A GRAIN-SIZE DISTRIBUTION MODEL FOR NONCATALYTIC GAS-SOLID REACTIONS

A new model to describe the non-catalytic conversion of a solid by a r eactant gas is proposed. This so-called grain size distribution (GSD) model presumes the porous particle to be a collection of grains of var ious sizes. The size distribution of the grains is derived from mercur y porosimetry measurements. The measured pore size distribution is con verted into a grain size distribution through a so-called pore-to-sphe re factor whose value is also derived from the porosimetry measurement s. The grains are divided into a number of size classes. For each clas s the conversion rate is calculated either according to the shrinking core model, involving core reaction and product layer diffusion as rat e-determining steps, or according to a new model in which some reactio n at the grain surface is assumed to be limiting. The GSD model accoun ts for the phenomenon of pore blocking by calculating the maximum atta inable conversion degree for each size class. In order to verify the m odel, two types of precalcined limestone particles with quite differen t microstructures were sulphided as well as sulphated. Furthermore, a single sample of sulphided dolomite was regenerated with a mixture of carbon dioxide and steam. For each reaction good agreement was attaine d between measured and simulated conversion vs. time behaviour.