2 replies to this topic

[jango]

Hi,

 

I'm a young engineer currently working on some optimisation of destillation columns in the recovery section of a polymer plant. These columns work on atmospheric pressure and have a high reflux ratio (not internal reflux ratio) RR=3. Our top temperature is 80°C but we have a condensor and this cools the reflux all the way to 30°C (90/10 ethanol water mixture). This low temperature of reflux creates a good amount of internal reflux. However I once read in a book that having very subcooled reflux decreases the efficiency of the trays below, because the vapour and liquid cannot reach their equilibrium.

At the moment we have no control on the cooling of our condensors. Could it maybe be beneficial to have temperature control on our condensor so that we only have 10°C of subcooling? Would this make sense energy wise (reboiler load, tray effiencies etc)? Does anyone have some references to this problem? Thank you in advance!

[Pilesar]

You would need to model the process in your column to see the effect of subcooling. Some condenser subcooling is unavoidable. If your subcooling is in a real plant, then it would likely not be worth changing anything. If your subcooling is an artifact of your theoretical model, then it may not be real. When vapor condenses on exchanger tubes, the liquid drips off to the bottom of the exchanger shell. The amount of subcooling is difficult to predict in advance with accuracy. Unless the liquid remains in contact with the exchanger tube, there is not much subcooling. How would you control subcooling without also affecting the condensing service? Condensers must condense. The subcooling performance is built into the exchanger design. There is not commonly a separate temperature controller for subcooling unless the subcooling is in a separate exchanger than the condenser. To force subcooling inside a condenser, the exchanger is designed so that the liquid in the bottom of the shell is not allowed to drain immediately and purposefully covers some exchanger tubes at the bottom. If the exchanger is designed for little subcooling and there is still too much subcooling that it causes a problem, then perhaps some tubes must be permanently plugged.

[breizh]

Hi,

You may want to study tis paper:

https://www.jstage.j...1/_pdf/-char/en

another resource :

Separation Processes, Second Edition

page 218

Breizh