Vertical Thermosyphon Reboiler 1

[Fattony11]

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.

 

[dcasto]

just ha a similar one come up. You'll need to calculate the pressure drop though the exchanger wich will be tricky because it will be a two-phase flow as you start vaporization. This pressure drop plust the pressure drop through the piping to and from the exchange will give total drop needed. Then you'll need to spply enough "head" pressure on the liquid going to the exchanger to overcome the loss. This head is literally a height of liquid built up over the height of the exchanger.

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.

 

[Fattony11]

Not exactly what I was looking for but still some very good information. I am not worried about the liquid level being pushed through the exchanger and back to the column.
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?

 

[dcasto]

I believe that when you calculate the pressure drop through the exchanger you'll need to cut the exchanger into at least 10 pieces and "integrate" to see how the boiling fluid passes from liquid to vapour. Finally your two phase flow will be 10% to 30% vapour by mole.

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.

 

[Longinthetooth]

Decasto is right, but it sounds as if you are not really sure of how a vertical thermosyphon works. There is always 2 phase flow back to the tower. It is only 20-30% vapour as Decasto says. The idea that the outlet stream is all vapour is a common misconception, especialy among operators.
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).

 

[sshep]

Fattony11, your rule of thumb is misleading because it implies placing the tubesheet at the lowest liquid level and then having longer vertical return pipe to the return nozzle. These long returns usually involve a cone type head (or worse an oversized return pipe) which can lead to a serious problem of having liquid slip back into the exchanger instead of returning to the column. A heavy residue can build in the exchanger and cause the boiling to eventually stop.

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