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High Reflux Effect On Bottom Product Purity

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    Junior Member

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Posted 18 May 2018 - 12:25 PM

Suppose I have a distillation column which separate C3 & C4. Following parameters are kept constant.

1.Feed rate, composition, temperature

2. Column top pressure is in auto control and kept fixed.

3. Column bottom temperature is in auto control and kept fixed.


Now in this condition if operator reduces top temperature by increasing reflux flow. Then top product purity will increase because amount of c4 will reduce. As at low top temperature C4 will condense in top part of column. What will happen to bottom product?


Since top reflux has been increased; So to maintain bottom temperature reboiler duty will increase. Weight flow of vapor will increase. Now since reflux rate and vapor flow rate both has been increased so tray pressure drop for each tray will increase. It means for fixed top pressure bottom pressure should increase. And correspondingly bubble point or boiling point of bottom product will increase. Now, since you are maintaining same bottom temperature. Does it reflect bottom product is now lighter?  


In that way bottom product purity will reduce. Is it true? It seems incorrect but can anyone tell me what is wrong in this given logic?


Thanks & Regards!



#2 Bobby Strain

Bobby Strain

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Posted 18 May 2018 - 12:43 PM

There are some free distillation programs. If you don't have one, get one.





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Posted 18 May 2018 - 01:51 PM

Sir, Please provide me details of those program. I am not aware of it.


Thanks & Regards!


#4 Saml


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Posted 18 May 2018 - 10:36 PM


#5 rdcrags


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Posted 19 May 2018 - 09:44 PM

Yes. You are correct. For a given bottoms composition, the bubble point temperature will change with pressure. The bottoms temperature set point would be changed to maintain target composition.

#6 breizh


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Posted 19 May 2018 - 10:35 PM




Consider this resource to support your work


Good luck,



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