Process for the recovery of n,n-dimethylacetamide

Abstract

The application provides a recovery method of N,N-dimethylacetamide. The method comprises the following steps: waste solvent enters a distillation tower, is taken out in a side line in a gas phase, is directly introduced into a membrane separation unit after being pressurized, is dehydrated, and N,N-dimethylacetamide product is obtained. The application is coupled by means of distillation tower side line gas phase taking out and membrane separation, simplifies the process, reduces process energy consumption, especially saves the material heating process before membrane separation, is green in process, is high in recycling rate, is small in waste discharge amount, and is beneficial to industrialization. Meanwhile, the obtained N,N-dimethylacetamide product is higher than 99.9% in purity and lower than 200 ppm in water content, and completely meets the requirements of solvent reuse.

Description

Technical Field

[0001] This invention relates to the field of chemical waste solvent recovery, specifically to a method for recovering N,N-dimethylacetamide waste solvent with a core process of coupled distillation and membrane separation, and particularly simplifies the process by using side-stream vapor phase extraction technology from the distillation tower. Background Technology

[0002] N,N-Dimethylacetamide, abbreviated as DMAC, is a colorless and transparent commonly used aprotic solvent. It can dissolve a wide variety of compounds and is completely miscible with water, ethers, ketones, esters, etc. It possesses high thermal stability, is not easily hydrolyzed, has low corrosivity, and low toxicity. Therefore, it has wide applications in many fields, such as polymerization spinning, pharmaceutical synthesis, petroleum processing, leather and coating production, often serving as a medium or solvent. Its excellent solubility in polyurethane resins and polyimide resins has further expanded its application in polymer compounds.

[0003] However, the above processes generate large amounts of low-concentration wastewater solvents. For example, in the process of polymer wet spinning, to control the quality of the filaments and improve product quality, N,N-dimethylacetamide coagulation baths of varying concentrations and low-concentration washing wastewater are generated. Similarly, in the pharmaceutical industry, N,N-dimethylacetamide serves as a synthesis medium and is discharged as waste solvent in the later stages, often accompanied by large amounts of water, inorganic salts, and unreacted monomers. As a high-value and somewhat toxic organic solvent, its untreated discharge will cause significant harm to the natural environment and society, violating national environmental protection policies and resulting in a substantial waste of N,N-dimethylacetamide solvent resources, indirectly increasing production costs for manufacturing enterprises. Therefore, this patent for the recovery of N,N-dimethylacetamide wastewater solvents, especially the recovery of low-concentration N,N-dimethylacetamide wastewater, has significant theoretical and practical implications.

[0004] Currently, the recovery of solvents from N,N-dimethylacetamide wastewater mainly involves distillation / distillation-distillation technology, extraction-distillation technology, and membrane separation technology. Among these, distillation / distillation-distillation technology is the most researched and relatively mature method. Patent documents such as CN 207210299 U, CN 101462977 B, CN 1631558 A, CN 102030672 A, and CN104926675 A propose relatively complete waste solvent recovery schemes covering pharmaceutical wastewater, textile fiber, and other fields. However, this process has the disadvantage of high energy consumption, especially for the treatment of low-concentration organic solvent wastewater, where a large amount of water with high specific heat capacity evaporates from the top of the column, resulting in enormous energy consumption.

[0005] The extraction-distillation process is a recovery method based on distillation. It concentrates N,N-dimethylacetamide in wastewater to an extractant phase with low specific heat capacity and low boiling point through extraction, and then recovers the extractant and N,N-dimethylacetamide product through distillation. For this process, the quality of the extractant directly affects the product quality, making it a focus of research for many researchers. For example, patent documents CN 101255122 A, CN 105646271 A, and CN 105645501 A report on recovery processes using chlorine-containing compounds as extractants, with chloroform showing the best extraction performance. Furthermore, patent documents US 2602817 and US 264981 report solvent recovery processes from N,N-dimethylacetamide wastewater using hydrophobic organic solvents such as dibutyl phosphite and nonanol as extractants, respectively. Patent document CN 101921204 B also reports a novel green solvent ionic liquid as an extractant.

[0006] Membrane separation technology is a relatively new separation method that has emerged in recent years. Especially with advancements in membrane production technology and R&D capabilities, many systems can now be separated using membrane separation. However, this process has high requirements for the raw materials being treated; the wastewater solvent cannot contain inorganic salts, impurities, or other fine particles, otherwise it will clog the membrane and cause membrane fouling. Therefore, its applicability to treating solvents is somewhat limited, and it is not well-suited for directly treating N,N-dimethylacetamide wastewater solvents generated in processes such as polymerization spinning and pharmaceutical synthesis.

