Process for the preparation of low ash polypropylene

Abstract

The application discloses a preparation method of low-ash polypropylene, and the method comprises the following steps: (1) mixing liquid-phase propylene and a catalyst to perform pre-polymerization; (2) mixing the slurry obtained in the step (1) with a mixed liquid of liquid-phase propylene and hydrogen to perform liquid-phase propylene homopolymerization; and (3) mixing the slurry obtained in the step (2) with a mixed liquid of liquid-phase propylene and hydrogen to perform gas-phase propylene homopolymerization, so as to obtain polypropylene. The ash content of the polypropylene prepared by the method is generally not more than 30 ppm, and under more optimal process conditions, the ash content is generally not more than 20 ppm.

Description

Technical Field

[0001] This invention belongs to the field of polypropylene preparation technology, and relates to a method for preparing low-ash polypropylene, and particularly to a method for preparing ultra-low-ash polypropylene. Background Technology

[0002] Polypropylene, with its molecular chain composed only of C and H elements and almost no polar groups, does not orient under electric field stress, exhibits low loss coefficient, high volume resistivity, and high temperature resistance. Therefore, it is widely used in film capacitors and high-voltage power transmission. Polypropylene is generally prepared using Ziegler-Natta catalysts. Chlorine, aluminum, titanium, magnesium, or silicon from the catalyst may remain in the polypropylene particles, increasing the polymer's conductivity. This makes the polymer unsuitable for applications requiring extremely low conductivity, but suitable for films or cables in high-voltage and ultra-high-voltage capacitors.

[0003] To reduce trace impurities introduced into polypropylene by the catalyst, it is often necessary to wash the polypropylene particles. For example, Chinese patent CN201910179618.1 uses a solvent containing alkanes, complexing agents, and alcohols to wash the polypropylene; or polymerization is carried out in an alkane solvent followed by washing, as described in Korean patent KR102624224. In existing technologies, low-ash polypropylene can also be prepared by increasing the polymerization activity of the catalyst or reducing the amount of catalyst used, as described in Chinese patents CN201380010849.0, CN201080045745.X, CN200910236103.7, and CN201010152791.1. These existing technologies increase the catalyst activity from 30-60 kg PP / g catalyst to 100-150 kg PP / g catalyst, which can significantly reduce the ash content in polypropylene resin.

[0004] Clearly, increasing catalyst activity is a lower-cost method for preparing ultra-low ash polypropylene. However, this method also has some drawbacks. For example, the polymerization reaction time of polypropylene in existing industrial plants is generally no more than 2 hours, which limits the catalyst's activity. Fluctuations in impurities in propylene feedstock can cause fluctuations in catalyst polymerization activity, preventing it from reaching the required value. Furthermore, for some olefin polymerization plants, ultra-high catalyst activity may cause polypropylene particles to break down, increasing fine powder, or causing localized overheating and agglomeration. Summary of the Invention

[0005] Based on this, the present invention provides a method for preparing low-ash polypropylene, particularly a method for preparing low-ash polypropylene with a liquid-gas phase composition. The method exhibits good heat removal during the initial reaction stage; and the long reaction time significantly improves catalyst activity. After polymerization, an auxiliary solvent washing process is employed to further reduce the ash content of the polypropylene, eliminating ash fluctuations caused by catalyst activity fluctuations. The low-ash polypropylene (even ultra-low-ash polypropylene) prepared using the method of the present invention is suitable for the electrical engineering field, especially for ultra-high voltage power transmission.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A method for preparing low-ash polypropylene, the method comprising:

[0008] (1) Prepolymerization is carried out by mixing liquid propylene and catalyst;

[0009] (2) The slurry obtained by prepolymerization in step (1) is mixed with a mixture of liquid propylene and hydrogen to carry out a liquid-phase homopolymerization reaction of propylene.

[0010] (3) The slurry obtained from the homopolymerization reaction in step (2) is mixed with a mixture of liquid propylene and hydrogen to carry out a gas-phase homopolymerization reaction of propylene to obtain polypropylene.

[0011] According to an embodiment of the present invention, the method further includes the following steps:

[0012] (4) The powder after the gas phase propylene homopolymerization reaction in step (3) is washed, the polypropylene powder after washing is separated into solid and liquid, and dried to obtain purified polypropylene.

