Experimental investigation of the separation of binary gaseous mixtures flowing through a capillary tube

Abstract

This paper presents an optical method to investigate the separation phenomenon for the flow of a near-equimolar mixture of carbon dioxide and helium through a hollow-core photonic crystal fiber using tunable diode laser absorption spectroscopy to measure the temporal evolution of the path-integrated absorption of carbon dioxide. The gas flow was initiated by a pressure difference between two gas cells, each connected to one end of the fiber under isothermal conditions. The change in path-integrated concentration of CO2 over time was used to infer the separation of the gases, defined as the dimensionless quantity Φ in this paper. To investigate the effects of pressure ratio and rarefaction on the separation phenomenon, these parameters were varied in the experiments. The separation Φ increases from zero with no pressure gradient, reaching an asymptotic maximum value for pressure ratios exceeding 20. To examine the effect of rarefaction on Φ, measurements have been conducted for the binary mixture flowing into near-vacuum, covering a range of inlet Knudsen numbers (Kn) between 0.016 and 2. The separation Φ increases with Kn for 0.01 < Kn < 0.1, reaching a peak value at Kn ≈ 0.1, and then decreases with a further increase in Kn. This effect has not previously been noted in the literature. The experimental results are compared with a numerical model, demonstrating good agreement. Based on these findings, we have summarized the necessary conditions for carbon dioxide/helium separation to occur inside a capillary tube, which can be used as a mechanism for small-scale gas separation applications.