A polymerization process for a polypropylene composition

Abstract

The application discloses a polymerization method of a polypropylene composition, and belongs to the polymerization field. In the method, olefin monomers containing propylene are contacted with a catalyst system in an upstream polymerization section of a gas-phase copolymerization reactor to form polymer particles, a condensing medium is introduced in at least one upstream polymerization section or between or after the upstream polymerization sections; the obtained polymer particles and the condensing medium of the upstream polymerization section enter the gas-phase copolymerization reactor in the form of wet particles, and propylene and alpha-olefins are polymerized in the gas-phase copolymerization reactor in the presence of the condensing medium to obtain a polypropylene composition. The condensing medium can effectively improve the heat dissipation of the upstream polymerization section, make the polymer particle distribution more uniform, avoid the generation of local hot spots to cause the phenomena of stuck and caking, improve the heat removal capacity of the gas-phase copolymerization reactor and the space-time yield, enhance the copolymerization activity of the olefin monomers in the gas-phase reactor, and improve the bonding condition of the particle surface.

Description

Technical Field

[0001] This invention belongs to the field of polymerization, and specifically relates to a polymerization method for a polypropylene composition. Background Technology

[0002] In existing multiphase copolymer polypropylene polymerization processes, a typical approach involves producing the product in two tandem reaction stages. First, isotactic polypropylene (iPP) particles are produced in liquid or gaseous propylene. These polypropylene particles are then transferred to a gas-phase fluidized bed reactor, where an elastomer phase is generated within the isotactic polypropylene matrix through ethylene-propylene gas-phase copolymerization. While isotactic polypropylene can be produced in a liquid loop reactor or a gas-phase reactor, ethylene-propylene gas-phase copolymerization always takes place in a gas-phase reactor to avoid the dissolution of the amorphous copolymer; therefore, a slurry process cannot be used.

[0003] Existing technologies for producing high-impact copolymer polypropylene often result in sticky polymers that can clog pipelines and adhere to reactor walls, disrupting production. Therefore, existing technologies often add an anti-sticking agent—LPN (low-purity nitrogen)—to prevent the formation of unwanted sticky copolymers on the polymer particle surface. However, adding large amounts of LPN introduces several problems. For instance, the oxygen in LPN and the molecular weight regulator hydrogen have a wide explosion limit, and combined with the electrostatic effect on the polymer surface, there is a potential hazard of oxyhydrogen explosion. For safety reasons, hydrogen cannot be added during copolymer production with the anti-sticking agent, limiting product performance optimization and the production of different grades of products.

[0004] Furthermore, the catalyst activity for ethylene-propylene gas-phase copolymerization decays rapidly after the previous stage of isotactic polypropylene production. Existing technologies primarily aim to increase the copolymerization activity of the catalyst or reduce the residence time in homopolymer polypropylene production to achieve higher copolymerization activity; however, this reduces the catalyst's production capacity. As is well known in the art, for the same polypropylene homopolymer matrix, the larger the molecular weight of the impact-resistant polypropylene copolymer, the higher the impact strength of the impact-resistant polypropylene. Therefore, to ensure higher impact strength in impact-resistant polypropylene, efforts should be made to increase the molecular weight of the ethylene-propylene copolymer. Existing technologies mainly involve selecting suitable polypropylene catalysts to match the ideal copolymer molecular weight, which places high demands on the selection of catalyst characteristics.

[0005] Gas-phase copolymerization reactors have limited heat removal capabilities due to the relatively low heat capacity of the ethylene and propylene reaction media. Although the heat of polymerization in ethylene-propylene copolymerization is about 33% lower than that of ethylene, the amorphous rubber of ethylene-propylene melts more easily when heated, and poor heat transfer control can lead to product sticking together, even causing reactor shutdown in severe cases. Therefore, good temperature control and effective enhancement of heat transfer capacity in gas-phase copolymerization reactions are crucial. US patents US 4543399 and US4588790 disclose a process that cools the circulating gas below the dew point to achieve liquid-bearing operation in a fluidized bed. This process is called the "condensation-mode polyolefin process," or simply "condensation mode" or "condensation process." Since the vaporization of the liquid stream absorbs a large amount of heat, it improves the heat removal capacity of the fluidized bed reaction, thereby increasing the space-time yield of the reactor.

