Method for batch preparation of polyethylene

Abstract

The present invention relates to the field of resins, and discloses a method for preparing polyethylene intermittently, which can adjust the density of polyethylene resin, including: the method includes: in the presence of carbon four saturated alkanes and catalysts, ethylene and alpha-olefins are contacted in a polymerization kettle for polymerization reaction; wherein, during the polymerization reaction, sampling and analysis are performed from the polymerization kettle at regular intervals to obtain the content ratio of ethylene to alpha-olefins in the polymerization kettle, and the content ratio is transmitted to the alpha-olefin feeding system; the alpha-olefin feeding system adjusts the feed amount of alpha-olefins according to the content ratio, so that the content ratio of ethylene to alpha-olefins in the polymerization kettle is maintained within a predetermined range; wherein, alpha-olefins do not include ethylene. The method can effectively solve the problem that it is difficult to adjust the density of polyethylene resin in an intermittent production ultra-high molecular weight polyethylene device, and improve the utilization rate and economic benefits of the production device.

Description

Technical Field

[0001] The invention relates to the field of resins, and in particular to a method for intermittently preparing polyethylene. Background Art

[0002] 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 with UHMWPE resin or UHMWPE resin is mixed with conventional low molecular weight polyethylene, the quality of the product is significantly reduced. Therefore, high-quality UHMWPE resin is generally produced by intermittent method.

[0003] 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 companies producing ultra-high molecular weight polyethylene, they often cannot produce at full capacity. These idle production 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.

[0004] The existing intermittent polyethylene production process has large fluctuations in polymerization rate and polymerization activity during the production process, making it difficult to directly inject comonomers into the reactor as used in the continuous polyethylene production process. Therefore, a control method that can be used in intermittent polyethylene production is needed to control the density of polyethylene resin and expand the performance range of polyethylene products. Summary of the invention

[0005] 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 intermittently.

[0006] The inventors of the present invention have found in experiments that, in the presence of a carbon tetra saturated alkane and a catalyst, ethylene is contacted with an alpha-olefin for polymerization reaction, and the density of the polyethylene produced 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 a carbon tetra saturated alkane 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.

[0007] In order to achieve the above object, the present invention provides a method for intermittently preparing polyethylene, which comprises:

[0008] In the presence of a C4 saturated alkane and a catalyst, ethylene and α-olefin are contacted in a polymerization reactor to carry out a polymerization reaction;

[0009] In which, during the polymerization reaction process, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to α-olefin in the polymerization kettle, and the content ratio is transmitted to the α-olefin feeding system; the α-olefin feeding system adjusts the feed amount of α-olefin according to the content ratio to maintain the content ratio of ethylene to α-olefin in the polymerization kettle within a predetermined range; wherein α-olefin does not include ethylene.

[0010] The method for intermittently preparing polyethylene provided by the present invention can effectively solve the problem that an intermittent ultra-high molecular weight polyethylene production device is difficult to produce ordinary polyethylene, can increase the variety of products of the intermittent ultra-high molecular weight polyethylene production device, and effectively improve the utilization rate and economic benefits of the production device.

[0011] In addition, the 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 technology of the invention can significantly improve the economic benefits of a small-scale polypropylene production plant. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic structural diagram of a system for intermittently preparing polyethylene according to one embodiment of the present invention;

[0013] Figure 2 It is a schematic structural diagram of a control system for intermittently preparing polyethylene according to an embodiment of the present invention.

[0014] Description of Reference Numerals

[0015] 100 polymerization kettle, 200 recovery tank, 300 gas storage equipment, 400 flash tank, 500 vacuum equipment, 600 gas chromatograph, 700 control equipment DETAILED DESCRIPTION

[0016] 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.

