CN113214417B - Method for preparing polypropylene by using metallocene catalyst - Google Patents

Method for preparing polypropylene by using metallocene catalyst Download PDF

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CN113214417B
CN113214417B CN202110257895.7A CN202110257895A CN113214417B CN 113214417 B CN113214417 B CN 113214417B CN 202110257895 A CN202110257895 A CN 202110257895A CN 113214417 B CN113214417 B CN 113214417B
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inert solvent
prepolymerization
reactor
metallocene catalyst
polymerization
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CN113214417A (en
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范小强
王靖岱
黄正梁
胡晓石
阳永荣
韩国栋
任聪静
孙婧元
苏洪
冷家厂
吴文清
历伟
杨遥
曹德成
梁秀文
陈毓明
王琳琳
王程
周亮
丁强
尹建
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China Petroleum and Chemical Corp
Sinopec Engineering Inc
Zhejiang University ZJU
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China Petroleum and Chemical Corp
Sinopec Engineering Inc
Zhejiang University ZJU
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
    • C08F10/04—Monomers containing three or four carbon atoms
    • C08F10/06—Propene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F110/00—Homopolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
    • C08F110/04—Monomers containing three or four carbon atoms
    • C08F110/06—Propene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F2/00—Processes of polymerisation
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F2/00—Processes of polymerisation
    • C08F2/01—Processes of polymerisation characterised by special features of the polymerisation apparatus used
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/50—Improvements relating to the production of bulk chemicals
    • Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts

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  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
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  • Transition And Organic Metals Composition Catalysts For Addition Polymerization (AREA)

Abstract

The invention discloses a method for preparing polypropylene by using a metallocene catalyst, which comprises the following steps: (1) Mixing a metallocene catalyst with an inert solvent A to obtain a slurry material; (2) Mixing the slurry material obtained in the step (1) with an inert solvent B, and conveying the mixture to a prepolymerization reactor to perform prepolymerization reaction with propylene monomer to obtain a prepolymer; (3) Polymerizing the prepolymer obtained in the step (2) with propylene monomers and optional hydrogen in a polymerization reactor to obtain a polymer material containing polypropylene, and separating to obtain polypropylene; wherein the inert solvent A is selected from C 10 ‑C 32 At least one of the alkanes; the inert solvent B is selected from C 3 ‑C 7 At least one of the alkanes. The method of the invention realizes high-efficiency and safe transportation of the catalyst and controllable pre-polymerization activity, thereby reducing the breakage of catalyst particles and improving the operability and reliability of the olefin polymerization process.

Description

Method for preparing polypropylene by using metallocene catalyst
Technical Field
The invention relates to the field of olefin polymerization, in particular to a method for preparing polypropylene by using a metallocene catalyst.
Background
For the bulk propylene polymerization process, the catalyst activity is very high due to high propylene monomer concentration, and the phenomena of catalyst particle breakage, polymer fine powder quantity and poor morphology are very easy to occur, so that the problems of scaling and blockage of a polymerization reactor, separation equipment, a conveying pipeline and the like are caused. Therefore, in the existing bulk propylene polymerization process, a common method is to polymerize the catalyst with propylene in a prepolymerization reactor at a relatively low temperature. Because the polymerization reaction rate is low, the carrier is coated by the polypropylene layer by layer, so that the strength of the particles is improved, and the breakage of the catalyst particles in the main polymerization reactor is reduced.
The supported metallocene catalyst has the problem that catalyst particles are easy to break in the bulk propylene polymerization process. To solve this problem, chinese patent CN 102050906A proposes that a metallocene catalyst is reacted with a class of α -olefin monomers having specific side groups in an inert solvent at a prepolymerization temperature to obtain a metallocene catalyst prepolymer, and then the metallocene catalyst prepolymer is contacted with propylene to prepare polypropylene. However, this method has the following disadvantages: after the metallocene catalyst reacts with the alpha-olefin monomer with a special side group, unreacted monomer needs to be separated to carry out polymerization reaction with propylene, thereby increasing the complexity of the flow. Chinese patent CN 109906234A proposes that the metallocene catalyst is supported on some special silica-based support at a concentration of 1wt% to 20wt% in the solvent of oil, aliphatic hydrocarbon or mixtures of the above, and then injected directly into the loop reactor. This process eliminates the prepolymerization process by introducing the supported catalyst directly into a solution or slurry polymerization reactor, however, it is only applicable to some metallocene catalysts based on silica support structures.
