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Formaldehyde is a highly reactive chemical that is usually sold and processed in the form of aqueous solutions, with methanol added for stability. In these solutions, formaldehyde reacts with the solvents to form a variety of reaction products, including oligomers. These chemical reactions can occur in the liquid and vapour phases and have a significant influence on the properties of formaldehyde-containing solutions. Of particular interest to industrial applications is the prediction of the vapour–liquid equilibria (VLE) in formaldehyde solutions, considering the chemical reactions. We use the SAFT-γ Mie group-contribution (GC) equation of state to obtain the fluid-phase behaviour of binary and ternary mixtures of formaldehyde with water and methanol. The oligomerisation reactions taking place in aqueous and methanolic solutions of formaldehyde are modelled implicitly using a physical approach, which is possible within the SAFT-γ Mie framework by adding association (reactive) sites that mediate the formation of the reaction products. Using this approach, the nature of the chemical speciation in formaldehyde + water, formaldehyde + methanol and formaldehyde + water + methanol mixtures is studied. A new group, CH 2 O, characterising formaldehyde within the SAFT-γ Mie GC approach, is developed. Experimental data for the VLE in binary mixtures of formaldehyde + water and formaldehyde + methanol are used to obtain the optimal unlike interaction parameters between the corresponding SAFT-γ Mie groups. The newly developed parameters are used to predict the VLE of ternary formaldehyde + water + methanol mixtures for a wide range of temperatures and pressures, with excellent agreement to experimental data. Additionally, the SAFT-γ Mie approach is shown to provide accurate predictions of the distribution of reaction species (oligomers) in binary and ternary mixtures containing formaldehyde. [ABSTRACT FROM AUTHOR]

Examines the crossover region from the swollen region to the semidilute regime using polymer chains of degree up to N=2000 via Monte Carlo simulation. Analysis of chain end-to-end distance as a function of the number of chain links expressed in terms of their connectivity; Use of scattering functions to study structure of polymer chains.

A methodology for obtaining molecular parameters of a modified statistical associating fluid theory for variable-range interactions of Mie form (SAFT-VR Mie) equation of state (EoS) from ab initio calculations is proposed for non-associative species that can be modeled as single spherical segments. The methodology provides a strategy to map interatomic or intermolecular potentials obtained from ab initio quantum-chemistry calculations to the corresponding Mie potentials that can be used within the SAFT-VR Mie EoS. The inclusion of corrections for quantum and many-body effects allows for an excellent, fully predictive description of the vapor–liquid envelope and other bulk thermodynamic properties of noble gases; this description is of similar or superior quality to that obtained using SAFT-VR Mie with parameters regressed in the traditional way using experimental thermodynamic-property data. The methodology is extended to an anisotropic species, methane, where similar levels of accuracy are obtained. The efficacy of using less-accurate quantum-chemistry methods in this methodology is explored, showing that these methods do not provide satisfactory results, although we note that the description is nevertheless substantially better than those obtained using the conductor-like screening model for describing real solvents (COSMO-RS), the only other fully predictive ab initio method currently available. Overall, the reliance on thermophysical data is completely dispensed with, providing the first extensible, wholly predictive SAFT-type EoSs. [ABSTRACT FROM AUTHOR]

The phase behavior and structure of a simple square-well bulk fluid with anisotropic interactions is described in detail. The orientation dependence of the intermolecular interactions allows for the formation of a nematic liquid-crystalline phase in addition to the more conventional isotropic gas and liquid phases. A version of classical density functional theory (DFT) is employed to determine the properties of the model, and comparisons are made with the corresponding data from Monte Carlo (MC) computer simulations in both the grand canonical and canonical ensembles, providing a benchmark to assess the adequacy of the DFT results. A novel element of the DFT approach is the assumption that the structure of the fluid is dominated by intermolecular interactions in the isotropic fluid. A so-called augmented modified mean-field (AMMF) approximation is employed accounting for the influence of anisotropic interactions. The AMMF approximation becomes exact in the limit of vanishing density. We discuss advantages and disadvantages of the AMMF approximation with respect to an accurate description of isotropic and nematic branches of the phase diagram, the degree of orientational order, and orientation-dependent pair correlations. The performance of the AMMF approximations is found to be good in comparison with the MC data; the AMMF approximation has clear advantages with respect to an accurate and more detailed description of the fluid structure. Possible strategies to improve the DFT are discussed. [ABSTRACT FROM AUTHOR]

