Characteristics of SSSF of rice straw and mass transfer of ethanol in a granular packed bed with N₂ sparging

In this work, a solid simultaneous saccharification and fermentation (SSSF) system with N-2 stripping was constructed for ethanol production, and the glucose inhibition to enzymatic saccharification of cellulose and the ethanol inhibition to anaerobic fermentation of yeast cells could be relieved. First, effects of flow rate of carrier gas and initial yeast concentration on mass transfer of ethanol and performance of SSSF were experimentally investigated. It is found that the peaks of total ethanol amount (6.32 g), cellulose consumption amount (13.29 g), ethanol yield (0.48 g/g substrate) and change rate of porosity of the packed bed (57.45 %) were obtained at an initial yeast concentration of 2% and a N-2 flow rate of 30 mL/min. The results reveal that mass transfer efficiency during SSSF could be improved at an appropriate N-2 flow rate and an optimal initial yeast concentration. Then, a novel kinetic model was established to simulate mass transfer of ethanol during SSSF. The data predicted by the model were in good accordance with the experimental results, which shows that the established model could reasonably reflect the characteristics of ethanol mass transfer in the packed bed. This work demonstrates that the produced ethanol could be effectively stripped out of the packed bed by convection of the sparged N-2, leading to improving performances of enzymatic hydrolysis and ethanol fermentation.