A production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product

Abstract

The present invention relates to a production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product. The method includes: before stopping the injection of the dual-active-site catalyst, stopping the feeding of the induced condensate and the recycle liquid, heating to raise the temperature of the reaction system, and injecting a bed pretreatment agent; stopping the injection of the dual-active-site catalyst and injecting CO2 when withdrawing from the condensing state; stopping the injection of the gas-phase components, reducing the discharging frequency, and maintaining the reactor bed weight to meet the start-up requirements of the unimodal metallocene polyethylene; displacing the reaction system with nitrogen and then displacing the reaction system with ethylene; pretreating the bed powder, re-establishing the gas-phase component concentration, and injecting a single-active-site metallocene catalyst to initiate the reaction. By using the method of the present invention, it is not necessary to pour out the seed bed of the bimodal metallocene polyethylene product and refill the seed bed of the unimodal metallocene polyethylene product, and the conversion from the bimodal metallocene polyethylene product to the unimodal metallocene polyethylene product can be completed, and the product fish-eye index is qualified.

Description

Technical Field

[0001] The present invention belongs to the technical field of petrochemical engineering, and particularly relates to a production method for converting a bimodal metallocene polyethylene product into a unimodal metallocene polyethylene product. Background Art

[0002] Metallocene polyethylene is a polyethylene resin polymerized by a catalyst composed of a metallocene organic compound and a cocatalyst, which includes bimodal metallocene polyethylene and unimodal metallocene polyethylene. Among them, bimodal metallocene polyethylene is prepared by a dual-active center catalyst and is mainly used for preparing pipes; unimodal metallocene polyethylene is prepared by a single-active center catalyst and is mainly used for preparing film materials. At present, foreign companies mainly adopt gas-phase and loop slurry process production technologies. The comonomer is mostly 1-hexene, and some use 1-octene. The metallocene catalysts used are all independently developed, and the products have been serialized. Domestic mPE production enterprises mainly adopt gas-phase production technology, and the comonomer is 1-hexene. The catalyst mainly uses the metallocene catalyst of Univation Company, and a few enterprises use the metallocene TH-5 series catalysts of GRACE and New Plastics Chemical Industry.

[0003] For example, the BMC-200 bimodal catalyst used in Univation's Unipol gas-phase fluidized bed process is a PRODIGY bimodal catalyst series. It can produce high-density polyethylene film and pipe resins with a dual molecular weight distribution in a single reactor. The corresponding PE product is a PE-100 grade pipe resin with a density of 0.947 - 0.951 g / cm 3 , and is widely used in water pipes, natural gas pipelines, large-diameter pressure pipelines, etc. The BMC-200 catalyst is a suspension of spherical particles in a mixture of mineral oil and isooctane, containing a high MW-PE catalyst, a low MW-PE catalyst, and a common activator (methylaluminoxane or "MAO"). It needs to be transported and stored at a low temperature (-10°C) to maintain stable performance.

[0004] With the annual increase in the market demand for metallocene products, domestic enterprises have put forward new requirements for optimizing product production configuration. In order to produce two products, bimodal metallocene polyethylene and unimodal metallocene polyethylene, using the same production equipment, thus avoiding the need to purchase production equipment again and saving production costs, generally, after the production of the bimodal metallocene polyethylene series products is completed, the production equipment is processed and then switched to produce unimodal metallocene polyethylene. However, the single-site metallocene catalyst consists of methylaluminoxane (MAO) and metallocene catalyst Cp2MCl2 (M = Ti, Zr, Hf) to form a homogeneous catalyst system, which has a single active site and extremely high catalytic activity, is sensitive to impurity components in the raw materials, and at the same time, has significant differences in the response to hydrogen and comonomers. At present, the mutual conversion between the bimodal metallocene polyethylene series products (used for preparing pipes) and unimodal metallocene polyethylene products (used for preparing film materials) both adopt a discontinuous production conversion method. After the reactor is emptied and overhauled, it is restarted, and the shutdown and startup time is about 5 days, which is a long time and there is no product output during this period, affecting the efficiency. Summary of the Invention

[0005] In view of this, the present invention provides a production method for converting bimodal metallocene polyethylene products into unimodal metallocene polyethylene products, so as to directly convert bimodal metallocene polyethylene series products into unimodal metallocene polyethylene products without emptying the bed layer, and at the same time reduce the crystal points of unimodal metallocene polyethylene to ensure that the fish-eye index of the unimodal metallocene polyethylene film material product is qualified.

[0006] In the first aspect, the present invention provides a production method for converting bimodal metallocene polyethylene products into unimodal metallocene polyethylene products, including the following steps:

[0007] 1) Before stopping the injection of the dual-active-site catalyst, stop the feeding of the induced condensate and the recycle liquid; heat to raise the temperature of the reaction system, inject a bed pretreatment agent, and make the content of the bed pretreatment agent reach 50 ppm to 80 ppm of the reactor production load, and reduce the reaction heat load to 0 t / h. Among them, the bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-200 and CA-300;

[0008] 2) Stop injecting the dual-active-site catalyst to make the dual-active-site catalyst deactivate naturally; inject CO2 when exiting the condensing state;

[0009] 3) Stop injecting the gas-phase components, reduce the discharging frequency of the reactor discharging system, and maintain the reactor bed weight to the quality required for the startup of unimodal metallocene polyethylene;

[0010] 4) Heat the reaction system to the reaction temperature of the single-site metallocene polyethylene product; maintain the reaction temperature, pressure, and fluidization state of the reaction system, and displace the reaction system with nitrogen; then maintain the reaction temperature of the reaction system and displace the reaction system with ethylene;

[0011] 5) Pretreat the bed powder in the reaction system with a bed pretreatment agent. The bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-300 and CA-200. The injection amount of the bed pretreatment agent is 15 ppm to 25 ppm of the bed weight. Inject the bed pretreatment agent and let it circulate in the fluidization state of the reaction system for 1 h to 2 h;

[0012] 6) Re-establish the gas-phase component concentration according to the initiation conditions of the single-site metallocene catalyst; inject the single-site metallocene catalyst to initiate the reaction for the conversion production of the single-site metallocene polyethylene product.

[0013] In the above production method, in step 1), the dual-site catalyst includes the BMC-200 catalyst.

[0014] In the present invention, the type of the dual-site catalyst is not specifically limited, and those skilled in the art can adopt conventional dual-site catalysts in the art. For example, the BMC-200 catalyst.

[0015] The BMC-200 catalyst is a suspension with spherical particles in a mixture of mineral oil and isooctane, containing a high MW-PE catalyst, a low MW-PE catalyst, and a common activator (methylaluminoxane or "MAO"). It needs to be transported and stored at a low temperature (-10°C) to maintain stable performance. To obtain a wide / bimodal molecular weight distribution of the polyethylene product, it is usually necessary to add a molecular weight regulator Trim, which is a newly added LWM-PE catalyst solution. During production operation, it is mixed online with the BMC-200 catalyst to control the polymer flow index (FI).

[0016] In the above production method, in step 1), before stopping the injection of the dual-site catalyst, stop the feeding of the induced condensing agent (ICA) and the recycle liquid to reduce the concentration of the induced condensing agent in the system to 50% to 80% of the original concentration.

