A method for recycling waste plastics

Abstract

The present invention discloses a method for recycling waste plastics. The method includes: (1) mixing the waste plastics with a first-stage pyrolysis catalyst, heating and reacting in an inert atmosphere to prepare a pyrolysis product and a pyrolysis solid residue; (2) mixing the pyrolysis product in step (1) with a second-stage pyrolysis catalyst, heating in an inert atmosphere to prepare a pyrolysis liquid product. The present invention adopts a two-stage combined catalytic pyrolysis method for waste plastics, namely the first-stage in-situ catalytic pyrolysis and the second-stage catalytic upgrading, which can prepare high-quality olefins and mono-aromatic hydrocarbon products with high yields. Moreover, the first-stage pyrolysis catalyst is a modified pyrolysis solid residue, and the non-condensable gas generated in the second-stage pyrolysis reaction product can be used for power generation or energy supply, realizing the clean and high-value utilization of all components of the pyrolysis products.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical recycling of waste plastics in solid waste, and specifically relates to a method for recycling waste plastics. Background Art

[0002] As an important material, plastics are increasingly used in the industrial field. With the increase in population, rapid economic growth, continuous urbanization, and changes in lifestyle, the production and consumption of plastics are increasing at an alarming rate. The extensive use of plastics generates a large amount of waste plastic garbage. Plastics are difficult to degrade under natural conditions, and the accumulated waste plastics over the years have caused serious environmental pollution. Improper treatment of waste plastics will not only cause white pollution from cities to rural areas, from land to sea, but also a great waste of resources.

[0003] For a long time, the recycling and environmental protection treatment of low-residual-value waste plastics has been a pain point and a missing point in the industry. With the implementation of the waste classification work in China, waste plastics, as one of the main components of a huge amount of garbage, will provide a large number of processing sources for the waste plastic catalytic pyrolysis industry after classified recycling. The waste plastic catalytic pyrolysis technology will also provide a good outlet for the harmless and resource-based disposal of a large amount of plastic garbage, which can not only save the increasingly scarce petrochemical resources, but also be a good example of the circular economy operation.

[0004] The waste plastic catalytic pyrolysis technology uses the principles of thermal pyrolysis and catalytic upgrading to convert high-molecular compounds (such as waste tires, waste plastics, waste rubbers, etc.) into a variety of low-molecular compounds in an inert atmosphere. After separation, refining and other treatments, new plastic monomers can be obtained for the production of new recycled plastic products. These low-molecular compounds are easy to store, easy to transport, have a high energy density and are convenient to use. Compared with traditional treatment technologies such as landfilling, recycling granulation and incineration, the waste plastic catalytic pyrolysis technology can not only reduce environmental pollution during the treatment process, but also be converted into secondary fuels and chemicals, achieving the effects of cleanliness, high value addition and recycling. Driven by the construction of a waste-free city, waste classification and resource-based disposal, and under the background of China's "carbon peak" and "carbon neutrality" policies, the catalytic pyrolysis disposal of waste plastics meets the social needs of global energy structure adjustment and environmental protection and is of great significance. However, at present, the waste plastic catalytic pyrolysis technology has problems such as complex product composition, low proportion of liquid products, and easy coking and blockage of reaction devices, and it is impossible to achieve continuous and high liquid yield treatment of mixed waste plastic garbage. Therefore, there is an urgent need to develop a new technology for the catalytic pyrolysis of mixed waste plastic garbage to direct the preparation of high-value-added liquid products. Summary of the Invention

[0005] In order to improve the above technical problems, the present invention provides a method for recycling waste plastics, and the method includes the following steps:

[0006] (1) Mix the waste plastics with the first-stage pyrolysis catalyst, and heat and react in an inert atmosphere to obtain pyrolysis products and pyrolysis solid residues.

[0007] (2) Mix the pyrolysis products in step (1) with the second-stage pyrolysis catalyst, and heat in an inert atmosphere to obtain pyrolysis liquid products.

[0008] According to the embodiments of the present invention, the inert atmosphere is, for example, a nitrogen or argon atmosphere.

