Simulation of adsorption and desorption phenomena in a gas chromatography microcolumn

International audienceThe main goal of the present work is to optimize the micro-column for the gas chromatography analysis device, which is the device used in the real-time monitoring of atmospheric BTEX concentration for indoor environments. The BTEX group of molecules (Benzene, Toluene, Ethylbenzene and Xylenes) is, among volatile organic compounds (VOC), the most widely found in indoor air and its main sources are the cleaning products, building materials and outdoor air supply [1]. Among the BTEX compounds, Benzene is the most dangerous one, since it has been proven as a carcinogenic component [2]. The European Commission report [3] stated that the limit concentration of Benzene in an indoor environment should be 5μg/m³. Therefore, it is really important to measure the concentration of these species in indoor environments. The separation of BTEX takes place inside the micro-column is due to the adsorption and desorption phenomena. The relative rates of adsorption and desorption onto and off the stationary phase inside the chromatographic column allow chemicals in the analyzed samples to be separated and then quantified. Aiming to have a good performance of chemical detection, it is always important to have well-separated analytes in the GC column, hence designing an efficient column is crucial to the success of the microchemical detection system [4]. Usually, the micro-columns used in gas-chromatographs are silica tubes of about 150 to 300 microns in diameter and about 20 meters long [5]. These columns have a thin stationary phase layer coated onto the walls of about 1μm thick. The different adsorption and desorption rates between the species to be separated with the stationary phase will generate different interactions between them, and each species will reach the outlet of the micro-column in a distinct instant. Placed after the micro-column, another component of the gas chromatography device, called detector, is accountable for measuring the concentration of each specie in tested air sample. The final output provided by the gas chromatograph analyzed in the present work is a graph containing the concentration of the BTEX compounds species in time, called chromatogram. In a very long and narrow channel and under isothermal conditions, the pressure and density could change considerably between the inlet and outlet in the function of the applied end conditions. Therefore, the velocity of the flow in the outlet cross-section of the column is higher than the velocity nearby the inlet. Consequently, even though the flow velocity is much smaller than the sound speed, the compressibility effect and the flow velocity variation with the position have to be considered. The working conditions of our micro-column are: outlet pressure of 1atm and temperature of 65°C. Under these conditions, the mean free path of Nitrogen, th