Method for recovering carboxylic acids

Abstract

This method for collecting a carboxylic acid is a method for obtaining an inorganic carboxylic acid by separating a carboxylic acid from an aqueous solution containing water and the carboxylic acid, and comprises: a) a first step for bringing the aqueous solution into liquid-liquid contact with a prepared extracting agent that contains a carboxylic acid-extracting component and a diluent; b) a second step for distilling the prepared extracting agent obtained in the first step, azeotropically distilling the diluent component in the prepared extracting agent and water, separating the resultant into an extracting agent component layer composed mainly of a diluent and an aqueous layer, then discharging the aqueous layer, returning the extracting agent component layer composed mainly of a diluent to the distillation step through reflux, and discharging a carboxylic acid-containing prepared extracting agent from the bottom; and c) a third step for re-distilling the carboxylic acid-containing prepared extracting agent discharged in the second step, and returning, to the first step, a prepared extracting agent from which a carboxylic acid and water are removed and which is discharged from the bottom, wherein the diluent in the prepared extracting agent has the lowest azeotrope with water.

Description

Technical Field

[0001] The present disclosure relates to a method for recovering carboxylic acids. More specifically, the present disclosure relates to a method for recovering carboxylic acids as anhydrous carboxylic acids from, for example, an aqueous solution containing a carboxylic acid and an inorganic salt.

Background Art

[0002] For example, aqueous solutions of carboxylic acids such as formic acid and acetic acid are generated as wastewater in many industrially important manufacturing or processing steps. In recent years, with the increasing concern about environmental pollution and the strengthening of various regulations, it is necessary to purify such waste aqueous solutions, and from the perspective of reusing economically valuable resources, it has become necessary to recover carboxylic acids contained as process by-products.

[0003] As methods for recovering carboxylic acids from solutions, not limited to the treatment of wastewater as described above, for example, looking at the purification technology of dilute aqueous solutions of carboxylic acids obtained by the production of lower fatty acids by microorganisms in the fermentation industry, a considerable number of treatment methods have been proposed conventionally.

[0004] It is known to use an organic solvent as a method for extracting carboxylic acids from dilute aqueous solutions. However, lower carboxylic acids such as formic acid and acetic acid have a high affinity for water, and although a very large number of compounds have been tried as extractants, no fully satisfactory ones have been obtained. That is, since the partition coefficient, which greatly affects the efficiency of the extraction method, is generally small for lower fatty acids, the amount of extractant used increases to increase the extraction efficiency, resulting in high energy consumption in the separation process.

[0005] For example, Patent Document 1 discloses a method for purifying acetic acid, characterized by introducing an aqueous acetic acid solution having an acetic acid concentration in the range of 10% to 50% by weight into an extraction apparatus as a raw material solution, supplying an extractant containing isopropyl acetate in an amount in the range of 0.6 to 3.0 times the weight of the raw material solution to the extraction apparatus and bringing it into contact with the raw material solution, extracting acetic acid into the extractant phase and separating it into an extract containing acetic acid and an extraction residue, supplying this extractant to an azeotropic distillation column, and recovering dehydrated purified acetic acid from the bottom of the azeotropic distillation column.

[0006] Patent Document 2 discloses a method for producing an organic carboxylic acid-enriched aqueous solution by concentrating an aqueous solution containing an organic carboxylic acid as a raw material, comprising: (a) a step of contacting an extraction solvent with an aqueous solution containing an organic carboxylic acid as a raw material to extract the organic carboxylic acid into an extraction phase; (b) a step of separating the extraction phase obtained from step (a) into a fraction enriched with the extraction solvent and a fraction enriched with the organic carboxylic acid; and (c) a step of separating the residual extraction phase discharged from step (a) into a fraction enriched with the extraction solvent and a fraction enriched with water.

[0007] The methods described in Patent Documents 1 and 2 both use extractants such as ethers, ketones, or carboxylic acid esters, which have lower boiling points than the target carboxylic acid. In a liquid-liquid extraction apparatus, the target carboxylic acid is extracted into the fraction rich in the extractant. Therefore, if the extractant has a low boiling point, it is necessary to burn the entire extractant in the distillation and purification process, resulting in a high energy load. For example, ethyl acetate is a commonly used extraction solvent because it has a relatively large partition coefficient with lower fatty acids, especially acetic acid, among organic solvents and is readily available. However, because its boiling point is lower than that of acetic acid, the entire amount of the extraction solvent used must be evaporated. Furthermore, a large amount of water dissolves in the extract, resulting in significant dissolution loss into water, which is unsatisfactory in terms of mutual solubility with water.

[0008] Furthermore, Patent Documents 3 and 4 disclose that when extracting lower fatty acids from aqueous solutions containing lower fatty acids such as formic acid, acetic acid, and propionic acid, a mixed solvent consisting of trioctylphosphine oxide and isophorone, and a mixed solvent consisting of trioctylphosphine oxide and trimethylcyclohexanone are used as extraction solvents.

[0009] Patent Document 5 discloses a method for recovering a carboxylic acid from a water-soluble solution containing a carboxylic acid, comprising: a contact step in which the water-soluble solution is contacted with a solvent substantially composed of mixed trialkylphosphine oxides in a counterflow liquid-liquid extraction stream to transfer the acid from the water-soluble solution to the solvent, thereby producing an extract residue with a relatively low acid content and a rich solvent with a relatively high acid content, wherein the acid-rich rich solvent contains some water; a dehydration step in which heat is applied to the rich solvent to dehydrate it by separating the water therefrom, thereby producing a water stream and a dehydrated rich solvent stream; and a stripping step in which heat is applied to the dehydrated rich solvent stream to strip the acid from the dehydrated rich solvent stream and produce an acid stream containing a solvent substantially composed of mixed trialkylphosphine oxides and the acid for recirculation to the liquid-liquid extraction stream.

[0010] Furthermore, Patent Documents 6 and 7 disclose methods for extracting lower fatty acids from aqueous solutions containing lower fatty acids such as formic acid, acetic acid, and propionic acid, characterized by using a mixed solvent of a tertiary amine with a higher boiling point than the lower fatty acid and a branched primary alcohol such as 2-ethylhexanol or 3,5,5-trimethylhexanol, which also has a higher boiling point than the lower fatty acid, or a tertiary amine with a higher boiling point than the lower fatty acid and a linear primary alcohol such as n-hexanol, n-heptanol, n-octanol, or n-nonanol, which also has a higher boiling point than the lower fatty acid. While these combinations result in a high extraction rate of carboxylic acids, in this case the alcohol dissolves in water. Dissolution means that a certain amount of alcohol is lost during the extraction operation, and the lost amount must be added from an external source. Even if this amount is small, if the carboxylic acid to be recovered is dilute, the recovery becomes meaningless.

[0011] Furthermore, Patent Document 8 discloses a method for recovering organic acids, characterized by contacting an aqueous solution containing organic acids with an organic solvent selected from the group of primary amines to quaternary amines and the group of phosphate esters, which is diluted with petroleum hydrocarbons, to extract the organic acids from the solution; then, the organic solvent containing the migrated organic acids is heated or heated and distilled, or contacted with a liquid containing Na, Mg, NH3, etc., to peel off and recover the organic acids, while also regenerating the organic solvent (A).

[0012] On the other hand, as described in Patent Documents 3 to 8, using extractants with higher boiling points than lower fatty acids, such as trialkylphosphine oxides and amine compounds, makes it easy to separate the carboxylic acid extracted into the extractant-rich fraction in the distillation purification process. However, the extraction rate using trialkylphosphine oxides and amine compounds is not high. As shown in Patent Documents 6 and 7, the extraction rate of carboxylic acids is high when combining amine compounds with branched primary alcohols or linear primary alcohols, but in this case, the alcohol dissolves in water. Dissolution means that a certain amount of alcohol is lost during the extraction operation, and the lost amount must be added from an external source. Even if this amount is small, if the carboxylic acid to be recovered is dilute, the recovery becomes meaningless. Furthermore, although the method described in Patent Document 8 does not show a specific extraction method, the extraction rate is about 50%, which is not very high in terms of extraction equilibrium. In addition, regarding heated distillation, separation is possible based on physical properties, but no specific results are shown.

