11 replies to this topic

[trungphu]

Hi,

I have calculated the number of theoretical stages for a distillation column that fractionate 3000kg/h of an acetone-benzene solution containing 0.3656 mole fraction acetone at bubble point by two methods: McCable-Thiele diagram and Fenske equation. It is desired to produce a distillate containing 0.9851 mole fraction acetone and a residue containing 0.006713 mole fraction acetone. I was wondering whether this two methods always give nearly the same result or not. In my case, I get a major different result from those methods. For the same reflux ratio of 3.518, I got 26 theoretical stages from McCable-Thiele diagram and 18 theoretical stages from Fenske equation. Attached is my McCable-Thiele diagram.
I believe something was wrong here. Could someone give me some advances?

Regards,
TrungPhu

Attached Files

•  Chart in Microsoft Word.xlsx   26.58KB   90 downloads

[breizh]

Trung Phu

Let you consider this resource to validate your work ( mac cabe &thiele) :

http://www.chemsof.com/vle/vle.htm

Hope this helps

Breizh

[trungphu]

I really appreciate your help, Mr.Breizh

[USR]

I remember using Fenske equation for calculating minimum number of stages hence it you are getting the minimum number of stages that is 18.Though I have not checked your calculations.

[S.AHMAD]

A quick check using PRO-2 simulation I got 24 theoretical trays inclusive reboiler and condenser with feed on tray 11 from top. Distillation at 15 psia.

Edited by S.AHMAD, 01 November 2012 - 01:52 AM.

[trungphu]

Dear Mr S.AHMAD,
Thank you so much for your help! Could you tell me which thermodynamic data you have chose for your simulation. Actually , I got the result that different from yours by using PRO-2.

Edited by trungphu, 02 November 2012 - 09:25 PM.

[S.AHMAD]

I am using Peng-Robinson

[trungphu]

thank you kindly. I will check it again.

[Dacs]

While I won't try to comment on your particular problem (i.e., how you came up with the numbers), I think Fenske method will only give you the minimum theoretical trays to meet the needed separation at total reflux (i.e., without any distillate and bottoms being drawn from the column)

I don't think they're comparable at all.

[trungphu]

Dacs
You are right. Actually, in this case, I mean the Fenske - Underwood - Gilliland (FUG) method, not Fenske method. I'm sorry for my mistake here.
FUG method consist of three main parts and can allow us determine the theoretical stages necessary to produce desires product.
So we can make a comparison for this two methods, McCable-Thiele and FUG.

Edited by trungphu, 07 November 2012 - 05:20 AM.

[Dacs]

Truth be told, it's been ages since I even touched a McCabe-Thiele diagram.

But from what I can tell, there are some underlying assumptions behind those methods. IIRC, there should be a constant molal overflow (or something) that has to be assumed for McCabe-Thiele method.

Maybe you can look into those basis and compare it with your system and see how it holds up.

[Ahsan67]

kindly check what relative volatility you used in your solution while using fenske equation , average or you calculated by correlation ??
because if there is a big difference between top & bottom relative volatility we can not use average relative volatility in fenske equation,be careful, use underwood's equation for minimum reflux and then Erbar-Maddox(stages Vs Reflux) correlation can be use for finding theoritical number of plates, fenske equation is used for finding minimum number of plates becase it assumed 100% reflux situation which will occur at 45 degree in McCabe-Thiele digram