System and method for recovering dichloromethane from waste gas

Abstract

According to the system, a retentate side outlet of a first membrane separation device is connected with an inlet of a second membrane separation device, and a permeation side outlet of the first membrane separation device and a permeation side outlet of the second membrane separation device are connected with an inlet of a condensing device; when the condensing device is a normal-pressure condensing device, a tail gas outlet of the condensing device is connected with an inlet of the first membrane separation device, and when the condensing device is a compression condensing device, the tail gas outlet of the condensing device is connected with an inlet of the second membrane separation device. The system and the method are efficient, environment-friendly, economical and feasible, and can be used for recovering high-concentration dichloromethane from the waste gas and ensuring that the treated gas reaches the emission standard.

Description

Technical Field

[0001] The invention relates to the technical field of environmental protection, and in particular to a system and method for recovering dichloromethane from waste gas. Background Art

[0002] Dichloromethane (DCM) has high polarity, low flammability and good solubility. It is often used as a solvent, extractant and reaction medium. It is widely used in pharmaceuticals, new energy lithium batteries, organic synthesis, semiconductors and other fields. However, dichloromethane has a low boiling point and is very easy to volatilize into the surrounding environment. Excessive inhalation can cause harm to the human body. It can also decompose under the action of light to produce highly toxic phosgene. Therefore, efficient treatment of dichloromethane waste gas has become one of the environmental problems that need to be solved urgently.

[0003] Commonly used dichloromethane treatment technologies mainly include combustion, biodegradation, condensation, adsorption, absorption and membrane separation. In practical applications, two or more combined technologies are sometimes used for treatment, such as compression condensation-adsorption. Combustion, absorption, condensation and adsorption are currently the more mature and widely used dichloromethane treatment technologies in the world. Traditional membrane separation technology is mainly used for the treatment of low-concentration dichloromethane waste gas. The basic principle of membrane separation technology is to use the different diffusion rates of gas components through the membrane to separate the target component from other components. Membrane separation technology works best when treating low-concentration dichloromethane waste gas (concentration less than 8%), and can efficiently separate dichloromethane from the waste gas.

[0004] However, the current membrane separation technology faces the following problems, which limit its application in high-concentration dichloromethane waste gas: (1) Although membrane separation can efficiently separate dichloromethane from low-concentration dichloromethane waste gas, when the concentration in the waste gas increases, due to concentration polarization, a denser dichloromethane molecular layer will form on the membrane surface, and the performance of the membrane will be greatly reduced. (2) During the treatment of high-concentration dichloromethane waste gas, the interaction between dichloromethane and other components may cause contamination or failure of the membrane material. Some traditional membrane materials may experience permeability degradation, or even corrosion and degradation under the erosion of high-concentration dichloromethane waste gas, affecting their separation effect and service life.

[0005] Therefore, a new method is urgently needed to overcome the shortcomings of existing membrane separation technology in treating medium and high concentration dichloromethane waste gas, improve recovery efficiency, reduce energy consumption, and extend the service life of the membrane. Summary of the invention

[0006] In view of the above problems, the present invention provides a system and method for recovering dichloromethane from waste gas. The system and method are efficient, environmentally friendly and economically feasible, and can be used to recover medium and high concentrations of dichloromethane from waste gas and ensure that the treated gas meets the emission standards.

[0007] To achieve the above object, the present invention adopts the following technical solution: A system for recovering dichloromethane from waste gas, wherein the retentate side outlet of a first membrane separation device is connected to the inlet of a second membrane separation device, the permeate side outlet of the first membrane separation device and the permeate side outlet of the second membrane separation device are connected to the inlet of a condensation device, When the condensing device is a normal pressure condensing device, the tail gas outlet of the condensing device is connected to the inlet of the first membrane separation device. When the condensing device is a compression condensing device, the tail gas outlet of the condensing device is connected to the inlet of the second membrane separation device. The design of the condenser takes into account the flexibility in different application scenarios and improves the overall recovery rate of dichloromethane.

[0008] The present invention realizes the efficient concentration and liquefaction recovery of dichloromethane by combining two-stage membrane separation and condensation technology, and ensures that the treated gas meets the emission standards.

