Method for preparing polyethylene by using hexene as comonomer through slurry method and high-molecular-weight polyethylene

Abstract

The invention discloses a method for preparing polyethylene by using hexene as a comonomer through a slurry method and high-molecular-weight polyethylene. Monomer ethylene and comonomer 1-hexene are used as raw materials; uniformly mixing a catalyst and supercritical ethane fluid to prepare a catalyst solvent; raw materials, a catalyst solvent, hydrogen and supercritical ethane fluid are continuously added into a first stirring reaction kettle for a prepolymerization reaction, slurry discharged from the first stirring reaction kettle enters a second stirring reaction kettle for a polymerization reaction, and after polymerization is completed, the slurry is subjected to post-treatment to obtain a finished product. According to the method for preparing the polyethylene by the slurry method by taking the hexene as the comonomer, the melt index, the density and the molecular weight of the product can be improved by the supercritical ethane, and the impact resistance of the material is enhanced.

Description

Technical Field

[0001] The invention relates to the field of new material preparation, in particular to a slurry method for preparing polyethylene. Background Art

[0002] Polyethylene (PE), as one of the world's largest-volume plastic materials, is widely used in packaging, construction, agriculture, automobile manufacturing and other fields. Traditional polyethylene production methods mainly include slurry method, gas phase method and solution method, among which slurry method is widely used due to its wide range of applications and relatively low equipment requirements.

[0003] In the traditional slurry polymerization process, since hexene is a liquid and ethylene is a gas, when polymerized by the slurry method, the two have poor miscibility, especially in large-scale autoclave reactors, the utilization rate of the catalyst active center is uneven, and ethylene polymerization is a strong exothermic reaction, and the problem of heat removal efficiency also seriously affects the polymerization rate and the molecular weight distribution of the product. Especially in large-scale reactors, catalyst particles are easy to agglomerate, resulting in uneven utilization of active centers, affecting the polymerization rate and the molecular weight distribution of the product. The viscosity of the polymer slurry is high, which increases the difficulty of subsequent separation, washing and drying. It is difficult to adjust the key performance indicators of the polymer, such as density, crystallinity, and melt index, especially in the production of high-performance, multifunctional polyethylene products, it is difficult to achieve precise performance control through simple process parameter adjustment. Summary of the invention

[0004] In order to solve the defects in the prior art, the present invention provides a method for preparing polyethylene by a slurry process using hexene as a comonomer and high molecular weight polyethylene.

[0005] The present invention first provides a method for preparing polyethylene by a slurry process using hexene as a comonomer, comprising the following steps:

[0006] The monomer ethylene and the comonomer 1-hexene are used as raw materials;

[0007] The catalyst and the supercritical ethane fluid are uniformly mixed to obtain a catalyst solvent;

[0008] The raw materials, catalyst solvent, hydrogen and supercritical ethane fluid are continuously added into the first stirred reactor in proportion to carry out a prepolymerization reaction, and the temperature and pressure of the first stirred reactor are controlled to be above the supercritical conditions of ethane and ethylene;

[0009] The slurry discharged from the first stirred reactor enters the second stirred reactor for polymerization reaction, and the temperature and pressure of the second stirred reactor are controlled above the supercritical conditions of ethane and ethylene, and the pressure is lower than that of the first stirred reactor;

[0010] After the polymerization is completed, the slurry discharged from the second stirred reactor is post-treated to obtain a finished product.

[0011] Further, the following steps are included:

[0012] S1 uses monomer ethylene and comonomer 1-hexene in a mass ratio of 80-100:1-10 as raw materials;

[0013] S2. stirring the catalyst and the supercritical ethane fluid in a mass ratio of 1-7:380-900 to form a catalyst solvent, wherein the catalyst is a mixture of TiCl4 and triethylaluminum in a mass ratio of 1:1-4;

[0014] S3: continuously adding raw materials, catalyst solvent, hydrogen and supercritical ethane fluid into the first stirred reactor in a mass ratio of 81-110:381-907:0.2-1:5-50 to carry out a prepolymerization reaction, wherein the temperature in the first stirred reactor is controlled at 32.2-40° C. and the pressure is controlled at 5.6-6.5 MPa;

[0015] S4 The slurry discharged from the first stirred reactor enters the second stirred reactor for polymerization reaction, and the temperature in the second stirred reactor is controlled at 70-85°C and the pressure is controlled at 5.1-5.5MPa;

[0016] After the completion of S5 polymerization, the slurry discharged from the second stirred reactor is separated by flash evaporation, and the separated gas phase is sent to the recovery unit, and the solid phase polymer enters the degassing and drying stage;

[0017] The polymer powder after S6 drying is sieved and homogenized to obtain the finished product.

