Gas dispersion and solid suspension in a hot sparged multi-impeller stirred tank

Despite wide application in the process industries, the effects of settling particle characteristics on gas dispersion and solid suspension at elevated temperature have received little attention. Power consumption, gas holdup, and critical just-suspension agitation speed for different concentrations of solid particles have been measured in a fully baffled dished-base vessel of 0.48 m diameter holding 0.145 m 3 liquid. The impeller configuration (a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT + 2WH U ) recommended in previous work has been used in this paper. Air, deionized water, and glass beads of ∼90 μm diameter and density 2500 kg.m -3 were used for all experiments. The operating temperatures were 24 and 81 °C, identified as cold and hot, respectively. Results show that the relative (gassed-to-ungassed ratio) power demand, RPD, in a hot sparged system decreases slightly with an increasing solid concentration. This becomes less evident at lower agitation speeds, where, at a given gas rate, unlike in the cold systems, RPD is higher in hot conditions than when cold. At a given total gas flow rate and power input, gas holdup in a hot sparged system is independent of solid concentration and only about one-half to three-quarters that in the cold conditions. The reasons for these differences between cold and hot systems are analyzed in this paper. The critical suspension speeds in both cold and hot systems increase at first and then level off or even decrease a little at volumetric concentration of solids above 15%. The introduction of gas leads to a higher critical suspension speed in both hot and cold sparged systems, though the effect of total gas flow rate is greater in cold conditions than in hot conditions. Preliminary correlations based on the present work are presented as a guide for future industrial applications in which knowledge of the power requirements, the critical suspension speed, and the actual liquid fraction retained in the reactor may be of commercial importance.