Distillation

Distillation is a physical separation process in which a liquid mixture is partly vaporized and the resulting vapor is condensed to recover a liquid with a different composition. Separation depends on differences in the components’ volatility—their tendency to enter the vapor phase under specified conditions. The collected condensate is called the distillate; the liquid remaining is the residue or bottoms. Distillation is a major operation in chemical engineering and is used to separate and purify liquids without necessarily changing their chemical identities. (archive.nptel.ac.in)

Physical principles

The governing relationship is vapor–liquid equilibrium: at a given temperature and pressure, a mixture’s liquid and vapor generally have different compositions. Vapor is usually enriched in the more volatile component. Condensing and revaporizing successive portions can therefore increase separation. Pure-component boiling points provide useful guidance, but they do not alone determine how a mixture behaves. (archive.nptel.ac.in)

For an ideal liquid solution with approximately ideal vapor behavior, Raoult’s law gives

where $x_i$ and $y_i$ are the liquid and vapor mole fractions of component $i$, $P$ is total pressure, and $P_i^{\mathrm{sat}}$ is its pure-liquid vapor pressure. Departures from ideality require more detailed thermodynamic descriptions. Relative volatility, conventionally written $\alpha_{AB}=(y_A/x_A)/(y_B/x_B)$, measures the ease of separating two components. Values near unity indicate difficult separation; at unity, an equilibrium stage provides no compositional separation between them. (archive.nptel.ac.in)

A common misconception is that heating between two pure substances’ boiling points removes only the lower-boiling substance. Both volatile components generally enter the vapor, so a single distillation seldom produces complete separation. (columbia.edu)

Principal methods

Simple distillation uses a boiling vessel, vapor path, condenser, and receiver, without a dedicated multistage fractionating column. It is useful for recovering a volatile liquid from nonvolatile impurities or separating liquids whose volatility differences are sufficiently large for the required purity. The condenser removes heat and causes condensation. (archive.nptel.ac.in)

Fractional distillation introduces a column that provides repeated contact between vapor and liquid. This improves separation when a single vaporization–condensation step is inadequate, especially for liquids with relatively similar boiling characteristics. Products may be collected as separate fractions rather than as one combined distillate. (columbia.edu)

Vacuum distillation operates below atmospheric pressure. Lower pressure reduces the boiling temperature, allowing high-boiling or thermally unstable substances to be processed at lower temperatures. Vacuum operation can be combined with either simple or fractional distillation; it describes the pressure regime rather than the number of separation stages. (columbia.edu)

Steam distillation uses steam or boiling water to carry volatile substances into the condenser. For practically immiscible liquids, their vapor-pressure contributions add, allowing the mixture to boil below either pure liquid’s boiling temperature at the same pressure. The condensate contains both substances and may separate into liquid layers. (columbia.edu)

Batch and continuous operation

In batch distillation, a fixed charge is introduced into a still. Its composition changes as vapor is removed, and successive fractions can differ substantially. For simple differential batch distillation, the Rayleigh equation relates the remaining liquid quantity to changing composition using equilibrium data. Batch processing is inherently unsteady. (archive.nptel.ac.in)

In continuous distillation, feed enters while products are withdrawn. Industrial columns commonly contain trays or packing that promote contact between rising vapor and descending liquid. Trays provide discrete contacting stages; packing provides contact along the column height. Equipment design therefore considers both separation performance and the physical behavior of the flowing phases. (archive.nptel.ac.in)

A conventional column system includes a reboiler supplying vapor and an overhead condenser. Some condensed overhead liquid returns as reflux, while the remainder leaves as distillate. Reflux supplies downward liquid flow and supports enrichment of the overhead product. Design calculations connect reflux, internal flow rates, and the number of separation stages. A theoretical plate represents an ideal stage whose exiting liquid and vapor are in equilibrium; actual equipment requires allowance for imperfect contacting efficiency. (archive.nptel.ac.in)

Azeotropes and separation limits

Nonideal mixtures can form an azeotrope, a composition at which equilibrium vapor and liquid have identical compositions. Ordinary distillation at fixed pressure cannot carry a mixture through this composition to achieve unrestricted separation. Azeotropes may exhibit minimum or maximum boiling temperatures relative to neighboring mixture compositions. Adding more stages does not remove this equilibrium limitation. (archive.nptel.ac.in)

Specialized separation schemes alter the equilibrium conditions or combine operations. Their suitability depends on the mixture: reducing pressure, for example, does not universally eliminate azeotropes. Pressure-dependent equilibrium data are therefore essential when evaluating such alternatives. (archive.nptel.ac.in)

Applications and energy requirements

In petroleum refining, distillation separates crude oil into broad boiling-range fractions containing many hydrocarbons, rather than isolating every compound individually. Lighter fractions are recovered higher in the tower and heavier fractions lower down. Subsequent conversion and treatment operations produce finished fuels and chemical feedstocks; distillation itself is distinct from molecular conversion processes such as cracking. Laboratories also use it to purify liquids and recover solvents. (eia.gov)

Repeated vaporization makes distillation energy intensive, especially when relative volatility is low. Process development includes improved column configurations, thermal coupling, and heat integration, which reuse thermal energy between separation duties. The appropriate configuration balances separation requirements, equipment complexity, and operating energy demand. (archive.nptel.ac.in)