Production method for converting chromium polyethylene into polyolefin elastomer

Abstract

The invention relates to a production method for converting chromium polyethylene into a polyolefin elastomer, and belongs to the technical field of petrochemical engineering. The production method comprises the following steps: 1) controlling the pressure difference of a distribution plate of a to-be-converted gas phase reactor, increasing the discharging frequency of the to-be-converted gas phase reactor, and reducing the bed height of chromium polyethylene product powder; 2) stopping injection of the chromium-based catalyst and the antistatic agent, and injecting polyol; 3) sequentially replacing moisture in the to-be-converted gas phase reactor with nitrogen and pentane; then enabling the gas-phase reactor to enter a condensation state, increasing the water supplementing amount of fluidizing gas cooling circulating water, and reducing the temperature of the gas-phase reactor; according to the method, the yellow index of a polyolefin elastomer product is prevented from being affected by using polyhydric alcohol, the polyolefin elastomer product enters a condensation state after moisture replacement, the production conversion time is shortened, and transition materials are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of petrochemical industry and relates to a production method of converting chromium series polyethylene into polyolefin elastomer. Background Art

[0002] Polyolefin elastomer (POE) refers to elastomers (Elastomers) formed by copolymerization of ethylene and alpha olefins such as ethylene-octene copolymers, ethylene-hexene copolymers and ethylene-butene copolymers. Polyolefin elastomer (POE) is a rapidly developing synthetic polymer. It is an elastomer polymerized under the action of homogeneous metal catalysts and has a wide range of uses. It can be used as rubber, thermoplastic elastomer, plastic impact modifier and toughening agent. It has been well applied in the toughening and modification of various plastics and has become a strong competitor to traditional toughening agents. POE can replace general polymers such as EPM, EPDM, EVA, SBC (styrene bulk copolymer), TPU, PVC, LDPE and EMA, and is widely used in medical packaging materials, automotive parts, wires and cables, daily necessities and toys. Since POE can omit processing links such as vulcanization during use, it can save 25% to 40% of energy consumption and improve processing efficiency by 10 to 20 times.

[0003] At present, the most commercialized POE is ethylene-octene copolymer and vinyl copolymer with octene mass fraction greater than 35% (octene mass fraction is less than 20% is a plastomer). Dow Chemical, Mitsui, and ExxonMobil, which have a relatively high market share, all use the solution method to produce. Although the performance of POE products prepared by the solution method has advantages, the solution method has the following disadvantages: ① High cost of solvent recovery: A large amount of organic solvents are required in the production process, and the recovery and treatment of these solvents require high costs and energy; ② Complex process flow: It involves multiple steps such as solvent addition, separation and recovery, making the entire production process more cumbersome; ③Great environmental impact: The use and discharge of solvents may cause certain pollution and harm to the environment; ④ Relatively low production efficiency: Compared with conventional polyolefin production methods, the solution method has a slower production speed and limited production capacity; ⑤ Large equipment investment: Due to the complexity of the process and the requirements for solvent treatment, more expensive and complex production equipment is required.

[0004] It is precisely the many shortcomings in the above-mentioned process methods and equipment facilities that lead to the high price of POE.

[0005] In order to solve the problems of complex process and equipment in the solution method for producing POE, some researchers use the gas phase method to produce polyolefin elastomers. For example, the Chinese patent document with the authorization announcement number CN108948248B discloses a method, device and application of polyolefin elastomers produced by the gas phase method. The method is to carry out a reaction in a reactor with polyethylene particles as a seed bed, and introduce a raw material mixed gas including ethylene, hydrogen and nitrogen into the reactor to make the polyethylene particles in the seed bed flow and reach a fluidized state, and then add a catalyst into the reactor and introduce atomized droplet-shaped comonomers to react. The comonomers are long-chain olefins with 6 to 18 carbon atoms, and the prepared polymer product has a density of 0.865 to 0.910 g / cm 3 , the long-chain olefin content is 6 to 40 wt%.

[0006] For gas phase polyethylene production equipment, the production grades are mainly titanium and chromium polyethylene, and the corresponding titanium seed beds and chromium seed beds are required during production. In order to make full use of gas phase polyethylene production equipment, some companies that are planning to produce metallocene polyethylene have studied the production method of converting conventional polyethylene (chromium polyethylene or titanium polyethylene) to metallocene polyethylene.

