The effect of electrolyte concentration on counter-current gas–liquid bubble column fluid dynamics: Gas holdup, flow regime transition and bubble size distributions

Electrolytes are well-known to suppress the coalescence, stabilize the homogeneous flow regime and increase the gas holdup; however, there is a lack of studies concerning the influence of electrolyte concentration on counter-current bubble column fluid dynamics. In this paper, we contribute to the existing discussion by experimentally investigating the effect of electrolyte concentration (sodium chloride, NaCl) on gas holdup, flow regime transition and bubble size distributions in a large-diameter and large-scale counter-current bubble column (5.3 m in height and 0.24 m inner diameter). We considered gas superficial velocities in the range of 0.004–0.20 m/s and liquid superficial velocities up to −0.06 m/s. Air was used as the dispersed phase and various aqueous solutions of NaCl—up to the critical concentration—were used as the liquid phase. The gas holdup measurements were used to investigate the bubble column fluid dynamics and to analyze the flow regime transition. The image analysis was applied, near the sparger and in the developed region, to study the bubble size distributions in batch and in counter-current modes. The presence of NaCl—up to a critical concentration—stabilizes the homogeneous flow regime, increases the gas holdup and shifts the bubble size distribution toward lower bubble diameters. The counter-current mode destabilizes the homogeneous flow regime when using tap water and stabilizes the homogeneous flow regime when using aqueous solution of NaCl. The results suggest that the changes in the bubble size distributions stabilize/destabilize the homogeneous flow regime and, thus, increase/decrease the gas holdup. © 2016 Institution of Chemical Engineers