A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as an equilibrium stage or a theoretical tray. The performance of many separation processes depends on having a series of equilibrium stages and is enhanced by providing more such stages. In other words, having more theoretical plates increases the efficacy of the separation process be it either a distillation, absorption, chromatographic, adsorption or similar process. In chemistry and chemical engineering, a separation process is a process that transforms a mixture of substances into two or more compositionally-distinct products. ... A liquid will usually assume the shape of its container. ... Vapor (US English) or vapour (British English) is the gaseous state of matter. ... A dynamic equilibrium will form if, at a given temperature, two reversible processes occur at the same rate. ... Laboratory distillation set-up using, without a fractionating column 1: Heat source 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed... Absorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. ... Pictured is a sophisticated gas chromatography system. ... Adsorption is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate). ...

Applications

The concept of theoretical plates and trays or equilibrium stages is used in the design of many different types of separation.

Distillation columns

The concept of theoretical plates in designing distillation processes has been discussed in many reference texts. ^[1][2][3] Any physical device that provides good contact between the vapor and liquid phases present in industrial-scale distillation columns or laboratory-scale glassware distillation columns constitutes a "plate" or "tray". Since an actual, physical plate is rarely a 100% efficient equilibrium stage, the number of actual plates is more than the required theoretical plates. Laboratory distillation set-up using, without a fractionating column 1: Heat source 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed... A fractionating 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. ...

So-called bubble-cap or valve-cap trays are examples of the vapor and liquid contact devices used in industrial distillation columns. Another example of vapor and liquid contact devices are the spikes in laboratory Vigreux fractionating columns. A Vigreux column is a type of fractionating column. ...

The trays or plates used in industrial distillation columns are fabricated of circular steel plates and usually installed inside the column at intervals of about 60 to 75 cm (24 to 30 inches) up the height of the column. That spacing is chosen primarily for ease of installation and ease of access for future repair or maintenance.

Typical bubble cap trays used in industrial distillation columns

For example, a very simple tray would be a perforated tray. The desired vapor and liquid contacting would occur as the vapor flowing upwards through the perforations would contact the liquid flowing downwards through the perforations. In current modern practice, as shown in the adjacent diagram, better contacting is achieved by installing bubble-caps or valve caps located at each perforation to promote the formation of vapor bubbles flowing through a thin layer of liquid maintained by a weir on each tray. Image File history File links Bubble_Cap_Trays. ...

To design a distillation unit or a similar chemical process, the number of theoretical trays or plates (that is, hypothetical equilibrium stages), N _t, required in the process should be determined, taking into account a likely range of feedstock composition and the desired degree of separation of the components in the output fractions. In industrial continuous fractionating columns, N _t is determined by starting at either the top or bottom of the column and calculating material balances, heat balances and equilibrium flash vaporizations for each of the succession of equilibrium stages until the desired end product composition is achieved. The calculation process requires the availability of a great deal of vapor-liquid equilibrium data for the components present in the distillation feed, and the calculation procedure is very complex.^[2][3] The flash (or partial) evaporation is one of the simplest unit operations. ...

In an industrial distillation column, the N _t required to achieve a given separation also depends upon the amount of reflux used. Using more reflux decreases the number of plates required and using less reflux increases the number of plates required. Hence, the calculation of N _t is usually repeated at various reflux rates. N _t is then divided by the tray efficiency, E, to determine the actual number of trays or physical plates, N_a, needed in the separating column. The final design choice of the number of trays to be installed in an industrial distillation column is then selected based upon an economic balance between the cost of additional trays and the cost of using a higher reflux rate. Diagram of typical reflux apparatus. ...

There is a very important distinction between the theoretical plate terminology used in discussing conventional distillation trays and the theoretical plate terminology used in the discussions below of packed bed distillation or absorption or in chromatography or other applications. The theoretical plate in conventional distillation trays has no "height". It is simply a hypothetical equilibrium stage. However, the theoretical plate in packed beds, chromatography and other applications is defined as having a height.

