1. Aspen Introduction
ASPEN is a process simulation software package widely used in industry today. Given a process design and an appropriate selection of thermodynamic models, ASPEN uses mathematical models to predict the performance of the process. This information can then be used in an iterative fashion to optimize the design. This accurate modeling of thermodynamic properties is particularly important in the separation of non-ideal mixtures, and ASPEN has a large data bases of regressed parameters. ASPEN can handle very complex processes, including multiple-column separation systems, chemical reactors, distillation of chemically reactive compounds, and even electrolyte solutions like mineral acids and sodium hydroxide solutions.
ASPEN does not design the process. It takes a design that the user supplies and simulates the performance of the process specified in that design. Therefore, a solid understanding of the underlying chemical engineering principles is required to supply reasonable values of input parameters and to evaluate the suitability of the results obtained. For instance, a user should have some idea of the column behavior before attempting to use ASPEN. This information could come from an approximate method, such as the McCabe-Thiele approach, general modeling of the T-x-y behavior, or residue curve maps.
ASPEN cannot tell you how many stages to use for a given separation. You must set the number of stages and see what type of separation results. Some preliminary or 'back of the envelope' calculations are generally recommended.
MSU has a variety of Aspen packages for different simulations. Briefly, here are the programs and capabilities:
Aspen Adsim - Fixed bed adsorption for pressure swing adsorption, etc.
Aspen Chromatography - Fixed bed adsorption, simulated moving bed chromatography. Runs independent of Aspen Plus.
Aspen Custom Modeler - A utility to permit the creation of user unit operations.
Aspen Distil - Aspen's 'Conceptual Engineering Product' for planning for processing schemes. Runs independent of Aspen Plus.
Aspen Dynamics - Unsteady-state simulator.
Aspen Plus - Steady-state process simulator.
Aspen Properties - Modeling of properties and phase equilibria. Incorporated into most other components, though it can be run as a stand-alone subset. All of the phase equilibria and mixture property methods discussed on this site are accessible in either Aspen Plus or Aspen Properties.
Aspen Polymers - Modeling of polymerization reactors and polymer thermodynamics. This package is available within Aspen Plus or Aspen Properties rather than via an external menu.
BatchSep - Batch distillations. Runs independently of Aspen Plus.
Normally undergraduate student projects will involve Aspen Plus or Aspen Properties . To start either of these packages, be sure to look for the corresponding User Interface on the start menu.
1.1 Getting more help
This document is intended to be an overview. ASPEN has extensive online help. Do not try to contact ASPEN directly. They do not respond to student requests. Work through your instructor and TA for getting answers to your questions. If your questions are not answered with online help, see the pdf documents available from the ASPEN documentation folder available on the START menu. Most common tasks are covered.
To find descriptions/equations for the thermodynamic models and parameter variables, are in online Properties Help, Chapter 3.
2. Getting Started with Aspen Plus or Aspen Properties
Normally undergraduate student projects will involve Aspen Plus or Aspen Properties . To start either of these packages, be sure to look for the corresponding User Interface on the start menu. When you are prompted to connect to the engine (license) configure the window as shown, and click OK.
Figure 2.1 - Connection dialog
3. Specification of Flowsheet in Aspen Plus
If you are working with Aspen Properties, you may skip to section 4 of this document.
To demonstrate how to build a process simulation using ASPEN, we will develop a distillation column for separation of ethanol and water.
The first step in developing a simulation is to develop the process flow diagram (PFD), which consists of the unit operations (blocks) and streams that feed and connect the blocks. The blocks are listed by category at the bottom of the main window (columns, reactors, etc.) in a toolbar known as the 'Model Library', a portion is shown in Figure 3.1. There are a wide variety of block available. Documentation for the algorithm for each block is provided in the ASPEN documentation.
For the ethanol + water system, the short-cut will not be appropriate since the system has an azeotrope. Choose 'RadFrac'. Click on the small arrow on the right side of 'RadFrac' to select the column icon that you want to use on the PFD. The menu will disappear; move the crosshairs to the desired location on the main flowsheet window and click the mouse button.
