Method for preparing polyethylene using intermittent production equipment

Abstract

The present invention relates to the field of polyethylene, and discloses a method for preparing polyethylene using intermittent production equipment, the method comprising: in the presence of propane and a catalyst, ethylene and a comonomer are contacted in a polymerization kettle for polymerization reaction; wherein, during the polymerization reaction, samples are taken from the polymerization kettle for analysis at regular intervals to obtain the content ratio of ethylene to comonomer in the polymerization kettle, and the content ratio is transmitted to a comonomer feeding system; the comonomer feeding system adjusts the feed amount of the comonomer according to the content ratio, so that the content ratio of ethylene to comonomer in the polymerization kettle is maintained within a predetermined range. The method can effectively solve the problem that it is difficult to adjust the density of polyethylene resin in intermittent production of ultra-high molecular weight polyethylene devices, and effectively improve the utilization rate and economic benefits of production equipment.

Description

Technical Field

[0001] The invention relates to the field of polyethylene, and in particular to a method for preparing polyethylene by using intermittent production equipment. Background Art

[0002] Polyethylene (PE) resin is the largest-volume variety of general synthetic resins. It is cheap and has good performance. It is widely used in industry, agriculture and other fields. Polyethylene production technology has always been a coexistence of multiple processes. At present, the solution process includes Nova's medium-pressure process, Dow Chemical's low-pressure cooling process and DSM's low-pressure adiabatic process. The slurry process is the ring tube process of Phillips and Solvay and the stirred tank process of Hoechst, Nissan Chemical and Mitsui Chemical. The gas phase process mainly includes Univation's Unipol process, BP's Innovene process and Basell's Spherilene process.

[0003] Ultra-high molecular weight polyethylene (UHMWPE) is a polyethylene resin with a molecular weight of 1.5 million to 8 million. UHMWPE has a larger molecular weight than ordinary polyethylene, so it has excellent properties such as impact resistance, wear resistance, self-lubrication, low temperature resistance, and chemical corrosion resistance, and is widely used in coal mining, chemical industry, machinery industry, textile industry, artificial prostheses and other fields. The production process of UHMWPE resin is similar to that of ordinary high-density polyethylene HDPE, and can be produced using the production technology of HDPE. The difference is that UHMW-PE production has no granulation process and the product is in powder form. The preparation method of ultra-high molecular weight polyethylene resin mainly uses Ziegler catalyst, triethyl aluminum as a co-catalyst, 60-120℃ saturated hydrocarbon as a dispersion medium, and ethylene is polymerized under certain temperature and pressure conditions to obtain products with different molecular weights. However, in the polyethylene production process, whether conventional low molecular weight polyethylene is mixed into UHMWPE resin or UHMWPE resin is mixed into conventional low molecular weight polyethylene, the quality of the product is significantly reduced. Therefore, high-quality UHMWPE resin is generally produced by intermittent method.

[0004] In addition, although ultra-high molecular weight polyethylene has excellent product performance, its difficult processing characteristics also restrict the application of ultra-high molecular weight polyethylene. This results in a small number of ultra-high molecular weight polyethylene users and low usage. For enterprises producing ultra-high molecular weight polyethylene, they often cannot produce at full capacity, and there is a considerable amount of idle production capacity. These idle capacities usually also need to produce ordinary polyethylene products. Ordinary polyethylene products generally require the resin density to be adjusted according to their use, but the equipment for producing ultra-high molecular weight polyethylene usually does not have this ability.

[0005] The prior art intermittent polyethylene production process has large fluctuations in polymerization rate and polymerization activity during the production process, resulting in a single polyethylene product with few varieties, and the density of the polyethylene resin is uncontrollable. In addition, it is difficult to directly inject comonomers into the reactor as used in the continuous polyethylene production process. The traditional slurry process uses traditional solvents such as hexane and solvent oil, and requires process steps such as centrifugation, filtration, and drying. Summary of the invention

[0006] The purpose of the present invention is to overcome the technical problems that the ultra-high molecular weight polyethylene prepared by the intermittent method in the prior art has a single product and few varieties, and the traditional slurry process uses traditional solvents such as hexane and solvent oil, and requires process steps such as centrifugation, filtration, and drying, and to provide a method for preparing polyethylene using intermittent production equipment.

