Page 70 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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(234)

From equations (228) and (229) and equations (232) to (234), it can be
easily obtained:

(235)
HTU m = HTU G +—HTU L
A
(236)
HTU 0L = A.HTU a + HTU L

HTU OL=A.HTU OG (237)

1,5.4.2.3. Piston flow model with number of theoretical stages and height of a
theoretical stage
Another value largely used for calculation of the packing height, and
which has not lost its importance yet, is the height equivalent to a theoretical
stage, often called also theoretical plate, A theoretical stage corresponds to a
part of the apparatus volume in which the concentration of the component at the
outlet is equal to the equilibrium concentration at the inlet.
It is clear that in case of a not equilibrium process, a packed bed column
has less than one theoretical stage.
The theoretical stage is used by McCabe and Thiele [35] for calculation
of a binary distillation in plate and packed columns for systems with
components for which the molar heat of vaporization and liquid heat capacities
are almost the same for both components and the heat losses can be neglected.
Under these conditions the operating line of the distillation column can be
represented by the material balance equations. For a column with a total
condenser, the overhead vapour and distillate are of the same composition. The
operating line, i.e. the line of the concentration of the light component in the gas
phase versus its concentration in the liquid phase at these conditions, is straight
one. For the rectifying section its line passes through the distillate composition.
If a partial condenser is used, the reflux composition can be considered as a
liquid in equilibrium with the condenser outlet vapour composition. The partial
condenser usually provides an additional theoretical stage. In this case the
condenser outlet vapour constitutes at least some of the distillate products.
Usually, the re-boiler is considered as one theoretical stage so that the vapour at