Ethanol Production From Sweet Sorghum Stalks By Yeast Cells Imobilization System

Due to the diminishing fossil fuel reserves and climate deterioration, there is a need to develop renewable and cost-effective alternative energy sources. Bioethanol is a leading consideration as an alternative fuel for the future. Nowadays, the corn and cane are used for the production of ethanol. Considering the world food crisis and China conditions, we must find a non-food based feedstock to produce ethanol, sweet sorghum has been got more attention increasingly. In this paper, the authors use sweet sorghum bagasse to produce ethanol in cells immobilization system and carried the fermentation in industrial scale.Using sweet sorghum bagasse as biomaterial for cell immobilization for ethanol production by repeated batch fermentation was investigated in this study. The results indicated that the concentration of immobilized cell for unsterilized sorghum and treated sorghum by cellulose enzyme (60U/g carrier) was 0.489 dry cell weight (DCW)/g dry bass weight (DBW and 0.68g (DCW)/g(DBW), respectively. The elasticity of the carrier would weaken as the temperature rising from 100℃to 121℃, the concentration of immobilized cell decreased from 0.12g(DCW)/g(DBW) to 0.07g(DCW)/g(DBW). Take cellulase-treated and untreated sweet sorghum as a carrier for cell immobilization, the two systems was examined in repeated batch fermentation with high sugar concentration, it found that when the alcohol concentration reaches 130 g·L-1, the former fermentation period is 15h shorter than the latter. Pretreatment by cellulase not only increased the surface area ratio of the carrier but also improved the capacity of the immobilization. Moreover, cellulose could degrade the carrier surface partly so that many tiny pores formed, which enhance the mass transfer efficiency in the process of ethanol fermentation. Therefore, the rate of ethanol fermentation was greatly improved. The reactors were filled with untreated sweet sorghum stalks for repeated batch fermentation and continuous fermentation, the filling density was about 50%. Free cell concentrations increased gradually in the earlier stage of a batch and reduced in later stage of the batch. This tendency nearly repeated cyclically in every batch. The rapid proliferation can be attributed to abundant nutrition in the early stage of a batch fermentation. The maximum free cell concentrations of each batch reached a valuable concentration of 2.4×108/ml on average. The exhaustion of total sugar and ethanol inhibition led to the reduction of free cell concentration. Slight oscillation of immobilized cell concentration occurred in each batch which followed the changing rhythm of free cell concentration. The immobilized cell concentration reached 0.22 g (DCW/DBW) on average during the repeated batch fermentations. In this system, the overall ethanol concentration and residual sugar concentration remained almost constant at 130.1 g·L-1 and 5 g·L-1 on average, respectively. The free cell concentrations of each stage decreased to nearly the same level in the continuous fermentation. This steady state was harmful for the immobilized cells in the last two stages, where high ethanol concentrations were observed. Surprisingly, the viability of the immobilized cells in the forth and the fifth reactors decreased sharply after 26days. Only 22.13% of the cells were assumed to produce ethanol in the last reactor and 38.09% in the fourth reactor. The budding ratios of each individual stage plateaued began to decrease. As a result, final ethanol concentration of the fifth reactor decreased from 126 g·L-1 to 113 g·L-1 after 21 days with the residual sugar concentration increasing from 8.9 g·L-1 to 18.9 g·L-1. The present work demonstrates that an ethanol concentration as high as 130 g·L-1 for 26 days can be achieved using combined VHG and multistage continuous culture technologies. Two process routes were prepared In this multistage continuous fermentation to produce 1000 tons ethanol of a year. The results show that using the fresh sweet sorghum stem juice to produce ethanol directly was a more feasible route route after evaluating the economic feasibility. There were many advantages such as high fermentation speed, saving energy, low cost.