Effect of electrolytes on interphase mass transfer in microbubble-sparged airlift reactors

Better understanding of the impact that electrolytes have on the hydrodynamic performance of airlift reactors, ALR, is critical to improving the design and operational performance of these units which are increasingly being used as chemical and biochemical reactors. Such knowledge is particularly important to the saline environment encountered in water and wastewater treatment, algae production, treatment of produced water, aquaculture applications, and the management of marine environment. The effect of salt concentration on the mass transfer characteristics of ALR was investigated using a 200 1 pilot scale internally circulated unit equipped with a dual-fluid transonic sparger capable of generating microbubbles. Saline solutions containing 0-58,500 ppm were tested. A critical analysis of the literature suggests that the liquid side mass transfer coefficient, k(L),, is significantly reduced in the presence of electrolytes. This adverse effect was however compensated for by the achievement of large gas holdups and the formation of very fine bubbles in the slowly-coalescent saline systems. The combined effect of these factors resulted in the achievement of k(L)a values as high as 0.028 s(-1) at a superficial velocity of 0.03 m/s while maintaining the benefits of the homogeneous flow regime. The values achieved are much higher than most ALR data reported for the rapidly-coalescent air/water system and are about 5-fold larger than those obtained using conventional spargers in the case of the slowly-coalescent saline solutions. This superior performance is amplified by the coalescence retardation effect of electrolytes; hence, the mass transfer performance data depicted a transition/critical concentration at about 0.1 M, a value that is slightly lower than that obtained in bubble columns but somewhat higher than that obtained using the lab-scale method for testing the coalescence of two adjacent bubbles (C) 2013 Elsevier Ltd. All rights reserved.