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 ( K L a) and the differing oxygen solubilities in the immiscible liquid phases. In this study, K L a 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 K L a and hence OTR, was shown to be markedly dependent on the agitation rate. At agitation rates of 800 rpm and above, K L a was enhanced with increasing alkane concentration up to 10%, after which K L a repression became evident. At agitation rates below 800 rpm, K L a was repressed below that in water, at all alkane concentrations. Maximum K L a 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%.