Recent Studies on Agitated Three-Phase (Gas–Solid–Liquid) Systems in the Turbulent Regime

Abstract

Earlier work using Rushton turbines and down-(MFD) and up-(MFU) pumping, 45°-pitched blade turbines at relatively low concentrations of solids of different size and density in vessels up to 1.8 m diameter enabled correlations to be developed for predicting the minimum speed for solid suspension, NJSg. Further work has been conducted with these impellers and with two modern impellers, a Scaba 6SRGT, a typical hollow-blade radial flow impeller, and a typical up-pumping, axial flow, wide-blade hydrofoil, the Lightnin’ A315, in a vessel of 0.45 m diameter with solids up to 40% by weight. The earlier correlations were validated for the new higher concentration conditions. In all cases, increases in solids concentration increased NJSg and the specific energy dissipation rate required to suspend solids, (ɛT)JSg On the other hand, the large increase (up to two orders of magnitude) in mixing time found at solids concentration >∼15% by weight in two phase systems at NJSg as compared with the case without solids, is essentially eliminated in the three-phase case. At low gassing rates, QGv (vvm), down-pumping impellers achieve suspension and relatively good vertical solids distribution at the lowest (ɛT)jsg-However, with increased QGv UP to 3 vvm, NJSg and (ɛT)JSg increase rapidly, solids maldistribution develops, the flow pattern is very unstable and gross torque fluctuations occur. For the 6SRGT, the operation is very stable and although NjSg and (ɛT)JSg are both high they are insensitive to QGv This high value of (ɛT)JSg may not be a disadvantage if high rates of gas–liquid mass transfer are required. For both the 6MFU and especially the A315(U), the flow pattern, NJSg and (ɛT)JSg are again all very insensitive to QGv and a good vertical solids distribution is maintained. For the A315(U) at high QGv values, (ɛT)JSg is the least amongst the impellers tested, so that wide-blade, up-pumping axial flow hydrofoils are considered to be the optimum impeller when just physical suspension is required or solid–liquid reactions are rate limiting.