A method and system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis

Abstract

The present invention discloses a method and system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis. Taking advantage of the fact that chlorine-containing plastics can significantly absorb microwave radiation compared to other plastics, and the initial thermal decomposition temperature is significantly lower than other common plastics, a microwave pyrolysis reactor with the function of measuring temperature field and regulating microwave power is designed. Chlorine-containing plastics can be selectively heated by microwaves to achieve dechlorination of chlorine-containing plastics by heating without significantly increasing the temperature of other plastics. Combined with material pretreatment and final temperature pyrolysis treatment, high-value products such as chlorine-free / low-chlorine liquid fuels and pyrolytic carbon can be further prepared. The method of the present invention can reduce the energy consumption of the dechlorination process of mixed waste plastics, and while producing chlorine-free / low-chlorine products, it can significantly reduce the generation of chlorine-containing organic pollutants such as dioxins, improve the utilization rate of mixed plastics, and reduce the comprehensive processing cost of plastic chemical recycling.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste resource utilization, and specifically relates to a method and system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis. Background Art

[0002] The world produces 300 million tons of waste plastics every year, of which about 150-200 million tons are eventually sent to landfills or the natural environment, and only about 33 million tons can be recycled. An important reason affecting plastic recycling is that physical recycling, as the mainstream recycling method, is only applicable to some plastic categories with relatively high quality and value among waste plastics. Highly polluting waste plastics, plastic film materials or composite plastic materials cannot be effectively recycled, and most of them are directly incinerated. Waste plastics include plastics containing high concentrations of chlorine elements such as polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) and chlorinated polyethylene (CPE). Secondary pollutants such as HCl and chlorine generated during heat treatment will corrode the furnace body, and as a precursor of dioxins, they will cause serious pollution to the environment and affect the heat recovery process. Therefore, it is particularly important to pre-treat them with dechlorination.

[0003] Chlorine-containing plastics such as PVC have strong polarity due to the presence of C-Cl bonds. Their dielectric constant and dielectric loss angle are significantly higher than other plastics, so their microwave absorption performance is much higher than other common plastics (such as polyethylene, polypropylene, etc.). In addition, the pyrolysis process of chlorine-containing plastics is roughly divided into two steps. First, between about 250℃-400℃, the pyrolysis of chlorine-containing plastics is mainly manifested in the breaking of C-Cl bonds and the corresponding large-scale release of HCl. The second stage is the breaking of the remaining CH and CC bonds to produce hydrocarbons and pyrolytic carbon. In contrast, the pyrolysis process of other common plastics has only one step, and most of them start at 400℃. According to the above pyrolysis mechanism of chlorine-containing plastics and the characteristics of microwave selective heating, in the microwave heating process of mixed plastics, chlorine-containing plastics will absorb microwave heating first, while other plastics will not significantly heat up due to absorbing microwaves, thereby achieving the dechlorination of chlorine-containing plastics in the low temperature section (250-400℃), avoiding HCl from reacting with the pyrolysis products of other plastics and entering the liquid phase product. Applying microwave selective pyrolysis to the dechlorination process of chlorine-containing mixed plastics can effectively solve the chlorine pollution problem caused by the treatment of waste plastics. As a clean material, the dechlorinated mixed plastics will have a wide range of application markets, whether as a fuel to replace coal, directly pyrolyze to recover oil products, or prepare solid derivative fuel (SRF). In addition, microwave heating has the characteristics of fast heating speed, overall heating, selective heating, energy saving and high efficiency.

[0004] Current research focuses on the pyrolysis of single-type plastics, and there is a gap in the research on the dechlorination of difficult-to-separate domestic waste plastics to promote their resource utilization. At present, there is no report on a method and system for dechlorination and upgrading of waste plastics by microwave selective pyrolysis. Summary of the invention

[0005] In view of the existing technical gap, the purpose of the present invention is to provide a method and system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis. The method utilizes the characteristics of chlorine-containing plastics such as PVC that can significantly absorb microwaves compared with other types of plastics and the initial thermal decomposition temperature is significantly lower than that of other plastics. Microwave selective pyrolysis is used to promote the dechlorination of mixed plastics in the form of HCl in the low temperature section, and alkaline agents are sprayed for out-of-furnace adsorption. The dechlorinated mixed plastics can be used to produce low-chlorine liquid fuels and pyrolytic carbon, thereby realizing efficient resource utilization of low-quality mixed plastics.

