Vapor Phase Densities Research Articles

Observation of vapor pressure enhancement of rare-earth metal-halide salts in the temperature range relevant to metal-halide lamps

Total vapor-phase densities of Dy in equilibrium with a DyI3/InI condensate and Tm in equilibrium with a TmI3/TlI condensate have been measured for temperatures between 900 K and 1400 K. The measurements show strong enhancements in rare-earth vapor densities compared to vapors in equilibrium with the pure rare-earth metal-halides. The measurements were made with x-ray induced fluorescence on the sector 1-ID beam line at the Advanced Photon Source. The temperature range and salt mixtures are relevant to the operation of metal-halide high-intensity discharge lamps.

Investigation of The Nucleation and Growth of Methanol Clusters from Supersaturated Vapor by Molecular Dynamics Simulations

Vapor-liquid nucleation and growth kinetics of methanol clusters were investigated by molecular dynamics simulations. Supersaturated states were generated by temperature quenches of a stable gas phase. The initial methanol vapor phase density varied from 0.056 to 0.446 mol/dm3, the target temperature after the quench ranged from 250 K to 290 K. The complete system consisted of methanol and argon, which is a carrier gas removing the latent heat of condensation from the system. The growth of the largest cluster in the system, the average cluster size, and the initial cluster size distributions were analyzed. The results were compared to calculations with the classical nucleation theory (CNT) using macroscopic properties obtained from simulations with the same molecular model. The rates calculated with the CNT and the simulation data at high supersaturation differed by two to three orders of magnitude. Simulation results and experimental data taken from the literature were consistently below the CNT values.

Data Consistency Tests through the Use of Neural Networks and Virial Equation. Application of the Proposed Methodology to Critical Study of Density Data

This paper focuses on a very important point which consists in evaluating experimental data prior to their use for chemical process designs. Hexafluoropropylene P, ρ, T data measured at 11 temperatures from 263 to 362 K and at pressures up to 10 MPa have been examined through a consistency test presented herein and based on the use of a methodology implying both neural networks and Virial equation. Such a methodology appears as very powerful to identify erroneous data and could be conveniently handled for quick checks of databases previously to modeling through classical thermodynamic models and equations of state. As an application to liquid and vapor phase densities of hexafluoropropylene, a more reliable database is provided after removing out layer data.

Testing the adequacy of simple water models at the opposite ends of the phase diagram

The transferability of a few simple rigid non-polarizable water models were tested by Gibbs Ensemble Monte Carlo simulations to predict their vapor–liquid phase equilibria, and by isothermal–isobaric (Parrinello–Rahman) Monte Carlo simulations of the 13 known crystalline phases of ice. The temperature dependence of the corresponding second virial coefficients was also determined and then used to test the internal consistency of the simulated vapor-phase densities. The model predictions appear satisfactory for liquid water for ambient conditions, but they fail to mimic accurately the properties of the ice polymorphs and the orthobaric vapor phase. The major shortcomings of the models were in the overestimation by a factor of two (∼ 4–6 kJ/mol) of the internal energy difference between the high-pressure ice phases and the hexagonal phase. This unacceptable discrepancy is caused by the parameterization to reproduce the density of liquid water at ambient conditions, that accounts for the significant polarization effects in the condensed phases in terms of augmented dipole moments, with the consequent detrimental effect on the estimations of the vapor-phase properties.

An advanced Gibbs-Duhem integration method: Theory and applications

The conventional Gibbs-Duhem integration method is very convenient for the prediction of phase equilibria of both pure components and mixtures. However, it turns out to be inefficient. The method requires a number of lengthy simulations to predict the state conditions at which phase coexistence occurs. This number is not known from the outset of the numerical integration process. Furthermore, the molecular configurations generated during the simulations are merely used to predict the coexistence condition and not the liquid- and vapor-phase densities and mole fractions at coexistence. In this publication, an advanced Gibbs-Duhem integration method is presented that overcomes above-mentioned disadvantage and inefficiency. The advanced method is a combination of Gibbs-Duhem integration and multiple-histogram reweighting. Application of multiple-histogram reweighting enables the substitution of the unknown number of simulations by a fixed and predetermined number. The advanced method has a retroactive nature; a current simulation improves the predictions of previously computed coexistence points as well. The advanced Gibbs-Duhem integration method has been applied for the prediction of vapor-liquid equilibria of a number of binary mixtures. The method turned out to be very convenient, much faster than the conventional method, and provided smooth simulation results. As the employed force fields perfectly predict pure-component vapor-liquid equilibria, the binary simulations were very well suitable for testing the performance of different sets of combining rules. Employing Lorentz-Hudson-McCoubrey combining rules for interactions between unlike molecules, as opposed to Lorentz-Berthelot combining rules for all interactions, considerably improved the agreement between experimental and simulated data.

