Method and equipment for recovering NMP (N-Methyl Pyrrolidone) solvent

Abstract

The invention relates to the technical field of solvent recovery, and discloses an NMP solvent recovery method and equipment, the recovery method comprises the following steps: the NMP solvent of a concentration unit is conveyed to a film evaporator, the operation temperature of the film evaporator is 90-180 DEG C, and the operation pressure is 3-40 kPa; one part of the heavy-component solvent extracted from the film evaporator is conveyed to the destructive distillation unit for destructive distillation, and the other part of the heavy-component solvent is mixed with the NMP solvent obtained after distillation and liquefaction through destructive distillation and then conveyed back to the film evaporator. According to the method, the NMP solvent is subjected to film evaporation treatment under the working conditions of 90-180 DEG C and 3-40 kPa, so that efficient vaporization separation of the NMP solvent is realized. Compared with a traditional flash evaporation mode, the thin film evaporation process improves the heat transfer and mass transfer efficiency by forming a uniform and stable liquid film, so that the recovery rate and evaporation efficiency of the NMP are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of solvent recovery, and specifically to a method and apparatus for recovering NMP solvent. Background Technology

[0002] Acetylene is an important organic chemical raw material, widely used in metal processing, welding, cutting, and other industrial fields, as well as in the preparation of chemical products such as ethylene, vinyl chloride, trichloroethylene, vinyl acetate, acrylonitrile, polyacrylonitrile, and 1,4-butanediol. The main methods for acetylene production include the calcium carbide method, the non-catalytic partial oxidation method, and the plasma method. Among these, the calcium carbide method has high energy consumption and severe pollution; the plasma method is complex and not yet mature; therefore, the non-catalytic partial oxidation method has become the mainstream process route for producing acetylene from natural gas.

[0003] In the non-catalytic partial oxidation of natural gas to acetylene, natural gas and oxygen undergo a partial oxidation reaction after preheating and mixing. The resulting cracked gas then undergoes pretreatment steps such as dust removal, cooling, and compression. The pretreated cracked gas requires further enrichment and separation of acetylene, typically entering a concentration system for purification. The concentration system uses N-methylpyrrolidone (NMP) as the absorbent solvent. Acetylene in the cracked gas is selectively absorbed in an absorption tower, and then desorbed in a desorption tower by heating or reducing pressure, thereby increasing the acetylene concentration and providing conditions for subsequent product separation and utilization. Throughout this process, the NMP solvent is continuously recycled between absorption and desorption, participating in the entire acetylene concentration process.

[0004] However, the cracked gas contains a large amount of higher alkynes, which are prone to self-polymerization or copolymerization during the NMP cyclic absorption and desorption process. The resulting polymers disperse in the NMP, leading to decreased solvent performance and reduced system stability. Simultaneously, moisture carried by the cracked gas and externally supplied gas also enters the circulation system, gradually increasing the water content of the NMP solvent and affecting absorption efficiency and separation performance. Therefore, a solvent recovery process is necessary to remove polymers and excess moisture to maintain the absorption performance and recycling capacity of NMP.

[0005] Currently, NMP solvent recovery typically employs a combination of continuous flash evaporation and batch distillation. This process treats polymer-containing contaminated solvents through continuous flash evaporation, with the recovered gaseous solvent being condensed and returned to the system. However, this method has low flash evaporation efficiency, requiring the processing of large quantities of solvent to maintain the circulation, leading to a significant increase in energy consumption. Furthermore, the high-temperature gaseous phase generated during flash evaporation is not fully utilized during condensation and recovery, resulting in substantial heat energy waste. Additionally, polymers easily deposit in the cooling system, causing blockages and affecting the stable operation of the unit.

[0006] Therefore, there is an urgent need to develop a more efficient, energy-saving, and stable method for NMP solvent recovery to solve the problems of low efficiency, high energy consumption, and easy clogging in existing technologies. Summary of the Invention

[0007] This invention provides a method and equipment for recovering NMP solvent, in order to solve the problems of low NMP solvent recovery efficiency, high energy consumption, insufficient thermal energy utilization, and easy deposition and blockage of cooling system by polymers in the existing process of partial oxidation of natural gas to acetylene.

