Effect of liquid phase composition on the experimental determination of binary gas diffusivities in an isothermal Stefan diffusion column

Abstract

The Stefan diffusion column was designed in the 19th Century to determine binary gas diffusivities, D_AB’s. Typically, pure liquid A is overlaid by gases A and stagnant B, with a steady gas B sweep at the top to remove the evaporated-diffused A. Experimental diffusivities D_AB,exp may be obtained from transient interfacial descent data, but column end effects impacting such determination have been ignored or neglected in the literature. This study addresses experimentally and theoretically for the first time the role played by liquid phase composition on D_AB determination using the Stefan column. Specifically, changes in interfacial curvature were sought by adding a nonvolatile liquid I (glycerol) to a volatile liquid A (ethanol). Mixtures of known initial composition x_A0 were tested in isothermal evaporation-diffusion experiments at ∼65 °C using atmospheric air (B). A one-dimensional transport model for gas A was used to analyze the interfacial descent data. The average D_AB,exp errors did not differ significantly between the liquid mixture groups and those for pure ethanol (range for the latter of +9.9 to +17.1%). However, considerable scatter in D_AB,exp occurred as the mixtures became more dilute in ethanol, with the coefficient of variation increasing more than ten-fold in the x_A0 = 0.146 group. This variability cannot be explained by differences in surface tension exclusively, but may also result from changes in liquid physical and transport properties as well as diffusion of liquid A. A more comprehensive modeling of the liquid phase in conjunction with gas phase transport is necessary to obtain accurate D_AB’s.

Keywords:

• Effect of liquid phase composition on binary gas diffusivity determination
• Ethanol evaporation-diffusion experiments
• Experimental determination of binary gas diffusivity
• Gas phase transport modeling
• Interfacial curvature
• Isothermal Stefan diffusion column
• Liquid mixture physical and transport properties
• Surface tension

Acknowledgements

Our group recognizes the Departments of Chemistry and Civil Engineering, University of Puerto Rico, Mayagüez, for making available their experimental facilities throughout this study. Special thanks are given to technicians E. J. Quiñones and P. M. Torres for their assistance and encouragement. In addition, the General Library’s Interlibrary Loans Section tracked down most of the archival references. Finally, a deserved acknowledgement goes to Professors R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Department of Chemical and Biological Engineering, University of Wisconsin, Madison, for inspiring our research and for giving science their luminous Transport Phenomena text.

Authors María del Sol Jaime and Shayra G. Maisonet contributed equally to this project and received undergraduate credit for conducting/analyzing the experiments reported.