[0017] In the above production method, in step 1), the induced condensing agent includes at least one of isopentane, hexane, and isobutane.

[0018] In the above production method, in step 1), the recycle liquid is a liquid formed by condensing some remaining components or incompletely reacted components in the reactor outside the reactor through a condensation technique. The recycle liquid is transported into the reactor for recycling and used to remove the reaction heat generated by the reaction system. The recycle liquid generally includes C6 organic matter, C8 organic matter, isopentane, etc.

[0019] In the present invention, after the injection of the dual-active center catalyst is stopped, the catalyst will be gradually consumed during the reaction, thereby reducing the load of the reaction system. During this process of reducing the reaction load, the reaction heat will gradually decrease. Therefore, there is no need for too much coolant to remove the reaction heat generated by the reaction system. Therefore, before stopping the injection of the dual-active center catalyst in the present invention, by reducing the content of the coolant in the reactor, the reaction system is withdrawn from the condensed state, avoiding that too much coolant causes the temperature of the reaction system to be too low, resulting in the coolant being in a liquid state in the reaction system and mixing in the powder in the reactor, causing the powder to become sticky and easily forming flakes and lumps to block the reactor.

[0020] In the above production method, in step 1), the reaction system is heated to 100 °C to 110 °C.

[0021] In the above production method, in step 1), the heating to raise the temperature of the reaction system includes the following steps:

[0022] Put into use the start-up heater, adjust the steam control valve according to the polymerization reaction temperature of the bimodal metallocene polyethylene product, and preheat in advance to prevent the temperature of the reaction system from dropping significantly after the dual-active center catalyst is deactivated.

[0023] In the present invention, by injecting a bed pretreatment agent and increasing the injection amount of the bed pretreatment agent to 50 ppm to 80 ppm, the electrostatic stability and wall temperature stability of the reactor are ensured, and the injection is maintained until the reaction heat load drops to 0 t / h.

[0024] In the above production method, in step 2), the withdrawal from the condensed state means that after the condensation rate of the reaction system reaches 3% to 5%, CO2 is injected to withdraw from the condensed state.

[0025] In the above production method, in step 2), when the condensation rate is 3% to 5%, CO2 is injected to make its concentration less than 2 ppm, and then CO2 is injected again to make its concentration 3 ppm to 8 ppm.

[0026] In the present invention, the dual-active-site catalyst can adopt conventional catalysts in the art, such as the BMC-200 bimodal catalyst. The reaction of the bimodal metallocene polyethylene catalyzed by it is very rapid (i.e., the BMC-200 bimodal catalyst has the characteristic of rapid manifestation of catalyst activity), and it is necessary to avoid the problem of reactor caking and flaking caused by too violent reaction. Therefore, in the present invention, when exiting the condensing state, CO2 is used to immediately deactivate the dual-active-site catalyst.

[0027] In the above production method, in step 2), when the condensation rate is 3% to 5%, open the angle valve of the CO2 cylinder and inject CO2 for 5 to 15 seconds to make the CO2 concentration less than 2 ppm; inject CO2 again for 5 to 10 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm.

[0028] In the present invention, stop injecting the dual-active-site catalyst to make the dual-active-site catalyst deactivate naturally, and then pay attention to the reactor condensation rate. When the condensation rate is 3% to 5%, open the angle valve of the CO2 cylinder and inject CO2 for 5 to 15 seconds to make the CO2 concentration less than 2 ppm, so that the dual-active-site catalyst deactivates quickly at a low condensation rate; inject CO2 again for 5 to 10 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm, so that the remaining dual-active-site catalyst is completely deactivated, and quickly exit the condensing state, so as to ensure the lowest CO2 concentration in the system under the condition that the original dual-active-site catalyst in the reaction system is deactivated, so as to facilitate the replacement of CO2 later.

[0029] In the above production method, in step 3), stop injecting the gas-phase components, reduce the discharging frequency of the reactor discharging system, maintain the reactor bed weight to the quality required for the start-up of single-peak metallocene polyethylene, and at the same time maintain the circulating gas flow rate at 0.6 m / s to 0.8 m / s, and the expanded section material level at 35% to 60%.

[0030] In the present invention, maintain the circulating gas flow rate at 0.6 m / s to 0.8 m / s, and the expanded section material level at 35% to 60%, so that the powder in the reaction system is in a fluidized state, and avoid the phenomenon of caking and agglomeration that may be caused by the powder falling to the bed.

[0031] In the above production method, in step 3), the gas-phase components include ethylene, hydrogen and comonomer.

[0032] In the above production method, in step 3), the quality required for the start-up of single-peak metallocene polyethylene is 110 t to 150 t.

[0033] In the present invention, when the reactor condensation rate is 3% to 5%, the injection of the dual-active-site catalyst is stopped, and CO2 is injected to deactivate the dual-active-site catalyst in the reaction system. During this process, when the polymerization reaction in the reactor no longer occurs (judged according to the bottom temperature of the reactor), the injection of gaseous components such as ethylene, hydrogen, and comonomer is stopped, the discharging frequency of the reactor discharging system is reduced, and the reactor bed weight is maintained at 110 t to 150 t, the mass required for the start-up of single-site metallocene polyethylene, so as to meet the conditions initiated by single-site metallocene. At the same time, the circulating gas velocity is maintained at 0.6 m / s to 0.8 m / s, and the expanded bed level is maintained at 35% to 60%.

[0034] In the above production method, in step 4), the reaction temperature of the single-site metallocene polyethylene product is 80°C to 90°C.

[0035] In the above production method, in step 4), the steam control valve of the start-up heater is adjusted to maintain the reaction temperature of the reaction system at 80°C to 90°C, so as to prepare for the initiation reaction of the single-active-site catalyst.

[0036] In the above production method, in step 4), the reaction system is purged with nitrogen to terminate the polymerization reaction of the bimodal metallocene series products and reduce the reaction pressure to the lowest pressure at which the compressor does not surge.

[0037] In step 4) of the present invention, under the conditions of maintaining the reaction temperature, pressure and fluidization state of the reaction system, each purge main pipe in the reaction system is switched from ethylene to HPPN (high-pressure purified nitrogen), so that the original dual-active-site catalyst components in the bed are completely deactivated, preparing for the replacement, pretreatment and the initiation conditions of the subsequent single-site metallocene catalyst.

[0038] In the above production method, in step 4), the reaction system is purged with ethylene by the method of raising and lowering pressure.

[0039] In the above production method, in step 4), the replacement pressure of the method of raising and lowering pressure is 900 kPa to 1100 kPa to 1400 kPa. That is, the replacement pressure rises from 900 kPa to 1100 kPa, then rises to 1400 kPa, then drops back to 900 kPa, and the above process is repeated.

[0040] In the above production method, in step 4), the reaction system is purged with ethylene to displace the materials (such as CO2 and N2) used to deactivate the dual-active-site catalyst in the reactor. The standard for the completion of the replacement is: CO2 in the reactor < 5 ppm.

