A membrane coupling recovery process for a fusible polytetrafluoroethylene production tail gas

Abstract

A kind of membrane coupling recovery process of fusible polytetrafluoroethylene production tail gas, by the coupling integration of unit operation such as staged compression, shallow cooling, membrane separation, low-temperature condensation, circulating rectification, the process can realize the efficient separation of tetrafluoroethylene polymerization tail gas, respectively obtain hydrogen, tetrafluoroethylene, and the mixture of octafluorocyclobutane and perfluoropropyl vinyl ether, realize the efficient use of various high-value components.The membrane coupling recovery process and system proposed in the application ensure that more than 92.8% of the polymerization tail gas enters the recovery device, wherein the effective recovery rate of tetrafluoroethylene is more than 90.0%, and the effective recovery rate of octafluorocyclobutane and perfluoropropyl vinyl ether is more than 92.0%.Taking a 10000 tons / year fusible polytetrafluoroethylene production device as an example, 516.9 tons of tetrafluoroethylene, 31.3 tons of octafluorocyclobutane and 115.6 tons of perfluoropropyl vinyl ether can be recovered per year, which greatly reduces the raw material cost of fusible polytetrafluoroethylene production device.

Description

Technical Field

[0001] This invention relates to a membrane-coupled recovery process for separating tail gas from the production of fusible polytetrafluoroethylene (PTFE), belonging to the fields of chemistry and chemical engineering. For tail gas emitted from the PTFE reactor (pressurized emulsion polymerization process), this invention integrates membrane separation, compression condensation, and circulating distillation technologies to achieve efficient recovery and utilization of PTFE, polymerization aids, and hydrogen. Background Technology

[0002] Fusible polytetrafluoroethylene (PTFE) is a special fluorinated functional material formed by copolymerizing tetrafluoroethylene (TFE) with a small amount of perfluoropropyl vinyl ether (PPVE). In addition to the high chemical stability, high electrical insulation, and good dielectric properties of traditional PTFE, fusible PTFE is also a meltable material with relatively low viscosity after liquefaction, resulting in better bonding and processing performance. The production of fusible PTFE typically employs a pressurized emulsion polymerization process. Besides the two comonomers, tetrafluoroethylene and perfluoropropyl vinyl ether, an emulsion system consisting of water and surfactants, the polymerization aid octafluorocyclobutane (C318), and hydrogen as the pressure-maintaining medium are added during polymerization. Therefore, the main components of the residual tail gas after the reaction include hydrogen, tetrafluoroethylene, octafluorocyclobutane, perfluoropropyl vinyl ether, and a small amount of water. According to the plant operating data, the polymerization reactor pressure is approximately 2.0 MPaG, and the residual monomer in the PTFE polymerization tail gas accounts for approximately 5-8% of the feed, while the octafluorocyclobutane auxiliary accounts for approximately 95% of the feed. In conclusion, developing an efficient separation and comprehensive utilization system for the tail gas from the production of fusible polytetrafluoroethylene is of great significance for reducing plant emissions and improving economic efficiency.

[0003] Table 1 Typical composition of tail gas from reactor in a fusible polytetrafluoroethylene (PTFE) production unit

[0004]

[0005] Table 1 shows the typical composition of the tail gas from the reactor of a fusible polytetrafluoroethylene (PTFE) production unit. Hydrogen and water are the key components restricting the recovery of PTFE polymerization tail gas. Due to the presence of hydrogen, conventional condensation operations cannot guarantee the liquefaction of most of the high-value-added components in the tail gas. Simultaneously, due to the presence of a small amount of moisture, low-temperature condensation operations can experience freezing blockage, making it impossible to guarantee the long-term stable operation of the polymerization tail gas recovery system. To address these issues, introducing temperature-independent non-phase equilibrium technology coupled with compression condensation and distillation is key to achieving efficient separation and comprehensive utilization of PTFE polymerization tail gas.

