Continuous method and system for waste polyolefin thermal cracking resource recovery

Abstract

The present invention relates to a continuous method and reaction device for resource recovery by thermal cracking of waste polyolefins. Specifically, a melt of waste polyolefins is mixed with low-pressure hot water. By utilizing the solubilization of low-pressure hot water in the polyolefin melt, the adverse effect of high viscosity of the system on the kinetics of free radical thermal cracking reaction is alleviated. The mixture of waste polyolefins and low-pressure hot water enters a tubular reaction device, where thermal cracking is initiated in the preheating section and rapid conversion into wax, liquid hydrocarbons and gaseous hydrocarbons is completed in the adiabatic section. The yield of liquid hydrocarbons in the product reaches more than 90%, and no solid coke is formed.

Description

Technical Field

[0001] The present invention relates to the resource recovery of waste plastics, and particularly to the continuous and efficient conversion of waste polyolefins into wax, liquid hydrocarbons and gaseous hydrocarbons by thermal cracking. Background Art

[0002] Polyolefin products represented by high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), poly-1-butene (PB-1), etc. are the most produced and used plastic products globally. In 2021 alone, the production of polyolefins in China has approached 90 million tons.

[0003] The service life of polyolefin products is relatively short, and they become waste after exiting the consumer market. For the recycling of waste plastics including waste polyolefins, there are currently three levels of recycling methods: primary recycling, secondary recycling and tertiary recycling (Waste Manag. 2016, 48, 300–314). Tertiary recycling specifically refers to the gasification, cracking and depolymerization of waste plastics based on chemical reactions. Among them, cracking can be further divided into hydrocracking, thermal cracking and catalytic cracking.

[0004] Thermal cracking based on the free radical mechanism is considered the most economically viable resource tertiary recycling method for waste polyolefins. The thermal cracking of polyolefins has the characteristic of random chain scission, and the products are wax, liquid hydrocarbons and gaseous hydrocarbons with a certain molecular weight distribution, and corresponding olefin monomers cannot be obtained. As early as the end of the last century, the direct heating of polyolefins for resource recovery has been widely studied. According to thermogravimetric analysis, the initial cracking temperature (T ini ) of LDPE and HDPE is about 390 - 400 °C (J. Anal. Appl. Pyrolysis 2018, 133, 185–197; J. Therm. Anal. Calorim. 2008, 91, 737–743); the T ini of PP is 304 °C (Energy Convers. Manag. 2010, 51, 1363–1369); the T ini of PS is 395 °C (Resour., Conserv. Recycl. 2011, 55, 772–781). Practice shows that it is necessary to use a temperature much higher than T iniThe effective decomposition and conversion of polyolefins can only be carried out at a certain temperature. For example, Bagri and Williams carried out the thermal cracking of LDPE in a fixed-bed reactor in batch mode at 500 °C, and obtained 95% oil, a small amount of carbon and gas after 20 min of reaction (J. Anal. Appl. Pyrolysis 2002, 63, 29–41). Abbas-Abadi et al. carried out the thermal cracking of HDPE in a semi-batch reactor at 450 °C and obtained 91.2% condensable products, 4.1% non-condensable products and 4.7% coke (Fuel Process. Technol. 2013, 109, 90–95). Miandad et al. carried out the thermal cracking of PP in a pilot-scale batch reactor at 450 °C, and obtained 42% oil, 54.6% gas products and 3.5% coke after 75 min of reaction (Int. Biodeterior. Biodegrad. 2017, 119, 239–252.). Sogancioglu et al. carried out intermittent operation in a fixed-bed reactor at 500 °C and obtained 67.2% liquid products, 26.6% gas and 6.2% coke (J. Clean. Prod. 2017, 165, 369–381).

[0005] The significant drawbacks of directly heating for the resource recovery of waste polyolefins are as follows: 1) The energy consumption of high-temperature operation increases significantly, which does not meet the technical and economic requirements of green and low-carbon; 2) The uncontrollable condensation caused by long-term high-temperature operation results in the appearance of solid coke in the products; 3) The low efficiency of the batch reaction mode does not meet the requirements of industrial scale treatment; 4) The viscosity of the polyolefin melt is too high, and it is difficult to carry out effective stirring in an industrial-scale batch reactor. Although related patents such as CN201410181493.3 reported the resource recovery of waste polyolefin thermal cracking based on the batch mode, there is still no case of industrial application so far.

