Page 176 - Separation process engineering

Now the middle operating line y=x intercept can be determined from Eq. (4-45)

This intersection point could be used, but it is off of the graph in Figure 4-19. Instead of using a larger
sheet of paper, we will calculate the slope L′/V′.

L′ = 300(5/4) + (1.0)(242.9) = 617.9 kmol/h

V′ = L′ + D − F = 617.9 + 242.9 − 300 = 560.8
2
L′/V′ = 1.10

Now the middle operating line is plotted in Figure 4-19 from the intersection of feed line 2 and the
top operating line, with a slope of 1.10. The bottom operating line then goes from y = x = x to the
B
intersection of the middle operating line and feed line 1 (see Figure 4-19).

Since the feed locations were desired as stages above the reboiler, we step off stages from the bottom
up, starting with the partial reboiler as the first equilibrium contact. The optimum feed stage for feed
1 is the first stage, while the optimum feed stage for feed 2 is the second stage. Six stages + partial
reboiler are more than sufficient.

(4-49)

We need 5½ stages + partial reboiler.
E. Check. The internal consistency checks all make sense. Note that L′/V′ can be greater than or less
than 1.0. Since the latent heats of vaporization per mole are close, CMO is probably a good
assumption. Our initial assumption that feed 2 enters the column higher up than feed 1 is shown to