Distillation takes advantage of differences in concentrations of components in the liquid and vapor phases. Distillation is a process used to separate or partially separate components in a mixture by boiling (vaporization) followed by condensation. Such vapor–liquid equilibrium information is useful in designing columns for distillation, especially fractional distillation, which is a particular specialty of chemical engineers. The VLE concentration data can be determined experimentally, approximated with the help of theories such as Raoult's law, Dalton's law, and Henry's law. 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. 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. 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 equilibrium vapor pressure of a liquid is in general strongly dependent on temperature. The concentration of a vapor in contact with 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 gas(es) are present with the vapor. In thermodynamics and chemical engineering, the vapor–liquid equilibrium (VLE) describes the distribution of a chemical species between the vapor phase and a liquid phase. 13-100 Example 10: Calculation of Multicomponent Batch Distillation. 13-100 Rigorous Comput er-Based Calculation Procedures. 13-99 Shortcut Meth ods for Multicomponent Batch Rectification. 13-98 Batch R ectification at Constant Overhead Composition. 13-98 Batch Rectification at Constant Reflux. 13-96 Approximate C alculation Procedures for Binary Mixtures. 13-96 Batch Distillation with Rectification. 13-81 Mechanical Design and Implement ation Issues. 13-81 Simulation, Modeling, and Design Feasibility. 13-79 Extractive Distillation by Salt Effects. 13-76 Extractive Distillation Design and Optimization. 13-75 Solvent Effects in Extractive Distillation. 13-73 Design and Ope ration o f A zeotropic Distillation Co lumns. 13-73 Exploitation of Azeotropy and L iquid-Phase Im miscibility. 13-72 Exploitation of Boundary Curvature. 13-69 Exploitation of Pressure Sensitivity. 13-68 Exploitation of Homogeneous Azeotropes. 13-54 Residue Curve Maps and Distillation Region Diagrams. 13-32 Intermediate Re boilers and Cond ensers.