Vapor-liquid equilibrium (VLE) describes the distribution of a chemical species between the gas and liquid phases at equilibrium. The concentration of a vapor in contact with its liquid depends on temperature, with vapor pressure strongly dependent on temperature. At equilibrium, the concentrations or partial pressures of vapor components and liquid component concentrations are related. VLE is described thermodynamically, with temperature, pressure, and chemical potentials equal between phases for single-component and multicomponent systems. VLE diagrams graphically represent vapor and liquid compositions. VLE is important for distillation column design in separation processes.

1. Vapour Liquid Equilibrium
 Definition:
The vapor–liquid equilibrium (VLE) describes the distribution of a
chemical species between the gas phase and a liquid phase.
 Explanation:
The concentration of a vapor in contactwith its liquid, especially at equilibrium, is
often expressed in terms of vapor pressure, which will be a partial pressure (a part
of the total gas pressure) if any other gases are present with the vapor. The
equilibrium vapor pressure of a liquid is in general strongly dependent on
temperature. At vapor–liquid equilibrium, a liquid with individual components in
certain concentrations will have an equilibrium vapor in which the concentrations
or partial pressures of the vapor components have certain values depending on all
of the liquid component concentrations and the temperature. The converse is also
true: if a vapor with components at certain concentrations or partial pressures is in
vapor–liquid equilibrium with its liquid, then the component concentrations in the
liquid will be determined dependent on the vapor concentrations and on the
temperature. The equilibrium concentration of each component in the liquid phase
is often different from its concentration (or vapor pressure) in the vapor phase, but
there is a relationship.
 Thermodynamic description of vapor–liquid equilibrium:
The field of thermodynamics describes when vapor–liquid equilibrium is possible,
and its properties. Much of the analysis depends on whether the vapor and liquid
consist of a single component, or if they are mixtures.
 Pure (single-component)systems:
If the liquid and vapor are pure, in that they consist of only one molecular
component and no impurities, then the equilibrium state between the two phases is
described by the following equations:

2. Pliq =Pvap
Tliq = Tvap
G-liq=G-vap
where , and , Pliq =Pvap are the pressures within the liquid and Tliq = Tvap are the
temperatures within the liquid and vapor, and G-liq=G-vap are the molar Gibbs free
energies (units of energy per amount of substance) within the liquid and vapor,
respectively.In other words, the temperature, pressure and molar Gibbs free energy
are the same between the two phases when they are at equilibrium.
 Multicomponent systems
In a multicomponent system, where the vapor and liquid consist of more than one
type of molecule, describing the equilibrium state is more complicated. For all
components i in the system, the equilibrium state between the two phases is
described by the following equations:
Pliq =Pvap
Tliq = Tvap
G-liq=G-vap
where P and T are the temperature and pressure for each phase & G-liq=G-vap are
the partial molar Gibbs free energy also called chemical potential (units of energy
per amount of substance) within the liquid and vapor respectively, for each phase.
 Vapor–liquid equilibrium diagrams:
For each component in a binary mixture, one could make a vapor–liquid
equilibrium diagram. Such a diagram would graph liquid mole fraction on a
horizontal axis and vapor mole fraction on a vertical axis. In such VLE diagrams,
liquid mole fractions for components 1 and 2 can be represented
as x1 and x2 respectively, and vapor mole fractions of the corresponding
components are commonly represented as y1 and y2.Similarly for binary mixtures
in these VLE diagrams:
x1 + x2 = 1 and y1 + y2 = 1

3. Such VLE diagrams are square with a diagonal line running from the (x1 = 0, y1 =
0) corner to the (x1 = 1, y1 = 1) corner for reference.
 Vapor-Liquid Equilibrium Diagram
 Experimental Determinationof VLE:
The VLE concentration data can be determined experimentally, or computed or
approximated with the help of theories suchas Raoult's law, Dalton's law, and
Henry's law.
 Raoult’s Law for VLE:
Vapour-liquid systems which are ideal in nature display a behaviour corresponding
to Raoult’s Law. In such a system, both the vapour and the liquid phases
essentially behave as ideal mixtures.

4. If we now hold temperature constant and plot the vapor pressure of a benzene-
toluene solution as a function of composition we would get a graph that looked
something like this,
 Applications of VLE:
Vapor–liquid equilibrium information is useful in designing columns for
distillation, especially fractional distillation, which is a particular specialty of
chemical engineers. Distillation is a process used to separate or partially separate
components in a mixture by boiling (vaporization) followed by condensation.
Distillation takes advantage of differences in concentrations of components in the
liquid and vapor phases.
 References:
 https://pubs.acs.org/doi/abs/10.1021/je60053a027
 https://en.wikipedia.org/wiki/Vapor%E2%80%93liquid_equilibrium
 https://www.researchgate.net/publication/265815099_VapourLiquid_Equilib
rium_Theory
 http://nptel.ac.in/courses/103101004/downloads/chapter-7.pdf
 https://en.wikibooks.org/wiki/Introduction_to_Chemical_Engineering_Proce
sses/Vapor-Liquid_equilibrium