[0017] One aspect of the present invention provides a method for intermittently preparing polyethylene, which can adjust the density of polyethylene resin, and the method comprises:

[0018] In the presence of a C4 saturated alkane and a catalyst, ethylene and α-olefin are contacted in a polymerization reactor to carry out a polymerization reaction;

[0019] In which, during the polymerization reaction process, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to α-olefin in the polymerization kettle, and the content ratio is transmitted to the α-olefin feeding system; the α-olefin feeding system adjusts the feed amount of α-olefin according to the content ratio to maintain the content ratio of ethylene to α-olefin in the polymerization kettle within a predetermined range; wherein α-olefin does not include ethylene.

[0020] In some embodiments of the present invention, 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.

[0021] In some embodiments of the present invention, the raw material of the carbon tetra saturated alkane contains α-olefin, wherein the content of α-olefin in the raw material of the carbon tetra saturated alkane is lower than the content of α-olefin in the carbon tetra saturated alkane in the polymerization reaction. The carbon tetra saturated alkane can be various carbon tetra saturated alkanes commonly found in the art, such as n-butane and / or isobutane.

[0022] In the present invention, the density of polyethylene is reduced by copolymerizing ethylene and α-olefin, and the content ratio of ethylene and α-olefin is stabilized by automatic feed control to prevent excessive soluble matter from being caused by excessive α-olefin 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 0.935-0.97 g / cm 3The molar ratio of ethylene to α-olefin (e.g., 1:0.06-0.08) is determined according to the copolymerization capacity of the catalyst so that the density of the polyethylene product reaches a predetermined requirement. It should be noted that in the present invention, the content ratio of ethylene to α-olefin refers to the molar ratio.

[0023] The amount of the carbon tetra saturated alkane 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 α-olefin 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 α-olefin. When discharging, the volume concentration of polyethylene in the carbon tetra saturated alkane should be lower than 70%, preferably lower than 50%, and too high a concentration will make discharging difficult. For safety reasons, when discharging, the total volume of the carbon tetra saturated alkane 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.

[0024] 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 C4 saturated alkane and the catalyst are added to the polymerization kettle, or after the C4 saturated alkane and the catalyst are added to the polymerization kettle and before the ethylene gas is introduced.

[0025] In some embodiments of the present invention, the polymerization reaction conditions include: a temperature of 50-100° C., preferably 60-90° C. A pressure of 2.3-3.8 MPa, preferably 2.8-3.6 MPa. In the present invention, the amount of ethylene relative to the carbon tetrasaturated alkane can control the pressure of the polymerization reaction system within the above range.

[0026] In some embodiments of the present invention, the method further comprises:

[0027] (a) After the polymerization reaction is completed, the pressure of the polymerization kettle is reduced to vaporize the unreacted ethylene, α-olefin and C4 saturated alkane 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.

[0028] In some embodiments of the present invention, preferably, the vaporization temperature is 40-100°C, preferably 50-90°C.

[0029] In some embodiments of the present invention, the method further comprises:

[0030] 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. Preferably, the flash evaporation pressure is 0-0.1 MPa.

[0031] In some embodiments of the present invention, the device used for sampling and analysis from the polymerization reactor is a gas chromatograph. The sampling and analysis interval is less than 30 minutes, preferably less than 10 minutes, and more preferably less than 5 minutes.

[0032] 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.

[0033] 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.

[0034] Available solid catalysts can be purchased from Beijing Aoda Branch of Sinopec Catalyst Co., Ltd., such as BCE catalyst and CM catalyst.

[0035] 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.

[0036] 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.

[0037] In the present invention, reference Figure 1 and Figure 2 , the specific implementation methods of the present invention are as follows:

[0038] 1) putting a C4 saturated alkane and a catalyst into a polymerization reactor 100;

[0039] 2) The polymerization kettle 100 is heated to a predetermined polymerization temperature, and ethylene gas and α-olefin are introduced into the polymerization kettle 100 to carry out a polymerization reaction; during the reaction, a gas chromatograph 600 takes samples from the reactor for analysis at regular intervals to obtain a content ratio of ethylene to α-olefin in the polymerization kettle 100, and transmits the content ratio to an α-olefin feeding system; an α-olefin feeding control device 700 adjusts the feed amount of α-olefin according to the content ratio, so that the content ratio of ethylene to α-olefin in the polymerization kettle 100 is maintained within a predetermined range, thereby producing a polyethylene product with a predetermined density;

[0040] 3) After the polymerization reaction is completed, the pressure of the polymerization kettle is reduced to vaporize the unreacted ethylene, α-olefin and C4 saturated alkane 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;

[0041] 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 polyethylene powder is discharged from the bottom of the polymerization kettle 100 to obtain the polyethylene.