Therefore, it is important to develop a polypropylene preparation method which is simple in process and applicable to various metallocene catalysts.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a method for preparing polypropylene by using a metallocene catalyst, which adopts two inert solvents to continuously convey the catalyst to a prepolymerization reactor, and realizes high-efficiency and safe conveying of the catalyst and controllable prepolymerization activity by adjusting the proportion of the inert solvents to the catalyst, thereby reducing the breakage of catalyst particles and improving the operability and reliability of a propylene polymerization process.
In a first aspect the present invention provides a process for preparing polypropylene using a metallocene catalyst comprising the steps of:
(1) Mixing a metallocene catalyst with an inert solvent A to obtain a slurry material;
(2) Mixing the slurry material obtained in the step (1) with an inert solvent B, and conveying the mixture to a prepolymerization reactor to perform prepolymerization reaction with propylene monomer to obtain a prepolymer;
(3) Polymerizing the prepolymer obtained in the step (2) with propylene monomers and optional hydrogen in a polymerization reactor to obtain a polymer material containing polypropylene, and separating to obtain polypropylene;
wherein the inert solvent A is selected from C 10 -C 32 At least one of the alkanes; the inert solvent B is selected from C 3 -C 7 At least one of the alkanes.
In the invention, the propylene monomer is propylene or a mixture of propylene and at least one of alpha-olefins.
According to some embodiments of the invention, the alpha-olefin has the formula CH 2 =chr, wherein R is hydrogen or C 2 -C 6 In some preferred embodiments, the alpha-olefin is selected from one or more of ethylene, butene, pentene, hexene, octene, and 4-methyl-1-pentene.
According to some embodiments of the present invention, the inert solvent A is preferably a non-toxic, environmentally friendly and economical mixture of alkanes, such as white oil and some non-toxic environmentally friendly C 10 -C 32 Or mixtures thereof. The invention is characterized in thatThe metallocene catalyst can be uniformly suspended for a long time after being mixed with the inert solvent A by using the inert solvent A.
According to some preferred embodiments of the invention, the melting point of the inert solvent a is lower than 15 ℃, preferably lower than 10 ℃. The inert solvent A selected by the invention can avoid the condition that the catalyst cannot be effectively dispersed due to the solidification of the solvent caused by low temperature in the prepolymerization reactor.
According to some preferred embodiments of the invention, the inert solvent B is selected from C 4 -C 6 Preferably at least one of n-butane, isobutane, n-pentane, isopentane, n-hexane and cyclohexane. According to the invention, when the inert solvent B is selected from C 4 -C 6 When at least one alkane is used, the content of the inert solvent B in the polypropylene product obtained when the polymer materials exported from the polymerization reactor are separated is lower, the removal is easier in the subsequent devolatilization process, and the agglomeration phenomenon of the particles of the reaction product is obviously reduced.
According to some embodiments of the invention, in step (1), the mass ratio of the metallocene catalyst to the inert solvent a is from 1:20 to 1:2, preferably from 1:9 to 1:4.
According to some embodiments of the invention, the mole fraction of inert solvent B in the prepolymerization reactor is 5-90%, preferably 10-80%, based on the total moles of feed.
According to the present invention, the feedstock in the prepolymerization reactor is all of the feedstock entering the prepolymerization reactor, in some embodiments the feedstock is a mixture of metallocene catalyst, inert solvent A and propylene monomer.
According to some embodiments of the invention, the polymer feed further comprises unreacted starting materials, the method further comprising: separating the polymer material obtained in the step (3) to obtain a propylene monomer enriched material and an inert solvent B enriched material, recycling the obtained propylene monomer enriched material to the prepolymerization reactor and/or the polymerization reactor, recycling the inert solvent B enriched material to the prepolymerization reactor and/or recycling the inert solvent B enriched material to the step (2) and mixing the inert solvent B in the step (2).
According to the present invention, the propylene monomer enriched material is the propylene monomer material separated from the polymer material in step (3), but it should be understood that the propylene monomer enriched material may further contain a small amount of inert solvent B, etc. in the reaction system, because it is difficult to separate a percentage of propylene from the polymer material.
According to some embodiments of the invention, the propylene monomer enriched feed is a feed comprising propylene monomer and inert solvent B.
