A method for converting polyethylene

Abstract

The application provides a polyethylene production switching method, which switches from producing low melt flow rate polyethylene to producing polyethylene in a multi-kettle series, and comprises the following steps: stopping the input of alpha-olefin into a first kettle; simultaneously increasing the feeding amount of main catalyst, auxiliary catalyst and hydrogen in the first kettle, so that the feeding amount of main catalyst is 25-80% higher than that in stable production of polyethylene in the multi-kettle series, the volume ratio of hydrogen to ethylene is 20-90% higher than the volume ratio of hydrogen to ethylene in the first kettle in stable production of polyethylene in the multi-kettle series, and the feeding amount ratio of auxiliary catalyst to main catalyst is 15-50% higher than the feeding amount ratio of auxiliary catalyst to main catalyst in the first kettle in stable production of polyethylene in the multi-kettle series; when the melt flow rate of polyethylene in the first kettle reaches 100-1000 g / 10 min, the feeding amount of main catalyst, the feeding amount ratio of auxiliary catalyst to main catalyst and the volume ratio of hydrogen to ethylene are adjusted to the range in stable production of polyethylene in the multi-kettle series. The method can shorten the production switching time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polyethylene process, in particular to a polyethylene production switching method. BACKGROUND

[0002] The common fields of polyethylene resin are packaging materials, containers, pipes, monofilaments, electric wires and cables, daily necessities, etc. Most of the polyethylene products have a melt flow rate range of less than 10 g / 10 min, and there are few high-flow polyethylene resin products. CN201310350586.X discloses a method for preparing high-melt-index low-density polyethylene with a melt index range of 42-60 g / 10 min. In a tubular high-pressure polymerization process device, by selecting appropriate initiators, molecular weight regulators and their appropriate amounts, selecting appropriate addition ratios of ethylene feed main stream and side stream, and selecting appropriate polymerization pressure and polymerization temperature, the purpose is achieved. The document "Industrialized production of coal-based high-flowability LLDPE DNDA-8320" (Synthetic Resin and Plastic, 2021, 38(2): 42) discloses a method for producing LLDPE with a melt flow rate of 20 g / 10 min by increasing the amount of H2 feed. The melt flow rate of high-flow HDPE special material on the market is generally below 30 g / 10 min.

[0003] With the increasing demand for diversification of polyolefin products in the market, in order to gain a favorable position in market competition, manufacturers no longer limit themselves to the production of a single resin grade, and often need to switch the product grade to a grade in high market demand. Bimodal high-density polyethylene (HDPE) has excellent processing performance and mechanical properties, and is widely used in the fields of film, hollow blow molding, high-performance pipe, etc. The use of double reactors in series to produce bimodal high-density polyethylene is a common technical means for domestic and foreign enterprises. For products with specific requirements for production process or structural properties, technical personnel have further improved the double reactor, such as the three-kettle series ethylene continuous polymerization production technology.

[0004] Patent CN201410400893.9 discloses a method for preparing a heat-resistant polyethylene (PE-RT) pipe material by using a three-reactor series slurry polymerization process. Under polymerization conditions, ethylene is contacted with a catalyst to generate a low molecular weight ethylene homopolymer in the first reactor, and ethylene is copolymerized with an alpha olefin containing 3-6 carbon atoms to generate a high molecular weight ethylene copolymer in the second reactor, and ethylene is copolymerized with an alpha olefin containing 3-6 carbon atoms to generate an ultrahigh molecular weight ethylene copolymer in the third reactor.

[0005] Patent CN201410487910.7 discloses a low-ash high-density polyethylene resin and its preparation method. The preferred preparation method utilizes three slurry reactors connected in series, employs a supported titanium-based main catalyst, and continuously polymerizes the resin in the presence of a co-catalyst.

[0006] CN201610158988.3 discloses a method for preparing polyethylene pipe materials resistant to slow crack growth using a three-reactor tandem slurry polymerization process. CN201610154345.1 discloses a method for preparing polyethylene pipe materials resistant to rapid crack propagation using a three-reactor tandem slurry polymerization process. CN201610221324.7 discloses a method for preparing a special resin for small hollow containers using a three-reactor tandem slurry polymerization process.

[0007] Bimodal or multimodal polyethylene produced by two or more polymerization reactors in series typically produces a high melt flow rate product in the first reactor, with a melt flow rate generally exceeding 100 g / 10 min for a 2.16 kg weight. Since most polyethylene resin grades have low melt flow rates, when switching from a low melt flow rate product to a series polymerization mode for bimodal or multimodal polyethylene, the performance indicators of the product before switching differ significantly from the product from the single reactor in a multi-reactor series. This results in a long switching time and the generation of transition material, reducing economic benefits. Therefore, effective product switching measures are needed to shorten the polyethylene product switching time and reduce transition material. The aforementioned series polymerization method for polyethylene does not address the switching issue.

[0008] CN202110120057.5 discloses a method for switching polyethylene polymerization in a low-pressure gas-phase fluidized bed polyethylene process, including process A for switching from the production of PE-ML-63D082 polyethylene to the production of PE-ML-57D075 polyethylene, and process B for switching from the production of PE-ML-57D075 polyethylene to the production of PE-L-FB-20D20 polyethylene; 1-butene is introduced in process A, and diethylaluminum chloride is introduced in process B, wherein processes A and B are achieved by adjusting the components and parameters. This method introduces an intermediate product with a melt flow rate of 7.5 g / 10 min during the process of switching from a product with a melt flow rate of 8.2 g / 10 min to a product with a melt flow rate of 2.0 g / 10 min.

[0009] CN202110623223.3 discloses a polymerization switching method for producing polyethylene. The method includes: conducting a copolymerization reaction in the presence of hydrogen, ethylene, 1-butene, a catalyst, and a first and a second co-catalyst to produce a first polyethylene; adjusting the ethylene feed rate, 1-butene feed rate, hydrogen / ethylene molar ratio, polymerization pressure, polymerization temperature, ethylene partial pressure, dew point temperature, catalyst feed rate, first co-catalyst feed rate, and second co-catalyst feed rate until the density and melt index of the obtained polymer product reach set values, then switching to polymerization to obtain a second polyethylene; wherein the density of the first polyethylene is 0.917-0.923 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16 kg is 1.5-2.5 g / 10 min; the density of the second polyethylene is 0.96-0.966 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16 kg is 6.2-10.2 g / 10 min. This method requires a pause in catalyst addition for a period of time during the switchover process, along with adjustments to the co-catalyst, which reduces the amount of transition material during the process, but the product switchover time is as long as 26 hours.

[0010] The above methods are all gas-phase fluidized bed grade switching technologies, and the product density varies greatly, while the melt flow rate is still far below 20g / 10min.

[0011] The existing publicly reported research results on slurry grade switching are mainly concentrated in university research units. The main research method is to establish a product quality model and then solve the dynamic optimization problem. There are also technical personnel engaged in polyethylene production who have published research on the melt flow rate control method of high-density polyethylene grade switching process based on production experience. For example, the literature "Control of Melt Flow Rate of High-Density Polyethylene" (Petrochemical Technology, 2021, (8): 109-111) introduces the general operation method of switching the melt flow rate from low to high in the slurry process as follows: 1) First, fully open the hydrogen regulating valve and introduce a large amount of hydrogen; 2) Then, increase the catalyst feed rate to 120% of the normal catalyst feed rate after switching for 30-40 minutes and continue to operate for 1.5-2.0 hours. At this time, the catalyst concentration in the polymerization reactor can reach the concentration required by the grade after switching; 3) Then, adjust the catalyst feed rate back to the normal value. 4) When the hydrogen-to-ethyl ratio (molar ratio) is gradually increased to about 10% higher than the normal operating value, the hydrogen feed rate is adjusted back to the normal value. This hydrogen-to-ethyl ratio (molar ratio) is kept basically unchanged for 2 hours. Then, the amount of catalyst is increased or decreased according to the pressure changes.

