Gas hold-up and oxygen transfer in a draft-tube airlift bioreactor with petroleum-based liquids

Gas hold-up (ɛg) and volumetric gas-liquid oxygen transfer coefficient (kLa) in a draft-tube airlift bioreactor (24 × 10−3 m3 in volume) were studied using pure kerosene and diesel and their water-in-oil microemulsions, as the model solutions for petroleum biodesulfurization. For comparison, experiments were also done with distilled water. The ɛg and the kLa values for kerosene and diesel systems were in most cases significantly higher than the water system. Increase in water-to-oil phase volume ratio (φ) of the microemulsion systems resulted in decrease in the values of ɛg, which was attributed to a decrease in the coalescence-inhibiting tendency of the petroleum liquids. Increase in the viscosity (v) of the microemulsion systems to around 32 × 10−6 m2/s resulted in the occurrence of the churn turbulent regime with an associated decrease in the value of ɛg, which could not be solely accounted for by the increase in φ. Furthermore, the kLa values decreased with increase in the viscosity of the petroleum-based liquids. However, when churn turbulent conditions prevailed, increase in viscosity alone could not account for the decrease observed in the kLa values. Empirical correlations were developed that related ɛg and kLa to φ and ν, respectively, under the bubbly flow regime inside the bioreactor.