what is "thermosyphon" in connection with a reboiler?

[JOM]

I've come across ther term "thermosyphon" in connection with reboilers on columns. I guess it implies flow of liquid induced by thermal gradient or something...

Can anyone give me a simple but correct explanation of "thermosyphon"?

John.

J.

 

[pennpiper]

From Wikipedia
Thermosiphon (alt. thermosyphon) refers to a method of passive heat exchange based on natural convection which circulates liquid in a vertical closed-loop circuit without requiring a conventional pump. Its intended purpose is to simplify the pumping of liquid and/or heat transfer, by avoiding the cost and complexity of a conventional liquid pump.
Convective movement of the liquid starts when liquid in the loop is heated, causing it to expand and become less dense, and thus more buoyant than the cooler water in the bottom of the loop. Convection moves heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity. In many cases the liquid flows easily because the thermosiphon is designed to have very little hydraulic resistance.

 

[25362]

McGraw-Hill Dictionary of Scientific Terms

A liquid reheater (as for distillation-column bottoms) in which natural circulation of the boiling liquid is obtained by maintaining a sufficient liquid head.

From the ChE guide to heat transfer (McGraw-Hill)

The operation of thermosiphon reboilers depends on a balance between two-phase friction and acceleration losses in the reboiler flow loops, and the net static head developed by the liquid in the distillation column. Vapor-to-liquid ratio is critical to the operation of the reboiler (usually 5 to 35% vapor).

 

[mbeychok]

JOM:

Read the article at

http://en.wikipedia.org/wiki/Reboiler

to see drawings of various types of reboilers ... and be sure to read the text of the article as well.


Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JOM]

Thanks All.

That Wikipedia link was useful, Milton, thanks.

This has helped me. The case I was looking at involves liquid leaving the bottom of the tower to an exchanger and then re-entering the tower sump. So the flow is downwards. So thermosyphon (correct spelling, BTW) can't be involved as that could only produce an upwards flow.



J.

 

[25362]

JOM, is the HE a reboiler as the post heading suggests ?

 

[JOM]

"JOM, is the HE a reboiler as the post heading suggests ?"

Great question. I've wondered about the definition of reboiler. My impression is it's a term liberally used for any HE at the base of a tower. This one has been called a reboiler.

In this case, the tower is an absorber. Condensate falls to a collection tray, flows to the HE/reboiler, then back into the absorber bottom. Gravity flow only. Thermal convection flow would seem to be undesirable, as it would be in opposite direction to gravity flow. Could thermally-induced flow be a problem?

Purpose is to heat the condensate. This causes some evaporation. Is it a reboiler? It's been called that. What do you think? Not a reboiler? What difference would it mean to you as an engineer?

John.

 

[25362]

JOM,

IMHO: as long as it provides heat to evaporate (boil), it could be called a reboiler and it doesn't matter whether it is attached to an absorber or to a distillation column.

Apart from reducing the density of the flowing liquid upon heating and partial vaporizing, the partial evaporation changes the monophasic flow into a two-phase type of r[é]gime, and it could affect the gravity (constantly downward) flow as described by you.

 

[mbeychok]

JOM:

Please excuse me if you already understand how a thermosyphon reboiler works:

Ths head of liquid in the inlet line from the column to the reboiler is higher than the head of liquid + vapor in the return line from the reboiler to the column. That difference in head provides the driving force to overcome the friction in the piping plus the friction across the reboiler tubes. In other words, that difference in head is what drives the circulation without using a pump.

Thermosyphon reboilers are particularly useful if the column bottoms product is very heat sensitive because such reboilers can be designed so that the liquid to vapor ratio in contact with the reboiler tubes is as high as about 6:1 or even 8:1. That reduces the probability of overheating the circulating fluid.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[JOM]

Hi Milton.

Yes, I believe I understand things correctly.

I'm examining the details of an accident that involved an absorber where the condensate level in its bottom was not under control. The investigation report is in the public domain. I don't represent any party.

The condensate level rose above its normal level. My undersatnding from the diagrams is that it is only the static head difference between level at the collection tray that feeds into the HE, and the level in the absorber bottom. (No pump)

As the level in the bottom rises, the static head difference drops so the flow rate and velocity through the HE drops. That reduces the heat transfer rate. In this case the level possibly rose until it reached the collection tray. That means zero static head difference and no gravity induced flow. Do I have that right?

Under that condition, would thermally induced flow develop?

I needed to clarify my understanding of how this worked and the answere above have helped. Thanks.

John.



J.

 

[25362]

John,

When you say

Under that condition, would thermally induced flow develop?

do you mean reverse flow by the heating action of the reboiler ?

 

[JOM]

Yes, with zero static head difference to induce flow, could thermally induced flow take over causing reverse flow through convection?

It doesn't really matter, 25362. The heat to the HE had been shutdown anyway. I just wanted to get the thermosyphon process clear in my head. You've all helped in that. It doesn't seem to be a factor.

The accident was at the Longford gas plant, 1998 (Australia). Run by Esso.

John.

J.