Process for the recovery of n-methylpyrrolidone from complex waste solutions

Abstract

The disclosed method for recovering N-methyl pyrrolidone from complex waste liquid includes the following processes: pumping the NMP-containing waste liquid into a rectification one-effect tower after vacuum dehydration in a dehydration tower, condensing and collecting NMP at the top of the tower, and pumping the tower bottom into a rectification two-effect tower; obtaining NMP at the top of the rectification two-effect tower, sending the liquid residue to a depolymerization reactor after evaporation of the gas phase in a thin film evaporator, and condensing and condensing; performing side line extraction on the rectification two-effect tower and injecting sulfolane to extract heavy components, separating after mixing, returning the light phase to the rectification two-effect tower, and sending the heavy phase to a regeneration tower; collecting NMP-water azeotrope at the top of the regeneration tower to return to the dehydration tower, collecting sulfolane at the side line to return to the extraction cycle, and sending the concentrated residue at the bottom to the depolymerization reactor; sending the depolymerization gas obtained from the depolymerization reactor to a quenching tower, sending the cooling liquid of the quenching tower to a light-removing tower, sending the overhead product of the light-removing tower to sewage treatment, and collecting NMP at the tower bottom. The recovery treatment method has reasonable and efficient process flow, and has higher NMP yield for complex NMP-containing waste liquid.

Description

Technical Field

[0001] This invention belongs to the field of waste liquid recovery, specifically relating to a method for recovering N-methylpyrrolidone from complex waste liquids. Background Technology

[0002] N-methylpyrrolidone (NMP) is a high-performance organic solvent with excellent solubility and stability. It can dissolve a wide variety of organic and inorganic substances and is widely used in industries such as lithium batteries, pharmaceuticals, pesticides, electronics, and petrochemicals. For example, in the chemical industry, it is used as a solvent in polymerization reactions, coatings, inks, and adhesives; in the electronics industry, it is used as a solvent for dissolving and dispersing electrode materials in lithium battery production, and is also applied in areas such as conductor precision instruments and circuit board cleaning, electronic cleaning, photoresists, and LCD liquid crystal materials; in the pharmaceutical industry, it is used as a solvent or reaction medium in drug synthesis for drug purification and crystallization; it can also be used in natural gas desulfurization and lubricating oil refining.

[0003] The waste solvents generated during the production process of the aforementioned industries, i.e., NMP-containing waste liquids, require the recovery of their NMP components and harmless treatment. Traditional recovery methods can achieve high recovery purity (NMP purity greater than 99%) through distillation systems. However, the NMP recovery rate depends on the complexity of the waste liquid. Simple waste liquids can achieve high recovery rates, but for complex NMP-containing waste liquids, since NMP exists not only in a free state but also in complex forms and is encapsulated and bound by polymers, conventional distillation systems struggle to separate and release these "bound" NMP components. Furthermore, the heavy components in the distillation column, after concentration, further bind the NMP, making this portion of NMP even more difficult to recover.

[0004] Therefore, to further improve the NMP recovery rate for NMP-containing waste liquids with complex compositions, it is necessary to improve the existing recovery process. Summary of the Invention

[0005] In view of the above, in order to improve the recovery rate of N-methylpyrrolidone in complex waste liquids, this application provides another recovery method.

[0006] A method for recovering N-methylpyrrolidone from complex waste liquid is disclosed, comprising the following steps:

[0007] (1) The NMP-containing waste liquid is dehydrated by vacuum in a dehydration tower and then pumped into the first-effect distillation tower. The first-effect distillation tower is operated at atmospheric pressure, with the top temperature controlled at 200-205℃, the bottom temperature controlled at 220-230℃, and the reflux ratio controlled at 1.5-2.0. The top of the first-effect distillation tower is condensed to collect industrial-grade NMP (purity ≥99.5%), and a small amount of electronic-grade NMP (purity ≥99.9%) is collected from the side stream. The heavy components in the bottom of the tower are pumped into the second-effect distillation tower.

[0008] (2) The distillation double-effect column is operated under negative pressure, with the operating pressure controlled at -0.094 to -0.096 MPa, the top temperature controlled at 195-200℃, the bottom temperature controlled at 208-212℃, and the reflux ratio controlled at 1.5-2.0; the vapor phase condensation at the top of the distillation double-effect column yields industrial-grade NMP (purity ≥99.5%), and the bottom liquid is sent to the thin-film evaporator;

[0009] Side-stream liquid is drawn from the 13th to 17th plates of the distillation column (32 plates in total, counting from the top of the column downwards), and heavy components are extracted by injecting sulfolane through a static mixer. After mixing, the mixture is separated in a coalescing separator. The light phase is returned to the 20th to 24th plates of the distillation column, and the heavy phase is sent to the regeneration column.