[0007] The inventors of the present invention have found in experiments that, in the presence of propane and a catalyst, ethylene is contacted with a comonomer for polymerization reaction, and the density of the produced polyethylene can be adjusted. The density of the polyethylene product can be controlled on an intermittent process device for producing ultra-high molecular weight polyethylene, and medium- and high-density polyethylene products can be produced, thereby expanding the performance range of the polyethylene product. In addition, the present application uses propane as a suspension solvent to produce ultra-high molecular weight polyethylene. Compared with the traditional slurry process using traditional solvents such as hexane and solvent oil, there is no need for process steps such as centrifugation, filtration, and drying, and there is no need to use equipment such as a centrifuge and a dryer, thereby reducing production costs.

[0008] In order to achieve the above object, the present invention provides a method for preparing polyethylene using an intermittent production device, the method comprising:

[0009] In the presence of propane and a catalyst, ethylene and a comonomer are contacted in a polymerization reactor to carry out a polymerization reaction;

[0010] During the polymerization reaction, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to comonomer in the polymerization kettle, and the content ratio is transmitted to the comonomer feeding system; the comonomer feeding system adjusts the feed amount of the comonomer according to the content ratio to maintain the content ratio of ethylene to comonomer in the polymerization kettle within a predetermined range.

[0011] The technical solution of the present invention also has the following advantages:

[0012] (1) It can effectively solve the problem that intermittent ultra-high molecular weight polyethylene production equipment is difficult to produce ordinary polyethylene, increase the variety of products produced by intermittent ultra-high molecular weight polyethylene production equipment, and effectively improve the utilization rate and economic benefits of the production equipment.

[0013] (2) The present invention is used to transform the polypropylene production equipment of a small-scale polypropylene production plant to produce ultra-high molecular weight polyethylene, which is very suitable for meeting the needs of downstream manufacturers for small batches of goods. The price of ultra-high molecular weight polyethylene is relatively high, and the technical solution of the present invention can significantly improve the economic benefits of a small-scale polypropylene production plant. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic structural diagram of a system for preparing polyethylene using intermittent production equipment according to one embodiment of the present invention;

[0015] Figure 2 It is a schematic structural diagram of a control system for preparing polyethylene using intermittent production equipment according to an embodiment of the present invention.

[0016] Description of Reference Numerals

[0017] 100 polymerization kettle, 200 recovery tank, 300 gas storage equipment, 400 flash tank, 500 gas chromatograph, 600 control equipment DETAILED DESCRIPTION

[0018] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0019] In one aspect, the present invention provides a method for preparing polyethylene using an intermittent production device, which can adjust the density of the polyethylene resin. The method comprises:

[0020] In the presence of propane and a catalyst, ethylene and a comonomer are contacted in a polymerization reactor to carry out a polymerization reaction;

[0021] During the polymerization reaction, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to comonomer in the polymerization kettle, and the content ratio is transmitted to the comonomer feeding system; the comonomer feeding system adjusts the feed amount of the comonomer according to the content ratio to maintain the content ratio of ethylene to comonomer in the polymerization kettle within a predetermined range.

[0022] In some embodiments of the present invention, the comonomer is an α-olefin; preferably, the α-olefin is selected from at least one of butene, pentene, hexene, heptene, octene, nonene, decene, undecene, dodecene and isomers thereof, more preferably propylene and / or n-butene.

[0023] In some embodiments of the present invention, the propane raw material contains a comonomer, wherein the content of the comonomer in the propane raw material is lower than the content of the comonomer in the propane during the polymerization reaction.

[0024] In the present invention, the density of polyethylene is reduced by copolymerizing ethylene and comonomers, and the content ratio of ethylene to comonomers is stabilized by automatic feed control to prevent excessive soluble matter from forming due to excessive comonomer content, which may cause the kettle to stick during the flash evaporation process. The polyethylene resin that can be produced is medium-high density polyethylene with a density range of 0.926-0.97 g / cm 3 , preferably 0.93-0.97 g / cm 3 , and preferably 0.935-0.970 g / cm 3 The molar ratio of ethylene to comonomer (e.g., 1:0.04-0.06) is determined according to the copolymerization capacity of the catalyst so that the density of the polyethylene product reaches the predetermined requirement. It should be noted that in the present invention, the content ratio of ethylene to α-olefin refers to the molar ratio.