[0013] Patent Document 9 describes a method for producing anhydrous or nearly anhydrous formic acid by hydrolyzing methyl formate, separating unreacted methyl formate and the resulting methanol by distillation in a first distillation step, and then removing formic acid from the bottom product by liquid-liquid extraction, wherein (a) water and methyl formate are used in a molar ratio of 1:1 to 30:1 during the hydrolysis of methyl formate, and (b) the effluent from the hydrolysis reactor is supplied to a distillation column to separate methanol and methyl formate from the formic acid-water-bottom product. (c) Separate the methyl formate and methanol into the upper part of the same column or into another column, and recirculate the partial flow of methyl formate downstream of the addition site of the hydrolysis mixture in the first distillation step; (c) extract aqueous formic acid from the bottom product of the first distillation step at a temperature of 20 to 100°C using at least a stoichiometric amount of a high-boiling point amine that forms a hydroformieate slightly soluble in water having a pKa- value of 4 to 9; (d) obtain the amine extract A method for producing anhydrous or nearly anhydrous formic acid is disclosed, characterized in that (c) and / or (d) a hydrophobic solvent is added in a distillation column at a temperature of 30 to 120°C and a pressure of 10 to 400 millibars, wherein in steps (c) and / or (d), a heteroazeotropic mixture is formed with water and formic acid, or a hydrophobic solvent whose boiling point is higher than that of water and formic acid but lower than that of the amine used is added; (e) the dehydrated extract is introduced into the upper stage of a decomposition column, where the amine hydroformeate is decomposed into formic acid and amine at a temperature of 110 to 240°C, thereby obtaining formic acid and solvent as the top product of the decomposition column, possibly as an azeotropic mixture, and amine and solvent as the bottom product, and the top product is separated into formic acid and solvent; and (f) the resulting stream of methanol, methyl formate, water, amine and solvent is recycled to a process and / or treatment step, thereby introducing the recycled amine from the bottom of the decomposition column for purification in an adsorption column.

[0014] The method described in Patent Document 9 involves extracting formic acid using a high-boiling point amine, removing water by distillation, and then adding an aliphatic, cyclic aliphatic, or aromatic hydrocarbon having 8 to 12 C-atoms as a hydrophobic solvent to decompose the formic acid and amine at high temperatures to purify the formic acid. However, since the decomposition of formic acid and amine requires high temperatures, there are concerns about corrosion inside the distillation column and the generation of decomposition gases.

[0015] Furthermore, Patent Document 10 describes the extraction of formic acid using N,N-di-n-butylformamide. However, this extractant accepts a considerable amount of water along with the formic acid (43% by weight based on the extracted formic acid in the described example), and therefore a large amount of water must be evaporated. Moreover, only 14% of the formic acid is obtained in an anhydrous state, with the remainder being obtained as 70% product. Thus, although Patent Document 10 used N,N-di-n-butylformamide, which has a high boiling point, along with the carboxylic acid, a large amount of water was extracted into the extractant-rich fraction, making it difficult to remove the water during the distillation process and thus difficult to recover formic anhydride.

[0016] Furthermore, in various manufacturing or processing steps, aqueous solutions of carboxylic acids generated as wastewater may have a dilute carboxylic acid concentration of approximately 0.01 to 0.3 by mass fraction, and may also contain inorganic salts in the range of 0.003 to 0.2 by mass fraction. It was unclear whether the methods described above could be applied as a method for recovering carboxylic acids in such cases where inorganic salts are present in the aqueous solution. [Prior art documents] [Patent Documents]

[0017] [Patent Document 1] Japanese Patent Application Publication No. 9-151158 [Patent Document 2] Japanese Patent Publication No. 2018-062512 [Patent Document 3] Japanese Patent Application Publication No. 61-176550 [Patent Document 4] Japanese Patent Application Laid-Open No. 61-176551 [Patent Document 5 Japanese Patent Application Laid-Open No. 08-283191 [Patent Document 6 Japanese Patent Application Laid-Open No. 61-176552 [Patent Document 7 Japanese Patent Application Laid-Open No. 61-176553 [Patent Document 8 Japanese Patent Application Laid-Open No. 55-154935 [Patent Document 9 Japanese Patent Application Laid-Open No. 61-043133 [Patent Document 10 German Patent Application Publication No. 2545658 [Summary of the Invention [Problems to be Solved by the Invention

[0018] Therefore, an object of the present disclosure is to provide a method for recovering carboxylic acid that solves the problems in the prior art as described above. The present disclosure also relates to, for example, a method for recovering carboxylic acid as anhydrous carboxylic acid from an aqueous solution containing carboxylic acid and inorganic salts. Another object of the present disclosure is to provide a method for recovering carboxylic acid that has low concerns such as corrosion inside the apparatus and can reduce costs such as those of the apparatus materials. [Means for Solving the Problems

[0019] As a result of intensive research and study to solve the above problems, the present inventors have separated carboxylic acid from an aqueous solution containing water and carboxylic acid and having a carboxylic acid concentration of 0.05 to 0.3% by mass fraction, and purified the separated carboxylic acid to obtain anhydrous carboxylic acid having a water content of less than 0.01% by mass fraction and a carboxylic acid content of 0.99 or more, as a method of a) The aqueous solution is brought into liquid-liquid contact with a prepared extractant containing a component for extracting carboxylic acid and a diluent, such that the components of the prepared extractant dissolve in the aqueous solution side at a mass fraction of less than 0.001, the carboxylic acid concentration in the aqueous solution becomes less than 0.005, and carboxylic acid separated from the aqueous solution at a recovery rate of 90% or more dissolves in the prepared extractant side, and water dissolves at a mass fraction of less than 0.05. A first step comprising a liquid-liquid extraction step is provided. b) The prepared extractant containing carboxylic acid and water that has been treated in the first step is distilled to azeotropically distill and remove the diluent component of the prepared extractant and water. After separating the extractant component layer mainly composed of the diluent and the water layer with a decanter provided in the distillation column, the water layer is discharged. The discharged water is mixed with the primary-side aqueous solution or the secondary-side aqueous solution of the first step, or discarded. On the other hand, the extractant component layer mainly composed of the diluent is returned to the distillation step as reflux, and a second step is provided in which the prepared extractant containing carboxylic acid is discharged from the bottom of the column. c) The prepared extractant containing carboxylic acid discharged in the second step is distilled again. When there is no azeotrope between the diluent and carboxylic acid in the prepared extractant, purified carboxylic acid containing water at a mass fraction of less than 0.01 and the prepared extractant at a mass fraction of less than 0.01 is discharged from the top of the column, and the prepared extractant from which carboxylic acid and water have been removed and discharged from the bottom of the column is returned to the first step. A third step is provided. When the diluent and carboxylic acid in the prepared extractant have a minimum azeotrope, it is distilled off by azeotropic distillation and separated into an extractant component layer mainly composed of the diluent and a carboxylic acid layer with a decanter provided in the distillation column. The carboxylic acid layer containing water at a mass fraction of less than 0.01 and the prepared extractant component at a mass fraction of less than 0.01 is discharged to obtain purified carboxylic acid. The extractant component layer mainly composed of the diluent is returned to the distillation step as reflux, and the prepared extractant from which carboxylic acid and water have been removed and discharged from the bottom of the column is returned to the first step. A third step is provided. The diluent of the prepared extractant has a minimum azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more in mass fraction, and the diluent is a hydrophobic solvent having a higher boiling point than water and a higher boiling point than carboxylic acid under atmospheric pressure, and has no azeotrope with carboxylic acid. Alternatively, a hydrophobic solvent having a higher boiling point than water and a higher boiling point than carboxylic acid at atmospheric pressure, characterized in that it has the lowest azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, the carboxylic acid has the lowest azeotrope with the diluent, the carboxylic acid and diluent separate into layers in any proportion, and the solubility of the diluent in the carboxylic acid is less than 0.002 by mass fraction, The carboxylic acid is extracted using an organic solvent that has a higher boiling point than the diluent, has a carboxylic acid partition ratio D (carboxylic acid in the extracted component / carboxylic acid in water) of 0.3 or higher within the operating range, and is a water-insoluble organic solvent that is perfectly miscible with the diluent. This is achieved by a method for recovering carboxylic acids.

