Patent Publication Number: US-2023133006-A1

Title: Method for producing purified acetic acid

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for producing purified acetic acid. More particularly, the present disclosure relates to a method for producing purified acetic acid, in which the acetic acid is recovered from a mixed solution containing acetic acid, an organic solvent, and water. The present application claims priority from the Japanese patent application No. 2020-063081, filed in Japan on Mar. 31, 2020, the contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     Cellulose acetate is used in various applications including cigarette filter tows, fibers, photographic films, and artificial kidneys. As a method of producing cellulose acetate, a method in which an acetylating agent is subjected to a reaction with raw material pulp or cellulose (hereinafter referred to as “raw material pulp or the like”) in an acetic acid solvent is known. In this method, an acetic acid aqueous solution (hereinafter referred to as “raw acid”) with an acetic acid concentration of 20 to 50 wt.% in which a certain amount of cellulose acetate is dissolved is discharged. Acetic acid is recovered from this raw acid and reused in the production of cellulose acetate. 
     In the process of recovering acetic acid from the raw acid, the raw acid is first extracted with an organic solvent, the resultant is separated into an extract containing acetic acid, an organic solvent, and water, and a raffinate mainly containing water, and then the extract is distilled to recover acetic acid. In this distillation, it is necessary to separate not only the acetic acid and the organic solvent but also the water. A known distillation method utilizing a boiling point difference is commonly used to separate acetic acid, an organic solvent, and water (Patent Documents 1 and 2). 
     Patent Document 3 discloses a method for recovering acetic acid including: extracting an acetic acid aqueous solution with an organic solvent; separating the extract by a separation portion including a water-selective permeable separation membrane; supplying a fraction that has passed through the separation portion to the extraction portion; and supplying a fraction that has not passed through the separation portion to a dehydration-desolvation column to continuously recover acetic acid. Patent Document 4 discloses a method for concentrating acetic acid including: distilling a raw material, which is a mixture of acetic acid and water, in a distillation column to obtain overhead vapor containing acetic acid and water; separating the overhead vapor by a vapor permeable separation membrane to remove water; condensing the acetic acid vapor by utilizing the acetic acid vapor after removing water to heat the raw material. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: JP 09-048744 
         Patent Document 2: JP 10-114699 
         Patent Document 3: JP 02-111404 
         Patent Document 4: JP 2014-226573 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the difference in boiling point between acetic acid and water is small, and it is difficult to completely separate water and acetic acid by a distillation method alone. Furthermore, in the separation of acetic acid and water, it is necessary to evaporate and distill water having high latent heat of vaporization, and thus there is an issue that the energy efficiency is not good. 
     In the method of Patent Document 3, since water is excessively taken, azeotropy is unlikely to occur in the distillation process, thereby causing issues such as the rise of energy requirement and the high flow rate that increases the equipment cost due to the large membrane area. Further, in the method of Patent Document 4, since the vapor at the top of the distillation column is dehydrated by the vapor permeation-separation membrane, the amount of vapor to be used itself is not reduced, and the issue still remains in that a large amount of energy is used. Furthermore, in this method, acetic acid cannot be efficiently separated from a mixed liquid containing acetic acid, an organic solvent, and water. 
     Therefore, an object of the present disclosure is to provide a method for producing purified acetic acid from a mixed solution containing acetic acid, an organic solvent, and water in an energy-efficient manner. 
     Solution to Problem 
     As a result of diligent research to solve the issues described above, the present inventors have studied and found that when purified acetic acid is produced from a mixed solution containing acetic acid, an organic solvent, and water, the membrane separation step that includes separating water in the purified liquid in the distillation column by the separation membrane is incorporated into the process at the subsequent distillation step, thereby minimizing the separation of acetic acid and water in the distillation step and reducing the energy load. The present disclosure has been completed based on these findings. 
     That is, the present disclosure provides the followings: 
     [1] A method for producing purified acetic acid from a mixed solution containing acetic acid, an organic solvent, and water, the method including: a distillation step of distilling the mixed solution and separating the mixed solution into a purified liquid rich in acetic acid and a separated liquid rich in an organic solvent; and a membrane separation step of separating the water from the purified liquid by a separation membrane, in which the purified liquid has a water concentration of 4 wt.% or less;   [2] The method for producing purified acetic acid according to [1], in which the purified liquid has an ethyl acetate concentration as measured by gas chromatography of equal to or less than a detection limit;   [3] The method for producing purified acetic acid according to [1] or [2], in which the purified liquid has an organic solvent concentration as measured by gas chromatography of equal to or less than a detection limit;   [4] The method for producing purified acetic acid according to any one of [1] to [3], in which the membrane separation step is performed by bringing the purified liquid into contact with the separation membrane under reduced pressure;   [5] The method for producing purified acetic acid according to any one of [1] to [4] above, in which the separation membrane is one or more zeolite separation membranes selected from the group consisting of A-type, FAU-type, CHA-type, MFI-type, MOR-type, and DDR-type zeolite separation membranes; and   [6] The method for producing purified acetic acid according to any one of [1] to [5], in which the mixed solution is derived from a liquid discharged during a process of producing cellulose acetate.   