[0009] Furthermore, a fan is provided at the inlet of the first membrane separation device. The exhaust gas as the raw material enters the first membrane separation device after being pressurized by the fan. The pressurization process not only increases the pressure of the gas, but also helps to improve the fluidity of the gas on the membrane surface and improve the membrane separation efficiency.

[0010] Furthermore, the fan is an explosion-proof centrifugal fan or a vortex fan to ensure safety in an environment where flammable gas may exist.

[0011] Furthermore, the compressor of the compression condensation device is a liquid injection screw compressor. The working pressure range of the compressor is 0.4~0.8MPaG, and through the adjustment of the liquid injection system, the gas compression process can be effectively controlled to avoid energy waste caused by excessive temperature or low efficiency. The compressor is used to increase the pressure of dichloromethane vapor and condense it, and at the same time provide power for the non-condensable gas to enter the second membrane separation system.

[0012] Furthermore, the membrane material of the first membrane separation device is a dichloromethane-resistant VOCs recovery organic membrane, which includes a base membrane layer, an intermediate layer and an active layer arranged in sequence, wherein the base membrane layer is polyvinylidene fluoride, the intermediate layer is polybenzimidazole dispersed with a hydrogen-bonded organic framework material, and the active layer is cross-linked polydimethylsiloxane. The base membrane layer material is polyvinylidene fluoride, which has good mechanical strength and chemical stability; the intermediate layer is polybenzimidazole dispersed with a hydrogen-bonded organic framework material, which enhances the selectivity and permeability of the membrane; the active layer is cross-linked polydimethylsiloxane, which has a high affinity for dichloromethane and allows dichloromethane to pass through preferentially.

[0013] Preferably, the hydrogen-bonding organic framework material is HIAM-JM; the hydrogen-bonding organic framework material is activated, and the HIAM-JM is prepared by the following method: 2,2-Dithiodibenzoic acid and acetic acid were evenly dispersed in a ratio of 0.05-0.1 g:50-80 mL, and then reacted at 70-90 °C for 10-14 h under vacuum conditions to obtain HIAM-JM.

[0014] Preferably, the dichloromethane-resistant VOCs recovery organic membrane is prepared by the following method: The first coating liquid is applied on the base film, and after pre-drying, the second coating liquid is applied thereon, and then cross-linked at high temperature to obtain; The first coating liquid is prepared by the following method: ultrasonically oscillating a hydrogen-bonded organic framework material in n-heptane to obtain a HOF dispersion, and then uniformly mixing the HOF dispersion with a polybenzimidazole solution to obtain the first coating liquid; The second coating liquid is prepared by the following method: uniformly mixing a n-heptane solution of hydroxy vinyl silicone oil with a crosslinking agent and a catalyst to react, thereby obtaining the second coating liquid.

[0015] Preferably, the high temperature cross-linking temperature is 120-140°C.

[0016] Preferably, the high temperature cross-linking time is 4 to 6 hours. After high temperature cross-linking, a dichloromethane-resistant VOCs recovery organic membrane is obtained, and the HOF also completes the activation process during the high temperature process.

[0017] Preferably, in the method for preparing the first coating solution, the mass ratio of the hydrogen-bonding organic framework material to n-heptane is 2-10:90-98.

[0018] Preferably, in the method for preparing the first coating solution, the solvent of the polybenzimidazole solution is DMAc. More preferably, in the polybenzimidazole solution, the mass ratio of polybenzimidazole to DMAc is 20-25:75-80.

[0019] Preferably, in the method for preparing the first coating solution, the mass ratio of the polybenzimidazole solution to the HOF dispersion is 5-10:90-95.

[0020] Preferably, in the preparation method of the second coating liquid, the mass ratio of hydroxy vinyl silicone oil to n-heptane is 7-12:88-93.

[0021] Preferably, in the method for preparing the second coating liquid, the mass ratio of the crosslinking agent to the n-heptane solution of the hydroxy vinyl silicone oil is 1-4:6-9.

[0022] Preferably, in the method for preparing the second coating liquid, the catalyst is 0.01-1% by weight of a n-heptane solution of hydroxy vinyl silicone oil.

[0023] Preferably, the cross-linking agent is tetraethoxysilane (TEOS), and the catalyst is tetrabutyltin.

[0024] Preferably, the viscosity of the second coating liquid is 18 mPa·s.

