Slurry process polymerization polypropylene

Most commercial processes produce polypropylene by a Hquid-phase slurry process. Hexane or heptane are the most commonly used diluents. However, there are a few examples in which Hquid propylene is used as the diluent. The leading companies involved in propylene processes are Amoco Chemicals (Standard OH, Indiana), El Paso (formerly Dart Industries), Exxon Chemical, Hercules, Hoechst, ICl, Mitsubishi Chemical Industries, Mitsubishi Petrochemical, Mitsui Petrochemical, Mitsui Toatsu, Montedison, Phillips Petroleum, SheU, Solvay, and Sumimoto Chemical. Eastman Kodak has developed and commercialized a Hquid-phase solution process. BASE has developed and commercialized a gas-phase process, and Amoco has developed a vapor-phase polymerization process that has been in commercial operation since early 1980. 

Gas phase olefin polymerizations are becoming important as manufacturing processes for high density polyethylene (HOPE) and polypropylene (PP). An understanding of the kinetics of these gas-powder polymerization reactions using a highly active TiCi s catalyst is vital to the careful operation of these processes. Well-proven models for both the hexane slurry process and the bulk process have been published. This article describes an extension of these models to gas phase polymerization in semibatch and continuous backmix reactors. 

Like the slurry process for polyethylene manufacture, this is also the first commercial process for the production of polypropylene. The basic process includes a series of CSTRs and polymerization takes places in an inert diluent. Many different variations of the slurry process were developed in the early 1970s. The design of the process was dictated by the available catalyst at the time, which typically had low activity and produced polymer with low isotacticity. Since catalyst activity was low, a series of reactors were needed to push the reaction to completion. It is not imusual to see processes with five reactors in series, and some with even seven reactors in series, designed in those earlier days. Deashing was required to remove the high level of catalyst residue from the product an atactic polypropylene removal step was also necessary. Different variations of the slurry process included the use of diluents ranging from Ce to C12 hydrocarbons. 

It is tempting to suggest that polymer processes will gradually evolve toward bulk. Recently, the suspension process for impact polystyrene has been supplanted by the bulk process, and the emulsion process for ABS may similarly be replaced. However, the modern gas-phase process for polyethylene appears to represent an opposite trend. It seems that polymerization technology tends to eliminate solvents and suspending fluids other than the monomers themselves. When the monomer is a solvent for the polymer, bulk processes are preferred. When the monomer is not a solvent, suspension and slurry processes like those for polyethylene and polypropylene are employed. 

Although the Phillips and Standard Oil processes can be used to prepare polypropylene, the polymer yields tend to be low and it appears that these processes have not been used for commercial production of polypropylene. Until about 1980, polypropylene has been produced commercially only by the use of Ziegler-Natta catalysts. Commonly a slurry process is used and is carried out in much the same manner as described previously for the preparation of polyethylene (see section 2.3.2(b)). In the case of polypropylene, some atactic polymer is formed besides the required isotactic polymer but much of this atactic material is soluble in the diluent (commonly heptane) so that the product isolated is largely isotactic polymer. Recently, there has been a marked shift towards processes involving gas phase polymerization and liquid phase polymerization. Few details of these newer processes have been published. Gas phase processes resemble those described previously for the preparation of polyethylene (see section (2.3.2(b)) and swing plants are now feasible. In liquid phase processes polymerization is conducted in liquid propylene, typically at 2 MPa (20 atmospheres) and 55°C. Concurrently with these developments, new catalyst systems have been introduced. These materials have very high activity and the reduced levels that are required make it unnecessary to remove catalyst from the final polymer. Also, the new catalyst systems lead to polypropylene with higher proportions of isotactic polymer and removal of atactic polymer is not necessary. 

Some processes are based upon soluble catalysts but gas phase and slurry processes use about 1-2 wt% catalyst impregnated onto a suitable form of silica Changes to the catalyst structure can modify catalytic activity, the molecular weight of the product, and the incorporation of co-monomers in the polymerization of ethylene. In the production of polypropylene, stractural changes within the catalyst also direct the formation of isotactic, syndiotactic, and noncrystalline atactic polymers. Better control of the polymerization process was therefore possible, as shown in Table 8.7. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

Another method of manufacturing polypropylene employs the liquid monomer as the polymerization solvent. This process, known as the liquid propylene or bulk-phase process, has a major advantage over the slurry method in that the concentration of the monomer is extremely high. The high concentration increases the rate of the reaction relative to that seen... 

Finally, in the fourth section the fundamentals of the modelling concerning two basic olefin polymerization processes are examined heterogeneous slurry polymerization and gas-phase polymerization. The SPERIPOL process for making High Impact PolyPropylene (HIPP) is then described as an illustrative example for combining fundamentals and elements of product and technology development. 

In only a few polymerization processes are metallocene catalysts used in a soluble form. Supported metallocene catalysts are preferred for the production of polyethylene or isotactic polypropylene on an industrial scale, especially in the slurry and gas-phase processes. To use them in existing technological processes (drop-in technology) as replacements for the conventional Ziegler-Natta catalysts, the metallocenes have to be anchored to an insoluble powder support, including silica, alumina, and magnesium dichloride (208-217). Various methods of anchoring catalysts to supports are possible (Fig. 25) ... 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

Commercial production of crystalline polypropylene (PP) was first put on stream in late 1959 by Hercules in the United States, by Montecatini in Italy, and by Farbenwerke Hoechst AG in Germany. The workhorse process for commercial production of PP has been slurry polymerizations in liquid hydrocarbon diluent, for example, hexane or heptane. These are carried out either in stirred batch or... 

Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

It may be that the polymer is insoluble in the monomer-solvent mixture from which it is formed. Polypropylene and PVC are two examples where the polymer has very limited solubility in the monomer. As polymerization proceeds, the polymer will precipitate from the reacting mass to form a dispersed phase of polymer swollen with the monomer-solvent mixture. This is called a slurry polymerization. (Phase inversion can occur at high conversions to give a bulk polymerization.) A typical slurry polymerization is autorefrigerated. The heat of polymerization causes the reacting mass to boil it is condensed and returned to the reactor. The gas-phase processes for polyethylene and polypropylene are conceptually similar to slurry polymerizations. The continuous phase is now a gas and the dispersed phase is a fluidized solid, but the heat of polymerization is still removed through the low-viscosity, continuous phase. 

Slurry-phase processes may involve either an inert diluent such as iso-butane or heptane, or condensed monomer such as propylene. In either case the catalyst particles are suspended and well mixed in the liquid medium. Monomer concentrations are high and the liquid provides good removal of the heat produced by the polymerization of the polymer particles. The two main reactors for slurry-phase olefin polymerization are the loop reactor and continuous-stirred tank. Slurry-phase processes are very attractive for high crystalline homopolymer products such as polypropylene and polyethylene. 

Ionic polymerizations are almost exclusively solution processes. Many Zieg-ler-Natta polymerizations are also. They can be run under conditions such that the polymer product stays in solution, as in the production of stereospecific rubbers. The crystalline polymers polyethylene and isotactic polypropylene are commonly produced at temperatures sufliciently below T so that the polymer product is a solid that grows on the catalyst particles as in gas-phase polymerizations. Such processes are known as slurry polymerizations. 

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