[0025] The amount of propane used in the present invention is determined according to the volume of the polymerization kettle used, and the general amount of feed is 40-90% of the volume of the polymerization kettle. Ethylene and comonomers are added continuously as the reaction proceeds, and the output of polyethylene in the kettle can be measured according to the consumption of ethylene and comonomers. The volume concentration of polyethylene in propane should be lower than 70% when discharging, preferably lower than 50%. Too high a concentration will make discharging difficult. For safety reasons, when discharging, the total volume of propane and polyethylene in the kettle should not exceed 95% of the volume of the polymerization kettle, preferably lower than 90%. The amount of catalyst fed is determined according to the activity of the catalyst and the output of a single kettle. The single kettle polymerization time is generally 0.5-12 hours according to the active life of the catalyst.

[0026] In some embodiments of the present invention, the contacting is carried out in the presence of hydrogen, and the amount of hydrogen used is such that the pressure in the system rises by 0.01-1.5 MPa (the pressure refers to the hydrogen partial pressure). The present invention does not limit the order of adding hydrogen, for example, the hydrogen can be added to the polymerization kettle before the propane and the catalyst are added to the polymerization kettle, or after the propane and the catalyst are added to the polymerization kettle and before the ethylene gas is introduced.

[0027] In some embodiments of the present invention, the polymerization reaction conditions include: a temperature of 50-90°C, preferably 60-80°C; a pressure of 2.3-3.8MPa, preferably 2.8-3.6MPa. In the present invention, the amount of ethylene relative to propane can control the pressure of the polymerization reaction system within the above range.

[0028] In some embodiments of the present invention, the method further comprises:

[0029] (a) After the polymerization reaction is completed, the pressure of the polymerization kettle is reduced to vaporize the unreacted ethylene, comonomer and propane in the polymerization kettle, and the obtained gas phase is condensed into liquid phase material through a recovery condenser, and the liquid phase material enters a recovery tank.

[0030] In some embodiments of the present invention, preferably, the vaporization pressure is 0.8-1.8 MPa, more preferably 1-1.5 MPa.

[0031] In some embodiments of the present invention, the method further comprises:

[0032] After step (a) is completed, the residual pressure in the polymerization reactor is used to spray the materials in the reactor into a flash tank for flash evaporation to obtain flash gas and polyethylene powder. Specifically, after the flash evaporation is completed, the flash tank is evacuated and replaced with nitrogen, and the replacement gas is also discharged from the flash tank into the gas storage device.

[0033] In some embodiments of the present invention, the flash evaporation conditions include: a pressure of 0-0.1 MPa.

[0034] In some embodiments of the present invention, the equipment used for sampling and analysis from the polymerization kettle is a gas chromatograph. Preferably, the sampling and analysis interval is less than 30 minutes, preferably less than 10 minutes, and more preferably less than 5 minutes.

[0035] In the present invention, the catalyst is a polyethylene catalyst, which can be any catalyst that can polymerize ethylene into high molecular weight polyethylene, and preferably a Ziegler-Natta catalyst is used.

[0036] According to the method of the present invention, the Ziegler-Natta catalyst contains: (1) a titanium-containing solid catalyst active component, whose main components are magnesium, titanium, halogen and internal electron donor; (2) an organic aluminum compound co-catalyst component; and (3) an optional external electron donor component.

[0037] Available solid catalysts can be purchased from Beijing Aoda Branch of Sinopec Catalyst Co., Ltd., such as BCE catalyst and CM catalyst.

[0038] The organic aluminum compound used as the co-catalyst component of the catalyst is preferably an alkyl aluminum compound, more preferably at least one selected from trialkyl aluminum (such as trimethyl aluminum, triethyl aluminum, triisobutyl aluminum, tri-n-butyl aluminum, trioctylaluminum, etc.), diethyl aluminum chloride, diisobutyl aluminum chloride, diethyl aluminum chloride, diisobutyl aluminum chloride, ethyl aluminum dichloride and ethyl aluminum dichloride.

[0039] The ratio of the titanium-containing solid catalyst active component to the organic aluminum compound co-catalyst component may be 0.1:25 to 0.1:1000 in terms of Ti / Al molar ratio.

