System and method for recovering dichloroethane by VOCs membrane process

Abstract

The application discloses a system and method for recovering dichloroethane by VOCs membrane method, which comprises a water washing tower, a temporary storage tank, a condenser, a gas-liquid separator, an air draught fan, a membrane assembly and a storage tank; the gas inlet of the water washing tower is connected with the VOCs tail gas, and the gas outlet at the top of the water washing tower is connected with the gas inlet of the temporary storage tank; the gas outlet at the top of the temporary storage tank is connected with the gas inlet of the condenser, the gas outlet of the condenser is connected with the gas-liquid separator, and the liquid outlet at the bottom of the temporary storage tank and the gas-liquid separator is used for discharging the condensate; the gas outlet at the top of the gas-liquid separator is connected with the air draught fan, and the air draught fan is connected with the membrane assembly; the first outlet at the bottom of the membrane assembly is communicated with the storage tank, the storage tank is kept in vacuum negative pressure, the second outlet at the upper end of the membrane assembly is used for discharging the treated tail gas, and the system and method adopt the process of water washing pretreatment, deep cold pre-recovery and vacuum negative pressure membrane separation, a stable transmembrane pressure difference is built on both sides of the membrane through vacuum negative pressure, the mass transfer driving force of dichloroethane is greatly increased, the recovery efficiency is remarkably improved, and the product purity is high.

Description

Technical Field

[0001] This invention relates to the field of waste gas recovery technology, and more specifically, to a system and method for recovering dichloroethane using a membrane method for VOCs. Background Technology

[0002] Dichloroethane is a widely used organic solvent and chemical intermediate in industries such as chemical processing, pharmaceuticals, rubber manufacturing, and electronic cleaning. During production and use, it easily volatilizes and releases, forming VOCs (volatile organic compounds) waste gas containing dichloroethane. With increasingly stringent domestic environmental emission standards, and given the high economic value of dichloroethane, direct emission not only wastes resources but also faces compliance pressures. Therefore, efficient recovery of dichloroethane-containing waste gas has become a common need in related industries.

[0003] Currently, the treatment of industrial waste gas containing dichloroethane mainly relies on traditional processes such as condensation, adsorption, and combustion. Condensation is simple in principle, but limited by gas-liquid phase equilibrium; increasing recovery rates requires deep freezing, significantly increasing energy consumption and operating costs. Adsorption is widely used, but adsorbents are easily clogged and deactivated by dust and moisture in the waste gas, resulting in high replacement costs for consumables. The desorption and regeneration process is energy-intensive and easily generates waste liquid and gas, posing a risk of secondary pollution; the overall recovery rate is also difficult to exceed 90%. Combustion is a destruction-type treatment, completely destroying the value of the raw materials. The combustion of chlorinated organic compounds also easily generates secondary pollutants such as hydrogen chloride and dioxins, causing severe equipment corrosion and high operating and maintenance costs.

[0004] In recent years, membrane separation technology has been gradually promoted to the field of VOCs treatment. However, existing conventional membrane methods mostly rely on concentration difference to drive mass transfer, which has weak mass transfer driving force, slow dichloroethane permeation rate, and low overall recovery efficiency. In summary, existing technologies generally have problems of insufficient recovery efficiency and high energy consumption. Therefore, a technical solution is needed to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and to provide a system and method for recovering dichloroethane using a VOCs membrane method in order to improve the recovery efficiency of dichloroethane.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention discloses a system for the membrane-based recovery of dichloroethane from VOCs, comprising a water scrubbing tower, a temporary storage tank, a condenser, a gas-liquid separator, an induced draft fan, a membrane module, and a storage tank;

[0008] The inlet of the water washing tower is supplied with VOCs exhaust gas, and the outlet at the top of the water washing tower is connected to the inlet of the temporary storage tank.

[0009] The air outlet at the top of the temporary storage tank is connected to the air inlet of the condenser, the air outlet of the condenser is connected to the gas-liquid separator, and the liquid outlet at the bottom of the temporary storage tank and the gas-liquid separator discharges condensate.

[0010] The gas outlet at the top of the gas-liquid separator is connected to an induced draft fan, and the induced draft fan is connected to the membrane module;

[0011] The first outlet at the bottom of the membrane module is connected to the storage tank, which is kept under vacuum. The second outlet at the top of the membrane module discharges the treated exhaust gas.