[0013] According to an embodiment of the present invention, the method further includes the following steps:

[0014] (5) The product obtained in step (4) is granulated after adding the additives.

[0015] In this invention, the purity of the raw material liquid propylene is above 99%, wherein the total content of water, alcohol, sulfur-containing compounds and carbon dioxide does not exceed 5 ppm, and the carbon monoxide content does not exceed 15 ppb.

[0016] According to an embodiment of the present invention, in step (1), the catalyst comprises (a) a MgCl2-supported titanium tetrachloride catalyst and an internal electron donor; (b) an activator alkylaluminum and (c) a polymerization regulator.

[0017] Preferably, the internal electron donor is selected from 1,3-diether compounds, or complexes of 1,3-diether compounds with diesters or glycol esters. Specifically, the 1,3-diether compounds are selected, for example, from at least one of 9,9-dimethoxymethylfluorene, 2,2-diisopropyl-1,3-dimethoxypropane, and 2,-isopropyl-2-isobutyl-1,3-dimethoxypropane. Specifically, the diesters are selected, for example, from phthalate diesters. Specifically, the glycol esters are those described in Chinese Patent CN201010152791.1.

[0018] For example, in the catalyst, the titanium tetrachloride catalyst supported on (a) MgCl2 and the internal electron donor are specifically a slurry containing 30 wt% Ziegler-Natta catalyst white oil, such as HAR catalyst selected from Auda Catalysts.

[0019] Preferably, the alkylaluminum includes, but is not limited to, at least one of triethylaluminum, triisobutylaluminum, and tri-n-hexylaluminum.

[0020] Preferably, the molar ratio of alkylaluminum to Ti in the catalyst is 10-300, more preferably 30-150.

[0021] Preferably, the polymerization regulator is an alkylalkoxysilane, including but not limited to at least one of cyclohexylmethyldimethoxysilane, diisopropyldimethoxysilane, diisobutyldimethoxysilane, and dicyclopentyldimethoxysilane.

[0022] Preferably, the molar ratio of Ti in the polymerization regulator and the catalyst is 10-100, more preferably 10-30.

[0023] According to an embodiment of the present invention, in step (1), the propylene polymerization activity (activity of propylene polymerization at 70°C for 2 hours) of the catalyst is not less than 60 kg PP / g catalyst, preferably not less than 100 kg PP / g catalyst, more preferably not less than 130 kg PP / g catalyst, and more preferably 130-150 kg PP / g catalyst.

[0024] According to an embodiment of the present invention, in step (1), the polymerization temperature of the prepolymerization is -20 to 50°C, preferably 10 to 35°C, the polymerization time of the prepolymerization is no more than 15 minutes, for example 1 to 15 minutes, preferably 2 to 8 minutes, and the polymerization pressure is no higher than 2 MPa, for example 0.1 to 2 MPa.

[0025] According to an embodiment of the present invention, in step (2), the liquid phase propylene homopolymerization reaction is carried out in a propylene liquid phase stirred tank.

[0026] Preferably, the propylene liquid phase stirred tank is composed of one or more tanks connected in series. Preferably, a small amount of comonomers such as ethylene and butene may also be added to the propylene liquid phase stirred tank.

[0027] According to an embodiment of the present invention, in step (2), the mass ratio of hydrogen to liquid propylene is 0.02-0.15:1000, preferably 0.04-0.1:1000.

[0028] According to an embodiment of the present invention, in step (2), the polymerization temperature is 60-90℃, preferably 70-90℃, for example 70℃, 72℃, 74℃, 76℃, 80℃, 82℃, 84℃, 86℃ or 90℃; the polymerization reaction time is 0.5-6h, preferably 0.5-3h.

[0029] According to an embodiment of the present invention, in step (2), the liquid phase propylene homopolymerization reaction is carried out sequentially in the first propylene liquid phase stirred tank R201 and the second propylene liquid phase stirred tank R202;

[0030] In the first propylene liquid phase stirred tank R201, the polymerization temperature is 60-90℃, preferably 70-90℃; the polymerization reaction time is 0.5-6h, preferably 0.5-3h, and even more preferably 0.5-1h.

