Syngas fermentation by Clostridium carboxidivorans P7 in a horizontal rotating packed bed biofilm reactor with enhanced ethanol production

Highlights: • A novel a horizontal rotating packed bed (h-RPB) reactor for syngas fermentation was reported. • The h-RPB reactor enhanced ethanol productivity by 3.3-folds compared to continuous stirred tank reactor (CSTR). • The h-RPB reactor has a unique feature of transfer gas from both bulk liquid phase and headspace phase. • The mass transfer in the headspace of h-PRB played an important role for enhanced ethanol production. - Abstract: Gasification of lignocellulosic biomass followed by syngas fermentation is a promising process for producing fuels and chemicals. Syngas fermentation, however, is commonly limited by low mass transfer rates. In this work, a horizontally oriented rotating packed bed (h-RPB) reactor was developed to improve mass transfer and enhance ethanol production. In the h-RPB reactor, cell attachment materials were packed in the reactor and half submerged in the liquid and half exposed to the headspace. With continuous rotation of the packing materials, the cells in biofilm were alternately in contact with liquid and headspace; thus, transport of syngas to the cells occurred in both the liquid phase and headspace. The volumetric mass transfer coefficient (k_La) of the h-RPB reactor was lower than that in a traditional continuous stirred tank reactor (CSTR), indicating the mass transfer in the liquid phase of h-PRB was lower than CSTR, and the mass transfer in the headspace phase played an important role in syngas fermentation. The syngas fermentation of Clostridium carboxidivorans P7 in h-RPB resulted in a 7.0 g/L titer and 6.7 g/L/day productivity of ethanol, respectively, 3.3 times higher than those obtained in a CSTR under the same operational conditions. The results demonstrate that the h-RPB reactor is an efficient system for syngas fermentation, making cellulosic ethanol biorefinery one step closer to technical and economic feasibility.