Generation of small bubbles and small bubble-liquid mass transfer in airlift reactors containing highly viscous liquids

Abstract

The time-dependent gas hold-up is investigated during the aeration of the Saccharomyces cerevisiae suspension, the aqueous saccharose solutions and the glycerol solutions in the external loop airlift reactor. Due to the time-dependent bubble size distribution the fraction of the small bubble hold-up in the total gas hold-up decreases with an increase of the gas¯ow rate and with a decrease of the viscosity. The course of the accumulation process of the small bubbles is described by the ®rst-order kinetic equation. The small bubble accumulation rate is investigated in the airlift reactor and the bubble column. It is showed that the small bubbles form and disappear exclusively in the riser of the airlift reactor. It is found that the small bubble-liquid mass transfer coef®cient is several times larger than the overall oxygen transfer coef®cient. List of symbolsA d m 2 cross-section of the downcomer A r m 2 cross-section of the riser B, C parameters in the equation (10) D r m diameter of the riser D d m diameter of the downcomer D BC m diameter of the equivalent bubble column with the cross-section area equal to the sum of the riser and the downcomer cross-section areas H m height of the unaerated liquid (suspension) k s A1 accumulation rate constant of the small bubbles k d s A1 disengagement rate constant of the small bubbles k L a SB s A1

Measurement of bubble and pellet size distributions: past and current image analysis technology

“…The validity of the correlation needs to be considered relative to the manual size analysis conducted from photographs by these researchers based on available technology. In the case of non-Newtonian fluids where l L changes as a function of shear, a suitable equation for the apparent viscosity is necessary to relate it to operating parameters such as agitation speed [3,43,64].…”