[0008] In a first aspect, the present invention provides a method for recovering NMP solvent, wherein the NMP solvent from the concentration unit is conveyed to a thin-film evaporator, the thin-film evaporator having an operating temperature of 90°C to 180°C and an operating pressure of 3 kPa to 40 kPa; for example: the operating temperature of the thin-film evaporator is 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, or 180°C; and the operating pressure of the thin-film evaporator is 3 kPa, 10 kPa, 15 kPa, 20 kPa, 25 kPa, 30 kPa, or 40 kPa.

[0009] Part of the heavy component solvent collected by the thin film evaporator is sent to the dry distillation unit for dry distillation, and the other part is mixed with the NMP solvent obtained after liquefaction from the dry distillation and then sent back to the thin film evaporator.

[0010] This invention feeds the NMP solvent from the concentration unit into a thin-film evaporator at 90°C to 180°C and 3 kPa to 40 kPa for processing, allowing the material to flow along the heated wall in the form of a thin liquid film and complete vaporization separation. Compared with the traditional continuous flash evaporation process, this significantly improves heat and mass transfer efficiency, achieves a high NMP recovery rate at a lower circulation rate, thereby reducing the energy consumption of the evaporation section and improving the operational stability of high-viscosity polymer-containing systems.

[0011] A portion of the heavy component solvent collected from the thin-film evaporator enters the dry distillation unit for deep removal of high-boiling impurities and polymers, while the other portion is mixed with the NMP solvent obtained after liquefaction from the dry distillation and then returned to the inlet of the thin-film evaporator. This creates an internal circulation loop in which both the heavy components and the recovered solvent participate, which helps maintain the appropriate composition and viscosity of the feed to the thin-film evaporator, inhibits local polymer enrichment and scaling and clogging, and reduces the processing load of the dry distillation unit. This achieves efficient recovery of NMP solvent and energy-saving, long-term stable operation of the entire system.

[0012] In one optional embodiment, the NMP solvent is preheated in a heat exchanger before being delivered to the thin-film evaporator; Preferably, the preheating temperature is 70℃ to 150℃; for example, the preheating temperature can be 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃ or 150℃.

[0013] This invention significantly reduces the viscosity of the feed by incorporating a preheating step before the NMP solvent enters the thin-film evaporator and controlling the preheating temperature between 70°C and 150°C. This makes it easier for the material to form a uniform and stable liquid film in the thin-film evaporator, thereby improving the heat and mass transfer efficiency of the evaporation section. Preheating brings the material close to its vaporization temperature, effectively reducing the temperature load on the thin-film evaporator and allowing NMP to vaporize and separate more quickly, improving overall recovery efficiency. Simultaneously, reducing viscosity decreases the risk of polymer accumulation and adhesion on the evaporator walls, which is beneficial for the long-term stable operation of the thin-film evaporator. When combined with heat exchange using heavy component solvents, it can further reduce system energy consumption, achieving excellent energy-saving effects.

[0014] In one alternative embodiment, a portion of the heavy component solvent discharged from the thin-film evaporator is transported to the heat exchanger to exchange heat with the NMP solvent.

[0015] This invention utilizes a portion of the heavy component solvent discharged from the thin-film evaporator to exchange heat with the NMP solvent entering the evaporator. This effectively recovers heat already at a high temperature within the device, allowing the feed to reach a relatively high temperature before entering the evaporation section. This significantly reduces the temperature rise load on the thin-film evaporator, decreases the amount of externally supplied steam or heat transfer oil, and improves the overall thermal efficiency of the system. Furthermore, this heat exchange process helps improve the flowability of the feed and the conditions for liquid film formation, making the thin-film evaporation process more stable. Simultaneously, it prevents heat loss from the heavy component solvent during discharge, achieving excellent energy-saving results.

[0016] In one optional embodiment, the operating temperature of the thin-film evaporator is 150°C to 170°C.

[0017] In one optional embodiment, the pyrolysis unit has a pyrolysis temperature of 80°C to 110°C; for example, the pyrolysis temperature is 80°C, 90°C, 100°C, or 110°C.

[0018] In one optional embodiment, the outflow rate of the heavy component solvent collected by the thin-film evaporator is 1t / h to 5t / h; for example, the outflow rate can be 1t / h, 2t / h, 3t / h, 4t / h or 5t / h; preferably, the outflow rate is 3t / h to 4t / h.

[0019] In one alternative embodiment, the heat recovered from the vapor NMP evaporated by the thin-film evaporator is reused in the concentration unit.