[0012] Furthermore, the temperature of the exhaust gas flowing through the condenser is controlled between 0°C and -2°C.

[0013] Furthermore, the vacuum negative pressure environment inside the storage tank is controlled at -50kPa to -80kPa.

[0014] Furthermore, a circulating cooling water flow path is formed between the condenser, the temporary storage tank, and the external cold source. The external cold source is connected to the cooling water inlet of the condenser, the cooling water outlet of the condenser is connected to the cooling water inlet of the temporary storage tank, and the cooling water outlet of the temporary storage tank is connected to the external cold source.

[0015] Furthermore, the membrane assembly includes a first housing, a second housing, a first cover plate, a permeable membrane, a mesh cylinder, and a second cover plate. The second housing is installed inside the first housing. The permeable membrane and the mesh cylinder are coaxially sleeved inside the second housing. The second cover plate is fixedly closed on the top of the permeable membrane and the mesh cylinder. The first cover plate is closed on the upper opening of the second housing. An inlet is provided at the lower end of the side wall of the first housing. A first chamber is formed between the second housing and the permeable membrane. The inlet communicates with the first chamber. The interior of the permeable membrane is a second chamber. A third chamber is formed between the first cover plate and the second cover plate. The second cover plate has multiple second through holes located above the first chamber. The center of the first cover plate has multiple first through holes. The top of the first housing has a second outlet. The first through holes communicate with the third chamber and the second outlet.

[0016] Furthermore, the bottom of the first housing is provided with a connecting part, and the lower ends of the permeation membrane and the mesh cylinder are fixedly installed on the connecting part. The portion of the permeation membrane located between the connecting part and the second cover plate is tapered, with the top being larger than the bottom, and the cross-sectional area of ​​the first chamber gradually decreases from bottom to top.

[0017] Furthermore, the center of the first cover plate is recessed downwards, the height of the third chamber gradually decreases radially from the outside to the inside, and the first through hole is located at the lowest point of the third chamber.

[0018] Furthermore, a baffle is installed at the bottom of the first housing, the baffle is located in the first cavity, the baffle is located at the inner end of the inlet and is spaced apart, the upper end of the baffle is higher than the upper end of the inlet, and the width of the baffle is greater than the diameter of the inlet.

[0019] Furthermore, the permeation membrane is a silicone rubber-based membrane or a polyimide-based membrane.

[0020] This invention also discloses a method for recovering dichloroethane from VOCs using a membrane process, comprising the following steps:

[0021] S1. Water washing: VOCs exhaust gas is fed into a water washing tower for water washing treatment;

[0022] S2, Buffer: VOCs exhaust gas treated by the water washing tower enters the temporary storage tank for temporary storage;

[0023] S3. Cooling: The VOCs exhaust gas in the temporary storage tank enters the condenser for cooling, and the temperature of the VOCs exhaust gas is controlled at -1℃.

[0024] S4. Gas-liquid separation: The VOCs exhaust gas cooled by the condenser is separated into gas and liquid in the gas-liquid separator. The condensate in the gas-liquid separator is collected together with the condensate in the temporary storage tank.

[0025] S5. Membrane treatment: The gas phase of the VOCs tail gas treated by S1 to S4 is sent to the membrane module by an induced draft fan. The vacuum environment of the storage tank applies negative pressure to the inside of the permeate membrane, so that the dichloroethane in the gas phase of the VOCs tail gas permeates through the permeate membrane and is separated and collected in the storage tank.

[0026] The beneficial effects of this invention are:

[0027] 1. This invention employs a process of water washing pretreatment, cryogenic pre-recovery, and vacuum negative pressure membrane separation. By using vacuum negative pressure to construct a stable transmembrane pressure difference across the membrane, the mass transfer driving force of dichloroethane is significantly enhanced compared to the traditional concentration difference driven membrane method. At the same time, the membrane module adopts a conical permeate membrane structure with a larger top and a smaller bottom, which allows the inlet gas flow rate to gradually increase along the membrane surface, effectively reducing the concentration polarization phenomenon on the membrane surface and further improving the permeate flux. Combined with the high selective permeation characteristics of silicone rubber-based or polyimide-based membranes, the overall dichloroethane recovery rate of the system can reach over 95%, which is more than 20% higher than the traditional condensation and adsorption methods. The purity of the recovered product is ≥98%, which can be directly reused in the production process, greatly improving resource utilization.

