ALTERNATIVE MECHANISMS OF DROP BREAKAGE IN STIRRED VESSELS

Increase of dispersed phase hold-up, phi, in a turbulent stirred dispersion is expected to result in increased value of d(max) because of the suppression of turbulence intensity. Experiments in stirred vessels however show that with increasing phi, the d(max) first increases and then decreases. This unexpected trend can be explained by exploring two mechanisms of drop breakage at the impeller, other than the hitherto recognized mechanism involving turbulent inertial stresses. These new mechanisms involve elongation flow breakage in the accelerating flow along the impeller length and the shear mechanism operating in the boundary layer at the impeller. In a stirred dispersion all the three mechanisms operate simultaneously, each with its own d(max) value. The experimentally obtained value corresponds to the smallest of the d(max) phi, can be predicted reasonably well by considering all the three mechanisms together.