Method for producing high-purity tetrafluoroethylene monomer by recycling polytetrafluoroethylene reclaimed material

Abstract

The invention provides a method for producing a high-purity tetrafluoroethylene monomer by recovering a polytetrafluoroethylene reclaimed material, and belongs to the field of waste recovery production, and the method specifically comprises the following steps: cleaning the polytetrafluoroethylene reclaimed material, crushing at low temperature, carrying out magnetic separation on the crushed reclaimed material, drying, and screening to obtain polytetrafluoroethylene micro powder; the method comprises the following steps: in a preset temperature range of a high-vacuum system, cracking polytetrafluoroethylene micro powder by adopting a thermal cracking method, sequentially carrying out heat exchange cooling, water washing, pressurization, cooling dehydration and adsorption deep dehydration on obtained high-temperature cracked gas, and then carrying out first-stage and second-stage deep cooling to remove light components so as to obtain a first stream; and carrying out cryogenic rectification on the first stream to obtain a heavy component stream and a light component stream, and removing heavy components except the tetrafluoroethylene monomer in the heavy component stream to obtain a high-purity tetrafluoroethylene monomer as a second stream. By means of the treatment scheme, the purity and the recovery rate of tetrafluoroethylene are improved, and meanwhile output of highly toxic substances such as octafluoroisobutylene and fluorophosgene can be controlled.

Description

Technical Field

[0001] The invention relates to the field of waste material recycling production, and in particular to a method for recycling polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer. Background Art

[0002] Polytetrafluoroethylene enjoys the reputation of "King of Plastics" due to its excellent performance, and its uses and usage in various fields are increasing. But at the same time, the waste products produced by polytetrafluoroethylene materials are also increasing. At present, radiation cracking, high-temperature cracking, and direct mechanical crushing are generally used for recycling at home and abroad. Among these three methods, the high-temperature cracking method has high product quality and the most thorough recycling. In addition, the high-temperature cracking method has no requirements for waste materials. Whether or not fillers are added, they can be recycled through high-temperature cracking.

[0003] However, the high-temperature pyrolysis method requires decomposing PTFE into small molecules at a high temperature of 425-700 °C, and then recovering useful components such as tetrafluoroethylene, hexafluoropropylene (HFP), octafluorocyclobutane (OFCB), etc. However, a small amount of highly toxic substances such as octafluoroisobutylene (OFIB) and fluorophosgene (COF2) will be produced during the decomposition process. These highly toxic substances cannot be easily separated from the above-mentioned useful components, which not only reduces the purity of the recovered tetrafluoroethylene, but also increases the processing cost.

[0004] Based on this, a new method needs to be developed. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a method for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer, which can control the output of highly toxic substances such as octafluoroisobutylene and fluorophosgene while improving the purity and recovery rate of tetrafluoroethylene.

[0006] In order to achieve the above object, the present invention provides a method for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer, comprising: S1, washing the polytetrafluoroethylene recycled material and then crushing it at low temperature, subjecting the crushed recycled material to magnetic separation, drying and sieving to obtain polytetrafluoroethylene powder; S2. Within a predetermined temperature range of a high vacuum system, the polytetrafluoroethylene powder is cracked by thermal cracking, and the obtained high-temperature cracked gas is subjected to heat exchange cooling, water washing, pressurization, cooling dehydration, adsorption deep dehydration in sequence, and then the light components are removed by one or two stages of deep cooling to obtain a first stream; the vacuum absolute pressure of the high vacuum system is 0.1-10 kPa, and the predetermined temperature range is 490-580 °C. S3, subjecting the first stream to cryogenic distillation to obtain a recombinant fraction stream and a light fraction stream, and removing the heavy components except tetrafluoroethylene monomer from the recombinant fraction stream to obtain a second stream of high-purity tetrafluoroethylene monomer.

[0007] In one embodiment, S3 further comprises treating the light component stream: using a tubular furnace for thermal destruction, alkaline solution absorption and activated carbon to sequentially treat the light component stream to obtain tail gas emissions as the third stream.

[0008] In one embodiment, the S2 adsorption tower is regenerated using hot nitrogen, and the regenerated tail gas enters the thermal destruction system in S3 for treatment.

[0009] In one embodiment, the thermal cracking method in S2 is carried out in a tubular furnace using electric heating to control the temperature.

[0010] In one embodiment, the particle size of the polytetrafluoroethylene powder is in the range of 1-100 μm.

