Method for preventing polymerization of (meth)acrylic acid and esters thereof and method for the production thereof

This method of preventing polymerization of methacrylic acid inn the process of its production is characterized by comprising using, in a refining column, a combination of N-nitrosophenylhydroxylamine or salt thereof with an N-oxyl compound, an N-hydroxy-,2,2,6,6-tetramethylpiperidine compound and a 2,2,6,6-tetramethylpiperidine compound. In this method, N-nitrosophenylhydroxylamine or a salt thereof rises inside a refining column under splitting part thereof to effectively inhibit polymerization of (meth)acrylic acid or the like. The above polymerization in both vapor and liquid phases can be inhibited more effectively by the combined use of the compounds mentioned above.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for preventing (meth)acrylic acid and their esters from polymerizing and a method for the production thereof and more particularly to a method for preventing polymerization of (meth)acrylic acid and their esters, characterized by supplying an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, a 2,2,6,6-tetramethylpiperidine compound, and an N-nitrosophenylhydroxylamine or a salt as a polymerization inhibitor to a distilling column and a method for the production of (meth)acrylic acid or their esters by including the method for the prevention of polymerization in the step of production. This invention also relates to a method for preventing polymerization of (meth)acrylic acid and their esters, characterized by using a nitroso compound as a polymerization inhibitor and performing the distillation in the column while supplying the polymerization inhibitor to the column from the bottom side of the column and a method for the production of (meth)acrylic acid and their esters, including the method for the prevention of the polymerization in the step of production.

2. Description of Related Arts

Such easily polymerizing compounds as acrylic acid and methacrylic acid are raw materials for commercial production and are chemical substances which are produced in a large amount at a plant of a large scale. In the case of (meth)acrylic acid, for example, the easily polymerizing compound is produced by the reaction of catalytic gas phase oxidation of propylene, isobutylene, t-butanol, methyl-t-butyl ether, or acrolein. The reaction gas obtained by the reaction of catalytic gas phase oxidation contains (meth)acrylic acid, the target product, as mixed with other by-products. For example, this reaction generates mainly non-condensable gases, namely unaltered propylene, isobutylene, and acrolein, low-boiling point organic compounds having lower boiling points than acrylic acid, namely steam and unaltered acrolein, formaldehyde produced by a secondary reaction, and impurities such as acetic acid, and high-boiling point compounds having higher boiling points than acrylic acid, namely maleic anhydride, furfural, benzaldehyde, benzoic acid, and acrylic acid dimer. For the purpose of purifying this reaction gas thereby producing the target product, therefore, it is generally for the reaction gas to wash with a counter flow water or a heavy solvent thereby effecting absorption of the gas and subsequently supplying the absorbed gas to a distilling column and purifying it therein.

(Meth)acrylic acid and esters thereof are compounds which possess a very easily polymerizing quality because of their structures. Moreover, since the process for the distillation of (meth)acrylic acid, for example, forms a system in which a gas phase part and a liquid phase part exist in a mixed state, it becomes necessary to prevent both the liquid phase part and the gas phase part in the distilling column effectively from polymerizing and enable the column to be continuously operated stably for a longtime. Generally, for the purpose of preventing the occurrence of such polymerization, various polymerization inhibitors are incorporated in the monomers either singly or in the form of a combination of several members to prevent the process of production from generating a polymer.

In the official gazette of U.S. Pat. No. 5,856,568, for example, discloses a method for preventing a vinyl compound from polymerizing by using an N-nitrosophenylhydroxylamine, i.e. one kinds of nitroso compound, or a salt, characterized by using the N-nitrosophenylhydroxylamine or the salt thereof in the presence of a copper salt compound. This method is claimed to prevent effectively the polymerization of acrylic acid or methacrylic acid and allow the process of production to be operated stably for a long time by introducing the both compounds mentioned above simultaneously or separately into the steps of distillation. In a working example adduced therein, the occurrence of a polymer was observed when a refluxing operation was performed after copper dibutyldithiocarbamate and N-nitrophenyl hydroxyl amine had been dissolved in acrylic acid.

