Mixing of three-phase systems at high solids content (up to 40% w/w) usingradial and mixed flow impellers

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 diff erent radial flow impellers (Scaba 6SRGT and a standard Rushton turbine) an d a six-bladed mixed flow impeller with pitch angle of 45 degrees, either i n its down ward (6MFD) or upward pumping (6MFU) mode were used. Power input , mixing time, the speed to achieve complete suspension of the solids (unga ssed and gassed) as well as the amount of suspended solids and the height o f the liquid-solid interface were measured. In addition, a new technique fo r measuring the amount of suspended solids in two-phase systems was extende d successfully to three phases, When compared to the situation without soli ds, the mixing time, tm, in the solid-liquid case at tile higher solid conc entrations was much greater, as previously reported, but for the three phas e case, the increase was relatively small, especially with the Scaba and 6M FU impellers. in general, the power and speed required to suspend the solid s increased with increasing solids concentration but once suspended, with t he 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 agita tor torque on gassing, solids suspension would be sustained. The present wo rk showed this to be broadly valid for the 6SRGT and the 6MFU but not for t he Rushton and the 6MFD impellers. Overall, the most stable impeller, requi ring the least specific energy dissipation rate for solids suspension and g as dispersion under the most demanding conditions, was the 6MFU.