Next you have to add streams to the block. Click on the small arrow to the right of the STREAMS button at the lower left corner of your screen (as shown in Figure 3.1), and choose the stream icon you want from the menu (material, energy or work). For this example, set up the feed stream: choose the Material stream by clicking on it. The column will now show arrows where the stream can be connected; red arrows indicate required streams as shown in Figure 3.2
Figure 3.2 - Required and optional stream connection points
To set up the feed stream to the column, move the crosshair on top of the red feed position and left click once. Now, move the mouse to the left and click again. You should now have a defined feed stream (Stream 1). For the outlet streams click the column outlet first to connect the bottoms (Stream 2) and liquid distillate (Stream 3).
If you make a mistake and want to delete a stream or block, click on the arrow (select) button at the upper left of the Model Library toolbar, then click on the stream or block you want to delete and hit the DELETE key.
Figure 3.3 - Column after connection of material streams
Now that you have defined the unit operations to be simulated and set up the streams into and out of the process, you must enter the rest of the information required to complete the simulation. Within Aspen Plus, the easiest way to find the next step is to use one of the following equivalent commands: (1) click the Next icon (blue N ->); (2) find 'Next' in the Tools menu; or (3) use keyboard shortcut F4. Any option will open the Data Browser.
4. Configuring Units and Settings
In the Data Browser, you are required to enter information at locations where there are red semicircles. When you have finished a section, a blue checkmark will appear. However, providing some 'Setup' settings is often desirable.
You can change default units by opening the ‘Setup’ Folder as shown below.
You can browse the unit sets to see the choices. The base 'unit-set' names shown above are reserved names and you cannot modify them. However, if you right-click on a unit set, you can ‘rename’ it and then modify it. Once you have viewed the units you can specify the choice by using the drop down boxes.
If you are running Aspen Plus, you may wish to have stream results summarized with mole fractions or some other basis that is not set by default. Use the 'Report Options' as shown below.
5. Specifying Components
Here you have to enter all the components you are using in the simulation. The opening screen is shown below.
5.1 Entering compound information
The easiest way to enter component information is to click on the 'Find' button and enter the name of the component. Start by typing 'ethanol', and then select ETHANOL from the list of components that appears. Click the 'Add' button to add it to the components list. Repeat to add water to the components list. The ‘Component ID’ is an arbitrary name of your choice that will be used to label the component in your calculations. The ‘Type’ is a specification of how ASPEN will calculate thermodynamic properties. For fluid processing of organic chemicals, it is usually appropriate to use ‘Conventional’. If you make a mistake adding a component, right click on the row to specify deletion.
6. Specification of Thermodynamic Methods
Aspen furnishes a "Property Method Selection Assistant" to assist in selection of a reasonable thermodynamic model, Tools>Property Method Selection Assistant. Also, Appendix D of the "Introductory Chemical Engineering Thermodynamics by Elliott and Lira furnishes a flowchart to assist with model selection.
You need to be aware of the manner in which Aspen implements parameter values because Aspen offers temperature-dependent functions in place of parameters, and sometimes uses different signs on parameters than the same models in the literature.
To find information on the property models, access the online help file, and on the page "Accessing other Help", use the link for "Aspen Properties Help". Then browse to "Aspen Properties Reference". Then, to find the model description and parameters implementation click in the help window, click on "Physical Property Methods and Models". Look in Chapter 3 for descriptions of the EOS and activity models. If you have trouble finding "Physical Property Methods and Models" via the online help links, load the correct help file C:\Program Files\AspenTech\APrSystem V7.1\GUI\Xeq\aprsystem.chm. You may also find a pdf file by browsing from the Start menu to C:\Program Files\AspenTech\Documentation\Aspen Engineering V7_1\Aspen Properties\AspenPhysPropModelsV7_1-Ref.pdf.
The screen to select the property method is shown below.
The ‘Process Type’ will narrow down the choices for thermodynamic methods. Often for undergraduate design, ‘Chemical’ will provide a wide range of methods. However to access the van Laar model, you must select 'all'. The ‘Base method’ will specify the default calculation method for all blocks though you can control which method is used in individual blocks by editing the setup for the individual blocks. You will generally not use ‘Henry Components’ or ‘Free water’. For the example here, select UNIQUAC, a well-accepted model for non-ideal multicomponent liquid mixtures at low pressure.
