ark: ark:/67531/metadc706000
Phase Transformations
Analytical Solution
Surface Tension
18 p.
United States. Department of Energy.
Condensation pressures in small pores: An analytical model based on density functional theory
grantno: AC04-94AL85000
1999-02-01
Other Information: PBD: 1 Feb 1999
Griffiths, S. K.
Nilson, R. H.
Adsorption
Pore Structure
Filtration
Porous Materials
Separation Processes
doi: 10.2172/751009
32 Energy Conservation, Consumption, And Utilization
Gases
rep-no: SAND99-8220
Vapor Condensation
Pressure Dependence
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.
Storage
osti: 751009
Sandia National Laboratories