Method and device for continuously degrading plastic based on Raman spectrometer and application

Abstract

The invention provides a method and a device for continuously degrading plastics based on a Raman spectrometer and application. The Raman spectrometer is used for feeding back the content ratio of alkane to aromatic hydrocarbon in the degradation product in real time and regulating and controlling feeding, so that the degradation product with relatively stable chemical components is obtained. According to the method for degrading the plastic, the problems that waste plastic resources are wasted and the environment is polluted are solved, on the basis, the method for preparing the low-carbon olefin from the plastic is provided, recycling of the waste plastic is achieved, pollution is reduced, and carbon emission is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of plastic degradation, and in particular relates to a method, a device and an application of continuously degrading plastic based on a Raman spectrometer. Background Art

[0002] Plastics are polymer materials formed by monomers through addition polymerization or condensation polymerization. They are widely used in daily life due to their easy processing and low cost, which greatly facilitates people's lives. However, due to the non-degradable nature of plastics, the "white pollution" caused by waste plastics has become a major problem facing mankind.

[0003] Waste plastics are misplaced resources. Turning waste into resources and changing "burden" into economic benefits is the only way to comprehensively utilize waste plastics. Using supercritical fluids to chemically recycle waste plastics can degrade plastics into low-carbon olefins, which can then be used as monomers to polymerize and produce plastics, rubber and other polymer materials, thus realizing the recycling of resources. This can not only avoid pollution, but also reduce production costs and improve corporate competitiveness.

[0004] Plastic recycling can be divided into physical recycling and chemical recycling. Physical recycling refers to a processing method that re-granulates waste plastics without destroying the polymer structure of the plastic. Physical recycling has simple processes and steps and is commercially feasible, so the current mainstream plastic recycling method is physical recycling. However, physical recycling also has corresponding limitations. Physical recycling is suitable for clean, single-variety materials, and waste plastics with complex components need to be sorted. In addition, the molecular chain of plastics may change after physical recycling, so the recycled materials can only be downgraded for use.

[0005] Chemical recycling is the process of destroying the polymer chains of waste plastics, converting them into small molecules, and producing fuel or chemical products. Chemical recycling is not only suitable for waste plastics with complex components, but can also "turn waste into treasure" and convert plastic waste into chemicals, reducing dependence on fossil energy, solving the plastic crisis and reducing carbon emissions. However, as of 2020, chemical recycling still accounts for less than 1% of waste plastic recycling. Benefiting from environmental protection policies and oil price levels, chemical recycling is expected to account for 17% of waste plastic recycling methods in 2030, becoming the most significant waste plastic recycling method.

[0006] Thermal degradation is a chemical recycling method that treats waste plastics as "crude oil" for refining. It uses the thermal instability of plastics to place plastics in a degradation reactor and decompose them into chemicals under heat. This method can directly separate high-value olefins and aromatics from the degradation products. In addition, the degraded oil can be added to the cracking reactor as a cracking raw material for cracking to produce low-carbon olefins. Summary of the invention

[0007] The degradation process of waste plastics faces the problems of complex raw materials and uneven molecular composition, so the products change dramatically under specific degradation conditions. In order to obtain degradation products with relatively stable chemical components, the reaction conditions need to be adjusted in real time according to the degradation products.

[0008] In order to solve the above technical problems, the present invention provides a method and device for continuously degrading plastics based on Raman spectrometer.

[0009] One of the purposes of the present invention is to provide a method for continuously degrading plastics based on a Raman spectrometer, comprising the step of using a Raman spectrometer to online monitor the content ratio of alkanes to aromatics in the plastic degradation products to regulate the feed flow rate.

[0010] According to the present invention, the method for continuous degradation of plastics based on Raman spectrometer comprises the following steps:

[0011] Step (1) continuously feeding plastic and water into a degradation reactor for degradation reaction to obtain degradation products;

[0012] Step (2) using a Raman spectrometer to detect the content ratio of alkanes and aromatics in the degradation products in real time, and adjusting the feed flow rates of the plastic and water according to the detected content ratio of alkanes and aromatics;

[0013] Step (3) cooling the degradation products and separating the gas and liquid;

[0014] Optionally, the step (3) includes the step (4): sending the liquid phase component obtained after gas-liquid separation into a cracking reactor, adding sulfur-containing compounds to carry out thermal cracking reaction, and obtaining a product rich in light olefins.

