Vertical Thermosyphon Reboiler Problem

[sshep]

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.

 

[25362]

Branan on Rules of Thumb for Chem. Engineers mentions a large inlet line as a source of troublesome unstable circulation because some tubes "backfire" individually impairing (or just altogether stopping) inflow on the process side. Reducing orifices are the solution he proposes, if there is no room for valves.

I cannot say this is the solution you are seeking, but it's worth considering it.

 

[sshep]

I should add this to my description:

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.

 

[25362]

As you say concentrations changes may be the reason. In particular when the system operates OK for some time, and then problems start w/o any apparent reason. Faulty hydraulics may be the direct culprits, but they may be influenced by process variations or transients. Can you describe where is the liquid feed taken from, how is its level controlled, and where isthe boiling mixture returned to the tower ? Some description of the vessel bottom section may help to clarify the picture.
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.

 

[25362]

Have also a look at the exchanger itself. When steam pressure is reduced, steam becomes superheated and the delta t and HTC may not be sufficient to boil a minimum of liquid on the cold side, and the average density on the boiling side may not be low enough to promote movement. When steam pressure (and temperature) is too high, you may encounter film boiling. How did the reboiler operate when it was under surface control ? Has the steam any uncondensables that may interfere with heat transfer ?

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.

 

[25362]

Since by steam priming you get the reboiler started and "going", then after a time it stops operating, is it possible that the BPR due to varying concentrations of the organic in water changes the performance ? Can you change tower pressures to induce boiling ? Is there a possible "analyzer" type of control that would indicate (feed forward) whether the BP of the bottom liquid is higher and needs a "warmer" (higher pressure) steam supply ?

As you may see questions abound.

 

[sshep]

Thanks for the interest 25362 and any others checking this out in the future. If you send me an email (sean.shepherd@sasol.com), I can provide better info on the system including perhaps a sketch and more detailed background. Thanks, sshep

 

[pmureiko]

Sean,

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.

 

[hacksaw]

Have experienced a similar problem with boilup where both high and low boilers were in the feed.

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.

 

[klem]

You mentioned foaming parenthetically. I ran into a case in a kettle reboiler where we looked for foam on the lab bench, and found it "insignificant" under lab conditions. However in the plant it was a major problem, requiring changing operating pressure to vacuum conditions. Luckily, the temperature was still in operable range. I can't be specific on how to test for it, as I don't know enough about your system, but include this in you analyses.