[0010] Although sulfolane and NMP are miscible, the heavier components in the extract are more soluble in sulfolane. After selective extraction of the heavier components by sulfolane, the heavier components reduce the interfacial energy between sulfolane and NMP, promoting phase separation. The sulfolane containing the heavy components forms a heterogeneous phase with the lighter components such as NMP. The coalescing separator can separate the heterogeneous liquid into liquid and liquid phases. The light phase rich in NMP is returned to the lower part of the second-effect column for further distillation, while the high-density condensed heavy phase is sent to the regeneration column for treatment.

[0011] Such side-stream extraction can prevent impurities from accumulating in the second-effect distillation column, avoid coking in the trays, and improve column efficiency; and removing some heavy components from the distillation system can reduce the concentration and viscosity of the bottom liquid, prevent the NMP binding of the bottom liquid from intensifying, and reduce the processing load of the subsequent thin-film evaporator and the difficulty of deep recovery.

[0012] (3) A small amount of NMP-water azeotrope is collected from the top of the regeneration tower and returned to the dehydration tower, while the regenerated sulfolane is collected from the side stream for extraction and recycling, and the concentrated residue at the bottom of the tower is sent to the depolymerization reactor.

[0013] (4) The operating pressure of the thin film evaporator is -0.093~-0.096MPa, the liquid phase heating temperature is 205-210℃, and the scraping speed is 180-220rpm. After the bottom liquid of the distillation second-effect column is scraped and evaporated, the evaporated gas phase is condensed and sent to the light removal column, and the liquid phase residue is sent to the depolymerization reactor. The liquid phase residue is a viscous paste and still contains a small amount of "bound" NMP to be recovered. The residual NMP needs to be completely released through subsequent catalytic depolymerization.

[0014] Due to the limitations of distillation separation, the bottom liquid of a distillation column is a heavy component concentrate, making it difficult to further separate the bound NMP entrained within it through distillation. However, in a thin-film evaporator, the use of a scraped film to form an extremely thin liquid film greatly improves heat and mass transfer efficiency. The shear force of mass transfer can break down as much of the residual NMP that has been evaporated as possible, releasing NMP in weakly complexed forms and NMP physically bound by polymer molecular chains. Furthermore, by separating as much weakly complexed and physically bound NMP from the concentrate through thin-film evaporation as possible, the subsequent depolymerization efficiency can be improved, avoiding the loss of these NMPs due to high-temperature decomposition caused by excessive residence time in the depolymerization reactor.

[0015] (5) After the concentrated residue at the bottom of the regeneration tower and the liquid phase residue of the thin film evaporator are sent into the depolymerization reactor, water vapor is injected. The operating temperature of the depolymerization reactor is 315-325℃. The depolymerization reaction is carried out under the catalysis of TiO2-Al2O3 loaded phosphotungstic acid. Water vapor can inhibit high-temperature coking and promote the dissociation and release of NMP by strong metal complexes in a high-temperature steam environment. The depolymerization reaction can break the CN or CO bonds of polymers such as PVDF and NVP through phosphotungstic acid catalysis, and release the bound NMP by selectively breaking the polymer coating.

[0016] The depolymerization gas obtained from the depolymerization reactor is separated into solid particles in a cyclone separator and then sent to a quench tower. The waste residue obtained from the depolymerization reactor and the cyclone separator is sent to a pyrolysis furnace for final treatment. The waste residue is then thermally pyrolyzed and incinerated in the pyrolysis furnace at high temperature to recover heat for use as a steam boiler. Finally, the solid waste is screened to recover metal components and then disposed of in a landfill.

[0017] (6) The quench tower is sprayed with alkaline spray liquid to cool down and neutralize the acidic gas in the depolymerization gas. The liquid phase cooled in the bottom of the quench tower is pumped into the light removal tower after the precipitate is removed in the sedimentation tank. The non-condensable gas at the top of the tower is collected and burned in the heat transfer oil furnace to recover heat. The heated heat transfer oil can provide heat for equipment such as reboilers.

[0018] (7) After the vapor phase condensate of the thin film evaporator and the coolant of the quench tower are sent to the light-removal tower, the top product of the light-removal tower is condensed and sent to the sewage treatment, and the bottom product of the tower is industrial-grade NMP (purity ≥99.5%).

[0019] Preferably, in process (1), the pressure of the dehydration tower is controlled at -0.090 to -0.095 MPa, the temperature at the top of the tower is controlled at 65-75℃, the temperature at the bottom of the tower is controlled at 180-190℃, and the reboiler of the dehydration tower is heated by heat transfer oil; the vapor phase at the top of the dehydration tower is condensed and sent to sewage treatment, and the liquid phase at the bottom of the tower is pumped into the first-effect distillation tower.

[0020] Furthermore, the NMP-containing waste liquid needs to be preheated to 75-85°C before being fed into the dehydration tower. The preheating is achieved by exchanging heat between the NMP-containing waste liquid and the bottom liquid of the first distillation column via a plate heat exchanger. Since the second distillation column is a negative pressure column, this heat exchange can also prevent the gas-liquid imbalance caused by excessively high feed temperature to the second distillation column.

