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PVT and Flow course - Two-Component Phase Behavior

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Now we're going to look at two components together and see how things change in regard to the the mapping of the pressure-temperature or space where you have two phases.

So, here we have a pressure temperature diagram again and on this diagram there is the vapor pressure curve of methane and the vapor pressure curve of some heavier hydrocarbons.
Phase behavior of two separate components

And now let's mix these components together, 50/50 on a molar basis. What happens to this pressure-temperature diagram of the two-component system is that you no longer get a line, you get an envelope. This envelope will have a critical point like the critical point of a pure component. Both along the lines and inside this envelope we have two phases: vapor and liquid. At any condition outside that envelope it's single phase.
So, with two components you get 2 pressure saturation curves: bubble point and dew point. These two curves meet at critical point.
Phase behavior of the 50/50 mixture

If you mix the same components but in different proportions, you'll get different envelopes, limited by the saturation pressure curves of the pure components.
And then, if we connect all of the critical points we get the curve called the critical locus. And only within this critical locus can you always find a mixture (any proportions of the components) that will be two-phase.
Phase behavior of mixtures

Let's look at Pressure - Volume diagram. With the single component when it hits the saturation pressure mother nature's kept the pressure constant when you increase the volume. It is not the same with two components. It doesn't get stuck on that constant pressure that only worked with pure compounds.

P-V diagram of mixture


You need two phases in equilibrium to rigorously and properly define or label phases as vapor and liquid. And when you have only one phase you can use arbitrary or well-founded definitions to label as vapor-like and liquid-like.

Watch the full video

Other lectures from the PVT and Flow course

Class notes developed during lectures are available as PDF files, named with the format yyyymmdd.pdf located on:

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