where the light ends are components having lower boiling points as compared with acetic acid. The vapors typically include methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, and formic acid. The vapors also include acetic acid. The vapors as above are continuously introduced through the line  24  into the condenser  3   a.    
     The condenser  3   a  cools and partially condenses the vapors from the distillation column  3  to separate the vapors into a condensate and a gas. The condensate typically includes methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The condensate is continuously introduced from the condenser  3   a  through the line  28  into the decanter  4 . The condensate introduced into the decanter  4  is liquid-liquid separated into an aqueous phase (upper phase) and an organic phase (methyl iodide phase; lower phase). 
     The aqueous phase includes water, and other components such as methyl iodide, hydrogen iodide, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The organic phase typically includes methyl iodide, and other components such as hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. 
     In the embodiment, of the aqueous phase, a portion is refluxed through the line  29  to the distillation column  3 ; and another portion is introduced and recycled through the lines  29 ,  30 , and  23  to the reactor  1 . A portion of the organic phase is introduced and recycled through the lines  31  and  23  to the reactor  1 . Another portion of the organic phase and/or another portion of the aqueous phase is introduced through the lines  31  and  50  and/or through the lines  30  and  51  into the acetaldehyde-removing system  9 . To the aqueous phase in the decanter  4 , the first or second solution containing water may be recycled. The water in the first or second solution is merged with the aqueous phase and is treated together with the aqueous phase. 
     In an acetaldehyde-removing step using the acetaldehyde-removing system  9 , acetaldehyde is removed from at least one of the organic phase and the aqueous phase by a known technique such as distillation, or extraction, or both in combination. The separated acetaldehyde is discharged through the line  53  out of the equipment. Useful components (such as methyl iodide) contained in at least one of the organic phase and the aqueous phase are recycled through the lines  52  and  23  to the reactor  1 . 
       FIG. 5  depicts a schematic flow chart illustrating an acetaldehyde-removing system according to an embodiment. For example, assume that the organic phase is treated in the acetaldehyde-removing step according to the flow. In this case, the organic phase is treated typically by a procedure as follows. The organic phase is fed through a line  101  to a distillation column (first acetaldehyde-removing column)  91  and is separated by distillation into an overhead stream rich in acetaldehyde (line  102 ), and a residue stream rich in methyl iodide (residual liquid stream; bottoms stream) (line  103 ). The overhead stream is condensed in a condenser  91   a  to give a condensate. Of the condensate, a portion (line  104 ) is refluxed to a column top portion of the distillation column  91 ; and the remainder (line  105 ) is fed to an extraction column  92 . 
     The condensate fed to the extraction column  92  is extracted with water introduced from a line  109 , to give an extract. The extract resulting from the extraction is fed through a line  107  to a distillation column (second acetaldehyde-removing column)  93  and is separated by distillation into an overhead stream rich in acetaldehyde (line  112 ) and a residue stream rich in water (line  113 ). The overhead stream rich in acetaldehyde is condensed in a condenser  93   a  to give a condensate. Of the condensate, a portion (line  114 ) is refluxed to a column top portion of the distillation column  93 ; and the remainder (line  115 ) is discharged out of the system. 
     The residue stream rich in methyl iodide as bottoms from the first acetaldehyde-removing column  91 , a raffinate rich in methyl iodide (line  108 ) from the extraction column  92 , and the residue stream rich in water as bottoms from the second acetaldehyde-removing column  93  are recycled respectively through the lines  103 ,  111 , and  113  to the reactor  1 , and/or to an appropriate site in the process and reused. For example, the raffinate rich in methyl iodide from the extraction column  92  can be recycled through a line  110  to the distillation column  91 . The liquid in the line  113  is generally discharged out as an effluent. Gases (lines  106  and  116 ), which have not been condensed in the condensers  91   a  and  93   a , are subjected to absorptive treatment in the scrubbing system  8 , or discarded. 
     Also assume that the aqueous phase is treated in the acetaldehyde-removing step according to the flow illustrated in  FIG. 5 . In this case, the aqueous phase is treated typically by a procedure as follows. The aqueous phase is fed through the line  101  to the distillation column (first acetaldehyde-removing column)  91 , and is separated by distillation into an overhead stream rich in acetaldehyde (line  102 ) and a residue stream rich in water (line  103 ). The overhead stream is condensed in the condenser  91   a  to give a condensate. Of the condensate, a portion (line  104 ) is refluxed to the column top portion of the distillation column  91 ; and the remainder (line  105 ) is fed to the extraction column  92 . 
     The condensate fed to the extraction column  92  is extracted with water introduced from the line  109 , to give an extract. The extract resulting from the extraction is fed through the line  107  to the distillation column (second acetaldehyde-removing column)  93 , and is separated by distillation into an overhead stream rich in acetaldehyde (line  112 ) and a residue stream rich in water (line  113 ). The overhead stream rich in acetaldehyde is condensed in the condenser  93   a  to give a condensate. Of the condensate, a portion (line  114 ) is refluxed to the column top portion of the distillation column  93 ; and the remainder (line  115 ) is discharged out of the system. 
     The residue stream rich in water as bottoms from the first acetaldehyde-removing column  91 , a raffinate rich in methyl iodide (line  108 ) from the extraction column  92 , and the residue stream rich in water as bottoms from the second acetaldehyde-removing column  93  are recycled respectively through the lines  103 ,  111 , and  113  to the reactor  1 , and/or to an appropriate site in the process and reused. For example, the raffinate rich in methyl iodide from the extraction column  92  can be recycled through the line  110  to the distillation column  91 . The liquid in the line  113  is generally discharged out as an effluent. Gases (lines  106  and  116 ), which have not been condensed in the condensers  91   a  and  93   a , are subjected to absorptive treatment in the scrubbing system  8 , or discarded. 
     Instead of, or in addition to the technique, acetaldehyde derived from the process stream including water, acetic acid (AC), methyl iodide (MeI), and acetaldehyde (AD) can also be removed by utilizing extractive distillation. For example, acetaldehyde can be discharged out of the system by the following procedure. The process stream is liquid-liquid separated into an organic phase and an aqueous phase, and at least one of the organic phase and the aqueous phase is fed as a charge liquid to a distillation column (extractive distillation column). With this, an extractant (generally, water) is introduced into a concentrated zone in the distillation column, where methyl iodide and acetaldehyde are concentrated in the concentrated zone. A non-limiting example of the concentrated zone is space ranging from the column top to the charge liquid feeding height. A liquid (extract) falling down from the concentrated zone is drawn as a side stream (sidecut stream), the side stream is liquid-liquid separated into an aqueous phase and an organic phase, and the aqueous phase is subjected to distillation. Thus, acetaldehyde is discharged out of the system. 
     When a relatively large amount of water is present in the distillation column, the liquid falling down from the concentrated zone may be drawn as a side stream without introduction of the extractant to the distillation column. For example, it is acceptable that this distillation column is provided with a unit (such as a chimney tray) capable of receiving the liquid (extract) falling down from the concentrated zone, and the liquid (extract) received by the unit is drawn as a side stream. 
     The extractant is preferably introduced into a portion at a height higher than the charge liquid feeding height and is more preferably introduced into a portion adjacent to the column top. The side stream is preferably drawn from the distillation column at a height lower than the extractant introducing height and higher than the charge liquid feeding height. This technique enables high-concentration extraction of acetaldehyde from the concentrate of methyl iodide and acetaldehyde, with the extractant (generally, water). In addition, the technique enables efficient extraction of acetaldehyde with a small amount of the extractant, because of using, as an extraction zone, space between the extractant introducing height and the sidecut height. The technique can therefore significantly reduce the number of plates in the distillation column and can reduce the required steam amount, as compared typically with a technique of drawing an extract resulting from extractive distillation from a column bottom portion of a distillation column (extractive distillation column). Further, the technique enables removal of acetaldehyde under such conditions as to restrain or minimize the loss of methyl iodide out of the system, because the technique, as using a small amount of the extractant, can reduce the ratio (MeI/AD ratio) of methyl iodide to acetaldehyde in the aqueous extract as compared with the technique illustrated in  FIG. 5 , which technique employs acetaldehyde removing distillation and aqueous extraction in combination. 
     The acetaldehyde concentration in the side stream is significantly higher than the acetaldehyde concentrations in the charge liquid and in the bottoms (bottom liquid). The ratio of acetaldehyde to methyl iodide in the side stream is higher than the ratios of acetaldehyde to methyl iodide in the charge liquid and in the bottoms. 
     An organic phase (methyl iodide phase) resulting from liquid-liquid separation of the side stream may be recycled to this distillation column. In this case, the organic phase resulting from liquid-liquid separation of the side stream is preferably recycled to the distillation column at a height lower than the side stream drawing height and higher than the charge liquid feeding height. 
     A miscible solvent may be introduced into the distillation column (extractive distillation column), where the miscible solvent is miscible with a component or components (such as methyl acetate) constituting the organic phase resulting from liquid-liquid separation of the process stream. Non-limiting examples of the miscible solvent include acetic acid and ethyl acetate. The miscible solvent is preferably introduced into the distillation column at a height lower than the side stream drawing height and higher than the charge liquid feeding height. When the organic phase resulting from liquid-liquid separation of the side stream is recycled to this distillation column, the miscible solvent is preferably introduced to a portion at a height lower the organic phase recycling height. 
     Recycling of the organic phase resulting from liquid-liquid separation of the side stream to the distillation column and/or the introduction of the miscible solvent to the distillation column can lower the methyl acetate concentration in the extract drawn as the side stream, can lower the methyl acetate concentration in the aqueous phase resulting from liquid-liquid separation of the extract, and, consequently, can restrain or minimize the contamination of the aqueous phase with methyl iodide. 
     The distillation column (extractive distillation column) has typically 1 to 100, preferably 2 to 50, furthermore preferably 3 to 30, and particularly preferably 5 to 20 theoretical plates. Thus, the distillation column according to the technique enables efficient separation and removal of acetaldehyde with a smaller number of plates (theoretical plates), as compared with the number of plates (80 to 100 theoretical plates) in distillation columns and extractive distillation columns for use in conventional acetaldehyde removal. 