[0021] Preferably, in process (2), the flow rate of the liquid being pumped is 12-17% of the total feed of the distillation double-effect column, the volume ratio of the injection amount of sulfolane to the liquid being pumped is 1.1-1.3, and the temperature is controlled at 79-81℃ when the heavy phase and light phase are separated in the coalescing separator to achieve the best separation effect.

[0022] Preferably, in process (3), the operating pressure of the regeneration tower is -0.078 to -0.085 MPa, the top temperature of the tower is controlled at 90-95℃, and the bottom temperature of the tower is controlled at 170-180℃.

[0023] Preferably, in process (6), the alkaline spray solution is a NaOH solution with a concentration of 3-5 wt%.

[0024] Preferably, in process (7), the operating pressure of the light-light removal tower is -0.090~-0.094MPa, the tower top temperature is controlled at 47-52℃, the tower bottom temperature is controlled at 100-105℃, and the reflux ratio is controlled at 0.7-0.9.

[0025] Through the above technical solutions, the present invention has at least the following beneficial effects:

[0026] The method for recovering NMP from complex waste liquid described in this application has a reasonable and efficient process flow. The provided recovery process of "two-tower distillation + liquid extraction + thin-film evaporation + catalytic depolymerization" is effective for complex NMP-containing waste liquids: atmospheric pressure separation in the first-effect distillation column and negative pressure separation in the second-effect distillation column can recover most of the free NMP. Furthermore, the side-stream extraction of heavy components in the second-effect distillation column not only improves the column efficiency but also removes some heavy components from the distillation system, reducing the concentration and viscosity of the bottom liquid and mitigating the effects of distillation concentration. The increased NMP binding caused by this process can reduce the load on the thin-film evaporator and lower the difficulty of subsequent deep NMP recovery. By separating weakly complexed and physically bound NMP from the concentrate through thin-film evaporation, the subsequent depolymerization efficiency can be improved, avoiding the loss of these NMP due to excessive residence time in the depolymerization reactor. The high-temperature steam environment for catalytic depolymerization can promote the dissociation and release of NMP by strong metal complexes under high-temperature steam, while the depolymerization reaction selectively breaks bonds to break down the polymer coating and release its bound NMP.

[0027] By achieving maximum NMP recovery through deep treatment of waste liquid, this application's treatment and recovery scheme has a higher NMP yield compared to conventional methods that are not effective in recovering complex waste liquids, with a total recovery rate of over 99%. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the recovery of N-methylpyrrolidone solvent in the embodiments of this application. Detailed Implementation

[0029] The following will be combined with the appendix Figure 1 The present invention will be further illustrated by the examples.

[0030] The NMP-containing waste liquid received by our company comes from NMP recovery liquid generated during the lithium battery electrode slurry coating process, waste positive electrode slurry recovery liquid, and cleaning waste liquid generated from cleaning pipelines and equipment. We need to recover the NMP-containing NMP-containing waste liquid and treat it to be harmless.

[0031] Because the waste liquid has a complex composition, NMP exists not only in a free state but also in complex forms and is encapsulated and bound by polymers. Conventional distillation separation can only recover free NMP and is insufficient to separate and release these bound NMP, resulting in poor treatment of complex waste liquids. Furthermore, the heavy components in the distillation column bottoms, after distillation and concentration, further bind NMP, making this portion even more difficult to recover. Therefore, the recovery process described in this application is needed to achieve a high NMP recovery rate for this waste liquid.

[0032] Example 1

[0033] The specific composition of a batch of combined waste liquid treated in this embodiment is as follows:

[0034]

[0035] The method for recovering N-methylpyrrolidone from complex waste liquid according to this application includes the following processes for recycling and treating the aforementioned complex waste liquid:

[0036] (1) The NMP-containing waste liquid is dehydrated by vacuum in a dehydration tower and then pumped into the first-effect distillation tower. The pressure of the dehydration tower is controlled at -0.092MPa, the top temperature is controlled at 70℃, and the bottom temperature is controlled at 184℃. The reboiler of the dehydration tower is heated by heat transfer oil. The vapor phase at the top of the dehydration tower is condensed and sent to wastewater treatment, and the liquid phase at the bottom of the tower is pumped into the first-effect distillation tower.

[0037] The NMP-containing waste liquid needs to be preheated to 80°C before being fed into the dehydration tower. The preheating is achieved by exchanging heat between the NMP-containing waste liquid and the bottom liquid of the first distillation column via a plate heat exchanger. Since the subsequent second distillation column is a negative pressure column, this heat exchange can also prevent the gas-liquid imbalance caused by excessively high feed temperature to the second distillation column.

[0038] The first-effect distillation column operates at atmospheric pressure, with the top temperature controlled at 203°C, the bottom temperature controlled at 225°C, and the reflux ratio controlled at 1.8. The top of the first-effect distillation column condenses and collects industrial-grade NMP (purity 99.8%), and a small portion of electronic-grade NMP (purity 99.95%) is collected from the side stream. The heavy components in the bottom of the column are pumped into the second-effect distillation column.