Salt formation is commonly used to increase the solubility of ionisable active pharmaceutical ingredients (APIs) and is monitored via a pH-solubility profile of the API and its salt. Due to the extremely low solubilities of many APIs in water, experiments are difficult to perform, and reliable predictive tools can be especially useful in this context. Here, we use the SAFT- γ Mie group-contribution equation of state to predict the phase diagrams and the pH-dependent solubility of two acidic APIs: ibuprofen and ketoprofen. We consider ibuprofen and ketoprofen in basic buffer solutions of NaOH, KOH, LiOH, RbOH, Ca (OH) 2 , Mg (OH) 2 , n -hexylamine, n -octylamine, benzylamine and tert -butylamine, and in acidic buffer solutions of HCl and acetate. A predictive approach is developed in which the unlike interactions involving charged groups are either taken from previous work, calculated using combining rules or, in the case of groups originating from organic groups (e.g., COO −), taken to be the same as the equivalent interaction involving the neutral (uncharged) group. A new group, NH 3 + , is characterised for the case of amine buffers and salts. Equilibrium constants for the dissociation of the APIs and the formation of salts (p K a and K sp , A ν A B ν B values) are also incorporated in the model using experimental values from literature. Predictions of the complete phase diagrams of the APIs in water are presented, including the vapour–liquid, liquid–liquid (oiling out), and solid–liquid (solubility) equilibria. The SAFT- γ Mie approach is shown to provide accurate predictions of the solid–liquid solubility of the compounds, as well as of the presence of liquid–liquid separation. Furthermore, the pH-solubility profiles of the APIs at T = 298.15 K and 310.15 K for the range of buffers and salts considered are predicted in good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

Characterization of the vapor pressure of a volatile liquid or azeotropic mixture, and its fluid phase diagram, can be achieved with an isoteniscope and an industrial grade digital pressure sensor using the experimental method reported in this study. We describe vapor-pressure measurements of acetone and n-hexane and their azeotrope, and how the data can be used to calculate thermodynamic properties of the test liquids, such as the molar heat of vaporization. This hands-on experience allows students to appreciate important thermodynamic concepts such as phase equilibrium, preparing them for more advanced studies of the subject. [ABSTRACT FROM AUTHOR]

Abstract: A detailed molecular-dynamics (MD) simulation study of the interfacial tension of the n-heptane+perfluoro-n-hexane (C 6F14 + C 7H16) binary mixture is carried out at different temperatures and compositions for the vapour–liquid and liquid–liquid interfaces. The molecules are represented as chains of united-atom segments using previously reported force-field pure-component parameters with no further modification, while the unlike interaction energy between the alkyl and perfluoroalkyl segments is adjusted to describe the experimental data for the fluid phase equilibria. In the case of the vapour–liquid interface a horizontal inflection at a critical composition, sometimes referred to as aneotropy, is represented by our model as found in experimental work. Interfacial tensions for both the liquid-liquid and vapour-liquid interfaces are determined using the test-area method. A prerequisite of the determination of the interfacial tension by direct MD simulation involves studies of the fluid phase equilibria (vapour–liquid (VLE) and liquid–liquid equilibria (LLE)). A unique unlike energetic interaction parameter is adjusted to improve the prediction of the LLE and used in all the other simulations. The resulting description of the fluid phase behaviour is compared with that calculated using an equation of state (SAFT), and with experimental data where available; good agreement is observed. [Copyright &y& Elsevier]

An efficient and versatile method to calculate the components of the pressure tensor for hard-body fluids of generic shape from the perspective of molecular simulation is presented. After due consideration of all the possible repulsive contributions exerted by molecules upon their surroundings during an anisotropic system expansion, it is observed that such a volume change can, for non-spherical molecules, give rise to configurations where overlaps occur. This feature of anisotropic molecules has to be taken into account rigorously as it can lead to discrepancies in the calculation of tensorial contributions to the pressure. Using the condition of detailed balance as a basis, a perturbation method developed for spherical molecules has been extended so that it is applicable to non-spherical and non-convex molecules. From a series of 'ghost' anisotropic volume perturbations the residual contribution to the components of the pressure tensor may be accurately calculated. Comparisons are made with prior methods and, where relevant, results are evaluated against existing data. For inhomogeneous systems this method provides a particularly convenient route to the calculation of the interfacial tension (surface free energy) from molecular simulations. [ABSTRACT FROM AUTHOR]