[0006] The specific technical solutions are as follows:

[0007] A system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis treatment comprises a metal outer shell, a ceramic cylinder is arranged in the metal outer shell, a microwave pyrolysis chamber is arranged in the ceramic cylinder, an acid gas outlet connected to a deacidification tower and a first vacuum pump in sequence and a pyrolysis gas outlet connected to a condensation collection device and a second vacuum pump in sequence are provided on the metal outer shell, the acid gas outlet and the pyrolysis gas outlet are connected to the microwave pyrolysis chamber, a microwave generator is also provided on the metal outer shell, a pyrolysis carbon outlet is provided at the bottom of the metal outer shell, a sealed cavity formed by a plastic sealing sleeve is provided above the metal outer shell, an electric hydraulic jack is provided at the upper end of the plastic sealing sleeve, the lower end of the electric hydraulic jack is connected to the upper surface of a steel plate, the lower surface of the steel plate is connected to the upper end of an absorbing column arranged in the sealed cavity, and the lower end of the absorbing column is movably connected to a microwave anti-leakage device arranged on the outer surface of the upper end of the metal outer shell.

[0008] Furthermore, an infrared thermal imager is provided on the inner surface of the upper end of the metal outer shell, a thermocouple is provided in the wave-absorbing column, and the thermocouple and the infrared thermal imager are respectively connected to the control system.

[0009] Furthermore, the metal outer shell is also provided with a temperature observation window for viewing the infrared thermal imager and a feed pipe connected to the sealed hopper and the screw feeder.

[0010] Furthermore, a reversible grate is provided in the pyrolytic carbon outlet, and the reversible grate is controlled by a rotating shaft and a motor. A retractable scraper is also provided below the reversible grate, and a carbon collecting box is provided below the pyrolytic carbon outlet.

[0011] Furthermore, the absorbing column comprises an alloy supporting column and a silicon carbide column, the upper end of the silicon carbide column is connected to the alloy supporting column, and the lower end is movably connected to the microwave leakage prevention device.

[0012] Furthermore, a heat-insulating filling interlayer is provided between the metal outer shell and the ceramic tube.

[0013] A method for dechlorinating and upgrading mixed waste plastics by using the above system for microwave selective pyrolysis treatment comprises the following steps:

[0014] 1) Pretreatment: Wash the mixed plastics with water to remove surface impurities, then use a solvent to dissolve the aluminum foil layer and adhesive on the surface of the mixed plastics, dry and crush them for later use;

[0015] 2) Microwave selective pyrolysis dechlorination: The mixed plastics treated in step 1) are fed into the microwave pyrolysis chamber through a feed pipe. Under the protection of inert gas, microwave radiation is used to achieve rapid heating of chlorine-containing plastics. The material surface temperature field is monitored in real time by an infrared thermal imager. When the hot spot temperature reaches the dechlorination temperature range of 250-350°C for chlorine-containing plastics, the control system is used to adjust the microwave output power to keep the hot spot area warm for 1-2 minutes to alleviate the process of local hot spots radiating heat outward. The generated gas enters the deacidification tower through the acid gas outlet, and the acid gas is adsorbed by spraying limestone-gypsum;

[0016] 3) Final temperature pyrolysis treatment: After the dechlorination step, the material moves downward under the action of the electric hydraulic jack, passes through the microwave leakage prevention device and enters the microwave pyrolysis chamber, absorbs microwaves, and continues to heat up to 500-550℃. It is kept warm for 5-10 minutes to ensure the complete pyrolysis reaction. The pyrolysis gas is discharged through the pyrolysis gas outlet into the condensation collection device to collect the liquid product and pyrolysis carbon. The reversible grate is rotated by the rotating shaft and the motor, and the pyrolysis carbon on the reversible grate is turned over and collected in the carbon collecting box through the pyrolysis carbon outlet. The pyrolysis carbon stuck on the reversible grate is scraped off by the retractable scraper.

[0017] Furthermore, in step 1), the proportion of chlorine in the mixed plastic is 3-20%.

[0018] Furthermore, the solvent in step 1) is toluene, hexane, ethanol, acetone or N,N-dimethylcyclohexylamine.