[0025] Furthermore, the membrane material of the second membrane separation device is a membrane that preferentially permeates organic matter. The active membrane layer material of the membrane that preferentially permeates organic matter is at least one of polyoctylmethylsiloxane, polydimethylsiloxane, polypropylmethylsiloxane, polyphenylmethylsiloxane or modified polymers of the foregoing materials, and the supporting layer is polyethersulfone, polyvinylidene fluoride, polytetrafluoroethylene or polyacrylonitrile. This type of membrane material has preferential selectivity for organic matter and is suitable for treating low concentrations of dichloromethane.

[0026] Furthermore, the retentate side outlet of the second membrane separation device is connected to an adsorption device or a combustion device to ensure that the final discharged gas meets environmental standards.

[0027] The adsorption device uses activated carbon, resin or other adsorbents to perform deep adsorption treatment on dichloromethane in the tail gas, ensuring that the concentration of dichloromethane in the exhaust gas is lower than the detection limit.

[0028] Furthermore, the inlet flow channel separators and the permeate side flow channel separators of the first membrane separation device and the second membrane separation device are made of flexible antistatic plastic mesh materials, which can effectively prevent charge accumulation and reduce the risk of static sparks between the methylene chloride waste gas and the equipment.

[0029] The liquid phase outlet of the condensing device is connected to a liquid storage tank for collecting liquefied dichloromethane liquid.

[0030] Furthermore, the permeate sides of the first membrane separation device and the second membrane separation device are connected to a vacuum device, which ensures that the permeate gas can be effectively collected and transported.

[0031] The second object of the present invention is to provide a method for recovering dichloromethane from waste gas using the above system, comprising the following steps: (1) sending the waste gas to the inlet of the first membrane separation device at a pressure of 8-12 kPa, and under the action of the partial pressure difference before and after the membrane, dichloromethane selectively permeates through the membrane layer to form a first permeate gas, and the first retentate gas is obtained at the retentate side outlet of the first membrane separation; (2) sending the first retentate gas to the inlet of the second membrane separation device, and under the action of the partial pressure difference before and after the membrane, dichloromethane selectively permeates through the membrane layer to form a second permeate gas; (3) The first permeate gas and the second permeate gas are sent to the condensation device for condensation at 0°C, dichloromethane is collected at the liquid phase outlet of the condensation device, and the tail gas of the condensation device is returned to the inlet of the first membrane separation device or the second membrane separation device for cascade treatment.

[0032] The waste gas in step (1) contains 10-20% v / v of dichloromethane.

[0033] Preferably, step (2) further comprises obtaining a second retentate gas at the retentate side outlet of the second membrane separation, and subjecting the second retentate gas to adsorption or combustion treatment, so as to ensure that the final discharged gas meets environmental standards. The second retentate gas contains 0.05-0.3% v / v of dichloromethane.

[0034] In step (3), when the condensing device is a normal pressure condensing device, the concentration of dichloromethane in the tail gas is about 20% v / v, and the tail gas is returned to the inlet of the first membrane separation device for cascade treatment; when the condensing device is a compression condensing device, the concentration of dichloromethane in the tail gas is 2.2~4.0% v / v, and the tail gas is returned to the inlet of the second membrane separation device for cascade treatment.

[0035] The beneficial effects of the present invention are: (1) The use of a dichloromethane-resistant organic membrane and a membrane that preferentially permeates organic matter, combined with a two-stage membrane separation system, effectively improves the separation efficiency of dichloromethane in waste gas. Under the synergistic effect of the first membrane separation device and the second membrane separation device, dichloromethane can be gradually concentrated at different concentrations and finally recovered through a condensation device. The overall recovery rate of this method can reach more than 90%, significantly improving the recovery efficiency of dichloromethane.

[0036] (2) Due to the high recovery rate of dichloromethane in the exhaust gas, the concentration of unrecovered dichloromethane is very low, and the concentration of dichloromethane in the tail gas is controlled at a low level. Through the efficient condensation and secondary cascade treatment of the condensation device, the final exhaust gas meets strict environmental standards and avoids pollution to the atmospheric environment.

[0037] (3) Combined with the cascade treatment design, it reduces energy waste and has lower operating costs. In particular, the use of condensation technology can not only reduce energy consumption through the atmospheric condensation device, but also optimize the recovery of dichloromethane by selecting the compression condensation device, further improving the economy and operational flexibility.

