natural thermosyphon reactor

[proceng234]

Dear all,

I’m working on a project related to a reactor in which natural thermosiphon boiling of an organic heat transfer medium occurs on the shellside of a reactor (11 m long, D = 3.5 m, 4500 tubes filled with catalyst). The organic fluid is returned to the reactor strongly subcooled, then undergoes convective heat transfer and warms up until its saturation point after which it is evaporated and leaves the reactor via an annular ring on the top. It is condensed again in a steam generator and is returned subcooled to the RX via 2 annular rings at 2/3 and 1/3 of the RX height.

Due to a catalyst change we will need to switch the heat transfer medium. The big question is how this will affect the heat transfer and thermosiphon operation/stability, mainly in respect to the different physical properties (thermal conductivity, viscosity, mass density, specific heat, enthalpy of vaporization, …)

Any suggestions on some background reading / rules of thumb / … for making a first approximation? How to best tackle this problem?

Any tips/advice is more than welcome.

Best regards,

proceng234

 

[georgeverghese]

Thermal - hydraulic design of a thermosyphon reboiler was a design project I was asked to work on at final year at Uni - you may find examples of this in some textbooks (I went by guidance from the heat transfer lecturer fortunately) - several trial and error iterations are required. Assume a flowrate of the HTF and see if this matches up from both a hydraulic and heat transfer perspective. Obviously, in this case you have the additional complication of finding out the length of tube required for convective heat transfer and the length required for boiling. Some estimate of the tube side htc also has to be made. Getting the pressure drop calcs correct is as important as the heat transfer calcs in this case.
It may be possible to run this thermosyphon unit in Simsci - Pro/II, however, memory tells me this unit operation is embedded in distillation column simulation. See if you can tweak the program to run this anyway.
Why do you have to change the HTF ? Right now, you have a circuit that works well with this HTF.

 

[proceng234]

Hi George,

Thanks for your answer.

The HTF has to be changed cause the new catalyst works at lower temperatures, thus lower pressures at saturation T leading to a too high pressure drop in the steam generator and so the liquid level drains it.
So i guess this is difficult to do by hand calculations, especially with the 2 liquid annular returns. I only have Aspen Exchanger design and rating to my disposal, can it be done by this or should I look for experienced consultance?

Can you recommened some books?
Best regards,

Stijn

 

[georgeverghese]

Since this is only a once through thermosyphon on the shellside, would expect it should be possible to do this with a rate check on Aspen with 2 shells in series to model the 2 annular feeds at the RX. You could model the tubeside in such a way with some dummy fluid so that it produces a tubeside htc equal to what you manually estimate it to be. Add on manually for other transfer line and feed pressure drops as required.
Looks like you've also got the additional task of thermal - hydraulic rating of the steam generator for the new HTF condensing duty. If you are running at lower shellside condensing temp at the steam generator, presume steam pressure will be lower than current. Manual calcs can be done but may be quite painful with all the iterations involved.
A manual check of the boiling htc on the RX side and the condensing htc on the steam generator side would be useful. Take care not to exceed recommended max permissible operating shellside film temp at the RX with this new HTF if it is thermally more sensitive than the current HTF.
It would be well worth getting a engg contractor's senior process engineer on board (or someone similar in skills sets in your organisation ) for this task so you have a second set of results you can discuss and compare with. Experience tells me it is a must that you do this completely yourself as thoroughly as you can so you can make full use of the peer review.
Presume Aspen has this new HTF in its component databank. Take a look in DQ Kern's Process Heat Transfer - see worked example 15.5, page 488.

 

[georgeverghese]

With the new HTF ,the heat removal load profile ( Q vs RX tube vertical length ) will be different in comparison to the current HTF. This is due to the changes in shellside sensible htc / boiling htc and the length of the convective heating section. RX yield may change depending on whether this is an exothermic or endothermic reaction, which may be masked somewhat by reaction kinetics.
With these added complications, it does seem like this is more a task for the process licensor who would have a more intimate knowledge of how reactions will proceed along the reactor length with the revised heat removal load profile. Yields may be affected, and so may handling capacity for recovery and recycle of unreacted components.