Loss of Thermosyphon Circulation

[EmmanuelTop]

Dear members of Eng-Tips,

Can anyone provide some information about possible causes for diminished thermosyphon circulation in horizontal recirculating reboiler (without baffle)? The tower is operating at 80% of design capacity, with regular feed and regular temperature level of reboiler heating fluid (250C).

The symptoms were:

1) Initial state: Tower bottoms temperature was 165C, while the reboiler outlet temperature was 170C. These parameters indicate normal reboiler operation.

2) After increasing heating fluid flowrate through the reboiler, reboiler outlet temperature increased to 175C and the tower bottoms temperature dropped to 160C, indicating that process fluid flow through reboiler was reduced. Moreover, bottoms product flowrate started to oscillate between 0 and 60m3/h, followed by column overhead receiver pressure swinging (from 5.5 to 6.0 barg). Column behaviour showed very erratic heat input, telling us that some kind of cycling is happening.
After decreasing heating fluid flowrate, everything came back and the tower operated in its initial state.

Since reboiler is designed to achieve tower bottoms temperature of 175C at 100% tower capacity, what are possible explanations for diminished thermosyphon circulation?

- Undersized reboiler?
- Slug flow in reboiler outlet piping (cycling in tower operation)?
- Changes in boiling regime, or something else?

 

[25362]

I don't know whether this applies to your case; nevertheless, here is the message taken from Shinskey's Distillation Control. When speaking of instability in reboilers created by excessive boiling, he says:

Columns containing substantial concentrations of only slightly volatile components may exhibit unstable operation for this reason.

This problem can be more severe in horizontal thermosiphon reboilers where the percent vaporization is higher. Trapping the liquid from the first tray then becomes essential for the reboiler to function.

Note that the liquid circuit between column base and thermosiphon reboiler forms a "U" tube. Liquid in a U tube has a capability of resonant oscillation like a pendulum.

Its period in seconds = 0.78 [√]l where l is the total length of the U tube, ft. The length of the U tube in most columns is about 15 ft. giving a 3-s period. In absence of friction, oscillation, once begun, would continue indefinitely. However, normal frictionl resistance of the piping provides some damping. Nonetheless, bubble formation in the reboiler produces enough random disturbances or "noise" to induce oscillation at the resonant period. Buckley describes unstable reboiler operation having a period of 3 to 4 s coincident with high heat-flux rates. A restriction in the liquid line can correct this problem by providing damping.