[0007] Therefore, it is necessary to develop an efficient and simplified process for membrane separation and recovery of N,N-dimethylacetamide wastewater generated in industries such as spinning and pharmaceuticals. Summary of the Invention

[0008] This invention proposes a method for recovering N,N-dimethylacetamide, specifically a recovery process coupled with vapor phase extraction from a distillation column side stream and membrane separation concentration, as well as an operational method for obtaining high-purity N,N-dimethylacetamide using this process. This process is simple, efficient, and yields N,N-dimethylacetamide with high purity and low water content, meeting the requirements of enterprises.

[0009] The objective of this invention can be achieved through the following measures:

[0010] The technical solution of the present invention includes the following steps: a mixture containing N,N-dimethylacetamide and water enters a distillation column, the side vapor phase is collected, and then after being pressurized, it directly enters a membrane separation unit for dehydration to obtain the N,N-dimethylacetamide product.

[0011] In this invention, the pH value of the mixture containing N,N-dimethylacetamide and water is 6-8; the pH value is adjusted by adding an acid or alkali solution. The acid solution is generally hydrochloric acid, and the alkali solution is generally sodium carbonate. Since N,N-dimethylacetamide undergoes slight hydrolysis under acidic or alkaline conditions, especially under acidic conditions where the acetic acid produced further promotes hydrolysis, the pH value is first adjusted to neutral to reduce the hydrolysis of N,N-dimethylacetamide. In this invention, the mass fraction of N,N-dimethylacetamide in the mixture containing N,N-dimethylacetamide and water is between 2% and 30%, preferably between 5% and 30%, and more preferably between 10% and 30%. The mixture containing N,N-dimethylacetamide and water includes wastewater and process water. The process water is a recovered liquid containing organic solvents from the process. The wastewater includes wastewater generated during synthesis processing, polymerization spinning, pharmaceutical processes, and paint production, but is not limited to wastewater generated in the aforementioned industries.

[0012] In this invention, the distillation column is a conventional rectification column with collection outlets at different locations along its length. The treated mixture containing N,N-dimethylacetamide and water is fed into the column from different feed points at the top, depending on the concentration. Then, in the lower middle section of the column, the vapor phase is collected from different locations via side streams, depending on the required concentration and temperature. The product from the top of the column can be used as water in spinning or pharmaceutical processes, while the product from the bottom of the column needs to be further treated in a wastewater treatment plant.

[0013] In this invention, the operating pressure of the distillation column is 0.5 atm to 1.5 atm, preferably 0.5 atm to 1 atm, more preferably 0.75 atm to 1 atm, and the operating reflux ratio is 0.2 to 2, preferably 0.2 to 1, more preferably 0.5 to 1; the top temperature of the distillation column is controlled at 81℃ to 111℃, preferably 81℃ to 100℃, more preferably 92℃ to 100℃.

[0014] In this invention, the mass fraction of N,N-dimethylacetamide collected from the side-stream vapor phase of the distillation column is controlled between 50% and 90%, preferably between 50% and 80%, and more preferably between 50% and 70%. Within this preferred mass fraction range, energy consumption can be reduced. The advantage of using side-stream vapor phase collection in this process is that it not only achieves preliminary concentration and removal of impurities and colored substances, but also the vapor phase collection temperature at the corresponding concentration is within the preferred temperature range of the membrane separation method used in this process. This eliminates the need for the step of superheating the original material to generate pressure, simplifying the original process.

[0015] In this invention, the pressure after liquefaction is controlled between 1.5 atm and 8 atm, preferably between 1.5 atm and 6 atm, and more preferably between 1.5 atm and 4 atm. The temperature of the liquefied liquid is between 120°C and 190°C, preferably between 120°C and 170°C, and more preferably between 120°C and 150°C. Within the above temperature range, the membrane separation component used in this invention has a better operating temperature. Higher temperatures and pressures increase the vapor pressure ratio of water and N,N-dimethylacetamide, thereby increasing the separation efficiency of the membrane separation. However, higher temperatures can also cause some damage to the membrane, therefore, it is necessary to control the process within a certain temperature range.

[0016] In this invention, the membrane separation adopts a pervaporation membrane, and the pressurized liquefied material directly enters the membrane separation unit; in the membrane separation, the gauge pressure on the permeate side is 0.01 kPa to 10 kPa, preferably 0.01 kPa to 5 kPa, and more preferably 0.01 kPa to 1 kPa.

[0017] In this invention, the membrane separation is carried out in a multi-stage series manner for dehydration, with 3 to 8 stages, preferably 4 to 7 stages, and more preferably 5 to 7 stages.

[0018] In this invention, the reflux ratio of the refining tower is 2 to 4, preferably 2.5 to 4, and more preferably 3 to 4.