[0007] A Chinese patent document with authorization publication number CN107207649B discloses a method for converting between incompatible catalysts, and relates to a method for converting a first continuous polymerization reaction carried out in a gas phase reactor in the presence of a first catalyst into a second polymerization reaction carried out in a gas phase reactor in the presence of a second catalyst, wherein the first catalyst and the second catalyst are incompatible, and the method comprises: (a) stopping the introduction of the first catalyst into the gas phase reactor; (b) introducing an effective amount of cyclohexylamine into the reactor to at least partially deactivate the first catalyst; (c) introducing an organic metal compound into the reactor and reacting the organic metal compound with the cyclohexylamine; (d) introducing a gas composition for the second polymerization reaction into the reactor; and (e) introducing the second catalyst into the reactor.

[0008] A Chinese patent document with authorization announcement number CN114621378B discloses a method for switching between different chromium series catalysts of gas phase polyethylene, comprising the following steps: stopping the feeding of the first chromium series catalyst, setting the discharge program according to the discharge interval, and reducing the bed material level of the gas phase reactor to not less than the lowest fluidization height of the bed that can be operated under the polymerization reaction conditions; adjusting the process conditions of the gas phase reactor, starting to add the second chromium series catalyst according to ≤ 1 / 2 of the normal consumption, and when the reaction is established, starting to inject oxygen, and gradually increasing the feed amount of the second chromium series catalyst and the oxygen injection amount until the ethylene load is controlled to reach 100%; during the switching process, observing the process parameters of the gas phase reactor that characterize the fluidization state, and adjusting the process conditions in time; fine-tuning the relevant process conditions according to the process control conditions of the second chromium series catalyst and the polymerization product indicators; when the polymerization product indicators are tested to be qualified, the switching process ends smoothly.

[0009] The Chinese patent document with the authorization announcement number CN 117567675 B discloses a production method of converting chromium-based polyethylene to metallocene polyethylene, which belongs to the field of petrochemical technology; the method includes: adjusting the gas phase reactor to be converted to the conversion state, and then injecting a terminator; replacing the gas phase reactor to be converted, and then injecting a pretreatment agent; re-establishing the gas phase component concentration according to the initiation conditions of the metallocene polyethylene, and then injecting a metallocene catalyst to initiate the reaction and complete the conversion; wherein the pretreatment agent includes an antistatic agent; the whole process of this method can complete the start-up of metallocene polyethylene without using a metallocene polyethylene seed bed, which minimizes the cost of extracting and filling the seed bed and avoids the construction cost of adding a metallocene polyethylene seed silo. At the same time, the start-up time of metallocene polyethylene is reduced. Since there is no need to extract the chromium-based seed bed and fill the metallocene polyethylene seed bed, the workload and working time are greatly reduced, and the inspection and maintenance costs of the device are reduced.

[0010] However, the above-mentioned prior art of using chromium catalyst to convert to metallocene catalyst still has the disadvantages of complex conversion process, long material qualification time, large amount of transition material, easy blockage of pipeline, short production cycle, etc. In addition, the above-mentioned prior art only involves the method of switching different catalyst systems when preparing polyethylene by gas phase method, and none of them solves the problem of large amount of transition material when converting to POE, and cannot effectively eliminate the negative impact of antistatic agent on the appearance of POE particles. Summary of the invention

[0011] In order to solve the problems existing in the prior art, the present invention provides a production method for converting chromium-based polyethylene into polyolefin elastomer. The production method can greatly reduce the conversion time of polyolefin elastomer, reduce the amount of transition materials, solve the problem that the preceding antistatic agent is difficult to replace cleanly, effectively eliminate the negative impact of the preceding antistatic agent on the particle appearance of the polyolefin elastomer, and reduce the yellowness index of the polyolefin elastomer product.