According to some preferred embodiments of the invention, the mole fraction of propylene monomer in the propylene monomer concentrate feed is greater than 80%.
According to the present invention, the inert solvent B enriched material is the inert solvent B separated from the polymer material, but it should be understood that the inert solvent enriched material may further contain a small amount of inert solvent a and propylene monomer, etc. in the reaction system, because it is difficult to separate the inert solvent B percentage from the polymer material.
According to some embodiments of the invention, the inert solvent B enriched stream is a stream comprising inert solvent B and a minor amount of propylene monomer, and in some preferred embodiments the mole fraction of inert solvent B in the inert solvent B enriched stream is greater than 20%.
According to the invention, the inert solvent B enrichment material is circulated to the step (2) and mixed with the inert solvent B in the step (2), and the reaction rate of monomers in the inert solvent B enrichment material and the catalyst in the catalyst conveying process can be reduced by reasonably selecting the concentration of the inert solvent B in the mixture and the proportion of the inert solvent B enrichment material to the catalyst, so that the pipeline blockage caused by polymerization in the pipeline is avoided, and the efficient and safe conveying of the metallocene catalyst is realized.
According to some embodiments of the invention, the metallocene catalyst is a supported metallocene catalyst, preferably the supported metallocene catalyst is prepared by immobilizing a homogeneous metallocene catalyst on a carrier. The preparation method of the supported metallocene catalyst can be a conventional method in the field, or can be a special preparation method disclosed in literature or patent. The support may be a support for a known metallocene catalyst. In some preferred embodiments of the invention, the support is silica or alumina.
According to the invention, the support, the supported metallocene catalyst and the polypropylene prepared are present in solid form in the inert solvent and in the propylene monomer in liquid phase.
According to some embodiments of the invention, the polymer feed is fed out intermittently or continuously from the polymerization reactor.
According to some embodiments of the invention, the temperature of the prepolymerization is 10-50 ℃, preferably 15-30 ℃. In some embodiments, the temperature of the prepolymerization reaction is 10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃, 50 ℃.
According to some embodiments of the invention, the pressure of the prepolymerization is 2.5-5.0MPa, preferably 3.0-4.5MPa.
According to some embodiments of the invention, the time of the prepolymerization is less than 30 minutes, preferably less than 20 minutes, more preferably less than 15 minutes.
According to some embodiments of the invention, the polymerization reaction temperature is 40-90 ℃, preferably 60-80 ℃.
According to some embodiments of the invention, the polymerization reaction is carried out at a pressure of 2.4-4.9MPa, preferably 3.0-4.5MPa.
According to some embodiments of the invention, the polymerization time is 0.5 to 5 hours, preferably 0.5 to 2 hours.
According to some embodiments of the invention, the prepolymerization reactor is a loop reactor or a stirred reactor.
According to some embodiments of the invention, the polymerization reactor is a loop reactor.
According to some embodiments of the invention, the prepolymerization reactor has a jacket into which water or chilled brine is introduced to remove the heat of polymerization.
According to the method, the polypropylene generated by contacting propylene with the metallocene catalyst coats the catalyst particles, so that the strength of the catalyst particles is improved. For metallocene catalysts without induction period or with short induction period, the aim of regulating and controlling the prepolymerization reaction rate is fulfilled by changing the concentration of the inert solvent B in the prepolymerization reactor.
Compared with the prior art, the invention has the following advantages:
1) By selecting two inert solvents A and B and adjusting the proportion of the inert solvent A and the inert solvent B to the catalyst, the high-efficiency and safe conveying of the metallocene catalyst is realized.
2) The metallocene catalyst is further diluted and dispersed by the inert solvent B, so that the catalyst can be rapidly dispersed after being injected into the prepolymerization reactor, and the problem of unstable device caused by poor dispersion of the catalyst is avoided.
3) By reasonably selecting the ratio of the inert solvent to the propylene monomer in the prepolymerization, the prepolymerization activity of the metallocene catalyst in the prepolymerization reactor can be controlled under the condition of not reducing the reaction temperature of the prepolymerization reactor. Therefore, the method can be suitable for metallocene catalysts with different activities, particularly for supported metallocene catalysts with short induction period and high activity, can reduce the breakage of metallocene particles, and improves the operability and reliability of propylene polymerization process.
