Vertical Thermosyphon Reboiler
Vertical Thermosyphon Reboiler
(OP)
We are in the process of installing a distillation tower with a vertical thermosyphon reboiler. It is a Shell and Tube Heat Exchanger; our product on the tube side and steam on the shell side.
Our liquid level in the distillation tower sump and Reboiler are controlled via a barometric leg. I am wondering how I can determine the buffer I require to ensure that the tubes are always flooded during boiling.
Our liquid level in the distillation tower sump and Reboiler are controlled via a barometric leg. I am wondering how I can determine the buffer I require to ensure that the tubes are always flooded during boiling.





RE: Vertical Thermosyphon Reboiler
For example, you do the pressure drop at it comes to say 2 psi and the fluid going into the reboiler has a density of 36 lbs/ft^3, heres what you need. 36/144 = .25 psi/ft. Since you calculated yo need 2 psi, then 2/.25 = 4 feet. You need to have at least 4 feet of liquid above the inlet to the exchanger to push its wat through the exchanger and back into the column.
RE: Vertical Thermosyphon Reboiler
We are more concerned that once the product starts boiling there will be a section of tubes which is not filled with liquid.
I read a rule of thumb in one of the threads that states:
"The upper tube sheet of a vertical natural circulation thermosyphon reobiler to be set at the lowest design liquid level to maintain a liquid head for the reboiler at all times"
Would anyone be able to confirm this and provide me a resource for this ROT?
RE: Vertical Thermosyphon Reboiler
Now, most engineers like to limit the vapourization rate to less than about 30%. The only control of this is by having side reboilers that each flash out less than say 20% each.
RE: Vertical Thermosyphon Reboiler
The rule of thumb was really designed for startup. Using it, you will always have the tubes full before you put the heat on. After that it is a hydraulic balance that determines the flowrate through the reboiler. The flowrate will vary depending on the heat load. Both HTFS and HTRI will do this calc for you under simulation mode (not design mode).
RE: Vertical Thermosyphon Reboiler
Inside the tubes you should have slug flow (seperate slugs of liquid and vapor). Obviously the return pipe should not be slug flow, but if the superficial vapor velocity is to low then you can get the slip back of liquid. What you really want is to have as little lift between the exchanger and the tower as possible- a horizontal return pipe ideally. Because horizontal thermosyphons generally are deliberately designed to operate with higher % vaporization, they can tolerate a far taller return line.
If you are really worried about the liquid level being too low, then consider putting in a baffle to hold the desired liquid level.
best wishes,
sshep