[0019] Furthermore, the drying temperature in step 1) is 80-100°C.

[0020] The beneficial effects of the present invention are:

[0021] 1) The present invention mainly processes low-quality mixed plastics that are difficult to recycle. Through washing-dissolving-drying-crushing and upgrading, microwave low-temperature pyrolysis and dechlorination, high-value products such as chlorine-free / low-chlorine liquid fuel and pyrolysis carbon are finally produced. The dechlorination process is carried out at low temperature (<350 ℃), avoiding the induction of dioxins and other pollutants at high temperatures. Therefore, this method improves the recovery rate and resource utilization efficiency of plastics, reduces pollutant emissions in the plastic recycling process, and has positive significance for promoting carbon emission reduction, solid waste pollution control and circular economy.

[0022] 2) By utilizing the selective heating characteristics of microwaves, the present invention can achieve selective heating and dechlorination of chlorine-containing plastics in mixed waste plastics without significantly increasing the temperature of other plastics. Compared with the step-by-step pyrolysis dechlorination process using traditional heating methods, the present invention can significantly reduce process energy consumption and improve the dechlorination effect.

[0023] 3) In the design of the specific system, by combining the infrared temperature monitoring system and the control system, accurate measurement of the non-uniform temperature field of the material and program control of the microwave power can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0025] In the figure: 1. metal outer shell; 2. ceramic cylinder; 3. microwave pyrolysis chamber; 4. deacidification tower; 5. first vacuum pump; 6. acid gas outlet; 7. condensation collection device; 8. second vacuum pump; 9. pyrolysis gas outlet; 10. microwave generator; 11. pyrolysis carbon outlet; 12. plastic sealing sleeve; 13. electric hydraulic jack; 14. steel plate; 15. absorbing column; 16. microwave leakage prevention device; 17. infrared thermal imager; 18. thermocouple; 19. control system; 20. temperature observation window; 21. sealed hopper; 22. screw feeder; 23. feeding pipe; 24. reversible grate; 25. retractable scraper; 26. carbon collecting box; 27. alloy support column; 28. silicon carbide column; 29. ​​thermal insulation filling interlayer. DETAILED DESCRIPTION

[0026] In order to facilitate those skilled in the art to understand the technical solution of the present invention, Figure 1 The technical solution of the present invention is further described.

[0027] like Figure 1As shown, a system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis treatment includes a metal outer shell 1, a ceramic cylinder 2, a deacidification tower 4, a first vacuum pump 5, a condensation collection device 7, a second vacuum pump 8, a microwave generator 10, a plastic sealing sleeve 12, an electric hydraulic jack 13, a steel plate 14, an absorbing column 15, a microwave leakage prevention device 16, an infrared thermal imager 17, a control system 19, a sealed hopper 21, a screw feeder 22, a feed pipe 23, a reversible grate 24, a retractable scraper 25 and a carbon collection box 26.

[0028] A ceramic tube 2 is arranged in the metal outer shell 1, and a heat-insulating filling interlayer 29 is arranged between the metal outer shell 1 and the ceramic tube 2. A microwave pyrolysis chamber 3 is arranged in the ceramic tube 2. An acid gas outlet 6 connected to a deacidification tower 4 and a first vacuum pump 5 in sequence is provided on the metal outer shell 1, a pyrolysis gas outlet 9 connected to a condensation collection device 7 and a second vacuum pump 8 in sequence, and a feed pipe 23 connected to a sealed hopper 21 and a screw feeder 22. The acid gas outlet 6, the pyrolysis gas outlet 9 and the feed pipe 23 are respectively communicated with the microwave pyrolysis chamber 3. A microwave generator 10 is arranged on the metal outer shell 1. A pyrolysis carbon outlet 11 is arranged at the bottom of the metal outer shell 1. The pyrolysis carbon outlet 11 remains sealed during the pyrolysis process. A reversible grate 24 is arranged in the pyrolysis carbon outlet 11. The reversible grate 24 is controlled by a rotating shaft and a motor. A retractable scraper 25 is also arranged below the reversible grate 24. A carbon collecting box 26 is provided below the pyrolytic carbon outlet 11, a sealed cavity formed by a plastic sealing sleeve 12 is provided above the metal outer shell 1, an electric hydraulic jack 13 is provided at the upper end of the plastic sealing sleeve 12, the lower end of the electric hydraulic jack 13 is connected to the upper surface of the steel plate 14, the lower surface of the steel plate 14 is connected to the upper end of an absorbing column 15 provided in the sealed cavity, the absorbing column 15 includes an alloy support column 27 and a silicon carbide column 28, the upper end of the silicon carbide column 28 is connected to the alloy support column 27, and the lower end is movably connected to a microwave leakage prevention device 16 provided on the outer surface of the upper end of the metal outer shell 1, an infrared thermal imager 17 is provided on the inner surface of the upper end of the metal outer shell 1, a thermocouple 18 is provided in the absorbing column 15, the thermocouple 18 and the infrared thermal imager 17 are respectively connected to the control system 19, and a temperature observation window 20 for observing the infrared thermal imager 17 is also provided on the metal outer shell 1.