In this contribution we provide a review and reformulation of perturbation theories (generalised van der Waals approaches) for the description of the fluid phase behaviour and orientational ordering transitions of thermotropic nematic liquid crystals. Free-energy functionals of the basic Onsager form are used as the platform for the development of a general formulation that reduces to the specific forms of the various theories that have found common use. A novel closed analytical description of the thermodynamic properties and degree of nematic order is then obtained by employing the Onsager trial function to represent the orientational distribution function in terms of a single parameter. The latter essentially constitutes an algebraic equation of state for the nematic phase appropriate for use in engineering applications. The description of ordering transitions with scaled-Onsager theories and suitable trial functions has already been illustrated by its application to systems of hard spherocylinders (HSCs), indicating that the approach provides an excellent representation of the orientational order of the hard-core (athermal) system [Mol. Phys. 106, 649 (2008)]. Here, the hard-body model is extended to account for attractive interactions (treated at the van der Waals level) of a general isotropic/anisotropic form (e.g., Lennard-Jonesium (LJ), square-well (SW), Maier-Saupe (MS), etc.). The adequacy of our generalised van der Waals-Onsager theory is exemplified by an analysis of the vapour-liquid, liquid-nematic, and vapour-nematic phase equilibria for hard spherocylinders with attractive square-well interactions (HSC-SW). The effect of the potential range and molecular aspect ratio on the vapour-liquid-nematic equilibria and orientational ordering transitions is examined to investigate the van der Waals limit (corresponding states) of the ordering phase behaviour. In the case of systems with an aspect ratio of ∼5 the corresponding-states limit is reached when the range is about 16 times the molecular diameter. For progressively longer molecules with an attractive range that follows their long dimension, the fluid-nematic equilibrium is enhanced to the point that the vapour-liquid boundary becomes metastable relative to fluid-nematic equilibria. In the case of molecules of very large aspect ratio (∼50), an additional region of nematic-nematic coexistence is exhibited by the system. [ABSTRACT FROM AUTHOR]

Abstract: We show how to extend the Hudson–McCoubrey combining rules, which were derived assuming the Lennard-Jones potential, for use with intermolecular potentials that are not of Lennard-Jones form. In particular, we illustrate the method by deriving combining rules for use with the square-well intermolecular potential. We show how these combining rules may be applied for equations of state such as the statistical associating fluid theory (SAFT), and that they perform at least as well as, and usually far better than, the standard Lorentz–Berthelot rules for all mixtures of simple molecules that we have considered. Liquid–liquid-equilibrium as well as vapour–liquid-equilibrium data are demonstrated to be important in judging the quality of combining rules. A methodology for the inclusion of the effect of polar interactions in our combining rules is suggested, however the success for these systems is mixed. The theory provides an excellent description of the thermodynamic properties of water and rationalises recent, apparently contradictory, models of the mixture of carbon , however the approach is less successful for mixtures such as carbon sulphide. This indicates that further work is still required for reliable prediction of binary-interaction parameters in mixtures of polar molecules. [Copyright &y& Elsevier]

The molecular theory of Onsager [L. Onsager, Ann. N.Y. Acad. Sci. 51, 627 (1949)] for liquid crystals is developed and extended to describe ordering transitions in systems of generic cylindrically symmetrical molecules. A number of new analytical results are discussed for particles characterised by a general form of the excluded-volume interaction. Our description makes use of the Onsager trial function (OTF) to represent the orientational distribution and degree of anisotropy. Algebraic expressions for the thermodynamic properties, which provide a particularly tractable description of the isotropic-nematic equilibria, are also presented. The degree of orientational order can be represented by a simple cubic equation in the molecular parameters (molecular diameter and aspect ratio) and thermodynamic variables (temperature and number density). Onsager's theory was originally developed at the level of the second virial coefficient; here the Parsons-Lee decoupling approximation is used to describe the higher-body contributions in a straightforward manner. The adequacy of treating the scaled Onsager (Parsons-Lee) free-energy functional within the OTF formalism to describe anisotropic states is illustrated by examining systems of hard spherocylinders. An excellent representation of the equation of state of the isotropic and nematic phases and the ordering transition is demonstrated for molecules of moderate aspect ratio (L/D = 5). Algebraic equations of state of the type developed here are suitable for practical engineering applications involving anisotropic fluids particularly in the case of multicomponent systems; our general analytical results for the averages of orientational functions will turn out to be useful in the development of a description of molecules with more realistic attractive and Maier-Saupe interactions. [ABSTRACT FROM AUTHOR]

