Information Technology Reference
In-Depth Information
IPA - H
2
O - 95 mmHg
1
0.9
Experimental
Calculated
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.5
x - mole fraction
0.4
0.6
0.7
0.8
0.9
1
Figure 11.13
Experimental versus Wilson equation data.
If no experimental data are available for a system, or data cannot be developed in
a timely manner, the Unifac group contribution method may be used to estimate the
data. Although these data may be used for preliminary design, it would be wise to
set in motion means for experimental verification. For example, Figure 11.15 shows
a comparison of experimental and Unifac estimated vapor-liquid equilibrium for an
isopropyl alcohol-water system at 95 mmHg. The prediction of the azeotrope compo-
sition is remarkable, but the lower composition range is acceptable at best, but again,
use of these data is application driven.
11.5.1 RadFrac
RadFrac is the rigorous distillation modeling workhorse of Aspen Plus. It is based on
the inside-out formulation of Boston and Sullivan (1974), which employs a reorgani-
zation of the basic equations and the sequence of calculation. A detailed description
of the method may be found in Seader and Henley (1998). For problems in which
convergence difficulties are experienced, five variations of the basic algorithm are
available.
The first example of RadFrac use here is the same as that used for DSTWU and
Distl. Since RadFrac is a rating or simulation program, the necessary specifications
are obtained from the results of the DSTWU example (i.e., feed stage location, reflux
ratio, and distillate rate). The primary input form for RadFrac is shown in Figure 11.16.
Most of the remaining process specifications are straightforward and accessed from the
tabs available. The tabs are Streams, in which the feeds and products are specified;
Pressure, in which the stage pressures are specified; Condenser, in which the condenser
details are specified; and Reboiler, in which the reboiler details are specified. If in
Figure 11.16 the reboiler specified is thermosyphon, the Thermosyphon Config and
