Page 71 - Packed bed columns for absorption, desorption, rectification and direct heat transfer
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the bottom of the column is in equilibrium with the liquid bottom product. The
operating line of the stripping section passes through the bottom composition.
In a binary distillation, the feed liquid should be introduced into the column at
the equilibrium stage represented by the intersection of the two operating lines.
The theoretical stages are determined using a stepwise procedure between the
operating lines and equilibrium curves. Fig. 9 demonstrates a separation
requiring three theoretical stages rectifying and four theoretical stages stripping.
0.2 0.4 x 0.6
X D
Mai Fraction in Liquid
Fig. 9. McCabe-Thiele diagram.
The height of the packing is calculated by the equation
= NTP.HETP , (238)
where NTP is the number of theoretical stages (theoretical plates) determined by
Fig. 9 and HETP is the height equivalent to one theoretical stage (theoretical
plate). The data for determining of HETP for different packings are presented in
Chapter 3.
It is easy to see that the maximal value of NTP in case of co-current
flow is always less than 1, tending to 1 when the height of the packing is