Drawings
FIG. 1 is a schematic diagram of a polymerization system according to an embodiment of the present invention.
Reference numerals illustrate: 1-a catalyst preparation tank; 2-a mixer; 3-a prepolymerization reactor; a 4-polymerization reactor; 5-high pressure separator; 6-a washing tower; 7-a compressor; 8-cyclone separator; 9-a low pressure separator; 10-a solvent recovery column; 11-a first feed pump; 12-a second feed pump; 13-an inert solvent B; 14-propylene monomer; 15-hydrogen; 16-an inert solvent A; 17-metallocene catalyst; 18-a mixed slurry of inert solvent A and metallocene catalyst; 19-prepolymerization of the reaction mass; 20-prepolymerization product; 21-a mixture of material 31 and fresh propylene; 22-a feed comprising feed 21 and hydrogen; 23-polymer mass; 24-a first polymer mass; 25-a second polymer mass; 26-a mixture containing a small amount of polypropylene fines, a large amount of propylene monomer and an inert solvent B; a mixture of 27-propene and an inert solvent B; 28-polypropylene fine powder; 29-polypropylene; 30-a mixture of propylene and an inert solvent a/B; 31. enriching materials of propylene monomers; 32-inert solvent B enrichment.
FIG. 2 is a graph of supported metallocene catalyst activity versus temperature.
Detailed Description
The invention aims to provide a novel method for preparing polypropylene by using a metallocene catalyst, which is particularly suitable for a propylene polymerization system, in particular a propylene homopolymerization system, with a supported metallocene catalyst.
The term "homo" as used herein means that only one polymerized monomer is included in the polymerization system.
The term "inert solvent" as used herein refers to an organic solvent, such as saturated hydrocarbons, that does not chemically react with the olefin monomer, catalyst and cocatalyst at the reaction pressure and reaction temperature. The "inert solvent" may be a single component or a mixture of components.
In order that the invention may be more readily understood, the invention will be described in detail below with reference to the following examples and the accompanying drawings, which are provided for illustration only and are not to be construed as limiting the scope of the invention. The specific conditions are not noted in the examples and are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or apparatus used are conventional products which are commercially available or which are obtainable using conventional or published methods, without the manufacturer's knowledge.
The endpoints and any values of the ranges disclosed herein are not limited to the precise range or value, and are understood to encompass values approaching those ranges or values. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point value, in combination with each other, and are to be considered as specifically disclosed herein.
FIG. 1 is a schematic diagram of a polymerization system according to an embodiment of the present invention. Comprises the following main equipment and units:
a catalyst configuration tank 1 for contacting the metallocene catalyst with an inert solvent a;
a mixer 2 for contacting the metallocene catalyst with the inert solvent B and/or the inert solvent B enriched material;
a prepolymerization reactor 3 for prepolymerization;
a polymerization reactor 4 for preparing polypropylene by contacting propylene monomer, hydrogen and a metallocene catalyst;
a separation unit Z for separating polypropylene from unreacted raw materials;
a first feed pump 11 for returning the inert solvent B enriched material to the mixer 2 and the prepolymerization reactor 3.
A second feed pump 12 for returning the propylene enriched material to the prepolymerization reactor 3 and the polymerization reactor 4.
In the operation of the polymerization system shown in FIG. 1, the inert solvent A is contacted with the metallocene catalyst in the catalyst preparation tank 1, and the catalyst is uniformly dispersed in the inert solvent A after stirring. The resulting mixed slurry material 18 of inert solvent A and metallocene catalyst is metered into mixer 2 where it is contacted with inert solvent B to form a prepolymerized reaction material 19 comprising inert solvent A, metallocene catalyst and inert solvent B which is introduced into prepolymerization reactor 3. In the prepolymerization reactor 3, the polypropylene produced by contacting the propylene monomer with the metallocene catalyst coats the catalyst particles, thereby improving the strength of the catalyst particles. For metallocene catalysts without induction period or with short induction period, the aim of regulating and controlling the prepolymerization reaction rate is fulfilled by changing the concentration of the inert solvent B in the prepolymerization reactor. The pre-polymerized product 20 is then fed to a polymerization reactor 4 for the preparation of polypropylene, optionally with the introduction of hydrogen for the modification of the molecular structure of the polypropylene. From the polymerization reactor 4, a polymer mass 23 is intermittently or continuously withdrawn, said polymer mass 23 comprising polypropylene, unreacted propylene monomer, hydrogen, inert solvent a, inert solvent B, and the polymer mass is separated in a separation unit Z, wherein the polypropylene is fed to a pelletization unit after removal of unreacted propylene and inert solvent B remaining in the polypropylene or directly as a product. The propylene monomer enriched material 31 containing a small amount of inert solvent B and a large amount of propylene obtained in the separation unit Z is mixed with fresh propylene and returned to the prepolymerization reactor 3 and/or the polymerization reactor 4 via the second feed pump 12. The inert solvent B enriched material 32 containing a large amount of inert solvent B and a small amount of propylene obtained in the separation unit Z is returned to the mixer 2 and/or the prepolymerization reactor 3 via the first feed pump 11.
