CHEMCAD Version 5.6
Add button: Clicking on the Add button will add the currently highlighted component to the end of the
flowsheet component list. Components may also be selected by double clicking on them. The
selected component will appear in the list displayed in the Selected Components Area.
Clear button: The Clear button is used to delete the entire component list. When this button is clicked,
all of the components listed in the Selected Components Area will be erased.
Delete button: The Delete button is used to delete individual components from the component list. To
use this option, highlight the component in the Selected Components Area and click Delete.
Selection string field: This field is used to locate a component in the databank. Once the desired
component has been located, it is added to the flowsheet component list by double clicking on it or
by highlighting it and then clicking on the Add button. The program locates components by
matching the string entered in this field. The string can be any alphanumeric combination. The
string will be matched to any part of an ID number, formula, or synonym (component name) listed
in the databank.
As you type in the string, the program automatically finds and highlights the closest match. The
highlighted component changes as the string changes.
Next button: If the current match is not the component you are looking for, click the Next button to find
the next matching string.
Copy Components from another job: The Copy Components button is used to import the component
list from another CHEMCAD job. When this button is clicked, the user is prompted to browse for
the location of the CHEMCAD job from which the components will be imported. These components
are then added to the Selected Components Area.
To show how a component is selected, let’s select Nitrogen. First, click on the Selection String Field,
then type in the string "Nitrogen". As you build the string, the program will locate and highlight the
component nitrogen. To add it to the component list, double click on it or click on the Add button. The
component will appear in the Selected Components Area.
Nitrogen could just as easily been located by entering the string "N
" in the field. The same thing could
have been done with the string "46" which is the CHEMCAD ID number for nitrogen. Strings, which
comprise only part of the word, number, or formula, can also be used. It is also possible to highlight the
desired component by clicking on it directly and then clicking the Add button.
Now select the remaining components in the component list. Remember, if the first search does not
locate the desired component; click the Next button to find the next matching string. Now find the
following components and add them to the component list:
CHEMCAD Version 5.5
The dialog box should now look like this:
Let’s save our list by clicking the [OK] button.
SELECTING THERMODYNAMIC OPTIONS
After the user first selects components, the ThermoWizard will appear. The ThermoWizard is a system
available in the CHEMCAD Suite to assist the user in making K-value and enthalpy choices.
The system works like this:
1. First, it looks at the component list and decides what general type of model is required, i.e.,
equation-of-state, activity model, etc.
2. Next, it looks at the temperature and pressure ranges input by the user and decides which equation
within a given category is best at the limits of those ranges.
3. If the method is an activity model, the program then looks at the BIP database to see which model
has the most data sets for the current problem. It then calculates the fractional completeness of the
BIP matrix. If that fraction is greater than the BIP threshold parameter, it uses the chosen activity
method; if not, it uses UNIFAC.
The K-Value Wizard is no replacement for engineering judgment. The Wizard uses an algorithm
based on general rules and might not always be correct. The suggested model might not be the ‘best’
model for the system.
Selecting thermodynamic options basically means selecting a model or method for calculating vapor-
liquid (or vapor-liquid-liquid) phase equilibrium (called the K-value option) and selecting a method or
model for calculating the heat balance (called the enthalpy option). You do this by clicking on the
ThermoPhysical command located on the menu bar. This is the same command you used to access
the Component List above. The ThermoPhysical menu should appear as before.
CHEMCAD Version 5.6
CHEMCAD has a library of about 50 K-value models with a variety of options and about twelve enthalpy
models. Making the proper selection from these libraries can sometimes be difficult. The issues
involved and the proper techniques for selection are described in the Thermodynamics section of the
Because the issues involved in making thermodynamic selections and in using the Thermo Wizard are
beyond the scope of this tutorial, let’s assume we know that we want to use the Peng-Robinson method
for both the K-value and enthalpy calculations. In this case, we would proceed as given below.
First, we need to accept the default temperature/pressure values for the ThermoWizard. The Wizard
will recommend using SRK, please accept this message box. Then, let us now make our own selection
in the K-value Options dialog box. In the upper-left corner of this screen will be a combo box labeled
Global K-value Option, which is currently highlighted. Please open this window by clicking on it.
You will notice the scroll bar at the right of this list indicating that all of the available options could not fit
into this area. The user may scroll through them using any of the methods previously described in this
tutorial. We would like to use the Peng-Robinson method. To make this choice, please scroll up through
list; point the arrow to the Peng-Robinson option; and click the left button on the mouse. The box will
close and Peng-Robinson will now be displayed in the field. Your screen should look like this:
The other options on this screen are not important to us at this time. Therefore, let’s save our selection
by pointing and clicking the [OK] button in the lower right corner. This will return you to the Simulation
Now, let’s select our enthalpy model. Do this by again clicking the ThermoPhysical command, then
clicking the Enthalpy option. The Enthalpy Options dialog box will be displayed. You will notice on
this screen that the Peng-Robinson method has already been chosen. The program automatically
makes this the enthalpy model when you choose the Peng-Robinson K-value method. If this assumption
is erroneous, you may override it at any time. In our case, we would like to use the Peng-Robinson
model, so let’s point and click the [OK] button on the lower right portion of the screen. This will close
the Enthalpy Options dialog box, saving the currently displayed options.
