A method for preparing polyethylene wax by continuous cracking of waste polyethylene

Abstract

The present invention provides a method for preparing polyethylene wax by continuous cracking of waste polyethylene, comprising the following steps: continuously cracking waste polyethylene at 300-500°C to obtain gas phase products and non-gas phase products, and separating and refining the non-gas phase products to obtain polyethylene wax. The technical solution of the present application can obtain polyethylene wax, micro-powder polyethylene wax and oxidized polyethylene wax with number average molecular weights of 1000-2000, 2000-3000, 3000-4000, 4000-5000 at low cost and high yield.

Description

Technical Field

[0001] The invention relates to the field of recycling of polymer waste, and in particular to a method for preparing polyethylene wax by continuous cracking of waste polyethylene. Background Art

[0002] Polyethylene wax is a low-molecular-weight polyethylene with a number average molecular weight of 1000-5000, a melting point above 90°C, and a relative density of 0.91-0.96 g / cm 3 , and the color is mostly white or light yellow. Because of its high melting point, low melt viscosity, and excellent wear resistance, thermal stability and lubricity, it can be used as a lubricant, dispersant and other widely used in various fields. Polyethylene wax products are divided into two types: polymerization type and cracking type. Their performance and application mainly depend on their molar mass, molecular weight distribution and molecular structure. There is no unified regulation for its various performance indicators so far, but the corresponding indicators are selected according to their different uses. At present, most of the high-performance polyethylene wax products used in China are imported from abroad, mainly polymerized polyethylene wax from Honeywell in the United States and Clariant in Germany, and cracked polyethylene wax from South Korea and Thailand.

[0003] Polyethylene wax products with controllable molar mass and narrow distribution can be obtained by polymerizing ethylene monomers, but this method requires large processing equipment and high production costs. The cracking production process is that the raw materials are subjected to high-temperature thermal cracking or catalytic cracking above 300°C under anaerobic conditions to generate polyethylene wax products. The properties of the produced polyethylene wax are greatly affected by the source of the cracking raw materials. The raw materials for producing polyethylene wax from polyethylene waste plastics are abundant and cheap, but the product quality is difficult to control. Due to the poor thermal conductivity of plastics, the viscosity is very high when heated, and it is difficult to form an easily transportable melt; secondly, carbon deposits will be produced when waste plastics are decomposed at high temperatures, which not only affects the wax production rate, but also adheres to equipment and pipelines to cause blockage, and is difficult to clean.

[0004] Patent CN201810569475.0 discloses a method for producing polyethylene wax from waste polyethylene: first, the waste polyethylene is classified, cleaned and dried for pretreatment, and then sent to the cracking device, and AL-MCM-48 catalyst of about 0.1% of the weight of the waste polyethylene is added, and it is heated and stirred; after the reaction is completed, the wax product is filtered and granulated, and the gas-liquid phase product enters the condensation system for cooling and recovery. There are the following problems in the actual production of this process: the classification and cleaning and drying of waste polyethylene is time-consuming and labor-intensive, and consumes a lot of water and energy; the added AL-MCM-48 solid acid catalyst will not only cause wear to the stirring equipment at high temperature, but also accelerate the process of polyethylene coking and carbon deposition, causing serious coking of the equipment, and also reducing the wax yield; after the reaction, the wax product filtration process faces the problem of high viscosity wax product and catalyst powder and difficulty in separation.

[0005] CN201210132597.6 discloses a method for preparing polyethylene wax containing phenoxy groups in chain segments by thermal degradation, wherein the method comprises first mixing hydroquinone and polyethylene pellets at a mass ratio of 1 to 4:100 and adding the mixture into a high-pressure stirred reactor, then introducing nitrogen, reacting at 330 to 380°C for 0.5 to 5 hours, and taking out the wax block and crushing it to obtain the product after the reaction is completed. The polyethylene used in this method is fresh polyethylene pellets, which has a high cost; the price of hydroquinone is also high, and aromatic groups are introduced into the system, which affects the color and stability of the product; the reaction time is long, and polyethylene will undergo a coking and carbon deposition process at a high temperature of 330 to 380°C, causing serious coking of the equipment and reducing the yield of polyethylene wax.

