An examination of Wen and Yu's formula for predicting the onset of fluidisation

Classical methods (measuring the pressure drop across a bed for different flowrates of air through the bed) were used to determine the superficial velocity for minimum fluidisation, U-mf, of sieved particles of alumina. The particles were characterised optically using an instrument (Morphologi G3, Malvern Instruments), which, from pixelated, enlarged photographs, measured the particles' mean diameter, d(p), to be 0.48 +/- 0.04 mm and their sphericity, phi, to be 0.77 +/- 0.09. Values of U-mf were measured for the temperature in the electrically heated bed varying from 14 to 920 degrees C. The results were analysed using Ergun's equation; one outcome was that the voidage, epsilon(mf), in an incipiently fluidised bed was found to be related to the particles' sphericity by: (1 - epsilon(mf))/phi(2)epsilon(3)(mf) = 12.2 +/- 0.4. The measured U-mf were significantly larger than the values predicted by Wen and Yu's equation, if the mean diameter of the particles was taken, as recommended, to be the geometric mean of the upper and lower sieve sizes, used when preparing the particles. Alternatively, the measured values of U-mf were over-predicted by Wen and Yu's equation, when using the optically measured mean size of the particles. The best predictions of U-mf were made by using the optically measured mean values of both d(p) and phi, together with Ergun's equation and the above equation coupling epsilon(mf) and phi. Such a procedure is proposed for estimating U-mf in general. No evidence was found for epsilon(mf) varying with temperature. (C) 2018 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.