Validation of a model for process development and scale-up of packed-bed solid-state bioreactors.

We have validated our previously described model for scale-up of packed-bed solid-state fermenters (Weber et al., 1999) with experiments in an adiabatic 15-dm(3) packed-bed reactor, using the fungi Coniothyrium minitans and Aspergillus oryzae. Effects of temperature on respiration, growth, and sporulation of the biocontrol fungus C. minitans on hemp impregnated with a liquid medium were determined in independent experiments, and the first two effects were translated into a kinetic model, which was incorporated in the material and energy balances of the packed-bed model. Predicted temperatures corresponded well with experimental results. As predicted, large amounts of water were lost due to evaporative cooling. With hemp as support no shrinkage was observed, and temperatures could be adequately controlled, both with C. minitans and A. oryzae. In experiments with grains, strong shrinkage of the grains was expected and observed. Nevertheless, cultivation of C. minitans on oats succeeded because this fungus did not form a tight hyphal network between the grains. However, cultivation of A. oryzae failed because shrinkage combined with the strong hyphal network formed by this fungus resulted in channeling, local overheating of the bed, and very inhomogeneous growth of the fungus. For cultivation of C. minitans on oats and for cultivation of A. oryzae on wheat and hemp, no kinetic models were available. Nevertheless, the enthalpy and water balances gave accurate temperature predictions when online measurements of oxygen consumption were used as input. The current model can be improved by incorporation of (1) gas-solids water and heat transfer kinetics to account for deviations from equilibrium observed with fast-growing fungi such as A. oryzae, and (2) the dynamic response of the fungus to changes in temperature, which were neglected in the isothermal kinetic experiments.

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