[0006] Based on the direct heating for the resource recovery of waste polyolefins, some studies have proposed improvements, mainly reflected in two aspects: 1) Pressurized operation; 2) Introduction of supercritical water (SCW) solvent. The relevant work is briefly described as follows.

[0007] Gu Jing et al. used an intermittent device to investigate the thermal cracking of LDPE at 380 ° C. Before the reaction, 0.1-0.5 MPa of nitrogen was filled into the reactor, and the temperature rose to 452 ° C during the experiment (Journal of Fuel Chemistry, 2021, 49, 395-406). Similarly, Cheng et al. used an intermittent device to investigate the thermal cracking of LDPE at an initial temperature of 330-380 ° C. Before the reaction, 0.1-5.1 MPa of nitrogen was filled into the reactor (Chem. Eng. J. 2020, 385, 123866). At an initial temperature of 380 ° C and an initial pressure of 3.1 MPa, the experiment observed a temperature rise to 487 ° C and a system pressure of 7.2 MPa. Patent CN202110660297.4 reports that waste polyolefins are placed in a high-pressure reactor, and an inert gas with an initial pressure value higher than 0.5 MPa and / or a hydrocarbon liquid with a boiling point lower than 125 ° C is introduced as a phase builder. Heat the reactor to 340-380℃ and make the pressure in the reactor reach 2-6MPa, and complete the thermal degradation of waste polyolefins under high temperature and high pressure conditions. The above-mentioned papers or patents that introduce nitrogen or liquid hydrocarbons for pressurized resource recovery are essentially intermittent operations. Accordingly, the pressurized method also has problems such as low intermittent operation efficiency, inability to effectively stir large-scale intermittent reaction devices, and the presence of coke in the product.

[0008] The introduction of SCW solvent in thermal cracking was once considered a universal method for treating waste plastics. As early as 1999, Moriya et al. discussed the cracking behavior of PE in SCW and the mechanism of SCW intervening in the PE cracking network (Polym. Degrad. Stab. 1999, 65, 373–386). Su et al. investigated the thermal cracking of PE in a SCW environment. At 460°C and a SCW / PE ratio of 6, the oil yield was higher than 90wt% (Fuel Process. Technol. 2004, 85, 1249–1258). Chen et al. investigated the thermal cracking of PP in a SCW environment and obtained an oil yield of 91wt% at 425°C, 2–4h or 450°C, 0.5–1h. Kwak et al. investigated the thermal cracking of PS in subcritical water and SCW environment (370-420℃, 24-32MPa), and the reaction reached equilibrium in 5-15min (J.Appl.Polym.Sci.2006,101,695-700). It should be pointed out that the research on resource recovery of waste polyolefins in SCW is carried out in small or even micro reactors without exception. The extreme reaction conditions make it difficult to realize the engineering application of SCW process so far. Summary of the invention

[0009] The basic requirements for realizing economical and large-scale thermal cracking resource recovery of waste polyolefins are: fast reaction rate, green and low-carbon, and continuous process. The purpose of the present invention is to provide a continuous method and system for waste polyolefin thermal cracking resource recovery in view of the deficiencies of the prior art.

[0010] To achieve the above object, the present invention provides a continuous method for waste polyolefin thermal cracking resource recovery. The main feature is that the waste polyolefin melt is mixed with low-pressure hot water, and the formed mixture initiates thermal cracking in the preheating section of the tubular reactor and completes the rapid conversion to wax, liquid hydrocarbons and gaseous hydrocarbons in the adiabatic section of the tubular reactor. The solubilization of water in the polymer melt is utilized to relieve the constraint of the high viscosity of the system on the thermal cracking reaction kinetics.

[0011] To effectively improve the continuous resource recovery effect of waste polyolefins, the type of waste polyolefins, the thermodynamic state of low-pressure hot water, the feeding of waste polyolefins and water, the type and structure of the thermal cracking reactor, the temperature distribution of the reaction device, and the space-time of the material in the reactor are optimized.

