Thermodynamic modelling of dissociating Al₂Br₆ and Al₂Cl₆ used as reactive working fluids in thermodynamic cycles

Abstract

To enhance the efficiency of thermodynamic cycles in heat pumps and power plants, we explore a novel approach: replacing conventional inert pure fluids or mixtures with reactive fluids that undergo reversible chemical reactions. A key step towards the implementation of this concept is the development of a fully predictive framework for determining the thermodynamic properties of such reactive working fluids. In this context, the present work extends a semi-empirical methodology previously proposed by the authors, aiming to address the challenge introduced by newly developed reactive fluids for which experimental data are unavailable. The methodology presented in this work requires only the critical-point properties and acentric factor of the molecules participating in the chemical reaction. As in the earlier approach from the authors, it combines ab-initio quantum mechanics calculations to determine the ideal gas properties of each molecule, the a-thermal version of the “Peng-Robinson + EoS/a_res^E,γ mixing rules” equation of state and molecular Monte Carlo simulations to assess real fluid properties and enable cross-validation between methods. This work, however, applies a simplification to the force fields used in Monte Carlo simulations consisting in employing single-particle force fields instead of all-atom models. This strategy decreases the amount of experimental data required to parametrise the force field of each molecule contained in the reactive mixture, and allows the use of the same inputs in equation of state modelling and Monte Carlo simulations (i.e., molecular critical parameters). Indeed, this work proposes to calculate force field parameters using either the critical temperature and pressure, or the critical temperature and density of each molecule. The methodology is applied to two reactive systems, Al₂Br₆ ⇌ 2AlBr₃ and Al₂Cl₆ ⇌ 2AlCl₃. The results show that Monte Carlo predictions, although less accurate than those from the equation of state, remain acceptably close to experimental data, while the equation of state results demonstrate significantly higher accuracy.