Vacuum powder feeding and dispersion analysis for a solar thermochemical drop-tube reactor

Abstract

Ultrasonic vibratory and rotary valve particle feeders have been designed, constructed, and investigated for application to feeding reactant powder to a solar thermochemical drop-tube reactor. Zinc oxide and carbon particles are fed continuously to the drop-tube under vacuum pressures as low as 1mbar. The particles are probed in situ by laser transmission measurements with the aim to characterize particle residence time, axial and radial dispersion as a function of operating pressure. The ultrasonic feeder disperses particles well and can be operated at mass flow rates in the range of 57–288mgmin−1. The rotary valve feeder operates in the mass flow range of 3.46–41.96gmin−1 and exhibits reduced particle dispersion due to discrete pulsing mass flow created from the rotating valve. The time resolved transmission signals reflect characteristic changes under different experimental vacuum conditions. Particles traveling through the measurement zone at 1mbar exhibit residence and clearance times of 0.05s and 0.52s, respectively. At 960mbar, residence and clearance times are increased to as much as 0.16s and 3.98s, respectively. Particles falling at 1mbar show radial dispersion three times less than those falling under ambient pressure. A critical result of the functional characterization of powder feeding under vacuum is a potential reaction capacity limitation at low vacuum pressures due to short particle residence time and narrow axial dispersion.