[0041] In the present invention, in step 4), the reaction temperature is continuously maintained in the reactor. The purge main pipe is switched back from HPPN to ethylene, and the ethylene feed rate into the purge main pipe is increased. By the method of pressure lifting and lowering, the materials used for the dual-active-site catalytic polymerization reaction in the reactor are displaced. The displacement pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then to 1400 kPa, then drops back to 900 kPa, and then the above process is repeated). The displacement standard is that CO2 in the reactor is < 5 ppm (at this time, the dual-active-site catalyst has been deactivated). Under such conditions, the materials used for the dual-active-site catalytic polymerization reaction have been completely displaced in the reaction system, and the residual CO2 < 5 ppm in the reactor will not deactivate the newly injected catalyst subsequently.

[0042] In the present invention, the bed powder in the reactor is pretreated with a bed pretreatment agent. Specifically, CA-300 and / or CA-200 are selected as the bed pretreatment agent to completely eliminate the influence of the residual poisons of the metallocene catalyst on the metallocene catalyst and prevent flaking in the reactor.

[0043] In the above production method, in step 6), the re-establishment of the gas-phase component concentration includes: establishing the ethylene concentration and the nitrogen concentration.

[0044] In the above production method, in step 6), after the gas-phase components are established in place, a single-active-site metallocene catalyst is injected to initiate the reaction, and the heat load can be observed within 20 min (i.e., the reaction has been initiated).

[0045] In the above production method, in step 6), the single-active-site metallocene catalyst includes an easy-to-process type metallocene catalyst (for example, the EZ series of metallocene catalysts).

[0046] In the above production method, the total switching time for converting the bimodal metallocene polyethylene product to the unimodal metallocene polyethylene product is 22 h to 24 h.

[0047] In the above production method, the melt index of the bimodal metallocene polyethylene product is 5 g·10 min -1 to 7 g·10 min -1 , and the density is 0.947 g / cm 3 to 0.951 g / cm 3 .

[0048] In the present invention, the bimodal metallocene polyethylene product belongs to high-density polyethylene (HDPE), and is prepared by using a conventional dual-active-site catalyst in the art.

[0049] In the above production method, the melt index of the single-peak metallocene polyethylene product is 0.2 g / 10 min to 20 g / 10 min, and the density is 0.910 g / cm 3 to 0.940 g / cm 3 .

[0050] In the present invention, due to the characteristic that the dual-active-site catalyst rapidly exhibits catalytic activity, during the conversion from a bimodal metallocene polyethylene product to a single-peak metallocene polyethylene product, the conversion process cannot directly withdraw the reaction system from the condensed state like a titanium-based or chromium-based catalyst. Instead, the dual-active-site catalyst must be immediately deactivated first, and then the condensed state is withdrawn. In addition, during the conversion process, the reaction system cannot be transferred from the condensed state to the dry state operation. If the production of bimodal metallocene polyethylene is transferred to the dry state operation for a period of time, the reactor is prone to caking, which causes the powder in the reactor to no longer be used as the seed bed for the start-up of single-peak metallocene.

[0051] Therefore, the present invention first stops the feeding of the inducing condensing agent (ICA) and the recycle liquid to reduce the concentration of the inducing condensing agent in the system to 50% to 80% of the original concentration. Without directly withdrawing from the condensed state, the reaction system is heated to make the temperature of the reaction system reach 100 °C to 110 °C, so that the remaining dual-active-site catalyst reacts rapidly and is completely consumed. At the same time, a bed pretreatment agent is injected to avoid caking of the polymerization product. Then, the injection of the dual-active-site catalyst is stopped, and CO2 is used to immediately deactivate the dual-active-site catalyst, ensuring that the reactor in the production process of bimodal metallocene can continue to be used as the seed bed for the start-up of single-peak metallocene.

[0052] In the prior art, the mutual conversion between the bimodal metallocene polyethylene series products (for preparing pipes) and the single-peak metallocene polyethylene products (for preparing film materials) all adopts a discontinuous conversion method, and the reactor is emptied, repaired, and then restarted after reconnecting the bed. Among them, during the production process of metallocene polyethylene products, metallocene polyethylene is used as the seed bed for start-up. Considering the compatibility between the metallocene polyethylene powder (belonging to mLLDPE) in the seed bed and the metallocene polyethylene product prepared by the single-active-site metallocene catalyst, the conventional method generally uses the high-strength metallocene catalyst HP series as the single-active-site metallocene catalyst for initiating polymerization to achieve the conversion of single-peak metallocene polyethylene, so that the fish-eye index of the polyethylene film material product produced from the converted single-peak metallocene polyethylene product as the raw material is qualified.

[0053] In the present invention, a bimodal metallocene polyethylene series product (belonging to HDPE) is directly converted into a unimodal metallocene polyethylene product without emptying the bed layer. Among them, the seed bed for the conversion of unimodal metallocene polyethylene is the powder remaining in the reactor after the completion of the bimodal metallocene polyethylene product (i.e., the powder is HDPE), which is incompatible with conventional unimodal metallocene polyethylene (LLDPE). Therefore, on the premise of not emptying the bed layer powder, directly adopting the technical solution in the prior art to convert the bimodal metallocene polyethylene series product into a unimodal metallocene polyethylene product easily results in unqualified fish-eye index of the finally converted polyethylene film material product. In order to make the fish-eye index of the polyethylene film material product qualified, in the present invention, an easy-to-process metallocene catalyst (for example, EZ series) is used instead of the conventional high-strength metallocene catalyst HP series when converting to a unimodal metallocene polyethylene product. The unimodal metallocene polyethylene product obtained by its initiation polymerization can be well miscible with the powder in the seed bed used for the conversion of unimodal metallocene polyethylene in the present invention, thereby reducing the crystal points of the unimodal metallocene polyethylene and ensuring the qualified fish-eye index of the unimodal metallocene polyethylene film material product.

[0054] In the above production method, the produced unimodal metallocene polyethylene has the following properties: tensile strength ≥ 11 MPa, dart impact breakage mass ≥ 300 g, number of fish-eyes with 0.8 mm < 6 per 1520 cm 2 , number of fish-eyes with 0.4 mm < 15 per 1520 cm 2 .

[0055] In the present invention, when the fish-eye level of the unimodal metallocene polyethylene product reaches the standard of number of fish-eyes with 0.8 mm < 6 per 1520 cm 2 , number of fish-eyes with 0.4 mm < 15 per 1520 cm 2 , the quality of this unimodal metallocene polyethylene product is comparable to that of foreign similar products.