[0012] Preferably, the polyolefin is one or a mixture of polymers obtained by homopolymerization or copolymerization of α-olefins and cycloolefins. For example, α-olefins such as ethylene, propylene, styrene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, etc.

[0013] Preferably, the low-pressure hot water is water with a temperature of 100–200 °C and a pressure of 0.3–1.6 MPa.

[0014] Preferably, the tubular reactor is divided into a preheating section and an adiabatic section, and is an empty tube or an internal component such as a static mixer is installed inside the tube.

[0015] Preferably, the operating pressure of the tubular reactor is controlled at 0.3–1.6 MPa.

[0016] Preferably, the waste polyolefin is heated to the molten state by a device providing shear mixing and heat and then enters the tubular reactor;

[0017] Water enters the tubular reactor through a pressurizing device and reaches the low-pressure hot water state through preheating;

[0018] The mixing of the waste polyolefin melt and low-pressure hot water can be carried out before entering the tubular reactor or in the preheating section of the tubular reactor.

[0019] Preferably, the mass ratio of the waste polyolefin to the low-pressure hot water is between 3:1 and 1:3;

[0020] A mixture of waste polyolefin melt and low-pressure hot water is heated to 350–400 °C in the preheating section of a tubular reactor;

[0021] The space time of the waste polyolefin in the tubular reactor is 10–60 min.

[0022] Preferably, the method is carried out in a continuous manner, and the waste polyolefin is continuously converted into wax, liquid hydrocarbons and gaseous hydrocarbons through treatment, and there is no solid coke in the product.

[0023] The present invention also provides a continuous system for waste polyolefin thermal cracking resource recovery. Its main features include a tubular reactor, a device for providing shear mixing and heat, and a pressurizing device. The device for providing shear mixing and heat, for example, a screw extruder, is used to heat the waste polyolefin to a molten state and transport it to the tubular reactor. The pressurizing device, for example, a plunger pump, is used to introduce water into the tubular reactor. A mixture formed by mixing the waste polyolefin melt and low-pressure hot water initiates thermal cracking in the preheating section of the tubular reactor and completes a rapid conversion into wax, liquid hydrocarbons and gaseous hydrocarbons in the adiabatic section of the tubular reactor. The mixing of the liquid waste polyolefin melt and low-pressure hot water promotes the solubilization of water into the polyolefin melt. Water can be continuously introduced into the reaction system as low-pressure hot water through preheating by the pressurizing device and subsequent pipelines.

[0024] Preferably, the tubular reactor is an empty tube or a static mixer is installed inside the tube.

[0025] A continuous method and reaction device for waste polyolefin thermal cracking resource recovery provided by the present invention utilize the solubilization of low-pressure hot water in the polyolefin melt to alleviate the adverse effect of high viscosity in the system on the kinetics of free radical thermal cracking reaction. A mixture of waste polyolefin and low-pressure hot water enters the tubular reaction device, initiates thermal cracking in the preheating section, and completes a rapid conversion into wax, liquid hydrocarbons and gaseous hydrocarbons in the adiabatic section. The yield of liquid hydrocarbons in the product reaches more than 90%, and no solid coke is generated. Description of the Drawings

[0026] Figure 1 It is a schematic structural diagram of the continuous system for waste polyolefin thermal cracking resource recovery of the present invention. Detailed Embodiments

[0027] In order to further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention and not for limiting the claims of the invention.

[0028] Polyolefins have a long carbon chain structure, and the energy required to initiate the thermal cracking network through C–C bond cleavage is relatively low. For PP, the presence of active methyl groups makes the energy required to initiate thermal cracking even lower. However, the viscosity of the polyolefin melt is on the order of 10 2 Pa·s (J. Phys.: Conf. Ser. 2018, 1045, 012030). Therefore, the alkyl carbon radicals formed by C–C bond cleavage can easily recombine in situ (activation energy is zero), and it is difficult for the alkyl carbon radicals to diffuse to complete the propagation of the cracking reaction network. Studies on the thermal cracking of heavy oil with a similar mechanism have confirmed that even when the viscosity of heavy oil is as low as 10 –3 Pa·s at high temperatures, diffusion constraints still force the operating temperatures of heavy oil thermal cracking processes (delayed coking and visbreaking) in industrial applications to be about 50–100 °C higher than the threshold temperature of C–C bond cleavage (J. Anal. Appl. Pyrolysis 2021, 159, 105296; Energy Fuels 2019, 33, 1074-1082). The key to increasing the thermal cracking rate of polyolefins and reducing the temperature of polyolefin thermal cracking lies in: alleviating the diffusion constraints of the high viscosity of the system on the initiation of thermal cracking and the reaction kinetics of chain propagation.