[0020] In one embodiment of the carboxylic acid recovery method according to this disclosure, the aqueous solution further contains an inorganic salt in a mass fraction of 0.003 to 0.2, and in the liquid-liquid extraction step of the first step, the inorganic salt in the aqueous solution dissolves in the prepared extractant at a rate of less than 0.0001 and in the water in the aqueous solution at a rate of less than 0.003 to 0.2.

[0021] In one embodiment of the method for recovering carboxylic acids according to this disclosure, the carboxylic acid contained in the aqueous solution is selected from the group consisting of formic acid, acetic acid, and propionic acid.

[0022] In one embodiment of the carboxylic acid recovery method according to this disclosure, the inorganic salt contained in the aqueous solution is selected from the group consisting of metal chlorides, metal sulfates, metal bisulfates, metal hydroxides, metal carbonates, metal bicarbonates, metal phosphates, metal hydrogen phosphates, and metal borates.

[0023] In one embodiment of the carboxylic acid recovery method according to this disclosure, the diluent of the prepared extractant is a hydrophobic solvent having a boiling point of 110 to 220°C at atmospheric pressure and a solubility in water of less than 0.001 by mass fraction at 25°C.

[0024] In one embodiment of the carboxylic acid recovery method according to the present disclosure, the diluent of the prepared extractant is further shown to be at least one selected from the group consisting of toluene, octane, isooctane, nonane, decane, undecane, dodecane, o-xylene, m-xylene, p-xylene, and ethylbenzene.

[0025] In one embodiment of the carboxylic acid recovery method according to this disclosure, the aqueous solution contains impurities that are soluble in the carboxylic acid but insoluble in water. It is shown that, before applying the aqueous solution to the first step, there is a step of removing impurities by filtering the aqueous solution, or, in the first step, the carboxylic acid is recovered on the prepared extractant side, and the impurities precipitated at the interface with the extractant phase are extracted together with the prepared extractant and aqueous solution from a nozzle provided at the top or bottom of the liquid-liquid extraction device, the liquid containing the extracted impurities is separated into impurities and liquid by filtration or centrifugation, and the recovered liquid is mixed with the primary aqueous solution of the first step, thereby removing the impurities.

[0026] In one embodiment of the carboxylic acid recovery method according to this disclosure, the component for extracting the carboxylic acid in the prepared extractant is selected from the group consisting of organophosphorus compounds and amide compounds.

[0027] In one embodiment of the carboxylic acid recovery method according to this disclosure, the mixing ratio of the carboxylic acid extraction component and the diluent in the prepared extractant is shown to be carboxylic acid extraction component:diluent = 1:5 to 9:1 by mass ratio.

[0028] In one embodiment of the carboxylic acid recovery method according to this disclosure, the liquid-liquid extraction apparatus is operated at a temperature of 10 to 90°C when the aqueous solution containing the inorganic salt is subjected to the first step.

[0029] In one embodiment of the carboxylic acid recovery method according to this disclosure, the second step is performed at a reduced pressure of 6.67 to 66.7 kPa.

[0030] In one embodiment of the carboxylic acid recovery method according to the present disclosure, the third step includes a filtering step in the line that supplies the bottom liquid to the distillation column in order to remove inorganic salts precipitated at the bottom of the distillation column by distilling off water from the prepared extractant containing the carboxylic acid and water in the second step.

[0031] In one embodiment of the carboxylic acid recovery method according to the present disclosure, the second step is shown to include a line for supplying the prepared extractant into the column from any stage above the raw material supply stage, separate from the reflux of the extractant component layer mainly consisting of a diluent, in order to improve the carboxylic acid recovery rate.

[0032] In one embodiment of the carboxylic acid recovery method according to this disclosure, the third step is performed at a reduced pressure of 6.67 to 66.7 kPa.

[0033] In one embodiment of the carboxylic acid recovery method according to the present disclosure, the third step includes a line that supplies the extractant component layer, mainly composed of a diluent, into the column from any stage below the raw material supply stage, separate from the reflux of the extractant component layer, mainly composed of a diluent, in order to improve the recovery rate of the carboxylic acid.

[0034] In one embodiment of the carboxylic acid recovery method according to the present disclosure, a fourth step is further provided in which the carboxylic acid containing water in a mass fraction of less than 0.01 and a prepared extractant component in a mass fraction of less than 0.01 discharged in the third step is distilled again to remove the water and prepared extractant component by distillation, thereby obtaining an anhydrous carboxylic acid having a carboxylic acid content of 0.99 or more and water content of 0.002 or less from the top of the distillation column or the middle of the distillation column.

[0035] One embodiment of the carboxylic acid recovery method according to the present disclosure is shown to further include a fifth step, characterized in that the wastewater discharged in the first step, which contains less than 0.005 mass fraction of carboxylic acid, less than 0.001 mass fraction of prepared extractant components, and 0.003 to 0.2 mass fraction of inorganic salts, is subjected to distillation or stripping treatment to remove the prepared extractant components to obtain an aqueous inorganic salt solution, and the removed prepared extractant components are returned to the primary wastewater of the first step. [Effects of the Invention]

[0036] In this disclosure, when recovering a carboxylic acid from an aqueous solution containing a carboxylic acid, and extracting the carboxylic acid by liquid-liquid contact with the aqueous solution, a prepared extractant consisting of a diluent that satisfies the specific conditions described above and a component for extracting the carboxylic acid is used, and the process including this extraction step and the subsequent distillation step is made appropriate, thereby enabling efficient recovery of the carboxylic acid as a highly purified anhydrous carboxylic acid. [Brief explanation of the drawing]

[0037] [Figure 1] This is a block diagram showing the steps in one embodiment of the carboxylic acid recovery method according to the present disclosure. [Figure 2] This is a block diagram showing the steps in another embodiment of the carboxylic acid recovery method according to the present disclosure. [Figure 3] This is a block diagram showing each step in yet another embodiment of the carboxylic acid recovery method according to the present disclosure. [Figure 4] This is a block diagram showing each step in yet another embodiment of the carboxylic acid recovery method according to the present disclosure. [Modes for carrying out the invention]

[0038] The following description will be based on preferred embodiments of this disclosure.

[0039] (Object to be processed) In this disclosure, carboxylic acids are recovered from aqueous solutions containing carboxylic acids. The aqueous solutions containing carboxylic acids used as the material to be treated are not particularly limited and include, but are not limited to, wastewater discharged from processes in various chemical and textile industries, and dilute carboxylic acid solutions produced by microorganisms in the fermentation industry, as well as wastewater containing carboxylic acids and inorganic salts.

[0040] The carboxylic acids covered in this disclosure are, for example, carboxylic acids having 1 to 3 carbon atoms, specifically formic acid, acetic acid, and propionic acid, and in particular formic acid.

[0041] The inorganic salts that may be contained in the wastewater are not particularly limited and may include, for example, metal chlorides, metal sulfates, metal hydrogen sulfates, metal hydroxides, metal carbonates, metal bicarbonates, metal phosphates, metal hydrogen phosphates, and metal borates, but sodium chloride and sodium sulfate are particularly noteworthy.