[0006] The latest technique used in condensation mode operation is to use nitrogen as a non-volatile component of the circulating gas stream, while allowing small amounts of ethane and methane to increase the heat capacity of the circulating gas stream. Inert condensing media such as isopentane and n-hexane typically serve as moderately volatile components, evaporating away most of the heat of polymerization and significantly increasing the heat capacity of the circulating gas stream. US patents US4588790, US5436304, US5405922, and US5352749 disclose methods for polymerizing α-olefins in a gas-phase reactor with a fluidized bed and fluidizing media, increasing the reactor's heat removal capacity by adding inert condensing liquids. WO2011147539 discloses a method for gas-phase polymerization of one or more olefin monomers, preferably ethylene, in a fluidized bed reactor using a gas stream containing inert gases in a dry mode or (super)condensation mode. These methods are characterized by the fact that the circulating gas flow in the gas-phase reactor contains up to 50% inert condensate, thereby increasing the space-time yield of the reactor by increasing the heat capacity of the circulating gas flow and removing a large amount of polymerization heat through the vaporization absorption of the condensate.

[0007] Most existing slurry polymerization processes employ a multi-reactor series configuration and can be used to produce propylene homopolymers, propylene-ethylene random copolymers, and propylene-ethylene block copolymers. Generally, homopolymers and random copolymers are produced in a slurry, while the production of block copolymers requires flash evaporation to remove the solvent from the upstream process. Therefore, the slurry method is typically more suitable for producing homopolymers or random copolymers with low comonomer content, and is not the optimal choice for producing high-performance copolymers. Due to the large amount of solvent used, slurry polymerization requires an efficient solvent recovery system to reduce operating costs and environmental pollution; however, the solvent recovery process is complex and energy-intensive. Furthermore, after long-term operation, scaling occurs inside the reactor, which not only reduces heat transfer efficiency but also leads to unstable product quality and increases equipment maintenance frequency. Therefore, the slurry method is gradually being replaced by bulk polymerization and gas-phase polymerization, or a combination of both. Currently, the world's slurry-process PP production capacity accounts for less than 10% of the total global PP production capacity.

[0008] While existing gas-phase polymerization or bulk polymerization technologies are more mature than slurry polymerization, they still face challenges such as uneven polymer particle dispersion in the upstream propylene homopolymerization or random copolymerization stage, leading to hot spots, agglomeration, and reactor sticking. Some patents mention introducing additional condensing media in the downstream gas-phase copolymerization stage to achieve advantages such as reduced fine powder content in the polypropylene composition, increased copolymer molecular weight, reduced sticking, significantly improved copolymerization activity, and increased ethylene content. However, this approach fails to address the problems encountered in the upstream propylene homopolymerization or random copolymerization stage. Furthermore, the downstream gas-phase copolymerization stage is a high-pressure gas-phase reaction, and in practice, ensuring the condensing media is added to the high-pressure gas-phase reactor and uniformly distributed within it is a significant challenge. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a polymerization method for a polypropylene composition. In the presence of at least one gas-phase copolymerization reactor, a condensing medium is introduced upstream of, between, or after the upstream polymerization stage. Olefin monomers containing propylene contact with a catalyst system in the upstream polymerization stage to form polymer particles. The polymer particles and the condensing medium enter the gas-phase copolymerization reactor in wet particle form. In the presence of the condensing medium, propylene and α-olefins polymerize in the gas-phase copolymerization reactor to obtain a polypropylene composition. The upstream stage, by introducing a condensing medium, effectively dissipates heat from the reactor and makes the polymer particles more uniformly distributed, avoiding localized hot spots that cause sticking and agglomeration. The condensing medium can be introduced not only within the upstream reactor but also between two upstream reactors or after the upstream polymerization stage through particle entrainment, thereby reducing energy consumption. The polymer enters the gas-phase reactor in wet particle form, eliminating the need for complete flash evaporation; only partial flash evaporation is required to remove light components (such as hydrogen), effectively saving energy. The condensing medium entering the gas-phase reactor can adjust the dew point of the circulating gas flow, thereby increasing the copolymerization activity of olefin monomers and the molecular weight of olefin copolymers in the gas-phase reactor. This method increases the reactor polymerization activity by 30-60%, making it suitable for producing high-ethylene-content, impact-resistant polypropylene. The content of polymer fines is effectively reduced, eliminating the need for anti-sticking agents, and the equipment can operate stably for extended periods.