     The mass ratio of the extractant flow rate to the charge liquid flow rate may be selected within the range of from 0.0001:100 to 100:100, but is generally from 0.0001:100 to 20:100, preferably from 0.001:100 to 10:100, more preferably from 0.01:100 to 8:100, and furthermore preferably from 0.1:100 to 5:100, where the charge liquid is at least one of the organic phase and the aqueous phase resulting from liquid-liquid separation of the process stream. 
     At the distillation column (extractive distillation column), the column top temperature is typically 15° C. to 120° C., preferably 20° C. to 90° C., more preferably 20° C. to 80° C., and furthermore preferably 25° C. to 70° C.; and the column top pressure is typically about 0.1 to about 0.5 MPa (absolute pressure). Other conditions for the distillation column (extractive distillation column) may be as with conditions for distillation columns and extractive distillation columns for use in conventional acetaldehyde removal. 
       FIG. 6  depicts a schematic flow chart illustrating an acetaldehyde-removing system using the extractive distillation, according to an embodiment. In this embodiment, at least one of the organic phase and the aqueous phase resulting from liquid-liquid separation of the process stream is fed as a charge liquid through a feed line  201  to an intermediate plate (at a height between the column top and the column bottom) of a distillation column  94 , and water is introduced through a line  202  into a portion adjacent to the column top. Thus, extractive distillation is performed in the distillation column  94  (extractive distillation column). 
     The distillation column  94  is provided with a chimney tray  200  at a height higher than the charge liquid feeding height, where the chimney tray  200  is capable of receiving a liquid (extract) falling down from a concentrated zone in the column, and where methyl iodide and acetaldehyde are concentrated in the concentrated zone. In this extractive distillation, a liquid on the chimney tray  200  is drawn, preferably in the whole quantity, through a line  208  and introduced into, and liquid-liquid separated in a decanter  95  to give an aqueous phase and an organic phase. 
     The aqueous phase (including acetaldehyde) is fed from the decanter  95  through a line  212  and introduced into and cooled in a cooler  95   a . Thus, methyl iodide dissolved in the aqueous phase is two-phase separated, followed by liquid-liquid separation in a decanter  96  to give an aqueous phase and an organic phase. The aqueous phase is fed from the decanter  96  through a line  216  to a distillation column  97  (acetaldehyde-removing column) and undergoes distillation. Vapors from the column top are fed through a line  217  and introduced into and condensed in a condenser  97   a  to give a condensate (mainly including acetaldehyde and methyl iodide). Of the condensate, a portion is refluxed to the column top of the distillation column  97 ; and the remainder is discarded, or fed through a line  220  to a distillation column  98  (extractive distillation column). 
     Water is introduced through a line  222  into the distillation column  98  at a portion adjacent to the column top, to perform extractive distillation. Vapors from the column top are brought through a line  223  into, and are condensed in a condenser  98   a  to give a condensate (mainly including methyl iodide). Of the condensate, a portion is refluxed to the column top portion; and the remainder is recycled through a line  226  to the reaction system, or may be removed (discharged) out of the system. The organic phase (methyl iodide phase) from the decanter  95  is recycled, preferably in the whole quantity, through lines  209  and  210  to the distillation column  94  at a height lower than the height of the chimney tray  200 . A portion of the aqueous phase from the decanter  95 , and the organic phase from the decanter  96  are recycled respectively through lines  213  and  210  and through lines  214  and  210  to the distillation column  94 , but this recycling is not always performed. A portion of the aqueous phase from the decanter  95  may be used as the extractant (water) in the distillation column  94 . A portion of the aqueous phase from the decanter  96  may be recycled through the line  210  to the distillation column  94 . 
     In some cases (such as the case where the charge liquid contains methyl acetate), the distillation efficiency can be improved by charging a miscible solvent through a line  215  to the distillation column  94 , where the miscible solvent is miscible with a component or components (such as methyl acetate) constituting the organic phase resulting from liquid-liquid separation of the process stream, and the miscible solvent herein is exemplified by acetic acid and ethyl acetate. The miscible solvent is fed to the distillation column  94  at a height higher than the charge liquid feeding height (line  201  coupling height) and lower than the recycle line  210  coupling height. Bottoms from the distillation column  94  are recycled to the reaction system. 
     Vapors from the column top of the distillation column  94  are fed through a line  203  and brought into and condensed in a condenser  94   a  to give a condensate, and the condensate is liquid-liquid separated in a decanter  99  into an aqueous phase and an organic phase. The organic phase is refluxed through a line  206  to the column top portion of the distillation column  94 , and the aqueous phase is brought through a line  207  to the decanter  95 . 
     Bottoms (mainly containing water) from the distillation column  97  and bottoms (water containing a small amount of acetaldehyde) from the distillation column  98  (extractive distillation column) are transferred respectively through lines  218  and  224  and removed from the system, or recycled to the reaction system. Gases (lines  211 ,  221 , and  227 ), which have not been condensed in the condensers  94   a ,  97   a , and  98   a , are each subjected to absorption treatment in the scrubbing system  8 , or discarded. 
       FIG. 7  depicts a schematic flow chart illustrating an acetaldehyde-removing system using the extractive distillation, according to another embodiment. In this embodiment, the condensate derived from the vapors from the column top of the distillation column  94  is brought into a hold tank  100 , and the whole quantity of the condensate is refluxed through a line  206  to a column top portion of the distillation column  94 . The other configurations than this are as in the embodiment illustrated in  FIG. 6 . 
       FIG. 8  depicts a schematic flow chart illustrating an acetaldehyde-removing system using the extractive distillation, according to yet another embodiment. In this embodiment, the whole quantity of a liquid on the chimney tray  200  is drawn, introduced through the line  208  directly into the cooler  95   a  without passing through the decanter  95 , and cooled in the cooler  95   a  and fed to the decanter  96 . The other configurations than this are as in the embodiment illustrated in  FIG. 7 . 
     Referring back to  FIG. 4 , the gas formed in the condenser  3   a  typically includes carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The gas is fed from the condenser  3   a  through the lines  32  and  15  to the scrubbing system  8 . Of the gas that reaches the scrubbing system  8 , components such as methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid are absorbed by the absorbing liquids (absorbents) in the scrubbing system  8 . Assume that an absorbing liquid including methanol or methyl acetate is used as the absorbing liquid (absorbent) in one of the first absorption step and the second absorption step. In this case, hydrogen iodide reacts with methanol or methyl acetate in the absorbing liquid to form methyl iodide. The resulting liquid (overhead stream from the column top of the distillation column  84 ) containing the methyl iodide and other useful components can be recycled from the scrubbing system  8  through the recycle lines  48  and  23  to the reactor  1  and reused. 
     The bottoms drawn from the column bottom portion of the distillation column  3  include larger amounts of heavy ends as compared with the overhead stream and the side stream from the distillation column  3 , and typically include propionic acid, as well as the catalyst and the promoter as being entrained, where heavy ends are components having higher boiling points as compared with acetic acid. The bottoms also include other components such as acetic acid, methyl iodide, methyl acetate, and water. In the embodiment, of the bottoms as above, a portion is continuously introduced and recycled through the lines  25  and  26  to the evaporator  2 ; and another portion is continuously introduced and recycled through the lines  25  and  23  to the reactor  1 . 
     The first acetic acid stream, which is continuously drawn as a side stream from the distillation column  3 , is enriched with acetic acid as compared with the vapor stream, which is continuously introduced into the distillation column  3 . Namely, the acetic acid concentration in the first acetic acid stream is higher than the acetic acid concentration in the vapor stream. The acetic acid concentration in the first acetic acid stream is typically 90 to 99.9 mass percent, and preferably 93 to 99 mass percent. In addition to acetic acid, the first acetic acid stream further includes other components such as methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, as well as alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, and decyl iodide. 
     At the distillation column  3 , the line  27  may be coupled to the distillation column  3  at a height higher than the coupling height of the line  21  as illustrated in the figure, but may also be coupled at a height lower than or equal to, the coupling height of the line  21  to the distillation column  3 . The first acetic acid stream from the distillation column  3  is introduced through the line  27  into the subsequent (downstream) distillation column  5  continuously at a predetermined flow rate. It is also acceptable that the first acetic acid stream drawn as a side stream from the distillation column  3 , the bottoms from the distillation column  3 , and/or a condensate of vapors in the column bottom portion of the distillation column  3  is continuously introduced into the after-mentioned distillation column  6  without passing through the distillation column  5  (dehydration step). 
     To the first acetic acid stream flowing through the line  27 , potassium hydroxide may be fed or added through the line  55  (potassium hydroxide inlet line). The potassium hydroxide may be fed or added typically as a solution such as an aqueous solution. The feeding or addition of potassium hydroxide to the first acetic acid stream can decrease hydrogen iodide in the first acetic acid stream. Specifically, hydrogen iodide reacts with potassium hydroxide to give potassium iodide and water. This can decrease hydrogen iodide-induced corrosion of the equipment such as distillation columns. In this process, potassium hydroxide can be fed or added to an appropriate site where hydrogen iodide is present. The potassium hydroxide added in the process also reacts with acetic acid to give potassium acetate. 
     The distillation column  5  is a unit with which the second distillation step is performed. The distillation column  5  in the embodiment is characterized as a so-called dehydration column. The second distillation step is the step of subjecting the first acetic acid stream, which is continuously introduced into the distillation column  5 , to distillation treatment to further purify acetic acid. 
     The distillation column  5  is selected typically from rectification columns such as a plate column and a packed column. The distillation column  5 , when being a plate column, typically has 5 to 50 theoretical plates and is operated at a reflux ratio of typically 0.2 to 3000, where the reflux ratio may be determined according to the number of theoretical plates. In the distillation column  5  during the second distillation step, the column top pressure is set typically to 150 to 250 kPaG; and the column bottom pressure is set typically to a pressure which is higher than the column top pressure and is typically 160 to 290 kPaG. In the distillation column  5  during the second distillation step, the column top temperature is set typically to a temperature which is higher than the boiling point of water and lower than the boiling point of acetic acid at the set column top pressure and is from 130° C. to 160° C.; and the column bottom temperature is typically set to a temperature which is equal to or higher than the boiling point of acetic acid at the set column bottom pressure and is from 150° C. to 175° C. 
