Have you encountered something like this before? Looking at this phase chart (a Txy diagram), it’s clear there is something very wrong with the predicted phase behavior:
Even though we’re told that best practice is to check thermodynamic charts during process simulation, it’s not always obvious what to do when the phase behavior looks questionable.
Here’s a helpful tip: Try checking your global phase settings.
In the chart above, CHEMCAD’s Global Phase Option is set to Vapor/Liquid/Solid. As it turns out, however, the two components in this binary system are at least partially immiscible. Because of this, we need to perform a more rigorous vapor-liquid-liquid flash calculation.
In CHEMCAD, the Thermodynamic Settings dialog lets you toggle between Vapor/Liquid/Solid (V/L/S) and Vapor/Liquid/Liquid/Solid (V/L/L/S) modes. This choice is easy to overlook, but it’s essential for correctly modeling systems with partial miscibility or liquid-phase separation. Knowing when—and why—to switch modes is key to getting accurate phase behavior predictions.
Why are there different settings for VLE and VLLE?
When the V/L/S global phase option is selected, CHEMCAD performs all flash calculations as two-phase, i.e., vapor-liquid flash calculation. (Technically, CHEMCAD will allow solids to be present as a third phase with this setting.) That means the algorithm will not search for a second liquid phase when calculating equilibrium. If you are modeling a second liquid phase, a more complex mathematical model is required to solve rigorous three-phase flash calculations and calculate the correct phase splitting.
Not all K-value models can predict two liquid phases, so be sure to select a model that can accurately predict the phase behavior for your system. The Non-Random Two-Liquid (NRTL) activity coefficient model and UNIQUAC are examples of widely used K-value methods that can predict liquid-liquid equilibria (LLE). To make accurate predictions, these models use binary interaction parameters (BIPs) to describe how components interact in a mixture. These parameters are derived from experimental VLE or LLE data, much of which is available in CHEMCAD; BIPs can also be regressed from user data.
Using LLE data is especially important for systems where two liquid phases may form, often due to the presence of an azeotrope or other immiscibility phenomena. Enabling the V/L/L/S global phase option allows CHEMCAD to search for a second liquid phase in all stream and unit operation flash calculations. For example, if you want to perform a three-phase distillation, all you need to do is invoke the VLLE option and set up the SCDS distillation UnitOp as you normally would. In this way, CHEMCAD can determine if and where two liquid phases form.
What does the Global Phase Option influence?
The implications of ignoring VLLE can be profound. In the case where two liquid phases should form, the model may incorrectly predict a single liquid composition — even if, thermodynamically, the system wants a phase split. This affects more than just liquid phase behavior; it also impacts vapor compositions, liquid separation efficiency, and product purity predictions.
Referring back to the impaired Txy diagram from the beginning of this article, switching the Global Phase Option in the Thermodynamic Settings menu from VLE to VLLE corrects the phase behavior prediction, resulting in a physically consistent phase behavior diagram.
Switching Global Phase Options in CHEMCAD
The Thermodynamic Settings command is prominently displayed on both the Home tab and the Thermophysical tab, and you can also access it on the Simulation tab of the Explorer Pane under Thermodynamics. From the Thermodynamic Settings dialog, you can find the Global Phase Option setting in the top right corner of the main tab.
If you change this setting or make other adjustments in the Thermodynamic Settings dialog, be sure to click OK to apply your changes.
Sometimes the solution isn’t more data—it’s smarter settings. The Global Phase Option is a small detail that can make a big difference in ensuring your simulations reflect real-world phase behavior and deliver accurate, meaningful results.