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Especially important in this connection are the still heads, fractionating columns, and condensers that permit the return of some of the condensed vapour toward the still. The objective is to achieve the closest possible contact between rising vapour and descending liquid so as to allow only the most volatile material to proceed in the form of vapour to the receiver while returning the less volatile material as liquid toward the still.

The purification of the more volatile component by contact between such countercurrent streams of vapour and liquid is referred to as rectification, or enrichment. Multiple-effect distillation , often called multistage-flash evaporation, is another elaboration of simple distillation. This operation, used primarily by large commercial desalting plants, does not require heating to convert a liquid into vapour.

The liquid is simply passed from a container under high atmospheric pressure to one under lower pressure. The reduced pressure causes the liquid to vaporize rapidly; the resulting vapour is then condensed into distillate. A variation of the reduced-pressure process uses a vacuum pump to produce a very high vacuum.


This method, called vacuum distillation , is sometimes employed when dealing with substances that normally boil at inconveniently high temperatures or that decompose when boiling under atmospheric pressure. Steam distillation is an alternative method of achieving distillation at temperatures lower than the normal boiling point.

It is applicable when the material to be distilled is immiscible incapable of mixing and chemically nonreactive with water. Examples of such materials include fatty acids and soybean oils. The usual procedure is to pass steam into the liquid in the still to supply heat and cause evaporation of the liquid.

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Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. See Article History. Start your free trial today for unlimited access to Britannica. Learn More in these related Britannica articles:. The same factors that lead to the enrichment of alcohol in the vapour above a solution of water and alcohol permit the enrichment of isotopes.

Special distillation processes are required for separation of mixtures close to boiling point or for forming azeotrope mixtures into their pure components. In Special Distillation Processes, the authors focus on latest developments in the field, such as separation methods that may prove useful for solving problems encountered during research. Topics include extraction, membrane and adsorption distillation involving the separation principle, process design and experimental techniques.

The relationship between the processes and the techniques are also presented. Comprehensive and easy-to-read, this book provides key information needed to understand the processes and is a valuable reference source for chemical engineers as well as students wishing to branch out in chemical engineering. Help Centre. My Wishlist Sign In Join. Be the first to write a review. Add to Wishlist. Ships in 7 to 10 business days. Depending on the reaction, the desired product may end up in the vapor stream or in the bottoms. Unreacted feed can be recycled back to the reactive zone within the distillation column.

This animation shows a typical reactive distillation column. The reactants, exemplified as the incoming blue and yellow streams, enter the reactive zone. The reaction takes place and the desired product is produced, shown as the outgoing red stream. The column operation dictates the separation of reactants and products. The reactants are more volatile than the products in this example, and therefore rise upward to the distillate stream, which is demonstrated by the movement of the green arrows.

The condensed reactants, also exemplified by blue and yellow, are fed back to the reactive zone. The products are less volatile, which fall downward and exit the bottom stream as liquid. Many refineries produce methyl-tert butyl ether MTBE. MTBE can be produced in a reactive distillation column from ethanol and isobutylene.

The unreacted materials are carried overhead as the distillate stream, while the less volatile MTBE product is removed as the bottoms. Extractive distillation involves an additional species that acts as a solvent to change the relative volatility of one of the components of a mixture. The animation below shows the typical two-column design. The first column is known as the extractive unit. In addition to a feed stream with two components, which is shown below in green, a solvent stream also enters the extractive unit, which is exemplifed below in red.

The component of the feed stream that is ultimately recovered becomes associated with the solvent and leaves in the bottoms stream of the extractive unit, as the purple stream demonstrates. The other component vaporizes and exits in the distillate, as the yellow stream exemplifies. In the second column, known as the solvent stripper, the desired product, shown in blue, is separated and the regenerated solvent, shown in red, is returned to the extractive unit to repeat the cycle.

