Thermosyphon Reboiler Outlet Line

[sshep]

I have a 10.5ft dia tower which is being changed from a vapor feed to a liquid feed resulting in a large additional reboiler duty. We are installing a new vertical thermosyphon reboiler. The existing tower has two 20" bottom nozzles located 180deg from each other on the column bottoms. If the column diameter is used as a basis for the new installation, a single 20" nozzle is looking too small. As the process engineer I wish to either install a single new larger reboiler return nozzle, or otherwise just design and operate up against the constraint of a 20" return nozzle using a vapor distributor in the tower to deal with the high entrance velocity.

Unfortunately my project manager doesn't want to pay for a proper size nozzle and operations would like to stay with the the higher vapor basis. Together they have cooked up an idea to split the reboiler outlet between the two available nozzles. Splitting the two phase return to the tower between the two 20" nozzles seems like an invatation to unforeseeable problems. It will be difficult to get a symetrical piping layout, but they are at this moment working on some sketches to convince me.

I am shopping for input in this forum, with exchanger and tower internals vendors, and elsewhere. My 20years of experience with tower revamps has made me generally conservative with respect to funny looking installations. Has anyone in this forum had experience with a similar situation? Any advice is appreciated. Thanks- sshep

 

[katmar]

My immediate reaction is to agree with you and avoid complex 2-phase piping.

What are the chances of leaving the existing reboiler in place and installing a second reboiler on the opposite side? This will simplify the piping and could save you a lot of money by requiring a reboiler of only the differential capacity in place of the new full load? I would not be unhappy with a split in the liquid line feeding the two reboilers, although you may need to increase the size of the liquid outlet from the column.

regards
Katmar

 

[roker]

The new reboiler should be installed between the two 20" return nozzles, the two nozzles will be used with two symmetrical lines from the reboiler itself or split the line from the reboiler to two symmetrical branches into the 20" nozzles. It is understood that the lines are smaller than 20" but an enlargement of pipe size before entering the column is quite common, to reduce the velocity and help the phases to separate.

regards,
roker

 

[pmureiko]

I'd look at how much the flow in each return leg changes versus differences in length, elbows, etc. If it is relatively small, then the "symetric pipes" have plenty of capacity to handle process and design variations.

Is the thermosyphon reboiler being considered a stage for mass transfer? If so will this return arrangement complicate the sump design in order for liquid suction to be separate from the return?

 

[sshep]

Hey Friends,

Thanks for the interest. I left it with the project guys that if we can get our exchanger vendors to guarentee this divided flow design then I would go for it. I have provided them the tower elevation details and conceptual piping layouts, etc and it looks like we will probably design such a split flow. It is pretty symmetrical- better than I previously thought. Discussion so far is that even though our exchanger designers have not seen this type layout before, most don't see any reason why it won't work. Thanks again for the advice. I will let you know how this turns out.

cheers, sshep

p.s. Good to hear from you again pmurieko- give me a call sometime. Because this is a retrofit with a single bottom sump and nozzle and possibly two return points, there is no practical way to baffle so I am not counting on any extra seperation from the reboiler.