= the vapor-liquid equilibrium concentration of component j in the liquid phase = the vapor-liquid equilibrium concentration of component j in the vapor phase = the vapor-liquid equilibrium concentration of component i in the liquid phase = the vapor-liquid equilibrium concentration of component i in the vapor phase = the relative volatility of the more volatile component i to the less volatile component j Relative volatilities are not used in separation or absorption processes that involve components reacting with each other (for example, the absorption of gaseous carbon dioxide in aqueous solutions of sodium hydroxide).įor a liquid mixture of two components (called a binary mixture) at a given temperature and pressure, the relative volatility is defined as An alternative measure of composition is the convergence pressure of the system, which is defined as that pressure at which the K values for all the components in an isothermal mixture converge to unity.Relative volatilities are used in the design of all types of distillation processes as well as other separation or absorption processes that involve the contacting of vapor and liquid phases in a series of equilibrium stages.

The Kellogg and DePriester charts and their subsequent extensions and generalizations use the molar average boiling points of the liquid and vapor phases to represent the composition effect. SI versions of these charts have been developed by Dadyburjor. For K as a function of T and P only, the DePriester charts provide good starting values for the iteration. One cannot calculate K values until phase compositions are known, and those cannot be known until the K values are available to calculate them. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state, which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question.Ī trial-and-error procedure is required with any K-value correlation that takes into account the effect of composition. These charts are a simplification of the Kellogg charts and include additional experimental data. The easiest to use are the DePriester charts, which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, n-butane, isopentane, n-pentane, n-hexane, and n-heptane). For example, several major graphical K-value correlations are available for light-hydrocarbon systems. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure.

4, the K value of a species is a complex function of temperature, pressure, and equilibrium vapor- and liquid-phase compositions.

Depriester chart isobutane