[0029] The ceramic tube 2 can be made of cordierite, high-purity alumina and other materials, the thermal insulation filling interlayer 29 can be made of wave-transmitting and high-temperature thermal insulation materials such as polycrystalline alumina fiber, and the temperature observation window 20 is made of quartz material with a window diameter less than 1 cm to prevent microwaves from leaking out of the pyrolysis chamber.

[0030] A method for dechlorinating and upgrading mixed waste plastics by microwave selective pyrolysis treatment comprises the following steps:

[0031] 1) Pretreatment: Wash the mixed plastics with water to remove surface impurities, then use a solvent to dissolve the aluminum foil laminate and adhesive on the surface of the mixed plastics, dry and crush them for later use. The chlorine content in the mixed plastics is controlled between 3% and 20%. The solvents can be toluene, hexane, ethanol and acetone, etc. Switchable hydrophilic solvents (such as N, N-dimethylcyclohexylamine) can also be used as green and sustainable solvents. The drying temperature is set between 80-100°C to prevent the plastic from melting and hindering the subsequent crushing step;

[0032] 2) Microwave selective pyrolysis dechlorination: The mixed plastic treated in step 1) is fed into the microwave pyrolysis chamber 3 through the feed pipe 23. Under the protection of inert gas, the microwave generated by the microwave generator 10 enters the microwave pyrolysis chamber 3 through the heat-insulating filling interlayer 29, and the chlorine-containing plastic is quickly heated by microwave radiation. The material surface temperature field is monitored in real time by the infrared thermal imager 17. When the hot spot temperature reaches the dechlorination temperature range of 250-350°C for the chlorine-containing plastic, the control system 19 is used to adjust the microwave output power to keep the hot spot area warm for 1-2 minutes to alleviate the process of the local hot spot radiating heat outward. The generated gas enters the deacidification tower 4 through the acid gas outlet 6, and the acid gas is adsorbed by spraying limestone-gypsum;

[0033] 3) Final temperature pyrolysis treatment: After the material has undergone the dechlorination step, the acid gas outlet 6 is closed. Under the action of the electric hydraulic jack 13, the absorbing column 15 moves downward, passes through the microwave leakage prevention device 16 and enters the microwave pyrolysis chamber 3, absorbs microwaves, and increases the microwave transmission power to carry out the final temperature pyrolysis process. The temperature is measured by the embedded thermocouple 18, and the temperature is raised to 500-550°C. The temperature is kept for 5-10 minutes to ensure that the pyrolysis reaction is complete. The pyrolysis gas is discharged through the pyrolysis gas outlet 9 into the condensation collection device 7 to collect the liquid product, pyrolysis carbon. The reversible grate 24 is rotated by the rotating shaft and the motor, and the pyrolysis carbon on the reversible grate 24 is turned over and collected in the carbon collecting box 26 through the pyrolysis carbon outlet 11. The pyrolysis carbon adhering to the reversible grate 24 is scraped off by the retractable scraper 25.

