A method for recovering DMAC solvent from waste liquid produced by meta-aramid production

Abstract

The invention relates to a method for recovering DMAC solvent in waste liquid from meta-aramid production, belongs to the technical field of waste liquid treatment, and solves one of the problems of environmental pollution and high energy consumption caused by the DMAC solvent recovery process in the waste liquid from meta-aramid production in the prior art. The invention discloses a method for recovering DMAC solvent in waste liquid from meta-aramid production, firstly removing oligomers and mechanical impurities in the waste liquid by a flocculation precipitation process; then removing most of the water in the waste liquid by an azeotropic distillation process; finally separating the residual entrainer and the DMAC solvent by a distillation process to obtain high-purity DMAC. The above-mentioned recovery method does not introduce an additional highly toxic extractant, selects an environmentally friendly entrainer to separate DMAC from water, and has low energy consumption and good separation effect. The DMAC recovery rate can reach 98%, and the purity is ≥99%, which is suitable for large-scale promotion and wide application.

Description

Technical Field

[0001] The invention relates to the technical field of waste liquid treatment, in particular to a method for recovering a DMAC solvent in waste liquid produced by meta-aramid production. Background Art

[0002] N,N-dimethylacetamide, abbreviated as DMAC, is a highly polar colorless transparent liquid that can dissolve a variety of compounds and is completely miscible with water, ether, ketones, esters, etc. It has the characteristics of high thermal stability, not easy to hydrolyze, low corrosiveness, and low toxicity. It is often used as a raw material for synthetic fibers and an excellent polar solvent for organic synthesis.

[0003] In the process of fiber forming, a large amount of waste liquid containing 20% ​​to 40% (mass fraction) of dimethylacetamide is generated, of which 0.1% to 2% oligomers and the rest is water; the above waste liquid is produced by the aramid production process without alkaline earth metal salts. DMAC is used in large quantities in the aramid spinning industry and has a high market price. It is also toxic. When discharged into water during production, use and disposal, it will have a great impact on the water quality environment, endanger people's health, and waste the organic raw material DMAC. Therefore, the recovery and treatment of DMAC waste liquid in aramid spinning production has important significance in protecting the environment and reducing the production cost of enterprises.

[0004] At present, the methods for recovering DMAC wastewater in meta-aramid spinning production mainly include extraction distillation and evaporation distillation. Solution extraction is an operation method that uses the principle of different solubility of solutes in immiscible solvents to transfer solutes from one solvent to another. The National Key Laboratory of Tsinghua University once simulated chloroform as an extractant to treat DMAC wastewater. It was found that after the residual liquid was stripped and recovered, the concentration of DMAC in the final discharged wastewater could be reduced to 180 mg / kg. The advantages of this method are low energy consumption and high recovery rate for low-concentration DMAC. The disadvantage is that the extractant is introduced, and the loss of the extractant is very likely to cause secondary pollution to the environment. Evaporation distillation is to recover dimethylacetamide from high-concentration wastewater, but if the wastewater containing medium and low concentrations of DMAC is treated by evaporation distillation, its energy consumption is huge, and if it is directly discharged, it will pollute the environment and cause waste of resources.

[0005] Therefore, it is necessary to develop a high recovery rate, low energy consumption, environmentally friendly method for recovering DMAC solvent in salt-free meta-aramid production wastewater. Summary of the invention

[0006] In view of the above analysis, the embodiment of the present invention aims to provide a method for recovering DMAC solvent in meta-aramid production waste liquid, so as to solve at least one of the problems of environmental pollution and high energy consumption caused by the DMAC solvent recovery process in the prior art of meta-aramid production waste liquid.

[0007] The present invention provides a method for recovering DMAC solvent in meta-aramid production waste liquid, which specifically comprises the following steps:

[0008] S1: Add flocculant to the waste liquid, let it stand, filter it, and remove oligomers and mechanical impurities in the waste liquid;

[0009] S2: adding an azeotropic agent to the filtrate obtained in S1, and conveying it to a primary azeotropic distillation tower for azeotropic distillation, after the overhead distillate F1 is separated by an oil-water separator, the organic phase is refluxed to the primary azeotropic distillation tower, and the water phase is conveyed to a secondary azeotropic distillation tower for secondary azeotropic distillation;

[0010] S3: The bottom effluent D1 of the primary azeotropic distillation tower in S2 is transported to a distillation tower for distillation. The top distillate is the azeotropic agent, which can be recovered and recycled; the bottom effluent is high-concentration DMAC, which realizes the recovery of DMAC.

