Recovery of acetic acid and dimethylformamide using pressure swing distillation

Abstract

The present invention relates to a method for separating and recovering acetic acid and dimethylformamide from a waste solvent, comprising the steps of: feeding a waste solvent containing water, acetic acid, and dimethylformamide into a low vacuum distillation tower, separating water at the top of the low vacuum distillation tower, and obtaining a mixture containing acetic acid and dimethylformamide at the bottom of the low vacuum distillation tower (S11); feeding the mixture containing acetic acid and dimethylformamide into a high vacuum distillation tower, recovering dimethylformamide at the top of the high vacuum distillation tower, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition at the bottom of the high vacuum distillation tower (S12); The present invention provides a method for recovering acetic acid and dimethylformamide, comprising the steps of: charging a mixture containing an amide into an atmospheric distillation column, recovering acetic acid at the top of the atmospheric distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition at the bottom of the atmospheric distillation column (S13); and discharging a portion of the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition obtained at the bottom of the atmospheric distillation column so that the ratio of dimethylformamide / (dimethylformamide+dimethylacetamide) in the second azeotropic composition is 0.4 to 0.9, and charging the remainder into the high vacuum distillation column or the low vacuum distillation column for circulation (S14).

Description

[Technical Field]

[0001] The present invention relates to a method for recovering useful components from waste solvents, and more specifically to a method for recovering the components acetic acid (AA) and dimethylformamide (DMF) and dimethylacetamide (DMAc) by utilizing the difference in azeotropic composition between acetic acid (AA) and dimethylformamide (DMF) and dimethylacetamide (DMAc) under high vacuum and atmospheric pressure conditions. [Background technology]

[0002] The components contained in the waste solvent (or DMF waste liquid) generated during the production of polyimide film and their typical composition are shown in Table 1 below. [Table 1]

[0003] The waste solvent is fed into a distillation column with a large number of plates, and water and 3MP can be removed from the top of the column, while a mixture of 20% acetic acid and 80% DMF can be obtained at the bottom of the column.

[0004] AA and DMF form a maximum temperature azeotrope, and according to literature, the azeotropic temperature under normal pressure conditions is 159°C, and the azeotropic composition at this time is 26% AA and 74% DMF.

[0005] Therefore, when the mixture of AA and DMF obtained at the bottom of the column is recharged into a conventional distillation column, some of the DMF is recovered at the top of the column, and an azeotropic mixture of 26% AA and 74% DMF is obtained at the bottom of the column. If the azeotropic mixture obtained in this way is recharged into a conventional distillation column, an azeotropic mixture of the same composition will simply be obtained at the top and bottom of the column.

[0006] Methods for separating azeotropes include extractive distillation, azeotropic distillation, and pressure swing distillation. Depending on the characteristics of the azeotrope, an efficient method can be selected and used to separate the azeotrope. However, such selection is not possible in all cases, and even if the selected method can separate the azeotrope, it may not be economical.

[0007] In particular, as a technique for separating acetic acid and DMF, Patent Document 1 proposes a method for separating acetic acid and dimethylformamide using toluene as an azeotropic agent, which forms an azeotropic distillate with acetic acid but not with dimethylformamide, and Patent Document 2 proposes a method for removing carboxylic acid from a tertiary amide-containing solution by contacting a solution containing carboxylic acid and tertiary amide with an extraction medium containing trilaurylamine. However, these conventional techniques require a third component, which makes them less economical and inefficient in the separation process, and a complementary separation technique is needed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japan Patent No. 2002-363150 [Patent Document 2] Korea Patent No. 10-2010-0130219 Summary of the Invention [Problem to be solved by the invention]