[0010] This invention is achieved through the following technical solution: a polymerization method for a polypropylene composition, comprising the following steps:

[0011] Olefin monomers containing propylene are contacted with a catalyst system in the upstream polymerization stage of a gas-phase copolymerization reactor to form polymer particles. The upstream polymerization stage includes at least one of propylene prepolymerization, propylene homopolymerization, and propylene random copolymerization. A condensing medium is introduced at least one upstream polymerization stage, between upstream polymerization stages, or after an upstream polymerization stage. The polymerization method of the upstream polymerization stage is gas-phase polymerization or bulk polymerization.

[0012] The polymer particles obtained from the upstream polymerization stage and the condensing medium enter the gas-phase copolymerization reactor in the form of wet particles. In the presence of the condensing medium, propylene and α-olefins polymerize in the gas-phase copolymerization reactor to obtain a polypropylene composition.

[0013] In a preferred embodiment of the present invention, the upstream polymerization stage includes a propylene prepolymerization stage and at least one polymerization stage downstream of the propylene prepolymerization stage, namely, propylene homopolymerization or propylene random copolymerization. The polymerization method for propylene prepolymerization, propylene homopolymerization, or propylene random copolymerization is gas-phase polymerization or bulk polymerization. A condensing medium can be added to one or more of the above polymerization stages, or it can be introduced between two polymerization stages to act on the polymerization stage downstream of the introduction point, or it can be introduced after the upstream polymerization stage. Preferably, the condensing medium is introduced into at least one upstream polymerization stage or between upstream polymerization stages, and more preferably, the condensing medium is introduced into at least one upstream polymerization stage.

[0014] To reduce the cost of condensate recovery and to prevent partial dissolution of the amorphous phase during the gas-phase copolymerization stage, in a preferred embodiment of the invention, the amount of condensate added is less than 95% of the saturated solubility of the condensate in the polymer particles. The condensate swells the polymer particles, causing them to exist as wet particles. Saturated solubility of the condensate in the polymer particles refers to the amount of condensate added that allows the condensate to fully swell the polymer particles without precipitating out of the wet particles; when this amount is exceeded, the condensate will precipitate out of the wet particles.

[0015] Preferably, the condensing medium has a molecular weight of 28 g / mol to 140 g / mol and is selected from at least one alkane, alkene, or haloalkane from C2 to C10. More preferably, it is one or more alkanes selected from propane, n-butane, isobutane, n-pentane, isopentane, cyclohexane, n-hexane, or n-heptane.

[0016] In order to enhance the uniformity of polymer particle dispersion and avoid the generation of local hot spots in the reactor and the occurrence of phenomena such as agglomeration and sticking, in a preferred embodiment of the present invention, the content of condensing medium in the wet particles is not less than 1% of the saturated solubility of the condensing medium in the polymer particles.

[0017] In a preferred embodiment of the present invention, the catalyst system comprises a solid catalyst component, alkylaluminum or alkylaluminoxane and at least one external electron donor; the solid catalyst component is selected from Ziegler-Natta catalysts, chromium-based catalysts, metallocene catalysts, post-transition metal catalysts or mixtures thereof.

[0018] In a preferred embodiment of the present invention, the polymer particles are prepolymer particles, random copolymer particles, or homopolymer particles formed by contacting a catalyst system with an olefin monomer; the α-olefin is selected from ethylene or butene.

[0019] In a preferred embodiment of the present invention, the condensing medium and polymer particles enter the gas-phase copolymerization reactor with or without flash evaporation.

