4 replies to this topic

[sliman666]

Hi

I have a question I struggle to find an answer to on the web.
Suppouse a process was changed in a way the required duty of a thermosyphon reboiler is now 20% of the design of the unit. As a result the steam side chest pressure is now throttled back and very low.
With very low differential pressure accros the steam trap will the reboiler become partly flooded?
I'm guessing this would reduce the heat exchange area to some extent but as the heat exchange area is much bigger than required this is not likely to trigger raise in the chest pressure?
Is there any way to avoid this problem?
How much of an impact this would have on the reboiler?

Many thanks for any answers in advance.

[Technical Bard]

You will probably have two problems - the first is that if you throttle the steam pressure too low, there will be insufficient head to get through the stream trap and return the condensate to wherever it is going. You may also get a cycling effect as the condensate builds up in the exchanger, then the trap finally opens and so forth.

The second and more serious problem is that even if the steam side can operate, the process side may not circulate effectively. The hydraulics around the thermosyphon are usually designed to have ~30% vapour fraction returning to the tower. At turndown, you won't be able to boil enough liquid to get to this vapour fraction. The result is the liquid to the reboiler and the return won't be different enough and the thermosyphon won't circulate. Thermosyphons are notorious in severe turndown situations.

[katmar]

I agree with what Technical Bard has said. Let me add some possible work-arounds.

In the first situation described by Technical Bard the reboiler steam chest will have a varying pressure and a varying (effective) heat transfer area. This certainly can lead to instability. A potential solution to this is to run the steam inlet valve fully open and to control the flow of steam with a control valve in the condensate outlet line. This will make the steam pressure constant and at least you now only have a varying heat transfer area. This is illustrated in Fig 17.1 b and c in "Distillation Operation" by Kister.

What I have experienced with the second situation described by Technical Bard is that when the circulation stops the liquid in the tubes does quite soon vaporize and force the liquid out - probably upwards and downwards - and then you are left with a huge level difference and the liquid rushes in and cools the reboiler and circulation stops again. This can lead to cycling in much the same way as described above. Controlling the condensate out rather than the steam in (as above) may solve it or you may be forced to throttle the inlet. This is described in the same book in Figure 15.7 where a successful case study is cited, but I have to say that I never like throttling the circulation on a reboiler.

Attached Files

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[sliman666]

Many thanks for your replies.

Do you think an orifice plate could be a solution?
I would imagine (with discharge proportional to pressure differential) there would be a balance achieved eventually between the heat transfer area (with partial flooding), heat requirement and pressure (the supply pressure would be reduced from 120psi to 30psi and control valve resized)

[jmukund]

Will it be helpful if we choke some tubes in the reboiler?