Mass transfer from a large sphere buried in a packed bed along which liquid flows

Consideration is given to the problem of dissolution of a buried solid sphere in the liquid flowing uniformly through the packed bed around it. The basic equations describing liquid movement and mass transfer in the interstices of the packed bed are presented and due consideration is given to the process of transverse convective dispersion. The resulting theoretical expressions for the coefficient of mass transfer are then compared with experimental data. Experiments were performed on the dissolution of spheres of benzoic acid (with diameters between 10.8 and 20.8 mm) buried in beds of sand, through which distilled water was forced to flow, at 293 and 303 K. Two closely sized batches of sand were used (0.219 and 0.322 mm), with interstitial water velocities covering the range 0.1-0.7 mm/s. The experimental data are shown to be in very good agreement with the predictions from our theory and the importance of accounting for convective transverse dispersion is well brought out by the results. The predictions of two alternative correlations, applicable to mass transfer in the conditions under study, are also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.