[0042] Furthermore, the wastewater may contain impurities characterized by being components that dissolve in carboxylic acid but not in water.

[0043] Furthermore, in the recovery method relating to this disclosure, the carboxylic acid concentration in the aqueous solution containing the target carboxylic acid is in the range of approximately 0.05 to 0.3 by mass fraction, and if an inorganic salt is present, the inorganic salt concentration is in the range of approximately 0.003 to 0.2 by mass fraction.

[0044] (first step) For aqueous solutions containing a carboxylic acid concentration of 0.05 to 0.3 by mass fraction as described above, the first step of the recovery method according to this disclosure comprises a liquid-liquid extraction step in which the aqueous solution and a prepared extractant containing a component for extracting carboxylic acid and a diluent are brought into liquid-liquid contact, the components of the prepared extractant dissolve in the aqueous solution at a mass fraction of less than 0.001, the carboxylic acid concentration in the aqueous solution becomes less than 0.005, and the carboxylic acid separated from the aqueous solution with a recovery rate of 90% or more dissolves in the prepared extractant, with water dissolving at a mass fraction of less than 0.05.

[0045] Furthermore, if the aqueous solution containing the carboxylic acid to be treated also contains an inorganic salt in a mass fraction of 0.003 to 0.2, then in the liquid-liquid extraction step of the first step, the inorganic salt in the aqueous solution will dissolve in the prepared extractant at a rate of less than 0.0001 and in the water of the aqueous solution at a rate of less than 0.003 to 0.2.

[0046] However, in the recovery method relating to this disclosure, in the liquid-liquid extraction step of the first step, a prepared extractant is used which includes a component for extracting carboxylic acid and a diluent that satisfies the following conditions as the extractant.

[0047] In other words, the diluent used in the prepared extractant has a minimum azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, more preferably 0.5 or more, and the diluent does not have an azeotrope with the carboxylic acid. Alternatively, the diluent has a minimum azeotrope with water, and the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, more preferably 0.5 or more, or it has a minimum azeotrope with a carboxylic acid, the carboxylic acid and the diluent separate in any proportion, and the dissolution of the diluent in the carboxylic acid is less than 0.002 by mass fraction. The solvent is a hydrophobic solvent that has a higher boiling point than water and a higher boiling point than carboxylic acids under atmospheric pressure (e.g., 10¹³ ± 20 hPa), preferably represented by hydrocarbons with a boiling point of 110 to 220°C. Here, "hydrophobic solvent" is not particularly limited, but it refers to a solvent whose solubility in water at 25°C is less than 0.01 by mass fraction, more preferably less than 0.001.

[0048] Such diluents include, for example, saturated or unsaturated aliphatic and aromatic hydrocarbons, as long as the above conditions are met. Preferably, these include toluene, octane, isooctane, nonane, decane, undecane, dodecane, o-xylene, m-xylene, p-xylene, and ethylbenzene, with toluene, octane, and decane being particularly preferred.

[0049] On the other hand, the component for extracting the carboxylic acid has a higher boiling point than the diluent, preferably 140 to 280°C at the operating pressure range of 6.67 kPa to 66.7 kPa, and is an insoluble organic solvent whose carboxylic acid partition ratio D (= carboxylic acid in the extracting component / carboxylic acid in water) between "water" and "component to be extracted" is 0.3 or higher within this operating range and is completely miscible with the diluent. Here, "completely miscible" in this specification means that it becomes a homogeneous single liquid without separation under the operating temperature conditions of the first step.

[0050] Furthermore, if the carboxylic acid partition ratio D (=carboxylic acid in the extracted component / carboxylic acid in water) between "water" and "component to be extracted" is 0.3 or higher within this operating range, it can be judged that the extraction performance of the component to be extracted is good, and it is more desirable that it be 0.6 or higher. The components used to extract carboxylic acids that satisfy these conditions are not particularly limited, but preferably include organophosphorus compounds such as tributyl phosphate and trioctylphosphine oxide, and amide compounds such as N,N-di-n-butylformamide and Nn-butyl-N-2-ethylhexylformamide, and more preferably tributyl phosphate and N,N-di-n-butylformamide.

[0051] In this disclosure, by adding the above-described diluent to the component for extracting carboxylic acids as the prepared extractant, the stratification during extraction in the first step can be improved, the movement of water to the extractant can be suppressed, and in the distillation in the second step described later, when removing water, the diluent acts as an azeotrope with water, preferentially pushing water to the top of the distillation column over carboxylic acids, thereby increasing the separation efficiency of carboxylic acids. Specific examples of combinations of the component for extracting carboxylic acids and the diluent are not limited to tributyl phosphate and toluene, tributyl phosphate and octane, tributyl phosphate and decane, N,N-di-n-butylformamide and octane, and N,N-di-n-butylformamide and decane.

[0052] While not particularly limited, the mixing ratio of the carboxylic acid extraction component to the diluent in the prepared extractant is preferably, by mass ratio, for example, carboxylic acid extraction component:diluent = 1:5 to 9:1, preferably carboxylic acid extraction component:diluent = 1:1 to 5:1, and more preferably carboxylic acid extraction component:diluent = 2:1 to 3:1. While not particularly limited, for example, the ratio of carboxylic acid extraction component to diluent can be 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.55:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3:1, etc. If the amount of the carboxylic acid extracting component and the diluent in the prepared extractant is extremely low compared to the mixing ratio range shown here, the efficiency of the first step may decrease, and the required amount of prepared extractant may increase. On the other hand, if the amount of diluent is extremely low compared to the mixing ratio range shown here, in the second step, the composition will shift from an azeotropic composition of water and diluent to an azeotropic composition of water and carboxylic acid, resulting in a significant decrease in the yield of carboxylic acid. Furthermore, if the amount of diluent is extremely low, the carboxylic acid extracting component has a high boiling point, so in the third step, the dilution of the diluent results in a small effect of reducing the boiling point at the bottom of the column, and the bottom temperature will rise, which may cause corrosion inside the column due to carboxylic acid or gas generation due to the decomposition of carboxylic acid.

[0053] The mechanism for liquid-liquid extraction performed in the first step is not particularly limited and may be a static liquid-liquid contact mechanism using packing material, or a dynamic liquid-liquid contact mechanism such as an RDC (rotating disc countercurrent continuous extractor) or a Karr column.

[0054] Furthermore, in the liquid-liquid extraction performed in the first step, the continuous phase may be either the carboxylic acid-containing aqueous solution (wastewater) or the prepared extractant, and accordingly, the dispersed phase may also be either the prepared extractant or the carboxylic acid-containing aqueous solution (wastewater).

[0055] Furthermore, if the carboxylic acid-containing aqueous solution (wastewater) to be treated contains an inorganic salt in addition to the carboxylic acid, when this aqueous solution is subjected to liquid-liquid extraction in the first step, water at a mass fraction of 0.05 or less is extracted into the prepared extractant, causing the inorganic salt to precipitate at the interface with the extractant phase. For this reason, in order to increase the solubility of the inorganic salt in the aqueous phase and suppress precipitation at the interface, it is desirable that the liquid-liquid extraction apparatus be operated at a temperature of 10 to 90°C, more preferably 30 to 50°C.

[0056] Furthermore, if the carboxylic acid-containing aqueous solution (wastewater) to be treated contains an inorganic salt, such as sodium sulfate, in addition to the carboxylic acid, the presence of the inorganic salt improves stratification in liquid-liquid extraction, making it more difficult for water to enter the extractant layer, thus yielding a more favorable effect. This effect is well exhibited when the carboxylic acid-containing aqueous solution (wastewater) to be treated contains the inorganic salt in a mass fraction range of 0.003 to 0.2, more preferably in a mass fraction range of 0.1 to 0.15, as described above. In embodiments where the carboxylic acid-containing aqueous solution (wastewater) to be treated does not contain an inorganic salt, it is not necessary to add an inorganic salt, but in some cases, it is possible to intentionally add an inorganic salt in a predetermined amount.

