There are various methods to store thermal energy from season to season. Because the density of energy storage in thermochemical materials is higher than the latent and sensible heat storage method, the thermochemical heat storage is considered the sufficient type. This paper provides a mathematical model of thermochemical storage process for thermochemical materials; potassium aluminum sulfate 12-hydrate (KAl(SO4)2.12H2O). The software used is COMSOL Multiphysics Modelling Software. The model data is validated and compared with the previous experimental result. Furthermore, the temperature difference and the pressure over the material inside the reactor are investigated. There are seven designs for the bed. However, the inlet and outlet diameter of the bed is changed; there is any change in the volume and height of the bed. The model results present that the rise in entrance area decreases the charging time and rises the pressure drop inside the material. Case 4 is the best design from the charge time.