     At the distillation column  5 , vapors as an overhead stream are continuously drawn from a column top portion to the line  33 ; and bottoms are continuously drawn from a column bottom portion to the line  34 . There is disposed the reboiler  5   b . A side stream (liquid or gas) may be continuously drawn to the line  34  from the distillation column  5  at a height position between the column top and the column bottom. 
     The vapors drawn from the column top portion of the distillation column  5  include larger amounts of light ends as compared with the bottoms from the distillation column  5  and typically include methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, where the light ends are components having lower boiling points as compared with acetic acid. The vapors as above are continuously introduced through the line  33  into the condenser  5   a.    
     The condenser  5   a  cools and partially condenses the vapors from the distillation column  5  to separate the vapors into a condensate and a gas. The condensate typically includes water and acetic acid. Of the condensate, a portion is continuously refluxed from the condenser  5   a  through the line  35  to the distillation column  5 ; and another portion is continuously introduced and recycled from the condenser  5   a  through the lines  35 ,  36 , and  23  to the reactor  1 . The gas from the condenser  5   a  typically includes carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The gas is fed from the condenser  5   a  through the lines  37  and  15  to the scrubbing system  8 . The gas from the condenser  5   a  may be fed to the scrubbing system  8  without being merged into the line  15 , as described above. Hydrogen iodide in the gas that reaches the scrubbing system  8  is absorbed by the absorbing liquids (absorbents) in the scrubbing system  8 , and then reacts with methanol or methyl acetate in the absorbing liquids, to give methyl iodide. The resulting liquid (overhead stream from the column top of the distillation column  84 ) containing the methyl iodide and other useful components is recycled from the scrubbing system  8  through the recycle lines  48  and  23  to the reactor  1  and reused. 
     The bottoms drawn from the column bottom portion of the distillation column  5  (or the side stream) include larger amounts of heavy ends as compared with the overhead stream from the distillation column  5 , where the heavy ends are components having higher boiling points as compared with acetic acid. The bottoms (or the side stream) typically include acetic anhydride, propionic acid, acetate salts, and iodide salts such as potassium iodide and metal iodides derived from metals such as corrosion metals, as well as the catalyst and the promoter as being entrained. Non-limiting examples of the acetate salts include metal acetates such as potassium acetate which is formed when potassium hydroxide or another alkali is fed typically to the line  27 . Non-limiting examples of the acetate salts also include metal acetates formed by acetic acid and metals such as corrosion metals, including metals liberated from the inner walls of constitutional elements of the acetic acid production equipment. A non-limiting example of the iodide salts is potassium iodide which is formed when potassium hydroxide or another alkali is fed typically to the line  27 . The bottoms may also include acetic acid. The bottoms as above are continuously introduced, as a second acetic acid stream, through the line  34  into the subsequent (downstream) distillation column  6 . The bottoms drawn from the column bottom portion of the distillation column  5  (or side stream) also include the metals such as corrosion metals; and compounds (iodide salts) between the metals such as corrosion metals and iodine derived from corrosive iodine. The bottoms as above are, in the embodiment, discharged out of the acetic acid production equipment. 
     The second acetic acid stream is enriched with acetic acid as compared with the first acetic acid stream, which is continuously introduced into the distillation column  5 . Specifically, the acetic acid concentration in the second acetic acid stream is higher than the acetic acid concentration in the first acetic acid stream. The acetic acid concentration in the second acetic acid stream is typically 99.1 to 99.99 mass percent, as long as being higher than the acetic acid concentration in the first acetic acid stream. The second acetic acid stream may include other components such as propionic acid and hydrogen iodide in addition to acetic acid, as described above. In the embodiment, the side stream, when to be drawn, is drawn from the distillation column  5  at a height lower than the height at which the first acetic acid stream is introduced into the distillation column  5 . 
     To the second acetic acid stream flowing through the line  34 , potassium hydroxide may be fed or added through the line  56  (potassium hydroxide inlet line). The potassium hydroxide may be fed or added as a solution such as an aqueous solution. The feeding or addition of potassium hydroxide to the second acetic acid stream can decrease hydrogen iodide in the second acetic acid stream. Specifically, hydrogen iodide reacts with potassium hydroxide to form potassium iodide and water. This can decrease corrosion of the equipment such as distillation columns, where the corrosion will be caused by hydrogen iodide. 
     The distillation column  6  is a unit with which the third distillation step is performed. The distillation column  6  in the embodiment is characterized as a so-called heavy ends column. The third distillation step is the step of subjecting the second acetic acid stream, which is continuously introduced into the distillation column  6 , to purification treatment to further purify acetic acid. 
     The distillation column  6  may be selected typically from rectification columns such as a plate column and a packed column. The distillation column  6 , when being a plate column, typically has 5 to 50 theoretical plates and is operated at a reflux ratio of typically 0.2 to 3000, where the reflux ratio may be determined according to the number of theoretical plates. In the distillation column  6  during the third distillation step, the column top pressure is set typically to −100 to 150 kPaG; and the column bottom pressure is set to a pressure which is higher than the column top pressure and is typically −90 to 180 kPaG. In the distillation column  6  during the third distillation step, the column top temperature is set typically to a temperature which is higher than the boiling point of water and lower than the boiling point of acetic acid at the set column top pressure and is from 50° C. to 150° C.; and the column bottom temperature is set typically to a temperature which is higher than the boiling point of acetic acid at the set column bottom pressure and is from 70° C. to 160° C. 
     At the distillation column  6 , vapors as an overhead stream are continuously drawn from a column top portion to the line  38 ; and bottoms are continuously drawn from a column bottom portion to the line  39 . There is disposed the reboiler  6   b . A side stream (liquid or gas) is continuously drawn, to the line  46 , from the distillation column  6  at a height position between the column top and the column bottom. The line  46  may be coupled to the distillation column  6  at a height higher than the coupling height of the line  34  to the distillation column  6 , as illustrated in the figure, but may be coupled to the distillation column  6  at a height lower than or equal to, the coupling height of the line  34  to the distillation column  6 . 
     The vapors drawn from the column top portion of the distillation column  6  include larger amounts of light ends as compared with the bottoms from the distillation column  6 , where the light ends are components having lower boiling points as compared with acetic acid. The vapors include acetic acid; and other components such as methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, and formic acid. The vapors as above are continuously introduced through the line  38  into the condenser  6   a.    
     The condenser  6   a  cools and partially condenses the vapors from the distillation column  6  to separate the vapors into a condensate and a gases. The condensate includes acetic acid; and other components such as methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, and formic acid. At least a portion of the condensate is continuously refluxed from the condenser  6   a  through the line  40  to the distillation column  6 . A portion (distillate) of the condensate can be recycled from the condenser  6   a  through the lines  40 ,  41 , and  42  to the first acetic acid stream in the line  27  before introduction into the distillation column  5 . In addition to or instead of this, a portion (distillate) of the condensate can be recycled from the condenser  6   a  through the lines  40 ,  41 , and  43  to the vapor stream in the line  21  before introduction into the distillation column  3 . 
     A portion (distillate) of the condensate may be recycled from the condenser  6   a  through the lines  40 ,  44 , and  23  to the reactor  1 . A portion of the distillate from the condenser  6   a  can be fed to the scrubbing system  8  and be used as the absorbing liquid or liquids in the system, as described above. At the scrubbing system  8 , gases from which useful components have been absorptively removed are discharged out of the equipment, whereas the liquid (overhead stream from the column top of the distillation column  84 ) containing the useful components is introduced or recycled from the scrubbing system  8  through the recycle lines  48  and  23  to the reactor  1  and reused. In addition, a portion of the distillate from the condenser  6   a  may be brought through lines (not shown) to various pumps (not shown) operated in the equipment and be used as a sealing liquid for the pumps. Further, a portion of the distillate from the condenser  6   a  may be drawn out of the system through a draw line attached to the line  40  steadily, or non-steadily at the time of need. 
     When a portion (distillate) of the condensate is removed from the distillation treatment system in the distillation column  6 , the amount of the distillate (distillate amount) is typically 0.01 to 30 mass percent, preferably 0.1 to 10 mass percent, more preferably 0.3 to 5 mass percent, and furthermore preferably 0.5 to 3 mass percent, of the condensate obtained by the working of the condenser  6   a . In contrast, the gas formed in the condenser  6   a  typically includes carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The gas is fed from the condenser  6   a  through the lines  45  and  15  to the scrubbing system  8 . The gas from the condenser  6   a  may be fed to the scrubbing system  8  without being merged into the line  15 , as described above. 
     The bottoms drawn from the column bottom portion of the distillation column  6  through the line  39  include larger amounts of heavy ends as compared with the overhead stream from the distillation column  6  and typically include acetate salts, acetic anhydride, and propionic acid, where the heavy ends are components having higher boiling points as compared with acetic acid. A non-limiting example of the acetate salts is potassium acetate formed when potassium hydroxide or another alkali is fed typically to the line  34 . Non-limiting examples of the acetates also include metal acetates formed between acetic acid and metals such as corrosion metals, which are exemplified by metals liberated from the inner walls of constitutional elements of the acetic acid production equipment. The bottoms drawn from the column bottom portion of the distillation column  6  through the line  39  further include the metals such as corrosion metals; and compounds between the metals such as corrosion metals and iodine derived from corrosive iodine. In the embodiment, the bottoms as above are discharged out of the acetic acid production equipment. 
