Distillation
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Distillation is a method of separation of substances based on differences in their volatilities.
Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid solution is perhaps the oldest form of distillation, in the course of producing distilled beverages. However, the technique is now widely used for a variety of liquids in the chemical industry and in the production of petroleum products, among other fields.
The liquid solution evaporates, such that the vapor has a composition determined by the chemical properties of the solution. In distillation, the mixture is boiled to effect sufficient evaporation. Distillation of a given component is possible if the vapor has a higher proportion of the given component than the solution. This is another way of saying that one component has a higher volatility, due to a higher vapor pressure in the operating temperature. Although a higher vapor pressure correlates with a lower boiling point, it is important to note that distillation does not use the different boiling points of pure components. Most mixtures are nonideal: they boil at different temperatures from their components, and significant evaporation takes place around the boiling point, not just at it. It is more accurate to say that given a boiling mixture, its composition is more important in predicting the separation efficiency than the boiling point of a free individual component.
Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in an azeotrope. At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95% at 78.2°C.
By the nature of the process, it is theoretically impossible to completely purify the components using distillation, as distillation only tends to approach purity, never reaching it. This is comparable to dilution, which never reaches purity. If ultra-pure products are the goal, then further chemical separation must be used.
The minimum in distillation is flash evaporation, where either the temperature is rapidly increased or pressure reduced, and vapor and liquid fractions are thus obtained, which may be processed as such.
The device used in distillation is referred to as a still and consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid is collected.
The equipment may effect separation by one of two main methods. Firstly the vapors given off by the heated solution may consist of two liquids with significantly different boiling points. Thus, the vapor that is given off is in the vast majority of one or the other liquid, which after condensation and collection effects the separation.
The second method (fractional distillation) is more effective at separating liquids with similar boiling points. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical plants and natural gas processing plants.
Industrial distillation is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. When the process feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column. Large-scale industrial towers also use reflux to achieve a more complete separation of products.
Design and operation of a distillation column depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as "plates") used in distillation columns are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation column needs more plates than the number of theoretical vapor-liquid equilibrium stages.
Distillation was developed into its modern form with the invention of the alembic by Persian alchemist Jabir ibn Hayyan c. 800; he is also credited with the invention of numerous other chemical apparatus and processes that are still in use today.
Chemists often use distillation in their work as a means of separating compounds or components. See at right a diagram of a simple distillation set-up without a fractionating column often used by chemists. A distillation apparatus sometimes used by chemists is a rotary evaporator to distill (or evaporate) away solvent from a solution.
An analogous method with freezing instead of evaporation is called freeze distillation. It is not distillation, and does not produce products equivalent to distillation.
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1. heat source (a Bunsen burner here)
2. distilling flask (a round bottom flask)
3. distilling head
4. thermometer
5. condenser
6. cooling water in
7. cooling water out
8. receiving flask collecting dripping distillate
9. vacuum source
10. vacuum adapterList of American distillers
See also
External links
References
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