High-purity purification process for N-methyl pyrrolidone

Abstract

The invention relates to the technical field of materials, particularly discloses a purification process of high-purity N-methyl pyrrolidone (NMP), and relates to the processes of pretreatment rectification, complexing rectification, membrane separation, catalytic hydrogenation, ion exchange and the like. NMP-water azeotropy is broken through heterogeneous azeotropic distillation, polar impurities and metal ions are synchronously removed by using a bifunctional complexing adsorbent, macromolecular impurities are intercepted by combining a ceramic membrane coupling technology, energy consumption is reduced by adopting a heat pump distillation system, and the problem of equipment corrosion is solved through material upgrading and process optimization. According to the process, the purity of NMP can be improved to 99.995% or above, impurities such as water, aromatic hydrocarbon and metal ions are remarkably removed, and the process is suitable for high-end fields such as lithium batteries and semiconductors and has the characteristics of high efficiency, low consumption and stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fine chemical separation and purification, and particularly relates to a high-purity purification process for N-methyl pyrrolidone (NMP). BACKGROUND

[0002] The existing technology has complex impurities and high purification difficulty. The NMP industrial crude product usually contains the following impurities: polar impurities: water (0.5%-3%), pyrrolidone (0.1%-1%), amines (such as dimethylamine, 0.05%-0.5%); non-polar impurities: aromatic hydrocarbons (such as benzene, 50-200 ppm), long-chain alkanes (C10-C16, 20-100 ppm); special impurities: metal ions (Fe 3+ , Cu 2 + , 1-5 ppm), polymer precursors (such as olefin oligomers, 0.01%-0.1%). The existing process (such as ordinary rectification, activated carbon adsorption) cannot simultaneously and efficiently remove multiple types of impurities, resulting in a low yield (<70%) of high-purity NMP (purity ≥ 99%).

[0003] High energy consumption and serious equipment corrosion: NMP forms azeotrope with water (azeotrope temperature 122℃, water content about 11%), and traditional atmospheric rectification needs repeated distillation, with energy consumption 40%-60% higher than the theoretical value; under acidic or alkaline conditions (such as using NaOH for drying), NMP is prone to hydrolysis to generate corrosive substances (such as pyrrolidone and methanol), resulting in an annual average corrosion rate of 0.3-0.5 mm of the equipment.

[0004] Insufficient product stability: trace impurities (such as metal ions, olefin oligomers) can accelerate the oxidation and deterioration of NMP during storage, experiments show that NMP containing Fe 3+ >1 ppm is stored at 60℃ for 3 months, the colority (APHA) increases from <10 to >50, and the acid value (mgKOH / g) increases from <0.01 to 0.12, affecting high-end application scenarios (such as semiconductor-grade NMP requires colority <5 and acid value <0.005). SUMMARY

[0005] Therefore, the application provides a high-purity N-methyl pyrrolidone (NMP) purification process, which is particularly suitable for high-purity NMP demand scenarios such as lithium battery electrolyte and semiconductor photoresist, and solves the key problems of incomplete impurity removal, high energy consumption, and equipment corrosion in the existing process. The process comprises the following steps:

[0006] (1) removing light and heavy boiling substances from the crude NMP through pretreatment rectification;

[0007] (2) removing water and polar impurities through complex rectification tower, and introducing n-hexane as an entrainer;

[0008] (3) Membrane separation unit removes polymer precursors and high molecular weight impurities;

[0009] (4) Catalytic hydrogenation removes aromatic impurities;

[0010] (5) Ion exchange resin removes acid value in depth;

[0011] (6) The finished product is stored after online detection.

[0012] Further, the complex rectification column uses a combination of packing and plate column structure, loaded with supported molecular sieve catalyst, operating pressure 0.05-0.1 MPa, temperature 90-110℃.

[0013] Further, the bifunctional complex adsorbent is porous silica gel loaded with organic amine, loading 15-20%, and containing 0.5-1% nano copper particles.

[0014] Further, the membrane separation unit uses Al2O3 ceramic microfiltration membrane (pore size 0.2 μm) combined with ZrO2 ceramic ultrafiltration membrane (molecular weight cut-off 500 Da), operating pressure 0.3-0.5 MPa.

[0015] Further, the catalytic hydrogenation process uses Pd / Al2O3 catalyst, Pd loading 0.3-0.5%, reaction temperature 150-180℃, pressure 1.0-1.5 MPa.