[0009] According to the embodiments of the present invention, in step (1), the source of the waste plastics can be a mixed low-value waste plastic composition sorted from municipal solid waste or industrial waste, or it can be agricultural films, low-value waste edge materials generated during the plastic production process, medical waste plastics, construction waste plastics, waste plastics from paper mills, etc. The composition of the waste plastics includes 80 wt%-100 wt% of polyolefins and 0-20 wt% of other components, and the other components are polyester, PVC, etc. For example, the waste plastics are selected from at least one of polyethylene (including HDPE and LDPE), polypropylene, polystyrene, etc., or the waste plastics can further contain one or more selected from polyvinyl chloride, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), pulp, sediment, etc.

[0010] According to the embodiments of the present invention, in step (1), the mass ratio of the waste plastics to the first-stage pyrolysis catalyst is 1-100:1, preferably 10-50:1. Exemplarily, it is 1:1, 5:1, 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1 or 100:1.

[0011] According to the embodiments of the present invention, in step (1), it is a one-stage pyrolysis reaction, and the one-stage pyrolysis reaction can be carried out in a rotary kiln type pyrolysis reactor.

[0012] According to the embodiments of the present invention, in step (1), the pressure of the reaction system is normal pressure or slightly negative pressure, and the pressure is 90-101.325 kpa.

[0013] According to the embodiments of the present invention, in step (1), the reaction temperature is 300-700 °C, preferably 450-600 °C. Exemplarily, it is 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C or 700 °C.

[0014] According to the embodiments of the present invention, in step (2), it is a two-stage pyrolysis reaction, and the two-stage pyrolysis reaction is carried out in a fixed bed pyrolysis upgrading reactor or a fluidized bed pyrolysis reactor.

[0015] According to an embodiment of the present invention, in step (2), the pressure of the reaction system is normal pressure or slightly negative pressure, and the pressure is 90 - 101.325 kpa.

[0016] According to an embodiment of the present invention, in step (2), the heating temperature is 250 - 600 °C, preferably 300 - 450 °C. Exemplarily, it is 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C or 600 °C.

[0017] According to an embodiment of the present invention, in step (2), after the reaction ends, two - stage pyrolysis non - condensable gas is generated in the system; the two - stage pyrolysis non - condensable gas is used for power generation or energy supply.

[0018] According to an embodiment of the present invention, in step (2), the pyrolysis liquid product is more than 50% of the total mass of the waste plastics, preferably more than 60%.

[0019] According to an embodiment of the present invention, in step (2), the pyrolysis liquid product includes at least one of alkanes, olefins, mono - aromatic hydrocarbons, polycyclic aromatic hydrocarbons, etc. with C3 and above. Exemplarily, the alkanes with C3 and above are at least one of propane, butane, isobutane, pentane, etc.; the olefins are at least one of ethylene, propylene, etc.; the mono - aromatic hydrocarbons are at least one of benzene, toluene, xylene; the polycyclic aromatic hydrocarbons are, for example, naphthalene or anthracene.

[0020] According to an embodiment of the present invention, the sum of the masses of the olefins and mono - aromatic hydrocarbons is more than 70% of the total mass of the pyrolysis liquid product, preferably more than 78%.

[0021] In step (2) of the present invention, the generated pyrolysis liquid product can be used as fuel or chemical raw material after separation, or can be incorporated into the refining unit to produce new plastic monomers, realizing the clean and high - value utilization of all components of mixed waste plastic garbage.

[0022] According to an embodiment of the present invention, in step (1), the waste plastics can be first crushed.

[0023] According to an embodiment of the present invention, in step (1), the pyrolysis solid residue can be used to prepare the first - stage pyrolysis catalyst, or can be used as building filler, roadbed filler, etc.

[0024] According to an embodiment of the present invention, in step (1), the first - stage pyrolysis catalyst is a metal - modified carbon - based catalyst, and the modified metal is one or more of Al, Ca, Zn, Mg, preferably Al and / or Zn, and the carbon - based material of the carbon - based catalyst is the pyrolysis solid residue.

[0025] According to an embodiment of the present invention, the metal is 0.1 - 10 wt% of the first - stage pyrolysis catalyst, preferably 0.5 - 3 wt%.

