A polyacrylonitrile polymer waste liquid recovery system and method

Abstract

The application provides a polyacrylonitrile polymerization waste liquid recovery system and method, and belongs to the technical field of solvent recovery and purification. The system comprises, in sequence in the material flow direction, an AN single-removing unit, a first dehydration unit, a second dehydration unit, a third dehydration unit, a DMSO rectification and purification unit and an evaporation and concentration unit. The first dehydration unit is operated at normal pressure, and the operation pressures of the second dehydration unit, the third dehydration unit, the DMSO rectification and purification unit and the evaporation and concentration unit decrease in sequence. The DMSO decomposition rate can be reduced to 0.1%-0.5%, the desalted water consumption is reduced by more than 70%, and the wastewater discharge amount is reduced by more than 50%.

Description

Technical Field

[0001] This invention relates to the field of solvent recovery and purification technology, and in particular to a polyacrylonitrile polymerization waste liquid recovery system and method. Background Technology

[0002] In the production of polyacrylonitrile (PAN)-based carbon fibers, the solution polymerization process using dimethyl sulfoxide (DMSO) as a solvent produces a polymerization solution containing acrylonitrile (AN) monomer, DMSO, and water. To control costs and meet emission standards, the DMSO in the polymerization solution must be separated, recovered, and recycled. The quality of the recovered DMSO directly affects the spinnability of the dope and the uniformity of the precursor fiber, making it a key quality control step for carbon fibers.

[0003] The existing recovery process utilizes the boiling point difference between AN (boiling point about 77°C), water (100°C) and DMSO (about 189°C) to sequentially achieve AN removal from monohydrate, dehydration and concentration, DMSO distillation purification and high-boiling-point concentration through distillation and evaporation. However, this process faces three prominent problems: First, the temperature of the atmospheric distillation column bottom needs to be maintained above 150℃. DMSO undergoes significant thermal decomposition above 120℃, generating sulfur-containing impurities such as dimethyl sulfide, which not only causes solvent loss but also enters the polymerization spinning system with the recovered solvent, affecting the quality of the precursor fiber. Second, although the introduction of vacuum operation can lower the operating temperature and inhibit DMSO decomposition, the separation factor between DMSO and water decreases under vacuum conditions, reducing dehydration efficiency. Moreover, a single vacuum scheme cannot match the dynamic changes in DMSO concentration from low to high and the gradually increasing risk of thermal decomposition during the dehydration process, resulting in internal friction between separation efficiency and decomposition inhibition. Third, the dehydration column reflux relies on external replenishment of demineralized water, which is costly, and the discharged aqueous distillate needs to be sent to wastewater treatment, forming a simultaneous scale-up cycle of discharge and replenishment.

[0004] In summary, how to reduce auxiliary material consumption and wastewater discharge through internal material circulation while ensuring separation efficiency is a technical challenge that urgently needs to be overcome in the field of DMSO recycling. Summary of the Invention

[0005] To address the technical challenge of reducing auxiliary material consumption and wastewater discharge through internal material recycling while maintaining separation efficiency, this invention provides a polyacrylonitrile polymerization waste liquid recovery system and method.

[0006] One objective of this invention is to provide a polyacrylonitrile polymerization waste liquid recovery system, comprising the following components connected sequentially in the material flow direction: The AN monomer removal unit is used to separate and recover AN monomer from the polymerization solution under a first reduced pressure condition to obtain the monomer removal reactor liquid; A primary dehydration unit is connected to the outlet of the AN dehydration unit and is used to initially remove water from the dehydration kettle liquid under normal pressure to obtain a first dehydration concentrate and a first aqueous distillate. A secondary dehydration unit is connected to the outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under a second reduced pressure to obtain a second dehydration concentrate and a second aqueous distillate. A third-stage dehydration unit is connected to the outlet of the second-stage dehydration unit and is used to remove water from the second dehydration concentrate under a third reduced pressure condition to obtain a third dehydration concentrate and a third aqueous distillate. The DMSO distillation and purification unit is connected to the outlet of the three-stage dehydration unit and is used to distill and purify the third dehydration concentrate under the fourth reduced pressure condition to obtain DMSO and residue. An evaporation and concentration unit is connected to the outlet of the DMSO distillation and purification unit, and is used to evaporate and concentrate the residue in the reactor under the fifth reduced pressure condition and recover the residual DMSO. The operating pressures under the first, second, third, fourth, and fifth pressure reduction conditions decrease sequentially.

[0007] Preferably, the operating pressure of the first decompression condition of the AN desiccant unit is 18~22 kPaA.

[0008] Preferably, the polyacrylonitrile polymerization waste liquid recovery system meets at least one of the following characteristics: (a) The operating pressure under the second pressure reduction condition is 13~17 kPaA; (b) The operating pressure of the third pressure reduction condition is 8~12 kPaA; (c) The operating pressure of the fourth pressure reduction condition is 5~7 kPaA; and, (d) The operating pressure of the fifth pressure reduction condition is 3~5 kPaA.

[0009] Preferably, the polyacrylonitrile polymerization waste liquid recovery system meets all of the following characteristics: (a) The operating pressure under the second pressure reduction condition is 13~17 kPaA; (b) The operating pressure of the third pressure reduction condition is 8~12 kPaA; (c) The operating pressure of the fourth pressure reduction condition is 5~7 kPaA; and, (d) The operating pressure of the fifth pressure reduction condition is 3~5 kPaA.

[0010] Preferably, the polyacrylonitrile polymerization waste liquid recovery system meets at least one of the following characteristics: (e) The temperature of the bottom of the AN column in the AN de-monograph unit is controlled at 50~90℃; (f) The operating temperature of the primary dehydration unit is controlled at 100~110℃; (g) The operating temperature of the secondary dehydration unit is controlled at 80~95℃; (h) The operating temperature of the three-stage dehydration unit is controlled at 75~125℃; (i) The operating temperature of the DMSO distillation and purification unit is controlled at 95~120℃; and, (j) The operating temperature of the evaporation and concentration unit is controlled at 80~115℃.

[0011] Preferably, the primary dehydration unit includes: A primary dehydration tower, connected to the outlet of the AN dehydration unit, is used to remove water from the dehydration solution under normal pressure. The primary dehydration tower is provided with a top outlet and a bottom outlet. The first dehydration concentrate is collected from the bottom outlet of the primary dehydration tower. A primary dehydration condenser, connected to the top outlet of the primary dehydration tower, is used to condense water-containing vapor to obtain a first aqueous distillate; and, A primary dehydration reflux tank, connected to the outlet of the primary dehydration condenser, is used to collect the first aqueous distillate; And / or, The secondary dehydration unit includes: A secondary dehydration tower is connected to the bottom outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under the second reduced pressure condition; the secondary dehydration tower is provided with a top outlet and a bottom outlet; the second dehydration concentrate is collected from the bottom outlet of the secondary dehydration tower; A secondary dehydration condenser, connected to the top outlet of the secondary dehydration tower, is used to condense water-containing vapor to obtain a second water-containing distillate; and, A secondary dehydration reflux tank, connected to the outlet of the secondary dehydration condenser, is used to collect the second aqueous distillate; And / or, The three-stage dehydration unit includes: A three-stage dehydration tower is connected to the bottom outlet of the two-stage dehydration unit and is used to remove water from the second dehydration concentrate under the third reduced pressure condition. The three-stage dehydration tower is provided with a top outlet and a bottom outlet. The third dehydration concentrate is collected from the bottom outlet of the three-stage dehydration tower. A three-stage dehydration condenser, connected to the top outlet of the three-stage dehydration tower, is used to condense water-containing vapor to obtain a third water-containing distillate; and A three-stage dehydration reflux tank is connected to the outlet of the three-stage dehydration condenser and is used to collect the third aqueous distillate.

[0012] Preferably, the DMSO distillation and purification unit includes: A DMSO distillation column is connected to the outlet of the three-stage dehydration unit and is used to distill the third dehydration concentrate under the fourth reduced pressure condition. The DMSO distillation column is provided with a top outlet and a bottom outlet. DMSO vapor is collected from the top outlet of the DMSO distillation column, and the bottom residue is collected from the bottom outlet of the DMSO distillation column. A distillation column condenser, connected to the top vapor outlet of the DMSO distillation column, is used to condense the DMSO vapor to obtain liquid DMSO; and A distillation column reflux tank, connected to the outlet of the distillation column condenser, is used to collect the liquid DMSO and provide reflux; The distillation column reflux tank is provided with a first gas phase outlet, which is connected to a first injection system for extracting non-condensable gases to control the vacuum level of the DMSO distillation column.

[0013] Preferably, the evaporation and concentration unit includes: An evaporator, connected to the bottom outlet of the DMSO distillation column, is used to heat the residue in the vessel, causing the residual DMSO in the residue to evaporate and obtain recovered DMSO vapor; An evaporator-condenser, connected to the outlet of the evaporator, is used to condense the recovered DMSO vapor to obtain liquid recovered DMSO; and An evaporator condensate tank, connected to the evaporator condenser outlet, is used to collect the liquid recovered DMSO; The evaporator condensate tank is provided with a second gas phase outlet, which is connected to a second injection system for extracting non-condensable gases to control the vacuum level of the evaporator.

[0014] Preferably, the primary dehydration unit, the secondary dehydration unit, and the tertiary dehydration unit each independently use the system reflux medium for reflux operation; The system reflux medium includes at least one of the first aqueous distillate, the second aqueous distillate, or the third aqueous distillate.

[0015] Preferably, the polyacrylonitrile polymerization waste liquid recovery system further includes: A solvent storage unit, connected to the outlet of the AN monomer removal unit and the outlet of the DMSO distillation and purification unit, is used to store the recovered AN monomer and DMSO. The exhaust gas scrubbing unit is connected to the gas phase outlet of the first injection system and the second injection system, and is used to treat the non-condensable gases generated by the system. An alkali solution preparation unit, connected to the primary, secondary, and tertiary dehydration units, is used to prepare and supply the alkali solution required for the process to the dehydration units; and The vacuum vacuum unit is connected to the AN dehydration unit, the secondary dehydration unit, the tertiary dehydration unit, the DMSO distillation purification unit, and the evaporation concentration unit, and is used to provide vacuum conditions.