[0028] 2. This invention adopts a cascaded cold energy utilization design. The refrigerant after heat exchange in the cryogenic condenser is returned to the temporary storage tank for exhaust gas pre-cooling, fully recovering the cold energy. High-efficiency pre-recovery can be achieved without deep freezing, significantly reducing refrigeration energy consumption compared to traditional cryogenic processes. On the other hand, the membrane separation process is a physical mass transfer process with no phase change energy consumption. Combined with the variable frequency speed regulation of the vacuum system to precisely control the negative pressure, energy waste caused by excessive vacuuming is avoided. Long-term operation can significantly reduce the enterprise's pollution control costs, resulting in significant economic benefits.

[0029] 3. The present invention features a front-end water washing pretreatment unit, which can effectively remove dust, acidic gases and soluble impurities from the exhaust gas, preventing impurities from clogging membrane pores and corroding equipment, thus protecting the core membrane element. An air inlet baffle is installed inside the membrane module to achieve uniform gas distribution and prevent high-speed airflow from directly scouring the membrane surface and causing damage. Combined with the inner mesh cylinder support structure, it prevents the membrane from deforming and failing under negative pressure, effectively extending the service life of the membrane element. The system has a simple structure and a long replacement cycle for the core membrane element. Routine maintenance is only required for the vacuum unit and blower, and the workload and cost of operation and maintenance are far lower than those of traditional adsorption and condensation processes. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of one embodiment.

[0031] Figure 2 This is a cross-sectional view of the membrane module in this embodiment.

[0032] Reference numerals: 1. Water washing tower; 2. Temporary storage tank; 3. Condenser; 4. Gas-liquid separator; 5. Exhaust fan; 6. Membrane module; 61. First shell; 611. Inlet; 612. First outlet; 613. Second outlet; 614. Connecting part; 62. Second shell; 63. First cover plate; 631. First through hole; 64. Permeable membrane; 65. Mesh tube; 66. Second cover plate; 661. Second through hole; 67. Baffle; 7. Storage tank; 101. First chamber; 102. Second chamber; 103. Third chamber. Detailed Implementation

[0033] The technical solutions in this embodiment will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 , Figure 2As shown in the figure, this embodiment discloses a VOCs membrane-based system for efficient recovery of dichloroethane, comprising a water washing tower 1, a temporary storage tank 2, a condenser 3, a gas-liquid separator 4, an induced draft fan 5, a membrane module 6, and a storage tank 7; the water washing tower 1, the temporary storage tank 2, the condenser 3, the gas-liquid separator 4, the induced draft fan 5, the membrane module 6, and the storage tank 7 are connected sequentially through process pipelines.

[0035] The water washing tower 1 is a pretreatment unit of the system for recovering dichloroethane. The tail gas containing dichloroethane and VOCs to be treated is introduced into the bottom inlet of the side wall of the water washing tower 1. A spray structure is installed inside the tower, with the tail gas coming into counter-current contact with the spray water from bottom to top. The water washing tower 1 has a packing layer inside to ensure more thorough washing. The outlet at the top of the water washing tower 1 is connected to the inlet of the temporary storage tank 2 via a pipeline. The water washing process removes particulate matter, acidic gases, and soluble impurities from the tail gas, preventing subsequent membrane element clogging and equipment corrosion, and ensuring the long-term operational stability of the system.

[0036] The temporary storage tank 2 serves as both an airflow buffer and pre-cooling unit, with a cooling water circulation jacket installed in its body. The inlet of the temporary storage tank 2 connects to the outlet at the top of the water scrubbing tower 1, and the outlet at the top of the temporary storage tank 2 connects to the inlet of the condenser 3. A liquid outlet is located at the bottom of the temporary storage tank 2 to discharge the condensate generated during pre-cooling. The temporary storage tank 2 stabilizes the inlet flow rate and pressure, uniformly mixes gas components, and eliminates the impact of gas flow fluctuations on subsequent units. Simultaneously, it pre-cools the exhaust gas using circulating cooling water, reducing the cooling load on the cryogenic unit.

