Thermodynamic Properties of Moist Gases from ab initio Potentials

摘要

The thermodynamic properties of gaseous mixtures containing water are of interest for the heating, ventilation, and air conditioning (HVAC) industries, for describing the properties of combustion gases, and for a variety of metrology applications (most notably the development of humidity standards). In most of these applications, the pressures are not too high, and the appropriate level of thermodynamic description is at the level of the second virial coefficient B. The virial equation provides a simple and rigorous framework for calculating the thermodynamic properties of gaseous mixtures. However, the experimental determination of second virial coefficients B_(ij) for binaries containing water is quite difficult. Typical methods using gas-phase PVT data are distorted by adsorption below roughly 500 K. Values can be backed out from measurements of the solubility of ice or liquid water in a gas, but these experiments cover a limited temperature range and typically produce second virial coefficients with large uncertainties. It is, however, possible to calculate B_(ij)(T) rigorously from statistical mechanics from the intermolecular pair potential. Our goal is to derive pair potentials for water/gas binaries of sufficient quality to produce second virial coefficients with accuracy and precision superior to experimentally obtained values. Our approach for deriving a high-quality pair potential without unreasonable computing time requirements was to rely primarily on intermolecular perturbation theory calculations, which can be performed at relatively little expense. These results were suitably scaled and calibrated using a smaller number of highly accurate supermolecule calculations at the CCSD(T) level with large basis sets. The first system examined was water/helium; work on watereon and water/argon is in progress, and the eventual goal is to describe the more challenging but more important cases involving gases like N_2, O_2, and CO_2.