[0039] (2) The distillation double-effect column is operated under negative pressure, with the operating pressure controlled at -0.095MPa, the top temperature controlled at 198℃, the bottom temperature controlled at 210℃, and the reflux ratio controlled at 1.7; the top vapor phase of the distillation double-effect column is condensed to obtain industrial-grade NMP (purity of 99.7%), and the bottom liquid is sent to the thin film evaporator;

[0040] A side stream was drawn from the 15th tray of the distillation column (counting from the top down, a total of 32 trays), and the heavy components were extracted by injecting sulfolane through a static mixer. After mixing, the mixture was separated in a coalescing separator. The light phase was returned to the 22nd tray of the distillation column, and the heavy phase was sent to the regeneration column.

[0041] The pumping flow rate is 14% of the total feed rate of the distillation double-effect column, the volume ratio of the injected sulfolane to the pumping flow rate is 1.2, and the temperature is controlled at 80°C during the separation of the heavy phase and the light phase in the coalescing separator to achieve the best separation effect.

[0042] Although sulfolane and NMP are miscible, the heavier components in the extract are more soluble in sulfolane. After selective extraction of the heavier components by sulfolane, the heavier components reduce the interfacial energy between sulfolane and NMP, promoting phase separation. The sulfolane containing the heavy components forms a heterogeneous phase with the lighter components such as NMP. The coalescing separator can separate the heterogeneous liquid into liquid and liquid phases. The light phase rich in NMP is returned to the lower part of the second-effect column for further distillation, while the high-density condensed heavy phase is sent to the regeneration column for treatment.

[0043] Such side-stream extraction can prevent impurities from accumulating in the second-effect distillation column, avoid coking in the trays, and improve column efficiency; and removing some heavy components from the distillation system can reduce the concentration and viscosity of the bottom liquid, prevent the NMP binding of the bottom liquid from intensifying, and reduce the processing load of the subsequent thin-film evaporator and the difficulty of deep recovery.

[0044] (3) The operating pressure of the regeneration tower is -0.082MPa, the temperature at the top of the tower is controlled at 92℃, and the temperature at the bottom of the tower is controlled at 175℃. A small amount of NMP-water azeotrope is collected from the top of the regeneration tower and returned to the dehydration tower. The regenerated sulfolane is collected from the side stream for extraction and recycling. The concentrated residue at the bottom of the tower is sent to the depolymerization reactor.

[0045] (4) The operating pressure of the thin film evaporator is -0.094MPa, the liquid phase heating temperature is 208℃, and the scraping speed is 200rpm. After the bottom liquid of the distillation second-effect column is scraped and evaporated, the evaporated gas phase is condensed and sent to the light removal column, and the liquid phase residue is sent to the depolymerization reactor. The liquid phase residue is a viscous paste and still contains a small amount of "bound" NMP to be recovered. The residual NMP needs to be completely released through subsequent catalytic depolymerization.

[0046] Due to the limitations of distillation separation, the bottom liquid of a distillation column is a heavy component concentrate, making it difficult to further separate the bound NMP entrained within it through distillation. However, in a thin-film evaporator, the use of a scraped film to form an extremely thin liquid film greatly improves heat and mass transfer efficiency. The shear force of mass transfer can break down as much of the residual NMP that has been evaporated as possible, releasing NMP in weakly complexed forms and NMP physically bound by polymer molecular chains. Furthermore, by separating as much weakly complexed and physically bound NMP from the concentrate through thin-film evaporation as possible, the subsequent depolymerization efficiency can be improved, avoiding the loss of these NMPs due to high-temperature decomposition caused by excessive residence time in the depolymerization reactor.

[0047] (5) After the concentrated residue at the bottom of the regeneration tower and the liquid phase residue of the thin film evaporator are sent into the depolymerization reactor, water vapor is injected. The operating temperature of the depolymerization reactor is 318°C. The depolymerization reaction is carried out under the catalysis of TiO2-Al2O3 loaded phosphotungstic acid. Water vapor can inhibit high-temperature coking and promote the dissociation and release of NMP by strong metal complexes in a high-temperature steam environment. The depolymerization reaction can break the CN or CO bonds of polymers such as PVDF and NVP through phosphotungstic acid catalysis, and release the bound NMP by selectively breaking the polymer coating.

[0048] The depolymerization gas obtained from the depolymerization reactor is separated into solid particles in a cyclone separator and then sent to a quench tower. The waste residue obtained from the depolymerization reactor and the cyclone separator is sent to a pyrolysis furnace for final treatment. The waste residue is then thermally pyrolyzed and incinerated in the pyrolysis furnace at high temperature to recover heat for use in a steam boiler. Finally, the solid waste is screened to recover metal components and then disposed of in a landfill.

[0049] (6) The quench tower is sprayed with alkaline spray liquid to cool down and neutralize the acidic gas in the depolymerization gas. The alkaline spray liquid is a 4wt% NaOH solution. The liquid phase cooled in the bottom of the quench tower is pumped into the light gas removal tower after the precipitate is removed in the sedimentation tank. The non-condensable gas at the top of the tower is collected and burned in the heat transfer oil furnace to recover heat. The heated heat transfer oil can provide heat for equipment such as reboilers.