 Note that the organic solvent does not contain acetic acid.
     Advantageous Effects of Invention 
     According to [1] above of the present disclosure, the membrane separation step that includes separating water in the purified liquid by the separation membrane is incorporated to the process at the subsequent distillation step, and thus it is possible efficiently separate water by the subsequent membrane separation step even in a case where the water concentration in the purified liquid in the distillation column reaches 4 wt.%. Thus, an excellent energy-saving effect is exerted such that the energy load required for water separation is greatly reduced in the distillation step. 
     According to [2] above of the present disclosure, even when the separation efficiency in the distillation step is decreased by reducing the energy load, the ethyl acetate concentration in the purified liquid is equal to less than a detection limit, and thus high-quality purified acetic acid free from ethyl acetate can be obtained by subjecting the purified liquid to the membrane separation step. 
     According to [3] above of the present disclosure, even when the separation efficiency in the distillation step is decreased by reducing the energy load, the detectable level of concentration of the organic solvent in the purified liquid is equal to less than a detection limit, and thus high-quality purified acetic acid free from contamination with the organic solvent can be obtained by subjecting the purified liquid to the membrane separation step. 
     According to [4] above of the present disclosure, separation between acetic acid and water can be achieved by the difference in pressure between the inlet side and the permeate side of the separation membrane remaining in a liquid state. Therefore, when water is separated from acetic acid, it is not necessary to vaporize the purified liquid by heating, and thus an excellent vapor-saving effect is given and the energy load can be greatly reduced. 
     According to [5] above of the present disclosure, a specific zeolite separation membrane is used, and thus an effect is exerted in which the operating conditions with excellent dehydration efficiency can be adjusted depending on the composition of the raw material. 
     According to [6] above of the present disclosure, it is possible to further improve the separation efficiency of acetic acid contained in the liquid (i.e., raw acid) discharged during the process of producing cellulose acetate. These result in an excellent vapor-saving effect in the process of recovering acetic acid, and the energy load can be greatly reduced. Recycling of the purified acetic acid obtained by the method of the present invention during the process of producing cellulose acetate can also contribute to a reduction in the production cost of cellulose acetate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic flow chart illustrating an example of a method for producing purified acetic acid of the present disclosure. 
         FIG.  2    is a schematic flow chart illustrating an apparatus for testing performance of a separation membrane in a preliminary test. 
         FIG.  3    is a graph showing a relationship between an acetic acid concentration X11 and a permeation rate Q in a model purified liquid in the preliminary test. 
         FIG.  4    is a graph showing a relationship between the acetic acid concentration X11 in the model purified liquid in the preliminary test and an acetic acid concentration X21 in a permeated liquid. 
         FIG.  5    is a graph showing a relationship between the acetic acid concentration X11 in the model purified liquid in the preliminary test and a permeability coefficient K. 
         FIG.  6    is a graph showing a relationship between the acetic acid concentration X11 in the model purified liquid in the preliminary test and a separation coefficient α. 
         FIG.  7    is a graph showing a vapor use rate (when the reference is 100) of a distillation column  3  in a case where the water concentration in the purified liquid in the distillation column  3  in Example 1 is varied from 0 to 5 wt.%. 
         FIG.  8    is a graph showing the acetic acid concentration (wt. ppm, indicated by a rhombus) in an upper layer liquid in a decanter and the ethyl acetate concentration (wt. ppm, indicated by a triangle) in the purified liquid in the distillation column  3 , in a case where the water concentration in the purified liquid in the distillation column  3  in Example 1 is varied from 0 to 5 wt.%. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Method for Producing Purified Acetic Acid 
     As a raw material to be used in the present disclosure, raw acid is commonly used, but the raw material is not limited to the raw acid. An acetic acid-containing solution containing at least acetic acid (e.g., an aqueous solution including acetic acid, a mixture of acetic acid, water, and various solvents, etc.) may be used. In the acetic acid-containing solution containing at least acetic acid, the total content of acetic acid and water is, for example, 50 wt.% or greater, preferably 80 wt.% or greater, more preferably 90 wt.% or greater, and still more preferably 95 wt.% or greater. 
     Hereinafter, an example of an embodiment using raw acid as a raw material will be described with reference to  FIG.  1   .  FIG.  1    is a schematic view illustrating a production flow of purified acetic acid of the present disclosure. A purified acetic acid production apparatus according to this production flow includes an extraction column  1 , an evaporator  2 , a distillation column  3 , a separation membrane  4 , reboilers  2   a  and  3   a , a capacitor  3   b , and lines  11  to  26 . Note that in Examples described below, an acetic acid-containing solution containing at least acetic acid other than raw acid can be used in place of the raw acid. 
     Extraction Step 
     Extraction step is a process of bringing raw acid into contact with an extractant and separating the raw acid into an extractant liquid phase mainly containing acetic acid, the extractant, and a small amount of water, and an extracted residual phase mainly containing water. In  FIG.  1   , the raw acid is introduced into the extraction column  1  through the line  11 . The acetic acid concentration in the raw acid is usually from 20 to 50 wt.%. This raw acid includes water, cellulose acetate, and the like, in addition to acetic acid. Impurities in the raw acid may be removed in advance with diatomaceous earth, activated carbon, or the like. 