[0006] Gas membrane separation technology is a typical non-phase equilibrium separation technology. Targeting the key components in the tetrafluoroethylene polymerization tail gas that restrict the separation process, this invention introduces a glassy polymer membrane module that preferentially permeates hydrogen and water molecules. This module is coupled and integrated with separation technologies such as compression condensation and circulating distillation. Membrane separation ensures the feasibility and stability of low-temperature operation, thereby achieving the efficient recovery and utilization of tetrafluoroethylene, polymerization aids, and hydrogen. Summary of the Invention

[0007] The purpose of this invention is to provide a membrane-coupled recovery process for the efficient separation and comprehensive utilization of tail gas from the production of meltable polytetrafluoroethylene (PTFE). This process achieves efficient separation of PTFE polymerization tail gas through the integrated coupling of unit operations such as staged compression, shallow cooling, membrane separation, low-temperature condensation, and cyclic distillation, yielding hydrogen, tetrafluoroethylene, octafluorocyclobutane, and perfluoropropyl vinyl ether, thus enabling the efficient utilization of various high-value components.

[0008] The technical solution of this invention:

[0009] A membrane-coupled recovery process for the tail gas from the production of fusible polytetrafluoroethylene (PTFE) involves the following steps: The residual polymerization tail gas S-1 in the polymerization reactor 1 gradually decreases in pressure during venting, and is divided into three pressure ranges: high-pressure polymerization tail gas S-2 (1.60–2.00 MPaG), medium-pressure polymerization tail gas S-3 (0.40–1.60 MPaG), and low-pressure polymerization tail gas S-4 (0.05–0.40 MPaG). High-pressure polymerization tail gas S-2 is collected via high-pressure gas collection pipe 2 and then enters buffer tank 7. Medium-pressure polymerization tail gas S-3 is collected via medium-pressure gas collection pipe 3 and then enters a second compressor 6, where it is pressurized to above 1.60 MPaG before merging with high-pressure polymerization tail gas S-2 and entering buffer tank 7. Low-pressure polymerization tail gas S-4 is collected via low-pressure gas collection pipe 4 and then enters… The first compressor 5 pressurizes the gas to above 0.40 MPaG, then combines it with the medium-pressure polymerization tail gas S-3 and enters the second compressor 6. The top of the buffer tank 7 collects constant-pressure polymerization tail gas S-5 with a pressure not lower than 1.60 MPaG, which is sent to the first cooler 8 to reduce the gas temperature to the range of 5-10°C, and then enters the three-phase separator 9. Condensate S-6 is collected from the heavy liquid phase outlet at the bottom of the three-phase separator 9 and used for circulating the emulsion system required for the polymerization process. The first recovered condensate S-7 is collected from the light liquid phase outlet at the bottom of the three-phase separator 9 and sent to the middle of the first distillation column 14. The first condensed tail gas S-8 is collected from the gas phase outlet at the top of the three-phase separator 9, heated to 40-60°C by the heater 10, and then enters the membrane separator. Unit 11; Water molecules and hydrogen preferentially permeate in membrane separation unit 11, obtaining hydrogen gas S-9 with a concentration exceeding 99.5 mol% on the low-pressure side of membrane separation unit 11, which is sent to the hydrogen storage tank; tetrafluoroethylene, octafluorocyclobutane, and perfluoropropyl vinyl ether are retained on the permeate side of membrane separation unit 11, forming membrane separation permeate gas S-10; the membrane separation permeate gas S-10 is cooled in the second cooler 12, with the condensation temperature controlled below -15℃, and after partial liquefaction, it is sent to gas-liquid separator 13; a second condensate tail gas S-11 is obtained at the top of gas-liquid separator 13, which is sent to the medium-pressure gas collection pipeline 3 for circulation separation; a second recovered condensate S-12 is obtained at the bottom of gas-liquid separator 13, which is sent to the upper middle part of the first distillation column 14; the first recovered condensate S-7 and the second recovered condensate S-12 undergo multi-stage phase equilibrium separation in the first distillation column 14, with the top pressure controlled at 0.60–1.20 MPaG. The first distillation column distillate S-13, with a tetrafluoroethylene concentration exceeding 99.0 mol%, is obtained at the condenser outlet of the first distillation column 14 and sent to the tetrafluoroethylene monomer storage tank. The first distillation column bottom liquid S-14, with a tetrafluoroethylene concentration below 20.0 mol%, is obtained at the reboiler outlet of the first distillation column 14. The first distillation column bottom liquid S-14 is then depressurized to 0.20–0.40 MPaG and enters the second distillation column 15 for multi-stage phase equilibrium separation. The second distillation column distillate S-15, with a tetrafluoroethylene concentration exceeding 60%, is obtained at the condenser outlet of the second distillation column 15.0 mol% was sent to the low-pressure gas collecting pipeline 4 for recycling and separation. The reboiler outlet of the second distillation column 15 yielded the second distillation column bottom liquid S-16, in which the total concentration of octafluorocyclobutane and perfluoropropyl vinyl ether exceeded 99.9 mol%. This solution was then sent to the perfluoropropyl vinyl ether refining unit.