[0006] CN201510665356.1 discloses a method for producing polyethylene wax from waste polyethylene: first, the waste polyethylene is cleaned and placed in a cracking furnace, and a petroleum ether solution is added to dissolve at room temperature for 2 to 3 hours; then steam is introduced into the furnace for 1 to 2 hours for cracking; 2,2,6,6-tetramethyl-N-piperidine oxide is added to the cracked solution, stirred continuously, and reacted for 30 to 50 minutes; the cracked solution is cooled to room temperature and discharged; and the product quality is tested after crushing. The actual production of this process has the following problems: the cleaning process of waste polyethylene is time-consuming and laborious, and a large amount of water is consumed; adding petroleum ether solution to dissolve at room temperature for 2 to 3 hours is time-consuming and laborious, and a large amount of petroleum ether is consumed; introducing steam into the furnace for 1 to 2 hours for cracking and adding 2,2,6,6-tetramethyl-N-piperidine oxide to the cracked solution and stirring continuously, and reacting for 30 to 50 minutes is also a time-consuming and laborious process; coupled with the subsequent cooling, discharging and crushing process, the whole process is too complicated and energy-consuming, and the technical economy is difficult to guarantee. Summary of the invention

[0007] In view of the shortcomings of the prior art described above, the purpose of the present invention is to provide a method for preparing polyethylene wax by continuous cracking of waste polyethylene, so as to solve the problems existing in the prior art of easy coking, low wax yield, poor product quality, inability to carry out continuous production, high safety risks, high labor intensity and high energy and material consumption in the cracking process of waste polyethylene, thereby providing a method for producing polyethylene wax which can be operated continuously, is safe, stable, has low energy consumption, has a high wax yield, has good product quality and is not prone to carbon deposition.

[0008] To achieve the above objectives and other related objectives, the present invention is achieved through the following technical solutions.

[0009] The invention provides a method for preparing polyethylene wax by continuous cracking of waste polyethylene, comprising the following steps: continuously cracking waste polyethylene at 300-500°C to obtain gas phase products and non-gas phase products, and separating and refining the non-gas phase products to obtain polyethylene wax.

[0010] Preferably, the waste polyethylene is cracked after being pretreated.

[0011] More preferably, the pretreatment comprises one or more selected from washing, granulation and grinding.

[0012] The cleaning is carried out by selecting a suitable cleaning technique according to the raw material conditions and local conditions. More preferably, the cleaning is selected from one or more of water cleaning, organic solvent cleaning, dry cleaning and ultrasonic cleaning.

[0013] The waste polyethylene raw material is waste. Preferably, the material of the waste polyethylene is one or more selected from LLDPE, LDPE, and HDPE.

[0014] From the perspective of raw material and product stability, it is best to have one material dominate after classification pretreatment, and the content of other elements other than carbon and hydrogen should not exceed 10wt%. Preferably, the content of any one selected from LLDPE, LDPE and HDPE in the waste polyethylene raw material exceeds 50wt%, and the content of other elements other than carbon and hydrogen does not exceed 10wt%. More preferably, the content of any one selected from LLDPE, LDPE and HDPE in the waste polyethylene raw material exceeds 90wt%.

[0015] Preferably, the waste polyethylene is uniformly and continuously fed into the cracking process for cracking.

[0016] Preferably, in the pyrolysis step, the pyrolysis time of the waste polyethylene is 1 to 60 minutes, more preferably, the pyrolysis time is 2 to 30 minutes.

[0017] Preferably, during cracking, nitrogen or an inert gas is used as the protective gas for cracking. The inert gas is helium or argon.

[0018] Preferably, during cracking, the heating method can be one or more selected from electric heating, steam heating, molten salt heating, microwave or infrared. More preferably, the heating method is electric heating or infrared heating. Electric heating or infrared heating has fast temperature rise, uniform heating and high energy utilization rate.

[0019] Preferably, the waste polyethylene is fed into the pyrolysis process via a feeder. More preferably, the feeder is a screw feeder, a conveyor belt or a feed pump.

[0020] Preferably, the cracking temperature is 350-450°C.

[0021] Preferably, the gas phase product comprises one or more of hydrogen, methane, ethane, ethylene, propane, propylene, butane and butene.

[0022] Preferably, the non-gaseous product includes a liquid phase and a solid phase. More preferably, the solid phase includes polyethylene wax and impurities. The impurities include one or more of waste polyethylene modified fillers or entrained impurities, solid products with a high carbon-to-hydrogen ratio in the reaction process, or additives. The high carbon-to-hydrogen ratio in this application refers to a carbon-to-hydrogen ratio higher than 1:2. 1:2 is the carbon-to-hydrogen ratio of polyethylene, and acetylenic bonds or aromatic structures will cause the carbon-to-hydrogen ratio to become higher.