The statistical associating fluid theory (SAFT) is now well established as an approach for the description of the thermodynamics and phase equilibria of a wide variety of fluid systems. Numerous SAFT studies of the fluid phase equilibria of pure water and aqueous mixtures have been made with the various incarnations of the theory, yet there is no consensus on what the 'optimal' values of the intermolecular parameters are for water, or what the association scheme should be (two-, three-, or four-site models). We show that the conventional use of vapour-pressure and saturated-liquid-density data on their own leads to a degeneracy in the values of the parameters, particularly in the relative values of the dispersion and hydrogen-bonding energies. A discretized energy-parameter space is examined and a long valley of 'optimal' parameter sets for the vapour-liquid equilibria is found for the various models, ranging from low dispersion (high hydrogen-bonding) to high dispersion (low hydrogen-bonding) energies. Other thermodynamic information such as the heat of vaporization does not allow one to discriminate between the values of the parameters or the association scheme. An examination of the degree of association (hydrogen bonding) allows such a discrimination to be made: the four-site model is found to provide the best overall description of the thermodynamics, fluid phase equilibria, and degree of association. The set of parameters obtained in this way also provides the best description of the fluid-phase equilibria for a mixture of water and methanol. This degeneracy of parameters could also be important when models of water are refined to vapour-liquid equilibria in simulation studies of aqueous systems, and for other systems of associating molecules. [ABSTRACT FROM AUTHOR]

Abstract: Absorption of light gases in polyethylene (PE) is studied using two versions of the Statistical Associating Fluid Theory (SAFT): SAFT for chain molecules with attractive potentials of variable range (VR) and simplified perturbed-chain (PC) SAFT. Emphasis is placed on the light gases typically present during ethylene polymerisation in the gas-phase reactor (GPR) process. The two approaches are validated using experimental binary-mixture data for gas absorbed in PE, and predictions are made for mixtures of more components. For most cases studied both SAFT versions perform equally well. For the case of ternary mixtures of two gases with PE, it is predicted that the less-volatile of the two gases acts to enhance the absorption of the more-volatile gas, while the more-volatile gas inhibits the absorption of the less-volatile gas. This general behaviour is also predicted in mixtures containing more gases, such as typical reactor mixtures. The magnitude of the effect may vary considerably, depending on the relative proximity of the gas-mixture saturation pressure to the reactor pressure; for example it is predicted that the absorption of ethylene may be approximately doubled if diluent gases, propane or nitrogen, are partially or completely replaced by less-volatile butane or pentane for a reactor pressure ∼2MPa. In the case of a co-polymerisation reaction, it is predicted that increases in absorption of both co-monomers may be obtained in roughly equal proportion. Our findings cast light on the so-called co-monomer effect, in which substantial increases in the rate of ethylene polymerisation are observed in the presence of hexene co-monomer, while suggesting that the effect is more general and not restricted to co-monomer. For example, similar rate increases may be expected in the presence of, e.g., pentane instead of hexene, but without the change in the branch structure of the produced polymer that is inevitable when the amount of co-monomer is increased. [Copyright &y& Elsevier]

We present an extension of the SAFT-VR Mie approach to model adsorption of molecular fluids based on a two-dimensional (2D) approximation to describe the adsorbed fluid. Analytical results are provided for the first- and second-order perturbation terms of the free energy for the 2D system. The adsorption model is based on the assumption that the particle pair interactions in the adsorbed and bulk phases are described with the same Mie potential exponents λ a and λ r , in contrast with the square-well version of the 2D-SAFT-VR approach in which it is considered necessary to modify the attractive ranges of the SW interactions. This important difference between the two approaches leads to a reduction in the number of molecular parameters to be determined. In order to demonstrate the performance of the 2D-SAFT-VR Mie approach, we present results for the the modelling of carbon dioxide (CO2) and methane (CH4) adsorbed onto dry coal. [ABSTRACT FROM AUTHOR]

The structural properties and interfacial tension of a fluid of rodlike hard-spherocylinder particles in contact with hard structureless flat walls are studied by means of Monte Carlo simulation. The calculated surface tension between the rod fluid and the substrate is characterized by a nonmonotonic trend as a function of the bulk concentration (density) over the range of isotropic bulk concentrations. As suggested by earlier theoretical studies, a surface-ordering scenario is confirmed by our simulations: the local orientational order close to the wall changes from uniaxial to biaxial nematic when the bulk concentration reaches about 85% of the value at the onset of the isotropic-nematic phase transition. The surface ordering coincides with a wetting transition whereby the hard wall is wetted by a nematic film. Accurate values of the fluid-solid surface tension, the adsorption, and the average particle-wall contact distance are reported (over a broad range of densities into the dense nematic region for the first time), which can serve as a useful benchmark for future theoretical and experimental studies on confined rod fluids. The simulation data are supplemented with predictions from second-virial density functional theory, which are in good qualitative agreement with the simulation results. [ABSTRACT FROM AUTHOR]