[0016] Further, the heat pump rectification system uses the vapor compression energy at the top of the column as the heat source at the bottom of the column, reducing energy consumption by 30-50% compared to traditional rectification.

[0017] Further, the entire process uses 316L stainless steel material and PTFE lining, the system pH is controlled at 6.5-7.5, the corrosion rate is ≤0.1 mm / year,

[0018] Beneficial technical effects:

[0019] Impurities are completely removed, and product purity is significantly improved

[0020] Using the "complex rectification-membrane separation-catalytic hydrogenation-ion exchange" multi-stage purification system, the water content in industrial crude NMP can be reduced from 1.5% to below 0.01%, the benzene content can be reduced from 100 ppm to below 0.1 ppm, the removal rate of metal ions (Fe 3+ , Cu 2+ ) is ≥99.9%, the acid value is reduced to below 0.002 mgKOH / g, the product purity is above 99.995%, meeting the stringent requirements of semiconductor-grade NMP on color ( <5 APHA), particle size ( >0.2 μm impurities <1 ppm), solving the problem of incomplete removal of multiple impurities in existing processes.

[0021] Energy consumption is greatly reduced, and process economy is significantly improved

[0022] The introduction of heat pump rectification energy integrated system uses the tower top vapor compression as the tower bottom heat source. The comprehensive energy consumption is reduced from 3500-4000 kJ / kg of traditional process to 1800-2200 kJ / kg, with a reduction of 48.6%-55%. At the same time, the entrainer (n-hexane) is recycled through the phase separator, with a loss rate of <3%, and combined with the adsorbent regeneration process (reuse ≥50 times), the consumption of raw and auxiliary materials is significantly reduced, and the economy of industrial production is improved.

[0023] Device corrosion control and stability improvement

[0024] The whole process uses 316L stainless steel material and PTFE lining, combined with precise pH control (6.5-7.5) and online corrosion monitoring, the annual average corrosion rate of the equipment is reduced from 0.3-0.5 mm to <0.1 mm, and the service life is extended by more than 3 times. Through catalytic hydrogenation and ion exchange process, metal ions and acidic substances that cause NMP oxidation are effectively removed. The color (APHA) of the product is <5, the acid value is <0.005 mgKOH / g, and the oxidation induction period is ≥200 hours after 3 months of storage at 60℃, which is 5 times more stable than the existing process, meeting the long-term storage and high-end application requirements. DETAILED DESCRIPTION

[0025] Example 1

[0026] Through the multi-dimensional technology integration of "multi-stage rectification-complexation adsorption-membrane separation-catalytic refining", a high-purity NMP purification process is provided to achieve the following goals:

[0027] Simultaneous removal of polar, non-polar and special impurities, product purity ≥99.995%;

[0028] Energy consumption is reduced by 30%-50%, and the equipment corrosion rate is reduced to <0.1 mm / year;

[0029] Improve product storage stability, oxidation induction period extended to ≥200 hours (120℃, air atmosphere).

[0030] (I) Innovative design of process route

[0031] Core process:

[0032] Crude NMP → Pretreatment section (light / heavy boiling) → Complexation rectification section (remove water and polar impurities) → Membrane separation section (retain high molecular weight impurities) → Catalytic adsorption section (remove metal ions and aromatic hydrocarbons) → Product storage tank

[0033] (II) Key innovation points

[0034] Heterogeneous azeotropic distillation column design

[0035] Innovative structure: "filling + plate tower combination" (lower section of the packing tower, the upper section of the plate tower), filled with new hydrophobic catalysts (such as supported molecular sieve, SiO2-Al2O3-MgO, specific surface area ≥800m 2 / g);

[0036] Process parameters:

[0037] Pressure: 0.05 MPa (absolute pressure), temperature: 90-110℃;

[0038] The entrainer (n-hexane, the amount of crude product is 5-8%) is introduced, and the ternary azeotrope (boiling point 62℃, water: n-hexane: NMP = 3:7:1, mass ratio) is formed with water;

[0039] Technical breakthrough:

[0040] The water content is reduced from 1.5% to less than 0.01%, and the efficiency is improved by 3 times compared with the traditional rectification;

[0041] The entrainer can be recycled through the phase separator, and the loss rate is less than 3%.