Vapor–liquid equilibria and densities of CO 2 with four unsaturated fatty acid esters at elevated pressures

In this study experimental P– T– x– y equilibrium data for four binary mixtures over a wide range of pressure (10.44–23.54 MPa) were determined at 313 K and 333 K. Liquid and vapor densities of carbon dioxide + four fatty acid ester systems, including oleic acid methyl ester, linoleic acid methyl ester, eicosapentaenoic acid ethyl ester, and docosahexaenoic acid ethyl ester were measured by using two circulating systems consisting of two vibrating tube densitometers. The vapor-phase density was observed to increase more significantly with pressure than the liquid-phase density at constant temperature. Experimental equilibrium constants of oleic acid methyl ester and linoleic acid methyl ester were found to be larger than those of eicosapentaenoic acid ethyl ester, docosahaxaenoic acid ethyl ester, linoleic acid, triolein, and tocopherol. This indicates that the two methylated fatty acids would be extracted easier than the other compounds in a separation process using supercritical CO 2 extraction. The equilibrium data were successfully correlated using the Peng–Robinson and modified Soave–Redlich–Kwong equations of state with quadratic (two parameters), Panagiotopoulos–Reid (three parameters), and Adachi–Sugie (three parameters) mixing rules. In general, the Soave–Redlich–Kwong equation of state with the Panagiotopoulos–Reid mixing rules and the quadratic mixing rules give the best correlation for methylated fatty acids and ethylated fatty acids, respectively.

Modeling vapor–liquid interfaces with the gradient theory in combination with the CPA equation of state

With the final purpose of describing the important aqueous + hydrocarbon liquid–liquid interfaces, the gradient theory was combined with the Cubic-Plus-Association equation of state (CPA EOS), taking advantage of the correct representation of interfacial tensions provided by the gradient theory and the correct phase equilibrium of water + hydrocarbon systems already obtained from CPA.In this work, preliminary studies involving the vapor–liquid interfacial tensions of some selected associating and non-associating pure components (water, ethanol, n-butane, n-pentane, n-hexane, n-heptane) are presented and discussed.The good description of equilibrium properties such as vapor pressure and liquid and vapor phase densities is shown in the full range of the vapor–liquid saturation line. For non-associating components, results are compared with those from the Soave–Redlich–Kwong and Peng–Robinson equations of state.A correlation for the influence parameter is presented from which surface tensions can be obtained in a broad temperature range with average errors smaller than 1%.

Response to “Comment on ‘Equations of state for fluids: The Dieterici approach revisited’ ” [J. Chem. Phys. 115, 1460 (2001)

A re-evaluation is reported of earlier calculations for vapor–liquid equilibria [R. J. Sadus, J. Chem. Phys. 115, 1460(E) (2001); 116, 5913 (2202)] using the Dieterici–Carnahan–Starling (DCS) equation of state. The results for liquid-phase densities and properties near the critical point are generally consistent with the original work. However, the recalculated vapor-phase densities and coexistence pressures are shifted to substantially higher values. The implications for real fluids are that the DCS equation is likely to yield accurate predictions of both near-critical equilibria and the temperature-density behavior of the liquid phase at the expense of poor agreement with experiment for both vapor densities and pressures.

Application of the holographic interferometer for modeling of processes in a vapor phase epitaxy reactor

Using holographic interferometry techniques, it is possible to visualize flows in the gas phase and obtain quantitative data about vapor phase density distribution in a given volume. The possibility of using this approach for monitoring processes in a vapor phase epitaxy (VPE) reactor has been studied using an experimental setup comprising a reactor chamber with two transparent windows, a real-time holographic interferometer, a gas supply system, and a system for visual observation of the gas flows. The results show good prospects of this method for optimization of the reactor design and the VPE process parameters.

Apparatus for phase equilibrium measurements at high temperatures and pressures

An apparatus for high pressure studies of phase equilibrium in multicomponent fluid systems is completely described. The apparatus has been designed to yield a complete thermodynamic description of fluid mixtures containing any number of components which have up to three coexisting phases in the pressure range up to 100 MPa and for temperatures up to 423 K. It therefore makes possible the determination of liquid and vapour phase densities and composition at prescribed pressures and temperatures for two phases as well as the determination of the same quantities for a second liquid phase should one exist. At present, owing to the temperature and pressure limitations imposed by some of the commercial parts, the apparatus works within the temperature range up to 393 K and the pressure range up to 40 MPa. The instrument has the capability for complete automation if it is desired. The results of preliminary measurements in the normal hydrocarbon systems CH 4–C 6H 14 and CH 4–C 6H 14–C 14H 30 are presented.

