Page 342 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

344 Chapter 11 Distillation

If y is reasonably more than x D , a single-stage distillation is sufficient. The calculation can follow

F
the Rayleigh’s equation (Eq. 11.44).
In case y < x D , reduce P op to increase y and check if y is reasonably more than x D .If P op needs

F F F
2
to be reduced below 0.1 kg/cm (abs), then distillation under vacuum will possibly turn out to be
uneconomic in moderate to large scale operation and more than one stage of fractionation is
required. The stages need to be provided by a packed bed or a plate contactor as shown in
Fig. 11.18.
4) Initial operation: Starting from empty overhead receiver, filling under no reflux till the desired
level in receiver is achieved.
Time taken, t i ¼ D i =V , where Vis the boil up rate decided considering the production capacity of
the plant.
Compute x Bi bottoms composition at end of receiver filling from

Fx F Fð1 x F Þ
log ¼ a log (11.67)
B i x Bi B i ð1 x Bi Þ
and, from component material balance,

(11.68)
Fðx F x Bi Þ
D i ¼
B i ðx Di x Bi Þ
or
x
Z B
B i dx B 1 ð1 x F Þx Bi ð1 x F Þ
ln ¼ ¼ ln þ ln (11.69)
F x Di x B a 1 x F ð1 x Bi Þ ð1 x Bi Þ
x F
Outputs: B i , D i , x Bi , x Di ;
ax Bi
x Di is in equilibrium with x Bi , i.e., x Di ¼
1þða 1Þx Bi
5) Selection of the mode of operation viz. (a) constant reflux ratio, (b) constant top product
composition (varying reflux ratio)
a) Operation with constant reflux
Inputs: B i , D i , x Bi , x Di
(i) Find N min from

x Di 1 x Bi
log
1 x Di x Bi
(11.70)
N min þ 1 ¼
log a
and check N > N min , for N that has been decided a priori. N can also be computed from Smoker’s
equation, not included here.