This article addresses some recurring difficulties and problems of computing phase equilibria involving supercritical fluid phases. These difficulties prevent a full automatization of thermodynamic ...calculations and require human interference. Examples are the wrong interpretation of experimental data, phase inversion phenomena, or overlooking phases. While none of these insights are knew, publications dealing with them are scattered and sometimes hard to obtain. This article gives a short overview over some of the most common difficulties and pitfalls.
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► Model-free estimation of K factors at infinite dilution. ► Useful for interpolation of phase boundary curves. ► Simple consistency test for isothermal phase diagrams. ► Special ...behaviour of phase boundary curves for limiting azeotropy.
The extended van’t Hoff equations for dilute mixtures relate the differences of initial slopes of phase boundary curves to the phase transition enthalpy or volume of the solvent. While the isobaric van’t Hoff equation is frequently used in mixture thermodynamics, its isothermal counterpart has received much less attention. We show that the isothermal van’t Hoff equation can be used to interpolate dew point curves in isothermal phase diagrams or to create a model-free consistency test for experimental phase equilibrium data. Furthermore, an expression for the initial curvature of phase boundary curves is derived.
We report first-principles calculations of the triple point that allow us to predict the triple point temperature of atomic fluids to an accuracy that has not been previously possible. This is ...achieved by proposing a molecular simulation technique that can be used for solid-liquid equilibria at arbitrarily low pressures. It is demonstrated that the triple point is significantly influenced by the choice of two-body, three-body and quantum interactions. An improved theoretical understanding of triple points is important for both science in general, and metrology in particular, as it links the Boltzmann constant and the Kelvin temperature scale to fundamental constants.
The Characteristic Curves of Water Neumaier, Arnold; Deiters, Ulrich K.
International journal of thermophysics,
09/2016, Letnik:
37, Številka:
9
Journal Article
Recenzirano
In 1960, E. H. Brown defined a set of characteristic curves (also known as ideal curves) of pure fluids, along which some thermodynamic properties match those of an ideal gas. These curves are used ...for testing the extrapolation behaviour of equations of state. This work is revisited, and an elegant representation of the first-order characteristic curves as level curves of a master function is proposed. It is shown that Brown’s postulate—that these curves are unique and dome-shaped in a double-logarithmic
p
,
T
representation—may fail for fluids exhibiting a density anomaly. A careful study of the Amagat curve (Joule inversion curve) generated from the IAPWS-95 reference equation of state for water reveals the existence of an additional branch.
Cubic equations of state are still much used in chemical engineering. Inverting such an equation, i.e., calculating molar volumes or densities for given pressure and temperature, is an important and ...frequently invoked operation. A new algorithm is proposed, whichon modern computersusually performs faster than other algorithms from the literature.
Superancillary equations are presented for the PC-SAFT equation of state of Gross and Sadowski for nonpolar pure fluids. The equations cover the range of a number of segments from 1 to 64. These ...equations, formed as nested sets of Chebyshev expansions, represent the densities for vapor–liquid equilibria better than can be achieved by iterative calculations in double precision arithmetic. Furthermore, the expressions are more than 16 times faster to evaluate than the iterative calculations from the fastest thermodynamic property library. By their construction, the superancillary equations are guaranteed to be reliable over the entire temperature range. For user-friendliness, the functions have been packaged into a Python package available on PYPI.
In this work, we have been carried out GEMC-NVT simulations in the temperature range 18 K–32 K for fluid hydrogen and in range 60 K–140 K for fluid fluorine using four our developed
5-site ...intermolecular potentials for dimers H
-H
and F
-F
, respectively. The thermodynamic properties of vapor-liquid equilibria and the critical points of fluids hydrogen and fluorine were calculated with the obtained densities of coexisting phases and vapor pressures. The simulation results drived from
pair potentials were compared with those from
potential plus three-body Axilrod-Teller potential and experimental data as well as those from Monte Carlo simulation using Lennard-Jones potentials, Deiters equation of state (D1-EOS) and Benedict-Webb-Rubin equation of state (EOS) reported in the literature.
The computation of distances between interactions sites while observing the minimum image convention is one of the most frequently performed operations in computer simulations of fluids. In this work ...several ways of encoding the minimum image convention are discussed and their performances on different computers compared. It turns out that the optimal choice for the algorithm depends on the computer type as well as the optimization level of the compiler, and that a well adapted algorithm can achieve a significant reduction of the computation time.
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► Simulation of DSC/transitiometer experiments. ► Prediction of apparent heat capacities by means of equations of state. ► Calorimetric responses of high-pressure phase equilibria of ...fluid mixtures. ► Demonstration of “negative signals”.
Equations of state can be used not only for the calculation of fluid phase equilibria, but also for the prediction of the accompanying changes of the internal energy or enthalpy. In this work, traces of DSC (differential scanning calorimetry) or transitiometry experiments on some fluid model systems at high pressures are simulated and discussed.
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