I am theoretically designing a distillation system in which acetonitrile and acetic acid are separated. The substance being separated is acetonitrile. I do my theoretical tray calculations with McCabe Thiele and Ponchon Savarit methods. In the vapor phase enthalpies used in the Ponchon$-$Savarit method, I found the slope of the vapor line to be positive because the enthalpy of pure acetonitrile in the vapor phase is higher than acetid acid. In general, the vapor enthalpy of the separated material is lower.

How can I interpret this? Can I do a tray calculation according to this graph? Since it is a completely theoretical system, you can explain all the disadvantages of the system $($efficiency value, column height, etc.$)$