Designing Reboiler for 2 Liquid Phases

[sshep]

Friends,

A replacement vertical thermosyphon reboiler in one of our plants suffers from instability and heat transfer limitations. On the theory that there is no liquid circulation (failure to thermosyphon), we are injecting some steam below the bottom tubesheet which has helped and we are surviving. The is the second such rebolier performance failure that I have seen which were both characterized by: 1) being a vertical thermosyphon variable area type on the shell (steam) side, and 2) the process is an aqueous & hydrocarbon mix.

In both cases the towers are at atmospheric pressure. The systems were involving MEK-water (this time), and C11 hydrocarbons-water (last time).

Can you tell me:
a) How does one normally design a reboiler for the uncommon situation of two liquid phases?
b) Can you think of a mechanism that would cause a failure to thermosyphon due to two liquid phases?

The first time I thought there was a reboiler return head design problem (cone type with high lift), but in the most recent case there is nothing unusual. Although this was a replacement, I can see that the very similar old one also struggled. Maybe new tubes are making the old problem even worse.

Any ideas could be great.

best wishes always,
Sean Shepherd

 

[ash9144]

It is very strange that you would have a tower with two phases like this. Why not just use a settler and physically separate the phases?

Thermosyphon depends on boiling liquid in the tubes. So when it turns to vapor it goes back into tower and allows more liquid to enter. One possibility is your bottom temp is below the boiling point of the aqueous phase so you aren't getting any flash and so no circulation.

 

[Jason5000]

I've seen upset conditions in a tower do exactly what ash9144 describes. The tower had a water wash in the overhead system. It was equipped with a thermosyphon reboiler, and the bottoms was not hot enough to boil water at operating pressure. When interface level failed in the overhead accumulator it wasn't long before you noticed. After refluxing enough water down the tower the thermosyphon would stop, and it was impossible to reestablish circulation until you got the water out of the bottoms.