[0020] In a preferred embodiment of the present invention, the gas-phase copolymerization reactor is selected from a horizontal stirred bed reactor, a vertical stirred bed reactor, or a fluidized bed reactor.

[0021] Compared with the prior art, the present invention finds that, under the premise of controlling the amount of condensing medium in the preceding process, the polymer particles discharged from the preceding process of the gas-phase copolymerization reactor in the form of wet particles (polymer particles and condensing medium form wet particles) will not cause the dissolution of amorphous ethylene-propylene copolymer. On the contrary, it will increase the copolymerization activity of olefin monomers. Furthermore, the amorphous ethylene-propylene copolymer tends to grow inside the polypropylene particles rather than flow out of the surface, thereby avoiding problems such as the formation of clumps between polypropylene composition particles during the polymerization process.

[0022] This invention, by introducing an appropriate amount of condensing medium in the upstream section, can effectively improve the uniformity of heat dissipation during the polymerization reaction in the upstream section, avoid the generation of local hot spots, and make the polymer particles more uniformly dispersed, preventing sticking and agglomeration. The polymer particles exiting the upstream section of the gas-phase copolymerization reactor enter the gas-phase reactor in wet particle form, eliminating the need for complete flash evaporation of the polymer particles; only light components (such as hydrogen) need to be removed, effectively saving energy. The polymer particles entering the gas-phase reactor undergo gas-phase copolymerization in the presence of the condensing medium. Compared with traditional gas-phase copolymerization, this invention has advantages such as reduced fine powder content in the polypropylene composition, increased copolymer molecular weight, less sticking, significantly improved copolymerization activity, and high ethylene content. Compared with existing technologies, this invention has significant inventiveness in both technical means and technical effects.

[0023] In a preferred embodiment of the present invention, polymer particles from the upstream stage of the gas-phase reactor enter the gas-phase reactor in wet particle form. These polymer particles do not require complete flash evaporation; only some light components are removed before they enter the gas-phase copolymerization reactor to participate in the reaction. Under conditions where the temperature of the reaction gas mixture is close to (slightly above) the dew point, the copolymerization activity is increased by 39.34%. With more precise temperature control, the copolymerization activity can be significantly improved, solving the problems of rapid activity decay and long residence time in ethylene-propylene gas-phase copolymerization. Furthermore, the addition of an inert condensing medium improves the reactor's heat removal capacity, thus also contributing to an increase in the reactor's space-time yield.

[0024] Compared to existing high-impact polypropylene production processes, this invention eliminates the need for the anti-sticking agent LPN. The condensation medium generated by the wet particles allows the sticky copolymer to preferentially grow inside the polypropylene particles while the polymer surface remains non-sticky. This invention significantly improves the copolymerization activity of propylene and ethylene without controlling the activity of the highly reactive substrate in the preceding gas-phase reaction stage, simply by adding a portion of flash-evaporated, liquid-laden polypropylene particles to the gas-phase reactor. Furthermore, this invention achieves a significant increase in the molecular weight of the copolymer without requiring catalyst replacement.

[0025] Existing technologies do not involve introducing a condensing medium in the upstream section of the polypropylene gas-phase copolymerization reaction. This invention effectively solves the defects of easy agglomeration and sticking in the homopolymerization or random copolymerization of propylene by introducing a condensing medium in the upstream section. By introducing the condensing medium into the gas-phase copolymerization reactor through polymer particles, the composition and concentration of comonomers at the active sites are changed, thereby increasing the copolymerization activity of olefin monomers and the molecular weight of olefin copolymers in the gas-phase reactor, and solving the problem of easy sticking of copolymerized polypropylene.