[0029] Under conventional conditions, water is a typical polar solvent with a dielectric constant of 80. As the temperature increases, the hydrogen bonds between water molecules are broken, and the dielectric constant of water drops significantly and tends to be a non-polar solvent. For example, the dielectric constant of low-pressure hot water at 200 °C and 1.6 MPa drops to 34. Based on the principle of like dissolves like, an increase in temperature is conducive to the solubilization of water in hydrocarbons (J. Chem. Thermodyn. 1991, 23, 105–112; Fluid Phase Equilibr. 1990, 59, 207-215; Fluid Phase Equilibr. 1994, 93, 317-336). The polyolefin melt belongs to a liquid non-polar hydrocarbon substance, and low-pressure hot water can form a solubilization in the polyolefin melt. The solubilization of small water molecules reduces the viscosity of the melt, thereby alleviating the adverse effects of diffusion constraints on the reaction kinetics of polyolefin thermal cracking.

[0030] In the continuous mode and system provided by the present invention, a tubular reactor is used, which is a suitable device type for continuous reaction processes. To promote the mixing of waste polyolefin melt and low-pressure hot water and further heat and mass transfer of the reaction materials, internal components such as static mixers for enhancing shear can be placed inside the tube.

[0031] In the present invention, the tubular reactor is divided into a preheating section and an adiabatic section. The waste polyolefin melt after solubilizing water is heated to the initiation temperature in the preheating section to initiate the reaction network, and a certain amount of alkyl carbon free radicals and terminal α-olefins are generated according to the bimolecular cracking mechanism of alkanes (Adv. Pet. Chem. Refin. 1964, 9, 157–201). After the reaction material enters the adiabatic section, the reactions between olefins and olefins, and between olefins and hydrocarbon free radicals in the system release a large amount of heat, promoting the rapid increase in the temperature of the system and accelerating the decomposition and conversion of the material. No external heat supply is required for the adiabatic section.

[0032] Based on the methods and devices proposed in the present invention, each example conducts continuous resource recovery of waste polyolefin by thermal cracking, and at the same time, refers to the inert gas pressurization method reported in Chem. Eng. J. 2020, 385, 123866 and Patent CN202110660297.4 to conduct batch resource recovery of thermal cracking of the same raw materials, thereby forming a comparison.

[0033] Example 1:

[0034] Taking a certain LDPE waste as a representative of waste PE, its basic properties are shown in Table 1.

[0035] Table 1 Basic properties of the waste LDPE used in the experiment

[0036]

[0037] · Reaction process of the present invention

[0038] According to Figure 1 the reaction device shown, the inner diameter of the reaction tube is 0.04 m, the length of the preheating section is 0.35 m, and the length of the adiabatic section is 0.35 m; a static mixer is installed inside the reaction tube, the effective volume of the preheating section is 0.4 L, and the effective volume of the adiabatic section is 0.4 L; LDPE is added to the reaction system by a screw extruder, and the temperature at the outlet of the screw is 120 °C; the feeding rates of LDPE are 20, 30, and 60 g / min, corresponding to the space times of 37, 24.7, and 12.3 min; water is added to the reaction system by a metering pump and heated to 120 °C through the preheating pipeline, and the feeding rate of water is 6.7, 10, and 20 g / min; the set temperature of the preheating section of the tubular reactor is 400 °C; the reaction system pressure is fixed at 0.3 MPa.