[0026] According to an embodiment of the present invention, in step (1), the preparation of the first-stage pyrolysis catalyst adopts the solution co-impregnation method, specifically as follows:

[0027] (S1) Mix a metal precursor, pyrolysis solid residue, and a solvent, and impregnate to prepare a first-stage catalyst precursor;

[0028] (S2) Heat and calcine the first-stage catalyst precursor in step (S1) to obtain the first-stage pyrolysis catalyst.

[0029] According to an embodiment of the present invention, in step (S1), the metal precursor is selected from at least one of AlCl3, AlCl3·6H2O, calcium chloride, zinc chloride, magnesium chloride, aluminum nitrate, calcium nitrate, Zn(NO3)2·6H2O, magnesium nitrate, etc.

[0030] According to an embodiment of the present invention, in step (S1), the solvent is, for example, water.

[0031] In step (S1) of the present invention, the mass ratio of the metal precursor to the pyrolysis solid residue is selected according to the mass ratio of the metal in the first pyrolysis catalyst. The impregnation temperature is not particularly limited, and is, for example, room temperature.

[0032] According to an embodiment of the present invention, in step (S2), the calcination temperature is 300-500°C; the calcination time is 6-18 h.

[0033] According to an embodiment of the present invention, before the catalyst precursor is calcined in step (S2), the catalyst precursor can also be dried; for example, the drying temperature is 80-120°C, and the drying time is 6-18 h.

[0034] According to an embodiment of the present invention, in step (2), the second-stage pyrolysis catalyst is a metal-modified silicon / aluminum oxide catalyst. The silicon / aluminum oxide can be zeolite molecular sieves such as MCM-41, MCM-22, HZSM-5, HZSM-35, HY, Hβ, SAPO-11, etc., preferably MCM-41 zeolite molecular sieve or MCM-22 zeolite molecular sieve; the modified metal is one or more of Cu, Zn, Al, Mg, In, Ga, Ni, preferably Cu, Ni, Mg, and / or In.

[0035] According to an embodiment of the present invention, in step (2), the metal is 0.05-10 wt% of the total mass of the second-stage pyrolysis catalyst, preferably 0.1-5 wt%. Exemplarily, it is 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%.

[0036] According to an embodiment of the present invention, in step (2), the second-stage pyrolysis catalyst is prepared by a solution co-impregnation method, specifically:

[0037] (K1) Mix a metal precursor, a silicon / aluminum oxide, and a solvent, impregnate them to prepare a second-stage catalyst precursor;

[0038] (K2) Heat and calcine the second-stage catalyst precursor in step (K1) to obtain the second-stage pyrolysis catalyst.

[0039] According to an embodiment of the present invention, in step (K1), the metal precursor is selected from at least one of copper chloride, zinc chloride, AlCl3, AlCl3·6H2O, NiCl2·6H2O, InCl3·4H2O, magnesium chloride, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, magnesium nitrate, Ni(NO3)2·3H2O, etc.

[0040] According to an embodiment of the present invention, in step (K1), the solvent is, for example, water.

[0041] In step (K1) of the present invention, the mass ratio of the metal precursor to the silicon / aluminum oxide is selected according to the mass ratio of the metal in the second pyrolysis catalyst. The impregnation temperature is not particularly limited and is, for example, room temperature.

[0042] According to an embodiment of the present invention, in step (K2), the calcination temperature is 250-500 °C; the calcination time is 6-18 h. Exemplarily, it is 250 °C, 300 °C, 350 °C, 400 °C, 450 °C or 500 °C.

[0043] According to an embodiment of the present invention, in step (K2), before the second-stage catalyst precursor is calcined, the second-stage catalyst precursor can also be dried; for example, the drying temperature is 80-120 °C and the drying time is 6-18 h.

[0044] Advantages of the present invention:

[0045] The present invention adopts a two-stage combined catalytic pyrolysis method for waste plastics, namely in-situ catalytic pyrolysis in the first stage and catalytic upgrading in the second stage, which can prepare high-quality olefins and monoaromatic products with high yields. Moreover, the pyrolysis catalyst in the first stage is modified pyrolysis solid residue, and the non-condensable gas generated in the pyrolysis reaction product of the second stage can be used for power generation or energy supply, realizing the clean and high-value utilization of all components of the pyrolysis products.

