Hydrodynamics and oxygen transfer characterization in a net draft tube airlift reactor with water-in-diesel microemulsion

The objective of the present work was to characterize gas holdup (εG), volumetric oxygen mass transfer coefficient (kLa), and specific gas-liquid interfacial area (a) in a water-in-diesel microemulsion (WDME) as a liquid model for aerobic biodesulfurization inside an airlift reactor with a net draft tube (ALR-NDT) when aerated at different rates ranging from 0.05 to 1 vvm. For comparison, the hydrodynamics of ALR (with solid draft tube) and bubble column reactor ('BCR', with no use of draft tube) were also studied for water, diesel and WDME systems. In all reactors, the εG and kLa values for diesel-based liquids were higher compared to the water system. This indicates the coalescence-inhibiting tendency of petroleum liquids mainly due to the lower surface tension which resulted to a decrease in bubble size distribution (i.e., 0.29-1.90 mm for the WDME versus 0.43-14.17 mm for water in the ALR-NDT). Although the kLa values in ALR-NDT were maintained between those values obtained in ALR and BCR for all fluids; however, the transition points from homogeneous to heterogeneous regime were shifted to higher aerations in the ALR-NDT. In this regard, empirical correlations were developed by considering the physicochemical properties of the liquid phase and superficial gas velocity.