[0015] According to the present invention, the water is preferably supercritical water. Supercritical water has good solubility for non-polar hydrocarbons, and can form a "water cage" to "wrap" polyolefin molecules therein, thereby reducing the concentration of reaction intermediates, inhibiting the formation of coke and reducing the yield of aromatics. After supercritical water is mixed with hydrocarbons, its critical temperature and pressure change, and in the near-critical region, supercritical water can already degrade plastics into oils.

[0016] According to the present invention, in the method for continuous degradation of plastics based on Raman spectrometer:

[0017] The plastic includes polyolefins, preferably, but not limited to, at least one of polyethylene, polypropylene, and polybutylene;

[0018] The light olefins include but are not limited to at least one of ethylene, propylene and butene;

[0019] The sulfur-containing compound is selected from at least one of inorganic sulfides, mercaptans, sulfur-containing aliphatic hydrocarbons, and sulfur-containing heterocyclic compounds, and is preferably selected from at least one of sodium sulfide, carbon disulfide, dimethyl disulfide, and thiophene.

[0020] According to the present invention, in step (1) of the method for continuous degradation of plastics:

[0021] The ratio of the feed rate of the plastic to that of water is 1:(2-5), preferably 1:(2.5-4);

[0022] The conditions for the degradation reaction are: pressure not less than 20 MPa, temperature not less than 370°C, and reaction residence time of 0.5 to 5 h; preferably, the conditions for the degradation reaction are: pressure of 22 to 40 MPa, temperature of 380 to 480°C, and reaction residence time of 1 to 2 h;

[0023] The temperature of the material at the inlet of the degradation reactor is not less than 140°C, preferably 150-250°C.

[0024] According to the present invention, in step (2) of the method for continuous degradation of plastics, the content ratio of alkanes and aromatics in the degradation product is controlled to be (2.5-5):1, preferably (3.5-5):1, by adjusting the feed flow rate of plastics and water. When the content ratio of alkanes and aromatics does not meet the above ratio range, the water inlet valve and the feed valve are adjusted until the above ratio is met. If the content ratio of alkanes and aromatics is lower than the above ratio, the water inlet amount is increased and / or the feed amount is reduced. If the content ratio of alkanes and aromatics is higher than the above ratio, the feed amount is increased and / or the water inlet amount is reduced.

[0025] The present invention uses a Raman spectrometer to perform real-time analysis on the degradation products in step 1. The specific analysis method includes: using a CCD to collect the Raman signal of the sample under a 532nm wavelength excitation light and a 40mW laser power, and using a 1400-1510cm -1 Characteristic peak of alkanes, 1550~1630cm -1 The characteristic peak of aromatics is used to calculate the contents of alkanes and aromatics according to the characteristic peak signal. Among them, alkanes include but are not limited to butane, pentane, and hexane, and aromatics include but are not limited to benzene, toluene, and xylene.

[0026] According to the present invention, the cooling operation in step (3) of the method for continuous degradation of plastics based on Raman spectrometer includes: first cooling to 360-370°C to separate part of the liquid water, and then cooling to below 350°C to separate the liquid phase component and the gas component containing at least one of hydrogen, methane, carbon monoxide, carbon dioxide, ethane and propane.

[0027] According to the present invention, in step (4) of the method for continuous degradation of plastics based on Raman spectrometer:

[0028] The conditions for the thermal cracking reaction are: pressure not higher than 0.5 MPa, temperature not lower than 700°C, and reaction residence time of 0.1 to 0.5 s; preferably, the conditions for the thermal cracking reaction are: pressure of 0.01 to 0.4 MPa, temperature of 750 to 870°C, and reaction residence time of 0.18 to 0.25 s;

[0029] In terms of mass percentage, the amount of the sulfur-containing compound is 0.001 to 0.02% of the total feed amount entering the thermal cracking reaction.