[0011] Specifically, the composition of the meta-aramid production wastewater is DMAC: 20-40% by mass, oligomers: 0.1%-2% by mass, and the rest is water.

[0012] Specifically, before step S1, an alkaline compound needs to be added to adjust the pH value of the waste liquid to be neutral or weakly alkaline.

[0013] Specifically, the alkaline compound is an alkaline earth metal hydroxide.

[0014] Specifically, the standing time in step S1 is ≥ 10 min.

[0015] Specifically, the flocculant is one of aluminum sulfate, potassium aluminum sulfate, aluminum chloride, ferrous sulfate, polyferric sulfate, and ferric chloride.

[0016] Preferably, the azeotropic agent is isobutanol.

[0017] Specifically, the mass ratio of the azeotropic agent to the filtrate is 0.1 to 2:1.

[0018] Specifically, the operating parameters of the first-stage azeotropic distillation tower are: a reflux ratio of 1 to 7, normal pressure, a top temperature of 89 to 95°C, a bottom temperature of 100 to 115°C, and a number of plates of 8 to 18; the reflux ratio of the second-stage azeotropic distillation tower is 0.2 to 1, normal pressure, a top temperature of 89 to 95°C, a bottom temperature of 90 to 115°C, and a number of plates of 8 to 18.

[0019] Specifically, the operating parameters of the distillation tower are: top temperature of 50-80°C, bottom temperature of 105°C-120°C, vacuum degree of 0.06-0.09Mpa, number of plates of 15-25, and reflux ratio of 2-10.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] 1. The recovery method provided by the present invention has high recovery efficiency and good product purity. The present invention first uses a flocculant to act on the oligomers in the wastewater, so that the oligomers flocculate and precipitate, and then filters and separates the oligomers and mechanical impurities, preventing the oligomers and mechanical impurities from precipitating, accumulating and clogging the equipment during further distillation of the waste liquid, which can greatly improve the distillation effect; it also prevents the oligomers from entering the final distillation tower kettle, resulting in a large amount of tower kettle residue that needs to be processed in the end, and the cost is greatly increased. An entrainer is introduced into the system to reduce the boiling point of the material. At the same time, only water and the entrainer in the material to be processed need to be distilled out of the tower according to the azeotropic ratio, and all materials do not need to reach the boiling point for distillation, which directly reduces energy consumption and improves processing efficiency. Finally, the distillation stage is mainly the separation of alcohol and organic solvent. The boiling points of the two materials are very different, and the separation is easy, the separation effect is good, and the product purity is high.

[0022] The DMAC recovery rate of the invention can reach 98%, the detection purity is ≥99.0%, the contents of water and alcohol are respectively less than 200 ppm, the chromaticity is ≤3°, the acetic acid content is ≤500 ppm, and the dimethylamine is ≤500 ppm.

[0023] 2. The recovery method provided by the present invention does not introduce an extractant, and the entrainer used has low toxicity and can be recycled, with a low risk of secondary pollution and is environmentally friendly.

[0024] Among the extraction and distillation routes that are currently more commonly used, chloroform with good extraction effect is often used. However, chloroform is harmful to both the human body and the environment. First, it is highly toxic and has certain teratogenic and mutagenic effects on humans and animals. It is a substance on the list of Class 2B carcinogens and a suspected human carcinogen. Secondly, it is a substance on the list of toxic and harmful water pollutants. It has an accumulation effect in groundwater, which will pollute drinking water, food and vegetables, and damage the respiratory system of sensitive aquatic organisms. It is also easy to make poisonous chemicals. In the residual phase of the extraction operation, there will be 100 to 5000 ppm of chloroform residues, which is harmful to humans or causes secondary pollution to the environment. The entrainer used in the present invention, such as isobutanol, has very low toxicity and can even be used as an edible flavoring. At the same time, it is relatively easy to be biodegraded.

[0025] In the evaporation-distillation process in the prior art, the wastewater needs to be completely evaporated to separate the high-boiling-point substances (oligomers, salts and other impurities) and low-boiling-point substances (mainly water and solvents) dissolved in the wastewater. Because the boiling point of the solvent is high, and the latent heat of evaporation of water is large and the content is also high, the evaporation time is long, the energy consumption is large, and the material will also decompose into small molecular acetic acid and dimethylamine after a long period of heating, which reduces the quality and yield of the product.

