Appendix B. Instructions for Fitting VLE and LLE Data with Aspen Plus
The vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) correlations in Aspen Plus are not always as accurate as possible. This can cause significant errors, particularly near pinch points in distillation columns. If data are available, Aspen Plus will find -values of the parameters for any of the VLE or LLE correlations by doing a regression against the data you input.
The regression method is illustrated to obtain an improved fit for the nonrandom two-liquid (NRTL) VLE correlation for the binary system water and isopropanol (IPA) (listed in Aspen Plus as isopropyl-alcohol). VLE data for water and isopropanol are listed in Table B-1. This system has a minimum-boiling azeotrope at ~80.46°C. The Aspen Plus fit to the data with NRTL is pretty good, but can be improved.
TABLE B-1. VLE Data for isopropanol-water at 1.0 atm. Perry’s Chemical Engineers Handbook (Perry and Green, 1984, p. 13–13)
In the components selection window enter component names water and IPA, which you identify as isopropyl-alcohol. Click Next and select NRTL as the property method. Click Next, and when the message Properties Input Complete appears select Enter experimental data and click OK. An alternative is to go to the All Items list, scroll down to Data, and click Data. In the Data window click New, chose an ID (the default is fine), and select Mixture for type. Click OK. In the setup window, the components listed should be water and IPA. Click the >> button to select both. Then click the Data Type dropdown menu, and select Txy (VLE data at constant pressure). At the bottom of the form, in the Composition menu select Mole fraction. In the Pressure menu input the desired units and pressure. Click the Data tab.
On the Aspen Plus data page in the first blank line in the Usage column select DATA from the menu. Input the temperature (you can select units) and the x and y values for either water or IPA (input the one listed first in your Aspen Plus table). Aspen will automatically calculate the values for x and y for the other component When all data are input, click the Generate Data button. In the pop-up window select NRTL and click Generate. Click Next, select Run Property Analysis/Setup. To look at the VLE in graphical form, use Analysis. Now you can go to Simulation and run any simulation you desire.
Fitting LLE data is similar. In the dropdown menu for data type, select TPXX (temperature, pressure, and two-liquid fractions). Input the temperature (you can select units), the pressure, and the x1 and x2 values for two of the components. Aspen automatically enters the third component. Ignore any error messages stating that three-phase flash results do not agree and that constraints cannot be met.
Sandler (2015) provides additional information on regression of VLE, LLE, and vapor-liquid-liquid equilibrium (VLLE) data in Aspen Plus.