Dynamic modelling of aerobic bioprocesses in gel particles with immobilised cells

A dynamic model for aerobic growing cells immobilised into gel beads is developed and its operation is illustrated for the case of gluconic acid production by a strictly aerophilic strain of Gluconobacter oxydans. The model consists of both kinetic and mass transfer equations predicting the time course of bulk and intraparticle concentrations of substrates, products, and biomass. The model includes a product inhibition term. The parameter values are taken from own studies and from the literature. A sensitivity analysis of the model shows that the most significant parameters for the process are the biotransformation rate constant, the specific cell growth rate in the bulk, and the Thiele modulus for glucose. The computer simulation reveals that depending on the parameter values the gel particles might perform as a source or a sink of the product, thus enhancing or retarding the net process. For a specific parameter selection, the biotransformation in the pellets can prevail compared with the bulk in the beginning of the process as long as the direction of the product diffusion flux is from the beads toward the bulk. Since the process in the free culture dominates, the system is more sensitive to parameters associated with the bulk phase (aeration rate, specific microbial growth rate, oxygen uptake rate). The model can be applied for prediction and fast evaluation of the performance of aerobic processes accomplished by immobilised growing cells.