Acetic acid recovery method

Abstract

To provide an acetic acid recovering method that can reduce the loss of acetic acid for an improved recovery rate and has superior vapor saving effect.SOLUTION: The present invention discloses an acetic acid recovering method for recovering acetic acid from an aqueous acetic acid solution in which cellulose acetate has been dissolved, the method including: a step (1) for adjusting the concentration of cellulose acetate dissolved in the aqueous acetic acid solution to 1-10000 ppm, a step 2 for bringing the aqueous acetic acid solution in which the concentration of the dissolved cellulose acetate is 1-10000 ppm, into contact with an organic solvent, and separating into an extracted liquid phase that primarily includes the acetic acid and the organic solvent, and an extraction residue phase that primarily includes the water, and a step 3 for distilling the extraction liquid phase to recover the acetic acid.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for recovering acetic acid and a method for producing cellulose acetate using the same. [Background technology]

[0002] Cellulose acetate has excellent chemical resistance, heat resistance, and flame retardancy and is therefore used in a variety of applications, such as liquid crystal optical films, cigarette filters, acetate fibers, photographic films, cellulose-based plastics, and filtration membranes. A known method for producing cellulose acetate involves reacting cellulose with an acetylating agent in an aqueous acetic acid solution. After obtaining cellulose acetate as a product from the reaction mixture, acetic acid is recovered from the residual liquid and reused.

[0003] The residual liquid contains water and cellulose acetate in addition to acetic acid, but because water and acetic acid have similar boiling points, it is difficult to separate acetic acid by distillation. For this reason, an organic solvent with a boiling point significantly different from that of acetic acid is used to separate the liquid into an extract phase containing mainly acetic acid and the organic solvent, and a raffinate phase containing mainly water. The extract phase is then subjected to distillation to separate the organic solvent and acetic acid, thereby recovering acetic acid. However, the extraction efficiency of acetic acid into the extract phase is low, and a large amount of acetic acid is discarded without being recovered, which is a problem.

[0004] Furthermore, if the extract liquid phase is introduced directly into a distillation column, the cellulose acetate dissolved in the extract liquid phase will precipitate at the bottom of the column, causing poor liquid transport and poor heat transfer, making it difficult to maintain stable operation of the distillation column. Therefore, the extract liquid phase is gasified using an evaporator or the like before being charged into the distillation column, thereby removing the cellulose acetate as a non-volatile component.

[0005] However, gasifying the extract liquid phase requires a huge amount of energy, and when the extract liquid phase is fed into the distillation column in a gasified state, the separation efficiency between the organic solvent and acetic acid deteriorates. While this can be addressed by increasing the reflux ratio, this increases the cost of recovering acetic acid, creating a problem. Therefore, the development of energy-saving technology was required.

[0006] As a solution to this problem, Patent Document 1 describes a method of removing cellulose acetate by subjecting the residual liquid to nanofiltration, and describes the addition of a surfactant, a lignosulfonate, to the residual liquid before filtration. This surfactant prevents the filtration membrane from passing through, and inhibits the formation of a layer containing high concentrations of cellulose acetate on the surface of the membrane. This method therefore yields high-purity acetic acid. However, even with this method, much of the acetic acid remains unrecovered and is discarded. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2003-508594 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present disclosure is to provide a method for recovering acetic acid that can reduce the loss of acetic acid, improve the recovery rate, and has an excellent steam-saving effect. Another object of the present disclosure is to provide a method for producing cellulose acetate by efficiently recovering high-purity acetic acid and reusing it, which is an excellent steam-saving method. [Means for solving the problem]

[0009] As a result of extensive research into solving the above problems, the present inventors have found that 1. If the concentration of cellulose acetate dissolved in the acetic acid aqueous solution (hereinafter sometimes referred to as "dissolved cellulose acetate") is reduced to 1 to 10,000 ppm before the acetic acid aqueous solution is added to the extraction process, the 1 to 10,000 ppm of cellulose acetate dissolved in the acetic acid aqueous solution acts as a surfactant to make the droplets of the acetic acid aqueous solution finer and increase the contact area with the organic solvent, thereby dramatically improving the extraction efficiency of acetic acid into the extraction liquid phase and reducing the amount of acetic acid to be discarded. 2. Adjusting the concentration of dissolved cellulose acetate in an aqueous acetic acid solution by membrane separation treatment is more effective in saving steam than adjusting the concentration by the conventional method of removing cellulose acetate as a non-volatile component through gasification. 3. When the extracted liquid phase with a reduced concentration of dissolved cellulose acetate is fed to a distillation column as a liquid, the separation efficiency is improved compared to feeding the gasified phase into a distillation column. The present disclosure was completed based on these findings.