[0042] Development of bifunctional complex adsorbent

[0043] Composition:

[0044] Main body: porous silica gel (pore size 5-10 nm) loaded with organic amine (such as triethanolamine, loading amount 15-20%);

[0045] Auxiliary agent: nano copper particles (particle size 20-50 nm, content 0.5-1%);

[0046] Mechanism of action:

[0047] Polar impurity adsorption: organic amine captures pyrrolidone and amine through hydrogen bond, and the adsorption capacity reaches 120mg / g adsorbent;

[0048] Metal ion chelation: nano copper chelates Fe 3+ , Cu 2+ through electron transfer, removal rate ≥99.9%;

[0049] Regeneration process: 150℃ nitrogen blowing for 4 hours, repeated use ≥50 times, adsorption efficiency retention rate >95%.

[0050] Ceramic membrane microfiltration-ultrafiltration combined technology

[0051] Membrane module:

[0052] Microfiltration (MF): Al2O3 ceramic membrane, pore size 0.2 μm, cut off polymer precursors (particle size > 0.1 μm);

[0053] Ultrafiltration (UF): ZrO2 ceramic membrane, molecular weight cut-off value 500 Da, remove olefin oligomers (molecular weight > 300 Da);

[0054] Operating conditions:

[0055] Pressure: 0.3-0.5 MPa, temperature: 40-50℃, cross-flow rate 3-5 m / s;

[0056] Effect: The content of particulate matter (> 0.2 μm) is reduced from 5000 ppm to < 1 ppm, and the concentration of high molecular weight impurities is reduced from 0.1% to < 0.001%.

[0057] Catalytic hydrogenation de-aromatization technology

[0058] Reactor design: fixed bed reactor, packed with Pd / Al2O3 catalyst (Pd loading 0.3-0.5%, particle size 2-3 mm);

[0059] Process parameters:

[0060] Temperature: 150-180℃, pressure: 1.0-1.5 MPa, hydrogen / oil ratio (volume ratio): 500:1;

[0061] Reaction mechanism: aromatics (such as benzene) undergo addition reaction with H2 under Pd catalysis to form cyclohexane (boiling point 80.7℃), which is subsequently separated by distillation;

[0062] Indicators: benzene content is reduced from 100 ppm to < 0.1 ppm, hydrogenation selectivity ≥ 99%.

[0063] Ion exchange resin deep deacidification

[0064] Resin selection: strong basic anion exchange resin (such as IRA-400, Cl-type), exchange capacity ≥ 350 mmol / L;

[0065] Operation mode: dynamic column chromatography, flow rate 5-10 BV / h (bed volume / hour);

[0066] Effect: acid value (calculated as HCl) is reduced from 0.05 mg KOH / g to < 0.002 mg KOH / g, reaching semiconductor grade standards.

[0067] Heat pump distillation energy integration system

[0068] Technical solution:

[0069] Adopting "tower top vapor compression-tower bottom liquid flashing" heat pump system, using compressor to increase pressure of low temperature vapor (95℃) at tower top to 0.3MPa (temperature 120℃), as heat source of tower bottom reboiler;

[0070] Energy consumption comparison:

[0071] Traditional rectification energy consumption: 3500-4000kJ / kg NMP;

[0072] Energy consumption of the application: 1800-2200kJ / kg NMP, reduction of 48.6%-55%.

[0073] Full-process anti-corrosion process design

[0074] Material upgrading:

[0075] Rectification tower and pipeline adopt 316L stainless steel (containing 2-3% molybdenum), surface electrolytic polishing (roughness Ra≤0.2μm);

[0076] Reaction kettle lining polytetrafluoroethylene (PTFE), thickness≥5mm;

[0077] Process optimization:

[0078] Avoid using strong acid / alkaline reagent, complex adsorption stage pH control at 7.0±0.2;

[0079] System is provided with online pH monitoring and automatic liquid supplementing device, real-time adjustment of corrosion potential (ORP) to below-100mV.

[0080] III. Process parameters and effects

[0081] (I) Impurity removal efficiency of each section

[0082]

[0083]

[0084] (II) Comparison of finished product quality indexes

[0085]

[0086] Example 2

[0087] 5000t / a high purity NMP purification production line

[0088] Process parameters

[0089] Pretreatment rectification tower:

[0090] Tower type: plate tower (30 layers of float valve tray), treatment capacity 200t / h;

[0091] Operating conditions: pressure 0.1 MPa, tower top temperature 102°C, tower bottom temperature 170°C, light distillate (mainly dimethylamine, acetone) recovery.