Distillation and absorption packed beds

Distillation and absorption separation processes using packed beds for vapor and liquid contacting have an equivalent concept referred to as the plate height or the height equivalent to a theoretical plate (HETP). ^[2][3] HETP arises from the same concept of equilibrium stages as does the theoretical plate and is numerically equal to the absorption bed length divided by the number of theoretical plates in the absorption bed (and in practice is measured in this way). Look up absorption in Wiktionary, the free dictionary. ... A packed bed is a hollow tube or pipe that is filled with a packing material. ...

The material in packed beds 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 contact the wetted surface, where mass transfer takes place. Raschig rings are referred to as random packing and are used in commercial fractional distillation columns to reduce pressure loss (versus using trays), while maintaining good efficiency and economy. ... The term Structured packing refers to a range of specially designed materials for use in distillation columns and chemical reactors. ... Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems. ...

Chromatographic processes

The theoretical plate concept was also adapted for chromatographic processes by Martin and Synge. The IUPAC's Gold Book provides a definition of the number of theoretical plates in a chromatography column.^[4] A comprehensive, detailed discussion is available on the Internet.^[5] Pictured is a sophisticated gas chromatography system. ... Archer John Porter Martin was a British chemist and Nobel Prize winner. ... Richard Laurence Millington Synge (born Liverpool, October 28, 1914, died Norwich, August 18, 1994) was a British biochemist, and winner of the 1952 Nobel Prize in Chemistry for the invention of partition chromatography. ... The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ... The Gold Book or Compendium of Chemical Terminology (ISBN 0865426848) is a book published by IUPAC containing internationally accepted definitions for terms in chemistry. ...

The same equation applies in chromatography processes as for the packed bed processes, namely:

In chromatography, the HETP may also be calculated with Van Deemter's equation. The Van Deemter equation gives the variance per unit length of a packed column in terms of the physical properties of the solute and phase system and the linear mobile phase velocity. ...

Other applications

The concept of theoretical plates or trays applies to other processes as well, such as capillary electrophoresis and some types of adsorption. Capillary electrophoresis (CE) can be used to separate ionic species by their charge and frictional forces. ... Adsorption is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate). ...

See also

• Batch distillation
• Continuous distillation
• Extractive distillation
• Fenske equation
• Fractional distillation
• McCabe-Thiele Method

Bold textBold textBatch distillation[1] refers to the use of distillation, the separation of components in a system by volatilities, in batches. ... Continuous distillation is a distillation process, which does not require interruption for adding raw material. ... Extractive Distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. ... The Fenske equation in fractional distillation is an equation relating the mole ratio of two gases in the vapor phase to the initial molar ratio, the partial pressures and the number of theoretical equilibrium stages. ... Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. ... The graphical approach presented by McCabe and Thiele in 1925, the McCabe-Thiele Method is considered the simplest and perhaps most instructive method for analysis of binary distillation. ...

References

1. ^ B.S. Furnis et al (1989). Vogel's Textbook of Practical Organic Chemistry, 5th Edition, Longman Scientific. ISBN 0-582-46236-3. 
2. ^ ^{a b c} Kister, H.Z. (1992). Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6. 
3. ^ ^{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. 
4. ^ Definition of the number of plates (in chromatography) IUPAC Gold Book
5. ^ Theoretical Plate Model (Node 8 of the Theory of Chromatography)

Distillation Design provides complete coverage of the design of industrial distillation columns for the petroleum refining, chemical and petrochemical plants, natural gas processing, pharmaceutical, food and alcohol distilling industries. ... Perrys Chemical Engineers Handbook was first published in 1934 and the seventh edition was published in 1997. ...

External links

• Distillation, An Introduction by Ming Tham, Newcastle University, UK
• Distillation Theory by Ivar J. Halvorsen and Sigurd Skogestad, Norwegian University of Science and Technology, Norway