[0012] The main problem with the above studies is that the melt flow rates of the different grades are relatively similar, resulting in poor applicability when switching to products with a flow rate higher than 100g / 10min. Furthermore, to avoid the risk of localized explosive polymerization due to increased reactor temperature caused by significantly increasing the catalyst quantity, the rate of catalyst addition needs to be controlled, which prolongs the time required for overshooting the catalyst feed rate. Additionally, adding a large amount of hydrogen in advance, then adjusting the overshooted hydrogen feed rate and maintaining the hydrogen-ethylene ratio for a period of time also prolongs the grade switching time. Summary of the Invention

[0013] The main objective of this invention is to provide a method for switching polyethylene production, which changes the production of polyethylene with low melt flow rate to the production of polyethylene in multiple reactors in series. Compared with existing methods, the method of this invention can shorten the switching time, reduce the amount of transition material during the switching process, and improve the switching efficiency.

[0014] To achieve the above objectives, the present invention provides a method for switching polyethylene production. This method involves switching from producing low melt flow rate polyethylene to producing polyethylene in multiple reactors connected in series. The melt flow rate of the low melt flow rate polyethylene is below 2.5 g / 10 min, and the melt flow rate of the polyethylene in the first reactor of the multi-reactor series production is between 100 and 1000 g / 10 min. The method includes:

[0015] Stop feeding α-olefins into the first reactor;

[0016] Simultaneously, the feed rates of the main catalyst, co-catalyst, and hydrogen in the first reactor are increased, so that the feed rate of the main catalyst is 25-80% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series, the volume ratio of hydrogen to ethylene is 20-90% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series, and the molar ratio of the feed rates of the co-catalyst and the main catalyst is 15-50% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series.

[0017] When the melt flow rate of polyethylene in the first reactor reaches 100-1000 g / 10 min, the feed rate of the main catalyst, the ratio of the feed rate of the co-catalyst and the main catalyst, and the volume ratio of hydrogen to ethylene are adjusted to the range of stable production when multiple reactors are connected in series to produce polyethylene.

[0018] The molar ratio of the feed of the co-catalyst and the main catalyst is expressed as the aluminum / titanium molar ratio.

[0019] The polyethylene conversion method of the present invention includes a slurry process for producing low melt flow rate polyethylene and a slurry process for producing polyethylene in multiple reactors in series.

[0020] The polyethylene conversion method of the present invention, wherein when increasing the feed rates of the main catalyst, co-catalyst and hydrogen, the feed rate of ethylene in the first reactor is also increased, so that the feed rate of ethylene in the first reactor is 5% to 20% higher than the feed rate of ethylene in the first reactor when multiple reactors are connected in series to produce polyethylene stably.

[0021] The polyethylene conversion method of the present invention includes a main catalyst comprising titanium tetrachloride, a co-catalyst comprising at least one of triethylaluminum, n-butylaluminum, isobutylaluminum, tripropylaluminum, dichlorohexylaluminum, and triisobutylaluminum, and an α-olefin having 3 to 8 carbon atoms.

[0022] In the polyethylene conversion method of the present invention, when producing polyethylene stably in multiple reactors in series, the feed amount of the main catalyst in the first reactor, calculated as Ti, is 0.036-0.075 mmol / L, the molar ratio of the feed amount of the co-catalyst and the main catalyst, calculated as the aluminum / titanium molar ratio, is 12-25, the hydrogen / ethylene volume ratio is 3.0-6.0, and the ethylene added in the first reactor accounts for 30-60% of the total ethylene added in the entire reactor.

[0023] The method for switching production of polyethylene according to the present invention, wherein when producing polyethylene with low melt flow rate, the feed amount of the main catalyst is 0.010 to 0.015 mmol / L (calculated as titanium), the molar ratio of the feed amount of the co-catalyst and the main catalyst is 30 to 50 (calculated as the aluminum / titanium molar ratio), and the volume ratio of hydrogen to ethylene is 0.4 to 0.7.

[0024] The polyethylene conversion method of the present invention, wherein the process conditions for switching from producing polyethylene with low melt flow rate to producing polyethylene in multiple reactors in series are as follows: the feed rate of the main catalyst in the first reactor, calculated as Ti, is 0.036 to 0.135 mmol / L; the molar ratio of the feed rate of the co-catalyst and the main catalyst, calculated as the aluminum / titanium molar ratio, is 12 to 37; and the hydrogen / ethylene volume ratio is 3.0 to 11.4.

[0025] The polyethylene conversion method of the present invention, wherein when the melt flow rate of polyethylene in the first reactor reaches 100-1000 g / 10 min, the feed rate of ethylene in the first reactor is adjusted to the range of stable production when multiple reactors are connected in series to produce polyethylene.

[0026] In the polyethylene conversion method of the present invention, when multi-reactor series production of polyethylene is stable, the proportion of ethylene feed in the first reactor to the total ethylene feed is 30-68%.

[0027] The polyethylene conversion method of this invention, wherein the density of the low melt flow rate polyethylene is 0.950-0.956 g / cm³. 3In the first reactor of a multi-reactor series polyethylene production process, the density of polyethylene is 0.960–0.980 g / cm³. 3 The density of polyethylene produced by multiple reactors connected in series is 0.945–0.952 g / cm³. 3 The melt flow rate of polyethylene produced in multiple reactors in series at 190°C and with a load of 5.0 kg is 0.15–0.70 g / 10 min.

[0028] The beneficial effects of this invention are:

[0029] (1) The present invention performs polymerization switching in a continuous state, which avoids the impact of shutdown license plate switching or production load reduction on ethylene balance, and can also avoid the generation of unqualified materials and the loss of output due to shutdown.

[0030] (2) By simultaneously adjusting process parameters such as the amount of main catalyst added in the first reactor, the amount of co-catalyst added, and the volume ratio of hydrogen to ethylene, the present invention effectively shortens the conversion time and reduces the generation of transition material.

[0031] (3) The present invention simultaneously adjusts the amount of main catalyst added in the first reactor, the amount of co-catalyst added, and the volume ratio of hydrogen to ethylene, so that the reactor temperature will not fluctuate significantly during the production switchover process, thus reducing the risk of reactor "runaway temperature". Detailed Implementation

[0032] The following provides a detailed description of the embodiments of the present invention. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and processes. However, the scope of protection of the present invention is not limited to the following embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions. For example, the melt flow rate (MFR) of plastics can be tested at different temperatures and loads. For the polyethylene resin described in this invention, the melt flow rate is measured at 190°C. Unless otherwise specified, the load used in the test is assumed to be 2.16 kg.

[0033] The endpoints and any values ​​of the ranges disclosed herein 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 the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0034] Common multi-reactor series polyethylene production processes include the Hostalen process from LondellBasell in the Netherlands, which uses two or three slurry reactors in series, and the CX process from Mitsui Chemicals in Japan, which uses two slurry reactors in series. In multi-reactor series polyethylene production, a catalyst is typically added to the first reactor to produce a product with a high melt flow rate. No catalyst is added to the second reactor, and the product from the first reactor is then transferred to the second reactor to continue the reaction, producing a product with a low melt flow rate. If there is a third or fourth reactor, the product from the second reactor is transferred to the third or fourth reactor to continue the reaction and obtain the final product. To improve the processing performance of the final product, the first reactor product needs to have a high melt flow rate, requiring the addition of a relatively large amount of catalyst. To control the condensed structure of the final product, the first reactor product needs to have a high density. These technical characteristics of the first reactor product make multi-reactor series polyethylene production processes significantly different from other polyethylene production processes in terms of performance indicators. Switching from conventional polyethylene production processes requires shutdowns or lengthy adjustments, generating a large amount of transition material.