[0019] In this invention, the operating pressure of the refining column is 0.25 atm to 1 atm, preferably 0.25 atm to 0.75 atm, and even more preferably 0.25 atm to 0.75 atm. The operating temperature at the top of the column is controlled at 143°C to 166°C, and the corresponding fractions are collected.

[0020] In this invention, the method for recovering N,N-dimethylacetamide can be used not only for the recovery of N,N-dimethylacetamide wastewater, but also for the recovery of N,N-dimethylformamide, methylpyrrolidone, and dimethyl sulfoxide solvents. The wastewater can originate from wastewater generated during polymerization spinning, pharmaceutical synthesis, and coating production, but is not limited to wastewater generated in these fields.

[0021] Without violating common sense in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0022] The significant advantages of this invention are as follows: It provides a method for recovering N,N-dimethylacetamide wastewater, employing a coupled technique of side-stream vapor extraction from a distillation column and membrane separation. By controlling process parameters, the method achieves the recovery of N,N-dimethylacetamide. This invention simplifies the process, is more energy-efficient than traditional distillation methods, and yields N,N-dimethylacetamide with a purity greater than 99.9% and a moisture content below 200 ppm, meeting the solvent reuse requirements of enterprises. Attached Figure Description

[0023] Figure 1 Flow chart of N,N-dimethylacetamide wastewater recovery process.

[0024] Where ① represents a distillation column, ② represents a compressor, ③ represents a membrane separation unit, and ④ represents a purification column.

[0025] 1 represents treated wastewater, 2 represents the top product of the distillation column, 3 represents the bottom product of the distillation column, 4 represents the vapor phase product from the side stream of the distillation column, 5 represents the liquid phase product after being compressed to a certain pressure by the compressor, 6 represents the permeate, 7 represents the N,N-dimethylacetamide concentrate, 8 represents the residue, and 9 represents the high-purity N,N-dimethylacetamide product.

[0026] The specific process is as follows: The treated wastewater stream 1 enters the distillation tower from a certain inlet according to its concentration. The top stream 2 discharges the light component water, and the bottom stream 3 contains the heavy impurity component. The concentrated product is collected by the vapor stream 4 and liquefied by the compressor, becoming a pressurized liquid stream 5 that enters the membrane separation unit. The liquid stream 6 that permeates from the permeate side is vaporized and carried away under negative pressure. Finally, the concentrated product stream 7 is obtained in the last stage of the multi-stage membrane separation and then enters the purification tower. The top of the tower contains the product stream 9, and the bottom contains some heavy component streams 8. Detailed Implementation

[0027] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0028] Example 1

[0029] A batch of wastewater solvent containing 2% N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 125°C, containing 55% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 150°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A four-stage membrane separation dehydration process was used, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 2, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.92%, and the moisture content was 175 ppm, which meets the requirements for solvent reuse.

[0030] Example 2

[0031] A batch of wastewater solvent containing 5% N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 122°C, containing 50% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 149°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 2.5, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 159 ppm, which meets the requirements for solvent reuse.

[0032] Example 3

[0033] A batch of wastewater solvent containing 10% N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A three-stage membrane separation dehydration process was used, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.92%, and the moisture content was 172 ppm, which meets the requirements for solvent reuse.

[0034] Example 4

[0035] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 135°C, containing 70% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 155°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. An 8-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 4, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.97%, and the moisture content was 108 ppm, which meets the requirements for solvent reuse.

[0036] Example 5

[0037] A batch of wastewater solvent containing 30% N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 143°C, containing 80% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 161°C, the vapor stream directly entered a membrane separation unit for further dehydration and concentration. A 7-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.95%, and the moisture content was 121 ppm, which meets the requirements for solvent reuse.

[0038] Example 6

[0039] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with hydrochloric acid. The operating pressure of the distillation column was 0.75 atm, the reflux ratio was 0.2, and the top temperature was controlled at 92°C. The concentrated product was collected via a side stream vapor stream at a temperature of 152°C, containing 90% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 173°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 128 ppm, which meets the requirements for solvent reuse.

[0040] Example 7

[0041] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1.5 atm, the reflux ratio was 0.5, and the top temperature was controlled at 111°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 131 ppm, which meets the requirements for solvent reuse.

[0042] Example 8

[0043] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 0.5 atm, the reflux ratio was 1, and the top temperature was controlled at 81°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A six-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.95%, and the moisture content was 125 ppm, which meets the requirements for solvent reuse.

[0044] Example 9

[0045] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 1.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C, the vapor stream directly entered a membrane separation unit for further dehydration and concentration. A six-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.95%, and the moisture content was 129 ppm, which meets the requirements for solvent reuse.

[0046] Example 10

[0047] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 2, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.5 atm, the reflux ratio was set to 3, and the top temperature was controlled at 141°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 151 ppm, which meets the requirements for solvent reuse.

