2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-0513Prepare a Flowsheet for Energy Analysis in AspenPlus and Aspen HYSYSAn Industry White PaperJack Zhang, Product Manager, Aspen Technology, Inc.Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-05132Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYSIntroductionActivated Energy Analysis, introduced in Aspen Plus and Aspen HYSYS V8, enables simulation users to quicklyevaluate the energy saving potentials and identify changes to a flowsheet to reduce process energy usage.ActivatedEnergy Analysis runs Aspen Energy Analyzer in the background to perform the energy target calculations and retrofittingstudies.Successful data extraction is the prerequisite of using this powerful feature.Although significant efforts have been spenton improving the data extraction engine, the simulation user is strongly encouraged to review the flowsheet to avoidpossible errors in extracting streams and heat exchangers into Aspen Energy Analyzer.Diagnose Data Extraction IssuesIn general it is a time consuming process to diagnose data extraction errors and identify problems in flowsheetconfiguration.To expedite the error diagnostics of data extraction, a warning icon indicating the status of the dataextraction is displayed on the Activated Energy Analysis panel.When the user clicks on the warning icon, details of theerrors will be listed in the heat exchanger details table located in the Energy Analysis environment.Figure 1. Warning Icon Displayed in Activated Energy Analysis DashboardIn the heat exchanger details table (Fig. 2) a blue circle indicates that a heat exchanger experienced successful dataextraction and is feasible in Aspen Energy Analyzer.A yellow triangle indicates that a heat exchangers data wasextracted, but is infeasible in Aspen Energy Analyzer.A red circle represents a heat exchanger that is not extracted inAspen Energy Analyzer.Figure 2. Heat Exchanger Details Table Showing Extraction Status 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-0513Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYS3Prerequisites for Energy Analysis Flowsheet Must Converge Without ErrorIn 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 haveconverged.If the main simulation converges but the energy dashboard is still disabled, the user should check thenavigation 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 HYSYSDo Not Use Multi-Stream Heat ExchangersMulti-stream heat exchangers cannot be extracted in Aspen Energy Analyzer.Thus, network design and retrofit cannot beperformed on a flowsheet containing these types of blocks.MHEATX model in Aspen Plus or LNG model in Aspen HYSYSshould be avoided in the flowsheet when using Activated Energy Analysis.Streams with Solid Phase Cannot Be ExtractedAspen Energy Analyzer currently cannot extract data from streams containing a solid phase. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-05134Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYSOvercome Data Extraction IssuesColumn Reboiler/Condenser Not ExtractedSometimes the reboiler or condenser of a column block is not extracted.This situation may occur if the feed stream to acolumn block contains components with a narrow range of boiling points.The error in data extraction is typically causedby 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. Thefollowing 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 simulationmust be set.The default number of HCurve data points is 10. Typically the number of data points needs to be increased toat least 25 for the reboiler and condenser because of phase transition.To navigate to the window in which the number ofdata points is specified, open the column blocks 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 isillustrated in Figure 4 below.Figure 4. Navigation to HCurve Addition Window in Aspen PlusAfter 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 byswitching 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 areillustrated in Figure 5. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-0513Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYS5Figure 5. Steps to Plot Heating CurveIf any non-monotonic behavior exists, check and adjust the column pressure or reboiler configuration to eliminate thenon-monotonic behavior.Only True Utilities Streams Can Be OptimizedUtility streams are typically modeled the same as process streams.In Aspen Plus and Aspen HYSYS, a material stream isused when temperature, pressure, and composition are specified in a flowsheet.However, Aspen Energy Analyzer cannotdistinguish whether a material stream is specifically a utility stream or a process stream.If a material stream is used, theheating/cooling requirement of the extracted utility stream will remain fixed.Aspen Energy Analyzer cannot optimize theutility flow. For example, 300 psig steam is used to heat stream 10 in the following figure from 146C to 150C 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_CONDwould be extracted as a process stream in Aspen Energy Analyzer and therefore its heat flow would be fixed and unable tobe reduced through optimization. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-05136Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYSFigure 6. Example Flowsheet Showing Steam Stream Modeled as Utility Rather than Process StreamIn order to work around this problem, in Aspen Plus, always create a utility stream for the flow of utilities that can beoptimized and specify the utility type in the required blocks. In Aspen HYSYS, always specify the utility type for theenergy stream in the associated blocks.From the example shown in Figure 6, create a folder named Utilities STM300 under the Utilities tree.Using either aheater 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 beoptimized.Figure 7. Setting a Stream as a Utility 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-0513Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYS7Exchanger with Bypass is Not ExtractedMany heat exchangers include a bypass so that the heat exchanger unit can be put offline for maintenance without majordisturbances in the production process.Simulation users typically model a bypass in Aspen Plus with splitter and mixerblock, 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 PlusHowever, 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 non-monotonic.As Aspen Energy Analyzer extracts the streams, it creates a process stream in Aspen Energy Analyzer with asupply temperature of 50C (i.e. the temperature of the material stream FEED highlighted in Figure 9) and targettemperature 120C (i.e. the temperature of material stream HOT-FEED highlighted in Figure 9).However, materialstream 3 in Figure 9, coming out of the block HTR, has a temperature of 140C.This often causes data extraction to fail.Figure 9. Streams in Bypass Setup that Can Cause Data Extraction ErrorsTo 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 MixingEnergy saving potential could be reduced with non-isothermal stream mixing.Thus, care must be taken to model thesemixings in the simulation model before carrying out energy analysis.In the following example in Figure 10, Stream A at 120C is mixed with stream B at 70C and then heated to 210C.Themixed stream temperature is 87C.Assuming that the process pinch temperature on the cold side is 110C, Aspen EnergyAnalyzer extracts the stream with supply temperature 87C and a target temperature 210C.However, non-isothermalmixing causes cross pinch heat transfer and thus increases the heating target, which would not be recognized whenperforming an energy analysis. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-05138Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYSFigure 10. Aspen Plus Flowsheet with Non-Isothermal Mixing Causing Data Extraction ErrorsIn order to work around this problem, separate the streams in the simulation model and use a heater/heat exchangerblock 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 ErrorsUnit Operation and Streams that Share the Same Name Cannot Be ExtractedUnit operations and streams can share the same names in a main flowsheet or subflowsheet in Aspen HYSYS. However, ifa 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 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved. 11-3457-0513Prepare a Flowsheet for Energy Analysis in Aspen Plusand Aspen HYSYS9Figure 13. Reworked HYSYS Flowsheet with Renamed BlockSummaryData extraction is often overlooked, yet it is an essential task in performing an energy analysis using Activated EnergyAnalysis in Aspen Plus and Aspen HYSYS.Identifying possible problems in the flowsheet causing data extraction errorscan be a time consuming process, therefore Activated Energy Analysis provides valuable guidance on the location of dataextraction errors.After sifting through potential flowsheet problems and known sources of data extraction error, thesimulation user is advised to carefully inspect the identified areas and make changes accordingly to correctly pass thedata into Aspen Energy Analyzer.Worldwide HeadquartersAspen Technology, Inc.20 Crosby DriveBedford, MA 01730United Statesphone: +17812216400fax: +17812216410info@aspentech.comRegional HeadquartersHouston, TX | USAphone: +12815841000So Paulo | Brazilphone: +551134436261Reading | United Kingdomphone: +44(0)1189226400Singapore | Republic of Singaporephone: +6563953900Manama | Bahrainphone: +97317503000For a complete list of offices, please visit www.aspentech.com/locationsAbout AspenTechAspenTech is a leading supplier of software that optimizes process manufacturingfor energy, chemicals,engineering and construction, and other industries that manufacture and produce products from achemical process. With integrated aspenONEsolutions, process manufacturers can implement bestpractices 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 becomemore energy efficient. To see how the worlds leading process manufacturers rely on AspenTech toachieve their operational excellence goals, visit www.aspentech.com. 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of AspenTechnology, Inc. All rights reserved. 11-3457-0513