Oxygen transfer in three phase inverse fluidized bed bioreactor during biosurfactant production by Bacillus subtilis

•The effect of main parameters on kLa was studied in fermentation conditions in TPIFB.•The strong influence on kLa of the “in situ” produced biosurfactants was elucidated.•First correlation using dimensional analysis allowing to estimate kLa in TPIFB.•The surface tension variations were considered in the proposed correlation for kLa.

Production of microbial biosurfactants requires development of bioreactors enabling sufficient oxygen supply and foaming control. Gas–liquid mass transfer in a three-phase inverse fluidized bed reactor designed for biosurfactant production by Bacillus subtilis was studied. The influence of main parameters on kLa was investigated in model and real fermentation conditions. Oxygen transfer increased up to 175% and up to 24% with the increase of superficial gas and liquid velocities, respectively. The oxygen transfer obtained in the TPIFB bioreactor (kLa up to 0.015 s−1) was higher than in the other bioreactors used for biosurfactant production. A significant decrease of kLa (up to 27%) was measured during fermentation process. It was shown that this decrease of kLa was a result of surface tension decrease due to the production of biosurfactants. However the oxygen transfer remained important and allowed a correct oxygen supply for the aerobic B. subtilis strains. A correlation based on dimensional analysis was proposed to estimate kLa in function of the influencing parameters, integrating surface tension effects. This correlation allowed estimating correctly kLa in the presence or not of solid particles. The correlation being based on dimensionless numbers, it could help for further process control and scale-up considerations.