[0009] The method for recovering acetic acid and dimethylformamide of the present invention has been devised to solve the above-mentioned problems, A step (S11) of feeding the waste solvent containing water, acetic acid, and dimethylformamide into a low-vacuum distillation column, separating water at the top of the low-vacuum distillation column, and obtaining a mixture containing acetic acid and dimethylformamide at the bottom of the low-vacuum distillation column; a step (S12) of charging the mixture containing acetic acid and dimethylformamide into a high vacuum distillation column, recovering dimethylformamide at the top of the high vacuum distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition at the bottom of the high vacuum distillation column; (S13) charging the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having the first azeotropic composition into an atmospheric distillation column, recovering acetic acid at the top of the atmospheric distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition at the bottom of the atmospheric distillation column; and The object of the present invention is to provide a method for recovering acetic acid and dimethylformamide, comprising the step (S14) of discharging a portion of a mixture containing acetic acid, dimethylformamide, and dimethylacetamide and having a second azeotropic composition obtained at the bottom of the atmospheric distillation column so that the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) in the second azeotropic composition is 0.4 to 0.9, and feeding the remainder into the high vacuum distillation column or the low vacuum distillation column for circulation.

[0010] The present invention also aims to achieve the above-mentioned clear objects, as well as other objects that can be easily conceived by those skilled in the art from the general teachings of this specification. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the method for recovering acetic acid and dimethylformamide of the present invention comprises the steps of: A step (S11) of feeding the waste solvent containing water, acetic acid, and dimethylformamide into a low-vacuum distillation column, separating water at the top of the low-vacuum distillation column, and obtaining a mixture containing acetic acid and dimethylformamide at the bottom of the low-vacuum distillation column; a step (S12) of charging the mixture containing acetic acid and dimethylformamide into a high vacuum distillation column, recovering dimethylformamide at the top of the high vacuum distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition at the bottom of the high vacuum distillation column; (S13) charging the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having the first azeotropic composition into an atmospheric distillation column, recovering acetic acid at the top of the atmospheric distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition at the bottom of the atmospheric distillation column; and (S14) comprising a step of discharging a portion of the mixture containing acetic acid, dimethylformamide, and dimethylacetamide and having a second azeotropic composition obtained at the bottom of the atmospheric distillation column so that the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) in the second azeotropic composition is 0.4 to 0.9, and introducing the remainder into the high vacuum distillation column or the low vacuum distillation column for circulation.

[0012] In addition, in the second azeotropic composition, the ratio of dimethylformamide / (dimethylformamide+dimethylacetamide) may be 0.6 to 0.8.

[0013] The waste solvent may also contain 3-methylpyridine.

[0014] The column bottom pressure of the low vacuum distillation column may be 700 torr or less, 600 torr or less, or 500 torr or less.

[0015] The bottom pressure of the high vacuum distillation column may be 200 torr or less, 150 torr or less, or 100 torr or less.

[0016] Furthermore, the bottom pressure of the atmospheric distillation column may be 700 to 1200 torr, 700 to 1000 torr, or 700 to 900 torr.

[0017] The product recovered from the top of the high vacuum distillation column may have a purity of 99.5% or more with respect to dimethylformamide.

[0018] The top recovered material of the high vacuum distillation column may have a moisture content of 200 ppm or less, 150 ppm or less, or 100 ppm or less.

[0019] The product recovered from the top of the high vacuum distillation column may not contain acetic acid.

[0020] The product recovered from the top of the atmospheric distillation column has a purity of 99.5% or more with respect to acetic acid.

[0021] The water content of the overhead recovery product of the atmospheric distillation column may be 1 wt% or less, 0.8 wt% or less, or 0.6 wt% or less.

[0022] The low vacuum distillation column may also be equipped with a forced circulation reboiler.

[0023] The atmospheric distillation column may also be equipped with a forced circulation reboiler. [Effects of the Invention]

