Tracer dispersion in packings of porous activated carbon grains

A new method based on radioactive dispersion is reported which permits the accurate determination of the parameters which describe the transport and exchange mechanisms for packed beds of porous particles: characteristic exchange time between the internal and external pores (including adsorption effects), mean tracer velocity, mobile fluid fraction and longitudinal hydrodynamic dispersion coefficient. The method relies upon the fitting of the Coats-Smith model to radioactive tracer transit time distributions at regularly spaced intervals along a packed column for low Peclet numbers. Relationships between the transport and exchange mechanism parameters and relatively easily measured properties of the porous particles (size, porosity, intraparticle diffusion coefficient, adsorption isotherm) and packed bed (packing density) have been determined. These relationships may be used to estimate the hydrodynamic parameters. It is shown that the transport and exchange mechanism parameters determined at low Pe numbers using the new method can be used to accurately predict behavior of the packed bed at high Pe numbers provided transport within the porous particles is explicitly included within the model. (C) 1999 Elsevier Science Ltd. All rights reserved.