rev. 050107
DISTILLATION COLUMNS IN CHEMCAD
METHOD TO CONVERGE RELUCTANT COLUMNS IN CHEMCAD
COLUMN MODELS IN CHEMCAD
There are three rigorous steady state distillation models in CHEMCAD: TOWR (Inside-out method), SCDS (simultaneous correction
distillation), and TPLS (TOWR unit with additional in / out connections).
The SHOR column is not a rigorous column. The SHOR column uses the Fenske-Underwood-Giliand model to calculate separations.
The liquid-liquid extractor (EXTR) is a modified form of the SCDS column. Mathematics for extraction are similar to those of distillation.
In general, the SCDS/EXTR column is better suited to activity coefficient K values and the TOWR is better suited to equations of state. The
TPLS is well suited for petroleum models. The SHOR column is generally used only in education, and should not be used to model an actual
column.
DIFICULTIES WITH COLUMNS
Most column difficulties occur for one of the following reasons:
-Inappropriate thermodynamic selection (using the wrong model for vapor liquid equilibria)
-Inappropriate column specification (specifying a result which can’t be achieved)
-Rigid constraints for convergence (specifying a result which is difficult to obtain)
TROUBLESHOOTING DIFFICULT COLUMNS
While trying to converge a column it is interesting to see why the model is failing. To see the results of the iterations that failed, follow these
steps.
•
In CHEMCAD, go to the RUN menu.
•
Choose Convergence.
•
In the convergence dialog, checkmark Display Trace Window and Generate Run History.
The first setting activates a display window which contains information on the current status of calculations when CHEMCAD runs. The latter
checkmark prevents this window from being refreshed with each unit / iteration. For a column, the trace window will show the predicted
composition profile from each iteration, and the error (relative to previous iteration) term. Often this information helps identify a problem. For
example, if a stage is drying up, either you have too much heat or too high a vapor flow in the column.
THERMODYNAMICS
Separation units are highly dependent on the selection of vapor liquid equilibrium (VLE) K value model. The user will remember that K value
models are used to calculate the composition of the liquid and vapor phases of a mixture at a given temperature and pressure. There are
several different K models which are designed for different chemical systems.
Example:
Figures 1 and 2 are x vs. y plots of the system water – ethanol using NRTL and SRK, respectively. There is a significant difference between
the separations you will obtain in a model using these two models. For this system, NRTL would be a reasonable choice while SRK would be
unrealistic (as well as inappropriate). Quite often you will not be able to run a column model if you have an unrealistic thermodynamic
selection. Choosing the correct thermodynamics model is typically the most significant challenge of a simulation. Some guidance can be
found from Reid / Sherwood Properties of Gases and Liquids, physical chemistry texts, and various literature articles.
Figure 1
Figure 2
COLUMN SPECIFICATION
Stage numbering convention in CHEMCAD is top to bottom, 1 to N. A stage is considered the space above a plate. If a condenser is present it
is stage 1. If a reboiler is present it is stage N. To model a column which has ten stages plus condenser and reboiler one must specify 12
stages in CHEMCAD (10+condenser+reboiler=12).
If a condenser is present, the feed must not enter stage 1, as that is the condenser. Top stage feeds should enter stage 2, the top stage
(plate), if a condenser is present. Likewise, if a reboiler is present a bottom plate feed is connected to stage (N-1), not stage N.
CONVERGING TO RIGID CONSTRAINTS
Typically the user has a specification to which they wish to desire a column. Quite often this is a restrictive composition, such as mass
fraction of a key component in either the bottoms or tops.
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rev. 050107
DISTILLATION COLUMNS IN CHEMCAD
Converging a column model in CHEMCAD is similar to converging a column in the real world; it is difficult to go directly to high purity
separation. It is best to start with an easy target, such as reflux ratio and bottoms flowrate. Once the column is converged to this simple
specification, we ‘tighten’ the specifications toward the goal.
Procedure for Converging from loose specifications: (see example, below)
1.
Set up the column
2.
Verify that the target is thermodynamically feasible with your selected VLE K model.
3.
On the SPECIFICATIONS page, set ‘loose’ specifications such as ‘Reflux Ratio’ and ‘bottoms mass/mole flowrate
4.
Run the column and converge. Change the specifications if necessary.
5.
Go to the CONVERGENCE page of the column dialog. Set the initial flag to 0 Reload Column Profile. This setting instructs CHEMCAD to
use the current converged profile as it’s starting point (initial conditions) in iterative calculations.
6.
On the SPECIFICATIONS page change to more tight specifications. Run the column.
7.
If the column converges, tighten the specifications and run again. If the column fails to converge, do not save the profile of the failed
attempt. Relax the specifications and run the column again.
8.
Repeat step 6 until you reach the target.