[0012] In a first aspect, the present invention provides a method for converting chromium-based polyethylene into polyolefin elastomer, the production method comprising the following steps: 1) Control the pressure difference of the distribution plate of the gas phase reactor (or gas phase fluidized bed) to be converted to make the pores of the distribution plate of the gas phase reactor to be converted to be unobstructed; increase the discharge frequency of the gas phase reactor to be converted to reduce the bed height of the chromium polyethylene product powder; the gas phase reactor includes a straight section and an expansion section; 2) Stop the injection of chromium catalyst and antistatic agent, and inject polyol; 3) Use nitrogen and pentane to replace the water in the gas phase reactor to be converted in turn; after the water replacement is completed, the gas phase reactor is put into a condensed state; increase the water replenishment amount of the fluidizing gas cooling circulating water and reduce the reactor temperature; 4) Re-adjusting the concentration of the gas phase component of the polyolefin elastomer according to the polymerization initiation conditions of the polyolefin elastomer, and then injecting the metallocene catalyst to initiate the reaction; 5) After the initiation reaction is completed and the polymerization product in the bed is replaced, the bed height is raised to the expansion section of the gas phase reactor, the material level is maintained, and the production conversion is completed.

[0013] As an optional embodiment, the polyolefin elastomer is obtained by polymerization of ethylene and α-olefin using a metallocene catalyst, wherein the α-olefin includes at least one of octene, hexene and butene; the density of the polyolefin elastomer is less than 0.89 g / cm 3 .

[0014] In the present invention, chromium-based polyethylene products refer to polyolefin products produced using chromium-based catalysts, such as polyethylene plastics and polyethylene-α-olefin copolymers. Polyolefin elastomers refer to elastomers obtained by polymerization of ethylene and α-olefin using metallocene catalysts.

[0015] As an optional embodiment, the gas phase reactor (or gas phase fluidized bed) includes a straight section and an expansion section.

[0016] The straight section of the gas phase reactor usually has a relatively regular cylindrical shape, and its diameter is relatively stable, providing the main reaction site for the polymerization reaction of polyethylene. The raw gas contacts the catalyst in the straight section, undergoes polymerization reaction, and generates polyethylene particles. The regular shape of the straight section helps to form a stable flow field, making the flow of gas and particles more uniform and reducing local turbulence and dead zones.

[0017] The expansion section of the gas phase reactor is located above the straight section and is shaped like a gradually expanding cone. After the polyethylene particles enter the expansion section, the gas flow rate decreases due to the increase in cross-sectional area. The polyethylene particles are separated from the gas under the action of gravity and return to the straight section. The expansion section helps to reduce the entrainment of polyethylene particles by gas and can reduce the load of subsequent separation equipment.

[0018] As an optional embodiment, in step 1), the pressure difference of the distribution plate of the gas phase reactor to be converted is controlled at 18 kPa to 24 kPa.

[0019] There are numerous small holes on the distribution plate of the gas-phase reactor to be converted, which may be clogged by powder or sticky materials. By adjusting the operating conditions of the gas-phase reactor to be converted, such as increasing the fluidizing gas velocity, or / and increasing the condensate concentration, the clogged materials in the small holes can be flushed away, thereby ensuring that the pores on the distribution plate of the gas-phase reactor to be converted are in an unobstructed state.

[0020] In the present invention, when the pressure difference of the distribution plate is too large, many small holes on the distribution plate are blocked by polymers, and the fluidization circuit is blocked, which leads to untimely heat removal and easy occurrence of implosion.

[0021] In the present invention, in step 1), the pores of the distribution plate of the gas phase reactor to be converted are kept in an unobstructed state, and the pressure difference of the distribution plate is controlled in the lowest range; when the pores of the distribution plate are in an unobstructed state, the discharge frequency of the discharge system of the gas phase reactor to be converted is increased to reduce the height of the bed of the chromium polyethylene product powder, so that the chromium polyethylene product powder to be replaced is at the lowest material level.

[0022] As an optional embodiment, in step 1), the bed height of the chromium polyethylene powder is 75% to 90% of the straight section of the gas phase fluidized bed (ie, the gas phase reactor).

[0023] As an optional embodiment, the polyol is at least one of ethylene glycol and glycerol.

[0024] In the present invention, in step 2), the injection of the chromium catalyst is stopped and the polyol is injected to quickly remove the antistatic agent in the gas phase reactor to be converted and deactivate the activity of the chromium catalyst (i.e., to terminate the activity of the catalyst). Here, the antistatic agent is the antistatic agent added during the production of chromium polyethylene.

