Method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene

Abstract

The present invention relates to the technical field of waste recycling and utilization. Provided is a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene. The present invention comprises: performing a plasma cracking reaction on waste polytetrafluoroethylene, and sequentially carrying out quenching and gas-solid separation on the obtained cracking product so as to obtain a cracking gas containing tetrafluoromethane, the temperature of the plasma cracking reaction being 1800-5000 K. The method provided by the present invention has the advantages of high conversion rate, short reaction time, large treatment capacity, safe reactions, easy purification of products, and being suitable for industrially continuous and scale-up production, etc., achieves low-energy-consumption and low-cost recycling and utilization of waste polytetrafluoroethylene, avoids waste of fluorine element caused by incineration treatment, and can produce the tetrafluoromethane product having high additional value.

Description

A method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on August 7, 2023, with application number 202310984261.0 and invention name “A method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The invention relates to the technical field of waste recycling, in particular to a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene. Background Art

[0003] Polytetrafluoroethylene (PTFE) is the most widely used fluorocarbon polymer. Known as the "King of Plastics," it exhibits relatively high chemical and thermal stability, wear resistance, and self-lubrication. It is primarily used in the petrochemical, electronics, electrical, and mechanical industries. The synthesis, processing, secondary processing, and application of PTFE all generate PTFE waste, and the treatment and recycling of waste PTFE has become a growing concern.

[0004] Currently, the main methods for recycling waste PTFE include mechanical pulverization, radiation degradation, and high-temperature pyrolysis. Mechanical pulverization uses mechanical force to transform the waste material, producing a relatively small powder. It requires no special requirements for the waste material, only that it be relatively pure and free of foreign substances, so that its excellent properties remain unchanged when reused. Radiation pyrolysis uses high-energy radiation to overcome the high-energy fluorine-carbon bonds, breaking the molecular chains and randomly scissoring the PTFE molecules to produce ultrafine PTFE powder. Scrap with added fillers is generally not suitable for radiation pyrolysis because such waste materials often contain fillers such as glass fiber, copper powder, and molybdenum disulfide, which affect the powder's color and particle size during the radiation process. Pyrolysis involves breaking down PTFE into smaller molecules such as tetrafluoroethylene and hexafluoropropylene at temperatures between 425 and 800°C. Currently, pyrolysis of PTFE is typically performed using tubular furnaces, two-stage reactors, or fluidized bed reactors, requiring low pressures and long residence times. While the primary pyrolysis products are hexafluoropropylene and tetrafluoroethylene, considerable byproducts, such as octafluorocyclobutane, octafluoroisobutylene, hexafluoroethane, octafluoropropane, and hexafluorobutyne, are also produced, making separation and purification of the pyrolysis gas difficult. Furthermore, tetrafluoroethylene forms tetrafluoroethylene polymers at around 550°C, which can easily clog the reactor and material pipelines, reducing the reactor's thermal efficiency. Maintaining the reaction temperature requires greater energy input and equipment investment, making it unsuitable for industrial scale-up.

[0005] Therefore, it is necessary to find a method for processing waste polytetrafluoroethylene (PTFE) and generating high-value gas that is suitable for continuous industrial scale-up and has no special requirements for waste materials, so as to achieve resource utilization of waste polytetrafluoroethylene. Tetrafluoromethane (R14) is currently the most widely used plasma etching gas in the microelectronics industry and has great development potential. However, there are few reports on the production of tetrafluoromethane from waste polytetrafluoroethylene.

[0006] Summary of the Invention

[0007] The object of the present invention is to provide a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene, which can efficiently convert waste polytetrafluoroethylene into tetrafluoromethane and is suitable for industrial scale-up.

[0008] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0009] The present invention provides a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene, comprising the following steps: subjecting the waste polytetrafluoroethylene to a plasma cracking reaction, and sequentially quenching and gas-solid separation of the obtained cracking products to obtain cracked gas containing tetrafluoromethane; the temperature of the plasma cracking reaction is 1800-5000K.

