Design-002H
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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 S7 into two streams; a purge stream and a 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 Tee block from the Model Palette. The tee block will fractionally split a stream into several streams according to user specifications. Note that you can rotate the tee using the Rotate button on the Flowsheet/Modify tab of the ribbon.
4.03.
Double click the tee (TEE-100) to open the property window. Select S7 as the Inlet stream and create two Outlet streams called Rec1 and Purge.
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4.04.
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In the Parameters form under the Design tab, enter a value of 0.01 for the Flow Ratio of the purge stream. This means that 1% of the S7 stream will be diverged to the purge stream. The tee should solve.
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4.05.
In order to recycle stream Rec1 back to the mixer, we must add a Recycle block to the flowsheet. The recycle 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 Rec1 as the Inlet stream and create an Outlet stream called 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 back to the mixer. Add a Compressor to the flowsheet. Select Rec2 as the Inlet stream, create an Oulet stream called Rec3, and create an Energy stream called Q-Comp2. Specify an outlet stream Pressure of 274 bar_g in the Worksheet tab. 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 click on the mixer (MIX-100) to open the property window. Add stream Rec3 to the Inlet streams. The flowsheet should solve.
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Check results. Double click on stream NH3. In the Composition form under the Worksheet tab you can see 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 0.96 ammonia. Add an adjust block to the flowsheet.
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4.11.
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Double click the adjust block (ADJ-1) to open the adjust window. We must define our adjusted and targeted variables. For the Adjusted Variable select Flow Ratio_2 of object TEE-100. To do this, click the Select Var… button and select the following options. When finished select OK.
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4.12.
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Next, select the targeted variable. Choose Master Comp Mole Frac of Ammonia in stream NH3. This is shown below. When finished click OK.
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4.13.
Next, we will specify the target of 0.96 for the Mole Fraction of Ammonia in the product stream. The adjust window should now look like the following.
4.14.
Go to the Parameters tab and enter a Step Size of 0.001, and a Maximum Iterations of 1000. Click Start to begin calculations. The adjust block should solve. Go to the Monitor tab 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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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 product composition. Changes in cooling efficiency or utility 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 Studies and click Add.
4.18.
A new case study called Case Study 1 will be created. In Case Study 1 click Add to add variables to the study. First we will select the Mole Fraction of Ammonia in the product stream NH3.
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4.19.
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Next, we will add the Temperature of stream S6.
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4.20.
We will vary the Temperature of stream S6 from 25°C to 100°C with a Step Size of 5°C.
4.21.
Click the Run button, and then go to the Plots tab to view the results.
4.22.
You will see that as temperature increases, the ammonia mole fraction decreases which means that when 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. Conclusion This 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. Copyright Copyright © 2012 by Aspen Technology, Inc. (“AspenTech”). All rights reserved. This work may not be reproduced or distributed in any form or by any means without the prior written consent of AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS WORK and assumes no liability for any errors or omissions. In no event will AspenTech be liable to you for damages, including any loss of profits, lost savings, or other incidental or consequential damages arising out of the use of the information contained in, or the digital files supplied with or for use with, this work. This work and its contents are provided for educational purposes only. AspenTech®, aspenONE®, and the Aspen leaf logo, are trademarks of Aspen Technology, Inc.. Brands and product names mentioned in this documentation are trademarks or service marks of their respective companies.
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