DIFFUSION COEFFICIENTS FOR HELIUM-NITROGEN AND NITROGEN-CARBON DIOXIDE AT ELEVATED TEMPERATURES C. S. E L L I S ' A N D J . N. H O L S E N Department of Chemical Engineering, Washington University, St. Louis, Mo. 63130
Gas diffusion coefficients for the systems helium-nitrogen and nitrogen-carbon dioxide have been measured over the approximate temperature range from 300" to 900" K. in a diffusion cell utilizing a barrier of porous stainless steel. An observed change in the effective length of the barrier as a function of the net flux is attributed to convective mixing and dispersion in the interstices of the relatively large pores of the porous medium.
XPERILIESTAL measurements of gas diffusion coef-
E ficients a t high temperatures are relatively few in number.
Yet such data are highly desirable for the evaluation of potential energy parameters characteristic of collisions between unlike molecules, as well as for the confirmation of theoretical relations which have been tested primarily near room temperature. Kestenberg (1966) reviewed the experimental data for high temperature diffusion coefficients covering the period t o 1966. Klibanov et al. (1942) determined diffusivities for HzO-air and C02-air at temperatures to 1500" K. in a capillary tube diffusion cell. Kalker and Westenberg (1958a, b ) developed the "point source technique" and applied it to the determination of diffusivities of He-N2 and Xz-CO2 as well as several other systems over the nominal temperature range from 300" to 1150" K. In their method, a trace quantity of one coniponent is introduced into a laminar stream of the second component flowing at uniform velocity. The diffusion coefficient is computed from the axial decay of the trace component concentration. Ember et al. (1962) extended the range of the point source technique to about 1800" K. by utilizing the hot gases generated in a flame. Scott and Cox (1960) used a diffusion cell of porous porcelain to measure diffusivities for Hz-K-2 and H2-XH3 from 300" to 600" K. I n this study, the diffusion coefficients for He-Sz and Nz-CO~ were measured between approximately 300" and 900" E
