Liquid Equilibrium Properties Research Articles

Flexible or rigid molecular models? A study on vapour–liquid equilibrium properties of ammonia

The influence of the intramolecular degrees of freedom on the vapour–liquid equilibrium properties of ammonia is studied for vapour pressure, saturated densities and enthalpy of vaporization. Molecular force fields with and without intramolecular degrees of freedom, keeping all other parameters unchanged, show significantly different phase envelopes. For ammonia, the angle potential is particularly important, because the hydrogen sites are more aligned in the liquid than in the vapour, leading to a significantly enhanced molecular dipole moment in the condensed phase. Based on a rigid force field for ammonia from prior work of our group [Eckl et al., Mol. Phys. 106, 1039 (2008)], a new accurate force field with intramolecular degrees of freedom is developed.

Validation of CFD simulation for ammonia emissions from an aqueous solution

In order to model and predict ammonia emissions from animal houses, it is important to determine the concentration on the emission surface correctly. In the current literature, Henry's law is usually used to model the mass transfer through the gas–liquid surface (e.g. manure or aqueous solution). The objective of this study is to investigate the accuracy of three models for Henry's law constant (HLC) as well as functions derived from experimental vapor–liquid equilibrium (VLE) properties of ammonia water to determine the concentration on the liquid ammonia solutions surface in order to be used as boundary condition for CFD prediction of ammonia emission. The accuracy of CFD simulation depends on many factors. In this study, the effects of appropriate geometry model, inlet turbulent parameters and three turbulence models (low-Reynolds number k– ɛ model, renormalization group k– ɛ model and Shear Stress Transport k– w model) on ammonia emissions are investigated and discussed. Then the concentration boundary condition determined by different HLC models and the VLE model is validated by ammonia emissions and concentration profiles measured in the boundary layer. The results indicate that the current HLC models generally over-predict the ammonia emissions from aqueous solution in this study whereas VLE gives better agreement between simulated and measured results. A linear relation is observed between ammonia mass transfer coefficient obtained from the VLE relation and those from HLC models.

Vapor–liquid and liquid–liquid equilibrium properties of ethane + methanol system at ambient temperature

The liquid–liquid and vapor–liquid equilibrium data for the binary system of ethane + methanol were measured at ambient temperature over a wide range of pressures using a designed PVT apparatus. The experimental liquid–liquid and vapor–liquid equilibrium data were compared with the modeling results obtained using the Peng Robinson and Soave–Redlich–Kwong equations of state.

Prediction of thermodynamic, transport and vapor–liquid equilibrium properties of binary mixtures of ethylene glycol and water

Equilibrium and non-equilibrium molecular dynamics and Monte Carlo simulation techniques were applied to predict various thermodynamic, transport and vapor–liquid equilibrium properties of binary mixtures of ethylene glycol and water (EG–W) based on OPLS-AA and SPC/E force fields. The properties predicted include density, vaporization enthalpy, enthalpy of mixing, heat capacities, diffusion coefficients, shear viscosities, thermal conductivities, vapor–liquid coexistence isotherms and isobaric curves, and saturation vapor pressures. Good agreements with experimental data were obtained for most of these properties. Errors are mostly related to inaccuracy found in predictions of pure fluids; a correction to prediction of pure substance can systematically improve prediction for the mixture. This work suggests that OPLS-AA and SPC/E force fields using the common combining rules are transferable for predicting multiple physical properties of EG–W mixtures.

Accuracy estimate of equilibrium flash vaporization calculation of crude oil and vacuum residue at high temperature

For the reliable design of crude oil and residua fractionators and fired heaters, accurate calculation method for vapor–liquid equilibrium (VLE), thermodynamic and physical properties is necessary. Recent trends using increasingly heavier crude oils, extracting the maximum light oils by deep-cut vaporization at high temperatures and high vacuum, and the decreasing demand for black oil products prompted this investigation. However, the accuracies of these methods in the calculation of vapor–liquid equilibrium of crude oils and their residues at high temperatures have not been thoroughly investigated and published by previous investigators. The purpose of this paper is to evaluate the accuracy of the currently available and often-used methods, and to investigate a new method for the VLE of high-boiling petroleum fractions. For this purpose, the equilibrium flash vaporization data (EFV) of four crude oils and their vacuum residues were used, which have been published by major oil companies.

