Change in surface area of silica-alumina catalysts caused by sintering in steam atmosphere

Abstract

A model is presented which describes the transient change in the surface area of silica-alumina catalysts at temperatures in the range of 751–1136 K and steam partial pressures in the range of 11.1-709.1 kPa. Silica-alumina catalysts comprise very fine spherical ultimate particles of about 5 nm in diameter, which are in contact with one another. At the point of contact of two such particles, solid material transfers from the convex surface of the particles and forms a neck with a small negative curvature. This kind of mass transfer enhances the coalescence of the two particles, and results in a decrease in the surface area of the catalysts. Rate equations for coalescence of the ultimate particles were derived under two mass transfer mechanisms: vapor transfer and surface diffusion mechanisms. The process of coalescence was well formulated by a population balance model, and the overall rate of decrease of surface area was expressed by an equation consisting of the sum of the sixth and eighth powers of the surface area. Experimental data including those of other investigators were well correlated by the proposed equation. © 1985, The Society of Chemical Engineers, Japan. All rights reserved.