[0037] Condenser 3 is a primary recovery unit. The inlet of condenser 3 is connected to the outlet at the top of storage tank 2, and the outlet of condenser 3 is connected to the inlet of gas-liquid separator 4. Condenser 3 is connected to an external cold source, which can control the exhaust gas temperature between 0℃ and -2℃, preferably -1℃ in this embodiment. This allows most of the dichloroethane and high-boiling-point components in the exhaust gas to condense and liquefy, achieving pre-recovery. This system adopts a cascaded cooling capacity utilization design: the low-temperature refrigerant from the external cold source is first connected to the cooling water inlet of condenser 3. After heat exchange, the refrigerant flows out from the outlet of condenser 3 and then into the cooling water inlet of storage tank 2 for exhaust gas pre-cooling. Finally, it flows back to the external cold source from the cooling water outlet of storage tank 2, fully improving the utilization efficiency of the cold source and reducing refrigeration energy consumption.

[0038] The gas-liquid separator 4 is designed based on the principles of gravity settling and inertial separation. The gas inlet of the gas-liquid separator 4 is connected to the outlet of the condenser 3. The liquid outlet at the bottom of the condenser 3 and the liquid outlet at the bottom of the temporary storage tank 2 converge to discharge the pre-recovered dichloroethane condensate. The condensate is collected and subjected to liquid phase separation for the first dichloroethane extraction. The gas phase outlet at the top of the gas-liquid separator 4 is connected to the inlet of the induced draft fan 5. After the gas-liquid mixture in the condenser 3 flows in, the flow rate drops sharply, the liquid component settles to the bottom, and the gas phase flows upward, achieving efficient separation of the two phases.

[0039] The induced draft fan 5 provides airflow power for the system. The outlet of the induced draft fan 5 is connected to the inlet of the membrane module 6. The airflow can be matched by frequency conversion adjustment to ensure stable air intake and gas pressure control of the membrane separation unit.

[0040] Membrane module 6 is the core separation unit of the system, including a first housing 61, a second housing 62, a first cover plate 63, a permeate membrane 64, a mesh cylinder 65, and a second cover plate 66. The second housing 62 is coaxially installed inside the first housing 61. The permeate membrane 64 and the mesh cylinder 65 are coaxially sleeved inside the second housing 62. The mesh cylinder 65 is located inside the permeate membrane 64 and is used to support the membrane body of the permeate membrane 64 to prevent deformation caused by negative pressure. The second cover plate 66 seals the top of the permeate membrane 64 and the mesh cylinder 65, and the first cover plate 63 covers the upper opening of the second housing 62.

[0041] The lower end of the side wall of the first housing 61 is provided with an inlet 611. A first chamber 101 is formed between the second housing 62 and the permeation membrane 64. The pipe of the inlet 611 passes through the side walls of the first housing 61 and the second housing 62 and communicates with the first chamber 101. The cavity inside the permeation membrane 64 is the second chamber 102. The bottom of the second chamber 102 communicates with the first outlet 612 at the bottom of the first housing 61. A third chamber 103 is formed between the first cover plate 63 and the second cover plate 66. The second cover plate 66 has a plurality of second through holes 661 arranged in a ring array, corresponding to the top of the first chamber 101. The second through holes 661 connect the first chamber 101 and the third chamber 103. The center of the first cover plate 63 is provided with a plurality of first through holes 631. The top of the first housing 61 is provided with a second outlet 613. The first through holes 631 connect the third chamber 103 and the second outlet 613.

[0042] The bottom of the first housing 61 is provided with an annular connecting part 614, and the lower ends of the permeation membrane 64 and the mesh cylinder 65 are sealed and fixed to the connecting part 614. The part of the permeation membrane 64 located between the connecting part 614 and the second cover plate 66 is conical with a larger upper part and a smaller lower part. That is, the main body of the permeation membrane 64 has a conical structure with a larger upper part and a smaller lower part, so that the annular cross-sectional area of ​​the first chamber 101 gradually decreases from bottom to top. When the exhaust gas flows upward, the flow velocity gradually increases, which can effectively reduce the concentration polarization of the membrane surface and gradually increase the permeation pressure of the exhaust gas upward, thereby improving the permeation flux and separation efficiency of dichloroethane.