[0056] The technical solution of the present invention has the following beneficial technical effects compared with the prior art:

[0057] (1) The switching method for converting a bimodal metallocene polyethylene product into a unimodal metallocene polyethylene product provided by the present invention takes a short time, which is shortened from 120 hours of the traditional method to within 24 h, effectively reducing the time cost and realizing the rapid switching from a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product;

[0058] (2) The switching method for converting a bimodal metallocene polyethylene product into a unimodal metallocene polyethylene product provided by the present invention does not require a complete reaction shutdown, does not need to open the reactor, and does not need to purchase metallocene polyethylene powder as the seed bed, but realizes the switching in the dual-active center catalyst bed layer, greatly reducing the development and production difficulty of metallocene polyethylene;

[0059] (3) By using the switching method for converting the bimodal metallocene polyethylene product provided by the present invention into a unimodal metallocene polyethylene product, it is not necessary to carry out large-scale transformation on the existing raw material storage tank, seed bin, refining system, catalyst injection system, raw material feeding control system, electrostatic control system, etc., greatly reducing the cost of transformation investment;

[0060] (4) By using the switching method for converting the bimodal series products provided by the present invention into unimodal metallocene products, the metallocene polyethylene produced has the following properties: the melt index ranges from 0.2 g / 10 min to 20 g / 10 min, the density ranges from 0.910 g / cm 3 to 0.940 g / cm 3 , the tensile strength ≥ 11 MPa, the dart impact breakage mass ≥ 300 g, the number of fish eyes (0.8 mm) < 6 per 1520 cm 2 , the number of fish eyes (0.4 mm) < 15 per 1520 cm 2 . Detailed implementation manners

[0061] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the scope of protection of the present application.

[0062] Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments, and equipment used in the present application can be obtained through market purchases or can be prepared by existing methods.

[0063] In the prior art, the mutual conversion between the bimodal metallocene polyethylene series products (for preparing pipes) and the unimodal metallocene polyethylene products (for preparing film materials) all adopts a discontinuous production conversion method. The reactor is emptied, repaired, and then restarted after reconnecting the bed. The start-up and shutdown time is about 5 days, which is long and there is no product output during this period, affecting the benefits. The present invention provides a production method for converting a bimodal metallocene polyethylene product into a unimodal metallocene polyethylene product, so as to directly convert the bimodal metallocene polyethylene series products into unimodal metallocene polyethylene products without emptying the bed layer, while reducing the crystal points of the unimodal metallocene polyethylene and ensuring that the fish eye index of the unimodal metallocene polyethylene film material product is qualified.

[0064] However, due to the incompatibility between the dual-active-site catalyst and the single-active-site metallocene catalyst, before injecting the single-active-site metallocene catalyst, it is necessary to completely consume or deactivate the dual-active-site catalyst before the production can be switched to single-peak metallocene polyethylene products.

[0065] Some embodiments of the present invention provide a production method for switching from a bimodal metallocene polyethylene product to a single-peak metallocene polyethylene product, comprising the following steps:

[0066] S1: Before stopping the injection of the dual-active-site catalyst, stop the feeding of the induced condensate and the recycle liquid; heat to raise the temperature of the reaction system, and inject a bed pretreatment agent to reduce the reaction heat load;

[0067] S2: Stop injecting the dual-active-site catalyst to allow the dual-active-site catalyst to deactivate naturally; inject CO2 when exiting the condensing state;

[0068] S3: Stop injecting the gas-phase components, reduce the discharging frequency of the reactor discharging system, and maintain the reactor bed weight to the mass required for the start-up of single-peak metallocene polyethylene;

[0069] S4: Heat to raise the temperature of the reaction system to the reaction temperature of the single-peak metallocene polyethylene product; maintain the reaction temperature, pressure, and fluidization state of the reaction system, and displace the reaction system with nitrogen; then maintain the reaction temperature of the reaction system and displace the reaction system with ethylene;

[0070] S5: Pretreat the bed powder in the reaction system with a bed pretreatment agent;

[0071] S6: Re-establish the gas-phase component concentration according to the initiation conditions of the single-active-site metallocene catalyst; inject the single-active-site metallocene catalyst to initiate the reaction for the production switch to single-peak metallocene polyethylene products.

[0072] In some embodiments, in step S1, the dual-active-site catalyst includes the BMC-200 catalyst.

[0073] In some embodiments, in step S1, before stopping the injection of the dual-active-site catalyst, stop the feeding of the induced condensate and the recycle liquid to (rapidly) reduce the concentration of the induced condensate in the system to 50% to 80% of the original concentration (for example, 55%, 60%, 65%, 70%, or 75%).

[0074] In some embodiments, in step S1, the induced condensate includes at least one of isopentane, hexane, and isobutane.

[0075] In some embodiments, in the step S1, the recycle liquid is a liquid formed by condensing some remaining components or incompletely reacted components in the reactor through a condensation technique outside the reactor. The recycle liquid is transported into the reactor for recycling and used to remove the reaction heat generated by the reaction system. The recycle liquid generally includes C6 organic matter, C8 organic matter, isopentane, etc.

[0076] In the present invention, after the injection of the dual-active-site catalyst is stopped, the catalyst will be gradually consumed during the reaction, thereby reducing the load of the reaction system. During this process of reducing the reaction load, the reaction heat will gradually decrease. Therefore, there is no need for too much coolant to remove the reaction heat generated by the reaction system. Therefore, before stopping the injection of the dual-active-site catalyst, the present invention reduces the coolant content in the reactor to make the reaction system exit the condensed state, avoiding the situation that too much coolant causes the temperature of the reaction system to be too low, resulting in the coolant remaining in a liquid state in the reaction system and mixing in the powder in the reactor, causing the powder to become sticky and easily forming flakes and lumps to block the reactor.

[0077] In some embodiments, in the step S1, heat is applied to raise the temperature of the reaction system to 100°C to 110°C; specifically, it includes the following steps:

[0078] Put into use the start-up heater, adjust the steam control valve according to the polymerization reaction temperature of the bimodal metallocene polyethylene product, and control the reaction temperature at 100°C to 110°C (for example, 102°C, 105°C or 108°C) by controlling the steam flow rate, so as to preheat the reaction system in advance and prevent the temperature of the reaction system from dropping significantly after the dual-active-site catalyst is deactivated.

[0079] In some embodiments, in the step S1, a bed pretreatment agent is injected to make the content of the bed pretreatment agent reach 50 ppm to 80 ppm (for example, 55 ppm, 60 ppm, 65 ppm, 70 ppm or 75 ppm) of the production load of the reactor (or reaction system).

[0080] In the present invention, by injecting a bed pretreatment agent and increasing the injection amount of the bed pretreatment agent to 50 ppm to 80 ppm, it is ensured that the static electricity of the reactor is stable, the wall temperature is stable, and the injection is maintained until the reaction heat load drops to 0 t / h.

[0081] In some embodiments, in the step S1, the bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-200 and CA-300.

[0082] The bed pretreatment agents CA-200 and CA-300 are common antistatic agents in the art. The main components of CA-200 are aluminum stearate, oil-soluble alkyl alkanolamine, and mineral oil solvent, with a ratio of 10% to 30%: 15% to 25%: 65% to 90%; the main components of CA-300 are aluminum stearate, oil-soluble alkanolamide, mineral oil solvent, and stearate salts, with a ratio of 10% to 30%: 15% to 25%: 65% to 90%: 1% to 3%.

[0083] In some embodiments, in step S2, the exit from the condensation state means that after the condensation rate of the reaction system reaches 3% to 5%, CO2 is injected to exit the condensation state; when the condensation rate is 3% to 5% (for example, 3.5%, 4%, 4.5%, or 4.8%), CO2 is injected to make its concentration less than 2 ppm, and then CO2 is injected again to make its concentration 3 ppm to 8 ppm (for example, 3.5 ppm, 4 ppm, 5 ppm, 6 ppm, 6.5 ppm, or 7.5 ppm).