By clicking the ‘N->’ button, you will be shown the binary parameters as shown in the screenshot below. When you close the window or click 'Next', you have provided approval of the values, and you will receive no further prompting for parameter values. If parameters are blank, zeros will be used. This does not imply that the ideal mixture assumption will be used because many models predict non-ideal behavior with parameter values of zero.
Understanding Aspen Binary Parameters
The form of the thermodynamic model parameters usually differs from the form in the published literature because ASPEN often replaces parameters with functions of temperature. To find the form of equation used in Aspen, open the Help file, and from the 'index' tab, search for the index for the model name (e.g. UNIQUAC), click on the resulting model name in the index pane, select the entry with the 'model' name (e.g. 'UNIQUAC activity coefficient Model'). You should then see equations very similar to the published literature. To understand where you are within the help file system, switch back to the Contents tab of the help folder and you will see links to the other activity coefficient methods. You will be in Chapter 3 of the Physical Properties Methods and Models Manual.
For UNIQUAC the typical published form of the parameters is
tau.ij = exp(-a.ij/T)
In ASPEN, this is implemented as
tau.ij = exp(a.ij + b.ij/T + c.ij*lnT + d.ij * T + e.ij/T^2)
So the published parameters are related to the aspen parameters:
b.ij (aspen) = -a.ij (published)
WHEN THE ASPEN UNITS ARE SET TO K (See the dialog box above, note the temperature units are specified in the top row of the table).
To verify the pure component values (e.g. UNIQUAC R and Q), in the data browser, click the 'Components' folder. Then in the right pane on the 'selection' tab, click the 'Review' button at the bottom right. The listing will include constants pulled from the Aspen databases, including GMUQR and GMUQQ and GMUQQ1. For our purposes GMUQQ and GMUQQ1 are the same. These should match the values from the textbook.
7. Specifying Stream and Block Information
This section applies to Aspen Plus; if you are working with Aspen Properties, skip to the Section 8.
Click on 'Next'. Stream specifications will appear. You must choose the stream composition, flow rate, and state for feed streams. The state is specified by pressure, temperature, and vapor fraction. For this example, for the feed stream (1) choose a pressure of 1 atm and a temperature of 25 oC. Now enter the component molar flow rates as 20 kmol/hr for EtOH and 980 kmol/hr for water. (If you enter feed composition as mole fractions, you also have to specify the total flow rate.)
Click on Next. The block (RadFrac) setup will appear. For this rigorous simulation, you must specify the column configuration. Enter the number of stages as 33 and specify total condenser. In the 'Operating Specifications' section, set the distillate flow rate to 23 kmol/hr, and set the boilup rate at 1500 kmol/hr as shown below.
Hit 'Next' and the 'Stream' page appears. Locate the feed stream (1) on stage 17. Hit 'Next' to get to the 'Pressure' page. Specify the 'Stage 1/Condenser' pressure as 1 atm. By leaving the other sections of the pressure page alone, pressure drop through the column will be ignored in this calculation.
7.1 Running the simulation
All required information should now be complete. Click 'Next'. You should now get a message that all required information has been entered. If you don't, complete the required form or look at the menu on the left for any red semicircles. To run the simulation, click OK on the message, or you can run the simulation on Run in the 'Run' pulldown menu.
7.2 Viewing Results
To view results, click on the blue folder in the toolbar. Choose 'Stream' to view stream properties, or 'Block' to view column properties. In 'Streams', you can look at the streams you wish and place a streams table on your PFD by clicking the 'Stream Table' button. (Note: pasted stream tables are NOT updated if you modify the simulation and rerun). To view the RadFrac Block properties, click on Blocks (B1) in the left pane of the data browser.
In a complex simulation, it is sometime more convenient to work with the PFD to find results. Right-click on a block or stream for a short-cut menu to results.
You can bring up compiled reports by going to the 'View' menu and clicking on the desired information. The information in the reports is controlled somewhat by the report options introduced in Section 4.
7.3 Reviewing Column Behavior
You can study the behavior of the column by looking at the column profiles as shown below from the 'Results' data browser. An example table is shown below.
