Hydrodynamics and oxygen transfer behaviour of water in diesel microemulsions in a draft tube airlift bioreactor

Determinations of pertinent hydrodynamic and mass transfer parameters were carried out inside draft tube internal loop airlift bioreactor (DTAB) containing water in diesel (W/D) microemulsions with water to diesel volume ratio (φ) in the range 2.5–40%. For comparative purposes, data for diesel and water was also obtained. Increase in φ of the microemulsion systems up to 40% or increase in the amount of surfactant used to make the microemulsions resulted in a decrease in the values of g and kLa, which was attributed to the increase in the viscosity of the microemulsions. This viscosity was found to be a more suitable correlating parameter than φ. Increase in the value of φ of the W/D microemulsion resulted in a decrease in the values of liquid velocity (vL) for ugr up to 0.032 m/s. At higher gas velocities, similar vL values were obtained for all the microemulsions. On the other hand, increase in φ of the W/D microemulsion resulted in increase in both the circulation time and mixing time values obtained inside the DTAB, although the difference was more pronounced at lower values of ugr. Suitable empirical correlations relating g, kLa, vL, circulation time and mixing time to riser superficial gas velocity (ugr) and kinematic viscosity were presented. The downcomer holdup data for diesel and W/D microemulsions was related to the riser holdup data using a single correlation.