[0030] A second object of the present invention is to provide a plastic degradation device, which uses the above-mentioned method for continuously degrading plastic based on Raman spectrometer to continuously degrade plastic.

[0031] According to the present invention, the plastic degradation device comprises a degradation reactor, a Raman spectrometer and a program control system connected in sequence, and optionally a pyrolysis reactor connected to the degradation reactor.

[0032] Wherein, the degradation reactor is one of a tubular reactor and a kettle reactor; the degradation reactor can be selected from a part of the convection section of the industrial cracking furnace as the degradation reactor according to actual needs;

[0033] The degradation reactor is connected to the extruder via a raw material delivery pipeline 1, a valve 1 is provided on the raw material delivery pipeline 1, and the valve 1 is connected to the program control system via a line; according to the present invention, the plastic to be degraded is heated to a molten state by the extruder and then extruded, and is delivered to the degradation reactor via the raw material delivery pipeline 1, and the program control system adjusts the feed flow of the plastic by controlling the valve 1 according to the alkane / aromatic ratio obtained by the Raman spectrometer;

[0034] The degradation reactor is connected to a water pump via a raw material delivery pipeline 2, a valve 2 is provided on the raw material delivery pipeline 2, and the valve 2 is connected to a program control system via a line; according to the present invention, water is delivered to the degradation reactor from the raw material delivery pipeline 2 via the water pump, and the program control system adjusts the feed flow of water by controlling the valve 2 according to the alkane / aromatic ratio obtained by the Raman spectrometer;

[0035] A high temperature and high pressure visual unit is optionally provided at the tail of the degradation reactor for observing the degradation reaction;

[0036] The Raman spectrometer is connected to the degradation product output pipeline of the degradation reactor through a line to test the alkanes and aromatics in the degradation products; the program control system adjusts the feed flow of plastic and / or water by controlling valve 1 and valve 2 according to the alkane / aromatic ratio measured by the Raman spectrometer;

[0037] The cracking reactor is a tubular reactor. In addition, the cracking reactor can be selected from the radiation section furnace tube of the industrial cracking furnace as the cracking reactor according to actual needs;

[0038] The cracking reactor comprises a material inlet and a cracking product discharge pipeline, and the material inlet of the cracking reactor is connected to the degradation product outlet of the degradation reactor through a pipeline.

[0039] The third object of the present invention is to provide a plastic degradation method or a plastic degradation device based on online monitoring of a Raman spectrometer, which is used in plastic degradation and is particularly suitable for the degradation of waste plastics to obtain products rich in low-carbon olefins.

[0040] In the present invention, the plastic thermal degradation is preferably carried out in supercritical water, which has good mass transfer and heat transfer properties. While reducing the hydrocarbon partial pressure, the plastic polymer can be protected by the "cage effect" of water molecules to prevent the molecular chain from breaking too quickly, resulting in excessive formation of aromatic hydrocarbons or increased coking. This method can greatly reduce pollution, reduce carbon emissions, reduce enterprise production costs, and improve efficiency.

[0041] Raman spectroscopy is a method of studying molecular structure using Raman scattering. Raman spectroscopy is very sensitive to molecular bonds, and each molecule will have a corresponding Raman spectrum. Using Raman spectroscopy, Raman spectra can be quickly generated to determine the chemical composition of the sample in real time.

[0042] Compared with the prior art, the process method for feedback controlling the content of waste plastic degradation products provided by the present invention has the following beneficial effects:

[0043] 1) The present invention uses a Raman spectrometer to adjust the reaction conditions in real time according to the degradation products, which is suitable for degrading waste plastics with complex components, obtaining degradation products with relatively stable components and contents, and avoiding excessive differences in degradation products caused by changes in raw materials;

[0044] 2) In the degradation method provided by the present invention, the degradation product has a low aromatic content and can be used as a raw material for cracking to produce low-carbon olefins, thereby increasing the source of cracking raw materials and reducing dependence on crude oil;

[0045] 3) The method for degrading plastics provided by the present invention solves the problem of waste plastic resources and environmental pollution, and on this basis provides a method for preparing low-carbon olefins from plastics, thereby realizing the recycling of waste plastics, reducing pollution and carbon emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a schematic diagram of the plastic degradation device used in Examples 1 and 2 of the present invention;

[0047] Figure 2This is a schematic diagram of the plastic degradation device used in Example 3 of the present invention. DETAILED DESCRIPTION

[0048] The present invention is described in detail below in conjunction with specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the contents of the present invention still fall within the scope of protection of the present invention.

