' An equilibrium still is purifying benzene and toluene from a small amount of essentially non-volatile impurity and is initially charged with 20 lbmoles of feed stock of composition xF = 0.32 mole fraction benzene. Feed is supplied at the rate of 10 lbmoles/hr, and the heat input is adjusted so that the total moles of liquid in the still remain constant at 20. Estimate the time required for the composition of overhead product yD to fall to 0.40 mole fraction benzene. No liquid stock is removed from the still during this peod. _ 2.48m, 3'15" \"1.43ng