[0031] In the second propylene liquid phase stirred tank R202, the polymerization temperature is 60-90℃, preferably 70-90℃; the polymerization reaction time is 0.5-6h, preferably 0.5-3h, and even more preferably 0.5-1h.

[0032] According to an embodiment of the present invention, in step (2), after the reaction is completed, the gaseous propylene is recovered, or the gaseous propylene is further condensed into liquid propylene, and the recovered liquid propylene is reused in step (2) as a partial replacement for the liquid propylene.

[0033] According to an embodiment of the present invention, in step (2), the pressure of the liquid phase propylene homopolymerization reaction is 2.8-4.5 MPa.

[0034] According to an embodiment of the present invention, in step (3), the gas phase propylene homopolymerization reaction is carried out in a gas phase plug flow reactor.

[0035] Preferably, the gas-phase plug flow reactor is composed of one or more reactors connected in series. Preferably, a small amount of comonomers such as ethylene and butene may also be added to the gas-phase plug flow reactor.

[0036] According to an embodiment of the present invention, in step (3), the mass ratio of hydrogen to liquid propylene is 0.02-0.15:1000, preferably 0.04-0.1:1000.

[0037] According to an embodiment of the present invention, in step (3), the polymerization temperature is 60-90℃, preferably 65-90℃, and even more preferably 80-90℃, for example 80℃, 82℃, 84℃, 86℃ or 90℃; the polymerization reaction time is 1-6h, preferably 1.5-3h; and the pressure during gas phase propylene polymerization is 2.5-3.0MPa.

[0038] According to an embodiment of the present invention, in step (3), the gas-phase propylene homopolymerization reaction is carried out sequentially in the first gas-phase polymerization reactor R301 and the second gas-phase polymerization reactor R302;

[0039] In the first gas-phase polymerization reactor R301, the polymerization temperature is 60-90℃, preferably 65-90℃, and even more preferably 80-90℃; the polymerization reaction time is 1-6h, preferably 1.5-3h; and the pressure during gas-phase propylene polymerization is 2.5-3.0MPa.

[0040] In the second gas-phase polymerization reactor R302, the polymerization temperature is 60-90℃, preferably 65-90℃, and even more preferably 80-90℃; the polymerization reaction time is 1-6h, preferably 1.5-3h; and the pressure during gas-phase propylene polymerization is 2.5-3.0MPa.

[0041] According to an embodiment of the present invention, in step (3), after the reaction is completed, the gaseous propylene is recovered, or the gaseous propylene is further condensed into liquid propylene, and the recovered liquid propylene is reused in step (2) as a partial replacement for the liquid propylene.

[0042] According to an embodiment of the present invention, in step (4), the washing is carried out in a washing tank.

[0043] Preferably, the washing vessel is composed of one vessel or multiple vessels connected in series.

[0044] According to an embodiment of the present invention, in step (4), the temperature of the washing process is room temperature to 80°C, preferably 50-80°C; the washing time is 2-6 hours, preferably 3-6 hours.

[0045] According to an embodiment of the present invention, in step (4), the polypropylene powder after polymerization is completed is separated from the unreacted propylene and then directly washed. Preferably, the washing agent used includes alkanes, alcohols, and metal complexing agents;

[0046] The alkanes include, but are not limited to, at least one of pentane, hexane, cyclohexane, and heptane;

[0047] The alcohols include, but are not limited to, at least one of methanol, ethanol, propanol, isopropanol, and butanol;

[0048] The metal complexing agent includes, but is not limited to, at least one of acetylacetone, 3-methyl-2,4-pentanedione, 3-ethyl-2,4-pentanedione, 3-butyl-2,4-pentanedione, and 1,1-dibenzoylmethane.

[0049] Preferably, the proportions of alcohol and metal complexing agent in the detergent are both no more than 30 wt%, more preferably 5-30 wt%, and even more preferably 15-20 wt% and 20-30 wt%, respectively.

[0050] In this invention, unreacted alkylaluminum reacts with alcohol to generate soluble alkoxyaluminum, Ti in the catalyst reacts with alcohol to generate soluble alkoxytitanium, and magnesium chloride reacts with complexing agent to dissolve in alkanes.