You can plot the column profiles using "Plot>Plot Wizard...". For compositions, choose the composition tool, specify liquid mole fractions. The analysis below shows that there may be more stripping stages than necessary for the given column 33 stages, flowrates, reflux and boilup. Naturally, compositions at the top of the column are limited by the azeotrope.
7.4 Printing your work
See the note about the ASPEN print bug workaround at the top of this web page.
You can print the process flowchart and include the stream table if you have pasted it onto the PDF. Go to the 'Setup' page, and click on 'Use Specified Font Size' in order to get a readable printout. Then select 'Print'. To print 'Input Summary', 'History', or results ('Report'), go to the 'View' menu and select your choice. Save the information as a Notepad (.txt text) file, which you can then import into Word or Excel and print much more efficiently. The default reports have more information than you typically need. Avoid printing reports without reviewing them or pasting them in a Word document or you will use up print quota quickly!
7.5 Saving your Work
As you work with Aspen plus and Aspen Properties, saving files in the 'backup' format will assure that they can be opened in the next version of Aspen. Currently it is not possible to open 'standard' files when upgrading Aspen. The backup format ends with an 'bkp' as the last part of the file extension.
7.6 Running the simulation again, and reinitializing
You will want to modify your process parameters to run the case again. After modifying, you can click the 'Next' button, or the 'Run' button. The 'Run' button is blue '>' triangle in the main toolbar.
Aspen will 'reuse' the last state to start the next simulation. When a case crashes, this is usually not desirable. To reinitialize, use the '|<' button in the main toolbar.
Be sure to explore the phase behavior of the systems in your design. It can be frustrating to try to get Aspen to give a physically impossible result, but many students have struggled with this, and blame Aspen. Not all separations are possible because of azeotropes, pinch points, and/or distillation boundaries.
8. Additional Features to Explore Thermodynamic Behavior
8.1 Obtaining a complete set of thermodynamic parameters.
The default folder views do not give you a full view of the parameters used by APSEN. To get a full view, use ‘Tools -> Retrieve Parameter Results…’.
8.2 Stream Reports with Additional Property Information
To see mole fractions of each phase in a mixed stream of multiple phases, you can add mole fractions as property sets for the specific phases. If you build you simulation from a specialty chemical template, the property sets XTRUE (liquid mole fraction) and VMOLFRAC (vapor mole fraction) are available. If these property sets are not available because your simulation did not use the template, you can create custom property sets that include the vapor and liquid mole fractions. (Properties->Prop-Sets->New... and then choose the mole fractions as the 'Physical Properties' and the appropriate phases on the 'Qualifiers' tab).
To add these property sets to a stream report, Setup -> Report Options -> Stream Report Tab -> Click the 'Property Sets' button and select the desired property sets to add to the stream report.
Note that it also possible to add activity coefficients, fugacity coefficients in this manner. To view special properties, create a custom view of the stream report.
8.3 Calculating Pure Properties, Binary Phase Behavior or Ternary Residue Curves
Once all data has been loaded, you may use ‘Tools -> Analysis -> Pure…’ or ‘Tools -> Analysis -> Binary…’ or ‘Tools -> Analysis -> Residue…’ to evaluate properties.
For example, after setting up a acetic acid + water system to use the Hayden-O’Connell method for vapor fugacities and the UNIQUAC method for liquid properties, a T-x-y diagram can be quickly generated using ‘Tools -> Analysis -> Binary…’. Be sure to edit the ‘Valid phases’ box if you expect there may be ‘VLL’ equilibria. (Do not use ‘Free Water’ unless you can safely assume that an aqueous liquid phase is pure water. This assumption can sometimes be used in petroleum processing of hydrocarbons, but is not valid for most functional organics).
The diagram is displayed:
When you close the diagram you will find the table with some intermediate calculations. If you would like to get the values into Excel, drag the mouse over the columns, and copy. Then paste into Excel.
Here is another example for methanol + benzene.
8.4 Calculating Mixture Properties
It is also possible to plot fugacity coefficients, activity coefficients, or other properties as a function of composition or temperature, etc. Mixture properties typically require that you specify a property set and then 'run' the case.
First, specify the components as shown in Section 5. To get properties as a function of composition at a fixed T and P, you will have to set up a property set and then request execution of the set.