[0020] By conveying the high-temperature gaseous NMP evaporated from the thin-film evaporator to the enrichment unit for heat exchange, the sensible heat generated in the evaporation section can be fully utilized, achieving heat recovery and cascade utilization within the device. On one hand, the gaseous NMP provides the necessary heat to the enrichment unit during heat exchange, reducing the consumption of externally supplied steam or other heating media, thus significantly reducing system energy consumption. On the other hand, the gaseous phase is condensed after heat exchange, avoiding the heat waste caused by direct steam condensation and reducing temperature fluctuations within the enrichment unit, which is beneficial for improving the stability and absorption-desorption efficiency of the enrichment process. This integrated heat management method improves the overall energy efficiency of the device, achieving energy-saving operation and optimizing system thermal balance.

[0021] Secondly, the present invention also provides an apparatus for recovering NMP solvent, comprising: a concentration unit, a thin-film evaporator, a dry distillation unit, and a circulation pipeline, wherein the outlet of the concentration unit is connected to the inlet of the thin-film evaporator, the heavy component outlet of the thin-film evaporator is connected to the inlet of the dry distillation unit and the circulation pipeline, the outlet of the circulation pipeline is connected to the inlet of the thin-film evaporator, and the outlet of the dry distillation unit is connected to the inlet of the thin-film evaporator and / or the circulation pipeline.

[0022] This invention constructs a compact NMP solvent recovery system with a clearly defined material flow by combining a concentration unit, a thin-film evaporator, a pyrolysis unit, and a circulation pipeline. The outlet of the concentration unit is directly connected to the thin-film evaporator, enabling rapid entry of contaminated solvent into the evaporation section for vaporization and separation. The heavy component outlet of the thin-film evaporator is connected to both the pyrolysis unit and the circulation pipeline, allowing for the diversion of heavy components. One portion undergoes pyrolysis to deeply remove high-boiling-point substances and polymers, while the other portion returns to the thin-film evaporator to form an internal circulation, which facilitates the adjustment of the feed composition and the maintenance of a stable liquid film. The outlet of the pyrolysis unit is connected to the inlet of the thin-film evaporator and / or the circulation pipeline, allowing the pyrolysis distillate to be promptly returned to the system, achieving a closed-loop circulation of the internal solvent.

[0023] The interconnection method of the present invention can reduce the retention and transmission resistance of materials in the system, improve the synergistic efficiency between the evaporation, distillation and circulation sections, make the NMP recovery process continuous and stable with lower energy consumption, and effectively avoid the blockage problem caused by local enrichment of polymers, thereby improving the overall operational reliability and recovery effect of the solvent recovery device.

[0024] In one optional embodiment, a heat exchanger is provided on the feed line connecting the concentration unit and the thin-film evaporator to preheat the NMP solvent entering the thin-film evaporator.

[0025] Preferably, the heat exchanger is selected from shell-and-tube heat exchangers or plate heat exchangers.

[0026] This invention adds a heat exchanger to the feed line between the concentration unit and the thin-film evaporator, enabling stable and controllable preheating of the NMP solvent before it enters the evaporation section. This significantly improves the solvent's flowability and film formation conditions, thereby enhancing the heat and mass transfer efficiency of the thin-film evaporator. The preheated material temperature is closer to the required evaporation temperature, effectively reducing the heating load on the thin-film evaporator and minimizing the consumption of external steam or heating media. Simultaneously, the heat exchanger employs a shell-and-tube or plate structure, offering high heat exchange efficiency, small footprint, and easy cleaning and maintenance. It enables rapid heat transfer with relatively low pressure drop, improving the overall energy efficiency and operational stability of the recovery system, and facilitating a continuous and reliable NMP evaporation and separation process.

[0027] In one optional embodiment, the heavy component outlet of the thin-film evaporator is connected to the heat exchanger for supplying heavy component solvent to the heat exchanger for heat exchange with NMP solvent flowing through the heat exchanger.

[0028] In one alternative embodiment, the concentration unit includes a vacuum degassing tower, and the gas phase outlet of the thin-film evaporator is connected to the vacuum degassing tower.

[0029] In one alternative embodiment, the gas phase outlet of the thin-film evaporator is connected to the gas phase inlet of a reboiler connected to a vacuum degassing tower.

[0030] In one alternative embodiment, the thin-film evaporator is a scraped-film evaporator.

[0031] In one alternative embodiment, the scraped film evaporator feed and circulation lines are equipped with flow regulating valves.

