Method for collecting carboxylic acid

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

Method for recovering carboxylic acid

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

[0002] Aqueous solutions of carboxylic acids, such as formic acid, acetic acid, etc., are generated as wastewater in many industrially important manufacturing or treatment processes. In recent years, with increasing concern about environmental pollution and strengthening of various regulations, it has become necessary to purify such waste aqueous solutions and, in order to reuse economically valuable resources, to recover the carboxylic acids contained as process by-products.

[0003] Methods for recovering carboxylic acids from solutions are not limited to the wastewater treatment methods described above. Considering, for example, purification techniques for diluted aqueous carboxylic acid solutions obtained by the production of lower fatty acids by microorganisms in the fermentation industry, a considerable number of treatment methods have been proposed.

[0004] The use of organic solvents to extract carboxylic acids from dilute aqueous solutions is well known. However, lower carboxylic acids such as formic acid and acetic acid have a high affinity for water, and although numerous compounds have been tried as extractants, no satisfactory results have been obtained. In other words, the partition coefficients of lower fatty acids, which significantly affect the efficiency of extraction methods, are generally small. Therefore, in order to increase the extraction efficiency, a large amount of extractant is used, resulting in high energy consumption during the separation process.

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

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

[0007] The methods described in Patent Documents 1 and 2 both use extractants such as ethers, ketones, and 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 extractant-rich fraction. Therefore, if the extractant has a low boiling point, the entire amount of extractant must be boiled during the distillation purification process, resulting in a large energy load. For example, ethyl acetate is a commonly used extractant because it has a relatively high partition coefficient with lower fatty acids, particularly 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 extractant used must be evaporated. Furthermore, a large amount of water dissolves in the extract, resulting in significant dissolution loss in water, making the process unsatisfactory in terms of mutual solubility with water.

[0008] Furthermore, Patent Documents 3 and 4 disclose the use of a mixed solvent of trioctylphosphine oxide and isophorone, and a mixed solvent of trioctylphosphine oxide and trimethylcyclohexanone as an extraction solvent when extracting lower fatty acids from an aqueous solution containing lower fatty acids such as formic acid, acetic acid, and propionic acid.

[0009] Patent Document 5 discloses a method for recovering carboxylic acids from an aqueous solution containing carboxylic acids, the method comprising: a contacting step in which the aqueous solution is contacted with a solvent consisting essentially of mixed trialkylphosphine oxides in a countercurrent liquid-liquid extraction stream to transfer the acid from the aqueous solution to the solvent and produce a raffinate having a relatively low acid content and a rich solvent having a relatively high acid content, the rich solvent containing some water; a dehydration step in which heat is applied to the rich solvent and water is separated therefrom to dehydrate the rich solvent, 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 a solvent consisting essentially of mixed trialkylphosphine oxides for recycle to the liquid-liquid extraction stream, and an acid stream containing the acid.

[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 having a higher boiling point than the lower fatty acids and a branched primary alcohol such as 2-ethylhexanol or 3,5,5-trimethylhexanol, which has a higher boiling point than the lower fatty acids, or a tertiary amine having a higher boiling point than the lower fatty acids and a linear primary alcohol such as n-hexanol, n-heptanol, n-octanol, or n-nonanol, which has a higher boiling point than the lower fatty acids. While these combinations increase the extraction rate of carboxylic acids, the alcohol dissolves in water. Dissolution means that the alcohol is lost during the extraction process, and the lost amount must be replaced externally. Even if the amount is small, if the carboxylic acid to be recovered is diluted, the recovery becomes meaningless.

[0011] Patent Document 8 also describes a method for extracting organic acids from an aqueous solution of an organic acid by contacting the aqueous solution with an organic solvent containing one or more amines selected from the group consisting of primary amines to quaternary amines and phosphate esters, and diluted with petroleum hydrocarbons, and then heating or heating and distilling the organic solvent containing the organic acid, or removing Na, Mg, NH ３ and the like, thereby stripping and recovering the organic acid and regenerating the organic solvent (A).

[0012] On the other hand, as described in Patent Documents 3 to 8, using an extractant with a boiling point higher than that of lower fatty acids, such as trialkylphosphine oxide or an amine compound, facilitates the separation of carboxylic acids extracted into the extractant-rich fraction in a distillation purification process; however, the extraction yield using trialkylphosphine oxide or an amine compound is not high. As shown in Patent Documents 6 and 7, the combination of an amine compound with a branched primary alcohol or a linear primary alcohol increases the extraction yield of carboxylic acids, but in this case the alcohol dissolves in water. Dissolution means that an amount of alcohol is lost in the extraction operation, and the lost amount must be replaced externally. Even if this amount is small, if the carboxylic acid to be recovered is diluted, the recovery becomes meaningless. Furthermore, while the method described in Patent Document 8 does not disclose a specific extraction method, the extraction yield is approximately 50%, which is not very high considering the extraction equilibrium. Furthermore, although thermal distillation is said to be possible to separate based on physical properties, no specific results are given.

[0013] Patent Document 9 discloses a method for producing anhydrous or nearly anhydrous formic acid by hydrolyzing methyl formate, separating unreacted methyl formate and produced methanol by distillation in a first distillation step, and then removing formic acid from the bottom product by liquid-liquid extraction, in which: (a) water and methyl formate are used in a molar ratio of 1:1 to 30:1 during the hydrolysis of methyl formate; (b) the effluent from the hydrolysis reactor is fed to a distillation column, and methanol and methyl formate are separated from the formic acid-water-bottom product. (c) extracting aqueous formic acid from the bottom product of the first distillation step at a temperature between 20 and 100°C with at least a stoichiometric amount of a high-boiling amine which forms a slightly water-soluble hydroformate having a pKa value between 4 and 9, and (d) recovering the amine extract obtained. in a distillation column at a temperature of 30 to 120°C and a pressure of 10 to 400 mbar, wherein in steps (c) and / or (d) a hydrophobic solvent is added which forms a heteroazeotrope with water and formic acid or whose boiling point is higher than those of water and formic acid but lower than that of the amine used; (e) introducing the dehydrated extract into the top stage of a decomposition column in which the amine hydroformate is decomposed into formic acid and the amine at a temperature of 110 to 240°C, obtaining formic acid and the solvent, optionally as an azeotrope, as the top product of the decomposition column and the amine and the solvent as the bottom product, and separating the top product into formic acid and the solvent; and (f) recycling the resulting streams of methanol, methyl formate, water, amine and solvent to the process and / or treatment steps, wherein the amine recycled from the bottom of the decomposition column is introduced for purification in an adsorption column.