[0035] For example, during a production switchover, the first reactor requires increased catalyst dosage and reaction temperature. Higher polymerization temperatures increase catalyst activity and ethylene molecule activity, accelerating polymerization and releasing a large amount of heat in a short time. If the reaction temperature rises too high, the resulting polymer powder begins to expand, causing a "burst polymerization" accident that disrupts production. Therefore, to ensure stable operation of the production unit, various measures must be taken to control the reaction and prevent it from spiraling out of control. Besides measures to enhance heat removal, slowly increasing the catalyst dosage is a standard operating procedure. Alternatively, a large amount of hydrogen can be introduced initially, followed by increasing the catalyst feed rate to 120% of the normal catalyst feed rate after the switchover, allowing the catalyst concentration in the polymerization reactor to quickly reach the required concentration for the new grade. For the TiCl4-Al(C2H5)3 catalyst system, activity gradually decreases with increasing H2 addition. This is because hydrogen adsorbs on the catalyst surface, making it difficult for the monomer ethylene to contact the catalyst surface, resulting in a reduced chain growth constant and hindering the reaction.

[0036] Based on this, the present invention proposes a method for switching polyethylene production. This method involves switching from producing low melt flow rate polyethylene to producing polyethylene in multiple reactors connected in series. The melt flow rate of the low melt flow rate polyethylene is below 2.5 g / 10 min, and the melt flow rate of the polyethylene in the first reactor of the multi-reactor series production is between 100 and 1000 g / 10 min. The method includes:

[0037] Stop feeding α-olefins into the first reactor;

[0038] Simultaneously, the feed rates of the main catalyst, co-catalyst, and hydrogen in the first reactor are increased, so that the feed rate of the main catalyst is 25-80% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series, the volume ratio of hydrogen to ethylene is 20-90% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series, and the molar ratio of the feed rates of the co-catalyst and the main catalyst is 15-50% higher than that of the first reactor in the stable production of polyethylene in multiple reactors in series.

[0039] When the melt flow rate of polyethylene in the first reactor reaches 100-1000 g / 10 min, the feed rate of the main catalyst, the molar ratio of the feed rate of the co-catalyst and the main catalyst, and the volume ratio of hydrogen to ethylene are adjusted to the range of stable production when multiple reactors are connected in series to produce polyethylene.

[0040] The molar ratio of the feed of the co-catalyst and the main catalyst is expressed as the aluminum / titanium molar ratio.

[0041] Before and after the production conversion, the present invention uses the slurry process to produce polyethylene. That is, the slurry process is used to produce polyethylene with low melt flow rate, and the slurry process is used to produce polyethylene in multiple reactors in series.

[0042] During the production transition, this invention simultaneously adds a significantly higher amount of main catalyst, co-catalyst, and hydrogen than is required for stable production of polyethylene using multiple reactors in series, and employs different over-adjustment ratios for the main catalyst, co-catalyst, and hydrogen. Compared to existing technologies that first add a large amount of hydrogen to suppress catalyst activity before adding a 20% excess catalyst, this invention simultaneously adds a 25-80% excess of main catalyst, with the feed molar ratio of co-catalyst and main catalyst exceeding the target by 15-50%, and the hydrogen / ethylene volume ratio exceeding the stable production ratio by 20-90%. Because the main catalyst, co-catalyst, and hydrogen are added synchronously, the co-catalyst and main catalyst form active centers. These complexed active centers initiate the polymerization of ethylene monomers and comonomers. The growing polymer chains terminate upon encountering hydrogen. The growing polymer chains also undergo chain transfer to the co-catalyst, temporarily terminating the polymerization reaction and preventing the formation of high molecular weight products. The co-catalyst is an important component of the ethylene polymerization catalytic system. By synergistically controlling the over-adjustment ratio of the co-catalyst and the main catalyst, the formation rate of active centers in the catalytic system can be regulated. At the same time, by matching an appropriate amount of hydrogen added, a balance can be formed between the active centers and polymer chain transfer and chain termination. Under the premise of stable release of catalytic activity, the degree of polymerization of polyethylene molecular chains can be controlled to obtain polymer products with high melt index.

[0043] This invention does not impose particular limitations on the process for producing low melt flow rate polyethylene before conversion; conventional slurry processing techniques in the field can be used. For example, a Ziegler-Natta catalyst system can be used, including a main catalyst and a co-catalyst. The main component of the main catalyst is titanium tetrachloride, and the main component of the co-catalyst is triethylaluminum, with small amounts of n-butylaluminum, isobutylaluminum, and tripropylaluminum. Dichlorohexylaluminum and triisobutylaluminum can also be used. High-purity ethylene and α-olefins are used as raw materials, hexane as a solvent, and hydrogen as a molecular weight regulator. The amount of catalyst added is adjusted and controlled according to the Ti content in the slurry reactor. The amount of co-catalyst is controlled according to the aluminum / titanium ratio in the slurry reactor. The amount of hydrogen added is controlled according to the hydrogen / ethylene volume ratio in the gas phase of the slurry reactor. The α-olefin is an α-olefin with 3 to 8 carbon atoms, preferably an α-olefin with 4 to 8 carbon atoms, such as propylene or butene-1.

[0044] In another embodiment, the catalyst used in this invention is any catalyst suitable for catalyzing ethylene polymerization, preferably the Ziegler-Natta catalyst commonly used in the art, such as the BCE catalyst of Beijing Aoda Company, the PZ or RZ catalyst of Mitsui Chemicals Corporation of Japan, and the Z501 or Z509 catalyst of Basel Company. The co-catalyst used is triethylaluminum. The amount of the main catalyst and the co-catalyst added in the reactor is adjusted according to the Ti content and the aluminum-titanium ratio.

[0045] In one embodiment, the process conditions for producing low melt flow rate polyethylene before conversion are as follows: the feed rate of the main catalyst (calculated as Ti) is 0.010–0.015 mmol / L, the feed ratio of the co-catalyst to the main catalyst (calculated as the aluminum / titanium ratio) is 30–50, the hydrogen / ethylene volume ratio is 0.4–0.7, the reactor temperature is 78–88°C, and the reactor pressure is 0.38–0.48 MPa.

[0046] In one embodiment, the low melt flow rate polyethylene produced before the conversion has a melt flow rate of less than 2.5 g / 10 min at 190°C and a load of 2.16 kg; in another embodiment, the low melt flow rate polyethylene produced before the conversion has a melt index of 0.8-1.3 g / 10 min, preferably 0.9-1.2 g / 10 min, and a density of 0.950-0.956 g / cm³ at 190°C and a load of 2.16 kg. 3 The preferred value is 0.951-0.954 g / cm³. 3 In another embodiment, the low melt flow rate polyethylene produced before the conversion has a melt index of 1.8-2.2 g / 10 min, preferably 1.9-2.0 g / 10 min, and a density of 0.950-0.954 g / cm³ at 190°C and a load of 2.16 kg. 3 The preferred value is 0.951-0.953 g / cm³.3 .