[0046] The preparation method of the present invention can effectively avoid the contamination of the second-stage pyrolysis catalyst by impurity components in waste plastics, extend the service life of the second-stage pyrolysis catalyst, and realize the continuous operation of the process. The pyrolysis liquid product prepared by the method of the present invention can be used again as fuel or chemical raw material, or directly incorporated into refining crude oil in a certain proportion for producing plastic raw material monomers to produce high-quality new plastics. Brief Description of the Drawings

[0047] Figure 1 It is the process flow chart of the recovery process of the present invention.

[0048] Figure 2 It is the GC-MS spectrum of the pyrolysis product in Example 1. Detailed Embodiments

[0049] The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only for illustrative and explanatory purposes of the present invention, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0050] Unless otherwise specified, the raw materials and reagents used in the following embodiments are commercially available products, or can be prepared by known methods.

[0051] Example 1

[0052] (1) Select the mixed waste plastic garbage sorted from municipal solid waste, and its composition is 17 wt% of polyethylene plastic (PE, including HDPE and LDPE), 66 wt% of polypropylene plastic, 5 wt% of polystyrene plastic, 4 wt% of polyvinyl chloride plastic, 6 wt% of polyethylene terephthalate (PET) polyester plastic, and 2 wt% of other impurities.

[0053] (2) The first-stage pyrolysis catalyst is an Al-modified carbon-based catalyst (Cat-I) (the Al content is 2 wt% of the first-stage pyrolysis catalyst), and the second-stage pyrolysis catalyst is an Ni- and In-modified MCM-22 zeolite molecular sieve catalyst (Cat-II) (the Ni and In contents are 1.5 wt% and 0.1 wt% of the second-stage pyrolysis catalyst, respectively). The preparation process of Cat I is as follows: Weigh 100 parts of pyrolysis solid residue, 17.90 parts of solid AlCl3·6H2O, and 200 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate at room temperature for 12 h, dry at 100 °C for 12 h in a N2 atmosphere, and calcine at 400 °C for 8.0 h to obtain Cat-I, that is, the first-stage pyrolysis catalyst. The preparation process of Cat II is as follows: Weigh 100 parts of MCM-22 zeolite molecular sieve, 6.07 parts of solid NiCl2·6H2O, 0.26 parts of solid InCl3·4H2O, and 300 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate for 12 h, dry at 100 °C for 12 h in a N2 atmosphere, and calcine at 350 °C for 8.0 h to obtain Cat II, that is, the second-stage pyrolysis catalyst.

[0054] (3) After the above mixed waste plastics are pretreated and crushed, they are mixed with Cat-I at a mass ratio of 30:1, then added to the first-stage rotary kiln pyrolysis reactor. After the system is purged and replaced with N2, it is heated to 500 °C in a N2 atmosphere, the reaction pressure is 98 Kpa, and continuous feeding is carried out for pyrolysis reaction to obtain pyrolysis products and pyrolysis solid residue. The pyrolysis solid residue is regularly discharged from the rotary kiln outlet and used to prepare the first-stage pyrolysis catalyst.

[0055] (4) A heat preservation pipeline is connected behind the first-stage rotary kiln pyrolysis reactor. The pyrolysis products directly enter the second-stage fixed-bed catalytic upgrading reactor under a N2 atmosphere. The temperature of the second-stage catalytic upgrading reactor is 350 °C. The reaction gas, together with the carrier gas, passes through the Cat-II bed from top to bottom. The lower end of the second reactor is connected to a condensation device to collect the high-value liquid products of pyrolysis, and the non-condensable gas is burned to recover energy for power supply.

[0056] (5) The comprehensive yield of the pyrolysis liquid products collected by the condensation device is 69%. Analyzed by GC-MS, the results show that the proportion of olefins and mono-aromatic hydrocarbon products is about 88%. The basic composition of the pyrolysis products is shown in Table 1 below.

[0057] Table 1 Basic composition of pyrolysis products in Example 1

[0058]

[0059]

[0060] Example 2

[0061] (1) Select the mixed waste plastics separated from municipal solid waste, which consists of 47 wt% of polyethylene plastics (PE, including HDPE and LDPE), 36 wt% of polypropylene plastics, 5 wt% of polystyrene plastics, 4 wt% of polyvinyl chloride plastics, 6 wt% of polyethylene terephthalate (PET) polyester plastics, and 2 wt% of other impurities.

