2 replies to this topic

[SP500]

How to decrease both reflux and the number of stages in a distillation column?

Edited by SP500, 02 January 2017 - 10:09 AM.

[colt16]

The Fenske shortcut method calculates the minimum number of stages.
The Underwood shortcut method calculates the minimum reflux ratio.

These are actually not the same thing. The minimum stages means the highest reflux ratio (highest energy consumption) whereas the minimum reflux ratio means the least stages. I am not sure if the question should be rephrased as "A short cut calculation predicts this column will have MINIMUM 41 stages..."

These two equations provide the "extremes". Hence our job is to find the balance. Often this is a decision based on CAPEX versus OPEX. Do you go with a low initial investment (CAPEX) and incur higher operating costs (OPEX), or do you go with a high initial investment (CAPEX) but lower your operating costs?

You are right that this is a difficult separation, of benzene and cyclohexane. These are the light keys and heavy keys. It is not clear from the question whether a different method is allowed. Typically aromatics (Benzene and Toluene) are separated from nonaromatics using a solvent extractive distillation method (sulfolane solvent) in the industry. However with the cyclohexane there, it could be a problem as the cyclohexane also has a bit of affinity with the solvent. This means in theory i could use an extractive distillation column to get a n-pentane + cyclohexane stream and a benzene-toluene stream. The products can then be separated easily.

Leaving the benzene-cyclohexane separation last doesn't really make a difference to the net energy consumption.

I am assuming the magnitude of the answer doesnt need to be so detailed and qualitative answers would do. If so, investigating feed location and lowering pressure definitely helps.

[breizh]

Hi ,

 

 

http://www.separatio...om/Mainmenu.htm

 

Consider this resource to support your work.

 

Good luck

Breizh

Edited by breizh, 26 December 2016 - 09:00 AM.