[0025] As an optional embodiment, the addition amount of the polyol is 13 kg / h to 23 (ie, 18±5) kg / h.

[0026] In the present invention, if the amount of polyol added is too little, the activity of the catalyst cannot be terminated; if the amount added is too much, the excess polyol becomes a poison to the metallocene catalyst in the gas phase reactor and needs to be replaced by multiple replacements to avoid the adverse effects of excess polyol on the activity of the metallocene catalyst in the production process of polyolefin elastomers.

[0027] As an optional implementation, in step 2), the injection of the chromium-based catalyst is stopped, and the reaction is continued for 3 to 5 hours before injecting the polyol.

[0028] As an optional implementation, when nitrogen is used to replace the gas phase reactor to be converted, the replacement adopts a pressure increase and decrease method.

[0029] As an optional implementation, the low pressure of the boost-and-drop pressure method is 600 kPa to 900 kPa; the high pressure of the boost-and-drop pressure method is 1100 kPa to 1400 kPa.

[0030] As an optional implementation, when nitrogen is used to replace the gas phase reactor to be converted, the replacement is completed when the water content in the reaction system is ≤50 ppm.

[0031] As an optional implementation, when pentane is used to replace the gas phase reactor to be converted, the replacement adopts a pressure increase and decrease method.

[0032] As an optional implementation, when pentane is used to replace the gas phase reactor to be converted, the replacement is completed when the water content in the reaction system is ≤20 ppm.

[0033] In the present invention, in step 3), nitrogen and pentane are used to replace the water in the gas phase reactor to be converted in sequence; before the water replacement, the reaction system is in a state where poisons are to be removed; there is toxic water in the reactor, and the water is the product of the reaction between polyols and polyethylene; after the water replacement is completed, the gas phase reactor enters a condensed state; the water replenishment amount of the fluidizing gas cooling circulating water is increased, and the reactor temperature is reduced.

[0034] As an optional embodiment, in step 3), after the water replacement is completed, the pentane concentration is maintained at 40 kPa to 70 kPa to allow the gas phase reactor to enter a condensed state.

[0035] After the water replacement is completed, the present invention adjusts the pentane concentration to 40 kPa to 70 kPa, and does not need to be converted in a dry state, and can directly enter a condensed state, thereby increasing the heat removal capacity of the polymerization reaction system.

[0036] As an optional implementation, in step 3), the opening of the cooling water inlet valve is increased by 15% to 20% to increase the replenishment amount of fluidizing gas cooling circulating water.

[0037] As an optional embodiment, in step 3), the reactor temperature is 75°C to 80°C.

[0038] In the present invention, in step 3), the replenishment amount of the fluidizing gas cooling circulating water is increased. Since the melting point of the polyolefin elastomer is lower than that of the chromium-based polyethylene product before the production change, the temperature of the reactor is lowered to avoid the POE product from becoming sticky and clogging.

[0039] As an optional embodiment, the polyolefin elastomer catalyst is a metallocene catalyst.

[0040] In the present invention, the polyolefin elastomer catalyst may be a metallocene catalyst commonly used in the art, for example, the polyolefin elastomer catalyst includes a bridged dioxadiene catalyst and a CGC catalyst.

[0041] As an optional implementation, in step 5), replacing the polymer product in the bed means replacing the chromium polyethylene powder produced in the reactor bed with polyolefin elastomer powder through a discharge system.

[0042] As an optional embodiment, the conversion time of the production method of chromium polyethylene to polyolefin elastomer is 12 hours to 20 hours. The conversion time refers to the time from the decrease of the bed height of the chromium polyethylene product powder to the time when the metallocene catalyst initiates the reaction and the bed height is raised to the expansion section of the gas phase reactor.

[0043] As an optional embodiment, the production method of converting chromium-based polyethylene to polyolefin elastomer produces 80 t to 330 t of transition material.

[0044] In the present invention, the above technical features can be freely combined to form a new technical solution without conflicting with each other.