[0010] Preferably, the plasma cracking reaction is carried out in a plasma reactor; the plasma reactor includes a radio frequency plasma reactor, a microwave plasma reactor or an arc plasma reactor.

[0011] Preferably, the waste polytetrafluoroethylene is introduced into the plasma reactor using a carrier gas.

[0012] Preferably, the waste polytetrafluoroethylene is in the form of granules.

[0013] Preferably, the carrier gas includes argon and / or nitrogen.

[0014] Preferably, the feed rate of the waste polytetrafluoroethylene is 0.5 to 3000 kg / h.

[0015] Preferably, the quenching is partition heat exchange quenching and / or direct quenching.

[0016] Preferably, the cooling medium used in the partition-type heat exchange quenching is water and / or refrigerant; the cooling medium used in the direct quenching is argon and / or nitrogen.

[0017] Preferably, after obtaining the cracked gas, the process further comprises adsorbing and rectifying the cracked gas to obtain pure tetrafluoromethane.

[0018] Preferably, the cracked gas further comprises hexafluoroethane and tetrafluoroethylene, and the total volume content of tetrafluoromethane, hexafluoroethane and tetrafluoroethylene in the cracked gas is ≥90%, with tetrafluoromethane having the highest volume content in the cracked gas.

[0019] The present invention provides a method for producing tetrafluoromethane by thermal plasma pyrolysis of waste polytetrafluoroethylene, comprising the following steps: subjecting waste polytetrafluoroethylene to a plasma pyrolysis reaction, sequentially quenching the resulting pyrolysis products, and performing gas-solid separation to produce a pyrolysis gas containing tetrafluoromethane; the plasma pyrolysis reaction temperature is 1800-5000K. The present invention utilizes plasma technology to pyrolyze waste polytetrafluoroethylene. Plasma heating offers higher thermal efficiency than conventional pyrolysis methods. Combined with the pyrolysis reaction temperature, the pyrolysis gas achieves high tetrafluoromethane selectivity. The resulting pyrolysis gas is primarily composed of tetrafluoromethane, with small amounts of hexafluoroethane and tetrafluoroethylene. This relatively simple product composition reduces the investment cost of subsequent separation processes and is suitable for industrial scale-up. Furthermore, after adsorption and distillation, the pyrolysis gas can be returned to the device and reused as a carrier gas for feeding and pyrolysis of waste polytetrafluoroethylene, thereby improving carrier gas utilization and product yield. The present invention avoids the generation of highly toxic octafluoroisobutylene to a great extent, and the separation and purification method adopted does not require the separate removal of octafluoroisobutylene. Octafluoroisobutylene is cracked to generate tetrafluoromethane without accumulation, thereby improving the yield of tetrafluoromethane. The present invention generates less tetrafluoroethylene, avoids the generation of autopolymers, does not block reactors and material pipelines, and improves production safety.

[0020] In addition, the present invention has the following advantages:

[0021] The conversion rate of waste polytetrafluoroethylene is high, the reaction time is short, the raw material processing capacity is large, and the energy consumption is low;

[0022] It can realize resource utilization of one or more materials in waste polytetrafluoroethylene products, unqualified products in polytetrafluoroethylene production process, and scraps;

[0023] No catalyst is used, which avoids the problems of catalyst coking and deactivation under high temperature conditions, as well as carbonization and device clogging. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 is a flow chart of a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene provided by the present invention;

[0025] FIG2 is a gas chromatogram of the cracked gas of Example 1;

[0026] FIG3 is a SEM image of the solid product of Example 1;

[0027] FIG4 is a gas chromatogram of the cracked gas of Example 2;

[0028] FIG5 is a SEM image of the solid product of Example 2;

[0029] FIG6 is a gas chromatogram of the cracked gas of Example 3;

[0030] FIG7 is a SEM image of the solid product of Example 3;

[0031] FIG8 is a gas chromatogram of the cracked gas of Comparative Example 1. DETAILED DESCRIPTION

[0032] The present invention provides a method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene, comprising the following steps: subjecting the waste polytetrafluoroethylene to a plasma cracking reaction, and sequentially quenching and gas-solid separation of the obtained cracking products to obtain cracked gas containing tetrafluoromethane; the temperature of the plasma cracking reaction is 1800-5000K.