[0014] In the method of Patent Document 9, formic acid is extracted using a high-boiling amine, and after water removal by distillation, an aliphatic, cycloaliphatic, or aromatic hydrocarbon having 8 to 12 C atoms is added as a hydrophobic solvent to decompose the formic acid and the amine at a high temperature, thereby purifying the formic acid. However, since the decomposition of the formic acid and the amine requires a high temperature, there are concerns about corrosion inside the distillation column and generation of decomposition gas.

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

[0016] Furthermore, in some cases, aqueous solutions of carboxylic acids generated as wastewater in various production or treatment processes have a low carboxylic acid concentration of about 0.01 to 0.3 in mass fraction, and contain inorganic salts in a mass fraction range of 0.003 to 0.2. In such cases where inorganic salts are present in the aqueous solution, it has been unclear whether the above-described methods are applicable as methods for recovering carboxylic acids.

[0017] Japanese Patent Laid-Open No. 9-151158 Japanese Patent Laid-Open No. 2018-062512 Japanese Patent Laid-Open No. 61-176550 Japanese Patent Laid-Open No. 61-176551 Japanese Patent Laid-Open No. 08-283191 Japanese Patent Laid-Open No. 61-176552 Japanese Patent Laid-Open No. 61-176553 Japanese Patent Laid-Open No. 55-154935 Japanese Patent Laid-Open No. 61-043133 German Patent Application Publication No. 2545658

[0018] Therefore, an object of the present disclosure is to provide a method for recovering carboxylic acid that solves the problems of the conventional technology as described above. The present disclosure also relates to a method for recovering carboxylic acid as carboxylic acid anhydride from, for example, an aqueous solution containing carboxylic acid and an inorganic salt. Another object of the present disclosure is to provide a method for recovering carboxylic acid that reduces concerns about corrosion inside the equipment and can reduce costs for equipment materials, etc.

[0019] As a result of extensive research and investigation to solve the above problems, the present inventors have discovered a method for separating carboxylic acid from an aqueous solution containing water and carboxylic acid, the carboxylic acid concentration being 0.05 to 0.3 in mass fraction, and purifying the separated carboxylic acid to obtain a carboxylic anhydride having a water content of less than 0.01 in mass fraction and a carboxylic acid content of 0.99 or more, comprising: a) a first step including a liquid-liquid extraction step in which the aqueous solution is brought into liquid-liquid contact with a prepared extractant containing a component for extracting carboxylic acid and a diluent, so that the component of the prepared extractant dissolves in the aqueous solution at a mass fraction of less than 0.001 and the carboxylic acid concentration in the aqueous solution is less than 0.005, and the carboxylic acid separated from the aqueous solution at a recovery rate of 90% or more dissolves in the prepared extractant, and water dissolves in a mass fraction of less than 0.05; and b) a second step in which the prepared extractant containing carboxylic acid and water that has been treated in the first step is distilled to azeotropically remove the diluent component of the prepared extractant and the water, and the diluent component and the water are separated in a decanter provided in the distillation column into an extractant component layer mainly composed of diluent and an aqueous layer, and the aqueous layer is then discharged, the discharged water is mixed with the primary side aqueous solution or the secondary side aqueous solution of the first step or is discarded, while the extractant component layer mainly composed of diluent is returned to the distillation step as reflux, and the prepared extractant containing carboxylic acid is discharged from the bottom of the column; and a third step in which the prepared extractant containing carboxylic acid discharged in the second step is again distilled, and if there is no azeotropy between the diluent and carboxylic acid in the prepared extractant, a purified carboxylic acid containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of prepared extractant is discharged from the top of the column, 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 step. a third step in which, when the diluent and carboxylic acid in the prepared extractant have a minimum azeotropic ratio, they are distilled off by azeotropic distillation, and the diluent is separated into an extractant component layer mainly composed of the diluent and a carboxylic acid layer in a decanter provided in the distillation column, the carboxylic acid layer containing 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, while 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;The diluent for the prepared extractant is a hydrophobic solvent having a boiling point higher than that of water and a boiling point higher than that of carboxylic acid under atmospheric pressure, characterized in that it has a minimum azeotrope with water, the water concentration in the azeotropic composition with the diluent is 0.2 or more in mass fraction, and the diluent does not form an azeotrope with carboxylic acid; or a hydrophobic solvent having a boiling point higher than that of water and a boiling point higher than that of carboxylic acid under atmospheric pressure, characterized in that it has a minimum azeotrope with water, the water concentration in the azeotropic composition with the diluent is 0.2 or more in mass fraction, the carboxylic acid has a minimum azeotrope with the diluent, the carboxylic acid and the diluent are phase-partitioned in any ratio, and the solubility of the diluent in the carboxylic acid is less than 0.002 in mass fraction; and the component extracting the carboxylic acid is an organic solvent having a boiling point higher than the diluent, a carboxylic acid partition ratio D between "water" and "component to be extracted" (= carboxylic acid in the component to be extracted / carboxylic acid in water) of 0.3 or more in this operating range, and is completely miscible with the diluent, but sparingly soluble in water.

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

[0021] In one embodiment of the method for recovering a carboxylic acid according to the present 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 method for recovering a carboxylic acid according to the present disclosure, the inorganic salt contained in the aqueous solution is selected from the group consisting of metal chlorides, metal sulfates, metal hydrogensulfates, metal hydroxides, metal carbonates, metal hydrogencarbonates, metal phosphates, metal hydrogenphosphates, and metal borates.

[0023] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, 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 in mass fraction at 25°C.

