A review of attrition of fluid cracking catalyst particles

Catalyst particles used for fluid catalytic cracking (FCC) in oil refineries can undergo attrition, which can contribute to the production of fines. Most FCC units thus have fines recovery systems (e.g. multi-stage cyclones) to control the loss of fines to the environment. However it is also necessary to ensure that the catalyst is sufficiently attrition resistant. This paper reviews the attrition of FCC catalysts. Catalyst particles have a variety of features which may be implicated in fines production — surface burrs, cracks and shells formed during the production or ageing. The level of attrition in an FCC unit is thus a function of the interactions between these features, particle properties and structure, and the hydrodynamic regimes prevailing in the unit. The latter depends on the geometry, solids concentration, flow rates and other operating conditions, and includes the fluidized state as well as dense and lean phase flows and impact of particle flows on stationary surfaces. A number of test methods exist, which have been very useful in tackling design issues. Bulk test methods are often designed to reproduce the hydrodynamic regimes in a unit on a smaller scale, so that a relative assessment of the attrition propensity of the particles can be made quickly. However, the analysis of the experimental data for relating the trend to the large-scale operation or particle properties is not straightforward. On the other hand, single-particle impact testing provides an unambiguous method for assessing the attrition propensity of particulate solids. Furthermore, it allows various mechanisms of attrition to be investigated in detail. However, the application of this method for the prediction of particle attrition on large-scale operations requires reliable models of the hydrodynamics of gas-solids flow for various flow regimes, a feature which has not been fully developed. Nevertheless, this has been done for one case, i.e. the analysis of attrition in jets in fluidized beds, and that is reported here. It is shown that the results from the bulk tests may not be easily applicable to attrition occurring in large-scale operations. The source of this discrepancy is discussed in this paper.