[0047] In a slurry-process polyethylene production unit, when switching from producing polyethylene with a low melt flow rate to producing polyethylene from multiple reactors connected in series, coordinating the feed rate of the main catalyst in the first reactor, the aluminum / titanium ratio, and the hydrogen / ethylene volume ratio can switch the melt flow rate of the polymer product obtained from the first reactor in the multiple reactors connected in series from below 2.5 g / 10 min to a density of 0.960–0.980 g / cm³. 3 Polyethylene with a melt flow rate of 100–1000 g / 10 min at 190 °C and a load of 2.16 kg.

[0048] In detail, at the start of the production switchover, the first reactor's α-olefin feed is stopped, and within 20 minutes, the feed rate of the main catalyst, the aluminum / titanium ratio, and the hydrogen / ethylene volume ratio of the first reactor are simultaneously increased. The main catalyst feed rate is 25-80% higher than that of the first reactor in stable production of polyethylene using multiple reactors in series. The molar ratio of the feed rates of the co-catalyst and the main catalyst is 15-50% higher than that of the first reactor in stable production of polyethylene using multiple reactors in series. The hydrogen / ethylene volume ratio is 20-90% higher than that of the first reactor in stable production of polyethylene using multiple reactors in series. After 2-3 hours, the melt flow rate of the first reactor reaches the target range. Then, the feed rate of the main catalyst, the molar ratio of the feed rates of the co-catalyst and the main catalyst, and the hydrogen / ethylene volume ratio of the first reactor are reduced to the stable production value. The entire switchover time is shortened to less than 8 hours, preferably less than 5 hours. Existing technologies gradually increase the amount of catalyst and hydrogen in the first reactor to the levels required for stable production, and wait for the reaction to reach the specified melt flow rate. The conversion time is more than 15 hours.

[0049] In one embodiment, when increasing the feed rates of the main catalyst, co-catalyst, and hydrogen, the present invention also increases the feed rate of ethylene in the first reactor, making the feed rate of ethylene in the first reactor 5% to 20% higher than the feed rate of ethylene in the first reactor during stable production of polyethylene in multiple reactors connected in series.

[0050] This invention adjusts the catalyst feed rate, aluminum / titanium ratio, and hydrogen / ethylene volume ratio at the start of production switchover, while simultaneously adjusting the ethylene feed rate of the first reactor. The ethylene feed rate of the first reactor is increased by 5% to 20% compared to stable production. In multi-reactor series polyethylene production, the ethylene feed rate of each reactor has a certain proportion, which directly determines the molecular weight and molecular weight distribution of the final product. When the melt flow rate of the first reactor product is lower than the target, increasing the ethylene feed rate of the first reactor can increase the proportion of low molecular weight components in the final product, enabling the product to meet the molecular weight and molecular weight distribution requirements more quickly and reducing the time required for the final product to meet the target.

[0051] In one embodiment, the process conditions for switching from producing polyethylene with a low melt flow rate to producing polyethylene in multiple reactors in series are as follows: the feed rate of the main catalyst in the first reactor, calculated as Ti, is 0.036–0.135 mmol / L; the molar ratio of the feed rate of the co-catalyst to the main catalyst, calculated as the aluminum / titanium molar ratio, is 12–37; the hydrogen / ethylene volume ratio is 3.0–11.4; the ethylene added in the first reactor accounts for 30–68% of the total ethylene added in the entire reactor; the temperature of the first reactor is 75–90°C; and the pressure of the first reactor is 0.55–0.90 MPa. In another embodiment, when the present invention switches from producing polyethylene with a low melt flow rate to producing polyethylene in multiple reactors in series, the process conditions are as follows: the feed rate of the main catalyst in the first reactor, calculated as Ti, is 0.045–0.135 mmol / L; the feed ratio of the co-catalyst to the main catalyst, calculated as the aluminum / titanium ratio, is 14–37; the hydrogen / ethylene volume ratio is 3.6–11.4; the ethylene added in the first reactor accounts for 32–68% of the total ethylene added in the entire reactor; the temperature of the first reactor is 75–90°C; and the pressure of the first reactor is 0.55–0.90 MPa.

[0052] In one embodiment, the process conditions for stable production of polyethylene using multiple reactors in series according to the present invention are as follows: the feed rate of the main catalyst in the first reactor, calculated as Ti, is 0.036–0.075 mmol / L; the molar ratio of the feed rate of the co-catalyst to the main catalyst, calculated as the aluminum / titanium molar ratio, is 12–25; the volume ratio of hydrogen to ethylene is 3.0–6.0; the proportion of ethylene added in the first reactor to the total ethylene added in the entire reactor is 30–60%; the temperature of the first reactor is 80–90°C; and the pressure of the first reactor is 0.55–0.90 MPa.

[0053] In one embodiment, in the multi-reactor series production of polyethylene of the present invention, the density of the first reactor product is 0.960–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 150–300 g / 10 min, and the density of the final product from multiple reactors in series was 0.945–0.949 g / cm³. 3 The melt flow rate at 190°C and a load of 5.0 kg is 0.35–0.70 g / 10 min. In another embodiment, in the multi-reactor series production of polyethylene of the present invention, the density of the first reactor product is 0.960–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 400–900 g / 10 min, and the density of the final product from multiple reactors in series was 0.948–0.952 g / cm³. 3 The melt flow rate at 190℃ and 5.0kg load is 0.15~0.35g / 10min.

[0054] Therefore, this invention provides a method for switching the production of polyethylene. By synergistically adjusting the feed rates of the main catalyst, co-catalyst, hydrogen / ethylene volume ratio, and α-olefin in the first reactor, the melt flow rate of the polymer product obtained from multiple reactors connected in series in the first reactor is switched from below 2.5 g / 10 min to a density of 0.960–0.980 g / cm³. 3 When producing polyethylene at 190℃ and a load of 2.16 kg with a melt flow rate of 100–1000 g / 10 min, the ethylene feed rate in the second reactor should be reduced by 5–15%. After the melt flow rate in the first reactor reaches the specified range, the feed rates of the main catalyst and co-catalyst in the first reactor, the hydrogen / ethylene volume ratio, and the ethylene feed rates in both the first and second reactors should be adjusted to a stable production range. Generally, during the conversion process and during stable production, small amounts of α-olefins and ethylene can be introduced into the second and third reactors.

[0055] The key points of the polyethylene conversion of this invention are: (1) the first batch of α-olefin feed is stopped quickly at the start of the conversion; (2) the feed rate of the main catalyst, the aluminum / titanium molar ratio and the hydrogen / ethylene volume ratio of the first batch are adjusted to the correct values ​​in a short time. The feed rate of the main catalyst, the aluminum / titanium molar ratio and the hydrogen / ethylene volume ratio must be adjusted to the correct values ​​at the same time. The main catalyst and the aluminum / titanium molar ratio cannot be adjusted first and then the hydrogen can be adjusted, or the hydrogen can be adjusted first and then the main catalyst and the aluminum / titanium molar ratio can be adjusted; (3) during the conversion process, the volume ratio of the main catalyst, the aluminum / titanium molar ratio and the hydrogen / ethylene volume ratio are large. (4) When adjusting the main catalyst, the amount of co-catalyst added should be adjusted simultaneously to ensure that the aluminum-titanium ratio in the reactor meets the requirements; (5) When adjusting the main catalyst feed rate, aluminum / titanium molar ratio, and hydrogen / ethylene volume ratio at the start of the production switchover, the ethylene feed rate of the first reactor should be adjusted simultaneously to reduce the time required for the final product to meet the index; (6) After the melt flow rate of the first reactor reaches the index range, the main catalyst feed rate, aluminum / titanium molar ratio, hydrogen / ethylene volume ratio, and ethylene feed rate of the first reactor should be adjusted simultaneously to the stable production range.

