Effect of Gas Recirculation Rate on Mass Transfer within Mechanically Agitated Vessels

The effect of gas recirculation on the mass transfer and gas dispersion within mechanically agitated vessels was examined by measuring the local mass transfer coefficients and estimating the gas recirculation rates through a developed CFD approach. Since the recirculated gas as well as the sparged gas affects the power drawn by the impeller, it also exerts a strong effect on the mass transfer. With the same rotational speed, as the gas is completely dispersed, the pitched blade impeller obtains a larger gas recirculation rate due to its stronger circulating flow, and can Provide 80 % of the mass transfer rate which the Rushton turbine impeller can produce. However, with a higher gassing rate, the Rushton turbine impeller demonstrates a better gas dispersion and recirculates more gas back into the impeller region, resulting in about a 50% higher mass transfer rate than the pitched blade impeller. Summarizing the calculated data shown in this study, the gas recirculation rate QR can be correlated with N, Qs and the D/T ratio as: QR/QS=1.3810-18N14.78VS-1.13(D/T)4.01 for a single Rushton turbine impeller system, and QR/QS=7.2110-19N14.21VS-1.25(D/T)5.11 for a single pitched blade impeller system, for the gas dispersion status beyond stage(c). Comparing the plots of (KLa) vs. the total gassing rate Qt, and the plot of (KLa) vs. the sparging gas rate Qs, it is found that ＜ KLa) always increases monotonically with Qt while a leveling-off value of (KLa is seen in the plot of (KLa ＞ vs. Qs. This finding implies that (KLa) is more appropnately correlated with Q, than with the sparging gas rate Qs. By corre-laring (KLa) with the rotational speed N and Qt, it is found that the Rushton turbine impeller and the pitched blade impeller perform similarly if the value of the modified aeration number N A' (= Qt/ND3) is the same, and the values of (KLa) for both systems can be estimated by a single equation as: ＜KLa＞=0.314(Qt/ND3)+0.00339 where the deviation of this correlation is always less than 15%.