[0049] It should also be noted that the various specific technical features described in the following specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0050] The calculation method of the content ratio of alkanes and aromatics in the degradation products of the present invention is as follows:

[0051] Raman spectrometer (B&T i-Raman laser spectrometer) test conditions: 532nm wavelength excitation light, 40mW laser power, using CCD to collect Raman signals of samples, with a wavelength of 1400-1510cm -1 Characteristic peak of alkanes, 1550~1630cm -1 The characteristic peak of aromatics is used to calculate the contents of alkanes and aromatics based on the characteristic peak signal. The signal intensity of alkanes and aromatics obtained by the Raman spectrometer can be calculated using a commonly used calculation method or program software, or can be calculated using the following method.

[0052] Preparation and testing of standard solutions: Taking pentane and toluene as examples, standard solutions with alkane to aromatic ratios of 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, and 5:1 were prepared respectively. The Raman spectra of standard solutions with different ratios were obtained after scanning with a Raman spectrometer. The Smooth function of the origin software was used to smooth the Raman spectrum curve of the standard solution, and the peak analysis function was used to flatten the Raman spectrum. The processed Raman spectrum showed a peak of 1400-1510 cm -1 The characteristic peaks of alkanes, and their Raman signal intensities are I A1 ,I A2 ,I A3 ,I A4 ,I A5 ,I A6 ,I A7 ,I A8 , and 1550~1630cm -1 The characteristic peaks of aromatic hydrocarbons, and their Raman signal intensities are I B1 ,I B2,I B3 ,I B4 ,I B5 ,I B6 ,I B7 ,I B8 . Among them I A1 / I B1 ~I A8 / I B8 The standard solution corresponding to the ratio of alkanes to aromatics is 1.5:1 to 5:1, that is, the ratio of the Raman signal intensity corresponding to the standard solution is obtained. In the above standard solution, the alkane is not limited to pentane, and one or more combinations of butane, pentane, and hexane can be used; the aromatics are not limited to toluene, and one or more combinations of benzene, toluene, and xylene can be used. The Raman signal intensities of the ratio of alkanes to aromatics at different ratios can be used to calculate the ratio of alkanes to aromatics in the degradation products of the present invention.

[0053] The test and calculation of the ratio of alkanes to aromatics in the embodiment: the degradation products are analyzed in real time by using a Raman spectrometer, the Raman spectrum curve of the degradation products is smoothed, and then the ratio of the characteristic peak intensity of alkanes to the characteristic peak intensity of aromatics in the processed Raman spectrum signal is calculated to obtain I A / I B , and use mathematical interpolation to calculate the ratio of alkane to aromatic concentration. For example: I A4 / I B4 is 2.9, I A5 / I B5 is 3.3, test result I A / I B is 3.2, then the ratio of alkanes to aromatics (X) is: (3.3-3.2) / (3-X)=(3.2-2.9) / (X-2.5), and X is calculated to be 2.875.

[0054] The calculation method of the liquid yield in the degradation product of the present invention is:

[0055] The liquid components in the degradation products include but are not limited to C5-C32 alkanes and C6-C22 aromatics. The calculation method of the liquid yield is as follows:

[0056] Degradation product liquid yield (%) = degradation reactor outlet liquid weight / plastic feed weight × 100%

[0057] Testing and calculation of triene yield in the cracking product of the present invention

[0058] "Triene" includes ethylene, propylene, and butadiene. The cracking reaction products were analyzed using a gas chromatograph (Agilent G2070AA Chemical Workstation A.10.01), and the gas yield, ethylene yield, propylene yield, and butadiene yield in the cracking products were calculated. The specific calculation method is as follows:

[0059] Gas yield test and calculation method: Cool the product to below 5°C, use a flow meter to measure the product gas volume, and use gas chromatography to analyze the product gas composition to obtain the gas density. Gas yield = gas volume × gas density / plastic feed weight;

[0060] Ethylene yield test and calculation method: gas chromatography analysis of ethylene content in product gas, ethylene yield = gas yield × ethylene content;

[0061] Propylene yield test and calculation method: gas chromatography analysis of propylene content in product gas, propylene yield = gas yield × propylene content;

[0062] Butadiene yield test and calculation method: gas chromatography analysis of butadiene content in product gas, butadiene yield = gas yield × butene content.