[0033] The invention performs plasma cracking reaction on waste polytetrafluoroethylene to obtain cracking products.

[0034] In the present invention, scrap polytetrafluoroethylene refers to waste primarily composed of polytetrafluoroethylene. The present invention has no particular requirements for the source of the scrap polytetrafluoroethylene; it can be any one or more of waste polytetrafluoroethylene products, unqualified products from polytetrafluoroethylene production processes, and scraps. In the present invention, the scrap polytetrafluoroethylene may or may not contain fillers.

[0035] In the present invention, the plasma cracking reaction is preferably carried out in a plasma reactor.

[0036] The present invention preferably uses a carrier gas to pass the waste polytetrafluoroethylene into the plasma reactor; the waste polytetrafluoroethylene is preferably in the form of granules. The present invention has no special requirements for the size of the waste polytetrafluoroethylene granules, as long as they can be smoothly fed.

[0037] In the present invention, the feed rate of the waste polytetrafluoroethylene is preferably 0.5 to 3000 kg / h. The present invention preferably selects an appropriate plasma reactor based on the feed rate of the waste polytetrafluoroethylene and the temperature required for the plasma cracking reaction. When the feed rate of the waste polytetrafluoroethylene is large, the corresponding processing capacity increases.

[0038] In the present invention, the carrier gas preferably comprises argon and / or nitrogen. When the carrier gas comprises both argon and nitrogen, the present invention has no particular requirements for the ratio of the two gases; any ratio may be used. The present invention has no particular requirements for the flow rate of the carrier gas; it only requires that the flow rate of the carrier gas be sufficient to fluidize the waste polytetrafluoroethylene.

[0039] In the present invention, the plasma reactor preferably includes a radio frequency plasma reactor, a microwave plasma reactor, or an arc plasma reactor. The arc plasma reactor is preferably a direct current arc plasma reactor, and the direct current arc plasma reactor is further preferably a magnetically rotating arc thermal plasma reactor. Magnetically rotating arc thermal plasma reactors have the advantages of high mass and heat transfer efficiency and uniform and easily controllable residence time.

[0040] In the present invention, the temperature of the plasma cracking reaction is preferably 1800-5000K, more preferably 2000-4500K, further preferably 2500-4000K, and most preferably 3000-3500K.

[0041] By controlling the temperature of the plasma pyrolysis reaction within the above-mentioned range, the present invention ensures that the resulting pyrolysis products include solid-phase products and pyrolysis gas; the pyrolysis gas is primarily tetrafluoromethane. In the present invention, the pyrolysis gas preferably also includes hexafluoroethane and tetrafluoroethylene, and the total volume content of tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene in the pyrolysis gas is preferably ≥90%, with tetrafluoromethane being the highest volume content in the pyrolysis gas. In the present invention, the pyrolysis gas inevitably also contains a large amount of carrier gas (and, when direct quenching is employed, a direct quenching medium). The above volume content of the present invention is calculated based on the gaseous products produced by the simple pyrolysis reaction (the carrier gas and direct quenching medium are not included as gaseous products of the pyrolysis reaction).

[0042] The cracked gas composition of the present invention is simple, and the relatively simpler product composition reduces the investment cost of subsequent separation processes, making it suitable for industrial scale-up. In addition, the production of tetrafluoroethylene is relatively low, avoiding the formation of self-polymers, preventing blockage of reactors and material pipelines, and improving production safety.