[0010] The beneficial effects of this invention are as follows: Through the coupled integration of unit operations such as staged compression, shallow cooling, membrane separation, low-temperature condensation, and circulating distillation, efficient separation of tetrafluoroethylene polymerization tail gas is achieved, enabling efficient recycling of high-value components such as hydrogen, tetrafluoroethylene, octafluorocyclobutane, and perfluoropropyl vinyl ether. The polymerization tail gas remaining in the reactor is released in stages according to the pressure range, and then converted into constant-pressure polymerization tail gas through staged compression. This ensures that most of the polymerization tail gas is transported to the membrane coupling recovery system, and that the separation process operates under constant high-pressure conditions, ultimately achieving efficient separation and full utilization of the tetrafluoroethylene polymerization tail gas. The combined operation of shallow cooling, membrane separation, and low-temperature condensation... The separation process satisfies the requirement that the membrane separation feed be far from the dew point (avoiding excessive concentration of octafluorocyclobutane and perfluoropropyl vinyl ether, and the resulting condensation and membrane damage), while also ensuring the feasibility and stability of low-temperature condensation through membrane separation, ultimately achieving long-term stable operation of the membrane-coupled recovery system. The first distillation column purifies tetrafluoroethylene under high pressure, while the second distillation column deeply removes tetrafluoroethylene from the bottom liquid under low pressure. This avoids the problem of freezing and blockage caused by small amounts of water in the separation system when the top operating temperature is below -30°C, and also avoids the problem of high-reactivity tetrafluoroethylene self-polymerization affecting production stability when the bottom operating temperature is above 65°C. The membrane-coupled recovery process and system proposed in this invention can ensure that more than 92.8% of the polymerization tail gas enters the recovery unit, with an effective recovery rate of over 90.0% for tetrafluoroethylene and over 92.0% for octafluorocyclobutane and perfluoropropyl vinyl ether. Taking a 10,000-ton / year fusible polytetrafluoroethylene (PTFE) production unit as an example, the separation device established based on this invention can recover 516.9 tons of tetrafluoroethylene, 31.3 tons of octafluorocyclobutane, and 115.6 tons of perfluoropropyl vinyl ether annually, generating economic benefits exceeding 25.5 million yuan. In summary, the membrane coupling recovery process for separating fusible PTFE production tail gas provided by this invention can safely, stably, and efficiently achieve the high-value separation and comprehensive utilization of high-value resources, significantly reducing raw material costs for fusible PTFE production units. Attached Figure Description

[0011] Figure 1 This is a simplified flow chart of the membrane coupling recovery process for the tail gas from the production of fusible polytetrafluoroethylene.

[0012] In the diagram: S-1 Polymerization tail gas; S-2 High-pressure polymerization tail gas; S-3 Medium-pressure polymerization tail gas; S-4 Low-pressure polymerization tail gas; S-5 Constant-pressure polymerization tail gas; S-6 Condensate; S-7 First recovered condensate; S-8 First condensate tail gas; S-9 High-concentration hydrogen; S-10 Membrane separation permeate; S-11 Second condensate tail gas; S-12 Second recovered condensate; S-13 First distillation column distillate; S-14 First distillation column bottom liquid. S-15, Distillate from the second distillation column; S-16, Bottom liquid from the second distillation column; 1, Polymerization reactor; 2, High-pressure gas collection pipeline; 3, Medium-pressure gas collection pipeline; 4, Low-pressure gas collection pipeline; 5, First compressor; 6, Second compressor; 7, Buffer tank; 8, First cooler; 9, Three-phase separator; 10, Heater; 11, Membrane separation unit; 12, Second cooler; 13, Gas-liquid separator; 14, First distillation column; 15, Second distillation column. Detailed Implementation