Prediction of vapor–liquid equilibrium and PVTx properties of geological fluid system with SAFT-LJ EOS including multi-polar contribution. Part I: Application to H 2O–CO 2 system

Molecular based equations of state (EOS) are attractive because they can take into account the energetic contribution of the main types of molecular interactions. This study models vapor–liquid equilibrium (VLE) and PVTx properties of the H 2O–CO 2 binary system using a Lennard-Jones (LJ) referenced SAFT (Statistical Associating Fluid Theory) EOS. The improved SAFT-LJ EOS is defined in terms of the residual molar Helmholtz energy, which is a sum of four terms representing the contributions from LJ segment–segment interactions, chain-forming among the LJ segments, short-range associations and long-range multi-polar interactions. CO 2 is modeled as a linear chain molecule with a constant quadrupole moment, and H 2O is modeled as a spherical molecule with four association sites and a dipole moment. The multi-polar contribution to Helmholtz energy, including the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole contribution for H 2O–CO 2 system, is calculated using the theory of Gubbins and Twu (1978). Six parameters for pure H 2O and four parameters for pure CO 2 are needed in our model. The Van der Waals one-fluid mixing rule is used to calculate the Lennard-Jones energy parameter and volume parameter for the mixture. Two or three binary parameters are needed for CO 2–H 2O mixtures, which are evaluated from phase equilibrium data of the binary system. Comparison with the experimental data shows that our model represents the PVT properties of CO 2 better than other SAFT EOS without a quadrupole contribution. For the CO 2–H 2O system, our model agrees well with the vapor–liquid equilibrium data from 323–623 K. The average relative deviation for CO 2 solubility (expressed in mole fraction) in water is within 6%. Our model can also predict the PVTx properties of CO 2–H 2O mixtures up to 1073 K and 3000 bar. The good performance of this model indicates that: (1) taking account of the multi-polar contribution explicitly improves the agreement of calculated properties with experimental data at high temperatures and high pressures, (2) the molecular-based EOS with just a few parameters fit to data in the sub-critical region can predict the thermodynamic properties of fluids over a wide range of P– T conditions.

Assessment of numerical optimization algorithms for the development of molecular models

In the pursuit to study the parameterization problem of molecular models with a broad perspective, this paper is focused on an isolated aspect: It is investigated, by which algorithms parameters can be best optimized simultaneously to different types of target data (experimental or theoretical) over a range of temperatures with the lowest number of iteration steps. As an example, nitrogen is regarded, where the intermolecular interactions are well described by the quadrupolar two-center Lennard-Jones model that has four state-independent parameters. The target data comprise experimental values for saturated liquid density, enthalpy of vaporization, and vapor pressure. For the purpose of testing algorithms, molecular simulations are entirely replaced by fit functions of vapor–liquid equilibrium (VLE) properties from the literature to assess efficiently the diverse numerical optimization algorithms investigated, being state-of-the-art gradient-based methods with very good convergency qualities. Additionally, artificial noise was superimposed onto the VLE fit results to evaluate the numerical optimization algorithms so that the calculation of molecular simulation data was mimicked. Large differences in the behavior of the individual optimization algorithms are found and some are identified to be capable to handle noisy function values.

Volatility of Aprotic Ionic Liquids — A Review

Research on the volatility of aprotic ionic liquids is reviewed. This topic is limited to aprotic liquids since measurements have led to a generalization that this class of substances possesses extremely low vapor pressures. For research related to this topic, the period covered is 2003 to 2008. The review begins with a discussion of the earliest successful study of thermal vaporization of aprotic ionic liquids, the earliest indirect measurements of their vapor pressure, the first proof of conventional distillation of ionic liquids, and competition of vaporization with thermal decomposition seen at moderately high temperatures of (200 to 300) °C. The review then briefly discusses the nature of the vapor phase and proceeds to analyze the various approaches used to measure or predict the vapor−liquid equilibrium properties of ionic liquids, including their normal boiling point, vapor pressure, and enthalpy of vaporization. It is proposed that the most reliable thermodynamic data exist solely for the enthalp...