[0092] Complex distillation system:

[0093] Tower type: combined tower of packing + plate (packing section height 8 m, plate section 15 layers);

[0094] Entrainment agent: n-hexane, flow rate 10 t / h;

[0095] Phase separator: volume 50 m 3 , temperature control 30°C, upper n-hexane recycled, lower water phase NMP content <0.5%.

[0096] Membrane separation unit:

[0097] Microfiltration membrane: flux 500 L / m 2 ·h, operating cycle 8 hours, backwash pressure 0.8 MPa;

[0098] Ultrafiltration membrane: flux 200 L / m 2 ·h, concentration factor 5 times, concentrated liquid sent to recovery system.

[0099] Catalytic hydrogenation reactor:

[0100] Specifications: Φ1.2 m x 6 m, catalyst loading 3 m 3 ;

[0101] Operating cycle: 1200 hours, catalyst regeneration method: air charring + hydrogen reduction.

[0102] Operating results

[0103] Energy consumption: comprehensive energy consumption 2050 kJ / kg NMP, reduced by 51.3% compared to traditional process;

[0104] Yield: high-purity NMP (≥99.995%) yield 85.6%, increased by 18.7% compared to traditional process;

[0105] Equipment corrosion: after 1 year of operation, the tower wall thickness was detected, the corrosion rate was 0.08 mm / year, meeting the requirements of long-term operation.

[0106] Example 3

[0107] I. Typical scheme of traditional NMP purification process (based on the background technology of the specification)

[0108] Scheme 1: single atmospheric distillation + activated carbon adsorption process

[0109] Process steps:

[0110] Pretreatment of crude product: crude NMP was distilled in a plate column (30 theoretical plates, reflux ratio 10:1) at atmospheric pressure, with the overhead temperature at 122°C (NMP-water azeotrope temperature), and the bottom product being the preliminary dehydrated NMP with water content reduced to 50-200 ppm;

[0111] Activated carbon adsorption: 5% (w / w) granular activated carbon was added to the distilled NMP, and stirred for 2 hours at room temperature to adsorb the aromatics and some polar impurities. The benzene residue was 1-5 ppm, and the metal ion (Fe 3+ , Cu 2+ ) residue was 0.1-0.5 ppm;

[0112] Alkaline washing and drying: NaOH solid was used for further dehydration, and the system pH was increased to 10-12, which led to the hydrolysis of NMP to form pyrrolidone and methanol, and the equipment corrosion rate was 0.3-0.5 mm / year.

[0113] Key parameters:

[0114]

[0115] Technical defects:

[0116] NMP-water azeotrope cannot be broken, and the water residue is high;

[0117] Activated carbon adsorption has poor selectivity, and the removal of aromatics and metal ions is not complete;

[0118] Basic conditions lead to serious equipment corrosion and NMP hydrolysis side reactions.

[0119] Scheme 2: ordinary complex distillation + single adsorption process

[0120] Process steps:

[0121] Complex distillation: ethylene glycol was introduced as an entrainer (10-15% of the mass of the crude product), and azeotropic distillation was carried out at 0.1 MPa, with the water content reduced to 200-500 ppm. However, ethylene glycol and NMP form a high-boiling azeotrope, which is difficult to separate;

[0122] Single adsorption impurity removal: activated carbon or molecular sieve adsorbents can only remove a single type of impurity (such as polar impurities or aromatics), and cannot simultaneously remove water, metal ions and high molecular weight impurities;

[0123] Simple filtration: cellulose filter membranes (pore size 1-5 μm) were used for filtration, which can only remove particles with a particle size >1 μm, and the removal rate of high molecular weight impurities (such as olefin oligomers) is <50%.

[0124] Key parameters:

[0125] Complexing rectification temperature 140-160℃, entrainer ethylene glycol loss rate 10-15%;

[0126] Adsorbent is discarded after single use and cannot be regenerated;

[0127] Low filtration flux (<100L / m 2 ·h) and easy to be blocked.