The official gazette of JP-A-08-48,650 (relevant to U.S. Pat. No. 5,504,243) discloses a method for (meth)acrylic acid or a salt thereof from polymerizing by using together with an N-oxyl compound at least one polymerization inhibitor selected from among manganese salt compounds, copper salt compounds, 2,2,6,6-tetramethylpiperidine compounds, and nitroso compounds.

The method described in the official gazette of JP-A-09-95,465 (relevant to U.S. Pat. No. 5,856,568), however, uses a copper salt compound as an essential component and consequently suffers the used water containing the copper compound to be released into the environment. This method, herefore, is at a disadvantage in newly necessitating a means for the disposal of this polluted used water.

Further, for the purpose of effectively preventing (meth)acrylic acid from polymerizing, it suffices to increase the amount of the polymerization inhibitor. This measure, however, entails the disadvantage of necessitating a step for the removal of the polymerization inhibitor at the stage of an operation for polymerizing (meth)acrylic acid, for example.

The official gazette of U.S. Pat. No. 5,504,243 described working examples using a plurality of polymerization inhibitors in combination. The compound stated therein as actually used in combination with an N-nitrosophenylhydroxylamine or a salt thereof is limited to 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl.

Generally, for the purpose of separating such by-products as low-boiling point substances like acetic acid and some aldehyde and high-boiling point substances like furfural and maleic anhydride which are contained in the (meth)acrylic acid synthesized by the catalytic gas phase oxidation of propylene, for example, the (meth)acrylic acid-containing solution is subjected to distillation, dissipation, extraction, and crystallization. The process production, however, is required to be further simplified. Such in the true status of affairs, when the number of acetic acid separating columns or the number of steps of distillation is decreased by way of simplification, however, the decrease entails the necessity for rigidifying the conditions of distillation. Though the simplification of the process of production constitutes a demand from a social cause, it will be added an occurrence of generation of a polymer because it requires at each step of purification severer conditions of distillation than today. When the polymer is generated, the continuous operation of the purifying column is disabled and the work of removing the polymer from the purifying column is rendered more difficult.

In the light of this true state of affairs, the need for developing in connection with the production of such an easily polymerizing substance as (meth)acrylic acid a method for effectively preventing the occurrence of a polymer in both the gas phase and the liquid phase in a purifying column and consequently attaining continuous operation of the column required inherently and a method for producing the easily polymerizing substance has been earnestly felt.

SUMMARY OF THE INVENTION

The present inventors, as a result of pursuing an elaborate study on the function of a nitroso compound manifested in the prevention of polymerization, have found that an N-nitrosophenylhydroxylamine or a salt thereof is decomposed at least partly after it has been added to the step of distillation, the product of this decomposition contains what is possessed of the function of inhibiting polymerization and what is possessed of the function of promoting polymerization in a mixed state, the use of a known N-oxyl compound and other polymerization inhibitor in combination with the decompositions can promote the effect of preventing the polymerization of (meth)acrylic acid, etc., and the supply of the aforementioned compound via a specified point during the introduction thereof into a distilling column can promote the effect of preventing polymerization in the gas phase and repress the effect of the polymerization-promoting substance. This invention has been perfected as a result. Specifically, the object mentioned above is accomplished by the following items (1) to (3).

(1) A method for preventing the polymerization of (meth)acrylic acid and esters thereof, characterized by using an N-nitrosophenylhydroxylamine or a salt thereof in combination with an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, and a 2,2,6,6-tetramethylpiperidine compound in a purifying column.

(2) The method described above, wherein the N-nitrosophenylhydroxylamine or the salt thereof, mentioned above is supplied to the purifying column from a level not higher than the position of 70% of the total number of theoretical plates, with the bottom side of the column as the base point.

(3) A method for the production of (meth)acrylic acid or an ester thereof, characterized by incorporating in the process of production the method for the prevention of the polymerization of (meth)acrylic acid and esters thereof as set forth in any of Items (1)-(2).