[0018] Preferably, in step S1, the ethylene and 1-hexene are refined high-purity ethylene and 1-hexene, which are first subjected to oxygen, carbon monoxide, water and carbon dioxide removal steps to remove oxygen, carbon monoxide, water and carbon dioxide. As a specific implementation, in actual industrial production, the raw materials are respectively passed through a deoxygenation tower, a CO removal tower, a dehydration tower and a carbon dioxide removal tower to remove corresponding impurities.

[0019] Preferably, in step S2, TiCl4, triethylaluminum and supercritical ethane fluid are evenly mixed in a catalyst preparation tank to obtain a fully dispersed and diluted catalyst solvent. Step S2 is carried out in an inert environment (such as nitrogen), and the catalyst preparation tank is maintained at supercritical conditions of ethane (temperature ≥32.2°C, pressure ≥4.87MPa).

[0020] Preferably, in step S3, at the initial stage of the reaction, a certain amount of supercritical ethane fluid is first added as the bottom liquid of the reactor, and then raw materials, catalyst solvent, hydrogen, and supercritical ethane fluid are continuously added;

[0021] Preferably, in step S3, each raw material is added from the bottom of the first stirred reactor, and the slurry after the reaction is discharged from the upper slurry of the first stirred reactor;

[0022] Preferably, in step S4, the slurry discharged from the first stirred reactor is introduced into the second stirred reactor from the lower part, the slurry after the reaction is discharged from the upper part of the second stirred reactor, and the gas component is discharged from the top of the second stirred reactor;

[0023] Preferably, the slurry after the polymerization in step S5 is subjected to gas-solid separation by decompression operation in a flash tank;

[0024] The present invention also discloses a high molecular weight polyethylene prepared according to the above preparation method, which has a molecular weight of about 850,000 to 950,000, a melt index of 0.27 to 0.32 g / 10 min, and a density of 0.92 to 0.93 g / cm 3 .

[0025] The beneficial effects of the present invention are:

[0026] The present invention uses 1-hexene as a comonomer and polymerizes ethylene. By adding unsaturated olefin hexene with a higher carbon number, the density of the polymer is effectively reduced, the melt flow index of the product is controlled, and the mechanical processing performance is improved. Hexene is a liquid and ethylene is a gas. When polymerized by a slurry method, the miscibility of the two is poor. The present invention optimizes the catalyst configuration, especially in a large-scale autoclave reactor, the utilization rate of the catalyst active center is more uniform, the heat removal efficiency is optimized, the polymerization rate is increased, and the molecular weight distribution of the product is optimized.

[0027] The present invention firstly configures the catalyst. In the catalyst configuration process, supercritical ethane fluid is used as a diluent to improve the dispersion and activity of the catalyst before the catalyst is added to the reaction system. Supercritical ethane has the properties of low viscosity and high diffusivity, can significantly improve the dispersion of the catalyst and the co-catalyst, reduce the agglomeration of catalyst particles, make the catalyst active centers more evenly distributed in the reaction medium, and improve the catalytic efficiency.

[0028] Secondly, the present invention adopts the continuous slurry method of two kettles in series to prepare polyethylene, which is easy to industrial production. In the first stirred reactor, ethane in a supercritical state is used as a solvent for low-temperature prepolymerization. The ethane used as the solvent and the ethylene of the raw material are both in a supercritical state. By utilizing the supercritical special fluid state, the viscosity of the reaction system is reduced, which not only reduces the shear stress of the reaction system, reduces energy consumption, but also helps to improve the mass transfer and heat transfer efficiency of the reactor, accelerates the reaction process, and improves the regularity and uniformity of the product. Then enter the second stirred reactor, increase the reaction temperature, improve the reaction conversion rate, and quickly complete the polymerization process. The pressure of the second stirred reactor is lower than that of the first stirred reactor. On the one hand, it helps the slurry to enter the second stirred reactor from the first stirred reactor through the pressure difference. On the other hand, after reducing the pressure, the low-boiling ethane in the second stirred reactor will be vaporized and discharged from the top of the reactor in the form of a large amount of gas, and a large amount of latent heat of vaporization is removed from the reaction system, which can effectively remove the heat of polymerization and ensure the smooth progress of the polymerization.

[0029] In addition, the supercritical ethane in the present invention can be conveniently recovered by reducing pressure after completing its function, which reduces the energy consumption and environmental pollution of recovery, improves the environmental protection and economy of the whole process; and the post-processing of the product is also easier to carry out. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the process flow of preparing polyethylene by a slurry process according to an embodiment of the present invention. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the technical solution of the invention, the present invention is further described in detail below in conjunction with specific implementation methods.

[0032] The invention discloses a method for preparing polyethylene by a slurry process using hexene as a comonomer.

