Method of polymerization of polypropylene (PP)

Method of polymerization of polypropylene (PP)

At present, polypropylene production process can be divided into five categories according to type of polymerization: solution method, suspension method, volumetric method and gas phase method, as well as combined process of volumetric and gas phase methods. Specific processes mainly include BP's Innovene gas phase process, Chisso gas phase process, Dow Unipol process, Novolene gas phase process and Sumitomo gas phase process, Basell volumetric process, Mitsui's Hypol process, Borstar's Borealis process, etc.

Method of polymerization of polypropylene (PP)

Molecular structure of polypropylene

A. Suspension process

The slurry process, also known as slurry or solvent process, is world's earliest polypropylene production technology. From opening of first industrial plant in 1957 until mid to late 1980s, slurry process was most important process in production of polypropylene for 30 years. Typical processes mainly include Italy's Montedison Process, United States Hercules Process, Japan's Mitsui Topress Chemical Process, United States' Amoco Process, Japan's Mitsui Petrochemical Process, and Sowell Process. The development of these processes was based on first generation of catalysts of time, using vertical stirred tank reactors, which required deashing and deregularization. Due to different solvents used, process sequence and operating conditions were different. In recent years, proportion of traditional slurry process in production has been greatly reduced, and remaining slurry products are mainly used in some important fields, such as specialty BOPP films, high molecular weight blown films and high strength pipes. . In recent years, people have improved this method. The improved slurry production process uses a second generation high activity catalyst which can eliminate catalyst deashing step and reduce formation of random polymers. It can be used to produce homopolymers, random copolymers and impact copolymer products, etc. At present, world's slurry polypropylene production capacity is about 13% of world's total polypropylene production capacity.

B. Solution Process

The solution process is an early crystalline polypropylene manufacturing process unique to Eastman. The process uses a specially improved catalyst system - lithium compounds (such as lithium aluminum hydride) to adapt to high temperature of solution polymerization. Catalyst components, monomers and solvents are continuously fed into polymerization reactor, and unreacted monomers are separated and circulated by depressurizing solvents. Additional solvent was added to reduce viscosity of solution, and residual catalyst was removed by filtration. The solvent is concentrated by passing through several evaporators and then passed through an extruder which removes volatiles to form a solid polymer. The solid polymer is further purified by extraction with heptane or similar hydrocarbons, which also removes amorphous polypropylene, eliminates use of ethanol and multi-stage distillation, and is mainly used to produce some lower modulus products than slurry products. higher strength. The process of this meThe method is complicated and cost is high, polymerization temperature is high, and scope of product is limited due to use of a special high-temperature catalyst, so it is no longer used for production. from crystalline PP.

C. Ontology Process

Research and development of volumetric process began in 1960s. In 1964, American Dart Company built world's first industrial facility for production of PP by volumetric production using a tank reactor. After 1970, companies such as Sumitomo, Phillips, and EI Psao in United States commercialized liquid phase bulk polypropylene technology. Compared with solvent slurry method, liquid phase bulk propylene polymerization method has advantages of no inert solvent, high monomer concentration in reaction system, high polymerization rate, high catalyst activity, high polymerization reaction conversion. , and spatio-temporal stability of reactor. Large production capacity, low energy consumption, simple process, less equipment, low production cost, and less "three waste", easy to remove heat of polymerization, and simplify control of heat dissipation, which can increase amount of polymerization in a single reactor; Low molecular weight random polymers and catalyst residues that adversely affect product properties can provide high quality products and other benefits. The disadvantage is that reaction gas must be vaporized and condensed before being returned back to reactor. The high pressure liquid hydrocarbon material in reactor has a large capacity and is potentially hazardous. In addition, concentration of ethylene in reactor should not be too high, otherwise a separate gas phase will be formed in reactor, hindering operation of reactor, so ethylene content in resulting copolymerized product will not be too high. .

The difference between various flow paths of volumetric method lies mainly in differences in reactor. Reactors can be divided into two categories: tank reactors and loop reactors. The reactor tank uses latent heat of vaporization of liquid to remove heat of reaction. Most of vaporized gas is condensed and then returned to reactor. The non-condensed gas is forced by a compressor and then circulated back to reactor. The loop reactor uses an axial flow pump to circulate slurry at high speed, cool and remove heat through jacket. Due to large heat transfer area and good heat dissipation effect, yield per unit volume of reactor is high and energy consumption is low. short.

