Thermosyphon Reboiler Return Line - Vertical 2 Phase Flow

[AggieCHEN04]

thread124-95533

I found this thread from a while back. Sshep had mentioned that the ideal mixed-mean velocity for vertical runs of pipe is 15 ft/sec or higher to avoid slugging. In one particular thermosyphon reboiler circuit I am evaluating, the velocities would be about 5 to 10 ft/sec.

The longest vertical run is about 10 ft for one reboiler return and about 50 ft for another return with a vapor fractions that ranges from 20% to 60%. This is a demethanizer system, so it involves light hydrocarbons at fairly low temperatures (-40F to 40F) and a high pressure (250 psig).

An accurate prediction of slug flow for this application is difficult and would depend on the layout of the piping and fluid characteristics.(ie. vapor fraction, slip density, etc)

Does anybody know of a good simulator for predicting two phase flow in pipe? I am currently using the standard HYSYS pipe segment, but I've found that the slug prediction calculations are highly inaccurate for this application.

Also, my boss had mentioned "lift plates" which are basically restriction orifices installed next to vertical runs of piping to help mix the two phases. Does anybody have experience with these?

How much turndown can you generally have for a thermosyphon reboiler in hydrocarbon distillation before hydraulic slugging becomes an issue?

 

[AggieCHEN04]

Some additional questions:

If the fluid is relatively close to it's critical point is it more or less likely to slug in two phase flow?

Does the liquids surface tension make a difference?

Is a two-phase hydrocarbon fluid more likely to slug than a two-phase aqueous fluid?

What is the 15 ft/sec standard based on?

 

[CJKruger]

AggieCHEN04, thermosyphon reboiler return lines should have low enough pressure drop to increase circulation, but high enough pressure drop to prevent slug flow.

I recommend you target 0.2-0.3 psi/100 ft and check the flow regime with the Dukler-Taitel maps.