[0039] After the thermal cracking product exits the reactor, it is cooled with 70 °C hot water and the gas-liquid separation is completed in a gas-liquid separation tank. The product obtained below the gas-liquid separation tank is mixed with dichloromethane. The white substance insoluble in dichloromethane is defined as wax, and the wax in the mixture is separated by filtration. Dichloromethane and water dissolving liquid hydrocarbons are separated by the method of standing and stratifying. Finally, at 30 °C and 10 4Dichloromethane was separated from liquid hydrocarbons by vacuum rotary evaporation under Pa conditions. The carbon number distribution of liquid hydrocarbons was determined on a gas chromatography-mass spectrometer (GC-MS).

[0040] The distribution of cracking products obtained at different reaction times and the carbon number distribution of liquid hydrocarbon products are shown in Tables 2 and 3.

[0041] Table 2 Distribution of products obtained from thermal cracking of waste LDPE at different reaction times

[0042]

[0043] Table 3 Carbon number distribution of liquid hydrocarbon products obtained by thermal cracking of waste LDPE at different reaction times

[0044]

[0045] ·Reaction process reported in literature

[0046] References reported the intermittent thermal cracking recovery of waste LDPE. The reaction was carried out in a 1L PARR high-pressure reactor equipped with a flat paddle with a stirring speed of 500rpm. The operation steps are as follows: 1) Add 200g LDPE to the reactor; 2) Vacuum the reactor after sealing it; 3) Fill the reactor with 0.5MPa of high-purity nitrogen; 4) Preset the reactor temperature to 400 or 420℃, heat the system, and start stirring at the same time; 5) When the reactor temperature reaches the preset value, it is defined as the zero point of the reaction time and the reaction time is set to 30, 60, 90 and 120min. The time required for the entire intermittent operation is shown in Table 4.

[0047] Table 4 Time required for each process of intermittent operation to treat waste LDPE

[0048]

[0049] Regardless of whether the preset temperature of thermal cracking is 400 or 420℃, it can be observed that the system temperature rises after exceeding the preset value, which is consistent with the literature reports. The former has a peak temperature of 430℃ and the latter is 453℃. When the preset value of the cracking temperature is 400℃, the product is still a block that wraps the entire stirring paddle at room temperature after 60 minutes of reaction. Only when the preset value of the cracking temperature is 420℃ can the effective decomposition and conversion of waste LDPE be achieved.

[0050] The liquid phase product and wax obtained by cracking are mixed with dichloromethane. The white substance insoluble in dichloromethane is defined as wax, and the wax in the mixture is separated by filtration. Dichloromethane in which liquid hydrocarbons are dissolved is heated at 30°C and 10 4Under the condition of Pa, dichloromethane was separated from liquid hydrocarbons by vacuum rotary evaporation. The carbon number distribution of liquid hydrocarbons was determined on a gas chromatography-mass spectrometry (GC-MS).

[0051] The distributions of cracking products and the carbon number distributions of liquid hydrocarbon products obtained at different reaction times are shown in Tables 5 and 6.

[0052] Table 5 Distribution of products obtained from thermal cracking of waste LDPE at different reaction times

[0053]

[0054] Table 6 Carbon number distribution of liquid hydrocarbon products obtained from thermal cracking of waste LDPE at different reaction times

[0055]

[0056] · Comparison of the results of two reaction methods

[0057] According to the continuous reaction method and reaction device claimed in the present invention, at a preheating section set temperature of 400 °C and a reaction space time of 37 min, waste LDPE can be converted into liquid hydrocarbons and gaseous hydrocarbons. The yield of liquid hydrocarbons is 93.7 wt%, and the carbon number distribution is concentrated in C11-C15 (mass fraction is 44.9 wt%).

[0058] According to the batch reaction method and reaction device provided in the literature, at a preset reaction system temperature of 420 °C and a reaction time of 120 min, waste LDPE can be converted into liquid hydrocarbons and gaseous hydrocarbons. The yield of liquid hydrocarbons is 92.5 wt%, and the carbon number distribution is concentrated in C11-C15 (mass fraction is 38.3 wt%). If the necessary auxiliary processes are included, the total duration of the entire batch operation reaches 200 min.