[0050] (7) After the vapor phase condensate of the thin film evaporator and the coolant of the quench tower are sent to the light-light removal tower, the operating pressure of the light-light removal tower is -0.092MPa, the top temperature is controlled at 49℃, the bottom temperature is controlled at 102℃, and the reflux ratio is controlled at 0.8. The top product of the light-light removal tower is condensed and sent to sewage treatment, and the bottom product of the tower is industrial-grade NMP (purity of 99.7%).

[0051] Through the above-described process in this embodiment, the total NMP recovery rate in the waste liquid is 99.41%.

[0052] Example 2

[0053] The specific composition of a batch of combined waste liquid treated in this embodiment is as follows:

[0054]

[0055] The method for recovering N-methylpyrrolidone from complex waste liquid according to this application includes the following processes for recycling and treating the aforementioned complex waste liquid:

[0056] (1) The NMP-containing waste liquid is vacuum dehydrated in a dehydration tower and then pumped into the first-effect distillation tower. The tower pressure of the dehydration tower is controlled at -0.090MPa, the top temperature is controlled at 75℃, and the bottom temperature is controlled at 190℃. The reboiler of the dehydration tower is heated by heat transfer oil. The vapor phase at the top of the dehydration tower is condensed and sent to wastewater treatment, and the liquid phase at the bottom of the tower is pumped into the first-effect distillation tower.

[0057] The NMP-containing waste liquid needs to be preheated to 85°C before being fed into the dehydration tower. The preheating is achieved by exchanging heat between the NMP-containing waste liquid and the bottom liquid of the first distillation column via a plate heat exchanger. Since the subsequent second distillation column is a negative pressure column, this heat exchange can also prevent the gas-liquid imbalance caused by excessively high feed temperature to the second distillation column.

[0058] The first-effect distillation column operates at atmospheric pressure, with the top temperature controlled at 205℃, the bottom temperature controlled at 230℃, and the reflux ratio controlled at 1.5. The top of the first-effect distillation column condenses and collects industrial-grade NMP (purity 99.6%), and a small portion of electronic-grade NMP (purity 99.96%) is collected from the side stream. The heavy components in the bottom of the column are pumped into the second-effect distillation column.

[0059] (2) The distillation double-effect column is operated under negative pressure, with the operating pressure controlled at -0.096MPa, the top temperature controlled at 195℃, the bottom temperature controlled at 208℃, and the reflux ratio controlled at 1.5; the top vapor phase of the distillation double-effect column is condensed to obtain industrial-grade NMP (purity of 99.5%), and the bottom liquid is sent to the thin film evaporator;

[0060] A side stream was drawn from the 13th tray of the distillation column (counting from the top down, a total of 32 trays), and the heavy components were extracted by injecting sulfolane through a static mixer. After mixing, the mixture was separated in a coalescing separator. The light phase was returned to the 20th tray of the distillation column, and the heavy phase was sent to the regeneration column.

[0061] The pumping flow rate is 17% of the total feed rate of the distillation double-effect column, the volume ratio of the injected sulfolane to the pumping flow rate is 1.3, and the temperature is controlled at 79°C during the separation of the heavy phase and the light phase in the coalescing separator to achieve the best separation effect.

[0062] Although sulfolane and NMP are miscible, the heavier components in the extract are more soluble in sulfolane. After selective extraction of the heavier components by sulfolane, the heavier components reduce the interfacial energy between sulfolane and NMP, promoting phase separation. The sulfolane containing the heavy components forms a heterogeneous phase with the lighter components such as NMP. The coalescing separator can separate the heterogeneous liquid into liquid and liquid phases. The light phase rich in NMP is returned to the lower part of the second-effect column for further distillation, while the high-density condensed heavy phase is sent to the regeneration column for treatment.

[0063] Such side-stream extraction can prevent impurities from accumulating in the second-effect distillation column, avoid coking in the trays, and improve column efficiency; and removing some heavy components from the distillation system can reduce the concentration and viscosity of the bottom liquid, prevent the NMP binding of the bottom liquid from intensifying, and reduce the processing load of the subsequent thin-film evaporator and the difficulty of deep recovery.

[0064] (3) The operating pressure of the regeneration tower is -0.085MPa, the temperature at the top of the tower is controlled at 90℃, and the temperature at the bottom of the tower is controlled at 170℃. A small amount of NMP-water azeotrope is collected from the top of the regeneration tower and returned to the dehydration tower. The regenerated sulfolane is collected from the side stream for extraction and recycling. The concentrated residue at the bottom of the tower is sent to the depolymerization reactor.

[0065] (4) The operating pressure of the thin film evaporator is -0.096MPa, the liquid phase heating temperature is 205℃, and the scraping speed is 180rpm. After the bottom liquid of the distillation second-effect column is scraped and evaporated, the evaporated gas phase is condensed and sent to the light removal column, and the liquid phase residue is sent to the depolymerization reactor. The liquid phase residue is a viscous paste and still contains a small amount of "bound" NMP to be recovered. The residual NMP needs to be completely released through subsequent catalytic depolymerization.