Molecular simulations of benzene and hexafluorobenzene using new optimized effective potential models: Investigation of the liquid, vapor–liquid coexistence and supercritical fluid phases

The development of new effective intermolecular potential models of benzene and hexafluororbenzene, capable in reproducing the thermodynamic and structural properties of molecular systems in a wide range of thermodynamic state points has been presented and discussed. Subsequently, the properties of the fluids have been investigated by employing molecular dynamics and Monte Carlo simulation techniques. The main purpose of this study was to reveal information concerning the liquid state, vapor–liquid equilibrium and supercritical phase properties of these fluids. In the case of the supercritical phase, we mainly focused on the behavior of local density inhomogeneities and related properties. Our calculations reveal that the local density augmentation is much more pronounced in the case of hexafluorobenzene. The origins of possible resemblances and discrepancies with available experimental data have been also systematically discussed and related to our conclusions reported in previous publications. The local density reorganization dynamics as a function of the bulk density and the size of the local region have been also studied, revealing a significant density and length scale dependence similar to the ones presented for other pure supercritical fluids in previous publication of our group.

A basis for the development of new ammonia–water–sodium hydroxide absorption chillers

Adding NaOH to ammonia/water improves the separation of ammonia in the generator and reduces both chiller driving temperature and rectification losses. In this paper the main disadvantages for the implementation of these new mixtures are addressed: a) few experimental data or reliable correlations of the fluid mixture properties exist; b) selection of an adequate system for separating the hydroxide; and c) evaluation of potential corrosion problems. Our results show that the separation of NaOH from an ammonia/water solution with a maximum approximate mass fraction of 0.05 (5% weight) is viable when reverse osmosis is used with membranes BW30 and SW30HR LE. Cycle simulation using experimental data to calculate the vapour–liquid equilibrium properties shows that the COP is approximately 20% higher than with a conventional ammonia/water chiller working under the same conditions and using a hydroxyl separation efficiency of 99% for NaOH, which is feasible according to the experimental tests.

Vapor−Liquid−Liquid Equilibria for Aqueous Systems with Methyl Acetate, Methyl Propionate, and Methanol

Vapor−liquid−liquid equilibrium (VLLE) properties were measured with a static apparatus for mixtures of water + methyl acetate, water + methyl propionate, water + methyl acetate + methanol, and water + methyl propionate + methanol at temperatures ranging from 313.24 to 348.15 K. Both ternary systems exhibit type I liquid−liquid equilibrium (LLE) behavior. By checking with the Othmer−Tobias equation, the experimental tie-line data of these two ternary systems appeared to be consistent. The binary VLLE data were correlated satisfactorily with the UNIQUAC model for water + methyl acetate and water + methyl propionate. While the UNIQUAC model with the parameters determined from the phase equilibrium data of the constituent binaries overestimated the two-liquid-phase regions of the ternary systems, the representation of the binodal loci was substantially improved as the six parameters of the UNIQUAC model were determined simultaneously from the ternary VLLE data.

Properties of Refrigerants from Cubic Equations of State

Vapour � liquid equilibrium, thermodynamic and volumetric properties were predicted for three pure hydrofluorocarbons: difluoromethane (R32), pentafluoroethane (R125) and 1,1,1,2 � tetrafluoroethane (R134a) as well as for binary and ternary mixtures of these refrigerants. Three cubic equations of state GEOS3C, SRK (Soave � Redlich � Kwong) and PR (Peng � Robinson) were used. A wide comparison with literature experimental data was made. For the refrigerant mixtures, classical van der Waals mixing rules without interaction parameters were used. The GEOS3C equation, with three parameters estimated by matching several points on the saturation curve (vapor pressure and corresponding liquid volumes), compares favorably to other equations in literature, being simple enough for applications.