[0016] The second objective of this invention is to provide a method for recovering polyacrylonitrile polymerization waste liquid, comprising the following steps: The polyacrylonitrile polymerization waste liquid is passed into the AN monomer removal unit, and the AN monomer is separated and recovered under the first depressurization condition to obtain the monomer removal kettle liquid. The dehydration solution is passed into the first-stage dehydration unit to initially remove water from the dehydration solution under normal pressure, resulting in a first dehydration concentrate. The first dehydrated concentrate is passed into a secondary dehydration unit, and the water in the first dehydrated concentrate is removed under a second reduced pressure condition to obtain a second dehydrated concentrate. The second dehydrated concentrate is passed into a three-stage dehydration unit, and the water in the second dehydrated concentrate is removed under the third reduced pressure condition to obtain the third dehydrated concentrate. The third dehydrated concentrate is passed into the DMSO distillation and purification unit, and the DMSO is purified by distillation under reduced pressure to obtain DMSO and residue. The residual liquid in the reactor is passed into the evaporation and concentration unit, where it is evaporated and concentrated under reduced pressure to recover the residual DMSO.

[0017] The beneficial effects of this invention are: This invention breaks down the dehydration process into a single stage of atmospheric pressure dehydration and a two-stage vacuum dehydration, sequentially connecting DMSO distillation purification and high-boiling-point evaporation concentration. This creates a separation sequence with progressively decreasing operating pressure, allowing the operating temperature of each separation stage to gradually decrease as the DMSO concentration in the feed solution increases. This achieves the goal of maintaining dehydration efficiency and DMSO distillation purification effect while suppressing high-temperature thermal decomposition of DMSO, reducing solvent loss and the generation of decomposition byproducts. By incorporating an evaporation concentration unit, the residue from DMSO distillation purification is further evaporated under vacuum conditions to recover residual DMSO, increasing the total DMSO recovery rate to over 98%.

[0018] The recovered DMSO can be reused in the coagulation bath preparation and stretching bath preparation processes of spinning, reducing production costs and improving resource utilization. Detailed Implementation

[0019] The present application will now be described in further detail with reference to embodiments. In the following description, certain specific details are included to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented without employing one or more of these specific details, but using other methods, components, materials, etc. Unless otherwise required by the present invention, the terms "comprising" and "including" should be interpreted in an open-ended, inclusive sense, meaning "including but not limited to". Throughout this specification, "an embodiment," "an embodiment," "a preferred embodiment," or "some embodiments" means that at least one embodiment includes a specific reference element, structure, or feature related to that embodiment. Therefore, the phrases "in an embodiment," "in an embodiment," "in a preferred embodiment," or "in some embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0020] According to a first aspect of the present invention, a polyacrylonitrile polymerization waste liquid recovery system is provided, comprising: connected sequentially in the material flow direction: The AN monomer removal unit is used to separate and recover AN monomer from the polymerization solution under a first reduced pressure condition to obtain the monomer removal reactor liquid; A primary dehydration unit is connected to the outlet of the AN dehydration unit and is used to initially remove water from the dehydration kettle liquid under normal pressure to obtain a first dehydration concentrate and a first aqueous distillate. A secondary dehydration unit is connected to the outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under a second reduced pressure to obtain a second dehydration concentrate and a second aqueous distillate. A third-stage dehydration unit is connected to the outlet of the second-stage dehydration unit and is used to remove water from the second dehydration concentrate under a third reduced pressure condition to obtain a third dehydration concentrate and a third aqueous distillate. The DMSO distillation and purification unit is connected to the outlet of the three-stage dehydration unit and is used to distill and purify the third dehydration concentrate under the fourth reduced pressure condition to obtain DMSO and residue. An evaporation and concentration unit is connected to the outlet of the DMSO distillation and purification unit, and is used to evaporate and concentrate the residue in the reactor under the fifth reduced pressure condition and recover the residual DMSO. The operating pressures under the first, second, third, fourth, and fifth pressure reduction conditions decrease sequentially.

[0021] This invention employs a tiered combination of an AN dehydration unit, a three-stage dehydration unit, a DMSO distillation and purification unit, and an evaporation and concentration unit, with the operating pressure of the latter five units decreasing in a gradient. This ensures that the operating temperature of the material decreases sequentially with decreasing pressure during each stage of dehydration, purification, and concentration. This gradient pressure reduction configuration confines the overall operating temperature below the DMSO thermal decomposition temperature, effectively preventing high-temperature decomposition of DMSO during recovery, reducing the amount of decomposition byproducts, and thus improving the recovery rate and quality of DMSO.

[0022] This invention employs a process architecture that couples atmospheric pressure primary dehydration with progressively reduced pressure deep dehydration and purification. As the DMSO concentration of the material increases from low to high, the operating pressure gradually decreases. Atmospheric pressure operation ensures high gas-liquid separation efficiency in the low-concentration range, while deep pressure reduction suppresses the risk of thermal decomposition in the high-concentration range. This overcomes the contradiction between reduced separation factor and suppression of thermal decomposition under single pressure reduction operation, achieving a synergistic improvement in separation efficiency and product quality.

[0023] The configuration of the operating pressure of the first to fifth decompression conditions in this invention, which decreases sequentially, matches the trend of gradually increasing DMSO concentration and gradually increasing heat sensitivity in the material. This avoids the energy waste caused by using a uniformly high vacuum degree in each unit, and also avoids the insufficient separation efficiency caused by a uniformly low vacuum degree, making the overall energy consumption of the system more reasonable while meeting the separation requirements.

[0024] In a preferred embodiment of the present invention, the operating pressure of the first decompression condition of the AN de-singling unit is 18~22 kPaA, for example, 18 kPaA, 19 kPaA, 20 kPaA, 21 kPaA or 22 kPaA.

[0025] Acrylonitrile has a boiling point of approximately 77°C at atmospheric pressure, while DMSO undergoes significant thermal decomposition above 120°C. Therefore, the operating pressure of the AN monomer removal unit is preferably set at 18–22 kPaA. Under reduced pressure conditions of 18–22 kPaA, the boiling point of AN can be significantly lowered, thereby controlling the operating temperature of the AN monomer removal unit within the range of 70–120°C. This ensures effective vaporization and separation of AN while keeping DMSO below its thermally stable temperature throughout the process, avoiding the formation of high-temperature decomposition byproducts and guaranteeing the quality of the recovered DMSO. Furthermore, this reduced pressure operation also helps prevent the high-purity AN monomer obtained at the top of the column from undergoing self-polymerization at high temperatures, ensuring the safety and quality of the recovered monomer.

[0026] In a preferred embodiment of the present invention, the polyacrylonitrile polymerization waste liquid recovery system satisfies at least one of the following characteristics: (a) The operating pressure of the second pressure reduction condition is 13~17 kPaA, for example, 13 kPaA, 14 kPaA, 15 kPaA, 16 kPaA or 17 kPaA; (b) The operating pressure of the third pressure reduction condition is 8~12 kPaA, for example, 8 kPaA, 9 kPaA, 10 kPaA, 11 kPaA or 12 kPaA; (c) The operating pressure of the fourth pressure reduction condition is 5~7 kPaA, for example, 5 kPaA, 6 kPaA or 7 kPaA; (d) The operating pressure of the fifth pressure reduction condition is 3~5 kPaA, for example, 3 kPaA, 4 kPaA or 5 kPaA.

[0027] After the first stage of dehydration, the concentration of DMSO in the material increases, raising the risk of thermal decomposition. Therefore, the preferred second decompression condition is 13-17 kPaA, appropriately reducing the pressure from atmospheric pressure to lower the operating temperature and begin inhibiting decomposition. As dehydration progresses, the DMSO concentration in the material further increases. The preferred third decompression condition is 8-12 kPaA to further lower the boiling point and control the temperature within a safer range. In the DMSO distillation and purification stage, the material is predominantly DMSO, posing the greatest risk of thermal decomposition. The preferred fourth decompression condition is 5-7 kPaA, a deep decompression to significantly lower the boiling point of DMSO and ensure the distillation process is carried out at ≤120℃. In the high-boiling-point concentration stage, the DMSO content in the material is already low, and the concentration of heavy components is high. The preferred fifth decompression condition is 3-5 kPaA to provide the highest vacuum, allowing residual DMSO to evaporate and be recovered at the lowest possible temperature, preventing prolonged thermal decomposition of DMSO in the reactor residue. Overall, the pressure of the four decompression conditions decreases sequentially, forming a pressure gradient that matches the progressively increasing concentration of DMSO in the material and the progressively increasing risk of thermal decomposition. Under the premise of ensuring the separation efficiency of each unit, the operating temperature of the entire process is constrained within the thermal stability range of DMSO.

[0028] In this invention, the polyacrylonitrile polymerization waste liquid contains three main components: acrylonitrile (AN), dimethyl sulfoxide (DMSO), and water. The AN content is typically 0.8 wt% to 10.2 wt%, the DMSO content is typically 72 wt% to 96 wt%, and the water content is typically 0.2 wt% to 4.5 wt%. After treatment in the AN monomer removal unit, AN monomer (purity ≥ 99.5%) is collected from the top of the column, and the AN content in the monomer removal residue collected from the bottom of the column is reduced to below 0.1 wt%, with DMSO and water as the main components. The monomer removal residue enters the primary dehydration unit, where most of the water is evaporated under normal pressure. The resulting first dehydration concentrate has a DMSO content increased to approximately 75 wt% to 85 wt% and a water content reduced to approximately 15 wt% to 25 wt%. It then enters the secondary dehydration unit for further dehydration under a second reduced pressure. The resulting second dehydration concentrate has a DMSO content increased to approximately 88 wt% to 93 wt% and a water content reduced to approximately 7 wt% to 12 wt%. The material undergoes further dehydration in a three-stage dehydration unit under a third reduced pressure condition. The resulting third dehydrated concentrate contains 95wt%–98wt% DMSO and 2wt%–5wt% water. At this point, the water content is essentially removed, and the main components are DMSO and a small amount of high-boiling-point heavy impurities. The third dehydrated concentrate then enters a DMSO distillation purification unit. After distillation, the DMSO collected from the top of the column has a purity of over 99.5%, while the bottom residue mainly consists of high-boiling-point heavy components and a small amount of residual DMSO (approximately 20wt%–40wt%). This residue then enters an evaporation concentration unit, where residual DMSO is evaporated and recovered under a fifth reduced pressure condition. The recovered DMSO has a purity of over 95wt%, and the concentrated high-boiling-point residue is discharged from the bottom of the evaporator.

