Stress History – a Concept for the Description of the Age-Dependent Attrition ofCatalysts in Fluidized Bed Systems

摘要

In fluidized bed reactors a catalyst is subjected to significant attrition. This attrition influencestogether with the efficiency of the solids recovery system the particle size distribution inside thefluidized bed system and thus the performance of the reactor and it is a major reason for the loss ofcatalyst. In previous work the attrition of catalyst has been studied in detail for catalysts which haveundergone attrition inside the fluidized bed for a long time already and therefore reached a constantattrition rate. But it is known that a fresh catalyst is much more fragile and shows up a much higherattrition rate at the beginning than at steady state. In a fluidized bed reactor a particle will besubjected to attrition due to different mechanisms in different parts of the system, namely attritionby gas jets near the bottom of the fluidized bed, bubble induced attrition in the fluidized bed itselfand the attrition during the passage through a cyclone. All these different attrition mechanisms aredescribed by different mathematical models and individually need different times to reach constantvalues of the attrition rate. In the fluidized bed system the ageing of the particle due to the differentmechanisms will be superposed. To be able to summarize the effect of the stresses on the particlewithin the different parts of the fluidized bed system the concept of the 'stress history' has beendeveloped, which allows a uniform treatment of the different attrition mechanisms. This concepthas been experimentally validated and been implemented into an existing population balancemodel. The experiments were carried out with fresh FCC catalyst.