[0066] Due to the limitations of distillation separation, the bottom liquid of a distillation column is a heavy component concentrate, making it difficult to further separate the bound NMP entrained within it through distillation. However, in a thin-film evaporator, the use of a scraped film to form an extremely thin liquid film greatly improves heat and mass transfer efficiency. The shear force of mass transfer can break down as much of the residual NMP that has been evaporated as possible, releasing NMP in weakly complexed forms and NMP physically bound by polymer molecular chains. Furthermore, by separating as much weakly complexed and physically bound NMP from the concentrate through thin-film evaporation as possible, the subsequent depolymerization efficiency can be improved, avoiding the loss of these NMPs due to high-temperature decomposition caused by excessive residence time in the depolymerization reactor.

[0067] (5) After the concentrated residue at the bottom of the regeneration tower and the liquid phase residue of the thin film evaporator are sent into the depolymerization reactor, water vapor is injected. The operating temperature of the depolymerization reactor is 325°C. The depolymerization reaction is carried out under the catalysis of TiO2-Al2O3 loaded phosphotungstic acid. Water vapor can inhibit high-temperature coking and promote the dissociation and release of NMP by strong metal complexes in a high-temperature steam environment. The depolymerization reaction can break the CN or CO bonds of polymers such as PVDF and NVP through phosphotungstic acid catalysis, and release the bound NMP by selectively breaking the polymer coating.

[0068] The depolymerization gas obtained from the depolymerization reactor is separated into solid particles in a cyclone separator and then sent to a quench tower. The waste residue obtained from the depolymerization reactor and the cyclone separator is sent to a pyrolysis furnace for final treatment. The waste residue is then thermally pyrolyzed and incinerated in the pyrolysis furnace at high temperature to recover heat for use in a steam boiler. Finally, the solid waste is screened to recover metal components and then disposed of in a landfill.

[0069] (6) The quench tower is sprayed with alkaline spray liquid to cool down and neutralize the acidic gas in the depolymerization gas. The alkaline spray liquid is a 5wt% NaOH solution. The liquid phase cooled in the bottom of the quench tower is pumped into the light gas removal tower after the precipitate is removed in the sedimentation tank. The non-condensable gas at the top of the tower is collected and burned in the heat transfer oil furnace to recover heat. The heated heat transfer oil can provide heat for equipment such as reboilers.

[0070] (7) After the vapor phase condensate of the thin film evaporator and the coolant of the quench tower are sent to the light-light removal tower, the operating pressure of the light-light removal tower is -0.090MPa, the top temperature of the tower is controlled at 52℃, the bottom temperature of the tower is controlled at 105℃, and the reflux ratio is controlled at 0.9. The top product of the light-light removal tower is condensed and sent to sewage treatment, and the bottom product of the tower is industrial-grade NMP (purity of 99.7%).

[0071] Through the above-described process in this embodiment, the total NMP recovery rate in the waste liquid is 99.23%.

[0072] Example 3

[0073] The specific composition of a batch of combined waste liquid treated in this embodiment is as follows:

[0074]

[0075] The method for recovering N-methylpyrrolidone from complex waste liquid according to this application includes the following processes for recycling and treating the aforementioned complex waste liquid:

[0076] (1) The NMP-containing waste liquid is vacuum dehydrated in a dehydration tower and then pumped into the first-effect distillation tower. The tower pressure of the dehydration tower is controlled at -0.095MPa, the top temperature is controlled at 65℃, and the bottom temperature is controlled at 180℃. The reboiler of the dehydration tower is heated by heat transfer oil. The vapor phase at the top of the dehydration tower is condensed and sent to wastewater treatment, and the liquid phase at the bottom of the tower is pumped into the first-effect distillation tower.

[0077] The NMP-containing waste liquid needs to be preheated to 75°C before being fed into the dehydration tower. The preheating is achieved by exchanging heat between the NMP-containing waste liquid and the bottom liquid of the first distillation column via a plate heat exchanger. Since the subsequent second distillation column is a negative pressure column, this heat exchange can also prevent the gas-liquid imbalance caused by excessively high feed temperature to the second distillation column.

[0078] The first-effect distillation column operates at atmospheric pressure, with the top temperature controlled at 200℃, the bottom temperature controlled at 220℃, and the reflux ratio controlled at 2.0. The top of the first-effect distillation column condenses and collects industrial-grade NMP (purity ≥99.8%), and a small portion of electronic-grade NMP (purity 99.96%) is collected from the side stream. The heavy components in the bottom of the column are pumped into the second-effect distillation column.

[0079] (2) The distillation double-effect column is operated under negative pressure, with the operating pressure controlled at -0.094MPa, the top temperature controlled at 200℃, the bottom temperature controlled at 212℃, and the reflux ratio controlled at 2.0; the top vapor phase of the distillation double-effect column is condensed to obtain industrial-grade NMP (purity of 99.7%), and the bottom liquid is sent to the thin film evaporator;

[0080] A side stream was drawn from the 17th tray of the distillation column (counting from the top down, a total of 32 trays), and the heavy components were extracted by injecting sulfolane through a static mixer. After mixing, the mixture was separated in a coalescing separator. The light phase was returned to the 24th tray of the distillation column, and the heavy phase was sent to the regeneration column.

