Diffusion and chemical reaction rates with nonuniform enzyme distribution:An experimental approach

The study of the effects of nonuniform distributions of immobilized P-galac tosidase on the overall reaction rate of the hydrolysis of lactose are pres ented. Diffusion inside the particles has been characterized by measuring t he diffusion rates of two P-galactosidase substrates: lactose and ONPG in a commercial silica-alumina support. Effective diffusivities have been deter mined by the chromatographic method under inert conditions. The results obt ained for tortuosity can be explained assuming that the transport only take s place in the macropores. The distribution of the immobilized enzyme has b een measured by means of confocal microscopy technique. The enzyme has been tagged with FITC and immobilized in particles of different diameters, the internal local concentrations of the enzyme have been determined with the a id of an image computer program. As expected, a more nonuniform internal pr ofile of the enzyme was found when the particle diameter was bigger. Experi ments under reaction conditions were carried out in batch reactors using la ctose and ONPG as substrates and particles of the immobilized P-galactosida se of different diameter (1 . 10(-4) to 5 . 10(-3) m) as catalyst, employin g a temperature of 40 degreesC for lactose and 25 and 40 degreesC for ONPG, respectively. The mass balance inside the particle for the substrates has been solved for the internal profiles of the immobilized enzyme inside part icles of different size and the enzymatic reactions considered. The calcula ted and the experimental effectiveness factor values were similar when part icles under 2.75 10-3 m in diameter were employed. For the same Thiele modu lus, a particle with nonuniform distribution of enzyme showed a higher effe ctiveness as a catalyst than particles with a more uniform distribution. (C ) 2001 John Wiley & Sons, Inc.