[0040] In the present invention, reference Figure 1 and Figure 2 , the specific implementation methods of the present invention are as follows:

[0041] 1) Propane and a catalyst are put into a polymerization reactor 100;

[0042] 2) The polymerization kettle 100 is heated to a predetermined polymerization temperature, and ethylene gas and comonomers are introduced into the polymerization kettle 100 to carry out a polymerization reaction; during the reaction, a gas chromatograph 500 takes samples from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to comonomer in the polymerization kettle 100, and transmits the content ratio to a comonomer feeding system; a comonomer feeding control device 600 adjusts the comonomer feed amount according to the content ratio to maintain the content ratio of ethylene to comonomer in the polymerization kettle 100 within a predetermined range, thereby producing a polyethylene product with a predetermined density.

[0043] 3) After the polymerization reaction is completed, the pressure of the polymerization kettle is reduced to vaporize the unreacted ethylene, comonomer and propane in the polymerization kettle 100, and the obtained gas phase is condensed into a liquid phase material through a recovery condenser, and the liquid phase material enters the recovery tank 200;

[0044] 4) After the recovery is completed, the residual pressure in the polymerization kettle is used to spray the material in the kettle into the flash tank 400 to obtain flash gas and polyethylene powder. After the flash gas is discharged, the gas is transported to the gas storage device 300, and the flash tank is evacuated and replaced with nitrogen. The replacement gas is also transported to the gas storage device 300. The polyethylene powder is discharged from the bottom of the polymerization kettle 100 to obtain the polyethylene.

[0045] In addition, optionally, hydrogen is added to the polymerization kettle 100 in step 1), or before the ethylene gas is introduced in step 2). The amount of hydrogen used is such that the pressure in the system rises by 0.01-1.5 MPa (the pressure refers to the hydrogen partial pressure). In step 2), the device for heating and removing heat from the polymerization kettle 100 is a conventional device. A typical device for heating the polymerization kettle 100 is to pass hot water into the jacket of the polymerization kettle; and the device for removing heat from the polymerization kettle 100 is to pass cold water into the jacket of the polymerization kettle. In step 3), the liquid phase material that enters the recovery tank 200 after vaporization and condensation in the polymerization kettle 100 is mainly propane, containing a small amount of ethylene and comonomers, and can be used as a polymerization raw material again.

[0046] In the method of the present invention, in step 3, the gas in the flash tank 400 is discharged into the gas storage device 300. The gas evacuated and replaced with nitrogen in the flash tank 400 also enters the gas storage device 300. The gas in the gas storage device 300 is recovered or harmlessly treated according to the production plant equipment.

[0047] More specifically, in the present invention, step 1) feeds, usually first adding a measured amount of hydrogen to the polymerization kettle, preferably, the hydrogen introduced makes the pressure in the system 0.01-1.0MPa. Then the Ziegler Natta catalyst component is added to the polymerization kettle 100 with liquid propane; finally, the propane required by the process is supplemented. Step 2) The polymerization kettle 100 is heated, and hot water is sent to the jacket of the polymerization kettle 100 by a hot water pump for heating and heating, and the kettle temperature is raised from room temperature to the polymerization temperature. The polymerization temperature is 50-90°C, preferably 60-80°C. At this time, ethylene gas and comonomer are introduced into the polymerization kettle 100 to start the polymerization reaction. The pressure of the polymerization kettle 100 is controlled by ethylene, the polymerization rate rises to the predetermined polymerization rate, the hot water (gradually closed in the automatic state) is switched to circulating cooling water (automatically adjusting the opening of the circulating cooling water valve), the heat released by the polymerization reaction is removed, and the reaction temperature is controlled to be stable. The content ratio of ethylene to comonomer in the polymerization kettle 100 is obtained by sampling and analyzing from the polymerization kettle at regular intervals through the gas chromatograph 500; the comonomer feeding system adjusts the feeding amount of the comonomer according to the content ratio, so that the content ratio of ethylene to comonomer in the polymerization kettle is maintained within a predetermined range. In step 3), when the polymerization time reaches the requirement, or the polyethylene output reaches the requirement, the recovery system is turned on, and the unreacted ethylene, comonomer and propane are vaporized in the polymerization kettle 100, and the gas phase is condensed into liquid phase material through the condenser, and the liquid phase material enters the recovery tank 200. The liquid phase propane entering the recovery tank 200 after vaporization and condensation in the polymerization kettle contains ethylene and comonomer, and this liquid phase material can be used as a raw material again. After the recovery is completed, the material in the kettle is sprayed into the flash tank 400 with the residual pressure in the kettle, and the gas in the flash tank 400 is discharged into the gas storage device 300. The flash tank 400 is evacuated and replaced with nitrogen, and the replacement gas is also discharged from the flash tank 400 into the gas storage device 300. The gas in the gas storage device 300 is recovered or harmlessly treated according to the factory equipment. In step 4), when the combustible gas content in the flash tank 400 is qualified, the polyethylene powder is discharged from the flash tank 400 and enters the powder bin or is packaged. The powder product is tested for melt index and the like, and the product is graded according to the test results. Thus, a polyethylene powder product is obtained.