Depending on polymerization process, mass production process can beo divided into two types: batch polymerization process and continuous polymerization process:

(a) Batch bulk process. Polypropylene polymerization technology in bulk batch mode is a production technology successfully developed in our country. It has advantages of reliable production technology, low quality requirements for propylene raw materials, domestically guaranteed catalysts, simple process, low investment, fast payback, easy operation, flexible product conversion, less "three waste", suitable for China's national conditions, and etc. The disadvantage is that scale of production is small, and it is difficult to achieve economies of scale, a lot of manual operation of device, discontinuous production, low level of automation control, unstable product quality, consumption rate of raw materials is relatively high, use is narrow. At present, polypropylene production capacity in my country using this method is about 24.0% of country's total production capacity.

(b) Continuous mass process. The process mainly includes American Rexall process, American Phillips process and Japanese Sumitimo process.

(1) Rexall process. The Rexall bulk polymerization process is a manufacturing process between solvent method and bulk method. It was successfully developed by Rexall in USA. Propylene polymerizes. An azeotropic mixture of hexane and isopropanol is used as a solvent in deashing of polymer, which simplifies rectification steps. The residual catalyst and random PP are dissolved in solvent together and removed from bottom of solvent distillation column. Later, a joint thermoplastic company formed by company and El Paso of United States developed a new manufacturing process called "liquid pool process". A feature of this process is that high-purity liquid-phase propylene is used as a raw material, a highly efficient HY-HS catalyst is used, there is no deashing process and removal of random substances. A continuously stirred reactor is used and heat of polymerization is dissipated by reactor jacket and overhead condenser. After slurry is separated by flashing, monomer is recycled back to reaction.

(2) Phillips process. This process was successfully developed by American Phillips Petroleum Company in 1960s. Its process is characterized by using a unique loop reactor. This simple loop reactor has a large heat transfer area per unit volume, high overall heat transfer coefficient, high single-pass conversion rate, high flow rate, good mixing, and no. The curing zone has advantages of fast plasticizing and short time for product changeover. The process can produce polymers with a wide range of flow rates.melt and random polymers.

(3) The craft is sumitimo. The process was successfully developed in 1974 by Japanese chemical company Sumitomo. This process is basically similar to Rexene bulk process, but Sumitimo bulk process includes some measures to remove stray particles and catalyst residue. These measures could lead to creation of superpolymers for some electrical and medical applications. The Sumitimo bulk process uses a complex SCC catalyst (titanium tetrachloride is reduced with monochlorodiethylaluminum and treated with n-butyl ether), and liquid phase propylene is polymerized at 50-80°C and 3.0 MPa with a high reaction rate. The isotactic index of polymer is also high, and it is also deashed by high efficiency extractor. The isotactic index of product is 96%-97%. The product is spherical particles, with high hardness, good thermal stability and excellent oil resistance. and electrical properties.

D. Polypropylene Gas-Phase Process

Research and development of gas-phase process for production of polypropylene began in 1960. In 1967, BASF built a pilot plant in Ludwigshafen for gas-phase production of polypropylene using a vertical stirred-tank reactor. In 1969, ROW, a joint venture between BASF and Shell, built world's first industrial gas phase polypropylene plant with a capacity of 25,000 tons per year in Wesseling, Germany, using a vertically agitated reactor called Novolen process. In 1970s, American company Amoco developed a gas-phase process for production of polypropylene using a nearly perfect displacement horizontal stirred-bed gas-phase reactor. In early 1980s, UCC used its proven Unipol PE gas phase fluidized bed process to produce polypropylene and launched Unipol gas phase polypropylene process. The Japanese Sumitomo Corporation also developed a gas-phase process using a gas-phase fluidized bed during same period. At present, world's gas-phase polypropylene production processes mainly include BP's Innovene process, Chisso process, United Carbon's U process, Nipol's process, BASF's Novolen process, and Sumitomo Chemical Company's Sumitomo process, etc.

E. Combination of volumetric and gas phase methods

Combination of volumetric method and gas phase method mainly includes Basel's Spheripol process, Mitsui Chemicals' Hypol process, and Borealis' Borstar process.