[0029] In a preferred embodiment of the present invention, the polyacrylonitrile polymerization waste liquid recovery system satisfies all of the following characteristics: (a) The operating pressure of the second pressure reduction condition is 13~17 kPaA, for example, 13 kPaA, 14 kPaA, 15 kPaA, 16 kPaA or 17 kPaA; (b) The operating pressure of the third pressure reduction condition is 8~12 kPaA, for example, 8 kPaA, 9 kPaA, 10 kPaA, 11 kPaA or 12 kPaA; (c) The operating pressure of the fourth pressure reduction condition is 5~7 kPaA, for example, 5 kPaA, 6 kPaA or 7 kPaA; (d) The operating pressure of the fifth pressure reduction condition is 3~5 kPaA, for example, 3 kPaA, 4 kPaA or 5 kPaA.

[0030] In a preferred embodiment of the present invention, the polyacrylonitrile polymerization waste liquid recovery system satisfies at least one of the following characteristics: (e) The temperature of the bottom of the AN column in the AN de-monograph unit is controlled at 50~90℃, for example, 50℃, 60℃, 70℃, 80℃ or 90℃; (f) The operating temperature of the primary dehydration unit is controlled at 100~110℃, for example, 100℃, 102℃, 105℃, 108℃ or 110℃; (g) The operating temperature of the secondary dehydration unit is controlled at 80~95℃, for example, 80℃, 85℃, 88℃, 90℃ or 95℃; (h) The operating temperature of the three-stage dehydration unit is controlled at 75~125℃, for example, 75℃, 78℃, 80℃, 85℃, 90℃, 100℃, 115℃, 120℃ or 125℃; (i) The operating temperature of the DMSO distillation purification unit is controlled at 95~120℃, for example, 95℃, 100℃, 105℃, 110℃, 115℃ or 120℃; (j) The operating temperature of the evaporation and concentration unit is controlled at 80~115℃, for example, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃ or 115℃.

[0031] The AN degassing unit operates under reduced pressure of 18-22 kPaA, controlling the reboiler temperature at 50-90°C. This ensures complete AN vaporization and separation while keeping DMSO below its thermal stability temperature throughout the process, preventing AN self-polymerization. The primary dehydration unit operates at atmospheric pressure, with the temperature controlled at 100-110°C. Utilizing the fact that water boils at approximately 100°C under atmospheric pressure, it efficiently removes most of the water, and DMSO has virtually no decomposition risk within this temperature range. As the pressure decreases in the secondary and tertiary dehydration stages, the operating temperature correspondingly drops to 80-95°C and 75-125°C, respectively. This further removes residual water while keeping the risk of DMSO thermal decomposition to a lower level. The DMSO distillation and purification unit operates under a high vacuum of 6 kPaA, controlling the temperature at 95-120°C. This ensures effective vaporization and separation of DMSO without exceeding its thermal decomposition threshold. The evaporation and concentration unit operates under a maximum vacuum of 4 kPaA, with the temperature controlled at 80-115°C. This allows residual DMSO to be evaporated and recovered at the lowest possible temperature, avoiding prolonged thermal decomposition. Overall, the above temperatures are matched one by one with the operating pressure of each unit, ensuring that the temperature of the entire process is kept within the thermal stability range of DMSO while ensuring that each separation process proceeds normally.

[0032] In a preferred embodiment of the present invention, the primary dehydration unit includes: A primary dehydration tower, connected to the outlet of the AN dehydration unit, is used to remove water from the dehydration solution under normal pressure. The primary dehydration tower is provided with a top outlet and a bottom outlet. The first dehydration concentrate is collected from the bottom outlet of the primary dehydration tower. A primary dehydration condenser, connected to the top outlet of the primary dehydration tower, is used to condense water-containing vapor to obtain a first aqueous distillate; and, A primary dehydration reflux tank, connected to the outlet of the primary dehydration condenser, is used to collect the first aqueous distillate; And / or, The secondary dehydration unit includes: A secondary dehydration tower is connected to the bottom outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under the second reduced pressure condition; the secondary dehydration tower is provided with a top outlet and a bottom outlet; the second dehydration concentrate is collected from the bottom outlet of the secondary dehydration tower; A secondary dehydration condenser, connected to the top outlet of the secondary dehydration tower, is used to condense water-containing vapor to obtain a second water-containing distillate; and, A secondary dehydration reflux tank, connected to the outlet of the secondary dehydration condenser, is used to collect the second aqueous distillate; And / or, The three-stage dehydration unit includes: A three-stage dehydration tower is connected to the bottom outlet of the two-stage dehydration unit and is used to remove water from the second dehydration concentrate under the third reduced pressure condition. The three-stage dehydration tower is provided with a top outlet and a bottom outlet. The third dehydration concentrate is collected from the bottom outlet of the three-stage dehydration tower. A three-stage dehydration condenser, connected to the top outlet of the three-stage dehydration tower, is used to condense water-containing vapor to obtain a third water-containing distillate; and A three-stage dehydration reflux tank is connected to the outlet of the three-stage dehydration condenser and is used to collect the third aqueous distillate.

[0033] In a preferred embodiment of the present invention, the DMSO distillation and purification unit includes: A DMSO distillation column is connected to the outlet of the three-stage dehydration unit and is used to distill the third dehydration concentrate under the fourth reduced pressure condition. The DMSO distillation column is provided with a top outlet and a bottom outlet. DMSO vapor is collected from the top outlet of the DMSO distillation column, and the bottom residue is collected from the bottom outlet of the DMSO distillation column. A distillation column condenser, connected to the top vapor outlet of the DMSO distillation column, is used to condense the DMSO vapor to obtain liquid DMSO; and A distillation column reflux tank, connected to the outlet of the distillation column condenser, is used to collect the liquid DMSO and provide reflux; The distillation column reflux tank is provided with a first gas phase outlet, which is connected to a first injection system for extracting non-condensable gases to control the vacuum level of the DMSO distillation column.

[0034] In a preferred embodiment of the present invention, the evaporation and concentration unit includes: An evaporator, connected to the bottom outlet of the DMSO distillation column, is used to heat the residue in the vessel, causing the residual DMSO in the residue to evaporate and obtain recovered DMSO vapor; An evaporator-condenser, connected to the outlet of the evaporator, is used to condense the recovered DMSO vapor to obtain liquid recovered DMSO; and An evaporator condensate tank, connected to the evaporator condenser outlet, is used to collect the liquid recovered DMSO; The evaporator condensate tank is provided with a second gas phase outlet, which is connected to a second injection system for extracting non-condensable gases to control the vacuum level of the evaporator.

[0035] In a preferred embodiment of the present invention, the primary dehydration unit, the secondary dehydration unit and the tertiary dehydration unit each independently use the system reflux medium for reflux operation; The system reflux medium includes at least one of the first aqueous distillate, the second aqueous distillate, or the third aqueous distillate.

[0036] All dehydration towers are distillation towers, and regardless of the medium used, a liquid reflux at the top is essential for gas-liquid mass transfer with the rising vapor, ensuring effective separation of water and DMSO. In traditional processes, the reflux liquid from each dehydration tower relies on externally supplied demineralized water, increasing demineralized water consumption. Furthermore, this water eventually distills off the top and enters the wastewater system, creating a simultaneous large-scale cycle of discharge and replenishment, leading to increased operating and wastewater treatment costs. The aqueous distillate from the top of each dehydration tower is primarily water, containing only trace amounts of DMSO and AN. Its composition and temperature meet the process requirements for reflux liquid returning to the top of the tower, making it a viable alternative to externally supplied demineralized water.

[0037] This invention preferably uses the system's own aqueous distillate instead of externally added demineralized water as the reflux liquid at the top of the tower. This reduces the amount of demineralized water used by more than 70%, while reducing wastewater discharge by more than 50%, significantly lowering wastewater treatment costs. Furthermore, trace amounts of DMSO and AN entrained in the aqueous distillate are returned to the dehydration tower with the reflux liquid and reintroduced into the system for recovery, preventing the loss of effective components and further improving the overall solvent recovery rate.

[0038] In a preferred embodiment of the present invention, a portion of the first aqueous distillate collected by the primary dehydration reflux tank is returned to the top of the primary dehydration tower via a reflux pipeline as reflux liquid, where it undergoes gas-liquid mass transfer with the rising vapor, while the remainder is discharged from the system or used as a reflux medium for other dehydration units.

[0039] A portion of the second aqueous distillate collected in the secondary dehydration reflux tank is returned to the top of the secondary dehydration tower via the reflux pipeline as reflux liquid, while the remainder is discharged from the system or used as reflux medium for other dehydration units.

[0040] A portion of the third aqueous distillate collected in the three-stage dehydration reflux tank is returned to the top of the three-stage dehydration tower as reflux liquid via the reflux pipeline, while the remainder is discharged from the system or used as reflux medium for other dehydration units.

[0041] In a preferred embodiment of the present invention, the reflux ratio of the primary dehydration tower is 0.1 to 0.5; and / or, the reflux ratio of the secondary dehydration tower is 0.2 to 0.8; and / or, the reflux ratio of the tertiary dehydration tower is 0.2 to 0.8. The reflux ratio, or reflux rate, is the ratio of the amount of liquid returned from the top of a distillation column to the amount of product collected from the top. It directly affects the separation efficiency and energy consumption. The reflux ratio of each stage of the dehydration column can be adjusted according to actual operating conditions. In the first-stage dehydration column, due to the high water content of the material and the operation at atmospheric pressure, the reflux ratio is preferably controlled between 0.1 and 0.5 to ensure effective separation of water and DMSO. In the second and third-stage dehydration columns, which operate under reduced pressure, the gas-liquid mass transfer driving force decreases, and the reflux ratio can be preferably increased to 0.2 to 0.8 to ensure the depth of dehydration.