[0011] In one embodiment, the adsorbent for deep dehydration in S2 is at least one of activated alumina molecular sieve, 13X molecular sieve, CaX molecular sieve and activated carbon.

[0012] In one embodiment, the first and second cryogenic devices in S2 are connected in series, the first-stage cryogenic device is mechanical refrigeration or recycles the cooling capacity of the second-stage cryogenic device, the second-stage cryogenic device uses refrigerant refrigeration, the first-stage cryogenic device refrigerates at a temperature range of 0°C to -50°C; the second-stage cryogenic device refrigerates at a temperature range of -30°C to -80°C.

[0013] In one embodiment, before the polytetrafluoroethylene powder is cracked by thermal cracking in S2, high-purity nitrogen is first used to purge and then vacuum the tube furnace to remove oxygen, so that the oxygen concentration is less than 0.1 ppmv.

[0014] In one embodiment, the pressure range of the cryogenic distillation tower in S3 is 0.1-0.7 MPa (G), the bottom temperature range is -50°C to -10°C, the top temperature range is -70°C to -50°C, the theoretical plate number is 10-30, and the reflux ratio is 3-20.

[0015] In one embodiment, the temperature of thermal destruction in S3 is 800-1200°C, the alkali solution is 5-15% NaOH or Na2CO3 aqueous solution, a packed absorption tower is used, and the packing is a ball ring made of PTFE or PP material with hydrophilic surface modification; the activated carbon filled in the adsorption bed is activated carbon impregnated with Na2CO3.

[0016] Compared with the prior art, the advantages of the present invention are: PTFE is cracked in a vacuum environment by vacuum thermal cracking to prepare high-purity TFE monomer, and the output of highly toxic substances such as octafluoroisobutylene and fluorophosgene can be controlled during the preparation process; in addition, the side reactions caused by subsequent direct water spray cooling are reduced by heat exchange cooling of the generated high-temperature cracking gas; a low-temperature condenser is used to perform distillation on the basis of improving the recovery rate to obtain high-purity tetrafluoroethylene monomer, and the yield of high-purity tetrafluoroethylene monomer reaches 98%. The entire production system is suitable for smaller-scale enterprises, such as enterprises with an annual output of 2 tons of perfluoroether elastomers and a required TFE of about 2 tons / year. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 These are the process steps of a method for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer in an embodiment of the present invention.

[0019] Figure 2 It is a system for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer in the embodiment of the present invention.

[0020] In the figure, 1 is the PTFE low-temperature crushing system; 2 is the tubular pyrolysis furnace; 3 is the partition wall cooling heat exchanger; 4 is the water washing and cooling system; 5 is the pressurized cooling and dehydration system; 5-1 is the condensate; 6 is the adsorption deep dehydration system; 7 is the deep cold distillation system; 8 is the tetrafluoroethylene TFE product; 9 is the tail liquid treatment system; H1 and H2 are the first and second level deep cold systems. DETAILED DESCRIPTION

[0021] The embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0022] The following describes the implementation methods of the present application through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The present application can also be implemented or applied through other different specific implementation methods, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present application. It should be noted that, in the absence of conflict, the following embodiments and the features in the embodiments can be combined with each other. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without making creative work belong to the scope of protection of the present application.

[0023] It should be noted that various aspects of the embodiments within the scope of protection of the present invention are described below. It should be apparent that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on the present application, it should be understood by those skilled in the art that an aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number and aspect described herein can be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described herein can be used to implement this device and / or practice this method.

[0024] It should also be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present application. The drawings only show components related to the present application rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0025] Additionally, in the following description, specific details are provided to facilitate a thorough understanding of the examples. However, it will be understood by those skilled in the art that the aspects described may be practiced without these specific details.

[0026] like Figure 1 and Figure 2 As shown, the embodiment of the present application provides a method for recycling polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer, comprising the following steps: S1. The polytetrafluoroethylene recycled material is washed and then crushed at low temperature. The crushed recycled material is subjected to magnetic separation, dried and sieved to obtain polytetrafluoroethylene powder.

[0027] The polytetrafluoroethylene recycled material is washed and dried, and then the polytetrafluoroethylene recycled material is crushed at low temperature. In one embodiment, the polytetrafluoroethylene recycled material can be frozen by a refrigerant to make it brittle, and then the polytetrafluoroethylene recycled material is quickly crushed. The crushed recycled material is subjected to magnetic separation to remove metal impurities. The polytetrafluoroethylene micropowder (PTFE micropowder) is dried and sieved, and the drying temperature is 160-200 °C.