     The extractant is introduced from the line  12  into the extraction column  1 . The extractant is preferably a solvent that is easily separated from acetic acid in the subsequent distillation step and has a boiling point (azeotropic temperature in the case of azeotropic with water) lower than the boiling point of acetic acid, for example, a solvent having a boiling point (azeotropic temperature in the case of azeotropic with water) lower than the boiling point of acetic acid by equal to or higher than 10° C. Preferable examples of the solvent include aromatic hydrocarbons such as benzene and toluene; aliphatic hydrocarbons such as n-hexane, cyclohexane, and heptane; esters such as methyl acetate, ethyl acetate, and propyl acetate; halides such as chloroform and carbon tetrachloride; and ether compounds such as 1,2-dimethoxyethane and dimethyl ether. One kind of the extractant can be used alone or two or more kinds in combination. 
     The amount of the extractant to be used is, for example, from about 1 to 2.5 times (volume ratio), and preferably from about 1.5 to 1.7 times (volume ratio) the amount of the raw acid. 
     The extraction column  1  may be a commonly used type of column, an example of which includes a mixer-settler type extraction column, a perforated-plate type extractor, a packed-column type extractor, a baffle column type extractor, a vibrating perforated-plate type extractor, a stirring and mixing type extractor, a pulsating packed type extractor, or a centrifugal extraction type extractor. The extraction operation in the extraction column  1  can be performed intermittently or continuously. In a case where the extraction effect is insufficient, repeated extraction operations can also be performed. 
     The contact temperature between raw acid and the extractant is, for example, in a range from 20 to 60° C., and preferably in a range from 30 to 50° C. The contact time is, for example, approximately from 0.1 to 10 hours. 
     The contact between raw acid and the extractant can be performed under normal pressure, under pressure, or under reduced pressure. 
     The extractant liquid phase obtained in this step mainly includes acetic acid and the extractant, and the water concentration is, for example, 15 wt.% or less, and preferably 10 wt.% or less. The cellulose acetate concentration is 200 wt. ppm or less, and preferably 150 wt. ppm or less. Meanwhile, the extracted residual phase mainly contains water and soluble cellulose acetate, and the acetic acid concentration is, for example, 0.5 wt.% or less, and preferably 0.1 wt.% or less. 
     Evaporation Step 
     Evaporation step is a process of introducing the extractant liquid phase into the evaporator  2  from the line  13 , and evaporating the extractant liquid phase to cause separation into vapor including acetic acid, the extractant, and water and an evaporated residual liquid containing soluble cellulose acetate. In this step, the separated soluble cellulose acetate is discharged to the outside of the system through the line 16. In this step, the cellulose acetate dissolved in the extractant liquid phase can be separated and removed. 
     In this step, it is sufficient that the extractant liquid phase can be separated into vapor including acetic acid, the extractant, and water and an evaporated residual liquid containing soluble cellulose acetate. For example, the separation can be achieved by subjecting the extractant liquid phase to a heating treatment. The temperature of heating treatment is, for example, from 80 to 100° C. The heating treatment can be performed under normal, pressurized, or reduced pressure. It may include a single step, or may include a combination of multiple steps. A known evaporator can be used as the evaporator  2 , and an evaporation tank in the form of distillation column may also be used. 
     The vapor mainly contains acetic acid, the extractant, and water. The acetic acid concentration in vapor is, for example, 10 wt.% or greater, and preferably 15 wt.% or greater. The soluble cellulose acetate concentration is 50 wt. ppm or less, and preferably 20 wt. ppm or less. The water concentration is, for example, 10 wt.% or less, and preferably 8 wt.% or less. 
     Distillation Step 
     The distillation step is a process of separating acetic acid, the extractant, and water, utilizing a boiling point difference, and the extractant and water are distilled off to obtain acetic acid as a purified liquid. In the distillation step, the extractant liquid phase obtained in the extraction step may be fed into the distillation column  3  in liquid phase (not illustrated), or the vapor obtained in the evaporating may be fed into the distillation column  3  from the line  17 . 
     When the extractant liquid phase or the vapor is fed into the distillation column  3 , large portions of the extractant and water contained in the extractant liquid phase or vapor pass through the line 18 from the column top and are cooled in a cooler 3b, being introduced into a tank (decanter) (not illustrated). The upper layer liquid of the liquid introduced into the tank mainly contains the extractant, and is partially refluxed into the distillation column  3 . The rest of the liquid in the tank is discharged outside the system. This discharged liquid can be reused again as an extractant to be introduced into the extraction column  1 . The lower layer liquid in the tank mainly contains water, and is discharged outside the system. Meanwhile, acetic acid is obtained from the column bottom of the distillation column  3  through the line  23 . 
     Examples of the distillation column  3  include a plate column or a packed column. The column top temperature is, for example, from 20 to 120° C., preferably from 30 to 100° C., and more preferably from 40 to 80° C. The pressure in the distillation column  3  is, for example, from 0.1 to 0.5 MPa (absolute pressure). Further, the distillation can also be performed under reduced pressure. The distilling may be comprised of a single step, or may be comprised of a combination of steps. 