A mixed-integer non-linear programming optimisation framework is formulated and developed that combines a molecular-based, group-contribution equation of state, SAFT- γ Mie, with a thermodynamic description of an organic Rankine cycle (ORC) power system. In this framework, a set of working fluids is described by its constituent functional groups ( e.g. , since we are focussing here on hydrocarbons: CH 3 , CH 2 , etc. ), and integer optimisation variables are introduced in the description the working-fluid structure. Molecular feasibility constraints are then defined to ensure all feasible working-fluid candidates can be found. This optimisation framework facilitates combining the computer-aided molecular design of the working fluid with the power-system optimisation into a single framework, thus removing subjective and pre-emptive screening criteria, and simultaneously moving towards the next generation of tailored working fluids and optimised systems for waste-heat recovery applications. SAFT- γ Mie has not been previously employed in such a framework. The optimisation framework, which is based here on hydrocarbon functional groups, is first validated against an alternative formulation that uses (pseudo-experimental) thermodynamic property predictions from REFPROP, and against an optimisation study taken from the literature. The framework is then applied to three industrial waste-heat recovery applications. It is found that simple molecules, such as propane and propene, are the optimal ORC working fluids for a low-grade (150 °C) heat source, whilst molecules with increasing molecular complexity are favoured at higher temperatures. Specifically, 2-alkenes emerge as the optimal working fluids for medium- and higher-grade heat-sources in the 250–350 °C temperature range. Ultimately, the results demonstrate the potential of this framework to drive the search for the next generation of ORC systems, and to provide meaningful insights into identifying the working fluids that represent the optimal choices for targeted applications. Finally, the effects of the working-fluid structure on the expander and pump are investigated, and the suitability of group-contribution methods for evaluating the transport properties of hydrocarbon working-fluids are considered, in the context of performing complete thermoeconomic evaluations of these systems. [ABSTRACT FROM AUTHOR]

A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an "aromatic-like" nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature-composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction energy parameter is employed for each solvent to reproduce experimental solubility behavior; this is sufficient for the quantitative prediction of both upper and lower critical solution points and the transition to the characteristic "hourglass" phase behavior for these systems. [ABSTRACT FROM AUTHOR]

We employ a validated first-order lumped dynamic model of the Up-THERM heat converter, a two-phase unsteady heat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which contain fewer moving parts than conventional engines and represent an attractive alternative for remote or off-grid power generation as well as waste-heat conversion applications. We investigate the performance of the Up-THERM with respect to working-fluid selection for its prospective applications. An examination of relevant working-fluid thermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play important roles in determining the performance of the Up-THERM – the device delivers a higher power output at high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore, working fluids with low critical temperatures, high critical pressures and exhibiting high values of reduced pressures and temperatures result in designs with high power outputs. For a pre-specified Up-THERM design corresponding to a target (CHP prime-mover) application with a heat-source temperature of 360 °C, water is compared with 45 other pure working fluids. When maximizing the power output, R113 is identified as the optimal fluid, followed by i -hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize the exergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-source temperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizing thermal energy at temperatures above 200 °C. Of all the working fluids considered here, ammonia, R245ca, R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over a wide range of heat-source temperatures. [ABSTRACT FROM AUTHOR]

The Up-THERM heat converter is an unsteady, two-phase thermofluidic oscillator that employs an organic working fluid, which is currently being considered as a prime-mover in small- to medium-scale combined heat and power (CHP) applications. In this paper, the Up-THERM heat converter is compared to a basic (sub-critical, non-regenerative) organic Rankine cycle (ORC) heat engine with respect to their power outputs, thermal efficiencies and exergy efficiencies, as well as their capital and specific costs. The study focuses on a pre-specified Up-THERM design in a selected application, a heat-source temperature range from 210 °C to 500 °C and five different working fluids (three n-alkanes and two refrigerants). A modeling methodology is developed that allows the above thermo-economic performance indicators to be estimated for the two power-generation systems. For the chosen applications, the power output of the ORC engine is generally higher than that of the Up-THERM heat converter. However, the capital costs of the Up-THERM heat converter are lower than those of the ORC engine. Although the specific costs (£/kW) of the ORC engine are lower than those of the Up-THERM converter at low heat-source temperatures, the two systems become progressively comparable at higher temperatures, with the Up-THERM heat converter attaining a considerably lower specific cost at the highest heat-source temperatures considered. [ABSTRACT FROM AUTHOR]