[0042] In addition, optionally, hydrogen is introduced into the polymerization kettle 100 in step 1), or before the ethylene gas is introduced into the polymerization kettle 100 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 conventional equipment. 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 C4 saturated alkanes, containing a small amount of ethylene and α-olefins, and can be used again as a polymerization raw material.

[0043] In step 3 of the method of the present invention, 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.

[0044] More specifically,

[0045] The feeding process of step 1) of the present invention is as follows: usually, a measured amount of hydrogen is first added to the polymerization kettle; then the Ziegler-Natta catalyst component is added to the polymerization kettle 100 using liquid C4 saturated alkane; and finally, the C4 saturated alkane required by the process is added.

[0046] The specific process of step 2) of the present invention is as follows: the polymerization kettle 100 is heated, and hot water is delivered 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 50-100°C, preferably 60-90°C. At this time, ethylene gas and α-olefin are introduced into the polymerization kettle 100 to start the polymerization reaction. The pressure of the polymerization kettle 100 is controlled by ethylene, and the pressure of the polymerization reaction is 2.3-3.8MPa, preferably 2.8-3.6MPa. When the polymerization temperature rises to a 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 gas chromatograph 600 is used to take samples from the reactor at regular intervals for analysis to obtain the content ratio of ethylene to α-olefin in the polymerization kettle 100; the α-olefin feed system adjusts the feed amount of α-olefin according to the content ratio, so that the content ratio of ethylene to α-olefin in the reactor is maintained within a predetermined range.

[0047] The specific process of step 3) of the present invention is as follows: after the polymerization reaction is completed (usually when the polymerization time reaches the requirement or the polyethylene output reaches the requirement), the unreacted ethylene and carbon four saturated alkanes in the system are vaporized by reducing the pressure of the polymerization kettle 100 to achieve the first step of separation and recovery. During the vaporization process in the polymerization kettle 100, the vaporization rate is controlled, and hot water is introduced into the jacket of the polymerization kettle to control the temperature of the polymerization kettle to be higher than 40°C, preferably higher than 45°C, and more preferably higher than 50°C; at the same time, the temperature of the polymerization kettle is lower than 100°C, preferably lower than 90°C. The lowering of the pressure of the polymerization kettle 100 is achieved by opening the valve connecting the polymerization kettle 100 and the recovery system (i.e., opening the recovery system), and the recovery system includes the recovery condenser and the recovery tank 200. The recovery condenser can use circulating cooling water at 20-25°C, and the pressure of the recovery tank is controlled to be 0.2-0.5MPa by discharging non-condensable gas. The discharged non-condensable gas is mainly ethylene and hydrogen, which can be uniformly recovered and used by the factory. Usually, the recovery tank 200 is placed at room temperature. In the initial recovery stage, the gas phase in the polymerization kettle 100 continuously enters the recovery tank 200 from the polymerization kettle 100. After equilibrium, the recovery is completed. At this time, there is still residual pressure in the polymerization kettle 100. Preferably, the residual pressure in the polymerization kettle 100 is 0.2-0.5 MPa.

[0048] It should be noted that the liquid phase material entering the recovery tank 200 after vaporization and condensation in the polymerization reactor 100 contains C4 saturated alkanes, ethylene and α-olefins, and this liquid phase material can be used as a raw material again.

