Modeling semi-simultaneous saccharification and fermentation of ethanol production from cellulose

Using a highly refined standard cellulose (microcrystalline cellulose, (Avicel PH101)), the kinetics of the simultaneous saccharification and fermentation (SSF) of cellulose to ethanol was studied with a prior hydrolysis phase (semi-simultaneous saccharification and fermentation (SSSF)) conducted under optimal conditions for enzymatic hydrolysis. Four cases have been studied: 24-h pre-hydrolysis + 48-h SSF (SSSF 24), 12-h pre-hydrolysis + 60-h SSF (SSSF 12), 72-h SSF, and 48-h hydrolysis + 24-h fermentation. SSSF 24 produced higher yield and higher productivity of ethanol than the other operating modes. A coupled set of differential equations were developed to describe the change rates of cellobiose, glucose, microorganism, ethanol, glycerol, acetic acid, and lactic acid concentrations in the batch operation of separate hydrolysis and SSF of ethanol production from Avicel PH101. The model parameters were determined by a MATLAB program based on the batch experimental data of the SSSF. The analysis of the reaction rates of cellobiose, glucose, cell, and ethanol using the model showed that the conversion of cellulose to cellobiose was the rate-controlling step in the SSSF process of ethanol production from cellulose. The batch SSSF model was extended to the continuous and fed-batch operating modes. For the continuous operation in the SSSF, the productivity of SSSF 24 was much higher than that of SSSF 12 though the ethanol concentrations of both cases have not a great difference.