Hydrodynamic and oxygen mass transfer studies in a three-phase (air–water–ionic liquid) stirred tank bioreactor

Recently, room temperature ionic liquids have been investigated as organic solvent replacements in multiphase partitioning bioreactors. They could prove particularly useful for oxidative bioconversions due to their non-volatile and non-flammable nature. In order to promote high mass transfer rates stirred tank bioreactors are normally used. However, for multiphase systems, particularly those featuring ionic liquids, there is little hydrodynamic information available to underpin bioreactor design and operation. The aim of this work was to study the hydrodynamic and oxygen mass transfer characteristics of a three-phase (gas–water–ionic liquid) stirred tank partitioning bioreactor. A central composite experimental design was used to investigate the interaction of three operational factors, agitation (400, 700, 1000 rpm) and aeration rates (0.5, 0.75, 1.0 vvm), and ionic liquid volume fraction (5, 12.5, 20%), on two response variables, the Sauter mean droplet diameter (d32) and the volumetric oxygen transfer coefficient (kLa). Three different ionic liquids with a range of physicochemical properties were also investigated. The experimental data obtained were fitted to empirical correlations relating d32 and kLa values to the measured gassed power input and other bioreactor operating parameters. In all cases the measured values of kLa (and in most cases d32 values) decreased as ionic liquid volume fraction increased as is the case when using conventional organic solvents. It was shown that different physicochemical properties of the ionic liquid tested gave important differences in the drop size behaviour as a function of agitation conditions. Both of the correlations established for describing the variation of kLa and d32 with bioreactor operating conditions gave reliable predictions of the measured values. These correlations will be useful for ongoing studies on the design and operation of partitioning bioreactors for oxidative bioconversions.