A step-forward in the characterization and potential applications of solid and liquid oxygen transfer vectors

Silicone oil 20 and 200 cSt, a perfluorocarbon (FC40TM), heptamethylnonane, Kraton, Elvax, and Desmopan were evaluated for their ability to enhance oxygen transfer in stirred tank and airlift reactors (STR and ALR, respectively). None of the vectors tested was either toxic or biodegradable and they exhibited a moderate affinity for oxygen (gas/vector partitioning coefficients K-g/v = C-g center dot C-v(-1) ranging from 3 to 5.1). FC40 was highly volatile, while KratonTM and ElvaxTM exhibited a low thermal stability, which constitutes a serious handicap for their implementation in fermentations. Silicone oil 20 cSt and Desmopan supported the highest oxygen transfer rates under abiotic conditions in both STR and ALR designs, with enhancement factors of up to 90% and 250%, respectively, compared to control tests (deprived of vector). The fact that these vectors showed the highest K-g/v proved that, besides the classical selection criteria, the in situ hydrodynamic behavior (which affects K(L)a) must be considered for vector selection. The use of silicone oil 20 cSt and Desmopan in glucose-supplemented Saccharomyces cerevisiae fermentations resulted in a two- and threefold increase in biomass productions, respectively. The better performance of Desmopan in terms of biomass growth enhancement, together with the absence of the operational problems inherent to the use of liquid vectors (such as intensive foaming, high cost, and difficult solvent recovery), make solid vectors a promising and cost-effective alternative in the future developments of two-phase partitioning bioreactors.