[0056] In one embodiment, the conversion method of the present invention further includes: adjusting the feed rate of the main catalyst, the molar ratio of the feed rate of the co-catalyst and the main catalyst, and the volume ratio of hydrogen to ethylene to the range of stable production when multiple reactors are connected in series to produce polyethylene. By adjusting the molar ratio of hydrogen to ethylene, the purpose of adjusting the melt flow rate of polyethylene within a small range is achieved.

[0057] In this invention, the process of switching from low melt flow rate polyethylene to multi-reactor series polyethylene production can be carried out in a slurry process polyethylene unit (preferably a reactor slurry process), and the adjustment of each parameter can be carried out stepwise in a continuous production state. For example, the method may include the following steps:

[0058] (1) Stop feeding the first batch of α-olefins.

[0059] (2) Simultaneously increase the feed rate of the main catalyst, the aluminum / titanium molar ratio, the hydrogen / ethylene volume ratio, and the ethylene feed rate of the first reactor within 20 minutes. The main catalyst feed rate is 25-80% higher than that used in stable multi-reactor series polyethylene production; the aluminum / titanium molar ratio is 15-50% higher; the hydrogen / ethylene volume ratio is 20-90% higher; and the ethylene feed rate of the first reactor is increased by 5%-20% compared to stable production. The frequency of polyethylene powder melt flow rate analysis is adjusted from 4-5 hours / time to 0.5 hours / time.

[0060] (3) After the melt flow rate in the first reactor reaches the target range, the feed rate of the main catalyst, the aluminum / titanium molar ratio, the hydrogen / ethylene volume ratio and the ethylene feed rate in the first reactor are adjusted simultaneously to achieve a stable production range for polyethylene production in multiple reactors connected in series.

[0061] (4) Adjust the polymerization parameters within a small range according to the production situation to ensure that the performance of the final product from the tandem polymerization is within the specified range. The frequency of analysis of the melt flow rate of polyethylene powder was adjusted from 0.5 hours / time to 4-5 hours / time.

[0062] The following embodiments provide a more detailed description of the technical solutions described in this invention. However, it should be noted that the following embodiments are only used to describe the content of the invention and do not constitute a limitation on the scope of protection of this invention.

[0063] Example 1

[0064] In the Mitsui Oilfield Chemicals CX process slurry polyethylene unit, a conversion process is carried out from parallel low melt flow rate polyethylene to three-reactor tandem polymerization of polyethylene, wherein:

[0065] The density of the low melt flow rate polyethylene product is 0.952 g / cm³. 3 The melt flow index at 190℃ and 2.16kg load is 1.0g / 10min.

[0066] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.960–0.970 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 150–250 g / 10 min, and the density of the final product from multiple reactors in series was 0.946–0.948 g / cm³. 3 The melt flow rate at 190℃ and 5.0kg load is 0.40~0.60g / 10min.

[0067] The main catalyst is BCE catalyst from Beijing Aoda Company, and the co-catalyst is triethylaluminum.

[0068] The production switchover process is as follows:

[0069] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of multi-reactor series polyethylene products, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.013 mmol / L (calculated as Ti), the aluminum-titanium ratio is 40, the hydrogen / ethylene volume ratio is 0.6, the propylene feed rate (relative to the ethylene feed) mass fraction is 1.3%, the reactor temperature is 85℃, and the reactor pressure is 0.45 MPa.

[0070] (2) Stop feeding propylene into the first reactor.

[0071] (3) Within 20 minutes, simultaneously increase the feed rate of the main catalyst, the feed rate of the co-catalyst, the volume ratio of hydrogen to ethylene, and the feed rate of ethylene in the first reactor to the grade switching control range. The feed rate of the main catalyst is 60% higher than that of the multi-reactor series production of polyethylene in stable production. The molar ratio of the feed rate of the co-catalyst and the main catalyst (i.e., the aluminum-titanium molar ratio) is 20% higher than that of the multi-reactor series production of polyethylene in stable production. The volume ratio of hydrogen to ethylene is 50% higher than that of the multi-reactor series production of polyethylene in stable production. The feed rate of ethylene is increased by 6% compared with the multi-reactor series production of polyethylene in stable production. The operating conditions include: the content of the main catalyst in the first reactor (calculated as Ti) is 0.064 mmol / L, the aluminum-titanium molar ratio is 22, the volume ratio of hydrogen to ethylene is 6.0, the proportion of ethylene added in the first reactor to the total ethylene added in the reactor is 48%, the temperature of the first reactor is 83℃, and the pressure of the first reactor is 0.60 MPa. In the second reactor, the ethylene addition accounted for 36% of the total ethylene addition in the reactor, the propylene feed rate (relative to ethylene feed) was 3.3% by mass, the reactor temperature was 80℃, and the reactor pressure was 0.51 MPa. In the third reactor, the ethylene addition accounted for 16% of the total ethylene addition in the reactor, the propylene feed rate (relative to ethylene feed) was 3.0% by mass, the reactor temperature was 78℃, and the reactor pressure was 0.35 MPa. The frequency of polyethylene powder melt flow rate analysis was adjusted from 4-5 hours / time to 0.5 hours / time.

[0072] (4) After the melt flow rate in the first reactor reaches the specified range, simultaneously adjust the feed rates of the main catalyst and co-catalyst, the hydrogen / ethylene volume ratio, and the ethylene feed rate in the first reactor to achieve a stable production range for polyethylene production in multiple reactors connected in series. Operating conditions include: the main catalyst in the first reactor (calculated as Ti) has a content of 0.040 mmol / L, an aluminum-to-titanium ratio of 18, a hydrogen / ethylene volume ratio of 4.0, the ethylene addition in the first reactor accounts for 45% of the total ethylene addition in the entire reactor, the reactor temperature is 85℃, and the reactor pressure is 0.70 MPa. In the second reactor, the ethylene addition accounts for 39% of the total ethylene addition in the entire reactor, the propylene feed rate (relative to the ethylene feed) has a mass fraction of 3.3%, the reactor temperature is 80℃, and the reactor pressure is 0.51 MPa. In the third reactor, the ethylene addition accounts for 16% of the total ethylene addition in the entire reactor, the propylene feed rate (relative to the ethylene feed) has a mass fraction of 3.0%, the reactor temperature is 78℃, and the reactor pressure is 0.35 MPa.

[0073] (5) Adjust the polymerization parameters within a small range according to the production situation to keep the product performance within the specified range. The frequency of polyethylene powder melt flow rate analysis was adjusted from 0.5 hours / time to 4-5 hours / time.

[0074] The production switchover was completed in 3.8 hours.

[0075] Example 2

[0076] In the Hearst slurry-process polyethylene plant, a conversion process is carried out from parallel low melt flow rate polyethylene to two-tandem series polymerization of polyethylene, wherein:

[0077] The density of the low melt flow rate polymer product is 0.951 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16kg is 2.2g / 10min.

[0078] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.970–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 750–850 g / 10 min, and the density of the final product from multiple reactors in series was 0.949–0.951 g / cm³. 3 The melt flow rate at 190℃ and a load of 5.0 kg is 0.20–0.30 g / 10 min.

[0079] The main catalyst is Basel Z501, and the co-catalysts are triethylaluminum (96% by mass) and tri-n-butylaluminum (4% by mass).

