Temperature Effects on the Gas Hold-up in Agitated Vessels

Abstract

Previous work found the gas hold-up in a tall, 0.15 m 3 vessel, agitated by three impellers, to decrease considerably with temperature. This was attributed to either a decrease in liquid viscosity, a decrease in surface tension or a decrease in gas density. The aim of this work was to investigate the influence of gas density on the gas hold-up. First of all, it was attempted to reproduce the data found in literature. It was shown that contaminants had a significant impact on the gas hold-up. Experiments with increasing and decreasing amounts of tap water (as source of the contamination) found the gas hold-up to be affected by a strongly absorbing nonionic surface active agent. Experiments in a system exhibiting low conductivity and low gas hold-up showed that the gas hold-up decreased 15 % for temperatures between 295 K and 350 K. Experiments with helium as sparging gas found that gas density does have a lowering effect on the gas hold-up. The gas hold-up increased again for temperatures above 350 K, which may be due to the increased presence of micro-bubbles in a near-boiling system.

The Effect of Impeller–Sparger Geometry on the Gas Holdup in an Oxygen–Water System Using an Agitated and Sparged Tank Contactor

“…Also, we observe relatively higher ε g values at θ = 303 K for oxygen compared to similar conditions reported for θ = 313 K (Supporting Information, Table S1b,c). The relatively lower ε g for gases at higher θ may be attributed to the decrease in gas density at increased θ . The effect is usually prominent in the region θ = 303–323 K .…”

“…The relatively lower ε g for gases at higher θ may be attributed to the decrease in gas density at increased θ. 25 The effect is usually prominent in the region θ = 303−323 K. 26 The extent of ε g reduction usually would depend on several factors (e.g., liquid viscosity, interfacial tension, liquid density, etc.) 25,26 including the degree of agitation (N 0 ).…”

“…25 The effect is usually prominent in the region θ = 303−323 K. 26 The extent of ε g reduction usually would depend on several factors (e.g., liquid viscosity, interfacial tension, liquid density, etc.) 25,26 including the degree of agitation (N 0 ). Taken together, it may be stated that ε g for an ASTC system with an impeller−sparger configuration under identical experimental conditions is essentially independent of the gases involved.…”

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

“…Extensive studies of cold-sparged three-phase systems, involving settling particles, have been reported during the last 30 years. – However, many of the processes named above are exothermic and “hot”, operating at higher temperatures, when the assumption of negligible vapor pressure is likely to be invalid. During the past decade, studies of hot gas–liquid systems have shown that hot-sparged conditions differ significantly from those in cold gassed systems. – 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 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.…”

“…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.…”