     The side stream continuously drawn from the distillation column  6  to the line  46  is continuously introduced, as a third acetic acid stream, into the subsequent (downstream) ion exchange resin column  7 . The third acetic acid stream is enriched with acetic acid as compared with the second acetic acid stream, which is continuously introduced into the distillation column  6 . Specifically, the acetic acid concentration in the third acetic acid stream is higher than the acetic acid concentration in the second acetic acid stream. The acetic acid concentration in the third acetic acid stream is typically 99.8 to 99.999 mass percent, as long as being higher than the acetic acid concentration in the second acetic acid stream. In the embodiment, the side stream is drawn from the distillation column  6  at a height higher than the height at which the second acetic acid stream is introduced into the distillation column  6 . In another embodiment, the side stream is drawn from the distillation column  6  at a height equal to or lower than the height at which the second acetic acid stream is introduced into the distillation column  6 . In place of the distillation column  6 , a simple distillator (evaporator) is usable. The distillation column  6  can be omitted when impurities are sufficiently removed by the working of the distillation column  5 . 
     The ion exchange resin column  7  is a purification unit with which the adsorptive removing step is performed. The adsorptive removing step is the step of removing, by adsorption, mainly alkyl iodides contained in trace amounts in the third acetic acid stream, to further purify acetic acid, where the third acetic acid stream is continuously introduced into the ion exchange resin column  7 . Non-limiting examples of the alkyl iodides include ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, and decyl iodide. 
     In the ion exchange resin column  7 , an ion exchange resin capable of adsorbing alkyl iodides is packed to form an ion exchange resin bed. Non-limiting examples of the ion exchange resin as above include cation-exchange resins with part of leaving protons in exchange groups being substituted or replaced with a metal such as silver or copper, where the exchange groups are exemplified typically by sulfonic groups, carboxy groups, and phosphonate groups. In the adsorptive removing step, the third acetic acid stream (liquid) passes through the inside of the ion exchange resin column  7  packed typically with the ion exchange resin as above, and, during the passing process, alkyl iodides and other impurities are adsorbed and removed from the third acetic acid stream by the ion exchange resin. In the ion exchange resin column  7  during the adsorptive removing step, the internal temperature is typically 18° C. to 100° C., and the acetic acid stream flow rate is typically 3 to 15 m 3 /h·m 3  (resin volume), where the acetic acid stream flow rate is the acetic acid throughput (m 3 /h) per cubic meter of the resin volume. 
     From a bottom portion of the ion exchange resin column  7 , a fourth acetic acid stream is continuously drawn to the line  47 . The fourth acetic acid stream has a higher acetic acid concentration as compared with the third acetic acid stream. Specifically, the fourth acetic acid stream is enriched with acetic acid as compared with the third acetic acid stream, which is continuously introduced into the ion exchange resin column  7 . The acetic acid concentration in the fourth acetic acid stream is typically 99.9 to 99.999 mass percent, or more, as long as being higher than the acetic acid concentration in the third acetic acid stream. In the production method, the fourth acetic acid stream can be stored in a product tank (not shown). 
     This acetic acid production equipment may include a so-called product column or finishing column as a purification unit for further purifying the fourth acetic acid stream from the ion exchange resin column  7 , where the product column or finishing column is a distillation column. The product column as above, when provided, may be selected typically from rectification columns such as a plate column and a packed column. The product column, when being a plate column, typically has 5 to 50 theoretical plates and is operated at a reflux ratio of typically 0.5 to 3000, where the reflux ratio may be determined according to the number of theoretical plates. In the product column during the purification step, the column top pressure is set typically to −195 to 150 kPaG; and the column bottom pressure is set to a pressure which is higher than the column top pressure and is typically −190 to 180 kPaG. In the product column, the column top temperature is typically set to a temperature which is higher than the boiling point of water and lower than the boiling point of acetic acid at the set column top pressure and is from 50° C. to 150° C.; and the column bottom temperature is set typically to a temperature which is higher than the boiling point of acetic acid at the set column bottom pressure and is from 70° C. to 160° C. In place of the product column or finishing column, a simple distillator (evaporator) is usable. 
     Into the product column, when provided, all or a portion of the fourth acetic acid stream (liquid) from the ion exchange resin column  7  is continuously introduced. At the product column as above, vapors as an overhead stream are continuously drawn from a column top portion, where the vapors include trace amounts of light ends such as methyl iodide, water, methyl acetate, dimethyl ether, crotonaldehyde, acetaldehyde, and formic acid. The vapors are separated, using a predetermined condenser, into a condensate and a gas. 
     Of the condensates, a portion is continuously refluxed to the product column; and another portion may be recycled to the reactor  1 , or be discarded out of the system, or both. The gas is fed to the scrubbing system  8 . At the product column, bottoms including trace amounts of heavy ends are continuously drawn from a column bottom portion, and are typically recycled to the second acetic acid stream in the line  34  before introduction into the distillation column  6 . At the product column, a side stream (liquid) as a fifth acetic acid stream is continuously drawn from a portion at a height position between the column top and the column bottom. The side stream is drawn from a portion of the product column typically at a height, lower than the height at which the fourth acetic acid stream is introduced into the product column. 
     The fifth acetic acid stream is enriched with acetic acid as compared with the fourth acetic acid stream, which is continuously introduced into the product column. Specifically, the acetic acid concentration in the fifth acetic acid stream is higher than the acetic acid concentration in the fourth acetic acid stream. The acetic acid concentration in the fifth acetic acid stream is typically 99.9 to 99.999 mass percent, or more, as long as being higher than the acetic acid concentration in the fourth acetic acid stream. The fifth acetic acid stream is stored typically in a product tank (not shown). Instead of, or in addition to being disposed downstream from the distillation column  6 , the ion exchange resin column  7  may be disposed downstream from the product column, for the treatment of the acetic acid stream from the product column. 
     EXAMPLES 
     The present invention will be illustrated in further detail with reference to several examples below. It should be noted, however, that the examples are by no means intended to limit the scope of the present invention. All percentages, parts per million (ppm), and parts per billion (ppb) are by mass. Hydrogen iodide concentrations were determined by the subtraction technique. 
     Comparative Example 1 
     An experiment was performed using the scrubbing system illustrated in  FIG. 9( a ) . A high-pressure charge gas (1) and a low-pressure charge gas (5) were charged respectively into a high-pressure absorption column A (having 5 theoretical plates) and a low-pressure absorption column B (having 5 theoretical plates), circulating acetic acid was introduced through the column tops of the two absorption columns, and dispersed from the upper portions of the absorption columns using dispersing plates to absorb condensable gases including an iodine compound or compounds. Thus, an absorption step was performed, and absorbing liquids were drawn from the column bottoms of the absorption columns. A high-pressure offgas (3) from the column top of the high-pressure absorption column A, and a low-pressure offgas (7) from the column top of the low-pressure absorption column B were merged with each other and discharged out of the system. Bottoms (4) from the high-pressure absorption column A and bottoms (8) from the low-pressure absorption column B were merged to give a charge liquid (9). The charge liquid (9) was charged to a central portion (between upper 2.5 theoretical plates and lower 2.5 theoretical plates) of a distillation column C (having 5 theoretical plates) to perform a stripping step. The charge liquid (9) was then heated with steam in the distillation column C, by which light ends other than acetic acid were concentrated at the column top to give an overhead stream (10), and this was distilled at a reflux ratio of 1 and recycled to the reactor, where the reflux ratio was defined as the ratio of the reflux amount to the distillate amount. The amount of steam used herein was defined as 100. Acetic acid (11) after the stripping was drawn from the column bottom of the distillation column C, cooled, combined with fresh acetic acid (12), and cycled as the absorbing liquids (absorbents) (2) and (6) to the high-pressure absorption column and the low-pressure absorption column. The two absorption columns and the distillation column were each packed with a structured packing Mellapak 250X supplied by Sulzer Chemtech Ltd. In this experiment, drawing of the bottoms from the distillation column out of the system was not performed. The hydrogen iodide concentration in the bottoms from the distillation column was 15 ppm by mass. The flow rates of, and the concentrations of individual components in, the elements (1) to (11) are presented in Table 1. 
     In the table, “AD” stands for acetaldehyde, “MeI” stands for methyl iodide, “MA” stands for methyl acetate, “AC” stands for acetic acid, and “PA” stands for propionic acid. The symbol “-” in the table indicates a component for which the measurement given in the table was not performed, or indicates that no measurement was performed for the concentration of the component in question. The components “Others” may include, for example, components in the tables whose concentrations had not been measured; methanol, dimethyl ether, alkanes, crotonaldehyde, and other substances that adversely affect the potassium permanganate test result (permanganate time); and organic iodine compounds. 
     In the bottom liquid at the column bottom of the distillation column C, metal test pieces were placed, subjected to the experiment at 147° C. continuously for 500 hours, and, after the completion of the experiment, retrieved from the bottoms. These test pieces were evaluated for corrosivity as the thickness reduction rates (corrosion rates) a year on the basis of the mass changes of the test pieces. The test pieces used herein were test pieces having a size of 36 mm by 25 mm by 2.5 mm and being made of zirconium (Zr), a nickel base alloy Hastelloy B2 (supplied as HB 2 by Oda Koki Co., Ltd.), a nickel base alloy Hastelloy C (supplied as HC 276 by Oda Koki Co., Ltd.), and a stainless steel SUS 316 (supplied as SUS 316 by UMETOKU Inc.). As indicated in Table 5, only the test piece made of zirconium had a thickness reduction rate of less than 0.05 mm/Y (millimeter per year), indicating complete corrosion resistance. This demonstrates that Hastelloys and lower-grade materials are not usable under the conditions of Comparative Example 1. In this connection, during the experiment, the test pieces as well as a test piece made of SUS 304 (supplied by UMETOKU Inc.) having the same size were placed in the bottom liquids at the column bottoms of the high-pressure absorption column A and the low-pressure absorption column B, both of which columns were in low-temperature regions, and were evaluated by a procedure as above. As a result, it was found that all the test pieces had thickness reduction rates of less than 0.05 mm/Y, indicating complete corrosion resistance. This demonstrated that these materials are usable for the absorption columns without problems. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 (2) 
                   
