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The process for the manufacture of polyethylene

 Polyethylene is now 0 produced by several processes that are more convenient and do not require the high capital investment and expertise associated with high-pressure operation. When catalysts were first 1____________, the most widely used processes were 2____________ on solution or slurry operation, in which polymerization 3____________ place on catalyst suspended in an inert liquid hydrocarbon. Since 1968, many new plants have 4_____________ a gas phase process in which the catalyst and polymer particles are suspended as a fluid bed, by the circulation of gaseous monomer.

 In the slurry process, a solution of about 2–6 wt % of the monomers in solution in a hydrocarbon such as isobutane, n-pentane or n-hexane is 5____________ with the catalyst, and the polymerization 6____________ in a loop reactor achieving conversions 97%. Typical co-monomers with ethylene can be butene-1, hexene-1, 4-methylpentene-1, and octene-1. The catalyst is 7____________ in the solution, which is then 8____________ as turbulent slurry through relatively narrow tubes. The polymer is essentially insoluble in the liquid diluent and forms a slurry containing up to 30 wt % of small polymer particles, which are 9____________ continuously.

 The operating temperature of the process must 10____________ below the melting or softening point of polymers and, depending on the type of polymer produced and the diluents used, is generally less than 100°C. A unique loop reactor was 11____________ by Phillips Petroleum Corporation to 12____________ the deposition of solids onto the reactor walls. It is 13____________ that loop reactors have 14____________ continuously for as long as three years without the need for cleaning. As an alternative, autoclave slurry reactors with efficient stirrers have been operated in batch or cascade processes. Changes in the partial pressure of the hydrogen moderator from one stage to another allow a range of different average molecular weights to be obtained within the product. Loop processes 15____________ Phillips chromium catalysts operate at pressures in the range 20–40 bar, whereas autoclave processes operate with Ziegler catalysts at lower pressures.

Residence times of up to 1 h are typical, with the continuous addition of monomer, catalyst, and fresh diluent. Productivity can 16____________ more than 10,000 lb of polymer per pound of catalyst. The use of low concentrations of catalyst allows the grade of the product to be changed within a few hours. The average molecular weight of the product can be 17____________ by the type of catalyst used, the activation procedure, operating conditions, and by changing the partial pressure of the hydrogen moderator.