By employing the SAFT-VR Mie equation of state, molecular-based models are developed from which the thermodynamic properties of pure (i.e., single-component) organic fluids and their mixtures are calculated. This approach can enable the selection of optimal working fluids in organic Rankine cycle (ORC) applications, even in cases for which experimental data relating to mixture properties are not available. After developing models for perfluoroalkane ( n -C 4 F 10 + n -C 10 F 22 ) mixtures, and validating these against available experimental data, SAFT-VR Mie is shown to predict accurately both the single-phase and saturation properties of these fluids. In particular, second-derivative properties (e.g., specific heat capacities), which are less reliably calculated by cubic equations of state (EoS), are accurately described using SAFT-VR Mie, thereby enabling an accurate prediction of important working-fluid properties such as the specific entropy. The property data are then used in thermodynamic cycle analyses for the evaluation of ORC performance and cost. The approach is applied to a specific case study in which a sub-critical, non-regenerative ORC system recovers and converts waste heat from a refinery flue-gas stream with fixed, predefined conditions. Results are compared with those obtained when employing analogue alkane mixtures ( n -C 4 H 10 + n -C 10 H 22 ) for which sufficient thermodynamic property data exist. When unlimited quantities of cooling water are utilized, pure perfluorobutane (and pure butane) cycles exhibit higher power outputs and higher thermal efficiencies compared to mixtures with perfluorodecane (or decane), respectively. The effect of the composition of a working-fluid mixture in the aforementioned performance indicators is non-trivial. Only at low evaporator pressures (<10 bar) do the investigated mixtures perform better than the pure fluids. A basic cost analysis reveals that systems with pure perfluorobutane (and butane) fluids are associated with relatively high total costs, but are nevertheless more cost effective per unit power output than the fluid mixtures (due to the higher generated power). When the quantity of cooling water is constrained by the application, overall performance deteriorates, and mixtures emerge as the optimal working fluids. [ABSTRACT FROM AUTHOR]

An accurate representation of molecular association is a vital ingredient of advanced equations of state (EOSs), providing a description of thermodynamic properties of complex fluids where hydrogen bonding plays an important role. The combination of the first-order thermodynamic perturbation theory (TPT1) of Wertheim for associating systems with an accurate description of the structural and thermodynamic properties of the monomer fluid forms the basis of the statistical associating fluid theory (SAFT) family of EOSs. The contribution of association to the free energy in SAFT and related EOSs is very sensitive to the nature of intermolecular potential used to describe the monomers and, crucially, to the accuracy of the representation of the thermodynamic and structural properties. Here we develop an accurate description of the association contribution for use within the recently developed SAFT-VR Mie framework for chain molecules formed from segments interacting through a Mie potential [T. Lafitte, A. Apostolakou, C. Avendaño, A, Galindo, C. S. Adjiman, E. A. Müller, and G. Jackson, J. Chem. Phys.139, 154504 (2013)]. As the Mie interaction represents a soft-core potential model, a method similar to that adopted for the Lennard-Jones potential [E. A. Müller and K. E. Gubbins, Ind. Eng. Chem. Res.34, 3662 (1995)] is employed to describe the association contribution to the Helmholtz free energy. The radial distribution function (RDF) of the Mie fluid (which is required for the evaluation of the integral at the heart of the association term) is determined for a broad range of thermodynamic conditions (temperatures and densities) using the reference hyper-netted chain (RHNC) integral-equation theory. The numerical data for the association kernel of Mie fluids with different association geometries are then correlated for a range of thermodynamic states to obtain a general expression for the association contribution which can be applied for varying values of the Mie repulsive exponent. The resulting SAFT-VR Mie EOS allows for a much improved description of the vapour-liquid equilibria and single-phase properties of associating fluids such as water, methanol, ammonia, hydrogen sulphide, and their mixtures. A comparison is also made between the theoretical predictions of the degree of association for water and the extent of hydrogen bonding obtained from molecular simulations of the SPC/E and TIP4P/2005 atomistic models. [ABSTRACT FROM PUBLISHER]