[0042] In a preferred embodiment of the present invention, a portion of the liquid DMSO in the DMSO distillation column is returned to the top of the DMSO distillation column to participate in gas-liquid mass transfer, while the remainder is collected as DMSO product.

[0043] In a preferred embodiment of the present invention, the reflux ratio of the DMSO distillation column is 0.2 to 1.0.

[0044] As a product purification unit, the DMSO distillation column should be kept within a reflux ratio of 0.2 to 1.0 to ensure that the purity of DMSO reaches 99.5% or higher. The above reflux ratio range can be optimized and adjusted according to the actual feed composition, throughput and product requirements.

[0045] In a preferred embodiment of the present invention, the polyacrylonitrile polymerization waste liquid recovery system further includes: A solvent storage unit, connected to the outlet of the AN monomer removal unit and the outlet of the DMSO distillation and purification unit, is used to store the recovered AN monomer and DMSO. The exhaust gas scrubbing unit is connected to the gas phase outlet of the first injection system and the second injection system, and is used to treat the non-condensable gases generated by the system. An alkali solution preparation unit, connected to the primary, secondary, and tertiary dehydration units, is used to prepare and supply the alkali solution required for the process to the dehydration units; and The vacuum vacuum unit is connected to the AN dehydration unit, the secondary dehydration unit, the tertiary dehydration unit, the DMSO distillation purification unit, and the evaporation concentration unit, and is used to provide vacuum conditions.

[0046] During vacuum distillation and evaporation, non-condensable gases from the reflux tank of the DMSO distillation column and the evaporator condensate tank are extracted by the injection system. These gases contain small amounts of uncondensed AN monomers, DMSO vapor, and sulfur-containing volatile organic compounds such as dimethyl sulfide and dimethyl disulfide produced by the high-temperature decomposition of DMSO. Direct emission of these gases not only causes material loss but also pollutes the environment. The tail gas scrubbing unit typically uses a packed scrubbing tower with demineralized water or dilute alkali as the scrubbing medium. Non-condensable gases enter from the bottom of the tower and come into countercurrent contact with the scrubbing liquid flowing from top to bottom in the packing layer. AN and DMSO vapors are absorbed by the scrubbing liquid. After scrubbing and purification, the AN and DMSO content in the tail gas can be reduced to levels that meet environmental emission standards before venting. The scrubbing liquid containing AN and DMSO can be returned to the system to recover useful components, thus achieving harmless treatment and resource utilization of the tail gas.

[0047] During the polymerization process, polymerization inhibitors are added to the polymer solution to prevent AN (acetic acid) from self-polymerizing during polymerization and storage. Simultaneously, small amounts of acidic substances, such as residual polymerization initiators and organic acids generated from AN oxidation, may be introduced during polymerization and subsequent processing. If these acidic substances and polymerization inhibitor residues enter the dehydration system with the material, they will corrode equipment such as the dehydration tower and reboiler, shortening their service life. Furthermore, AN is more prone to self-polymerization under acidic conditions, forming polyacrylonitrile gels that clog trays and pipes, affecting the stable operation of the system. Adding an appropriate amount of alkali solution to the dehydration unit can effectively neutralize the acidic substances in the material, adjusting the system pH to a neutral or slightly alkaline range, thereby slowing down equipment corrosion, inhibiting the self-polymerization reaction of AN, and ensuring the long-term stable operation of the dehydration unit.

[0048] Typically, but not limitingly, the alkaline solution includes a sodium hydroxide solution.

[0049] In a preferred embodiment of the present invention, the secondary dehydration reflux tank is provided with a third gas phase outlet, which is used to connect to a vacuum pumping system to control the vacuum level of the secondary dehydration tower. The three-stage dehydration reflux tank is equipped with a fourth gas phase outlet, which is used to connect to a vacuum pumping system to control the vacuum level of the three-stage dehydration tower.

[0050] In a preferred embodiment of the present invention, the vacuum decompression unit is connected via a vacuum pipeline to the AN column vapor phase outlet of the AN dehydration unit, the third vapor phase outlet of the secondary dehydration reflux tank, the fourth vapor phase outlet of the tertiary dehydration reflux tank, the first vapor phase outlet of the distillation column reflux tank, and the second vapor phase outlet of the evaporator condensate tank, respectively. This allows for independent adjustment of the vacuum degree of each decompression unit, ensuring that the operating pressures of the AN dehydration unit, the secondary dehydration unit, the tertiary dehydration unit, the DMSO distillation purification unit, and the evaporation concentration unit are maintained at their respective set values ​​without interference.

[0051] Typically, but not limitingly, the vacuum evacuation unit includes a liquid ring vacuum pump and / or a steam jet pump. Specifically, for the AN dehydration unit and the secondary dehydration unit, which have relatively low vacuum requirements (pressure ≥ 8 kPaA), a liquid ring vacuum pump can be used alone for evacuation. Liquid ring vacuum pumps have the advantages of simple structure, reliable operation, and insensitivity to entrained liquids. For the tertiary dehydration unit, DMSO distillation purification unit, and evaporation concentration unit, which have higher vacuum requirements (pressure ≤ 7 kPaA), a combination of a steam jet pump and a liquid ring vacuum pump can be used. That is, the steam jet pump acts as the main pump, handling most of the evacuation load, while the liquid ring vacuum pump acts as the final stage booster pump, and the two are connected in series.

[0052] Typically, but not limitingly, the working fluid of the liquid ring vacuum pump can be demineralized water or internal circulating water of the system, and the driving steam of the steam jet pump is derived from low-pressure steam provided by the plant utilities. Each vacuum device is connected to the gas phase outlet of the reflux tank or the gas phase pipeline at the top of the tower of the corresponding unit through vacuum pipelines. Pressure regulating valves are installed on the vacuum pipelines, and the vacuum level of each unit can be precisely controlled by adjusting the opening of the valves.

[0053] In a preferred embodiment of the present invention, the polyacrylonitrile polymerization waste liquid recovery system includes an AN monomer removal unit, a primary dehydration unit, a secondary dehydration unit, a tertiary dehydration unit, a DMSO distillation purification unit, and an evaporation concentration unit connected sequentially in the material flow direction. The units are connected to each other through material conveying pipelines. The polymer liquid flows through each unit sequentially from upstream to downstream, thereby completing the removal of AN monomers, three-stage gradient dehydration, DMSO distillation purification, and high-boiling-point concentration and recovery in sequence.

[0054] Specifically, the polyacrylonitrile polymerization waste liquid first enters the AN monomer removal unit, which includes an AN tower. Under reduced pressure of 18~22kPaA, AN is separated from the polymerization liquid by utilizing the boiling point difference between AN and DMSO and water. The AN monomer collected from the top of the tower is sent to the solvent storage unit for recovery, and the monomer removal liquid collected from the bottom of the tower is sent to the primary dehydration unit.

[0055] The primary dehydration unit includes a primary dehydration tower, a primary dehydration condenser, and a primary dehydration reflux tank. The dehydrated liquid enters the primary dehydration tower and is heated and distilled under atmospheric pressure and an operating temperature of 100-110°C. Utilizing the fact that the boiling point of DMSO is much higher than that of water, the water vapor is vaporized. The water-containing vapor is discharged from the top of the tower and condensed in the primary dehydration condenser into the first aqueous distillate, which is then collected in the primary dehydration reflux tank. A portion of the first aqueous distillate collected in the primary dehydration reflux tank is returned to the top of the primary dehydration tower as reflux liquid through a reflux pipeline to undergo gas-liquid mass transfer with the rising vapor. The remainder is discharged from the system or used as a reflux medium for other dehydration units. The first dehydrated concentrate extracted from the bottom of the tower is sent to the secondary dehydration unit.

[0056] The secondary dehydration unit includes a secondary dehydration tower, a secondary dehydration condenser, and a secondary dehydration reflux tank. The first dehydration concentrate enters the secondary dehydration tower and is further distilled and dehydrated under the second reduced pressure conditions of 13~17 kPaA and an operating temperature of 80~95℃. The water-containing vapor is discharged from the top of the tower and condensed by the secondary dehydration condenser to obtain the second water-containing distillate, which is collected in the secondary dehydration reflux tank. A portion of the second water-containing distillate collected in the secondary dehydration reflux tank is returned to the top of the secondary dehydration tower as reflux liquid through the reflux pipeline, and the remainder is discharged from the system or used as reflux medium for other dehydration units. The second dehydration concentrate collected from the bottom of the tower is sent to the tertiary dehydration unit.

[0057] The three-stage dehydration unit includes a three-stage dehydration tower, a three-stage dehydration condenser, and a three-stage dehydration reflux tank. The second dehydration concentrate enters the three-stage dehydration tower and continues to be distilled and dehydrated under the third reduced pressure conditions of 8~12 kPaA and an operating temperature of 75~90℃. The water-containing vapor is discharged from the top of the tower and condensed by the three-stage dehydration condenser to obtain the third water-containing distillate, which is collected in the three-stage dehydration reflux tank. A portion of the third water-containing distillate collected in the three-stage dehydration reflux tank is returned to the top of the three-stage dehydration tower as reflux liquid through the reflux pipeline, and the remainder is discharged from the system or used as reflux medium for other dehydration units. The third dehydration concentrate collected from the bottom of the tower is sent to the DMSO distillation and purification unit.

[0058] After three stages of dehydration, the moisture in the material has been largely removed. The main components of the third dehydration concentrate are DMSO and a small amount of high-boiling-point heavy component impurities. It then enters the DMSO distillation purification unit. This unit includes a DMSO distillation column, a distillation column condenser, and a distillation column reflux tank. The third dehydration concentrate enters the DMSO distillation column for distillation purification under the fourth reduced pressure conditions of 5-7 kPaA and an operating temperature of 95-120℃. After vaporization, the DMSO is discharged from the top of the column and condensed into liquid DMSO in the distillation column condenser, which then enters the distillation column reflux tank. The outlet of the distillation column reflux tank is connected to the top reflux inlet of the DMSO distillation column and the product collection line, respectively. Part of the liquid DMSO is returned to the top of the column to participate in gas-liquid mass transfer to ensure separation efficiency, while the remainder is sent to the solvent storage unit as DMSO product. The bottom residue from the DMSO distillation column is sent to the evaporation and concentration unit.

