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Selection of a thermodynamic model
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General advice
The weakest
link in simulating chemical engineering operations is uncertainty in the
physical properties, particularly phase equilibria required for modeling
distillation, stripping, absorption and extraction. See, for example, the excellent
paper: “Uncovering
the realities of simulation,” part 1 Part 2
Experimental
data
When possible one should either use experimental data to check
the predictions of simulation engines such as HYSYS and
Aspen, or to use these data to fit suitable thermodynamic models.
- An excellent source of binary and ternary phase
equilibria data as of 1975 is Band 3, Gruppe IV
of the Neue Serie of
the Landolt-Börnstein Zahlenwerte
und Functionen aus Naturwissenschaften und Technik,
Springer-Verlag (1975) Ref R541/9 U58nt.
- A newer comprehensive source is DETHERMA from DECHEMA. Search and then pay for data
found. (Professor Taylor has
most volumes in his laboratory.)
- Recent original publications with data often can
be found by searching the Engineering
Village.
- Search also Fluid Phase Equilibria , the Journal of Chemical & Engineering Data, and the Journal of Chemical Thermodynamics
Expert
systems and decision trees (flow charts)
- Decision
tree and property models from Aspen Plus help
- Aspen Plus
models with decision tree
- Decision tree
from a thermodynamics text
- AIChE
student tool “Selecting Thermodynamic Models for Process
Simulation of Organic VLE and LLE Systems” (also at Suppes advice).
- Aspen advice
- Figure
8.4 on p 471 of Towler
& Sinnott
- “Don’t
Gamble with Physical Properties for Simulations,” by E.C. Carlson,
Chem. Eng. Prog. (October 1996) pp 35-46. Includes a decision tree, which is
one of two found in the Encyclopedia on the CD accompanying the text
“Product and Process Design Principles: Synthesis, Analysis, and
Evaluation,” 2nd ed., by W.D. Seider,
J.D. Seader, D.R. Lewin
(Wiley, NY, 2004), with a needed revision at http://www.seas.upenn.edu/~dlewin/Upgrade_2004.htm
.
Henry’s
Law
Henry’s
Law states that the solubility of a gas is proportional to its partial
pressure, with the constant of proportionality called “Henry’s Law
Constant.”
- Unfortunately,
Henry’s Law Constant varies, particularly with temperature. How to
correct for the variation.
Environmental engineering implications. - Henry’s Law should not be used when there is
dissociation or reaction in either phase. A common example is the
dissociation of diatomic molecules (hydrogen, oxygen, nitrogen) when they
dissolve in a liquid or solid metal.
When this occurs, the solubility is proportional to the square
root of the partial pressure, a dependence known at “Sievert’s Law.” One consequence of Sievert’s Law is that the driving force for mass
transfer of a diatomic gas across a metal is proportional to the
difference in square roots of the partial pressures in the contacting gas.
Below are four methods to select
thermodynamic models in Aspen. Note
that Aspen includes several varieties of the NRTL, UNIQUAC and Wilson activity
coefficient models. With these
names alone, an activity coefficient model is used for the liquid phase while
the vapor phase is taken to be ideal. The following varieties account
for non-idealities in the vapor phase due to high pressure and
association: NRTL-HOC, NRTL-NTH,
NRTL-RK, UNIQ-HOC, UNIQ-NTH, UNIQ-RK, WILS-HOC, WILS-NTH, WILS-RK, WILS-HF,
WILS-LR and WILS-GLR. Tools/Property Method Selection Assistant
·
Listing of
Thermodynamic Property Models. Excellent! From Aspen’s Help. The methods are organized by type, with
links to pages clearly explaining each.
However, the links in this pdf copy do not function. To access the original, with functioning
links, do the following while in Aspen Plus. Help, Contents, Accessing Other Help,
click on the Aspen Physical Properties System Help link, in the Contents select
Aspen Physical Property System Reference, Physical Property Methods and Models
Reference Manual, Chapter 3 Property Model Description, Thermodynamic Property
Models, Overview.
·
Help,
Contents, Using Aspen Plus, Entering Data for Simulation, Physical Property
Methods, Available Property Methods, Overview. For activity coefficient models it
specifies what method is used for vapor fugacity coefficients. For more information about any method,
use the Help Search.