[0059] Compared with the batch reaction method and reaction device provided in the literature, the continuous reaction method and reaction device claimed in the present invention can achieve efficient and complete continuous conversion of waste LDPE with a lower preheating temperature, a significantly shortened treatment time, and a lighter product distribution.

[0060] Example 2:

[0061] A certain PP waste was used as a representative of waste PP, and its basic properties are shown in Table 7.

[0062] Table 7 Basic properties of waste PP for testing

[0063]

[0064] · Reaction process of the present invention

[0065] According to Figure 1The reaction device shown has a reaction tube with an inner diameter of 0.04 m, a preheating section with a length of 0.35 m, and an adiabatic section with a length of 0.35 m. No static mixer is installed inside the reaction tube. The effective volume of the preheating section is 0.44 L, and the effective volume of the adiabatic section is 0.44 L. PP is added to the reaction system using a screw extruder, and the temperature at the screw outlet is 190 °C. The PP feeding rates are 20, 30, and 60 g / min, corresponding to reaction space times of 40.0, 26.5, and 13.3 min. Water is added to the reaction system using a metering pump and heated to 190 °C through a preheating pipeline. The water feeding rates are 20, 30, and 60 g / min. The set temperature of the preheating section of the tubular reactor is 380 °C. The reaction system pressure is fixed at 1.6 MPa.

[0066] After the thermal cracking products leave the reactor, they are cooled with 70 °C hot water and gas-liquid separation is completed in a gas-liquid separation tank. The product obtained below the gas-liquid separation tank is mixed with dichloromethane. The white substance insoluble in dichloromethane is defined as wax, and the wax in the mixture is separated by filtration. The dichloromethane and water dissolving the liquid hydrocarbons are separated by the method of static settling. Finally, dichloromethane is separated from the liquid hydrocarbons using vacuum rotary evaporation at 30 °C and 10 4 Pa.

[0067] The distributions of the cracking products obtained at different reaction space times are shown in Table 8.

[0068] Table 8 Distributions of the products obtained from the thermal cracking of waste PP at different reaction space times

[0069]

[0070] · Literature-reported reaction process

[0071] The batch thermal cracking recovery of waste PP was carried out as reported in the references. The reaction was carried out in a 1 L PARR high-pressure reaction kettle equipped with a flat paddle with a stirring speed reaching 500 rpm. The operation steps are as follows: 1) Add 200 g of PP to the reaction kettle; 2) After closing the reaction kettle, perform a vacuum treatment; 3) Fill the reaction kettle with high-purity nitrogen at 1.0 MPa; 4) Preset the reaction temperature to 380, 400, or 420 °C, heat up the system, and start stirring at the same time; 5) When the system temperature reaches the preset value, define it as the zero point of the reaction time and start timing. The reaction duration is fixed at 30 min. The time required for the entire batch operation is shown in Table 9.

[0072] Table 9 Time required for each process of treating waste PP by batch operation

[0073]

[0074] The liquid products and wax obtained from cracking are mixed with dichloromethane. The white substances insoluble in dichloromethane are defined as wax, and the wax in the mixture is separated by filtration. The dichloromethane dissolving the liquid hydrocarbons is separated from the liquid hydrocarbons by vacuum rotary evaporation at 30 °C and 10 4 Pa.

[0075] The distributions of the cracking products obtained at different temperatures are shown in Table 10.

[0076] Table 10 Distribution of products obtained from thermal cracking of waste PP at different reaction temperatures; reaction time 30 min

[0077]

[0078] · Comparison of the results of the two reaction methods

[0079] According to the continuous reaction method and reaction device claimed in the present invention, at a preheating section set temperature of 380 °C and a reaction space time of 37 min, waste PP can complete the continuous conversion into liquid hydrocarbons and gaseous hydrocarbons. The yield of liquid hydrocarbons is 94.8 wt%, and the yield of gaseous hydrocarbons is 5.2 wt%.

[0080] According to the batch reaction method and reaction device provided in the literature, at a preset reaction temperature of 380 °C and a reaction time of 30 min in the system, the wax content in the product is as high as 92.1 wt%. Even when the preset reaction temperature of the system is 420 °C, there is still 2.5 wt% of wax in the product after reacting for 30 min. At this time, the yield of liquid hydrocarbons is 90.3 wt%, and the yield of gaseous hydrocarbons is 7.2 wt%. If the necessary auxiliary processes are included, the total duration of the entire batch operation reaches 110 min.