[0032] In one optional embodiment, the heating medium of the scraped film evaporator may be high-pressure steam or high-temperature heat transfer oil.

[0033] In one optional embodiment, the scraper of the scraper film evaporator is a hinged scraper made of carbon steel, 304 stainless steel, or polytetrafluoroethylene (PTFE).

[0034] In one optional embodiment, the lower section of the scraped film evaporator is provided with a liquid storage buffer section, and is equipped with liquid level control and temperature control.

[0035] In one optional embodiment, the operating pressure of the scraped film evaporator is the gas phase outlet pressure, and the operating temperature is the liquid phase temperature of the liquid storage buffer section.

[0036] In one alternative embodiment, the thin-film evaporator is configured to use a centrifugal pump for the extraction pump.

[0037] The technical solution of this invention has the following advantages: This invention achieves efficient vaporization and separation of NMP solvent by performing thin-film evaporation at 90℃~180℃ and 3kPa~40kPa. Compared with the traditional flash evaporation method, the thin-film evaporation process improves heat and mass transfer efficiency by forming a uniform and stable liquid film, thereby significantly improving the recovery rate and evaporation efficiency of NMP.

[0038] The heavy component solvent collected from the thin-film evaporator is divided into two streams: one stream enters the dry distillation unit for deep separation, which further removes high-boiling impurities and polymers from the system, improving the purity of the recovered solvent; the other stream is mixed with the NMP solvent obtained after liquefaction from the dry distillation unit and then refluxed back to the thin-film evaporator, forming an internal circulation reflux. This reflux method helps maintain a suitable liquid phase composition in the thin-film evaporator, avoiding viscosity increases and scaling / clogging problems caused by local solvent concentration or polymer enrichment, thereby ensuring the continuous and stable operation of the evaporation process.

[0039] In this invention, the NMP contaminated solvent is preheated by a heat exchanger before entering the thin-film evaporator, and a portion of the heavy component solvent discharged from the thin-film evaporator is used as a heat source for heat exchange. This forms a feedforward energy-saving and stabilization measure for the evaporation section. By using the heavy component solvent, which is already at a higher temperature inside the device, for preheating, the heating load of the main evaporation section and the external steam demand can be significantly reduced, thereby improving the system's thermal efficiency and reducing operating energy consumption. Attached Figure Description

[0040] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the assembly structure of the NMP solvent recovery device of the present invention.

[0042] Explanation of reference numerals in the attached figures: 1. Thin-film evaporator; 2. Discharge pump; 3. Vacuum degassing tower; 4. Dry distillation unit; 5. Heat exchanger; 6. Concentration unit. Detailed Implementation

[0043] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0044] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0045] like Figure 1 As shown, this embodiment provides an apparatus for recovering NMP solvent. The apparatus includes a concentration unit 6, a thin-film evaporator 1, a dry distillation unit 4, a vacuum degassing tower 3, a feed heat exchanger 5, a collection pump 2, and a material circulation pipeline. The units are connected by pipelines to form a complete recovery system.

[0046] The outlet of the concentration unit 6 is connected to the inlet of the thin-film evaporator 1 via a feed pipe. A heat exchanger 5 is installed on the feed pipe to preheat the NMP waste solvent from the concentration unit, so that the material reaches a suitable temperature before entering the thin-film evaporator.

[0047] The thin-film evaporator 1 adopts a scraped-film evaporation structure, with a heavy component outlet at its lower part. This outlet is connected to the inlet of the pyrolysis unit 4 and the circulation pipeline via an outlet pipeline. A collection pump 2 is installed on the heavy component outlet pipeline to transport the heavy component solvent to the pyrolysis unit 4 or the circulation pipeline. The outlet of the pyrolysis unit 4 returns to the inlet of the thin-film evaporator 1 via a reflux pipeline, or merges with the inlet pipeline via the circulation pipeline, realizing the mixing and reflux of the pyrolysis distillate with some of the heavy components.

[0048] In addition, the heavy component outlet of the thin film evaporator 1 is connected to the heat exchanger 5 through a heat exchange connection branch, so that the high-temperature heavy component solvent discharged from the thin film evaporator can be used as a heat source to exchange heat with the feed, thereby improving the system energy efficiency.

[0049] The thin-film evaporator 1 has a gas phase outlet at its upper part, which is connected to the vacuum degassing tower 3 via a gas phase pipeline for degassing the evaporated gaseous NMP. Optionally, the gas phase outlet can also be connected to a reboiler installed on the vacuum degassing tower 3 to further utilize the heat of the gas phase.