[0084] In the present invention, the dual-active-site catalyst can adopt conventional catalysts in the art, such as the BMC-200 bimodal catalyst. The reaction of bimodal metallocene polyethylene catalyzed by it is very rapid (that is, the BMC-200 bimodal catalyst has the characteristic of rapid manifestation of catalyst activity), and it is necessary to avoid the problem of reactor caking and flaking caused by too violent reaction. Therefore, in the present invention, when exiting the condensation state (that is, when the condensation rate is 3% to 5%), CO2 is used to immediately deactivate the dual-active-site catalyst.

[0085] In some embodiments, in step S2, when the condensation rate is 3% to 5%, the angle valve of the CO2 cylinder is opened to inject CO2 for 5 seconds to 15 seconds (for example, 6 seconds, 8 seconds, 10 seconds, or 12 seconds) to make the CO2 concentration less than 2 ppm, and then CO2 is injected again for 5 seconds to 10 seconds (for example, 6 seconds, 7 seconds, 8 seconds, or 9 seconds) to ensure that the CO2 concentration is 3 ppm to 8 ppm.

[0086] In the present invention, the injection of the dual-active-site catalyst is stopped to make the dual-active-site catalyst deactivate naturally, and then the condensation rate of the reactor is monitored. When the condensation rate is 3% to 5%, the angle valve of the CO2 cylinder is opened to inject CO2 for 5 seconds to 15 seconds, and the CO2 concentration is less than 2 ppm, so that the dual-active-site catalyst can be quickly deactivated at a low condensation rate (or when exiting the condensation state); CO2 is injected again for 5 seconds to 10 seconds to ensure that the CO2 concentration is 3 ppm to 8 ppm, so that the remaining dual-active-site catalyst can be completely deactivated, so as to ensure the lowest CO2 concentration in the reaction system under the condition of deactivation of the original dual-active-site catalyst in the reaction system, which is convenient for later replacement of CO2 completely.

[0087] In some embodiments, in step S3, the injection of the gas-phase components is stopped, the discharging frequency of the reactor discharging system is reduced, the reactor bed weight is maintained to the mass required for the start-up of single-site metallocene polyethylene, and at the same time, the circulating gas velocity is maintained at 0.6 m / s to 0.8 m / s (for example, 0.65 m / s, 0.7 m / s or 0.75 m / s), and the expanded section level is at 35% to 60% (for example, 42%, 45% or 48%).

[0088] In some embodiments, in step S3, the gas-phase components include ethylene, hydrogen and comonomer.

[0089] In some embodiments, in step S3, the mass required for the start-up of single-site metallocene polyethylene is 110 t to 150 t (for example, 115 t, 120 t, 125 t, 130 t, 135 t, 140 t or 145 t).

[0090] In the present invention, the injection of the dual-active-site catalyst is stopped. When the reactor condensation rate is 3% to 5%, CO2 is injected to deactivate the dual-active-site catalyst in the reaction system; during this process, when the polymerization reaction in the reactor no longer occurs (judged according to the bottom temperature of the reactor), the injection of gas-phase components such as ethylene, hydrogen and comonomer is stopped, the discharging frequency of the reactor discharging system is reduced, and the reactor bed weight is maintained to the mass required for the start-up of single-site metallocene polyethylene, which is 110 t to 150 t, so as to meet the conditions initiated by single-site metallocene; at the same time, the circulating gas velocity is maintained at 0.6 m / s to 0.8 m / s, and the expanded section level is at 35% to 60%, so that the powder in the bed is in a fluidized state and will not fall on the bed to form flakes and agglomerates.

[0091] In some embodiments, the reaction temperature of the single-site metallocene polyethylene product in step S4 is 80°C to 90°C (for example, 82°C, 85°C or 88°C); specifically, in step 4), the steam control valve of the start-up heater is adjusted to maintain the reaction temperature of the reaction system at 80°C to 90°C to prepare for the initiation reaction of the single-active-site catalyst.

[0092] In some embodiments, in step S4, the reaction system is purged with nitrogen to terminate the polymerization reaction of the bimodal metallocene series products and reduce the reaction pressure to the lowest pressure at which the compressor does not surge.

[0093] In step 4) of the present invention, under the conditions of maintaining the reaction temperature, pressure and fluidized state of the reaction system, each purge main pipe in the reaction system is switched from ethylene to HPPN (high-pressure purified nitrogen) to completely deactivate the original dual-active-site catalyst components in the bed and prepare for the replacement, pretreatment and the initiation conditions of the subsequent single-site metallocene catalyst.

[0094] In some embodiments, in step S4, ethylene is used to displace the reaction system by the method of raising and lowering pressure; the displacement pressure of the method of raising and lowering pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then rises to 1400 kPa, then drops back to 900 kPa, and the above process is repeated).

[0095] In some embodiments, in step S4, ethylene is used to displace the reaction system to displace the materials (such as CO2 and N2) used to deactivate the dual-active-site catalyst in the reactor. The criterion for the completion of displacement is that CO2 in the reactor is less than 5 ppm.

[0096] In the present invention, in step S4, the reaction temperature is continuously maintained in the reactor. The purge main pipe is switched back from HPPN to ethylene, and the ethylene inlet amount into the purge main pipe is increased. By the method of raising and lowering pressure, the materials used for the dual-active-site catalytic polymerization reaction in the reactor are displaced. The displacement pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then rises to 1400 kPa, then drops back to 900 kPa, and the above process is repeated). The displacement criterion is that CO2 in the reactor is less than 5 ppm (at this time, the dual-active-site catalyst has been deactivated). Under such conditions, the materials used for the dual-active-site catalytic polymerization reaction have been completely displaced in the reaction system, and the residual CO2 less than 5 ppm in the reactor will not deactivate the newly injected catalyst subsequently.

[0097] In some embodiments, in step S5, the bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-300 and CA-200; the injection amount of the bed pretreatment agent is 15 ppm to 25 ppm of the bed weight (for example, 18 ppm, 20 ppm, 22 ppm or 24 ppm); the bed pretreatment agent is injected and circulated in the fluidized state of the reaction system for 1 h to 2 h (for example, 1.5 h).

[0098] In some embodiments, in step S6, the re-establishment of the gas-phase component concentration includes: establishing the ethylene concentration and the nitrogen concentration; after the gas-phase components are established in place, a single-active-site metallocene catalyst is injected to initiate the reaction, and the heat load can be seen in 20 min (that is, the reaction has been initiated).

[0099] In some embodiments, in step S6, the single-active-site metallocene catalyst includes an easy-to-process metallocene catalyst (for example, the metallocene catalyst EZ series).

[0100] In some embodiments, the total switching time for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product is from 22 h to 24 h (e.g., 22.5 h, 23 h, or 23.5 h).

[0101] In some embodiments, the produced unimodal metallocene polyethylene has the following properties: a melt index in the range of 0.2 g / 10 min to 20 g / 10 min, a density in the range of 0.910 g / cm 3 to 0.940 g / cm 3 , a tensile strength ≥ 11 MPa, a dart impact break mass ≥ 300 g, the number of 0.8 mm fish eyes < 6 per 1520 cm 2 , and the number of 0.4 mm fish eyes < 15 per 1520 cm 2 .