[0010] That is, the present disclosure provides a method for recovering acetic acid from an aqueous acetic acid solution in which cellulose acetate is dissolved, comprising the steps of: Step 1: adjusting the concentration of cellulose acetate dissolved in an aqueous acetic acid solution to 1 to 10,000 ppm; a step 2 of contacting an aqueous acetic acid solution having a dissolved cellulose acetate concentration of 1 to 10,000 ppm with an organic solvent to separate the solution into an extract phase mainly containing acetic acid and the organic solvent and a raffinate phase mainly containing water; A method for recovering acetic acid is provided, comprising step 3 of subjecting the extract liquid phase to distillation to recover acetic acid.

[0011] The present disclosure also provides the method for recovering acetic acid, wherein step 1 comprises subjecting an aqueous acetic acid solution having cellulose acetate dissolved therein to membrane separation treatment using a microfiltration membrane having a pore size of 0.01 to 10 μm, to obtain, as a membrane permeate, an aqueous acetic acid solution having a cellulose acetate concentration of 1 to 10,000 ppm.

[0012] The present disclosure also provides the method for recovering acetic acid, wherein step 1 comprises subjecting an aqueous acetic acid solution having cellulose acetate dissolved therein to membrane separation treatment using a microfiltration membrane having a pore size of 0.01 to 10 μm, and further subjecting the resulting membrane permeate to membrane separation treatment using a reverse osmosis membrane to obtain, as the membrane permeate, an aqueous acetic acid solution having a cellulose acetate concentration of 1 to 10,000 ppm.

[0013] The present disclosure also provides a method for recovering acetic acid, which uses a reverse osmosis membrane having a surface layer containing a polymer having phosphorylcholine groups (hereinafter, sometimes referred to as "PC polymer") as the reverse osmosis membrane.

[0014] The present disclosure also provides a method for producing cellulose acetate by acetylating cellulose in an aqueous acetic acid solution to obtain cellulose acetate, comprising recovering cellulose acetate from the reaction solution after the acetylation, recovering acetic acid from the residual solution by the method for recovering acetic acid as described above, and reusing the recovered acetic acid as a raw material for the aqueous acetic acid solution. [Effects of the Invention]

[0015] According to the method of the present disclosure, the extraction rate of acetic acid in the extraction step can be improved and the recovery rate of acetic acid can be improved (resource-saving effect) compared to conventional methods (i.e., methods in which, after extraction with an organic solvent, the extracted liquid phase is gasified to reduce the concentration of dissolved cellulose acetate, and then the cellulose acetate is subjected to a distillation treatment). Moreover, the acetic acid recovered by the above method can be reused for the production of cellulose acetate. Furthermore, in the distillation step, the liquid phase can be charged into the distillation column, resulting in excellent separation efficiency, and even if the reflux ratio is lowered, acetic acid can be recovered with a purity comparable to that of conventional methods. Furthermore, if the concentration of dissolved cellulose acetate in the aqueous acetic acid solution is reduced by membrane separation treatment, the enormous amount of energy required for gasification can be reduced (steam saving effect). [Brief explanation of the drawings]

[0016] [Figure 1]FIG. 1 is a schematic flow diagram showing an example of the acetic acid recovery method of the present disclosure. [Figure 2] FIG. 1 is a diagram showing the relationship between the operation time of the membrane separation treatment and the circulation flow rate for a reverse osmosis membrane (0), a reverse osmosis membrane (1) having a surface layer containing a PC polymer, and a reverse osmosis membrane (1) after alkali washing. [Figure 3] FIG. 1 is a graph showing the relationship between the operating time of membrane separation treatment and the rate of decrease in circulation flow rate for a reverse osmosis membrane (0), a reverse osmosis membrane (1) having a surface layer containing a PC polymer, and a reverse osmosis membrane (1) after alkali washing. DETAILED DESCRIPTION OF THE INVENTION

[0017] [Acetic acid recovery method] The method for recovering acetic acid according to the present disclosure is a method for recovering acetic acid from an aqueous acetic acid solution in which cellulose acetate has been dissolved, and includes the following steps 1 to 3. Step 1: A step of adjusting the concentration of cellulose acetate dissolved in an aqueous acetic acid solution to 1 to 10,000 ppm Step 2: A step of contacting an aqueous solution of acetic acid having a dissolved cellulose acetate concentration of 1 to 10,000 ppm with an organic solvent to separate the solution into an extract phase containing mainly acetic acid and the organic solvent and a raffinate phase containing mainly water. Step 3: subjecting the extract liquid phase to distillation to recover acetic acid

[0018] (Step 1: Step of adjusting the concentration of cellulose acetate dissolved in an aqueous acetic acid solution) Step 1 is a step of adjusting the concentration of cellulose acetate dissolved in an aqueous acetic acid solution to 1 to 10,000 ppm. The upper limit of the cellulose acetate concentration is preferably 5,000 ppm, more preferably 3,000 ppm, even more preferably 2,000 ppm, even more preferably 1,500 ppm, even more preferably 1,250 ppm, particularly preferably 1,000 ppm, most preferably 500 ppm, and especially preferably 100 ppm. The lower limit of the cellulose acetate concentration is preferably 10 ppm.