[0026] In addition, the method of this invention has high heat transfer efficiency, enabling precise control of the gas-phase copolymerization temperature. This invention can reduce the content of polymer fines and significantly increase the molecular weight of ethylene-propylene copolymers. This invention also improves the dispersion of rubber in polypropylene particles, making it suitable for producing impact-resistant polypropylene with high rubber content that is less prone to sticking to the reactor. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of a method for producing an olefin polymerization in one embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of a process for producing an olefin polymerization method in another embodiment of the present invention. Detailed Implementation

[0029] The present invention will be described in detail below with reference to the embodiments and accompanying drawings. The embodiments and accompanying drawings are only used to illustrate the present invention and do not constitute any limitation on the scope of protection of the present invention. All reasonable modifications and combinations included within the scope of the inventive spirit of the present invention fall within the scope of protection of the present invention.

[0030] Figure 1 and Figure 2 These are schematic diagrams illustrating two processes for implementing the olefin polymerization method of the present invention in embodiments of the present invention. It should be noted that... Figure 1 and Figure 2 The processes described are merely illustrative, and the implementation methods of this invention are not limited to these two processes. Figure 1 and Figure 2The reactor forms and types used in each polymerization unit are only illustrative; technicians can freely choose other reactor types as needed. Figure 1 The polymerization method shown includes three polymerization units: propylene prepolymerization, propylene homopolymerization, and gas-phase copolymerization. The propylene prepolymerization reaction is carried out in a prepolymerization reactor; propylene homopolymerization is conducted in a loop reactor; and the gas-phase copolymerization reactor is a fluidized bed reactor. The propylene gas-phase prepolymerization reaction and propylene bulk homopolymerization serve as upstream polymerization stages of the gas-phase copolymerization reaction. Figure 2 The polymerization method shown includes two polymerization units, both of which are fluidized bed reactors. The upstream fluidized bed reactor mainly performs gas-phase homopolymerization of propylene, while the downstream fluidized bed reactor is the gas-phase copolymerization reactor described in this invention, used for copolymerization reaction.

[0031] Example 1:

[0032] use Figure 1 The olefin polymerization method shown produces multiphase polypropylene.

[0033] Propylene, Zn catalyst, triethylaluminum, dicyclopentyldimethoxysilane, and hydrogen were added to a prepolymer reactor for reaction. The temperature of the prepolymer reactor was controlled at 20°C, and the reactor pressure was controlled at approximately 0.2 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor was 0.02 mol / mol, and the propylene concentration was 88 mol%. The prepolymer reactor intermittently fed the prepolymerized catalytically active wet particles to a settling tank through a switch valve on the discharge bottom pipe. After settling, the catalytically active wet particles were fed into a gas lock device. After pressurizing the gas lock device, the catalytically active wet particles were pressurized and sent to the loop reactor.

[0034] The loop reactor temperature is controlled at 75℃, and the reactor pressure is maintained at approximately 3.0 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio inside the reactor is 0.08 mol / mol, the propylene concentration is 85 mol%, and the isobutane concentration of the condenser is 8%. The loop reactor intermittently feeds slurry or wet particles to the settling tank via a switch valve on the discharge bottom pipe, while excess liquid is circulated back into the loop reactor via a metering pump. After settling, the slurry or wet particles are fed into an airlock. The airlock is pressurized using circulating gas from the gas-phase copolymerization reactor, and the catalytically active wet particles are then pumped into the gas-phase copolymerization fluidized bed reactor.

[0035] Propylene, ethylene, and hydrogen from the raw material refining unit are fed into the reactor, where the temperature is controlled at 70°C and the reactor pressure is controlled at approximately 2.5 MPaG. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, the ethylene / propylene molar ratio is 0.21 mol / mol, and the propylene concentration is 77 mol%. The main phase generated in this reactor is ethylene-propylene rubber. The content of the circulating gas condensate medium is adjusted by the wet particle liquid holding capacity. In this embodiment, the wet particle liquid holding capacity is 12 wt% of the total particle mass, and the condensate medium content in the wet particles is 18% of the saturated solubility of this condensate medium in the polymer particles.

[0036] The melt index I of the obtained heterogeneous polypropylene 2.16 The strength of the beam is 34.1 g / 10 min, the tensile yield strength is 20.0 MPa, the flexural strength is 21.9 MPa, the flexural modulus is 0.94 GPa, and the notched impact strength of the simply supported beam is 16.3 kJ / m. 2 It has a heat distortion temperature of 77.5℃. It can be used as a high-rubber-content polypropylene grade for automotive parts.