[0102] In the present invention, when the fish eye level of the unimodal metallocene polyethylene product reaches the standard that the number of 0.8 mm fish eyes < 6 per 1520 cm 2 , and the number of 0.4 mm fish eyes < 15 per 1520 cm 2 , the quality of the unimodal metallocene polyethylene product is comparable to that of similar foreign products.

[0103] The following further elaborates the present application with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods without specific conditions noted in the following embodiments are generally determined according to national standards. If there is no corresponding national standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0104] Example 1

[0105] A production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product, comprising the following steps:

[0106] S1: Before stopping the injection of the dual-active-site catalyst, stop the fresh isopentane (i.e., ICA) and the recycle liquid feed, and quickly reduce the ICA concentration in the system to 60% of the original concentration; put into use the start-up heater, adjust the steam control valve according to the reaction temperature, and control the reaction temperature at 100°C to 110°C by controlling the steam flow rate to preheat the reaction system in advance to prevent a large drop in temperature after the dual-active-site catalyst is deactivated; increase the injection amount of the bed pretreatment agent to 60 ppm to ensure the electrostatic stability and wall temperature stability of the reactor, and maintain the injection until the reaction heat load drops to 0 t / h.

[0107] S2: Stop injecting the dual-active-site catalyst to allow it to deactivate naturally. Monitor the reactor condensation rate. When the condensation rate approaches 5%, open the angle valve of the CO2 cylinder to inject CO2 for 10 seconds. If the CO2 concentration is less than 2 ppm, inject CO2 again for 5 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm.

[0108] S3: When the polymerization reaction no longer occurs, stop injecting gaseous components such as ethylene, hydrogen, and comonomer, reduce the discharging frequency of the reactor discharging system, maintain the reactor bed weight at 110 t, which is the required quality for metallocene startup, and at the same time maintain the circulating gas velocity at 0.7 m / s and the expanded section inventory level between 40% and 50%.

[0109] S4: Adjust the steam control valve of the startup heater to make the reactor temperature between 80°C and 90°C; maintain the reaction temperature, pressure, and fluidization state inside the reactor. Switch each purge main pipe inside the reactor from ethylene to HPPN (high-pressure purified nitrogen) to completely deactivate the original dual-active-site catalyst components in the bed; when the reaction is completely terminated, reduce the reactor pressure to the lowest pressure at which the compressor does not surge; continue to maintain the reaction temperature inside the reactor, switch the purge main pipe back from HPPN to ethylene, increase the ethylene intake of the purge main pipe, and displace the materials used for deactivating the dual-active-site catalytic polymerization reaction inside the reactor by the method of raising and lowering the pressure. The displacement pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then to 1400 kPa, then drops back to 900 kPa, and then repeats the above process). The displacement standard is that CO2 in the reactor < 5 ppm.

[0110] S5: Conduct pre-treatment operations on the bed powder in the reactor. Select CA-300 or CA-200 as the bed pre-treatment agent to completely eliminate the influence of the dual-active-site catalyst residues and bed impurities on the metallocene catalyst; among them, the injection amount of the bed pre-treatment agent is 20 ppm of the bed weight, and after injection, it circulates for 1.5 h under the fluidization state of the reactor.

[0111] S6: According to the initiation conditions of the metallocene catalyst, re-establish the gaseous component concentrations, usually only need to establish the ethylene and nitrogen concentrations; after the gaseous components are established, inject the single-active-site metallocene catalyst to initiate the reaction and produce single-peak metallocene polyethylene products. Usually, the heat load can be seen in 20 minutes. Among them, the single-active-site metallocene catalyst uses the easy-to-process metallocene catalyst EZ series.

[0112] Example 2

[0113] A production method for converting a bimodal metallocene polyethylene product to a single-peak metallocene polyethylene product, comprising the following steps:

[0114] S1: Before injecting the dual-active-site catalyst (using the same catalyst as in Example 1), stop the feeding of fresh isopentane (i.e., ICA) and the recycle liquid, and quickly reduce the ICA concentration in the system to 60% of the original concentration; put into use the start-up heater, adjust the steam control valve according to the reaction temperature, and maintain the reaction temperature at 100 °C to 110 °C to preheat the reaction system in advance to prevent a large temperature drop after the dual-active-site catalyst is deactivated; increase the injection amount of the bed pretreatment agent (using the same bed pretreatment agent as in Example 1) to 70 ppm, ensure the electrostatic stability and wall temperature stability of the reactor, and maintain the injection until the reaction heat load drops to 0 t / h.

[0115] S2: Stop injecting the dual-active-site catalyst to let the dual-active-site catalyst deactivate naturally. Pay attention to the reactor condensation rate. When the condensation rate is close to 5%, open the angle valve of the CO2 cylinder and inject CO2 for 12 seconds. If the CO2 concentration is less than 2 ppm, inject CO2 again for 5 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm.

[0116] S3: When the polymerization reaction no longer occurs, stop injecting gas-phase components such as ethylene, hydrogen, and comonomer, reduce the discharging frequency of the reactor discharging system, maintain the reactor bed weight at 110 t, the mass required for metallocene start-up, and at the same time maintain the circulating gas velocity at 0.75 m / s and the expanded section liquid level at 45% to 55%.

[0117] S4: Adjust the steam control valve of the start-up heater to make the reactor temperature 80 °C to 90 °C; maintain the reaction temperature, pressure, and fluidization state in the reactor, switch each purge main pipe in the reactor from ethylene to HPPN (high-pressure purified nitrogen) to completely deactivate the original dual-active-site catalyst components in the bed; when the reaction is completely terminated, reduce the reactor pressure to the lowest pressure at which the compressor does not surge; continue to maintain the reaction temperature in the reactor, switch the purge main pipe back from HPPN to ethylene, increase the ethylene inlet amount of the purge main pipe, and displace the materials used for the dual-active-site catalytic polymerization reaction in the reactor by the pressure-lifting and pressure-lowering method. The displacement pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then to 1400 kPa, then drops back to 900 kPa, and then repeats the above process). The displacement standard is that CO2 in the reactor is < 5 ppm.

[0118] S5: Conduct the pretreatment operation on the bed powder in the reactor, select CA-300 or CA-200 as the bed pretreatment agent to completely eliminate the influence of the dual-active-site catalyst residue and bed impurities on the metallocene catalyst; among them, the injection amount of the bed pretreatment agent is 25 ppm of the bed weight, and after injection, it circulates for 1 h under the fluidization state of the reactor.

[0119] S6: According to the initiation conditions of the metallocene catalyst, re - establish the concentrations of the gas - phase components. Usually, only the concentrations of ethylene and nitrogen need to be established. After the gas - phase components are established, inject a single - site metallocene catalyst (the EZ series of metallocene catalysts for easy processing, using the same catalyst as in Example 1) to initiate the reaction. Usually, heat load can be observed within 20 min.