[0019] Through step 1, an aqueous acetic acid solution (hereinafter sometimes referred to as "aqueous acetic acid (a)") having a dissolved cellulose acetate concentration within the above-mentioned range is obtained. The obtained aqueous acetic acid solution (a) is subjected to the subsequent step 2 (extraction step). In the method for recovering acetic acid of the present disclosure, the aqueous acetic acid solution (a) to be subjected to the extraction step contains a dissolved cellulose acetate concentration within the above-mentioned range, and therefore, the surfactant effect of the dissolved cellulose acetate can be used to improve extraction efficiency.

[0020] The method for obtaining the aqueous acetic acid solution (a) is not particularly limited, and examples thereof include membrane separation treatment methods, distillation treatment methods, etc. Among these methods, membrane separation treatment methods are preferred because of their excellent steam-saving effect.

[0021] Among the methods for obtaining the aqueous acetic acid solution (a) by membrane separation, membrane separation using a microfiltration membrane with a pore size of 0.01 to 10 μm (preferably 1 to 10 μm) is preferred. Normally, the aqueous acetic acid solution, which is the residual liquid after the production of cellulose acetate, contains a large amount of insoluble cellulose acetate along with dissolved cellulose acetate (the insoluble cellulose acetate concentration is, for example, 100 ppm or more). However, if membrane separation using a microfiltration membrane with a pore size of 0.01 to 10 μm is performed, the insoluble cellulose acetate can be separated and removed into a membrane concentrate, and the aqueous acetic acid solution (a) is obtained as a membrane permeate.

[0022] Furthermore, when the membrane permeate obtained through membrane separation treatment using a microfiltration membrane contains dissolved cellulose acetate at more than 10,000 ppm, it is preferable to further perform membrane separation treatment using a reverse osmosis membrane. Since dissolved cellulose acetate is hardly able to pass through a reverse osmosis membrane, the membrane permeate obtained through membrane separation treatment using a reverse osmosis membrane has a dissolved cellulose acetate concentration reduced to a range of 1 to 10,000 ppm. Note that the membrane permeate is the liquid that has passed through the reverse osmosis membrane, and the membrane concentrate is the liquid that has not passed through the reverse osmosis membrane. Dissolved cellulose acetate is mainly contained in the membrane concentrate.

[0023] The membrane separation treatment can be carried out using a filtration device equipped with a filtration membrane module. The filtration membrane module is not particularly limited as long as it has a configuration that can separate the membrane permeate that has permeated the filtration membrane from the membrane concentrate that has not permeated the filtration membrane.

[0024] The filtration membrane module may be, for example, a spiral-type filtration membrane module in which a filtration membrane is wound around a water collection pipe. The filtration membrane may be made of a material such as polyamide, polysulfone, or cellulose acetate. Among these, polyamides such as aromatic polyamides and crosslinked aromatic polyamides are preferred.

[0025] As the membrane separation treatment, a cross-flow filtration method is preferable. By using the cross-flow filtration method, it is possible to perform the membrane separation treatment while preventing a decrease in the filtration rate. Note that the cross-flow filtration method is a method in which the water to be treated flows parallel to the surface of the filter membrane, and a part of the water to be treated is filtered on the side of the flow of the water to be treated while preventing contamination of the filter membrane due to the deposition of filter cake.

[0026] The membrane separation using a filtration membrane is preferably carried out at a temperature in the range of, for example, 30 to 50°C, preferably 35 to 45°C.

[0027] The filtration pressure of membrane separation using a filtration membrane is, for example, about 0.001 to 6.0 MPaG, preferably 0.01 to 5.0 MPaG, and particularly preferably 0.1 to 4.0 MPaG.

[0028] The circulation rate of the liquid flowing across the membrane surface in membrane separation using a filtration membrane is, for example, 10 to 40 L / min, preferably 15 to 30 L / min, and particularly preferably 20 to 25 L / min, when a 4-inch membrane module is used. When a 2-inch membrane module is used, the circulation rate is, for example, 5 to 22 L / min, preferably 6 to 17 L / min, and particularly preferably 8 to 12 L / min. If the size of the membrane module is different, the circulation flow rate is also changed in accordance with the above.

[0029] The membrane separation operation using a filtration membrane is preferably repeated using a circulating membrane filtration system or the like until the concentration of dissolved cellulose acetate in the membrane permeate falls within the above range.

[0030] The acetic acid aqueous solution (a) contains dissolved cellulose acetate at a concentration of 1 to 10,000 ppm, but may also contain other soluble components, such as inorganic salts (for example, magnesium sulfate).

[0031] The reverse osmosis membrane may also be a reverse osmosis membrane having a surface layer containing a PC polymer (=a polymer having a phosphorylcholine group, or a polymer having a phospholipid polar group).

[0032] As the PC polymer, for example, a PC polymer having a cationic hydrophilic group, such as 2-methacryloyloxyethyl phosphorylcholine (=MPC polymer), can be suitably used.

