Multicomponent gas transport in a Stefan diffusion column containing an azeotropic liquid mixture of acetone-n-hexane

Abstract

The Stefan column was designed in the 19^th Century to estimate binary gas diffusivities by placing a pure volatile liquid at the bottom overlaid with a stagnant gas. A steady gas sweep at the top removed the diffused species. Previous validation, by other research groups, of the Maxwell-Stefan multicomponent mass transport equations in the Stefan column prompted a study to test an azeotropic binary liquid as the source of both gaseous species, maintaining the composition at the liquid-gas interface constant. Azeotropic acetone (A)-n-hexane (B) mixtures were placed in a vertical glass pipette in preparation for an isothermal evaporation-diffusion experiment in atmospheric air (C). The pure solvents were also tested separately. The experimental interfacial descent, the solvents’ diffusivities in air, and their molar fluxes were provided as input to the Maxwell-Stefan equations to obtain the binary gas diffusivity D_AB,exp, which is unavailable in the literature. The pure solvent diffusivities in air showed good agreement with the Chapman-Enskog kinetic theory for low-density gases, but the experimental gas diffusivity of acetone in n-hexane exceeded consistently the theoretical predictions by at least two orders of magnitude. A possible explanation for this unexpected result is the high affinity between the solvent molecules in the liquid state, which undoubtedly affects their gas transport behavior. Further studies are necessary to elucidate this finding. This is the first attempt to use a liquid azeotrope in a Stefan column. By combining this approach with the Maxwell-Stefan transport equations, the gas diffusivity of acetone in n-hexane was obtained.

Keywords:

• acetone-n-hexane azeotropic liquid mixture
• Chapman-Enskog kinetic theory for low-density gases
• constant interfacial composition
• experimental binary gas diffusivity determination
• gas diffusivity of acetone in n-hexane
• Maxwell-Stefan multicomponent gas transport equations
• Stefan column evaporation-diffusion experiment

Acknowledgements

The authors acknowledge Prof. I. Y. Padilla, Director, and P. M. Torres, technician, Environmental Engineering Laboratory, Department of Civil Engineering and Surveying, University of Puerto Rico, Mayagüez, for use of their experimental facilities. The Interlibrary Loans Section of the General Library obtained electronic copies of the archival references. Finally, the University of Wisconsin Professors R. B. Bird, W. E. Stewart, and E. N. Lightfoot are recognized for their contributions to science and for their timeless Transport Phenomena text.

Dr. F. J. Braña generously provided valuable background information on the thermodynamic properties of the acetone-n-hexane azeotrope and is hereby thanked. Prof. S. P. Hernández, Department of Chemistry, is acknowledged for his help with the characterization of the azeotrope. Student authors I. Moreno and M. Moreno received undergraduate research credit for performing the evaporation-diffusion experiments, contributing in equal proportion to the project.

This paper is dedicated to the memory of John Wade, Richard William Merriman, and the students they trained at Guy’s Hospital, London.