[0120] Comparative Example 1

[0121] A production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product, comprising the following steps:

[0122] S1: Before stopping the injection of the dual - site catalyst (using the same dual - site catalyst as in Example 1), stop the feeding of fresh isopentane (i.e., ICA) and the recycle liquid, and quickly reduce the ICA concentration in the system to 60% of the original concentration. Put into use the start - up heater, adjust the steam control valve according to the reaction temperature to make the reaction temperature 100 °C to 110 °C, so as to pre - heat the reaction system in advance to prevent a large - scale temperature drop after the dual - site catalyst is deactivated. Increase the injection amount of the bed pretreatment agent (the same as the bed pretreatment agent in Example 1) to 70 ppm, ensure the electrostatic stability of the reactor and the stability of the wall temperature, and maintain the injection until the reaction heat load drops to 0 t / h.

[0123] S2: Stop the injection of the dual - site catalyst to make the dual - site catalyst deactivate naturally. Pay attention to the reactor condensation rate. When the condensation rate is close to 5%, open the angle valve of the CO2 cylinder and inject CO2 for 12 seconds. If the CO2 concentration is less than 2 ppm, inject CO2 again for 5 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm.

[0124] S3: When the polymerization reaction no longer occurs, stop the injection of gas - phase components such as ethylene, hydrogen, and comonomer, reduce the discharging frequency of the reactor discharging system, maintain the reactor bed weight at 110 t, the mass required for metallocene start - up, and at the same time maintain the circulating gas velocity at 0.75 m / s and the expanded - section material level at 45% to 55%.

[0125] S4: Close the guide vane opening, set the PDS system parameters, and increase the dumping speed until the bed powder is emptied.

[0126] S5: Adjust the steam control valve of the start - up heater to maintain the reactor temperature at 80 °C to 90 °C.

[0127] S6: Release the reactor pressure to 300 kPa, fully open the on - site steam manual valve, add steam for 30 min, about 400 ppm, corresponding dew point: - 30 °C, and circulate for 1 h.

[0128] S7: Open the crude nitrogen process, purge the reactor with the maximum amount of gas in the main pipe, and flow and displace the reactor at about 500 kPa (ensure that the pressure difference between nitrogen and the reactor is more than 100 kPa) until the hydrocarbon content ≤ 0.2% v / v.

[0129] S8: After the system replacement is qualified, complete the blind flange isolation of the reaction system, disconnect the pipeline, and open the manhole of the reactor for maintenance.

[0130] S9: After the maintenance is completed, replace with nitrogen until the oxygen content ≤ 2% v / v, and perform operations such as filling, replacement, heating up, and establishing gas phase for the metallocene polyethylene seed bed.

[0131] S10: According to the initiation conditions of the metallocene catalyst, re - establish the gas - phase component concentration. Usually, only the concentrations of ethylene and nitrogen need to be established. After the gas - phase components are established in place, inject the high - strength metallocene catalyst HP series to initiate the reaction. Usually, the heat load can be seen within 20 minutes.

[0132] Comparative Example 2

[0133] A production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product, comprising the following steps:

[0134] S1: Before stopping the injection of the dual - active - site catalyst (the same as the dual - active - site catalyst in Example 1), stop the feeding of fresh isopentane (i.e., ICA) and the recycle liquid to quickly reduce the ICA concentration in the system to 60% of the original concentration; put into use the start - up heater, adjust the steam control valve according to the reaction temperature to make the reaction temperature 100°C to 110°C to pre - heat the reaction system in advance to prevent a large - scale temperature drop after the dual - active - site catalyst is deactivated; increase the injection amount of the bed pretreatment agent (the same as the bed pretreatment agent in Example 1) to 70 ppm to ensure the electrostatic stability of the reactor and the stability of the wall temperature, and maintain the injection until the reaction heat load drops to 0 t / h.

[0135] S2: Stop injecting the dual - active - site catalyst to make the dual - active - site - 200 catalyst deactivate naturally. Pay attention to the reactor condensation rate. When the condensation rate is close to 5%, open the angle valve of the CO2 cylinder and inject CO2 for 12 seconds. If the CO2 concentration is less than 2 ppm, inject CO2 again for 5 seconds to ensure that the CO2 concentration is between 3 ppm and 8 ppm.

[0136] S3: When the polymerization reaction no longer occurs, stop injecting gas - phase components such as ethylene, hydrogen, and comonomer, reduce the discharging frequency of the reactor discharging system, maintain the reactor bed weight at 110 t, the required mass for metallocene start - up, and at the same time maintain the circulating gas velocity at 0.75 m / s and the expanded - section material level at 45% to 55%.

[0137] S4: Adjust the steam control valve of the start-up heater to maintain the reactor temperature at 80 °C to 90 °C; maintain the reaction temperature, pressure and fluidization state in the reactor, switch the purge headers of the reactor from ethylene to HPPN (high-pressure purified nitrogen) to completely deactivate the original dual-active-site catalyst components in the bed; after the reaction is completely terminated, reduce the reactor pressure to the minimum pressure at which the compressor does not surge; continue to maintain the reaction temperature in the reactor, switch the purge header back from HPPN to ethylene, increase the ethylene inlet volume of the purge header, and displace the materials used for the dual-active-site catalytic polymerization reaction in the reactor by the pressure-lifting and lowering method. The displacement pressure is 900 kPa to 1100 kPa to 1400 kPa (the displacement pressure rises from 900 kPa to 1100 kPa, then to 1400 kPa, then drops back to 900 kPa, and the above process is repeated), and the displacement standard is CO2 < 5 ppm in the reactor.

[0138] S5: Conduct pretreatment operations on the bed powder in the reactor. Select CA-300 or CA-200 as the bed pretreatment agent to completely eliminate the influence of the dual-active-site catalyst residues and bed impurities on the metallocene catalyst; the injection amount of the bed pretreatment agent is 25 ppm of the bed weight, and after injection, it is circulated for 1 h under the fluidization state of the reactor.

[0139] S6: Re-establish the gas-phase component concentration according to the initiation conditions of the metallocene catalyst. Usually, only the ethylene and nitrogen concentrations need to be established; after the gas-phase components are established, inject the single-active-site metallocene catalyst, here it is the HP series of high-strength metallocene catalysts, to initiate the reaction. Usually, the heat load can be seen within 20 min.

[0140] Table 1 shows the conversion time of the production method for converting the bimodal metallocene polyethylene product to the unimodal metallocene polyethylene product provided in Examples 1 to 2 and Comparative Examples 1 to 2 and the fish-eye level of the unimodal metallocene polyethylene product prepared.

[0141] As can be seen from Table 1, the fish-eye indexes of the metallocene polyethylene film materials prepared by the methods provided in Examples 1, 2 and Comparative Example 1 of the present invention can meet the requirements, and the time used is less than 24 h; and Examples 1, 2 and Comparative Example 1 respectively meet the requirements that the fish-eye (0.8 mm) ≤ 6 pieces / 1520 cm 2 , and the fish-eye (0.4 mm) ≤ 15 pieces / 1520 cm 2The requirements are equivalent to the quality of similar foreign products. For the production conversion method provided in Comparative Example 1, although the fish-eye index of the product can meet the requirements, the entire production conversion adopts a discontinuous production conversion method, which requires emptying the reactor, overhauling, and then restarting the reactor bed, taking 120 h; for the production conversion method provided in Comparative Example 2, although the production conversion time is also less than 24 h, when producing single-peak metallocene polyethylene products, the high-strength metallocene catalyst HP series is used, and the metallocene polyethylene product obtained by its catalytic polymerization is incompatible with the particulate powder in the seed bed when not emptying the bed. There are more fish-eyes in the polyethylene film material prepared from the metallocene polyethylene product obtained thereby. Specifically, the number of fish-eyes (0.4 mm) is greater than 15 per 1520 cm 2 , and the number of fish-eyes (0.8 mm) is greater than 6 per 1520 cm 2 , which is significantly inferior to the production conversion methods provided in Examples 1-2.