[0033] A reverse osmosis membrane having a surface layer containing a PC polymer can be prepared, for example, by passing an aqueous PC polymer solution through the reverse osmosis membrane. The PC polymer concentration in the aqueous PC polymer solution is, for example, 100 to 3000 ppm, preferably 300 to 2000 ppm, and particularly preferably 500 to 1000 ppm.

[0034] The amount of the liquid circulating across the membrane surface of the reverse osmosis membrane having the surface layer is the same as above.

[0035] When the reverse osmosis membrane is clogged with soluble cellulose acetate, the circulation flow rate decreases, and when the circulation flow rate decreases to a certain level, it is necessary to stop the membrane separation process and remove the clog. However, in the reverse osmosis membrane to which the polymer is adsorbed, the electrostatic interaction between the polymer and the reverse osmosis membrane prevents the soluble cellulose acetate from approaching the reverse osmosis membrane. This allows for a dramatic reduction in the amount of soluble cellulose acetate adhering to the membrane surface, and suppresses a decrease in the circulation flow rate over time.

[0036] A reverse osmosis membrane having the surface layer can maintain the effect of suppressing adhesion of soluble cellulose acetate to its surface even in the presence of an alkaline chemical or after contact with an alkaline chemical, i.e., the reverse osmosis membrane having the surface layer has excellent alkali resistance.

[0037] Furthermore, the reverse osmosis membrane having the surface layer is highly durable, and the surface layer is not damaged even after 100 hours of use.

[0038] Furthermore, since the polymer constituting the surface layer does not permeate the reverse osmosis membrane, the polymer does not get mixed into the recovered acetic acid.

[0039] (Step 2: Extraction step 1) In step 2, the aqueous acetic acid solution (a) is contacted with an organic solvent to separate the solution into an extract phase containing mainly acetic acid and the organic solvent, and a raffinate phase containing mainly water. In the method for recovering acetic acid according to the present disclosure, the aqueous acetic acid solution (a) subjected to the extraction step contains dissolved cellulose acetate at a concentration within the above range, and therefore high extraction efficiency is achieved due to the surfactant effect of the dissolved cellulose acetate.

[0040] The organic solvent preferably has a boiling point lower than that of acetic acid (for example, at least 25°C lower, preferably at least 30°C lower, and particularly preferably at least 35°C lower than that of acetic acid), has high solubility for acetic acid, and low solubility in water. Examples of the organic solvent include aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as n-hexane, cyclohexane, heptane, octane, and nonane; esters such as ethyl acetate, n-amyl acetate, cyclohexyl acetate, isoamyl propionate, and methyl benzoate; halides such as chloroform and carbon tetrachloride chlorobenzene; and ether compounds such as 1,2-dimethoxyethane and dimethyl ether. These may be used alone or in combination of two or more.

[0041] As the organic solvent, a mixed solvent of an ester such as ethyl acetate (particularly preferably, acetic acid ester) and an aromatic hydrocarbon such as benzene is preferred, and the mixing ratio of the ester to the aromatic hydrocarbon (former / latter; weight ratio) is, for example, 5 / 95 to 95 / 5, preferably 20 / 80 to 90 / 10, particularly preferably 40 / 60 to 85 / 15, and most preferably 55 / 45 to 85 / 15.

[0042] The amount of the organic solvent used is about 1.0 to 5.0 times (volume ratio) the amount of the aqueous acetic acid solution (a) used.

[0043] The extraction operation preferably uses an apparatus capable of contacting the aqueous acetic acid solution (a) with an organic solvent and separating the solution into an extract liquid phase (organic phase) and a raffinate liquid phase (aqueous phase). Such an apparatus is not particularly limited, and examples thereof include known methods using extraction apparatuses such as extraction towers. The extraction towers include commonly used types such as mixer-settler extraction towers, perforated plate types, packed towers, baffle towers, vibrating perforated plate types, stirring and mixing types, pulsating packed types, and centrifugal extraction types. When such an extraction apparatus is used, the aqueous acetic acid solution (a) and the organic solvent come into countercurrent contact within the extraction tower.

[0044] The contact temperature between the aqueous acetic acid solution (a) and the organic solvent is, for example, in the range of 20 to 60° C., and preferably 30 to 50° C. The contact time is, for example, about 0.1 to 10 hours.

[0045] The contact of the aqueous acetic acid solution (a) with the organic solvent can be carried out under normal pressure, elevated pressure, or reduced pressure, and the gauge pressure is, for example, about −30 to 30 kPa.

[0046] The extraction operation can be carried out intermittently or continuously. If the extraction effect is insufficient, the extraction operation can be repeated.

[0047] By carrying out the extraction operation under the above conditions, the extraction rate of acetic acid in the aqueous acetic acid solution (a) into the extract liquid phase can be increased, and the efficiency of water separation and removal can be improved.