[0028] The particle size and distribution of PTFE powder have a great influence on the cracking reaction speed and recovery rate. When the particle size distribution is wide, the small particle size may have been completely cracked, and the core of some large particle size PTFE powder has not been cracked, which reduces the recovery rate. Therefore, the particle size of polytetrafluoroethylene powder should be uniform to control the subsequent high temperature cracking time.

[0029] S2. Within the predetermined temperature range of the high vacuum system, thermal cracking method is used to crack polytetrafluoroethylene powder, and the obtained high-temperature cracking gas is subjected to heat exchange cooling, water washing, pressurization, cooling dehydration, adsorption deep dehydration in sequence, and then the light components are removed by one or two stages of deep cooling to obtain the first stream; the vacuum absolute pressure of the high vacuum system is 0.1-10 kPa, and the predetermined temperature range is 490-580 ℃.

[0030] The pyrolysis composition of PTFE is closely related to the pyrolysis temperature and pressure. By controlling the vacuum absolute pressure of the high vacuum system to 0.1-10 kPa and the predetermined temperature range to 490-580 °C, the generation of perfluorobutylene compounds can be effectively controlled. The pyrolysis products of PTFE are a mixture, including TFE / hexafluoropropylene HFP, or TFE / hexafluoropropylene HFP / octafluorocyclobutane OFCB.

[0031] The high-temperature cracking gas is cooled by inter-wall heat exchange, which can avoid the cracking gas from directly spraying water at a high temperature to cool it down and reacting with water to form oxides, which will corrode instruments and equipment. Water washing is used to further reduce the temperature of the cracking gas and absorb some water-soluble compounds. The cracking gas is pressurized to improve the transportation efficiency of the cracking gas, and some components that appear in gaseous state under low vacuum can also be condensed into liquid phase to reduce the subsequent gas processing volume. The dew point temperature of the cracking gas is then reduced by cooling dehydration and adsorption deep dehydration. In one embodiment, the dew point temperature is -60 ℃. Deep cooling generally refers to the temperature range of 233K-77K. The first and second levels of deep cooling refer to deep cooling at different temperatures, and the first level temperature is higher than the second level temperature.

[0032] S3, subjecting the first stream to cryogenic distillation to obtain a recombinant fraction stream and a light fraction stream, and removing the heavy components except tetrafluoroethylene monomer from the recombinant fraction stream to obtain a second stream of high-purity tetrafluoroethylene monomer.

[0033] The first stream is distilled under cryogenic conditions to obtain a recombinant stream and a light fraction. The system for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer includes: a tubular furnace, a heat exchanger, a spray water washing device, a pressurizing, a cooling and dehydration system, an adsorption deep dehydration system, a first and second cryogenic system, a cryogenic distillation system and an exhaust gas treatment system. The entire system is controlled by a PLC control system through pressure sensors, temperature sensors, flow controllers and concentration signals.

[0034] The tubular furnace is made of nickel. The polytetrafluoroethylene recycled material is placed in the tubular furnace, and the high-temperature cracking gas produced enters and passes through the above system in turn to obtain the second stream of high-purity tetrafluoroethylene monomer.

[0035] GC-MS (Agilent 5977B) was installed at the outlets of the primary and secondary cryogenic systems, cryogenic distillation system and tail gas treatment system to analyze the tail gas of each stream online.

[0036] The above method uses vacuum thermal cracking to crack PTFE in a vacuum environment to prepare high-purity TFE monomer, and highly toxic substances such as octafluoroisobutylene and fluorophosgene can be controlled during the preparation process; in addition, the high-temperature cracking gas produced is cooled by interwall heat exchange to reduce the side reactions caused by subsequent direct water spraying of high-temperature tail gas; and then a low-temperature condenser is used to improve the recovery rate and perform distillation to obtain high-purity tetrafluoroethylene monomer, with a high-purity tetrafluoroethylene monomer yield of 98%. The entire production system is fully applicable to smaller-scale enterprises, such as enterprises with an annual output of 2 tons of perfluoroether elastomers and a required TFE of about 2 tons / year. The produced high-purity tetrafluoroethylene monomer can be used to synthesize polymers or intermediates containing tetrafluoroethylene.

