Gas-Liquid Mass Transfer Behavior In A Surface-Aerated Vessel Stirred By A Novel Long-Short Blades Agitator

The critical rotational speed for the onset of gas entrainment (NC), the distribution of bubbles in the vessel, power number (NP) and gas-liquid mass transfer coefficients (kLa) were measured in a flat-bottomed, surface aerated vessel equipped with a novel long-short blades (LSB) agitator. The effect of the agitator configurations (e.g. the long blades fixed on the outer or inner connector rings, the width of the long blades), the aspect ratio of HIT, the submergence of the long blades, and the agitator clearance have been investigated in detail.The volumetric mass transfer coefficient, kLa, was determined by Feeding Steady-state Method (FSM). It is found that two necessary conditions for the effective gas entrainment and high mass transfer coefficient are that the long blades are fixed on the outer connector ring and they are partially submerged in the liquid. In this case, the gas entrainment starts at a low NC value of 1.5～2.0 rps, and the NC values do not change dramatically when HIT and CIT change. Moreover, the power number NP decreases with the stirring speed, and the exponent of the power-law for PIV=αNβ is less than 3, and β become smaller with the decreasing of HIT. Furthermore, the kLa increase linearly with the stirring speed after a critical aeration speed, and compared with the standard turbines, the kLa has only a little decrease with the liquid level, thus can be used for systems where the liquid level increases with the time.When the long blades are completely submerged, the NC values increase sharply with the increase of HIT or S/T. When the long blades are fixed on the inner connector ring, the NC values are about 50% larger than the long blades are fixed on the outer connector ring, Furthermore, a flooding phenomenon is found in this case when the long blades are partially submerged.When the long blades are fixed on the outer connector ring and they are partially submerged in the liquid, the kLa values significantly higher than other conditions, also a highest kLa value is found when C/T=0.25, and the mass transfer coefficients are well correlated by the equation of kLa=0.0346(P/V)1.596(H/T)-0.512 (C/T=0.25, WLB/T=0.05, P/V=0.5-6kW/m3). Moreover, the effect of different width of long blades on mass transfer coefficients are investigated, the highest exponent of the power-law is found when WLB/T=0.06, and for this type of impeller, the exponent of the power-law is much larger than those of standard turbines (almost less than 1).