[0038] (4) The system has a compact design, small footprint, low operating cost, and can be modularized according to different application scenarios and needs, with good scalability and adaptability. It is especially suitable for the treatment of methylene chloride waste gas in the chemical, pharmaceutical, printing, new energy and other industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the structure of the system for recovering dichloromethane from waste gas used in Example 1.

[0040] Figure 2 This is a schematic diagram of the structure of the system for recovering dichloromethane from waste gas used in Example 2. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is described in detail below in conjunction with specific embodiments. Example 1

[0042] Use Figure 1 A system for recovering dichloromethane from waste gas is shown, wherein the retentate side outlet of a first membrane separation device is connected to the inlet of a second membrane separation device, the permeate side outlet of the first membrane separation device and the permeate side outlet of the second membrane separation device are connected to the inlet of a condensation device, the condensation device is a normal pressure condensation device, and the tail gas outlet of the condensation device is connected to the inlet of the first membrane separation device.

[0043] A fan is also provided at the inlet of the first membrane separation device, and the fan is an explosion-proof centrifugal fan.

[0044] The compressor of the compression condensing device is a liquid injection screw compressor.

[0045] The membrane material of the first membrane separation device is a dichloromethane-resistant VOCs recovery organic membrane, which includes a base membrane layer, an intermediate layer and an active layer arranged in sequence, wherein the base membrane layer is polyvinylidene fluoride, the intermediate layer is polybenzimidazole dispersed with hydrogen-bonded organic framework materials, and the active layer is cross-linked polydimethylsiloxane. The base membrane layer material is polyvinylidene fluoride, which has good mechanical strength and chemical stability; the intermediate layer is polybenzimidazole dispersed with hydrogen-bonded organic framework materials, which enhances the selectivity and permeability of the membrane; the active layer is cross-linked polydimethylsiloxane, which has a high affinity for dichloromethane and allows dichloromethane to pass through preferentially.

[0046] The hydrogen-bonded organic framework material is HIAM-JM; the hydrogen-bonded organic framework material is activated, and the HIAM-JM is prepared by the following method: 2,2-Dithiodibenzoic acid and acetic acid were evenly dispersed in a ratio of 0.05-0.1 g:50-80 mL, and then reacted at 70-90 °C for 10-14 h under vacuum conditions to obtain HIAM-JM.

[0047] The dichloromethane-resistant VOCs recovery organic membrane is prepared by the following method: The first coating liquid is applied on the base film, and after pre-drying, the second coating liquid is applied thereon, and then cross-linked at high temperature to obtain; The first coating liquid is prepared by the following method: ultrasonically oscillating a hydrogen-bonded organic framework material in n-heptane to obtain a HOF dispersion, and then uniformly mixing the HOF dispersion with a polybenzimidazole solution to obtain the first coating liquid; The second coating liquid is prepared by the following method: uniformly mixing a n-heptane solution of hydroxy vinyl silicone oil with a crosslinking agent and a catalyst to react, thereby obtaining the second coating liquid.

[0048] The temperature of the high temperature crosslinking is 120-140°C.

[0049] The high temperature cross-linking time is 4 to 6 hours. After the high temperature cross-linking, a dichloromethane VOCs recovery organic membrane is obtained, and the HOF also completes the activation process during the high temperature process.

[0050] In the preparation method of the first coating solution, the mass ratio of the hydrogen-bonded organic framework material to n-heptane is 2-10:90-98.

[0051] In the method for preparing the first coating solution, the solvent of the polybenzimidazole solution is DMAc. More preferably, in the polybenzimidazole solution, the mass ratio of polybenzimidazole to DMAc is 20-25:75-80.

[0052] In the preparation method of the first coating solution, the mass ratio of the polybenzimidazole solution to the HOF dispersion is 5-10:90-95.

[0053] In the preparation method of the second coating liquid, the mass ratio of hydroxy vinyl silicone oil to n-heptane is 7~12:88~93.

[0054] In the preparation method of the second coating liquid, the mass ratio of the crosslinking agent to the n-heptane solution of the hydroxy vinyl silicone oil is 1-4:6-9.

[0055] In the preparation method of the second coating liquid, the catalyst is 0.01-1% by weight of the n-heptane solution of hydroxy vinyl silicone oil.

[0056] The cross-linking agent is tetraethoxysilane (TEOS), and the catalyst is tetrabutyltin.

