Phase Diagrams

In the Phase Diagrams tab, you can add one or more fluids and visualize their phase behavior. A phase diagram shows where different phases of a substance coexist in equilibrium.

The most common line is the vapor–liquid equilibrium (VLE) line. It shows the boundary of regions where vapor and liquid can exist together. For a pure fluid, this is a one-dimensional curve in temperature pressure space; for mixtures, it becomes a two-dimensional region, and the curve marks the boundary of that region.

Different phases appear under different conditions. For example, solid phases usually become stable at low temperatures. Knowing when solids form is important, since they can precipitate in pipes and cause blockages.

For a typical hydrate-screening diagram, you might add a mixture of carbon dioxide, nitrogen, and a trace amount of water. That lets you compare the fluid phase envelope with the free-water, ice, dry-ice, and hydrate curves.

To choose fluids, use the Fluids list on the left:

• Add Fluid opens the fluid editor so you can define a new fluid for this diagram.
• Import pulls one or more fluids from the Fluid Library.
• You can edit or remove items in the list, and add up to 15 fluids total.

The fluids you add here are local to the phase diagram tool (stored in your session) and do not change the Fluid Library unless you explicitly save them there.

The diagram can currently show these curves:

• Fluid: Marks the boundary of the two-phase vapor–liquid equilibrium region. To the left, vapor and liquid coexist.
• Free water: Marks where a water-rich liquid phase becomes stable. To the left of both this curve and the Fluid curve, one gas phase and two liquid phases can coexist.
• Ice: Marks where solid water becomes stable. It is stable to the left of the curve.
• Dry ice: Marks where solid CO₂ (dry ice) becomes stable. It is stable to the left of the curve.
• Hydrate: Marks where a solid hydrate phase becomes stable. It is stable to the left of the curve.

You can also plot these curves in other coordinates by clicking the settings icon to the right of the Export button.