Procedure for Initial convergence of a difficult column:
Often it is difficult to obtain the first convergence on a column. If the column is run with no condenser or reboiler, one does not have the
option of ‘loose’ specifications. If the column has a condenser or reboiler, relaxing specifications does not always help.
1.
On the convergence page of the column dialog, specify estimates if you can make reasonable estimates. Note that a bad guess will make
the column more difficult to converge than no estimate.
2.
Remove non-key components from the feed(s) to obtain the first convergence.
Example:
If the feed is primarily ethanol, water, and nitrogen but also has oxygen, ethane, and carbon dioxide present remove the latter components
from the feed stream. Converge the column with a feed of ethanol, water, and nitrogen. Set the intial flag to 0 Reload Column Profile,
return the other component to the feeds, and run the unit again.
3.
4.
5.
Specify a larger number of iterations on the convergence page of the column dialog. The default is 50, but often 52 iterations will find
the answer, as per Murphy’s Law.
Try an alternate column model. If you are currently using the SCDS try the same separation with a TOWR or vice versa. The two models
use extremely different mathematical models to get to the same answer; often one will find an answer in 10 iterations while the other is
difficult to converge with fewer than 60 iterations.
Use the correct condenser type. If you are rating an existing unit, verify that the condenser mode matches the unit. . The default
condenser type, total, requires that no vapor leaves stage 1 (condenser). If light ends are present, this may not be possible without
cryogenic temperatures when using a total condenser. A partial condenser allows vapor and liquid to leave the condenser. Using a partial
condenser the light ends gases will slip past the condenser without cryogenics. Do not change condenser mode arbitrarily; consider
the actual process.
Example: Recovery of high purity ethanol
An SCDS column with 8 stages, a condenser, and reboiler is used to separate a mixture of 1000 lb/hr ethanol, 1000 lb/hr water. Temperature
is 70°F and pressure is 15psia. The goal is to obtain high purity ethanol in the distillate.
The K model is set to NRTL (Thermodynamics menu > K Value), which is capable of accurately modeling the nonideal system wateralcohol. It is important to note that there is an azeotrope in this system at high levels of alcohol.
Specify 10 stages for the column, with feed on stage 3.
Now set ‘loose’ specifications for the condenser and reboiler. For columns with total condensers, a reflux ratio of 1.2 is a good specification for
the condenser. For partial condensers, reflux ratio of 5 is often a decent start. A good initial specification for reboilers is mass flowrate. There
are 2000 lb/hr entering the column; set the bottoms to have 1500 lb/hr.
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rev. 050107
DISTILLATION COLUMNS IN CHEMCAD
Run the flowsheet. The column will converge. It is now time to tighten the specifications toward the target. Visual inspection of the tops
stream shows that mass fraction of ethanol is 0.86. Change the condenser specification to distillate component mass fraction, set ethanol as
the component, and set the spec as 0.91. Visual inspection of the bottoms shows that ~950 lb/hr of water is here. Change the reboiler
specification to bottom component mass flowrate, set the component to Water, and specify 980 lb/hr.
The next step is crucial: go to the specifications page. On the right hand side is the field initial flag. Set this to mode 0: reload column
profile. This setting instructs the model to use the current temperature, pressure, and composition profile of the column as the starting
values for the next calculation. By starting from the current converged profile, the numerical method packages can find a tighter solution.
Note that there are other modes which allow you to specify a profile yourself. These are particularly useful in starting a model of an existing
unit; you already have a rough profile across the stages.
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rev. 050107
DISTILLATION COLUMNS IN CHEMCAD
Run the column. It will converge. Now change the setting on the condenser to 0.95 and run the column. The following message will appear.
The column has not converged. CHEMCAD is asking if you want to save the current profile, which is not a converged result. If you save the
profile you may run more iterations of the column methods; sometimes this leads to a solution. If more iterations does not converge the
column, you will have to start this column over from the beginning. This can be tedious; it is often easier to make small changes rather than
jumps. Press [NO] to discard the current profile, CHEMCAD will revert to the converged profile from before (top specification 0.91, bottom
specification 980).
The jump from 0.91 to 0.95 was too large to be solved in 40 iterations (default number of iterations for SCDS). We will try a smaller change.
Set the top specification to 0.93 and the bottom specification to 990. Run the column.
We return to Thermodynamics to check the feasibility of distilling for higher purity. Zooming in on a TPXY plot (Plot menu > TPXY) of the
system we see that we are quite close to the azeotrope. It will be difficult to obtain higher purity; more stages are likely needed. The reboiler
specification should be changed to a bottoms component mass fraction. We end the exercise here; the user may wish to continue towards
0.95 mass fraction as an exercise.
http://www.chemstations.net
Chemstations, Inc.
2901 Wilcrest Drive, Suite 305 • Houston, TX 77042 USA
713.978.7700 Voice • 713.978.7727 FAX • 800.243.6223 Tollfree