                 (4) 
                   
                 (6) 
                   
                 (8) 
                 (9) 
                 (10) 
                   
               
               
                   
                 (1) 
                 High- 
                   
                 High- 
                 (5) 
                 Low- 
                   
                 Low- 
                 Dis- 
                 Dis- 
                 (11) 
               
               
                   
                 High- 
                 pressure 
                 (3) 
                 pressure 
                 Low- 
                 pressure 
                 (7) 
                 pressure 
                 til- 
                 til- 
                 Dis- 
               
               
                   
                 pres- 
                 absorption 
                 High- 
                 absorp- 
                 pres- 
                 absorption 
                 Low- 
                 absorp- 
                 lation 
                 lation 
                 til- 
               
               
                   
                 sure 
                 column 
                 pres- 
                 tion 
                 sure 
                 column 
                 pres- 
                 tion 
                 column 
                 column 
                 lation 
               
               
                   
                 charge 
                 absorbing 
                 sure 
                 column 
                 charge 
                 absorbing 
                 sure 
                 column 
                 charge 
                 distil- 
                 column 
               
               
                   
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 liquid 
                 late 
                 bottoms 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Flow 
                 part 
                 1.2 
                 18.6 
                 0.9 
                 18.9 
                 16.0 
                 74.6 
                 9.5 
                 81.1 
                 100.0 
                 11.1 
                 88.9 
               
               
                 rate 
                 by 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 mass 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 H 2   
                 mass % 
                 0.3 
                 — 
                 0.4 
                 — 
                 0.5 
                 — 
                 0.8 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CO 
                 mass % 
                 67.1 
                 — 
                 86.2 
                 — 
                 42.2 
                 — 
                 71.2 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CO 2   
                 mass % 
                 0.9 
                 — 
                 1.2 
                 — 
                 4.7 
                 — 
                 8.0 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CH 4   
                 mass % 
                 3.0 
                 — 
                 3.9 
                 — 
                 5.3 
                 — 
                 9.0 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 N 2   
                 mass % 
                 4.8 
                 — 
                 6.1 
                 — 
                 5.7 
                 — 
                 9.6 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 AD 
                 mass % 
                 0.3 
                 — 
                 — 
                 0.0 
                 0.1 
                 — 
                 — 
                 0.0 
                 0.0 
                 0.2 
                 — 
               
               
                 MeI 
                 mass % 
                 21.4 
                 0.0 
                 — 
                 1.4 
                 37.3 
                 0.0 
                 — 
                 7.3 
                 6.2 
                 56.1 
                 — 
               
               
                 MA 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 1.9 
                 — 
                 — 
                 0.4 
                 0.3 
                 2.8 
                 — 
               
               
                 H 2 O 
                 mass % 
                 0.1 
                 0.3 
                 — 
                 0.3 
                 0.1 
                 0.3 
                 — 
                 0.3 
                 0.3 
                 0.2 
                 0.3 
               
               
                 AC 
                 mass % 
                 0.1 
                 99.0 
                 0.1 
                 97.6 
                 0.0 
                 99.0 
                 0.0 
                 91.0 
                 92.3 
                 38.4 
                 99.0 
               
               
                 PA 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 HI 
                 mass % 
                 0.0111 
                 0.0014 
                 0.0029 
                 0.0020 
                 0.0084 
                 0.0014 
                 0.0028 
                 0.0026 
                 0.0025 
                 0.0106 
                 0.0015 
               