[0026] The entrainer used in the present invention is mostly alcohols containing 3 to 6 carbon atoms, and a few benzenes and ethylene dichloride. The boiling point is between 80 and 140 degrees, and the stability is good under normal conditions. The azeotropic point of the entrainer with water is lower than the boiling point of water, about 70 to 95 degrees, and the azeotropic ratio is high (the water content is 15 to 50%). It is not necessary to use evaporation during operation, and only the water or alcohol in the mixture needs to be taken out by azeotropy. At the same time, after azeotropy with water, the entrainer itself has low intermiscibility with water, which is conducive to subsequent separation and reuse (such as simple separation by an oil-water separator), and the entrainer is continuously recycled, thereby reducing energy consumption and improving the recovery rate.

[0027] Taking the above aspects into consideration, the azeotropic agent used in the present invention has a high azeotropic ratio, a low azeotropic point, and relatively low energy consumption. In addition, the azeotropic agent has low solubility in water at 40 to 60 degrees, can be quickly separated by stratification, and has a good separation effect. The toxicity is low, and isobutanol with low environmental impact is preferred.

[0028] 3. The recovery method provided by the present invention has reasonable process settings and significantly reduces energy consumption.

[0029] In the process of the present invention, a flocculant is first used to flocculate and filter the oligomers in the waste liquid to separate the oligomers, thereby preventing the oligomers from clogging in the recovery equipment, such as the filling material of the distillation tower, and improving the azeotropic separation effect and separation efficiency. In addition, the cost of processing the final distillation residue is reduced. If the oligomers are left in the final distillation tower as the bottom residue, the amount of the bottom residue will be increased many times. These aspects can significantly reduce the operating energy consumption and cost.

[0030] After the oligomers are removed, the present invention takes out the simple materials for recycling, and the first stage of azeotropic separation of solvent and salt is extracted from the bottom of the tower; the top of the tower is separated by an oil-water separator, and the entrainer is recovered, and the materials containing a small amount of entrainer are further separated from water and the entrainer by one more stage of simple azeotropy, and then the entrainer is recovered; after rectification, a high-purity solvent product is obtained, and the entrainer is recovered at the same time. The whole process is efficient, reasonable and energy-saving.

[0031] 4. The relevant reagents and devices / equipment of the recovery method provided by the present invention are easily available and can be combined or appropriately modified on the existing basis to meet the implementation requirements, which is conducive to large-scale production / processing and wide promotion.

[0032] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0034] Figure 1 The present invention is a process flow chart of a method for recovering DMAC solvent from waste liquid in meta-aramid production. DETAILED DESCRIPTION

[0035] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0036] The present invention provides a method for recovering DMAC solvent in meta-aramid production waste liquid, which specifically comprises the following steps:

[0037] S1: Add flocculant to the waste liquid, let it stand, filter it, and remove oligomers and mechanical impurities in the waste liquid;

[0038] S2: adding an azeotropic agent to the filtrate obtained in S1, and conveying it to a primary azeotropic distillation tower for azeotropic distillation, after the overhead distillate F1 is separated by an oil-water separator, the organic phase is refluxed to the primary azeotropic distillation tower, and the water phase is conveyed to a secondary azeotropic distillation tower for secondary azeotropic distillation;

[0039] S3: The bottom effluent D1 of the primary azeotropic distillation tower in S2 is transported to a distillation tower for distillation. The top distillate is the azeotropic agent, which can be recovered and recycled; the bottom effluent is high-concentration DMAC, which realizes the recovery of DMAC.

[0040] Specifically, the oligomers and mechanical impurities in the waste liquid may affect the stability of the azeotropic structure, and may block the gaps between the tower plates in the subsequent azeotropic distillation / rectification process, so they need to be removed in advance. Process section S1 adds a flocculant to flocculate / precipitate the oligomers, and removes the waste solid oligomers / filter residues (mechanical impurities) through a filtration process.

[0041] Specifically, the main components of the bottom effluent D1 obtained in S2 are the entrainer, the organic solvent DMAC and a trace amount of water.

[0042] Specifically, the composition of the meta-aramid production wastewater is DMAC: 20-40% by mass, oligomers: 0.1%-2% by mass, and the rest is water.

[0043] Specifically, an alkaline compound needs to be added before step S1 to adjust the pH value of the waste liquid to be neutral or weakly alkaline. During the synthesis of aramid polymers, an acidic byproduct, hydrogen chloride, is produced. In the subsequent treatment, some are neutralized with alkali, while others are not neutralized or not completely neutralized. Therefore, the waste liquid generated in the production will be neutral or acidic. In order to reduce the impact (corrosion) on equipment or processes, these waste liquids need to be treated with alkaline substances before treatment to make the waste liquid neutral or weakly alkaline.