[0050] In another optional configuration, the feed branch and circulation branch of the scraped thin-film evaporator 1 are equipped with flow regulating valves to adjust the material flow rate and reflux ratio entering the thin-film evaporator, thereby stabilizing the liquid film thickness and controlling the evaporation load. In this embodiment, the heating medium of the thin-film evaporator 1 can be high-pressure steam or high-temperature heat transfer oil.

[0051] This invention establishes a recovery process whereby the concentrate unit 6 is transported to the thin-film evaporator 1 via the heat exchanger 5. The gaseous material generated by the thin-film evaporator enters the vacuum degassing tower 3 for processing, while the unvaporized heavy components flow to the pyrolysis unit 4 or the circulation pipeline, and are returned to the feed end of the thin-film evaporator after pyrolysis or circulation. This closed-loop circulation method enables continuous separation and purification of NMP contaminated solvent in the evaporation, pyrolysis, and circulation paths. The equipment has a clear structure and well-defined material flow direction, which is conducive to the long-term stable operation of the system and suitable for continuous industrial operation.

[0052] Example 1 As attached Figure 1 As shown, the present invention provides a method and apparatus for recovering NMP solvent, comprising: The NMP waste solvent from the concentration unit 6 of the natural gas to acetylene unit is transported to the feed heat exchanger 5 for preheating at a preheating temperature of 145℃ and a transport flow rate of 10t / h. After preheating, the material is conveyed to thin-film evaporator 1, which operates at a temperature of 170°C and a pressure of 30 kPa. After being distributed by the upper distributor in the thin-film evaporator 1, the material enters the lower high-temperature wall. The waste solvent continuously evaporates through the wall as it descends. NMP becomes a gas phase and is extracted from the upper gas phase outlet. The heavy components in the NMP waste solvent gradually accumulate, and the solvent becomes viscous. Finally, it is collected by the lower collection pump 2 at a flow rate of 3 t / h. Part of it is sent to the dry distillation unit 4 to remove polymers. The dry distillation temperature of the dry distillation unit is 110°C, and the flow rate into the dry distillation unit 4 is 1 t / h. The other part is recycled back into the thin-film evaporator 1 at a circulation flow rate of 2 t / h. The evaporation rate of the thin-film evaporator 1 is 90%.

[0053] The NMP solvent distilled from the dry distillation unit 4 is returned to the feed end of the thin film evaporator 1 and mixed with the NMP waste solvent circulating in the thin film evaporator 1 in the distributor before entering the thin film evaporator together for evaporation and recovery of NMP. The vapor solvent evaporated from the thin film evaporator 1 is sent to the vacuum degassing tower 3 to recover the vapor heat. After the heat is recovered, it is reused in the concentration unit 6.

[0054] Under the above operating conditions, the system operates stably, and the power consumption in this embodiment is 15 kW.

[0055] Example 2 The only difference between this embodiment and Embodiment 1 is that the NMP waste solvent from the concentration unit 6 of the natural gas to acetylene unit is directly transported to the thin-film evaporator 1 without preheating.

[0056] Example 3 The only difference between this embodiment and Embodiment 1 is that the operating temperature of the thin-film evaporator is 150°C.

[0057] Example 4 The only difference between this embodiment and Embodiment 1 is that the operating temperature of the thin-film evaporator is 180°C.

[0058] Example 5 As attached Figure 1 As shown, the present invention provides a method and apparatus for recovering NMP solvent, comprising: The NMP waste solvent from the concentration unit 6 of the natural gas to acetylene plant is transported to the feed heat exchanger 5 for preheating at a temperature of 70°C and a flow rate of 10t / h. After preheating, the material is conveyed to thin-film evaporator 1, which operates at a temperature of 90°C and a pressure of 3 kPa. After being distributed by the upper distributor in the thin-film evaporator 1, the material enters the lower high-temperature wall. The sludge solvent continuously evaporates through the wall as it descends. NMP becomes a gaseous phase and is extracted from the upper gas phase outlet. The heavy components in the NMP sludge solvent gradually accumulate, and the solvent becomes viscous. Finally, it is collected by the lower collection pump 2 at a flow rate of 3.6 t / h. Part of it is sent to the dry distillation unit 4 to remove polymers. The dry distillation temperature of the dry distillation unit is 80°C, and the flow rate into the dry distillation unit 4 is 1.6 t / h. The other part is recycled back into the thin-film evaporator 1 at a circulation flow rate of 2 t / h. The evaporation rate of the thin-film evaporator 1 is 84%.