[0037] Example 2:

[0038] exist Figure 1 This paper describes a method for producing multiphase polypropylene through olefin polymerization. Propylene, Zn catalyst, triethylaluminum, dicyclopentyldimethoxysilane, and hydrogen are added to a prepolymer reactor for reaction. The prepolymer reactor temperature is controlled at 20°C, and the reactor pressure is maintained at approximately 0.2 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, and the propylene concentration is 88 mol%. The prepolymer reactor intermittently feeds the prepolymerized catalytically active particles to a settling tank via a valve on the discharge bottom pipe. After settling, the catalytically active particles are fed into a gas lock. The gas lock is pressurized, and the catalytically active particles are then forced into a loop reactor.

[0039] The loop reactor temperature is controlled at 75℃, and the reactor pressure is maintained at approximately 3.0 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio inside the reactor is 0.08 mol / mol, the propylene concentration is 83 mol%, and the isobutane concentration in the condenser is 11.2%. The loop reactor intermittently feeds wet particles to the settling tank via a switch valve on the discharge bottom pipe, while excess liquid is circulated back into the loop reactor via a metering pump. After settling, the slurry or wet particles are fed into an airlock. The airlock is pressurized using circulating gas from the gas-phase copolymerization reactor, and the catalytically active wet particles are then pumped into the gas-phase copolymerization fluidized bed reactor.

[0040] Propylene, ethylene, and hydrogen from the raw material refining unit are fed into the reactor, with the temperature controlled at 70°C and the reactor pressure controlled at approximately 2.5 MPaG. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, the ethylene / propylene molar ratio is 0.21 mol / mol, and the propylene concentration is 77 mol%. The main phase generated in this reactor is ethylene-propylene rubber. The content of the circulating gas condensate medium is adjusted by the wet particle liquid holding capacity. In this embodiment, the wet particle liquid holding capacity is 18 wt% of the total particle mass, and the condensate medium content in the wet particles is 27% of the saturated solubility of this condensate medium in the polymer particles.

[0041] The melt index I of the obtained heterogeneous polypropylene 2.16 The yield strength is 27.5 g / 10 min, the tensile yield strength is 18.2 MPa, the flexural strength is 19.0 MPa, the flexural modulus is 0.92 GPa, and the notched impact strength of the simply supported beam is 22.5 kJ / m. 2 It has a heat distortion temperature of 74.1℃. It can be used as a high-rubber-content polypropylene grade, applied in automotive parts and waterproof membranes.

[0042] Example 3:

[0043] exist Figure 1 This paper describes a method for producing multiphase polypropylene through olefin polymerization. Propylene, Zn catalyst, triethylaluminum, dicyclopentyldimethoxysilane, and hydrogen are added to a prepolymer reactor for reaction. The prepolymer reactor temperature is controlled at 20°C, and the reactor pressure is maintained at approximately 0.2 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, and the propylene concentration is 88 mol%. The prepolymer reactor intermittently feeds the prepolymerized catalytically active particles to a settling tank via a valve on the discharge bottom pipe. After settling, the catalytically active particles are fed into a gas lock. The gas lock is pressurized, and the wet catalytically active particles are then forced into a loop reactor.

[0044] The loop reactor temperature is controlled at 75℃, and the reactor pressure is maintained at approximately 3.0 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio inside the reactor is 0.08 mol / mol, the propylene concentration is 82 mol%, and the isobutane concentration of the condenser is 14.5%. The loop reactor intermittently feeds slurry or wet particles to the settling tank via a switch valve on the discharge bottom pipe, while excess liquid is circulated back into the loop reactor via a metering pump. After settling, the slurry or wet particles are fed into an airlock. The airlock is pressurized using circulating gas from the gas-phase copolymerization reactor, and the catalytically active wet particles are then pumped into the gas-phase copolymerization fluidized bed reactor.

