The thermodynamics of high shear blending

As a key constituent in many aerosol powder formulations, the properties of inhaled lactose greatly influence the overall performance of the pharmaceutical powder in inhalers. The interparticle forces defining pharmaceutical performance are markedly affected by the energy input during the high shear blending process. By conducting a number of lactose blending experiments in a small-scale pharmaceutical high shear blender, heat generation in lactose during blending was studied and shown to be consistently predictable. Experiments were then conducted in a larger high shear blender to quantify the energy input into powder using an energy balance equation. A set of relations were developed requiring only the powder temperature and jacket water temperature to approximate energy input into lactose at a given impeller speed and blend time in a high shear blending process. Additionally, this study sets out to substantiate the assumptions made in the proposed energy balance model. These experiments could not identify all possible sources of error; however, evidence is presented suggesting that energy input due to impeller shaft heating is negligible. As a result, the set of relations should be applicable for any high shear blender, given a constant blade size and shape. This analysis of powder heating with respect to blending parameters could be applied to tune blending parameters to minimize powder heating and the resulting change in powder properties. These experiments have provided the framework to understand power input in high shear blending for a range of blend parameters.