Local bubble size distributions in agitated vessels

Abstract

Local bubble size distributions (BSD) were measured from 12, 14 and 194 dm3 agitated vessels with capillary suction probe (CSP) technique. The four investigated systems were air–water, air–aqueous NaCl, air–aqueous starch and CO2–n-butanol. Several gassing rates and stirring speeds were investigated to find their effect on local BSDs. The reproducibility tests and the comparison against photography showed that Sauter mean bubble sizes and volumetric BSDs can be measured accurately from the air–water system with the capillary probe. In the CO2–n-butanol experiments the variation of BSDs with the agitation conditions was reasonable, although the majority of bubbles was likely smaller than the capillary diameter. In the air–water system, bubble size varied more in the Rushton turbine than in the flat-blade turbine (FBT) agitated vessel because of differences in flow fields. The addition of a small amount of NaCl caused a sharp decrease of bubble size in the vicinity of liquid surface. In the impeller discharge flow, bubble size decreased less. The lower coalescence rates due to higher apparent viscosity likely explain the smaller bubbles in the air–aqueous starch system than in the air–water system. In the CO2–n-butanol system, bubbles were notably smaller compared to other systems and the bubble size decreased unexpectedly in the flow from the impeller discharge to the liquid surface. The measured local BSDs are useful for the validation of population balance models that are, as connected to a computational fluid dynamic code, useful tools for the simulation of local mass transfer areas in agitated gas–liquid reactors.