Fractional distillation



Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating simple distillation is used.

Laboratory setup

Fractional distillation in a laboratory makes use of common laboratory glassware, as well as some single-purpose items like a fractionating column.

Apparatus

 

Method

As an example, consider the distillation of a mixture of water and volatile than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.

The apparatus (the diagram represents a batch apparatus, as opposed to a continuous apparatus) is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few thermometer.

In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense.

Alternate set-ups may utilize a "cow" or "pig" which is connected to three or four receiving flasks. By turning the "cow" or "pig", the distillates can be channeled into the appropriate receiver.

Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling point of the materials.

Industrial distillation

  Distillation is the most common form of separation technology used in continuous steady state. New feed is always being added to the distillation column and products are always being removed. Unless the process is disturbed due to changes in feed, heat, ambient temperature, or condensing, the amount of feed being added and the amount of product being removed are normally equal. This is known as continuous, steady-state fractional distillation.

Industrial distillation is typically performed in large, vertical cylindrical columns known as "distillation or fractionation 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. The distillation towers have liquid outlets at intervals up the column which 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.

For example, fractional distillation is used in oil refineries to separate crude oil into useful substances (or fractions) having different hydrocarbons of different boiling points. The crude oil fractions with higher boiling points:

  Large-scale industrial towers use theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required.

Fractional distillation is also used in air separation, producing liquid semiconductor.

In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum. This packing material can either be random dumped packing (1-3" wide) such as "theoretical plates" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

Design of industrial distillation columns

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 Fenske equation[2] 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 or trays) used in distillation columns, as seen in Figure 2, 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.

An indication of numbers: the separation of two compounds with boiling point difference of 30°C requires 12 theoretical plates and, for a difference of 3°C, the number of plates increased to 1000.[6]

The reflux ratio is the ratio of the amount of moles returned as refluxed liquid to the fractionating column and the amount of moles of final product, both per unit time.

See also

References

  1. ^ Kister, Henry Z. (1992). Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6. 
  2. ^ a b c Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition, McGraw-Hill. ISBN 0-07-049479-7. 
  3. ^ Beychok, Milton (May 1951). "Algebraic Solution of McCabe-Thiele Diagram". Chemical Engineering Progress.
  4. ^ Seader, J. D., and Henley, Ernest J. (1998). Separation Process Principles. New York: Wiley. ISBN 0-471-58626-9. 
  5. ^ Editors: Jacqueline I. Kroschwitz and Arza Seidel (2004). Kirk-Othmer Encyclopedia of Chemical Technology, 5th Edition, Hoboken, NJ: Wiley-Interscience. ISBN 0-471-48810-0. 
  6. ^ Arthur I. Vogel and Brian S. Furnis (1988). Vogel's Textbook of Practical Organic Chemistry, 5th Edition, London: Longman Scientific & Technical. ISBN 0-582-46236-3. 
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Concepts in Salt-effect distillation
 
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