[0048] The extract liquid phase obtained in this step mainly contains acetic acid and the organic solvent, and has a water content of, for example, 15% by weight or less, preferably 10% by weight or less, and a cellulose acetate content of 200 ppm or less (preferably 150 ppm or less).

[0049] As described above, the cellulose acetate content in the extract liquid phase obtained through this step is reduced. Therefore, even if the extract liquid phase is subjected to a distillation step as is (for example, even if the extract liquid phase is charged directly into a distillation column without providing a gasification step or the like), precipitation of cellulose acetate at the column bottom can be suppressed, and safe operation of the distillation column can be maintained.

[0050] (Step 2': Extraction step 2) The membrane concentrate obtained through the membrane separation treatment in step 1 contains water, acetic acid, and a large amount of dissolved cellulose acetate. This membrane concentrate is subjected to the extraction step in the same way as the membrane-permeated aqueous acetic acid solution (a). The resulting extract phase (containing acetic acid, the organic solvent used in the extraction, and a large amount of dissolved cellulose acetate) is gasified using an evaporator or the like, allowing the cellulose acetate to be separated and removed as a non-volatile component, yielding an aqueous acetic acid solution (a') with an extremely low concentration of dissolved cellulose acetate. High-purity acetic acid can also be recovered from the resulting aqueous acetic acid solution (a') by subjecting it to the subsequent steps in the same way as the aqueous acetic acid solution (a).

[0051] (Step 3: Distillation process) Step 3 is a step of recovering acetic acid by distilling the extract liquid phase obtained through step 2 and the gas obtained through step 2'. In this step, the extract liquid phase obtained through step 2 and the gas obtained through step 2' are distilled using a distillation column, and high-purity acetic acid is recovered by separating acetic acid and the organic solvent by utilizing the difference in boiling points. For example, when an organic solvent with a lower boiling point than acetic acid is used, the organic solvent is discharged as a distillate to the outside of the system, and high-purity acetic acid is recovered as a bottom product.

[0052] Examples of the distillation column include a plate column and a packed column. The column top temperature is, for example, 20 to 120°C, preferably 30 to 100°C, and more preferably 40 to 80°C. The pressure inside the distillation column can be adjusted appropriately, for example, within the range of 0 to 60 kPa. The distillation step may consist of a single step or a combination of multiple steps.

[0053] The actual number of plates in the distillation column is, for example, 1 to 100, and is preferably 10 to 100, particularly preferably 30 to 80, and most preferably 40 to 60, in terms of excellent separation efficiency.

[0054] In the method for recovering acetic acid according to the present disclosure, the liquid extract phase obtained through step 2 can be charged to a distillation column as it is. Therefore, compared to when all the charging to the distillation column is performed in the gas phase, energy costs can be reduced and steam saving effects are excellent.

[0055] Through the above steps, high-purity acetic acid is obtained as a bottom product, which has an acetic acid concentration of, for example, 90% by weight or more, preferably 98% by weight or more, and more preferably 99.9% by weight or more.

[0056] The concentration of acetic acid in the distillate can be controlled by adjusting the reflux ratio.

[0057] An example of the method for recovering acetic acid according to the present disclosure will be described in detail below with reference to FIG. In FIG. 1, the residual liquid after cellulose acetate production is supplied to membrane separation apparatus A via line 1 and subjected to membrane separation treatment. The aqueous acetic acid solution (a) obtained by permeation through the reverse osmosis membrane is introduced into the top of extraction column B-1 via line 2. An organic solvent (i.e., an organic solvent with a boiling point lower than that of acetic acid) is introduced into the bottom of extraction column B-1 via line 3 and comes into countercurrent contact with the aqueous acetic acid solution (a) within extraction column B-1, resulting in separation into an extract liquid phase containing mainly acetic acid and organic solvent, and a raffinate liquid phase containing mainly water. The extract liquid phase is fed (as a liquid) into distillation column C via line 4. The raffinate liquid phase is discharged to the outside of the system via line 13. When the extract liquid phase is charged into distillation column C, most of the organic solvent contained in the extract liquid phase passes from the top of the column through line 8, is cooled in cooler D, and is introduced into tank E through line 9. Of the liquid introduced into tank E, the upper layer liquid mainly contains the organic solvent and is discharged to the outside of the system through line 10. This can be reused as the organic solvent to be introduced again into extraction column B-1 or B-2. The lower layer liquid from tank E mainly contains water and is discharged to the outside of the system through line 11. A portion of the upper layer liquid from tank E may be recycled to distillation column C via line 12 as a liquid phase. Meanwhile, high-purity acetic acid is recovered as bottoms from the bottom of distillation column C via line 5.