[0013] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0014] Example 1

[0015] Example 1 addresses the polymerization tail gas from a 10,000-ton / year fusible polytetrafluoroethylene (PTFE) production unit of a fluorochemical company. Using the membrane coupling recovery process specifically proposed in this invention, three recyclable products were separated: a mixture of high-concentration hydrogen, tetrafluoroethylene, octafluorocyclobutane, and perfluoropropyl vinyl ether. All three products can be recycled for the production of fluorinated specialty materials, fusible PTFE. Following a batch reactor production model, the annual production time is approximately 3000 hours, and the total flow rate of the polymerization tail gas is approximately 64.0 Nm³. 3 The gas concentration is 235 kg / h, and during the release process, approximately 12.3 Nm³ of high-pressure polymerized exhaust gas is formed. 3 / h, the medium-pressure polymerization exhaust gas is approximately 37.0 Nm³. 3 / h, the low-pressure polymerization exhaust gas is approximately 10.9 Nm³. 3 / h. High-pressure, medium-pressure, and low-pressure polymerization tail gases are converted into constant-pressure polymerization tail gases through staged compression, with a total flow rate of 60.2 Nm³. 3 / h. The motor power of the first compressor is 1.0kW, and the motor power of the second compressor is 3.5kW.

[0016] The pressure of the constant-pressure polymerization tail gas is 1.60 MPaG. After being cooled to 5°C in the first cooler, the first recovered condensate volume is approximately 54.5 kg / h; the first condensate tail gas flow rate is approximately 63.1 Nm³. 3 The gas is heated to 40°C per hour and then enters the membrane separation unit, where a high concentration of hydrogen (approximately 99.5 mol%) is obtained on the low-pressure permeation side at a volumetric flow rate of approximately 17.2 Nm³. 3 / h; the membrane separation permeate gas is cooled to -17℃ in the second cooler, and the resulting second recovered condensate is approximately 173.0 kg / h; the second condensate tail gas is sent to the medium-pressure gas collection pipeline, with a circulation flow rate of approximately 9.5 Nm³. 3 / h; The first and second recovered condensates undergo multi-stage phase equilibrium separation in the first distillation column, with a top operating pressure of 1.00 MPaG and an operating temperature of -17℃, and a bottom operating pressure of 1.03 MPaG and an operating temperature of 60℃; The tetrafluoroethylene concentration in the distillate from the first distillation column is 99.5 mol%, the main impurity is hydrogen, and the total concentration of octafluorocyclobutane and perfluoropropyl vinyl ether is less than 0.01 mol%; The tetrafluoroethylene concentration in the bottom liquid of the first distillation column is 18.7 mol%, and it is sent to... Multi-stage phase equilibrium separation was performed in the second distillation column. The operating pressure at the top of the column was 0.28 MPaG, and the operating temperature was 0 °C. The operating pressure at the bottom of the column was 0.30 MPaG, and the operating temperature was 59 °C. The distillate from the second distillation column contained 69.5 mol% tetrafluoroethylene, with the remaining component being octafluorocyclobutane. The bottom liquid from the second distillation column contained less than 0.01 mol% tetrafluoroethylene, approximately 26.5 mol% octafluorocyclobutane, and approximately 73.5 mol% perfluoropropyl vinyl ether.

[0017] Based on the process design and simulation optimization results, 516.9 tons of tetrafluoroethylene, 31.3 tons of octafluorocyclobutane, and 115.6 tons of perfluoropropyl vinyl ether can be separated from the polymerization tail gas annually. The main utilities are the power consumption for compression and refrigeration, with a total power of approximately 10 kW. The techno-economic evaluation results indicate that the high-efficiency separation and comprehensive utilization system for polymerization tail gas constructed based on the membrane coupling recovery process described in this invention can generate economic benefits exceeding 25.5 million yuan annually, with an estimated investment payback period of no more than 6 months.

