Evaluating strategies for overcoming overheating problems during solid-state fermentation in packed bed bioreactors

Abstract

Normal operation of packed bed bioreactors used for solid-state fermentation involves a static bed aerated from the bottom throughout the fermentation. This leads to axial temperature profiles with the highest temperature, sometimes over 20°C higher than the inlet air temperature, occurring at the top of the bed. An axial heat transfer model was used to explore two strategies designed to prevent the temperature reaching undesirable levels, namely periodic reversal of the direction of airflow, and periodic mixing. Simulations were done for the growth of Aspergillus niger on a starchy substrate. The bed was assumed to be wide enough such that the radial heat transfer could be ignored. With hourly air reversal or mixing events, the maximum temperature predicted during the fermentation is higher than the maximum temperature predicted for normal operation. Increasing the frequency of air reversals leads to temperatures close to the maximum for growth in the middle of the bed. However, for a 0.345 m high bed and a superficial air velocity of 0.0236 m s −1, mixing 10–60 times per hour can lead to lower maximum temperatures in the column compared to normal operation, because the frequent mixing distributes the benefits of the effective cooling at the bottom of the column throughout the bed. The degree of reduction in maximum temperature increases with increasing specific growth rate of the microorganism. Modelling is a useful tool in guiding experimental programs for the design and scale-up of bioreactors for solid-state fermentation, since it can be used to identify promising strategies and eliminate unfruitful strategies.