[0081] Compared with the batch reaction method and reaction device provided in the literature, the continuous reaction method and reaction device claimed in the present invention can achieve the efficient and complete continuous conversion of waste PP into liquid hydrocarbons and gaseous hydrocarbons with a lower preheating temperature, a shortened treatment time, and a lighter product distribution.

[0082] Example 3:

[0083] A mixture of waste polyolefins is taken as a representative of the mixed waste polyolefins, and its composition is shown in Table 11. The density of the mixture is 0.967 g / mL (25 °C).

[0084] Table 11 Composition of the mixed waste polyolefins

[0085]

[0086]

[0087] · Reaction process of the present invention

[0088] According to Figure 1 the reaction device shown, the inner diameter of the reaction tube is 0.04 m, the length of the preheating section is 0.35 m, and the length of the adiabatic section is 0.35 m; a static mixer is installed inside the reaction tube, the effective volume of the preheating section is 0.4 L, and the effective volume of the adiabatic section is 0.4 L; the mixed waste polyolefin is added to the reaction system by a screw extruder, the temperature at the outlet of the screw is 180 °C, the feeding rate is 13 g / min, and the corresponding reaction space time is 60 min; water is added to the reaction system by a metering pump and heated to 180 °C through a preheating pipeline, and the feeding rate of water is 39 g / min; the set temperature of the preheating section of the tubular reactor is 350 °C; the reaction system pressure is fixed at 1.0 MPa;

[0089] After the thermal cracking products leave the reaction device, they are cooled with 70 °C hot water and gas-liquid separation is completed in a gas-liquid separation tank. The product obtained below the gas-liquid separation tank is mixed with dichloromethane. The white substance insoluble in dichloromethane is defined as wax, and the wax in the mixture is separated by filtration. The dichloromethane and water dissolving the liquid hydrocarbon are separated by the method of static settling. Finally, dichloromethane is separated from the liquid hydrocarbon by vacuum rotary evaporation at 30 °C and 10 4 Pa conditions.

[0090] The distribution of the cracking products obtained after treating the mixed waste polyolefin is shown in Table 12.

[0091] Table 12 Distribution of thermal cracking products of mixed waste polyolefin

[0092]

[0093] · Literature-reported reaction process

[0094] Intermittent thermal cracking recovery of mixed waste polyolefin was carried out according to the literature report. The reaction was carried out in a 1 L PARR high-pressure reactor equipped with a flat paddle with a stirring speed of 500 rpm. The operation steps are as follows: 1) Add 200 g of mixed waste polyolefin to the reactor; 2) After closing the reactor, perform vacuum treatment; 3) Fill the reactor with 0.5 MPa of high-purity nitrogen; 4) Preset the reaction temperature to 360, 380, 400 or 420 °C, heat up the system, and start stirring at the same time; 5) When the system temperature reaches the preset value, it is defined as the zero point of the reaction time and timing is started, and the reaction duration is fixed at 60 min. The time required for the entire intermittent operation is shown in Table 13.

[0095] Table 13 Time required for each process of treating mixed waste polyolefin by intermittent operation

[0096]

[0097] The liquid product and wax obtained from cracking are mixed with dichloromethane. The white substance insoluble in dichloromethane is defined as wax, and the wax in the mixture is separated by filtration. Dichloromethane dissolving liquid hydrocarbons is separated from the liquid hydrocarbons by vacuum rotary evaporation at 30 °C and 10 4 Pa.

[0098] When the preset reaction temperature is 360 °C, the product obtained is a lump wrapping the stirring paddle. The product distributions obtained at other preset reaction temperatures are shown in Table 14.

[0099] Table 14 Product distribution of thermal cracking of mixed waste polyolefins at different preset reaction temperatures; reaction time 60 min

[0100]

[0101] · Comparison of the results of two reaction methods

[0102] According to the continuous reaction method and reaction device claimed in the present invention, at a preset temperature of 350 °C in the preheating section and a reaction space time of 60 min, the mixed waste polyolefins can be continuously and completely converted into liquid hydrocarbons and gaseous hydrocarbons. The yield of liquid hydrocarbons is 91.4 wt%, and the yield of gaseous hydrocarbons is 8.6 wt%.