[0045] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects: (1) The method provided by the present invention does not need to kill the activity of the chromium catalyst in the gas phase reactor (that is, it does not need to completely terminate the polymerization reaction of the preceding chromium polyethylene), so that the distribution plate of the gas phase reactor is kept unobstructed. Without emptying the bed powder or cleaning the gas phase reactor, the bed height of the chromium powder in the gas phase reactor is reduced, and polyol is injected to eliminate the influence of the residual antistatic agent in the bed on the polyolefin elastomer catalyst. After the water in the gas phase reactor is replaced with nitrogen and pentane (under dry operation mode), the gas phase reactor enters a condensed state and maintains low temperature operation. The gas phase components are adjusted, and the polyolefin elastomer catalyst is injected to initiate the reaction, thereby completing the production conversion; (2) The method provided by the present invention does not require emptying the bed powder, cleaning the reactor, or killing the polymer activity during the entire process. The use of polyols can eliminate the residual antistatic agent in the chromium catalyst system, thereby avoiding affecting the yellow index of subsequent polyolefin elastomer products. During the conversion, the gas phase reactor enters a condensed state after water replacement, which greatly shortens the conversion time and reduces the amount of transition material. (3) The production method provided by the present invention does not need to kill the activity of the gas phase reactor. Without emptying the bed powder or cleaning the gas phase reactor, polyol is injected to eliminate the influence of the residual antistatic agent in the bed on the polyolefin elastomer catalyst. After nitrogen and pentane are used to replace the water in the gas phase reactor, the gas phase reactor enters a condensed state; the yellowness index of the subsequent polyolefin elastomer product will not be negatively affected; (4) Since the production method provided by the present invention does not have the steps of killing and re-establishing the gas phase, the amount of transitional materials during the conversion process is greatly reduced. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0047] The catalysts commonly used in the production of polyolefin elastomers are mainly constrained geometry metallocene catalysts (CGC), which are generally a type of metal organic complex formed by the interaction of transition metal elements (such as Ti, Zr, Hf, etc., group IVB elements) or rare earth metal elements and at least one cyclopentadienyl or its derivatives through covalent bonds or coordination bonds. The catalysts currently used in the commercial production of polyolefin elastomers are mainly bridged dicyclopentadienyl catalysts and CGC catalysts (i.e., metallocene catalysts), which are very different from the components of traditional Ziegler-Natta catalysts and chromium-based catalyst systems and are more sensitive to impurity components in the raw materials. Therefore, in the process of converting chromium-based catalysts to metallocene catalysts in the gas phase polymerization process to produce polyolefin elastomers, the domestic main method for conversion is semi-continuous production, but this production method has the following disadvantages: ① After the polymerization activity is killed, it needs to be replaced and the gas phase components need to be re-established, which will produce a large amount of transition material; ② The residual antistatic agent in the chromium-based polymerization system will affect the appearance of subsequent polyolefin elastomer products and make the particles yellow.

[0048] To this end, the inventor intends to provide a method for reducing the transition material in the polyolefin elastomer produced by the chromium polyethylene conversion. When the pores of the gas phase fluidized bed (i.e., gas phase reactor) distribution plate are kept unobstructed, there is no need to kill the activity of the gas phase reactor, the bed powder is not emptied, and the gas phase reactor is not cleaned. By reducing the bed height of the chromium powder in the gas phase reactor, polyols are injected to eliminate the influence of the residual antistatic agent in the bed on the polyolefin elastomer catalyst; after nitrogen and pentane are used to replace the water in the gas phase reactor, the gas phase reactor enters a condensed state, keeps low temperature operation, adjusts the gas phase components, injects a polyolefin elastomer catalyst to initiate the reaction, and completes the conversion. The method provided by the present invention does not empty the bed powder, clean the gas phase reactor, or kill the polymer activity during the whole process. The use of polyols can eliminate the residual antistatic agent in the chromium catalyst system and avoid affecting the yellow index of the subsequent polyolefin elastomer product; in the conversion, pentane (which can be used as a condensing agent) is used to replace the water in the gas phase reactor, and the gas phase reactor is adjusted to enter a condensed state, so that the conversion time is greatly shortened, and the amount of transition material can be reduced. During the conventional conversion process, the product must first be dried and then condensed.

[0049] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0050] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0051] Example 1 A method for producing polyolefin elastomer by converting chromium polyethylene into polyolefin elastomer comprises the following steps.

