CONVECTION-DIFFUSION OF SOLUTES IN MEDIA WITH PIECEWISE-CONSTANT TRANSPORT-PROPERTIES

Motivated by applications to electrophoretic techniques for bioseparat ions, we consider transient one-dimensional convection-diffusion throu gh a medium in which the solute diffusivity and convective velocity un dergo step changes at a prescribed position. An exact method of soluti on of the governing transport equations is formulated in terms of a la rgely analytical approach representing a novel alternative to the self -adjoint formalism advanced by Ramkrishna and Amundson (1974, Chem. En gng Sci. 29, 1457-1464), and applied recently by Locke and Arce (1993, Chem. Engng Sci. 48, 1675-1686) and Locke et al. (1993, Chem Engng Sc i. 48, 4007-4022). A concentration boundary layer of O(Pe(-1)) thickne ss is found to form at the upstream side of the interface. No concentr ation boundary layer exists on the downstream side. The exact solution is supplemented with an asymptotic analysis for large Peclet numbers, Pe. Detailed study of the boundary layer reveals interesting features of the local dynamical processes whereby the interface-infinitesimall y thin macroscopically-appears as an effective source or sink of the s olute content. The asymptotic analysis has direct utility in accurate prediction of concentration profiles for high Peclet number operations where analytical approaches break down and finite-difference methods require tremendous computational time to achieve sufficient accuracy a nd resolution. Copyright (C) 1996 Elsevier Science Ltd