[0024] According to the method for recovering acetic acid and dimethylformamide of the present invention, high-purity dimethylformamide can be recovered from waste solvents through pressure swing distillation, and at the same time, dimethylformamide or impurity-free acetic acid or an acetic acid aqueous solution can be economically recovered. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a graph showing the change in azeotropic composition of AA and DMF and AA and DMAc depending on the pressure. [Figure 2] 1 is a graph showing changes in boiling points of DMAc and DMF with pressure. [Figure 3] FIG. 1 is a process diagram for recovering DMF from DMF waste liquid according to one embodiment of the present invention. [Figure 4]This shows the results of measuring the DMF thermal decomposition reaction rate at different temperatures depending on the residence time in an oil bath. [Figure 5] FIG. 1 is a diagram illustrating a distillation experiment for removing water and 3-methylpyridine from DMF waste liquid according to an embodiment of the present invention. [Figure 6] FIG. 2 is a diagram illustrating a distillation experiment using a vacuum distillation column and an atmospheric distillation column in combination according to an embodiment of the present invention. [Figure 7] FIG. 1 is a diagram showing a prediction calculation tool for DMF yield that reflects the DMF thermal decomposition reaction rate. [Figure 8] This shows the results of calculating the DMF recovery rate based on the ratio of DMF / (DMF+DMAc). BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Preferred embodiments of the present invention will now be described in detail.

[0027] However, the following will be described in detail by way of example of specific embodiments, and the present invention can be modified in various ways and can have various forms, so the present invention is not limited to the specific embodiments exemplified. It should be understood that the present invention includes all modifications, equivalents, and alternatives that fall within the spirit and technical scope of the present invention.

[0028] Furthermore, in the following description, many specific details such as specific components are described, but these are provided to facilitate a more comprehensive understanding of the present invention, and it would be obvious to a person skilled in the art that the present invention can be practiced without these specific details. Furthermore, in describing the present invention, if it is determined that a detailed description of related well-known functions or configurations unnecessarily obscures the gist of the present invention, the detailed description will be omitted.

[0029] Furthermore, the terms used in this application are merely used to describe specific embodiments and are not intended to limit the present invention. Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention belongs. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning they have in the context of the relevant art, and should not be interpreted as having an ideal or overly formal meaning unless expressly defined in this application.

[0030] In this application, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0031] In this application, terms such as "first," "second," etc. may be used to describe various components, but the components are not limited by these terms. These terms are used only to distinguish one component from another. For example, a first component may be designated a "second component," and similarly, a second component may be designated a "first component," without departing from the scope of the present invention.

[0032] In this application, the terms "comprise," "contain," or "have" refer to the presence of a feature, component (or constituent), etc. described in the specification, but do not imply that one or more other features, components, etc. are not present or cannot be added.

[0033] In this specification, % may be wt%.

[0034] Considering the change in the composition ratio of the azeotropic mixture of acetic acid (AA) and dimethylformamide (DMF) with pressure, as shown in Figure 1, if the pressure at the bottom of the low-pressure distillation column is 200 torr or less and the pressure at the bottom of the high-pressure distillation column is 700 torr or more, the difference in the azeotropic composition required for pressure swing distillation (5% or more based on AA) can be ensured, and AA and DMF can be expected to be recovered smoothly. Therefore, based on the concept of pressure swing distillation, a diagram of the process for recovering DMF and AA from DMF waste liquid can be derived, as shown in Figure 3.

[0035] However, when attempting to recover each useful component by feeding a mixture of AA and DMF into a pressure swing distillation process, thermal decomposition of DMF occurs in the section of the high-pressure distillation column where the temperature exceeds 155°C, and the thermal decomposition products dimethylacetamide (DMAc) and water are produced. This makes it uncertain whether the DMF and AA recovered at the top of the column will meet their respective product specifications, and whether their economic recovery is possible.

[0036] As shown in Figure 4, DMF, a heat-sensitive substance, begins to decompose at approximately 100°C. The higher the temperature, the greater the decomposition, with the degree of decomposition doubling for every 10°C increase in temperature. As shown in Reaction Scheme 1 below, DMF is thermally decomposed into dimethylamine (DMA) and carbon monoxide (CO), and the dimethylamine combines with surrounding acetic acid to produce dimethylacetamide (DMAc) and water. The amount of water produced can be indirectly calculated as 18 / 87 = 20.7% of the amount of DMAc produced.

[0037] (Reaction Scheme 1) [ka]

[0038] The internal temperature of the high-vacuum distillation column is generally below 110°C, which means that thermal decomposition of DMF hardly occurs and almost no water is produced. However, the water produced by thermal decomposition in the atmospheric distillation column, which has a relatively high internal temperature, not only increases the water content of the acetic acid recovered at the top of the atmospheric distillation column by several thousand ppm, but can also increase the water content of the bottoms stream (the recycle stream in Figure 3) from the atmospheric distillation column by several tens of ppm. Although the amount of water contained in this recycle stream is small, it is the main cause of the increase in the water content of the DMF recovered at the top of the high-vacuum distillation column, potentially making it difficult to meet the DMF moisture specification.