               
                 Others 
                 mass % 
                 2.0 
                 0.8 
                 2.1 
                 0.8 
                 2.1 
                 0.8 
                 1.4 
                 1.0 
                 0.9 
                 2.3 
                 0.8 
               
               
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Tem- 
                 ° C. 
                 26.0 
                 28.8 
                 35.9 
                 33.2 
                 14.8 
                 23.5 
                 24.0 
                 32.4 
                 24.0 
                 24.0 
                 146.6 
               
               
                 per- 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 ature 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Pres- 
                 kPaG 
                 2760 
                 2800 
                 2750 
                 2750 
                 128 
                 122 
                 122 
                 123 
                 122 
                 119 
                 133 
               
               
                 sure 
               
               
                   
               
            
           
         
       
     
     Comparative Example 2 
     An experiment was performed by a procedure similar to that in Comparative Example 1, except for not preforming the absorption step using the high-pressure absorption column A. The amount of steam used in the distillation column in the stripping step was 90. The flow rates and the concentrations of individual components in the elements (5) to (11) are given in Table 2. The results of the corrosivity test are given in Table 5. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 (6) 
                   
                   
                   
                   
                   
               
               
                   
                   
                 Low- 
                   
                 (8) 
                   
                   
                   
               
               
                   
                 (5) 
                 pressure 
                   
                 Low- 
                 (9) 
                   
                   
               
               
                   
                 Low- 
                 absorption 
                 (7) 
                 pressure 
                 Distillation 
                 (10) 
                 (11) 
               
               
                   
                 pressure 
                 column 
                 Low- 
                 absorption 
                 column 
                 Distillation 
                 Distillation 
               
               
                   
                 charge 
                 absorbing 
                 pressure 
                 column 
                 charge 
                 column 
                 column 
               
               
                   
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 liquid 
                 distillate 
                 bottoms 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Flow rate 
                 part by mass 
                 19.5 
                 92.0 
                 11.6 
                 100.0 
                 100.0 
                 13.2 
                 86.8 
               
               
                 H 2   
                 mass % 
                 0.5 
                 — 
                 0.8 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CO 
                 mass % 
                 42.2 
                 — 
                 71.2 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CO 2   
                 mass % 
                 4.7 
                 — 
                 8.0 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 CH 4   
                 mass % 
                 5.3 
                 — 
                 9.0 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 N 2   
                 mass % 
                 5.7 
                 — 
                 9.6 
                 0.0 
                 0.0 
                 0.0 
                 — 
               
               
                 AD 
                 mass % 
                 0.1 
                 — 
                 — 
                 0.0 
                 0.0 
                 0.2 
                 — 
               
               
                 MeI 
                 mass % 
                 37.3 
                 0.0 
                 — 
                 7.3 
                 7.3 
                 55.1 
                 — 
               
               
                 MA 
                 mass % 
                 1.9 
                 — 
                 — 
                 0.4 
                 0.4 
                 2.9 
                 — 
               
               
                 H 2 O 
                 mass % 
                 0.1 
                 0.3 
                 — 
                 0.2 
                 0.2 
                 0.2 
                 0.1 
               
               
                 AC 
                 mass % 
                 0.0 
                 99.0 
                 0.0 
                 91.0 
                 91.0 
                 39.3 
                 98.9 
               
               
                 PA 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 HI 
                 mass % 
                 0.0084 
                 0.0014 
                 0.0028 
                 0.0029 
                 0.0029 
                 0.0099 
                 0.0019 
               
               
                 Others 
                 mass % 
                 2.1 
                 0.8 
                 1.4 
                 1.1 
                 1.1 
                 2.3 
                 0.9 
               
               
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Temperature 
                 ° C. 
                 14.8 
                 23.5 
                 24.0 
                 32.4 
                 24.0 
                 24.0 
                 146.8 
               
               
                 Pressure 
                 kPaG 
                 128 
                 122 
                 122 
                 123 
                 122 
                 119 
                 133 
               
               
                   
               
            
           
         
       
     
     Example 1 
     An experiment was performed using the scrubbing system illustrated in  FIG. 9( b ) . A high-pressure charge gas (1) and a low-pressure charge gas (5) were charged respectively to a high-pressure absorption column A (having 5 theoretical plates) and to a low-pressure absorption column B (having 5 theoretical plates); whereas circulating water was introduced through the column tops of the two absorption columns, dispersed, using dispersing plates, from the upper portions of the absorption columns to allow the water to absorb a condensable gas including an iodine compound. Thus, a first absorption step was performed. Absorbing liquids (4) and (8) were drawn from the column bottom of the absorption columns. The absorbing liquids (4) and (8) drawn from the column bottoms of the two absorption columns were merged with each other; a portion (9) of the merged absorbing liquid was drawn, cooled, and combined with fresh water (10) to give an absorbing liquid (11). The absorbing liquid (11) was cycled and used as the absorbing liquids (2) and (6) in the high-pressure absorption column A and the low-pressure absorption column B. A portion of the drawn absorbing liquid was recycled through a decanter to the reactor, where the decanter was used for reserving an overhead stream condensate from a light ends column. A high-pressure offgas (3) from the column top of the high-pressure absorption column A and a low-pressure offgas (7) from the column top of the low-pressure absorption column B were merged with each other and charged (12) to a bottom portion of a low-pressure absorption column D (having 5 theoretical plates). From the column top of the low-pressure absorption column D, circulating acetic acid was introduced and dispersed, using a dispersing plate, from the upper portion of the absorption column to absorb a condensable gas including an iodine compound. Thus, a second absorption step was performed. An absorbing liquid (absorbed liquid) (15) was drawn from the column bottom of the absorption column. An offgas (14) from the column top of the low-pressure absorption column D was discharged out of the system. The bottoms (15) from the low-pressure absorption column D were charged to a central portion (between upper 2.5 theoretical plates and lower 2.5 theoretical plates) of a distillation column C (having 5 theoretical plates) to perform a stripping step. The bottoms were heated with steam in the distillation column to concentrate light ends other than acetic acid at the column top, to give an overhead stream (16). This was distilled at a reflux ratio of 1 and recycled to the reactor, where the reflux ratio was defined as the ratio of the reflux amount to the distillate amount. The amount of steam used herein was 100. A solution (17) after the stripping, which mainly contained acetic acid, was drawn from the column bottom of the distillation column, cooled, combined with fresh acetic acid (18), and cycled and used as the absorbing liquid (13) in the low-pressure absorption column D. Each of the three absorption columns and the distillation column employed a structured packing Mellapak 250X supplied by Sulzer Chemtech. In this experiment, drawing of the bottoms from the distillation column C out of the system was not performed. The hydrogen iodide concentration in the distillation column bottoms was less than the detection limit. The flow rates and the concentrations of individual components in the elements (1) to (9) and (11) to (17) are given in Table 3. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                   
                   
                   
                 (2) 
                   
                   
                   
                 (6) 
                   
                   
                   
               
               
                   
                   
                 (1) 
                 High- 
                   
                 (4) 
                 (5) 
                 Low- 
                   
                 (8) 
                   
               
               
                   
                   
                 High- 
                 pressure 
                 (3) 
                 High- 
                 Low- 
                 pressure 
                 (7) 
                 Low- 
                   
               
               
                   
                   
                 pres- 
                 adsorption 
                 High- 
                 pressure 
                 pres- 
                 absorption 
                 Low- 
                 pressure 
                 Merged 
               
               
                   
                   
                 sure 
                 column 
                 pres- 
                 absorption 
                 sure 
                 column 
                 pres- 
                 absorption 
                 bottoms 
               
               
                   
                   
                 charge 
                 absorbing 
                 sure 
                 column 
                 charge 
                 absorbing 
                 sure 
                 column 
                 (4) and 
               
               
                   
                   
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 (8)) 
               
               
                   
               
               
                 Flow rate 
                 part by mass 
                 1.2 
                 35.8 
                 1.5 
                 35.5 
                 16.0 
                 143.1 
                 15.7 
                 143.4 
                 178.9 
               
               
                 H 2   
                 mass % 
                 0.3 
                 — 
                 0.2 
                 — 
                 0.5 
                 — 
                 0.5 
                 — 
                 — 
               
               
                 CO 
                 mass % 
                 67.1 
                 — 
                 53.4 
                 — 
                 42.2 
                 — 
                 43.1 
                 — 
                 — 
               
               
                 CO 2   
                 mass % 
                 0.9 
                 — 
                 0.7 
                 — 
                 4.7 
                 — 
                 4.8 
                 — 
                 — 
               