[0043] In the present invention, the solid phase product is mainly carbon material, and the mass content of carbon black in the solid phase product is preferably ≥90%, and the remainder is mainly inorganic components such as graphene or other heavy metals (if waste polytetrafluoroethylene contains heavy metals, the solid phase product will contain heavy metal components).

[0044] In the present invention, the plasma cracking reaction is a millisecond-level cracking reaction. After the cracking reaction, the particles automatically enter the quenching device under the driving effect of the flow of the carrier gas.

[0045] In the present invention, the quenching is preferably performed by partition-type heat exchange quenching and / or direct quenching. The cooling medium used in the partition-type heat exchange quenching is preferably water and / or a refrigerant. The present invention has no particular requirements for the specific type of the refrigerant; any refrigerant with a cooling function known in the art, such as ethylene glycol, may be used. In the present invention, the cooling medium used in the direct quenching is preferably argon and / or nitrogen.

[0046] The present invention utilizes quenching to ensure subsequent gas-solid separation. Because existing gas-solid separation devices cannot withstand the temperature of the cracking reaction products, quenching is necessary.

[0047] The present invention has no special requirements on the temperature of the cracking product after quenching, and it is sufficient to ensure that the subsequent gas-solid separation can withstand the temperature.

[0048] The present invention has no particular requirements for the gas-solid separation method; any method known in the art for achieving gas-solid separation may be used, such as cyclone separation. In the present invention, as a preferred embodiment, the gas-solid separation utilizes a two-stage gas-solid separation method, wherein the first stage separation device is a cyclone separator for capturing larger carbon black particles or agglomerates of small carbon black particles; and the second stage separation device is a filter for capturing smaller carbon black particles.

[0049] After gas-solid separation, the present invention obtains a solid phase product and a cracked gas containing tetrafluoromethane. The compositions of the solid phase product and the cracked gas have been discussed above and will not be repeated here.

[0050] After obtaining the cracked gas, the present invention preferably further comprises subjecting the cracked gas to adsorption and distillation to obtain pure tetrafluoromethane. In the present invention, the adsorption and distillation are performed to separate tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene. The present invention has no particular requirements for the conditions of the adsorption and distillation; any method known in the art capable of separating tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene can be employed. The present invention simultaneously obtains pure tetrafluoromethane and pure hexafluoroethane and tetrafluoroethylene.

[0051] In the present invention, the light components obtained by distillation of the cracked gas include carrier gas and direct quenching medium, and are preferably divided into two streams, one of which is used to transport the waste polytetrafluoroethylene into the thermal plasma reactor for cracking, and the other is used as the direct quenching medium to quench the cracked gas. The ratio of the two gases can meet the flow requirements of the carrier gas and the direct quenching medium. The heavy components obtained by distillation include other by-products, and are preferably used to transport the waste polytetrafluoroethylene into the thermal plasma reactor for cracking.

[0052] Figure 1 is a flow chart of the method for producing tetrafluoromethane by thermal plasma pyrolysis of waste polytetrafluoroethylene according to the present invention. As shown in Figure 1, the method introduces waste polytetrafluoroethylene into a plasma reactor via a feed system for pyrolysis. The resulting pyrolysis products are sequentially quenched and subjected to gas-solid separation to obtain solid-phase products and gas-phase products (i.e., pyrolysis gas containing tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene). A portion of the gas-phase products is adsorbed and distilled to obtain pure tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene. A portion of the light components (primarily argon and / or nitrogen) obtained after distillation is circulated to a quenching device for direct quenching and used to cool the pyrolysis gas. (When using partition-type heat exchange quenching, the light components can be entirely used as carrier gas and circulated to the feed system.) The remaining light components (primarily argon and / or nitrogen) and all heavy components are circulated to the feed system for feeding and pyrolysis of the waste polytetrafluoroethylene.

[0053] The method for producing tetrafluoromethane by thermal plasma cracking of waste polytetrafluoroethylene provided by the present invention is described in detail below with reference to the examples, but they should not be construed as limiting the scope of protection of the present invention.