[0037] In one embodiment, S3 further includes treating the light component stream: using a tubular furnace for thermal destruction, alkaline solution absorption and activated carbon to sequentially treat the light component stream to obtain a third stream of tail gas emissions.

[0038] In one embodiment, the temperature of thermal destruction in S3 is 800-1200°C, the alkali solution is 5-15% NaOH or Na2CO3 aqueous solution, a packed absorption tower is used, and the packing is a ball ring made of PTFE or PP material with hydrophilic surface modification; the activated carbon filled in the adsorption bed is activated carbon impregnated with Na2CO3.

[0039] In one embodiment, the S2 adsorption tower is regenerated using hot nitrogen, and the regenerated tail gas enters the tail gas thermal destruction system for treatment.

[0040] In one embodiment, the thermal cracking process in S2 is carried out in a tubular furnace using electric heating to control the temperature.

[0041] In one embodiment, the particle size of the polytetrafluoroethylene powder is in the range of 1-100 μm.

[0042] In one embodiment, the adsorbent for deep dehydration in S2 is at least one of activated alumina molecular sieve, 13X molecular sieve, CaX molecular sieve and activated carbon.

[0043] In one embodiment, the first and second cryogenic devices in S2 are connected in series, the first-stage cryogenic device is mechanical refrigeration or recycles the cooling capacity of the second-stage cryogenic device, the second-stage cryogenic device uses refrigerant refrigeration, the temperature range of the first-stage cryogenic device is 0 ℃ to -50 ℃; the temperature range of the second-stage cryogenic device is -30 ℃ to -80 ℃.

[0044] In one embodiment, the cryogen includes at least one of liquid nitrogen, liquid argon, liquid air or liquid carbon dioxide.

[0045] In one embodiment, before the thermal cracking method is used to crack the polytetrafluoroethylene powder in S2, high-purity nitrogen is first used to purge and then vacuum the tube furnace to remove oxygen, so that the oxygen concentration in the tube furnace is less than 0.1 ppmv.

[0046] Oxygen can act as an inhibitor and initiator of free radical polymerization. In the case of TFE, the presence of oxygen not only inhibits polymerization, but also worsens the decomposition reaction. If there is enough oxygen in TFE, the oxygen may trigger a spontaneous decomposition reaction under pressure. Molecular oxygen can also cap the PTFE free radicals to form unstable peroxides. Therefore, in order to improve the recovery of TFE, it is necessary to remove free oxygen in the cracking tube furnace to less than 0.1 ppmv.

[0047] In one embodiment, the pressure range of the deep cold distillation tower in S3 is 0.1-0.7 MPa (G), the bottom temperature range is -50°C to -10°C, the top temperature range is -70°C to -50°C, the theoretical plate number is 10-30, and the reflux ratio is 3-20.

[0048] In one embodiment, the primary and secondary cryogenic systems in S2 are regenerated using high-temperature nitrogen, and the regenerated tail gas enters the thermal destruction system in S3 for treatment.

[0049] Example 1 The PTFE recycled material was crushed at low temperature in a PTFE low temperature crushing system, washed with deionized water, dried at 180 °C for 12 h, sieved after cooling, magnetically separated and sieved, and 1000 g of 1250-625 mesh (10-20 μm) powder was collected and loaded into a 5.0 L nickel stainless steel tube furnace (inner diameter φ10 cm, effective length 64 cm).

[0050] Thermal cracking is performed in the tubular pyrolysis furnace 2. The entire gas path including the tubular furnace is purged with high-purity nitrogen and evacuated. The cracking pressure is 0.5 kPa (absolute pressure), the temperature is controlled at 510 ± 2 °C, and the main composition of the cracking mixed gas is shown in Table 1. The high-temperature cracking gas is cooled to about 95 °C through the interlayer cooling heat exchanger 3, and enters the closed packed spray tower 4 from the bottom for water washing and cooling.

[0051] Table 1 Main composition of the cracked gas mixture in Example 1

[0052] The packed tower spray tower 4 has an inner diameter of φ10 cm and is filled with PTFE ball rings. Water is sprayed continuously on the top. The water soaks the ball rings and flows downward to counter-flow the rising gas. It enters the closed water storage tank and is lifted and sprayed by a peristaltic pump. A demister is installed on the top of the packed spray tower 4.