(a) Spheripol process. The Spheripol process has been successfully developed by Basell Polyolefins. Since technology was first introduced to industry in 1982, it is by far most successful and widely used process for production of polypropylene. The Spheripol process is a polymerization process that combines liquidphase prepolymerization with liquid-phase homopolymerization and gas-phase copolymerization. The process uses high-performance catalysts, and particle size of polypropylene powder obtained with catalyst is spherical, with large and uniform particles. Both wide and narrow can be adjusted. Can produce a full range of multipurpose products. Its homopolymer and random copolymer products are characterized by high transparency, good optical properties and no peculiar smell. The liquid loop reactor used in Spheripol process has following advantages:

(1) It has a high spatio-temporal productivity of reactor (up to 400 kg PP/h.m3), volume of reactor is small, and investments are small.

(2) The reactor has a simple design and low material requirements. Low temperature carbon steel can be used, and design and manufacture are simple. Due to small pipe diameter (DN500 or DN600), even if pressure is high, pipe wall is thin.

(3) The straight part of jacketed reactor can be used to support reactor frame, and this structural design reduces capital investment.

(4) Due to small volume of reactor, residence time is short, switching of products is fast, and there is less transition material.

(5) The polymer particles are suspended in propylene liquid, and there is good heat exchange between polymer and propylene. The cooling jacket is used to dissipate reaction heat, heat transfer area per unit volume is large, and heat transfer coefficient is large. The total heat transfer coefficient of loop reactor reaches 1600W/(m2℃).

(6) The slurry in loop reactor is circulated at high speed by an axial pump, and liquid flow rate reaches 7 m/s, so polymer slurry can be stirred evenly, catalyst system is uniformly distributed, and polymerization reaction conditions are easy to control and can be control. It is very accurate, product quality is same, not easy to create hot spots, not easy to stick to wall and consume power. axial pump is also low.

(7) The concentration of polymer slurry in reactor is high (more than 50% mass fraction), and single-pass conversion rate of reactor is high, and single-pass conversion rate of homopolymerized propylene is 50%-60%.

The above characteristics make loop reactor very suitable for production of homopolymers and random copolymers. High performance catalysts such as GF-2A, FT-4S and UCD-104 are used at start of Spheripol process. Catalyst activity reaches 40 kg PP/CAT and product isotacticity is 90%-99%.

The technology is now developed to second generation. Compared to first generation technology using a single loop reactor, second generation technology uses a double loop reactor andwhose pressure and temperature are significantly increased to obtain a bimodal PP. The catalytic system adopts fourth or fifth generation high-performance Z-N catalyst, adding hydrogen separation and recovery units, improving high and low pressure polymer degassing equipment, and improving propylene steaming, drying and emergency discharge devices, increasing operational flexibility improves efficiency, and also significantly reduced consumption of monomer raw materials and various public works. The particle size of resulting product is more uniform, and range of melt flow index of product is wider (from 0.3 to 1600.0 g/10 min), which allows production of new grades of polypropylene with high rigidity, high crystallinity and low thermal deformation. temperature. The impact copolymerization reaction of Spheripol process is carried out by gas-phase method, and reactor is one or two dense phase fluidized bed reactors connected in series. The reactor uses a dense phase gas-phase fluidized bed. The gas phase reactor system can be used to produce impact copolymers with an ethylene content of 8%-12% (w/w). low stress), it is necessary to design two gas phase reactor systems, keep gas phase composition and operating conditions in two gas phase reactor systems independent, and two different copolymers can be added to homopolymer.

