Critical impeller speed for solid suspension in multi-impeller agitated contactors: Solid-liquid system

The critical impeller speed for suspension of solid particles (N-js) has been measured in multi-impeller mechanically agitated contactors of 0.15 and 0.30 m 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. The number of impellers used in the 0.3 m and 0.15 m id reactors were three and four, respectively. The distance maintained between two impellers was equivalent to the tank diameter. The effect of impeller type and diameter, particle size and loading, and clearance of the bottom impeller from the reactor bottom was studied and results compared with those of single impeller agitated contactors. PTD impeller was found to be more efficient for solid suspension. The N-js values obtained in reactors with multiple impeller are essentially the same as those observed in single impeller reactor and the bottom impeller plays dominant role in determining the N-js values. An empirical correlation has been proposed for estimation of N-js and an attempt has been made to explain the mechanism of suspension in multi-impeller agitated reactor.