      The number of theoretical plates and the reflux ratio in the distillation column 3 can be set as appropriate in consideration of the separation efficiency and the vapor cost. In the present invention, since the purified liquid in the distillation column  3  is subjected to the membrane separation step described below, water contained in the purified liquid is separated, and thus the liquid to be supplied to the distillation column  3  does not need to be completely distilled the water contained in the vapor from the top of the column. Consequently, this allows the number of actual plates and the reflux ratio in the distillation column  3  to be greatly reduced as compared with those in the related art, thereby achieving saving of energy. 
     After undergoing the distillation step, acetic acid containing a trace or small amount of water as purified liquid is obtained from the line  24 . The lower limit of the acetic acid concentration in the purified liquid is, for example, 96 wt.%, preferably 96.5 wt.%, and more preferably 97 wt.%. In the present invention, the purified liquid is subjected to the membrane separation step described below, and thus it is possible to set the acetic acid concentration in the purified liquid to a concentration lower than the known concentration. 
     The water concentration in the purified liquid is, for example, 4 wt.% or less. In the present invention, since acetic acid and water are separated by the membrane separation step described below, it is possible to set conditions in which the water concentration in acetic acid after being subjected to the distillation step is reduced to 4 wt.% or less. This makes it possible to minimize the separation of acetic acid and water in the distillation step, an excellent vapor-saving effect is given, and the energy load can be greatly reduced. 
     The concentration of an organic solvent (e.g., an ester such as ethyl acetate, an aromatic hydrocarbon such as benzene, etc.) in the purified liquid is, for example, 0.1 wt. ppm or less, preferably 0.05 wt. ppm or less, and more preferably 0.01 wt. ppm or less. Preferably, the concentration of each of the organic solvents in the purified liquid is equal to or less than the detection limit. More preferably, the concentration of all the organic solvents is equal to or less than the detection limit. The term “detection limit” used herein means that the concentration of an organic solvent in the purified liquid measured by the following measurement method is equal to or less than the detection limit. 
     Method for Measuring Organic Solvent 
     The purified liquid to be measured is analyzed by gas chromatography (GC-2010, Shimazu Corporation), and the concentration of an organic solvent is measured by quantifying the organic solvent in the purified liquid (detector: FID). 
     The column top distillate is mainly a solution rich in an organic solvent such as ethyl acetate or benzene. The acetic acid concentration in the column top distillate is, for example, 0.02 wt.% or less, preferably 0.01 wt.% or less, and more preferably 0.005 wt.% or less. The acetic acid concentration in the column top distillate can be controlled by adjusting the reflux ratio of the distillation column  3 . 
     When the column top distillate is divided into an organic phase and an aqueous phase, the acetic acid concentration in the organic phase is, for example, 0.05 wt.% or less, preferably 0.02 wt.% or less, more preferably 0.01 wt.% or less, and still more preferably 0.005 wt.% or less. 
     Membrane Separation Step 
     The membrane separation step is a process of separating water from the purified liquid by the separation membrane  4 . The separating by the membrane can be performed by introducing the purified liquid into the separation membrane  4  from the line 24. 
     In the membrane separation step, for example, the purified liquid is directly brought into contact with the separation membrane  4 , causing water to permeate selectively from a mixture of acetic acid and water. This method is referred to as pervaporation. In  FIG.  1   , the purified liquid is supplied to the inlet side of the separation membrane  4 , the permeate side is depressurized, and thus a transmembrane pressure difference is generated, thereby obtaining a driving force for moving the purified liquid from the inlet side to the permeate side. The separation is caused by a difference in permeation rate between acetic acid and water. 
     The method for depressurizing the permeate side is not particularly limited, and the permeate side can be depressurized by condensing water passing through the permeate side of the separation membrane  4  with a cooling source (not illustrated). In this case, there is an advantage that a partial pressure difference, which is a driving force for causing water to permeate the membrane, is secured, whereby the area of the membrane can be reduced and electric power such as a vacuum pump becomes unnecessary. 
      The pressure on the permeate side of the separation membrane  4  is, for example, in a range of 100 to 400 Pa, and preferably in a range of 200 to 300 Pa. 
     The temperature at which the purified liquid is fed into the separation membrane  4  is not particularly limited, and is usually from 20 to 160° C., preferably from 50 to 150° C., more preferably from 70 to 145° C., and still more preferably from 80 to 140° C. As for the flow rate for feeding the purified liquid into the separation membrane  4 , for example, in the case of using a flow rate of 1 m/hr as an indicator, the flow rate per one is approximately 0.6 m 3 /hr. 
     The area of the separation membrane  4  is not particularly limited, and is, for example, from 800 to 2500 m 2 . 
     Water is discharged to the permeate side by the line  26 , and acetic acid as a non-permeable component is separated by the line  25 . The acetic acid concentration in the recovered acetic acid is determined by the required quality, and is, for example, 98 wt.% or greater, preferably 99 wt.% or greater, and more preferably 99.5 wt.% or greater. The acetic acid concentration in the permeated water is, for example, 1 wt.% or less. 
     The performance of the separation membrane  4  is not particularly limited, and can be determined as appropriate depending on the acetic acid concentration and water concentration in the purified liquid to be introduced. 
     The separation coefficient α of the separation membrane  4  with respect to water is, for example, 1000 or greater, and preferably 2000 or greater. The separation coefficient α herein can be obtained from the following Formula (1): 
     