[0063] The plastics used in the following examples and comparative examples are polyethylene PE and polypropylene PP powders, and their GPC test results are shown in Table 1:

[0064] Table 1 GPC test results of PE and PP powders

[0065] Mn M Mz Mw / Mn PE powder 28474 151504 660649 5.32 PP powder 49947 397084 1328004 7.95

[0066] Example 1

[0067] Combine the following Figure 1 The plastic degradation device and the method for continuously degrading plastic used in Example 1 are described.

[0068] like Figure 1 As shown, the plastic degradation device used includes a degradation reactor, a Raman spectrometer, a program control system and a cracking reactor. The Raman spectrometer is connected to the program control system and is connected to the degradation product outlet of the degradation reactor through a line, and the degradation product outlet of the degradation reactor is connected to the cracking reactor through a pipeline. The degradation reactor is a tubular reactor, and the degradation reactor is connected to the plastic extruder through a plastic delivery pipeline, and a valve 1 is arranged on the plastic delivery pipeline, and the valve 1 is connected to the program control system through a line; the degradation reactor is connected to the water pump through a water delivery pipeline, and a valve 2 is arranged on the water delivery pipeline, and the valve 2 is connected to the program control system through a line, and valves 1 and valves 2 connected to the program control system are respectively arranged on the pipeline, which are used to control the input amount of plastic and water in the continuous degradation reaction process according to the ratio of alkanes and aromatics in the degradation reaction process.

[0069] The method for continuously degrading plastic PE using the above-mentioned plastic degradation device is specifically described, including:

[0070] (1) using a screw extruder to heat the plastic PE to a molten state and feed it into a degradation reactor, and using a water pump to feed water into the degradation reactor. The plastic undergoes a degradation reaction under high temperature and high pressure conditions in the degradation reactor to obtain a degradation product;

[0071] (2) using a Raman spectrometer to detect the content ratio of alkanes and aromatics in the degradation products in real time, and adjusting the flow rates of plastic and water according to the detected content ratio of alkanes and aromatics;

[0072] (3) cooling the degradation products to separate gas from liquid, first cooling to below 370° C. to separate part of the liquid water, and then cooling to below 350° C. to separate the liquid component and the gas component rich in hydrogen, methane, carbon monoxide, carbon dioxide, ethane, and propane;

[0073] (4) The liquid phase component obtained after gas-liquid separation is sent to a cracking reactor, and sulfur-containing compounds are added to carry out thermal cracking reaction to obtain a product rich in light olefins.

[0074] The operating conditions in the above steps (1) to (4) and the analysis results of the pyrolysis products are shown in Table 2.

[0075] Example 2

[0076] The degradation device and the method for degrading plastics in Example 1 are used, except that: after continuously degrading the plastic PE, the plastic to be degraded is changed to a mixed plastic of PE and PP (the mass ratio of PE and PP is 1:1). Under the operating conditions of Example 1, the ratio of alkane to aromatic content in the degradation reaction product is reduced to 1.77, and the degradation product has a large aromatic content, which is not suitable for preparing light olefins as a cracking raw material. The water inlet is adjusted by the program control system after feedback from the Raman spectrometer. The specific operating conditions refer to the conditions shown in Example 2 in Table 2. The ratio of alkane to aromatic content is increased to 3.67, and the aromatic content is reduced. It can continue to be used as a cracking raw material to prepare light olefins. The analysis results of the cracking products after cracking are shown in Table 2.

[0077] Example 3

[0078] Combine the following Figure 2 The plastic degradation device and the method for continuously degrading plastic used in Example 3 are described.

