281496 Prediction of Single Drop Granule Formation Mechanisms for Regime Separated Granulation
Wet granulation is the process of adding a liquid binder to a fine powder in order to get larger granules for improved particle properties. This process is used in a variety of industries, including pharmaceuticals, food, agricultural chemicals, and detergents. In typical granulation equipment, many of the granulation rate processes (wetting and nucleation, consolidation and growth, and breakage and attrition) occur simultaneously, making it notoriously difficult to control and predict product properties, such as size and shape. Recently, a new granulation approach, regime separated granulation, has been proposed as a way to physically separate the different rate processes to get dramatically better control of the product granule attributes.
The first and most important stage in any regime separated granulation process is nucleation, where new granules are formed by the addition of liquid to the powder bed. Drop controlled nucleation, where one drop forms one granule, is the most desirable operating regime. In order to apply regime separated granulation in practice, more knowledge is necessary regarding the mechanisms by which granules are formed from drop impact and penetration into a static powder bed.
Single drop granule experiments with a syringe and a dish filled with powder are used to simulate drop controlled nucleation in static beds. From high speed camera videos of drop impact and penetration, three different granule formation mechanisms were identified: Tunneling, Spreading, and Crater Formation. Each mechanism produced distinctly different granule shapes. To quantify the conditions under which each mechanism will occur, dimensional analysis was performed and a new regime map was created that plots the powder bed porosity (ε) against the modified granular Bond number (Bog*), which is a ratio of the capillary force acting on a particle to the gravitational force acting on a particle. Subsequently, a mechanistic model based on a force balance was derived for the Tunneling mechanism and compared to the regime map.
The results of this study have important implications for controlling the granule formation mechanism via formulation and process properties to produce the desired granule shape.