[0057] Furthermore, as described above, if the aqueous carboxylic acid solution (wastewater) to be treated contains impurities, it is desirable to remove the impurities by filtering the wastewater before supplying it to the first step, or, in the first step, when the carboxylic acid is recovered on the prepared extractant side, the impurities precipitated at the interface with the extractant phase are extracted together with the prepared extractant and wastewater from a nozzle provided at the top or bottom of the liquid-liquid extraction device, the extracted liquid containing the impurities is separated into impurities and liquid by centrifugation, and the recovered liquid is mixed with the primary wastewater of the first step to remove the impurities. These treatments may be performed individually or in combination.

[0058] (Second process) The second step of the recovery method according to this disclosure is characterized by comprising the steps of distilling the prepared extractant containing the carboxylic acid and water that has been treated in the first step, removing the diluent component and water of the prepared extractant by azeotrope, separating the extractant component layer mainly consisting of the diluent and the aqueous layer in a decanter provided in the distillation column, discharging the aqueous layer, mixing the discharged water with the primary or secondary aqueous solution of the first step, or discarding it as wastewater, while the extractant component layer mainly consisting of the diluent is returned to the distillation step as reflux, and the prepared extractant from which water has been removed is discharged from the bottom of the column. In this specification, "decanter" does not have any particular form as long as it can separate the diluent layer and the aqueous layer, and is not limited to, for example, one that separates them using a slope.

[0059] It should be noted that the "extractant layer, mainly composed of diluents," in the decanter of the distillation column may, strictly speaking, contain trace amounts of carboxylic acids and components that extract carboxylic acids, in addition to the diluents. However, since it is returned to the distillation process as reflux, the presence of carboxylic acids and components that extract carboxylic acids does not pose any particular problem.

[0060] In this disclosure, as described above, the diluent blended in the prepared extractant has a minimum azeotrope with water, the concentration of water in the azeotropic composition is 0.2 or more by mass fraction, and the diluent does not have an azeotrope with the carboxylic acid, or the diluent has a minimum azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, and the carboxylic acid has a minimum azeotrope with the diluent, and when the prepared extractant containing the carboxylic acid and water after treatment in the first step is subjected to distillation, the component that extracts the carboxylic acid in the prepared extractant has a higher boiling point than the diluent, and has a high affinity for the carboxylic acid, so the extractant effect in distillation works, suppressing the vaporization of the carboxylic acid and allowing water to be preferentially removed by distillation.

[0061] It should be noted that even in the aqueous layer, which is separated from the extractant layer (primarily the diluent) by decanting in this manner and using a decanter in the distillation column, there is a slight possibility that trace amounts of carboxylic acid may still be present. However, since the separated water can be mixed with the primary or secondary aqueous solution from the first step and subjected to each step again, the presence of carboxylic acid in the aqueous layer does not pose a particular problem.

[0062] The distillation in the second step is not particularly limited, but it is desirable to operate under reduced pressure of, for example, 6.67 to 66.7 kPa, more preferably 13.3 to 26.7 kPa. Distilling under reduced pressure in the second step lowers the column bottom temperature, which helps to suppress the decomposition of carboxylic acids and corrosion inside the column.

[0063] While there are no particular limitations on the distillation conditions, examples include using a tray column with 5 to 10 stages and a reflux ratio of approximately 1 to 3. Furthermore, when the compositional balance within the column is disrupted, supplying the prepared extractant, more preferably the regenerated prepared extractant from the third step described later, from any stage above the raw material supply stage has the effect of causing formic acid to fall to the bottom of the column due to the extraction effect of the carboxylic acid extracting component, thereby further reducing the loss of formic acid.

[0064] Furthermore, if the carboxylic acid-containing aqueous solution (wastewater) to be treated contains inorganic salts in addition to the carboxylic acid, the removal of water from the prepared extractant containing the carboxylic acid and water in this second step may cause the inorganic salts dissolved in the water to precipitate at the bottom of the distillation column. To remove these precipitates, it is desirable to include a filter treatment step in the line that supplies the bottom liquid to the third step, which will be described later.

[0065] (Third step) The third step of the recovery method relating to this disclosure is characterized in that the prepared extractant containing the carboxylic acid discharged in the second step is distilled again, and if there is no azeotrope between the diluent and the carboxylic acid in the prepared extractant, the purified carboxylic acid containing less than 0.01 by mass fraction of water and less than 0.01 by mass fraction of the prepared extractant is discharged from the top of the column, and the prepared extractant from which the carboxylic acid and water have been removed discharged from the bottom of the column is returned to the first step. If the diluent and carboxylic acid in the prepared extractant have a minimum azeotrope, the extractant layer mainly composed of the diluent and the carboxylic acid layer are separated by azeotropic distillation in a decanter equipped in the distillation column. The carboxylic acid layer, which contains less than 0.01 mass fraction of water and less than 0.01 mass fraction of the prepared extractant component, is discharged to obtain purified carboxylic acid. The extractant layer mainly composed of the diluent is returned to the distillation process as reflux, and the prepared extractant, from which the carboxylic acid and water discharged from the bottom of the column have been removed, is returned to the first step.

[0066] In the recovery method of this disclosure, in addition to removing water from the prepared extractant by distillation in the second step, the carboxylic acid can be obtained as purified carboxylic acid from the prepared extractant by distillation again in this third step. Furthermore, since the rise in the bottom temperature of the distillation column is suppressed by the presence of a diluent at the bottom of the distillation column, the heat resistance requirements of the materials constituting the apparatus are relaxed, and the range of material choices is expanded.

[0067] The distillation in the third step is not particularly limited, but it is desirable to operate under reduced pressure of 6.67 to 66.7 kPa, more preferably 13.3 to 26.7 kPa. Distilling under reduced pressure in the third step lowers the azeotropic point of the carboxylic acid and the diluent, thereby suppressing corrosion in the column and decomposition of the carboxylic acid. Furthermore, since the prepared extractant regenerated in the third step is reused in the first step, the risk of decomposition due to thermal history and generation of impurities can be suppressed.

[0068] While there are no particular limitations on the distillation conditions, examples include using a tray column with 5 to 10 stages and a reflux ratio of approximately 3 to 5. Furthermore, in order to improve the recovery rate of carboxylic acids, by supplying an extractant component layer, mainly the diluent obtained from the decanter in this third step, into the column from any stage below the raw material supply stage, separate from reflux, it is possible to promote azeotropy between the carboxylic acid and the diluent, which is suppressed by the extraction effect of the components that extract the carboxylic acid.

[0069] (Fourth step) In one embodiment of the carboxylic acid recovery method according to the present disclosure, a fourth step may be further included as needed, in which the carboxylic acid containing water in a mass fraction of less than 0.01 and a prepared extractant component in a mass fraction of less than 0.01 discharged in the third step is distilled again to remove the water and prepared extractant component by distillation, thereby obtaining an anhydrous carboxylic acid having a carboxylic acid content of 0.99 or more, more preferably 0.995 or more, and water content of 0.002 or less.

[0070] If necessary, the purity of the carboxylic acid obtained in the third step can be increased by distilling it again. Note that the number of distillations in this fourth step can be multiple times, not just once.

[0071] The distillation in this fourth step, which may be performed as needed, is not particularly limited, but it is desirable to operate it at a reduced pressure of 6.67 to 66.7 kPa, more preferably 26.7 to 40.0 kPa.

[0072] While there are no particular limitations on the distillation conditions, examples include using a tray column with 5 to 10 stages and a reflux ratio of approximately 3 to 5, with the product withdrawn from the top of the column, or withdrawn from the middle of the column by side-cutting under full reflux. This concentrates the water and carboxylic acid extractants at the bottom of the column and the diluent components at the top, allowing for the recovery of high-purity carboxylic acid. While not particularly limited, for example, anhydrous formic acid containing 0.998 mass fraction of formic acid and 0.002 mass fraction of water can be obtained.

