Boiling point in thermosyphon reboiler

[Waggeveld]

I've a question regarding the boiling point in a horizontal thermosyphon reboiler.
The once through horizontal reboiler is located at ground level, while the liquid level in the column connected to this reboiler is some 10 meters above the reboiler inlet. For the sake of easiness let’s assume the liquid is water.

The pressure in the column is 1.5 bara. Is it true that the pressure in the reboiler is something like 2.5 bara, because of the 10 meters liquid head?

Also, the temperature in the column is 115°C (at boiling point), what is the temperature at inlet and outlet of the reboiler (provided only partial vaporization takes place)? Is it 115°C at the inlet and 127°C (boiling point at 2.5 bara) at the outlet?

Your help is highly appreciated

 

[25362]

As I see it, assuming the tower pressure is equal at the liquid level and at the vapor-liquid mix return, the pressure at the outlet of the reboiler would be this tower pressure plus the two-phase friction drop in the return pipe.

The liquid head would be needed to overcome the friction drops in the supply pipe, the reboiler and the return pipe.

Boiling temperatures would correspond to those pressures. Do you agree ?

 

[Waggeveld]

Thanks a lot for your quick reply

I do almost agree with your reasoning. I do agree with your assumption that tower pressure is equal at the liquid level and at the vapor-liquid mix return.
However, I think that the outlet pressure in the reboiler is the tower pressure + friction pressure drop + gravitational pressure drop (+ momemtum pressure drop, although I neglect this). Temperature at reboiler outlet would indeed be boiling point at this pressure.

Thanks for helping me to understand it clearly.

BTW: Probably you meant with friction drop the total pressure drop in the line from the reboiler outlet to column inlet, in that case I do agree.

 

[25362]

You are entirely right. A thermosyphon design gets its moving force from the density (gravitational) differences.

For natural circulation the density at the returning arm is generally much less than one tenth that of the "pushing" liquid leg, especially when considering that steam at the suggested conditions has a specific volume about 1000 times larger than liquid water.