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

再循環氣體對機械式氣液攪拌槽中質傳速率之影響

呂維明(Wei-Ming Lu) ； 巫鴻章(Hong-Zhang Wu) ； 周呈霙(Chen-Ying Chou) ； 張珍永(Jen-Yung Chang)

《Journal of the Chinese Institute of Chemical Engineers》 31卷1期 (2000/01) Pp. 67-78

https://doi.org/10.6967/JCICE.200001.0067

摘要

本研究探討攪拌翼值周圍之再循環氣體量對具有Rushton及pitched blade兩種不同攪拌翼之氣液攪拌槽之KLa之影響。在同迴轉速及低通氣量下，pitched blade攪拌翼雖可以約1/2之功率消耗而其總平均KLa可達Rushton攪拌翼之80%；然而當通氣量大於0.88vvm時。Pitched blade攪拌翼所產生之KLa就遠低於Rushton攪拌翼之KLa值。本研究結果亦指出以總通氣量Qt〈亦即通氣量QS+氣體再循環量QR〉來回歸〈KLa〉時。可得單一直線〈KLa〉=0.134(Qt/ND3)+0.00339。較以往單以通氣量QS去回歸〈KLa〉來得簡單而且更符合其實際之物理意義。

並列摘要

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%.

並列關鍵字

Gas Recirculation Rale ； Mass Transfer Rale ； Energy Dissipation Rate ； Stirred Vessel