[0024] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the diluent for the prepared extractant is 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 method for recovering a carboxylic acid according to the present disclosure, the aqueous solution contains impurities that are soluble in the carboxylic acid but not in water, and the method further comprises a step of filtering the aqueous solution to remove the impurities before subjecting the aqueous solution to the first step; alternatively, the impurities that precipitate at the interface with the extractant phase as a result of recovering the carboxylic acid on the prepared extractant side in the first step are extracted together with the prepared extractant and the aqueous solution from a nozzle provided on the upper or lower part of the liquid-liquid extraction device, the extracted liquid containing the impurities is separated into the impurities and the 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 method for recovering a carboxylic acid according to the present disclosure, the component that extracts a carboxylic acid in the prepared extractant is selected from the group consisting of an organic phosphorus compound and an amide compound.

[0027] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the mixing ratio of the component extracting a carboxylic acid to the diluent in the prepared extractant is, in mass ratio, component extracting a carboxylic acid:diluent=1:5 to 9:1.

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

[0029] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the second step is operated at a reduced pressure of 6.67 to 66.7 kPa.

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

[0031] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the second step includes a line for supplying a prepared extractant into the column from any plate above the feed plate, separate from the reflux of the extractant component layer mainly containing the diluent, in order to improve the recovery rate of the carboxylic acid.

[0032] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the third step is operated at a reduced pressure of 6.67 to 66.7 kPa.

[0033] In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the third step includes a line for supplying an extractant component layer mainly containing a diluent into the column from any stage lower than the raw material supply stage, separately from the reflux of the extractant component layer mainly containing a diluent, in order to improve the recovery rate of the carboxylic acid.

[0034] One embodiment of the method for recovering a carboxylic acid according to the present disclosure further includes a fourth step of distilling again the carboxylic acid, which contains water and a prepared extractant component in a mass fraction of less than 0.01 and which has been discharged in the third step, to remove the water and the prepared extractant component by distillation, thereby obtaining a carboxylic acid anhydride 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 from the middle of the distillation column.

[0035] One embodiment of the method for recovering carboxylic acid according to the present disclosure is shown to further include a fifth step of distilling or stripping the wastewater discharged from the first step, which contains less than 0.005 carboxylic acid, less than 0.001 preparation extractant component, and 0.003 to 0.2 inorganic salt in a mass fraction, to distill off the preparation extractant component to obtain an aqueous solution of inorganic salt, and returning the distilled preparation extractant component to the primary wastewater from the first step.

[0036] In the present disclosure, in recovering a carboxylic acid from an aqueous solution containing the carboxylic acid, a prepared extractant comprising a diluent that satisfies the specific conditions as described above and a component that extracts the carboxylic acid is used to extract the carboxylic acid from the aqueous solution by liquid-liquid contact, and further, the process including this extraction step and the subsequent distillation step is made appropriate, thereby enabling the carboxylic acid to be efficiently recovered as a highly purified carboxylic anhydride.

[0037] Fig. 1 is a block diagram showing the steps in one embodiment of the method for recovering carboxylic acid according to the present disclosure. Fig. 2 is a block diagram showing the steps in another embodiment of the method for recovering carboxylic acid according to the present disclosure. Fig. 3 is a block diagram showing the steps in yet another embodiment of the method for recovering carboxylic acid according to the present disclosure. Fig. 4 is a block diagram showing the steps in yet another embodiment of the method for recovering carboxylic acid according to the present disclosure.

[0038] The present disclosure will be described in more detail below based on preferred embodiments.

[0039] (Material to be treated) In the present disclosure, carboxylic acids are recovered from an aqueous solution containing carboxylic acids. The aqueous solution containing carboxylic acids as the material to be treated is not particularly limited, and includes wastewaters containing carboxylic acids and inorganic salts, such as wastewaters discharged from processes in various chemical industries, spinning industries, etc., and dilute carboxylic acid solutions produced by microorganisms in the fermentation industry, but is not limited to these.

[0040] Carboxylic acids that are the subject of the present disclosure include, for example, carboxylic acids having 1 to 3 carbon atoms, specifically formic acid, acetic acid, and propionic acid, and particularly formic acid.

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

[0042] The wastewater may also contain impurities that are components that dissolve in carboxylic acid but do not dissolve in water.

[0043] The carboxylic acid concentration in the aqueous solution containing the target carboxylic acid in the recovery method according to the present disclosure is in the range of about 0.05 to 0.3 in mass fraction, and when an inorganic salt is contained, the inorganic salt concentration is in the range of about 0.003 to 0.2 in mass fraction.

[0044] (First Step) For an aqueous solution containing a carboxylic acid at a mass fraction of 0.05 to 0.3 as described above, the first step of the recovery method according to the present disclosure includes a liquid-liquid extraction step of bringing the aqueous solution into liquid-liquid contact with a prepared extractant containing a component that extracts carboxylic acid and a diluent, so that the components of the prepared extractant dissolve in the aqueous solution at a mass fraction of less than 0.001 and the carboxylic acid concentration in the aqueous solution is less than 0.005, and the carboxylic acid separated from the aqueous solution at a recovery rate of 90% or more dissolves in the prepared extractant, and water dissolves in a mass fraction of less than 0.05.

[0045] In addition, when the aqueous solution containing a carboxylic acid to be treated further contains an inorganic salt in a mass fraction range of 0.003 to 0.2, the inorganic salt in the aqueous solution is dissolved in a proportion of less than 0.0001 in the preparation extractant and in a proportion of 0.003 to 0.2 in the water of the aqueous solution in the liquid-liquid extraction step in the first step.

[0046] Therefore, in the recovery method according to the present disclosure, a prepared extractant containing a component that extracts carboxylic acid and a diluent that satisfies the following conditions is used as the extractant in the liquid-liquid extraction step in the first step.

[0047] That is, the diluent used in the prepared extractant has a minimum azeotropic property with water, and the water concentration in the azeotropic composition with the diluent is 0.2 or more, more preferably 0.5 or more, by mass fraction, and the diluent does not form an azeotrope with a carboxylic acid; alternatively, the diluent has a minimum azeotropic property with water, and the water concentration in the azeotropic composition with the diluent is 0.2 or more, more preferably 0.5 or more, by mass fraction, or has a minimum azeotropic property with a carboxylic acid, and the carboxylic acid and the diluent are layered in any ratio, and the solubility of the diluent in the carboxylic acid is less than 0.002 by mass fraction. The diluent is a hydrophobic solvent, such as hydrocarbons, having a boiling point higher than water and carboxylic acid at atmospheric pressure (e.g., 1013±20 hPa), preferably 110 to 220°C. Here, the "hydrophobic solvent" is not particularly limited, but is, for example, a solvent whose solubility in water at 25°C is less than 0.01, more preferably less than 0.001, by mass fraction.

