(1977a), is probably the most useful for process design. The objective has been to enable the prediction of phase equilibrium data for the tens of thousands of possible mixtures of interest to the process designer to be made from the contributions of the relatively few functional groups which made up the compounds. Group contribution methods have been developed for the prediction of liquid-phase activity coefficients.
Treybal (1963) has shown that the Van-Laar equation should be used for predicting activity coefficients from mutual solubility limits. The methods used are described by Reid et al. (1987).Ĭalculation of activity coefficients from mutual solubility data For systems that are only partially miscible in the liquid state, the activity coefficient in the homogeneous region can be calculated from experimental values of the mutual solubility limits. Their method, and the data needed for predictions, is described by Treybal (1963) and Reid et al. (1955) have developed correlations for the prediction of the activity coefficientsĪt infinite dilution for systems containing water, hydrocarbons and some other organic compounds. The methods used are described by Null (1970) and Conder and Young (1979). Relatively simple experimental techniques, using ebulliometry and chromatography, are available for the determination of the activity coefficients at infinite dilution. The constants in any of the activity coefficient equations can be readily calculated from experimental values of the activity coefficients at infinite dilution. Horsley (1973) gives an extensive collection of data on azeotropes.Īctivity coefficients at infinite dilution The values of the activity coefficients determined at the azeotropic composition can be used to calculate the coefficients in the Wilson equation (or any other of the three-suffix equations) and the equation used to estimate the activity coefficients at other compositions. Where P °i is determined at the azeotropic temperature. At the azeotropic point the composition of the liquid and vapour are the same, so from equation: If a binary system forms an azeotrope, the activity coefficients can be calculated from a knowledge of the composition of the azeotrope and the azeotropic temperature. Some of the techniques most useful in design are given in the following paragraphs.Įstimation of activity coefficients from azeotropic data The same confidence cannot be placed on the prediction of equilibrium data as that for many of the prediction techniques for other physical properties given in this chapter. Caution must be used in the application of these techniques in design and the predictions should be supported with experimentally determined values whenever practicable. Some techniques are available for the prediction of vapour liquid equilibria (v l e) data and for the extrapolation of experimental values.
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The designer will often be confronted with the problem of how to proceed with the design of a separation process without adequate experimentally determined equilibrium data.
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