Kwangseok Choi

Experimental studies on the effect of moisture content and volume resistivity on electrostatic behaviour of pharmaceutical powders

Pharmaceutical powders are mainly organic materials and are likely to be charged due to repeated inter-particle and particle-wall contacts during industrial processes. This study experimentally investigated the effect of moisture content (ranging from approximately 1.8 to 30wt.%) on tribocharging behaviour of pharmaceutical granules, as well as their apparent volume resistivity. The tribocharging behaviour of pharmaceutical granules was investigated using a rotating device and apparent volume resistivity was measured in a conventional volume resistivity test cell. Additional measurements were performed on individual ingredients, each having the same moisture content as that of the granules, in order to investigate the effect of each single ingredient on the apparent volume resistivity of granules. In this work, the individual ingredients used for granules were: α-Lactose Monohydrate (α-LMH), Microcrystalline Cellulose (MCC), Hydroxypropyl Methylcellulose (HPMC), and Croscarmellose Sodium (CCS). The results showed that the specific charge of granules began to increase at the moisture contents below 5wt.%, which can be referred as critical moisture content of granules. The apparent volume resistivity showed the same behaviour, indicating that the specific charge could be due to an increase in apparent volume resistivity of granules at reduced moisture content. Finally, it was shown that the apparent volume resistivity measured for granules was mainly affected by that of the α-LMH, the major component of granules accounting for 40wt.%.

Experimental study on detection of electrostatic discharges generated by polymer granules inside a metal silo.

To detect electrostatic discharges generated by polymer granules within a metal silo, we developed a novel and simple electrostatic discharge detector that utilizes a photosensor. The novel detector consists of a photosensor module in a metal cylinder, an optical band-pass filter, a quartz glass, a power supply, an amplifier for the photosensor module, and a digital oscilloscope. In this study, we conducted experiments at a real pneumatic powder transport facility that includes a metal silo to evaluate the novel detector using polypropylene granules. To determine the performance of the novel detector, we observed the electrostatic discharge within the metal silo using a conventional image intensifier system. The results obtained from the experiments show that the novel detector worked well in this study. The signals obtained with the novel detector were identical to the electrostatic discharges obtained with the conventional image intensifier system. The greatest advantage of this novel detector is that it is effective even when placed under external lights. In addition, the influence of various optical band-pass filters on the performance of the novel detector was discussed. Our study confirmed that an optical band-pass filter with a center wavelength of λ 330 nm (λ1/2: 315-345 nm) was the best performer among the optical band-pass filters used in this study.

Electrostatic hazards of polypropylene powders in the fluidized bed reactor

This paper reports on electrostatic hazards related to a powder fluidized bed reactor. The electric field and the space charge density were monitored over the fluidizing time. We also investigated the minimum ignition energy of sample powders with a Hartman vertical-tube apparatus. A batch-type fluidized bed system containing 1 kg of commercial grade polypropylene resin powder was used in the experiments. The superficial air velocity with zero relative humidity was in the range of 0.2 to 0.32 m/s. Results obtained from the experiments show that the bi-polar charging existed in the reactor during operation. As the superficial air velocity increased, the space charge density in the lower column increased, whereas, the space charge density decreased in the upper column. The electric field generated from charged powders reached a maximum of 250 V/cm, safe level for incendiary electrostatic discharge from charged powders. The powders in the upper column section can be more easily ignited by electrostatic discharges compared to powders in the lower column.