Vapor–liquid equilibrium

The vapor-liquid equilibrium ( VLE usually called by English Vapor - Liquid Equilibrium ) is a phase equilibrium, in which a liquid and a vapor or gas in thermodynamic equilibrium are. The vapor-liquid equilibrium is also characterized in that the quantity of material which is evaporated, the amount of fuel corresponding to the condensed. In addition, the chemical potential is equal in both phases.

Basics

A vapor-liquid equilibrium is uniquely determined for pure substances by the values ​​of pressure and temperature, the compositions of the liquid and vapor phase have yet to be specified for mixtures. Compositions are usually expressed as amount of substance ( mole fraction ), x and y for the liquid phase to the vapor phase. A vapor-liquid equilibrium exists in the region between the triple and the critical point. The pressure which is established in the vapor-liquid equilibrium or set, is the saturation vapor pressure.

Mixtures

Mixture vapor -liquid equilibria are characterized in that the composition of the liquid and the vapor phase differ mostly. This effect is caused by the different volatility and thus the different partial pressure of the substances involved and is used in separation processes, in particular the rectification to separate mixtures. The low boilers enriched in the vapor phase, the high-boiling, however, in the liquid phase. The rectification is currently the most common method employed for separating substances such as in oil refineries. Systems in which the composition is no different, hot azeotropically and can not be separated by evaporation.

Parameters

The description of the vapor-liquid equilibrium of a mixture of a number of parameters have been defined:

• Separation factor which describes the ratio of the saturation vapor pressure of the components involved
• K factor which describes the ratio of the material of a component in vapor and liquid
• Relative volatility, which describes the ratio of two K- factors

These measures are all designed to be able to recognize in a simple manner whether a rectification shall be carried out in a meaningful way.

Typical representations

Pure component equilibria are usually simply represented as temperature-pressure diagrams, but often the logarithm of the pressure are plotted against the reciprocal of the temperature, since this representation approximate a straight line results.

Mixed vapor -liquid equilibria can be determined experimentally for the most part at a constant temperature and constant pressure. Therefore VLE binary mixtures are typically represented with a constant pressure and temperature to the composition and at a constant temperature and pressure to the composition in the liquid and the vapor phase. The concentrations in the vapor are referred to as dew line, while the sequence of liquid compositions is known as boiling. A common alternative representation is the plot of the two compositions in liquid and vapor x against y.

Modeling

A pure substance vapor - liquid equilibrium can be described by simple, often derived from the Clausius -Clapeyron equation equations such as the Antoine equation. These equations using substance-specific parameters that were fitted to experimental vapor pressure data.

A mixture of vapor - liquid equilibrium can be described as a first approximation by Raoult's Law. However, this assumes ideal behavior of the materials involved. To account for the real behavior is usually on the activity coefficient models such as non- random two- liquid model (NRTL ) or UNIQUAC (Universal Quasi Chemical) resorted describing the excess Gibbs energy.