An improved formulation of the extension of the statistical associating fluid theory for potentials of variable range to electrolytes (SAFT-VRE) is presented, incorporating a representation for the dielectric constant of the solution that takes into account the temperature, density and composition of the solvent. The proposed approach provides an excellent correlation of the dielectric-constant data available for a number of solvents including water, representative alcohols and carbon dioxide, and it is shown that the methodology can be used to treat mixed-solvent electrolyte solutions. Models for strong electrolytes of the metal-halide family are considered here. The salts are treated as fully dissociated and ion-specific interaction parameters are presented. Vapour pressure, density, and mean ionic activity coefficient data are used to determine the ion–ion and solvent–ion parameters, and mixed-salt electrolyte solutions (brines) are then treated predictively. We find that the resulting intermolecular potential models follow physical trends in terms of energies and ion sizes with a close relationship observed with well-established ionic diameters. A good description is obtained for the densities, mean ionic activity coefficients, and vapour pressures of the electrolyte solutions studied. The theory is also seen to provide excellent predictions of the osmotic coefficient and of the depression of the freezing temperature, and provides a qualitative estimate of the solvation free energy. The vapour pressure of aqueous brines is predicted accurately, as is the density of these solutions, although not at the highest pressures considered. Calculations for the vapour–liquid and liquid–liquid equilibria of salts in water+methanol and water+n-butan-1-ol are presented. In addition, it is shown that the salting-out of carbon dioxide in sodium chloride solutions is captured well using a predictive model. [ABSTRACT FROM AUTHOR]

A non-linear dynamic framework is presented for the modelling of a novel two-phase heat engine termed ‘Up-THERM’, which features a single solid moving-part (piston). When applied across the device, a constant temperature difference between an external (low- to medium-grade) heat source and an external heat sink is converted into sustained and persistent oscillations of pressure and volumetric fluid displacement. These oscillations are transformed in a load arrangement into a unidirectional flow from which power is extracted by a hydraulic motor. The Up-THERM engine is modelled using a system of first-order differential equations that describe the dominant thermal/fluid processes in each component of the device. For certain components where the deviations from a linear approximation are non-negligible (gas spring in the displacer cylinder, check valves and piston valve, and heat exchangers), a non-linear description is employed. A comparison between the linear and non-linear descriptions of the gas spring at the top of the displacer cylinder reveals that the non-linear description results in more realistic predictions of the oscillation frequency compared to experimental data from a similar device. Furthermore, the shape of the temperature profile over the heat-exchanger surfaces is modelled as following a hyperbolic tangent function, based on findings from an experimental investigation. Following the validation of these important device components, a parametric study is performed on the Up-THERM engine model with the aforementioned non-linear component descriptions, aimed at investigating the effects of important geometric parameters and of the heat-source temperature on key performance indicators, namely the oscillation frequency, power output and exergy efficiency of the engine. The results indicate that the geometric design of the displacer cylinder, including the height of the gas spring at the top of the cylinder, and the heat-source temperature have the most significant influence on the performance of the engine. A maximum exergy efficiency of 2.8% and a maximum power output of 175 W are observed at the proposed operating temperature of 450 °C for a nominal Up-THERM design (based on the physical dimensions of a device prototype and water as the working fluid; the role of the working fluid is explored in follow-up paper Ref. [1]) but with shorter displacer cylinder gas-spring lengths relative to a nominal design. The results and insight can assist the further development of this technology, in particular as a prime mover in combined heat and power applications. [ABSTRACT FROM AUTHOR]

We present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimation. The inclusion of the Born free-energy contribution, together with appropriate estimates for the size of the ionic cavity, allows for accurate predictions of the Gibbs free energy of solvation of the ionic species considered. The solubility limits are also predicted for a number of salts; in cases where reliable reference data are available the predictions are in good agreement with experiment. [ABSTRACT FROM AUTHOR]

