5 replies to this topic

[rectification24]

Greetings everyone!

 

I have one question concerning rectification process (countercurrent distillation).

For example; if you have a process in which a feed of water-methanol solution (F) (22%mol) with a flow of 6.5 kg/h enters a column. Inside a column a feed of vapours (V) has value of 7 kg/h with 95%mol. If reflux ratio is 2.5, what is the value of reflux going back into the column (L) and fraction of distillation residue (xw)?

 

If my knowledge is correct then you get reflux (L) from reflux ratio and mass balance of vapours:

 

R=L/D   2.5=L/D

 

V=L+D    7=L+D

 

From this you get:   D=2 kg/h  and L=5 kg/h

 

Then from the mass balance of feed you would eventually get a distillation residue:

 

F=W+D              6.5=W+2   W=4.5kg/h

 

F*xf=W*xw+D*xd    6.5*0.22=4.5*xw+2*0.95         xw=-0.104   (?????)

 

Question I want to ask is, why do I get negative value of distillation residue? Is my procedure correct? Do you in this case consider absolute value of xw?

 

Could someone please help me?

 

Regards,

Filip 

 

 

[breizh]

Filip ,

You are mixing carrots and apples .

Either you perform a moles  balance or a mass balance .

 

Hope this helps

 

Breizh

Edited by breizh, 24 July 2014 - 09:05 AM.

[rectification24]

Dear Breizh,

 

yes, It is true, I made a mistake in writing, but nevertheless mole balance is the same as mass balance, you just have to insert then molar values instead of mass values.

If I ask you in other direction, how would you solve this problem?

[Bobby Strain]

If you search Google, you will quickly find your answer. And many more resources, too.

 

Bobby

[PingPong]

Dear Breizh,

 

yes, It is true, I made a mistake in writing, but nevertheless mole balance is the same as mass balance, you just have to insert then molar values instead of mass values.

If I ask you in other direction, how would you solve this problem?

As Breizh already tried to make clear to you: you are multiplying mass flowrates with mol%, which is just plain wrong.

[rectification24]

Yes of course, obviously you can't multiply mass flowrates with mol%. What a stupidity from me. I should know that.

Anyway, thanks for your answers.