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In-Depth Information
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
x - mole fraction
Figure 11.9
Preliminary design: three-product distillation example.
is one of several possible solutions, since there is a trade-off between the number of
stages, the reflux ratio, the equipment cost, and energy considerations.
11.4 PRELIMINARY DESIGN AND RATING MODELS
11.4.1 DSTWU
The DSTWU model is applicable to both binary and multicomponent systems and
was developed for systems that exhibit approximately constant relative volatility. The
DSTWU model is based on the Gilliland correlation shown in Figure 11.10. This
correlation was developed from the operating data of many columns and relates the
minimum and operating reflux ratios,
R
min
and
R
, to the minimum and actual number
of theoretical stages,
N
min
and
N
. Gilliland's data were correlated by Molokanov
et al. (1972), which resulted in an analytical version of the Gilliland correlation:
exp
1
X
0
.
5
N
−
N
min
N
+
1
54
.
4
X
11
+
117
.
2
X
+
X
−
1
=
−
1
(11.8)
where
R
−
R
min
R
+
1
X
=
The minimum reflux ratio is obtained by applying Underwood's (1946) method, which
is applicable to binary or multicomponent systems. In the latter case it is necessary to
select light and heavy key components and to solve for the roots of
