Solar carbothermal reduction of aerosolized ZnO particles under vacuum: Modeling, experimentation, and characterization of a drop-tube reactor

- Solar vacuum drop-tube reactor extensively characterized at 1, 100 and 960 mbar.
- Zinc production is higher at 100 mbar compared to ambient pressure.
- Reactor model featuring radiative heat transfer with Monte Carlo ray tracing.
- Reaction at 1 mbar is inhibited due to insufficient particle residence time.
- Maximum zinc production predicted to be 52 mmol·min−1, at a feed rate of 68 g·min−1.

A vacuum aerosol particle reactor was tested for the carbothermal reduction of zinc oxide using concentrated solar power as a heat source. A steady state reactor model was developed to investigate the effect of pressure dependent particle residence time and radiative input power on the zinc production rate. Radiative heat transfer to the particle cloud is solved by Monte Carlo ray tracing, accounting for spectral and directional optical properties and temperature dependent chemical kinetics. Experiments with the solar drop-tube reactor were conducted to ascertain the reaction capacity of the system at pressures between 1 and 960 mbar by varying the reactant feed rate between 4 and 56 g·min−1. Experiments show that the zinc production rate is maximal at around 100 mbar and significantly diminishes under high vacuum. Model and experimental results indicate that the reaction at 1 mbar is inhibited due to insufficient residence time and heat up of the particles in the reaction zone. Maximum experimental zinc production rate was 51.4 mmol·min−1, while feeding 56 g·min−1 of solid reactants and operating the reactor at 100 mbar with 9.8 kW of radiative input power. Extrapolation to higher feed rates with the reactor model predicts a peak zinc production capacity of 52.1 mmol·min−1 at a feed rate of 68 g·min−1, achieving a net thermal efficiency of 3.2%.

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