[0045] Propylene, ethylene, and hydrogen from the raw material refining unit are fed into the reactor, with the temperature controlled at 70°C and the reactor pressure controlled at approximately 2.5 MPaG. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, the ethylene / propylene molar ratio is 0.21 mol / mol, and the propylene concentration is 77 mol%. The main phase generated in this reactor is ethylene-propylene rubber. The content of the circulating gas condensate medium is adjusted by the liquid holding capacity of the wet particles. In this embodiment, the liquid holding capacity of the wet particles is 22.5 wt% of the total particle mass, and the condensate medium content in the wet particles is 33.8% of the saturated solubility of this condensate medium in the polymer particles.

[0046] The melt index I of the obtained heterogeneous polypropylene 2.16 The tensile yield strength is 18.6 g / 10 min, the flexural strength is 19.6 MPa, the flexural modulus is 20.7 MPa, the flexural modulus is 0.94 GPa, and the notched impact strength of the simply supported beam is 35.4 kJ / m. 2 Its heat distortion temperature is 72.0℃. It can be used as a high-rubber-content polypropylene grade for applications in special fields such as waterproof membranes.

[0047] Example 4:

[0048] exist Figure 2 This paper describes a method for producing multiphase polypropylene through olefin polymerization. Propylene, Zn catalyst, triethylaluminum, dicyclopentyldimethoxysilane, and hydrogen are added to a gas-phase homopolymer reactor for reaction. A mixed solvent of isopentane and n-hexane is added as a condensing medium to remove the heat generated during polymerization. The temperature of the gas-phase fluidized bed reactor is controlled at 80°C, and the reactor pressure is controlled at approximately 2.8 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor is 0.06 mol / mol, and the propylene concentration is 84 mol%. The gas-phase homopolymer reactor intermittently feeds polymer particles to a settling tank via a valve on the discharge bottom pipe. After settling, the wet particles are fed into an airlock. The airlock is pressurized using circulating gas from the gas-phase copolymerization reactor, and the catalytically active wet particles are then pressurized into the gas-phase copolymerization fluidized bed reactor.

[0049] Propylene, ethylene, and hydrogen from the raw material refining unit are fed into a gas-phase copolymerization fluidized bed reactor. The temperature is controlled at 70°C, and the reactor pressure is controlled at approximately 2.5 MPaG. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, the ethylene / propylene molar ratio is 0.21 mol / mol, and the propylene concentration is 77 mol%. The main phase generated in this reactor is ethylene-propylene rubber. The content of the circulating gas condensate is adjusted by the liquid holding capacity of the wet particles. In this embodiment, the liquid holding capacity of the wet particles is 14.2 wt% of the total particle mass, and the condensate content in the wet particles is 26.3% of the saturated solubility of the condensate in the polymer particles.

[0050] The melt index I of the obtained heterogeneous polypropylene2.16 The yield strength is 32.8 g / 10 min, the tensile yield strength is 19.4 MPa, the flexural strength is 21.1 MPa, the flexural modulus is 0.95 GPa, and the notched impact strength of the simply supported beam is 16.8 kJ / m. 2 It has a heat distortion temperature of 75.0℃. It can be used as a high-rubber-content polypropylene grade for automotive parts.

[0051] Comparative Example 1:

[0052] Comparative Example 1 Figure 1 In the process of producing multiphase polypropylene by olefin polymerization shown, a flash evaporation section is added after the loop reactor to completely remove the condensate in the loop reactor. The specific operation process is as follows:

[0053] Propylene, Zn catalyst, triethylaluminum, dicyclopentyldimethoxysilane, and hydrogen were added to the prepolymer reactor for reaction. The prepolymer reactor temperature was controlled at 20°C, and the reactor pressure was maintained at approximately 0.2 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor was 0.02 mol / mol, and the propylene concentration was 88 mol%. The prepolymer reactor intermittently fed the catalytically active particles to the settling tank through a switch valve on the discharge bottom pipe.