[0052] S1: Control the pressure difference of the distribution plate to 23 kPa, so that the pores of the distribution plate of the gas phase reactor to be converted are in an unobstructed state; increase the discharge frequency of the discharge system of the gas phase reactor to be converted to reduce the bed height of the chromium polyethylene product powder to 85% of the height of the straight section of the gas phase reactor, so that the chromium powder to be replaced is at the lowest material level.

[0053] S2: Stop the injection of chromium catalyst and antistatic agent, and inject ethylene glycol at 18±5 kg / h to neutralize the antistatic agent in the gas phase reactor; S3: Nitrogen was used to replace the water in the reaction system to 45 ppm, and then pentane was used to continue replacing the water in the reaction system to 10 ppm, so that the gas phase reactor entered a condensed state and the pentane concentration was maintained at 68 kPa.

[0054] S4: Lower the temperature in the gas phase reactor to 78°C and readjust the gas phase components. After the gas phase components are in place, inject the polyolefin elastomer catalyst (i.e., metallocene catalyst) to initiate the reaction. Usually, after the gas phase reactor is stabilized, the temperature of the reaction system can be seen to rise within 10 minutes, indicating that the polyolefin elastomer catalyst has initiated the reaction.

[0055] S5: 3 hours after the initiation reaction in step S4 is completed, the chromium polyethylene powder in the gas phase reactor bed is completely replaced with polyolefin elastomer powder through the discharge system, that is, the bed replacement is completed, and the bed height is rapidly raised to the junction of the expansion section and the straight section of the gas phase reactor, and the material level of the polyolefin elastomer powder in the bed is kept unchanged, thereby completing the conversion of polyolefin elastomer.

[0056] Example 2 A method for producing polyolefin elastomer by converting chromium polyethylene into polyolefin elastomer comprises the following steps.

[0057] S1: Control the pressure difference of the distribution plate to 22 kPa, so that the pores of the distribution plate of the gas phase reactor to be converted are in an unobstructed state; increase the discharge frequency of the discharge system of the gas phase reactor to be converted to reduce the height of the chromium polyethylene product powder bed to 80% of the height of the straight cylinder section, so that the chromium powder to be replaced is at the lowest material level.

[0058] S2: Stop the injection of chromium catalyst and antistatic agent, and inject 18±5 kg / h of glycerol to neutralize the antistatic agent in the gas phase reactor; S3: nitrogen was used to replace the water in the reaction system to 40 ppm, and then pentane was used to continue replacing the water in the reaction system to 6 ppm, so that the gas phase reactor entered a condensed state and the pentane concentration was maintained at 50 kPa.

[0059] S4: Lower the temperature in the gas phase reactor to 76°C and readjust the gas phase components. After the gas phase components are in place, inject the polyolefin elastomer catalyst to initiate the reaction. Usually, after the gas phase reactor is stabilized, the temperature of the reaction system can be seen to rise within 16 minutes, indicating that the polyolefin elastomer catalyst has initiated the reaction.

[0060] S5: After the initiation reaction in step S4 is completed, 4.5 hours later, the chromium polyethylene powder in the gas phase reactor bed is completely replaced with polyolefin elastomer powder through the discharging system, that is, the bed replacement is completed, and the bed height is rapidly raised to the junction of the expansion section and the straight section of the gas phase reactor, keeping the material level of the polyolefin elastomer powder in the bed unchanged, thereby completing the conversion of polyolefin elastomer.

[0061] Example 3 A method for producing polyolefin elastomer by converting chromium polyethylene into polyolefin elastomer comprises the following steps.

[0062] S1: Control the pressure difference of the distribution plate to 19 kPa, so that the pores of the distribution plate of the gas phase reactor to be converted are in an unobstructed state; increase the discharge frequency of the discharge system of the gas phase reactor to be converted to reduce the height of the chromium polyethylene product powder bed to 75% of the height of the straight cylinder section, so that the chromium powder to be replaced is at the lowest material level.