The separation unit Z comprises a high-pressure separator 5 for separating the polymer and unreacted materials at a relatively high pressure, a washing column 6 for washing minute amounts of fine powder, a low-pressure separator 9 for removing unreacted materials and inert solvent B from the polymer, and a solvent recovery column 10 for separating propylene and inert solvent B. In order to increase the economics of the recovery system, the high pressure separator 5, the scrubber 6 and the solvent recovery column 10 are typically maintained at relatively high pressures, preferably the pressures of the high pressure separator 5 and the scrubber 6 are greater than 1.0MPa, more preferably the pressures of the high pressure separator 5 and the scrubber 6 are greater than 1.5MPa. The pressure of the low pressure separator 9 is usually maintained at normal pressure to minimize the residual amounts of propylene and inert solvents in the polypropylene.
The polymer mass withdrawn from the polymerization reactor 3 is separated by a high-pressure separator 5 to obtain a first polymer mass 24, which is a mixture comprising inert solvent B, propylene monomer and a small amount of polypropylene, and a second polymer mass 25, which is a mixture comprising a large amount of polypropylene, a small amount of propylene monomer and inert solvent B. The first polymer material is passed to a washing column 6 for separation, the polypropylene obtained is used later, and the mixture of propylene and inert solvent B obtained is sent to a solvent recovery column 10. And sending the second polymer material to a low-pressure separator for separation to obtain a polypropylene product.
FIG. 2 is a graph of supported metallocene catalyst activity versus temperature for the present invention. The activity of the supported metallocene catalyst is the average activity over a reaction time including a catalyst activity up-period and an activity down-period. The "activity-increasing period" refers to a period in which the catalyst activity increases with time until the maximum activity is reached as the polymerization reaction proceeds. By "activity decrease period" is meant the period after which the activity of the polymerization reaches a maximum and the catalyst activity decreases with time until the polymerization process ends. The relation between the average activity of the supported metallocene catalyst and the reaction temperature accords with an Arrhenius equation.
According to some embodiments of the invention, the sum of the mole fractions of propylene monomer and inert solvent B in the prepolymerization reactor is approximately 1, and the residence time of the metallocene catalyst in the prepolymerization reactor is constant in one embodiment of the invention, so that the average reactivity of the metallocene catalyst decreases linearly with increasing concentration of inert solvent B in the prepolymerization reactor.
Example 1
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil, and the inert solvent B is isobutane. The average activity of the supported metallocene catalyst is 50000gPP/g catalyst, the mass ratio of the supported metallocene catalyst to the inert solvent A is 1:4, the reaction temperature of the prepolymerization reactor 3 is 25 ℃, and the reaction pressure is 3.6MPa. The mole fraction of inert solvent B was 0.57 based on the total moles of starting materials. The prepolymerization reaction product was introduced into a polymerization reactor, the temperature of the polymerization reactor 4 was 70℃and the reaction pressure was 3.5MPa, and the mole fraction of the inert solvent B was 0.16 based on the total mole number of the starting materials. As a result, the polymer yield was 8t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 0.56%. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.3% by weight. The content of the inert solvent B in the polymer at the outlet of the low-pressure separator 9 is 0.15wt percent, the content of the inert solvent in the polypropylene is lower, and the polymer is easy to remove in the subsequent devolatilization process.