               
                 CH 4   
                 mass % 
                 3.0 
                 — 
                 2.4 
                 — 
                 5.3 
                 — 
                 5.4 
                 — 
                 — 
               
               
                 N 2   
                 mass % 
                 4.8 
                 — 
                 3.8 
                 — 
                 5.7 
                 — 
                 5.8 
                 — 
                 — 
               
               
                 AD 
                 mass % 
                 0.3 
                 — 
                 0.2 
                 — 
                 0.1 
                 — 
                 0.1 
                 — 
                 — 
               
               
                 MeI 
                 mass % 
                 21.4 
                 0.6 
                 16.9 
                 0.6 
                 37.3 
                 0.6 
                 38.0 
                 0.6 
                 0.6 
               
               
                 MA 
                 mass % 
                 — 
                 0.3 
                 1.3 
                 0.3 
                 1.9 
                 0.3 
                 1.8 
                 0.3 
                 0.3 
               
               
                 H 2 O 
                 mass % 
                 0.1 
                 97.9 
                 1.0 
                 98.7 
                 0.1 
                 97.9 
                 0.0 
                 97.7 
                 97.9 
               
               
                 AC 
                 mass % 
                 0.1 
                 0.2 
                 0.0 
                 0.2 
                 0.0 
                 0.2 
                 0.0 
                 0.2 
                 0.2 
               
               
                 PA 
                 mass % 
                 — 
                 — 
                 0.0 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 HI 
                 mass % 
                 0.0111 
                 0.1423 
                 0.0001 
                 0.1440 
                 0.0084 
                 0.1423 
                 0.0001 
                 0.1429 
                 0.1431 
               
               
                 Others 
                 mass % 
                 2.0 
                 0.8 
                 20.0 
                 0.1 
                 2.1 
                 0.8 
                 0.3 
                 1.0 
                 0.9 
               
               
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Temperature 
                 ° C. 
                 25.8 
                 28.6 
                 35.5 
                 33.0 
                 14.2 
                 232 
                 23.9 
                 32.2 
                 25.1 
               
               
                 Pressure 
                 kPaG 
                 2750 
                 2990 
                 2740 
                 2740 
                 127 
                 121 
                 121 
                 122 
                 121 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 (9) 
                   
                 (12) 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Part of 
                   
                 Second 
                   
                 (14) 
                 (15) 
                 (16) 
                 (17) 
               
               
                   
                   
                   
                 First 
                 (11) 
                 absorp- 
                   
                 Second 
                 Second 
                 Distil- 
                 Distil- 
               
               
                   
                   
                   
                 absorption 
                 Circulating 
                 tion 
                 (13) 
                 absorp- 
                 absorp- 
                 lation 
                 lation 
               
               
                   
                   
                   
                 merged 
                 first 
                 charge 
                 Second 
                 tion 
                 tion 
                 column 
                 column 
               
               
                   
                   
                   
                 bottoms 
                 absorbent 
                 gas 
                 absorbent 
                 oilgas 
                 bottoms 
                 distillate 
                 bottoms 
               
               
                   
               
               
                   
                 Flow rate 
                 part by mass 
                 1.0 
                 178.9 
                 17.2 
                 94.7 
                 10.5 
                 101.3 
                 15.1 
                 86.2 
               
               
                   
                 H 2   
                 mass % 
                 — 
                 — 
                 0.5 
                 — 
                 0.8 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CO 
                 mass % 
                 — 
                 — 
                 44.0 
                 — 
                 72.0 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CO 2   
                 mass % 
                 — 
                 — 
                 4.5 
                 — 
                 7.3 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CH 4   
                 mass % 
                 — 
                 — 
                 5.2 
                 — 
                 8.4 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 N 2   
                 mass % 
                 — 
                 — 
                 5.6 
                 — 
                 9.2 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 AD 
                 mass % 
                 — 
                 — 
                 0.2 
                 — 
                 0.0 
                 0.0 
                 0.2 
                 — 
               
               
                   
                 MeI 
                 mass % 
                 0.6 
                 0.6 
                 36.2 
                 0.0 
                 0.0 
                 6.1 
                 41.2 
                 — 
               
               
                   
                 MA 
                 mass % 
                 0.3 
                 0.3 
                 1.8 
                 — 
                 0.0 
                 0.3 
                 2.0 
                 — 
               
               
                   
                 H 2 O 
                 mass % 
                 97.9 
                 97.9 
                 0.1 
                 0.2 
                 0.0 
                 0.2 
                 0.2 
                 0.2 
               
               
                   
                 AC 
                 mass % 
                 0.2 
                 0.2 
                 0.0 
                 99.1 
                 0.0 
                 92.6 
                 55.7 
                 99.0 
               
               
                   
                 PA 
                 mass % 
                 — 
                 — 
                 0.0 
                 — 
                 0.0 
                 — 
                 — 
                 — 
               
               
                   
                 HI 
                 mass % 
                 0.1431 
                 0.1423 
                 0.0001 
                 — 
                 0.0000 
                 0.0000 
                 0.0001 
                 0.0000 
               
               
                   
                 Others 
                 mass % 
                 0.9 
                 0.8 
                 2.0 
                 0.7 
                 2.2 
                 0.8 
                 0.8 
                 0.7 
               
               
                   
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 Temperature 
                 ° C. 
                 23.7 
                 32.1 
                 25.0 
                 33.2 
                 31.8 
                 32.9 
                 32.9 
                 146.5 
               
               
                   
                 Pressure 
                 kPaG 
                 121 
                 121 
                 121 
                 135 
                 119 
                 123 
                 119 
                 133 
               
               
                   
               
            
           
         
       
     
     Metal test pieces as above were placed in the bottom liquid at the column bottom of the distillation column C, and a corrosivity test was performed by a procedure similar to that in Comparative Example 1. As indicated in Table 5, not only the test piece made of zirconium, but also the test piece made of HC 276 had thickness reduction rates of less than 0.05 mm/Y, which indicates complete corrosion resistance. This demonstrated that the present invention enables the use of Hastelloys or lower-grade materials for the distillation column, in addition to zirconium. In this connection, the test pieces as well as a test piece made of SUS 304 (supplied by UMETOKU Inc.) having the same size were placed in the column bottoms of the high-pressure absorption column A and the low-pressure absorption column B, both of which were in low-temperature regions, and these test pieces were evaluated by a procedure as above. As a result, all the test pieces had thickness reduction rates of less than 0.05 mm/Y. This demonstrated that these materials are usable for the absorption columns without problems. 
     Example 2 
     An experiment was performed by a procedure similar to that in Example 1, except for not performing the absorption step using the high-pressure absorption column A. The amount of steam used at the distillation column in the stripping step was 90. The flow rates and the concentrations of individual components in the elements (5) to (9) and (11) to (17) are given in Table 4. The results of the corrosivity test are given in Table 5. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
             
            
               
                   
                   
                   
                 (6) 
                   
                   
                   
                   
                   
               
               
                   
                   
                 (5) 
                 Low- 
                   
                 (8) 
                   
                   
                   
               
               
                   
                   
                 Low- 
                 pressure 
                 (7) 
                 Low- 
                   
                 (9) 
                   
               
               
                   
                   
                 pres- 
                 absorption 
                 Low- 
                 pressure 
                   
                 Part of First 
                 (11) 
               
               
                   
                   
                 sure 
                 column 
                 pres- 
                 absorption 
                 Merged 
                 absorption 
                 Circulating 
               
               
                   
                   
                 charge 
                 absorbing 
                 sure 
                 column 
                 bottoms (4) 
                 merged 
                 first 
               
               
                   
                   
                 gas 
                 liquid 
                 offgas 
                 bottoms 
                 and (8)) 
                 bottoms 
                 absorbent 
               
               
                   
               
               
                 Flow rate 
                 part by mass 
                 16.0 
                 143.4 
                 16.0 
                 143.4 
                 143.4 
                 1.0 
                 143.4 
               
               
                 H 2   
                 mass % 
                 0.5 
                 — 
                 0.5 
                 — 
                 — 
                 — 
                 — 
               
               
                 CO 
                 mass % 
                 42.2 
                 — 
                 42.3 
                 — 
                 — 
                 — 
                 — 
               
               
                 CO 2   
                 mass % 
                 4.7 
                 — 
                 4.7 
                 — 
                 — 
                 — 
                 — 
               
               
                 CH 4   
                 mass % 
                 5.3 
                 — 
                 5.3 
                 — 
                 — 
                 — 
                 — 
               
               
                 N 2   
                 mass % 
                 5.7 
                 — 
                 5.7 
                 — 
                 — 
                 — 
                 — 
               
               
                 AD 
                 mass % 
                 0.1 
                 — 
                 0.1 
                 — 
                 — 
                 — 
                 — 
               
