Estimation of heat and mass transfer coefficients in a pilot packed-bed solid-state fermentation bioreactor

Abstract

Although mathematical models of heat and water transfer in packed-bed bioreactors for solid-state fermentation have been available for several decades, there have been few efforts to determine the heat and mass transfer coefficients between the solids and air phases experimentally. In the current work, we undertook drying, cooling and heating experiments in a pilot packed-bed bioreactor, using a solid substrate prepared as for a fermentation, but uninoculated. We then used the solids-air heat and mass transfer coefficients as fitting parameters to adjust a heat and mass transfer model to temperature data obtained at different bed heights. The model fit the data well, but only if the air flow rate was allowed to vary as a function of position, indicating that the air flow in the bed was not uniform. In a sensitivity analysis, the air flow rate had a greater effect on the predicted temperature profiles than did the solids-air heat and mass transfer coefficients themselves. However, despite the lesser effect of these coefficients, it is not possible to treat the solids and air phases as being in thermal and hydric equilibrium. Rather, it is essential to use models that recognize the solids and air as separate phases and use expressions based on driving forces to describe evaporation and convective heat removal. The experimentally determined heat and mass transfer coefficients can be substituted into a mathematical model that also describes microbial growth and metabolic heat production and this model can be used to guide the scale-up from our pilot bioreactor to larger scales.