Preferential reboiling

[Cassyg]

We are doing a design for a stripper column in a DCU unit and the licensors recommend the use of preferential reboiling. My understanding is that there is a hole in the plate which separates the reboiler return from the product leaving the tower. I tried to find some information on this without success, can anyone direct me to a book or website that would shed some light on this?

 

[sshep]

Perhaps this may be what I call an underflow baffle. The reboiler inlet is taken from the bottom tray downcomer side, and will also include excess liquid fraction from reboiler via the baffle underflow. The reboiler discharge is to the other side of the baffle, and the product is also taken from that side.

Since the reboiler return is a large flow that includes product, the product will be taken, and the rest of the liquid will underflow to get back to the reboiler inlet. The net result is that all of the product has been once through the reboiler. I thought it simple and clever the first time I saw it.

I tried to make a picture to explain. Sorry if it is lame.
best wishes,
sshep
=============================
from
btm tray
|
v <-- reb return
| |
| | |
| | |
| | |
\ /
----------
| |
v v
to reb to product
==============================

 

[Cassyg]

Thank you Sshep for responsing to my post.
Do you know where I could find some literature on the underflow baffle?

 

[sshep]

Hey Cassyg,

I encountered a design like this is a plant start-up some years back. I don't think there is any difficult sizing involved with the underflow (so long as it is big enough not to back-up) because if the reboiler liquid circulation is large, the product side draw-off must have been through the reboiler.

I see that my version of Kister "Distillation Operation" does not cover this arrangement, but has more complicated arrangements to get a similar advantage. Maybe his more recent revisions mention such a configuration.

Another source to check is Andrew Sloley who in my opinion runs the best free internet distillation resources located at

http://www.distillationgroup.com

. Andrew is a really nice guy. If he hasn't published something on this already, he may be interested in something new or novel like this.

best wishes,
sshep