[0059] The NMP solvent distilled from the dry distillation unit 4 is returned to the feed end of the thin film evaporator 1 and mixed with the NMP waste solvent circulating in the thin film evaporator 1 in the distributor before entering the thin film evaporator together for evaporation and recovery of NMP. The vapor solvent evaporated from the thin film evaporator 1 is sent to the vacuum degassing tower 3 to recover the vapor heat. After the heat is recovered, it is reused in the concentration unit 6.

[0060] Example 6 As attached Figure 1 As shown, the present invention provides a method and apparatus for recovering NMP solvent, comprising: The NMP waste solvent from the concentration unit 6 of the natural gas to acetylene unit is transported to the feed heat exchanger 5 for preheating at a preheating temperature of 150℃ and a transport flow rate of 10t / h. After preheating, the material is conveyed to thin-film evaporator 1, which operates at a temperature of 180℃ and a pressure of 40kPa. After being distributed by the upper distributor in the thin-film evaporator 1, the material enters the lower high-temperature wall. The sludge solvent continuously evaporates through the wall as it descends. NMP becomes a gaseous phase and is extracted from the upper gas phase outlet. The heavy components in the NMP sludge solvent gradually accumulate, and the solvent becomes viscous. Finally, it is collected by the lower collection pump 2 at a flow rate of 3t / h. Part of it is sent to the dry distillation unit 4 to remove polymers. The dry distillation temperature of the dry distillation unit is 110℃, and the flow rate into the dry distillation unit 4 is 1.2t / h. The other part is recycled back into the thin-film evaporator 1 at a circulation flow rate of 1.8t / h. The evaporation rate of the thin-film evaporator 1 is 88%.

[0061] The NMP solvent distilled from the dry distillation unit 4 is returned to the feed end of the thin film evaporator 1 and mixed with the NMP waste solvent circulating in the thin film evaporator 1 in the distributor before entering the thin film evaporator together for evaporation and recovery of NMP. The vapor solvent evaporated from the thin film evaporator 1 is sent to the vacuum degassing tower 3 to recover the vapor heat. After the heat is recovered, it is reused in the concentration unit 6.

[0062] Example 7 As attached Figure 1 As shown, the present invention provides a method and apparatus for recovering NMP solvent, comprising: The NMP waste solvent from the concentration unit 6 of the natural gas to acetylene unit is transported to the feed heat exchanger 5 for preheating at a preheating temperature of 120℃ and a transport flow rate of 10t / h. After preheating, the material is conveyed to thin-film evaporator 1, which operates at a temperature of 150°C and a pressure of 20 kPa. After being distributed by the upper distributor in the thin-film evaporator 1, the material enters the lower high-temperature wall. The sludge solvent continuously evaporates through the wall as it descends. NMP becomes a gas phase and is extracted from the upper gas phase outlet. The heavy components in the NMP sludge solvent gradually accumulate, and the solvent becomes viscous. Finally, it is collected by the lower collection pump 2 at a flow rate of 4 t / h. Part of it is sent to the dry distillation unit 4 to remove polymers. The dry distillation temperature of the dry distillation unit is 80°C, and the flow rate into the dry distillation unit 4 is 1.4 t / h. The other part is recycled back into the thin-film evaporator 1 at a circulation flow rate of 2.6 t / h. The evaporation rate of the thin-film evaporator 1 is 86%.

[0063] The NMP solvent distilled from the dry distillation unit 4 is returned to the feed end of the thin film evaporator 1 and mixed with the NMP waste solvent circulating in the thin film evaporator 1 in the distributor before entering the thin film evaporator together for evaporation and recovery of NMP. The vapor solvent evaporated from the thin film evaporator 1 is sent to the vacuum degassing tower 3 to recover the vapor heat. After the heat is recovered, it is reused in the concentration unit 6.

