Nutzen und Grenzen der Anwendung kinetischer Modelle für die Reaktorberechnung, dargestellt am Beispiel des Hochdruck-Polyäthylen-Verfahrens

Abstract

The design of a reactor for complex reactions by means of the generally adapted balances of mass, energy and momentum is only possible if the reaction mechanism can be described by a kinetic model.

For this the polymerization of ethylene under high pressure is a characteristic example. The reaction could be described by the well known simple model of the radical polymerization comprising the reaction steps initiation, propagation and termination. It was necessary to determine the degree of the ethylene conversion in industrial reactors with sufficient accuracy. Without erecting a pilot plant, a factory of 24.000 t/y capacity and 100 million DM investment could be designed and built.

The applicational proporties of the polymerisate depend on the molecular weight distribution and the branching of the molecules. In order to determine these structural values the model had to be extended to eight kinds of reactions. The mathematical solution was difficult and lengthy, but absolutely justified.

Only by changing the reaction conditions according to time and location, the limits of mathematical and experimental possibilities were reached.

The high pressure polymerization of ethylene is one of the big modern processes whose development can be followed and calculated in all its manifold steps. In order to illustrate these relationships, the origin of the fundamental idea from the p-V-T-experiments of the Van der Waals-Institute, the accidental discovery of the high pressure polymerization, the first intuitive conceptions of the reactor, the later kinetic laboratory experiments, the exact reactor design, the transfer of the enormous reaction heat, the total construction of the process and the presently reached reactor capacity of 100.000 t/y are described.