[0054] After settling, the catalytically active particles are fed into a gas lock. The gas lock is pressurized, and the catalytically active particles are then pumped into a loop reactor. The loop reactor temperature is controlled at 75°C, and the reactor pressure is maintained at approximately 3.0 MPaG by adjusting the fresh propylene feed rate. The hydrogen / propylene molar ratio in the reactor is 0.08 mol / mol, the propylene concentration is 85 mol%, and the isobutane concentration in the condenser is 8%. The loop reactor intermittently feeds wet particles to the settling tank via a valve on the discharge bottom pipe. Excess liquid is circulated back into the loop reactor via a metering pump, and the wet particles then enter the flash evaporation section, where isobutane is completely removed. After flash evaporation, the polymer particles are fed into a gas lock. The gas lock is pressurized using circulating gas from the gas-phase copolymerization reactor, and the flash-obtained particles are then pumped into the gas-phase copolymerization fluidized bed reactor.

[0055] Propylene, ethylene, and hydrogen from the raw material refining unit are fed into the reactor in a specific ratio. The temperature is controlled at 70°C, and the reactor pressure is controlled at approximately 2.5 MPaG. The hydrogen / propylene molar ratio in the reactor is 0.02 mol / mol, the ethylene / propylene molar ratio is 0.22 mol / mol, and the propylene concentration is 78 mol%. The main phase formed in this reactor is ethylene-propylene rubber. This comparative example does not contain liquid-holding particles.

[0056] The melt index I of the obtained heterogeneous polypropylene 2.16The yield strength is 35.7 g / 10 min, the tensile yield strength is 18.5 MPa, the flexural strength is 19.0 MPa, the flexural modulus is 0.93 GPa, and the notched impact strength of a simply supported beam is 9.52 kJ / m. 2 The heat distortion temperature is 76.0℃.

[0057] While the specific embodiments of the present invention have been described in detail above, it should be noted that the scope of protection of the present invention is not limited to these specific embodiments, but is determined by the claims. Those skilled in the art can make appropriate modifications to these embodiments without departing from the technical concept and spirit of the present invention, and these modified embodiments are obviously also included within the scope of protection of the present invention.

Claims

1. A polymerization method for a polypropylene composition, characterized in that, Includes the following steps: Olefin monomers containing propylene and a catalyst system are contacted in the upstream polymerization section of a gas-phase copolymerization reactor to form polymer particles. A condensing medium is introduced in at least one upstream polymerization section or between upstream polymerization sections. The polymerization method of the upstream polymerization section is gas-phase polymerization or bulk polymerization. The upstream polymerization section includes a propylene prepolymerization section and at least one polymerization section of propylene homopolymerization or propylene random copolymerization located downstream of the propylene prepolymerization section. The polymer particles obtained from the upstream polymerization stage and the condensing medium are flashed or not and enter the gas-phase copolymerization reactor in wet particle form. In the presence of the condensing medium, propylene and α-olefins are polymerized in the gas-phase copolymerization reactor to obtain a polypropylene composition.

2. The method according to claim 1, characterized in that, The amount of the condensing medium added is 1% to 95% of the saturated solubility of the condensing medium in the polymer particles.

3. The method according to claim 1, characterized in that, The condensing medium has a molecular weight of 28 g / mol to 140 g / mol and is selected from at least one alkane, olefin, or haloalkane from C2 to C10.

4. The method according to claim 1, characterized in that, The condensing medium is preferably one or more alkanes selected from propane, n-butane, isobutane, n-pentane, isopentane, cyclohexane, n-hexane, or n-heptane.

5. The method according to claim 1, characterized in that, The catalyst system comprises a solid catalyst component, alkylaluminum or alkylaluminoxane and at least one external electron donor; the solid catalyst component is selected from Ziegler-Natta catalysts, chromium-based catalysts, metallocene catalysts, post-transition metal catalysts or mixtures thereof.

6. The method according to claim 1, characterized in that, The polymer particles are prepolymer particles, random copolymer particles, or homopolymer particles formed by the contact between the catalyst system and olefin monomers.

7. The method according to claim 1, characterized in that, The gas-phase copolymerization reactor is selected from a horizontal stirred bed reactor, a vertical stirred bed reactor, or a fluidized bed reactor.

8. The method according to claim 1, characterized in that, The α-olefin is selected from ethylene or butene.

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