Binary gas diffusivity estimates from transient, one-dimensional sublimation–diffusion experiments in a spherical enclosure

Abstract

ABSTRACTBinary gas diffusivities DAB’s are extremely useful in the analysis/design of mass transfer systems and to develop correlations. This study used an unsteady experimental method to determine DAB’s in gas pairs starting with a sublimating solid (A) such as naphthalene or camphor and air (B). The cumulative fractional mass transferred from the surface of a solid A sphere placed concentrically within an isothermal spherical enclosure was followed gravimetrically with time. The experimental DAB,exp for the gas pair was determined by nonlinear regression using the solution to a transient, one-dimensional (radial) diffusion model. The model’s Case 1 option assumed impermeability (no flux of gas A) at the enclosure’s outer surface, while Case 2 assumed zero concentration of gas A at the same location. For naphthalene–air, DAB,exp overestimated the literature values, the errors ranging from −110 to −185% for Case 1 and −21 to −65% for Case 2. For camphor–air, the error in DAB,exp was −36% for Case 1 and −16% for Case 2. DAB,exp for camphor in atmospheric air is herein reported for the first time. Potential improvements to the experiments include automation of the sphere melt-casting process and tighter control of the enclosure’s environmental conditions. Likewise, the theoretical model could be extended to three dimensions with multicomponent diffusion to assess the effect of air humidity on the transport of gas A. This is the first attempt to determine DAB,exp for naphthalene–air and camphor–air from an unsteady sublimation–diffusion experiment and to model the results using rigorous mass transport theory.