8.4.1 Establishing the property set
Open the folder for 'Properties>Prop-Sets'.
Give the 'Property Set' a name that will help you remember the calculated properties. In this example the property set is called 'PHIMIX'. On the 'Properties' tab, select the APSEN name for the property that you want to tabulate. You will probably need to consult the documentation to find the ASPEN name for the property. In this case, I will select 'PHIMX', the ASPEN name for the component fugacity coefficient in a mixture. Enter the units if appropriate for your property.
On the 'Qualifiers' tab, set the other details for the calculation. In the case of fugacity coefficients, I chose to calculate them for the vapor phase.
To instruct ASPEN how to use the property set, you next specify the analysis to be performed.
8.4.2 Specifying the Analysis to run for the property set.
Select the folder for 'Properties>Analysis'. The screen will look much like the 'Property Set' page in Section 8.4.1 'Establishing the Property Set'. Click 'New' and name the analysis set. I will call mine 'PhiCalc'. Also for most properties you will want to select 'Generic' unless it is clearly an envelope or residue curve. T-x-y, P-x-y and residue curves are accessible more easily as shown in Section 8.3 'Calculating Pure Properties, Binary Phase Behavior or Ternary Residue Curves'.
On the 'Systems' tab, if you intend to specify the temperature and pressure, specify 'Point(s) without a flash'. It will be necessary to set the flow rates even though there isn't any real process stream. If not necessary for the calculation, ASPEN will ignore them.
On the 'Tabulate' tab, specify the Property Sets for the analysis, and move them to the right list box:
For summary of the output, click the button on the page for 'Table specifications' and give the table a name and specify the precision desired, as shown below.
On the variable tab of the 'Property Analysis' set, you will specify the fixed state variables and the adjusted variables as shown below. Note that the upper section of the form is for the Fixed state variables, in this case set to be 120C and 1 atm. The lower table has been edited to vary the mole fraction of acetic acid. Before leaving the form, the values or range for the adjusted variables must be specified. To provide this information, first put the cursor in the variable field (e.g. the variable 'Mole fraction' is selected below), and then click the form button named 'Range/List' to specify the range/list for that variable.
Specify the Range or List of Values to be varied as shown below. Here the range will be from 0 to 1 at intervals of 0.05.
8.4.3 Generating the calculated values
At this point, ASPEN has enough information to calculate the desired information. Click the 'Run' button on the toolbar. The 'Run' button is the blue triangle in the top tool bar (it is 'grayed out' on all screen shots on this web page). You can tell that results are available when the 'Analysis' folder changes to blue as shown below. Note that the blue 'PHICALC' folder has 'Results' available. The columns of calculations as shown below can be copied to the Windows clipboard by dragging the mouse over the column titles, using the Edit menu (or Ctrl-C). The clipboard contents can be pasted into Excel.
8.5 Sharing parameter values between simulation files
When a significant number of user parameters have been entered, it is convenient to transfer them to another file in a more efficient method than a copy/paste method. This section discusses a method to export parameters and import them into a new simulation.
As an overview, Aspen Properties files hold all the pure component and binary parameter information, but none of the process schematic information. They also include information about the property 'methods' including customization of how the vapor phase fugacity is calculated, etc., and all reaction chemistry, etc. Plus they hold user parameters that have been used to specify property information and binary interaction parameters.
So it is possible to export an Aspen Properties file from one aspen simulation using File>Export, and then import it into the other simulation. When you export, choose the Aspen Properties backup format, *.aprbkp for greatest compatibility. I also strongly suggest that you open the exported file using the Aspen Properties interface and enter a good description of the properties file in the description window (Setup>Description). This description is viewable when using the File>Open dialog box which is helpful. Resave the properties file after documenting the file.
When you import to a new simulation using File>Import, you must select from a list the properties that you wish to import, and there are two options: merge or replace. I have not studied these closely, but it in my trials, I had to use 'replace' to overwrite the binary parameters. Also, I did not have the patience to figure out which row in the property list imports the binary interaction parameters. I just used shift-click to select all rows and used the 'replace' button.
*If you notice any errors or outdated information on this page, please contact Professor Lira who maintains this content.