Fractionating column
A fractionating column or fractional column is equipment used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on their differences in volatility. Fractionating columns are used in small-scale laboratory distillations as well as large-scale industrial distillations.
Laboratory fractionating columns
A laboratory fractionating column is a piece of glassware used to separate vaporized mixtures of liquid compounds with close volatility. Most commonly used is either a
In a typical fractional distillation, a liquid mixture is heated in the distilling flask, and the resulting vapor rises up the fractionating column (see Figure 1). The vapor condenses on glass spurs (known as
Industrial fractionating columns
Distillation is one of the most common and energy-intensive separation processes. Effectiveness of separation is dependent upon the height and diameter of the column, the ratio of the column's height to diameter, and the material that comprises the distillation column itself.[6] In a typical chemical plant, it accounts for about 40% of the total energy consumption.[7] Industrial distillation is typically performed in large, vertical cylindrical columns (as shown in Figure 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.
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. The heat entering a distillation column is a crucial operating parameter, addition of excess or insufficient heat to the column can lead to foaming, weeping, entrainment, or flooding.
Figure 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columns have outlets at intervals up the column so that multiple products having different boiling ranges may be withdrawn from a column distilling a multi-component feed stream.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 Figure 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 as well as the composition of the desired products. Given a simple, binary component feed, analytical methods such as the McCabe–Thiele method[5][8][9] or the Fenske equation[5] can be used. For a multi-component feed, simulation models are used both for design, operation, and construction.
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 Figures 4 and 5.
The efficiency of a tray or plate is typically lower than that of a theoretical 100% efficient
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 in or 2.5–7.6 cm 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
- Laboratory glassware
- Steam distillation
- Theoretical plate
- Vacuum distillation
- Fractional distillation
References
- ISBN 0-471-48810-0.
- ISBN 0-07-284823-5.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - ^ ISBN 0-07-034909-6.
- ISBN 0-07-034612-7.
- ^ ISBN 0-07-049479-7.
- ^ "Distillation Columns". Brewhaus. Retrieved 4 August 2015.
- ISBN 978-0-471-68757-3.
- ^ Beychok, Milton (May 1951). "Algebraic Solution of McCabe-Thiele Diagram". Chemical Engineering Progress.
- ISBN 0-471-58626-9.
External links
- Use of distillation columns in Oil & Gas
- More drawings of glassware including Vigreux columns
- 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 Archived 2014-07-13 at the Wayback Machine by the Distillation Group, USA
- Distillation simulation software
- Fractional Distillation Explained for High School Students