[0103] According to the batch reaction method and reaction device provided in the literature, only when the preset reaction temperature of the reaction system reaches 380 °C or higher, an effective decomposition and conversion of the mixed waste polyolefins can be formed after reacting for 60 min. Even when the preset reaction temperature of the reaction system is increased to 420 °C, there is still 1.3 wt% of wax in the product after reacting for 60 min. At this time, the yield of liquid hydrocarbons is 89.5 wt%, and the yield of gaseous hydrocarbons is 9.2 wt%. If the necessary auxiliary processes are included, the total duration of the entire batch operation reaches 140 min.

[0104] Compared with the batch reaction method and reaction device provided in the literature, the continuous reaction method and reaction device claimed in the present invention can achieve an efficient, complete and continuous conversion of mixed waste polyolefins into liquid hydrocarbons and gaseous hydrocarbons at a lower preheating temperature, a shorter processing time, and a lighter product distribution.

[0105] In this specification, the present invention has been described with reference to its specific embodiments. However, it is obvious that various modifications and variations can still be made without departing from the spirit and scope of the present invention. Therefore, the specification should be considered illustrative rather than restrictive.

Claims

1. A continuous method for the resource recovery of waste polyolefins by thermal cracking, characterized in that, Mix the waste polyolefin melt with low-pressure hot water. The formed mixture initiates thermal cracking in the preheating section of the tubular reactor and completes the rapid conversion to wax, liquid hydrocarbons, and gaseous hydrocarbons in the adiabatic section of the tubular reactor. The low-pressure hot water is water with a temperature of 100–200 °C and a pressure of 0.3–1.6 MPa. The operating pressure of the tubular reactor is controlled at 0.3–1.6 MPa. The mixture of the waste polyolefin melt and low-pressure hot water is heated to 350–400 °C in the preheating section of the tubular reactor, and there is no external heat supply in the adiabatic section.

2. The continuous method according to claim 1, characterized in that The polyolefin is one or a mixture of polymers obtained by homopolymerization or copolymerization of α-olefins and cycloolefins.

3. The continuous method according to claim 1, characterized in that The tubular reactor is an empty tube or is equipped with internal components inside the tube.

4. The continuous method according to claim 1, characterized in that, The waste polyolefin is heated to the molten state by a device that provides shear mixing and heat and then enters the tubular reactor; Water enters the tubular reactor through a pressurizing device; The mixing of the waste polyolefin melt and low-pressure hot water can be carried out before entering the tubular reactor or in the preheating section of the tubular reactor.

5. The continuous method according to claim 1, characterized in that, The mass ratio of the waste polyolefin to the low-pressure hot water is between 3:1 and 1:3; The space time of the waste polyolefin in the tubular reactor is 10–60 min.

6. The continuous method according to claim 1, characterized in that, The described method is carried out in a continuous manner.

7. A continuous system for waste polyolefin thermal cracking resource recovery, characterized in that, It includes a tubular reactor, a device that provides shear mixing and heat, and a pressurizing device. The device that provides shear mixing and heat is used to heat the waste polyolefin to the molten state and transport it to the tubular reactor. The pressurizing device is used to let water enter the tubular reactor. The mixture formed by mixing the waste polyolefin melt and low-pressure hot water initiates thermal cracking in the preheating section of the tubular reactor and completes the rapid conversion to wax, liquid hydrocarbons, and gaseous hydrocarbons in the adiabatic section of the tubular reactor. The low-pressure hot water is water with a temperature of 100–200 °C and a pressure of 0.3–1.6 MPa. The operating pressure of the tubular reactor is controlled at 0.3–1.6 MPa. The mixture of the waste polyolefin melt and low-pressure hot water is heated to 350–400 °C in the preheating section of the tubular reactor, and there is no external heat supply in the adiabatic section.

8. The continuous system according to claim 1, characterized in that The tubular reactor is an empty tube or is equipped with a static mixer inside the tube.

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