[0051] According to an embodiment of the present invention, in steps (4) and (5), the mixture after washing is centrifuged or filtered to obtain polypropylene granules. The granules are heated and dried, fed into an extruder, and granulated with additives such as antioxidants.

[0052] Preferably, the additives of the present invention can be any additives known in the art. Preferably, the additives are generally composed of compounds such as hindered phenols, phosphites, and thioethers. Further, the additives are required to have low impurities, and the ash introduced into the polypropylene by the additives should not exceed 2 ppm.

[0053] The present invention also provides a low-ash polypropylene prepared by the above method.

[0054] Preferably, the ash content of the polypropylene is no more than 30 ppm, and more preferably no more than 20 ppm.

[0055] The beneficial effects of this invention are:

[0056] (1) The catalyst used in this invention has very high activity in the early stage of polymerization. The polymerization releases a lot of heat. A propylene liquid phase stirred tank is used. A large amount of liquid propylene absorbs heat and then vaporizes, carrying away the heat and preventing the reaction from bursting or the polymer from agglomerating.

[0057] (2) In the propylene liquid-phase stirred tank, the catalyst is added while the polymer is discharged. A small amount of catalyst is discharged from the reactor before it has fully reacted, which reduces the reactivity and increases the ash content in individual polypropylene particles. Then, a plug flow reactor is used to ensure that each catalyst particle has sufficient reaction time, thereby reducing the ash content of each polypropylene particle.

[0058] (3) The ash content of polypropylene prepared by the method of the present invention is generally no more than 30 ppm, and under better process conditions, the ash content is generally no more than 20 ppm. Attached Figure Description

[0059] Figure 1This is a process flow diagram of Embodiment 1 of the present invention.

[0060] Figure 2 This is a process flow diagram of Embodiment 2 of the present invention. Detailed Implementation

[0061] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0062] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0063] Test method:

[0064] 1. Polymer melt index: determined according to GB / T 3682.1-2018.

[0065] 2. Catalyst activity: determined according to HG / T 5402-2018.

[0066] 3. Polypropylene isotacticity: determined according to GB / T 2412-2008.

[0067] 4. Determination of polymer ash content: determined according to GB / T 9345-1988.

[0068] Example 1: Preparation of Polypropylene Homopolymer

[0069] The process employs a three-reactor tandem polymerization process, and the process flow is shown in the attached figure. Figure 1 As shown.

[0070] (1) Prepolymerization: Liquid propylene, a white oil slurry containing 30 wt% Ziegler-Natta catalyst (such as HAR catalyst from Auda Catalysts, with a polymerization activity of approximately 150 kg PP / g catalyst over 2 hours), triethylaluminum and dicyclopentyldimethoxysilane (the molar ratio of triethylaluminum to Ti in the catalyst is 150, and the molar ratio of dicyclopentyldimethoxysilane to Ti in the catalyst is 10) are all directly fed into the prepolymerization reactor R101 to form a polymerization slurry. The temperature of the prepolymerization reactor is 35°C, and the material residence time is 4 minutes. Propylene prepolymerization is carried out. The polymerization ratio of polypropylene in the resulting raw material slurry (i.e., catalyst activity gPP / g catalyst) is 50 times that of the catalyst.

[0071] (2) Liquid-phase bulk polymerization: The raw material slurry obtained from the prepolymerization reactor is evenly fed into R201 through a three-way valve. The liquid-phase propylene and hydrogen mixed liquid-phase polymerization raw material is fed into R201 (the amount of recovered liquid-phase propylene added is about 25 wt% of the new liquid-phase propylene, and the amount of hydrogen added is 0.08 kg / 1000 kg propylene). The temperature of R201 is controlled at 82℃, and the pressure of the polymerization reactor is controlled between 4.0-4.5 MPa. Propylene liquid-phase homopolymerization is carried out. The material level in the reactor is 40-50% by volume, and the residence time is 40 min. During the propylene liquid-phase homopolymerization process, part of the liquid-phase propylene vaporizes and carries away part of the polymerization reaction heat. The vaporized propylene gas is condensed and recovered, and returned to the propylene liquid-phase homopolymerization in either gas or liquid phase.