[0058] In addition, the membrane concentrate that did not pass through the reverse osmosis membrane in membrane separation apparatus A is introduced into the top of extraction column B-2 via line 14. The organic solvent is introduced into the bottom of extraction column B-2 via line 15 and comes into countercurrent contact with the membrane concentrate in extraction column B-2, separating it into an extract phase containing mainly acetic acid, organic solvent, and soluble cellulose acetate, and an extraction residue phase containing mainly water and soluble cellulose acetate. The extract phase is introduced into evaporator G via line 16. The extraction residue phase is discharged to the outside of the system via line 17. The extract liquid phase is evaporated in evaporator G and separated into vapor containing acetic acid and the organic solvent and evaporation residue containing soluble cellulose acetate, and the vapor containing acetic acid and the organic solvent is fed in the vapor phase to distillation column C via line 18. The subsequent flow is the same as that of the extract liquid phase fed in the liquid phase to distillation column C. The evaporation residue containing soluble cellulose acetate is discharged to the outside of the system via line 19.

[0059] [Method of manufacturing cellulose acetate] The method for producing cellulose acetate according to the present disclosure comprises acetylating cellulose in an aqueous acetic acid solution to obtain cellulose acetate, recovering cellulose acetate from the reaction solution after the acetylation, and recovering acetic acid from the residual solution by the above-described method for recovering acetic acid, and reusing the recovered acetic acid as a raw material for the aqueous acetic acid solution.

[0060] The production of cellulose acetate includes, for example, the following steps 1 to 3. Step 1: A process in which the cellulose raw material is disintegrated and crushed, and then acetic acid or acetic acid containing a small amount of acid catalyst is sprayed onto the cellulose to activate it. Step 2: Acetylation step in which the activated cellulose is acetylated in an aqueous acetic acid solution to obtain cellulose acetate. Process 3: Purification process

[0061] The reaction liquid after the acetylation contains insoluble cellulose acetate, soluble cellulose acetate, and an aqueous acetic acid solution obtained by the acetylation.

[0062] The aqueous acetic acid solution may contain other components in addition to acetic acid and water, such as an acetylating agent (for example, acetic anhydride) and an acid catalyst (for example, sulfuric acid).

[0063] The primary cellulose acetate obtained through the acetylation step can be hydrolyzed to obtain secondary cellulose acetate with a desired degree of acetylation.

[0064] In the method for producing cellulose acetate according to the present disclosure, acetic acid is recovered from the reaction solution after the acetylation step and reused, thereby reducing the production cost of cellulose acetate.

[0065] The above-described configurations and combinations of the invention according to the present disclosure are merely examples, and additions, omissions, substitutions, and modifications of the configurations are possible as appropriate within the scope of the invention according to the present disclosure. Furthermore, the invention according to the present disclosure is not limited to the embodiments, but is limited only by the description of the claims. [Example]

[0066] The present disclosure will be explained in more detail below using examples, but the present disclosure is not limited to these examples.

[0067] Example 1 [Membrane separation process - extraction process - distillation process] (Membrane separation process) A 30% aqueous acetic acid solution (1) (soluble components: dissolved cellulose acetate 0.4%, inorganic salts 0.6%, insoluble components: 100 ppm) was filtered using a microfiltration membrane (MF membrane manufactured by POREX Corporation, pore size: 5 μm, material: polyethylene) to obtain a membrane permeate (1) (soluble components: dissolved cellulose acetate 0.4%, inorganic salts 0.6%, insoluble components: zero). The membrane permeate (1) was subjected to cross-flow filtration using a reverse osmosis membrane (2-inch membrane module, ES15 manufactured by Nitto Denko Corporation, material: polyamide) at 40°C, a circulation flow rate of 8 L / min, and an operating pressure of 2.4 MPaG to obtain a membrane permeate (2) (soluble components: dissolved cellulose acetate 20 ppm, inorganic salts 30 ppm, insoluble components: zero).

[0068] (Extraction process 1) The membrane permeate (2) was subjected to countercurrent multistage extraction (112 plates in the extraction column: 28 plates / section x 4 sections) using an organic solvent (ethyl acetate: 75%, benzene: 25%) at an S / F ratio of 1.6 (v / v), extraction temperature of 40°C, and superficial velocity of 0.4 cm / sec. The extraction time was 0.2 hours. The extraction operation was stable throughout. This yielded an extract liquid phase. The acetic acid concentration in the obtained extract liquid phase was measured using an automatic titrator (Kyoto Electronics Manufacturing Co., Ltd., "NZH77774"), and the plate efficiency (= number of theoretical plates / number of actual plates) was calculated, which was taken as the extraction efficiency.

[0069] (Distillation process) The obtained extract liquid phase was charged to the 24th plate from the top of a distillation column (Oldershaw 60 plates). The charge liquid rate was 233 g / h. The column top pressure was atmospheric pressure, and thermometers were installed at the column top (first plate from the top), the 4th, 8th, 14th, 18th, 24th, 28th, 38th, 44th, 48th, 54th, and 58th plates from the top, and the reboiler liquid (BTM liquid). The reflux ratio (reflux amount / (volume of upper layer liquid + volume of lower layer water)) was 0.6. Distillation was carried out under the above conditions, and high-purity acetic acid was obtained as the bottoms.