[0048] Such diluents include, for example, saturated or unsaturated aliphatic and aromatic hydrocarbons as long as they satisfy the above conditions. Among these, preferred are 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 that extracts the carboxylic acid is a water-insoluble organic solvent that has a boiling point higher than that of the diluent, preferably a boiling point of 140 to 280°C at the operating pressure range of 6.67 kPa to 66.7 kPa, a carboxylic acid partition ratio D (=carboxylic acid in the component to be extracted / carboxylic acid in water) of 0.3 or more in this operating range, and is completely miscible with the diluent. Here, "completely miscible" in this specification means that the component becomes a homogeneous single liquid without separation under the operating temperature conditions of the first step.

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

[0051] In the present disclosure, by adding the above-described diluent to the component that extracts carboxylic acid as the prepared extractant, it is possible to improve the layer separation during extraction in the first step, suppress the migration of water to the extractant side, and when removing water by distillation in the second step described below, it acts as an azeotropic agent with water to push water to the top of the distillation column in preference to the carboxylic acid, thereby improving the separation efficiency of the carboxylic acid. Specific examples of combinations of the component that extracts carboxylic acid and the diluent include, but 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] Although not particularly limited, the mixing ratio of the component that extracts carboxylic acids to the diluent in the prepared extractant is, for example, a mass ratio of 1:5 to 9:1, preferably 1:1 to 5:1, and more preferably 2:1 to 3:1. For example, the mass ratio of the component that extracts carboxylic acids to the diluent may be, but is not particularly limited to, 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 diluent in the prepared extractant is too small compared to the range of the mixing ratio shown here, the efficiency of the first step may decrease, and the amount of the prepared extractant required may increase. On the other hand, if the amount of diluent is too small compared to the range of the mixing ratio shown here, the azeotropic composition of water and diluent will shift to that of water and carboxylic acid in the second step, resulting in a significant decrease in the yield of carboxylic acid. Furthermore, if the amount of diluent is too small, the boiling point of the carboxylic acid extracting component will be high, and therefore the effect of lowering the boiling point at the bottom of the column will be small in the third step due to the low concentration of diluent, and the bottom temperature will increase, which may cause corrosion inside the column due to carboxylic acid or gas generation due to decomposition of carboxylic acid.

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

[0054] In the liquid-liquid extraction performed in the first step, the continuous phase may be either the carboxylic acid-containing aqueous solution (wastewater) to be treated 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] In addition, when the carboxylic acid-containing aqueous solution (wastewater) to be treated contains inorganic salts in addition to carboxylic acids, when the aqueous solution is subjected to the liquid-liquid extraction in the first step, water is extracted into the prepared extractant at a mass fraction of 0.05 or less, causing inorganic salts to precipitate at the interface with the extractant phase. Therefore, in order to increase the solubility of inorganic salts in the aqueous phase and suppress precipitation at the interface, it is desirable to operate the liquid-liquid extraction apparatus at a temperature of 10 to 90°C, more preferably 30 to 50°C.

[0056] Furthermore, when 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 during liquid-liquid extraction, making it more difficult for water to enter the extractant layer, resulting in a more favorable effect. This effect is effectively achieved when the carboxylic acid-containing aqueous solution (wastewater) to be treated contains the inorganic salt in a mass fraction of 0.003 to 0.2, more preferably in a mass fraction 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 intentionally add an inorganic salt, but in some cases it is possible to intentionally add a predetermined amount of inorganic salt.

[0057] Furthermore, as described above, if the carboxylic acid-containing aqueous 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 to remove the impurities that precipitate at the interface with the extractant phase when the carboxylic acid is recovered on the prepared extractant side in the first step, by extracting the impurities together with the prepared extractant and wastewater from a nozzle provided at the top or bottom of the liquid-liquid extractor, separating the extracted liquid containing the impurities into impurities and liquid by centrifugation, and mixing the recovered liquid with the primary wastewater from the first step. These treatments may be used alone or in combination.

[0058] (Second Step) The second step of the recovery method according to the present disclosure is characterized by comprising a step of distilling the prepared extractant containing carboxylic acid and water after the first step, azeotropically removing the diluent component of the prepared extractant and the water, separating the diluent-based extractant layer and the water layer in a decanter installed in the distillation column, discharging the water layer, and mixing the discharged water with the primary or secondary aqueous solution from the first step or discarding it as wastewater, while returning the diluent-based extractant layer to the diluent distillation step as reflux, and discharging the prepared extractant from the bottom of the column. Note that, in this specification, the term "decanter" refers to any type of decanter capable of separating the diluent layer and the water layer, and is not limited to, for example, a decanter using inclination for separation.

[0059] Strictly speaking, the "extractant component layer mainly composed of diluent" in the decanter installed in the distillation column may contain trace amounts of carboxylic acids and components that extract carboxylic acids in addition to the diluent components. However, since the extractant 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 the present disclosure, as described above, when the diluent blended in the prepared extractant has a minimum azeotrope with water, the water concentration in the azeotropic composition is 0.2 or more in mass fraction, and the diluent does not form an azeotrope with carboxylic acid, or when the diluent has a minimum azeotrope with water, the water concentration in the azeotropic composition with the diluent is 0.2 or more in mass fraction, and the carboxylic acid has a minimum azeotrope with the diluent, and the prepared extractant containing carboxylic acid and water that has been treated in the first step is subjected to distillation, the component in the prepared extractant that extracts carboxylic acid has a higher boiling point than the diluent and has a high affinity with the carboxylic acid, so that an extractant effect in the distillation works, suppressing evaporation of the carboxylic acid and allowing water to be preferentially distilled off.