[0142] In summary, for the production method of converting a bimodal metallocene polyethylene product to a single-peak metallocene polyethylene product provided by the present invention, the dual-active center catalyst is naturally deactivated, assisted by injecting a small amount of CO2, without emptying the bed powder, without cleaning the reactor, and at the same time using a bed pretreatment agent to avoid caking of the bed caused by electrostatic fluctuations in the reactor. After passing the replacement, a gas phase is established, and a single-active center metallocene catalyst is injected to initiate the reaction to produce a single-peak metallocene polyethylene product. At the same time, the present invention ensures that the fish-eye index of the metallocene film product is qualified by setting a reasonable production conversion sequence and using a reasonable single-active center metallocene catalyst. This method has the following advantages: ① It is possible to complete the conversion of the dual-active center product to the metallocene product without pouring out the dual-active center product seed bed and without reloading the metallocene polyethylene seed bed, minimizing the cost brought by the extraction and loading of the seed bed, greatly reducing the workload and working time, and reducing the inspection and maintenance cost of the device. ② It greatly reduces the influence of the dual-active center series products on the fish-eye level of the metallocene film product, and the fish-eye index of the produced metallocene film product is qualified.

[0143] In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this article, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following (items)" or similar expressions refer to any combination of these items, including any combination of single item (s) or plural item (s).

[0144] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather will conform to the widest scope consistent with the principles and novel features claimed herein.

[0145] Table 1

[0146] 。

Claims

1. A production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product, characterized in that, The production method includes the following steps: 1) Before stopping the injection of the dual-active-site catalyst, stop the feeding of the induced condensate and the recycle liquid; heat to raise the temperature of the reaction system, inject the bed pretreatment agent to make the content of the bed pretreatment agent reach 50 ppm to 80 ppm of the reactor production load, and reduce the reaction heat load to 0 t / h. Among them, the bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-200 and CA-300. The dual-active-site catalyst is the BMC-200 catalyst; 2) Stop injecting the dual-active-site catalyst to make the dual-active-site catalyst deactivate naturally; inject CO2 when exiting the condensation state; the exiting the condensation state means after the condensation rate of the reaction system is 3% to 5%; 3) Stop injecting the gas-phase components, reduce the discharging frequency of the reactor discharging system, and maintain the reactor bed weight to the quality required for the start-up of single-site metallocene polyethylene; 4) Heat to make the temperature of the reaction system reach the reaction temperature of the single-site metallocene polyethylene product; maintain the reaction temperature, pressure and fluidization state of the reaction system, and displace the reaction system with nitrogen; then maintain the reaction temperature of the reaction system and displace the reaction system with ethylene; 5) Pretreat the bed powder in the reaction system with the bed pretreatment agent. Among them, the bed pretreatment agent includes an antistatic agent, and the antistatic agent includes at least one of CA-300 and CA-200. The injection amount of the bed pretreatment agent is 15 ppm to 25 ppm of the bed weight, inject the bed pretreatment agent and make it circulate in the fluidization state of the reaction system for 1 h to 2 h; 6) According to the initiation conditions of the single-active-site metallocene catalyst, re-establish the gas-phase component concentration; inject the single-active-site metallocene catalyst to initiate the reaction for the conversion to the single-site metallocene polyethylene product; the single-active-site metallocene catalyst is the EZ series easy-to-process metallocene catalyst.

2. The production method for converting a bimodal metallocene polyethylene product to a single-site metallocene polyethylene product according to claim 1, characterized in that in the step 1), the induced condensate includes at least one of isopentane, hexane and isobutane; or / and in the step 1), heat to raise the temperature of the reaction system to 100°C to 110°C.

3. The production method for converting a bimodal metallocene polyethylene product to a single-site metallocene polyethylene product according to claim 1 or 2, characterized in that in the step 1), the heating to raise the temperature of the reaction system includes the following steps: Put into use the start-up heater, adjust the steam control valve according to the polymerization reaction temperature of the bimodal metallocene polyethylene product, and preheat in advance to prevent the reaction temperature of the system from dropping significantly after the dual-active-site catalyst is deactivated.

4. The production method for converting a bimodal metallocene polyethylene product to a single-site metallocene polyethylene product according to claim 1, characterized in that in the step 2), when the condensation rate of the reaction system is 3% to 5%, inject CO2 to make its concentration less than 2 ppm, and then inject CO2 again to make its concentration 3 ppm to 8 ppm.

5. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 1 or 2 or 4, characterized in that, In step 3), the injection of the gas-phase components is stopped, the discharging frequency of the reactor discharging system is reduced, the reactor bed weight is maintained to the mass required for the start-up of the unimodal metallocene polyethylene, and at the same time, the circulating gas flow rate is maintained at 0.6 m / s to 0.8 m / s, and the expanded section level is maintained at 35% to 60%; or / and, In step 3), the gas-phase components include ethylene, hydrogen, and comonomer; or / and, In step 3), the mass required for the start-up of the unimodal metallocene polyethylene is 110 t to 150 t.

6. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 5, characterized in that, In step 4), the reaction temperature of the unimodal metallocene polyethylene product is 80°C to 90°C; or / and, In step 4), the reaction system is purged with nitrogen to terminate the polymerization reaction of the bimodal metallocene series products, and the reaction pressure is reduced to the lowest pressure at which the compressor does not surge; or / and, In step 4), the reaction system is purged with ethylene by the method of raising and lowering the pressure.

7. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 6, characterized in that, In step 4), the replacement pressure of the method of raising and lowering the pressure is 900 kPa to 1100 kPa to 1400 kPa.

8. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 7, characterized in that, In step 4), the reaction system is purged with ethylene to displace the material used to deactivate the dual-active-site catalyst in the reactor, and the completion criterion for the replacement is: CO2 < 5 ppm in the reactor.

9. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 1 or 2 or 4 or 6 or 7 or 8, characterized in that, In step 6), the re-establishment of the gas-phase component concentration includes: establishing the ethylene concentration and the nitrogen concentration.

10. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 9, characterized in that, In step 6), a single-active-site metallocene catalyst is injected to initiate the reaction, and the heat load can be seen within 20 min.

11. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 10, characterized in that, The total switching time for converting the bimodal metallocene polyethylene product to the unimodal metallocene polyethylene product is 22 h to 24 h.

12. The production method for converting a bimodal metallocene polyethylene product to a unimodal metallocene polyethylene product according to claim 11, characterized in that, The melt index of the produced single-peak metallocene polyethylene is 0.2 g / 10 min to 20 g / 10 min, and the density is 0.910 g / cm 3 to 0.940 g / cm 3 .