Gas channelling and heat transfer in moving beds of spherical particles

摘要

Gas-to-solid heat transfer coefficients measured in moving beds of solid particles are an order of magnitude smaller than those predicted by conventional 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 coefficients. In the present work, end effects have been removed from measured gas RTDs using a Fourier Transform technique applied to data from beds of different lengths. This refinement of the data analysis reveals gas RTDs for moving beds to be characteristically bimodal, conclusively demonstrating the predicted channelling. Radial profiles of axial gas velocities have also been measured, using the time-of-flight of tracer gas pulses injected and sampled at known radial positions in the packing. Two distinct channels are identified: an annular zone near the bed wall of thickness six to ten particle diameters, and a cylindrical zone in the bed centre where the gas velocity is 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 channelling observed is of the correct order of magnitude to account for the erroneous heat transfer coefficients reported. [References: 21]