[0063] S2: Stop the injection of chromium catalyst and antistatic agent, and inject 18±5 kg / h of glycerol to neutralize the antistatic agent in the gas phase reactor; S3: Nitrogen was used to replace the water in the reaction system to 25 ppm, and then pentane was used to continue replacing the water in the reaction system to 4 ppm, so that the gas phase reactor entered a condensed state and the pentane concentration was maintained at 45 kPa.

[0064] S4: Lower the temperature in the gas phase reactor to 75°C and readjust the gas phase components. After the gas phase components are in place, inject the polyolefin elastomer catalyst to initiate the reaction. Usually, after the gas phase reactor is stabilized, the temperature of the reaction system can be seen to rise within 20 minutes, indicating that the polyolefin elastomer catalyst has initiated the reaction.

[0065] S5: After the initiation reaction in step S4 is completed, 4 hours later, the chromium polyethylene powder in the gas phase reactor bed is completely replaced with polyolefin elastomer powder through the discharge system, that is, the bed replacement is completed, and the bed height is rapidly raised to the junction of the expansion section and the straight section of the gas phase reactor, keeping the material level of the polyolefin elastomer powder in the bed unchanged, thereby completing the conversion of polyolefin elastomer.

[0066] Comparative Example 1 This comparative example provides a method for producing polyolefin elastomer from chromium-based polyethylene, which differs from Example 1 only in step S1, and other processes and conditions are the same as those of Example 1. Specifically, S1: Make the pores of the distribution plate of the gas phase reactor to be converted to an unobstructed state, and control the pressure difference of the distribution plate to the lowest range of 26 kPa; keep the discharge frequency of the gas phase reactor discharge system unchanged, so that the powder bed height of the gas phase reactor is still maintained at the highest position, located at the expansion section of the gas phase reactor.

[0067] Comparative Example 2 This comparative example provides a method for producing polyolefin elastomer from chromium-based polyethylene, which differs from Example 2 only in step S2, and other processes and conditions are the same as those of Example 2. Specifically, S2: Stop injecting chromium catalyst and antistatic agent, and add deactivating agent CO, CO2 or N2 or O2.

[0068] Comparative Example 3 This comparative example provides a method for producing polyolefin elastomer from chromium-based polyethylene, which differs from Example 3 only in step S4, and other processes and conditions are the same as those of Example 3. Specifically, S4: The temperature of the gas phase reactor is maintained at 85°C, the gas phase components are readjusted, and the polyolefin elastomer catalyst is injected to carry out the reaction.

[0069] The polyolefin elastomer product particles prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were tested, and the results are shown in Table 1.

[0070] Table 1: .

[0071] Note: The conversion time is the time from when the bed height of the chromium polyethylene product powder drops to when the polyolefin elastomer catalyst initiates the reaction and the bed height is raised to the expansion section of the gas phase reactor.

[0072] As shown in Table 1, the conversion time of chromium-based polyethylene to polyolefin elastomer by the methods provided in Examples 1 to 3 of the present invention is shorter, less transition material is used, and the yellowness index of the product is lower.

[0073] Compared with Example 1, in Comparative Example 1, the pressure difference of the distribution plate is larger, resulting in the distribution plate being not smooth enough and the bed replacement being slow, thereby resulting in more transition materials being generated during the replacement process; in addition, the bed height of the chromium-based polyethylene powder is relatively high, and there is more preceding powder (i.e., chromium-based polyethylene powder) in the gas phase reactor, so the conversion time is longer and more transition materials are generated.

[0074] Compared with Example 2, in Comparative Example 2, after stopping the injection of the chromium-based catalyst and the antistatic agent, a deactivating agent CO, CO2, N2, or O2 is added. Since deactivating agents such as CO, CO2, N2, or O2 will directly kill the preceding chromium-based polyethylene reaction, the reaction needs to be reactivated and the gas phase needs to be rebuilt, which increases the time consumption and the amount of transition material.

[0075] Compared with Example 3, in Comparative Example 3, the temperature of the gas phase reactor in step S4 is relatively high (maintained at 85° C.), and the polyolefin elastomer product generated during the conversion process will be sticky and easily clog the pipeline. Therefore, it is necessary to adjust other process parameters (such as fluidizing gas velocity and condensate concentration) to flush the sticky material, which takes a long time to solve the clogging problem, thereby resulting in an increase in transition material.