[0023] Preferably, the separation is selected from one or more of hydrocyclone separation, centrifugal separation, filtration and sedimentation.

[0024] Higher quality polyethylene wax products are obtained through refining. Preferably, the refining is selected from one or more of filtration, oxidation modification, hydrogenation refining, solvent refining, adsorption refining and spray drying. The refining process can also be selected to be oxidized or grafted according to product characteristics and market demand.

[0025] More preferably, the solvent in the solvent refining is one or more selected from n-hexane, petroleum ether, n-dodecane, methylcyclohexane, decahydronaphthalene, toluene, xylene, ethanol, acetone, butanone and methyl isobutyl ketone.

[0026] More preferably, the filter screen used in the filtration is made of metal or ceramic. Preferably, the filtration is carried out by melt filtering. More preferably, the filtration precision is 0.01 to 100 μm, and more preferably, the filtration precision is 0.1 to 50 μm.

[0027] More preferably, the cracking step further uses a cracking catalyst, and the cracking catalyst is selected from one or more of MCM-41, MCM-48, SAPO molecular sieve, ZSM molecular sieve, CaCO3, MgO, ZnO, and aluminosilicate.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1. The process is simple and continuous: the whole process includes three steps: raw material pretreatment, continuous cracking and product separation and purification, which avoids the complicated operation process of repeated heating and cooling and adding and discharging of materials in the intermittent production process. It has a high degree of automation, a safe and reliable process, stable product properties, time-saving and labor-saving, energy-saving and environmentally friendly.

[0030] 2. Flexible product solutions: The operating conditions such as the type of additives, reaction temperature, reaction time, etc. can be adjusted according to the target product to obtain products such as polyethylene wax with different molecular structures and molecular weight distributions, which have strong market adaptability.

[0031] 3. The special design improves the heat transfer, mass transfer, momentum transfer and chemical reaction uniformity in the reactor, avoiding local overheating or material unevenness leading to messy product distribution and coke deposition, with high yield of target products and simple and efficient operation.

[0032] The technical solution of the present application can be used to obtain polyethylene wax, micro-powder polyethylene wax and oxidized polyethylene wax with number average molecular weights of 1000-2000, 2000-3000, 3000-4000, 4000-5000 at low cost and high yield. DETAILED DESCRIPTION

[0033] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0034] It should be noted that the process equipment or devices not specifically specified in the following embodiments are all conventional equipment or devices in the art.

[0035] In addition, it should be understood that one or more method steps mentioned in the present invention do not exclude the existence of other method steps before or after the combination step or the insertion of other method steps between the steps explicitly mentioned, unless otherwise specified. Moreover, unless otherwise specified, the numbering of each method step is only a convenient tool for identifying each method step, and is not intended to limit the order of arrangement of each method step or the scope of the present invention. The change or adjustment of their relative relationship shall also be regarded as the scope of the present invention without substantially changing the technical content.

[0036] A specific embodiment is provided in the examples of the present application, which is a method for preparing polyethylene wax by continuous cracking of waste polyethylene, comprising the following steps: continuously cracking the waste polyethylene raw material at 300-500°C to obtain gas phase products and non-gas phase products, and separating and refining the non-gas phase products to obtain polyethylene wax. The waste polyethylene raw material is waste. It is specifically a method that can continuously and massively process waste polyethylene into polyethylene wax with practical use value.

[0037] In one embodiment, the waste polyethylene raw material is cracked after pretreatment. In a more preferred embodiment, the pretreatment includes one or more selected from cleaning, granulation, and grinding. The cleaning is selected from a suitable cleaning technology depending on the raw material conditions and local conditions. More preferably, the cleaning is selected from one or more selected from water washing, organic solvent cleaning, dry cleaning, and ultrasonic cleaning.

[0038] In a preferred embodiment, the waste polyethylene is made of one or more materials selected from LLDPE, LDPE and HDPE.

