Effect Of Gas Recirculation Research Articles

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%.

Modeling of high pressure, liquid-encapsulated Czochralski growth of InP crystals

A high resolution numerical scheme based on multizone adaptive grid generation and curvilinear finite volume discretization has been implemented to simulate the high pressure, liquid-encapsulated Czochralski (HPLEC) growth of InP crystals and study the effect of gas recirculation on melt flow and crystal/melt interface shape. The model incorporates flows induced by buoyancy and capillary forces and by crystal and crucible rotations, as well as the radiation heat loss from the melt and the crystal surfaces. It is demonstrated that the thermal interaction between the gas and the melt must be accounted for to predict the interface shape and dynamics accurately. The numerical results demonstrate that the transport phenomena in a high pressure growth system is very complex. The temperature distribution in the crystal and the shape of the melt/crystal interface also change significantly with the size of the crystal. This has a strong influence on dislocations in the crystal as shown by the experiments of Kohiro et al. [J. Crystal Growth 158 (1996) 197] and Jordan et al. [J. Crystal Growth 70 (1984) 555].

Air‐lift reactor analysis: Interrelationships between riser, downcomer, and gas‐liquid separator behavior, including gas recirculation effects

AbstractAn approach to air‐lift reactor analysis is presented that examines the three parts of the air‐lift—the riser, downcomer, and gas‐liquid separator—as three interconnected and interrelated elements. Downcomer flow configurations are described and characterized by either straight or oscillating bubble flow patterns. The downcomer gas flow rate is determined using a two‐sparger system, and a correlation is presented relating the downcomer superficial gas velocity to the downcomer gas holdup. Gas recirculation is calculated and related to the geometric and operating conditions of the gas‐liquid separator. It is shown that an increase in residence time in the gas‐liquid separator decreases the gas recirculation rate. Correlations are presented relating gas holdup and liquid velocities to superficial gas velocity.A sparger was added near the entrance of the downcomer, in addition to the conventional placement near the entrance of the riser. Results of such a two‐sparger system are discussed in relation to reactor stability and liquid velocities.