[0080] The production switchover process is as follows:

[0081] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of polyethylene products in multiple reactors in series, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.015 mmol / L (calculated as Ti), the aluminum-titanium ratio is 31, the hydrogen / ethylene volume ratio is 0.7, the propylene feed rate (relative to the ethylene feed) mass fraction is 1.4%, the reactor temperature is 86℃, and the reactor pressure is 0.40 MPa.

[0082] (2) Stop feeding propylene into one reactor.

[0083] (3) Within 20 minutes, simultaneously increase the feed rate of the main catalyst, the feed rate of the co-catalyst, the volume ratio of hydrogen to ethylene, and the feed rate of ethylene in the first reactor to the grade switching control range. The feed rate of the main catalyst is 30% higher than that of the multi-reactor series production of polyethylene in stable production. The molar ratio of the feed rate of the co-catalyst and the main catalyst (i.e., the aluminum-titanium molar ratio) is 50% higher than that of the multi-reactor series production of polyethylene in stable production. The volume ratio of hydrogen to ethylene is 90% higher than that of the multi-reactor series production of polyethylene in stable production. The feed rate of ethylene is increased by 18% compared with the multi-reactor series production of polyethylene in stable production. The operating conditions include: the content of the main catalyst in the first reactor (calculated as Ti) is 0.091 mmol / L, the aluminum-titanium ratio is 20, the volume ratio of hydrogen to ethylene is 9.9, the proportion of ethylene added in the first reactor to the total ethylene added in the reactor is 62.5%, the reactor temperature is 82℃, and the reactor pressure is 0.85 MPa. In the second reactor, the ethylene content accounted for 37.5% of the total ethylene content in the reactor, the propylene feed rate (relative to ethylene feed) was 2.8% by mass, the reactor temperature was 80℃, and the reactor pressure was 0.56 MPa. The frequency of polyethylene powder melt flow rate analysis was adjusted from 4-5 hours / time to 0.5 hours / time.

[0084] (4) After the melt flow rate in the first reactor reaches the specified range, simultaneously adjust the feed rates of the main catalyst and co-catalyst, the hydrogen / ethylene volume ratio, and the ethylene feed rate in the first reactor to achieve a stable production range for polyethylene production in multiple reactors connected in series. Operating conditions include: the main catalyst in the first reactor (calculated as Ti) has a content of 0.070 mmol / L, an aluminum-to-titanium ratio of 13, a hydrogen / ethylene volume ratio of 5.2, the ethylene addition in the first reactor accounts for 53% of the total ethylene addition in the entire reactor, the reactor temperature is 84℃, and the reactor pressure is 0.80 MPa. In the second reactor, the ethylene addition accounts for 47% of the total ethylene addition in the entire reactor, the propylene feed rate (relative to the ethylene feed) has a mass fraction of 2.8%, the reactor temperature is 80℃, and the reactor pressure is 0.56 MPa.

[0085] (5) Adjust the polymerization parameters within a small range according to the production situation to keep the product performance within the specified range. The frequency of polyethylene powder melt flow rate analysis was adjusted from 0.5 hours / time to 4-5 hours / time.

[0086] The production switchover was completed in 4.7 hours.

[0087] Example 3

[0088] In the Mitsui Oilfield Chemicals' slurry-process polyethylene plant, a parallel low melt flow rate polyethylene production conversion to a two-reactor tandem polymerization polyethylene production process is carried out, wherein:

[0089] The density of the low melt flow rate polymer product is 0.952 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16kg is 0.50g / 10min.

[0090] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.970–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 400–600 g / 10 min, and the density of the final product from multiple reactors in series was 0.949–0.951 g / cm³. 3 The melt flow rate at 190℃ and 5.0kg load is 0.25~0.40g / 10min.

[0091] The main catalyst is the Mitsui Chemicals RZ catalyst from Japan, and the co-catalyst is triethylaluminum.

[0092] The production switchover process is as follows:

[0093] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of multi-reactor series polyethylene products, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.011 mmol / L (calculated as Ti), the aluminum-titanium ratio is 21, the hydrogen / ethylene volume ratio is 0.4, the mass fraction of butene-1 feed (relative to ethylene feed) is 1.0%, the reactor temperature is 82℃, and the reactor pressure is 0.40 MPa.

[0094] (2) Stop feeding butene-1 into the first batch.

[0095] (3) Within 20 minutes, simultaneously increase the feed rate of the main catalyst, the feed rate of the co-catalyst, the volume ratio of hydrogen to ethylene, and the feed rate of ethylene in the first reactor to the grade switching control range. The feed rate of the main catalyst is 80% higher than that of the multi-reactor series production of polyethylene in stable production. The molar ratio of the feed rate of the co-catalyst and the main catalyst (i.e., the aluminum-titanium molar ratio) is 40% higher than that of the multi-reactor series production of polyethylene in stable production. The volume ratio of hydrogen to ethylene is 20% higher than that of the multi-reactor series production of polyethylene in stable production. The feed rate of ethylene is increased by 10% compared with the multi-reactor series production of polyethylene in stable production. The operating conditions include: the content of the main catalyst in the first reactor (calculated as Ti) is 0.090 mmol / L, the aluminum-titanium ratio is 22.4, the volume ratio of hydrogen to ethylene is 5.5, the proportion of ethylene added in the first reactor to the total ethylene added in the reactor is 55%, the reactor temperature is 82℃, and the reactor pressure is 0.70 MPa. In the second reactor, the ethylene addition rate accounts for 45% of the total ethylene addition in the reactor, the butene-1 feed rate (relative to the ethylene feed) is 3.0% by mass, the reactor temperature is 81℃, and the reactor pressure is 0.60MPa. The frequency of polyethylene powder melt flow rate analysis was adjusted from 4-5 hours / time to 0.5 hours / time.

[0096] (4) After the melt flow rate in the first reactor reaches the specified range, simultaneously adjust the feed rates of the main catalyst and co-catalyst, the hydrogen / ethylene volume ratio, and the ethylene feed rate in the first reactor to achieve a stable production range for polyethylene production in multiple reactors connected in series. Operating conditions include: the main catalyst in the first reactor (calculated as Ti) has a content of 0.050 mmol / L, an aluminum-titanium ratio of 16, a hydrogen / ethylene volume ratio of 4.6, the ethylene addition in the first reactor accounts for 50% of the total ethylene addition in the reactor, the reactor temperature is 83℃, and the reactor pressure is 0.68 MPa. In the second reactor, the ethylene addition accounts for 50% of the total ethylene addition in the reactor, the butene-1 feed rate (relative to the ethylene feed) has a mass fraction of 3.0%, the reactor temperature is 79℃, and the reactor pressure is 0.50 MPa.

[0097] (5) Adjust the polymerization parameters within a small range according to the production situation to keep the product performance within the specified range. The frequency of polyethylene powder melt flow rate analysis was adjusted from 0.5 hours / time to 4-5 hours / time.

[0098] The production switchover was completed in 4.2 hours.

[0099] Comparative Example 1

[0100] In the Mitsui Oilfield Chemicals CX process slurry polyethylene unit, a conversion process is carried out from parallel low melt flow rate polyethylene to three-reactor tandem polymerization of polyethylene, wherein:

[0101] The density of the low melt flow rate polymer product is 0.952 g / cm³. 3 The melt flow index at 190℃ and 2.16kg load is 1.0g / 10min.

[0102] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.960–0.970 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 150–250 g / 10 min, and the density of the final product from multiple reactors in series was 0.946–0.948 g / cm³. 3 The melt flow rate at 190℃ and 5.0kg load is 0.40~0.60g / 10min.