[0039] From the perspective of raw material preparation and product stability, it is best to have a certain material dominate after classification pretreatment, and the content of other elements other than carbon and hydrogen should not exceed 10wt%. In a preferred embodiment, the content of any one selected from LLDPE, LDPE and HDPE in the waste polyethylene exceeds 50wt%, and the content of other elements other than carbon and hydrogen does not exceed 10wt%. In a more preferred embodiment, the content of other elements other than carbon and hydrogen in the waste polyethylene does not exceed 5wt%. In a more preferred embodiment, the waste polyethylene does not contain other elements other than carbon and hydrogen.

[0040] In a more preferred embodiment, the content of any one selected from LLDPE, LDPE and HDPE in the waste polyethylene exceeds 60wt%. In a more preferred embodiment, the content of any one selected from LLDPE, LDPE and HDPE in the waste polyethylene exceeds 90wt%. Thus, the processing process can be made more stable and easy to control, and the molecular weight distribution of the polyethylene wax with the final target number average molecular weight formed is narrower.

[0041] In a preferred embodiment, the waste polyethylene is uniformly and continuously fed into the cracking process for cracking.

[0042] In a preferred embodiment, in the pyrolysis process, the pyrolysis time of the waste polyethylene raw material is 1 to 60 minutes. More preferably, the pyrolysis time is 2 to 30 minutes. For example, the pyrolysis time is 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes or 30 minutes.

[0043] In a preferred embodiment, during cracking, nitrogen or an inert gas is used as the protective gas for cracking. The inert gas is helium or argon.

[0044] In a preferred embodiment, during cracking, the heating method can be one or more selected from electric heating, steam heating, molten salt heating, microwave or infrared. More preferably, the heating method is electric heating or infrared heating. Electric heating or infrared heating has fast temperature rise, uniform heating and high energy utilization rate.

[0045] In a preferred embodiment, the waste polyethylene is fed into the pyrolysis process via a feeder. More preferably, the feeder is a screw feeder, a conveyor belt or a feed pump.

[0046] The pyrolysis temperature and the residence time of the pyrolysis process will affect the molecular weight and molecular weight distribution of the polyethylene wax finally obtained. In a preferred embodiment, the pyrolysis temperature is 350-450°C, such as 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C or 450°C.

[0047] In a preferred embodiment, the gas phase product includes one or more of hydrogen, methane, ethane, ethylene, propane, propylene, butane and butene.

[0048] In a preferred embodiment, the non-gaseous product includes a liquid phase and a solid phase. In a more preferred embodiment, the solid phase includes polyethylene wax and impurities. The impurities include one or more of waste polyethylene modified fillers or entrained impurities, solid products with a high carbon-hydrogen ratio in the reaction process, or additives. The high carbon-hydrogen ratio in this application refers to a carbon-hydrogen ratio higher than 1:2. 1:2 is the carbon-hydrogen ratio of polyethylene, and acetylenic bonds or aromatic structures will cause the carbon-hydrogen ratio to become higher.

[0049] In a preferred embodiment, the separation is selected from one or more of hydrocyclone separation, centrifugal separation, filtration, and sedimentation.

[0050] When the impurities are larger solids, filtration and sedimentation separation can be used to remove the larger solid impurities. Larger solids can be straw, leaves, mud, and stones. This will reduce the cost. These impurities are often found in recycled waste polyethylene mulch.

[0051] When the impurities are small solids, centrifugation and hydrocyclone separation can be used to remove the small solid impurities. The small solids can be nano or micro calcium carbonate.

[0052] Higher quality polyethylene wax products are obtained through refining. In a preferred embodiment, the refining is selected from one or more of filtration, oxidation modification, hydrogenation refining, solvent refining, adsorption refining and spray drying. The refining process can also choose whether to perform oxidation and grafting modification according to product characteristics and market demand.

[0053] In a preferred embodiment, the filter screen used in the filtration is made of metal or ceramic. Preferably, the filtration is carried out by melt filtering. In a preferred embodiment, the filtration precision is 0.01 to 100 μm, and in a more preferred embodiment, the filtration precision is 0.1 to 50 μm. For example, it can be 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 37 μm, 40 μm, 43 μm, 45 μm, 47 μm or 50 μm.

[0054] In a preferred embodiment, the oxidative modification is carried out by soaking the mixture in a hydrogen peroxide solution at 75 to 95° C., such as 75° C., 76° C., 77° C., 78° C., 79° C., 80° C., 81° C., 82° C., 83° C., 84° C., 85° C., 86° C., 87° C., 88° C., 89° C., 90° C., 91° C., 92° C., 93° C., 94° C. or 95° C. In a preferred embodiment, the concentration of the hydrogen peroxide solution is 25 to 35wt%. For example, it can be 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, 31wt%, 32wt%, 33wt%, 34wt% or 35wt%.