[0048] In the present invention, pressure refers to gauge pressure.

[0049] The present invention will be described in detail below through examples.

[0050] The experimental results in the examples were obtained according to the following test methods, which were all performed at room temperature unless otherwise specified:

[0051] The main catalyst was BCE catalyst, which was obtained from Beijing Aoda Branch of Sinopec Catalyst Co., Ltd.

[0052] The co-catalyst component is triethylaluminum, which is prepared with dehydrated hexane to a concentration of 0.35 mol / L.

[0053] Melt Flow Rate (MFR): measured according to GB / T 3682.1-2018 at 190°C and 2.16 kg load.

[0054] Density: Determined according to the method in GB / T 1033.2-2010.

[0055] Example 1

[0056] The polymerization reaction was carried out in a 5-liter intermittent liquid phase polymerization kettle. Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.4 MPa. 23.5 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid propane at 20°C. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 70°C, the gauge pressure of the polymerization kettle rose to 3.2 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.5 MPa. Propylene was added and the molar ratio of propylene to ethylene was controlled to be 0.06 according to the analysis value of gas chromatography (sampling and analysis every 3 minutes). The flow rate of cooling water was adjusted to maintain the polymerization reaction temperature at 70°C, thereby maintaining the polymerization rate. After the polymerization reaction was carried out for 150 minutes, the polymerization kettle was depressurized to recover propane in the kettle, so that the unreacted ethylene, propylene and propane were vaporized in the polymerization kettle, and the obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank. When the pressure drop of the polymerization kettle dropped to 1.5 MPa, the recovery was stopped. The gas and polyethylene powder in the polymerization kettle were discharged into the flash kettle for flash evaporation (pressure was 0.01 MPa gauge pressure), and the gas after flash evaporation was discharged into the gas cabinet, and the gas in the flash tank was replaced with nitrogen. Specifically, the flash kettle was vented to 0.01 MPa; the flash kettle was filled with nitrogen to 0.4 MPa, vented to normal pressure, and the replaced gas was also discharged into the gas cabinet. After nitrogen was filled and replaced 3 times, the powder in the kettle was discharged and weighed to obtain 414g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0057] Example 2

[0058] The polymerization reaction was carried out in a 5-liter intermittent liquid phase polymerization kettle. Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.6 MPa. 24.2 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid propane at 20°C. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 65°C, the gauge pressure of the polymerization kettle rose to 3 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.3 MPa. Propylene was added and the molar ratio of propylene to ethylene was controlled to be 0.05 according to the analysis value of gas chromatography (sampling and analysis every 3 minutes). The flow rate of cooling water was adjusted to maintain the polymerization reaction temperature at 65°C. After the polymerization reaction was carried out for 150 minutes, the polymerization kettle was depressurized to recover propane in the kettle, and the unreacted ethylene, propylene and propane were vaporized in the polymerization kettle. The obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank. When the pressure drop of the polymerization kettle dropped to 1.5 MPa, the recovery was stopped. The gas and polyethylene powder in the polymerization kettle were discharged into the flash kettle for flash evaporation (gauge pressure 0.01MPa), and the gas after flash evaporation was discharged into the gas cabinet, and the gas in the flash tank was replaced with nitrogen. Specifically, the flash kettle was vented to 0.01MPa; the flash kettle was filled with nitrogen to 0.4MPa, vented to normal pressure, and the replaced gas was also discharged into the gas cabinet. After nitrogen was filled and replaced 3 times, the powder in the kettle was discharged and weighed to obtain 315g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0059] Example 3

