Continuous production of calcium gluconate crystals in an integrated bioreaction-crystallization process using external loop airlift bubble columns with immobilized glucose oxidase gel beads

The integrated bioreaction-crystallization process proposed previously was experimentally studied for efficient continuous production of calcium gluconate crystals. The process consisted of two external loop airlift bubble columns in series with 2.5 l reaction solution in each column, one being bioreactor and the other crystallizer. The bioreactor was used to produce calcium gluconate solution through the air oxidation of glucose catalyzed by the immobilized glucose oxidase plus manganese dioxide gel beads at 30degreesC with simultaneous neutralization of the oxidation product, gluconic acid, by calcium hydroxide. The solution in the bioreactor was then fed into the crystallizer operated at 10degreesC to produce calcium gluconate crystals which was recovered through filtration from the calcium gluconate slurry in a recycling tube from the crystallizer to bioreactor with the successive addition of solid glucose to the bioreactor. In addition to the optimal operating conditions determined in our previous work, the more favorable pH, type of neutralizer and gel beads size were examined to keep the MnO2 activity as high as possible in the gel beads by minimizing the dissolution of manganese dioxide into the liquid bulk throughout the reaction period. The 14-h and 60-h operations for the continuous production of calcium gluconate crystals were performed under the optimal operating conditions determined, which gave the crystal recovery yields of 77% and 72%, respectively at the filtration unit. Such low recovery yields were attributed mainly to the incomplete crystal recovery system. In conclusion, it has clearly been demonstrated that the feed of solid glucose and calcium hydroxide to the bioreactor with simultaneous removal of calcium gluconate crystals from the crystallizer realizes the continuous production of the crystals in the environment-friendly bioprocess.