The transition region of diffusion for a binary gas mixture was experimentally investigated in an isobaric open system of fine glass capillaries in parallel. Results were obtained over a 675/1 pressure range from 0.444 to 300.2 mm of Hg absolute in the transition region between Knudsen and molecular (ordinary) diffusion and confirm the theoretical equation for the flux in true capillaries in an open system. This equation has been derived by others using a momentum balance with various assumptions, such as additivity of the momentum transfer rates for the Knudsen and molecular regions, etc. These assumptions seem to be confirmed. This appears to be the first known experimental confirmation of this equation for capillaries over a wide pressure range. Experimental flux ratios obtained in the transition region confirm the predicted inverse square root relationship with the molecular weights. Data for counterdiffusion in an open system near the Knudsen region tend to confirm the Knudsen equation and data in the molecular region also substantiate the Stefan-Maxwell relations.
A numerical solution to the equations for steady-state diffusion in a three-component gas mixture of helium, neon, and argon was obtained by trial and error in the transition region between Knudsen and molecular diffusion in an open system. The shapes of plots of the fluxes of helium or neon having large concentration gradients vs. pressure are very similar to those of a binary mixture, increasing with pressure and approaching constant values at high pressures. Osmotic diffusion was found for argon and, unexpectedly, the shape of its curve of flux vs. pressure was somewhat similar to that of a binary mixture. Reasons were given for this behavior. Osmotic or reverse diffusion cannot occur in the Knudsen region. Under certain conditions the binary flux equations can be used to approximate the fluxes in a ternary mixture and reduce computational time. The existence of a maximum or minimum point in the plot of concentration vs. distance was indicated for argon. Such a point was shown for molecular diffusion in a closed ternary system. The flux ratios of helium to neon or helium to argon can change slightly or markedly with changes in pressure, depending on the concentration gradient of each component. This is contrary to the cases of diffusion in binary systems or multicomponent molecular diffusion in a closed system where the flux ratios are independent of pressure.
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