·
Help,
Contents, Using Aspen Plus, Entering Data for Simulation, Physical Property
Methods, Choosing a Property Method, Overview.
First read
the material in HYSYS’s help Index under
“Property Package Descriptions” and “property
package.” Also see AspenHYSYSSimulationBasis.pdf, which is normally located at
C:\Program Files\AspenTech\Documentation 2004.2\Aspen
Engineering Suite 2004.2\Aspen HYSYS on a computer with HYSYS.
- The expert system
developed by Hyprotech
- If you have data for vapor-liquid or
liquid-liquid equilibria, select the thermodynamic model that agrees best
with those data using predictions
of HYSYS. (See also isopropyl
alcohol – di-isopropyl ether examples.)
Activity coefficient models
Activity coefficient models are needed for non-ideal
liquid mixtures. The following is
from a HYSYS web seminar on 2 August 2005, as well as AspenHYSYSSimulationBasis.pdf. The binary interaction parameters (BIPs) provided by HYSYS for activity coefficient models
were found by regression of binary vapor-liquid data assuming equilibrium with
an ideal gas. These BIPs are not expected to be valid for very high
pressures. For low to moderate
pressures, use an activity coefficient model for the liquid, and take the gas
as ideal. When the pressure
exceeds 5 atm , use an EOS model such as PR, SRK or RK
for the vapor phase. If there are strong vapor-phase interactions between the molecules a
vapor-phase model must be used that accounts for these interactions. For example, “HYSYS recommends you
use the Virial option for organic acid components
(like formic acid, acetic acid, propionic acid,
butyric acid, and heptonic acid.” For operations involving only gases,
e.g. compression, it may be desirable to use an equation of state model just
for that one unit.
Advanced equation
of state models introduces the
“Twu” models, which are included in the latest
version of HYSYS but not in the expert systems or decision trees mentioned
above.
Missing BIPs
It is common for some BIPs to be missing for a multicomponent
mixture. If, on the Binary Coeffs page, a coefficient estimation method is given,
return to the Setup page and change the temperature to a value suitable for
your process, return to the Binary Coeffs page, then
click on VLE (vapor-liquid equilibria) or LLE (for a liquid-liquid extraction
column) and then Unknowns Only. If
you change your mind, hit Reset Params.
Multiple
thermodynamic models.
The following is taken from the
Aspen support
site. "Beginning
with HYSYS 3.0, you no longer need to use Sub-Flowsheets
when creating model with multiple Fluid Packages. Each Unit Operation and Stream can have
its own associated Fluid Package. A Stream Cutter Operation is automatically
inserted into the flowsheet at the point where a transition from one Fluid Package
to another occurs. The Stream
Cutter defines the Component mapping and Transition Basis for the transfer.
Among other things, this capability allows the use of separate Fluid Packages
for the Shell and Tube sides of a Heat Exchanger." For a
vapor-liquid separation such as distillation, you may want to select a model
specifically for the light and heavy keys, e.g. with the most number of BIPs.
Here’s more information
from Aspen. Following are the steps
to follow:
1.
Add the new
equipment to your pfd, but do not connect it to the streams
2.
In the Basis
Environment, define the desired new Fluid Package.
3.
On the Fluid Pkgs page indicate the package to be used for each piece of
equipment.
4.
If, while
returning to the Simulation Environment, you encounter a warning, then select a
more suitable transfer basis.
5.
Connect the
streams to the equipment. Run the
simulation.
6.
While, in
principle, manual insertion of stream cutters is not required, in practice that
sometimes doesn’t work and an error message persists, probably that the
transfer basis isn’t known.
When this happens, delete the equipment and insert stream cutters in
both inlet and outlet streams. Then
reinsert and connect the equipment to the streams. If the transfer basis error recurs, go
to the Flowsheet Setup page of the equipment and select a reasonable transfer
basis for all streams. At some
point, HYSYS may offer to delete the stream cutters. Decline this invitation.
Last modified June 20, 2008. Please submit all questions, comments
and suggestions to W.R.
Wilcox
Disclaimer: The
material on these pages is intended for instructional purposes by Clarkson
University students only. Neither
Clarkson University nor Professor Wilcox is responsible for problems caused by
using this information.
Wilcox
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