[0049] Step 4) of the present invention needs to discharge the flash gas to reduce the adsorption amount of the combustible gas in the polyethylene powder, so as to realize the second step of separation and recovery. The specific method of discharging the flash gas is as follows: first, the pressure is released to the gas storage device 300: the valve between the flash tank 400 and the gas storage device 300 is opened, so that the gas in the flash tank 400 is discharged into the gas storage device 300, and the pressure of the flash tank 400 is reduced to the pressure of the gas storage device 300; then, the vacuum device 500 is used to further reduce the combustible gas in the polyethylene powder, and the extracted gas also enters the gas storage device 300. The gas in the gas storage device 300 is recovered according to the equipment of the small body polypropylene production plant. The vacuum time is based on the qualified combustible gas content in the flash tank 400. When the combustible gas content in the flash tank 400 is qualified, nitrogen is filled into the flash tank 400, and the polyethylene powder is discharged from the flash tank 400 and enters the powder bin or is packaged. After that, the powder product can be tested for melt index, melt mass flow rate, etc., and the product is graded according to the test results.

[0050] In the present invention, pressure refers to gauge pressure.

[0051] The present invention will be described in detail below through examples.

[0052] The experimental results in the examples were obtained according to the following test methods, which were all performed at room temperature unless otherwise specified:

[0053] The main catalyst was BCE catalyst, which was obtained from Beijing Aoda Branch of Sinopec Catalyst Co., Ltd.

[0054] The co-catalyst component is triethylaluminum, which is prepared with dehydrated hexane to a concentration of 0.35 mol / L.

[0055] Isobutane contains a small amount of other tetra-saturated alkanes, and its purity is greater than 99 mol%.

[0056] Melt Flow Rate (MFR): measured according to GB / T 3682.1-2018 at 190°C and 2.16 kg load.

[0057] Density: Determined according to the method described in GB / T 1033.2-2010.

[0058] The volume of the polymerization kettle is 5 L, and a jacket (not shown) is provided on the outside. The recovery condenser uses circulating cooling water at 20-25°C.

[0059] Example 1

[0060] 15.1 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of liquid isobutane. The reactor 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 0.87 MPa. Hydrogen was added to the polymerization kettle to raise the pressure to 2.07 MPa. Ethylene was added to raise the pressure to 3.27 MPa. Propylene was added and the molar ratio of gas phase propylene and 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 according to the temperature of the polymerization kettle to maintain the polymerization temperature. After the polymerization reaction was carried out for 60 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, and the unreacted ethylene and isobutane 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. Control the depressurization speed and pass hot water into the jacket of the polymerization kettle to keep the temperature in the kettle at 50°C. When the kettle pressure drops to 0.32MPa, stop recycling. Discharge the gas and polyethylene powder in the kettle into the flash tank for flash evaporation (gauge pressure 0.01MPa), open the valve between the flash tank and the gas cabinet, and discharge the gas in the flash tank into the gas cabinet; the pressure of the flash tank drops to normal pressure, turn on the vacuum pump to evacuate the flash tank to -0.08MPa for 60 minutes to remove the isobutane adsorbed by the polyethylene powder in the flash tank. Then fill the flash tank with nitrogen to normal pressure, release the powder in the tank and weigh it, and obtain 242g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0061] Example 2

[0062] 21.4 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with a mixture of 2 liters of liquid isobutane at 20°C. The reactor was heated by the jacket hot water of the polymerization kettle. When the temperature of the polymerization kettle reached 70°C, the gauge pressure of the polymerization kettle rose to 1.1 MPa. Hydrogen was added to the polymerization kettle to raise the pressure to 2.3 MPa. Ethylene was added to raise the pressure to 3 MPa. Propylene was added and the molar ratio of gas phase propylene and ethylene was controlled to be 0.07 according to the analysis value of the chromatography (sampling and analysis every 3 minutes). 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 60 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, and the unreacted ethylene and isobutane 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. Control the depressurization speed and introduce hot water into the jacket of the polymerization kettle to keep the temperature in the kettle at 55°C. When the kettle pressure drops to 0.32MPa, stop recycling. Discharge the gas and polyethylene powder in the kettle into the flash tank for flash evaporation (gauge pressure 0.01MPa), open the valve between the flash tank and the gas cabinet, and discharge the gas in the flash tank into the gas cabinet; the pressure of the flash tank drops to normal pressure, turn on the vacuum pump to evacuate the flash tank to -0.08MPa for 60 minutes to remove the isobutane adsorbed by the polyethylene powder in the flash tank. Then fill the flash tank with nitrogen to normal pressure, release the powder in the tank and weigh it, and obtain 293g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0063] Example 3

