Changes in particle size distribution play an important role in fluidized bed processes, and these changes are dominated by elutriation and carryover of fines and by attrition or agglomeration. In this study on attrition in gas-fluidized beds, we found that the attrition is a function of the particle size distribution, the jet velocities and the overall superficial gas velocity. Empirical equations have been developed to predict the attrition rate. Les Keywords: attrition, fluidized bed, grid velocity.he particle size distribution in a gas-fluidized bed is con-T tinuously changing as the result of (i) elutriation and carryover of fines, and (ii) attrition or agglomeration. Whereas, the effect of carryover on the bed composition can be limited by recycling the collected fines to the bed, attrition and agglomeration will produce extra amounts of respectively, fine or coarse particles, both resulting in the modification of the bed composition.Agglomeration occurs mostly with sticky particles, in moist air or at high temperature (Ray et al., 1987) and is not studied in this work.Particle attrition in fluid-bed systems was first studied to characterize catalysts for fluid-cracking systems. Forsythe and Hertwig (1 949) illustrate early efforts to select catalysts to resist the effect of high-velocity jets. In spite of subsequent research, as illustrated below, attrition is not fully understood yet. Evaluating attrition is important in many fluid-bed systems for process operation (e.g. bed loss during start-up), control of fines production to minimize downstream deposits or erosion, controlled attrition to maintain material reactivity, low operating cost, or environmental regulations (e.g. minimize fine particle collection cost). Vaw and Keairns (1980) tested several sources, i.e. particle heating, calcination, sulfatation, low and high velocity impacts, and their importance for particle size reduction through wear, fracture, decrepitation, abrasion, splitting, shattering, chipping and desintegration.Attrition of fluidized-bed solids can be caused by several mechanisms, including thermal stress (when particles are being heated or cooled and unequal temperatures in the particles cause uneven expansion, intra-particle stress and decrepitation); chemical stress (when during chemical reaction of a particle its surface will generally react first); static mechanical stress (when a bed particle is stressed at its surface *Author to whom correspondence should be addressed. E-mail address: jan haeyens@cit.kuleuven.ac.be e.g. at the bottom of a bed, or from within e.g. formation of gas by vaporization of hydrated water); and kinetic stress (when slow moving particles suffer surface abrasion upon collision or fast-moving particles shatter completely e.g. in cyclones or by high-velocity (grid) jets). Baeyens et al. (1989) illustrated thermal and chemical stresses during the calcination of limestone where evaporation, hydrate decomposition and/or calcination enhanced production of fines. The magnitude of attrition depends mostly...