       
         
           
             Separation coefficient 
               
             α 
             = 
             
               
                 
                   
                     100 
                     − 
                     X 
                     21 
                   
                 
                 / 
                 X 
                 21 
               
             
             / 
             
               
                 
                   
                     100 
                     − 
                     X 
                     11 
                   
                 
                 / 
                 X 
                 11 
               
             
           
         
       
     
      where X21 indicates an acetic acid concentration in the permeated liquid (wt.%), and X11 indicates an acetic acid concentration in the purified liquid (wt.%). 
     The water permeation rate Q of the separation membrane  4  is, for example, 0.5 kg/m 2  ·hr or greater (e.g., from 0.5 to 5 kg/m 2  ·hr), preferably 0.8 kg/m 2  ·hr or greater (e.g., from 0.8 to 3 kg/m 2 · hr), more preferably 0.9 kg/m 2 · hr or greater (e.g., from 0.9 to 2.5 kg/m 2 · hr) when the water content in the purified liquid is 4 wt.%, and is, for example, 0.01 kg/m 2 · hr or greater (e.g., from 0.01 to 0.8 kg/m 2 ·hr), preferably 0.02 kg/m 2  ·hr or greater (e.g., from 0.02 to 0.5 kg/m 2 ·hr), more preferably 0.025 kg/m 2 ·hr or greater (e.g., from 0.025 to 0.4 kg/m 2 ·hr) when the water content in the purified liquid is 0.5 wt.%. 
     The permeability coefficient K of the separation membrane  4  is, for example, 0.00001 kg/m 2 ·hr-Pa or greater (e.g., from 0.00001 to 0.00008 kg/m 2 · hr·Pa), preferably 0.00002 kg/m 2 ·hr·Pa or greater (e.g., from 0.00002 to 0.00006 kg/m 2 · hr·Pa), and more preferably 0.000025 kg/m 2 ·hr·Pa or greater (e.g., from 0.000025 to 0.00005 kg/m 2 ·hr·Pa). The permeability coefficient K herein can be obtained from the following Formula (2): 
     
       
         
