Introduction

To enable reactor design for immobilized enzyme catalytic systems, it is important to examine in some detail the kinetic properties of immobilized enzymes. the observed catalytic properties of a single catalyst particle containing immobilized enzyme depend upon interaction of substrate transport and the catalytic activity of the enzyme. Hence the need to study mass-transport-catalytic reaction coupling and its effect on a single particle of immobilized enzyme.

To begin our examination of the ways in which mass transport and reaction processes interact to determine the overall activity, deactivation, and parametric dependencies of an immobilized enzyme catalyst, consider the following schematic. The diagram shows a cross section of immobilized enzyme in contact with substrate solution which is flowing over both sides of the sheet. Far from the catalyst, the substrate concentration and other process variables, such as pH, have values characteristic of the bulk reaction mixture. These are the compositions that we would measure by conventional analytical chemistry methods.

In the above schematic, the phenomena summarized in the top section cause the substrate and product concentration profiles sketched in the middle section which can be described quantitatively in terms of the parameters indicated in the bottom section.

Because substrate is consumed within the immobilized enzyme catalyst and product is formed there, concentration gradients arise between the bulk solution far from the catalyst and reaction events which are occurring at the active sites of the immobilized enzyme molecules. substrate, for example, must be transported from the bulk solution to the outer surface of the catalyst. If the reaction mixture is stagnant, this transport occurs by molecular diffusion. Ordinarily, mixing or flow of substrate solution adds convective transport contributions to the movement of substrate from bulk solution to the external surface. If the immobilized enzyme formulation does not contain enzyme in its internal volume or if substrate cannot penetrate into the internal volume, this external mass transport process is the only one which has to be considered. Often, however, the enzyme is entrapped or impregnated within a permeable matrix. In such a circumstance, most of the catalytic activity is located in and distributed through the interior of the catalyst formulation. Consequently, for such a system, the substrate must diffuse from the outer surface of the catalyst to some internal location where reaction occurs. All of this scenario is inverted for the products. In this type of situation, the intraparticle diffusion processes must be considered in addition to external mass transport.

Development of an analytical framework for description of these interacting processes and of criteria for assessing their importance is one of the central achievements of chemical reaction engineering. These phenomena may be conveniently discussed under the following heads:

• Effects of External Mass-Transfer Resistance

• Analysis of Intraparticle Diffusion and Reaction

• Simultaneous Film and Intraparticle Mass-Transfer Resistances

Page created by Asif Ladiwala and Shripad Gokhale.

Last modified : 13 December, 2000.