Prepare a Flowsheet for Energy Analysis in Aspen
Plus ® and Aspen HYSYS ®
An Industry White Paper
Jack Zhang, Product Manager, Aspen Technology, Inc.
Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc.
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Introduction
Activated Energy Analysis, introduced in Aspen Plus® and Aspen HYSYS® V8, enables simulation users to quickly
evaluate the energy saving potentials and identify changes to a flowsheet to reduce process energy usage. Activated
Energy Analysis runs Aspen Energy Analyzer in the background to perform the energy target calculations and retrofitting
studies.
Successful data extraction is the prerequisite of using this powerful feature. Although significant efforts have been spent
on improving the data extraction engine, the simulation user is strongly encouraged to review the flowsheet to avoid
possible errors in extracting streams and heat exchangers into Aspen Energy Analyzer.
Diagnose Data Extraction Issues
In general it is a time consuming process to diagnose data extraction errors and identify problems in flowsheet
configuration. To expedite the error diagnostics of data extraction, a warning icon indicating the status of the data
extraction is displayed on the Activated Energy Analysis panel. When the user clicks on the warning icon, details of the
errors will be listed in the heat exchanger details table located in the Energy Analysis environment.
Figure 1. Warning Icon Displayed in Activated Energy Analysis Dashboard
In the heat exchanger details table (Fig. 2) a blue circle indicates that a heat exchanger experienced successful data
extraction and is feasible in Aspen Energy Analyzer. A yellow triangle indicates that a heat exchanger’s data was
extracted, but is infeasible in Aspen Energy Analyzer. A red circle represents a heat exchanger that is not extracted in
Aspen Energy Analyzer.
Figure 2. Heat Exchanger Details Table Showing Extraction Status
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Prerequisites for Energy Analysis
Flowsheet Must Converge Without Error
In Aspen Plus, the Activated Energy Analysis dashboard is disabled until the simulation run has completed without error,
i.e. the run status shows ‘Results Available’ or ‘Results Available with Warnings’.
In Aspen HYSYS, the Activated Energy Analysis dashboard is disabled until all streams and blocks in the flowsheet have
converged. If the main simulation converges but the energy dashboard is still disabled, the user should check the
navigation pane to ensure no item is listed under the ‘Not Solved’ or ‘Under-Specified’ folders as shown below in figure 3.
Figure 3. Navigation to the Not Solved and Under-Specified Folders in Aspen HYSYS
Do Not Use Multi-Stream Heat Exchangers
Multi-stream heat exchangers cannot be extracted in Aspen Energy Analyzer. Thus, network design and retrofit cannot be
performed on a flowsheet containing these types of blocks. MHEATX model in Aspen Plus or LNG model in Aspen HYSYS
should be avoided in the flowsheet when using Activated Energy Analysis.
Streams with Solid Phase Cannot Be Extracted
Aspen Energy Analyzer currently cannot extract data from streams containing a solid phase.
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Overcome Data Extraction Issues
Column Reboiler/Condenser Not Extracted
Sometimes the reboiler or condenser of a column block is not extracted. This situation may occur if the feed stream to a
column block contains components with a narrow range of boiling points. The error in data extraction is typically caused
by non-monotonic heating curves for the reboiler or condenser.
The first solution in debugging this problem is to create HCurves for the condenser and reboiler in the simulation. The
following paragraphs describe the steps to accomplish this in Aspen Plus.
In order to construct useful HCurves for the reboiler or condenser, the number of data points taken from the simulation
must be set. The default number of HCurve data points is 10. Typically the number of data points needs to be increased to
at least 25 for the reboiler and condenser because of phase transition. To navigate to the window in which the number of
data points is specified, open the column block’s tree menu and select ‘Configuration’. This will open a separate tree,
allowing the user to select either ‘Condenser Hcurves’ or ‘Reboiler Hcurves’. Then, create a new case. This process is
illustrated in Figure 4 below.
Figure 4. Navigation to HCurve Addition Window in Aspen Plus
After increasing the number of data points acquired, run the simulation to ensure that it converges without errors.
Next, plot the HCurves and inspect the heating curve for any non-monotonic behavior. To plot the heating curve, begin by
switching to the ‘Results’ tab of the HCurve form. Then, select the ‘Custom’ plotting option from the Home ribbon.
Specify ‘Heat duty’ as the X-axis and Temperature as the Y-axis, and click ‘OK’ to draw the heating curve. These steps are
illustrated in Figure 5.
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Figure 5. Steps to Plot Heating Curve
If any non-monotonic behavior exists, check and adjust the column pressure or reboiler configuration to eliminate the
non-monotonic behavior.
Only “True” Utilities Streams Can Be Optimized
Utility streams are typically modeled the same as process streams. In Aspen Plus and Aspen HYSYS, a material stream is
used when temperature, pressure, and composition are specified in a flowsheet. However, Aspen Energy Analyzer cannot
distinguish whether a material stream is specifically a utility stream or a process stream. If a material stream is used, the
heating/cooling requirement of the extracted utility stream will remain fixed. Aspen Energy Analyzer cannot optimize the
utility flow.
