$7\mathrm{.}4($Binary flash distillation$-$yield vs composition$)\text{'}$ Ethanol forms a nearly ideal solution with iso$-$butanol and has a relative volatility $2\mathrm{.}2.$ A heated feed containing $40$ mole$\%$ ethanol and $60$ mole$\%$ iso$-$butanol enters a flash drum at a rate of $50$ kmol$/$h$.$ Prepare a plot of the fractional yield of the distillate versus its composition. What fraction of the feed should be vaporized in order to have a bottom product containing not more than $10\%$ ethanol?