[0128] II. Comparison between existing process and the process of the present application

[0129]

[0130]

[0131] III. Analysis of technical bottlenecks of existing process

[0132] Limitation of azeotropic system: traditional rectification does not introduce high-efficiency entrainer, and NMP-water azeotropy leads to low dehydration efficiency, and repeated distillation is required (energy consumption is 40-60% higher than the theoretical value);

[0133] Insufficient adsorption selectivity: active carbon, single molecular sieve and other adsorbents cannot simultaneously remove polar impurities (water, amines), non-polar aromatic hydrocarbons and metal ions, and the residual amount of impurities exceeds the semiconductor grade standard;

[0134] Corrosion and side reaction: using NaOH drying or NMP hydrolysis under acidic conditions generates corrosive substances, shortens the service life of equipment (annual corrosion rate is 0.3-0.5mm), and the side reaction leads to an increase in product acid value;

[0135] Absence of high-molecular impurity treatment: lack of high-efficiency interception means such as membrane separation, and the residual polymer precursor accelerates the oxidation and deterioration of NMP, and the storage stability is poor (colority increases by >50APHA after 3 months of storage at 60℃).

[0136] Impurity removal efficiency verification experiment

[0137] Method: take industrial crude NMP (purity 98.5%, water 1.2%, benzene 120ppm, Fe 3+ 0.8ppm), and process according to the process of the present application, and detect the impurity content of each section.

[0138] Results:

[0139]

[0140] Conclusion: each section has a significant removal effect on the target impurities, and the final product meets the semiconductor grade NMP standard.

[0141] Summary

[0142]

[0143]

[0144] Comparative Example: High-purity NMP Purification Process Synergy Comparison Experiment

[0145] Experimental Purpose

[0146] To verify the synergy of the process (multi-stage rectification-complex adsorption-membrane separation-catalytic refining integration) of the present application compared to the traditional process (single rectification + activated carbon adsorption) in terms of impurity removal efficiency, energy consumption, product stability, etc.

[0147] Experimental Design

[0148] Three groups of comparison processes were designed to process the same batch of industrial crude NMP (purity 98.5%, water 1.2%, benzene 120 ppm, Fe 3+ 0.8 ppm, acid value 0.045 mgKOH / g), and the following indicators were tested:

[0149] Impurity removal rate: water, benzene, Fe 3+ , residual concentration of acid value;

[0150] Energy consumption: energy consumption per kilogram of NMP purification (kJ / kg);

[0151] Product stability: change in color (APHA) and acid value after 3 months of storage at 60°C.

[0152] Comparison of Process and Parameters

[0153]

[0155] Comparison of Experimental Results

[0156]

[0157] Result Analysis

[0158] Impurity removal efficiency synergy

[0159] Limitations of traditional process (Group A):

[0160] Water and NMP azeotrope, resulting in incomplete dehydration, residual amount 180 ppm, far exceeding the standard for semiconductor grade (<10 ppm);

[0161] Activated carbon has poor selectivity for benzene and metal ion adsorption, with benzene residue of 85 ppm and Fe 3+ residual 0.55 ppm;

[0162] Acid value only decreased by 37.8%, which cannot meet the needs of high-end applications.

[0163] Improvements and bottlenecks of improved process (Group B):

[0164] Complexation rectification reduces water content to 25 ppm, and catalytic hydrogenation removes benzene to 5.2 ppm, but Fe 3+ The residue still reaches 0.12 ppm (semiconductor grade requires <0.1 ppm);

[0165] Lack of membrane separation step, high molecular impurities are not effectively removed, and the color rises to 12 APHA after storage (original value 8).

[0166] The process of the present application (group C) has the following advantages:

[0167] Multi-stage separation synergy: complexation rectification (dehydration) + membrane separation (remove high molecules) + catalytic hydrogenation (dearomatic) + ion exchange (remove acid) to form a closed loop of impurity removal, and each impurity residue meets the semiconductor grade standard;

[0168] Adsorption-catalysis synergy: dual-function adsorbent (organic amine + nano copper) simultaneously removes polar impurities and metal ions, with efficiency improved by more than 90% compared to traditional activated carbon.

[0169] Energy consumption optimization mechanism

[0170] The traditional process (group A) has a high energy consumption of 3800 kJ / kg due to repeated distillation of azeotrope;

[0171] The present application (group C) recovers tower top steam energy through a heat pump system, reducing energy consumption by 46.1% compared to group A and 21.2% compared to group B, confirming the effectiveness of energy integration design.