[0053] The mixed gas coming out from the top of the packed spray tower enters the pressurization and cooling dehydration system 5, and is first stored in a vacuum buffer tank. The outlet of the vacuum buffer tank is connected to a Leno small oil-free vacuum pump to control the vacuum degree in the nickel stainless steel tube furnace to an absolute pressure of 0.2 kPa. The other side of the vacuum pump is connected to a normal pressure buffer tank. When the mixed gas pressure in the normal pressure buffer tank exceeds 102 kPa (absolute pressure), it is pressurized to 0.25 MPa by the Leno oil-free compressor. When the mixed gas pressure in the normal pressure buffer tank drops to 98 kPa, the compressor stops working.

[0054] Due to the relatively small compression ratio, the temperature rise is about 15 ℃. After the mixed gas is pressurized and cooled to remove the condensate, it enters the adsorption device 6 from the bottom for deep dehydration. The adsorption device is a double-tower temperature and pressure swing adsorption. The adsorbents filled in the tower are activated alumina, 13X and activated carbon from bottom to top. During desorption, 5N (99.999%) N2 at a temperature of 120 ℃ is used for purging for 2 hours, and 5N N2 at room temperature is used for purging for 4 hours to cool to room temperature for use.

[0055] After adsorption treatment, the temperature of the mixed gas is about 35 ℃, and the dew point temperature is about -65 ℃. The first stream is obtained by removing the light components through the first and second stage cryogenic systems (H1, H2); the first stage uses the cooling capacity of the second stage condenser to cool to -20 ℃; the second stage is cooled by liquid nitrogen to -70 ℃. The composition of the mixed liquid is shown in Table 2.

[0056] 7 is a cryogenic distillation system, and tetrafluoroethylene TFE product 8 is obtained after distillation. The temperature of thermal destruction of the first stream of non-condensable gas is 800 °C, the alkali solution is a 10% NaOH aqueous solution, and the tail liquid is treated by a packed absorption tower 9, the packing is a ball ring of PTFE with hydrophilic surface modification; the activated carbon filled in the adsorption bed is impregnated with Na2CO3. After high-temperature cracking, the alkaline adsorbent is adsorbed and the non-condensable gas is removed.

[0057] Table 2 Composition of the condensate of cracked gas after primary and secondary condensation in Example 1

[0058] The secondary liquid nitrogen condenser H2 is a partition-type heat exchanger. The inner layer consists of two parts, one is a central tube through which liquid nitrogen LN2 passes, and the lower end of the central tube is a free end, allowing it to expand and contract with heat. After the liquid nitrogen is partially vaporized and discharged from the lower end of the central tube, it continues to vaporize and flow upward along the closed heat exchange gap along the outer side of the central tube. The central tube and the closed heat exchange unit form the cold end. At the same time, the cracked mixed gas exchanges heat with the closed heat exchange unit, flows from top to bottom along the other side of the partition, gradually liquefies, and enters the bottom of the kettle of the deep-cold distillation tower 7. The gaseous nitrogen discharged from the secondary liquid nitrogen condenser is the cold source of the primary condenser or cooler. The pressure of the gaseous nitrogen discharged from the primary condenser is about 3.5 bar, which is stored in a storage tank as a regeneration gas source.

[0059] The secondary condenser needs to be regenerated regularly, and the regeneration condition is that the pressure difference at both ends of the secondary liquid nitrogen condenser is greater than 0.2 bar.

[0060] The main component of the mixed liquid after liquefaction of the first stream is tetrafluoroethylene monomer TFE, which contains a small amount of hexafluoropropylene monomer HFP and octafluorocyclobutane monomer OFCB. It is subjected to deep cold distillation to remove the heavy components HFP and OFCB to obtain the second stream of high-purity tetrafluoroethylene monomer. At the same time, the non-condensable gas is thermally destroyed and merged into the non-condensable gas of stream 1, and is discharged as the tail gas of the third stream after absorption by alkali solution and adsorption by activated carbon.

[0061] The pressure of the cryogenic distillation tower is 0.25 MPa (G), the temperature of the tower bottom reboiler is -40 °C, the tower top reflux temperature is -60 °C, the theoretical plate number is 15, and the reflux ratio is 7.2.

[0062] The purity of the high-purity tetrafluoroethylene monomer obtained is 99.99%, and the yield is 98%.

[0063] Example 2 The PTFE recycled material was crushed at low temperature in a PTFE low temperature crushing system, washed with deionized water, dried at 180 °C for 12 h, sieved after cooling, magnetically separated and sieved, and 1000 g of 1250-625 mesh (10-20 μm) powder was collected and loaded into a 5.0 L nickel stainless steel tube furnace (inner diameter φ10 cm, effective length 64 cm).

