Information Technology Reference
In-Depth Information
Under the subject Analysis and Tab System, select “generate points along a flash
curve” and valid phases “vapor - liquid - liquid” even though there will be no vapor. Set
up a mixture for equilibration as described using the following procedure. Under the tab
Variable, select pressure as a fixed state variable and temperature as the adjusted vari-
able. Select and highlight temperature and the Range/List button will become available
to enter temperature data.
Workshop 6.2a
The fit is exact because a single mutual solubility point is described
by two equilibrium equations like equation (6.30) which will contain as unknowns
only the two parameters of the activity coefficient equation.
Workshop 6.2b and 6.2c
As expected, use of the
a
ij
parameter produces values
of solubility that are independent of temperature; however, even when using the
b
ij
parameters, the data are not well represented.
Workshop 6.3a
An exact solution is obtained as in Workshop 6.2a.
Workshop 6.3b and 6.3c
When viewing results using the Plot Wizard's est vs. exp
plot, there are small differences between the two approaches, but this is not a universal
result.
Use of the mutual solubility data point as in Workshop 6.3b establishes the location
of the phase envelope termini (on a ternary diagram) on the toluene - water edge of
the diagram and will force the composition at very dilute phases to approach the
experimental mutual solubility. If the mutual solubility data are not consistent with the
other experimental tie lines, the phase envelope in that area of the phase diagram will
not appear smooth. If the data are fit as in Workshop 6.3c, the phase envelope will
be smooth, but the mutual solubility predicted from the ternary data may not conform
to the experimental mutual solubility. Because of this potential problem, an Aspen
Plus user must be cognizant of the expected compositions of the simulation for which
the regression is being prepared and make the appropriate choice when the data are
regressed.
Workshop 6.4a
The fit of the data to the van Laar equation suggests that there is a
heterogeneous region that lies between 0.1 to 0.25 mole fraction TbOH. A careful look
at the experimental data shows that this may be so, but it is a judgment call. More
experimental data may be needed. Extreme caution is advised.
The Wilson equation fit by its nature cannot have a heterogeneous region and is
otherwise satisfactory. The Uniquac fit exhibits behavior similar to that of van Laar.
The NRTL fit with the
c
parameter
=
0
.
2 exhibits similar behavior to that of van Laar.
Workshop 6.4b
The NRTL fit with the
c
parameter
=
0
.
3 exhibits behavior similar
to that of van Laar.
REFERENCES
Anderson, T. F., Abrams, D. D., and Grens, E. A., II,
AIChE. J.
, 24(1), 20 - 39 (1976).
Aspen Plus version 7.0, Physical Properties System documentation.