[0081] The pumping flow rate is 12% of the total feed rate of the distillation double-effect column, the volume ratio of the injected sulfolane to the pumping flow rate is 1.1, and the temperature is controlled at 81°C during the separation of the heavy phase and the light phase in the coalescing separator to achieve the best separation effect.

[0082] Although sulfolane and NMP are miscible, the heavier components in the extract are more soluble in sulfolane. After selective extraction of the heavier components by sulfolane, the heavier components reduce the interfacial energy between sulfolane and NMP, promoting phase separation. The sulfolane containing the heavy components forms a heterogeneous phase with the lighter components such as NMP. The coalescing separator can separate the heterogeneous liquid into liquid and liquid phases. The light phase rich in NMP is returned to the lower part of the second-effect column for further distillation, while the high-density condensed heavy phase is sent to the regeneration column for treatment.

[0083] Such side-stream extraction can prevent impurities from accumulating in the second-effect distillation column, avoid coking in the trays, and improve column efficiency; and removing some heavy components from the distillation system can reduce the concentration and viscosity of the bottom liquid, prevent the NMP binding of the bottom liquid from intensifying, and reduce the processing load of the subsequent thin-film evaporator and the difficulty of deep recovery.

[0084] (3) The operating pressure of the regeneration tower is -0.078MPa, the temperature at the top of the tower is controlled at 95℃, and the temperature at the bottom of the tower is controlled at 180℃. A small amount of NMP-water azeotrope is collected from the top of the regeneration tower and returned to the dehydration tower. The regenerated sulfolane is collected from the side stream for extraction and recycling. The concentrated residue at the bottom of the tower is sent to the depolymerization reactor.

[0085] (4) The operating pressure of the thin film evaporator is -0.093MPa, the liquid phase heating temperature is 210℃, and the scraping speed is 220rpm. After the bottom liquid of the distillation second-effect column is scraped and evaporated, the evaporated gas phase is condensed and sent to the light removal column, and the liquid phase residue is sent to the depolymerization reactor. The liquid phase residue is a viscous paste and still contains a small amount of "bound" NMP to be recovered. The residual NMP needs to be completely released through subsequent catalytic depolymerization.

[0086] Due to the limitations of distillation separation, the bottom liquid of a distillation column is a heavy component concentrate, making it difficult to further separate the bound NMP entrained within it through distillation. However, in a thin-film evaporator, the use of a scraped film to form an extremely thin liquid film greatly improves heat and mass transfer efficiency. The shear force of mass transfer can break down as much of the residual NMP that has been evaporated as possible, releasing NMP in weakly complexed forms and NMP physically bound by polymer molecular chains. Furthermore, by separating as much weakly complexed and physically bound NMP from the concentrate through thin-film evaporation as possible, the subsequent depolymerization efficiency can be improved, avoiding the loss of these NMPs due to high-temperature decomposition caused by excessive residence time in the depolymerization reactor.

[0087] (5) After the concentrated residue at the bottom of the regeneration tower and the liquid phase residue of the thin film evaporator are sent into the depolymerization reactor, water vapor is injected. The depolymerization reactor operates at a temperature of 315°C. The depolymerization reaction is carried out under the catalytic action of TiO2-Al2O3 loaded phosphotungstic acid. Water vapor can inhibit high-temperature coking and promote the dissociation and release of NMP by strong metal complexes in a high-temperature steam environment. The depolymerization reaction can break the CN or CO bonds of polymers such as PVDF and NVP through phosphotungstic acid catalysis, and release the bound NMP by selectively breaking the polymer coating.

[0088] The depolymerization gas obtained from the depolymerization reactor is separated into solid particles in a cyclone separator and then sent to a quench tower. The waste residue obtained from the depolymerization reactor and the cyclone separator is sent to a pyrolysis furnace for final treatment. The waste residue is then thermally pyrolyzed and incinerated in the pyrolysis furnace at high temperature to recover heat for use in a steam boiler. Finally, the solid waste is screened to recover metal components and then disposed of in a landfill.

[0089] (6) The quench tower is sprayed with alkaline spray liquid to cool down and neutralize the acidic gas in the depolymerization gas. The alkaline spray liquid is a 3wt% NaOH solution. The liquid phase cooled in the bottom of the quench tower is pumped into the light gas removal tower after the precipitate is removed in the sedimentation tank. The non-condensable gas at the top of the tower is collected and burned in the heat transfer oil furnace to recover heat. The heated heat transfer oil can provide heat for equipment such as reboilers.

[0090] (7) After the vapor phase condensate of the thin film evaporator and the coolant of the quench tower are sent to the light-light removal tower, the operating pressure of the light-light removal tower is -0.094MPa, the top temperature is controlled at 47℃, the bottom temperature is controlled at 100℃, and the reflux ratio is controlled at 0.7. The top product of the light-light removal tower is condensed and sent to sewage treatment, and the bottom product of the tower is industrial-grade NMP (purity of 99.6%).