[0055] In a preferred embodiment, the oxidative modification refers to grafting modification with maleic anhydride and / or acrylic acid monomer at 120-180°C, such as 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C, 175°C or 180°C.

[0056] In a preferred embodiment, the hydrofining refers to hydrofining at 250-350°C using hydrogen, such as 250°C, 255°C, 260°C, 265°C, 270°C, 275°C, 280°C, 285°C, 290°C, 295°C, 300°C, 305°C, 310°C, 315°C, 320°C, 325°C, 330°C, 335°C, 340°C, 345°C or 350°C.

[0057] In a preferred embodiment, the solvent refining refers to a solvent immersion treatment.

[0058] In a preferred embodiment, the adsorption refining refers to adsorption treatment using an adsorbent. The adsorbent is activated carbon. In a preferred embodiment, the solvent in the solvent refining is one or more selected from n-hexane, petroleum ether, n-dodecane, methylcyclohexane, decahydronaphthalene, toluene, xylene, ethanol, acetone, butanone, and methyl isobutyl ketone.

[0059] In a preferred embodiment, the cracking also uses a cracking catalyst, and the cracking catalyst is selected from one or more of MCM-41, MCM-48, SAPO molecular sieve, ZSM molecular sieve, CaCO3, MgO, ZnO, and aluminosilicate. In the present application, the catalyst is used to reduce the reaction temperature and to improve the product distribution, that is, to narrow the molecular weight distribution.

[0060] In a specific embodiment, a spray dryer is used for granulation, specifically, a polyethylene wax solution is treated with a spray dryer, and the polyethylene wax as a heavy component is precipitated and collected after passing through the spray dryer, while light components such as solvents or impurity fine powders are taken away in the gas as atomized and then processed.

[0061] The technical solution and technical effects of the present application are further explained and illustrated by means of specific embodiments below.

[0062] In Examples 1 to 6 of the present application, a specific method for continuously preparing polyethylene wax is adopted, which uses pretreated waste polyethylene as raw material, continuously feeds it into a continuous cracking reactor, and then enters a melt filter for filtration to separate gas phase products and non-gas phase products. The non-gas phase products can also be mixed with an auxiliary agent and then refined. The refined polyethylene wax is granulated to obtain the final product.

[0063] Pretreatment refers to cleaning the waste polyethylene to remove impurities attached to the surface or inside of the waste polyethylene, and can also be further ground to make its morphology meet the device feed requirements. The cleaned impurities include soil, straw, calcium carbonate, etc.

[0064] The wax yield described in this application is the ratio of the polyethylene wax finally obtained to the polyethylene content in the raw material. The polyethylene content in the waste polyethylene in this application is obtained by infrared testing.

[0065] Example 1

[0066] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0067] In the cracking process, inert gas nitrogen is used to provide a protective gas environment, and the waste polyethylene powder purchased on the market is evenly mixed with the powdered cracking catalyst MCM-41, and then fed into a continuous cracking reactor through a feeder. The cracking reactor is a tubular reactor. After preliminary cracking at 350°C for 30 minutes, it enters a melt filter for filtration with a filtration accuracy of 10 microns. The gas phase product enters a gas recovery tank after being separated from the non-gas phase product in the melt filter; the non-gas phase product is mixed with auxiliary hydrogen at a volume ratio of 100:1 and then enters a crude wax fixed bed hydrogenation refining tank for treatment at 300°C for 60 minutes. The obtained hydrogenated polyethylene wax is mixed with p-xylene at a mass ratio of 1:1 while hot, and ethanol is used as a precipitant to precipitate the polyethylene wax. The precipitated polyethylene wax is dried and ground to obtain refined polyethylene wax.

[0068] In this embodiment, the waste polyethylene powder purchased on the market is waste polyethylene grinding powder with a HDPE content of more than 90 wt%.

[0069] The obtained polyethylene wax has a snow-white appearance and no odor, a number average molecular weight of 4950, a melting point of 123.80° C., and a molecular weight distribution index of 2.68.

