Flash Vaporization: If a liquid of known composition is vaporized in one stage, then the composition of the liquid and vapor after separation can be found as

A component balance gives

Which can be rearranged to give

This line passes through point 1(x_F,x_F) and has a slope of -W/D. Intersection of this line with the equilibrium curve yields point 2(x,y) that gives composition of vapor y, and a liquid of a composition x leaving the separator.

Knowing the equilibrium composition, we can find the temperature in the separator. Both liquid and vapor are at same temperature. We can either use bubble- or dew point equation to find the temperature.

Example 6.5: A liquid mixture contains two components. Component A is present at a concentration of 50 mol %. Component B is also present at a concentration of 50 mol %. The feed is at a temperature of 80 °F. This feed is continuously flash vaporized at a pressure of 1 atm to vaporize 60 mol % of the feed. What will be the composition of the vapor and liquid leaving the flash separator? Relative volatility at conditions within separator is 2.16. Component A is more volatile.

Solution: Basis: 100 lbmol of feed.

For 60 % vaporization, amount of distillate obtained after condensation of vapors is

Draw operating line having a slope -0.667 and passing through point (0.5, 0.5). We can draw equilibrium curve using the following equation.

It can be seen that intersection of operating line and equilibrium line occurs at (0.386, 0.756). This gives us a composition of vapor (distillate) as 75.6 % A and 24.4 % B. The composition of liquid is 38.6 % A and 61.4 % B.