Page 295 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 297

The pressure estimated on the feed tray (P Fd ) and the column bottom (P Btm ) are

P Fd ¼ P Top þ (9 0.8 60 / 10000) ¼ P Top þ 0.0432 atm.

P Btm ¼ P Top þ (19 0.8 60 / 10000) ¼ P Top þ 0.091 atm.
If this column is expected to be operating at around 7 atm, then

P Top ¼ 7 atm; P Fd ¼ 7.043 atm; P Btm ¼ 7.091 atm.
The closeness of the pressure values (within 1.3%) suggests that the selection of the pressure for the
equilibrium data does not make much practical difference in computing the number of stages.
On the other hand, if the separation is carried out under vacuum with say

P Top ¼ 120 mm Hg(abs.),

P Fd ¼ P Top þ (9 0.8 60/13.6) ¼ P Top þ 31.8 mm Hg(abs.) ¼ 151.8 mm Hg(abs.)
and

P Btm ¼ P Top þ (19 0.8 60 / 13.6) ¼ P Top þ 87.4 mm Hg(abs.) ¼ 207.4 mm Hg(abs.)
In this case the estimated variation of pressure (36.6%) within the column is large, and a more
conservative (higher) pressure may be adopted for the equilibrium data.
While predicting equilibrium data, it is wise to determine the possibility of azeotrope formation of
a given system. In such cases, both the minimum and maximum boiling azeotrope compositions can be
modified by changing the system pressure and/or addition of a third component which forms a min-
imum boiling azeotrope with one of the original pair and the new azeotrope boils well below or above
the original azeotrope. By this, one of the original components can often be recovered as a nearly pure
product, while the second azeotrope obtained needs to be separated in an additional step.
Such thermodynamic calculations are not included here.
Number of ideal stages/trays
Most real systems are multicomponent. Compared to multicomponent systems, the analysis and
design calculations for binary systems are much simpler and fairly well developed. In order to un-
derstand the fundamentals of distillation, binary distillation is discussed first.

11.4.2 Binary fractionation

The number of theoretical stages required for separating a feed into a distillate and a bottom product of
specified composition depends on the reflux ratio.
Binary fractionator design based on equal molar counterflow
McCabeeThiele graphical procedure is drawn on the equilibrium x-y diagram at the design column
pressure. This method is applicable when both components have nearly the same molar heat of
vaporisation. It does not include any detailed thermal balances and neglects sensible heat change, heat
of mixing and heat losses from the column. Energy balances are needed only to determine condenser
and reboiler duties.