[0043] The center of the first cover plate 63 is recessed downwards, causing the height of the third chamber 103 to gradually decrease radially from the outside to the inside. The first through hole 631 is opened at the center position where the height of the third chamber 103 is the lowest, which can guide the intercepted airflow to converge towards the center, reduce the dead zone in the chamber, ensure smooth exhaust, and at the same time further pressurize the first chamber 101 and the third chamber 103, further improving the permeation flux and separation efficiency of dichloroethane.

[0044] A baffle 67 is fixedly installed at the bottom of the first housing 61. The baffle 67 is located in the first chamber 101, facing the inner end of the inlet 611 with a gap. The baffle 67 is an arc-shaped plate coaxial with the first housing 61. The upper end of the baffle 67 is higher than the upper end of the inlet 611, and the width is greater than the diameter of the inlet 611. It can buffer and distribute the air intake, prevent the high-speed airflow from directly scouring the membrane surface and causing damage, and at the same time make the air intake uniformly distributed circumferentially in the first chamber 101.

[0045] In this embodiment, the permeation membrane 64 is a silicone rubber-based membrane or a polyimide-based membrane. This type of membrane material has high selective permeability to dichloroethane molecules, which can effectively retain non-target components such as air and ensure the purity of the recovered liquid.

[0046] The storage tank 7 is directly connected to the first outlet 612 at the bottom of the membrane module 6. The top of the storage tank 7 is connected to a vacuum unit to maintain a vacuum negative pressure environment of -50kPa to -80kPa inside the tank, providing a negative pressure environment for the second chamber 102, so as to provide a stable transmembrane pressure difference for the separation process and enhance the mass transfer driving force of dichloroethane. After the dichloroethane gas phase through the permeate membrane 64 enters the storage tank 7, it is condensed and liquefied to obtain a high-purity dichloroethane recovery liquid, which can be directly reused in the production process.

[0047] This embodiment also discloses a method for efficient recovery of dichloroethane from VOCs using a membrane process, which specifically includes the following steps:

[0048] S1. Water Washing: The VOCs tail gas containing dichloroethane to be treated is introduced into water washing tower 1 from the bottom. The tail gas flows from bottom to top, making full countercurrent contact with the washing water sprayed from top to bottom inside the tower. Through physical washing, dust particles, acidic gases and other soluble volatile impurities entrained in the tail gas are removed by the washing water, preventing impurities from entering subsequent units and causing problems such as membrane pore blockage and equipment corrosion. This provides qualified inlet conditions for subsequent membrane separation units. The washed tail gas is discharged from the top of water washing tower 1 and enters a temporary storage tank.

[0049] S2. Buffer: The exhaust gas treated by the water washing tower 1 is introduced into the temporary storage tank 2. The ample volume of the tank buffers and stabilizes the fluctuating airflow, ensuring thorough mixing of gas components and maintaining a uniform and stable inlet flow rate and pressure. This eliminates the impact of upstream gas flow fluctuations on the operation of subsequent cryogenic and membrane separation units. Simultaneously, cooling water returning from the condenser 3 is introduced into the jacket of the temporary storage tank 2 to pre-cool the exhaust gas, initially reducing its temperature and the saturated vapor pressure of dichloroethane. Some easily condensable components are pre-liquefied and settle to the bottom of the tank, while also reducing the cooling energy consumption of subsequent cryogenic units.

[0050] S3. Cooling: The buffered and pressure-stabilized exhaust gas flows out from the top of the temporary storage tank 2 and enters the condenser 3 for deep cooling. Through indirect heat exchange between the exhaust gas and the refrigerant provided by an external cold source, the exhaust gas temperature is precisely controlled between 0℃ and -2℃. Under these low-temperature conditions, the high concentration of dichloroethane and other high-boiling-point components in the exhaust gas condenses and liquefies after reaching the saturated vapor pressure threshold, achieving primary pre-recovery of most of the dichloroethane and significantly reducing the processing load of the subsequent membrane separation unit. The cooled gas-liquid two-phase mixture is discharged from the outlet of the condenser 3 and enters the gas-liquid separator 4.

