A two‐phase model for water and heat transfer within an intermittently‐mixed solid‐state fermentation bioreactor with forced aeration

Abstract

A two-phase dynamic model is developed that describes heat and mass transfer in intermittently-mixed solid-state fermentation bioreactors. The model predicts that in the regions of the bed near the air inlet there can be significant differences in the air and solid temperatures, while in the remainder of the bed the gas and solid phases are much closer to equilibrium, although there can be differences in water activity of around 0.05. The increase in the temperature of the gas as it flows through the bed means that it is impossible to prevent the bed from drying out, even if saturated air is used at the air inlet. The substrate can dry to water activities that severely limit growth, unless the bed is intermittently mixed, with the addition of water to bring the water activity back to the desired value. Under the conditions assumed for the simulation, which was designed to mimic the growth of Aspergillus niger on corn, two mixing events were necessary, one at 17.4 and the other at 27.9 h. Even though such a strategy can minimize the restriction of growth by water-limitation, temperature-limitation remains a problem due to the rapid heating dynamics. The model is obviously a useful tool that can be used to guide scale-up and to test control strategies. Such a model, describing the non-equilibrium situation between the gas and solid phases, has not previously been proposed for solid-state fermentation bioreactors. Models in the literature that assume gas-solid temperature and moisture equilibrium cannot describe the large temperature differences between the gas and solid phase which occur within the bed near the air inlet.

Modeling Menaquinone 7 production in tray type solid state fermenter

“…The symbol µ T denotes that the equation describes specifically the effect of temperature on the specific growth rate parameter. The dependence of the specific growth rate parameter µ W on water activity was described by Equation (3) of von Meien and Mitchell [11], and the geometric mean of the individual specific growth rates were calculated for their combined effect in Equation (4).…”

“…The assumption of an isothermal system was used to develop the model for determining the effect of temperature. A similar concept was also applied by von Meien and Mitchell [11] for predicting the effect of water activity on bacterial growth which can be depicted as µ W µ opt = exp D 1 (a 3 ) ws + D 2 (a 2 ) ws + D 3 (a) ws + D 4 ,…”

Estimation of Transfer Coefficients for SSF Bioreactors

“…In their two-phase model of heat and mass transfer in a packed-bed bioreactor, von Meien and Mitchell (2002) used correlations that had been determined for the drying of corn. The coefficient that they used for convective heat transfer between the solid and gas phases, which appears in (Eq.…”

Coeficientes De Interface Para Transferência De Calor E Água Em Biorreatores De Fermentação Sólida

“…Von Meien e Mitchell (2002) foram os primeiros a sugerir um modelo de duas fases para a predição de gradientes de temperatura e umidade em um biorreator de FES agitado intermitentemente com aeração forcada, tendo modelado apenas os estágios em que o biorreator operava como leito empacotado, entre os sucessivos estágios de agitação, usando a mesma aproximação aplicada para a secagem de grãos em leitos fixos. Embora a implementação dos modelos de duas fases requeira as informações fluido-partícula, esses modelos são mais realistas e são os únicos capazes de representar adequadamente a transferência de água, e consequentemente de calor evaporativo, da fase sólida para a gasosa.…”

“…Nas Equações 1 e 2, os termos contendo o parâmetro βa representam a transferência de água na interface sólido-gás, enquanto nas Equações 3 e 4 os termos contendo o parâmetro ha representam a transferência de calor na interface. No entanto, apesar da complexidade do modelo de Von Meien e Mitchell (2002), os coeficientes de transferência na interface sólido-gás βa e ha, que regem os termos de transferência de massa e de calor entre as fases, respectivamente, foram obtidos da literatura de secagem de grãos em leito fixo e não necessariamente correspondem ao sistema de FES. Schutyser et al (2004) também empregaram um modelo de duas fases para prever perfis térmicos e de umidade durante os estágios da FES em que o substrato permanecia estático em seu biorreator de tambor rotativo, e também não deixaram clara a obtenção dos coeficientes de transferência na interface.…”