The productivity of bioethanol from the synthetic gas anaerobic fermentation by Clostridium jungdahlii is still very low when compared to other bioethanol fermentation methods. The low mass transfer rate of CO, CO2, and H2 gases to the liquid fermentation broth has been considered a major bottleneck in the overall process. Another possible bottleneck is the low concentration of biomass as the real catalyst for bioethanol production. A repeated batch fermentation configuration is proposed to solve the biomass concentration problem. This paper presents the evaluation of the repeated batch configuration for syngas anaerobic fermentation. A model for syngas fermentation has been developed and was used to simulate the effects of repeated batch configurations on bioethanol productivity. The results indicated more than a 50% increase in bioethanol productivity can be achieved by running this fermentation configuration.

Syngas fermentation is an alternative route that combines the advantages of thermochemical and biochemical processes have been proposed for biomass conversion to ethanol. One of the main obstacles to syngas fermentation is the low yield of ethanol, caused by the limited utilization of the syngas substrate due to low microbial cell concentration in the fermentation system. This research examined the modification of fermentation medium to improve microbial cell growth. The modifications were to increase the concentration of micronutrients/trace metals and macronutrients in the medium. The results showed that the maximum mass cell and maximum growth rate produced by microbial growth in the modified trace metal medium were 0.63 g/L and 0.0076 h-1, while in a modified macronutrient medium were 0.97 g/L and 0.0298 h-1. Modification of the macronutrient medium was able to increase the yield of biomass, but the opposite occurred in the modification of the trace metals. Meanwhile, the maximum concentration of ethanol from syngas fermentation in the modified macronutrient medium was lower than the concentration of ethanol in the standard medium.