           
             Permeation rate K 
             = 
               
             W 
             / 
             A 
             / 
             ΔP 
           
         
       
     
      where W indicates a permeated liquid volume (kg/hr), A indicates a membrane surface area (m 2 ), ΔP indicates a water partial pressure difference (Pa). 
     Examples of the material of the separation membrane  4  can include a polymer membrane, a ceramic membrane, or a zeolite membrane. Among these materials, a zeolite membrane is preferable from the viewpoint of hydrophilicity, permeation rate, water separation selectivity, and acid resistance. The zeolite membrane is not particularly limited, and can be selected as appropriate. For example, preferred are one or more zeolite separation membranes selected from the group consisting of A-type, FAU-type, CHA-type, MFI-type, MOR-type, and DDR-type zeolite separation membranes, and the CHA-type is more preferred from the viewpoint of being excellent in hydrophilicity, permeation rate, water separation selectivity, and acid resistance, and having durability. The CHA-type zeolite membrane can be sieved according to the difference between the molecular size of acetic acid and the molecular size of water by a size exclusion method, and thus the membrane has high separation performance. 
     The configuration of the separation membrane  4  (micropore size, shape, porosity and non-porosity, presence or absence of a support, and the like) is not particularly limited, and can be determined as appropriate in consideration of the temperature of feed, the flow rate of feed, the separation coefficient α, the permeation rate Q, the permeability coefficient K, and the like. For example, the CHA-type zeolite membrane has a three-dimensional pore structure, and has a high diffusion rate, high hydrophilicity, and a large adsorption amount of water. Accordingly, it is possible to obtain a high permeation flux (Flux). The separation membrane  4  may be used alone, or may be used in the form of a multitubular membrane, a so-called separation membrane module. 
      The separation membrane  4  may be a commercially available product. Examples thereof can include a CHA-type zeolite membrane (trade name “HZM-4”, Hitachi Zosen Corporation). 
     After undergoing the membrane separation step, acetic acid and water in the purified liquid can be efficiently separated. Thus, highly purified acetic acid (hereinafter referred to as “purified acetic acid”) can be recovered by the line 26. According to the method of the present invention, when separating water from acetic acid, the distillation step is combined with the membrane separation step, so the amount of heat injected into the distillation column  3  can be greatly reduced (vapor-saving effect). Accordingly, this makes it possible to achieve saving of energy, and to construct an industrially efficient method for producing purified acetic acid. Further, after the membrane separation step, the purified acetic acid can be incorporated in the process of producing cellulose acetate, and can also be fed into a reactor together with a raw material pulp or the like. Therefore, a reduction in the production cost of cellulose acetate can be achieved. 
     Each aspect disclosed in the present specification can be combined with any other feature disclosed herein. Note that each of the configurations, combinations thereof, and the like in each of the embodiments are an example, and various additions, omissions, and other changes may be made as appropriate without departing from the spirit of the present disclosure. The present disclosure is not limited by the embodiments and is limited only by the claims. 
     EXAMPLES 
     Hereinafter, the present invention will be specifically described with reference to examples. 
     Preliminary Test 
     To examine the performance of the separation membrane  4 , the following performance testing of the separation membrane was performed using a single separation membrane module. 
     The model purified liquid was used to evaluate the performance of a single module of a CHA-type zeolite membrane as the separation membrane  4 . The apparatus used for testing the performance is illustrated in  FIG.  2   . In  FIG.  2   , the apparatus for testing performance includes a TCV 50, a TIC 51, a membrane module  52  (separation membrane), heat exchangers  53   a ,  53   b , and  53   c , a tank (purified liquid)  54 , a tank (permeated liquid)  55 , pumps  56   a  and  56   b , a pressure transmitter  57 , a purified liquid sample  58 , a permeated liquid sample  59 , vapor  60 , cooling water  61 , and cold water  62 . 
     Test Method 
     After feeding the model purified liquid into the tank  54 , the vapor  60 , the cooling water  61 , and the cold water  62  were supplied, and the operation was started under the conditions below. Thereafter, the vacuum pump was operated to start permeation. The concentration in the acetic acid concentrated liquid, the acetic acid concentration in the permeated liquid, and the amount of the permeated liquid were measured every 1 hour from the start of the test. Based on these measured values, the acetic acid concentration X21, the permeation rate Q, the separation coefficient α, and the permeability coefficient K were evaluated in the permeated liquid from the start of the test until 258 hours after the start of the test. The results are shown in the graphs of  FIGS.  3  to  6   . 
     Test Conditions
     Model purified liquid: acetic acid concentration 87 wt.% (1 specimen in total)   Data collection: 30 times in total   Membrane module: trade name “HZM-4”, body diameter: 16 mm × length 60 mm, membrane surface area: 0.05 m 2 , CHA-type zeolite membrane, manufactured by Hitachi Zosen Corporation   Temperature of feed: 130° C.   Flow rate of feed: 120 kg/hr   Pressure in a space at the permeate side of the separation membrane: 100 to 200 Pa   

