Assessment of predicting methods for the air assisted discharge of pharmaceutical powders

Dosage of fine powders from storage bin to process vessels can represent a significant source of troubles due to the cohesive arching and piping phenomena especially under low consolidation conditions. Distributed injection of air from the hopper walls was proven to be an efficient flow promotion technique to prevent cohesive arching and piping [1-2]. Moreover, aeration can be applied also to increase the solid discharge rate with respect to that achievable under the gravity alone [3]. In this study, the aerated silo discharge is experimentally assessed for fine powders of pharmaceutical interest. In particular, experiments were carried out on a laboratory scale cylindrical bin with a 45°conical hopper. The discharged solids were collected in a bin placed on a load cell. The air flow rate through the distributor was regulated by mass thermal flowmeters. Three powder samples of different flowability (from easy flowing to cohesive according to the Jenike classification) made of microcrystalline cellulose and crystalline lactose were tested. Results show a beneficial effect of aeration on the solid discharge rate and confirm the phenomenology observed in previous studies with different powders. In particular, the solid discharge rate of the easy flowing microcrystalline cellulose powder A increases almost linearly with increasing air fow rate up to a plateau value. Differently, the discharge of microcrystalline cellulose B and of crystalline lactose could not be achieved under the gravity alone. As expected for cohesive powders, a minimum aeration was necessary to prevent arching at the hopper outlet and determine the powder discharge. This critical value was compared with that evaluated according to the design criteria previously proposed by the research group [1-2]. Further increase of the air flow rate caused less significant increase of the solid discharge rate before attaining a plateau value. A model [3] was applied to predict the solid discharge rate as a function of the air flow rate. In the case of the cohesive powders the formation of aggregates was properly taken into account. In particular, the relevant aggregate size was evaluated by means of cohesion values obtained by conventional and innovative experimental methods. Model predictions were in good agreement with experimental discharge rates. The plateau values corresponded to the attainment of a fluidized state for the powder inventory during the discharge.