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Topic: Fractional Distillation_required gap in boiling points  (Read 4895 times)

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Offline zeiks

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Fractional Distillation_required gap in boiling points
« on: March 14, 2008, 01:51:52 PM »
i am a chemistry lecturer preparing kids for the CIE exams. i would like to know the required gap in the boiling points (between the two liquids)in order to carry out a successful fractional distillation? will a difference of 2 degrees work? i am talking about simple fractional distillation that can be carried out in the lab using a fractionating column. i cant seem to find a reliable asnwer.  cheers
« Last Edit: March 15, 2008, 10:57:15 AM by zeiks »

Offline LQ43

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Re: Fractional Distillation_required gap in boiling points
« Reply #1 on: March 16, 2008, 09:46:57 AM »
I am not familiar with the CIE exam requirements but it seems that a 25 deg difference would give a successful separation

It also depends on the mole ratio of your mixture and the purity desired of the distillate
this seems to give a good example of this exercise

Offline zeiks

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Re: Fractional Distillation_required gap in boiling points
« Reply #2 on: March 16, 2008, 11:39:42 AM »
Hey, thanks for your answer

CIE = Cambridge International Examinations

i am aware of the 25 degrees mentioned in some sites..
however, one of the examples covered in the syllabus is the fractional distillation of liquid air where the difference between the boiling points of oxygen (about 182) and Argon ( about 185) has only 3 degrees difference. i understand that in the separation of liquid air using complex apparatus would allow the separation...when compared to a normal fractional distillation

but as this example has been mentioned i need to come up with an explanation for why or why not a normal mixture of 2 substances (that needs to be heated) of 3 degrees gap between the boiling points can or cannot be seprated using fractional distillation.

would be great if you could help out.. 

Offline ARGOS++

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Re: Fractional Distillation_required gap in boiling points
« Reply #3 on: March 16, 2008, 05:53:07 PM »
Dear Zeiks;

Even only one Centigrade can be enough!

Why?:   Your “Definition” of a “Simple Fractional Distillation with a Fractionating Column”(SFD/FD) is nearly in all directions TOO, too weak!

That’s why also the O2/Argon is an SFD/FD only at the correct low Temperature! (Of course, optimised!)

Do you know what the “Height Equivalent to a Theoretical Plate” (HETP) is and how it is defined?
But the HETP is by far not the only parameter who influences the Effectivity of  every SFD/FD!

There are also such Parameters like:  “Diameter”, “Length”, “Burden/Loading”, “Reflux Ratio”, etc., etc.

But the Base of all is the “Rauolt’s Law” you may read on:   "Rauolt’s Law

Only as an Example: HETP Parameter for only a “Vigreux Column”:
        Diameter [cm]         HETP [cm]
          2.4                       ~ 11.5
          1.2                       ~   5.4
          0.6                       ~   2.9
But even these values are strong influenced by “Burden/Loading” in mL per minutes!

And in the Attachment you can see the Influence of the “Reflux Ratio” for three different Ratios (A1, A2, and A3).

     y     =   Ratio of partial pressures in the gas phase.
     x     =   Concentration of the liquid phase.
     XB   =   Starting Concentration.
     XD   =   Preferred End Concentration.
     A1   =   Situation, where you need infinite Number of plates.
     A2   =   Situation, where success is assured in the Lab.
     A3   =   Situation, where Rectification is impossible.

In A.72. you see the Temperature Situation for Benzene Toluene during Distillation:
    a.)   SFD without  FD.
    b.)   SFD/FD with a FD of ~12 “Theoretical Plates” and “Reflux Ratio” of 1:10.
    c.)   SFD/FD with a FD of ~12 “Theoretical Plates”, but condensing all Vapour.
          (Also in this case a minimal “Reflux Ratio” exists.)

For  HETP you may also read on:   "Theoretical Plate

I hope this is giving you at least an Idea, what you should all define before!

Good Luck!

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