It sounds like you are asking about calculating theoretical stages for distillation of a binary mixture using the McCabe-Thiele method, which includes calculation of the q line from the composition of the feed. The parameter q is the mole fraction of liquid in the feed and the slope of the q-line is q / (q - 1). The q line passes through the point defining the composition of the feed on the x=y line drawn on the diagram. For more details on the McCabe-Thiele method, see the attached link.

Assuming you have binary components the McCabe Thiele Method should be used. Otherwise a process simulator such as Aspen, Pro/II, and/or KGTower should be used.

For a binary set of components you calculate the feed tray using the McCabe Thiele method. For more complex systems you would need to use a process simulator and perform a sensitivity analysis to determine the optimum feed tray.

In order to locate the feed tray in distillation, you need to calculate the field tray by using MCcabe Thiele field method. For more complex systems you need to use process simulators and perform a sensitivity analysis to determine the optimum feed tray.

Brett McCabe is 6' 1".