For the purpose of our tutorial, the thermodynamic selections are now complete.
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CHEMCAD Version 5.5
DEFINING THE FEED STREAMS
We are now in a position to define the feed streams. This can be done by using the Specifications
command on the menu bar or by double clicking directly on the stream. Double clicking directly on the
stream is obviously simpler, so let’s do that now. Double click stream 1. The Edit Streams dialog box
will appear like this:
Please note the following points about this dialog box.
The first field, called Stream Name, allows you to enter a stream label or name. This name
may be up to 16 alphanumeric characters.
Note: Entering and/or editing such stream labels can also be done a number of
The next four fields; temperature, pressure, vapor fraction, and enthalpy are the thermodynamic
properties of the stream. According to the Gibbs Phase Rule, once the composition is given,
specifying any two of the four thermodynamic properties of a mixture will define the other two.
Thus, defining the composition, temperature, and pressure uniquely defines the vapor fraction
and enthalpy (for a mixture). Alternately, defining the composition, pressure, and enthalpy will
uniquely define temperature and vapor fraction.
Since enthalpies are calculated relative to a datum, the calculation of any given stream enthalpy
is an involved process which is prone to errors. For this reason, CHEMCAD does not permit
you to enter stream enthalpy as a constraint.
To uniquely define any stream in CHEMCAD, the user must define the composition of that
stream and any two of temperature, pressure, and vapor fraction. The two variables you
specify will be typed in red. The third variable and enthalpy will be displayed in black when you
flash the stream.
An exception to this convention allows you to add heat duty with an empty stream. If you
specify a total component flowrate of zero, you may specify a temperature, pressure, and
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CHEMCAD Version 5.6
enthalpy rate. A stream defined this way is treated as a heat duty and added to the heat
balance of the unit. The temperature and pressure are arbitrary for this situation.
The fields Total Flow Units and Comp Units work together to provide the user with a variety of
ways to define stream compositions. If the Comp Units are set to mole, mass, or volume
fraction (which can be done globally or locally), then the Total Flow Units combo box will
become available. If Comp Units is set as a flow or amount option, then the total flowrate
becomes the sum of the component flowrates and the Total Flow Units combo box will not be
available for editing.
If the Comp Units is set to an amount flow unit (as opposed to mole, mass, or volume fractions),
then as the component flowrate values are entered, they are automatically summed and the
current sum is displayed in the field labeled Total Flow.
In the upper left hand corner of the dialog box is the Flash button. Clicking this button at
anytime will cause the program to perform a flash calculation using the currently specified
composition and thermodynamic properties. This allows quick flash calculations to be made
without exiting the dialog box.
Fractions that do not sum to 1.0 are automatically normalized upon flashing or exiting the dialog
Now let’s enter the data. Let’s start with temperature. Please move to the temperature field by clicking
Once the highlight is on the proper field, type 75 [down arrow]. When you press the [down arrow] key,
the highlight will move down to the pressure field. Please enter 200 in this field.
Next, let’s move down to the Nitrogen field. Please move the cursor down until it points to the field to
the right of Nitrogen and click the left button on the mouse. Please enter 100.19 in this field.
In a similar fashion, please enter the following numbers in their corresponding fields:
Save this stream information by pointing to the [OK] button in the upper-left corner of the dialog box. We
are now ready to input the equipment parameters.
CHEMCAD Version 5.5
INPUTTING EQUIPMENT PARAMETERS
Specifying the first heat exchanger…
In a fashion similar to streams, equipment parameters may be input using either the Specifications
command on the menu bar or by double clicking directly on the UnitOp we wish to specify. Again, the
latter is easier, therefore, please point and double click on the first heat exchanger now. An equipment-
specific input dialog box will appear:
Data entry screens can be more than one page long. The buttons called Specifications, Misc. Settings,
and Cost Estimations indicate the pages for this dialog box. You may browse through them by clicking
on the tab.
S FILL IN THE SCREEN
Pressure drop for both sides of the exchanger is 5 psi; therefore, click on the "Stream 1" pressure drop
field and press the 5 key, then click on the other pressure drop field which is labeled Stream 4. Likewise,
enter 5 in this field.
We need to specify the first stream outlet to be at its dewpoint. We do this by specifying an outlet vapor
fraction of 1. Therefore, move to the Vapor Fraction stream 2 field either by tabbing down to it or by
clicking on it. Then enter 1.
This completes the input for this unit. We now need to save this information and close the dialog box.
You do this by clicking on the [OK] button.
CHEMCAD Version 5.6
Specifying the second heat exchanger…
We can now select the next unit for data input. Move the cursor to the second heat exchanger and
double click. The menu for a single-sided heat exchanger will appear.