[0033] The following is a specific process flow of the polyethylene slurry preparation method using hexene as a comonomer (refer to Figure 1 ):

[0034] Step S1: Raw material preparation:

[0035] Prepare monomer ethylene and comonomer 1-hexene as raw materials in a mass ratio of 80-100:1-10, and ethylene and 1-hexene need to be refined raw materials from which oxygen, carbon monoxide, water and carbon dioxide are removed;

[0036] Step S2 Catalyst Solvent Preparation

[0037] Under a nitrogen atmosphere, in a catalyst preparation tank, TiCl4, triethylaluminum and supercritical ethane fluid are uniformly mixed, wherein TiCl4 and triethylaluminum are mixed in a mass ratio of 1:1-4 to form a catalyst; the catalyst and supercritical ethane fluid are mixed in a mass ratio of 1-7:380-900 to form a catalyst solvent containing supercritical ethane.

[0038] Step S3 Low Temperature Prepolymerization

[0039] The raw materials, catalyst solvent, hydrogen and supercritical ethane fluid are continuously added into a first stirred reactor to which a certain amount of supercritical ethane is added in a mass ratio of 81-110:381-907:0.2-1:5-50 to carry out a prepolymerization reaction, wherein the temperature in the first stirred reactor is controlled at 32.2-40°C and the pressure is controlled at 5.6-6.5MPa. The raw materials are added from the bottom of the first stirred reactor, and the slurry after the reaction is discharged from the upper part of the slurry in the first stirred reactor;

[0040] Step S4 High temperature polymerization

[0041] The slurry discharged from the first stirred reactor enters the second stirred reactor from the lower part for polymerization reaction. The temperature in the second stirred reactor is controlled at 70-85°C and the pressure is controlled at 5.1-5.5MPa. The slurry after the reaction is completed is discharged from the upper part of the slurry in the second stirred reactor, and the gas component is discharged from the top of the second stirred reactor, taking away a large amount of reaction heat;

[0042] S5 step product separation:

[0043] After the polymerization reaction is completed, the slurry is moved to a flash tank for separation, the separated gas phase is sent to a recovery unit, and the separated solid polymer enters a degassing and drying unit.

[0044] S6 step degassing and drying:

[0045] The dried polymer powder is sieved and homogenized to obtain the finished product.

[0046] According to the above process flow, five groups of experiments were designed, and the specific amounts of reagents added were added according to Table 1.

[0047] Table 1 Specific component dosage of the embodiment

[0048]

[0049]

[0050] Comparative Example 1

[0051] The ethane in steps S2 and S3 is replaced by hexane, that is, supercritical fluid ethane is not used, and the remaining steps are the same as in Example 3.

[0052] Comparative Example 2

[0053] Without performing the prepolymerization reaction in step S3, all the raw materials were directly added into the second stirred reactor for polymerization reaction, and the remaining steps were the same as those in Example 3.

[0054] Comparative Example 3

[0055] The temperature condition in step S3 was changed to control the temperature in the first stirred reactor to 28° C. The remaining steps were the same as those in Example 3.

[0056] Comparative Example 4

[0057] In step S1, the comonomer 1-hexene is not added to the raw material, but is replaced by an equal amount of ethylene, and the remaining steps are the same as those in Example 3.

[0058] Test product melt index, density, molecular weight test

[0059] Experimental Preparation

[0060] Sample selection: At least three representative samples were randomly selected from each experimental batch to ensure the reliability of the test results.

[0061] Sample preparation: Grind the dried polymer powder through a grinder to ensure uniform sample particle size for the accuracy of subsequent tests.

[0062] The melt index is tested according to standard GB / T 3682.1-2018 (test conditions 190°C / 21.6kg).

[0063] The density is tested according to the standard ASTM D1505-2010.

[0064] The viscosity average molecular weight is measured according to standard GB 1632.3-2010 (α=1.49).

[0065] Table 2 Product test results

[0066] Melt index (g / 10min) <![CDATA[Density (g / cm 3 )]]> Viscosity average molecular weight Molecular weight distribution (PDI) Example 1 0.27 0.931 915192 3.6 Example 2 0.28 0.929 934158 3.5 Example 3 0.32 0.911 904921 3.1 Example 4 0.30 0.922 883147 2.9 Example 5 0.29 0.926 896175 3.8 Comparative Example 1 0.23 0.953 917235 7.1 Comparative Example 2 0.24 0.951 923683 6.2 Comparative Example 3 0.25 0.956 914247 5.5 Comparative Example 4 0.23 0.961 895450 3.4

