Comparison of oxygen mass transfer coefficient in simple and extractive fermentation systems

Aeration and agitation are important variables to ensure effective oxygen transfer rate during aerobic bioprocesses; therefore, the knowledge of the volumetric mass transfer coefficient (kLa) is required. In view of selecting the optimum oxygen requirements for extractive fermentation in aqueous two-phase system (ATPS), the kLa values in a typical ATPS medium were compared in this work with those in distilled water and in a simple fermentation medium, in the absence of biomass. Aeration and agitation were selected as the independent variables using a 22 full factorial design. Both variables showed statistically significant effects on kLa, and the highest values of this parameter in both media for simple fermentation (241 s−1) and extractive fermentation with ATPS (70.3 s−1) were observed at the highest levels of aeration (5 vvm) and agitation (1200 rpm). The kLa values were then used to establish mathematical correlations of this response as a function of the process variables. The exponents of the power number (N3D2) and superficial gas velocity (Vs) determined in distilled water (α = 0.39 and β = 0.47, respectively) were in reasonable agreement with the ones reported in the literature for several aqueous systems and close to those determined for a simple fermentation medium (α = 0.38 and β = 0.41). On the other hand, as expected by the increased viscosity in the presence of polyethylene glycol, their values were remarkably higher in a typical medium for extractive fermentation (α = 0.50 and β = 1.0). A reasonable agreement was found between the experimental data of kLa for the three selected systems and the values predicted by the theoretical models, under a wide range of operational conditions.