[0044] Specifically, the alkaline compound is an alkaline earth metal hydroxide.

[0045] Specifically, the standing time in step S1 is ≥10 min to ensure that the oligomers and mechanical impurities in the waste liquid are fully aggregated and settled.

[0046] Specifically, the flocculant is one of aluminum sulfate, potassium aluminum sulfate, aluminum chloride, ferrous sulfate, polyferric sulfate, and ferric chloride. The amount of flocculant added is 0.01% to 0.1% of the total mass of the waste liquid, preferably 0.01 to 0.02%.

[0047] The above flocculant has a good flocculation effect on this waste liquid system. After the waste liquid is neutralized and the pH is adjusted, more oligomers are precipitated from the solution to form very small suspended particles. The added flocculant quickly disperses in the solution and adsorbs or interacts with these suspended particles, so that these suspended particles can quickly aggregate into large particles or floccules. These floccules also have good filterability and can stay on the filter cake to achieve a good separation effect. When the amount of flocculant added is less than 0.01%, the flocculation time is long, the flocculent particles obtained are not large, and suspended matter is still found in the filtrate after filtration; if the amount of flocculant added is greater than 0.1%, the flocculation effect is somewhat reduced.

[0048] Exemplarily, the flocculation process can be carried out in a stirring tank, preferably a corrosion-resistant stainless steel stirring tank or a PP plastic stirring tank, with a stirring speed of 50 to 150 rpm.

[0049] Specifically, in process section S2, an entrainer is added to mix with waste water, and the heating temperature rises to the azeotropic point. The steam formed by the azeotropic ratio of the entrainer and water comes out from the top of the tower, and then is cooled into a liquid in the condenser. The liquid enters the stratifier. Since the entrainer and water are not miscible (the azeotropic structure automatically collapses under low temperature conditions), the stratification is performed in the stratifier; the upper layer is an entrainer containing a small amount of water, and the lower layer is water containing a small amount of entrainer. The upper layer can be refluxed into the tower to enter azeotropy with the water in the system again and then take water out, and the lower layer of water can be discharged after further simple azeotropy, and the simple azeotropic entrainer in the secondary azeotropic distillation tower can also be recycled and reused. Compared with the extraction-distillation process, the present invention does not use an extractant that is highly toxic and harmful to humans and the environment. Compared with the evaporation-distillation process, the present invention does not require all the wastewater to be evaporated, and only requires the entrainer to take the water out of the tower at a lower azeotropic point. The material operation time is short, thereby reducing energy consumption and improving product quality.

[0050] Specifically, the azeotropic agent is one or more of isobutyl alcohol, n-butyl alcohol, isopentanol, n-pentanol, xylene, and ethylene dichloride.

[0051] Preferably, the azeotropic agent is isobutanol.

[0052] The selected entrainer has a moderate boiling point, stable physical properties within the operating temperature range, and less decomposition over a long period of time; the material has low toxicity and is less harmful to people and the environment; and at the same time, the azeotropic point is low, the water content in the azeotropic composition is high, the mutual solubility of the entrainer and water is not high, and it is easy to separate. Among them, isobutanol has low toxicity, a stable azeotropic system and a low boiling point (89.9℃), and is the best choice considering the separation effect and economic benefits.

[0053] In actual implementation, if multiple entrainers are mixed and used, it may cause the azeotropic system to be unstable or the boiling point of the azeotrope to fluctuate, which is not conducive to stable operation and separation and recovery effects. It is preferred to use one entrainer for azeotropic distillation.

[0054] Specifically, the mass ratio of the entrainer to the filtrate is 0.1 to 2: 1. Too low an entrainer to waste liquid ratio cannot fully azeotropize with the water in the waste liquid and take the water out of the tower top for separation; too high a ratio will increase the burden on the tower and increase energy consumption.

[0055] Specifically, the operating parameters of the first-stage azeotropic distillation tower are: a reflux ratio of 1 to 7, normal pressure, a top temperature of 89 to 95°C, a bottom temperature of 100 to 115°C, and a number of plates of 8 to 18; the reflux ratio of the second-stage azeotropic distillation tower is 0.2 to 1, normal pressure, a top temperature of 89 to 95°C, a bottom temperature of 90 to 115°C, and a number of plates of 8 to 18.

