Continuous-phase mass-transfer coefficients for liquid extraction in agitated vessels



Area-free continuous-phase mass-transfer coefficients for continuous flow of ethyl acetate (dispersed) and water (continuous) were measured in a 9.5 in. diameter baffled vessel, agitated with 3 and 5 in. diameter flat-blade turbines, and with the 3 in. turbine in the absence of baffles. Specific interfacial area was determined by a light-transmission technique which permitted a vertical traverse of the vessel.

The specific area is greatest, and mean drop diameter smallest, near the impeller. The mean area is lower than that predicted from existing correlations for batch operation in the absence of mass transfer. Continuous-phase mass-transfer coefficients increase with increased impeller Reynolds number and dispersed-phase holdup, and are about 2.5 times larger in the unbaffled vessel at the same impeller power. The coefficients are generally larger than those predicted by existing correlations based on suspended solids, and it is suggested that this and the effect of holdup is the result of drop coalescence and redispersion, which causes a renewal of the continuous phase surrounding each drop. Rough estimates of the minimum coalescence frequency in the baffled vessel and application of the renewal concept to the data appear to confirm this view. Lack of information on circulation rates prevents immediate test of the data from the unbaffled vessel.