[0054] Example 1

[0055] Waste polytetrafluoroethylene passes through the feeding system, is fluidized by argon, and then transported into the microwave plasma reactor. The flow rate of waste polytetrafluoroethylene is 2.4 kg / h and the flow rate of argon is 2 Nm 3 / h, and the temperature in the plasma reactor is 2000K. After a millisecond-level cracking reaction, the resulting cracking products are passed into a cooling device, using water as the cooling medium and partition heat exchange to cool the cracking products to 300K. After gas-solid separation, cracked gas and solid-phase products are obtained. The cracked gas is sampled and analyzed by gas chromatography (see Figure 2, where the horizontal axis in Figure 2 is time in minutes). After adsorption and distillation, high-purity tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene are obtained. The light components (mainly argon) and all heavy components obtained after distillation are again transported to the waste polytetrafluoroethylene and entered into the thermal plasma reactor for cracking.

[0056] Gas chromatography detection and analysis showed that the volume content of tetrafluoromethane, hexafluoroethane and tetrafluoroethylene in the cracked gas was 66.78%, 26.82% and 6.39% respectively. FIG3 is a SEM image of the solid phase product of Example 1. As shown in FIG3 , the solid phase product is mainly composed of nano-carbon black particles.

[0057] Example 2

[0058] After the waste polytetrafluoroethylene is crushed, it is fed into the RF plasma reactor through the feeding system and gasified with argon. The flow rate of waste polytetrafluoroethylene is 5kg / h and the flow rate of argon is 4Nm 3 / h, and the temperature in the plasma reactor is 3200K. After a millisecond-level cracking reaction, the resulting cracking products are passed into a cooling device, using water as the cooling medium and partition heat exchange to cool the cracked gas to 520K. The cracking products undergo gas-solid separation to obtain solid-phase products and cracked gas. The cracked gas is sampled for gas chromatography analysis (see Figure 4, where the abscissa in Figure 4 represents time in minutes). After adsorption and rectification, high-purity tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene are obtained. The light components (mainly argon) and all heavy components obtained after rectification are again transported to the waste polytetrafluoroethylene and enter the hot plasma reactor for cracking.

[0059] Gas chromatography detection and analysis revealed that the volume content of tetrafluoromethane, hexafluoroethane, and tetrafluoroethylene in the cracked gas was 83.24%, 12.08%, and 2.13%, respectively. FIG5 is a SEM image of the solid phase product of Example 2. As can be seen from FIG5 , the solid phase product is primarily composed of nano-carbon black particles containing a small amount of graphene sheet structures.

[0060] Example 3

[0061] After the waste polytetrafluoroethylene is crushed, it is transported into the DC arc plasma reactor through the feeding system and the carrier gas. The flow rate of the waste polytetrafluoroethylene is 10kg / h and the argon flow rate is 7.2Nm 3 / h, and the temperature in the plasma reactor is 4400K. After a millisecond-level cracking reaction, the resulting cracking products are passed into a cooling device, using water as the cooling medium and partition heat exchange to cool the cracking products to 980K. After gas-solid separation, the cracking products are separated to obtain solid-phase products and cracking gas. The cracking gas is divided into two paths. One path is used with argon as a carrier gas to preheat and fluidize the waste polytetrafluoroethylene, and the other path is sampled for gas chromatography analysis (see Figure 6, where the horizontal axis in Figure 6 is time, unit is min). After adsorption and distillation, high-purity tetrafluoromethane, hexafluoroethane and tetrafluoroethylene are obtained. The light components (mainly argon) and all heavy components obtained after distillation are again transported to the waste polytetrafluoroethylene into the hot plasma reactor for cracking.