[0060] The polymerization reaction was carried out in a 5-liter intermittent liquid phase polymerization kettle. Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.2 MPa. 25.3 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid propane at 20°C. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 65°C, the gauge pressure of the polymerization kettle rose to 3 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.3 MPa. N-butene was added and the molar ratio of n-butene to ethylene was controlled to be 0.04 according to the analysis value of gas chromatography (sampling and analysis every 3 minutes). The flow rate of cooling water was adjusted to maintain the polymerization reaction temperature at 65°C. After the polymerization reaction was carried out for 150 minutes, the polymerization kettle was depressurized to recover propane in the kettle, so that the unreacted ethylene, n-butene and propane were vaporized in the polymerization kettle. The obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank. When the pressure drop of the polymerization kettle dropped to 1.5 MPa, the recovery was stopped. The gas and polyethylene powder in the polymerization kettle were discharged into the flash kettle for flash evaporation (gauge pressure 0.01MPa), and the gas after flash evaporation was discharged into the gas cabinet, and the gas in the flash tank was replaced with nitrogen. Specifically, the flash kettle was vented to 0.01MPa; the flash kettle was filled with nitrogen to 0.4MPa, vented to normal pressure, and the replaced gas was also discharged into the gas cabinet. After nitrogen replacement 3 times, the powder in the kettle was discharged and weighed to obtain 367g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0061] Example 4

[0062] The polymerization reaction was carried out in a 5-liter intermittent liquid phase polymerization kettle. Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.8 MPa. 21.9 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid propane at 20°C. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 60°C, the gauge pressure of the polymerization kettle rose to 3 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.5 MPa. N-butene was added and the molar ratio of n-butene to ethylene was controlled to be 0.06 according to the analysis value of gas chromatography (sampling and analysis every 3 minutes). The flow rate of cooling water was adjusted to maintain the polymerization reaction temperature at 60°C. After the polymerization reaction was carried out for 150 minutes, the polymerization kettle was depressurized to recover propane in the kettle, so that the unreacted ethylene, n-butene and propane were vaporized in the polymerization kettle. The obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank. When the pressure drop of the polymerization kettle dropped to 1.5 MPa, the recovery was stopped. The gas and polyethylene powder in the polymerization kettle were discharged into the flash kettle for flash evaporation (gauge pressure 0.01MPa), and the gas after flash evaporation was discharged into the gas cabinet, and the gas in the flash tank was replaced with nitrogen. Specifically, the flash kettle was vented to 0.01MPa; the flash kettle was filled with nitrogen to 0.4MPa, vented to normal pressure, and the replaced gas was also discharged into the gas cabinet. After nitrogen replacement 3 times, the powder in the kettle was discharged and weighed to obtain 292g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0063] Example 5

[0064] The polymerization reaction was carried out in a 5-liter intermittent liquid phase polymerization kettle. Hydrogen was added to the polymerization kettle to raise the kettle pressure by 1 MPa. 24.5 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid propane at 20°C. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 55°C, the gauge pressure of the polymerization kettle rose to 3 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.4 MPa. N-butene was added and the molar ratio of n-butene to ethylene was controlled to be 0.05 according to the analysis value of gas chromatography (sampling and analysis every 3 minutes). The flow rate of cooling water was adjusted to maintain the polymerization reaction temperature at 55°C. After the polymerization reaction was carried out for 150 minutes, the polymerization kettle was depressurized to recover propane in the kettle, and the unreacted ethylene, propylene and propane were vaporized in the polymerization kettle. The obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank. When the pressure drop of the polymerization kettle dropped to 1.5 MPa, the recovery was stopped. The gas and polyethylene powder in the polymerization kettle were discharged into the flash kettle for flash evaporation (gauge pressure 0.01MPa), and the gas after flash evaporation was discharged into the gas cabinet, and the gas in the flash tank was replaced with nitrogen. Specifically, the flash kettle was vented to 0.01MPa; the flash kettle was filled with nitrogen to 0.4MPa, vented to normal pressure, and the replaced gas was also discharged into the gas cabinet. After nitrogen replacement 3 times, the powder in the kettle was discharged and weighed to obtain 306g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0065] Comparative Example 1