[0072] (3) Gas-phase homopolymerization: The polypropylene slurry obtained in R201 is fed into the gas-phase polymerization reactor R301. Liquid propylene and hydrogen are mixed and fed into R301 as liquid-phase polymerization raw materials (the amount of recovered liquid propylene added is approximately 25 wt% of the new liquid propylene, and the amount of hydrogen added is 0.08 kg / 1000 kg propylene). The reaction temperature of R301 is 90℃, and the pressure is maintained at 2.5-3.0 MPa for propylene gas-phase homopolymerization. The residence time is 1.5 h, and the material level in the reactor is controlled at 50-60 mL / min. The polypropylene slurry obtained in R301 is fed into the gas-phase polymerization reactor R302 via a gas lock. Liquid propylene and hydrogen are mixed and fed into R302 as liquid-phase polymerization feedstock (approximately 25 wt% of the new liquid propylene is recovered, and 0.08 kg / 1000 kg propylene is added). The reaction temperature in R302 is 90℃, and the pressure is maintained at 2.5-3.0 MPa for propylene gas-phase homopolymerization. The residence time is 1.5 h, and the material level in the reactor is controlled at 50-60% by volume. During the propylene gas-phase homopolymerization process, some unreacted propylene gas is condensed and recovered, returning to the propylene gas-phase homopolymerization process in either gas or liquid phase. After the propylene gas-phase homopolymerization is completed, the resulting product containing propylene homopolymer is subsequently dried and propylene is recovered. Polypropylene and propylene are then separated, and the recovered propylene is reused for propylene liquid-phase homopolymerization.

[0073] The polypropylene sampled from R302 had a melt index of 3.0 g / 10 min, a melting point (DSC test) of 165 °C, a polypropylene ash content of 45 ppm, and an isotacticity of 98.5%.

[0074] (4) Washing: The polypropylene particles obtained in R301 were fed into washing tank 401. The washing agent in the washing tank consisted of hexane, isopropanol, and 1,1-dibenzoylmethane, with isopropanol and 1,1-dibenzoylmethane accounting for 15 wt% and 30 wt% of the total solvent, respectively. The washing temperature was 60°C. The average residence time of the material in the washing tank was 3 hours and 40 minutes. A sample of the washed polypropylene was taken, and its ash content was 15 ppm. The isotacticity of the polypropylene was 99.3%.

[0075] After washing, the polypropylene is filtered through a pressure filter 501. The filter cake is dried with hot nitrogen. The dried polypropylene is then fed into a granulator and a composite antioxidant consisting of antioxidant 1010 and secondary antioxidant 168 is added.

[0076] Example 2

[0077] The process employs a four-reactor series polymerization process, and the process flow is shown in the attached figure. Figure 2 As shown.

[0078] The catalyst was prepared using the method described in Chinese invention CN201310176782.X, and its polymerization activity over 2 hours was approximately 130 kg PP / g catalyst.

[0079] (1) The prepolymerization process is the same as in Example 1.

[0080] (2) Liquid-phase bulk polymerization: The raw material slurry obtained from the prepolymerization reactor is fed into R201. Liquid-phase propylene and hydrogen mixed liquid-phase polymerization raw materials are fed into R201 (the amount of recycled liquid-phase propylene added is about 25 wt% of the new liquid-phase propylene, and the amount of hydrogen added is 0.08 kg / 1000 kg propylene). The temperature of R201 is controlled at 68℃ for propylene liquid-phase homopolymerization. The material level in the reactor is 40-50% by volume, and the residence time is 1 hour. The material in R201 is fed into R202, where liquid-phase propylene and hydrogen are mixed. The mixed liquid-phase polymerization feedstock is fed into R202 (where the amount of recovered liquid-phase propylene added is approximately 25 wt% of the amount of new liquid-phase propylene, and the amount of hydrogen added is 0.08 kg / 1000 kg propylene). The temperature of R202 is controlled at 72℃ for propylene liquid-phase homopolymerization. The material level in the reactor is 40-50% by volume, and the residence time is 1 hour. The pressure of the polymerization reactors R201 and R202 is controlled between 2.8-3.6 MPa. During the propylene liquid-phase homopolymerization process, some liquid-phase propylene vaporizes, carrying away some of the heat of polymerization. The vaporized propylene gas is condensed and recovered, and returned to the propylene liquid-phase homopolymerization in either gas or liquid phase.

