Gas separation by means of the Knudsen compressor

A possibility of making use of the Knudsen compressor as a gas separator is investigated. Starting from the description at the microscopic level on the basis of the kinetic theory of gases, a fluid-dynamic model describing the behaviour of the mixture in the Knudsen compressor is derived. Then, by the use of this model, it is numerically demonstrated that the Knudsen compressor works certainly as a gas separator. The separation performance is shown to reach a practical level by increasing the number of elemental units in the device. The numerical simulation is carried out for various molecular models, not only for fundamental models, the hard-sphere and Maxwell molecules, but also for more realistic models such as the inverse power-law potential and Lennard-Jones models, assuming the McCormack model equation at the microscopic level. The results show that the modelling by the celebrated Maxwell molecule (or the BGK-type model equation) fails to capture the phenomenon of the gas separation in the device. This presents a remarkable contrast to the capability of the other fundamental model, the hard-sphere molecule, even though this model exaggerates the phenomenon to some extent.