Modeling the high-pressure ammonia-water system with WATAM and the Peng-Robinson equation of state for Kalina cycle studies

The Kalina power cycle uses an ammonia-water mixture as the working fluid in the 283-866 K temperature range and at pressures up to 22 MPa. Modeling of these cycles, typically accomplished with a process simulator, requires accurate descriptions of the phase behavior of this binary. Therefore 58 previously published Pxy, isotherms in the 203-618 K temperature range were used to evaluate the predictive capabilities of the Peng-Robinson (PR) equation of state (EOS) and WATAM. WATAM provided a much better correlation of saturated liquid densities than the PR EOS. Although both models adequately correlated liquid phase compositions, WATAM provided a better fit of the near-critical vapor phase data. The Peng-Robinson EOS consistently overestimated the mixture critical pressures. Several correlations, based on different objective functions, for the temperature-dependent binary interaction parameter were developed. Slight improvements in the predictive capabilities of the PR EOS were realized using the Panagiotopolous and Reid composition-dependent mixing rule, which required two temperature-dependent parameters. Temperature-dependent correlations for each parameter were regressed from PTxy data. Guidelines for using the PR EOS in a process simulator to model Kalina cycles were developed.