Absorption Refrigeration Cycles with Ammonia-Ionic Liquid Working Pairs Studied by Molecular Simulation

Abstract

For absorption refrigeration, it has been shown that ionic liquids have the potential to replace conventional working pairs. Due to the huge number of possibilities, conducting lab experiments to find the optimal ionic liquid is infeasible. Here, we provide a proof-of-principle study of an alternative computational approach. The required thermodynamic properties, i.e., solubility, heat capacity, and heat of absorption, are determined via molecular simulations. These properties are used in a model of the absorption refrigeration cycle to estimate the circulation ratio and the coefficient of performance. We selected two ionic liquids as absorbents: [emim][Tf₂N], and [emim][SCN]. As refrigerant NH₃ was chosen due to its favorable operating range. The results are compared to the traditional approach in which parameters of a thermodynamic model are fitted to reproduce experimental data. The work shows that simulations can be used to predict the required thermodynamic properties to estimate the performance of absorption refrigeration cycles. However, high-quality force fields are required to accurately predict the cycle performance.

Figure 1

(a) Schematic diagram of a single-effect absorption refrigeration cycle. (b) ln(P)–1/T diagram of the same absorption refrigeration cycle. Q_GEN, Q_CON, Q_ABS, and Q_EVA are respectively the transferred heat at the generator, the condenser, the absorber, and the evaporator. T_GEN, T_CON, T_ABS, and T_EVA are the corresponding temperatures, and P_CON and P_EVA the pressures. ṁ_s and ṁ_r are the mass flow rates of the strong NH₃ solution and of the refrigerant, respectively.

Figure 2

Representation of the simulated system consisting of NH₃, [emim]⁺, and [Tf₂N]⁻. Exemplary, the molecules are marked by dashed lines. White, red, gray, purple, yellow, and green spheres represent hydrogen, oxygen, carbon, nitrogen, sulfur, and fluorine atoms, respectively.

Figure 3

Computed NH₃ solubilities (blue/green/cyan/magenta) in (a) [emim][Tf₂N] and (b) [emim][SCN], compared to solubilities calculated with the NRTL model (black), experimental data (○), and simulation results of Shi and Maginn (red) at 308.15 (▼), 347.15 (●), 373.15 (■), and 393.15 K (⧫). The determined standard error is smaller than the size of the symbols.

Figure 4

Comparison between computed total heat capacities (blue), computational results of Tenney et al. (black), and experimental measurements of Ge et al. (green), Paulechka et al. (cyan), Ferreira et al. (magenta), Navarro et al. (purple), and Ficke et al. (orange) for (a) [emim][Tf₂N] and (b) [emim][SCN].

Figure 5

Comparison between f values calculated with NH₃ solubilities from MC simulations (colors) and from the NRTL model (black) for [emim][Tf₂N] (●) and [emim][SCN] (■). The cycle conditions in this work are T_CON = 35 °C, T_ABS = 30 °C, T_EVA = 10 °C, T_GEN = 74–120 °C, P_EVA = 6.15 bar, and P_CON = 13.5 bar.

Figure 6

Comparison between the COP values calculated from simulations (colors) and from the NRTL/EoS model (black) for [emim][Tf₂N] (●) and [emim][SCN] (■). The cycle conditions in this work are T_CON = 35 °C, T_ABS = 30 °C, T_EVA = 10 °C, T_GEN = 74–120 °C, P_EVA = 6.15 bar, and P_CON = 13.5 bar.