Enhancement and repression of the volumetric oxygen transfer coefficient through hydrocarbon addition and its influence on oxygen transfer rate in stirred tank bioreactors

The oxygen transfer rate (OTR) is a key parameter in aerobic bioprocesses, and in particular, in hydrocarbon-based bioprocesses where a proportionately higher rate is required due to the oxygen deficient structure of the substrate. The OTR is dependent on the oxygen transfer coefficient (KLa) and the differing oxygen solubilities in the immiscible liquid phases. In this study, KLa was evaluated at six discrete alkane concentrations (0, 2.5, 5, 10, 15 and 20 vol.%) over a range of agitation rates (200–1200 rpm) and aeration rates (0.5–1.5 vvm). The influence of alkane on KLa and hence OTR, was shown to be markedly dependent on the agitation rate. At agitation rates of 800 rpm and above, KLa was enhanced with increasing alkane concentration up to 10%, after which KLa repression became evident. At agitation rates below 800 rpm, KLa was repressed below that in water, at all alkane concentrations. Maximum KLa was obtained at an agitation rate between 1000 and 1200 rpm and an alkane concentration between 5 and 10%. Maximum OTR was similarly attained between 1000 and 1200 rpm although the high oxygen solubility in the alkane shifted the optimum alkane concentration to between 15 and 20%.