[0057] The viscosity of the second coating liquid was 18 mPa·s.

[0058] The membrane material of the second membrane separation device is an organic-preferentially permeable membrane with polyoctylmethylsiloxane as the active layer.

[0059] The retentate side outlet of the second membrane separation device is connected to the adsorption device.

[0060] The adsorption device uses activated carbon to perform deep adsorption treatment on dichloromethane in the tail gas.

[0061] The inlet flow channel separators and the permeate side flow channel separators of the first membrane separation device and the second membrane separation device are made of flexible antistatic plastic mesh materials.

[0062] The liquid phase outlet of the condensing device is connected to a liquid storage tank for collecting liquefied dichloromethane liquid.

[0063] The permeate sides of the first membrane separation device and the second membrane separation device are connected to a vacuum device.

[0064] During operation, the waste gas containing 20% ​​v / v dichloromethane is pressurized to 8 kPa and then sent to the first membrane separation device by the fan. Dichloromethane selectively permeates the first membrane separation device to form the first permeate gas (about 40% v / v dichloromethane); the gas that does not permeate the membrane enters the second membrane separation device as the first retentate gas (about 5% v / v dichloromethane), further separates dichloromethane to form the second permeate gas (about 15% v / v dichloromethane); the remaining gas is the second retentate gas, in which the dichloromethane content is reduced to 0.3% v / v, and then adsorbed to ensure that the final discharged gas meets environmental standards. The first permeate gas and the second permeate gas are mixed and enter the atmospheric condensation device, condensed into liquid dichloromethane at 0°C, and collected by the liquid storage tank. The condensed non-condensable gas (about 20% v / v dichloromethane) is returned to the inlet of the first membrane separation device for cascade treatment, and the overall recovery rate of dichloromethane finally reaches 90.5%. Example 2

[0065] Use Figure 2 A system for recovering dichloromethane from waste gas is shown. Compared with the system in Example 1, the only difference is that the condensing device is a compression condensing device, and the retentate side outlet of the second membrane separation device is connected to the combustion device.

[0066] During operation, the waste gas containing 10% v / v dichloromethane is pressurized to 12 kPa and then sent to the first membrane separation device by the fan. Dichloromethane selectively permeates the first membrane separation device to form the first permeate gas (about 30% v / v dichloromethane); the gas that does not permeate the membrane enters the second membrane separation device as the first retentate gas (about 3% v / v dichloromethane), further separates dichloromethane to form the second permeate gas (about 10% v / v dichloromethane); the remaining gas is the second retentate gas, in which the dichloromethane content is reduced to 0.05% v / v, and then burns to ensure that the final emission gas meets environmental standards. The first permeate gas and the second permeate gas are mixed and enter the compression condensation device, which is compressed by a liquid injection screw compressor. The working pressure range of the compressor is 0.8MPaG, and the gas compression process can be effectively controlled by adjusting the liquid injection system to avoid energy waste caused by excessively high temperature or too low efficiency. It is condensed into liquid dichloromethane at 0℃ and collected by a liquid storage tank. The condensed non-condensable gas (2.2% v / v dichloromethane) is returned to the inlet of the second membrane separation device for cascade treatment, and the final overall recovery rate of dichloromethane reaches 92%. Example 3

[0067] The same device as in Example 2 was used to pressurize the waste gas containing 18% v / v dichloromethane to 10 kPa and then sent to the first membrane separation device by a blower. Dichloromethane selectively permeates the first membrane separation device to form a first permeate gas (about 36% v / v dichloromethane); the gas that did not permeate the membrane entered the second membrane separation device as the first retentate gas (about 4% v / v dichloromethane), and dichloromethane was further separated to form a second permeate gas (about 12% v / v dichloromethane); the remaining gas was used as the second retentate gas, in which the dichloromethane content was reduced to 0.1% v / v, and then adsorbed or burned to ensure that the final discharged gas met environmental standards. The first permeate gas and the second permeate gas were mixed and entered the compression condensation device, which was compressed by a liquid injection screw compressor. The working pressure range of the compressor was 0.4MPaG, and the gas compression process could be effectively controlled by adjusting the liquid injection system to avoid energy waste caused by excessively high temperature or too low efficiency. It was condensed into liquid dichloromethane at 0°C and collected by a liquid storage tank. The condensed non-condensable gas (about 4% v / v dichloromethane) is returned to the inlet of the second membrane separation device for cascade treatment, and the final overall recovery rate of dichloromethane reaches 95%.