[0056] The selection of the number of plates and the reflux ratio is mainly based on whether the tower can meet the material separation effect. Secondly, after meeting the material separation effect, energy consumption should be reduced as much as possible. At the same time, the investment in equipment should not be too high.

[0057] The mixed gas (tower top distillate D1) azeotropically distilled from the top of the primary azeotropic distillation tower is liquefied into a liquid state after passing through a condenser and enters an oil-water separator. Since the entrainer and water are immiscible (the azeotropic system automatically disintegrates under low temperature conditions), the mixture is phase-separated into two liquid layers in the oil-water separator. The aqueous solution layer with a trace of entrainer and the entrainer layer containing a small amount of water, the entrainer layer is refluxed to the primary azeotropic distillation tower for recycling. The aqueous solution layer with a trace of entrainer is extracted and enters the secondary azeotropic distillation tower, and is further separated by azeotropic distillation. The secondary azeotropic distillation tower top distillate D2 enters after condensation and liquefaction, and the water layer of the oil-water separator is refluxed, and then discharged as waste liquid (tower bottom discharge liquid F2) in the tower bottom. The entrainer can be extracted to the entrainer storage tank for recycling / or directly refluxed to the primary azeotropic distillation tower. The recycling of the entrainer reduces the production / processing cost and reduces the discharge of waste liquid.

[0058] Specifically, the operating parameters of the distillation tower are: the top temperature is 50-80°C, the bottom temperature is 105-120°C, the vacuum degree is 0.06-0.09Mpa, the number of plates is 15-25, and the reflux ratio is 2-10. The distillation tower is the main equipment for separating the entrainer and the solvent, and has high requirements; the plates need to be more than 15 to meet the separation requirements, and the reflux is relatively small when there are more than 15 or 20 plates. Negative pressure operation is used to reduce energy consumption.

[0059] Example 1

[0060] A method for recovering DMAC solvent in meta-aramid spinning waste liquid specifically comprises the following steps:

[0061] Step 1: Add 1200kg of aramid waste liquid into a stirring tank, the waste liquid contains 23.6% DMAC, 75.9% water, and 0.45% oligomers, and the stirring speed is 80 rpm. First, add a small amount of Ca(OH)2 to the aramid spinning waste liquid in the stirring tank to adjust the pH value to neutral, then add 0.015% aluminum chloride and flocculants to concentrate the oligomers in the waste liquid together, let it stand for 30 minutes, filter out the oligomers through a filter, and obtain a mixed solvent of DMAC and water.

[0062] Step 2: Add a certain amount of azeotropic agent isobutanol to the mixed solution of DMAC and water according to the ratio, the mass ratio of isobutanol to the mixed solution of DMAC and water is 0.5:1, and after isobutanol and water are mixed, they enter the azeotropic distillation tower 1 (primary azeotropic distillation tower), and isobutanol and water form an azeotrope, and the mixed gas of isobutanol and water comes out from the top of the tower. Isobutanol and DMAC containing trace water are obtained at the bottom of the tower. The top temperature of the azeotropic distillation tower bottom 1 (primary azeotropic distillation tower) is set at 89.8°C, the bottom temperature is set at 111.9°C, the pressure is set to normal pressure, the number of plates is 15, and the reflux ratio of the azeotropic distillation tower is 4.

[0063] Step 3: In step 2, the mixed gas formed by the azeotropic azeotropy at the top of the tower is transported to the inside of the condenser 1 through the gas phase pipe, and condensed and liquefied, so that the mixed gas of isobutanol and water is recondensed into a liquid state, and then enters the oil-water sharing device 1. Because the solubility between isobutanol and water is not large, and the density of isobutanol is relatively small, the solution is divided into an upper layer of isobutanol with a trace of water and a lower layer of an aqueous solution with a trace of isobutanol through the oil-water separator 1. Most of the upper isobutanol flows back to the azeotropic distillation tower 1, and the lower layer of the aqueous solution with a trace of isobutanol enters the next azeotropic distillation tower 2, and the isobutanol is further separated by azeotropy. The lower layer of water of the oil-water separator 2 flows back to the tower reactor of the azeotropic distillation tower 2 (secondary azeotropic distillation tower) and is directly discharged from the bottom of the tower reactor. The upper isobutanol enters the azeotropic distillation tower 1 as an entrainer for recycling. The top temperature of the azeotropic distillation tower bottom 2 was set at 89.8°C, the bottom temperature was set at 99.9°C, the tower pressure was atmospheric pressure, the number of plates was 9, and the reflux ratio was 0.4.