[0091] Through the above-described process in this embodiment, the total NMP recovery rate in the waste liquid is 99.28%.

[0092] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Based on the present invention, and through the above description, those skilled in the art can make various changes and modifications without departing from the technical concept of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recovering N-methylpyrrolidone from complex waste liquid, characterized in that, The process includes the following: (1) The NMP-containing waste liquid is dehydrated by vacuum in a dehydration tower and then pumped into the first-effect distillation tower. The first-effect distillation tower is operated at atmospheric pressure, with the top temperature controlled at 200-205℃, the bottom temperature controlled at 220-230℃, and the reflux ratio controlled at 1.5-2.

0. The industrial-grade NMP is collected from the top of the first-effect distillation tower and the heavy components in the bottom of the tower are pumped into the second-effect distillation tower. (2) The distillation double-effect column is operated under negative pressure, with the operating pressure controlled at -0.094~-0.096MPa, the top temperature controlled at 195-200℃, the bottom temperature controlled at 208-212℃, and the reflux ratio controlled at 1.5-2.0; the top vapor phase of the distillation double-effect column is condensed to obtain industrial-grade NMP, and the bottom liquid is sent to the thin film evaporator; the liquid is drawn off from the side stream at the 13th-17th plates of the distillation double-effect column, and the heavy components are extracted by injecting sulfolane through a static mixer. After mixing, the components are separated in the coalescence separator. The light phase is returned to the 20th-24th plates of the distillation double-effect column, and the heavy phase is sent to the regeneration column. (3) The NMP-water azeotrope is collected from the top of the regeneration tower and returned to the dehydration tower, the regenerated sulfolane is collected from the side stream and used for extraction recycling, and the concentrated residue at the bottom of the tower is used for depolymerization reactor. (4) The operating pressure of the thin film evaporator is -0.093~-0.096MPa, the liquid phase heating temperature is 205-210℃, and the scraping speed is 180-220rpm; after the bottom liquid of the distillation second-effect column is scraped and evaporated, the evaporated gas phase is condensed and sent to the light removal column, and the liquid phase residue is sent to the depolymerization reactor. (5) The concentrated residue at the bottom of the regeneration tower and the liquid phase residue of the thin film evaporator are sent to the depolymerization reactor and steam is injected. The depolymerization reactor operates at a temperature of 315-325℃ and undergoes depolymerization under the catalysis of TiO2-Al2O3 supported phosphotungstic acid. The depolymerization gas obtained from the depolymerization reactor is sent to the quench tower after the solid particles are separated in the cyclone separator. The waste residue obtained from the depolymerization reactor and the cyclone separator is sent to the pyrolysis furnace for final treatment. (6) The quench tower is sprayed with alkaline spray liquid to cool down and neutralize the acidic gas in the depolymerization gas. The liquid phase cooled by the bottom of the quench tower is pumped into the light gas removal tower. The non-condensable gas at the top of the tower is collected and burned to recover heat. (7) The vapor condensate from the thin film evaporator and the coolant from the quench tower are sent to the light-light removal tower. The product from the top of the light-light removal tower is condensed and sent to wastewater treatment. The industrial-grade NMP is produced from the bottom of the tower.

2. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 1, characterized in that: In process (1), the pressure of the dehydration tower is controlled at -0.090 to -0.095 MPa, the temperature at the top of the tower is controlled at 65-75℃, and the temperature at the bottom of the tower is controlled at 180-190℃. The vapor phase at the top of the dehydration tower is condensed and sent to sewage treatment, and the liquid phase at the bottom of the tower is pumped into the first-effect distillation tower.

3. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 2, characterized in that: The NMP-containing waste liquid needs to be preheated to 75-85°C before being sent to the dehydration tower. The preheating is achieved by exchanging heat between the NMP-containing waste liquid and the bottom liquid of the first-effect distillation column via a plate heat exchanger.

4. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 1, characterized in that: In process (2), the flow rate of the liquid being pumped is 12-17% of the total feed of the distillation double-effect column, the volume ratio of the injection amount of sulfolane to the liquid being pumped is 1.1-1.3, and the temperature is controlled at 79-81℃ when the heavy phase and light phase are separated in the coalescing separator.

5. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 1, characterized in that: In process (3), the operating pressure of the regeneration tower is -0.078 to -0.085 MPa, the temperature at the top of the tower is controlled at 90-95℃, and the temperature at the bottom of the tower is controlled at 170-180℃.

6. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 1, characterized in that: In process (6), the alkaline spray solution is a 3-5 wt% NaOH solution.

7. The method for recovering N-methylpyrrolidone from complex waste liquid according to claim 1, characterized in that: In process (7), the operating pressure of the light-light removal tower is -0.090~-0.094MPa, the tower top temperature is controlled at 47-52℃, the tower bottom temperature is controlled at 100-105℃, and the reflux ratio is controlled at 0.7-0.9.

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