Mass transfer to large particles in fluidised beds of smaller particles

Abstract

A theory is proposed for predicting the transfer of a gas through a fluidised bed of small particles to a large particle. It is proposed that non steady-state mass transfer of the gas occurs by two mechanisms: (i) mass transfer of gas in clusters or packets of the smaller particles approaching the large particle; and (ii) gas convection. The theory developed enables prediction of the Sherwood number (N sh, the dimensionless mass transfer coefficient) for a large particle, diameter d: N sh=2ε mf+ 4 g3 mfd(U mfε mf+ uinb )/π D A 1 2 where U mf is the minimum fluidising velocity, ε mf is the bed voidage at U mf-0 u b is the mean bubble rise velocity and D A is the gas diffusivity. This equation is shown to be in excellent agreement with Sherwood numbers determined from combustion experiments in which single large particles of petroleum coke were burned in air fluidised beds over a wide range of operating conditions. It is also shown that predictions using this expression are in close agreement with those from an empirical expression previously proposed by the autho