Time‐dependent behavior of a catalyst in a fluidized bed/cyclone circulation system

The performance of a catalytic fluidized‐bed reactor is strongly dependent on the properties of the catalyst of which the particle‐size distribution is one. The main influences on the particle‐size distribution are attrition of the catalyst particles, and the classifying effect of the solids recovery system. In a fluidized‐bed reactor a particle will be subjected to attrition due to different mechanisms in different parts of the system, namely attrition by gas jets near the bottom of the fluidized bed, bubble‐induced attrition in the fluidized bed itself, and attrition during the passage through a cyclone. All these different attrition mechanisms are considered in this work by different mathematical models. It is known that a fresh catalyst is much more fragile and exhibits a much higher attrition rate at the beginning of exposure to mechanical stress than under steady‐state conditions. Depending on the mechanism the particles need different times or in the case of attrition in a cyclone a certain number of passages to reach a constant value of the attrition rate. In the fluidized‐bed system a particle will during its aging experience all the different attrition mechanisms. In order to summarize the effect of these stresses on the particle within the different parts of the fluidized‐bed system, the concept of the “stress history” has been developed, which allows a uniform treatment of the different attrition mechanisms. This concept has been implemented into an existing population balance model. Experiments with a FCC‐catalyst in a fluidized/bed‐cyclone circulation system are well described by this model. © 2007 American Institute of Chemical Engineers AIChE J, 2007