(309h) Thermodynamic Properties of Humid Gases from First Principles

Numerous scientific and industrial situations require thermodynamic properties of gas mixtures containing water. Examples include humidity standards (where the dominant correction is the nonideality of water in the vapor phase) and synthesis and combustion gases in advanced power-generation cycles. In many of these cases, the conditions of temperature and pressure are such that the appropriate level of description is that of the second virial coefficient. However, experimental measurements for water/gas binaries are difficult, due to adsorption of water on apparatus and/or difficulty in measuring small concentrations of water in the vapor phase. As a result, the available experimental data for cross second virial coefficients involving water have large uncertainties and cover an insufficient temperature range for many applications.

Our approach uses ab initio quantum chemistry to develop quantitatively accurate potential-energy surfaces for the binaries of interest. Integration of these pair potentials (with quantum corrections) then yields second virial coefficients. This calculation can provide second virial coefficients at any temperature, and our calculated values have uncertainties that are in most cases smaller than those obtained by experiment. We will show the results of this approach for several binary systems, including water with argon, hydrogen, nitrogen, and oxygen. Applications are discussed for humidity standards (where our results can be used to produce an effective second virial coefficient for water with "air") and for a key step in the integrated gasification combined cycle (IGCC) advanced power cycle.