[0039] The present invention clearly identifies the problems caused by thermal decomposition during pressure swing distillation and derives a solution to overcome these problems. It then proposes the configuration, operating conditions, and method of a pressure swing distillation process that can economically and highly efficiently recover DMF, which meets the DMF product standard of "purity of 99.5% or more and a water content of 200 ppm or less," and AA, which also meets the AA product standard of "purity of 99.5% or more and a water content of 1% or less," from DMF waste liquid containing AA and DMF.

[0040] The method for recovering acetic acid and dimethylformamide of the present invention includes the steps of: charging a waste solvent containing water, acetic acid, and dimethylformamide into a low-vacuum distillation column, separating water at the top of the low-vacuum distillation column, and obtaining a mixture containing acetic acid and dimethylformamide at the bottom of the low-vacuum distillation column (S11); charging the mixture containing acetic acid and dimethylformamide into a high-vacuum distillation column, recovering dimethylformamide at the top of the high-vacuum distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition at the bottom of the high-vacuum distillation column (S12); and a step (S14) of discharging a portion of the mixture containing acetic acid, dimethylformamide, and dimethylacetamide and having a second azeotropic composition obtained at the bottom of the atmospheric distillation column so that the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) at the second azeotropic composition is 0.4 to 0.9, and charging the remainder into the high-vacuum distillation column or the low-vacuum distillation column for circulation.

[0041] Each step will be described in detail below with reference to FIG.

[0042] First, waste solvent containing water, acetic acid, and dimethylformamide is charged into a low vacuum distillation column, and water is separated at the top of the low vacuum distillation column, and a mixture containing acetic acid and dimethylformamide is obtained at the bottom of the low vacuum distillation column (S11).

[0043] The waste solvent to be treated in the present invention contains water, acetic acid, and dimethylformamide, and may further contain 3-methylpyridine (3MP). For example, the waste solvent may be generated during the production of polyimide films for semiconductors and displays.

[0044] Step (S11) may be a step of removing substances other than the carboxylic acid and the amide compound that are the target of separation in the present invention. Specifically, step (S11) may be a step of feeding a waste solvent containing water, a carboxylic acid, and an amide compound into a distillation column before step (S12), separating water and 3-methylpyridine at the top of the distillation column, and obtaining a mixture containing a carboxylic acid and an amide compound at the bottom of the distillation column.

[0045] In this case, the bottom pressure of the low vacuum distillation column may be 700 torr or less, 600 torr or less, or 500 torr or less. As with the high vacuum distillation column, most of the water generated by thermal decomposition in the low vacuum distillation column rises to the top of the column, but some is diverted to the bottom stream and rises to the top of the high vacuum distillation column, potentially affecting the water content of DMF. When the bottom pressure of the low vacuum distillation column is 700 torr or less or within this bottom pressure range, the temperature of the bottom stream having a composition of 20% AA and 80% DMF does not exceed 150°C, so the water content of the bottom stream is 40 ppm or less. As a result, the increase in the water content of DMF, the top stream of the high vacuum distillation column, is 50 ppm or less, which is within the range where the water content specification can be met.

[0046] Next, the mixture containing acetic acid and dimethylformamide recovered at the bottom of the low-vacuum distillation column in step (S11) is charged into a high-vacuum distillation column, dimethylformamide is recovered at the top of the high-vacuum distillation column, and a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition is obtained at the bottom of the high-vacuum distillation column (S12).

[0047] The bottom pressure of the high-vacuum distillation column may be 200 torr or less, 150 torr or less, or 100 torr or less. The azeotropic composition ratio under this pressure range is sufficiently different from the azeotropic composition ratio under atmospheric pressure, making it possible to separate components by pressure swing distillation. The lower limit is not particularly limited, but process efficiency and economy should be taken into consideration.