[0061] Strictly speaking, the aqueous layer separated from the extractant layer, which is mainly composed of diluent, in the decanter installed in the distillation column may also contain trace amounts of carboxylic acid. However, since the separated water can be mixed with the primary or secondary aqueous solution from the first step and then used again in each step, the presence of carboxylic acid in the aqueous layer does not pose any particular problem.

[0062] The distillation in the second step is not particularly limited, but is desirably carried out, for example, at a reduced pressure of 6.67 to 66.7 kPa, more preferably 13.3 to 26.7 kPa. By distilling under reduced pressure in the second step, the column bottom temperature is lowered, making it possible to suppress decomposition of the carboxylic acid and corrosion inside the column.

[0063] The distillation conditions are not particularly limited, but examples include a plate tower with 5 to 10 plates and a reflux ratio of about 1 to 3. When the composition balance in the tower is lost, a prepared extractant, more preferably a regenerated prepared extractant from the third step described below, is supplied from any plate above the raw material supply plate, whereby the extraction effect of the component that extracts carboxylic acid causes formic acid to fall to the bottom of the tower, further reducing the loss of formic acid.

[0064] In addition, when the carboxylic acid-containing aqueous solution (wastewater) to be treated contains inorganic salts in addition to carboxylic acids, the inorganic salts dissolved in the water may precipitate at the bottom of the distillation column by distilling off water from the prepared extractant containing carboxylic acids and water in the second step. In order to remove these inorganic salts, it is desirable to provide a filtering step in the line that sends the column bottom liquid to the third step described below.

[0065] (Third Step) The third step of the recovery method according to the present disclosure is characterized in that the prepared extractant containing carboxylic acid discharged in the second step is distilled again, and if there is no azeotropy between the diluent in the prepared extractant and the carboxylic acid, a purified carboxylic acid containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of prepared extractant is discharged from the top of the column, 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 step; and if there is a minimum azeotropy between the diluent and the carboxylic acid in the prepared extractant, the prepared extractant is separated by azeotropic distillation in a decanter equipped in the distillation column into an extractant component layer mainly composed of the diluent and a carboxylic acid layer, and the carboxylic acid layer containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of prepared extractant components is discharged and used as purified carboxylic acid, while the extractant component layer mainly composed of the diluent is returned to the distillation step as reflux, 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 step.

[0066] In the recovery method of the present disclosure, in addition to removing water from the prepared extractant by distillation in the second step, the carboxylic acid can be obtained from the prepared extractant as a purified carboxylic acid by distillation again in the third step. Note that, since the presence of a diluent at the bottom of the distillation column during distillation suppresses an increase in the column bottom temperature, the heat resistance requirements for the materials constituting the apparatus are relaxed, and there is also an effect of expanding the material options.

[0067] The distillation in the third step is not particularly limited, but is desirably carried out at a reduced pressure of, for example, 6.67 to 66.7 kPa, more preferably 13.3 to 26.7 kPa. Distillation under reduced pressure in the third step lowers the azeotropic point of the carboxylic acid and the diluent, thereby suppressing corrosion inside the column and decomposition of the carboxylic acid, and the prepared extractant regenerated in the third step is reused in the first step, thereby suppressing the risk of decomposition due to thermal history and the generation of impurities.

[0068] The distillation conditions are not particularly limited, but examples include a plate tower with 5 to 10 plates and a reflux ratio of about 3 to 5. In order to improve the recovery rate of the carboxylic acid, an extractant component layer mainly composed of the diluent obtained from the decanter in the third step can be supplied into the tower from any plate below the raw material supply plate, separately from the reflux, to promote azeotropy of the carboxylic acid with the diluent against the carboxylic acid that is suppressed by the extraction effect of the component that extracts the carboxylic acid.

[0069] (Fourth Step) In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, the method may further include, as necessary, a fourth step of again distilling the carboxylic acid, which contains water and a preparation extractant component in a mass fraction of less than 0.01 and which has been discharged in the third step, to distill off the water and the preparation extractant component, thereby obtaining a carboxylic acid anhydride 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 degree of purification of the carboxylic acid obtained in the third step can be increased by distilling the carboxylic acid again. The number of distillations in the fourth step can be not only one but also multiple times.

[0071] The distillation in the fourth step, which is carried out as needed, is not particularly limited, but is desirably carried out at a reduced pressure of, for example, 6.67 to 66.7 kPa, more preferably 26.7 to 40.0 kPa.

[0072] The distillation conditions are not particularly limited, but examples include using a tray tower with 5 to 10 plates and a reflux ratio of about 3 to 5, with the product withdrawn from the top of the tower, or withdrawing the product as a side cut from the middle of the tower under total reflux conditions. This allows the components extracting water and carboxylic acid to concentrate at the bottom of the tower, while the diluent components concentrate at the top of the tower, making it possible to recover a highly pure carboxylic acid. Although not particularly limited, formic anhydride containing formic acid in a mass fraction of 0.998 and water in a mass fraction of 0.002 can be obtained.

[0073] (Fifth Step) In one embodiment of the method for recovering a carboxylic acid according to the present disclosure, it is possible to further provide, as necessary, a fifth step in which the wastewater discharged from the first step, which contains less than 0.005 carboxylic acid, less than 0.001 preparation extractant component, and 0.003 to 0.2 inorganic salt in a mass fraction, is subjected to distillation or stripping treatment to distill off the preparation extractant component and carboxylic acid to obtain an aqueous solution of inorganic salt, and the distilled off preparation 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 production or treatment process that generates this waste water uses the inorganic salt in the process, providing such a fifth step makes it possible to recover the inorganic salt as an aqueous solution of the inorganic salt in addition to recovering the carboxylic acid, thereby further promoting the reuse of raw material resources.

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

[0076] The distillation conditions are not particularly limited, but examples include using a plate tower with 5 to 10 plates under total reflux conditions, and withdrawing the product as an aqueous solution of inorganic salts from the bottom of the tower.

[0077] The present disclosure will be described in more detail below with reference to examples. Figures 1 to 4 are block diagrams each showing each step in one embodiment of a method for recovering carboxylic acid according to the present disclosure. Note that Figure 1 is a block diagram showing a general aspect including all of steps 1 to 5 in one embodiment of a method for recovering carboxylic acid according to the present disclosure, Figure 2 is a block diagram showing an aspect comprising steps 1 to 3 in one embodiment of a method for recovering carboxylic acid according to the present disclosure, Figure 3 is a block diagram showing an aspect comprising steps 1 to 4 in one embodiment of a method for recovering carboxylic acid according to the present disclosure, and Figure 4 is a block diagram showing an aspect including steps 1 and 5 in one embodiment of a method for recovering carboxylic acid according to the present disclosure.