[0048] Example 11

[0049] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 122°C, containing 50% N,N-dimethylacetamide by mass. After being pressurized to 1.5 atm and cooled to 120°C, the vapor stream directly entered a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 154 ppm, which meets the requirements for solvent reuse.

[0050] Example 12

[0051] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 2 atm by a compressor, while maintaining the temperature at 128°C, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.75 atm, the reflux ratio was set to 3, and the top temperature was controlled at 156°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.94%, and the moisture content was 147 ppm, which meets the requirements for solvent reuse.

[0052] Example 13

[0053] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 6 atm and heated to 167°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A six-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 1 atm, the reflux ratio was set to 3, and the top temperature was controlled at 166°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.95%, and the moisture content was 141 ppm, which meets the requirements for solvent reuse.

[0054] Example 14

[0055] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 143°C, containing 80% N,N-dimethylacetamide by mass. After being pressurized to 8 atm and heated to 190°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.93%, and the moisture content was 162 ppm, which meets the requirements for solvent reuse.

[0056] Example 15

[0057] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C, the vapor stream directly entered a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 0.01 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.97%, and the moisture content was 113 ppm, which meets the requirements for solvent reuse.

[0058] Example 16

[0059] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 0.1 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.96%, and the moisture content was 122 ppm, which meets the requirements for solvent reuse.

[0060] Example 17

[0061] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 5 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.92%, and the moisture content was 166 ppm, which meets the requirements for solvent reuse.

[0062] Example 18

[0063] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by distillation in a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C by a compressor, the product was directly fed into a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 10 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.90%, and the moisture content was 188 ppm, which meets the requirements for solvent reuse.

[0064] Example 19

[0065] A batch of wastewater solvent containing 20% ​​N,N-dimethylacetamide was first concentrated and impurities removed by a distillation column after the pH was adjusted to between 6 and 8 with sodium carbonate. The operating pressure of the distillation column was 1 atm, the reflux ratio was 0.5, and the top temperature was controlled at 100°C. The concentrated product was collected via a side stream vapor stream at a temperature of 128°C, containing 60% N,N-dimethylacetamide by mass. After being pressurized to 4 atm and heated to 152°C, the vapor stream directly entered a membrane separation unit for further dehydration and concentration. A five-stage membrane separation process was used for dehydration, with the permeate-side pressure controlled at 0.5 kPa. The resulting concentrate was then sent to a purification column for further dehydration. The operating pressure of the purification column was 0.25 atm, the reflux ratio was set to 3, and the top temperature was controlled at 120°C. The corresponding fractions were collected. Finally, high-purity N,N-dimethylacetamide was obtained at the top of the column. The concentration of N,N-dimethylacetamide was determined to be 99.95%, and the moisture content was 130 ppm, which meets the requirements for solvent reuse.

[0066] In this invention, the purity of the products in all embodiments was obtained by HPLC detection, and the moisture content was determined by a moisture analyzer.

[0067] It should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the relevant conditions of the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for recovering N,N-dimethylacetamide, characterized in that, The method involves feeding a mixture containing N,N-dimethylacetamide and water into a distillation column, collecting the side vapor phase, then pressurizing and liquefying it. The liquefied material is then directly fed into a membrane separation unit for dehydration to obtain the N,N-dimethylacetamide product. The mass fraction of N,N-dimethylacetamide in the mixture is 2% to 30%. The pH value of the mixture is 6 to 8; The mixture containing N,N-dimethylacetamide and water enters from the top of the distillation column and is vaporized from different side stream positions in the middle and lower parts of the distillation column, controlling the mass fraction of N,N-dimethylacetamide in the vapor phase to be 50%~90%. The operating pressure of the distillation column is 0.5 atm to 1.5 atm, and the operating reflux ratio is 0.2 to 2; the top temperature of the distillation column is controlled at 81ºC to 111ºC. The vapor-phase pressurized liquefaction process is performed, with the liquefied pressure controlled at 1.5 atm to 8 atm and the temperature at 120 ºC to 190 ºC. The membrane separation employs a pervaporation membrane; in the membrane separation, the gauge pressure on the permeate side is 0.01 kPa ~ 10 kPa.

2. The recycling method as described in claim 1, characterized in that, The membrane separation process employs a multi-stage series dehydration method, with 3 to 8 stages.

3. The recycling method as described in claim 1, characterized in that, The dehydration is carried out in a refining tower with a reflux ratio of 2 to 4.

4. The recycling method as described in claim 3, characterized in that, The refining column operates at a pressure of 0.25 atm to 1 atm, and the top operating temperature is controlled at 143°C to 166°C, collecting the corresponding fractions.

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