[0059] The evaporation and concentration unit includes an evaporator, an evaporator condenser, and an evaporator condensate tank. The residue in the reactor enters the evaporator and is heated and evaporated under the fifth reduced pressure condition of 3~5 kPaA and an operating temperature of 80~115℃. The residual DMSO vaporizes and enters the evaporator condenser to be condensed into liquid DMSO for recovery and collection in the evaporator condensate tank. It is then sent to the solvent storage unit for recycling. The concentrated high-boiling residue is discharged from the bottom of the evaporator.

[0060] Furthermore, the DMSO distillation column reflux tank is equipped with a first gas phase outlet, connected to a first injection system for extracting non-condensable gases to control the vacuum level of the DMSO distillation column; the evaporator condensate tank is equipped with a second gas phase outlet, connected to a steam-driven second injection system for extracting non-condensable gases to control the vacuum level of the evaporator. The primary, secondary, and tertiary dehydration towers use a system reflux medium for reflux operation. The reflux liquid at the top of each dehydration tower originates from at least one of the first, second, or third aqueous distillate. By returning a portion of the aqueous distillate generated by each dehydration tower itself to the top as reflux liquid, replacing the externally added demineralized water, internal material circulation within the system is achieved.

[0061] Meanwhile, the secondary dehydration reflux tank is equipped with a third gas phase outlet, and the tertiary dehydration reflux tank is equipped with a fourth gas phase outlet, which are used to connect to the vacuum pumping system to control the vacuum level of the secondary and tertiary dehydration towers respectively. The above-mentioned auxiliary systems ensure the precise control of the vacuum level of each pressure-reducing operation unit and the stable operation of the system, together forming a complete, efficient, and closed-loop polyacrylonitrile polymerization waste liquid recovery system.

[0062] A second aspect of the present invention provides a method for recovering polyacrylonitrile polymerization waste liquid, comprising the following steps: The polyacrylonitrile polymerization waste liquid is passed into the AN monomer removal unit, and the AN monomer is separated and recovered under the first depressurization condition to obtain the monomer removal kettle liquid. The dehydration solution is passed into the first-stage dehydration unit to initially remove water from the dehydration solution under normal pressure, resulting in a first dehydration concentrate. The first dehydrated concentrate is passed into a secondary dehydration unit, and the water in the first dehydrated concentrate is removed under a second reduced pressure condition to obtain a second dehydrated concentrate. The second dehydrated concentrate is passed into a three-stage dehydration unit, and the water in the second dehydrated concentrate is removed under the third reduced pressure condition to obtain the third dehydrated concentrate. The third dehydrated concentrate is passed into the DMSO distillation and purification unit, and the DMSO is purified by distillation under the fourth reduced pressure condition to obtain DMSO and residue. The residual liquid in the reactor is passed into the evaporation and concentration unit, and evaporated and concentrated under the fifth reduced pressure condition to recover the residual DMSO.

[0063] In a preferred embodiment of the present invention, the method for recovering polyacrylonitrile polymerization waste liquid specifically includes: The polyacrylonitrile polymerization waste liquid is fed into the AN monomer removal unit and distilled under reduced pressure of 18~22kPaA. The bottom temperature of the column is controlled at 70~120℃. The AN is separated from the polymerization liquid by taking advantage of the boiling point difference between AN and DMSO and water. The AN monomer is collected from the top of the column and the monomer removal liquid with the AN content reduced to less than 0.1wt% is collected from the bottom of the column. The dehydration solution is fed into the first-stage dehydration unit and heated and distilled under normal pressure and an operating temperature of 100~110℃. The water vapor is vaporized by taking advantage of the fact that the boiling point of DMSO is much higher than that of water. The water vapor is condensed into the first water-containing distillate by the first-stage dehydration condenser. A portion of the water is returned to the top of the first-stage dehydration tower as reflux liquid. The reflux ratio of the first-stage dehydration tower is controlled at 0.1~0.5. The first dehydration concentrate with the DMSO content increased to 75wt%~85wt% is collected from the bottom of the tower. The first dehydrated concentrate is passed into a secondary dehydration unit, where it is further distilled and dehydrated under the second reduced pressure conditions of 13~17 kPaA and an operating temperature of 80~95℃. The water-containing vapor is condensed into a second water-containing distillate by the secondary dehydration condenser, and a portion of it is returned to the top of the secondary dehydration tower as reflux. The reflux ratio of the secondary dehydration tower is controlled at 0.2~0.8. The second dehydrated concentrate with an increased DMSO content of 88wt%~93wt% is collected from the bottom of the tower. The second dehydrated concentrate is fed into a three-stage dehydration unit, where it is further distilled and dehydrated under the third reduced pressure conditions of 8-12 kPaA and an operating temperature of 75-90°C. The water vapor is condensed into the third water-containing distillate by the three-stage dehydration condenser, and a portion of it is returned to the top of the three-stage dehydration tower as reflux. The reflux ratio of the three-stage dehydration tower is controlled at 0.2-0.8, and the third dehydrated concentrate with a DMSO content of 95wt%-98wt% is collected from the bottom of the tower. The third dehydrated concentrate is fed into the DMSO distillation purification unit, where it is purified by distillation under the fourth reduced pressure conditions of 5-7 kPaA and an operating temperature of 95-120°C. After vaporization, the DMSO is condensed into liquid DMSO in the distillation column condenser. A portion of the liquid is returned to the top of the DMSO distillation column to participate in gas-liquid mass transfer. The reflux ratio of the DMSO distillation column is controlled at 0.2-1.0. The remainder is collected as DMSO product (purity ≥99.5%), and the bottom residue is collected from the bottom of the column. The residual liquid in the reactor is fed into the evaporation and concentration unit and heated and evaporated under the fifth pressure reduction condition of 3~5 kPaA and the operating temperature of 80~115℃. The residual DMSO is vaporized and then condensed into liquid DMSO (purity ≥95wt%) by the evaporator condenser and collected. The concentrated high-boiling residue is discharged from the bottom of the evaporator.

[0064] Example 1 The polyacrylonitrile polymerization waste liquid recovery system of Example 1 of this invention is used to recover and treat polyacrylonitrile polymerization waste liquid. The composition of the polymerization liquid feed is: AN content 5.6wt%, DMSO content 88.2wt%, water content 6.2wt%, and feed rate is 1000kg / h.

[0065] The polymerization solution first enters the AN monomer removal unit. The operating pressure of the AN tower is 20 kPaA, and the tower bottom temperature is controlled at 75℃. The AN monomer is collected from the top of the tower, condensed, and recovered. The AN recovery rate is 99.2%, and the AN purity reaches 99.6%. The monomer removal liquid (AN content reduced to 0.08 wt%) collected from the bottom of the tower enters the primary dehydration unit.

[0066] The primary dehydration unit operates under atmospheric pressure at an operating temperature of 105℃, with the reflux ratio of the primary dehydration tower controlled at 0.3. Water-containing vapor is condensed into the first aqueous distillate by the primary dehydration condenser; a portion is returned to the top of the primary dehydration tower as reflux, and the remainder is discharged from the system. The first dehydration concentrate is collected at the bottom of the tower, where the DMSO content is increased to 82.5 wt% and the water content is reduced to 17.1 wt%.

[0067] The first dehydration concentrate enters the second-stage dehydration unit, operating at a pressure of 15 kPaA and a temperature of 88°C. The reflux ratio of the second-stage dehydration tower is controlled at 0.5. The water-containing vapor is condensed in the second-stage dehydration condenser into the second water-containing distillate. A portion of this distillate is returned to the top of the second-stage dehydration tower as reflux, while the remainder is discharged from the system or used as reflux medium for other dehydration units. The second dehydration concentrate is collected from the bottom of the tower, where the DMSO content has increased to 90.8 wt% and the water content has decreased to 8.9 wt%.

[0068] The second dehydration concentrate enters the tertiary dehydration unit, operating at a pressure of 10 kPaA and a temperature of 105°C. The reflux ratio of the tertiary dehydration tower is controlled at 0.6. The water-containing vapor is condensed in the tertiary dehydration condenser into the third aqueous distillate. A portion of this distillate is returned to the top of the tertiary dehydration tower as reflux, while the remainder is discharged from the system or used as reflux medium for other dehydration units. The third dehydration concentrate is collected at the bottom of the tower, containing 97.2 wt% DMSO and a water content reduced to 2.5 wt%.

[0069] The third dehydrated concentrate enters the DMSO distillation purification unit, operating at a pressure of 6 kPaA and a temperature of 112°C. The reflux ratio of the DMSO distillation column is controlled at 0.8. After vaporization, the DMSO is condensed into liquid DMSO in the distillation column condenser. A portion of the liquid is returned to the top of the DMSO distillation column to participate in gas-liquid mass transfer, while the remainder is collected as DMSO product with a purity of 99.7% and a recovery rate of 98.5%. The bottom residue (approximately 35 wt% DMSO) is collected and enters the evaporation and concentration unit.

[0070] The residue from the reactor enters the evaporation and concentration unit, operating at a pressure of 4 kPaA and a temperature of 105℃. The residual DMSO vaporizes and is condensed into liquid DMSO in the evaporator condenser, which is then collected. The purity of the recovered DMSO is 96.2%. High-boiling-point residue after concentration is discharged from the bottom of the evaporator. Overall, the total DMSO recovery rate is over 99.1% (98.5% DMSO product recovery rate + DMSO recovered from high-boiling-point evaporation), and the AN recovery rate is 99.2%. Testing shows that the content of dimethyl sulfide, a decomposition byproduct, in the DMSO is less than 10 ppm, and the DMSO decomposition rate is approximately 0.2%.

[0071] In this embodiment, the reflux liquid at the top of each dehydration tower preferentially uses the aqueous distillate generated by the system itself, with only a small amount of external demineralized water added when the system's water balance is insufficient. Compared with the traditional process (100% external demineralized water addition) at the same throughput, the amount of demineralized water used is reduced by approximately 75% to 80%, meaning the amount of external demineralized water added is only 20% to 25% of that in the traditional process. Simultaneously, since most of the aqueous distillate is recycled back to the top of the dehydration tower, the final wastewater discharge is reduced by approximately 55% to 60% compared to the traditional process. Trace amounts of DMSO and AN entrained in the aqueous distillate are returned to the dehydration tower with the reflux liquid and re-enter the recovery system, further improving the overall solvent recovery rate.

