Activity Coefficients Determination from Vapor-Liquid Equilibrium Measurements

5 Activity Coefficients Determination from Vapor-Liquid Equilibrium Measurements 

To this purpose, we present first a typical method for such measurements, and Aen discuss how the activity coefficients are evaluated from them. 

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The illustrations of this section were meant to demonstrate how one can determine activity coefficients from measurements of temperature, pressure, and the mole fractions in both phases of a vapor-liquid equilibrium system. An alternative procedure is at constant temperature, to measure the total equilibrium pressure above liquid mixtures of known (or measured) composition. This replaces time-consuming measurements of vapor-phase compositions with a more detailed analysis of the experimental data and more complicated calculations. ... 

Thus, the activity coefficient of component i in solution is determined from knowledge derived from measurements of the vapor fugacity of component i in equilibrium with the liquid solution. If the vapor fugacities of components other than i are known, the vapor fugacity of / can be determined by application of the Gibbs-Duhem equation and, again, the activity coefficient of component i can be found. 

Activity coefficients are generally either determined from Equation 6.18a and Equation 6.18b using measured vapor- and liquid-equilibrium compositions or are estimated from the UNIQUAC equation mentioned earlier. A third method arises when the two components in question have low mutual solubilities, y values are then confined to the Henry s law regions near x = 0 and X = 1 (Eigure 6.19) and cease to exist outside those regions. This provides a way of calculating activity coefficients using classical chemical thermodynamics. 

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