#### Partially-Miscible Liquids Phase Diagrams: Interactive Simulations

##### Simulation: Vapor-Liquid-Liquid Equilibrium (VLLE)

This Demonstration shows the phase equilibrium for a binary system of two partially-miscible liquids, A and B, on a T-x-y diagram. Because of the partial miscibility, vapor-liquid equilibrium (VLE), liquid-liquid equilibrium (LLE), and vapor-liquid-liquid equilibrium (VLLE) are present on the phase diagram. The amount of of heat added is used to illustrate the system behavior. The relative amounts of each phase are determined by the lever rule (mole balance) and are shown in the bar graph. The mole fraction(s) of component B in each phase are also displayed on the bar graph for the phases present.

**Try to answer these questions before determining the answer with the simulation**. We suggest that you write down the reasons for your answers.

- At what mole fraction will the temperature stay constant when heat is added? Is there more than one?
- What vapor mole fraction can be in equilibrium with two liquid phases in this binary system?
- Can a point on a the partially-miscible phase diagram represent more than one condition (amounts of phases)?

The second simulation was prepared using Mathematica. Download the free Wolfram player, and then download the simulation CDF file (link given below or click on figure to download).

A temperature-composition phase diagram is shown for two liquids (A, B) that are only partially miscible within the region enclosed by the orange/purple curve. Each phase in the two-phase region contains both A and B; the α phase (represented by the purple line) is enriched in A and the β phase (orange line) is enriched in B. Outside the phase envelope, A and B are completely miscible. The sizes of the rectangles at the top for pure A and pure B are proportional to the overall mole fraction of that component. The size(s) of the container(s) on the right is/are proportional to the amounts of the phase(s) (either α and β or a single miscible phase) in equilibrium, and the mole fractions are represented by the relative numbers of green(A) and blue(B) circles.

**Try to answer these questions before determining the answer with the simulation**. We suggest that you write down the reasons for your answers.

- When the temperature increases above about 405 K, what happens?
- Suppose a mixture with a mole fraction of A equal to 0.2 is at 390 K and more of component A is added to the system. What happens?
- Suppose a mixture with a mole fraction of A equal to 0.6 is at 390 K and more of component A is added to the system. What happens?