[0070] Comparative Example 1

[0071] The difference from Example 1 is that the purchased waste polyethylene powder in this example contains HDPE, PET and PVC, and the mass ratio of HDPE, PET and PVC is 1: (0.8-1.2): (0.8-1.2). The rest is the same as Example 1.

[0072] The physical and chemical properties of the obtained polyethylene wax are as follows: the obtained polyethylene wax has a yellow-brown appearance, a pungent smell, a number average molecular weight of 4730, a melting point of 120.50° C., and a molecular weight distribution index of 5.68.

[0073] Comparative Example 2

[0074] The difference from Example 1 is that the initial cracking is performed at 550°C.

[0075] The rest is the same as Example 1.

[0076] The physical and chemical properties of the obtained polyethylene wax are as follows: the obtained polyethylene wax has a dark yellow appearance, a diesel-like smell, a number average molecular weight of 730, a melting point of 60.50° C., and a molecular weight distribution index of 7.14.

[0077] Example 2

[0078] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0079] In the cracking process, inert gas nitrogen is used to provide a protective gas environment, and the pretreated waste polyethylene film fragments are mixed evenly with the powdered cracking catalyst MCM-48, and sent to a continuous cracking reactor through a feeder. The cracking reactor is a reactor with stirring. The temperature of the cracking reactor is 370°C, and the average residence time of the material is 60 minutes; after the material is discharged from the reactor, it enters a melt filter with a filtration accuracy of 100 microns. The gas phase product enters a gas recovery tank after being separated from the non-gas phase product in the melt filter; the non-gas phase product is mixed with the auxiliary agent while hot and enters a product solvent refining tank to obtain a hot mixed solution, and the solvent is a mixture of p-xylene and ethylene glycol in a mass ratio of 1:1; the refining processing time is 10 minutes, the temperature is 90°C, and the mixed solution enters a granulator (spray dryer) for granulation to obtain polyethylene wax microparticles or powder.

[0080] In this embodiment, the waste polyethylene film fragments are waste mulch films with an LLDPE content of more than 95 wt%. The waste mulch films are pretreated by using a waterless cleaning technology to separate and remove impurities such as straw, leaves, mud, and stones; and then a high-temperature horizontal centrifuge is used to centrifuge and separate and remove nanometer or micrometer-sized solid impurities (such as micrometer or nanometer calcium carbonate).

[0081] The obtained polyethylene wax has a number average molecular weight of 4280, a melting point of 116.50° C., and a molecular weight distribution index of 3.92.

[0082] Example 3

[0083] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0084] In the cracking process, inert gas nitrogen is used to provide a protective gas environment. After the purging is completed, the pretreated waste polyethylene pellets are sent to a continuous cracking reactor through a feeder. The cracking reactor is composed of a tubular reactor and a fixed bed reactor filled with a cracking catalyst MCM-48. The material is first preliminarily cracked at 350°C in the tubular reactor, and then transported to a fixed bed reactor filled with a cracking catalyst MCM-48. The average bed temperature is 370°C and the average residence time of the material is 20 minutes. The material at the reactor outlet enters a melt filter with a filtration accuracy of 3 microns. The gaseous product is separated from the non-gas phase product and enters a gas recovery tank. The non-gas phase product is mixed with petroleum ether at a mass ratio of 1:1 while hot and enters a crude wax adsorption refining tank. The adsorbent is activated carbon, and the amount of the adsorbent used is 3 times the mass of the product to be refined. The refined polyethylene wax enters a granulator for granulation to obtain polyethylene wax pellets or powder.

[0085] In this embodiment, the LDPE content in the waste polyethylene pellets is above 95 wt%. Impurities are removed by washing.

[0086] The obtained polyethylene wax has a number average molecular weight of 2820, a melting point of 108.70° C., and a molecular weight distribution index of 3.26.

[0087] Example 4

[0088] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0089] In the cracking process, inert gas nitrogen is used to provide a protective gas environment, the pretreated waste polyethylene powder is evenly mixed with the powdered cracking catalyst MCM-41, and is sent to a continuous cracking reactor through a feeder. The reactor is a tubular reactor, and after preliminary cracking at 400°C for 10 minutes, it enters a melt filter with a filtration accuracy of 20 microns. The gas phase product and the non-gas phase product are separated and enter a gas recovery tank; the non-gas phase product is mixed with a 30wt% hydrogen peroxide solution in a mass ratio of 1:1 and enters a polyethylene wax oxidation refining tank at 90°C for reaction for 2 hours. The refined polyethylene wax enters a granulator (spray dryer) for granulation to obtain oxidized polyethylene wax microparticles or powder.

