The Slurry Polymerization Process with Super-Active Ziegler-Type Catalyst Systems: From the 2 L Glass Autoclave to the 200 m³ Stirred Tank Reactor

Abstract

Since the discovery of the ethene polymerization with transition metal catalysts of group IV of the periodic table in combination with aluminum alkyl compounds as cocatalysts at low pressures and moderately high temperatures by Ziegler and colleagues in 1953, this catalytic polymerization process has been developed over six decades in an outstanding way and is now a mature technology for the production of high-density polyethylene grades with excellent properties for wide fields of application. Today, super-active heterogeneous catalysts are available. The catalyst must be designed to achieve high activity over a long polymerization time, be able to control average molecular mass over a wide range using hydrogen, to copolymerize ethene with higher 1-olefins, and to produce an unimodal polymer with a relative narrow molecular mass distribution. It is of greatest importance to avoid overheating of the growing polymer particle, especially when the polymerization starts at the virgin catalyst particle. This is not easy to achieve because the polymerization process is highly exothermic. The transformation of a catalyst particle into a polymer grain can be described and is well understood by the microreactor model. The technical process can be divided into three clear distinguishable levels: the microscale, the mesoscale, and the macroscale. The microscale level comprises all processes inside and at the surface of the growing polymer particle, i.e., the microreactor behavior. The mesoscale level deals with all processes inside the three-phase reactor content comprising gas bubbles, hydrocarbon diluent and the solid growing polymer particles. It is important to achieve reproducible and stable conditions on the basis of a detailed chemical engineering on this mesoscale level throughout the reactor. If this is the case, then this polymerization process can be well controlled on the macroscale level, comprising the polymerization vessel as a whole. By controlling a limited number of process data, the slurry polymerization process can be operated with excellent stability over a long time and can be controlled within narrow ranges. The modern slurry technology process is very flexible in controlling product properties by using the cascaded reactor technology. This technology involves two or even three reactors operated in series under different process conditions. The catalyst is only introduced into the first reactor. The polymerizing particle then passes through all reactors, producing different types of macromolecules to form a polymer blend within each polymer grain. A further enormous advantage of this cascade technology is the high flexibility in product change and product development. Just by changing process parameters of the different reactors, products with different average molecular mass, different molecular mass distributions, different copolymer compositions, and different comonomer distributions can be produced without changing the catalyst system.

In former time the author L.L. Böhm was working with Hoechst AG (Frankfurt (M), Germany), Hostalen Polyethylen GmbH (Frankfurt (M), Germany), Elenac GmbH (Kehl, Germany) and Basell Polyolefins (Wesseling, Germany).