Claims

1. A membrane coupling recovery process for tail gas from the production of meltable polytetrafluoroethylene, characterized in that: The polymerization tail gas (S-1) remaining in the polymerization reactor (1) gradually decreases in pressure during the venting process. According to the pressure range, it is divided into high-pressure polymerization tail gas (S-2) of 1.60-2.00 MPaG, medium-pressure polymerization tail gas (S-3) of 0.40-1.60 MPaG, and low-pressure polymerization tail gas (S-4) of 0.05-0.40 MPaG. The high-pressure polymerization tail gas (S-2) is collected by the high-pressure gas collection pipe (2) and then enters the buffer tank (7). The medium-pressure polymerization tail gas (S-3) is collected by the medium-pressure gas collection pipe (3) and then enters the second compressor (6), where it is pressurized to above 1.60 MPaG. Then it is combined with the high-pressure polymerization tail gas (S-2) and enters the buffer tank (7). The low-pressure polymerization tail gas (S-4) is collected by the low-pressure gas collection pipe (4). The gas enters the first compressor (5) and is pressurized to above 0.40 MPaG. Then it is combined with the medium-pressure polymerization tail gas (S-3) and enters the second compressor (6). The constant-pressure polymerization tail gas (S-5) with a pressure not lower than 1.60 MPaG is collected from the top of the buffer tank (7) and sent to the first cooler (8) to reduce the temperature of the gas to the range of 5-10℃. Then it enters the three-phase separator (9). The condensate (S-6) is collected from the heavy liquid phase outlet at the bottom of the three-phase separator (9) and used to circulate and prepare the emulsion system required for the polymerization process. The first recovered condensate (S-7) is collected from the light liquid phase outlet at the bottom of the three-phase separator (9) and sent to the middle of the first distillation column (14). The first condensate (S-7) is collected from the gas phase outlet at the top of the three-phase separator (9). -8), heated to 40-60℃ by heater (10), and then enters membrane separation unit (11); water molecules and hydrogen preferentially permeate in membrane separation unit (11), and hydrogen gas with a concentration of more than 99.5 mol% (S-9) is obtained on the low-pressure side of membrane separation unit (11), and sent to hydrogen storage tank; tetrafluoroethylene, octafluorocyclobutane, and perfluoropropyl vinyl ether are retained on the permeate side of membrane separation unit (11) to form membrane separation permeate gas (S-10); membrane separation permeate gas (S-10) is cooled in second cooler (12), the condensation temperature is controlled below -15℃, and after partial liquefaction, it is sent to gas-liquid separator (13); second condensed tail gas (S-11) is obtained at the top of gas-liquid separator (13) and sent to medium-pressure gas collection pipeline (3). The gas-liquid separator (13) is used for circulation separation. The second recovered condensate (S-12) is obtained at the bottom of the gas-liquid separator (13) and sent to the upper middle part of the first distillation column (14). The first recovered condensate (S-7) and the second recovered condensate (S-12) are separated in a multi-stage phase equilibrium in the first distillation column (14). The pressure at the top of the column is controlled at 0.60-1.20 MPaG. The distillate (S-13) of the first distillation column is obtained at the condenser outlet of the first distillation column (14). The tetrafluoroethylene concentration exceeds 99.0 mol%, and it is sent to the tetrafluoroethylene monomer storage tank. The bottom liquid (S-14) of the first distillation column is obtained at the reboiler outlet of the first distillation column (14). The tetrafluoroethylene concentration is less than 20.0 mol%. The bottom liquid (S-14) of the first distillation column is depressurized to 0.20-0.The solution is 40 MPaG, and then enters the second distillation column (15) for multi-stage phase equilibrium separation. The distillate (S-15) is obtained at the condenser outlet of the second distillation column (15), with a tetrafluoroethylene concentration exceeding 60.0 mol%. It is sent to the low-pressure gas collection pipeline (4) for recycling separation. The bottom liquid (S-16) is obtained at the reboiler outlet of the second distillation column (15), with a total concentration of octafluorocyclobutane and perfluoropropyl vinyl ether exceeding 99.9 mol%. It is sent to the perfluoropropyl vinyl ether refining unit.

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