[0048] The overhead recovery product of the high vacuum distillation tower may have a purity of 99.5% or more with respect to dimethylformamide. The overhead recovery product of the high vacuum distillation tower may have a moisture content of 200 ppm or less, 150 ppm or less, or 100 ppm or less, and may be free of acetic acid. The overhead recovery product of the high vacuum distillation tower may have a DMAc content of less than 0.5 wt%, less than 0.3 wt%, or less than 0.1 wt%.

[0049] A mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition is obtained at the bottom of the high-vacuum distillation column. The composition ratio of the azeotropic mixture containing all of acetic acid, dimethylformamide, and dimethylacetamide can be calculated from the graphs of the DMF-AA azeotrope and the DMAc-AA azeotrope shown in Figure 1. For example, at 100 torr, the acetic acid ratio of the DMF-AA azeotrope is 34%, and the acetic acid ratio of the DMAc-AA azeotrope is 28%. However, the acetic acid ratio of the azeotropic mixture with an 80:20 DMF-DMAc ratio is approximately (34 x 80 + 28 x 20) / 100 = 32.8%.

[0050] Next, the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having the first azeotropic composition is charged into an atmospheric distillation column, acetic acid is recovered at the top of the atmospheric distillation column, and a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition is obtained at the bottom of the atmospheric distillation column (S13).

[0051] In this case, the bottom pressure of the atmospheric distillation column may be 700 to 1200 torr, 700 to 1000 torr, or 700 to 900 torr. In the case of an atmospheric distillation column located next to a high-vacuum distillation column, the higher the bottom pressure, the greater the pressure difference with the high-vacuum distillation column, making it easier to separate acetic acid and dimethylformamide. However, if the bottom pressure exceeds 1000 torr or above this range, as shown in Figure 2, the boiling point of DMF exceeds 165°C, the azeotropic temperature of DMF and AA exceeds 170°C, and the thermal decomposition rate of DMF increases by more than two times compared to atmospheric pressure. As a result, the water content of the acetic acid recovered at the top of the atmospheric distillation column increases significantly, making it difficult to meet the specification (1% or less).

[0052] The overhead recovery product of the atmospheric distillation column may have a purity of 99.5% or more with respect to acetic acid, may be free of dimethylformamide, and may have a water content of 1 wt% or less, 0.8 wt% or less, or 0.6 wt% or less.

[0053] The bottom of the atmospheric distillation column yields a mixture containing acetic acid, dimethylformamide, and dimethylacetamide, which has a second azeotropic composition. As shown in Figure 1, the acetic acid ratio of the DMF-AA azeotrope at atmospheric pressure (760 torr) is 26%, and the acetic acid ratio of the DMAc-AA azeotrope is 21%. However, the acetic acid ratio of the azeotrope of DMF and DMAc at an 80:20 ratio is approximately (26 x 80 + 21 x 20) / 100 = 25.0%.

[0054] Next, a portion of the mixture containing acetic acid, dimethylformamide, and dimethylacetamide and having a second azeotropic composition obtained at the bottom of the atmospheric distillation column is discharged (discharge stream), and the remainder is introduced into the high vacuum distillation column or the low vacuum distillation column (circulation stream) and circulated (S14), so that the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) in the second azeotropic composition is 0.4 to 0.9.

[0055] In this method, a portion of the mixture having the second azeotropic composition obtained at the bottom of the atmospheric distillation column is fed to the high-vacuum distillation column or the low-vacuum distillation column for recirculation. Therefore, it can be mixed and fed together with the feed fed to the distillation column in step (S11) or (S12). In this case, the second azeotropic composition can be adjusted depending on the circulation ratio, and in particular, it is necessary to adjust the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) in the second azeotropic composition to 0.4 to 0.9.

[0056] If the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) is below this range, the DMAc content in the DMF recovered at the top of the vacuum distillation column increases, making it very difficult to meet the DMF purity standard (99.5% or higher).On the other hand, if the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) exceeds this range, the proportion of the bottom stream from the atmospheric distillation column having the second azeotropic composition that is discharged without being recycled increases, making it difficult to achieve the DMF recovery rate target (90% or higher) required by DMF waste liquid providers.