[0090] In this embodiment, the LDPE content in the waste polyethylene powder is above 95wt%.

[0091] The obtained oxidized polyethylene wax has a number average molecular weight of 2360, a melting point of 105.50° C., a molecular weight distribution index of 3.39, and an acid value of 26 mgKOH / g.

[0092] Example 5

[0093] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0094] First, purge with inert gas nitrogen to provide a protective gas environment. After the purge, the pretreated waste polyethylene powder is sent to a continuous cracking reactor through a feeder. The reactor is a slurry bed reactor. After preliminary cracking at 450°C for 15 minutes, it enters a ceramic membrane melt filter with a filtration accuracy of 0.1 microns. The gas phase product and the non-gas phase product are separated and enter the gas recovery tank; the non-gas phase product and the auxiliary agent maleic anhydride are mixed in a mass ratio of 1:1 and enter the product grafting modification refining tank. It is treated at 150°C for 3 hours. The grafted modified polyethylene wax enters a granulator for granulation to obtain grafted modified polyethylene wax microparticles or powder.

[0095] In this embodiment, the HDPE content in the waste polyethylene powder is above 90 wt%.

[0096] The obtained grafted modified polyethylene wax has a number average molecular weight of 1130, a melting point of 97.20° C., a molecular weight distribution index of 4.71, and an acid value of 37 mgKOH / g.

[0097] Example 6

[0098] This embodiment provides a specific method for preparing polyethylene wax by continuous cracking of waste polyethylene:

[0099] First, purge with inert gas nitrogen to provide a protective gas environment. After the purge, the pretreated waste polyethylene powder is sent to a continuous cracking reactor through a feeder. The reactor is a tubular reactor. After preliminary cracking at 500°C for 2 minutes, it enters a ceramic membrane melt filter with a filtration accuracy of 0.5 microns. The gas phase product and the non-gas phase product are separated and enter the gas recovery tank; the non-gas phase product and the auxiliary agent hydrogen are mixed in a volume ratio of 1:200 and enter the crude wax hydrogenation refining tank. The obtained hydrogenated polyethylene wax is mixed with decahydronaphthalene in a mass ratio of 1:1 while hot, and the light components are washed off. The heavy components are cooled at 70°C and crystallized. The crystals are dried and crushed to obtain polyethylene wax powder.

[0100] In this embodiment, the HDPE content in the waste polyethylene powder is above 90 wt%.

[0101] The obtained polyethylene wax had a number average molecular weight of 1510, a melting point of 103.40° C., and a molecular weight distribution index of 3.96.

[0102] The wax formation rates in Examples 1 to 6 of the present application are all higher than 95%.

[0103] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing polyethylene wax by continuous cracking of waste polyethylene, comprising the following steps: continuously cracking the waste polyethylene at 350-450°C to obtain gas phase products and non-gas phase products, and separating and refining the non-gas phase products to obtain white polyethylene wax; The waste polyethylene is cracked after being pretreated, wherein the pretreatment is selected from one or more of cleaning, granulation and grinding; The material of the waste polyethylene is one or more selected from LLDPE, LDPE and HDPE; The content of any one of LLDPE, LDPE and HDPE in the waste polyethylene exceeds 50wt%, and the content of other elements in the waste polyethylene except carbon and hydrogen does not exceed 10wt%; the cracking catalyst is selected from MCM-41; the refining is hydrogenation refining, and the hydrogenation refining refers to hydrogenation refining at 250-350°C using hydrogen; the separation is filtration; the filtration is carried out using a melt filter, and the filtration accuracy is 0.01-100μm.

2. The method according to claim 1, It is characterized in that The waste polyethylene is uniformly and continuously fed into the cracking process for cracking. In the cracking process, the cracking time of the waste polyethylene raw material is 1 to 60 minutes.

3. The method according to claim 1, It is characterized in that During cracking, nitrogen or inert gas is used as the protective gas for cracking.

4. The method according to claim 1, It is characterized in that The gas phase product includes one or more of hydrogen, methane, ethane, ethylene, propane, propylene, butane and butene.

5. The method according to claim 1, It is characterized in that The non-gas phase product includes a liquid phase and a solid phase, and the solid phase includes polyethylene wax and impurities.