Effect of Particle Size Distribution and Flow Non-Uniformities on Dispersion in Porous Media: Application to Oil Shale Retorts

摘要

This paper considers the effects of particle size distribution and flow nonuniformities on the dispersion of an inert tracer in porous media using some simplified models, and compares the results to recent dispersion data taken on external retorts filled with oil shale particles. The effect of the particle size distribution is analyzed by studying the dispersion of a tracer in a series of infinite parallel pores of varying diameters. The pores are considered to be in either laminar or turbulent flow. The predicted dispersion coefficient of the porous medium is shown to be a function of the Peclet number and the higher moments of the distribution of pore radii. The Peclet number dependence is in agreement with experimental measurements, as is the qualitative dependence on particle size distribution. The effect of flow nonuniformities is studied by analyzing the dispersion of an inert tracer in a packed bed with two annular regions of different interstitial velocities. The dispersion coefficient for the area-averaged concentration is shown to be a very sensitive function of the parameters in the flow distribution. The dispersion coefficients measured in external retorts or packed beds filled with oil shale particles are much higher than those previously measured using uniform particles of the same average size. These differences can be accounted for if one allows for the possibility of having a significant contribution from flow nonuniformities. (ERA citation 05:005386)