[0073] (Fifth step) In one embodiment of the carboxylic acid recovery method according to the present disclosure, a fifth step may be further provided as needed, characterized in that the wastewater discharged in the first step, which contains less than 0.005 mass fraction of carboxylic acid, less than 0.001 mass fraction of the prepared extractant component, and 0.003 to 0.2 mass fraction of the inorganic salt, is distilled or stripped to remove the prepared extractant component and carboxylic acid to obtain an aqueous inorganic salt solution, and the removed prepared extractant component and carboxylic acid are returned to the primary wastewater of the first step.

[0074] In an embodiment in which the aqueous solution (waste liquid) containing a carboxylic acid as the material to be treated further contains an inorganic salt, for example, if any manufacturing or treatment process that generates this wastewater uses the inorganic salt in that process, by providing such a fifth step, in addition to recovering the carboxylic acid, the inorganic salt can be recovered as an aqueous solution of the inorganic salt, thereby further promoting the reuse of raw material resources.

[0075] The distillation conditions in this fifth step, which is performed as needed, are not particularly limited, but for example, distillation can be carried out under atmospheric pressure of approximately 1013 ± 20 hPa.

[0076] While there are no particular limitations on the distillation conditions, one example is to use a tray column with 5 to 10 stages and under total reflux conditions, extracting the product as an aqueous solution of inorganic salt from the bottom of the column. [Examples]

[0077] The present disclosure will be described in more detail below based on examples. Figures 1 to 4 are block diagrams showing each step in one embodiment of the carboxylic acid recovery method according to the present disclosure. Figure 1 is a block diagram showing an overall configuration including all of the first to fifth steps in one embodiment of the carboxylic acid recovery method according to the present disclosure; Figure 2 is a block diagram showing a configuration consisting of the first to third steps in one embodiment of the carboxylic acid recovery method according to the present disclosure; Figure 3 is a block diagram showing a configuration consisting of the first to fourth steps in one embodiment of the carboxylic acid recovery method according to the present disclosure; and Figure 4 is a block diagram showing a configuration including the first and fifth steps in one embodiment of the carboxylic acid recovery method according to the present disclosure.

[0078] In the following examples, the analysis of each component was performed according to the following method.

[0079] (moisture) The moisture content was determined by quantification using gas chromatography under the following conditions. Equipment: GC-2014 (manufactured by Shimadzu Corporation) Detector: Thermal conductivity detector Column: Chromosorb 101 (Inner diameter: 2mm, Length: 1.83m) Column heating conditions: 90°C → 10°C / min heating → 220°C (hold for 47 minutes) Inlet conditions: 250°C, carrier gas flow rate: 4 ml / min

[0080] (Carboxylic acid) When the carboxylic acid concentration was 0.5 or less by mass fraction, the carboxylic acid concentration was determined by neutralization titration using a potentiometric automatic titrator under the following conditions. When the carboxylic acid concentration was 0.5 or greater by mass fraction, it was calculated by subtracting each component from 100. Equipment: AT-710 (manufactured by Kyoto Electronics Corporation) Titration reagent: 0.1 mol / L ethanolic potassium hydroxide aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0081] (Other volatile components) Other volatile components were quantified and determined by gas chromatography under the following conditions. Equipment: GC-2014 (manufactured by Shimadzu Corporation) Detector: Flame ion detector Column: G-100 (Inner diameter: 1.2 mm, Length: 40 m, Film thickness: 3.0 μm) Column heating conditions: 80°C → 10°C / min heating → 240°C (hold for 44 minutes) Inlet conditions: 250°C, carrier gas flow rate: 15 ml / min

[0082] (Non-volatile components) Non-volatile components were determined by weighing approximately 5.0 g of the liquid into a φ100 ml petri dish, letting it stand for 24 hours under reduced pressure of approximately 1.3 kPa in a vacuum dryer set to 140°C, and then measuring its weight.

[0083] Example 1 As the carboxylic acid-containing aqueous solution to be treated, wastewater was prepared containing 0.2 mass fractions of formic acid, 0.14 mass fractions of sodium sulfate, 0.0005 mass fractions of impurities, with the remainder being water. The specific gravity of the wastewater was 1.18 at 20°C.

[0084] (first step) As shown in Figures 1 and 2, the wastewater 10 was introduced into a packed extraction column liquid-liquid extraction apparatus 20 with a column diameter of 65 mm, packed with ordered packing material and having a height equivalent to 3 theoretical stages. A prepared extractant 12, a mixture of tributyl phosphate (TBP) as a component for extracting carboxylic acids and decane as a diluent in a mass ratio (TBP:decane=) of 3:1, was introduced opposite the liquid-liquid extraction apparatus 20. Under treatment conditions of 8 hours at 40°C, with a wastewater supply flow rate of 40 kg / h and a prepared extractant supply flow rate of 130 kg / h, 320 kg of wastewater was subjected to liquid-liquid contact with 40 kg of prepared extractant 10. The specific gravity of the prepared extractant was 0.89 at 20°C. The prepared extractant 12 was used as the continuous phase and the wastewater 10 as the dispersed phase.

[0085] The extractant layer (light liquid) 22 from which formic acid was extracted using a liquid-liquid extraction device was withdrawn from a nozzle located on the upper side of the liquid-liquid extraction device 20, while the aqueous layer (heavy liquid) 24 from which formic acid had been removed was withdrawn from a nozzle located on the lower side. Furthermore, impurities precipitated at the interface with the aqueous layer (intermediate layer 26) during this liquid-liquid extraction operation were withdrawn together with the aqueous layer 24 from a nozzle located on the lower side of the liquid-liquid extraction device. The liquid containing the withdrawn impurities was then separated into impurities and liquid by centrifugation to remove the impurities 26.

[0086] When the composition of the 1125 kg extractant layer (light liquid) 22 extracted from the first process was examined, it was found to consist of 0.056 mass fractions of formic acid, 0.231 mass fractions of decane, 0.020 mass fractions of water, 0.00001 mass fractions of sodium sulfate, and the remainder being TBP.

[0087] (Second process) The extractant layer (light liquid) 22 was then fed into a distillation column (continuous tray column) 30 with 8 stages, with the 4th stage from the top serving as the raw material supply stage at a rate of 200 kg / h. Distillation was carried out under reduced pressure of 13.3 kPa, with a bottom temperature of 118-120°C and a top temperature of 52-53°C. Decane and water were removed azeotropically from the top of the column, and the extractant layer, mainly composed of diluents, and the aqueous layer were separated in a decanter 32 provided in the distillation column 30, after which the aqueous layer was discharged. The discharged water 34 was mixed with the primary aqueous solution (wastewater 10) from the first step via the return line 72. Meanwhile, the extractant layer 36, mainly composed of diluents, was refluxed back into the distillation column 30 via the reflux line 37 at a reflux ratio of 2. The prepared extractant 38, from which water had been removed at a rate of 180 kg / h, was then taken from the bottom of the distillation column 30.

[0088] When the composition of the prepared extract 38, obtained by removing 1085 kg of water extracted from the second process, was examined, it was found to contain 0.055 mass fraction of formic acid, 0.236 mass fraction of decane, less than 0.0003 mass fraction of water, and the remainder being TBP.

[0089] Furthermore, the line 39 that sends the prepared extractant 38, from which the water extracted in the second step has been removed, to the third step described later is equipped with a filter (not shown) consisting of a bag filter with a filtration accuracy of 0.5 μm. In this second step, by distilling off the water from the prepared extractant containing formic acid and water, there is a possibility that sulfates dissolved in the water may precipitate at the bottom of the distillation column. However, due to the installation of the above-mentioned filter, no precipitates were observed at the bottom of the distillation column after approximately 5 hours of operation.

