What is a phase envelope and how is it constructed?

A phase envelope (or phase diagram) for a mixture shows the two-phase region on a P-T plot. Bounded by: bubble point curve (liquid just starts vaporizing), dew point curve (vapor just starts condensing), and cricondentherm/cricondenbar (maximum temperature/pressure for two-phase). Construction: at each T, calculate bubble pressure; at each T, calculate dew pressure; plot the curves. Critical point is where bubble and dew curves meet. Used for: pipeline design (avoid two-phase), separator operating conditions, and understanding mixture phase behavior. Generated by process simulators using EOS.