[0078] In the following examples, the analysis of each component was carried out according to the following methods.

[0079] (Moisture content) Moisture content was determined by quantitative analysis using gas chromatography under the following conditions: Apparatus: GC-2014 (Shimadzu Corporation) Detector: Thermal conductivity detector Column: Chromosorb 101 (inner diameter: 2 mm, length: 1.83 m) Column temperature increase conditions: 90°C → 10°C / min temperature increase → 220°C (hold for 47 minutes) Injection port conditions: 250°C, carrier gas flow rate: 4 ml / min

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

[0081] (Other Volatile Components) Other volatile components were determined by gas chromatography under the following conditions: Apparatus: GC-2014 (Shimadzu Corporation) Detector: Hydrogen flame ionization detector Column: G-100 (inner diameter: 1.2 mm, length: 40 m, film thickness: 3.0 μm) Column temperature increase conditions: 80°C → 10°C / min temperature increase → 240°C (hold for 44 minutes) Injection port conditions: 250°C, carrier gas flow rate: 15 ml / min

[0082] (Non-volatile components) The non-volatile components were determined by weighing out about 5.0 g of the liquid into a 100 ml diameter petri dish, leaving it to stand for 24 hours in a vacuum dryer set at 140° C. under a reduced pressure of about 1.3 kPa, and then measuring the weight.

[0083] Example 1: Wastewater containing formic acid at a mass fraction of 0.2, sodium sulfate at a mass fraction of 0.14, impurities at a mass fraction of 0.0005, and the remainder water was prepared as a carboxylic acid-containing aqueous solution to be treated. The specific gravity of the wastewater was 1.18 at 20°C.

[0084] (First Step) As shown in Figures 1 and 2, this wastewater 10 was introduced into a packed-type extraction column liquid-liquid extractor 20, packed with structured packing and having a column diameter of 65 mm and a height equivalent to three theoretical plates. 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 = 3:1), was introduced opposite the liquid-liquid extractor 20. The wastewater was subjected to liquid-liquid contact with 1,040 kg of the prepared extractant at 40°C, with a wastewater feed rate of 40 kg / h and a prepared extractant feed rate of 130 kg / h for 8 hours. The specific gravity of the prepared extractant was 0.89 at 20°C. The prepared extractant 12 served as the continuous phase, and the wastewater 10 served as the dispersed phase.

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

[0086] The composition of 1,125 kg of the extractant layer (light liquid) 22 withdrawn from the first step was investigated, and it was found to be as follows: formic acid at a mass fraction of 0.056, decane at a mass fraction of 0.231, water at a mass fraction of 0.020, sodium sulfate at a mass fraction of 0.00001, and the remainder being TBP.

[0087] (Second Step) The extractant layer (light liquid) 22 was then fed to an 8-plate distillation column (continuous plate column) 30 at a feed rate of 200 kg / h, with the fourth plate from the top serving as the feed tray. The distillation was carried out under reduced pressure of 13.3 kPa, with a column bottom temperature of 118-120°C and a column top temperature of 52-53°C. Decane and water were distilled off azeotropically from the top of the column. The distillation column 30 was separated into an extractant component layer mainly composed of diluent and an aqueous layer in a decanter 32, and the aqueous layer was then discharged. The discharged water 34 was mixed with the primary-side aqueous solution (wastewater 10) from the first step via a return line 72. Meanwhile, the extractant component layer 36 mainly composed of diluent was returned to the distillation column 30 as reflux via a reflux line 37 at a reflux ratio of 2. A prepared extractant 38 from which water had been removed was then extracted at 180 kg / h from the bottom of the distillation column 30.

[0088] The composition of the prepared extractant 38 extracted in the second step from which 1,085 kg of water had been removed was examined, and it was found to contain formic acid at a mass fraction of 0.055, decane at a mass fraction of 0.236, water at a mass fraction of less than 0.0003, and the remainder being TBP.

[0089] The line 39, through which the water-removed prepared extractant 38 extracted in the second step is sent to the third step described below, is equipped with a filter (not shown) consisting of a bag filter with a filtration accuracy of 0.5 μm. By distilling off water from the prepared extractant containing formic acid and water in the second step, sulfate dissolved in the water may precipitate at the bottom of the distillation column. However, by installing the filter, no precipitate was observed at the bottom of the distillation column throughout the approximately 5 hours of operation.

[0090] (Third Step) The prepared extractant 38 removed from the second step was then introduced into a distillation column (continuous packed column) 40 with eight theoretical plates, with the fourth plate from the top serving as the feed plate, at a feed rate of 180 kg / h. The distillation operation was carried out for approximately six hours under conditions of a column bottom temperature of 130-131°C and a column top temperature of 85-90°C under a reduced pressure of 11.3 kPa. The diluent (decane) and carboxylic acid (formic acid) in the prepared extractant were distilled off by azeotropic distillation. The diluent was separated into an extractant component layer 46, mainly composed of the diluent, and a formic acid layer 44 in a decanter 42 equipped 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 returned to the distillation column 40 as reflux via a reflux line 47 at a reflux ratio of 4. The prepared extractant 48 from which formic acid and water have been removed and discharged at a rate of 150 kg / h from the bottom of the distillation column 40 is returned to the first step via a return line 74, and its composition is adjusted as necessary to be combined with the prepared extractant 12 for reuse.

[0091] The composition of the 60 kg formic acid layer 44 obtained as purified formic acid in the third step was examined to find that it contained formic acid at a mass fraction of 0.99, water at a mass fraction of 0.003, TBP at a mass fraction of 0.004, and decane at a mass fraction of 0.003, indicating a high degree of purification. Furthermore, the composition of the prepared extractant 48 discharged from the bottom of the column in the third step, from which formic acid and water had been removed, was examined to find that it contained formic acid at a mass fraction of 0.0002, decane at a mass fraction of 0.251, water at a mass fraction of 0.0002, and the remainder was TBP.