[0057] Meanwhile, atmospheric distillation columns and low vacuum distillation columns can be equipped with forced circulation reboilers suitable for heat-sensitive substances such as DMF. This type of equipment is preferable because it prevents water generated by thermal decomposition of DMF in the reboiling section of the atmospheric distillation column and low vacuum distillation column from rising to the top of the column and reduces the amount of water that flows into the bottom stream.

[0058] On the other hand, when the entire recycle stream is fed to the high vacuum distillation column, the trace amount of water contained in the recycle stream will end up in the overhead stream of the high vacuum distillation column, but when part or all of the recycle stream is fed to the low vacuum distillation column, the proportion of the trace amount of water contained in the recycle stream that ends up in the overhead stream of the high vacuum distillation column can be reduced. Therefore, feeding part or all of the recycle stream to the low vacuum distillation column is preferred in order to meet the water content specification for DMF.

[0059] Examples of the present invention will be described below. Example Production Examples 1-1 to 1-4: Confirmation of DMAc and water production by thermal decomposition of DMF After filling the water bath of a rotary evaporator with silicone oil, a 250 cc round-bottom flask filled with 100 g of a mixed solution (26 g of AA acid, 74 g of DMF) was attached to the rotary evaporator while the bath temperature was maintained at 110°C. Samples were then taken at 60 rpm at 30-minute intervals, and the changes in the components of the mixed solution were analyzed (Production Example 1-1).

[0060] The same experiment was repeated at bath temperatures of 120°C (Production Example 1-2), 135°C (Production Example 1-3), and 150°C (Production Example 1-4), and the results are shown in Figure 4. It can be seen that the proportion of DMAc increases rapidly as the temperature increases.

[0061] Production Example 2-1: Removal of water and 3-methylpyridine (3MP) from DMF waste liquid A 9600 Packed Column manufactured by B / R (USA) with a column inner diameter of 25 mm, 45 theoretical plates, and a 5-liter heating flask was configured as shown in Figure 5, and a continuous distillation experiment was conducted to remove water and 3MP from DMF waste liquid.

[0062] Actual DMF waste liquid (water 17.5%, AA 15.8%, DMF 64.8%, 3MP 1.8%, DMAc 0.1%) was introduced into the T2 position of the distillation apparatus at a flow rate of 500 cc / h. With the pressure fixed at 450 torr and the reflux ratio fixed at 4, the load on the heating mantle was adjusted so that the 3MP content in the stream (anhydrous AA-DMF mixture) discharged to the bottom of the column was maintained at approximately 500 ppm.

[0063] 25 hours after the start of the DMF waste liquid introduction, the continuous distillation apparatus reached a normal state. At this time, approximately 100 cc / h of 3MP waste water was continuously discharged from the top of the column, the column bottom temperature (T5) was 150°C, and approximately 400 cc / h of 30 ppm water and an AA-DMF mixture (AA 19.1%, DMF 80.1%, 3MP 500 ppm, DMAc 0.7%) were steadily obtained from the column bottom.

[0064] Production Example 2-2: Removal of water and 3-methylpyridine (3MP) from DMF waste liquid The distillation apparatus was operated in the same manner as in Preparation Example 2-1, except that the vacuum pump was turned off and the distillation conditions were changed to normal pressure. Ten hours after the pressure change, normal conditions were reached. At this time, the column bottom temperature (T5) was 160°C, and approximately 400 cc / h of a mixture of 60 ppm water and AA-DMF (18.8% AA, 79.6% DMF, 500 ppm 3MP, 1.5% DMAc) was stably obtained at the bottom. This confirmed the tendency for the DMAc content and water content in the bottom stream to increase as the column bottom temperature of the distillation experiment apparatus increased.

[0065] Production Example 3: Recovery of DMF and AA in a combined vacuum and atmospheric distillation experiment Two sets of 9600 Packed Columns (manufactured by B / R, USA) with a column inner diameter of 25 mm, 45 theoretical plates, and a 5-liter heated flask were configured as shown in Figure 6, and pressure swing distillation experiments were conducted to recover DMF and AA from an anhydrous AA-DMF mixture.