       FIG.  3    is a graph showing the relationship between the acetic acid concentration in the model purified liquid and the permeation rate Q. The permeation rate Q tended to decrease as the acetic acid concentration increased, indicating that the amount of water molecules permeating the separation membrane decreased. The results were as expected. 
       FIG.  4    is a graph showing the relationship between the acetic acid concentration in the model purified liquid and the acetic acid concentration X21 in the permeated liquid. It was confirmed that the acetic acid concentration X21 in the permeated liquid showed a stable value (about 0.08 wt.%) even when the acetic acid concentration in the model purified liquid varied, and dehydration was stably performed. 
       FIG.  5    is a graph showing the relationship between the acetic acid concentration in the model purified liquid and the permeability coefficient K. It was confirmed that the permeability coefficient K showed a stable value (in a range of 0.00003 to 0.00004) even when the acetic acid concentration in the model purified liquid varied, and dehydration was stably performed. 
       FIG.  6    is a graph showing the relationship between the acetic acid concentration X11 in the model purified liquid and the separation coefficient α. It was confirmed that the separation coefficient α showed a constant value in a stable range even when the acetic acid concentration in the model purified liquid varied, and stable membrane separation was performed. 
     From the performance test results of the membrane module described above, it was confirmed that the membrane separation step by the separation membrane was introduced, thereby allowing for industrially efficient separation between acetic acid and water. 
     Example 1 
     A test was performed to demonstrate the energy-saving effect in a case where the method of the present invention was used in the acetic acid recovery plant, based on the results of the performance test of the separation membrane. The test conditions are as follows. 
     Distillation Step 
     Composition fed to the distillation column  3     Acetic acid: 17 wt.%   Water: 8 wt.%   Ethyl acetate: 56 wt.%   Benzene: 19 wt.%   Temperature of feed: 91.7° C.   Column top pressure: 1 atm   The number of plates of the distillation column  3 : 29 plates   Position of feed: at the 10th plate from the top   Feed of liquid and feed of vapor: feed of vapor   Reflux ratio: 0.74   

     Membrane Separation Step Conditions
     Material of separation membrane: CHA-type zeolite membrane   Temperature of feed: 130° C.   Concentration of water fed: 4 wt.%   Flow rate of feed: 10.1 t/hr   Pressure in a space at the permeate side of the separation membrane: 1 kPa   Concentration of water at the outlet: 0.5 wt.%   Outlet flow rate: 9.74 t/hr   Water concentration in the permeated liquid: 99 wt.%   Amount of permeated liquid: 0.36 t/hr   

