Equations of state based on intermolecular potentials are often developed about the Lennard-Jones (LJ) potential. Many of such EOS have been proposed in the past. In this work, 20 LJ EOS were ...examined regarding their performance on Brown’s characteristic curves and characteristic state points. Brown’s characteristic curves are directly related to the virial coefficients at specific state points, which can be computed exactly from the intermolecular potential. Therefore, also the second and third virial coefficient of the LJ fluid were investigated. This approach allows a comparison of available LJ EOS at extreme conditions. Physically based, empirical, and semi-theoretical LJ EOS were examined. Most investigated LJ EOS exhibit some unphysical artifacts.
An isoplethic phase envelope is a locus of two-phase equilibria where one of the phases is kept at constant composition. The thermodynamic conditions for such a phase envelope are expressed as a set ...of first-order ordinary differential equations in which molar concentrations appear as central variables instead of mole fractions. The resulting formalism is applicable to multicomponent mixtures, well-suited for machine calculations, and allows a very rapid calculation of phase envelopes with arbitrary equations of state.
Application to the formalism to pure fluids yields simple differential equations that are analoga of Clapeyron's equation, but permit the calculation of orthobaric densities.
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•Differential equations of isoplethic phase envelopes.•Applicable to multicomponent systems.•Can be rapidly integrated; avoids convergence problems or internal volume calculations.•For pure fluids: density-based Clapeyron equation (gives orthobaric densities).
The role of interatomic interactions on the solid–liquid and vapor–liquid equilibria of neon is investigated via molecular simulation using a combination of two-body ab initio, three-body, and ...quantum potentials. A new molecular simulation approach for determining phase equilibria is also reported and a comparison is made with the available experimental data. The combination of two-body plus quantum influences has the greatest overall impact on the accuracy of the prediction of solid–liquid equilibria. However, the combination of two-body + three-body + quantum interactions is required to approach an experimental accuracy for solid–liquid equilibria, which extends to pressures of tens of GPa. These interactions also combine to predict vapor–liquid equilibria to a very high degree of accuracy, including a very good estimate of the critical properties.
A novel, particularly robust method for the calculation of critical curves of fluid mixtures is proposed that makes use of differential equations representing the critical conditions (isochoric ...thermodynamics formalism). These differential equations are integrated with adaptive numerical integration methods, thus avoiding the convergence problems that so often afflict methods using algebraic equations. The novel method can be used with all Helmholtz energy-explicit equations of state, including models that can return unphysical results when applied to thermodynamic states within a two-phase region, for example, the GERG equations of state. In combination with the “parametric marching” technique, the new approach is able to follow critical curves of arbitrary shape. The Supporting Information provides an implementation of this approach for the GERG-2008 and Peng–Robinson models in the Python language.
Power generation from low-temperature heat sources (80–300 °C) like thermal solar, geothermal, biomass or waste heat has been becoming more and more significant in the last few decades. Organic ...Rankine Cycle (ORC) uses organic working fluids, obtaining higher thermal efficiency than with water used in traditional Rankine Cycles, because of the physical (thermodynamic) properties of these fluids. The traditional classification of pure (one-component) working fluids is based on the quality of the expanded vapour after an isentropic (adiabatic and reversible) expansion from saturated vapour state, and distinguishes merely three categories: wet, dry and isentropic working fluids. The purpose of this paper is to show the deficiencies of this traditional classification and to introduce novel categorisation mostly to help in finding the thermodynamically optimal working fluid for a given heat source.
•The need for a refined working fluid classification (beyond the classical categories wet/isentropic/dry) is demonstrated.•A novel classification based on characteristic points is introduced.•Potential technical applications for the new classification are presented.•Categories and characteristic points for 57 pure working fluids are provided.
Calculation of thermodynamic phase equilibrium is error-prone and can fail both near the critical point and at very low temperatures because of the limited precision available in double precision ...arithmetic. Most importantly, these calculations frequently represent a computational bottleneck. In this work, we extend the “superancillary” equation approach developed for reference multiparameter equations of state to classical cubic equations of state (van der Waals, Redlich–Kwong–Soave, Peng–Robinson). Iterative calculations in double precision are replaced by noniterative evaluation of prebuilt Chebyshev expansions constructed with extended precision arithmetic. Exact solutions for the equation of state constants are given. The Chebyshev expansions are shown to reproduce the equation of state values to within nearly double precision (aside from in the very near vicinity of the critical point) and are more than 40 times faster to evaluate than the VLE calculations from the fastest computational library. In this way we further expand the domains in which iterative calculations for pure fluid phase equilibria may be rendered obsolete. A C++ header implementing these expansions (and with no external dependencies) is provided as deposited data.
The thermodynamic conditions for isothermal or isobaric two-phase boundaries of fluid mixtures are expressed as sets of first-order ordinary differential equations. In contrast to the Gibbs–Konovalov ...equations, the new equations use molar densities instead of mole fractions. The resulting formalism is applicable to multicomponent mixtures, well-suited for machine calculations, and allows a very rapid calculation of phase diagrams.
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•Rapid calculation method for two-phase equilibria in mixtures.•Applicable to multicomponent mixtures.•Equilibrium conditions expressed as ordinary differential equations.•Using density-based formalism.
The ability of modern ab initio potentials to predict the thermophysical properties of helium is investigated. A new interatomic potential for helium is reported that is based on the latest available ...ab initio data and that is much more computationally efficient than other ab initio potentials, without sacrificing accuracy. The role of both two-body and three-body interactions is evaluated using classical Monte Carlo and molecular dynamics simulations. Data are reported for the second virial coefficient, vapor–liquid equilibria, acentric factor, compressibility factor, enthalpy, speed of sound, and isobaric heat capacity. Three-body interactions have a minor influence on the properties of helium with the exception of the estimated critical properties. The influence of quantum particle behavior is relevant at temperatures typically below 200 K. For example, the experimental maximum in the isobaric heat capacities (along isobars) of helium is not observed in the classical simulations and can be attributed to quantum particle behavior. However, above this temperature, helium behaves like a classical fluid and its thermodynamic properties can be adequately predicted by determining only two-body interactions.
In this work, we demonstrate that tracing the vapor–liquid equilibria for binary mixtures also includes all the information needed to obtain the three-phase line if one is present. This simple ...observation serves as a launching point for calculation of the liquid–liquid equilibrium and three-phase equilibria, even when one component is supercritical. We close with the demonstration of the automatic construction of an isothermal pressure–composition figure, a capability enabled by solving for the three-phase state. The scripts in the Supporting Information demonstrate in detail how the method is applied.
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