Investigation on drying kinetics and tribocharging behaviour of pharmaceutical granules in a fluidized bed dryer

Abstract

Among various methods for drying of pharmaceutical granules in the pharmaceutical industry, fluidized bed drying is a frequently used method due to its high rate of moisture removal, excellent performance in solids mixing, and heat and mass transfer. As pharmaceutical powders are typically organic materials with high resistivity, they can easily be charged due to repeated collision and separation of particles along with particle-wall friction in a fluidized bed dryer. This phenomenon, also called “tribocharging”, could adversely affect the process performance. In this work, experiments were conducted to investigate the effects of drying air temperature and drying air velocity on drying performance, as well as electrostatic charges generated during the drying process. Experimental results showed that decreasing the drying air temperature increased the drying time, as expected. The drying rate was observed to be lower at lower operating temperatures. The specific charge of pharmaceutical granules was found to be a function of moisture content and drying air velocity, rather than drying air temperature. With a decrease in moisture content, the specific charge of pharmaceutical granules increased. There was a sudden increase in measured specific charge when the moisture content decreased to approximately 5wt.%. In addition, regardless of operating conditions, the charge polarity of granules within the bed at the endpoint of the drying process was positive. The current findings indicate that the specific charge is directly indicative of moisture content in the fluidized bed dryer, and monitoring its dynamic changes could be used to monitor the drying process in the pharmaceutical industry.