[0103] The main catalyst is BCE catalyst from Beijing Aoda Company, and the co-catalyst is triethylaluminum.

[0104] The production switchover process is as follows:

[0105] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of low melt flow rate products and multi-reactor series products, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.013 mmol / L (calculated as Ti), the aluminum-titanium molar ratio is 40, the hydrogen / ethylene volume ratio is 0.6, the propylene feed rate (relative to the ethylene feed) mass fraction is 1.3%, the reactor temperature is 85℃, and the reactor pressure is 0.45 MPa.

[0106] (2) Stop feeding propylene.

[0107] (3) Gradually increase the amount of main catalyst in the first reactor so that the content of main catalyst in the first reactor of slurry is 0.040 mmol / L (calculated as Ti) and the aluminum-titanium ratio is 18. Monitor the reactor temperature to keep it at 80-88℃ to avoid excessive temperature rise and "runaway" polymerization. During the process, the frequency of polyethylene powder melt flow rate analysis was adjusted from 4-5 hours / time to 2 hours / time.

[0108] (4) After 6 hours, the amount of the main catalyst in the first reactor reached the required level, and the amount of hydrogen added was increased to make the hydrogen / ethylene volume ratio 4.0. During the process, the frequency of analysis of the melt flow rate of polyethylene powder was adjusted from 4-5 hours / time to 0.5 hours / time.

[0109] (5) After 9 hours, the melt flow rate of the first batch of polyethylene powder reached the specified range.

[0110] (6) Adjust the polymerization parameters of the second and third reactors according to the production situation to ensure that the performance of the final product is within the specified range. The frequency of polyethylene powder melt flow rate analysis was adjusted from 0.5 hours / time to 4-5 hours / time.

[0111] The production switchover was completed in 16 hours.

[0112] Comparative Example 2

[0113] In the Hearst slurry-process polyethylene plant, a conversion process is carried out from parallel low melt flow rate polyethylene to two-tandem series polymerization of polyethylene, wherein:

[0114] The density of the low melt flow rate polymer product is 0.951 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16kg is 2.2g / 10min.

[0115] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.970–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 750–850 g / 10 min, and the density of the final product from multiple reactors in series was 0.949–0.951 g / cm³. 3 The melt flow rate at 190℃ and a load of 5.0 kg is 0.20–0.30 g / 10 min.

[0116] The main catalyst is Basel Z501, and the co-catalysts are triethylaluminum (96% by mass) and tri-n-butylaluminum (4% by mass).

[0117] The production switchover process is as follows:

[0118] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of low melt flow rate products and multi-reactor series products, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.015 mmol / L (calculated as Ti), the aluminum-titanium ratio is 31, the hydrogen / ethylene volume ratio is 0.7, the propylene feed rate (relative to the ethylene feed) mass fraction is 1.4%, the reactor temperature is 86℃, and the reactor pressure is 0.40 MPa.

[0119] (2) Stop propylene feed. First, fully open the regulating valve of the first reactor for hydrogen, and introduce a large amount of hydrogen. Then, for 30-40 minutes, increase the feed rate of the main catalyst to 120% of the normal feed rate of the switched grade (the main catalyst content in the slurry reactor is 0.084 mmol / L based on Ti). Simultaneously over-adjust the aluminum-titanium molar ratio (120% of the normal feed rate, value 15.6), and continue this operation for 1.5-2.0 hours. At this point, the catalyst concentration in the polymerization reactor will reach the concentration required for the switched grade. Then, adjust the catalyst feed rate back to normal. The main catalyst content in the slurry reactor is 0.070 mmol / L based on Ti, the aluminum-titanium molar ratio is 13, and the hydrogen / ethylene volume ratio is 5.2.

[0120] (3) When the hydrogen-to-ethylene volume ratio gradually increases to about 10% higher than the normal operating value (5.7), the hydrogen feed rate is then adjusted back to the normal value. This hydrogen-to-ethylene ratio remains basically unchanged for 2 hours. Then, the amount of catalyst is increased or decreased according to the pressure changes.

[0121] (4) After the melt flow rate of polyethylene powder reaches the target of 80 g / 10 min, the amount of catalyst, co-catalyst, and hydrogen introduced is simultaneously reduced to the stable production control range of the grade. The operating conditions include: Ti content in the slurry reactor is 0.070 mmol / L, aluminum-titanium molar ratio is 13, hydrogen / ethylene volume ratio is 5.2, reactor temperature is 84℃, and reactor pressure is 0.80 MPa.

[0122] (5) Adjust the polymerization parameters of the second reactor according to the production situation so that the performance of the final product is within the specified range.

[0123] The production switchover was completed in 10.6 hours.

[0124] Example 4

[0125] In the Mitsui Oilfield's slurry-process polyethylene plant, a conversion process is carried out from parallel low melt flow rate polyethylene to three-reactor tandem polymerization of polyethylene, wherein:

[0126] The density of the low melt flow rate polymer product is 0.950 g / cm³. 3 The melt flow index at 190℃ and a load of 2.16kg is 0.60g / 10min.

[0127] The density of the first batch of polyethylene products produced in a multi-reactor series is 0.970–0.980 g / cm³. 3 The melt flow rate at 190℃ and a load of 2.16 kg was 460–550 g / 10 min, and the density of the final product from multiple reactors in series was 0.948–0.951 g / cm³. 3The melt flow rate at 190℃ and 5.0kg load is 0.30~0.50g / 10min.

[0128] The main catalyst is Mitsui Chemicals Ltd.'s RZ catalyst, and the co-catalyst is dichloroethylaluminum.

[0129] The production switchover process is as follows:

[0130] (1) After refining, the polymer grade ethylene is pressurized and fed into the reactor. Hexane is added as a solvent, and the prepared main catalyst, co-catalyst and hydrogen are added at the same time to carry out the polymerization reaction. Before switching to the production of low melt flow rate products in series with polyethylene products, the operating conditions for producing low melt flow rate products are as follows: the content of the main catalyst in the slurry reactor is 0.010 mmol / L (calculated as Ti), the aluminum-titanium ratio is 22, the hydrogen / ethylene volume ratio is 0.5, the mass fraction of butene-1 feed (relative to ethylene feed) is 1.1%, the reactor temperature is 84℃, and the reactor pressure is 0.38 MPa.

[0131] (2) Stop feeding butene-1 into the first batch.

[0132] (3) Within 20 minutes, simultaneously increase the feed rate of the main catalyst, the feed rate of the co-catalyst, and the volume ratio of hydrogen to ethylene in the first reactor to the grade switching control range. The catalyst feed rate is 80% higher than that used in stable production of polyethylene in multiple reactors in series. The molar ratio of the feed rate of the co-catalyst and the main catalyst (i.e., the aluminum-titanium molar ratio) is 40% higher than that used in stable production of polyethylene in multiple reactors in series. The volume ratio of hydrogen to ethylene is 20% higher than that used in stable production of polyethylene in multiple reactors in series. The ethylene feed rate is the same as that used in stable production. The operating conditions include: the main catalyst content in the first reactor (calculated as Ti) is 0.090 mmol / L, the aluminum-titanium ratio is 22.4, the hydrogen to ethylene volume ratio is 5.5, the ethylene added in the first reactor accounts for 42% of the total ethylene added in the entire reactor, the reactor temperature is 85℃, and the reactor pressure is 0.75 MPa. In the second reactor, the ethylene addition accounted for 40% of the total ethylene addition in the reactor, the butene-1 feed rate (relative to the ethylene feed) was 3.6% by mass, the reactor temperature was 80℃, and the reactor pressure was 0.55 MPa. In the third reactor, the ethylene addition accounted for 18% of the total ethylene addition in the reactor, the butene-1 feed rate (relative to the ethylene feed) was 3.2% by mass, the reactor temperature was 78℃, and the reactor pressure was 0.30 MPa. The frequency of polyethylene powder melt flow rate analysis was adjusted from 4-5 hours / time to 0.5 hours / time.

