Mixing of Three-Phase Systems at High Solids Content (up to 40% w/w) Using Radial and Mixed Flow Impellers.

Abstract

A liquid-gas-solid system has been studied at higher solids concentration (up to 40% by wt) and gas flow rates (up to 2 vvm) than previously. Two different radial flow impellers (Scaba 6SRGT and a standard Rushton turbine) and a six-bladed mixed flow impeller with pitch angle of 45°, either in its downward (6MFD) or upward pumping (6MFU) mode were used. Power input, mixing time, the speed to achieve complete suspension of the solids (ungassed and gassed) as well as the amount of suspended solids and the height of the liquid-solid interface were measured. In addition, a new technique for measuring the amount of suspended solids in two-phase systems was extended successfully to three phases. When compared to the situation without solids, the mixing time, t m , in the solid-liquid case at the higher solid concentrations was much greater, as previously reported, but for the three-phase case, the increase was relatively small, especially with the Scaba and 6MFU impellers. In general, the power and speed required to suspend the solids increased with increasing solids concentration but once suspended, with the Scaba and the 6MFU impellers, even the highest solids concentration and gas flow rates only required a very small further increase. It has recently been proposed (Pantula and Ahmed, 1997) that by maintaining constant agitator torque on gassing, solids suspension would be sustained. The present work showed this to be broadly valid for the 6SRGT and the 6MFU but not for the Rushton and the 6MFD impellers. Overall, the most stable impeller, requiring the least specific energy dissipation rate for solids suspension and gas dispersion under the most demanding conditions, was the 6MFU.