[0076] Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present invention; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0077] In the present invention, in the description of the specification, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second", etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" means one or more, and "plurality" means two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items.

[0078] The above are only specific embodiments of the present invention, which enable those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for converting chromium-based polyethylene into polyolefin elastomer, characterized in that: The following steps are involved: 1) Controlling the pressure difference of the distribution plate of the gas phase reactor to be converted to make the pores of the distribution plate of the gas phase reactor to be converted to be unobstructed, increasing the discharge frequency of the gas phase reactor to be converted to be converted, and reducing the bed height of the chromium polyethylene product powder, wherein the gas phase reactor includes a straight section and an expansion section; 2) Stop the injection of chromium catalyst and antistatic agent, and inject polyol; 3) Nitrogen and pentane are used to replace the water in the gas phase reactor to be converted in turn. After the water replacement is completed, the gas phase reactor enters a condensed state, the water replenishment amount of the fluidizing gas cooling circulating water is increased, and the temperature of the gas phase reactor is reduced; 4) According to the polymerization initiation conditions of the polyolefin elastomer, the concentration of the gas phase components is readjusted and then the metallocene catalyst is injected; 5) After the initiation reaction is completed and the polymerization product in the bed is replaced, the bed height is raised to the expansion section of the gas phase reactor, the material level is maintained, and the production conversion is completed.

2. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1, characterized in that: The polyolefin elastomer is obtained by polymerization of ethylene and α-olefin using a metallocene catalyst, wherein the α-olefin includes at least one of octene, hexene and butene, and the density of the polyolefin elastomer is less than 0.89 g / cm 3 .

3. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1 or 2, characterized in that: In step 1), the pressure difference of the distribution plate of the gas phase reactor to be converted is 18 kPa to 24 kPa.

4. The method for producing polyolefin elastomer from chromium polyethylene according to claim 3, characterized in that: In step 1), the bed height of the chromium polyethylene powder is 75% to 90% of the straight section of the gas phase reactor.

5. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1, 2 or 4, characterized in that: In step 2), the polyol is at least one of ethylene glycol and propylene glycol.

6. The method for producing polyolefin elastomer from chromium polyethylene according to claim 5, characterized in that: In step 2), the amount of polyol added is 13 kg / h to 23 kg / h.

7. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1, 2, 4 or 6, characterized in that: In step 2), the injection of the chromium catalyst is stopped, and the reaction is continued for 3 to 5 hours before injecting the polyol.

8. The method for producing polyolefin elastomer from chromium polyethylene according to claim 7, characterized in that: In step 3), when nitrogen and pentane are used to replace the gas phase reactor to be converted, the replacement adopts the pressure increase and decrease method.

9. The method for producing polyolefin elastomer from chromium polyethylene according to claim 8, characterized in that: When nitrogen is used to replace the gas phase reactor to be converted, the replacement is completed when the water content in the reaction system is ≤50 ppm.

10. The method for producing polyolefin elastomer from chromium polyethylene according to claim 8 or 9, characterized in that: When pentane is used to replace the gas phase reactor to be converted, the replacement is completed when the water content in the reaction system is ≤20 ppm.

11. The method for converting chromium polyethylene into polyolefin elastomer according to claim 10, characterized in that: In step 3), after the water replacement is completed, the pentane concentration is maintained at 40 kPa to 70 kPa to allow the gas phase reactor to enter a condensed state.

12. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1, 2, 4, 6, 8, 9 or 11, characterized in that: In step 3), the opening of the cooling water inlet valve is increased by 15% to 20% to increase the replenishment amount of fluidizing gas cooling circulating water.

13. The method for producing polyolefin elastomer from chromium polyethylene according to claim 12, characterized in that: In step 3), the reactor temperature is 75°C to 80°C.

14. The method for producing polyolefin elastomer from chromium polyethylene according to claim 1 or 2 or 4 or 6 or 8 or 9 or 11 or 13, characterized in that: The conversion time of the production method of converting chromium-based polyethylene to polyolefin elastomer is 12 hours to 20 hours.

15. The method for producing polyolefin elastomer from chromium polyethylene according to claim 14, characterized in that: The production method of converting chromium-based polyethylene to polyolefin elastomers produces 80 t to 330 t of transition material.