Effect of potassium citrate salts on the vapor–liquid equilibrium properties of aqueous solutions of alanine at different temperatures

Water activity measurements by isopiestic method have been carried out on the aqueous solutions of alanine + potassium di-hydrogen citrate (KH 2Cit) and alanine + tri-potassium citrate (K 3Cit) over a range of temperatures at atmospheric pressure. From these measurements, values of the vapor pressure of solutions were determined. The effect of temperature and charge on the anion of salts on the vapor–liquid equilibrium of the investigated systems has been studied. The experimental water activities have been correlated successfully with the segment-based local composition Wilson and NRTL models. The agreement between the correlations and the experimental data is good.

Molecular model for formic acid adjusted to vapor–liquid equilibria

A new molecular model for formic acid is proposed which favorably describes vapor–liquid equilibrium properties in the full temperature range. The geometry of the model was determined by quantum chemical calculations of the cis-conformer. The model is rigid and consists of anisotropic united atom Lennard–Jones sites and point charges, so that it requires relatively little computational effort.

Correlation for the Carbon Dioxide and Water Mixture Based on The Lemmon–Jacobsen Mixture Model and the Peng–Robinson Equation of State

Models representing the thermodynamic behavior of the CO2–H2O mixture have been developed. The single-phase model is based upon the thermodynamic property mixture model proposed by Lemmon and Jacobsen. The model represents the single-phase vapor states over the temperature range of 323–1074 K, up to a pressure of 100 MPa over the entire composition range. The experimental data used to develop these formulations include pressure–density–temperature-composition, second virial coefficients, and excess enthalpy. A nonlinear regression algorithm was used to determine the various adjustable parameters of the model. The model can be used to compute density values of the mixture to within ±0.1%. Due to a lack of single-phase liquid data for the mixture, the Peng–Robinson equation of state (PREOS) was used to predict the vapor–liquid equilibrium (VLE) properties of the mixture. Comparisons of values computed from the Peng–Robinson VLE predictions using standard binary interaction parameters to experimental data are presented to verify the accuracy of this calculation. The VLE calculation is shown to be accurate to within ±3 K in temperature over a temperature range of 323–624 K up to 20 MPa. The accuracy from 20 to 100 MPa is ±3 K up to ±30 K in temperature, being worse for higher pressures. Bubble-point mole fractions can be determined within ±0.05 for CO2.

Interactions of Nitrous Oxide with Fluorinated Liquids

The interactions of nitrous oxide with fluorinated liquids are investigated by reporting original experimental results on gas solubility and interpreting them using molecular simulation. Nitrous oxide is highly soluble in the three fluorinated liquids studied-perfluorooctane, 1-bromoperfluorooctane (perfluorooctylbromide), and perfluorohexylethane-with mole fraction solubilities on the order of 0.03 under ambient conditions. An intermolecular potential model was developed for nitrous oxide, with a functional form of the Lennard-Jones plus point charges type, adjusted to the experimental multipole moments and to vapor-liquid equilibrium properties. The solubility of nitrous oxide in perfluorocarbon liquids was calculated by molecular simulation methods, and a dissimilar interaction parameter of 0.92 in the Lennard-Jones well-depths between solute and solvent had to be introduced to reach agreement with the experimental results, similar to what is found for hydrocarbon-fluorocarbon interactions. The structure of the solutions was studied by analysis of solute-solvent radial distribution functions, showing that, although electrostatic interactions are not predominant, a small orientational effect is still present between the dipole of nitrous oxide and those of the substituted fluorinated liquids.

Liquid–liquid equilibria of aqueous mixtures containing selected dibasic esters and/or methanol

The liquid–liquid equilibrium (LLE) properties were measured for the ternary systems of water + methanol + dimethyl adipate, water + adipic acid monomethyl ester + dimethyl adipate and water + methanol + dimethyl glutarate at temperatures ranging from 298.15 to 318.15 K under atmospheric pressure. Each ternary system exhibited type-I LLE behavior and the region of heterogeneity was found to increase with a decrease of temperature. These new LLE data were correlated with the NRTL model and also used to examine the validity of various versions of the UNIFAC models.

