Vertical Thermosyphon Reboiler Problem
Vertical Thermosyphon Reboiler Problem
(OP)
Description- We have difficulty getting heat into a vertical thermosyphon reboiler on a solvent recovery tower. The process side is aqueous saturated with oxygenated organic compounds. The operating pressure is 150 kPaa. Originally the exchanger was variable area with steam pressure at 2100 kPaa. This has since been revised to control steam flow- chest pressure indication is provided. When the problem manifests, the chest pressure can go to supply pressure with maximum exposed area and still no steam condenses- normally we keep max area exposed to minimize the required heat flux and keep tubewall temps low (to address film boiling theory). A direct steam injection below the tubesheet has been successful to get the thermosyphon started (and provide the main heat input if required), but the reboiler can later die again for no apparent reason. There are no obvious problems with the reboiler design (it has been checked many times by various engineers usually assuming water as the boiled medium), but we have 3 reboilers in this type service all struggling to various degrees. We think that the problem is related to composition of the column bottoms. This is a closed system with respect to water residue in the column bottoms (designed for make-up only). Taking a blowdown during problem periods seems to help, but external boiling tests of the same material do not indicate a problem with normal boiling (although the boiling is slightly more foamy than straight water).
Any ideas are appreciated.
Any ideas are appreciated.





RE: Vertical Thermosyphon Reboiler Problem
I cannot say this is the solution you are seeking, but it's worth considering it.
RE: Vertical Thermosyphon Reboiler Problem
There is a butterfly valve in the inlet line. Pinching this valve has never helped the reboiler to work. This may help rule out some causes.
RE: Vertical Thermosyphon Reboiler Problem
Sometimes while explaining in detail the design and the operations, you suddenly find the reason(s) by yourselves. At least, that was my experience in troubleshooting plant problems in the past.
RE: Vertical Thermosyphon Reboiler Problem
In short you must do a in-depth analysis, probably a brainstorming session with colleagues in the plant to clarify the situation.
If you find that the problems are of a hydraulic character, switching over to pumping and forced circulation, rather than natural, may be one -added cost- solution.
RE: Vertical Thermosyphon Reboiler Problem
As you may see questions abound.
RE: Vertical Thermosyphon Reboiler Problem
RE: Vertical Thermosyphon Reboiler Problem
I think your reboiler is undersized because it was designed with too high of an overal heat transfer coefficient. Check the design calculations to see if this is the case.
In the meantime the pinch back on the butterfly valve on the inlet to promote circulation. You'll won't get as much heat duty as designed, but it will be better than what you have now.
Paul.
RE: Vertical Thermosyphon Reboiler Problem
the reboiler was setup for vaporizing operation, this worked until the high boiler concentration built up to a point where no vaporization of the low boilers could occur.
running the reboilers in the flooded mode worked as if kept the high boilers from building up, but process was hungup in there design egos. we had column trips about ever ten days, when the repoilers were operated in the vaporization mode.
RE: Vertical Thermosyphon Reboiler Problem