Example 2
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil, and the inert solvent B is isobutane. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 40℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0.57 based on the total mole of the starting materials. The reaction temperature of the polymerization reactor 4 was 70℃and the reaction pressure was 3.5MPa, and the mole fraction of the inert solvent B was 0.16 based on the total mole of the raw materials. Other conditions were the same as in example 1. The polymer yield was 8t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 1.42%. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.5% by weight. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.6% by weight. The content of the inert solvent B in the polymer at the outlet of the low-pressure separator 9 is 0.15wt percent, the content of the inert solvent in the polypropylene is lower, and the polymer is easy to remove in the subsequent devolatilization process.
Example 3
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil, and the inert solvent B is isopentane. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 30℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0.1 based on the total mole of the starting materials. The amount of the materials other than the prepolymerized product in the polymerization reactor 4 was the same as in example 1, the reaction temperature in the polymerization reactor 4 was 70℃and the reaction pressure was 3.5MPa, and the mole fraction of the inert solvent B based on the total mole number of the raw materials was 0.03. Other conditions were the same as in example 1. The polymer yield was 9.2t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 1.40%. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.6% by weight. The inert solvent B content in the polymer at the outlet of the low-pressure separator 9 is 0.36wt%, and the inert solvent content in the polypropylene is low, so that the polymer is easy to remove in the subsequent devolatilization process.
Example 4
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil, and the inert solvent B is isopentane. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 30℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0.2 based on the total mole of the starting materials. The amount of the materials other than the prepolymerized product in the polymerization reactor 4 was the same as in example 1, the reaction temperature in the polymerization reactor 4 was 70℃and the reaction pressure was 3.5MPa, and the mole fraction of the inert solvent B based on the total mole number of the raw materials was 0.056. Other conditions were the same as in example 1. The polymer yield was 9t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 1.27%. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.5% by weight. The inert solvent B content in the polymer at the outlet of the low-pressure separator 9 is 0.62wt percent, the inert solvent content in the polypropylene is lower, and the polymer is easy to remove in the subsequent devolatilization process.
Example 5
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil, and the inert solvent B is n-heptane. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 30℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0.2 based on the total mole of the starting materials. The amount of the materials other than the prepolymerized product in the polymerization reactor 4 was the same as in example 4, the reaction temperature in the polymerization reactor 4 was 70℃and the reaction pressure was 3.5MPa, and the mole fraction of the inert solvent B based on the total mole number of the raw materials was 0.056. Other conditions were the same as in example 4. The polymer yield was 9t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 1.21%. The content of fine powder having a particle diameter of less than 105 μm in the final product was 0.2% by weight. The inert solvent B content of the polymer at the outlet of the low-pressure separator 9 was 4.97% by weight. The content of the inert solvent B n-heptane in the polypropylene is high, the n-heptane is difficult to devolatilize, the n-heptane is difficult to remove in the subsequent devolatilization process, and the devolatilization cost is obviously increased. In addition, the problem of pipeline blockage caused by particle agglomeration is remarkably increased due to the fact that the oligomer is dissolved in n-heptane to reduce the heat transfer coefficient of the reactor.
Comparative example 1
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 25℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0. The reaction temperature of the polymerization reactor 4 was 70℃and the reaction pressure 3.5MPa, and the mole fraction of the inert solvent B was 0. Other conditions were the same as in example 1. The polymer yield was 9.5t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 1.1%. The prepolymerization rate of comparative example 1 was about 2 times that of example 1 at the same prepolymerization temperature and pressure. As the prepolymerization rate is high, the content of fine powder is increased compared with that of the embodiment 1, and the content of fine powder with the particle size smaller than 105 mu m in the final product is 1.1wt%, which shows that the method can effectively control the prepolymerization rate and greatly improve the operability of the process for preparing polypropylene by using the metallocene catalyst.
Comparative example 2
Polypropylene was produced in the polymerization system shown in fig. 1. The inert solvent A is white oil. The average activity of the supported metallocene catalyst was 50000gPP/g catalyst. The reaction temperature of the prepolymerization reactor 3 was 40℃and the reaction pressure was 3.6MPa, and the mole fraction of the inert solvent B was 0. The reaction temperature of the polymerization reactor 4 was 70℃and the reaction pressure 3.5MPa, and the mole fraction of the inert solvent B was 0. Other conditions were the same as in example 1. The polymer yield was 9.5t/h. The ratio of the polypropylene yield in the prepolymerization reactor 3 to the polymerization yield in the polymerization reactor 4 was 2.78%. The prepolymerization rate of comparative example 2 was 2 times that of example 2 at the same prepolymerization temperature and pressure. Since the prepolymerization rate was high, the content of fine powder was increased as compared with example 2, and the content of fine powder having a particle diameter of less than 105 μm in the final product was 1.2% by weight.