[0092] 1 and 3 , the formic acid layer 44 obtained in Example 1 was again introduced into a distillation column (continuous packed column) 50 having seven theoretical plates at a feed rate of 45 kg / h, and distilled again under reduced pressure of 30 kPa at a column bottom temperature of 68 to 70° C. and a column top temperature of 63 to 64° C., and distilled from the middle of the column under total reflux conditions to distill off water and prepared extractant component 56. As a result, formic anhydride containing formic acid in a mass fraction of 0.998 and water in a mass fraction of 0.002 was obtained as a purified carboxylic acid product 52.

[0093] Example 3 (Fifth Step) In Example 1, the composition of 235 kg of the aqueous layer (heavy liquid) 24 extracted from the nozzle provided on the bottom side of the liquid-liquid extraction apparatus 20 in the first step was examined and found to be sodium sulfate at a mass fraction of 0.19, formic acid at a mass fraction of 0.002, TBP at a mass fraction of 0.0008, decane at a mass fraction of less than 0.00003, and the remainder being water. As shown in Figures 1 and 4, this aqueous layer 24 was fed at a rate of 60 kg / h into a distillation column (continuous plate column) 60 with an actual number of six plates, with the third plate from the top serving as the raw material supply plate, and distilled under total reflux conditions at 101.3 kPa, a column bottom temperature of 102 to 103°C, and a column top temperature of 100°C. The composition of the bottoms 64 withdrawn from the bottom of the distillation column (continuous plate column) 60 at a rate of 55 kg / h was examined to find that it contained sodium sulfate at a mass fraction of 0.191, formic acid at a mass fraction of 0.002, and the remainder was water, making it suitable for reuse as an industrial sodium sulfate aqueous solution. Meanwhile, the composition of the distillate 62 removed from the top of the distillation column (continuous plate column) 60 was examined to find that it contained formic acid at a mass fraction of 0.0002, TBP at a mass fraction of 0.002, decane at a mass fraction of less than 0.0001, and the remainder was water. Therefore, the distillate 62 could be recycled to the primary aqueous solution (wastewater 10) of the first step.

[0094] Example 4 As a carboxylic acid-containing aqueous solution to be treated, simulated wastewater containing formic acid at a mass fraction of 0.2 and the remainder being water was prepared. The simulated wastewater contained sodium sulfate at mass fractions of 0.01, 0.1, and 0.2, with the remainder being water, with the formic acid at a constant mass fraction of 0.2.

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

[0096] After standing for 5 minutes, the water layer (heavy liquid) was removed from the separatory funnel, and the extractant layer (light liquid) was extracted. The water composition and separation properties of the extracted extractant layer (light liquid) were examined, and the results are shown in Table 1.

[0097]

[0098] As shown in Table 1, the presence of inorganic salts in the wastewater suppressed the movement of water (reduced the load on the second step) and improved the stratification.

[0099] Examples 5 to 13: The carboxylic acid-containing aqueous solution to be treated was simulated wastewater containing formic acid at a mass fraction of 0.18, sodium sulfate at a mass fraction of 0.15, impurities at a mass fraction of 0.0005, and the remainder being water. This wastewater was transferred to a separatory funnel, and a prepared extractant shown in Table 2 was used in a mass ratio of 1 part wastewater to 1 part prepared extractant, and the mixture was thoroughly shaken to perform simple extraction.

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

[0101]

[0102]

[0103] Lab-scale distillation steps were carried out for Examples 7, 9, 10, 11, and 13. All of the distillation steps used were performed using a glass Oldershaw distillation apparatus with 30 theoretical plates, a mantle heater as the heating source, and a reflux ratio of 5 under reduced pressure.

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

[0105]

[0106]

[0107] For the formic acid solution obtained in Example 13, which contained water at a mass fraction of 0.04, TBP at a mass fraction of 0.0001, and decane at a mass fraction of 0.001, with the remainder being formic acid, purified formic acid was recovered by side cut under reduced pressure conditions of 26.5 kPa in a total reflux state using a glass Oldershaw column with 10 theoretical plates and a mantle heater as a heating source.

[0108] As a result, when the composition of the side cut fraction obtained from the middle of the column was examined, it was found that formic anhydride containing formic acid in a mass fraction of 0.998 and water in a mass fraction of 0.002 was obtained.

[0109] The inorganic salt-containing aqueous solution to be treated in the fifth step contained simulated wastewater containing formic acid at a mass fraction of 0.004, TBP at a mass fraction of 0.0008, and sodium sulfate at a mass fraction of 0.15, with the remainder being water. A laboratory-scale distillation test was carried out using a glass Oldershaw reactor with five theoretical plates and a mantle heater as the heating source under a total reflux condition at 1,013 kPa.

[0110] As a result, the composition of the column bottom liquid was examined, and it was found that an aqueous solution of inorganic salts containing formic acid in mass fractions of 0.004 and sodium sulfate in mass fractions of 0.18, with the remainder being water.

[0111] 10 Wastewater 12 Prepared extractant 20 Liquid-liquid extraction apparatus 22 Extractant layer (light liquid) 24 Water layer (heavy liquid) 26 Intermediate layer 30, 40, 50, 60 Distillation column 32, 42 Decanter 34 Discharged water 36 Extractant component layer mainly consisting of diluent 37 Reflux line 38 Prepared extractant from which water has been removed 44 Formic acid layer 46 Extractant component layer mainly consisting of diluent 48 Prepared extractant from which formic acid and water have been removed 52 Purified carboxylic acid product 62 Distillate 64 Bottoms 72, 74 Return lines