[0172] Product stability is improved

[0173] Fe 3+ , aromatic hydrocarbons and other impurities become oxidation reaction catalysts, and the acid value increases by 203% after storage;

[0174] The present application removes Fe 3+ <0.01 ppm, benzene <0.1 ppm) and corrosion prevention design, and the acid value only increases by 33% after 3 months of storage, with almost no change in color, and the stability is improved by more than 5 times.

[0175] Conclusion

[0176] Synergistic hierarchy:

[0177] Separation technology synergy: complexation rectification solves the azeotrope problem, membrane separation makes up for the short board of traditional filtration, catalytic hydrogenation overcomes the bottleneck of aromatic removal, and ion exchange realizes precise control of acid value;

[0178] Energy and material synergy: heat pump system reduces energy consumption, corrosion-resistant materials and process pH control reduce side reactions, and together guarantee product purity and stability;

[0179] Impurity removal path coordination: design special removal modules for polarity, non-polarity, metal ions and macromolecular impurities, to avoid "trade-off" in single process.

[0180] Data support: compared with traditional process, the total impurity removal rate of the process is increased by 40-60%, the energy consumption is reduced by 46.1%, and the storage stability is increased by 5 times, which confirms the significant synergistic effect of multi-dimensional technology integration.

Claims

1. A purification process for high-purity N-methylpyrrolidone (NMP), characterized in that, Includes the following steps: (1) Crude NMP is pretreated by distillation to remove light and heavy boiling substances. The pretreatment distillation adopts a plate column structure with 30 floating valve plates, operating pressure of 0.1 MPa, top temperature of 102°C, and bottom temperature of 170°C. The light boiling substances include dimethylamine and acetone. (2) Water and polar impurities are removed by complexation distillation column, and n-hexane is introduced as an entrainer. The complexation distillation column adopts a combination structure of packing and plate column, with the lower section being a packed column and the upper section being a plate column, which is filled with supported molecular sieve catalyst. (3) The membrane separation unit uses Al2O3 ceramic microfiltration membrane and ZrO2 ceramic ultrafiltration membrane in combination to remove polymer precursors and high molecular impurities; (4) The catalytic hydrogenation process uses a Pd / Al2O3 catalyst to remove aromatic impurities; (5) The acid value is removed by deep removal of ion exchange resin, wherein the ion exchange resin is a strong base anion exchange resin; (6) The finished product is stored after online testing of its purity, water content and acid value.

2. The process according to claim 1, characterized in that, The complexation distillation column operates at a pressure of 0.05-0.1 MPa and a temperature of 90-110°C. The amount of n-hexane entrainer is 5-8% of the mass of crude NMP and is recycled through a phase separator.

3. The process according to claim 1 or 2, characterized in that, The supported molecular sieve catalyst is a SiO2-Al2O3-MgO composite molecular sieve with a specific surface area ≥800 m². 2 / g, the organic amine-loaded porous silica adsorbent has an organic amine loading of 15-20% and contains 0.5-1% nano copper particles.

4. The process according to claim 1, characterized in that, The Al2O3 ceramic microfiltration membrane has a pore size of 0.2 μm, the ZrO2 ceramic ultrafiltration membrane has a molecular weight cutoff of 500 Da, the membrane separation operation pressure is 0.3-0.5 MPa, the temperature is 40-50℃, and the crossflow velocity is 3-5 m / s.

5. The process according to claim 1, characterized in that, The Pd / Al2O3 catalyst has a Pd loading of 0.3-0.5%, a catalytic hydrogenation reaction temperature of 150-180℃, a pressure of 1.0-1.5MPa, and a hydrogen-to-oil volume ratio of 500:

1.

6. The process according to claim 1, characterized in that, The ion exchange resin is IRA-400 type strong basic anion exchange resin Cl- type, with an exchange capacity ≥350mmol / L and an operating flow rate of 5-10BV / h (bed volume / hour).

7. The process according to claim 1, characterized in that, The online detection indicators include NMP purity, water content, acid value, and color.

8. The process according to claim 1, characterized in that, The reboiler in the pretreatment rectification section is a polymer precursor with a boiling point higher than NMP, which is collected from the bottom of the column.

9. The process according to claim 1, characterized in that, The entire process uses 316L stainless steel and PTFE lining, and the system pH is controlled between 6.5 and 7.

5.

10. The process according to claim 1, characterized in that, The complexation distillation column is used in conjunction with a heat pump distillation system, utilizing the vapor compression energy at the top of the column as the heat source at the bottom of the column.

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