Using a two-stage steaming and drying process for processing polymers, steam in steaming tail gas can be easily condensed to separate pure hydrocarbon monomers, allowing hydrocarbons in tail gas to be completely recycled and reducing monomer consumption. The closed nitrogen drying system also reduces nitrogen consumption of unit. In addition, Spheripol process uses a modular design method, which can meet requirements of different users, and is easy to build in stages (for example, homopolymer production system is installed first, and then gas phase reaction system is added). of course), and production capacity of device is also easily expandable. The Spheripol process has a strict and perfect safety system design, which ensures high operational stability and safety of device. The new generation of Spheripol process uses a system of adding pure additives, which makes quality of product more uniform and stable and makes it easier to switch to another product. Spheripol's process technology offers a complete range of products including homopolymers, random copolymers, impact copolymers, terpolymers (ethylene-propylene-butene copolymers). The MFR range of its homopolymer products is 0,1-2000g/10min, MFR of industrial products reaches 1860g/10min (special non-granular products), and flexural modulus of high rigidity products reaches 2300MPa. The ethylene content of industrially produced random copolymer products reaches 4.5% (mass fraction), and ethylene-propylene-butylene terpolymer products are also available, and initial film sealing and sealing temperature is as low as 110°C, which can be produced by gas phase process High ethylene content competes with random copolymerization. Impact copolymer products have a good combination of stiffness and impact resistance. The ethylene content of product can reach 25% (40% rubber phase) and can reach 40% ethylene content (60% rubber phase). In addition, Spheripol process allows flexible adjustment of product molecular weight distribution between polymer dispersion index (PI) 3.2-12 by adding peroxide and bypass reactor according to product needs, and can also directly produce MFR products up to 1800g/10min . and large particles without granulation make Spheripol process extremely competitive. Another feature of Spheripol process is advanced catalyst technology. Basell offers a variety of catalyst systems that can be used in Spheripol process to produce various types of products, such as MC-GF2A catalyst for production of homopolymers, MC-M1 catalyst for production of impact copolymers and homopolymers, and large spherical random copolymers. materials, while some high modulus homopolymers require use of D-electron donors (dicyclopentyldimethoxysilane, referred to as DCPMC), and some special purpose high ethylene impact copolymers also use a special purpose catalyst. Diester catalysts, for which Basell has filed a number of patents, also have commercial products such as MC-126 and MC-127. Diester-based catalysts have high polymerization activity (up to 100 tl/kg cat) and long service life, good control of isotactic index, high sensitivity to hydrogen, and product has a narrow molecular weight distribution. At present, there are about one hundred sets of polypropylene plants produced by Spheripol technology in world, with a total production capacity of about 14.6 million tons per year, which is about 36.8% of total polypropylene production capacity in world. Among them, production capacity in North America is 4.03 million tons/year, total in Asia is 4.19 million tons/year, production capacity in Western Europe is 4.105 million tons/year, production capacity in Central and Eastern Europe is 620,000 tons. tons/year, and in Middle East and Africa, production capacity is 1.315 million tons/year.

(b) Hypol process. The Hypol process was successfully developed by Japaneseand Mitsui Chemicals in early 1980s. The process uses a HY-HS-II (TK-II) catalyst, which is a multi-stage polymerization process. It combines advantages of bulk propylene polymerization process and gas phase polymerization process. This is a combined technology that allows production of various grades of polypropylene without deashing and removing random impurities. This process is basically similar to Spheripol process. The main difference is that in Hypol process, homopolymer cannot be bypassed from gas phase reactor, and part of flash gas from high pressure degassing vessel is returned to gas phase reactor. In production of homopolymer, first gas phase reactor actually also acts as an evaporator. Gas-phase reactors are specially designed based on fluidized bed apparatus and stirring (scrapper) vessels. The reactor is free from fouling and does not require cleaning when producing impact copolymers. During production of homopolymer, gas phase reactor can be used as final polymerization tank, which improves productivity, and operation of gas phase reactor is flexible, and it can produce ethylene with 25% high-impact copolymer.

Based on technology provided by diethyl ether, Mitsui has produced 5th generation RK-RH catalyst, whose activity is 2-3 times higher than that of 4th generation catalyst. The Hypol process allows production of a full range of homopolymerized, randomized and high impact polypropylene products. The MFR range is 0.30-80 kg/10 min. The resulting products have high stereoregularity and rigidity, and produced films have good optical properties (transparency and gloss); oriented varieties such as monofilaments, tapes and fibers have good workability (orientation), so that finished product can have good characteristics, high melt flow rate, and varieties for high-speed injection molding can be obtained by direct polymerization without heat treatment and others. events. Currently, there are 22 sets of polypropylene plants produced by Hypol technology in world, with a total production capacity of about 2.51 million tons per year, which is about 6.3% of total polypropylene production capacity in world.

