Effects of SDS surface-active agents on hydrodynamics and oxygen mass transfer in a square bubble column reactor: Experimental and CFD modeling study

This work examined experimentally and numerically the effects of sodium dodecyl sulfate (SDS) surfactant agent on mass transfer coefficients k_l a_l and k_l in a high-aspect square bubble column reactor. To this end, SDS aqueous solutions at concentrations between 5 ppm and 30 ppm were prepared. Superficial gas velocity was varied from 0.74 cm/s to 9.44 cm/s covering all flow regimes to analyze the interaction between different phases and the effect of liquid surface tension on gas-liquid interfaces. Experimental data indicated that the presence of the SDS in the liquid phase augmented the overall gas holdup up to 25% at a constant gas velocity (Ug = 7.3 cm/s) in the transition regime, which is due to the role of SDS in bubble interactions. The impact of SDS addition on the mass transfer is twofold. Firstly, it reduced k_l because the SDS molecules migrate towards the bubble-liquid interface thanks to the lower O2 diffusivity in water, thus hindering the O2 transfer from one phase to the other. Secondly, it increased a_l owing to the inhibition of bubbles coalescence and, therefore, the lower mean bubble size. Among these two effects, the latter is prevailing, which led to an increase in k_l a_l in contaminated systems. In the second part of this work, a population balance model (PBM) was coupled with a computational fluid dynamics (CFD) model based on large-eddy simulation (LES) to analyze the effect of both tap water and SDS contaminated systems on the breakage phenomena, local flow pattern, and the resulting hydrodynamics and mass transfer features in the reactor. Ultimately, experiments and numerical simulations agreed well in terms of global gas holdup and k_l a_l coefficient. In particular, the transition points in the case of the SDS media (except for foam formation) provided the model with adequate PBM definitions for the dispersed phase.