[0062] (2) The first-stage pyrolysis catalyst is a carbon-based catalyst modified with metal Zn (Cat-III) (Zn is 2 wt% of the first-stage pyrolysis catalyst), and the second-stage pyrolysis catalyst is an MCM-41 molecular sieve catalyst modified with Cu, Ni, and In (Cat-IV) (Cu, Ni, and In are 1.5 wt%, 1.5 wt%, and 0.12 wt% of the second-stage pyrolysis catalyst, respectively). The preparation process of Cat III is as follows: Weigh 100 parts of pyrolysis solid residue, 9.10 parts of Zn(NO3)2·6H2O solid, and 200 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate for 16 h, dry at 80 °C for 14 h in an N2 atmosphere, and calcine at 450 °C for 8.0 h to obtain Cat-III, that is, the first-stage pyrolysis catalyst. The preparation process of Cat IV is as follows: Weigh 100 parts of MCM-41 molecular sieve, 5.70 parts of Cu(NO3)2·3H2O solid, 7.43 parts of Ni(NO3)2·3H2O solid, 0.31 parts of InCl3·4H2O solid, and 250 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate for 12 h, dry at 110 °C for 12 h in an N2 atmosphere, and calcine at 380 °C for 10 h to obtain Cat-IV, that is, the second-stage pyrolysis catalyst.

[0063] (3) After the above-mentioned mixed waste plastics are pretreated and crushed, they are mixed with Cat-III according to a mass ratio of 35:1 and then added to the first-stage rotary kiln pyrolysis reactor. After purging and replacing with N2 in the system, the temperature is raised to 550 °C in an N2 atmosphere, the reaction pressure is 98 Kpa, and continuous feeding is carried out for pyrolysis reaction to obtain pyrolysis products and pyrolysis solid residue. The pyrolysis solid residue is regularly discharged from the rotary kiln outlet and used to prepare the first-stage pyrolysis catalyst.

[0064] (4) A heat preservation pipeline is connected behind the first-stage rotary kiln pyrolysis reactor. The pyrolysis products directly enter the second-stage fixed-bed catalytic upgrading reactor in an N2 atmosphere. The temperature of the second-stage catalytic upgrading reactor is 400 °C, the reaction gas passes through the Cat-IV bed layer from top to bottom, and the lower end of the second reactor is connected to a condensation device to collect the high-value liquid products of pyrolysis, and the non-condensable gas is burned to recover energy.

[0065] (5) The comprehensive yield of the liquid products collected by the condensation device is 71%. Analyzed by GC-MS, the results show that the proportion of olefins and mono-aromatic hydrocarbons among them is 78%.

[0066] Example 3

[0067] (1) Select the mixed waste plastic garbage from a paper mill, whose composition is that the total of polyethylene and polypropylene plastics accounts for 95 wt%, pulp 1.5 wt%, plastic residues and sediment 2.5 wt%, and other impurities 1 wt%. The selection of the first-stage pyrolysis catalyst and the second-stage pyrolysis catalyst is the same as that in Example 2: that is, the first-stage pyrolysis catalyst is a carbon-based catalyst modified by metal Zn (Cat-III) (Zn is 2 wt% of the first-stage pyrolysis catalyst), and the second-stage pyrolysis catalyst is an MCM-41 molecular sieve catalyst modified by Cu, Ni and In (Cat-IV) (Cu, Ni, and In are 1.5 wt%, 1.5 wt%, and 0.12 wt% of the second-stage pyrolysis catalyst respectively).

[0068] (2) After the above-mentioned mixed waste plastic garbage is pretreated and crushed, it is mixed with Cat-III according to a mass ratio of 30:1 and then added into the first-stage rotary kiln pyrolysis reactor. After the system is purged and replaced with N2, it is heated to 550 °C under an N2 atmosphere, the reaction pressure is 98 Kpa, and continuous feeding is carried out for pyrolysis reaction to obtain pyrolysis products and pyrolysis solid residues. The pyrolysis solid residues are regularly discharged from the outlet of the rotary kiln and used to prepare the first-stage pyrolysis catalyst.

