Gas/Liquid mass transfer in a four-phase stirred fermentor: Effects of organic phase hold-up and surfactant concentration

Abstract

The aerobic microbial production of fine chemicals from liquid hydrocarbon substrates that are poorly soluble in water usually asks for a four-phase stirred tank fermentor (a continuous aqueous phase containing the cells, a dispersed organic phase and a dispersed gas phase necessary for oxygen supply). Depending on the operating conditions the gas/liquid oxygen mass transfer can be the rate controlling factor in the fermentation rate. The already complex behaviour of such four-phase reactors can be further complicated by the fact that hydrocarbon converting microorganisms commonly produce biosurfactants which can significantly influence gas/liquid mass transfer. Moreover, for hydrocarbon drop diameters below ▪ (diffusion coefficient/ mass transfer coefficient ), the mass transfer coefficient for oxygen transfer from the gas to liquid can be significantly enhanced by the presence of the dispersed organic liquid phase. One of the fermentation systems investigated at our laboratory is the growth of Pseudomonasoleovorans cells on n-octane as the organic substrate. The biosurfactant produced in this systems is a lipopolysaccharide from the outer membrane of the cell.A dynamic method was developed to investigate the effect of the dispersed organic phase and of the biosurfactant on the volumetric gas/liquid mass transfer coefficient of oxygen.In the absence of the organic phase small amounts of biosurfactant were found to lower the volumetric mass transfer coefficient (kwa) and relatively large amounts of the emulsifier increase this coefficient. For surfactant concentrations typically found in the fermentation mentioned above approximately 70 % of the hydrocarbon droplets were found to have diameters below 0.5 * 10−6 m. It could be shown that these small droplets are responsible for a significant enhancement of the effective gas/liquid mass transfer coefficient (kw) of oxygen into the aqueous phase. As theoretically expected, the enhancement factor increases with the concentration of small octane droplets. As far as the authors know, such a mass transfer enhancement effect so far has been observed in stirred cells with a flat gas/liquid interface only where kw values usually are an order of magnitude lower relative to the stirred tanks used in the present investigation. The results indicate that in hydrocarbon fermentation significant enhancement of mass transfer is possible, and that surfactants can be used to increase the relatively low fermentation rates.