[0070] Example 2 [Membrane separation step - extraction steps 1 and 2 - distillation step] (Membrane separation process) The membrane permeate (1) obtained in the same manner as in Example 1 was subjected to cross-flow filtration using a reverse osmosis membrane (2-inch membrane module, ES15 manufactured by Nitto Denko Corporation, material: polyamide) at 40°C, a circulation flow rate of 8 L / min, and an operating pressure of 2.4 MPaG to obtain a membrane permeate (3) (soluble components: dissolved cellulose acetate 20 ppm, inorganic salts 30 ppm, insoluble components: zero) and a membrane concentrate (1).

[0071] (Extraction process 1) The membrane permeate (3) was subjected to countercurrent multistage extraction (112 plates in the extraction column: 28 plates / section x 4 sections) using an organic solvent (ethyl acetate: 75%, benzene: 25%) under conditions of S / F: 1.6 (v / v), extraction temperature: 40°C, and superficial velocity: 0.4 cm / sec. The extraction operation was stable throughout. This yielded the extract liquid phase (1). The acetic acid concentration in the obtained extract liquid phase (1) was measured using an automatic titrator (Kyoto Electronics Manufacturing Co., Ltd., "NZH77774"), and the plate efficiency (= number of theoretical plates / number of actual plates) was calculated, which was used as the extraction efficiency.

[0072] (Extraction process 2) The membrane-concentrated liquid (1) was subjected to countercurrent multistage extraction (112 actual plates: 28 plates / section × 4 sections) using an organic solvent (ethyl acetate: 75%, benzene: 25%) under conditions of S / F: 1.6 (v / v), extraction temperature: 40°C, and superficial velocity: 0.4 cm / sec. The extraction operation was stable throughout. As a result, an extract liquid phase (2) containing cellulose acetate was obtained. The obtained extract liquid phase (2) was then gasified in an evaporator (distillate yield: 99%) to obtain a gas (2) not containing cellulose acetate.

[0073] (Distillation process) The obtained extract liquid phase (1) was charged to the 24th plate from the top of a distillation column (60 Oldershaw plates). The charge rate was 185 g / h. The obtained gas (2) was charged to the 34th plate from the top. The charge rate was 615 g / h. The column top pressure was atmospheric pressure, and thermometers were installed at the column top (first plate from the top), the 4th, 8th, 14th, 18th, 24th, 28th, 38th, 44th, 48th, 54th, and 58th plates from the top, and the reboiler liquid (BTM liquid). The reflux ratio (reflux volume / (volume of upper layer liquid + volume of lower layer water)) was 0.65. Distillation was carried out under the above conditions, and high-purity acetic acid was obtained as the bottoms.

[0074] Example 3 [Membrane separation process - extraction process] An extract liquid phase was obtained by carrying out the same (membrane separation step) and (extraction step) as in Example 1, except that the superficial velocity was changed from 0.4 cm / sec to 0.6 cm / sec.

[0075] Comparative example 1 [extraction process] The same procedure as in Example 1 (extraction step) was carried out to obtain an extract liquid phase, except that a 30% aqueous acetic acid solution (1) (soluble components: dissolved cellulose acetate 0.4%, inorganic salts 0.6%, insoluble components: 100 ppm) was used instead of the membrane permeate (2).

[0076] Comparative example 2 [extraction process] The same procedure as in Comparative Example 1 was carried out except that the superficial velocity was changed from 0.4 cm / sec to 0.6 cm / sec. After significant cloudiness was observed in the extraction tower, separation became poor and it became impossible to continue operation.

[0077] Comparative example 3 [extraction process] The same procedure as in Example 1 (extraction step) was carried out to obtain an extract liquid phase, except that a 30% aqueous acetic acid solution (2) (dissolved cellulose acetate and inorganic salts: zero, insoluble components: zero) was used instead of the membrane permeate (2).

[0078] The above results are summarized in the table below. [Table 1]

[0079] Preparation Example 1 (Preparation of reverse osmosis membrane with surface layer containing MPC polymer) A 0.075% aqueous solution of MPC polymer (2-methacryloyloxyethyl phosphorylcholine, manufactured by NOF Corporation) was placed in a tank and passed through a reverse osmosis membrane (0) (2-inch membrane module, manufactured by Nitto Denko Corporation, ES15, material: polyamide) at room temperature for 30 minutes at a flow rate of 1 mL / min, causing the MPC polymer to adsorb onto the membrane surface, thereby obtaining a reverse osmosis membrane (1) with a surface layer containing the MPC polymer.