[0069] (3) A heat preservation pipeline is connected behind the first-stage rotary kiln pyrolysis reactor. The pyrolysis products directly enter the second-stage fixed-bed catalytic upgrading reactor under an N2 atmosphere. The temperature of the second-stage catalytic upgrading reactor is 400 °C, the reaction gas passes through the Cat-IV bed layer from top to bottom, and the lower end of the second reactor is connected to a condensation device to collect the high-value liquid products of pyrolysis, and the non-condensable gas is burned to recover energy.

[0070] (4) The comprehensive yield of the liquid products collected by the condensation device is 78%. Analyzed by GC-MS, the results show that the proportion of olefins and mono-aromatic hydrocarbons among them is 87%.

[0071] Example 4

[0072] (1) Select the mixed waste plastic garbage from a paper mill, whose composition is that the total of polyethylene and polypropylene plastics accounts for 95 wt%, pulp 1.5 wt%, plastic residues and sediment 2.5 wt%, and other impurities 1 wt%.

[0073] (2) The first-stage pyrolysis catalyst is a carbon-based catalyst modified by Zn and Al (Cat-V) (Zn and Al are 1.5 wt% and 1.5 wt% of the first-stage pyrolysis catalyst respectively), and the second-stage pyrolysis catalyst is an MCM-22 zeolite molecular sieve catalyst modified by Ni, Mg and In (Cat-VI) (Ni, Mg and In are 1.5 wt%, 3 wt% and 0.1 wt% of the second-stage pyrolysis catalyst respectively). The preparation process of CatV is as follows: Weigh 100 parts of pyrolysis solid residue, 6.83 parts of Zn(NO3)2·6H2O solid, 20.86 parts of Al(NO3)3·9H2O and 250 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate for 15 h, dry at 80 °C for 14 h in an N2 atmosphere, and calcine at 500 °C for 8.0 h to obtain Cat-V, that is, the first-stage pyrolysis catalyst. The preparation process of Cat IV is as follows: Weigh 100 parts of MCM-22 zeolite molecular sieve, 7.43 parts of Ni(NO3)2·3H2O solid, 25.10 parts of MgCl2·6H2O solid, 0.34 parts of In(NO3)3·5H2O solid and 280 parts of water, add them to a mixing kettle, stir and mix evenly, impregnate for 8 h, dry at 120 °C for 8 h in an N2 atmosphere, and calcine at 350 °C for 12 h to obtain Cat-VI, that is, the second-stage pyrolysis catalyst.

[0074] (3) After the above mixed waste plastic garbage is pretreated and crushed, it is mixed with Cat-V according to a mass ratio of 30:1 and then added to the first-stage rotary kiln pyrolysis reactor. After the system is purged and replaced with N2, it is heated to 600 °C in an N2 atmosphere, the reaction pressure is normal pressure, and continuous feeding is carried out for pyrolysis reaction to obtain pyrolysis products and pyrolysis solid residue. The pyrolysis solid residue is regularly discharged from the rotary kiln outlet and used to prepare the first-stage pyrolysis catalyst.

[0075] (4) A heat preservation pipeline is connected behind the first-stage rotary kiln pyrolysis reactor. The pyrolysis products directly enter the second-stage fluidized bed catalytic upgrading reactor in an N2 atmosphere. The temperature of the second-stage catalytic upgrading reactor is 380 °C. The reaction gas passes through the Cat-VI bed layer from top to bottom. The lower end of the second reactor is connected to a condensation device to collect the high-value liquid products of pyrolysis, and the non-condensable gas is burned to recover energy.

[0076] (5) The comprehensive yield of the liquid products collected by the condensation device is 81%. Analyzed by GC-MS, the results show that the proportion of olefins and mono-aromatic hydrocarbons in them is 90%.

