Fragmentation, particle growth and single particle modelling

Abstract

In processes that rely on the use of heterogeneous catalysis as the major means of production, it should be quite obvious that understanding how the catalyst particles evolve will play an important role in many aspects related to quality and reactor performance. At the risk of oversimplifying things, the principal roles of the heterogeneous catalyst particles used in olefin polymerisation can be seen as being (1) to carry the active sites upon which the polymer is formed; and (2) to provide a structure for creating “solid” particles that can be easily transported, recovered and processed. It is therefore important for us to understand how the process used to make the polymer impacts the particle and the active sites (and vice versa!). From the schema in Fig. 3.1, where these concepts are applied to a heterogeneously catalysed olefin polymerisation process, it can be seen that one needs to consider many different length scales, from the reactor which has volumes on the order of several tens of cubic metres, to the catalyst and polymer particles with characteristic diameters on the order of 10−6 to 10−3 m, and finally the active sites with characteristic sizes on the order of Ångströms. The figure also suggests that in many ways one can consider the catalyst and polymer particles as being at the heart of a polymerisation process. This is of course not to over-simplify the technological challenges of correctly operating the reactors, nor to assume that we have totally mastered the behaviour of the active sites either. However, as we shall see below, the very fact that we are using heterogeneous catalysts implies that mass transfer limitations can eventually limit the concentrations of active species at the active sites, or that the quality of the polymer (sticky/hard, brittle/flexible) can have a major impact on reactor behaviour. For these, and many other related reasons it is therefore of importance to understand what happens to the particles injected into the reactor during the polymerisation.