[0051] S4. Gas-Liquid Separation: The gas-liquid mixture, after being deeply cooled by condenser 3, flows into gas-liquid separator 4. The sudden increase in the container's cross-sectional area significantly reduces the gas velocity. Combined with the inertial separation effect of the internal guiding structure, the denser liquid dichloroethane settles to the bottom of the separator under gravity, while the uncondensed low-temperature gas phase continues to flow upward, achieving efficient separation of the gas and liquid phases. The condensate collected at the bottom of gas-liquid separator 4 and the condensate at the bottom of temporary storage tank 2 are collected and discharged together. After collection, it is separated into dichloroethane as the first-stage recovered product through liquid phase separation. The separated low-temperature dried gas phase flows out from the top of the separator and is pressurized and transported to the membrane separation unit by induced draft fan 5.

[0052] S5. Membrane Treatment: The gaseous phase of the VOCs tail gas treated in S1 to S4 is sent to the membrane module 6 by the induced draft fan 5. The induced draft fan 5 pressurizes the tail gas after gas-liquid separation and sends it into the lower inlet 611 on the side wall of the membrane module 6. After entering the first chamber 101, the tail gas flows from bottom to top along the outer side of the conical permeate membrane 64. At the same time, the recovery storage tank 7 maintains a vacuum negative pressure of -50kPa to -80kPa through an external vacuum unit, so that the second chamber 102 inside the permeate membrane 64 forms a stable low-pressure side, creating a significant transmembrane pressure difference on both sides of the membrane, which greatly enhances the mass transfer driving force of dichloroethane.

[0053] Based on the dissolution and diffusion separation mechanism of the membrane material, dichloroethane molecules preferentially and selectively permeate through the permeation membrane 64 into the inner second chamber 102 due to their higher permeability coefficient. Under the action of negative pressure suction, they flow to the first outlet 612 at the bottom of the membrane module 6 and finally enter the recovery storage tank 7 for condensation and liquefaction to obtain high-purity dichloroethane recovery liquid. Non-target components such as air and nitrogen are retained by the membrane and flow upward along the first chamber 101. They enter the third chamber 103 through the second through hole 661 on the second cover plate 66 and are then discharged from the second outlet 613 at the top of the membrane module 6 through the first through hole 631 of the first cover plate 63, meeting the emission standards.

[0054] The following is a comparison between traditional recycling methods and this embodiment:

[0055]

[0056] Through the aforementioned integrated system and process, this technology can achieve a dichloroethane recovery rate of over 95%, which is more than 20% higher than the traditional condensation and adsorption methods, and reduces overall energy consumption by about 30%. Moreover, the entire process is a physical separation process, requiring no chemical reagents and generating no secondary pollutants, thus possessing significant environmental and economic benefits.

[0057] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A system for VOCs membrane recovery of dichloroethane, characterized in that, It includes a water washing tower (1), a temporary storage tank (2), a condenser (3), a gas-liquid separator (4), an induced draft fan (5), a membrane module (6), and a storage tank (7); The inlet of the water washing tower (1) is supplied with VOCs exhaust gas, and the outlet at the top of the water washing tower (1) is connected to the inlet of the temporary storage tank (2). The air outlet at the top of the temporary storage tank (2) is connected to the air inlet of the condenser (3), the air outlet of the condenser (3) is connected to the gas-liquid separator (4), and the liquid outlet at the bottom of the temporary storage tank (2) and the gas-liquid separator (4) discharges condensate. The gas outlet at the top of the gas-liquid separator (4) is connected to the induced draft fan (5), and the induced draft fan (5) is connected to the membrane module (6); The first outlet (612) at the bottom of the membrane module (6) is connected to the storage tank (7), the storage tank (7) is kept under vacuum negative pressure, and the second outlet (613) at the top of the membrane module (6) discharges the treated exhaust gas.

2. The system for VOCs membrane recovery of dichloroethane according to claim 1, characterized in that, The temperature of the exhaust gas flowing through the condenser (3) is controlled between 0°C and -2°C.

3. The system for VOCs membrane recovery of dichloroethane according to claim 1, characterized in that, The vacuum negative pressure environment inside the storage tank (7) is controlled at -50kPa to -80kPa.

