Oxygen transfer in a gas–liquid system containing solids of varying oxygen affinity

An air sparged, mechanically agitated bioreactor containing spherical solids was studied in order to determine the effect of the solid phase on oxygen mass transfer. It was found that both nylon 6,6 and glass beads cause an enhancement of the volumetric mass transfer coefficient of up to 268%, whereas particles of silicone rubber and styrene–butadiene copolymer reduce the volumetric mass transfer coefficient by up to 63%, relative to a system without a solid phase. A simple transport in series model has been proposed to account for the observed phenomena, which includes both the physical enhancement effects of particles on gas–liquid mass transfer as well as absorption of oxygen into the polymer. Even though volumetric mass transfer coefficient reductions were observed in the system containing silicone rubber, it was demonstrated that an increased oxygen transfer rate into the working volume of a two-phase system occurs relative to a system without a solid phase. This study provides an explanation for previous results regarding the enhanced effect of the presence of a styrene–butadiene copolymer phase in reducing oxygen limitations in a solid–liquid two-phase partitioning bioreactor. Results from this study can be applied to two-phase aerobic fermentation systems that will benefit from reducing oxygen limiting conditions.