For example, 300 psig steam is used to heat stream ‘10’ in the following figure from 146°C to 150°C in a heat exchanger.
If the user modeled the steam as a material stream in Aspen Plus as shown in Figure 6, stream ‘STM300_to_COND’
would be extracted as a process stream in Aspen Energy Analyzer and therefore its heat flow would be fixed and unable to
be reduced through optimization.
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Figure 6. Example Flowsheet Showing Steam Stream Modeled as Utility Rather than Process Stream
In order to work around this problem, in Aspen Plus, always create a utility stream for the flow of utilities that can be
optimized and specify the utility type in the required blocks. In Aspen HYSYS, always specify the utility type for the
energy stream in the associated blocks.
From the example shown in Figure 6, create a folder named ‘Utilities STM300’ under the Utilities tree. Using either a
heater or a HEATX block with one side stream connection, specify the STM300 as a utility in the heater or HEATX block.
By doing so, STM300 will be extracted as a utilities stream in Aspen Energy Analyzer and its duty will be able to be
optimized.
Figure 7. Setting a Stream as a Utility
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Exchanger with Bypass is Not Extracted
Many heat exchangers include a bypass so that the heat exchanger unit can be put offline for maintenance without major
disturbances in the production process. Simulation users typically model a bypass in Aspen Plus with splitter and mixer
block, as shown in Figure 8. In this representation, the user specifies the splitting fraction to control the bypass flow.
Figure 8. Modeling a Bypass in Aspen Plus
However, this representation also makes it difficult for Aspen Energy Analyzer to understand the temperature variation.
For example, if the splitting fraction is between 0 and 1, the temperature profiles for the streams highlighted are nonmonotonic. As Aspen Energy Analyzer extracts the streams, it creates a process stream in Aspen Energy Analyzer with a
supply temperature of 50°C (i.e. the temperature of the material stream ‘FEED’ highlighted in Figure 9) and target
temperature 120°C (i.e. the temperature of material stream ‘HOT-FEED’ highlighted in Figure 9). However, material
stream ‘3’ in Figure 9, coming out of the block ‘HTR’, has a temperature of 140°C. This often causes data extraction to fail.
Figure 9. Streams in Bypass Setup that Can Cause Data Extraction Errors
To work around this problem, either completely bypass the heat exchanger with a small flow to the heat exchanger branch
(e.g. 1.0e-6), or model without bypass flow at all.
Non-Isothermal Mixing
Energy saving potential could be reduced with non-isothermal stream mixing. Thus, care must be taken to model these
mixings in the simulation model before carrying out energy analysis.
In the following example in Figure 10, Stream A at 120°C is mixed with stream B at 70°C and then heated to 210°C. The
mixed stream temperature is 87°C. Assuming that the process pinch temperature on the cold side is 110°C, Aspen Energy
Analyzer extracts the stream with supply temperature 87°C and a target temperature 210°C. However, non-isothermal
mixing causes cross pinch heat transfer and thus increases the heating target, which would not be recognized when
performing an energy analysis.
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Figure 10. Aspen Plus Flowsheet with Non-Isothermal Mixing Causing Data Extraction Errors
In order to work around this problem, separate the streams in the simulation model and use a heater/heat exchanger
block to heat/cool each stream to the target temperature and then mix the streams together at that target temperature.
Figure 11. Reworked Aspen Plus Flowsheet to Eliminate Non-Isothermal Mixing Errors
Unit Operation and Streams that Share the Same Name Cannot Be Extracted
Unit operations and streams can share the same names in a main flowsheet or subflowsheet in Aspen HYSYS. However, if
a block and stream have the same name, they cannot be processed by the data extraction in Aspen Energy Analyzer.
To work around this problem, rename the unit operation block with a different name.
Figure 12. Example HYSYS Flowsheet with Shared Stream and Block Naming
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Prepare a Flowsheet for Energy Analysis in Aspen Plus ® and Aspen HYSYS ®
Figure 13. Reworked HYSYS Flowsheet with Renamed Block
Summary
Data extraction is often overlooked, yet it is an essential task in performing an energy analysis using Activated Energy
Analysis in Aspen Plus and Aspen HYSYS. Identifying possible problems in the flowsheet causing data extraction errors
can be a time consuming process, therefore Activated Energy Analysis provides valuable guidance on the location of data
extraction errors. After sifting through potential flowsheet problems and known sources of data extraction error, the
simulation user is advised to carefully inspect the identified areas and make changes accordingly to correctly pass the
data into Aspen Energy Analyzer.
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About AspenTech
AspenTech is a leading supplier of software that optimizes process manufacturing—for energy, chemicals,
engineering and construction, and other industries that manufacture and produce products from a
chemical process. With integrated aspenONE® solutions, process manufacturers can implement best
practices for optimizing their engineering, manufacturing, and supply chain operations. As a result,
AspenTech customers are better able to increase capacity, improve margins, reduce costs, and become
more energy efficient. To see how the world’s leading process manufacturers rely on AspenTech to
achieve their operational excellence goals, visit www.aspentech.com.
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Technology, Inc. All rights reserved.
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