[0064] Step 4: The mixed liquid of DMAC and isobutanol in the bottom of the azeotropic distillation tower 1 in step 2 is further distilled and separated, and the mixed gas of alcohol and water at the top of the tower is liquefied through the condenser 3 and enters the storage tank, and then is extracted and recycled into the batching tank. The DMAC at the bottom of the tower is cooled by the condenser 4 to obtain high-purity DMAC. The top temperature of the distillation tower is 61.7°C, the bottom temperature is 112.9°C, the vacuum degree is 0.08Mpa, the reflux ratio is 6, and the number of plates is 15.

[0065] Step 5: The dimethylacetamide after liquefaction and separation is transported to the interior of a storage tank for storage in a vacuum environment.

[0066] The recovery rate of DMAC is 98%, the detection purity is ≧99.95%, the contents of water and alcohol are respectively <200ppm, the chromaticity is ≤3°, the acetic acid content is ≤300ppm, and the dimethylamine is ≤300ppm.

[0067] Example 2

[0068] Step 1: Add 1500kg of aramid waste liquid into a stirring tank at a stirring speed of 80 rpm. The waste liquid contains 29.8% DMAC, 70% water, and 0.5% oligomers. First, add a small amount of Ca(OH)2 to the aramid spinning waste liquid in the stirring tank until the pH meter test is neutral, then add 0.015% (about 0.225kg) of aluminum chloride, and the flocculant concentrates the oligomers in the waste liquid in one place. After standing for 30 minutes, about 12.5kg of oligomers (containing about 50% liquid) are filtered out through a filter. The filtrate is clarified to obtain a mixed solvent of DMAC and water of about 1493kg, of which DMAC is 30% and water is 70%.

[0069] Step 2: Add the azeotropic agent isobutanol to the filtrate solution to be treated at a mass ratio of 0.4:1, mix thoroughly, and then enter the azeotropic distillation tower 1 (primary azeotropic distillation tower), and the entry amount is 140kg / hour. Isobutanol and water in the tower form an azeotrope, and the top distillate material is a mixed gas of isobutanol and water. The main components of the bottom material are isobutanol and DMAC, wherein the isobutanol content is 55.9%, the DMAC content is 41.94%, the water content is 2.13%, and the flow rate is 71.52kg. The top temperature of the azeotropic distillation tower reactor 1 (primary azeotropic distillation tower) is set at 89.8°C, the bottom temperature is set at 112.0°C, the pressure is normal, the number of plates is 15, and the reflux ratio of the azeotropic distillation tower is 3.1.

[0070] Step 3: In step 2, the mixed gas formed by the azeotropic reaction at the top of the tower is transported to the inside of the condenser 1 through the gas phase pipe for condensation and liquefaction treatment, so that the mixed gas of isobutanol and water is recondensed into a liquid state. Since the solubility of isobutanol and water is not large and the density of isobutanol is small, the solution is separated into an upper layer of isobutanol with a trace amount of water and a lower layer of aqueous solution with a trace amount of isobutanol through the oil-water separator 1. Most of the upper isobutanol is refluxed to the azeotropic distillation tower 1, wherein the isobutanol content is 82.2%, the DMAC content is trace, the water content is 17.78%, and the flow rate is 214.7kg; the lower aqueous solution with trace isobutanol enters the next azeotropic distillation tower 2, wherein the isobutanol content is 5.3%, the DMAC content is trace, the water content is 94.6%, and the flow rate is 73.2kg; the isobutanol is further separated by azeotropy, and the lower water of the oil-water separator 2 is refluxed to the bottom of the azeotropic distillation tower 2 (secondary azeotropic distillation tower), wherein the isobutanol content is 5.3%, the DMAC content is trace, the water content is 94.6%, and the flow rate is 73.2kg. AC content is trace, water content is 94.6%, and the flow rate is 0.87kg; the material separated from the bottom of the tower is discharged, wherein the isobutanol content is trace, several ppm to dozens of ppm, the DMAC content is trace, several ppm to dozens of ppm, the water content is 99.99%, and the flow rate is 68.48kg; the upper material of the oil-water separator 2 is rich in isobutanol and is used as an azeotropic agent to enter the azeotropic distillation tower 1 for recycling, wherein the isobutanol content is 82.2%, the DMAC content is trace, the water content is 17.78%, and the flow rate is 4.72kg. The top temperature of the azeotropic distillation tower 2 is set at 89.8℃, the tower bottom temperature is set at 99.9℃, the tower pressure is atmospheric pressure, the number of plates is 10, and the reflux ratio is 0.3.

