Fractionating column

A fractionating column or fractionation column is an essential item used in the distillation of liquid mixtures so as to separate the mixture into its component parts, or fractions, based on the differences in their volatilities. Fractionating columns are used in small-scale laboratory distillations as well as for large-scale industrial distillations.

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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, condense, and vaporize again in accordance with Raoult's law. With each 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 trays or plates) inside the column, and runs back down into the liquid below and refluxes the upflowing distillate vapor. The hottest tray is at the bottom of the column and the coolest tray is at the top. At steady state conditions, the vapor and liquid on each tray is at equilibrium. Only the most volatile of the vapors stays in gas form all the way to the top. The vapor at the top of the column, then flows through the water-cooled 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

Fractional distillation is one of the unit operations of chemical engineering.^[1][2] Fractionating columns are widely used in the chemical process industries where large quantities of liquids have to be distilled.^[3][4][5] Such industries are the petroleum processing, petrochemical production, natural gas processing, coal tar processing, brewing, liquified air separation, and hydrocarbon solvents production and similar industries but it finds its widest application in petroleum refineries. In such refineries, the crude oil feedstock is a very complex multicomponent mixture that must be separated and yields of pure chemical compounds are not expected, only groups of compounds within a relatively small range of 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 McCabe-Thiele method^[5][6][7] or 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 equilibrium stage. Hence, a fractionating column almost always needs more actual, physical plates than the required number of theoretical vapor-liquid equilibrium stages.  

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 Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where 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