[0090] (Third step) After being removed from the second step, the prepared extractant 38, from which water had been removed, was then fed into a distillation column (continuous packed column) 40 with 8 theoretical stages, with the fourth stage from the top serving as the raw material supply stage at a rate of 180 kg / h. Distillation was carried out for approximately 6 hours under reduced pressure of 11.3 kPa, with a column bottom temperature of 130-131°C and a column top temperature of 85-90°C. This removed the diluent (decane) and carboxylic acid (formic acid) from the prepared extractant by azeotropic distillation. The extractant component layer 46, mainly composed of the diluent, and the formic acid layer 44 were separated in a decanter 42 provided in the distillation column 40. The formic acid layer 44 was then discharged to obtain purified formic acid. Meanwhile, the extractant component layer 46, mainly composed of the diluent, was refluxed back into the distillation column 40 via a reflux line 47 at a reflux ratio of 4. Furthermore, the prepared extract 48, from which formic acid and water were removed from the bottom of the distillation column 40 at a rate of 150 kg / h, was returned to the first process via the return line 74, and its composition was adjusted as needed to match the prepared extract 12 for reuse.

[0091] When the composition of the 60 kg formic acid layer 44 obtained as purified formic acid from the third step was examined, it was found to contain 0.99 mass fractions of formic acid, 0.003 mass fractions of water, 0.004 mass fractions of TBP, and 0.003 mass fractions of decane, indicating a high degree of purification. Furthermore, when the composition of the prepared extract 48 obtained by removing the formic acid and water discharged from the bottom of the column in the third step was examined, it was found to consist of 0.0002 mass fractions of formic acid, 0.251 mass fractions of decane, 0.0002 mass fractions of water, and the remainder being TBP.

[0092] Example 2 (Fourth step) As shown in Figures 1 and 3, the formic acid layer 44 obtained in Example 1 was again fed into a distillation column (continuous packed column) 50 with 7 theoretical stages at a feed rate of 45 kg / h, and distilled again under reduced pressure of 30 kPa, with a column bottom temperature of 68-70°C and a column top temperature of 63-64°C. Water and the prepared extractant components 56 were removed by distillation from the middle stage of the column under total reflux. As a result, the purified carboxylic acid product 52 obtained was formic anhydride containing 0.998 mass fraction of formic acid and 0.002 mass fraction of water.

[0093] Example 3 (Fifth step) In Example 1, the composition of 235 kg of aqueous layer (heavy liquid) 24 extracted from a nozzle located at the bottom of the liquid-liquid extraction apparatus 20 in the first step was examined. The composition was found to be 0.19 mass fraction of sodium sulfate, 0.002 mass fraction of formic acid, 0.0008 mass fraction of TBP, less than 0.00003 mass fraction of decane, with the remainder being water. As shown in Figures 1 and 4, this aqueous layer 24 was fed into a distillation column (continuous tray column) 60 with 6 stages, using the third stage from the top as the raw material supply stage at a rate of 60 kg / h. Distillation was performed under full reflux conditions at 101.3 kPa, with a bottom temperature of 102-103°C and a top temperature of 100°C. When the composition of the bottom liquid 64, which was drawn from the bottom of the distillation column (continuous tray column) 60 at a rate of 55 kg / h, was examined, it was found to contain 0.191% sodium sulfate by mass fraction, 0.002% formic acid by mass fraction, and the remainder being water, indicating that it can be reused as an industrial sodium sulfate aqueous solution. On the other hand, when the composition of the distillate 62 removed from the top of the distillation column (continuous tray column) 60 was examined, it was found to contain 0.0002 mass fraction of formic acid, 0.002 mass fraction of TBP, less than 0.0001 mass fraction of decane, and the remainder being water. Therefore, the distillate 62 could be treated to be returned to the primary aqueous solution (wastewater 10) of the first step.

[0094] Example 4 As the aqueous solutions containing carboxylic acids to be treated, simulated wastewater was prepared containing 0.2% formic acid by mass fraction with the remainder being water, while simulated wastewater was prepared with a constant formic acid content of 0.2% by mass fraction and sodium sulfate content of 0.01%, 0.1%, and 0.2% by mass fraction with the remainder being water.

[0095] The wastewater was transferred to a separatory funnel, and a prepared extractant 12, which was a mixture of TBP and decane in a mass ratio (TBP:decane=) of 2:1, was used in a mass ratio of 1 part prepared extractant to 1 part wastewater, and a single extraction was performed at (23)°C with sufficient shaking.

[0096] After standing for 5 minutes, the aqueous layer (heavy liquid) was removed from the separatory funnel, and the extractant layer (light liquid) was also removed. The composition and stratification of the water in the removed extractant layer (light liquid) were examined and obtained as shown in Table 1.

[0097] [Table 1]

[0098] As shown in Table 1, the presence of inorganic salts in the wastewater resulted in the suppression of water movement (reducing the load on the second process) and improved stratification.

[0099] Examples 5-13 Using a simulated wastewater solution containing carboxylic acid, which was to be treated, the solution consisted of 0.18% formic acid by mass fraction, 0.15% sodium sulfate by mass fraction, 0.0005% impurities by mass fraction, with the remainder being water. This wastewater was transferred to a separatory funnel, and the prepared extractant shown in Table 2 was used in a mass ratio of 1 part wastewater to 1 part extractant. After shaking thoroughly, a single extraction was performed.

[0100] After standing, the aqueous layer (heavy liquid) was removed from the separatory funnel, and the extractant layer (light liquid) was removed. Upon examining the composition of the extracted extractant layer (light liquid), the formic acid composition, water composition, and formic acid distribution ratio of the extracted extract were as shown in Table 3 below.

[0101] [Table 2]

[0102] [Table 3]

[0103] For Examples 7, 9, 10, 11, and 13, the distillation process was carried out on a laboratory scale. All the apparatuses used were glass Aldershaws with 30 theoretical stages, and the heating source was a mantle heater under reduced pressure conditions with a reflux ratio of 5.

[0104] As a result, the water and formic acid compositions at the top of the column and at the bottom of the column were obtained as shown in Tables 4 and 5, respectively.

[0105] [Table 4]

[0106] [Table 5]

[0107] In Example 13, a formic acid solution was obtained in which water had a mass fraction of 0.04, TBP had a mass fraction of 0.0001, decane had a mass fraction of 0.001, and the remainder was formic acid. Purified formic acid was recovered by side cut under total reflux conditions at a reduced pressure of 26.5 kPa using a glass Aldershaw with 10 theoretical plates and a mantle heater as the heat source.

[0108] As a result, when the composition of the side-cut fraction obtained from the middle section of the column was examined, it was possible to obtain anhydrous formic acid containing 0.998 mass fraction of formic acid and 0.002 mass fraction of water.

[0109] In the fifth step described above, a simulated wastewater solution containing inorganic salts to be treated was prepared by using a glass Aldershaw with 5 theoretical plates, a mantle heater as the heat source, and distillation tests were conducted on a laboratory scale under total reflux conditions at 1013 kPa.