(c) Borstar process. Borealis (Borealis) Borstar (North Star Double Peaks) polypropylene production process is a new polypropylene production process successfully developed in 1998. The North Star Double Peaks PE process, as well as gas phase reactors designed with Z-N catalysts, can also use single site catalysts that are being pilot tested. Its basic configuration is to use twin reactors, i.e. loop reactors connected in series with gas phase reactors to producehomopolymers and random copolymers, followed by one or two gas phase reactors in series to produce impact copolymers, depending on final product. For example, production of high rubber phase impact copolymers requires a second gas phase copolymerization reactor. In May 2000, Borealis built world's first bimodal polypropylene plant with a production capacity of 200,000 tons per year in Schwechat, Austria. In conventional polypropylene process, polymerization reaction is carried out below critical point of propylene. In order to prevent formation of bubbles of light components (such as hydrogen, ethylene) and inert components, polymerization temperature is controlled at 70-80°C. . The loop reactor of Beixing bimodal polypropylene process can operate at high temperature (85-95°C) or under condition of exceeding supercritical point of propylene. The polymerization temperature and pressure are very high, which can prevent formation of bubbles. This is only process for polymerization of polypropylene under supercritical conditions. The main characteristics of Beixing PP Bimodal High Temperature Bimodal Technology can be summarized as: advanced catalyst technology, wide polymerization reaction conditions, wide product range, and excellent product performance.

(1) Use of more active carrier catalyst MgCl2 (BC1). The higher reaction temperature of Z-N series catalyst used, higher activity. The activity at 80°C is 60,000 kg PP/kg cat, so catalyst residue in product is very small. In addition, Borstar process allows production of all types of products using a single catalyst system.

(2) The combined process flow of loop reactor and gas-phase fluidized bed reactor allows flexible control of MWD, isotactic index and comonomer content in product. The high temperature or supercritical operation of loop reactor not only improves activity of catalyst, but also improves heat transfer capacity of reactor, lowers liquid density, increases solids concentration, and improves production efficiency of reactor. The outlet of loop reactor is directly fed into gas phase reactor without using steam to gasify monomer, and liquid phase monomer is gasified by reaction heat of gas phase polymerization, which reduces steam consumption. The reaction conversion rate per pass is high and can reach more than 80%, and amount of circulating monomer is small.

(3) Since loop reactor operates under supercritical conditions, there are practically no restrictions on concentration of hydrogen added, and gas phase reactor is also suitable for operation with a high hydrogen concentration. This combination of reactors makes it possible to obtain products with very high melt flow rates and high comonomer content in reactor. At present, fibrous products with an MFR of more than 1000 g/10 min and random copolymers with an ethylene content of 6% (mass fraction) have been developed.

(4) It can produce narrow molecular weight distribution unimodal products and wide molecular weight distribution bimodal products, which can broaden molecular weight distribution of polymers and improve processability of products. The molecular weight distribution of polymer decreases with increasing reactor temperature, and a product with desired molecular weight distribution can be obtained. Even for very high MFR products, MWD can be controlled resulting in unique properties such as low creep and high melt strength.

(5) Due to higher polymerization temperature, resulting polymer has a higher crystallinity and isotactic index, and xylene-soluble substance is very low, about 1% (mass fraction). Rigidity at same toughness is 10% higher than traditional polypropylene products.

(6) The distribution of comonomers in random copolymer is very uniform, so it has very good heat sealing and optical properties. Since reaction conditions are above critical point, only a small amount of polymer is dissolved in propylene, which reduces sticking phenomenon at high random copolymer content, and system can add more comonomer to random copolymer. maximum content of ethylene can reach 10% (mass fraction).

(7) Allows production of high-impact copolymers with a higher content of rubber phase. Impact copolymers with up to 25% rubber phase content (15% ethylene) can be produced using a single copolymerization reactor, and up to 50% rubber phase impact copolymer (15% ethylene content) can be produced using two 30% copolymerization reactors. The overall performance of impact resistance and rigidity of product is better.

(8) Borealis has developed and applied BorAPC technology to its Borstar process. Various process control methods can be realized by adopting a patented process controller, realizing advanced control and card edge operation, increasing productivity by 2%-3%, improving stability of controlling reaction conditions and product quality, and reducing time. production time.Product transit time reduces transport material.