Aqueous Ammonia Vapor-Liquid Equilibria: Entropy and Temperature Dependence of Wilson Coefficients

Abstract

This study extends a model for nonideal solution behavior by considering the temperature dependence of the coefficients in the Wilson equation for the aqueous ammonia system. Twenty-seven isothermal sets of experimental PXY data up to 2 MPa (20 atm) pressure (284 points) were analyzed using an objective function based on the excess Gibbs free energy to determine the pair of Wilson coefficients for each data set. Evaluation of these results supports the interpretation of the interaction parameters in Wilson's equation as temperature-dependent entropy functions. Comparison of computed results is made with four categories of vapor–liquid equilibrium (VLE) data: (1) primary PTXY, (2) refined PTXY, (3) secondary PTXY or PTM, and (4) partial. Excellent agreement is found with computed results for all but two of these VLE data sets in the region of rapidly changing vapor composition up to 90 mole % of ammonia. A comparison is also made to the only three previously published single-temperature (isothermal) pairs of Wilson coefficients with better agreement in Y, for all three cases, and in P, for two cases. A straightforward procedure is outlined to estimate any set of PTXY values (in the range P < 2 MPa, Y < 0.9) for the aqueous ammonia system.