[0081] (3) Gas-phase homopolymerization: The polypropylene slurry obtained in R202 is fed into the gas-phase polymerization reactor R301. Liquid propylene and hydrogen are mixed and fed into R301 as liquid-phase polymerization raw materials (the amount of recovered liquid propylene added is about 25 wt% of the new liquid propylene, and the amount of hydrogen added is 0.08 kg / 1000 kg propylene). The reaction temperature of R301 is 80℃, and the pressure is maintained at 2.5-3.0 MPa for propylene gas-phase homopolymerization. The residence time is 1.5 h, and the material level in the reactor is controlled at 50-60% by volume. The polypropylene slurry obtained in R301 is then transported through an airlock. The liquid-phase propylene and hydrogen feedstock are fed into gas-phase polymerization reactor R302 (approximately 25 wt% of the new liquid-phase propylene is recovered, and 0.08 kg / 1000 kg propylene is added). The reaction temperature in R302 is 85°C, and the pressure is maintained at 2.5-3.0 MPa for propylene gas-phase homopolymerization. The residence time is 1.5 h, and the material level in the reactor is controlled at 50-60% by volume. During propylene gas-phase homopolymerization, some unreacted propylene gas is condensed and recovered, returning to the propylene gas-phase homopolymer in either gas or liquid phase. After propylene gas-phase homopolymerization, the resulting propylene-containing homopolymer product undergoes subsequent drying and propylene recovery to separate polypropylene and propylene. The recovered propylene is then reused for propylene liquid-phase homopolymerization.

[0082] The polypropylene sample obtained from R302 had a melt index of 3.2 g / 10 min, a melting point (DSC test) of 166 °C, a polypropylene ash content of 30 ppm, and an isotacticity of 98.6%.

[0083] (4) Washing: The polypropylene particles obtained in R301 are fed into washing tank 401; the washing agent in the washing tank is hexane, isopropanol and 1,1-dibenzoylmethane, wherein isopropanol and 1,1-dibenzoylmethane account for 20 wt% and 20 wt% of the total solvent, respectively. The washing temperature is 60℃. The average residence time of the material in the washing tank is 2.5 hours.

[0084] After washing, the polypropylene is filtered through a pressure filter (501), and the filter cake is then fed into a washing tank (402). The washing agent in the tank consists of hexane, isopropanol, and 1,1-dibenzoylmethane, with isopropanol and 1,1-dibenzoylmethane accounting for 10 wt% and 30 wt% of the total solvent, respectively. The washing temperature is 60°C. The average residence time of the material in the washing tank is 2.5 hours. A sample of the washed polypropylene was taken, and its ash content was 10 ppm. The isotacticity of the polypropylene is 99.5%.

[0085] After washing, the polypropylene is filtered through a pressure filter 502. The filter cake is dried with hot nitrogen. The dried polypropylene is then fed into a granulator and a composite antioxidant consisting of antioxidant 1010 and secondary antioxidant 168 is added.

[0086] The embodiments of the present invention have been described above by way of example. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing low-ash polypropylene, characterized in that, The method includes: (1) Prepolymerization is carried out by mixing liquid propylene and catalyst; (2) The slurry obtained by prepolymerization in step (1) is mixed with a mixture of liquid propylene and hydrogen to carry out a liquid-phase homopolymerization reaction of propylene. (3) The slurry obtained from the homopolymerization reaction in step (2) is mixed with a mixture of liquid propylene and hydrogen to carry out a gas-phase homopolymerization reaction of propylene to obtain polypropylene.

2. The method according to claim 1, characterized in that, The method further includes the following steps: (4) The powder after the homopolymerization reaction in step (3) is washed, the washed powder is separated into solid and liquid, and dried to obtain purified polypropylene.

3. The method according to claim 2, characterized in that, The method further includes the following steps: (5) The product obtained in step (4) is granulated after adding the additives.