[0133] (4) After the melt flow rate in the first reactor reaches the specified range, simultaneously adjust the feed rates of the main catalyst and co-catalyst, as well as the hydrogen / ethylene volume ratio, to the stable production range. Operating conditions include: the main catalyst in the first reactor has a Ti content of 0.050 mmol / L, an aluminum-titanium ratio of 16, a hydrogen / ethylene volume ratio of 4.6, and the ethylene addition in the first reactor accounts for 42% of the total ethylene addition in the reactor; the reactor temperature is 83°C, and the reactor pressure is 0.68 MPa. In the second reactor, the ethylene addition accounts for 40% of the total ethylene addition in the reactor; the butene-1 feed rate (relative to the ethylene feed) is 3.6% by mass; the reactor temperature is 80°C, and the reactor pressure is 0.55 MPa. In the third reactor, the ethylene addition accounts for 18% of the total ethylene addition in the reactor; the butene-1 feed rate (relative to the ethylene feed) is 3.2% by mass; the reactor temperature is 78°C, and the reactor pressure is 0.30 MPa.

[0134] (5) Adjust the polymerization parameters of the second and third reactors according to the production situation to ensure that the performance of the final product is within the specified range. The frequency of polyethylene powder melt flow rate analysis was adjusted from 0.5 hours / time to 4-5 hours / time.

[0135] The production switchover was completed in 7.8 hours.

[0136] As can be seen from the comparison of Examples 1, 2, 3, 4 and Comparative Examples 1, 2, it is evident that using the polyethylene conversion method of the present invention, in the process of switching from a low melt flow rate product to a series polyethylene product with a high melt flow rate product in the first reactor, by simultaneously and synergistically adjusting the feed rate of the main catalyst, the feed rate of the co-catalyst, and the volume ratio of hydrogen / ethylene in the first reactor, i.e. rapidly increasing the amount of the main catalyst, co-catalyst, and hydrogen entering the first reactor, not only can a significant increase in reactor temperature be avoided, but the polymerization reaction can also be established quickly. Compared with the existing technology of gradually increasing the catalyst and adding excess hydrogen first and then adding catalyst for over-adjustment, the conversion time is greatly shortened.

[0137] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A method for switching polyethylene production, wherein the method switches from producing polyethylene with a low melt flow rate to producing polyethylene in multiple reactors connected in series, characterized in that, The low melt flow rate polyethylene has a melt flow rate below 2.5 g / 10min at 190℃ and a load of 2.16 kg, and the polyethylene in the first batch of a multi-reactor series polyethylene production has a melt flow rate of 100~1000 g / 10min at 190℃ and a load of 2.16 kg; the conversion method includes: Stop feeding α-olefins into the first reactor; Simultaneously, the feed rates of the main catalyst, co-catalyst, hydrogen, and ethylene in the first reactor are increased, so that the feed rate of the main catalyst is 25-80% higher than that of the first reactor in the stable production of polyethylene using multiple reactors in series, the feed rate of ethylene is 5-20% higher than that of the first reactor in the stable production of polyethylene using multiple reactors in series, the volume ratio of hydrogen to ethylene is 20-90% higher than that of the first reactor in the stable production of polyethylene using multiple reactors in series, and the molar ratio of the feed rates of the co-catalyst and the main catalyst is 15-50% higher than that of the first reactor in the stable production of polyethylene using multiple reactors in series. When the melt flow rate of polyethylene in the first reactor reaches 100~1000 g / 10min, the feed rate of the main catalyst, the molar ratio of the feed rate of the co-catalyst and the main catalyst, and the volume ratio of hydrogen to ethylene are adjusted to the range of stable production when multiple reactors are connected in series to produce polyethylene. The molar ratio of the feed of the co-catalyst and the main catalyst is expressed as the aluminum / titanium molar ratio.

2. The polyethylene conversion method according to claim 1, characterized in that, The production of low melt flow rate polyethylene adopts the slurry process, and the production of polyethylene in multiple reactors in series adopts the slurry process; the feed rate of the main catalyst, co-catalyst and hydrogen in the first reactor is increased within 20 minutes.

3. The polyethylene conversion method according to claim 1, characterized in that, While increasing the ethylene feed rate in the first reactor, the ethylene feed rate in the second reactor is kept 5-15% lower than the ethylene feed rate in the second reactor during stable production of polyethylene using multiple reactors connected in series.

4. The polyethylene conversion method according to claim 1, characterized in that, The main catalyst includes titanium tetrachloride, and the co-catalyst is at least one of triethylaluminum, n-butylaluminum, isobutylaluminum, tripropylaluminum, dichlorohexylaluminum, and triisobutylaluminum. The α-olefin is an α-olefin having 3 to 8 carbon atoms.

5. The polyethylene conversion method according to claim 4, characterized in that, When producing polyethylene stably in a multi-reactor series, the feed rate of the main catalyst in the first reactor, calculated as Ti, is 0.036–0.075 mmol / L. The molar ratio of the feed rate of the co-catalyst and the main catalyst, calculated as the aluminum / titanium molar ratio, is 12–25. The volume ratio of hydrogen to ethylene is 3.0–6.

0. ​​The amount of ethylene added in the first reactor accounts for 30–60% of the total amount of ethylene added in the entire reactor.

6. The polyethylene conversion method according to claim 4, characterized in that, When producing polyethylene with low melt flow rate, the feed rate of the main catalyst is 0.010 to 0.015 mmol / L (calculated as titanium), the molar ratio of the feed rate of the co-catalyst and the main catalyst is 30 to 50 (calculated as aluminum / titanium molar ratio), and the volume ratio of hydrogen to ethylene is 0.4 to 0.

7.

7. The polyethylene conversion method according to claim 4, characterized in that, In the process of switching from producing polyethylene with low melt flow rate to producing polyethylene in multiple reactors in series, the process conditions are as follows: the feed rate of the main catalyst in the first reactor is 0.036 to 0.135 mmol / L (calculated as Ti), the molar ratio of the feed rate of the co-catalyst and the main catalyst is 12 to 37 (calculated as the aluminum / titanium molar ratio), and the volume ratio of hydrogen to ethylene is 3.0 to 11.

4.

8. The polyethylene conversion method according to claim 1, characterized in that, When the melt flow rate of polyethylene in the first reactor reaches 100~1000 g / 10min, the feed rate of ethylene in the first reactor is adjusted to the range of stable production during multi-reactor series polyethylene production.

9. The polyethylene conversion method according to claim 8, characterized in that, When producing polyethylene stably in multiple reactors connected in series, the proportion of ethylene feed in the first reactor to the total ethylene feed is 30-68%.

10. The polyethylene conversion method according to claim 1, characterized in that, The density of polyethylene with low melt flow rate is 0.950-0.956 g / cm³. 3 In the first reactor of a multi-reactor series polyethylene production process, the density of polyethylene is 0.960~0.980 g / cm³. 3 The density of polyethylene produced by multiple reactors connected in series is 0.945~0.952 g / cm³. 3 The melt flow rate of polyethylene produced in multiple reactors in series at 190°C and with a load of 5.0 kg is 0.15 ~ 0.70 g / 10 min.