Condensation pressures in small pores: An analytical model based on density functional theory
Nilson, R. H.
Griffiths, S. K.
United States. Department of Energy.
Sandia National Laboratories
1999-02-01
English
Adsorption and condensation are critical to many applications of porous materials including filtration, separation, and the storage of gases. Integral methods are used to derive an analytical expression describing fluid condensation pressures in slit pores bounded by parallel plane walls. To obtain this result, the governing equations of Density Functional Theory (DFT) are integrated across the pore width assuming that fluid densities within adsorbed layers are spatially uniform. The thickness, density, and energy of these layers are expressed as composite functions constructed from asymptotic limits applicable to small and large pores. By equating the total energy of the adsorbed layers to that of a liquid-full pore, the authors arrive at a closed-form expression for the condensation pressure in terms of the pore size, surface tension, and Lennard-Jones parameters of the adsorbent and adsorbate molecules. The resulting equation reduces to the Kelvin equation in the large-pore limit. It further reproduces the condensation pressures computed by means of the full DFT equations for all pore sizes in which phase transitions are abrupt. Finally, in the limit of extremely small pores, for which phase transitions may be smooth and continuous, this simple analytical expression provides a good approximation to the apparent condensation pressure indicated by the steepest portion of the adsorption isotherm computed via DFT.
Pressure Dependence
Pore Structure
Storage
Filtration
Separation Processes
Porous Materials
Adsorption
Gases
32 Energy Conservation, Consumption, And Utilization
Surface Tension
Phase Transformations
Analytical Solution
Vapor Condensation
Other Information: PBD: 1 Feb 1999
Report
18 p.
Text
rep-no: SAND99-8220
grantno: AC04-94AL85000
doi: 10.2172/751009
osti: 751009
https://digital.library.unt.edu/ark:/67531/metadc706000/
ark: ark:/67531/metadc706000