Solubilities of salicylic acid in supercritical carbon dioxide with ethanol cosolvent

A circulation apparatus for simultaneously measuring the solubilities of solids in supercritical fluids and the densities of supercritical phases has been constructed. The apparatus can be used at pressures up to 20 MPa. The solubilities of salicylic acid in supercritical carbon dioxide with ethanol cosolvent and the densities of the fluid phase have been measured at 308.15 K and 318.15 K and at pressures up to 16 MPa. The concentration of ethanol in the vapor phase was in the range from 0 to 7 mol %. The effects of temperature, pressure, and cosolvent concentration on the solubility and the density have been investigated. The results show that the solubility of salicylic acid in supercritical carbon dioxide increases markedly with the concentration of the cosolvent. The solubility is a strong function of vapor-phase density when the concentration of the cosolvent is lower, and is nearly independent of the density at higher cosolvent concentrations. The interaction between the solute and the mixed solvent is very strong, especially through hydrogen bonding. The density of the fluid phase increases considerably with pressure and the concentration of the cosolvent.

Modified SPHCT equation of state for improved predictions of equilibrium and volumetric properties of pure fluids

Segment-segment interaction models, such as the simplified-perturbed-hard-chain theory (SPHCT) equation of state (EOS), offer the potential of improved volumetric and equilibrium property predictions. Our previous evaluations of the SPHCT EOS and similar evaluations in the literature lead us to believe that proper modifications can make the SPHCT more accurate. In the present study, we report our results for pure-fluid property predictions. The SPHCT EOS is reduced from a three-parameter to a one-parameter equation of state by using classical critical point constraints. The temperature dependence of the attractive portion of the equation is modified to provide improved vapor pressure predictions. In addition, volumetric predictions are enhanced by incorporating a phenomenological volume translation strategy. The modified SPHCT equation of state is compared with the orginal version and with the Peng-Robinson (PR) equation using experimental data for a variety of chemical species. The modified SPHCT equation yields better vapor pressure predictions than either the original SPHCT or PR equations (1.2% average absolute percent deviation for the modified SPHCT, 3.4% for the original SPHCT, and 4.2% for PR). Improved results are also obtained for liquid and vapor phase densities (2.7% and 1.9% for the modified SPHCT, 6.8% and 6.2% for the original SPHCT, and 6.6% and 3.1% for PR for liquid and vapor phases respectively).

Phase equilibria of ternary and quaternary systems containing carbon dioxide, water, isopropanol, and glucose

Vapor-liquid equilibrium data have been measured for the ternary carbon dioxide-isopropanol-water system and for the quaternary carbon dioxide-isopropanol-water-glucose system at 313 and 333 K and at pressures between 13.8 and 30.0 MPa. For the quaternary system, densities were also obtained. A special high-pressure apparatus with vapor-phase recirculation and'a sample bomb placed in the recirculation loop was used for the measurements. The results show a phase behavior in the ternary system similar to that described in the literature at lower pressures. In the quaternary system, glucose concentrations in the vapor phase vary between 4.4 × 10 -4 and 0.11 mol %. While the glucose concentration is strongly dependent on the vapor-phase concentration of isopropanol and water, pressure and vapor-phase density do not influence the solubility of glucose in the vapor phase significantly. Comparing the effect of isopropanol and ethanol on the entrainment of glucose in the vapor phase, it is evident that while the measured maximum glucose solubilities in the vapor phase are of the same order of magnitude (0.11 mol % at 13.79 MPa, 313.15 K for the isopropanol system and 0.17 mol % at 25.0 MPa, 313.15 K for the ethanol system), higher pressures are needed with ethanol as a cosolvent to achieve the same glucose solubilities possible with isopropanol.

Method for the steady solution of a nonhyperbolic system of equation for the motion of a two-phase flow

An approximate method for solving an incorrect Cauchy problem for a nonhyperbolic system of equations of a vapor-liquid flow barotropic model has been developed. The method is based on the approximation of a fluctuating parameter—the vapor phase density—by a random delta-correlated process and subsequent averaging of the stochastic equations obtained by its realization. Such a procedure allows one to reduce the matrix of the nonhyperbolic system coefficients to Hermitian form and to obtain a characteristic polynomial all the roots of which are real, i.e., it enable one to eliminate the incorrectness of the problem. The results of numerical realization have shown the steady-state character of the solution, which is in good agreement with experimental data.

Application of a generalized multiproperty apparatus to measure phase equilibrium and vapor phase densities of supercritical carbon dioxide in n-hexadecane systems up to 26 MPa

A high pressure multi-property apparatus has been developed to measure several thermodynamic and transport properties. In this study the apparatus has been applied to measure phase and volumetric properties of supercritical carbon dioxide in hexadecane from low pressures to the very vicinity of the critical pressure. The experimental data have been successfully fitted over the entire range of the experiment using the Wilson—Wegner expansion including the use of the latter to obtain an improved estimate of the critical pressure and composition. The relation between the occurrence of multiple condensed phases (at some lower temperature) and supercritical extraction is discussed.