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Use of Aspen Plus and SPLIT to calculate phase equilibria and ternary residue curves

 


 

For each of the following procedures it is first necessary to select components and a thermodynamic package.  See Selection of a thermodynamic model.  If you have experimental data for phase equilibria you should compare these data with the predictions of Aspen or HYSYS.  Alternately, you can fit (regress) data to a property methods to obtain, for example, binary interaction parameters. 

 

Binary phase diagrams and ternary residue curves

  1. Open the Aspen Plus User Interface.
  2. Select a suitable template, probably “General with Metric Units.”
  3. Go to Data/Setup, Specifications, Global.  Give your case a name, probably the components you are investigating.  For Run type, select Property Analysis.  (For SPLIT, you need to select Flowsheet, as described below.  Selecting Flowsheet also works here, but then streams and at least one operation must be specified and convergence obtained.)   Choose a pressure (this can be changed later) and the phases you expect to be present at equilibrium.  Select Vapor-Liquid-Liquid if there’s any possibility of liquid-phase immiscibility.  To go to the next setup page, Components Specification, click on the N̉ icon.
  4. Components, Specification, Selection.  Click on Find.  Type in the name or chemical formula of the first components, Find now, click on the right one, Add.  Continue until you have entered all components (must be at least 2).  Close.  N̉
  5. Properties, Specification, Global.  Process type ALL.  Select a suitable Base thermodynamic property method.  N̉
  6. Properties, Parameters, Binary Interaction.  If it appears, click on “Estimate all missing parameters by UNIFAC.  N̉
  7. PT PHASE ENVELOPE:  Aspen now asks which property you want to estimate.  (You can skip this step and go directly to step 9, if you have no interest in generating a PT phase envelope.)  Click on New, give it a suitable name, select PTENVELOPE.  OK.  Insert any flow rates.  N̉  Go to Next required input step.  Should then indicate that input is complete and load the simulation engine.
  8. To see the results, Data/Setup, Properties, Analysis, your name for the simulation just run (previous step), Results.  To plot table, Plot/PlotWizard.  This shows P versus T for saturated liquid (VFRAC = 0.0), saturated vapor (VFRAC = 1.0) plus any other vapor fractions you specified in step 7.  The composition is that specified via the flow rates in the previous step.  If an equation-of-state model was chosen in step 5, the saturated liquid and vapor curves will meet at the critical point for this composition.

BINARY PHASE DIAGRAMS: Continue with step 2 of Properties for Binary Systems:

  1. Tools/Analysis/Property/Binary.  Select Txy, Pxy or Gibbs energy of mixing for Analysis type.  Select the components you wish, Basis (probably mole fraction), Component, Valid phases, T or P.  Go.  This should produce the desired phase diagram.  Right-click on the figure to select properties other than the defaults.
  2. Close the figure to see the table of results.  Click on Plot Wizard to select other related phase diagrams.  (Gamma is the liquid activity coefficients.  If an activity-coefficient thermodynamic model was whose, this assumes the vapor is an ideal gas, i.e. compressibility near 1; far from critical conditions.)
  3. TERNARY RESIDUE CURVES: See Aspen help instructions and residue maps.  Tools/Analysis/Property/Residue, select P and number of curves.  Go.  A residue curve is a plot of the composition of a liquid as it is evaporated, assuming that the liquid is completely mixed (uniform composition), the vapor and liquid are in equilibrium when the vapor is produced, and the vapor is instantly removed once it is produced.  It approximates the liquid composition in batch distillation without reflux, and the liquid composition versus height in a packed column at total reflux.  Unfortunately, the plot created by Aspen Plus does not explicitly show any azeotropes, liquid-liquid tie lines (if any), distillation boundaries across which one cannot cross with equilibrium stages, or arrows on the residue curves.  For this sort of information, one must access SPLIT within Aspen Plus (see below).  Right-click on plot, Grid, Cartesian to convert to cartesian coordinates.
  4. TRYING OTHER THERMODYNAMIC MODELS (“Base method”):  Data/Setup, Properties, Specifications.  Select all of those you wish to investigate.  As you select each, it will show up under the Setup Properties, Property Methods.  When finished, repeat steps 9, 10, 11 for each method.  Compare with experimental data to select the best.  In the absence of data, select those with the most BIPs and reasonable phase diagrams.

 

Azeotropes, residue curves, distillation boundaries (See Aspen instructions for SPLIT)

For these, we use Aspen Split, which is accessed within Aspen Plus by Tools/Conceptual Design.  However, we must first specify one or more streams on a flowsheet and obtain convergence.  The following is a suggested procedure:

  1. Open the Aspen Plus User Interface.
  2. Select a suitable template, probably “General with Metric Units.”
  3. View/Model Library (unless units are already displayed on the bottom of the screen).
  4. Add a Separator, Flash 2 unit to the flowsheet (PFD) by clicking on the icon and then clicking again where you want it.  Connect Material Stream 1 to the flash unit.
  5. Go to Data/Setup, Specifications, Global.  Give your case a name, probably the components you are investigating.  For Run type, select Flowsheet, indicate the Valid phases you expect to be present.  N̉
  6. Components, Specification, Selection.  Click on Find.  Type in the name or chemical formula of the first components, Find now, click on the right one, Add.  Continue until you have entered all components (must be at least 2).  Close.  N̉
  7. Properties, Specification, Global.  Process type ALL.  Select a suitable Base thermodynamic property method.  N̉
  8. Properties, Parameters, Binary Interaction.  If it appears, click on “Estimate all missing parameters by UNIFAC.  N̉
  9. Flowsheet, Section, GLOBAL, Specifications.  Input stream 1 at port F(IN), output streams 2 at V(OUT) and 3 at L(OUT). N̉
  10. Streams, 1, Input, Specifications.  Choose composition (via flowrates), T, P.  N̉
  11. Blocks, B1, Input.  Choose P.  N̉  OK to run.  Click on >> to see more results.  If this has converged, you can skip to step M.
  12. To change stream 1, go to Streams, 1, Input, Specifications.  To change the flash conditions go to Blocks, B2, Input.
  13. Tools, Conceptual Design, Azeotrope search.  Select components, pressure, phases (VLL suggested).  Click on Azeotropes, Report.
  14. Tools, Conceptual Design, Ternary Maps.  Select P.  Click on Ternary Plot.  Right-click on diagram to convert to equilateral formate, flip and rotate to put components at desired vertices.
  15. Return to step G to select another thermodynamic model.

 

Three of the Aspen Plus tutorials online

University of Texas

Michigan State University

University of Washington


Created July 2, 2007.   Comments and corrections should be sent to Professor William R. Wilcox

 

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