Claims

1. A system for dechlorination and upgrading of mixed waste plastics by microwave selective pyrolysis, characterized in that The invention comprises a metal outer shell (1), a ceramic tube (2) is arranged inside the metal outer shell (1), a microwave pyrolysis chamber (3) is arranged inside the ceramic tube (2), an acid gas outlet (6) connected in sequence to a deacidification tower (4) and a first vacuum pump (5), and a pyrolysis gas outlet (9) connected in sequence to a condensation collection device (7) and a second vacuum pump (8), the acid gas outlet (6) and the pyrolysis gas outlet (9) are connected to the microwave pyrolysis chamber (3), a microwave generator (10) is also arranged on the metal outer shell (1), and the metal outer shell (1) is provided with a microwave generator (10). The bottom of the shell (1) is a pyrolytic carbon outlet (11), a sealed cavity formed by a plastic sealing sleeve (12) is provided above the metal outer shell (1), an electric hydraulic jack (13) is provided at the upper end of the plastic sealing sleeve (12), the lower end of the electric hydraulic jack (13) is connected to the upper surface of a steel plate (14), the lower surface of the steel plate (14) is connected to the upper end of an absorbing column (15) provided in the sealed cavity, and the lower end of the absorbing column (15) is movably connected to a microwave leakage prevention device (16) provided on the outer surface of the upper end of the metal outer shell (1); An infrared thermal imager (17) is provided on the inner surface of the upper end of the metal outer shell (1), a thermocouple (18) is provided in the wave absorbing column (15), and the thermocouple (18) and the infrared thermal imager (17) are respectively connected to a control system (19); The metal outer shell (1) is also provided with a temperature observation window (20) for viewing the infrared thermal imager (17) and a feed pipe (23) connected to the sealed hopper (21) and the screw feeder (22); A reversible grate (24) is provided in the pyrolytic carbon outlet (11), the reversible grate (24) is controlled by a rotating shaft and a motor, a retractable scraper (25) is provided below the reversible grate (24), and a carbon collecting box (26) is provided below the pyrolytic carbon outlet (11); A method for dechlorinating and upgrading mixed waste plastics by using the above system for microwave selective pyrolysis treatment comprises the following steps: 1) Pretreatment: Wash the mixed plastics with water to remove surface impurities, then use a solvent to dissolve the aluminum foil layer and adhesive on the surface of the mixed plastics, dry and crush them for later use; 2) Microwave selective pyrolysis dechlorination: the mixed plastics treated in step 1) are fed into a microwave pyrolysis chamber (3) through a feed pipe (23), and the chlorine-containing plastics are rapidly heated by microwave radiation under the protection of an inert gas. The temperature field on the surface of the material is monitored in real time by an infrared thermal imager (17). When the hot spot temperature reaches the dechlorination temperature range of 250-350° C. for the chlorine-containing plastics, the control system (19) is used to adjust the microwave output power so that the hot spot area is kept warm for 1-2 minutes to alleviate the process of heat radiation from the local hot spot. The generated gas enters the deacidification tower (4) through the acid gas outlet (6), and the acid gas is adsorbed by spraying limestone-gypsum; 3) Final temperature pyrolysis treatment: After the material has undergone the dechlorination step, under the action of the electric hydraulic jack (13), the absorbing column (15) moves downward, passes through the microwave leakage prevention device (16) and enters the microwave pyrolysis chamber (3), absorbs microwaves, and continues to heat up to 500-550° C., and is kept warm for 5-10 minutes to ensure that the pyrolysis reaction is complete. The pyrolysis gas is discharged through the pyrolysis gas outlet (9) and enters the condensation collection device (7) to collect the liquid product, pyrolysis carbon. The reversible grate (24) is rotated by the rotating shaft and the motor, and the pyrolysis carbon on the reversible grate (24) is turned over and collected in the charcoal collecting box (26) through the pyrolysis carbon outlet (11). The pyrolysis carbon adhering to the reversible grate (24) is scraped off by the retractable scraper (25).

2. The system according to claim 1, characterized in that The microwave absorbing column (15) comprises an alloy supporting column (27) and a silicon carbide column (28); the upper end of the silicon carbide column (28) is connected to the alloy supporting column (27), and the lower end is movably connected to the microwave leakage prevention device (16).

3. The system according to claim 1, characterized in that A heat-insulating filling interlayer (29) is provided between the metal outer shell (1) and the ceramic cylinder (2).

4. The system according to claim 1, characterized in that In step 1), the proportion of chlorine in the mixed plastic is 3-20%.

5. The system according to claim 1, characterized in that The solvent in step 1) is toluene, hexane, ethanol, acetone or N,N-dimethylcyclohexylamine.

6. The system of claim 1, wherein The drying temperature in step 1) is 80-100°C.

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