The enhancement of synthesis gas fermentations using microbubble dispersions

Synthesis gas, a mixture of primarily carbon monoxide and hydrogen, can be used in a variety of bioconversions. It is well suited as a carbon and electron source for production of liquid fuels and chemicals or as a source of reducing equivalents for biological sulfur reductions. Synthesis gas fermentations have typically been gas-to-liquid mass transfer limited due to the low aqueous solubility of hydrogen and carbon monoxide, the primary components of synthesis gas. A novel technique to increase mass transport without increased power expenditure is to use microbubbles, also known colloidal gas aphrons. Microbubbles are small, surfactant-coated gas bubbles with diameters on the order 60 mum that have colloidal properties, such as an electric double layer, that impart stability and useful properties such as the ability to be pumped without collapse. Surfactants that are biocompatible with the microbial biocatalysts used in synthesis gas fermentations and that are capable of forming stable microbubbles have been identified. The effects of these surfactants on the metabolic activity of Butyribacterium methylotrophicum were characterized. Non-ionic surfactants were relatively non-toxic, while ionic surfactants were toxic at concentrations necessary to form microbubbles. The mass transfer properties of microbubbles were determined in an abiotic bubble column. The local mass transfer coefficient, KL, and the volumetric mass transfer coefficient, KLa were determined as a function of surfactant concentration in the surrounding bulk liquid. Synthesis-gas Fermentations were run using the biocompatible surfactants for both the production of liquid fuels and chemicals and for the biological reduction of sulfur. The KLa value for carbon monoxide transport in a Butyribacterium methylotrophicum fermentation was measured with both conventional and microbubble sparging. A six-fold increase in KLa was obtained upon switching from conventional bubbles to microbubbles. A sulfate reducing bacteria culture was used to convert sulfur dioxide to hydrogen sulfide using synthesis gas as a source of reducing equivalents. The productivity using microbubble sparging was two-fold higher than that using conventional sparging. Dynamic microbubble coalescence was measured using video microscopy in a stirred vessel. Coalescence was modeled using a population balance approach with a film drainage model to describe the coalescence efficiency. The model predicted that the reduced coalescence rate observed at higher surfactant concentrations is due to a thickening of the liquid film between adjacent bubbles.