When the N-nitrosophenylhydroxylamine or the salt thereof is used in combination with an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, and a 2,2,6,6-tetramethylpiperidine compound in accordance with this invention, the N-nitrosophenylhydroxylamine or the salt thereof is decomposed in a distilling column and the gas component consequently obtained effectively prevents the polymerization of the easily polymerizing substance existing in the gas phase of the interior of a purifying column and, at the same time, inhibits the polymerization of the easily polymerizing substance by allowing the presence of the N-oxyl compound in the liquid phase thereof. When the N-nitroso compounds mentioned above are supplied to the purifying column at a level not higher than the position of 70% of the total number of theoretical plates in this case, these compounds are decomposed in the distilling column and the gas component consequently obtained effectively prevents the polymerization of the easily polymerizing substance existing in the gas phase of the interior of a purifying column and, at the same time, inhibits the effect of the aforementioned polymerization promoting substance. Thus, the polymerization of the easily polymerizing substance in both the liquid phase and the gas phase can be synergistically inhibited.

The easily polymerizing compounds contemplated by this invention are (meth)acrylic acid and esters thereof which form both the gas and liquid phases in the interior of the purifying column.

When the (meth)acrylic acid and the esters thereof are produced by implementing the method of this invention for preventing the polymerization, the occurrence of a polymer the process of production is inhibited to permit continuous operation of the column and improve the yield of the product.

EXAMPLES

Now, this invention will be described more specifically with reference to working examples.

Acrylic acid was produced in accordance with the production process for acrylic acid which is illustrated in Figure.

First, propylene and a molecular oxygen-containing gas were supplied to a catalytic gas phase reaction vessel 10 provided with an intermediate tube sheet partitioning the reaction vessel into an upper and a lower chamber to obtain an acrylic acid-containing gas in consequence of catalytic gas phase oxidation, introducing this acrylic acid-containing gas into an acrylic acid absorption column 20 , and brought therein into contact with water to absorb the acrylic acid in an aqueous solution. This acrylic acid-containing solution contained acrolein as an impurity. The acrylic acid-containing solution mentioned above was introduced into an acrolein dissipating column 30 to effect dissipation of acrolein and obtain an aqueous acrylic acid solution containing 30 wt. % of water and 3.0 wt. % of acetic acid.

This aqueous acrylic acid solution was introduced into an azeotropic separating column 40 measuring 105 mm in inside diameter and provided with 50 stepped stainless steel-made sieve trays spaced at intervals of 147 mm and provided in the top part of the column with distillation pipes and reflux supply pipes, in the central part of steps (20 steps) with a material supplying pipe and a polymerization inhibiting agent inlet pipe, and in the bottom part of the column with a column bottom extraction pipe and a polymerization inhibitor inlet pipe to distill the aqueous acrylic acid solution with toluene as an azeotropic solvent.

The amount of the polymerization inhibitor to be used was 100 ppm in the case of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl, 20 ppm in the case of 1,4-dihydroxy-2,2,6,6-tetramethylpiperizine, 20 ppm in the case of 4-hydroxy-2,2,6,6-tetramethylpiperidine, and 20 ppm in the case of the ammonium salt of N-nitrosophenylhydroxylamine respectively, relative to the amount of the vapor of acrylic acid formed by evaporation. The ammonium salt of N-nitrosophenylhydroxylamine was supplied into the interior of the column in the form dissolved in water via the bottom of the column and other polymerization inhibitor in the form dissolved in a reflux liquid via the top of the column. Further,the molecular oxygen was supplied to the bottom part of the column in an amount of 0.3 vol. % based on the amount of the vapor of the acrylic acid formed by evaporation. The expression amount of the vapor of acrylic acid formed by evaporation as used herein means the total amount of the vapor of the monomer expelled by evaporation through the bottom of the column in proportion to the amount of heat applied to a reboiler 42 of an azeotropic dehydrating column 40 .

As respects the operating conditions during the stationary operation, the temperature of the top of the azotropic separating column 40 was 50 C., the temperature of the bottom of the column was 105 C., the pressure in the top of the column was 170 hPa, the reflux ratio (the total number of mols of the reflux liquid per unit time/the total number of mols of the distillate per unit time) was 1.43, and the amount of the aqueous acrylic acid solution supplied was 8.5 liters/hr. The water phase expelled by distillation via the top of the azeotropic dehydrating column 40 contained 7.5 wt. % of acetic acid and 1.8 wt. % of acrylic acid and the liquid extracted via the bottom of the column contained 97.5 wt. % of acrylic acid, 0.03 wt. % of acetic acid, 0.02 wt. % of water, and 2.45 wt. % of other components. The content of toluene was less than the limit of detection (1 ppm).