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Extractive distillation is used mostly for the separation of mixtures of close-boiling species or those that form azeotropes. Extractive distillation is not as widely used in industry as conventional distillation because column simulation and design is difficult. For example, extractive distillation units are used in pulp-making processes. In the extractive unit an organic solvent separates cellulose from lignin. The solvent is then separated, purified and recycled in the second column. The extractive distillation unit being installed in the picture below can be used for solvent recovery, extraction of aromatic compounds, and purification of organic acids.

Pressure swing distillation is a multi-column process that exploits the effect of pressure on the composition of many azeotropes. The animation below shows one example of a variety of possible designs for pressure swing distillation systems. The feed stream and a recycle stream from the second column are fed into the first column. The first column operates at a specific pressure that separates the first component, shown in blue, as a distillate from an azeotropic mixture bottoms stream, shown in green.

The azeotropic bottoms stream is fed into a second column that operates at a different pressure. At this pressure the original azeotrope is "broken" and a separation can take place, resulting in the second component, shown in yellow, as a distillate and an azeotropic bottoms stream of different composition, shown in aqua. This bottoms product is recycled back to the first column. Pressure swing distillation can be used to break an ethanol-water mixture that forms an azeotrope. The columns pictured below to the left are used to distill proof ethanol to proof, so it can be used as a fuel grade additive, enhancing octane levels in gasoline.

The process consists of three or more columns operating at different pressures. Pressure swing distillation is used in the plant pictured below to the right to recover hydrogen from a methane or methanol feed. A homogeneous azeotropic distillation system is used to separate an azeotropic feed. This animation shows how a typical system works. An entrainer, exemplified by the pink component in the recycle stream, is added to the binary feed azeotrope, shown entering the first column in green. The entrainer forms another azeotrope with one of the feed components and moves into the second column, as the purple stream exiting the top of the first column demonstrates.

The other feed component separates from the azeotrope in the first column and exits through the bottom of the column, as the yellow stream demonstrates. In the second column, the pressure is set to break the entrainer-feed azeotrope, which results in the second component, shown in blue, and a third azeotrope containing the entrainer, as the pink recycle stream demonstrates. Homogeneous azeotropic distillation is not commonly used because it is so complex. Heterogeneous azeotropic distillation incorporates liquid-liquid separation into the separation of an azeotropic feed, as shown in the animation.

An azeotropic feed stream enters the first column, and is split into two streams: The bottoms contain the first component, as the blue stream demonstrates, and a second azeotrope exits through the top of the column as the distillate.


The distillate is condensed and sent to a decanter, where it is separated into two liquids: One is rich in component one and is returned to the first column, as the blue recycle stream exemplifies. The second liquid, which is rich in component two, is sent to a second column. The second column operates at a different pressure than the first column and is used to separate the stream into pure component two, indicated by the yellow bottoms stream, and a third azeotrope distillate.

This distillate is also condensed and sent to the decanter. In addition, an entrainer is often added to improve the separation. Heterogeneous azeotropic distillation is used in chemical processing industries to separate close-boiling or azeotropic systems. This type of distillation can be used as an alternative to extractive distillation, pressure swing distillation, or homogeneous azeotropic distillation. These columns can be used to separate water-ethanol mixtures. Air Science, Inc. Odfjell , Norway. Quincy Compressor , Quincy, IL. Scanning Technologies Inc.

Sigma-Aldrich Co. LLC , St. Louis, MO. Sulzer Chemtech Ltd. Vendome Copper and Brass Works, Inc.


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  • The Forgotten Master;
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Fair, James R. Geankoplis, Christie J. Transport Processes and Unit Operations. Englewood Cliffs, N. Humphrey, Jimmy L. Jenkins, John and Ken Porter. Jensen, B. Kister, Henry Z. Kunesh, John G. Laikins, Robert P. U of Michigan. Lee, C. Lockett, M.

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Distillation Tray Fundamentals. New York: Cambridge University Press, Meyers, Robert A, ed. Encyclopedia of Physical Science and Technology. Vol 4. Orlando, Fl: Academic Press,