Mathematical models can provide estimates for the shelf life of water-sensitive products like food or pharmaceuticals. This study presents a simple kinetic model using two first-order reactions for the evaporation and condensation of water. Furthermore, the model can be simplified to contain only one free parameter, the reaction rate constant k, which has been validated for silica gel at a relative humidity between 0% and 80% with experimental data. The experimental data shows the hysteresis effect of the silica gel in the region between 30% and 80% relative humidity and its dependence on the relative humidity earlier in the process. It also shows there are multiple equilibrium water contents at a relative humidity of 40%, depending on the previous relative humidity. The relative humidity barely influences the fitted reaction rate constant during adsorption. However, during the desorption process, not only the current relative humidity but also the history of relative humidity have an influence. A higher relative humidity in the previous step can slow down the desorption rate in the following step.
Keywords: hysteresis; kinetics; model; silica; sorption; water.