Axial distributions of bubble–liquid mass transfer coefficient in laboratory-scale stirred tank with viscous Newtonian and non-Newtonian fluids

Splitting of the volumetric mass transfer coefficient into a bubble–liquid mass transfer coefficient and interfacial area is essential to quantify the mass transfer rate of stirred tanks precisely. Axial distributions of the bubble–liquid mass transfer coefficient were determined in viscous Newtonian and non-Newtonian fluids using a laboratory-scale stirred tank. A detailed knowledge of the bubble–liquid mass transfer coefficient was obtained by using dedicated in situ oxygen and bubble size endoscope probes simultaneously. The volumetric mass transfer coefficient was estimated from recorded local dissolved oxygen concentrations in liquids. The interfacial area was calculated by measuring the bubble size and gas hold-up. The bubble–liquid mass transfer coefficient was then obtained by combining the estimated volumetric mass transfer coefficient and interfacial area. The bubble–liquid mass transfer coefficient was evaluated with effects of fluid rheology (concentrations of fluids), operating conditions (power input and superficial gas velocity), and axial liquid height. Bubble breakage is higher close to the stirrer due to intensive turbulence. The bubble–liquid mass transfer coefficient depended directly on operating conditions and indirectly on fluid rheology and liquid height.