Fractionating column

Laboratory fractionating columns

A laboratory fractionating column is a piece of glassware used to separate vaporized mixtures of liquid compounds with close volatilities. It can also be called a fractional column. Most commonly used is either a Vigreux column or a straight column packed with glass beads or metal pieces such as Raschig rings.

  Fractionating columns help to separate the mixture by allowing the mixed vapors to cool, condensation-vaporization cycle, the vapors are enriched in a certain component. A larger surface area allows more cycles, improving separation. This is the rationale for a Vigreux fractionating column or a packed fractionating column. Spinning band distillation achieves the same outcome by using a rotating band within the column to force the rising vapors and descending condensate into close contact, achieving equilibrium more quickly.

As shown in Image 1, as a liquid mixture in the round bottomed flask is boiled, vapor rises up the fractionating column. The vapor condenses on the glass platforms (known as condenser, which cools the vapor down until it condenses into a liquid distillate. The separation may be enhanced by the addition of more trays (to a practical limitation of heat, flow, etc.)

Industrial fractionating columns

boiling points, also called fractions and that is the origin of the name fractional distillation or fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements and economics.

Industrial distillation is typically performed in large, vertical cylindrical columns (as shown in image 2) 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.     Industrial distillation towers are usually operated at a continuous steady state. Unless disturbed by changes in feed, heat, ambient temperature, or condensing, the amount of feed being added normally equals the amount of product being removed.

It should also be noted that the amount of heat entering the column from the reboiler and with the feed must equal the amount heat removed by the overhead condenser and with the products.

Image 3 depicts an industrial fractionating column separating a feed stream into one distillate fraction and one bottoms fraction. However, many industrial fractionating columns have outlets at intervals up the column so that multiple products having different boiling ranges may be withdrawn from a column distilling a multi-component feed stream. The "lightest" products with the lowest boiling points exit from the top of the columns and the "heaviest" products with the highest boiling points exit from the bottom.

Industrial fractionating columns use external reflux to achieve better separation of products.^[3][5] Reflux refers to the portion of the condensed overhead liquid product that returns to the upper part of the fractionating column as shown in Image 3.

Inside the column, the downflowing reflux liquid provides cooling and condensation of upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux and/or more trays provided, the better is the tower's separation of lower boiling materials from higher boiling materials.

The design and operation of a fractionating column depends on the composition of the feed and as well as the composition of the desired products. Given a simple, binary component feed, analytical methods such as the Fenske equation^[5] can be used. For a multi-component feed, simulation models are used both for design and operation.

Bubble-cap "trays" or "plates" are one of the types of physical devices which are used to provide good contact between the upflowing vapor and the downflowing liquid inside an industrial fractionating column. Such trays are shown in Images 4 and 5.

The efficiency of a tray or plate is typically lower than that of a theoretical 100% efficient vapor-liquid equilibrium stages.  

In industrial uses, sometimes a mass transfer takes place. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

See also

• Azeotropic distillation
• Batch distillation
• Continuous distillation
• Extractive distillation
• Fractional distillation
• Laboratory glassware
• Steam distillation
• Theoretical plate
• Vacuum distillation

References

1. ^ 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. 
2. ^ McCabe, W., Smith, J. and Harriott, P. (2004). Unit Operations of Chemical Engineering, 7th Edition, McGraw Hill. ISBN 0-07-284823-5. 
3. ^ ^{a b} Kister, Henry Z. (1992). Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6. 
4. ^ King, C.J. (1980). Separation Processes. McGraw Hill. 0-07-034612-7. 
5. ^ ^{a b c d} Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition, McGraw-Hill. ISBN 0-07-049479-7. 
6. ^ Beychok, Milton (May 1951). "Algebraic Solution of McCabe-Thiele Diagram". Chemical Engineering Progress.
7. ^ Seader, J. D., and Henley, Ernest J.. Separation Process Principles. New York: Wiley. ISBN 0-471-58626-9. 

• Distillation Theory by Ivar J. Halvorsen and Sigurd Skogestad, Norwegian University of Science and Technology, Norway
• Distillation, An Introduction by Ming Tham, Newcastle University, UK
• Distillation by the Distillation Group, USA
• Distillation simulation software
• Fractional Distillation Explained for High School Students