UH-20-10 Comparison of Models of NH3-CO2-H2O Mixture

In the past several attempts have been made to fit experimental data of the mixture NH3-CO2-H2O making use of thermodynamic models. The most commonly used model for process calculations is the electrolyte NRTL model. The model is available in the Aspen Plus software (version 8.8) both in its original form as well as modified versions where parameters have been fitted especially for the NH3-CO2-H2O system. In this paper these models and a new fit are compared to the Extended UNIQUAC model.

The NH3-CO2-H2O mixture has been identified as a possible working fluid for compression-resorption heat
pumps (CRHP). The advantages of CRHP for low grade waste heat recovery compared to alternative technologies have been reported by Van de Bor et al. (2015). In the research the working fluid for the CRHP
was ammonia water. Preliminary calculations suggest that adding small amount of CO2 to the system can
increase the coefficient of performance (COP) of the heat pump. To further investigate the performance of
the CRHP with NH3-CO2-H2O mixture as a working fluid an accurate thermodynamic model is needed. Comparison of the models shows a clear improvement of the modified models over the original e-NRTL model. The better fitted models compare well to the extended UNIQUAC model which gives good results for its application range.

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