Critical impeller speed for solid suspension (Ncs), has been measured in gas inducing mechanically agitated contactors (GIMAC) fitted with two impellers. Five different impeller designs were employed. It was found that the multiple impeller system having both pitched blade downflow turbines was the most energy efficient for the suspension of solid particles. The interimpeller clearance (C-3), and the impeller bottom clearance (C-l) were found to be important parameters, Further, for this optimum combination, the design of the : lower impeller was optimized in terms of impeller diameter (D), blade width (W), blade angle (A), and the number of blades (n(b)). An empirical model has been proposed based on the mechanism of fluidization.