Theoretical plate

[1][2] The concept of theoretical plates in designing distillation processes has been discussed in many reference texts.

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.

In current modern practice, as shown in the adjacent diagram, better contacting is achieved by installing bubble-caps or valve caps at each perforation to promote the formation of vapor bubbles flowing through a thin layer of liquid maintained by a weir on each tray.

To design a distillation unit or a similar chemical process, the number of theoretical trays or plates (that is, hypothetical equilibrium stages), Nt, 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, Nt 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.

[2][3] In an industrial distillation column, the Nt required to achieve a given separation also depends upon the amount of reflux used.

The empirical formula known as Van Winkle's Correlation can be used to predict the Murphree plate efficiency for distillation columns separating binary systems.

[6] The IUPAC's Gold Book provides a definition of the number of theoretical plates in a chromatography column.

The concept of theoretical plates or trays applies to other processes as well, such as capillary electrophoresis and some types of adsorption.