HETP (Height Equivalent to a Theoretical Plate)
As we have noted, instead of a tray (plate) column, a packed column can be used for various unit operations such as continuous or batch distillation, or gas absorption.
With a tray column, the vapours leaving an ideal plate will be richer in the more volatile component than the vapour entering the plate by one equilibrium "step".
When packings are used instead of trays, the same enrichment of the vapour will occur over a certain height of packings, and this height is termed the height equivalent to a theoretical plate (HETP). As all sections of the packings are physically the same, it is assumed that one equilibrium (theoretical) plate is represented by a given height of packings. Thus the required height of packings for any desired separation is given by (HETP x No. of ideal trays required).
HETP values are complex functions of temperature, pressure, composition, density, viscosity, diffusivity, pressure drop, vapour and/or liquid flowrates, packing characteristics, etc. Empirical correlations, though available to calculate the values of HETP, are restricted to limited applications. The main difficulty lies in the failure to account for the fundamentally different action of tray and packed columns.
["Chemical Engineering Vol.2", 4th Ed., Coulson & Richardson, p.508-509]
In industrial practice, the HETP concept is used to convert empirically the number of theoretical trays to packing height. Most data have been derived from small-scale operations and they do not provide a good guide to the values which will be obtained on full-scale plant.
[ For more information on HETP prediction, see pp.1-355 of "Handbook of Separation Techniques for Chemical Engineers", 3rd Ed., P.A. Schweitzer , or pp.335, "Separation Process Principles", J.D. Seader & E.J. Henley ]
This method had been largely replaced by the Method of Transfer Units.
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