[0072] Example 2 The difference from Example 1 is that the operating parameters of each unit are all at their lower limits. The operating pressure of the AN dehydration unit is 18 kPaA, and the reboiler temperature is controlled at 50°C; the first-stage dehydration unit operates at 100°C under atmospheric pressure; the second-stage dehydration unit operates at 13 kPaA, at 80°C, and the reflux ratio is controlled at 0.2; the third-stage dehydration unit operates at 8 kPaA, at 75°C, and the reflux ratio is controlled at 0.2; the DMSO distillation purification unit operates at 5 kPaA, at 95°C, and the reflux ratio is controlled at 0.2; the evaporation and concentration unit operates at 3 kPaA, and at 80°C. All other operating conditions are the same as in Example 1.

[0073] In this embodiment, the purity of DMSO was 99.5%, the total recovery rate of DMSO was 98.6%, the recovery rate of AN was 99.0%, the decomposition rate of DMSO was approximately 0.15%, and the dimethyl sulfide content in DMSO was less than 8 ppm. Because the operating temperature and pressure of each unit were at relatively low levels, DMSO decomposition was suppressed to the greatest extent. However, due to the reduced reflux ratio, the separation efficiency decreased slightly, and the product purity and total recovery rate were slightly lower than in Example 1.

[0074] This embodiment also prioritizes using the system's own aqueous distillate as the reflux liquid at the top of each stage of the dehydration tower, supplementing only a small amount of external demineralized water to maintain the system's water balance. Compared with the traditional process at the same throughput, the amount of demineralized water used is reduced by approximately 70% to 75%. Because the reflux ratio of each unit in this embodiment is controlled at a low level (0.2), the amount of aqueous distillate recycled is slightly lower than in other embodiments, but it still significantly reduces the amount of external replenishment and wastewater discharge, ultimately reducing the amount of wastewater discharged by approximately 50% to 55% compared to the traditional process. The trace amounts of DMSO and AN entrained in the aqueous distillate are returned to the system with the reflux liquid, effectively reducing solvent loss.

[0075] Example 3 The difference from Example 1 is that the operating parameters of each unit are all at their upper limits. The operating pressure of the AN dehydration unit is 22 kPaA, and the reboiler temperature is controlled at 90°C; the first-stage dehydration unit operates at 110°C under atmospheric pressure; the second-stage dehydration unit operates at 17 kPaA, at 95°C, and the reflux ratio is controlled at 0.8; the third-stage dehydration unit operates at 12 kPaA, at 125°C, and the reflux ratio is controlled at 0.8; the DMSO distillation purification unit operates at 7 kPaA, at 120°C, and the reflux ratio is controlled at 1.0; the evaporation and concentration unit operates at 5 kPaA, and at 115°C. All other operating conditions are the same as in Example 1.

[0076] In this embodiment, the purity of DMSO was 99.6%, the total recovery rate of DMSO was 98.8%, the recovery rate of AN was 99.3%, the decomposition rate of DMSO was approximately 0.35%, and the dimethyl sulfide content in DMSO was approximately 15 ppm. Due to the increased reflux ratio, the separation effect was improved, but the increased operating temperature and pressure led to a slight increase in the DMSO decomposition rate compared to Examples 1 and 2, which remained at an extremely low level.

[0077] In this embodiment, the reflux liquid at the top of each dehydration tower is primarily derived from the aqueous distillate generated by the system itself, with only a small amount of external demineralized water added during the initial stage of system operation or when the liquid level is insufficient. Because the reflux ratio of each unit in this embodiment is controlled at a relatively high level (0.8~1.0), the amount of aqueous distillate reused is maximized, and the amount of demineralized water used is reduced by approximately 80%~85% compared to traditional processes. The amount of external demineralized water added is only 15%~20% of that in traditional processes; ultimately, the amount of wastewater discharged is reduced by approximately 60%~65% compared to traditional processes, demonstrating the most significant emission reduction effect. Simultaneously, trace amounts of DMSO and AN entrained in the reflux liquid are returned to the system and further recovered.

[0078] Example 4 The difference from Example 1 lies in the use of multiple intermediate value combinations for the operating parameters of each unit. The operating pressure of the AN dehydration unit is 19 kPaA, and the reboiler temperature is controlled at 60°C; the first-stage dehydration unit operates at 102°C under atmospheric pressure; the second-stage dehydration unit operates at 14 kPaA, at 85°C, and the reflux ratio is controlled at 0.4; the third-stage dehydration unit operates at 9 kPaA, at 78°C, and the reflux ratio is controlled at 0.5; the DMSO distillation purification unit operates at 5.5 kPaA, at 100°C, and the reflux ratio is controlled at 0.6; the evaporation and concentration unit operates at 3.5 kPaA, and at 90°C. All other operating conditions are the same as in Example 1.

[0079] In this embodiment, the purity of DMSO was 99.6%, the total recovery rate of DMSO was 99.0%, the recovery rate of AN was 99.1%, the decomposition rate of DMSO was approximately 0.18%, and the content of dimethyl sulfide in DMSO was less than 10 ppm.

[0080] In this embodiment, each dehydration tower uses the system's own aqueous distillate as the preferred reflux liquid, replacing externally demineralized water. Only a small amount of externally demineralized water is added when the system's water balance is insufficient. Because the reflux ratio in this embodiment is at a relatively low level (0.4~0.6), the amount of demineralized water used is reduced by approximately 72%~77% compared to traditional processes, and the final amount of wastewater discharged is reduced by approximately 52%~57% compared to traditional processes. Trace amounts of DMSO and AN entrained in the aqueous distillate are returned to the dehydration tower with the reflux liquid and then re-enter the recovery system, further improving resource utilization efficiency.

[0081] Example 5 The difference from Example 1 lies in the use of a different set of intermediate values ​​for the operating parameters of each unit. The operating pressure of the AN dehydration unit is 21 kPaA, and the reboiler temperature is controlled at 80°C; the operating temperature of the primary dehydration unit is 108°C under atmospheric pressure; the operating pressure of the secondary dehydration unit is 16 kPaA, the operating temperature is 92°C, and the reflux ratio is controlled at 0.6; the operating pressure of the tertiary dehydration unit is 11 kPaA, the operating temperature is 115°C, and the reflux ratio is controlled at 0.7; the operating pressure of the DMSO distillation purification unit is 6.5 kPaA, the operating temperature is 115°C, and the reflux ratio is controlled at 0.9; the operating pressure of the evaporation and concentration unit is 4.5 kPaA, and the operating temperature is 110°C. All other operating conditions are the same as in Example 1.

[0082] In this embodiment, the purity of DMSO was 99.7%, the total recovery rate of DMSO was 99.2%, the recovery rate of AN was 99.4%, the decomposition rate of DMSO was approximately 0.28%, and the content of dimethyl sulfide in DMSO was approximately 12 ppm.

[0083] In this embodiment, the reflux liquid from the top of each dehydration tower is preferentially composed of internal aqueous distillate, with only a small amount of external demineralized water added when the system's water balance is insufficient. Because the reflux ratio in this embodiment is at a relatively high level (0.6~0.9), the amount of aqueous distillate recycled is large, reducing the amount of demineralized water used by approximately 78%~83% compared to traditional processes, and ultimately reducing the amount of wastewater discharged by approximately 58%~63% compared to traditional processes. Trace amounts of DMSO and AN entrained in the aqueous distillate are returned to the dehydration tower with the reflux liquid, preventing the loss of effective components.

[0084] Example 6 The difference from Example 1 lies in the use of a lower pressure combination for the DMSO distillation and purification unit and the evaporation and concentration unit. The operating pressure of the AN dehydration unit is 20 kPaA, and the reboiler temperature is controlled at 75°C; the primary dehydration unit operates at 105°C under atmospheric pressure; the secondary dehydration unit operates at 15 kPaA, at 88°C, and the reflux ratio is controlled at 0.5; the tertiary dehydration unit operates at 10 kPaA, at 110°C, and the reflux ratio is controlled at 0.6; the DMSO distillation and purification unit operates at 5 kPaA, at 98°C, and the reflux ratio is controlled at 0.7; the evaporation and concentration unit operates at 3 kPaA, and at 85°C. All other operating conditions are the same as in Example 1.

[0085] In this embodiment, the DMSO purity was 99.6%, the total DMSO recovery rate was 99.0%, the AN recovery rate was 99.2%, the DMSO decomposition rate was approximately 0.12%, and the dimethyl sulfide content in the DMSO was less than 5 ppm. Further deep depressurization lowered the temperature during the DMSO distillation purification and high-boiling-point evaporation concentration stages, maximally suppressing DMSO decomposition. However, due to the lower temperature of the DMSO distillation column, the DMSO volatilization rate decreased, resulting in slightly lower product purity and recovery rate compared to Example 5.

[0086] In this embodiment, the system's own aqueous distillate is preferentially used as the reflux liquid at the top of each dehydration tower, replacing the externally added demineralized water. Only a small amount of externally added demineralized water is added when the system's water balance is insufficient. Because the reflux ratio parameters of each unit in this embodiment are similar to those in Embodiment 1, the amount of demineralized water used is reduced by approximately 75% to 80% compared to the traditional process, and the final amount of wastewater discharged is reduced by approximately 55% to 60% compared to the traditional process. Trace amounts of DMSO and AN are recovered and returned to the system with the reflux liquid, preventing the loss of effective components.

[0087] Example 7 The difference from Example 1 is that the operating pressures of the second, third, fourth and fifth decompression conditions in this example are all set outside their respective preferred limits, while the other operating conditions are the same as in Example 1.

[0088] Specifically, the operating pressure of the AN dehydration unit is 20 kPaA, and the reboiler temperature is controlled at 75°C; the primary dehydration unit operates under atmospheric pressure at an operating temperature of 105°C; the operating pressure of the secondary dehydration unit is set at 20 kPaA, and the operating temperature is raised to 100°C; the operating pressure of the tertiary dehydration unit is set at 15 kPaA, and the operating temperature is raised to 90°C; the operating pressure of the DMSO distillation purification unit is set at 10 kPaA, and the operating temperature is raised to 128°C; the operating pressure of the evaporation concentration unit is set at 8 kPaA, and the operating temperature is raised to 122°C. The reflux ratios of each dehydration column and the DMSO distillation column are consistent with those in Example 1.