[0064] 23.1 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with a mixture of 2 liters of liquid isobutane at 20°C. The reactor was heated by the jacket hot water of the polymerization kettle. When the temperature of the polymerization kettle reached 70°C, the gauge pressure of the polymerization kettle rose to 1.1 MPa. Hydrogen was added to the polymerization kettle to raise the pressure to 2.3 MPa. Ethylene was added to raise the pressure to 3 MPa. n-butene was added and the molar ratio of gas phase n-butene to ethylene was controlled to be 0.07 according to the chromatographic analysis value (sampling and analysis every 3 minutes). 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 60 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, and the unreacted ethylene and isobutane 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. Control the depressurization speed and introduce hot water into the jacket of the polymerization reactor to keep the temperature in the reactor at 55°C. When the reactor pressure drops to 0.32MPa, stop recovery. Discharge the gas and polyethylene powder in the reactor into the flash tank for flash evaporation (gauge pressure 0.01MPa), open the valve between the flash tank and the gas cabinet, and discharge the gas in the flash tank into the gas cabinet; the pressure of the flash tank drops to normal pressure, turn on the vacuum pump to evacuate the flash tank to -0.08MPa for 60 minutes to remove the isobutane adsorbed by the polyethylene powder in the flash tank. Then fill the flash tank with nitrogen to normal pressure, release the powder in the tank and weigh it, and obtain 317g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0065] Example 4

[0066] 18.6 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with a mixture of 2 liters of liquid isobutane at 20°C. The reactor was heated by the jacket hot water of the polymerization kettle. When the temperature of the polymerization kettle reached 70°C, the gauge pressure of the polymerization kettle rose to 1.5 MPa. Hydrogen was added to the polymerization kettle to raise the pressure to 2.3 MPa. Ethylene was added to raise the pressure to 3 MPa. N-butene was added and the molar ratio of n-butene to ethylene in the gas phase was controlled to be 0.07 according to the analysis value of the chromatography (sampling and analysis every 3 minutes). 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 60 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, and the unreacted ethylene and isobutane 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. Control the depressurization speed and introduce hot water into the jacket of the polymerization reactor to keep the temperature in the reactor at 55°C. When the reactor pressure drops to 0.32MPa, stop recycling. Discharge the gas and polyethylene powder in the reactor into the flash tank for flash evaporation (gauge pressure 0.01MPa), open the valve between the flash tank and the gas cabinet, and discharge the gas in the flash tank into the gas cabinet; the pressure of the flash tank drops to normal pressure, turn on the vacuum pump to evacuate the flash tank to -0.08MPa for 60 minutes to remove the isobutane adsorbed by the polyethylene powder in the flash tank. Then fill the flash tank with nitrogen to normal pressure, release the powder in the tank and weigh it, and obtain 277g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0067] Example 5

