Best Elevation for Thermosiphon Reboiler? 4

[ai4092]

The literature that I have read on the design of vertical thermosiphon reboilers (shell & tube vertical heat exchanger) recommends setting the top tubesheet of the reboiler at the same elevation as the liquid level in the column bottom. My question is then regarding the design of the upper piping that connects the reboiler back to the column. Is there a maximum elevation difference between the top of the tubesheet and the return nozzle on the column bottom that should not be exceeded? For example, if the discharge nozzle is tee'd into the side of the reboiler top channel/bonnet, then you can keep this elevation diff. quite small. If you go with an inline discharge nozzle on top of the bonnet followed by an elbow and connecting piping, you can end up with a considerable additional vertical elevation. Is there a point where this additional elevation could be enough to stall the two-phase return to the column (ie. stagnant pool of liquid stalled out in the top bonnet, with only the vapourized material exiting the reboiler and returning to column).

Thanks, Tom

 

[sshep]

The return piping geometry is accounted for in the circulation rate portion of the design calculation so in theory there could be a large lift allowed by the density differences and pressure drops, however, in practice the shorter the better. Conventional design calculations do not consider a phenomenon known as liquid slip which can make a thermosyphon reboiler unstable or be especially troublesome in start-up. Liquid slip is the tendency of liquid to fall backwards (due to gravity). During start-up this can lead to a heavy boiling residue accumulating in the reboiler. I have direct experience with these type circulation problems.

Presumably you are familiar Henry Kister's "Distillation Design" and "Distillation Operation". If not it is well worth getting a set. In the reboiler section of the operation book he recommends keeping the velocity in any vertical lift to be 15ft/s or higher in order to avoid slug flow in the return line- a velocity which results in additional considerations on impact to flow patterns inside the column. These texts can also give some input to designing the reboiler inlet including the decision to have a reboiler side compartment (i.e. fixed level to reboiler) or not.

 

[25362]

As sshep says, on low vaporization rates, bubble flow in the outlet line may occur, with small bubbles of vapor rising through a relatively still liquid. This is a kind of "slip flow" with discontinuities between the vapor and liquid velocities in the outlet line. Since the bulk phase density will be higher than expected, this may lead to circulation problems. Vapor or flashable liquid injection lines into the return riser may be required for startup.

Some ROT's on vertical reboilers taken from the literature:

1. The return nozzle on the tower should be a minimum of 6 in. above the highest design liquid level (HLL) to be encountered.

2. The upper tubesheet of a vertical natural circulation thermosyphon reboiler to be set at the lowest design liquid level (LLL) to maintain a liquid head for the reboiler at all times.

3. Normal liquid level (NLL): 2-3 minutes residence time; HHL: 1 to 2 minutes above NLL, or a minimum of NLL + 6 in.; LLL: 1 to 2 minutes below NLL, or a minimum of 6 in. below NLL.

Thermosyphon reboiler heads are generally TEMA B, L and M with site-specific slight variations. For comments on four possible configurations of top heads see Good design simplifies reboiler maintenance by G.S. Mason (Badger), an article that appeared in Hydrocarbon Processing, January 1983, p.82+. In this article the author discusses piping, structural and thermal design considerations.