Claims

1. A method for obtaining a carboxylic acid anhydride having a water content of less than 0.01 and a carboxylic acid content of 0.99 or more in mass fraction by separating a carboxylic acid from an aqueous solution containing water and a carboxylic acid and having a carboxylic acid concentration of 0.05 to 0.3 in mass fraction and purifying the separated carboxylic acid, comprising the steps of: a) a first step including a liquid-liquid extraction step of bringing the aqueous solution into liquid-liquid contact with a prepared extractant containing a component for extracting carboxylic acid and a diluent, so that the component of the prepared extractant is dissolved in the aqueous solution at a mass fraction of less than 0.001, the carboxylic acid concentration in the aqueous solution is less than 0.005, and the carboxylic acid separated from the aqueous solution at a recovery rate of 90% or more is dissolved in the prepared extractant, and water is dissolved in a mass fraction of less than 0.05; b) a second step comprising a step of distilling the prepared extractant containing carboxylic acid and water after the treatment in the first step, diluting the diluent component of the prepared extractant and water by azeotropic distillation, separating the diluent-based extractant component layer and the water layer in a decanter provided in the distillation tower, discharging the water layer, mixing the discharged water layer with the primary side aqueous solution or the secondary side aqueous solution of the first step, or discarding it as waste water, while returning the diluent-based extractant component layer to the dilution step as reflux, and discharging the prepared extractant containing carboxylic acid from the bottom of the tower; c) a third step in which the preparation extractant containing the carboxylic acid discharged in the second step is distilled again, and when there is no azeotropy between the diluent in the preparation extractant and the carboxylic acid, a purified carboxylic acid containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of the preparation extractant is discharged from the top of the tower, and the preparation extractant from which the carboxylic acid and water have been removed and discharged from the bottom of the tower is returned to the first step; In the case where the diluent and carboxylic acid in the prepared extractant have a minimum azeotropic potential, the diluent is distilled off by azeotropic distillation and separated into an extractant component layer mainly composed of the diluent and a carboxylic acid layer in a decanter equipped in a distillation tower, the carboxylic acid layer containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of prepared extractant is discharged to obtain a purified carboxylic acid, and the extractant component layer mainly composed of the diluent is returned to the distillation step as reflux, and the prepared extractant from which the carboxylic acid and water have been removed and discharged from the bottom of the tower is returned to the first step; The diluent for the prepared extractant is a hydrophobic solvent having a boiling point higher than that of water and higher than that of a carboxylic acid under atmospheric pressure, characterized in that the diluent has a minimum azeotropic composition with water, the concentration of water in the azeotropic composition with the diluent is 0.2 or more in mass fraction, and the diluent does not have an azeotropic composition with a carboxylic acid; Alternatively, a hydrophobic solvent having a boiling point higher than water and a boiling point higher than a carboxylic acid under atmospheric pressure, characterized in that the solvent has a minimum azeotrope with water, the concentration of water in an azeotropic composition with the diluent is 0.2 or more in mass fraction, the carboxylic acid has a minimum azeotrope with the diluent, the carboxylic acid and the diluent are layered in any ratio, and the dissolution of the diluent in the carboxylic acid is less than 0.002 in mass fraction, The method for recovering a carboxylic acid is characterized in that the component from which the carboxylic acid is extracted has a boiling point higher than that of the diluent, has a carboxylic acid distribution ratio D of "water" to "component to be extracted" (= carboxylic acid in the component to be extracted / carboxylic acid in water) of 0.3 or more in the present operating range, and is an organic solvent that is poorly soluble in water and completely compatible with the diluent.

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

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

4. 3. 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 hydrogen sulfates, metal hydroxides, metal carbonates, metal hydrogen carbonates, 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. under atmospheric pressure and a solubility in water of less than 0.001 in mass fraction at 25° C.

6. 3. The method according to claim 1 or 2, wherein the diluent of the prepared extractant is at least one selected from the group consisting of toluene, octane, isooctane, nonane, decane, undecane, dodecane, o-xylene, m-xylene, p-xylene and ethylbenzene.

7. the aqueous solution contains impurities that are soluble in carboxylic acid and insoluble in water, The method according to claim 1 or 2, further comprising a step of filtering the aqueous solution to remove impurities before subjecting the aqueous solution to the first step, or a step of recovering the carboxylic acid on the prepared extractant side in the first step, thereby removing the impurities precipitated at the interface with the extractant phase from a nozzle provided on the upper or lower part of a liquid-liquid extraction apparatus together with the prepared extractant and the aqueous solution, separating the extracted liquid containing the impurities into the impurities and the liquid by filtration or centrifugation, and mixing the recovered liquid with the primary aqueous solution in the first step, thereby removing the impurities.

8. 3. The method according to claim 1, wherein the component that extracts carboxylic acids in the prepared extractant is selected from the group consisting of organic phosphorus compounds and amide compounds.

9. 3. The method according to claim 1 or 2, wherein the mixing ratio of the component extracting a carboxylic acid to the diluent in the prepared extractant is, in mass ratio, component extracting a carboxylic acid:diluent=1:5 to 9:

1.

10. The method according to claim 2, wherein in subjecting the aqueous solution containing the inorganic salt to the first step, a liquid-liquid extraction apparatus is operated at a temperature of 10 to 90° C. in a state in which the inorganic salt is dissolved.

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

12. 3. The method according to claim 2, further comprising a step of treating the bottom liquid with a filter provided in a line for sending 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 a carboxylic acid and water in the second step.

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

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

15. 3. The method according to claim 1 or 2, wherein the third step includes a line for supplying an extractant component layer mainly containing the diluent from an arbitrary stage lower than the raw material supply stage into the column, separately from the reflux of the extractant component layer mainly containing the diluent, in order to improve the recovery rate of the carboxylic acid.

16. 3. The method according to claim 1 or 2, further comprising a fourth step of distilling again the carboxylic acid layer discharged in the third step, the carboxylic acid layer containing less than 0.01 mass fraction of water and less than 0.01 mass fraction of the prepared extractant component, to distill off the water and the prepared extractant component, thereby obtaining a carboxylic anhydride having a mass fraction of 0.99 or more of carboxylic acid and 0.002 or less of water from the top of the distillation column or the middle stage of the distillation column.

17. The method according to claim 2, further comprising a fifth step of subjecting the aqueous solution discharged in the first step, the aqueous solution containing less than 0.005 carboxylic acid, less than 0.001 preparation extractant component, and 0.003 to 0.2 inorganic salt in mass fractions, to a distillation or stripping treatment to distill off the preparation extractant component to obtain an aqueous inorganic salt solution, and returning the distilled preparation extractant component to the primary wastewater of the first step.