In order to determine the kinetic parameters of oxygen exchange between nonstoichiometric oxides and the gas phase, the rate constant of surface exchange k and the coefficient of bulk oxygen diffusion in the oxide D, one needs to model the kinetics of relaxation occurring under mixed diffusion-reaction control. It is shown that the widely used Crank model can misestimate the k and D values as it does not take into account such factors as (i) changes in oxygen partial pressure pO2 in the reactor caused by vigorous reaction, (ii) the non-uniform distribution of pO2 over the elongated oxide sample. Models taking into account these factors are discussed sequentially in this work: the ideal continuous stirred-tank reactor (CSTR) model where pO2 varies over time as reaction proceeds; the tanks-in-series (TIS) model imitating the non-uniform distribution of pO2 over the sample under approximation of convective gas transfer; and the model of tanks-in-series with diffusion-convective gas transfer (TSD). The TIS and TSD models (the latter one being used for low gas flow rates), which take into account all of the above factors, give the most reasonable results: the scatter of the values of the kinetic constants k and D determined at different rates of gas flow in the reactor does not exceed the reasonable experimental error. An expression is proposed for estimating kTIS, which corrects the k values obtained from the Crank model.
Предметные области OECD FOS+WOS
- 2.11.IF ИНЖЕНЕРИЯ, МУЛЬТИДИСЦИПЛИНАРНАЯ