Thermogravimetry – Kinetic study of reactions in solid-gas systems

Thermogravimetry is the most widely used technique for determining the reaction rate of a solid or divided solid as it transforms through thermal decomposition or reaction with one or more gases. It is used in a wide range of applications, from chemical processes for divided solids to high-temperature corrosion of materials. Modeling the course of a heterogeneous reaction based on the fundamental processes – germination and growth – calls on heterogeneous kinetics, a demanding but inescapable discipline for anyone wishing to predict the behavior of a real solid-gas system of any kind, such as powders in a calcination kiln or a material placed under extreme corrosive conditions. The scientific progress achieved over the last hundred years is essentially due to the existence of two parallel communities of researchers: those working in the field of divided solid transformation processes ("process engineering" in French laboratories and "chemical engineering" in Anglo-Saxon laboratories) and those working in the field of materials (metallurgists, ceramists). The current split between the two groups is such that the former use mathematical methods to interpret kinetic data obtained in temperature ramps, while the latter have laid the foundations of metal oxidation theory by describing reaction mechanisms very precisely on the basis of isothermal and isobaric tests. The former paid little heed to the physical characteristics of the powders under study, and sought the best rate law appearing in a "list" of models reproduced from article to article, while the latter developed sophisticated physical models generally limited to plane symmetry. However, the transposition of knowledge from one community to the other enriches their respective approaches and, above all, generalizes the concepts of chemical kinetics by proposing a methodology adapted to the establishment of predictive rate laws while elucidating reaction mechanisms.

The aim of this article is to bridge the gap between the two approaches by revisiting heterogeneous kinetics to provide engineers and researchers with the tools they need to understand and model heterogeneous reactions in solid-gas systems.

The approach developed is designed to interpret experimental kinetic curves describing mass variations over time of a solid placed under given thermodynamic conditions (isothermal thermogravimetry and gas mixing at fixed partial pressures). For this purpose, pseudo-stationary analytical models based on the determining step approximation are preferred, as numerical resolution methods for systems of differential equations are difficult to use due to the multitude of unknown parameters. The velocity equation can then be decomposed into a product of two functions with separate thermodynamic and morphological variables,...