[0079] like Figure 2As shown, the plastic degradation device used includes a degradation reactor, a Raman spectrometer, and a program control system. The Raman spectrometer is connected to the program control system and is connected to the degradation product outlet of the degradation reactor through a line. The degradation reactor is a tubular reactor, and the degradation reactor is connected to the plastic extruder through a plastic delivery pipeline, and a valve 1 is provided on the plastic delivery pipeline, and the valve 1 is connected to the program control system through a line; the degradation reactor is connected to the water pump through a water delivery pipeline, and a valve 2 is provided on the water delivery pipeline, and the valve 2 is connected to the program control system through a line, and valves 1 and 2 connected to the program control system are respectively provided on the pipeline, which are used to control the input amount of plastic and water in the continuous degradation reaction process according to the ratio of alkanes and aromatics in the degradation reaction process.

[0080] The method for continuously degrading plastic PE using the above-mentioned plastic degradation device is specifically described, including:

[0081] (1) using a screw extruder to heat the plastic PE to a molten state and feed it into a degradation reactor, and using a water pump to feed water into the degradation reactor. The plastic undergoes a degradation reaction under high temperature and high pressure conditions in the degradation reactor to obtain a degradation product;

[0082] (2) using a Raman spectrometer to detect the content ratio of alkanes and aromatics in the degradation products in real time, and adjusting the flow rates of plastic and water according to the detected content ratio of alkanes and aromatics;

[0083] (3) The degradation products are cooled and then separated into gas and liquid. The temperature is firstly lowered to below 370°C to separate part of the liquid water, and then lowered to below 350°C to separate the liquid phase component and the gas component rich in hydrogen, methane, carbon monoxide, carbon dioxide, ethane, and propane.

[0084] The operating conditions in the above steps (1) to (3) and the analysis results of the degradation products are shown in Table 2.

[0085] Comparative Example 1

[0086] The degradation device and the method for degrading plastics in Example 2 are different in that: during the degradation reaction, a Raman spectrometer is not used to detect the content ratio of alkanes and aromatics in the degradation products, and the original feeding rates of plastics and water are maintained.

[0087] Comparative Example 2

[0088] The degradation device and the method for degrading plastics in Example 3 are different in that: during the degradation reaction, a Raman spectrometer is not used to detect the content ratio of alkanes and aromatics in the degradation products, and the original feeding rates of plastics and water are maintained.

[0089] The operating conditions of Comparative Example 2 and the analysis results of the degradation products are shown in Table 2, and it can be seen that the oil yield in the degradation products is low.

[0090] Table 2 Changes in the content of degradation products when the raw materials and reaction conditions change

[0091]

[0092] Note: " / " in Table 2 means not performed

[0093] It can be seen from the test results in Table 2 that in Examples 1 to 3, by adjusting the feed rate of plastic and water within a certain range of the content ratio of alkanes to aromatics, the yield of liquid components in the degradation product and the yield of light olefins after cracking can be improved. In particular, in Examples 1 to 2, a cracking reactor is connected in series after the degradation reactor, and the degradation product can be used to prepare light olefins. When the Raman spectrometer is used for feedback adjustment, the influence of the change in feed composition can be minimized, the relative stability of the cracking raw material is ensured, and the coking of the cracking furnace is suppressed. Comparative Examples 1 and 2 do not adjust the feed rate according to the content ratio of alkanes to aromatics, and the yield of oil in the degradation product and the yield of light olefins after cracking are both low, which are not suitable for preparing light olefins as cracking raw materials.

[0094] It can be seen that the present invention uses a Raman spectrometer to control the reaction conditions in real time according to the components of the degradation products, so as to obtain degradation products with relatively stable components and contents, avoid excessive differences in degradation products caused by changes in raw materials, and effectively solve the problems of waste plastic resources and environmental pollution.

Claims

1. A method for continuous degradation of plastics based on Raman spectrometer, comprising the step of using Raman spectrometer to online monitor the content ratio of alkanes to aromatics in plastic degradation products to regulate the feed flow rate.

