On the measurement of the mutual diffusivity of binary gas mixtures with FTIR Spectroscopy

The diffusion of carbon dioxide and either methane, ethane or propane is investigated in situ with FTIR Spectroscopy in the transmission mode. The apparatus is a closed measuring system divided into two parts separated by a shut-off valve. The concentration evolution during the diffusion process is monitored with IR gas signals calibrated according to the Beer-Lambert law. The mutual diffusivity is determined by modelling the diffusion process as in a two-bulb system using Fick's law for binary diffusion. The geometry of the theoretical setup is calibrated by analysing the diffusion kinetics of C3H8 in CO2. Then, the mutual diffusivity of the binary systems CO2–CH4, CO2–C2H6 and of CO2 in C3H8are evaluated. The results are in agreement with those reported in the literature and deviate by 9 % from the values predicted with the Chapman–Enskog theory when the molecular interactions are modelled with the Lennard-Jones potential.