J.T.M. Pater

Polymerization of liquid propylene with a 4th generation Ziegler-Natta catalyst-influence of temperature, hydrogen and monomer concentration and prepolymerization method on polymerization kinetics

In a batch-wise operated autoclave reactor, liquid propylene was polymerized using a 4th generation, TiCl4/MgCl2/phthalate ester-AlEt3-R2Si(OMe)2, Ziegler-Natta catalyst system. By using a calorimetric principle it was possible to measure full reaction rate versus time curves for obtaining data on polymerization kinetics, under industrially relevant conditions. The influence of polymerization temperature, the hydrogen and monomer concentration, and the prepolymerization method on reaction kinetics were investigated. A new method for prepolymerization, the so-called non-isothermal prepolymerization, is described. In this short prepolymerization procedure featuring an increasing polymerization temperature the thermal runaway on particle scale was avoided. It was shown that this prepolymerization method can relatively easily be applied to an industrial process, with the introduction of a continuous plug flow reactor, giving a narrow residence time distribution, acceptable yield-in-prepolymerization and a method for monitoring catalyst activity. Using different methods for calculating the monomer concentration at the active site of the catalyst, the influence of polymerization temperature was determined. It was shown that at high polymerization temperatures, the reaction rate is barely influenced by polymerization temperature, when no prepolymerization is used. This is ascribed to thermal runaway on particle scale of a fraction of the catalyst particles. When a prepolymerization is used, this effect disappears and thermal runaway is avoided. When systematically reducing the monomer concentration (Cm,bulk) in the bulk by replacing part of the liquid propylene by hexane, the reaction rate proved to be remarkably independent of the monomer concentration. With reducing Cm,bulk, reaction rate decreased very slowly until Cm,bulk=150 g/l. When further decreasing Cm,bulk, reaction rate dropped rapidly. The hydrogen concentration was varied over a wide range at 60°C and 70°C. For both temperatures it was shown that reaction rates increased rapidly with increasing hydrogen concentration at low hydrogen concentrations. At higher hydrogen amounts, this effect disappeared and a maximum reaction rate was found.