ans = 6 theoretical plates plus the reboiler; feed on stage 4 b) 6 or 7 theoretical plates plus reboiler - depends on how you draw the diagram; feed on stage 4 or 5

2) Atmospheric air ( 79%N2 and 21%O2 on a molar basis) is to be distilled at 1atm to produce 98%N2 (top product) and 98%O2 (bottom product). A total condenser will be used. Assuming the reflux ratio R=2, determine the number of theoretical stages required (using the graph below) if the feed is, a) liquid at its bubble-point, b) vapour at its dew point