[0089] In this embodiment, the purity of the DMSO product was 97.8%, the total recovery rate of DMSO was 91.5%, the recovery rate of AN was 98.0%, the decomposition rate of DMSO was approximately 5.8%, and the dimethyl sulfide content in the DMSO was approximately 4200 ppm. Due to the generally high operating pressures of the secondary and tertiary dehydration units, as well as subsequent purification and concentration units, the operating temperatures of each corresponding unit increased accordingly. This led to significant thermal decomposition of DMSO in the later stages of dehydration and during the distillation and evaporation stages, causing the product purity to drop below 99.0% and the total recovery rate to decrease significantly compared to Example 1. Simultaneously, the risk of decomposition byproducts entering the spinning system along with the recovered DMSO increased significantly, resulting in poorer system stability. In this embodiment, each stage of the dehydration tower still preferentially used the system's own aqueous distillate as the top reflux liquid, reducing the amount of demineralized water used by approximately 75% to 80% compared to the traditional process. However, due to the increased decomposition byproducts, the pH of the aqueous distillate decreased, requiring additional alkali to neutralize it, increasing alkali consumption by approximately 30% compared to Example 1.

[0090] Example 8 The difference from Example 1 is that in this example, only the fifth decompression condition is set outside the preferred range, while the second, third, and fourth decompression conditions are all kept within their respective preferred ranges, and the remaining operating conditions are the same as in Example 1.

[0091] Specifically, the AN dehydration unit operates at a pressure of 20 kPaA and a reboiler temperature of 75°C; the primary dehydration unit operates at atmospheric pressure and a temperature of 105°C; the secondary dehydration unit operates at a pressure of 15 kPaA and a temperature of 88°C; the tertiary dehydration unit operates at a pressure of 10 kPaA and a temperature of 82°C; the DMSO distillation purification unit operates at a pressure of 6 kPaA and a temperature of 112°C; and the evaporation and concentration unit operates at a pressure of 8 kPaA and a temperature of 122°C.

[0092] In this embodiment, the purity of the DMSO product is 99.4%, the total DMSO recovery rate is 98.2%, the AN recovery rate is 99.2%, the DMSO decomposition rate is approximately 0.55%, and the dimethyl sulfide content in the DMSO is approximately 40 ppm. Compared with Example 1, due to the higher pressure in the evaporation and concentration unit, the operating temperature of this unit increased, leading to partial decomposition of the residual DMSO in the reactor at a higher temperature. Simultaneously, the evaporation efficiency decreased slightly, and the purity of the DMSO recovered in the evaporation section dropped to 94.8%, resulting in a decrease of approximately 0.9 percentage points in the overall recovery rate compared to Example 1. However, thanks to the fact that the first three stages of dehydration and the DMSO distillation purification unit are still within the optimal pressure range, the purity of the main DMSO product can still reach 99.4%, meeting the requirements for recycling at the spinning grade. The amount of demineralized water used is reduced by approximately 75%–80% compared to the traditional process, and the wastewater discharge is reduced by approximately 55%–60%.

[0093] Example 9 The difference from Example 1 is that the operating pressures of the fourth and fifth decompression conditions in this example are set outside their respective preferred limits, while the second and third decompression conditions are kept within their preferred limits, and the remaining operating conditions are the same as in Example 1.

[0094] Specifically, the AN dehydration unit operates at a pressure of 20 kPaA and a reboiler temperature of 75°C; the primary dehydration unit operates at atmospheric pressure and a temperature of 105°C; the secondary dehydration unit operates at a pressure of 15 kPaA and a temperature of 88°C; the tertiary dehydration unit operates at a pressure of 10 kPaA and a temperature of 82°C; the DMSO distillation purification unit operates at a pressure of 10 kPaA and a temperature of 128°C; and the evaporation and concentration unit operates at a pressure of 8 kPaA and a temperature of 122°C.

[0095] In this embodiment, the purity of the DMSO product was 98.3%, the total recovery rate of DMSO was 94.8%, the recovery rate of AN was 99.1%, the decomposition rate of DMSO was approximately 3.5%, and the dimethyl sulfide content in the DMSO was approximately 2200 ppm. Due to the significantly high pressure in the DMSO distillation purification unit, the bottom temperature of the column rose sharply to 128°C, exceeding the thermal decomposition threshold of DMSO, resulting in significant decomposition during the distillation process and a decrease in product purity to 98.3%. Simultaneously, the high pressure in the evaporation and concentration unit further exacerbated the decomposition and reduced the recovery efficiency of residual DMSO, with the overall recovery rate decreasing by approximately 4.3 percentage points compared to Example 1. The sulfur-containing byproducts such as dimethyl sulfide generated from DMSO decomposition not only cause solvent loss, but also, in some cases, affect the spinnability of the subsequent spinning solution when partially collected with the recovered DMSO. In this embodiment, the amount of demineralized water used can still be reduced by approximately 75% to 80% compared to the traditional process, but due to the cumulative effect of decomposition byproducts, the load on the tail gas scrubbing unit increases, and the steam consumption increases by approximately 15% compared to Example 1.

[0096] Example 10 The difference from Example 1 is that the operating pressures of the third, fourth and fifth decompression conditions in this example are all set outside their respective preferred limits, while the second decompression condition is kept within the preferred range, and the remaining operating conditions are the same as in Example 1.

[0097] Specifically, the AN dehydration unit operates at a pressure of 20 kPaA and a reboiler temperature of 75°C; the primary dehydration unit operates at atmospheric pressure and a temperature of 105°C; the secondary dehydration unit operates at a pressure of 15 kPaA and a temperature of 88°C; the tertiary dehydration unit operates at a pressure of 15 kPaA and a temperature of 96°C; the DMSO distillation purification unit operates at a pressure of 10 kPaA and a temperature of 128°C; and the evaporation and concentration unit operates at a pressure of 8 kPaA and a temperature of 122°C.

[0098] In this embodiment, the DMSO product purity was 97.5%, the total DMSO recovery rate was 90.2%, the AN recovery rate was 97.7%, the DMSO decomposition rate was approximately 6.8%, and the dimethyl sulfide content in the DMSO was approximately 5800 ppm. Due to the high pressure in the three continuous units of tertiary dehydration, DMSO distillation, and evaporation concentration, the material already had a high DMSO concentration and was subjected to prolonged superheated conditions during subsequent purification and concentration processes, resulting in DMSO thermal decomposition throughout the entire post-processing flow. The product purity dropped significantly to 97.5%. Simultaneously, AN underwent partial self-polymerization in the tertiary dehydration section due to increased temperature, reducing the AN recovery rate to 97.7%. During system operation, significant coking occurred on the surfaces of the reboiler and retort, shortening the continuous operating cycle to less than 60% of that in Example 1. Although the amount of demineralized water used could still be reduced by approximately 75%–80%, the system required frequent shutdowns for cleaning, resulting in higher overall operating costs compared to traditional processes.

[0099] Comparative Example 1 The polyacrylonitrile polymerization waste liquid with the same composition as in Example 1 (AN content 5.6wt%, DMSO content 88.2wt%, water content 6.2wt%, feed rate 1000kg / h) was recycled using a conventional atmospheric distillation process.

[0100] The polymerization solution first enters the AN removal column, where AN is removed by distillation under atmospheric pressure. Since AN and water form an azeotrope under atmospheric pressure, the bottom temperature of the AN column needs to be maintained at approximately 85°C. The AN-water azeotrope collected from the top of the column is condensed and separated to recover AN, with an AN recovery rate of 97.5% and an AN purity of 98.1%. The removal solution collected from the bottom of the column enters the dehydration column.

[0101] Dehydration and concentration are achieved using a single-stage atmospheric distillation column at an operating pressure of 101 kPaA. The bottom temperature must be maintained above 150°C (actual operating temperature is 152~158°C) to effectively distill off the water from the DMSO solution. The reflux ratio is controlled at 1.5. Water vapor is partially refluxed and partially discharged after condensation. The dehydrated concentrate collected from the bottom of the column contains approximately 92.5 wt% DMSO and approximately 7.2 wt% water.

[0102] The dehydrated concentrate enters a DMSO distillation column for purification under atmospheric pressure, with the bottom operating temperature at approximately 155–162°C. DMSO vapor is partially refluxed after condensation, and partially collected as product. The DMSO product purity is 98.3%, and the DMSO recovery rate is 92.7%. The bottom residue is further treated in an evaporator to recover residual DMSO before being discharged.

[0103] Testing revealed that the content of dimethyl sulfide, a decomposition byproduct, in DMSO was approximately 850 ppm, and the DMSO decomposition rate was approximately 3.6%. The AN recovery rate was 97.5%.

[0104] In this comparative example, the reflux liquid at the top of each dehydration tower and the DMSO distillation tower relies entirely on externally supplied demineralized water, with no internal water-containing distillate reused. The amount of demineralized water used is reduced by 0% compared to the embodiment of this invention, meaning it cannot be reduced further. Since all supplied demineralized water is ultimately distilled off from the top of the tower and enters the wastewater system, the wastewater discharge is equivalent to approximately 95% or more of the total supplied amount (with a small amount lost due to entrainment with the bottom product), meaning the wastewater discharge is reduced by 0%. Compared to the embodiment of this invention, this comparative example not only requires a continuous consumption of a large amount of externally supplied demineralized water but also generates an equivalent amount of wastewater that needs to be treated, forming a synchronized scale-up cycle of discharge and supply. The operating costs and environmental pressure are significantly higher than those of this invention.

[0105] Comparative Example 2 A single pressure reduction scheme was adopted, that is, the operating pressure of each unit was the same, and the polyacrylonitrile polymerization waste liquid with the same composition as in Example 1 (AN content 5.6wt%, DMSO content 88.2wt%, water content 6.2wt%, feed rate 1000kg / h) was recycled and treated.

