Bioreactor design for synthesis gas fermentations

Abstract

Bacterial cultures have been isolated for the conversion of synthesis gas (CO, H 2 and CO 2) into ethanol or methane. These heterogeneous reactions require the transport of substrate through the gas phase, across the interface into the liquid phase, and to the solid micro-organisms. The reactions are generally mass transfer limited due to very low gas solubilities. Bioreactors must maximize mass transport, while achieving high cell densities to promote fast reaction. This paper examines the performance of both dispersed gas phase systems (continuous stirred-tank reactor) and dispersed liquid phase systems (immobilized cell reactor) under mass transfer controlled and non-mass transfer controlled conditions. Mass transfer coefficients are determined and models are developed to predict bioreactor behaviour. Retention times of a few minutes are achieved for these gaseous substrate fermentations.