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Ammonia Synthesis with Aspen HYSYS V8.0

Part 1 Open Loop Simulation of Ammonia Synthesis

1. Lesson Objectives Become comfortable and familiar with the Aspen HYSYS graphical user interface

Explore Aspen HYSYS flowsheet handling techniques

Understand the basic input required to run an Aspen HYSYS simulation

Determination of Physical Properties method for Ammonia Synthesis

Apply acquired skill to build an open loop Ammonia Synthesis process simulation

Enter the minimum input required for an simplified Ammonia Synthesis model

Examine the open loop simulation results

2. Prerequisites Aspen HYSYS V8.0

3. Background Ammonia is one of the most highly produced chemicals in the world and is mostly used in fertilizers. In 1913

Fritz Haber and Carl Bosch developed a process for the manufacture of ammonia on an industrial scale (Haber-

Bosch process). This process is known for extremely high pressures which are required to maintain a reasonable

equilibrium constant. Today, this process produces 500 million tons of nitrogen fertilizer per year and is

responsible for sustaining one-third of the Earths population.

Ammonia is produced by reacting nitrogen from air with hydrogen. Hydrogen is usually obtained from steam

reformation of methane, and nitrogen is obtained from deoxygenated air. The chemical reaction is shown

below:

Our goal is to produce a simulation for the production of ammonia using Aspen HYSYS. We will create a very

simplified version of this process in order to learn the basics of how to create a flowsheet in the Aspen HYSYS

V8.0 user interface. A diagram for this process is shown below.

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Knowledge Base: Physical Properties for Ammonia Process

Equation-of-state models provide an accurate description of the thermodynamic properties of the high-

temperature, high-pressure conditions encountered in ammonia plants. The Peng-Robinson equation of state

was chosen for this application.

The examples presented are solely intended to illustrate specific concepts and principles. They may not

reflect an industrial application or real situation.

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4. Aspen HYSYS Solution

Build a Process Simulation for Ammonia Synthesis

4.01. Start Aspen HYSYS V8.0. Select New on the Start Page to create a new simulation.

4.02. Create a component list. In the Component Lists folder, select Add. Add the following components to

the component list.

4.03. Create a fluid package. In the Fluid Packages folder, select Add. Select the Peng-Robinson property

package.

4.04. Define reactions. Go to the Reactions folder, and click Add. This will create a new reaction set called

Set-1. In Set-1, select Add Reaction and select Hysys, Conversion. This will create a new reaction called

Rxn-1.

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4.05. Double click on Rxn-1 to open the Rxn-1 window. Enter the following information. Close this window

when complete.

4.06. In Set-1, we must now attach the reaction set to a fluid package. Click the Add to FP button and select

Basis-1. The reaction set should now be ready.

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4.07. Go to the simulation environment. Click on the Simulation button in the bottom left of the screen. Then

find the Flowsheet Main tab. The Flowsheet Main is the main simulation flowsheet where you will

create a simulation.

4.08. From the Model Palette, add a Compressor to the main flowsheet.

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4.09. Double click the compressor (K-100) to open the property window. Create an Inlet stream called

SynGas, an Outlet stream called S2, and an Energy stream called Q-Comp1.

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4.10. We must define our SynGas feed stream. In K-100, go to the Worksheet tab. For the stream SynGas,

enter a Temperature of 280C, a Pressure of 25.5 bar_g, and a Molar Flow of 7000 kgmole/h. In the

Composition form enter the following mole fractions. Stream SynGas should now solve.

4.11. Specify the compressor outlet pressure. In the Worksheet tab of K-100, enter a Pressure of 274 bar_g

for stream S2. The compressor should now solve.

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4.12. The flowsheet should look like the following.

4.13. Next, we will add a mixer. Add a Mixer to the flowsheet from the Model Palette.

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4.14. Double click on the mixer (MIX-100) to open the mixer window. Select stream S2 as the Inlet and create

an Outlet stream called S3. The mixer should solve. We will eventually use this mixer to connect a

recycle stream to the process.

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4.15. Next, add a heater to the flowsheet.

4.16. Double click on the heater (E-100) to open the heater window. Select S3 as the Inlet stream, create an

Outlet stream called S4, and create an Energy stream called Q-Heater. In the Parameters form in the

Design tab, enter a Delta P of 0. In the Worksheet tab, specify an outlet Temperature of 775 K

(481.9C). Note that this heater is currently acting as a cooler, but once we connect the recycle stream

this block will in fact add heat and raise the temperature of the stream.

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4.17. Next, we will add a reactor to the flowsheet. This process uses plug flow reactors to accomplish

synthesis reaction, but for this simplified simulation we will use a conversion reactor. To use a plug flow

reactor, we would need to have detailed kinetics describing the reaction. Press F12 to open the UnitOps

window. Select the Reactors radio button and select Conversion Reactor. Click Add.

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4.18. After clicking Add, the conversion reactor window will open. Select an Inlet stream of S4 and create a

Vapour Outlet stream of S5V, a Liquid Outlet stream of S5L, and an Energy stream called Q-Reac.

4.19. In the conversion reactor window (CRV-100), go to the Reactions tab. Select Set-1 for Reaction Set. In

the Worksheet tab enter an outlet Temperature of 481.9C for stream S5L. This value will copy over to

S5V. The reactor should then solve. Notice that the contents of the reactor are entirely vapor;

therefore the liquid outlet stream has a flowrate of zero.

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4.20. The flowsheet should now look like the following.

4.21. We will now add a cooler to cool the vapor stream leaving the reactor.

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4.22. Double click the cooler (E-101) to open the cooler window. Select stream S5V as the Inlet stream,

create an Outlet stream called S6, and create an Energy stream called Q-Cooler.

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4.23. In the Parameters form under the Design tab, enter a Delta P of 100 bar. We want to lower the

pressure in order to allow an easier separation of ammonia. In the Worksheet tab, specify an outlet

stream Temperature of 300 K (26.85C). The cooler should solve.

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4.24. Add a separator block to the flowsheet.

4.25. Double click on the separator (V-100). Select an Inlet stream of S6, create a Vapour Outlet called S7,

and create a Liquid Outlet called NH3. The separator should solve.

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4.26. The flowsheet should now look like the following.

4.27. Review simulation results. Double click stream NH3. In the Conditions form under the Worksheet tab

you can view the stream flowrate and conditions. In the Composition form you can view the stream

composition. Here you can see that the mole fraction of ammonia is equal to 0.9754.

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4.28. After completing this simulation, you should save the file as a .hsc file. It is also good practice to save

periodically as you create a simulation so you do not risk losing any work. The open loop simulation is

now ready to add a recycle stream, which we will then call a closed loop simulation. See module Design-

002H for the closed loop design.

5. Copyright

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