4. The system for VOCs membrane recovery of dichloroethane according to claim 1, characterized in that, A circulating cooling water flow path is formed between the condenser (3), the temporary storage tank (2), and the external cold source. The external cold source is connected to the cooling water inlet (611) of the condenser (3), the cooling water outlet of the condenser (3) is connected to the cooling water inlet (611) of the temporary storage tank (2), and the cooling water outlet of the temporary storage tank (2) is connected to the external cold source.

5. A system for VOCs membrane recovery of dichloroethane according to claim 1, characterized in that, The membrane assembly (6) includes a first housing (61), a second housing (62), a first cover plate (63), a permeate membrane (64), a mesh cylinder (65), and a second cover plate (66). The second housing (62) is installed inside the first housing (61). The permeate membrane (64) and the mesh cylinder (65) are coaxially sleeved inside the second housing (62). The second cover plate (66) is fixedly closed on the top of the permeate membrane (64) and the mesh cylinder (65). The first cover plate (63) closes to the upper opening of the second housing (62). An inlet (611) is provided at the lower end of the side wall of the first housing (61). The second housing (62) and the permeate membrane (64) A first chamber (101) is formed between the first cover plate (63) and the second cover plate (66). The inlet (611) is connected to the first chamber (101). The inside of the permeation membrane (64) is a second chamber (102). A third chamber (103) is formed between the first cover plate (63) and the second cover plate (66). The second cover plate (66) is provided with a plurality of second through holes (661). The second through holes (661) are located above the first chamber (101). The center of the first cover plate (63) is provided with a plurality of first through holes (631). The top of the first shell (61) is provided with a second outlet (613). The first through holes (631) are connected to the third chamber (103) and the second outlet (613).

6. A system for VOCs membrane recovery of dichloroethane according to claim 5, characterized in that, The bottom of the first housing (61) is provided with a connecting part (614). The lower ends of the permeation membrane (64) and the mesh cylinder (65) are fixedly installed on the connecting part (614). The part of the permeation membrane (64) located between the connecting part (614) and the second cover plate (66) is tapered with a larger top and a smaller bottom. The cross-sectional area of ​​the first chamber (101) gradually decreases from bottom to top.

7. A system for VOCs membrane recovery of dichloroethane according to claim 5, characterized in that, The center of the first cover plate (63) is recessed downwards, the height of the third chamber (103) gradually decreases radially from the outside to the inside, and the first through hole (631) is located at the lowest point of the height of the third chamber (103).

8. A system for VOCs membrane recovery of dichloroethane according to claim 5, characterized in that, A baffle (67) is installed at the bottom of the first housing (61). The baffle (67) is located in the first chamber (101). The baffle (67) is located at the inner end of the inlet (611) and is spaced apart. The upper end of the baffle (67) is higher than the upper end of the inlet (611). The width of the baffle (67) is greater than the diameter of the inlet (611).

9. A system for VOCs membrane recovery of dichloroethane according to claim 5, characterized in that, The permeation membrane (64) is made of silicone rubber or polyimide.

10. A method for recovering dichloroethane from VOCs using a membrane process, characterized in that, Includes the following steps: S1, Water washing: VOCs exhaust gas is fed into a water washing tower (1) for water washing treatment; S2, Buffer: VOCs exhaust gas treated by water washing tower (1) enters temporary storage tank (2) for temporary storage; S3, Cooling: The VOCs tail gas in the temporary storage tank (2) enters the condenser (3) for cooling, and the temperature of the VOCs tail gas is controlled at -1℃; S4, gas-liquid separation: VOCs tail gas cooled by condenser (3) is separated into gas and liquid in gas-liquid separator (4), and the condensate in gas-liquid separator (4) and the condensate in temporary storage tank (2) are collected together. S5. Membrane treatment: The gas phase of VOCs tail gas treated by S1 to S4 is sent to the membrane module (6) by the induced draft fan (5). The vacuum environment of the storage tank (7) applies negative pressure to the inside of the permeate membrane (64), so that the dichloroethane in the gas phase of VOCs tail gas passes through the permeate membrane (64) and is separated and collected in the storage tank (7).

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