[0071] Step 4: The DMAC-rich material discharged from the bottom of the azeotropic distillation tower 1 in step 2 enters the next distillation tower for distillation to share DMAC and isobutanol. The mixed gas of rich alcohol and a small amount of water at the top of the tower is liquefied through the condenser 3 and enters the storage tank, part of which is refluxed and part of which is extracted and recycled into the batching tank. The isobutanol content is 96.2%, the DMAC content is trace, the water content is 3.6%, and the flow rate is 43.4kg. The DMAC gas phase near the bottom of the tower is extracted and cooled by the condenser 4 to obtain high-purity DMAC, with a flow rate of 29.5kg. The top temperature of the distillation tower is 61.7-61.9℃, the bottom temperature is 112-115℃, the vacuum degree is 0.08Mpa, the reflux ratio is 5, and the number of plates is 20.

[0072] Step 5: The dimethylacetamide after liquefaction and separation is transported to the interior of a storage tank for storage in a vacuum environment.

[0073] The recovery rate of DMAC is 98.3%, the detection purity is ≧99.99%, water <150ppm and alcohol <200ppm, chromaticity ≤2.6°, acetic acid content ≤300ppm, dimethylamine ≤300ppm.

[0074] Example 3

[0075] Step 1: Add 1500kg of aramid waste liquid into a stirring tank at a stirring speed of 80 rpm. The waste liquid contains 34.5% DMAC, 64.7% water, and 0.8% oligomers. First, add a small amount of Ca(OH)2 to the aramid spinning waste liquid in the stirring tank until the pH meter test is neutral, then add 0.015% (about 0.225kg) of aluminum chloride and a flocculant to concentrate the oligomers in the waste liquid. After standing for 30 minutes, filter out about 21.6kg of oligomers (containing about 50% liquid) through a filter, and clarify the filtrate to obtain a mixed solvent of DMAC and water of about 1490kg, including 34.8% DMAC, 65% water, and trace salt.

[0076] Step 2: Add the azeotropic agent isobutanol to the filtrate solution to be treated at a mass ratio of 0.4:1, mix thoroughly, and then enter the azeotropic distillation tower 1 (primary azeotropic distillation tower), and the entry amount is 280kg / hour. Isobutanol and water in the tower form an azeotrope, and the top distillate material is a mixed gas of isobutanol and water. The main components of the bottom material are isobutanol and DMAC, wherein the isobutanol content is 50.23%, the DMAC content is 43.946%, the water content is 1.62%, and the flow rate is 152.29kg. The top temperature of the azeotropic distillation tower reactor 1 (primary azeotropic distillation tower) is set at 89.8°C, the bottom temperature is set at 115.0°C, the pressure is normal, the number of plates is 15, and the reflux ratio of the azeotropic distillation tower is 3.05.

[0077] Step 3: In step 2, the mixed gas formed by the azeotropic reaction at the top of the tower is transported to the inside of the condenser 1 through the gas phase pipe for condensation and liquefaction treatment, so that the mixed gas of isobutanol and water is recondensed into a liquid state. Since the solubility of isobutanol and water is not large and the density of isobutanol is small, the solution is separated into an upper layer of isobutanol with a trace amount of water and a lower layer of aqueous solution with a trace amount of isobutanol through the oil-water separator 1. Most of the upper isobutanol is refluxed to the azeotropic distillation tower 1, wherein the isobutanol content is 82.2%, the DMAC content is trace, the water content is 17.79%, and the flow rate is 412.1kg; the lower aqueous solution with trace isobutanol enters the next azeotropic distillation tower 2, wherein the isobutanol content is 5.3%, the DMAC content is trace, the water content is 94.6%, and the flow rate is 135.8kg; the isobutanol is further separated by azeotropy, and the lower water of the oil-water separator 2 is refluxed to the bottom of the azeotropic distillation tower 2 (secondary azeotropic distillation tower), wherein the isobutanol content is 5.3%, the DMAC content is trace, the water content is 94.6%, and the flow rate is 135.8kg. AC content is trace, water content is 94.6%, and the flow rate is 1.46kg; the material separated from the bottom of the tower is discharged, wherein the isobutanol content is trace, several ppm to dozens of ppm, the DMAC content is trace, several ppm to dozens of ppm, the water content is 99.99%, and the flow rate is 126.91kg; the upper material of the oil-water separator 2 is rich in isobutanol and is used as an azeotropic agent to enter the azeotropic distillation tower 1 for recycling, wherein the isobutanol content is 82.2%, the DMAC content is trace, the water content is 17.79%, and the flow rate is 8.898kg. The top temperature of the azeotropic distillation tower 2 is set at 89.8℃, the tower bottom temperature is set at 99.9℃, the tower pressure is normal pressure, the number of plates is 10, and the reflux ratio is 0.4.

