Gas channelling and heat transfer in moving beds of spherical particles

Gas-to-solid heat transfer coefficients measured in moving beds of solid pa rticles are an order of magnitude smaller than those predicted by conventio nal correlations which work well for fixed beds. The authors have elsewhere proposed a cause: gas flow in moving beds involves gross channelling which falsifies the plug-flow heat transfer analysis which yields the coefficien ts. In the present work, end effects have been removed from measured gas RT Ds using a Fourier Transform technique applied to data from beds of differe nt lengths. This refinement of the data analysis reveals gas RTDs for movin g beds to be characteristically bimodal, conclusively demonstrating the pre dicted channelling. Radial profiles of axial gas velocities have also been measured, using the time-of-flight of tracer gas pulses injected and sample d at known radial positions in the packing. Two distinct channels are ident ified: an annular zone near the bed wall of thickness six to ten particle d iameters, and a cylindrical zone in the bed centre where the gas velocity i s higher, by a factor of some 1.14-1.25, than that in the annulus. A simple model of flow and heat transfer is used to show that the degree of channel ling observed is of the correct order of magnitude to account for the erron eous heat transfer coefficients reported.