The ammonia-water mixture (AWM) has been successfully used in many absorption refrigeration systems (ARS). AWM has played an important role in the use of heat pumps or as ARS applied or coupled to diverse external systems, such as distillation towers ^[1], Kalina cycles, which have higher pressures than a conventional system ^[2,3], air conditioning systems, and combustion air cooling for gas turbines and streams in extractive industries such as coal and petroleum ^[4]. Most of the design and analysis of ARS until a little after the first half of the last century was based on graphical predictions. However, the range of pressures was small, and based only on few experimental data points and theoretical-empirical deductions ^[5,6,7].

The precise thermodynamic properties of the ammonia-water mixture are very important for the calculations in the design of ARS equipment and the prediction of their behavior; this also applies to the operation of existing systems and their possible improvements. In the past few decades, the analysis of these systems has covered their performance based on both the first and the second law of thermodynamics; this analysis involves the use of the specific entropy value, which is fundamental to improve the prediction and optimization of cycle performance ^[1,3,8,42]. The more than forty-five published methods for predicting the thermodynamic properties of AWMs have been classified into seven main groups according to Thorin et al. ^[9]. In these groups, there are combinations of cubic equations of state, excess Gibbs energy, corresponding states, perturbation theory, group contributions, and polynomial functions. Other developed methods are the Helmholtz free energy ^[14], the PC-SAFT equation ^[26], neural networks ^[19,27], and others that mix two or more groups ^{[10-13,15-18,20-25,28-30]}.

Due to the above, in most cases, the precise estimation of the water concentration in the generator exit (desorber and rectifier) results in over or under design of these components ^[5,31]. The disadvantage of some models is the restricted range over which they are valid, and in other cases is the lack of entropy prediction ^[32]. Currently, due to the need of predicting exergy balances, the cubic equations of state, which are incorporated in commercial process simulators for such systems analyses, have been used ^[1,8]. However, because the mixture in question is highly non-ideal, these equations of state lack the necessary adjustments to accurately predict its properties at ammonia concentrations close to unity. For this reason, many researchers have taken on the task of modifying or generating specialized equations. Nevertheless, there are few models with a wide range of applicability ^[9,26,33].

Here, an ARS has been evaluated according to both the first and the second law of thermodynamics, and the energy and irreversibility, with the aim of finding differences between three mathematical models for predicting thermodynamic properties ^[14,18,35]. A base case from the literature ^[34], in which the thermodynamic properties were calculated via a mathematical model based on Gibbs free energy, was employed to compared to the results obtained from the three methods herein considered.

We used three different methods to obtain the thermodynamic properties of the operating conditions given in the proposed base case ^[34]. M1, a model developed by Ibrahim and Klein ^[35], is implemented in the commercial program Engineering Equation Solver (EES); M2, proposed by Tillner-Roth and Friend ^[14], is implemented in REFPROP v.8.0 developed by the National Institute of Standards and Technology (NIST); and M3, proposed by Xu and Goswami ^[18], was developed from work by Ibrahim and Klein, El-Sayed and Tribus, and Ziegler and Trepp ^[35-38,40]. To carry out this analysis, the latter methodology ^[18] was programmed and validated ^[5,12,39,43].

The analysed ARS ^[34] consisted of a simple single effect ammonia-water cycle with a solution heat exchanger and a pre-cooler (super heater), which had fourteen streams and ten components (Fig. 1).

FIG. 1

The initial conditions and mass and energy balance equations for the corresponding analyses under the first and second laws of thermodynamics are given in tables 1 and 2.

Table 1

*Where α and β are random constants; x is the mass fraction of ammonia in the corresponding stream ^[41].

Table 2

The matrix shown in Fig. 2 was used to solve the system mass, energy and exergy balances with vectors corresponding to the streams in the system.

FIG. 2

The values of the properties calculated by the three methods (M1, M2 and M3) compared to the base case showed significant differences for only some of the streams, and particularly for the pressure prediction in the case of nearly pure ammonia in the vapour phase. However, when the overall system heat and mass balances were analysed, the differences were small, and in particular, the coefficient of performance was reasonably consistent. When the second law analysis was considered, differences were found in the exergy values for the process streams calculated by the models used in this study. The orders of magnitude of the percentage contributions to the overall irreversibility of the system were consistent among models; that is, desorber, absorber, and condenser were the components with the greatest irreversibilities. Notwithstanding, the observed differences in the overall irreversibility were the same in all cases.