Page 338 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

340 Chapter 11 Distillation

The term on the right-hand side of Eq. 11.55 is integrated graphically by plotting the still con-
centration x B on the abscissa and the term R þ 1 2 on the ordinate. The area (A) under the curve
ðx D x B Þ
between x F and x B gives the value of the integral. Thus, the heat requirement Q is found from
Q
¼ðx D x F Þ A (11.56)
F l F
It is evident from Eqs. 11.52 and 11.55 that the two operating modes require different quantities of
heat Q to obtain the same D and x D . The optimum energy requirement is expected to be a combination
of the two modes of operation, i.e., combination of distillate concentration decrease and increase in the
reflux ratio.

(A) (B) 0.8

1.0 0.7
0.6
0.8 (0.8, 1,1' (0.512,
1
0.5) 0.5 3 2 0.572)
y → 0.6 3 2 y → 0.4 R = 0.70 2'
0.4 4 (0.4,0.4) 3'
1 0.3
R = 2.8
2
0.2 3 0.2
4
x = 0.067
B 0.16
= 0.25 0.1
x F
0.25
0.0 0.2 0.4 0.6 0.8 1.0
0
x → 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
x →
FIGURE 11.19
McCabeeThiele construction for (A) constant reflux ratio; (B) constant distillate composition.
Multistage batch distillation resembles a fractionator with only the rectification section and is
usually designed assuming negligible holdup in the column and condenser as compared to the receiver
and kettle. The liquid holdup always present in the column can greatly influence both the yield and
purity of the products. Column hold up affects the yield by hindering the sharp decrease of distillate
composition and the off specification intermediate fractions are large. The foregoing analysis ignores
the holdup in the column and the condenser compared to the holdup in the still.
Case of ternary mixtures

i
Rayleigh equation can also be applied to multicomponent batch distillation. For component i, y ¼
K i x i which gives
(11.57)
dLðK i x i Þ¼ dðLx i Þ
Since the amount of liquid L in the still and its concentration (x i ) change during operation, it is
difficult to evaluate the term dðLx i Þ. In this case, it is advantageous to use component amount (l i )