Vertical vs Horizontal Thermosyphon Reboiler 2

[sshep]

Much of my 20yrs distillation experience using circulating thermosyphon reboilers has been with vertical units, and very little with horizontal ones- at least I have never specified a horizontal one. I am currently working at a site where horizontal thermosyphons are the norm. A tower is being changed from a vapor to a liquid feed. The existing reboiler is only 20% of what is required and must be replaced. Space is precious. This is an atmospheric tower seperating benzene from decane and heavier components.

I envision replacing with a vertical thermosyphon for reasons of space, cost, and prior experience. Before I do this I would like to better understand what advantages come from using a horizontal thermosyphon reboiler- the only thing that comes to mind is the ease of pulling the bundle which is not a big issue in my clean service.

This tower and equipment is less than 5 years old. What other advantages might have have been considered when the previous designer specified a horizontal over a vertical reboiler? There is a good chance that this was just the human factor of copying what was already common, but I would like your input anyway. Thanks in advance, sshep

 

[25362]

Quoting from various sources in the literature:

1. Horizontal thermosyphon reboilers have a less-sensitive
operation than vertical types.

2. More area can be placed in a single shell than with
vertical units. Especially for large sizes and high
duties. Sizing is not limited by constructional
considerations.

3. Easier maintenance due to horizontal orientation.

4. May be more suitable for greater than 2:1 turndown
ratios.

5. More suitable for wide boiling mixtures.

The main disadvantages:

1. Piping must be carefully designed to equalize flows in
all parallel branches.

2. More expensive if fixed tubesheet construction cannot be
used because of fouling on the shell-side.

3. If the available head is limited, vertical units are
preferred.

4. More expensive due to the complicated nature of the
piping and supporting structure.

It appears that horizontal units became more popular in recent years because the design of two-phase V/L flows from the reboiler to the column can now be calculated more accurately than previously with better models, and due to a better prediction of thermo-physical properties of the boiling fluids.

 

[Leclerc]

I have also changed reboiler types from flooded vertical tube(with large central downcomer tube)'thermosyphon'reboilers to vertical thermosyphon. Different scenario, I know.

The reason given for the previous need for flooded tubes was that "at high vaporisation rates, part of the tube surface might dry out and gross fouling by polymerising might occur". A certain amount of this type of fouling did occur, and an installed spare reboiler had been the answer.

Nevertheless, on changing, a single vertical thermosyphon reboiler, no spare, worked a treat, since the high velocities in the tubes kept them scoured clean.

 

[sshep]

Hey thanks,
One more question:
What is the typical % vaporization for these type units? My previous experience with verticals is in the range of 15% to 40%, but today I am looking at some horz unit spec sheets showing design values of 70%. I can see where the increased piping dP due to the more distant placement of a horizontal unit might tend to result in higher values, but am wondering what are normal % vaporizations for horz reboilers. Thanks again, sshep

 

[25362]

On thermosyphon reboilers, and the effects that a large V/L (% vaporization) might have on instabilities, you may be interested in reading Properly Design Thermosyphon Reboilers by A. W. Sloley. An article that appeared in the March 1997 issue of CEP.

 

[sshep]

Hey 25362, thanks for reminding me that Andrew Sloley has a nice website packed with distillation papers. It looks like the paper you referenced is there.

should others wish to reference please see

http://www.distillationgroup.com/technical.htm

-sshep