When the azeotropic dehydrating column 40 was continuously operated under the conditions mentioned above for 60 days, a constantly stable state was obtained. When the operation was stopped and the interior of the distilling column was inspected, absolutely no sign of the occurrence of a polymer was recognized.

The kinds and the amounts of polymerization inhibitor were 100 ppm of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl, 20 ppm of 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine, 20 ppm of 4-hydroxy-2,2,6,6-tetramethylpiperidine, and 10 ppm of the ammonium salt of N-nitrosophenylhydroxylamine, respectively relative to the amount of the vapor of acrylic acid formed by evaporation. Acrylic acid was produced by following the procedure of Example 1 while having the ammonium salt of N-nitrosophenylhydroxylamine dissolved in water and then supplied to the azeotropic dehydrating column 40 via the bottom thereof and the other polymerization inhibitors invariably dissolved in the reflux liquid and then supplied to the column via the top thereof. Further, molecular oxygen was supplied to the bottom part of the column in an amount of 0.3 vol. % relative to the amount of the vapor of acrylic acid formed by evaporation. Incidentally, the expression amount of the vapor of acrylic acid formed by evaporation as used herein means the total amount of the vapor of the monomer expelled by evaporation through the bottom of the column proportionately to the amount of heat applied from the reboiler 42 of the azeotropic dehydrating column 40 .

As respects the operating conditions during the stationary operation, the temperature of the top of the azeotropic separating column 40 was 50 C., the temperature of the bottom of the column was 105 C., the pressure in the top of the column was 170 hPa, the reflux ratio (the total number of mols of the reflux liquid per unit time/the total number of mols of the distillate per unit time) was 1.20, and the amount of the aqueous acrylic acid solution supplied was 9.0 liters/hr. The liquid extracted from the bottom of the column contained 97 wt. % of acrylic acid, 0.02 wt. % of water, and 2.98 wt. % of other components.

When the azeotropic dehydrating column 40 was continuously operated under the conditions mentioned above for 30 days, a constantly stable state was obtained. When the operation was stopped and the interior of the distilling column was inspected, a small amount of a polymer was recognized in the column and the column was found to be capable of further continuing operation.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 2 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the position of 25% of the total number of theoretical plates with the bottom side of the column as the basic point.

When the azeotropic separating column 40 was continuously operated under the conditions for 30 days, the same state of separation as formed in Example 2 was obtained. When the operation was stopped and the interior of the distilling column was inspected, virtually no sign of the occurrence of a polymer was recognized.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 2 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the position of 60% of the total number of theoretical plates with the bottom side of the column as the basic point.

When the azeotropic separating column 40 was continuously operated under the conditions for 30 days, the same state of separation as formed in Example 2 was obtained. When the operation was stopped and the interior of the distilling column was inspected, absolutely no sign of the occurrence of a polymer was recognized.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while using 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl in an amount of 50 ppm, 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine in an amount of 10 ppm, 4-hydroxy-2,2,6,6-tetramethylpiperidine in an amount of 10 ppm, and the ammonium salt of N-nitrosophenylhydroxylamine in an amount of 5 ppm respectively instead.

When the azeotropic separating column 40 was continuously operated under the conditions for 10 days, the same state of separation as formed in Example 1 was obtained. When the operation was stopped and the interior of the distilling column was inspected, absolutely no sign of the occurrence of a polymer was recognized.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 5 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the position of 25% of the total number of theoretical plates with the bottom side of the column as the basic point.

When the azeotropic separating column 40 was continuously operated under the conditions for 10 days, the same state of separation as formed in Example 1 was obtained. When the operation was stopped and the interior of the distilling column was inspected, substantially no sign of the occurrence of a polymer was recognized.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 5 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the position of 60% of the total number of theoretical plates with the bottom side of the column as the basic point.

When the azeotropic separating column 40 was continuously operated under the conditions for 10 days, the same state of separation as formed in Example 1 was obtained. When the operation was stopped and the interior of the distilling column was inspected, absolutely no sign of the occurrence of a polymer was recognized.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 5 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the position of 80% of the total number of theoretical plates with the bottom side of the column as the basic point.

