A kinetic study on crystallization of calcium gluconate in external loop airlift column and stirred tank for an immobilized glucose oxidase reaction with crystallization

The batch crystallization of calcium gluconate from the reaction solution of the immobilized glucose oxidase catalyzed oxidation of glucose was kinetically studied not only in an external loop airlift column but also in a small scale stirred tank. The solubility of calcium gluconate under a wide range of the glucose oxidation conditions was measured and correlated. The crystallization was started by charging successively a saturated calcium gluconate solution prepared at 30 degreesC and a known amount of its seed crystals in either crystallizer being thermostated at a constant temperature between 5 and 15 degreesC. The time courses of the dissolved calcium gluconate concentration during crystallization were observed to be unaffected either by superficial gas velocity in the airlift column or by impeller speed in the stirred tank. The time courses in the column were found to be identical with those in the tank under the same operating conditions of pH, temperature, and glucose and seed crystals concentrations. Almost no decrease in the dissolved calcium gluconate concentration was observed during the crystallization without any seed crystals, while negligible change in the crystal size distribution was microscopically found during the crystallization with seed crystals. These results obtained suggested that the crystallization proceeded under the surface reaction control and no secondary nucleation occurred in both crystallizers. A crystal growth model was proposed to reproduce the time courses of the dissolved calcium gluconate concentration during crystallization under the various operating conditions. The order of the crystal growth rate with respect to the calcium gluconate supersaturation was found to be 3.2. The calculated time courses of dissolved calcium gluconate concentration agreed well with the observed ones in both airlift and stirred tank crystallizers. (C) 2002 Elsevier Science B.V. All rights reserved.