A saturated liquid mixture containing 60 mole % benzene and 40 mole % toluene is to be distilled continuously into a distillate product containing 90 mole % benzene and the bottom product containing 5 mole % benzene. The fractional distillation column will operate at approximately constant pressure of 1 atm. The reflux ratio is 2. How many theoretical plates must the column have if the feed is introduced into the eighth plate? Equilibrium data are:
zF = 0.6
xD = 0.9
xW = 0.05
R = 2
xD/(R + 1) = 0.9/(2 + 1) = 0.3
Feed is saturated liquid
For saturated liquid feed, slope of 'q-line' is µ .
The equilibrium data are plotted and by McCabe-Thiele method, number of theoretical plates is found to be = 11 for feed introduced at 8th plate.