Mixing and drift in gas-fluidised beds

One of the most important aspects of bubbling fluidised beds is the mixing they promote. Rowe et al. demonstrated experimentally that the dominant mechanisms of mixing in the bulk of the bed are drift, or permanent displacement of particles, induced by the bubbles, and the return flow it generates [P. Rowe, B. Partridge, A. Cheney, G. Henwood, E. Lyall, The mechanics of solids mixing in fluidised beds, Trans. Inst. Chem. Eng. 43 (1965) 271-286] [1]. We show how, by combining new results relating drift and dispersion, practical estimates of mixing rates consistent with Rowe's view of mixing can be obtained [P. Rowe, B. Partridge, A. Cheney, G. Henwood, E. Lyall, The mechanics of solids mixing in fluidised beds, Trans. Inst. Chem. Eng. 43 (1965) 271-286] [1]. We also develop Batchelor's vortex model of bubbles in fluidised beds to elucidate aspects of bubble structure.