[0064] Thermal cracking is performed in the tubular pyrolysis furnace 2. The entire gas path including the tubular furnace is purged with high-purity nitrogen and evacuated. The cracking pressure is 0.5 kPa (absolute pressure), the temperature is controlled at 560 ± 2 °C, and the main composition of the cracking mixed gas is shown in Table 3. The high-temperature cracking gas is cooled to about 95 °C through the inter-wall cooling heat exchanger 3, and enters the closed packed spray tower 4 from the bottom for water washing and cooling.

[0065] Table 3 Main components of the cracking gas mixture in Example 2

[0066] The packed tower spray tower 4 has an inner diameter of φ10 cm and is filled with PTFE ball rings. Water is sprayed continuously on the top. The water soaks the ball rings and flows downward to counter-flow the rising gas. It enters the closed water storage tank and is lifted and sprayed by a peristaltic pump. A demister is installed on the top of the packed spray tower 4.

[0067] The mixed gas coming out from the top of the packed spray tower enters the pressurization and cooling dehydration system 5, and is first stored in a vacuum buffer tank. The outlet of the vacuum buffer tank is connected to a Leno small oil-free vacuum pump to control the vacuum degree in the nickel stainless steel tube furnace to an absolute pressure of 0.2 kPa. The other side of the vacuum pump is connected to a normal pressure buffer tank. When the mixed gas pressure in the normal pressure buffer tank exceeds 102 kPa (absolute pressure), it is pressurized to 0.25 MPa by the Leno oil-free compressor. When the mixed gas pressure in the normal pressure buffer tank drops to 98 kPa, the compressor stops working.

[0068] Due to the relatively small compression ratio, the temperature rise is about 15 ℃. After the mixed gas is pressurized and cooled to remove the condensate, it enters the adsorption device 6 from the bottom for deep dehydration. The adsorption device is a double-tower temperature and pressure swing adsorption. The adsorbents filled in the tower are activated alumina, 13X and activated carbon from bottom to top. During desorption, 5N (99.999%) N2 at a temperature of 120 ℃ is used for purging for 2 hours, and 5N N2 at room temperature is used for purging for 4 hours to cool to room temperature for use.

[0069] After adsorption treatment, the temperature of the mixed gas is about 35 ℃, and the dew point temperature is about -65 ℃. The first stream is obtained by removing the light components through the first and second stage cryogenic systems (H1, H2); the first stage uses the cooling capacity of the second stage condenser to cool to -20 ℃; the second stage is cooled by liquid nitrogen to -70 ℃. The composition of the mixed liquid is shown in Table 2.

[0070] 7 is a cryogenic distillation system, and tetrafluoroethylene TFE product 8 is obtained after distillation. The temperature of thermal destruction of the first stream of non-condensable gas is 800 °C, the alkali solution is a 10% NaOH aqueous solution, and the tail liquid is treated by a packed absorption tower 9, the packing is a ball ring of PTFE with hydrophilic surface modification; the activated carbon filled in the adsorption bed is impregnated with Na2CO3. After high-temperature cracking, the alkaline adsorbent is adsorbed and the non-condensable gas is removed.

[0071] Table 4 Composition of the condensate of cracked gas after primary and secondary condensation in Example 2

[0072] The secondary liquid nitrogen condenser H2 is a partition-type heat exchanger. The inner layer consists of two parts, one is a central tube through which liquid nitrogen LN2 passes, and the lower end of the central tube is a free end, allowing it to expand and contract with heat. After the liquid nitrogen is partially vaporized and discharged from the lower end of the central tube, it continues to vaporize and flow upward along the closed heat exchange gap along the outer side of the central tube. The central tube and the closed heat exchange unit form the cold end. At the same time, the cracked mixed gas exchanges heat with the closed heat exchange unit, flows from top to bottom along the other side of the partition, gradually liquefies, and enters the bottom of the kettle of the deep-cold distillation tower 7. The gaseous nitrogen discharged from the secondary liquid nitrogen condenser is the cold source of the primary condenser or cooler. The pressure of the gaseous nitrogen discharged from the primary condenser is about 3.5 bar, which is stored in a storage tank as a regeneration gas source.

