Gas−Liquid Dispersion with Buoyant Particles in a Hot-Sparged Stirred Tank

Abstract

Many industries use gas-liquid stirred-tank reactors with a third phase of buoyant particles. Few studies have looked at the effects of buoyant particle characteristics on solid-liquid suspension and gas-liquid dispersion, and even less have considered operation at elevated temperatures. This paper reports power consumption and gas holdup measurements made in a hot-sparged three-phase system. The vessel was a dished-base reactor of diameter 0.476 m (T) holding 0.145 m 3 of liquid, agitated by a multi-impeller agitator (a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT+2WH U ). This configuration has been recommended in previous work. Air, deionized water, and polypropylene (PP) particles were used in this work. The temperature of the water was in the range 80.5-82.5 °C. The results show that, as in a cold system, the effect of the solid concentration on aerated agitator power demand is negligible. There is little difference between the agitator power demand in cold-and hotsparged systems, except that the relative (gassed-to-ungassed ratio) power demand, RPD, is a little higher in hot-sparged conditions. The influence of power consumption and gas rate on gas holdup is slightly less when hot than when cold. Gas holdup in the hot-sparged system, which, with similar total gas rates and power input, is only about one-half or two-thirds of that at room temperature, is slightly increased at higher solids concentrations. The reasons for the differences in gas dispersion between cold-and hot-sparged systems are analyzed and discussed in this paper, with further experimental and CFD studies planned for future work.

Temperature Effects on Gas Dispersion and Solid Suspension in a Three-Phase Stirred Reactor

“…The detailed calculation of the total power input has been reported in our previous paper. 14 With the same power input and total gas flow rate, including the contribution of the evaporation from the liquid phase, the gas holdup decreases with increasing temperature. Without solids, the gas holdup in cold conditions is over twice that at the highest temperature.…”

“…14,15 The effects of the temperature on the RPD at different solid concentrations are shown in Figure 4a-c. It is clear that when no particles were present, the higher the temperature, the greater the RPD.…”

“…During the past decade, studies of hot gas-liquid systems have shown that hot-sparged conditions differ significantly from those in cold gassed systems. [8][9][10][11][12][13][14][15] The agitator power draw is somewhat greater at higher temperatures, although bulk and micromixing times are essentially unaltered while retained gas fractions are substantially reduced. The temperature dependence of the void fraction retained in a sparged reactor was measured by Smith et al 10 The reduction in the gas holdup over the range from 290 to 367 K could be correlated with the absolute temperature to the power -3.2.…”

Gas-liquid mass transfer for coalescent system in a hot-sparged triple-impeller stirred reactor

“…Moreover, recent studies of hot gas–liquid systems have shown that the behaviors of dispersed gas under the hot‐sparged conditions differ significantly from those of cold gassed systems . Gas holdup was found to decrease rapidly with increasing temperature according to a power law with an exponent of −3.2 for the absolute temperature from 290 to 368 K, and the overall Sauter mean bubble size in hot systems was about 21% larger than that for cold systems . Both effects can change the gas–liquid interfacial area ( a ) to a great extent.…”

“…However, all those reports mainly focused on k L a at ambient temperature when the vapor pressure can be neglected compared with the operating pressure, while many of the processes mentioned above are exothermic and ‘hot’, operating at higher temperatures when the assumption of negligible vapor pressure is likely to be invalid. Moreover, recent studies of hot gas–liquid systems have shown that the behaviors of dispersed gas under the hot‐sparged conditions differ significantly from those of cold gassed systems . Gas holdup was found to decrease rapidly with increasing temperature according to a power law with an exponent of −3.2 for the absolute temperature from 290 to 368 K, and the overall Sauter mean bubble size in hot systems was about 21% larger than that for cold systems .…”