# Distillation question need help on finding q!

1. Mar 17, 2009

### arsalan_y

hi guys
ive been given a problem and ive worked most of it out but im not sure whether my answer for the feed line (q) is correct can i get somebody to re assure me on how to plot it onto the graph and get it correctly. heres the question below.

A mixture containing 40 mole% heptane and 60 mole% octane is to be separated in a distillation column operating at a pressure of 1 bar. The distillate and the bottom product will contain 97 mole% heptane and 4 mole% heptane respectively. The feed rate will be 200 kmol h-1. The feed will be a mixture of 85% liquid at its boiling point and 15% saturated vapour. A reflux ratio of 2.1 is to be used. The plate efficiency is 75%.

Using the given information, determine:

(i) the top and bottom flowrates [kmol h-1],
which i found to be W= 122.58 and D= 78.42

(ii) the number of actual plates required for the separation using the McCabe Thiele graphical method,
i got 13 stages from the graph but i dont know if its right because i dont know if my q is right and im not sure how to exactly plot it on the graph. so then i get actual plates to be 18
(iii) the minimum reflux ratio, and
i got this to be 1.62
(iv) the number of stages at total reflux.
i think its N+1 which is 13+1 so 14

2. Mar 18, 2009

### alxm

Hmm. I used to know how to do this.. once. :)

Drawing the q line.. well you're told:
"The feed will be a mixture of 85% liquid at its boiling point and 15% saturated vapour"

So you start at the x=y line at your feed concentration. q is the mol fraction of liquid, and from the above you have q=0.85.

The slope of the q line is q/(q-1), vertical for saturated liquid, horizontal for saturated vapor.

3. Mar 18, 2009

### arsalan_y

i know that dude but i dont understand how to put the q line on the graph? i kno its going to haev a negative gradient but i dont understand on how plot it. i think u put the Xf going up to the 45 degree line then from there what do i do?