The outlet temperature from this heat exchanger will determine how much of the liquid is removed in the
flash drum. This, in turn, will determine the cricondentherm dewpoint of the product gas. Therefore, this
specification is one of the key parameters of our design. As a first attempt, let’s estimate an outlet
temperature of -5º F. Therefore, please enter 5 in the Pressure Drop field and a –5 in the Temperature
of Stream 3 field. Point and click on the [OK] button to save the data and close the dialog box.
Specifying the flash drum…
In our example, the flash drum is a vapor liquid separator and requires no specification. Therefore, we
do not need to enter any input for this unit.
Specifying the valve…
Let’s enter the outlet pressure for the valve. Please move the cursor to the valve and double click. The
Valve dialog box will appear. Since our outlet pressure for this unit is 125 psia, enter 125 in the Pressure
Out field, then point and click on the [OK] button. This completes the input for the valve unit.
Specifying the stabilizer tower…
Now double click the tower unit. The TOWR Distillation Column dialog box will appear. There are five
pages to this screen as indicated by the tabs.
On the first page, we need to enter the top pressure, which is 125 psia; the column pressure drop, which
is 5 psi; the number of stages, which is 12, and the feed stage location, which is stage number 1.
Therefore, please complete your screen as shown below:
CHEMCAD Version 5.5
Let’s continue our data input on the next page. Please click on the Specifications tab. We make our
specifications for the column on this screen. We have no condenser or side streams on this column so
we will only be making specifications for the reboiler.
First, we need to specify our reboiler mode. In order to determine what our options are, please point the
arrow to the field below the label Select Reboiler Mode and click the left button on the mouse. A list
should open displaying the available options.
We need to specify the bottom flowrate for this unit, which is Mode No. 4. Therefore, please point the
cursor to 4 Bottom mole flowrate and click the left button on the mouse. The list will close and the field
should now read 4 Bottom mole flowrate.
Now we need to specify the numeric value of that flowrate. This is done in the field immediately to the
right, which opens after we make our mode selection. Please point the arrow to that field, and click the
left button on the mouse. This field should be highlighted and we can enter a value of 30.
Now, let’s move to the next page of this dialogue box by clicking the Convergence tab. As you can see
from the screen, all of the entries on this page are optional. However, for the purpose of demonstration,
let’s enter an estimate of 50º F for the top temperature and 150º F for the bottom temperature. Please
point and click on the T top field and enter a value of 50 in this field. Now move to the field labeled T
bottom immediately below. Enter a value of 150 in this field.
Now we have completed the data input for the tower unit. Let’s save this data by pointing and clicking on
the [OK] button. When you do this, you will get a warning message saying you have not entered an
estimate for the distillate rate. The program will ask you if you want to ignore this warning. Warning
messages are for your information and can normally be ignored; therefore, please click [YES] now. This
will return you to the Simulation Window. All of the data entry for the flowsheet is now complete.
CHEMCAD Version 5.6
RUNNING THE SIMULATION
To run the simulation, point and click on the RUN command on the menu bar. This will cause the RUN
Menu to open up like this:
We want to run a steady state analysis, so select the Run All option.
The program will first recheck the data and list any errors and/or warnings on the screen. In this case,
we should have no errors, although we will have warnings saying we have not given certain estimates.
We can ignore these warnings and proceed by clicking the [YES] button. The calculation will then
Upon finishing, the CC5 Message Box will appear with the message “Recycle calculation has
converged”. To close this dialog box and clear the screen, you must click on the [OK] button.
REVIEWING THE RESULTS INTERACTIVELY
Now that the simulation is complete, we will want to review the results before printing a hardcopy. We
do this using the Results and Plot commands on the menu bar, or even more quickly by pointing at
streams and UnitOps when the default Flowsheet Quickview option of the View menu is active. With
these commands, let’s check whether we have met our design criteria.
Checking the cricondentherm dewpoint…
If we have chosen the proper outlet temperature for heat exchanger number 2, the cricondentherm
dewpoint for the product gas stream (Stream 5) will be 20º F or less. The cricondentherm dewpoint, you
will remember, is the highest dewpoint temperature a mixture will ever see at any pressure. The
simplest way for us to identify the highest dewpoint temperature of the product gas is to plot all of the
dewpoint temperatures of the product gas, i.e., to plot a phase envelope. Let’s do that now.
To plot anything inside CHEMCAD, we must start with the P
lot command on the menu bar. Please
select this option by clicking on it. When you do this, the PLOT MENU will appear. On this menu, you
will see a list of the general categories of plots available within CHEMCAD. Please select Envelopes by
clicking on it.
CHEMCAD Version 5.5
The Select streams dialog box will appear. Move the cursor to stream 5 (the product gas stream
leaving the bottom of heat exchanger number 1) and click it once. The number 5 will appear in the
dialog box field. Now click [OK] to close the streams selection process. The Phase Envelope Options
dialog box will appear.
No entries on this screen are required since we only need to look at the dew point line. To make the plot
more interesting, let’s display the 0.25 and the 0.5 vapor fraction lines in addition to the normal phase
envelope boundaries. Therefore, complete the dialog box as shown:
Now click the [OK] button to confirm these entries.
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