Measurements of intra-diffusion coefficients for gaseous binary mixtures

Benchtop pulsed field gradient (PFG) nuclear magnetic resonance (NMR) measurements of the intra-diffusion coefficient ($D_i^{\ast }$$D_i^{\ast }$) for binary gaseous mixtures are presented as a function of composition, for temperature and pressure conditions broadly relevant to industrial and geological processes. This required the design, construction, and application of a novel NMR-compatible sapphire sample cell. Measurements were performed for methane–nitrogen, methane-helium, and methane-hydrogen mixtures, with compositions down to 0.5 mol% methane that were resolvable in a reasonable time frame. Consequently, extrapolation to infinite dilution was enabled, with the resultant values of $D_i^{\ast }$$D_i^{\ast }$(x_i = 0) compared with relevant mutual diffusion coefficients (D₁₂) from both literature and as estimated using kinetic theory (Thorne-Enskog equation). In the case of methane-helium mixtures, agreement was overwhelmingly within experimental uncertainty across the temperature–pressure parameter space explored, whereas in the case of methane–nitrogen, the determined values of $D_i^{\ast }$$D_i^{\ast }$(x_i = 0) were slightly larger than D₁₂ data as predicted by kinetic theory. In the case of methane-hydrogen mixtures, simultaneous measurements of both methane and hydrogen intra-diffusion coefficients were possible. Agreement between $D_i^{\ast }$$D_i^{\ast }$(x_i = 0) and kinetic theory was comfortably within experimental uncertainty in the case of hydrogen but deviated in the case of methane.