Among various methods for drying pharmaceutical granules in the pharmaceutical industry, fluidized bed drying is commonly used 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, called “tribocharging”, could adversely affect the process performance. Effects of drying air temperature and drying air velocity on drying performance as well as electrostatic charge generation during the drying process were investigated in this project. In order to elucidate the effect of moisture content (ranging from approximately 30 wt. % to 1.8 wt. %) on tribocharging behaviour, the specific charge of granules was measured in a rotary device and apparent volume resistivity was investigated in a self-designed resistivity testing cell. Limited drying data was available in the literature for drying pharmaceutical granules. Therefore, experimental work also was conducted to investigate drying kinetics under different operating conditions of relevance to industry processes.
Experimental results showed that decreasing the drying air temperature increased the drying time, as expected. 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 5 wt. %. The same behaviour was observed in tribocharging experiments conducted in the rotary device. It also was revealed that the increase in specific charge could be due to an increase in apparent volume resistivity of granules at reduced moisture.
Drying data also indicated that the effective diffusion coefficient could be correlated satisfactorily to drying temperature by the typical Arrhenius equation