[0066] Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.6MPa, and 23.5mg of BCE catalyst and 10mL of triethylaluminum solution were charged into the polymerization kettle with 2L of propane. The polymerization kettle was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 65°C, the gauge pressure of the polymerization kettle rose to 3MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.3MPa, and the polymerization reaction was started. The flow rate of cooling water was adjusted according to the temperature of the polymerization kettle to maintain the polymerization temperature. After the polymerization reaction was carried out for 150 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, the unreacted ethylene and propane were vaporized in the polymerization kettle, and the obtained gas phase was condensed into liquid phase material through the recovery condenser, and the liquid phase material entered the recovery tank; when the kettle pressure dropped to 1.5MPa, the recovery was stopped. The gas in the kettle and the polyethylene powder were discharged into the flash tank (gauge pressure 0.01MPa), the valve between the flash tank and the gas storage device was opened, the gas in the flash tank was discharged into the gas storage device, the pressure of the flash tank was reduced to 0.01MPa, and then nitrogen was filled to 0.4MPa, and the gas after nitrogen replacement was also discharged into the gas storage device, and after nitrogen replacement was performed 3 times, the powder in the tank was released and weighed to obtain 272g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0067] Table 1

[0068]

[0069] It can be seen from the results in Table 1 that Examples 1-5 can obtain polyethylenes of different densities by adopting the technical solution of the present application, which indicates that the method of the present invention has a significant effect on the density of polyethylene, thereby effectively improving the utilization rate and economic benefits of the production equipment.

[0070] In addition, the present invention uses propane as a suspension solvent to produce polyethylene. Compared with the traditional slurry process using traditional solvents such as hexane and solvent oil, it does not require process steps such as centrifugation, filtration, and drying, and does not require the use of equipment such as centrifuges and dryers, thereby reducing production costs.

[0071] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing polyethylene using an intermittent production device, characterized in that: The method includes: In the presence of propane and a catalyst, ethylene and a comonomer are contacted in a polymerization reactor to carry out a polymerization reaction; Wherein, during the polymerization reaction, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to comonomer in the polymerization kettle, and the content ratio is transmitted to the comonomer feeding system; the comonomer feeding system adjusts the feeding amount of the comonomer according to the content ratio to maintain the content ratio of ethylene to comonomer in the polymerization kettle within a predetermined range; the comonomer is α-olefin; The method further comprises: (a) after the polymerization reaction is completed, reducing the pressure of the polymerization kettle to vaporize the unreacted ethylene, comonomer and propane in the polymerization kettle, and condensing the obtained gas phase into a liquid phase material through a recovery condenser, and the liquid phase material enters a recovery tank; After step (a) is completed, the material in the polymerization kettle is sprayed into a flash tank for flash evaporation by utilizing the residual pressure in the polymerization kettle to obtain flash gas and polyethylene powder; Wherein, the vaporization pressure is 0.8-1.8 MPa; the flash evaporation conditions include: the pressure is 0-0.1 MPa.

2. The method according to claim 1, wherein: The α-olefin is at least one selected from butene, pentene, hexene, heptene, octene, nonene, decene, undecene, dodecene and isomers thereof.

3. The method according to claim 2, wherein: The α-olefin is selected from propylene and / or n-butene.

4. The method according to any one of claims 1 to 3, wherein: The contacting is carried out in the presence of hydrogen, and the amount of hydrogen used is such that the pressure in the system increases by 0.01-1.5 MPa; And / or, hydrogen is added into the polymerization kettle before the propane and the catalyst are added into the polymerization kettle, or is added into the polymerization kettle after the propane and the catalyst are added into the polymerization kettle and before the ethylene gas is introduced into the polymerization kettle.

5. The method according to any one of claims 1 to 3, wherein: The polymerization reaction conditions include: temperature of 50-90° C.; pressure of 2.3-3.8 MPa.

6. The method according to claim 5, wherein: The polymerization reaction conditions include: temperature of 60-80° C.; pressure of 2.8-3.6 MPa.

7. The method according to claim 1, wherein: The vaporization pressure is 1-1.5 MPa.

8. The method according to any one of claims 1 to 3, 6 and 7, wherein: The equipment used for sampling and analysis from the polymerization kettle is a gas chromatograph.

9. The method according to claim 1, wherein: The sampling and analysis interval is less than 30 minutes.

10. The method according to claim 9, wherein: The sampling and analysis interval is less than 10 minutes.

11. The method according to claim 8, wherein: The sampling and analysis interval is less than 30 minutes.

12. The method according to claim 11, wherein: The sampling and analysis interval is less than 10 minutes.

13. The method according to any one of claims 1-3, 6, 7, 9-12, wherein: The catalyst is a Ziegler-Natta catalyst, which contains: (1) a titanium-containing solid catalyst active component, which contains magnesium, titanium, halogen and an internal electron donor; (2) an organic aluminum compound co-catalyst component; and (3) an optional external electron donor component.

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