# Kinetics Study of Gas Pollutant Adsorption and Thermal Desorption on Silica Gel
## 1 Introduction  
With the development of civil aviation transportation, passengers and crews are becoming more demanding for comfort and safety. In a cramped and crowded airliner cabin, passengers and crews have long complained about air quality problems, which have caused fatigue, dizziness, headache, ear disease, throat pain, and even occasional neurological disorders [1]. As an important issue for the cabin environment, gaseous contaminants directly influence the comfort, health, and working efficiency of passengers and crews. The air quality in the aircraft cabin is maintained directly by the Environment Control System (ECS), which supplies fresh air into the cabin and exhausts cabin air through an outflow valve [2]. In a typical civil aircraft, the air supplied into the cabin by the ECS consists of approximately 50% outside air and 50% filtered recirculation air [3]. Usually, the High Efficiency Particulate Air (HEPA) filter for the recirculation air has an efficiency of 99.97% to remove 0.3-micrometer particles, but the HEPA filter is incapable of removing gaseous pollutants. With the innovation of \"Cleaner Engine\" launched globally, future civil aircrafts need more efficient engines and less bleed air [4]. In order to maintain a healthy cabin environment with a low concentration of gaseous pollutants, novel filters are being explored and developed continuously. In recent years, researchers have conducted experimental and theoretical studies on various contaminant removal techniques, such as adsorption [5; 6; 7; 8; 9], photocatalytic oxidation [10; 11; 12], catalytic converters, non-thermal plasma oxidation [13], etc. Activated carbon adsorption has long been usedin cabin purification systems due to its excellent stability, limited secondary pollution, and portability. However, [PERSON] [14] found that the adsorbed volatile organic compounds (VOCs) could react with ozone and produce secondary pollution, such as ultrafine particles, formaldehyde, and some other harmful by-products. In addition, humidity control requirements and difficulties in regenerating high-boiling point solvents and flammability may impede the application of activated carbon. Hence, activated carbon is not used in any airlines. Different alternative adsorbents need to be developed to overcome these problems. In fact, the major requirements for a regenerative adsorbent are its adsorption capacity and its regeneration efficiency, which determine the contaminants' removal efficiency in a whole adsorption and desorption cycle. As an alternative to activated carbon, silica gel shows several advantages. Particularly, the use of silica gel in air purification is attracting more and more attention due to its high removal efficiency and stability to varying humidity levels [15; 16; 17; 18]. An average removal efficiency up to 94% was obtained in the experiments of [PERSON] et al. [15]. In their experiments, human subjects, flooring materials, and four pure chemicals (i.e., toluene, formaldehyde, ethanol, and 1,2-dichloroethane) were used as gas pollutants to investigate the effect of a desiccant wheel in the ventilation system. In their experiments, it was worth noting that the VOCs were nearly 100% removed from the recirculation air at about 333 K-343 K, regardless of the moisture load. The competition between VOCs and water vapor reduced when the regeneration temperature increased to 333 K-343 K. Their experiments confirmed the potential air-cleaning performance of silica, which also helps to confirm the aims of this paper. However, the desiccant wheel VOC removal capacity was far from reaching its limit during their eight-hour experiments. Further studies on the equilibrium uptake and kinetics need to be carried out to promote the investigation of silica gel air-cleaning properties. At the same time, they conducted passenger perception experiments in a simulated aircraft cabin in which the air purifier had a certain effect on improving the cabin air quality [19].