4. The method according to any one of claims 1-3, characterized in that, In step (1), the catalyst comprises (a) a MgCl2-supported titanium tetrachloride catalyst and an internal electron donor; (b) an activator alkylaluminum and (c) a polymerization regulator; Preferably, the internal electron donor is selected from 1,3-diether compounds, or complexes of 1,3-diether compounds with diester compounds or diol ester compounds; For example, in the catalyst, the titanium tetrachloride catalyst supported on MgCl2 and the internal electron donor are specifically a white oil slurry containing 30 wt% Ziegler-Natta catalyst. Preferably, the alkylaluminum is selected from at least one of triethylaluminum, triisobutylaluminum, and tri-n-hexylaluminum; Preferably, the molar ratio of alkylaluminum to Ti in the catalyst is 10-300; Preferably, the polymerization regulator is an alkylalkoxysilane; Preferably, the molar ratio of Ti in the polymerization regulator and the catalyst is 10-100.

5. The method according to any one of claims 1-4, characterized in that, In step (1), the propylene polymerization activity of the catalyst (the activity of propylene polymerization at 70°C for 2 hours) is not less than 60 kg PP / g catalyst.

6. The method according to any one of claims 1-5, characterized in that, In step (2), the mass ratio of hydrogen to liquid propylene is 0.02-0.15:1000; Preferably, in step (2), the polymerization temperature is 60-90℃; the polymerization reaction time is 0.5-6h. Preferably, in step (2), the liquid phase propylene homopolymerization reaction is carried out sequentially in the first propylene liquid phase stirred tank R201 and the second propylene liquid phase stirred tank R202; In the first propylene liquid phase stirred tank R201, the polymerization temperature is 60-90℃; the polymerization reaction time is 0.5-6h. In the second propylene liquid phase stirred tank R202, the polymerization temperature is 60-90℃; the polymerization reaction time is 0.5-6h. Preferably, in step (2), after the reaction is completed, the gaseous propylene is recovered, or the gaseous propylene is further condensed into liquid propylene, and the recovered liquid propylene is reused in step (2) as a partial replacement for the liquid propylene. Preferably, in step (2), the pressure of the liquid phase propylene homopolymerization reaction is 2.8-4.5 MPa.

7. The method according to any one of claims 1-6, characterized in that, In step (3), the mass ratio of hydrogen to liquid propylene is 0.02-0.15:1000; Preferably, in step (3), the polymerization temperature is 60-90℃; the polymerization reaction time is 1-6h; and the pressure during gas-phase propylene polymerization is 2.5-3.0MPa. Preferably, in step (3), the gas-phase propylene homopolymerization reaction is carried out sequentially in the first gas-phase polymerization reactor R301 and the second gas-phase polymerization reactor R302; In the first gas-phase polymerization reactor R301, the polymerization temperature is 60-90℃; the polymerization reaction time is 1-6h; and the pressure during gas-phase propylene polymerization is 2.5-3.0MPa. In the second gas-phase polymerization reactor R302, the polymerization temperature is 60-90℃; the polymerization reaction time is 1-6h; and the pressure during gas-phase propylene polymerization is 2.5-3.0MPa.

8. The method according to any one of claims 1-7, characterized in that, In step (3), after the reaction is completed, the gaseous propylene is recovered, or the gaseous propylene is further condensed into liquid propylene, and the recovered liquid propylene is reused in step (2) as a partial replacement for the liquid propylene.

9. The method according to claim 2, characterized in that, In step (4), the polypropylene powder after polymerization is completed is separated from the unreacted propylene and then washed directly. Preferably, the detergent used in the washing process includes alkanes, alcohols, and metal complexing agents; Preferably, the alkane is selected from at least one of pentane, hexane, cyclohexane, and heptane; Preferably, the alcohol is selected from at least one of methanol, ethanol, propanol, isopropanol, and butanol; Preferably, the metal complexing agent is selected from at least one of acetylacetone, 3-methyl-2,4-pentanedione, 3-ethyl-2,4-pentanedione, 3-butyl-2,4-pentanedione, and 1,1-dibenzoylmethane.

10. Low-ash polypropylene prepared by the method according to any one of claims 1-9. Preferably, the ash content of the polypropylene is no more than 30 ppm, and more preferably no more than 20 ppm.

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