Polyethylene process reactor configurations

It is useful to separate the discussion into processes for polyethylene and polypropylene, as the requirements for these two polymers are different and have led to similar, but by no means identical, processes. A short discussion on the various reactor configurations will be presented first, followed by descriptions on how each reactor configuration is used in different polymerization processes throughout the world. Also listed are a few keys references at the end of the chapter for further reading [72-85]. Finally, the chapter will be concluded with a few considerations on the mathematical modeling of industrial olefin polymerization reactors. 

There are many parallels between polypropylene and polyethylene manufacturing processes. The reactor configurations are similar, but, due to the different requirements of the polymer, it will be seen that there are significant differences between the processes as well. While propylene homopolymer can be produced in reactors of various configurations, for impact copolymer production, gas phase is the reactor of choice because of the stickiness of the polymer and the solubility of the copolymer in the monomer and diluent. 

The low-pressure polymerization processes were originally used in a single reactor configuration, but since the 1970s more complex polymerization systems have been developed that utilize two or more reactors that are in parallel and/or in series. Consequently, the polymerization conditions in each reactor may be varied over a wide range so that the final polyethylene material manufactured has a complex molecular structure, which is designed to provide various premium-grades of polyethylene that are suited for specific markets and applications. 

Borealis introduced a dual reactor configuration in 1995 in Finland combining a sliury-loop reactor with a gas-phase reactor under the trade-name of Borstar Polyethylene that is available for license. The 280 million poimd per year reactor is imique in that the first reactor is a slurry-loop process utilizing propane as diluent imder supercritical conditions rather than the usual slurry diluent isobutane. Supercritical propane offers lower polymer solubility than isobutane so that the risk of reactor fouling is lower with supercritical propane. 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Exxon and Phillips manufacture polypropylene in tubular reactors where the monomer is in the liquid form (see Section 6.8.2). One of the manufacturing processes for polyethylene involves the use of a loop reactor that has a recycle configuration. Here, under elevated pressure and temperature, a mixture of the catalyst, comonomer, hydrogen, and a solvent are introduced from one end of the reactor. The product and the unreacted starting materials are collected at the other end, and recycled back into the reactor. 

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