Investigations on Drying and Tribocharging Behaviour of Pharmaceutical Powders in a Fluidized Bed Dryer

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

Investigations on tribocharging behavior of pharmaceutical powders in a fluidized bed dryer

Fluidized bed drying is a commonly used methods in pharmaceutical industry due to its excellent performance in solids mixing, heat transfer, and mass transfer. As pharmaceutical powders are typically organic materials, 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. This work, investigates effects of inlet air with different temperature and velocity on drying performance and electrostatic charges generated during the drying process. Samples of approximately 6 g at discrete time intervals were taken via a sample thief; half of which was used to determine the moisture content, and the rest was used to measure charge density by a Faraday cup connected to an electrometer. Experimental results showed that, decreasing the drying air temperature increases the drying time as expected. The drying rate is lower at lower operating temperatures. Charge density of granules is found to be a function of moisture content rather than drying temperatures. With a decrease in the moisture content, charge density of pharmaceutical granules increases. At the end of drying process, granules carry similar amount of charges. Moreover, regardless of operating conditions, charge polarity of granules within the bed at the end of the process is positive. The current findings show that charge density is indicative of moisture content in the fluidized bed dryer and monitoring dynamics changes could be used to monitor the drying process in pharmaceutical industry

Measurement of moisture content in a fluidized bed dryer using an electrostatic sensor array

Fluidized bed dryers have been widely applied to dry raw materials or final products due to the advantages of good mixing efficiency and high heat and mass transfer rate. In order to control and optimize the drying process of fluidized bed dryers, it is necessary to develop reliable methods to measure the moisture content of solid particles in the bed. Because of the advantages of non-intrusiveness, simple structure and high sensitivity, an electrostatic sensor array has been developed to monitor the drying process. Experimental investigations were conducted on a lab-scale fluidized bed dryer. The moisture content during the drying process was measured using the sampled particles as reference. It is found that the fluctuation of the electrostatic signals can reflect the change in moisture content. However, the relationship between the fluctuation of the electrostatic signal and the moisture content depends on the air velocity in the dryer. To eliminate the velocity effect on moisture content measurement, a model between the moisture content and the root-mean-square magnitude of the electrostatic signal is established. The effectiveness of the model is validated using experimental results under a range of conditions. The findings indicate that the electrostatic sensor array can measure the moisture content in the bed with a maximum error of ±15%