ASPEN Exercise 3
Ammonia synthesis process with a recycle stream
for Aspen Plus 7.3.2
Flow diagram
Gas purge 5%
Recycle 95 %
Heat Q kW
Qchiller kW
25 oC
400 oC
Reactor
0 oC
Separator
Pressure: 125 atm
N2(g) + 3H2(g)
2NH3(g)
(Equilibrium)
Feed
N2: 1000 lbmol/h
H2: 2000 lbmol/h
CH4:
8 lbmol/h
Chiller
Liquid NH3
Compute
flow rate of NH3 product
heat duty for the reactor Q,
heat duty for chiller Qchiller
Getting started
Click on START  All Programs  AspenTech  Process Modeling V7.3.2 
Aspen Plus  Aspen Plus v7.3.2
Click on “New”, “Blank Simulation”, and
“Create”.
A new document will open. Click
the “Simulation”
Select “Mixer” and drop one on the
flowsheet.
Select “Material Streams” and add
two input streams and one outlet
stream.
As before, click on the flowsheet and
then on the red input arrow to add
the required input stream. Then click
on the flowsheet and click where the
red input arrow has turned to blue to
add a second input. Then click the
red output arrow and then on the
flowsheet to add the output stream.
Change the names of your streams
to “Feed”, “Recycle”, and “FR-MIX”,
and your mixer to “MIXER”.
You’ll need to click on the arrow
button in order to then right-click on
your streams and rename them.
Select the “Reactors” tab, then choose “REquil”
for an equilibrium reactor, and drop it on your
sheet.
Rename your reactor “REACTOR”.
Double-click “FR-MIX”. Your cursor
becomes an arrow into a line. Click
the red input arrow in the reactor.
Zoom if necessary
Now add a material output stream to
the top of the reactor (“VAP-REAC”),
and one to the bottom (“LIQ-REAC”).
Also add an output heat stream (“QREAC”).
We need a chiller. We use a heater block
for that. Click the “Heat Exchangers” tab,
then “Heater”, add one.
You can rename this
“Chiller”.
Double-click the end of the “VAP-REAC”
stream and connect it to the chiller.
Add an output material stream
(“CHILLED”) and output heat stream (“QCHILL”) to the chiller.
Click the “Separators” tab, click “Flash2” for twocomponent flash (liquid/vapor), and drop it on
your sheet. Name it “FLASH”.
Connect “CHILLED” to the Flash input. Make
two material output streams for the flash,
“VAP-FL” (top) and “AMMONIA” (bottom).
Click the “Mixers/Splitters” tab, then
“FSplit”, and drop a tee splitter on your
sheet; name it “TEE”.
Connect “VAP-FL” to TEE’s input, and
make an output material stream for TEE
called “PURGE”.
Now connect the beginning of the
“RECYCLE” stream to an output of the
TEE.
The flowsheet is done. Click the
“Next” button, then click OK to go to
the data browser.
Name your flowsheet “Example 3:
Recycle” and select SI units.
Click “Next”, then add Nitrogen,
Hydrogen, Ammonia, and Methane.
Click “Next”, and set “Base method”
to be “PENG-ROB” for the PengRobinson equation of state.
Click “Next”, and our interaction parameters
are fine, so click “Next” and then “OK” to
“Go to Simulation environment”.
Set your feed stream specifications:
T = 25 C, P = 125 atm, total flow =
3008 lbmol/hr.
Set the feed components: Nitrogen =
1000, Hydrogen = 2000, Methane = 8.
Click “Next” and set the chiller specs.
T = 0 C, P = 0 atm (no pressure drop).
Click “Next” and set the flash specs.
T = 0 C, P = 0 (no pressure drop).
Click “Next” and confirm the mixer spec
is set to no pressure drop (P = 0).
Click “Next” and set the reactor specs.
T = 400 C, P = 125 atm
Click “Next”, then “New”. Set the
reactants as Nitrogen (-1 coefficient)
and Hydrogen (-3 coefficient)
Set the product as Ammonia (2
coefficient).
Click “Next”, then “Next”, and set the
PURGE value as 0.05. This is the split
fraction.
Note that everything
else gets grayed out;
you are done here.
Click “Next”. It looks like you are done!
Click OK. Watch the calculations.
Note the warning.
The warning is about an inconsistency, but it’s not
too bad, and may be due to a big recycle stream.
In the Data Browser, go to “Result Summary” . Click on
“Streams”. Check the total amount in the recycle stream.
Click on “Heat” tab to confirm
the cooling duty
Exercises:
1.Calculate the net production rate of ammonia from the process. What fraction
of the ammonia produced in the reaction is lost in the purge stream? Why
is it crucial to have a purge stream in this process?
2. Look at the heat flows in the process. What general idea could be considered to
reduce the overall energy consumption of the process?
3. It is expensive to pressurize gases to high pressures. Determine how the
production rate of ammonia would be affected by reducing the reactor
pressure to 100, 75, or 50 atm. Can you explain the physical principle
that causes this change?
4. It is also expensive to heat gases up to high temperatures. Determine how the
production rate of ammonia would be affected by holding the reactor pressure
at 125 atm but reducing the temperature to 350, 300, and 200 oC.