[0078] Step 4: The DMAC-rich material discharged from the bottom of the azeotropic distillation tower 1 in step 2 enters the next distillation tower for distillation to share DMAC and isobutanol. The mixed gas of rich alcohol and a small amount of water at the top of the tower is liquefied through the condenser 3 and enters the storage tank, part of which is refluxed and part of which is extracted and recycled into the batching tank. The isobutanol content is 96.3%, the DMAC content is trace, the water content is 3.3%, and the flow rate is 90.3kg. The DMAC gas phase near the bottom of the tower is extracted and cooled by the condenser 4 to obtain high-purity DMAC, with a flow rate of 68.29kg. The top temperature of the distillation tower is 61.7-63.5℃, the bottom temperature is 115-120℃, the vacuum degree is 0.08Mpa, the reflux ratio is 4.1, and the number of plates is 23.

[0079] Step 5: The dimethylacetamide after liquefaction and separation is transported to the interior of a storage tank for storage in a vacuum environment.

[0080] The recovery rate of DMAC was 98.1%, the detection purity was ≧99.99%, water <180ppm and alcohol <200ppm, chromaticity ≤3.0°, acetic acid content ≤250ppm, and dimethylamine ≤230ppm.

[0081] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for recovering DMAC solvent in meta-aramid production wastewater, characterized in that: The specific steps include: S1: Add flocculant to the waste liquid, let it stand, filter it, and remove oligomers and mechanical impurities in the waste liquid; S2: adding an azeotropic agent to the filtrate obtained in S1, and conveying it to a primary azeotropic distillation tower for azeotropic distillation, after the overhead distillate F1 is separated by an oil-water separator, the organic phase is refluxed to the primary azeotropic distillation tower, and the water phase is conveyed to a secondary azeotropic distillation tower for secondary azeotropic distillation; S3: The bottom effluent D1 of the primary azeotropic distillation tower in S2 is transported to a distillation tower for distillation. The bottom effluent is a high-concentration DMAC, thereby realizing the recovery of the DMAC solvent. The flocculant is one of aluminum sulfate, potassium aluminum sulfate, aluminum chloride, ferrous sulfate, polyferric sulfate, and ferric chloride; The azeotropic agent is one or more of isobutyl alcohol, n-butyl alcohol, isopentanol, n-pentanol, xylene and ethylene dichloride; N,N-dimethylacetamide is abbreviated as DMAC.

2. The recycling method according to claim 1, characterized in that: The composition of the meta-aramid production waste liquid is DMAC: 20-40% by mass, oligomer: 1%-2% by mass, and the rest is water.

3. The recycling method according to claim 1, characterized in that: Before step S1, an alkaline compound needs to be added to adjust the pH value of the waste liquid to be neutral or weakly alkaline.

4. The recycling method according to claim 3, characterized in that: The alkaline compound is an alkaline earth metal hydroxide.

5. The recycling method according to claim 1, characterized in that: The standing time in step S1 is ≥10 min.

6. The recycling method according to claim 1, characterized in that: The azeotropic agent is isobutanol or n-butanol.

7. The recycling method according to claim 1, characterized in that: The mass ratio of the azeotropic agent to the filtrate is 0.1-2:

1.

8. The recycling method according to claim 1, characterized in that: The operating parameters of the first-stage azeotropic distillation tower are: a reflux ratio of 1 to 7, normal pressure, a tower top temperature of 89 to 95°C, a tower bottom temperature of 100 to 115°C, and a number of tower plates of 8 to 18; the operating parameters of the second-stage azeotropic distillation tower are: a reflux ratio of 0.2 to 1, normal pressure, a tower top temperature of 89 to 95°C, a tower bottom temperature of 90 to 115°C, and a tower plate number of 8 to 18.

9. The recycling method according to claim 1, characterized in that: The operating parameters of the distillation tower are: tower top temperature of 50-80°C, tower bottom temperature of 105-120°C, vacuum degree of 0.06-0.09Mpa, number of plates of 15-25, and reflux ratio of 2-10.

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