[0066] In Production Example 2-1, the 30 ppm water content obtained in the bottom stream and the AA-DMF mixture (30 ppm water content, 19.1% AA, 80.1% DMF, 500 ppm 3MP, 0.7% DMAc) were introduced into the T3 position of the vacuum distillation apparatus at a flow rate of 180 cc / h. With the vacuum set to 100 torr and the reflux ratio fixed at 3, the load on the heating mantle was adjusted so that the DMF stream discharged to the top of the column did not contain AA and the AA content in the stream discharged to the bottom of the column (vacuum azeotrope) was kept as high as possible.

[0067] The vacuum azeotropic mixture discharged from the bottom of the vacuum distillation apparatus was introduced into the atmospheric distillation apparatus at position T2, and with the reflux ratio fixed at 8, the load on the heating mantle was adjusted so that the AA content in the stream discharged from the bottom (atmospheric azeotropic mixture) was kept as low as possible.

[0068] Example 1: Changes in recovery rate and purity with DMF / (DMF+DMAc) Of the bottom stream (atmospheric azeotropic mixture) of the atmospheric distillation apparatus in Preparation Example 3, 260 cc / h was recycled to the T3 position of the vacuum distillation apparatus, and the remainder was discharged to the discharge stream.

[0069] After 20 hours from the start of the experiment, the atmospheric azeotropic mixture had a DMF / (DMF+DMAc) ratio of 0.84, the discharge stream rate was stabilized at 30 cc / h, the overhead temperature (T1) of the vacuum distillation unit stabilized at 89°C, and DMF meeting the purity standard was recovered at a flow rate of 125 cc / h, with a DMF recovery rate of approximately 86%. Meanwhile, the overhead temperature (T1) of the atmospheric distillation unit stabilized at 114°C, and AA containing only water but no DMF and meeting the purity standard was recovered at a flow rate of 25 cc / h.

[0070] Example 2: Changes in recovery rate and purity with DMF / (DMF+DMAc) Of the bottom stream (atmospheric azeotropic mixture) of the atmospheric distillation apparatus in Production Example 3, the amount recycled to the T3 position of the vacuum distillation apparatus was adjusted to 310 cc / h, and the remainder was discharged to the discharge stream.

[0071] After 17 hours had passed since the circulation rate was adjusted, the atmospheric azeotropic mixture had a DMF / (DMF+DMAc) ratio of 0.74, the discharge stream rate was stabilized at 20 cc / h, the top temperature (T1) of the vacuum distillation unit remained stable at 89°C, DMF meeting the purity standard was recovered at the top at a flow rate of 130 cc / h, and the DMF recovery rate was approximately 91%. Meanwhile, the top temperature (T1) of the atmospheric distillation unit remained stable at 114°C, and AA meeting the purity standard, containing only water but no DMF, was recovered at a flow rate of 30 cc / h.

[0072] Example 3: Changes in recovery rate and purity with DMF / (DMF+DMAc) Of the bottom stream (atmospheric azeotropic mixture) of the atmospheric distillation apparatus in Production Example 3, the amount recycled to the T3 position of the vacuum distillation apparatus was adjusted to 330 cc / h, and the remainder was discharged to the discharge stream.

[0073] After 15 hours had passed since the circulation rate was adjusted, the atmospheric azeotropic mixture had a DMF / (DMF+DMAc) ratio of 0.60, the discharge stream rate was stabilized at 10 cc / h, the top temperature (T1) of the vacuum distillation unit remained stable at 89°C, DMF meeting the purity standard was recovered at the top at a flow rate of 140 cc / h, and the DMF recovery rate was approximately 95%. Meanwhile, the top temperature (T1) of the atmospheric distillation unit remained stable at 115°C, and AA meeting the purity standard, containing only water but no DMF, was recovered at a flow rate of 30 cc / h.

[0074] Example 4: Changes in recovery rate and purity with DMF / (DMF+DMAc) Of the bottom stream (atmospheric azeotropic mixture) of the atmospheric distillation apparatus in Production Example 3, the amount recycled to the T3 position of the vacuum distillation apparatus was adjusted to 350 cc / h, and the remainder was discharged to the discharge stream.

