Multicomponent Boiling—Vapor-Liquid Equilibrium

The vapor composition in equilibrium with the liquid usually has a different composition than the liquid. For ideal gas and liquid mixtures the equilibrium relations are given by combining Dalton s and Raoult s laws. 

To compute the fugacity of the vapor phase for nonideal gases, we use the following procedure. At low pressures, the equation of state for an ideal gas is 

Considering a system at constant temperature and using equation (5.3.4) we have 

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The Non-Random, Two Liquid Equation was used in an attempt to develop a method for predicting isobaric vapor-liquid equilibrium data for multicomponent systems of water and simple alcohols—i.e., ethanol, 1-propanol, 2-methyl-l-propanol (2-butanol), and 3-methyl-l-butanol (isoamyl alcohol). Methods were developed to obtain binary equilibrium data indirectly from boiling point measurements. The binary data were used in the Non-Random, Two Liquid Equation to predict vapor-liquid equilibrium data for the ternary mixtures, water-ethanol-l-propanol, water-ethanol-2-methyl-1-propanol, and water-ethanol-3-methyl-l-butanol. Equilibrium data for these systems are reported. 

EXAMPLE 11.7-1. Boiling Point of a Multicomponent Liquid A liquid feed to a distillation tower at 405.3 kPa abs is fed to a distillation tower. The composition in mol fractions is as follows n-butane(x = 0.40), n-pentane [xg = 0.25), n-hexanefx = 0.20), n-heptane(xp = 0.15). Calculate the boiling point and the vapor in equilibrium with the liquid. 

Distillation is a process in which a liquid mixture is separated into its component parts by vaporization. The vapor evolved from a boiling liquid mixture is normally richer in the more volatile components than the liquid with which it is in equilibrium. Distillation rests upon this fact. Although multicomponent mixtures are most common in distillation processes, an understanding of the operation can be based on the vapor pressure characteristics of two-component or binary mixtures. 

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