design 002h ammoniasynthesis closedloop

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Design-002H Revised: Nov 7, 2012

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Ammonia Synthesis with Aspen HYSYS V8.0

Part 2 Closed Loop Simulation of Ammonia Synthesis

1. Lesson Objectives Build upon the open loop Ammonia Synthesis process simulation

Insert a purge stream Learn how to close recycle loops Explore closed loop convergence methods

Optimize process operating conditions to maximize product composition and flowrate Learn how to utilize the model analysis tools built into Aspen HYSYS Find the optimal purge fraction to meet desired product specifications Determine the effect on product composition of a decrease in cooling efficiency of the pre-flash

cooling unit

2. Prerequisites Aspen HYSYS V8.0 Design-001 Module (Part 1 of this series)

3. Background; Recap of Ammonia Process


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

reflect an industrial application or real situation.

4. Aspen HYSYS Solution:In Part 1 of this series (Design-001H), the following flowsheet was developed for an open loop Ammonia

Synthesis process.

This process produces two outlet streams; a liquid stream containing the ammonia product and a vapor stream

containing mostly unreacted hydrogen and nitrogen. It is desired to capture and recycle these unreacted

materials to minimize costs and maximize product yield.

Add Recycle Loop to Ammonia Synthesis Process

Beginning with the open loop flowsheet constructed in Part 1 of this series, a recycle loop will be constructed to

recover unreacted hydrogen and nitrogen contained in the vapor stream named S7, shown below.

4.01. The first step will be to add a tee to separate the vapor stream S7into two streams; a purge stream anda recycle stream. As a rule of thumb, whenever a recycle stream exists, there must be an associated

purge stream to create an exit route for impurities or byproducts contained in the process. Often times

if an exit route does not exist, impurities will build up in the process and the simulation will fail to

converge due to a mass balance error.


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4.02. On the main flowsheet add a Teeblock from the Model Palette. The tee block wil l fractionally split astream into several streams according to user specifications. Note that you can rotate the tee using the

Rotatebutton on the Flowsheet/Modifytab of the ribbon.

4.03. Double click the tee (TEE-100) to open the property window. Select S7as the Inletstream and createtwo Outletstreams called Rec1and Purge.


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4.04. In the Parametersform under the Designtab, enter a value of 0.01 for the Flow Ratioof the purgestream. This means that 1% of the S7stream will be diverged to the purge stream. The tee should solve.


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4.05. In order to recycle stream Rec1back to the mixer, we must add a Recycleblock to the flowsheet. Therecycle block is a theoretical block which acts to compare and modify the values of the outlet stream

until the inlet and outlet streams are equal to a specified tolerance.

4.06. Double click the recycle block (RCY-1). Select Rec1as the Inletstream and create an Outletstreamcalled Rec2. The flowsheet should now look like the following.


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4.07. We need to add a compressor to raise the pressure of the recycle stream before we can connect it backto the mixer. Add a Compressorto the flowsheet. Select Rec2as the Inletstream, create an Oulet

stream called Rec3, and create an Energystream called Q-Comp2. Specify an outlet streamPressureof

274 bar_g in the Worksheettab. The compressor should solve and the flowsheet should look like the

following.

4.08. The recycle stream is now ready to be connected back to the mixer block to close the loop. Double clickon the mixer (MIX-100) to open the property window. Add stream Rec3to the Inletstreams. The

flowsheet should solve.


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4.09. Check results. Double click on stream NH3. In the Composition form under the Worksheettab you cansee that the mole fraction of ammonia is now 0.9581. This is below our desired mole fraction of 0.96.

Optimize the Purge Rate to Deliver Desired Product

4.10. We now wish determine the purge rate required to deliver a product with a mole fraction of 0.96ammonia. Add an adjust block to the flowsheet.


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4.11. Double click the adjust block (ADJ-1) to open the adjust window. We must define our adjusted andtargeted variables. For the Adjusted Variableselect Flow Ratio_2of object TEE-100. To do this, click

the Select Varbutton and select the following options. When finished select OK.


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4.12. Next, select the targeted variable. ChooseMaster Comp Mole Fracof Ammoniain streamNH3. This isshown below. When finished click OK.


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4.13. Next, we will specify the target of 0.96for the Mole Fractionof Ammoniain the product stream. Theadjust window should now look like the following.

4.14. Go to the Parameterstab and enter a Step Sizeof 0.001, and a Maximum Iterationsof 1000. Click Startto begin calculations. The adjust block should solve. Go to the Monitortab to view results. You can see

that the mole fraction of ammonia in the product stream reached 0.96 at a purge fraction of 0.019.


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4.15. The flowsheet should now look like the following.

Investigate the Effect of Flash Feed Temperature on Product Composition

4.16. We would now like to determine how fluctuations in flash feed temperature will affect the productcomposition. Changes in cooling efficiency or utili ty fluid temperature can change the temperature of

the flash feed stream. This change in temperature will change the vapor fraction of the stream, thus

changing the composition of the product and recycle streams. First, we need to deactivate the adjust

block. Double click the adjust block and check Ignored.

4.17. In the navigation pane go to Case Studiesand click Add.4.18. A new case study called Case Study 1will be created. In Case Study 1click Addto add variables to the

study. First we will select the Mole Fractionof Ammoniain the product stream NH3.


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4.19. Next, we will add the Temperatureof stream S6.


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4.20. We wil l vary the Temperatureof stream S6from 25Cto 100Cwith a Step Sizeof 5C.

4.21. Click the Runbutton, and then go to the Plotstab to view the results.

4.22. You will see that as temperature increases, the ammonia mole fraction decreases which means thatwhen operating this process it will be very important to monitor the flash feed temperature in order to

deliver high quality product.


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5. ConclusionThis simulation has proved the feasibility of this design by solving the mass and energy balances. It is now ready

to begin to analyze this process for its economic feasibility. See module Design-003H to being the economic

analysis.

6. CopyrightCopyright 2012 by Aspen Technology, Inc. (AspenTech). All rights reserved. This work may not be

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AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH

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liable to you for damages, including any loss of profits, lost savings, or other incidental or consequential

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design 002h ammoniasynthesis closedloop

Documents

h ammoniasynthesis closedloop

series design

tee tee

ammonia product

liquid stream

stream rec1back

associatedpurge stream

purge stream anda