Enhancement of liquid phase splitting of water + ethanol + ethyl acetate mixtures in the presence of a hydrophilic agent or an electrolyte substance

The enhancement of phase splitting was investigated for the liquid mixtures of water + ethanol + ethyl acetate in the presence of a hydrophilic agent (glycerol) or an electrolyte compound (potassium acetate, KAc, or calcium chloride, CaCl 2). The liquid–liquid equilibrium (LLE) properties were measured for ternary systems of water + ethanol + ethyl acetate and water + ethyl acetate + CaCl 2, and three quaternary systems, including water + ethanol + ethyl acetate + auxiliary agent (glycerol, potassium acetate, or calcium chloride), at temperatures ranging from 283.15 K to 313.15 K. The region of heterogeneity was found to increase as one of the auxiliary agents was introduced into the aqueous mixtures. By adding the same percentage of auxiliary agents, the magnitudes of phase-splitting enhancement follow the order of CaCl 2 > KAc > glycerol. The Electrolyte-NRTL model was used to correlate the new LLE data for the investigated mixtures. In general, the model reproduces reasonably for the LLE phase envelops.

Extension of the anisotropic united atoms intermolecular potential to amines, amides and alkanols: Application to the problems of the 2004 Fluid Simulation Challenge

A combined study by quantum chemistry and Monte Carlo simulation is conducted to extend the anisotropic united atoms (AUA) intermolecular potential to alkylamines, alkylamides and alkanols. Ab initio simulations are used to derive electrostatic point charges of the investigated molecules. The amplitude and location of these charges are optimized to best reproduce the electrostatic field around the molecule. The Lennard–Jones parameters of the amine NH 2 and alcohol OH groups are regressed to match the vapor–liquid equilibrium properties of methylamine, propylamine, methanol and ethanol. The Lennard–Jones parameters of the CH 2 and CH 3 groups are taken from previously published work [P. Ungerer, C. Beauvais, J. Delhommelle, A. Boutin, B. Rousseau, A.H. Fuchs, Optimization of the anisotropic united atoms intermolecular potential for n-alkanes, J. Chem. Phys. 112 (2000) 5499–5510]. The three problems of the Fluid Simulation Challenge are investigated with a unique AUA parametrization for the organic solvents as well as intermolecular potentials from the literature to treat gases or water. In the first problem, AUA potential parameters for the carbonyl C O group [S. Kranias, D. Pattou, B. Levy, A. Boutin, An optimized potential for phase equilibria computation for ketone and aldehyde molecular fluids, Phys. Chem. Chem. Phys. 5 (2003) 4175] are used to simulate the vapor–liquid equilibrium of acetone and butyramide in the Gibbs ensemble. A good agreement is found with experimental vapor pressures and vaporization enthalpies. In the second problem, the Henry constants of four gases in ethanol are determined by biased Widom test insertions. This property is predicted within 15% on average, and its variations with temperature agree with experiments. In the third problem, the heats of mixing are computed through simulations in the NPT ensemble. Good agreement with experimental data is found for the heptane–butylamine system but not for the water–butylamine system. Possible ways to improve prediction are briefly discussed.

Vapor−Liquid Equilibrium and Mixing Properties of Methanol + Diethyl Carbonate and Vinyl Acetate + Diethyl Carbonate Systems

Different mixing properties (speed of sound, density, and refractive index on mixing) of the binary mixtures methanol + diethyl carbonate and vinyl acetate + diethyl carbonate have been measured at 298.15 K and atmospheric pressure. Corresponding derived properties have been calculated by experimental data. Values of the physical properties were fitted to Redlich−Kister polynomials. The isobaric vapor−liquid equilibrium (VLE) experimental data at 101.3 kPa were satisfactorily tested for thermodynamic consistency by means of a modified Dechema test. Activity coefficients were correlated with Margulles, Van Laar, UNIQUAC, NRTL, and Wilson equations. The ASOG group contribution method was used for prediction.