Excess enthalpy calculations by means of equations of state

Abstract

The suitability of existing equations of state for the prediction of heat of mixing data is invest igated.Classical cubic equations of state, such as those of Redlich-Kwong and Peng-Robinson, are compared with the more theoretically based equation developed by Carnaham and Starling. Calculations are reported for exothermic and endothermic systems.It is shown that equations of state can be used to describe heat of mixing data and that the Carnaham-Starling repulsive term seems to give better results than the other models invest igated.The calculation of thermodynamic properties for mixtures has been investigated by many authors.Christensen [1] and Nagata [2] paid particular attention to the correlation and the prediction of heat of mixing data from vapour-liquid equilibrium (VLE) data: they used different semiempirical excess Gibbs energy models to take into account the nonideality of the liquid phase.All the existing excess Gibbs energy models need two or more adjustable parameters obtained from the fitting of experimental data: in order to represent excess enthalpy data, these parameters have to be temperature-dependent since the solution theories on which the models are based do not take into account explicitly a temperature-dependence for the interaction energy. All the temperature-dependent expressions are therefore essentially empirical (linear or quadratic) and have the effect of increasing the number of parameters in the models (4 or 6 in some cases).Unfortunately, this is not the only limitation in the excess Gibbs energy models: none of the theories for the liquid state has the basic physical meaning necessary to describe the behaviour of different kinds of liquid mixtures.We therefore turned our attention to thermodynamic models which do not suffer from these basic weaknesses: the equations of state. Many authors are investigating equations of state in order to apply them to high and low-pressure phase equilibria calculations [3][4]. In fact, the thermodynamic models based on an equation of state

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

“…However, it is known that it is problematic to describe the phase behavior of polar nonvolatile compounds such as ILs with a cubic EoS. 81 Hence, the enthalpies of absorptions predicted from molecular simulations might be more reliable.…”

“…In addition, the EoS predicts positive heats of absorption for [emim]­[SCN] and temperatures higher than 373 K. Without experimental data, it is difficult to comment on the accuracy of the obtained data and further experiments are necessary. However, it is known that it is problematic to describe the phase behavior of polar nonvolatile compounds such as ILs with a cubic EoS . Hence, the enthalpies of absorptions predicted from molecular simulations might be more reliable.…”

Prediction of Excess Enthalpy Using Volume-Translated Peng–Robinson Equation of State

“…Excess enthalpies (h E ) can then be calculated with equations of state using the so-called residual enthalpies (h − h id ) of mixture and pure components (Formula (15)) [5,17].…”