[0110] As a result, when the composition of the bottom liquid was examined, an inorganic salt aqueous solution was obtained containing 0.004 mass fractions of formic acid, 0.18 mass fractions of sodium sulfate, and the remainder being water. [Explanation of symbols]

[0111] 10 Wastewater 12. Prepared Extracts 20 Liquid-liquid extraction equipment 22 Extractant layer (light liquid) 24 Water layer (heavy liquid) 26 Middle Class 30, 40, 50, 60 distillation columns 32, 42 Decanters 34 Discharged water 36. Extraction layer mainly composed of diluents 37 Recirculation Line 38. Prepared extract from which water has been removed. 44 Formic acid layer 46 Extraction layer mainly composed of diluents 48. Prepared extract from which formic acid and water have been removed. 52. Purified Carboxylic Acid Products 62 Distillate 64 cans of liquid 72, 74 Return lines

Claims

1. A method for separating a carboxylic acid from an aqueous solution containing water and a carboxylic acid with a carboxylic acid concentration of 0.05 to 0.3 by mass fraction, and purifying the separated carboxylic acid to obtain an anhydrous carboxylic acid with a water content of less than 0.01 and a carboxylic acid content of 0.99 or more by mass fraction, a) A first step comprising a liquid-liquid extraction process in which the aqueous solution and a prepared extractant containing a component for extracting carboxylic acid and a diluent are brought into liquid-liquid contact, the components of the prepared extractant dissolve in the aqueous solution at a mass fraction of less than 0.001, the concentration of carboxylic acid in the aqueous solution becomes less than 0.005, and the carboxylic acid separated from the aqueous solution with a recovery rate of 90% or more dissolves in the prepared extractant, and water dissolves at a mass fraction of less than 0.05, b) A second step comprising the following steps: distilling the prepared extract containing carboxylic acid and water after processing in the first step, removing the diluent component and water of the prepared extract by azeotrope, separating the extract component layer mainly consisting of the diluent and the aqueous layer in a decanter provided in the distillation column, discharging the aqueous layer, mixing the discharged aqueous layer with the primary or secondary aqueous solution of the first step, or discarding it as wastewater; on the other hand, returning the extract component layer mainly consisting of the diluent to the distillation step as reflux, and discharging the prepared extract containing carboxylic acid from the bottom of the column; c) The third step comprises distilling the prepared extract containing the carboxylic acid discharged in the second step again, and if there is no azeotrope between the diluent and the carboxylic acid in the prepared extract, discharging the purified carboxylic acid containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of the prepared extract from the top of the column, and returning the prepared extract from which the carboxylic acid and water have been removed, discharged from the bottom of the column, to the first step, If the diluent and carboxylic acid in the prepared extractant have a minimum azeotrope, they are removed by azeotropic distillation and separated into an extractant layer mainly composed of the diluent and a carboxylic acid layer in a decanter provided in the distillation column. The carboxylic acid layer, which contains less than 0.01 mass fraction of water and less than 0.01 mass fraction of the prepared extractant, is discharged to obtain purified carboxylic acid. The extractant layer mainly composed of the diluent is refluxed back to the distillation process, and the prepared extractant, from which the carboxylic acid and water have been removed and discharged from the bottom of the column, is returned to the first process in a third process. The diluent for the prepared extractant is a hydrophobic solvent with a higher boiling point than water and a higher boiling point than a carboxylic acid at atmospheric pressure, characterized in that it has the lowest azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, and the diluent does not have an azeotrope with a carboxylic acid. Alternatively, a hydrophobic solvent having a higher boiling point than water and a higher boiling point than a carboxylic acid at atmospheric pressure, characterized in that it has the lowest azeotrope with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more by mass fraction, the carboxylic acid has the lowest azeotrope with the diluent, the carboxylic acid and diluent separate into layers in any proportion, and the solubility of the diluent in the carboxylic acid is less than 0.002 by mass fraction, The diluent for the prepared extractant is at least one selected from the group consisting of toluene, octane, isooctane, nonane, decane, undecane, and dodecane. The component for extracting carboxylic acid has a higher boiling point than the diluent, and its carboxylic acid partition ratio D (carboxylic acid in the extracted component / carboxylic acid in water) between "water" and "component to be extracted" is 0.3 or higher within the operating range, and it is an organic solvent that is completely miscible with the diluent but poorly soluble in water. A method for recovering a carboxylic acid, characterized in that the component for extracting the carboxylic acid in the prepared extractant is selected from the group consisting of tributyl phosphate, trioctylamine, and N-di-n-butylformamide.

2. The method according to claim 1, wherein the aqueous solution further contains an inorganic salt in a mass fraction of 0.003 to 0.2, and in the liquid-liquid extraction step of the first step, the inorganic salt in the aqueous solution dissolves in the prepared extractant side in a mass fraction of less than 0.0001 and in the water in the aqueous solution in a mass fraction of less than 0.003 to 0.

2.

3. The method according to claim 1 or 2, wherein the carboxylic acid contained in the aqueous solution is selected from the group consisting of formic acid, acetic acid, and propionic acid.

4. The method according to claim 1 or 2, wherein the inorganic salt contained in the aqueous solution is selected from the group consisting of metal chlorides, metal sulfates, metal bisulfates, metal hydroxides, metal carbonates, metal bicarbonates, metal phosphates, metal hydrogen phosphates, and metal borates.

5. The method according to claim 1 or 2, wherein the diluent for the prepared extractant is a hydrophobic solvent having a boiling point of 110 to 220°C at atmospheric pressure and a solubility in water of less than 0.001 by mass fraction at 25°C.

6. The method according to claim 1 or 2, wherein the diluent for the prepared extractant is at least one selected from the group consisting of toluene, octane, and decane.

7. The aforementioned aqueous solution contains impurities that dissolve in carboxylic acid but not in water. The method according to 1 or 2, wherein, before applying the aqueous solution to the first step, the aqueous solution is filtered to remove impurities, or, in the first step, the carboxylic acid is recovered on the prepared extractant side, and impurities precipitated at the interface with the extractant phase are extracted together with the prepared extractant and aqueous solution from a nozzle provided at the top or bottom of the liquid-liquid extraction device, the liquid containing the extracted impurities is separated into impurities and liquid by filtration or centrifugation, and the recovered liquid is mixed with the primary aqueous solution of the first step to remove the impurities.

8. The method according to 1 or 2, characterized in that the mixing ratio of the carboxylic acid extracting component and the diluent in the prepared extractant is carboxylic acid extracting component:diluent = 1:5 to 9:1 by mass ratio.

9. The method according to claim 2, wherein, in the first step, the liquid-liquid extraction apparatus is operated at a temperature of 10 to 90°C while the inorganic salt is dissolved.

10. The method according to claim 1 or 2, wherein the second step is performed at a reduced pressure of 6.67 to 66.7 kPa.

11. The method according to claim 2, further comprising the step of treating the bottom liquid in a filter installed in the line that supplies the bottom liquid to the third step in order to remove inorganic salts precipitated at the bottom of the distillation column by distilling off water from the prepared extractant containing carboxylic acid and water in the second step.

12. The method according to claim 1 or 2, wherein the second step includes a line for supplying the prepared extractant into the column from any stage above the raw material supply stage, separate from the reflux of the extractant component layer mainly consisting of a diluent, in order to improve the recovery rate of carboxylic acid.

13. The method according to claim 1 or 2, wherein the third step is performed at a reduced pressure of 6.67 to 66.7 kPa.

14. The method according to claim 1 or 2, wherein the third step includes a line for supplying an extractant component layer mainly composed of a diluent into the column from any stage below the raw material supply stage, separate from the reflux of the extractant component layer mainly composed of a diluent, in order to improve the recovery rate of carboxylic acid.

15. The method according to claim 1 or 2, further comprising a fourth step of distilling the carboxylic acid layer discharged in the third step, the carboxylic acid layer containing less than 0.01 by mass fraction of water and less than 0.01 of the prepared extractant component, to remove the water and prepared extractant component by distillation, thereby obtaining an anhydrous carboxylic acid having a mass fraction of 0.99 or more carboxylic acid and 0.002 or less water from the top or middle of the distillation column.

16. The method according to claim 2, further comprising a fifth step of distilling or stripping the aqueous solution discharged in the first step, which contains a carboxylic acid in mass fraction of less than 0.005, a prepared extractant component in mass fraction of less than 0.001, and an inorganic salt in mass fraction of 0.003 to 0.2, thereby removing the prepared extractant component by distillation to obtain an inorganic salt aqueous solution, and returning the distilled prepared extractant component to the primary wastewater of the first step.

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