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Distillation question need help on finding q!

  1. Mar 17, 2009 #1
    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. jcsd
  3. Mar 18, 2009 #2


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    Science Advisor

    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.
  4. Mar 18, 2009 #3
    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?
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