[0075] After 18 hours had passed since the circulation rate was adjusted, the atmospheric azeotropic mixture had a DMF / (DMF+DMAc) ratio of 0.39, the discharge stream rate was stabilized at less than 10 cc / h, and the overhead temperature (T1) of the vacuum distillation unit remained stable at 89°C. However, the DMAc content in the overhead increased, and DMF that did not meet the purity standard was recovered at a flow rate of 140 cc / h, with a DMF recovery rate of approximately 97%. Meanwhile, the overhead temperature (T1) of the atmospheric distillation unit remained stable at 115°C, and AA containing only water but no DMF was recovered at a flow rate of 30 cc / h.

[0076] The DMAc content in DMF, DMF recovery rate, and DMF content in AA of the atmospheric pressure azeotropic mixture DMF / (DMF+DMAc) in Examples 1 to 4 are shown in Table 2. [Table 2]

[0077] As a result, when the DMF / (DMF+DMAc) ratio in the atmospheric pressure azeotropic mixture is low, there is an advantage in that the DMF recovery rate is high, but there is a disadvantage in that the DMAc content increases. In particular, when the DMF / (DMF+DMAc) ratio is less than 0.4, the DMF product specification (purity of 99.5% or more) is not met.

[0078] Example 5: Prediction of DMF recovery rate by DMF / (DMF+DMAc) ratio After modeling the process for recovering acetic acid and dimethylformamide from DMF waste liquid as shown in Figure 3, a DMF yield prediction calculation tool was created in Microsoft Excel that reflected the DMF thermal decomposition reaction rate, and the format is shown in Figure 7. This tool was then used to calculate the DMF recovery rate based on the DMF / (DMF+DMAc) ratio, and the results are shown in Figure 8.

[0079] While the preferred embodiments of the present invention have been described above, the present invention is not limited to the specific embodiments described above, and it is obvious that those skilled in the art can make various modifications without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited to the above embodiments, but should be determined by the following claims and their equivalents.

Claims

1. A step (S11) of feeding the waste solvent containing water, acetic acid, and dimethylformamide into a low-vacuum distillation column, separating water at the top of the low-vacuum distillation column, and obtaining a mixture containing acetic acid and dimethylformamide at the bottom of the low-vacuum distillation column; charging the mixture containing acetic acid and dimethylformamide into a high vacuum distillation column, recovering dimethylformamide at the top of the high vacuum distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a first azeotropic composition at the bottom of the high vacuum distillation column (S12); Charging the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having the first azeotropic composition into an atmospheric distillation column, recovering acetic acid at the top of the atmospheric distillation column, and obtaining a mixture containing acetic acid, dimethylformamide, and dimethylacetamide having a second azeotropic composition at the bottom of the atmospheric distillation column (S13); and A step (S14) of discharging a part of the mixture containing acetic acid, dimethylformamide, and dimethylacetamide having the second azeotropic composition obtained at the bottom of the atmospheric distillation column so that the ratio of dimethylformamide / (dimethylformamide + dimethylacetamide) in the second azeotropic composition is 0.4 to 0.9, while charging the remainder into the high vacuum distillation column or the low vacuum distillation column for circulation; A method for recovering acetic acid and dimethylformamide comprising:

2. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the waste solvent contains 3-methylpyridine.

3. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the pressure at the bottom of the low vacuum distillation column is 700 torr or less.

4. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the bottom pressure of the high vacuum distillation column is 200 torr or less.

5. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the bottom pressure of the atmospheric distillation column is 700 to 1,200 torr.

6. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the product recovered from the top of the high vacuum distillation column has a purity of 99.5% or more relative to dimethylformamide.

7. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the top recovered product of the atmospheric distillation column has a purity of 99.5% or more with respect to acetic acid.

8. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the low vacuum distillation column is equipped with a forced circulation reboiler.

9. 2. The method for recovering acetic acid and dimethylformamide according to claim 1, wherein the atmospheric distillation column is equipped with a forced circulation reboiler.

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