# Kinetics Study of Gas Pollutant Adsorption and Thermal Desorption on Silica Gel
## 1 Introduction
Silica gel not only adsorbs VOCs, but also ensures that the humidity of the aircraft cabin remains in a safe range. In order to simulate the silica gel adsorption and desorption processes, [PERSON] et al. conducted a theoretical study based on the mass conservation and diffusion equation [18]. They calculated the coefficient of convective mass transfer derived by analogy to the coefficient of convective heat transfer. An air purification system based on a silica gel-coated desiccant wheel is shown in Figure 1. This conceived system can be extended to a ventilation system for a semi-enclosed cabin, such as an aircraft cabin, basement, tank or armored vehicle. In this example, the exhaust air out of the cabin is divided into two parts: regeneration air and recirculation air. The recirculation air is purified first by a high-efficiency filter and then by a desiccant wheel. In an air mixing unit, the purified air is mixed with the fresh air cooled by an ECS, and then returned to the cabin. In addition, the regeneration air is heated to regenerate a part of the desiccant wheel. The VOCs adsorption and desorption performances on silica gel are generally measured at equilibrium conditions, so the kinetics and the equilibrium properties are the important factors for  
Figure 1: Desiccant wheel-based air purification system diagram. silica gel application. However, only a few kinetics studies on adsorption and desorption performances of silica gel have been conducted [20; 21; 22]. Most recently, the effect of silica gel microphysical properties on the moisture sorption capacity and kinetics was investigated through small-scale desiccant-coated energy exchanger experiments by [PERSON] et al. [20]. Furthermore, they conducted a novel transient thermal response test on a silica gel-packed bed to release the convective heat transfer coefficient, which provided a reference for the packed bed design with porous materials [21]. The transient numerical model of both heat and moisture transfer in a thin silica gel-packed bed was developed by [PERSON] [22]. Their studies offered a great help for the affinity study of adsorption on silica gel. In this paper, thermogravimetric analysis (TGA) experiments were conducted to investigate the performance and kinetics of adsorption and desorption of silica gel. Three contaminants are considered in our study. They are carbon dioxide (primarily considered in a closed and crowded cabin), methane (leakage of fuel oil and slide oil could generate various alkane steams [23]), and toluene (high-toxicity gas detected in most studies [24]). Based on the measured data, the adsorption characteristics are revealed by using a Langmuir isotherm-based kinetic model. As the thermal regeneration result decides the initial concentration of adsorbate during the adsorption and desorption cycle, it is necessary to discuss the thermal regeneration process. The [PERSON]n't [PERSON] equation, which explicates the temperature dependence of the equilibrium constant, is used to evaluate the enthalpy and entropy of thermal regeneration on silica gel. Analysis shows that the adsorption mechanism controls the silica gel adsorption processes, but both adsorption and desorption mechanisms affect the thermal regeneration process. The increase of temperature is beneficial for a desorption-terms process.