Details

A feed with a composition is flashed (vaporized). On the - diagram, the slope of the purple line is , where and are the numbers of moles of liquid and vapor leaving, respectively. The intersection of the line with the black equilibrium curve gives the mole fractions of methanol in the exiting vapor (green) and liquid (blue) streams. Because VLE is assumed, the mole fractions of methanol in these streams correspond to the horizontal line on the -- diagram. The purple dot on the -- diagram represents the feed mole fraction of methanol, and the lever rule is used to determine the liquid/vapor molar ratio.

Reference

[1] P. C. Wankat, Separation Process Engineering: Includes Mass Transfer Analysis, 3rd ed., Upper Saddle River, NJ: Prentice Hall, 2012.