In the following study, a basis has been developed for a pseudo-permanent model, taking into account the average functioning characteristics of a solid-gas reactor during an interval of time. This model is of interest thanks to its analytical form and allows simple calculations to be made concerning the dimensioning, and thereby helps in the decision-making process. The industrial development of thermochemical reactors requires the design and construction of demonstration machines and thus calls for dimensioning models. The already existing models are based on dynamic simulation linked to the discontinuous and transient functioning mode of such processes. Hence, these dynamic models require initially arbitrary choices, which are tested successively to achieve a correct sizing of the machine. The comparison of the analytical model developed in this study with a dynamic model, and with the results of experiments, demonstrates the limits of the analytical model, as well as its reliability compared to the dynamic model. Elsewhere, the flexibility linked to its analytical form shows the scale of the field of application.