Pharmaceutical powders are very prone to electrostatic charging by colliding and sliding contacts with walls and other particles. In pharmaceutical formulation processes, particle charging is often a nuisance and can cause problems in the manufacture of products, such as affecting powder flow, and reducing fill and dose uniformity. For a fundamental understanding of the powder triboelectrification, it is essential to study charge transfer due to a single contact of a particle with a target plane under well-defined physical, mechanical and electrical conditions. In this study, charge transfer due to a single impact of a particle against a stainless steel target was measured for alpha-lactose monohydrate, aspirin, sugar granules and ethylcellulose. The amount of transferred charge is expressed as a function of impact velocity and impact angle as well as the initial charge. The maximum contact area during impact between a particle and a target plane is estimated by an elastic-plastic deformation model. It is found that the transferred charge is a linear function of the contact area. For a given material, there is an initial particle charge for which no charge transfer occurs due to impact. This is found to be independent of impact velocity and angle, and is hence viewed as a characteristic property, which is related to the contact potential difference and tribo-electric series of the sample powders.
Electrostatic characteristics in gas-solids flow in a metal pipe are studied both theoretically and experimentally with particular attention to the collision between particles and pipe wall. Effects of gas velocity and particle diameter on the electrification are also examined.
ABSTRACT. Particle reentrainment from a fine powder layer was investigated both in a steady-state flow and in an unsteady-state (accelerated) flow. Experiments were conducted in a rectangular channel, where a powder layer of fly ash was placed. The average air velocity was increased at a constant rate in the range of 0.01-0.6 m / s 2 up to a certain velocity and, thereafter, it was maintained at the velocity. The reentrainment flux was measured automatically by an electrostatic method. Microscopic observation showed that small aggregates were reentrained randomly from the surface of the powder layer, and then the reentrainment gradually progressed through the depth of the powder layer. Through these processes, surface renewal of the powder occurred. The experimental results also showed that the distribution of adhesive strength (wall shear stress) was approximated by a log-normal distribution. Further, the time-delay of the reentrainment was found to be represented by two simple exponential functions with different time constant. A new reentrainment model is presented to explain the time-dependence of the reentrainment flux, which is based on the adhesive strength distribution, surface renewal of the powder layer, and the time-delay of the reentrainment. The reentrainment flux increases with time elapsed in an accelerated flow, while it decreases in a steady state flow. Air acceleration has a significant effect on the reentrainment flux not only in the accelerated flow but also in the steady-state flow. Furthermore, the critical (incipient) reentrainment velocity decreases with increasing air acceleration. The experimental results agreed well with the results calculated based on the new model.
Micro-feeding of fine powders has been studied experimentally by use of a vibrating capillary tube whose diameter is varied from 0.58 to 1.26 mm. Ultrasonic vibration of 20 kHz was generated by a piezoelectric transducer and applied to the tube so as to discharge micron or sub-micron particles. The mechanism of the powder flow in the vibrating capillary tube is deduced, such that a thin layer of particles near the inner wall acts as a lubricant through their micro-vibrations. Therefore, the inner powder easily passes through the tube by the force of gravity. The powder flow rate, velocity and packing fraction of particles in the capillary tube were also obtained. Furthermore, a new factor corresponding to the viscosity of fluid was introduced in characterizing the powder discharge. From the experiments using various fine powders, it was found that the continuous operation of micro-feeding was possible even at a rate of milligrams per second.
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