Experimental interfacial tensions of the systems (H 2 O + CO 2 ), (H 2 O + N 2 ), (H 2 O + Ar), (H 2 O + CO 2 + N 2 ) and (H 2 O + CO 2 + Ar) are compared with calculations based on the statistical associating fluid theory for variable range potentials of the Mie form (SAFT-VR Mie) in combination with the square-gradient theory (SGT). Comparisons are made at temperatures from (298 to 473) K and at pressures up to 60 MPa. Experimental data for the systems (H 2 O + CO 2 ), (H 2 O + N 2 ) and (H 2 O + CO 2 + N 2 ) are taken from the literature. For the (H 2 O + Ar) and (H 2 O + CO 2 + Ar) systems, we report new experimental interfacial-tension data at temperatures of (298.15–473.15) K and pressures from (2 to 50) MPa, measured by the pendant-drop method. The expanded uncertainties at 95% confidence are 0.05 K for temperature, 70 kPa for pressure, 0.016 × γ for interfacial tension in the binary (Ar + H 2 O) system and 0.018 × γ for interfacial tension in the ternary (CO 2 + Ar + H 2 O) system. The parameters in the SAFT-VR Mie equation of state are estimated entirely from phase-equilibrium data for the pure components and binary mixtures. For pure water, the SGT influence parameter is determined from vapour–liquid surface-tension data, as is common practice. Since the other components are supercritical over most or the entire temperature range under consideration, their pure-component influence parameters are regressed by comparison with the binary interfacial-tension data. A geometric-mean combining rule is used for the unlike influence parameter in mixtures without incorporation of adjustable binary parameters. The SAFT-VR Mie + SGT approach is found to provide an excellent correlation of the surface tension of water and of the interfacial tensions of the binary systems comprising H 2 O with CO 2 or Ar or N 2 . When applied to predict the interfacial tensions of the two ternary systems, generally good results are found for (H 2 O + CO 2 + N 2 ) while, for (H 2 O + CO 2 + Ar), the theory performs well at high temperatures but significant deviations are found at low temperatures. Overall, the SAFT-VR Mie + SGT approach can be recommended as a means of modelling the interfacial properties of systems comprising water, carbon dioxide and diluent gases. [ABSTRACT FROM AUTHOR]

Using both theory and continuum simulation, we examine a system comprising a mixture of polymer chains formed from 100 hard-sphere (HS) segments and HS colloids with a diameter which is 20 times that of the polymer segments. According to Wertheim's first-order thermodynamic perturbation theory (TPT1) this athermal system is expected to phase separate into a colloid-rich and a polymer-rich phase. Using a previously developed continuous pseudo-HS potential [J. F. Jover, A. J. Haslam, A. Galindo, G. Jackson, and E. A. Muller, J. Chem. Phys.137, 144505 (2012)], we simulate the system at a phase point indicated by the theory to be well within the two-phase binodal region. Molecular-dynamics simulations are performed from starting configurations corresponding to completely phase-separated and completely pre-mixed colloids and polymers. Clear evidence is seen of the stabilisation of two coexisting fluid phases in both cases. An analysis of the interfacial tension of the phase-separated regions is made; ultra-low tensions are observed in line with previous values determined with square-gradient theory and experiment for colloid–polymer systems. Further simulations are carried out to examine the nature of these coexisting phases, taking as input the densities and compositions calculated using TPT1 (and corresponding to the peaks in the probability distribution of the density profiles obtained in the simulations). The polymer chains are seen to be fully penetrable by other polymers. By contrast, from the point of view of the colloids, the polymers behave (on average) as almost-impenetrable spheres. It is demonstrated that, while the average interaction between the polymer molecules in the polymer-rich phase is (as expected) soft-repulsive in nature, the corresponding interaction in the colloid-rich phase is of an entirely different form, characterised by a region of effective intermolecular attraction. [ABSTRACT FROM PUBLISHER]

In this work we integrate the statistical associating fluid theory for fluids interacting through potentials of variable range (SAFT-VR) into a traditional van der Waals and Platteeuw framework for modelling clathrate hydrates. We incorporate a new water–guest cell potential for the hydrate phase that can be related to the potential adopted in the familiar SAFT-VR equation of state for modelling fluids. We show how the ability of this equation of state to treat a wide range of complex fluids increases the scope of hydrate modelling to incorporate, in a single framework, the presence of various inhibitors (alcohols, glycols) or brines – or, indeed, any fluid for which a model is available (for use within SAFT-VR) or can be conveniently obtained. Agreement with experimental results is good throughout and, in many cases, excellent. [ABSTRACT FROM PUBLISHER]

Abstract: We present a simplified but consistent picture of asphaltene precipitation from crude oil from a thermodynamic perspective, illustrating its relationship to the familiar bubble curve via the calculation of constant-composition p–T phase diagrams that incorporate both the bubble curve and the asphaltene precipitation boundary. Using the statistical associating fluid theory (SAFT) we show that the position of the precipitation boundary can be explained using a very simple fluid model including relatively few components. Our results support the view that the precursor to asphaltene precipitation is a liquid–liquid phase separation due to a demixing instability in the fluid. Moreover, the bubble curve for these systems is seen to represent a boundary between regions of two-phase (liquid–liquid) and three-phase (vapour–liquid–liquid) equilibria. [Copyright &y& Elsevier]