[0077] The above has given an exemplary description of the embodiments of the present invention. However, the protection scope of the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made by those skilled in the art within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for recycling waste plastics, characterized in that, The method includes the following steps: (1) Mix waste plastics with a first-stage pyrolysis catalyst, and heat and react in an inert atmosphere to obtain pyrolysis products and pyrolysis solid residues; in step (1), the reaction temperature is 450 - 700 °C; In step (1), the first-stage pyrolysis catalyst is a metal-modified carbon-based catalyst, and the modified metal is one or more of Al, Ca, Zn, and Mg. The carbon-based material of the carbon-based catalyst is the pyrolysis solid residue; (2) Mix the pyrolysis products in step (1) with a second-stage pyrolysis catalyst, and heat in an inert atmosphere to obtain pyrolysis liquid products; in step (2), the heating temperature is 250 - 600 °C; In step (2), the second-stage pyrolysis catalyst is a metal-modified silicon / aluminum oxide catalyst, and the silicon / aluminum oxide is one of MCM-41, MCM-22, HZSM-5, HZSM-35, HY, Hβ, and SAPO-11 zeolite molecular sieves; the modified metal is one or more of Cu, Zn, Al, Mg, In, Ga, and Ni.

2. The method according to claim 1, characterized in that In step (1), the waste plastics include 80 wt% - 100 wt% of polyolefins and 0 - 20 wt% of other components, and the other components are polyester or PVC.

3. The method according to claim 2, wherein In step (1), the waste plastics are selected from at least one of polyethylene, polypropylene, and polystyrene.

4. The method according to claim 3, wherein The waste plastics also contain one or more selected from polyvinyl chloride, polyethylene terephthalate, polybutylene terephthalate, pulp, and sediment.

5. The method according to claim 1, characterized in that In step (1), the mass ratio of waste plastics to the first-stage pyrolysis catalyst is 10 - 100:

1.

6. The method according to claim 1, wherein In step (1), the pressure of the reaction system is 90 - 101.325 kPa.

7. The method according to claim 1, characterized in that In step (1), the reaction temperature is 450 - 600 °C.

8. The method according to claim 1, wherein In step (2), the pressure of the reaction system is normal pressure or slightly negative pressure, and the pressure is 90 - 101.325 kPa.

9. The method according to claim 1, wherein In step (2), the pyrolysis liquid products are more than 50% of the total mass of the waste plastics.

10. The method according to claim 1, characterized in that, In step (1), the metal is 0.1 - 10 wt% of the first-stage pyrolysis catalyst.

11. The method according to claim 1, characterized in that, In step (1), the first-stage pyrolysis catalyst is prepared by the solution co-impregnation method, specifically: (S1) Mix a metal precursor, pyrolysis solid residue, and a solvent, and impregnate to prepare a first-stage catalyst precursor; (S2) Heat and calcine the first-stage catalyst precursor in step (S1) to obtain the first-stage pyrolysis catalyst.

12. The method according to claim 10, wherein In step (S1), the metal precursor is selected from at least one of AlCl3, AlCl3·6H2O, calcium chloride, zinc chloride, magnesium chloride, aluminum nitrate, calcium nitrate, Zn(NO3)2·6H2O, and magnesium nitrate; In step (S2), the calcination temperature is 300 - 500 °C; the calcination time is 6 - 18 h.

13. The method according to claim 1, wherein In step (2), the heating temperature is 300 - 450 °C.

14. The method according to claim 1, characterized in that, In step (2), the metal is 0.05 - 10 wt% of the total mass of the second-stage pyrolysis catalyst.

15. The method according to claim 1, wherein In step (2), the second-stage pyrolysis catalyst is prepared by the solution co-impregnation method, specifically: (K1) Mix a metal precursor, a silicon / aluminum oxide, and a solvent, and impregnate them to prepare a second-stage catalyst precursor. (K2) Heat and calcine the second-stage catalyst precursor in step (K1) to obtain a second-stage pyrolysis catalyst.

16. The method according to claim 15, wherein In step (K1), the metal precursor is selected from at least one of copper chloride, zinc chloride, AlCl3, AlCl3·6H2O, NiCl2·6H2O, InCl3·4H2O, magnesium chloride, Cu(NO3)2•3H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, magnesium nitrate, and Ni(NO3)2•3H2O.

17. The method according to claim 15, characterized in that, In step (K2), the calcination temperature is 250-500 °C; the calcination time is 6-18 h.

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