Intraparticle diffusion alters the dynamic response of immobilized cell/enzyme columns

Abstract

The effect of intra-particle tracer diffusion on the transient response of a fixed-bed column containing porous Ca-alginate beads is investigated experimentally. Dynamic response experiments are carried out using acetone solutions as tracer, whose concentration in the effluent stream is monitored on-line using a UV analyzer. Comparison of the response of such columns for two particle sizes (radii of 2 and 1 mm) to that of otherwise identical beds filled with solid glass particles of equal size reveals a substantial difference, mainly characterized by a trailing of the effluent concentration-vs-time curves at longer times. This cannot be explained by assuming a change in the effective porosity of the system, nor a change in the hydrodynamic dispersion. Following independent experimental determination of the pertinent physical parameters, a model based on the tanks-in-series concept [1, 2] is implemented on a personal computer and its predictions are compared to the experimental data. The essential assumption of the model is that tracer convection in the bulk liquid is substantially faster than intra-particle diffusion and, as a result, a bead is always surrounded by fluid of uniform (albeit time-dependent) concentration. In the absence of external mass transfer resistances and assuming that the porous nature of the Ca-alginate beads does not substantially alter the hydrodynamic dispersion in the column, the agreement between model and experiment is remarkably good. This leads to the suggestion that the observed response can be explained as the combined result of (fast) tracer convection in the bulk liquid and (slower) tracer diffusion from the bulk liquid to the beads.