[0064] Example 8 As attached Figure 1 As shown, the present invention provides a method and apparatus for recovering NMP solvent, comprising: The NMP waste solvent from the concentration unit 6 of the natural gas to acetylene unit is transported to the feed heat exchanger 5 for preheating at a preheating temperature of 140℃ and a transport flow rate of 10t / h. After preheating, it is transported to the thin-film evaporator 1. The operating temperature of the thin-film evaporator is 170 °C and the operating pressure is 35 kPa. After being fed by the distributor in the upper section of the thin-film evaporator 1, it enters the lower high-temperature wall surface. The contaminated solvent is continuously evaporated through the wall surface during the downward process; NMP becomes a gas phase and is extracted from the upper gas-phase outlet. The heavy components in the NMP contaminated solvent are gradually enriched, the solvent becomes viscous, and finally it is extracted by the lower extraction pump 2 with an extraction flow rate of 5 t / h. Part of it is sent to the dry distillation unit 4 to remove polymers. The dry distillation temperature of the dry distillation unit is 100 °C, and the flow rate sent to the dry distillation unit 4 is 1.3 t / h; the other part is recycled into the thin-film evaporator 1 with a recycling flow rate of 2.7 t / h. The evaporation rate of the thin-film evaporator 1 is 87%.

[0065] The NMP solvent dry-distilled from the dry distillation unit 4 returns to the feed end of the thin-film evaporator 1 and is mixed with the recycled NMP contaminated solvent in the thin-film evaporator 1 in the distributor and then enters the thin-film evaporator together for evaporation to recover NMP; the vapor-phase solvent evaporated from the thin-film evaporator 1 is sent to the vacuum degassing tower 3 to recover the heat of the vapor phase; after recovering the heat, it is recycled to the concentration unit 6.

[0066] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: The thin-film evaporator 1 is replaced with a continuous flash tank, specifically as follows: The NMP contaminated solvent in the concentration unit of the acetylene plant from natural gas is transported to the feed heat exchanger for preheating, and the transport flow rate is 7 t / h; After preheating, it is transported to the continuous flash tank. The evaporation rate of the continuous flash tank is 70%. NMP becomes a gas phase and is extracted from the upper gas-phase outlet, transported to the condenser for condensation, and then recycled to the concentration unit; the heavy components in the NMP contaminated solvent in the continuous flash tank are gradually enriched, the solvent becomes viscous, and finally it is extracted by the lower extraction pump. Part of it is sent to the dry distillation unit to remove polymers, and the flow rate sent to the dry distillation unit 4 is 2.1 t / h; the other part is recycled into the continuous flash tank with a recycling flow rate of 700 t / h; the NMP solvent dry-distilled from the dry distillation unit returns to the continuous flash tank and is mixed with the recycled NMP contaminated solvent in the continuous flash tank and then enters the continuous flash tank together for evaporation to recover NMP; the vapor-phase solvent evaporated from the continuous flash tank is extracted from the upper gas-phase outlet, transported to the condenser for condensation, and then recycled to the concentration unit.

[0067] Under the above conditions of operation, the system power consumption is 120 kW, and the matching cooler of the continuous flash tank shows obvious blockage after operating for about two months.

[0068] Test Example 1[[ID=第十九]] This test example compares the recovery effects of Examples 1 to 8 and Comparative Example 1. The recovery rate is calculated according to the following formula: Recovery rate = (1 - (flow rate sent to the dry distillation unit / feed transport flow rate)) × 100%.

[0069] The comparison results are shown in Table 1; Table 1: Recovery Rate Test Results

[0070] As shown in Table 1 and the implementation conditions, in the NMP waste solvent treatment scenario of Example 1, the recovery rate reached 90%, significantly higher than the 70% recovery rate of Comparative Example 1, which used a continuous flash evaporation process. This indicates that the process of thin-film evaporation combined with dry distillation and reflux is superior to the traditional flash evaporation scheme in terms of NMP vaporization separation and solvent recovery efficiency. Meanwhile, under the above conditions, the system power consumption of Example 1 was only 15kW, while the system power consumption of Comparative Example 1 was as high as 120kW. While achieving a higher recovery rate, it significantly reduced the energy consumption per unit recovery volume, demonstrating excellent energy-saving effects.

[0071] Furthermore, in Comparative Example 1, due to the need to process a large amount of circulating material during continuous flash evaporation, and the direct cooling and reuse of the gaseous solvent via the condenser, significant clogging of the condenser occurred after about two months of operation. In contrast, Example 1, by forming a stable liquid film in the thin-film evaporator and constructing an internal circulation loop between the dry distillation unit and the thin-film evaporator, and by recovering the gaseous heat and reusing it to the concentration unit, effectively controlled the polymer and high-boiling components in the system, ensuring stable system operation and avoiding the problem of easy clogging of the condenser.