     This result showed that the separation coefficient α was 2376, which was sufficient as the performance of the membrane. 
     Next, the water concentration in the purified liquid in the distillation column was varied from 0 to 5 wt.% by changing the reflux ratio of the distillation column. Then, it was examined how the vapor use rate of the distillation column, the acetic acid concentration in the upper layer liquid in the overhead vapor condensate (stored in the decanter), and the ethyl acetate concentration in the purified liquid in the distillation column were changed. 
       FIG.  7    is a graph showing the vapor use rate of the distillation column  3  in a case where the water concentration in the purified liquid in the distillation column is varied from 0 to 5 wt.% by changing the reflux ratio of the distillation column. In the known method for producing purified acetic acid, the water concentration in the purified liquid in the distillation column  3  was set to as close as possible to 0 wt.% (100 based on the vapor use rate in this case). From this result, it can be seen that the water concentration in the purified liquid is reduced to 4 wt.% or less, and as a result, the vapor use rate of the distillation column  3  can be reduced to about 74. 
       FIG.  8    is a graph showing the acetic acid concentration (wt. ppm, indicated by a rhombus) in an upper layer liquid in a decanter and the ethyl acetate concentration (wt. ppm, indicated by a triangle) in the purified liquid in the distillation column, in a case where the water concentration in the purified liquid in the distillation column is varied from 0 to 5 wt.% by changing the reflux ratio in the distillation column. Lowering of the vapor use rate of the distillation column  3  leads to lowering of the separation efficiency between acetic acid and the organic solvent. From this result, it was confirmed that even when the water concentration in the purified liquid in the distillation column was reduced to 4 wt.%, the acetic acid concentration in the upper layer liquid in the decanter did not change, and the ethyl acetate concentration in the purified liquid was equal to or less than the detection limit (0.01 ppm or less). Note that the value of concentration of ethyl acetate in the purified liquid when the concentration of water in the purified liquid in the distillation column is changed from 0 wt.% to 4 wt.% is a value obtained by the simulation. 
     Based on the results described above, the membrane separation step that includes separating water in the purified liquid by the separation membrane is incorporated into the process at the subsequent distillation step, and thus it is possible efficiently separate water by the membrane separation step even in a case where the water concentration in the purified liquid in the distillation column is 4 wt.% at a maximum. Therefore, it was confirmed that a significant vapor-saving effect was obtained in the distillation step, and an excellent energy-saving effect was exerted. 
     In the embodiment shown in  FIG.  1   , an aspect is illustrated in which one distillation column  3  and one separation membrane  4  are disposed, but the number of each of the distillation column and the separation membrane is not limited to one. The number of the distillation column  3  or the separation membrane  4  can be determined as appropriate depending on the required concentration of the purified acetic acid. In addition, the present invention is not limited to the embodiments described above, but variations and modifications within the scope where the object of the present invention can be achieved are included in the present invention. 
     As a summary of the above, configurations and variations of the present disclosure are described below. 
     [1] A method for producing purified acetic acid from a mixed solution containing acetic acid, an organic solvent, and water, the method including: a distillation step of distilling the mixed solution and separating the mixed solution into a purified liquid rich in acetic acid and a separated liquid rich in an organic solvent; and a membrane separation step of separating the water from the purified liquid by a separation membrane, in which the purified liquid has a water concentration of 4 wt.% or less;   [2] The method for producing purified acetic acid according to [1] above, in which the mixed solution is derived from a liquid discharged during a process of producing cellulose acetate.   [3] A method for producing purified acetic acid, the method including: an extraction step (A) of bringing an acetic acid-containing solution containing at least acetic acid (or an acetic acid-containing solution discharged during a process of producing cellulose acetate) into contact with a extractant and separating the acetic acid-containing solution into an extractant liquid phase mainly containing acetic acid, the extractant, and a small amount of water, and an extracted residual phase mainly containing water and soluble cellulose acetate; an evaporation step (B) of, by introducing at least part of the extractant liquid phase into an evaporator, evaporating and separating the extractant liquid phase into vapor containing acetic acid, the extractant, and water, and an evaporated residual liquid containing soluble cellulose acetate; a distillation step (C) of supplying the vapor (or the extractant liquid phase and the vapor) to a distillation column and separating the vapor into a distillate containing the extractant and water and a purified liquid containing acetic acid; and a membrane separation step (D) of separating the water from the purified liquid containing acetic acid by a separation membrane to obtain purified acetic acid.   [4] A method for producing purified acetic acid, the method including: an extraction step (A) of bringing an acetic acid-containing solution containing at least acetic acid (or an acetic acid-containing solution discharged during a process of producing cellulose acetate) into contact with a extractant and separating the acetic acid-containing solution into an extractant liquid phase mainly containing acetic acid, the extractant, and a small amount of water, and an extracted residual phase mainly containing water and soluble cellulose acetate; a distillation step (C) of supplying the extractant liquid phase to a distillation column and separating the extractant liquid phase into a distillate containing the extractant and water and a purified liquid containing acetic acid; and a membrane separation step (D) of separating water from the purified liquid containing acetic acid by a separation membrane to obtain purified acetic acid.   [5] The method for producing purified acetic acid according to any one of [1] to [4] above, in which the purified liquid has an ethyl acetate concentration as measured by gas chromatography of 0.1 wt. ppm or less (or 0.05 wt. ppm or less, 0.01 wt. ppm or less, or equal to or less than a detection limit).   [6] The method for producing purified acetic acid according to any one of [1] to [5] above, in which the purified liquid has a benzene concentration as measured by gas chromatography of 0.1 wt. ppm or less (or 0.05 wt. ppm or less, 0.01 wt. ppm or less, or equal to or less than a detection limit).   [7] The method for producing purified acetic acid according to any one of [1] to [6] above, in which the purified liquid has an organic solvent concentration as measured by gas chromatography of equal to or less than a detection limit.   [8] The method for producing purified acetic acid according to any one of [1] to [7] above, in which the membrane separation step is performed by bringing the purified liquid into contact with the separation membrane under reduced pressure.   [9] The method for producing purified acetic acid according to any one of [1] to [8] above, in which the separation membrane is one or more zeolite separation membranes selected from the group consisting of A-type, FAU-type, CHA-type, MFI-type, MOR-type, and DDR-type zeolite separation membranes.   

     INDUSTRIAL APPLICABILITY 
     The method for producing purified acetic acid of the present disclosure is appropriate as a method for producing purified acetic acid from a mixed solution containing acetic acid, an organic solvent, and water in an energy-efficient manner. 
     REFERENCE SIGNS LIST 
     
         
           1  Extraction column 
           2  Evaporator 
           2   a ,  3   a  Reboiler 
           3   b  Capacitor 
           3  Distillation column 
           4  Separation membrane 
           11  to  26  Line 
           50  TCV 
           51  TIC (temperature indicating controller) 
           52  Membrane module (separation membrane) 
           53   a ,  53   b ,  53   c  Heat exchanger 
           54  Tank (purified liquid) 
           55  Tank (permeated liquid) 
           56   a ,  56   b  Pump 
           57  Pressure transmitter 
           58  Purified liquid sample 
           59  Permeated liquid sample 
           60  Vapor 
           61  Cooling water 
           62  Cold water