Investigation of mixing and gas-liquid mass transfer in a thin-gap bubble column photobioreactor with Newtonian or non-Newtonian liquid phase

Thin-gap bubble column photobioreactors (PBRs), with a gap of only a few millimeters, offer the potential for increased microalgal biomass productivity. In this study, mixing and gas-liquid mass transfer were experimentally characterized in a confined bubble column (2 mm thickness), using Newtonian (aqueous glycerol solution) and non-Newtonian (xanthan gum) fluids to mimic Chlorella vulgaris suspensions at high concentrations (≈ 42 g.L -1 ). The aim was to investigate the effects of confinement and liquid rheology on mixing time (t m ) and global gas-liquid mass transfer coefficient ( k L a ). Results show poor mixing in viscous and non-Newtonian liquids at low superficial gas velocity (U Gsup ). t m increases with the degree of confinement, particularly at low U Gsup , due to the reduction of agitation capacity with confinement since bubble dynamics is mostly 2D. It is also shown that increasing viscosity has limited impact on k L a at low U Gsup , due to compensatory effects between ‘ k L ’ and ‘ a ’ and that gas-liquid transfer efficiency decreases with confinement, potentially due to a lower ‘ k L ’, caused by reduced agitation under confinement condition. • Mixing and gas-liquid transfer characterized in a 2 mm thin-gap bubble column • Viscous, non-Newtonian fluids show poor mixing at low superficial gas velocity (U Gsup ) • Mixing time increases with confinement due mostly 2D bubble motion • Viscosity has little effect on k L a at low U Gsup due to compensatory ‘ k L ’ and ‘ a ’ effects • k L a decreases with confinement due to low k L