On liquid-liquid mass transfer in two-liquid-phase fermentations

Almost all two-liquid phase bioprocesses are characterized by the presence of surface active materials (biosurfactants), which significantly influence the interaction between the phases. In order to predict mass transfer rates during cultivations of Pseudomonas oleovorans biosurfactant was isolated from the biosuspension and added in defined amounts to n-octane/water model-dispersions. Effects of biosurfactant concentration on interfacial tension, mean Sauter-diameter, drop size distribution, dispersion stability and liquid-liquid mass transfer coefficients were studied. A comparison was made between calculated solvent transfer rates (STR) and measured solvent uptake rates (SUR) of P. oleovorans cultures. With increasing interfacial surfactant concentration interfacial tension and mean Sauter-diameter decreased until a minimum for both, interfacial tension and mean Sauter-diameter, were reached. Interfacial tension measurements indicate that these minima have to be attributed to a maximum monomolecular surfactant concentration and the formation of polymolecular adsorption layers. Drop size distributions showed that, coalescence and droplet break-up disappear because droplets are stabilized by the biosurfactant adsorption layers at the interface. Mass transfer regime shifted from forced convection and surface renewal to diffusion. Comparison of solvent uptake rates (SUR) and solvent transfer rates (STR) showed that n-octane transfer usually will not be limiting P. oleovorans cultures, however, can become dominant in cultures where solvents with very low miscibilities like n-decane are used.