Studies On Ethanol Fermentation With Cycled Gas Stripping Technics

The ethanol industry accounts for important role in national economy. With the increasing crisis of energy source and strengthening power of environmental protection, the demands of fuel ethanol consumption have grown very quickly in recent years. Ninety percent of the whole ethanol production in the world is generated by fermentation, but a main problem in fermentation is the product inhibition effect by the ethanol metabolized from the yeast in the process. One of the most effective method for alleviating product inhibition is in-situ ethanol separation, which can not only improve the efficiency of fermentation, but simplify the process of separation and purification. For improving the traditional ethanol fermentation technics of current alcohol industry of our country, the characteristics of Saccharomices cerevisiae and Kluyveromyces marxianus were systematically studied in this thesis, then the mixed fermentation were carried out by two yeast strains. Combined with cycled gas stripping to separate products in situ and the previous studies, the cycled gas stripping equipment were improved—a condenser was installed, so that the circumfluence equipment was all under a low temperature environment, the aim is to improve the disadvantages in the traditional fermentation with gas stripping. The study is mainly based on the characteristics of chemical engineering separation, and fermentation engineering to reduce the energy cost for ethanol distillation and the amount of the liquor wastes, the final goal is to decrease the cost. The results suggested that the new process could greatly improve the ethanol fermentation efficiency.Firstly, the characteristics of S. cerevisiae and K. marxianus, which were employed in all experiments, were studied, and mixed-yeasts fermentation was suggested based on the results. The ethanol production rate was expected to be improved by investigation of the mixed fermentation inoculation mode, air flow flux and fed-batch strategies The basal conditions of mixed-yeasts ethanol fermentation were obtained as: natural pH conditions, mixed cultivation, 35℃of fermentation temperature and 0.5 vvm of airflow flux for fermentation.Secondly, CO₂ cycle gas-stripping for separation of ethanol in situ was studied in ethanol batch or fed-batch fermentation. In this process, CO₂ produced in ethanol fermentation was circulated between fermentor and separation instrument. From the results in a gas stripping experiment with ethanol-water solution as medium, we concluded that the recycle rate of CO₂ or the temperature in the condenser was linear correlation to the efficiency of gas stripping respectively, while the concentration of ethanol in fermentor presented as a parabolic curve. In the condenser for collection of returning ethanol at low-temperature, the concentration of ethanol can reach to 400 g·L-1, which is significantly higher than that at normal temperature. Combined above results, the optimal conditions for gas stripping is: 70g·L-1 of ethanol concentration when gas-stripping started, the gas flow for the cycle at 0.5vvm, the condenser temperature at 2℃, if all the conditions were met, it could be guaranteed a higher ethanol productivity and a minimum energy consumption, thereby reducing the cost in ethanol production.The results of simulated gas stripping experiments indicated that when collecting ethanol at low temperature, the ethanol concentration in the condenser can reach to 400g·L-1. In ethanol fermentation with gas stripping, the ethanol productivity was 4.32g·L-1 h-1, which was 2.29 times to that in the fed-batch fermentation without gas stripping.