[0062] Gas chromatography detection and analysis showed that the volume content of tetrafluoromethane, hexafluoroethane and tetrafluoroethylene in the cracked gas was 90.17%, 6.08% and 0.93% respectively. FIG7 is a SEM image of the solid phase product of Example 1. As shown in FIG7 , the solid phase product is mainly composed of nano-carbon black particles.

[0063] Comparative Example 1

[0064] After the waste polytetrafluoroethylene is crushed, it is transported into the fluidized bed reactor through the feeding system and the carrier gas. The flow rate of the waste polytetrafluoroethylene is 85g / h and the nitrogen flow rate is 0.7Nm 3 / h, with the reactor temperature at 650-800°C and a residence time of 3 seconds. Gas chromatography analysis of the cracked gas (see Figure 8, where the horizontal axis represents time in minutes) revealed a mass fraction of 81.9% of recovered materials, including 72% tetrafluoroethylene, 4.1% hexafluoropropylene, 5.8% octafluorocyclobutane, and 13.1% byproducts. No tetrafluoromethane was detected.

[0065] As demonstrated by the above examples and comparative examples, the present invention utilizes plasma technology to pyrolyze waste polytetrafluoroethylene and controls the pyrolysis temperature. Compared to conventional pyrolysis methods, the resulting pyrolysis products have a simpler composition and higher selectivity for tetrafluoromethane. Furthermore, the examples and comparative examples also demonstrate that the present invention's plasma treatment of polytetrafluoroethylene offers advantages over conventional pyrolysis methods, including higher throughput and shorter residence times.

[0066] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for producing tetrafluoromethane by thermal plasma pyrolysis of waste polytetrafluoroethylene, comprising the following steps: Waste polytetrafluoroethylene is subjected to plasma cracking reaction, and the obtained cracking products are sequentially quenched and gas-solid separated to obtain cracking gas containing tetrafluoromethane; the temperature of the plasma cracking reaction is 1800-5000K.

2. The method according to claim 1, characterized in that: The plasma cracking reaction is carried out in a plasma reactor; the plasma reactor includes a radio frequency plasma reactor, a microwave plasma reactor or an arc plasma reactor.

3. The method according to claim 2, characterized in that The waste polytetrafluoroethylene is introduced into the plasma reactor by using a carrier gas.

4. The method according to claim 1 or 3, characterized in that: The waste polytetrafluoroethylene is in the form of granules.

5. The method according to claim 3, characterized in that: The carrier gas includes argon and / or nitrogen.

6. The method according to any one of claims 1 to 3, characterized in that: The feed rate of the waste polytetrafluoroethylene is 0.5 to 3000 kg / h.

7. The method according to claim 1, characterized in that The quenching is wall heat exchange quenching and / or direct quenching.

8. The method according to claim 7, characterized in that The cooling medium used in the partition heat exchange quenching is water and / or refrigerant; the cooling medium used in the direct quenching is argon and / or nitrogen.

9. The method according to claim 1, characterized in that: The cracked gas also includes hexafluoroethane and tetrafluoroethylene. The total volume content of tetrafluoromethane, hexafluoroethane and tetrafluoroethylene in the cracked gas is ≥90%, and the volume content of tetrafluoromethane in the cracked gas is the highest.

10. The method according to claim 1 or 9, characterized in that: After the cracked gas is obtained, the method further includes adsorbing and rectifying the cracked gas to obtain pure tetrafluoromethane.

11. The method according to claim 10, characterized in that After the distillation, the light components obtained include carrier gas and direct quenching medium, which are divided into two streams, one of which is used to transport the waste polytetrafluoroethylene into the thermal plasma reactor for cracking, and the other is used as a direct quenching medium to quench the cracked gas; the heavy components obtained by distillation include other by-products and are used to transport the waste polytetrafluoroethylene into the thermal plasma reactor for cracking.

12. The method according to claim 1, characterized in that The waste polytetrafluoroethylene may or may not contain filler.

13. The method according to claim 1, characterized in that The temperature of the plasma cracking reaction is 2000-4500K.

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