The method can effectively control the prepolymerization reaction rate, and greatly improves the operability of the process for preparing polypropylene by using the metallocene catalyst.
It should be noted that the above-described embodiments are only for explaining the present invention and do not constitute any limitation of the present invention. The invention has been described with reference to exemplary embodiments, but it is understood that the words which have been used are words of description and illustration, rather than words of limitation. Modifications may be made to the invention as defined in the appended claims, and the invention may be modified without departing from the scope and spirit of the invention. Although the invention is described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein, as the invention extends to all other means and applications which perform the same function.

Claims (14)

1. A process for preparing polypropylene using a metallocene catalyst comprising the steps of:
(1) Mixing a metallocene catalyst with an inert solvent A to obtain a slurry material;
(2) Mixing the slurry material obtained in the step (1) with an inert solvent B, and conveying the mixture to a prepolymerization reactor to perform prepolymerization reaction with propylene monomer to obtain a prepolymer;
(3) Polymerizing the prepolymer obtained in the step (2) with propylene monomers and optional hydrogen in a polymerization reactor to obtain a polymer material containing polypropylene, and separating to obtain polypropylene;
wherein the inert solvent A is selected from C 10 -C 32 At least one of the alkanes; the inert solvent B is selected from C 3 -C 7 At least one of the alkanes;
in the step (1), the mass ratio of the metallocene catalyst to the inert solvent A is 1:20-1:2; in the prepolymerization reactor, the mole fraction of the inert solvent B is 5-90 percent based on the total mole number of the raw materials.
2. The process according to claim 1, wherein the inert solvent a has a melting point lower than 15 ℃.
3. The process according to claim 2, wherein the inert solvent a has a melting point lower than 10 ℃.
4. A process according to any one of claims 1 to 3, wherein the inert solvent B is selected from C 4 -C 6 At least one of the alkanes.
5. The method of claim 4, wherein the inert solvent B is at least one of n-butane, isobutane, n-pentane, isopentane, n-hexane, and cyclohexane.
6. A process according to any one of claims 1 to 3, wherein in step (1) the mass ratio of the metallocene catalyst to the inert solvent a is from 1:9 to 1:4; and/or the mole fraction of the inert solvent B in the prepolymerization reactor is 10-80% based on the total mole number of the raw materials.
7. A method according to any one of claims 1-3, wherein the polymer mass further comprises unreacted starting materials, the method further comprising: separating the polymer material obtained in the step (3) to obtain a propylene monomer enriched material and an inert solvent B enriched material, recycling the obtained propylene monomer enriched material to the prepolymerization reactor and/or the polymerization reactor, recycling the inert solvent B enriched material to the prepolymerization reactor and/or recycling the inert solvent B enriched material to the step (2) and mixing the inert solvent B in the step (2).
8. The process of claim 7, wherein the mole fraction of propylene monomer in the propylene monomer-enriched feed is greater than 80%, and/or the mole fraction of inert solvent B in the inert solvent B-enriched feed is greater than 20%.
9. A process according to any one of claims 1 to 3, wherein the metallocene catalyst is a supported metallocene catalyst.
10. A process according to any one of claims 1 to 3, wherein the temperature of the prepolymerization is 10 to 50 ℃; and/or the pressure of the prepolymerization reaction is 2.5-5.0MPa; and/or the time of the prepolymerization is less than 30 minutes.
11. The method of claim 10, wherein the temperature of the prepolymerization is 15-30 ℃; and/or the pressure of the prepolymerization is 3.0-4.5MPa.
12. A process according to any one of claims 1 to 3, wherein the temperature of the polymerization reaction is 40 to 90 ℃; and/or the pressure of the polymerization reaction is 2.4-4.9MPa; and/or the polymerization reaction time is 0.5 to 5 hours.
13. The method of claim 12, wherein the polymerization reaction temperature is 60-80 ℃; and/or the pressure of the polymerization reaction is 3.0-4.5MPa; and/or the polymerization reaction time is 0.5 to 2 hours.
14. A process according to any of claims 1-3, characterized in that the prepolymerization reactor is a loop reactor or a stirred reactor; and/or the polymerization reactor is a loop reactor.
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