[0073] The secondary condenser needs to be regenerated regularly, and the regeneration condition is that the pressure difference at both ends of the secondary liquid nitrogen condenser is greater than 0.2 bar.

[0074] The main component of the mixed liquid after liquefaction of the first stream is tetrafluoroethylene monomer TFE, which contains a small amount of hexafluoropropylene monomer HFP and octafluorocyclobutane monomer OFCB. It is subjected to deep cold distillation to remove the heavy components HFP and OFCB to obtain the second stream of high-purity tetrafluoroethylene monomer. At the same time, the non-condensable gas is thermally destroyed and merged into the non-condensable gas of stream 1, and is discharged as the tail gas of the third stream after absorption by alkali solution and adsorption by activated carbon.

[0075] The pressure of the cryogenic distillation tower is 0.25 MPa (G), the temperature of the tower bottom reboiler is -40 °C, the tower top reflux temperature is -65 °C, the theoretical plate number is 15, and the reflux ratio is 8.5.

[0076] The purity of the high-purity tetrafluoroethylene monomer obtained is 99.94%, and the recovery rate is 97%.

[0077] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed in the present application should be included in the protection scope of the present application.

Claims

1. A method for recovering polytetrafluoroethylene recycled materials to produce high-purity tetrafluoroethylene monomer, characterized in that: include: S1, washing the polytetrafluoroethylene recycled material and then crushing it at low temperature, subjecting the crushed recycled material to magnetic separation, drying and sieving to obtain polytetrafluoroethylene powder; S2. In a high vacuum system and within a predetermined temperature range, the polytetrafluoroethylene powder is cracked by thermal cracking, and the obtained high-temperature cracked gas is subjected to heat exchange cooling, water washing, pressurization, cooling dehydration, adsorption deep dehydration in sequence, and then the light components are removed by one or two stages of deep cooling to obtain the first stream; the vacuum absolute pressure of the high vacuum system is 0.1-10 kPa, and the predetermined temperature range is 490-580 °C. S3, subjecting the first stream to cryogenic distillation to obtain a recombinant fraction stream and a light fraction stream, and removing the heavy components except tetrafluoroethylene monomer from the recombinant fraction stream to obtain a second stream of high-purity tetrafluoroethylene monomer.

2. The method according to claim 1, characterized in that S3 also includes treating the light component stream: using a tubular furnace for thermal destruction, alkaline solution absorption and activated carbon to sequentially treat the light component stream to obtain a third stream of tail gas emissions.

3. The method according to claim 1, characterized in that The thermal cracking process in S2 is carried out in a tubular furnace, using electric heating to control the temperature.

4. The method according to claim 1, characterized in that: The particle size of the polytetrafluoroethylene powder is in the range of 1-100 μm.

5. The method according to claim 1, characterized in that The adsorbent for deep dehydration in S2 is at least one of activated alumina molecular sieve, 13X molecular sieve, CaX molecular sieve and activated carbon.

6. The method according to claim 1, characterized in that The S2 adsorption tower is regenerated by hot nitrogen, and the regenerated tail gas enters the thermal destruction system for treatment.

7. The method according to claim 1, characterized in that The first and second cryogenic devices in S2 are connected in series. The first cryogenic device is mechanical refrigeration or recycles the cooling capacity of the second cryogenic device. The second cryogenic device uses refrigerant refrigeration. The temperature range of the first cryogenic device is 0 ℃ to -50 ℃; the temperature range of the second cryogenic device is -30 ℃ to -80 ℃.

8. The method according to claim 1, characterized in that In S2, before the polytetrafluoroethylene powder is cracked by thermal cracking, high-purity nitrogen is first used to purge and then vacuum the tube furnace to remove oxygen, so that the oxygen concentration is less than 0.1 ppmv.

9. The method according to claim 1, characterized in that: The pressure range of the deep cold distillation tower described in S3 is 0.1-0.7 MPa (G), the bottom temperature range is -50 ℃ to -10 ℃, the top temperature range is -70 ℃ to -50 ℃, the theoretical plate number is 10-30, and the reflux ratio is 3-20.

10. The method according to claim 2, characterized in that The temperature of thermal destruction in S3 is 800-1200 ℃, the alkali solution is 5-15% NaOH or Na2CO3 aqueous solution, and a packed absorption tower is used. The packing is a ball ring made of PTFE or PP material with hydrophilic surface modification; the activated carbon filled in the adsorption bed is activated carbon impregnated with Na2CO3.

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