[0067] From the experimental results in Table 2 above, it can be seen that the average viscosity-average molecular weight of the products of Examples 1 to 5 is about 900,000, the melt index is 0.27 to 0.32, the density is between 0.91 and 0.93, and the molecular weight distribution is between 2.9 and 3.8. Compared with the conventional high molecular weight polyethylene with a viscosity-average molecular weight of about 900,000 (melt index is about 0.22, density is about 0.95, and molecular weight distribution is about 7.0), the high molecular weight polyethylene prepared by the embodiments of the present invention significantly improves the melt index of the product, reduces the density and molecular weight distribution of the product, and greatly improves the processing performance of the product. The melt index of the high molecular weight polyethylene obtained in Comparative Examples 1 to Comparative Examples 4 is low, and the density of the product is large. It can be seen that the present invention adds the comonomer hexene under the action of supercritical ethane and performs a prepolymerization reaction, which not only makes the polymerization more sufficient, but also ensures good processing performance of the product.

[0068] The above are only preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be regarded as limiting the present invention, and the protection scope of the present invention should be based on the scope defined by the claims. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing polyethylene by a slurry process using hexene as a comonomer, characterized in that: The following steps are involved: The monomer ethylene and the comonomer 1-hexene are used as raw materials; The catalyst and the supercritical ethane fluid are uniformly mixed to obtain a catalyst solvent; The raw materials, catalyst solvent, hydrogen and supercritical ethane fluid are continuously added into the first stirred reactor in proportion to carry out a prepolymerization reaction, and the temperature and pressure of the first stirred reactor are controlled to be above the supercritical conditions of ethane and ethylene; The slurry discharged from the first stirred reactor enters the second stirred reactor for polymerization reaction, and the temperature and pressure of the second stirred reactor are controlled above the supercritical conditions of ethane and ethylene, and the pressure is lower than that of the first stirred reactor; After the polymerization is completed, the slurry discharged from the second stirred reactor is post-treated to obtain a finished product.

2. The method for preparing polyethylene by slurry process using hexene as a comonomer according to claim 1, characterized in that: The following steps are involved: S1 uses monomer ethylene and comonomer 1-hexene in a mass ratio of 80-100:1-10 as raw materials; S2. stirring the catalyst and the supercritical ethane fluid in a mass ratio of 1-7:380-900 to form a catalyst solvent, wherein the catalyst is a mixture of TiCl4 and triethylaluminum in a mass ratio of 1:1-4; S3: continuously adding raw materials, catalyst solvent, hydrogen and supercritical ethane fluid into the first stirred reactor in a mass ratio of 81-110:381-907:0.2-1:5-50 to carry out a prepolymerization reaction, wherein the temperature in the first stirred reactor is controlled at 32.2-40° C. and the pressure is controlled at 5.6-6.5 MPa; S4 The slurry discharged from the first stirred reactor enters the second stirred reactor for polymerization reaction, and the temperature in the second stirred reactor is controlled at 70-85°C and the pressure is controlled at 5.1-5.5MPa; After the completion of S5 polymerization, the slurry discharged from the second stirred reactor is separated by flash evaporation, and the separated gas phase is sent to the recovery unit, and the solid phase polymer enters the degassing and drying stage; The polymer powder after S6 drying is sieved and homogenized to obtain the finished product.

3. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: In step S1, the ethylene and 1-hexene are refined high-purity ethylene and 1-hexene, which are first treated by removing oxygen, carbon monoxide, water and carbon dioxide.

4. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: In step S2, TiCl4, triethylaluminum and supercritical ethane fluid are uniformly mixed in a catalyst preparation tank under an inert environment, and the catalyst preparation tank is maintained at a supercritical condition of ethane.

5. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: In step S3, at the initial stage of the reaction, a certain amount of supercritical ethane fluid is first added as the bottom liquid of the reactor, and then raw materials, catalyst solvent, hydrogen, and supercritical ethane fluid are continuously added.

6. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: In step S3, the raw materials are added from the bottom of the first stirred reactor, and the slurry after the reaction is discharged from the upper slurry of the first stirred reactor.

7. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: In step S4, the slurry discharged from the first stirred reactor is introduced into the second stirred reactor from the lower part, the slurry after the reaction is discharged from the upper part of the second stirred reactor, and the gas component is discharged from the top of the second stirred reactor.

8. The method for preparing polyethylene by slurry process using hexene as comonomer according to claim 2, characterized in that: After the polymerization is completed in step S5, the slurry is decompressed to perform gas-solid separation.

9. A high molecular weight polyethylene prepared by the polyethylene slurry process using hexene as a comonomer according to any one of claims 1 to 8.

10. The high molecular weight polyethylene prepared by the polyethylene slurry process using hexene as a comonomer according to claim 9, characterized in that: The high molecular weight polyethylene has a viscosity average molecular weight of 850,000-950,000, a melt index of 0.27-0.32 g / 10 min, and a density of 0.92-0.93 g / cm 3 .

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