Assessing the feasibility of separating a stoichiometric mixture of zinc vapor and oxygen by a fast quench – Model calculations

An aerosol kinetic model is applied to evaluate the feasibility of separating a stoichiometric mixture of zinc vapor and oxygen by a fast quench as proposed in the solar thermochemical Zn/ZnO cycle. The model is able to explain the experimental evidence collected so far in our laboratory: high zinc yields are only obtained if the zinc/oxygen mixture is strongly diluted with inert gas. This behavior can be understood because the desired condensation of zinc is slow compared to its oxidation when liquid zinc is involved due of its high vapor pressure. The oxidation reaction, however, is not limited by this as the equilibrium partial pressures of zinc and oxygen above ZnO are both negligible. For the same reason, the oxidation reaction continues even if the zinc saturation drops below S = 1. Here, initially condensed zinc begins to evaporate further decreasing the zinc yield. The model predicts that increasing the quench rate has a negligible effect even at an initial zinc vapor pressure of 11,000 Pa but that higher zinc yields can be expected if the quench is applied to more dilute mixtures.