Ulrich K. Deiters

Systematic investigation of the phase behavior in binary fluid mixtures. I. Calculations based on the Redlich–Kwong equation of state

The original Redlich–Kwong equation, together with the usual quadratic mixing rules, has been used to calculate phase diagrams for binary fluid mixtures and to classify them according to the system of van Konynenburg and Scott. Global phase diagrams (maps) showing the extent of the various phase diagram classes in the space of the Redlich–Kwong parameters are presented. While for molecules of equal size the results are very similar to those known for the van der Waals equation, the maps become topologically different for molecules of unequal sizes; some complicated phase diagram classes, which otherwise cover small domains on the maps or cannot be found at all, become quite important.

A new semiempirical equation of state for fluids—I: Derivation

Abstract A semiempirical equation of state has been developed for non-polar and weakly polar fluid substances from the square well model of the inter-molecular

Systematic investigation of the phase behavior in binary fluid mixtures. II. Calculations based on the Carnahan–Starling–Redlich–Kwong equation of state

Phase diagrams of binary fluid mixtures have been calculated from the Carnahan–Starling–Redlich–Kwong equation of state in connection with standard quadratic mixing rules. The phase diagrams were classified according to the system of van Konynenburg and Scott and then used to construct global phase diagrams showing the extent of the various phase diagram classes in the space of the parameters of the equation of state. For molecules of equal size, the global phase diagram is rather similar to that of the Redlich–Kwong or the van der Waals equation. For molecules of different sizes, however, a new tricritical line appears. Such a behavior is observed for cubic equations of state only if nonadditive covolumes are assumed. Along this new tricritical line, some unusual phase diagrams involving four‐phase states and high‐density instabilities can be found. The influence of molecular size ratios on the global phase diagrams and the relationship of the equation of state of this work to the ternary symmetric lattice gas and the van der Waals lattice gas are discussed.

Excess enthalpies for (ethanol + water) at 298.15 K and pressures of 0.4, 5, 10, and 15 MPa

Abstract The design and construction of a modified isothermal-flow calorimeter with a reproducibility of better than 0.5 per cent is described. This apparatus was used to measure H m E for (ethanol + water) at 298.15 K and pressures of 0.4, 5, 10, and 15 MPa. The 0.4 MPa values are in excellent agreement with published values at atmospheric pressure. A fitting equation was developed which gives a good fit of the H m E results over the composition and pressure ranges investigated. We propose that H m E measurements be made on (ethanol + water) at high temperatures and pressures to establish it as a reference system for testing calorimeters. Agreement of our results with existing literature values suggests that it could now be used for this purpose at 298.15 K and atmospheric pressure.

Application of the Taylor dispersion method in supercritical fluids

This paper describes some of the experimental and theoretical problems encountered when the Taylor dispersion method is applied to the measurement of diffusion coefficients near gas-liquid critical points. We have used our own measurements of diffusion of benzene and toluene in supercritical carbon dioxide, along with measurements from several other sources, to illustrate some of the experimental challenges. Special attention is given to the peak shape. The intercomparisons are greatly simplified by comparing the experimental data as functions of density, rather than pressure. We find large and unexplained discrepancies between the various experimental sources. We discuss the theoretical predictions for the relationships between the diffusion coefficients and diffusivities obtained from Taylor dispersion and dynamic light scattering in fluids near critical points. We conclude that there is no strong reason to press for Taylor dispersion measurements near the gas-liquid critical point of the carrier gas.

Nomenclature for phase diagrams with particular reference to vapour–liquid and liquid–liquid equilibria (Technical report)

The phase diagrams of binary fluid mixtures are classified with regard to the topology of their critical curves and three-phase curves. A new nomenclature for the phase diagram classes is proposed, which can be applied to the previously known as well as to recently discovered phase diagram classes. The class names of the new nomenclature are systematic and descriptive; the (P,T) projection of a binary fluid phase diagram can always be constructed qualitatively from the class name. INTRODUCTION Phase equilibria in binary systems are usually depicted graphically in pressure, temperature, composition (P,T,x) space, in (P,T) and (T,x) projections or in isothermal (PJ) sections, isobaric (TJ ) sections and in constant composition (P,T) sections, so called isopleths. In these diagrams the different types of phase equilibria are represented by areas, curves or points. Some of these equilibrium states are characterized by special values of thermodynamic variables and will be referred to as special states. In the scope of this work a special state or point is defined as a state or point where 1. Either a phase is added or disappears. 2. Where the composition of two phases become identical. The composition of two phases can become identical in three different ways (Rowlinson & Swinton, 1982). The curves representing the composition of two coexisting phases can intersect at x = 0 or x = 1. This is shown in Fig. la, which shows a vapour-liquid equilibrium gl where the curve that represents the composition of the liquid phase 1 intersects the curve for the vapour phase g in the boiling points of both pure components. Also these curves can merge in a horizontal tangent point (dT/dx), = 0 or ( d P / d ~ ) ~ = 0. This type of special point is a critical point, where not only the composition but all thermodynamic properties of the two phases become identical. A diagram with a critical point is shown in Fig. lb. The third possibility is that the two curves have a common horizontal tangent. In this case only the compositions of the two phases are equal, but not the other thermodynamic properties. An example is an azeotropic point (Fig. lc).

High pressure phase equilibria: experimental methods

Abstract The experimental investigation of high pressure phase equilibria, especially the measurement of properties of coexisting phases, is not only of fundamental scientific interest, but is also an intriguing and fa s cinating technical problem. This work reviews some recently developed experimental methods and classifies them with respect to the observed thermodynamic properties. Several devices and experimental procedures for the determination of phase equilibria are briefly explained. The advantages and limitations of these different methods are discussed.

Prediction of the thermophysical properties of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton by Monte Carlo simulation using ab initio potentials

Gibbs ensemble Monte Carlo simulations were used to test the ability of intermolecular pair potentials derived ab initio from quantum mechanical principles, enhanced by Axilrod-Teller triple-dipole interactions, to predict the vapor-liquid phase equilibria of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton. The interaction potentials for Ne-Ne, Ar-Ar, Kr-Kr, and Ne-Ar were taken from literature; for Ar-Kr a different potential has been developed. In all cases the quantum mechanical calculations had been carried out with the coupled-cluster approach [CCSD(T) level of theory] and with correlation consistent basis sets; furthermore an extrapolation scheme had been applied to obtain the basis set limit of the interaction energies. The ab initio pair potentials as well as the thermodynamic data based on them are found to be in excellent agreement with experimental data; the only exception is neon. It is shown, however, that in this case the deviations can be quantitatively explained by quantum effects. The interaction potentials that have been developed permit quantitative predictions of high-pressure phase equilibria of noble-gas mixtures.

Closed-loop critical curves in simple hard-sphere van der Waals-fluid models consistent with the packing fraction limit

Two new hard-sphere equations are proposed which, in combination with a van der Waals attraction term, lead to a biquadratic, respectively a cubic, equation of state. The new equations show the correct limiting behavior at low as well as at high densities; their poles are close to the physical packing fraction of hard spheres. Both equations of state were extended towards mixtures by one-fluid mixing rules, and their global phase behavior was investigated for the special case of equal-sized molecules. Both equations are able to predict closed-loop liquid–liquid immiscibility; the topology of the phenomenenon is the same as for the Carnahan–Starling equation. It appears the occurrence of closed-loop liquid–liquid immiscibility does not depend on the location of the pole nor on the degree of the equation of state used.

A new semiempirical equation of state for fluids—II: Application to pure substances

Abstract The semiempirical equation of state proposed in our previous paper is applied to the calculation of thermodynamic properties of pure fluid substances.