Nonideality Of Liquid Phase Research Articles

Polyazeotropy in Binary Systems. 2. Association Effects

Polyazeotropy in binary systems is a singular case of vapor−liquid equilibrium where at a given temperature or pressure various azeotropes of alternating deviations from ideality are observed. These deviations are generally caused by liquid-phase nonidealities, but in associated systems and even at low pressures, the vapor-phase nonidealities are remarkable. Associated systems are analyzed using chemical theory, where associated species are considered to be products of polymerization reactions; in those systems, azeotropy can be achieved without the usual requirement of equal composition in the phases. The purpose of this work is to show how the combination of nonidealities in both liquid and vapor phases can induce polyazeotropy and to establish its thermodynamical conditions.

On the prediction of evaporation rates—With special emphasis on aqueous solutions

The performance of a theoretically based model for the prediction of evaporation rates of liquid mixtures is investigated with special emphasis on aqueous solutions. The model is an extension of a previously suggested evaporation model for pure volatile organic compounds. Liquid phase nonideality is corrected for by use of activity coefficients. A comparison with experimental data on evaporation of various liquid mixtures shows that the inclusion of activity coefficients is often mandatory for occupational hygiene applications. The potential risk of exposure associated with hydrophobic compounds in minute concentrations in water is demonstrated. Activity coefficients for a wide range of mixtures can be estimated accurately using the group-contribution method UNIFAC. For certain strongly non-ideal aqueous mixtures UNIFAC is less accurate and experimental infinite dilution activity coefficients should be applied if available. For liquid mixtures, measured and predicted evaporation rates were in satisfactory agreement, indicating that liquid phase diffusional resistance can be neglected also in strongly non-ideal aqueous solutions. It was found, however, that the presence of water-soluble polymers in aqueous solutions suppresses the evaporation of volatile organic compounds, even at low concentrations, owing to liquid phase diffusional resistance and reduced activity. Liquid phase diffusional resistance due to polymers was found to increase with liquid viscosity and volatility of the dilute organic compound. This effect cannot be predicted by the evaporation model.

Equilibrium and thermodynamics for 2-methyl-2-methoxybutane liquid-phase decomposition

The chemical equilibrium for the liquid-phase decomposition of 2-methyl-2-methoxybutane ( tert-amyl methyl ether) was studied experimentally in the temperature range 313–353K and at 700 kPa. To reach decomposition equilibrium, the macroporous acid resin K-2631 (Bayer) was used as the catalyst. Equilibrium constants and standard enthalpies, free energies, and entropies for addition of methanol to 2-methyl-1-butene and 2-methyl-2-butene (forward reactions) were determined as a temperature function and compared with literature data. The unifac estimates of activity coefficients were used to describe the liquid-phase non-ideality.

Evaluation of activity coefficient models in prediction of alkane solid-liquid equilibria

In the petroleum industry, the absence of a proper model to describe the liquid phase non-ideality for mixtures of alkanes with large size differences is still one of the main problems in solid-liquid equilibrium calculations. A search is made for a reliable model for the prediction of activity coefficients in these systems. The models investigated were originally developed for the polymer mixtures. The performances of original Flory-Huggins, modified UNIFAC, GCFLORY model, Flory free-volume, Entropic free-volume and some empirical modifications of these last two models are analysed extensively and compared using the deviations between experimental and predicted values of solid appearance temperature as criteria. A comprehensive experimental SLE data base with around 60 binary systems and more than 1000 data points for mixtures of linear, branched and cyclic alkanes is used. An analysis of the errors introduced by the simplifying assumptions more commonly used is also performed. Activity coefficient models that have been used in wax formation predictions, but which are not included in this comparison, are briefly discussed. It is shown that the original Flory-Huggins activity coefficient model, the regular solution theory and the ideal solution behavior, used in several wax formation models, as well as the modified UNIFAC and original Entropic free-volume models, are not appropriate for the description of the liquid phase in alkane systems. The importance of a free-volume contribution to the phase behavior description of liquid mixtures whose components have significant size differences is evident. The Flory free-volume and a modified version of Entropic free-volume seem to be the simplest and most reliable models.

Ionization and Henry’s law constants for volatile, weak electrolyte water pollutants

Data base on ionization and Henry’s law constants up to 673 K are presented by temperature dependent equations for volatile, weak electrolyte water pollutants; ammonia, carbon dioxide, hydrogen sulfide, sulfur dioxide and hydrogen cyanide. In the fitting of ionization constants, various recent sources of experimental isothermal data are mainly utilized, however. Henry’s constants are correlated by two steps: first the vapor pressures, the partial molar volumes at infinite dilution, and the vapor and liquid phase nonidealities based on the vapor-liquid equilibrium conditions are calculated from P-T-x data. And then the calculated Henry’s constants at different isotherms are treated as a function of temperature. Both correlations reproduce actual data accurately up to high temperatures for reliable phase equilibrium calculations.

Thermodynamics of adsorption from binary liquid mixtures on heterogeneous adsorbents

The surface excess isotherms for adsorption of n-heptane–cyclohexane mixtures on graphon and benzene–cyclohexane mixtures on silica gel at three different temperatures have been analysed by a simple analytical model which accounts for the effects of the liquid-phase non-ideality, the dissimilarity of the adsorbate sizes and the adsorbent heterogeneity on the isotherm. The adsorbents are found to be substantially heterogenous for these mixtures. An analytical expression for the heat of immersion of the heterogeneous adsorbent as a function of the equilibrium liquid-phase composition has been derived. It is shown that the adsorbent heterogeneity has a much more pronounced effect on the shape of the surface excess isotherm than on the heat of immersion.