Page 346 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

348 Chapter 11 Distillation

A curve of 1/(x D x B ) versus x B is drawn in Fig. 11.21 to tentatively calculate the area under the
curve from x B to x F . As a first trial, x B ¼ 0.067 is considered.

3.6

3.4
3.2
3
1/ (x D – x B ) 2.8

2.6
2.4
2.2
2

1.8
0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26

FIGURE 11.21 x B

Plot of 1/(x D x B ) versus x B
0:25
x F
Z Z
dx B dx B
The area under the curve from the graph ¼ ¼ 0:4493
0:067
x B ðx D x B Þ ðx D x B Þ
Hence, (F/B) ¼ exp(0.4493), i.e., B/F ¼ 0.6381 and
0:25 0:6381 0:067
x F ðB=FÞ x B
¼ 0:572, highly acceptable as a product.
1 0:6381
x D;avg ¼ ¼
1 ðB=FÞ
Toluene recovery ¼ 100 ð0:25 0:6381 0:067Þ=0:25 ¼ 82:9%
For the distillate; l avg ¼ 0:572 38:06e3 þð1 0:572Þ 36.24e3 ¼ 37:28e3kJ=kg mol
Total heat required for boil up ¼
ðR þ 1Þ ð1 ðB = FÞÞ l avg ¼ 3:8 ð1 0:6381Þ 37:28e3 ¼ 51268 kJ=kg mol charge
Therefore, it is feasible to have a batch distillation unit with three ideal stages in a column,
operating with constant reflux ratio 2.8, and recover about 82.9% toluene with 57.2% mol purity.
However, during operation the trays or packed tower will have a hold up that would drain into the still
increasing toluene content in the final hold up in the still. This may slightly reduce the recovery
percentage. In any case with this basic design scheme, 80% recovery of toluene with 55% purity can
safely be committed.