[0106] The polymerization solution enters the AN demonolysis tower, operating at a pressure of 20 kPaA and a bottom temperature of 75°C, achieving an AN recovery rate of 99.1% and an AN purity of 99.5%. The demonolysis solution then enters a primary dehydration tower, operating at a pressure of 10 kPaA and a bottom temperature of 82°C for dehydration. Due to the reduced separation factor between DMSO and water under reduced pressure, the DMSO content after single-stage dehydration only increases to approximately 88.5 wt%, while the water content remains at 11.2 wt%. Further dehydration requires increasing the reflux ratio to above 2.0, but even then, it is difficult to achieve the 97.2 wt% DMSO content achieved after the three-stage gradient dehydration of this invention.

[0107] The dehydrated concentrate was fed into a DMSO distillation column for purification at 10 kPaA and 118 °C. The DMSO product purity was 98.8%, and the DMSO recovery rate was 94.1%. The bottom liquid was fed into an evaporator for evaporation at 10 kPaA and 115 °C to recover residual DMSO.

[0108] Testing revealed that the content of dimethyl sulfide, a decomposition byproduct, in DMSO was approximately 120 ppm, and the DMSO decomposition rate was approximately 0.8%. Although the reduced pressure operation inhibited DMSO decomposition to some extent, the separation efficiency of the dehydration stage decreased significantly due to the uniform reduced pressure conditions used in each unit. The excessively high water content in the DMSO distillation feed led to an increased separation load on the distillation column, resulting in lower product purity and total recovery rate compared to Example 1.

[0109] In this comparative example, the reflux liquid from the top of each dehydration tower and the DMSO distillation tower also relies entirely on externally supplied demineralized water, without utilizing the internal aqueous distillate. The amount of demineralized water used is reduced by 0% compared to the embodiment of this invention. Although the single vacuum reduction scheme inhibits DMSO decomposition to some extent, since the reflux liquid is still externally supplied demineralized water, its supply is ultimately entirely discharged as wastewater, with wastewater discharge accounting for over 95% of the total supply, resulting in a 0% reduction in wastewater discharge. Compared to this invention, which uses the system's own aqueous distillate to replace externally supplied demineralized water to achieve a closed-loop cycle of waste-to-supply, this comparative example still suffers from high demineralized water consumption and high wastewater discharge, making it inferior to this invention in terms of both economy and environmental friendliness.

[0110] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A polyacrylonitrile polymerization waste liquid recovery system, characterized in that, Including those connected sequentially according to the material flow direction: The AN monomer removal unit is used to separate and recover AN monomer from the polymerization solution under a first reduced pressure to obtain the monomer removal reactor liquid; A primary dehydration unit is connected to the outlet of the AN dehydration unit and is used to initially remove water from the dehydration kettle liquid under normal pressure to obtain a first dehydration concentrate and a first aqueous distillate. A secondary dehydration unit is connected to the outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under a second reduced pressure to obtain a second dehydration concentrate and a second aqueous distillate. A third-stage dehydration unit is connected to the outlet of the second-stage dehydration unit and is used to remove water from the second dehydration concentrate under a third reduced pressure condition to obtain a third dehydration concentrate and a third aqueous distillate. The DMSO distillation and purification unit is connected to the outlet of the three-stage dehydration unit and is used to distill and purify the third dehydration concentrate under the fourth reduced pressure condition to obtain DMSO and residue. An evaporation and concentration unit is connected to the outlet of the DMSO distillation and purification unit and is used to evaporate and concentrate the residue in the reactor under the fifth reduced pressure condition and recover the residual DMSO. The operating pressures under the first, second, third, fourth, and fifth pressure reduction conditions decrease sequentially.

2. The system according to claim 1, characterized in that, The operating pressure of the first decompression condition of the AN single-unit is 18~22 kPaA.

3. The system according to claim 1, characterized in that, Satisfy at least one of the following characteristics: (a) The operating pressure under the second pressure reduction condition is 13~17 kPaA; (b) The operating pressure of the third pressure reduction condition is 8~12 kPaA; (c) The operating pressure of the fourth pressure reduction condition is 5~7 kPaA; and, (d) The operating pressure of the fifth pressure reduction condition is 3~5 kPaA.

4. The system according to claim 1, characterized in that, The system satisfies at least one of the following characteristics: (e) The temperature of the bottom of the AN column in the AN de-monograph unit is controlled at 50~90℃; (f) The operating temperature of the primary dehydration unit is controlled at 100~110℃; (g) The operating temperature of the secondary dehydration unit is controlled at 80~95℃; (h) The operating temperature of the three-stage dehydration unit is controlled at 75~125℃; (i) The operating temperature of the DMSO distillation and purification unit is controlled at 95~120℃; and, (j) The operating temperature of the evaporation and concentration unit is controlled at 80~115℃.

5. The system according to any one of claims 1 to 4, characterized in that, The primary dehydration unit includes: A primary dehydration tower, connected to the outlet of the AN dehydration unit, is used to remove water from the dehydration solution under normal pressure. The primary dehydration tower is provided with a top outlet and a bottom outlet. The first dehydration concentrate is collected from the bottom outlet of the primary dehydration tower. A primary dehydration condenser, connected to the top outlet of the primary dehydration tower, is used to condense water-containing vapor to obtain a first aqueous distillate; and, A primary dehydration reflux tank, connected to the outlet of the primary dehydration condenser, is used to collect the first aqueous distillate; And / or, The secondary dehydration unit includes: A secondary dehydration tower is connected to the bottom outlet of the primary dehydration unit and is used to remove water from the first dehydration concentrate under the second reduced pressure condition; the secondary dehydration tower is provided with a top outlet and a bottom outlet; the second dehydration concentrate is collected from the bottom outlet of the secondary dehydration tower; A secondary dehydration condenser, connected to the top outlet of the secondary dehydration tower, is used to condense water-containing vapor to obtain a second water-containing distillate; and, A secondary dehydration reflux tank, connected to the outlet of the secondary dehydration condenser, is used to collect the second aqueous distillate; And / or, The three-stage dehydration unit includes: A three-stage dehydration tower is connected to the bottom outlet of the two-stage dehydration unit and is used to remove water from the second dehydration concentrate under the third reduced pressure condition. The three-stage dehydration tower is provided with a top outlet and a bottom outlet. The third dehydration concentrate is collected from the bottom outlet of the three-stage dehydration tower. A three-stage dehydration condenser, connected to the top outlet of the three-stage dehydration tower, is used to condense water-containing vapor to obtain a third water-containing distillate; and A three-stage dehydration reflux tank is connected to the outlet of the three-stage dehydration condenser and is used to collect the third aqueous distillate.

6. The system according to any one of claims 1 to 4, characterized in that, The DMSO distillation and purification unit includes: A DMSO distillation column is connected to the outlet of the three-stage dehydration unit and is used to distill the third dehydration concentrate under the fourth reduced pressure condition. The DMSO distillation column is provided with a top outlet and a bottom outlet. DMSO vapor is collected from the top outlet of the DMSO distillation column, and the bottom residue is collected from the bottom outlet of the DMSO distillation column. A distillation column condenser, connected to the top vapor outlet of the DMSO distillation column, is used to condense the DMSO vapor to obtain liquid DMSO; and A distillation column reflux tank, connected to the outlet of the distillation column condenser, is used to collect the liquid DMSO and provide reflux; The distillation column reflux tank is provided with a first gas phase outlet, which is connected to a first injection system for extracting non-condensable gases to control the vacuum level of the DMSO distillation column.

7. The system according to claim 6, characterized in that, The evaporation and concentration unit includes: An evaporator, connected to the bottom outlet of the DMSO distillation column, is used to heat the residue in the vessel, causing the residual DMSO in the residue to evaporate and obtain recovered DMSO vapor; An evaporator-condenser, connected to the outlet of the evaporator, is used to condense the recovered DMSO vapor to obtain liquid recovered DMSO; and An evaporator condensate tank, connected to the evaporator condenser outlet, is used to collect the liquid recovered DMSO; The evaporator condensate tank is provided with a second gas phase outlet, which is connected to a second injection system for extracting non-condensable gases to control the vacuum level of the evaporator.

8. The system according to any one of claims 1 to 4, characterized in that, The primary dehydration unit, the secondary dehydration unit, and the tertiary dehydration unit each independently use the system reflux medium for reflux operation; The system reflux medium includes at least one of the first aqueous distillate, the second aqueous distillate, or the third aqueous distillate.

9. The system according to claim 7, characterized in that, Also includes: A solvent storage unit, connected to the outlet of the AN monomer removal unit and the outlet of the DMSO distillation and purification unit, is used to store the recovered AN monomer and DMSO. The exhaust gas scrubbing unit is connected to the gas phase outlet of the first injection system and the second injection system, and is used to treat the non-condensable gases generated by the system. An alkali solution preparation unit, connected to the primary, secondary, and tertiary dehydration units, is used to prepare and supply the alkali solution required for the process to the dehydration units; and The vacuum vacuum unit is connected to the AN dehydration unit, the secondary dehydration unit, the tertiary dehydration unit, the DMSO distillation purification unit, and the evaporation concentration unit, and is used to provide vacuum conditions.

10. A method for recovering polyacrylonitrile polymerization waste liquid, characterized in that, The system described in any one of claims 1 to 9 is implemented by comprising the following steps: The polyacrylonitrile polymerization waste liquid is passed into the AN monomer removal unit, and the AN monomer is separated and recovered under the first depressurization condition to obtain the monomer removal kettle liquid. The dehydration solution is passed into the first-stage dehydration unit to initially remove water from the dehydration solution under normal pressure, resulting in a first dehydration concentrate. The first dehydrated concentrate is passed into a secondary dehydration unit, and the water in the first dehydrated concentrate is removed under a second reduced pressure condition to obtain a second dehydrated concentrate. The second dehydrated concentrate is passed into a three-stage dehydration unit, and the water in the second dehydrated concentrate is removed under the third reduced pressure condition to obtain the third dehydrated concentrate. The third dehydrated concentrate is passed into the DMSO distillation and purification unit, and the DMSO is purified by distillation under reduced pressure to obtain DMSO and residue. The residual liquid in the reactor is passed into the evaporation and concentration unit, where it is evaporated and concentrated under reduced pressure to recover the residual DMSO.