Experimental Investigations of Suspension, Dispersion, Power, Gas Hold Up and Flooding Characteristics in Stirred Gas-Solid-Liquid Systems (Slurry Reactors)

Review on Mixing Characteristics in Solid‐Liquid and Solid‐Liquid‐Gas Reactor Vessels

“…Several research reports (Zlokarnik and Judat, 1969;Queneau et al, 1975;Arbiter et al, 1976;Subbarao and Taneja, 1979;Wiedmann et al, 1980;Chapman et al, 1983c;Wong et al, 1987;Rewatkar et al, 1991a) have shown that the solid suspension mechanism in a three-phase system is mainly determined by the gas-liquid hydrodynamics of the impeller. In a stirred gasliquid or gas-liquid-solid system the gas fl ow rate, distributable by the stirrer, is limited and dependent on the stirrer speed.…”

“…It is much more diffi cult to lift the particles from the centre than to drive them towards the corners (as the case with axial fl ow impeller) from where they can be picked up easily. In the past, up to 1980, the solid suspension in both two-and three-phase stirred vessels was studied by using conventional impellers such as the Rushton turbine and propeller (Zwietering, 1958;Subbarao and Taneja, 1979;Wiedmann et al, 1980). Wiedmann et al (1980) have reported that propeller is more susceptible to the gassing rate and results in poor gas dispersion as compared to Rushton turbine, resulting in higher values of N jsg N jsg N .…”

“…In the past, up to 1980, the solid suspension in both two-and three-phase stirred vessels was studied by using conventional impellers such as the Rushton turbine and propeller (Zwietering, 1958;Subbarao and Taneja, 1979;Wiedmann et al, 1980). Wiedmann et al (1980) have reported that propeller is more susceptible to the gassing rate and results in poor gas dispersion as compared to Rushton turbine, resulting in higher values of N jsg N jsg N . Hence, they concluded that the Rushton turbine is superior over a propeller for both gas dispersion and solid suspension in jsg over a propeller for both gas dispersion and solid suspension in jsg a three-phase system.…”

“…This may be due to the fact that at very low clearance and higher gassing rates, stagnant areas form near the periphery of the tank bottom where the fi llet settle thus, requiring additional power for the solid suspension. Wiedmann et al (1980) have reported that because of the opposing direction of the liquid fl ow generated by the ascending gas bubbles and the liquid fl ow generated by the down-fl ow impeller, the most favourable position of the stirrer in two-phase system does not match that in the three-phase system. They have reported that optimal conditions for the solids suspension in the three-phase system exist within the range of 1/12 < C/T <1/1.33.…”

“…One of the most important parameters for the assessment of the performance of the impellers for solid suspension (both in the absence and in the presence of gas), is the minimum impeller speed required for the complete offbottom suspension of the solids in the vessel. (Zwietering, 1958;Wiedmann et al, 1980;Chapman et al, 1981 Chapman et al, , 1983Frijlink et al, 1990;Rewatkar et al, 1991a; Mechanically agitated reactors with single and multiple impeller systems are used in the industry for the various three-phase mixing processes such as crystallization, fermentation, and hydrogenation, etc. The paper reviews the experimental work reported in the literature along with different techniques used for the measurement of the specifi c quantities such as minimum or critical impeller speed for solid suspension.…”