Thermodynamics of Pure Dipolar Fluids. 1. The Water and Ammonia Cases

Abstract

A molecular-based equation of state derived from perturbation theory is used to study the phase diagram and thermodynamic properties of water and ammonia in the fluid phase region. The molecular model that represents these substances consists of spherical particles interacting via a square-well potential with an embedded point dipole. The equation of state is an analytical expression, which depends explicitly on density, on temperature, and on the four adjustable parameters of the potential model: the particle's diameter, the energy depth and range of the square-well interaction, and the dipolar moment strength. Although the associating behavior due to hydrogen bonding that characterizes water and ammonia is not modeled by the perturbation approach followed in this paper, the theory is able to predict (except in the critical region) the vapor-liquid phase diagram and the saturation pressures, with accuracy close to that of theories that include a modeling of the H-bonding effects. A detailed comparison of experimental data from our theory and from three different approaches based on the Wertheim perturbation theory for associating fluids is presented. This comparison indicates that none of the later theories is able to give an accurate prediction within experimental error of the phase diagram of water, for the whole region of densities and temperatures for the fluid phase. This paper presents the ranges of values for density and temperature where each theory is accurate. Since the theory presented gives comparable predictions to those of the other equations, the dipolar square-well potential used in this work could be considered a good effective potential for associating polar fluids if the dipolar moment of the substance is taken slightly higher than its real value, an indication that by increasing the dipolar moment strength it is possible to mimic, at least in relation to thermodynamic properties, the H-bonding effects. The simplicity of this model can be useful as an important ingredient in the building of better equations of state for polar associating fluids.

Perturbed Chain-Statistical Associating Fluid Theory Extended to Dipolar and Quadrupolar Molecular Fluids

“…However, for both fluids, the model overpredicted the critical point significantly. In the same spirit, a dipolar square-well theoretical EoS (DSWTEOS) and a unique octopolar square-well theoretical EoS (OSWTEOS) were proposed. , Hydrogen bonding exhibited by water and ammonia molecules was accounted implicitly in DSWTEOS by using increased values for the dipolar moments of the two fluids compared to the experimental values (by 13 and 26%, respectively). A similar increase in the dipole moment of methanol was assumed by Twu and Gubbins when modeling the liquid−liquid immiscibility of the methanol/carbon dioxide/ethane mixture justified by the neglect of higher-order electrostatic terms in methanol …”

“…A number of theoretical models have been proposed over the last three decades for polar fluids based on perturbation theory. [3][4][5][6][7][8][9][10][11][12] Larsen et al 3 proposed a simple Pade ´approximant to calculate polar interactions in spherical fluids by using the second-and third-order perturbation terms accounting combined for two-body and three-body contributions. Gubbins and Twu 4 further extended the theory to polyatomic fluids, and Vimalchand and Donohue 5 incorporated this theory into the perturbed anisotropic chain theory.…”

Predicting the Phase Behavior of Nitrogen + n-Alkanes for Enhanced Oil Recovery from the SAFT-VR Approach: Examining the Effect of the Quadrupole Moment

“…Of particular relevance to the proposed work is that of Benavides and co-workers, − who developed a perturbation theory for pure polar fluids, the so-called multipolar square-well theoretical equation of state (MSWTEOS), that is based on an accurate equation of state for square-well fluids due to Gil-Villegas et al and Stell's Padé expression for the polar contribution to the Helmholtz free energy. This theory has been successful in describing the vapor- and liquid-phase properties of pure systems such as methane, nitrogen, water, and carbon dioxide. − In this work, we extend the SAFT-VR equation by incorporating a quadrupolar contribution into the expression for the Helmholtz free energy that is based on an extension of the framework developed in the MSWTEOS to mixtures of chain molecules. The modified SAFT-VR equation obtained is then applied to study the phase behavior of nitrogen and its binary mixtures with the n - alkanes.…”

“…This theory has been successful in describing the vapor-and liquid-phase properties of pure systems such as methane, nitrogen, water, and carbon dioxide. [59][60][61] In this work, we extend the SAFT-VR equation by incorporating a quadrupolar contribution into the expression for the Helmholtz free energy that is based on an extension of the framework developed in the MSWTEOS to mixtures of chain molecules. The modified SAFT-VR equation obtained is then applied to study the phase behavior of nitrogen and its binary mixtures with the n-alkanes.…”

Ανάπτυξη Και Αξιολόγηση Ενός Θερμοδυναμικού Μοντέλου Βασισμένου Στη Θεωρία Διαταραχών Για Συστήματα Πολικών Μορίων: Καθαρά Συστατικά Και Πολυσυστατικά Μίγματα

“…PR αδυνατούν να προβλέψουν την θερμοχωρητικότητα της μεθανόλης και διαφόρων φθοριωμένων υδρογονανθράκων[126], ενώ και η θεωρητική Κ.Ε. τετραγωνικού φρέατος SWTEOS παρέχει προβλέψεις που αποκλίνουν σημαντικά (περίπου 35%) από τις πειραματικές τιμές της ισόχωρης θερμοχωρητικότητας συμπιεσμένου υγρού διοξειδίου του άνθρακα στο εύρος πιέσεων (7-32) MPa[65]. Εξαίρεση αποτελεί η Κ.Ε.…”