Claims

1. A system for recovering dichloromethane from waste gas, characterized in that: The retentate side outlet of the first membrane separation device is connected to the inlet of the second membrane separation device, and the permeate side outlet of the first membrane separation device and the permeate side outlet of the second membrane separation device are connected to the inlet of the condensation device. When the condensing device is a normal pressure condensing device, the tail gas outlet of the condensing device is connected to the inlet of the first membrane separation device. When the condensing device is a compression condensing device, the tail gas outlet of the condensing device is connected to the inlet of the second membrane separation device.

2. The system according to claim 1, characterized in that A fan is also provided at the inlet of the first membrane separation device. Preferably, the fan is an explosion-proof centrifugal fan or a vortex fan.

3. The system according to claim 1, characterized in that The compressor of the compression condensing device is a liquid injection screw compressor.

4. The system according to claim 1, characterized in that The membrane material of the first membrane separation device is a dichloromethane-resistant VOCs recovery organic membrane, which includes a base membrane layer, an intermediate layer and an active layer arranged in sequence. The base membrane layer is polyvinylidene fluoride, the intermediate layer is polybenzimidazole dispersed with hydrogen bond organic framework material, and the active layer is cross-linked polydimethylsiloxane.

5. The system according to claim 4, characterized in that The hydrogen-bonded organic framework material is HIAM-JM; the hydrogen-bonded organic framework material is activated, and the HIAM-JM is prepared by the following method: 2,2-Dithiodibenzoic acid and acetic acid were evenly dispersed in a ratio of 0.05-0.1 g:50-80 mL, and then reacted at 70-90 °C for 10-14 h under vacuum conditions to obtain HIAM-JM.

6. The system according to claim 4, characterized in that The dichloromethane-resistant VOCs recovery organic membrane is prepared by the following method: The first coating liquid is applied on the base film, and after pre-drying, the second coating liquid is applied thereon, and then cross-linked at high temperature to obtain; The first coating liquid is prepared by the following method: ultrasonically oscillating a hydrogen-bonded organic framework material in n-heptane to obtain a HOF dispersion, and then uniformly mixing the HOF dispersion with a polybenzimidazole solution to obtain the first coating liquid; The second coating liquid is prepared by the following method: uniformly mixing a n-heptane solution of hydroxy vinyl silicone oil with a crosslinking agent and a catalyst to react, thereby obtaining the second coating liquid.

7. The system according to claim 1, characterized in that The membrane material of the second membrane separation device is a membrane that preferentially permeates organic matter, and the active membrane layer material of the membrane that preferentially permeates organic matter is at least one of polyoctyl methylsiloxane, polydimethylsiloxane, polypropyl methylsiloxane, polyphenyl methylsiloxane or modified polymers of the foregoing materials, and the supporting layer is polyether sulfone, polyvinylidene fluoride, polytetrafluoroethylene or polyacrylonitrile.

8. The system according to claim 1, characterized in that The retentate side outlet of the second membrane separation device is connected to an adsorption device or a combustion device.

9. The system according to claim 1, characterized in that The inlet flow channel separators and the permeate side flow channel separators of the first membrane separation device and the second membrane separation device are made of flexible antistatic plastic mesh materials.

10. A method for recovering dichloromethane from waste gas using any system of claims 1 to 9, characterized in that: The steps include: (1) The waste gas is fed into the inlet of the first membrane separation device at a pressure of 8-12 kPa. Under the action of the partial pressure difference before and after the membrane, dichloromethane selectively permeates through the membrane layer to form a first permeate gas, and the first retentate gas is obtained at the retentate side outlet of the first membrane separation; (2) sending the first retentate gas to the inlet of the second membrane separation device, and under the action of the partial pressure difference before and after the membrane, dichloromethane selectively permeates through the membrane layer to form a second permeate gas; (3) The first permeate gas and the second permeate gas are sent to the condensation device for condensation at 0°C, dichloromethane is collected at the liquid phase outlet of the condensation device, and the tail gas of the condensation device is returned to the inlet of the first membrane separation device or the second membrane separation device for cascade treatment.

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