2. The method according to claim 1, characterized in that The method comprises the following steps: Step (1) continuously feeding plastic and water into a degradation reactor for degradation reaction to obtain degradation products; Step (2) using a Raman spectrometer to detect the content ratio of alkanes and aromatics in the degradation products in real time, and adjusting the feed flow rates of the plastic and water according to the detected content ratio of alkanes and aromatics; Step (3) cooling the degradation products and separating the gas and liquid; Optionally, the step (3) includes the step (4): sending the liquid phase component obtained after gas-liquid separation into a cracking reactor, adding sulfur-containing compounds to carry out thermal cracking reaction, and obtaining a product rich in light olefins.

3. The method according to claim 2, characterized in that The plastic comprises polyolefin, preferably at least one of polyethylene, polypropylene and polybutylene; and / or, The low-carbon olefins include at least one of ethylene, propylene and butene; and / or, The sulfur-containing compound is selected from at least one of inorganic sulfides, mercaptans, sulfur-containing aliphatic hydrocarbons, and sulfur-containing heterocyclic compounds, and is preferably selected from at least one of sodium sulfide, carbon disulfide, dimethyl disulfide, and thiophene.

4. The method according to claim 2, characterized in that: In the step (1): The feed rate ratio of the plastic to water is 1:(2-5), preferably 1:(2.5-4); and / or, The conditions for the degradation reaction are: pressure not less than 20 MPa, temperature not less than 370° C., and reaction residence time of 0.5 to 5 h; preferably, the conditions for the degradation reaction are: pressure of 22 to 40 MPa, temperature of 380 to 480° C., and reaction residence time of 1 to 2 h; and / or, The temperature of the material at the inlet of the degradation reactor is not less than 140°C, preferably 150-250°C.

5. The method according to claim 2, characterized in that: In the step (2), the content ratio of alkanes to aromatics in the degradation products is controlled within the range of (2.5-5):1, preferably (3.5-5):1, by adjusting the feed flow rates of plastic and water.

6. The method according to claim 2, characterized in that The cooling operation in step (3) includes: first cooling to 360-370°C to separate part of the liquid water, and then cooling to below 350°C to separate the liquid phase component and the gas component containing at least one of hydrogen, methane, carbon monoxide, carbon dioxide, ethane and propane.

7. The method according to claim 2, characterized in that In the step (4): The conditions for the thermal cracking reaction are: pressure not higher than 0.5 MPa, temperature not lower than 700°C, and reaction residence time of 0.1 to 0.5 s; preferably, the conditions for the thermal cracking reaction are: pressure of 0.01 to 0.4 MPa, temperature of 750 to 870°C, and reaction residence time of 0.18 to 0.25 s; and / or, In terms of mass percentage, the amount of the sulfur-containing compound is 0.001 to 0.02% of the total feed amount entering the thermal cracking reaction.

8. A plastic degradation device, which uses the method described in any one of claims 1 to 7 to continuously degrade plastic.

9. The device according to claim 8, characterized in that The invention comprises a degradation reactor, a Raman spectrometer and a program control system which are connected in sequence, and an optional cracking reactor which is connected with the degradation reactor.

10. The device according to claim 9, characterized in that The degradation reactor is a tubular reactor or a tank reactor; and / or, The degradation reactor is connected to the extruder via a raw material delivery pipeline 1, a valve 1 is provided on the raw material delivery pipeline 1, and the valve 1 is connected to the program control system via a line; and / or, The degradation reactor is connected to a water pump via a raw material delivery pipeline 2, a valve 2 is provided on the raw material delivery pipeline 2, and the valve 2 is connected to a program control system via a line; and / or, The Raman spectrometer is connected to the degradation product output pipeline of the degradation reactor through a line to test alkanes and aromatics in the degradation products; and / or, The cleavage reactor is a tubular reactor; and / or, The cracking reactor comprises a material inlet and a cracking product discharge pipeline, and the material inlet of the cracking reactor is connected to the degradation product outlet of the degradation reactor through a pipeline.

11. Use of the plastic degradation method based on online monitoring of Raman spectrometer according to any one of claims 1 to 7 or the plastic degradation device according to any one of claims 8 to 10 in plastic degradation.

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