When the azeotropic separating column 40 was continuously operated under the conditions for 10 days, the same state of separation as formed in Example 1 was obtained. When the operation was stopped and the interior of the distilling column was inspected, the occurrence of 11 g of a polymer was recognized in the column. The column was capable of further continuing the operation.

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 5 while changing the position for the introduction of the aqueous solution of the ammonium salt of N-nitrosophenylhydroxylamine from the bottom of the column to the top of the column.

When the azeotropic separating column 40 was continuously operated under the conditions for 10 days, the same state of separation as formed in Example 1 was obtained. When the operation was stopped and the interior of the distilling column was inspected, the occurrence of 25 g of a polymer was recognized in the column. The column was capable of further continuing the operation.

Comparative Example 1

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while changing the amount of 4-hydoxy-2,2,6,6-tetramethylpiperidinooxyl to be used to 20 ppm and omitting the use of 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine.

When the azeotropic separating column 40 was continuously operated under the conditions, the same state of separation as formed in Example 1 was obtained during the initial stage of operation. On the 17th day of starting the operation, the pressure drop of the column inside was recognized. On the 17th day of starting the operation, the pressure loss was recognized in the column. On the 20th day of the operation, the operation could be continued only with difficulty. When the operation was stopped and the distilling column was disassembled and inspected, the occurrence of a large amount of a polymer was recognized in the stripping section of the interior of the column.

Comparative Example 2

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while changing the amount of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl to 120 ppm and omitting the use of 4-dihydroxy-2,2,6,6-tetramethylpiperidine.

When the azeotropic separating column 40 was continuously operated under the conditions, the same state of separation as formed in Example 1 was obtained. On the 20th day of starting the operation, the pressure drop of the column in side was recognized. On the24th day of the operation, the operation could be continued only with difficulty. When the operation was stopped and the distilling column was disassembled and inspected, the occurrence of a large amount of a polymer was recognized in the stripping section of the interior of the column.

Comparative Example 3

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while changing the amount of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl to 120 ppm and omitting the use of ammonium salt of N-nitrosophenylhydroxylamine.

When the azeotropic separating column 40 was continuously operated under the conditions, the same state of separation as formed in Example 1 was obtained. On the 10th day of starting the operation, the pressure drop of the column inside was recognized. On the 12th day of the operation, the operation could be continued only with difficulty. When the operation was stopped and the distilling column was disassembled and inspected, the occurrence of a large amount of a polymer was recognized in the stripping section of the interior of the column.

Comparative Example 4

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while changing the amount of 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine to be used to 60 ppm, that of 4-hydroxy-2,2,6,6-tetramethylpiperidine to 50 ppm, and that of the ammonium salt of N-nitrosophenylhydroxylamine to 50 ppm and omitting the use of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl.

When the azeotropic separating column 40 was continuously operated under the conditions, the same state of separation as formed in Example 1 was obtained during the first stage of operation. On the 8th day of starting the operation, the pressure drop of the column inside was recognized. On the 10th day of the operation, the operation could be continued only with difficulty. When the operation was stopped and the distilling column was disassembled and inspected, the occurrence of a large amount of a polymer was recognized in the stripping section of the interior of the column.

Comparative Example 5

An aqueous acrylic acid solution was subjected to an operation of azeotropic distillation by following the procedure of Example 1 while changing the amount of 4-hydroxy-2,2,6,6-tetramethylpiperidiooxyl to be used to 140 ppm and omitting the use of 1,4-dihydroxy-2,2,6,6-tetramethylpiperidine and 4-hydroxy-2,2,6,6-tetramethylpiperidine. When the azeotropic separating column 40 was continuously operated under the conditions, the same state of separation as formed in Example 1 was obtained during the initial stage of operation. On the 15th day of starting the operation, the pressure drop of the column inside was recognized. On the 18th day of the operation, the operation could be continued only with difficulty. When the operation was stopped and the distilling column was disassembled and inspected, the occurrence of a large amount of a polymer was recognized in the stripping section of the interior of the column.