[0068] 17.4 mg of BCE catalyst and 10 mL of triethylaluminum solution were charged into the polymerization kettle with a mixture of 2 liters of liquid isobutane at 20°C. The reactor was heated by hot water in the jacket of the polymerization kettle. When the temperature of the polymerization kettle reached 80°C, the gauge pressure of the polymerization kettle rose to 1.7 MPa. Hydrogen was added to the polymerization kettle to raise the pressure to 1.9 MPa. Ethylene was added to raise the pressure to 3 MPa. N-butene was added and the molar ratio of n-butene to ethylene in the gas phase was controlled to be 0.07 according to the analysis value of the chromatography (sampling and analysis every 3 minutes). 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 60 minutes, the valve connecting the polymerization kettle and the recovery system was opened, the polymerization kettle was depressurized, and the unreacted ethylene and isobutane 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. Control the depressurization speed and introduce hot water into the jacket of the polymerization reactor to keep the temperature in the reactor at 55°C. When the reactor pressure drops to 0.32MPa, stop recycling. The gas in the reactor and the polyethylene powder are discharged into the flash tank for flash evaporation (gauge pressure 0.01MPa). The valve between the flash tank and the gas cabinet is opened to discharge the gas in the flash tank into the gas cabinet; the pressure of the flash tank is reduced to normal pressure, and the vacuum pump is turned on to evacuate the flash tank to -0.08MPa for 60 minutes to remove the isobutane adsorbed by the polyethylene powder in the flash tank. Then fill the flash tank with nitrogen to normal pressure, release the powder in the tank and weigh it, and obtain 283g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0069] Comparative Example 1

[0070] Hydrogen was added to the polymerization kettle to raise the kettle pressure by 0.6 MPa, and 25.7 mg of BCE catalyst and 10.0 mL of triethylaluminum solution were charged into the polymerization kettle with 2 liters of isobutane. The reactor was heated by the jacket hot water of the polymerization kettle. When the temperature of the polymerization kettle reached 65°C, the gauge pressure of the polymerization kettle rose to 3.0 MPa. Ethylene was added to raise the pressure of the polymerization kettle to 3.3 MPa, and the polymerization reaction began. 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 isobutane 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 1.5 MPa, the recovery was stopped. The gas in the kettle and the polyethylene powder were discharged into the flash tank (gauge pressure 0.01MPa), and the valve between the flash tank and the gas storage device was opened to discharge the gas in the flash tank into the gas storage device; the pressure of the flash tank was reduced to normal pressure, and then nitrogen was filled to 0.4MPa, and the gas after nitrogen replacement was also discharged into the gas storage device. After nitrogen replacement for 3 times, the powder in the tank was released and weighed to obtain 285g of polyethylene powder. The powder was tested, and the results are shown in Table 1.

[0071] Table 1

[0072]

[0073] 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.

[0074] In addition, the present invention uses tetra-carbon saturated alkanes as suspension solvents 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.

[0075] 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 intermittently preparing polyethylene, characterized in that: The method includes: In the presence of a C4 saturated alkane and a catalyst, ethylene and α-olefin are contacted in a polymerization reactor to carry out a polymerization reaction; In the polymerization process, samples are taken from the polymerization kettle at regular intervals for analysis to obtain the content ratio of ethylene to α-olefin in the polymerization kettle, and the content ratio is transmitted to the α-olefin feeding system; the α-olefin feeding system adjusts the feed amount of α-olefin according to the content ratio to maintain the content ratio of ethylene to α-olefin in the polymerization kettle within a predetermined range; wherein the α-olefin does not include ethylene; The method further comprises: (a) after the polymerization reaction is completed, reducing the pressure of the polymerization kettle to vaporize the unreacted ethylene, α-olefin and C4 saturated alkane 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 temperature is 40-100°C; the flash evaporation conditions include: the pressure is 0-0.1MPa.

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 adding the C4 saturated alkane and the catalyst into the polymerization kettle, or is added into the polymerization kettle after adding the C4 saturated alkane and the catalyst into the polymerization kettle and before introducing ethylene gas.

5. The method according to any one of claims 1 to 3, wherein: The polymerization reaction conditions include: temperature of 50-100° C.; pressure of 2.3-3.8 MPa.

6. The method according to claim 5, wherein: The polymerization reaction conditions include: temperature of 60-90° C.; pressure of 2.8-3.6 MPa.

7. The method according to claim 1, wherein: The vaporization temperature is 50-90°C.

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 any one of claims 1 to 3, 6, 7, 9 and 10, 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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