[0080] (Usability evaluation of reverse osmosis membranes) The reverse osmosis membrane (1) obtained in Preparation Example 1 and the reverse osmosis membrane (0) before the surface layer was formed were evaluated for their clogging prevention effects by measuring the operating time of membrane separation treatment and the circulation flow rate under the following conditions. The results are shown in Figures 2 and 3. Membrane separation treatment conditions: Treatment solution composition: 70% water, 29.925% acetic acid, 0.075% MPC polymer Flow rate: 0.15mL / min Temperature: 40℃ Operating pressure: 2.2 MPaG

[0081] 2 and 3, it can be seen that the reverse osmosis membrane (1) dramatically suppresses the decrease in circulation flow rate compared to the reverse osmosis membrane (0), and can maintain a constant circulation flow rate for a long period of time.

[0082] (Durability evaluation of reverse osmosis membranes with a surface layer containing MPC polymer) The amounts of phosphorus and oxygen atoms on the membrane surface of the reverse osmosis membrane (0) and the reverse osmosis membrane (1) before and after 100 hours of membrane separation treatment were analyzed by wide spectrum and narrow spectrum measurement using XPS. The results are shown in Table 2 below.

[0083] [Table 2]

[0084] Table 2 confirms the presence of phosphorus atoms on the surface of reverse osmosis membrane (1) before membrane separation treatment, indicating that passing an MPC polymer aqueous solution through reverse osmosis membrane (0) causes the MPC polymer to adsorb to the reverse osmosis membrane. Furthermore, the amount of oxygen atoms on the surface of reverse osmosis membrane (1) after 100 hours of membrane separation treatment increased, indicating that oxidation was progressing over time. However, the amount of phosphorus atoms after 100 hours of membrane separation treatment remained almost unchanged from before membrane separation treatment. This indicates that the MPC polymer adsorbed to the reverse osmosis membrane is highly durable and remains attached without peeling even after 100 hours of membrane separation treatment.

[0085] Furthermore, the results of narrow spectrum measurement confirmed that the MPC polymer was adsorbed on the membrane surface (depth: 3 to 5 nm region) of the reverse osmosis membrane (1) before it was subjected to membrane separation treatment.

[0086] Furthermore, the reverse osmosis membrane (1) was subjected to alkaline washing using an aqueous NaOH solution at pH 12. After alkaline washing, the reverse osmosis membrane (1) was subjected to membrane separation treatment in the same manner as above, and the effect on the surface layer was confirmed. The results are shown in Figures 2 and 3.

[0087] 2 and 3, it was confirmed that the reverse osmosis membrane (1) after alkaline cleaning can also suppress a decrease in circulation flow rate over time. This indicates that the reverse osmosis membrane (1) can maintain its surface layer even after alkaline cleaning.

[0088] Furthermore, the elution of the MPC polymer present on the surface of the reverse osmosis membrane (1) was evaluated by the following method. That is, a 26 wt% aqueous solution of acetic acid (phosphorus not detected) was used as a model liquid, and the model liquid was filtered using a reverse osmosis membrane (1) to obtain a permeate and a concentrated liquid, and the phosphorus concentration in the permeate was quantified by ICP-AES. As a result, no phosphorus was detected in the permeate, confirming that the recovered acetic acid was almost entirely free of MPC polymer. [Explanation of symbols]

[0089] A Membrane separation device B-1, B-2 Extraction tower C. Distillation column D Cooler E-Tank F Reboiler G Evaporator

Claims

1. A method for recovering acetic acid from an aqueous acetic acid solution in which cellulose acetate is dissolved, comprising the steps of: Step 1: subjecting an aqueous acetic acid solution having cellulose acetate dissolved therein to membrane separation treatment to obtain, as a membrane permeate, an aqueous acetic acid solution having a cellulose acetate concentration dissolved therein of 1 to 100 ppm; Step 2, in which an aqueous acetic acid solution having a dissolved cellulose acetate concentration of 1 to 100 ppm, obtained as a membrane permeate through the membrane separation treatment in Step 1, is contacted with an organic solvent to separate the aqueous acetic acid solution into an extract phase mainly containing acetic acid and the organic solvent and a raffinate phase mainly containing water; a step 2' of subjecting the membrane concentrate obtained through the membrane separation treatment of the step 1 to an extraction step in which the membrane concentrate is contacted with an organic solvent, and gasifying the resulting extract liquid phase containing acetic acid, the organic solvent, and dissolved cellulose acetate using an evaporator; A method for recovering high-purity acetic acid, comprising step 3, in which the dissolved cellulose acetate obtained through step 2 and an extract liquid phase mainly containing acetic acid and an organic solvent are subjected to liquid-phase distillation using a distillation column, and the gas obtained through step 2' is subjected to distillation using a distillation column to recover acetic acid.

2. 2. The method for recovering high-purity acetic acid according to claim 1, wherein step 1 is a step of subjecting an aqueous acetic acid solution having cellulose acetate dissolved therein to membrane separation treatment using a microfiltration membrane having a pore size of 0.01 to 10 μm, and further subjecting the resulting membrane permeate to membrane separation treatment using a reverse osmosis membrane to obtain, as the membrane permeate, an aqueous acetic acid solution having a cellulose acetate concentration dissolved in the aqueous acetic acid solution of 1 to 100 ppm.

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