[0072] Comparing Example 1 and Example 2, Example 1 preheated the NMP contaminated solvent before it entered the thin-film evaporator, while Example 2 omitted the preheating step and directly fed the material into the thin-film evaporator 1. Preheating ensures the feed reaches a suitable temperature before entering the evaporation section, reducing the temperature load on the thin-film evaporator and resulting in a more uniform and stable liquid film, which is beneficial for improving evaporation efficiency and vaporization rate. Conversely, unpreheated material has a lower temperature, requiring the evaporator to provide more heat to establish a stable liquid film, easily leading to uneven liquid film temperature distribution and insufficient evaporation intensity, thus affecting the overall evaporation and recovery effect.

[0073] Example 3 lowered the operating temperature of the thin-film evaporator compared to Example 1. A lower evaporation temperature directly weakens the driving force for material vaporization, reducing the evaporation rate of the liquid film and consequently decreasing the amount of NMP effectively vaporized. Simultaneously, the liquid film viscosity is higher at lower temperatures, leading to decreased heat and mass transfer efficiency and making it easier for heavy components and polymers to accumulate on the evaporation surface, thus inhibiting the separation process. Therefore, lowering the evaporation temperature is detrimental to the full progress of the thin-film evaporation process, resulting in a relatively weaker overall recovery effect.

[0074] Example 4 employed a higher thin-film evaporation temperature than Example 1. The higher evaporation temperature significantly enhances the vaporization driving force, making it easier for NMP in the liquid film to vaporize and separate. Simultaneously, the higher temperature helps reduce the liquid film viscosity and improve fluidity, resulting in a more uniform distribution of the liquid film on the heated wall surface, thereby enhancing heat and mass transfer efficiency. In this case, lighter components in the contaminated solvent are more easily and rapidly vaporized and carried away, further improving the separation efficiency of the thin-film evaporator.

[0075] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for recovering NMP solvent, characterized by: The NMP solvent from the concentration unit is sent to a thin film evaporator, which has an operating temperature of 90-180°C and an operating pressure of 3-40 kPa; Part of the heavy component solvent taken out from the thin film evaporator is sent to the dry distillation unit for dry distillation, and the other part is mixed with the NMP solvent obtained after dry distillation and evaporation, and then sent back to the thin film evaporator.

2. The method of claim 1, wherein: The NMP solvent is sent to a heat exchanger for preheating before being sent to the thin film evaporator; Preferably, the preheating temperature is 70-150°C.

3. The method of claim 2, wherein: Part of the heavy component solvent discharged from the thin film evaporator is sent to the heat exchanger for heat exchange with the NMP solvent.

4. The method according to any one of claims 1 to 3, characterized in that: The operating temperature of the thin film evaporator is 150-170°C; And / or, the dry distillation temperature of the dry distillation unit is 80-110°C.

5. The method according to any one of claims 1 to 4, characterized in that: The take-out flow rate of the heavy component solvent taken out from the thin film evaporator is 1-5 t / h; Preferably, the take-out flow rate is 3-4 t / h.

6. The method according to any one of claims 1 to 5, characterized in that: The gaseous NMP evaporated from the thin film evaporator is recycled after heat recovery for the concentration unit.

7. An apparatus for recovering NMP solvent, characterized by: Comprise: a concentration unit, a thin film evaporator, a dry distillation unit, and a circulation pipeline, wherein, The outlet of the concentration unit is in communication with the inlet of the thin film evaporator, the heavy component outlet of the thin film evaporator is in communication with the inlet of the dry distillation unit and the circulation pipeline, the outlet of the circulation pipeline is in communication with the inlet of the thin film evaporator, and the outlet of the dry distillation unit is in communication with the inlet of the thin film evaporator and / or the circulation pipeline.

8. The apparatus for recovering NMP solvent according to claim 7, wherein: A heat exchanger is arranged on the feed pipeline connecting the concentration unit and the thin film evaporator, for preheating the NMP solvent entering the thin film evaporator.

9. The apparatus for recovering NMP solvent according to claim 8, wherein: The heavy component outlet of the thin film evaporator is in communication with the heat exchanger, for sending the heavy component solvent to the heat exchanger for heat exchange with the NMP solvent flowing through the heat exchanger.

10. The apparatus for recovering NMP solvent according to any one of claims 7-9, characterized in that: The thin film evaporator is a scraper thin film evaporator.

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