               
                 MeI 
                 mass % 
                 37.3 
                 0.6 
                 37.3 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
               
               
                 MA 
                 mass % 
                 1.9 
                 0.3 
                 1.9 
                 0.3 
                 0.3 
                 0.3 
                 0.3 
               
               
                 H 2 O 
                 mass % 
                 0.1 
                 97.7 
                 0.1 
                 97.7 
                 97.8 
                 97.8 
                 97.7 
               
               
                 AC 
                 mass % 
                 0.0 
                 0.2 
                 0.0 
                 0.2 
                 0.1 
                 0.1 
                 0.2 
               
               
                 PA 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 HI 
                 mass % 
                 0.0084 
                 0.1281 
                 0.0001 
                 0.1290 
                 0.1291 
                 0.1291 
                 0.1281 
               
               
                 Others 
                 mass % 
                 2.1 
                 1.0 
                 2.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Temperature 
                 ° C. 
                 14.2 
                 23.2 
                 23.9 
                 32.2 
                 23.7 
                 23.7 
                 32.0 
               
               
                 Pressure 
                 kPaG 
                 127 
                 121 
                 121 
                 122 
                 121 
                 121 
                 121 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 (12) 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Second 
                   
                 (14) 
                 (15) 
                 (16) 
                 (17) 
               
               
                   
                   
                   
                 absorp- 
                   
                 Second 
                 Second 
                 Distil- 
                 Distil- 
               
               
                   
                   
                   
                 tion 
                 (13) 
                 absorp- 
                 absorp- 
                 lation 
                 lation 
               
               
                   
                   
                   
                 charge 
                 Second 
                 tion 
                 tion 
                 column 
                 column 
               
               
                   
                   
                   
                 gas 
                 absorbent 
                 offgas 
                 bottoms 
                 distillate 
                 bottoms 
               
               
                   
               
               
                   
                 Flow rate 
                 part by mass 
                 16.0 
                 94.7 
                 9.5 
                 101.1 
                 14.8 
                 86.2 
               
               
                   
                 H 2   
                 mass % 
                 0.5 
                 — 
                 0.8 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CO 
                 mass % 
                 42.3 
                 — 
                 70.7 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CO 2   
                 mass % 
                 4.7 
                 — 
                 7.9 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 CH 4   
                 mass % 
                 5.3 
                 — 
                 8.9 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 N 2   
                 mass % 
                 5.7 
                 — 
                 9.5 
                 0.0 
                 0.0 
                 — 
               
               
                   
                 AD 
                 mass % 
                 0.1 
                 — 
                 0.0 
                 0.0 
                 0.2 
                 — 
               
               
                   
                 MeI 
                 mass % 
                 37.3 
                 0.0 
                 0.0 
                 5.9 
                 40.1 
                 — 
               
               
                   
                 MA 
                 mass % 
                 1.9 
                 — 
                 0.0 
                 0.3 
                 2.1 
                 — 
               
               
                   
                 H 2 O 
                 mass % 
                 0.1 
                 0.2 
                 0.0 
                 0.2 
                 0.2 
                 0.2 
               
               
                   
                 AC 
                 mass % 
                 0.0 
                 99.1 
                 0.0 
                 92.8 
                 56.7 
                 99.0 
               
               
                   
                 PA 
                 mass % 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 HI 
                 mass % 
                 0.0001 
                 — 
                 0.0001 
                 0.0000 
                 0.0001 
                 0.0000 
               
               
                   
                 Others 
                 mass % 
                 2.0 
                 0.7 
                 2.1 
                 0.8 
                 0.8 
                 0.7 
               
               
                   
                 Total 
                 mass % 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 Temperature 
                 ° C. 
                 24.8 
                 33.0 
                 31.5 
                 32.7 
                 32.7 
                 146.5 
               
               
                   
                 Pressure 
                 kPaG 
                 121 
                 135 
                 119 
                 123 
                 119 
                 133 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
                 Corrosivity evaluation results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Zr 
                 HB 2 
                 HC 276 
                 SUS 316 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Partial 
                   
                 Partial 
                   
                 Partial 
                   
                 Partial 
               
               
                   
                 mm/y 
                 corrosion 
                 mm/y 
                 corrosion 
                 mm/y 
                 corrosion 
                 mm/y 
                 corrosion 
               
               
                   
               
               
                 Comparative Example 1 
                 0.00 
                 absent 
                 0.06 
                 present 
                 0.10 
                 present 
                 0.35 
                 present 
               
               
                 Comparative Example 2 
                 0.00 
                 absent 
                 0.06 
                 present 
                 0.11 
                 present 
                 0.37 
                 present 
               
               
                 Example 1 
                 0.00 
                 absent 
                 0.02 
                 absent 
                 0.04 
                 absent 
                 0.11 
                 absent 
               
               
                 Example 2 
                 0.00 
                 absent 
                 0.02 
                 absent 
                 0.04 
                 absent 
                 0.12 
                 absent 
               
               
                   
               
               
                 Partial corrosion includes bead corrosion, pitting corrosion, and spot corrosion (pitting). 
               
            
           
         
       
     
     In general, the price is, from highest to lowest, Zr, HB 2, HC, and SUS. In consideration of this, a material or materials for apparatuses can be selected on the basis of corrosion rate according to rough criteria as follows, while the selection may be affected typically by the wall thickness of the material and renewal frequency of the apparatus. 
     Corrosion rate of 0.05 mm/Y or less: suitable for use; 
     Corrosion rate of from greater than 0.05 mm/Y to 0.1 mm/Y: at employable level; 
     Corrosion rate of from greater than 0.1 mm/Y to 0.2 mm/Y: usable under some conditions; and 
     Corrosion rate of greater than 0.2 mm/Y: unusable. 
     Discussion of Results 
     Example 1 employed two different absorbents containing specific components and having different compositions in the first and second absorption steps. This configuration allowed the first absorption step in the high-pressure absorption column and the low-pressure absorption column to absorb approximately no methyl iodide from the charge gas, but to efficiently selectively absorb approximately only hydrogen iodide from the charge gas. In contrast, the configuration allowed the second absorption step in the low-pressure absorption column to efficiently absorb methyl iodide from the charge gas. The configuration also allowed the stripping step in the distillation column to sufficiently concentrate methyl iodide at the column top of the distillation column and to give bottoms from which approximately no methyl iodide was detected. As a result, Example 1 enabled sufficient recovery of hydrogen iodide, efficient separation and obtaining of a methyl iodide-rich solution, and thus efficient separation between, and recovery of, hydrogen iodide and methyl iodide. In addition, Example 1 enabled sufficient recovery of hydrogen iodide in the first absorption step. This could restrain or impede hydrogen iodide from flowing or migrating into the absorption column for the second absorption step, and into the distillation column for the stripping step; and restrain hydrogen iodide from causing corrosion of the test pieces. Similar results to those of Example 1 were obtained even when only one absorption column was used in the first absorption step, as in Example 2. 
     In contrast, Comparative Example 1 performed the treatment of the offgas including hydrogen iodide and methyl iodide by the one-stage absorption step. This configuration failed to selectively recover one of hydrogen iodide and methyl iodide. In addition, the configuration caused the overhead stream from the distillation column in the stripping step to include hydrogen iodide and methyl iodide both in larger amounts as compared with Example 1. Thus, Comparative Example 1 failed to efficiently separate between, and recover hydrogen iodide and methyl iodide. Further, the configuration failed to sufficiently recover hydrogen iodide in the first absorption step and thereby failed to restrain hydrogen iodide from flowing into the distillation column for the stripping step. This tended to cause corrosion of the test pieces. 
     INDUSTRIAL APPLICABILITY 
     The acetic acid production methods according to the present invention enable industrial production of acetic acid by the methanol carbonylation process (methanol-acetic acid process). 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  reactor 
               2  evaporator 
               3 ,  5 ,  6  distillation column 
               4  decanter 
               7  ion exchange resin column 
               8  scrubbing system 
               9  acetaldehyde-removing system 
               16  reaction mixture feed line 
               17  vapor stream discharge line 
               18 ,  19  residue stream recycle line 
               54  carbon monoxide-containing gas inlet line 
               55 ,  56  potassium hydroxide inlet line 
               57  catalyst circulating pump 
               81 ,  82 ,  83  absorption column 
               84  distillation column (stripper) 
               91  distillation column (first acetaldehyde-removing column) 
               92  extraction column 
               93  distillation column (second acetaldehyde-removing column) 
               94  distillation column (extractive distillation column) 
               95  decanter 
               96  decanter 
               97  distillation column (acetaldehyde-removing column) 
               98  distillation column (extractive distillation column) 
               99  decanter 
               200  chimney tray