CRITICAL IMPELLER SPEED FOR SOLID SUSPENSION IN MULTI-IMPELLER 3-PHASE AGITATED CONTACTORS

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0-15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n(jsg)) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle-liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.