15 replies to this topic

[daraj]

Hi,

 

Iam trying to understand the usually listed limitationS of thermosiphon vs kettle when selecting reboiler type for a particular preliminary design of column exchangers.

 

Two of the commonly listed points in literature are:

 

1. Thermosiphon reboiler is usually used in applications where the vaporization is not very high. i.e only about a max of 20-30% of the inlet liquid to the reboiler is vaporized. The reason given is too much vaporization can cause low heat transfter coefficient and make areas high because of bubbling, vapor blanketing and such. 

 

Iam trying to understand why is this not happening in kettle and only with thermosiphon?

 

2. for a wide boiling mixture thermos are preferred to kettle. again Iam trying to understand the physics behind these observations/thumb rules. why is it advantageous to go with thermo in this case? 

 

a simple explanation of the physics behind this would be helpful, thanks

 

Thanks

Vijay

[Bobby Strain]

It is a choice based on cost, space, or other. The kettle is probably the economic choice for small systems with low vapor flow. You see a lot of kettles in gas processing plants. But not many in refining or chemicals/petrochemicals.

 

Bobby

[xavio]

daraj,

 

1. Kettle reboiler is pool boiling vaporizer where tubes are immersed under boiling liquid, that's why it doesn't have vaporization limit.

    You can have 100% vaporization if you want, but it would mean no bottom liquid from the column.

    Kettle reboiler returns only vapor to column, excess liquid is extracted as bottom product.

    Imagine the kettle you use for boiling water in your kitchen.

    The problems you mentioned above only happen to flow boiling vaporizer, where two-phase fluid is returned to column.

2. Again imagine the kettle in your kitchen, if you boil wide-boiling mixture, light components will be preferentially boiled-off, leaving heavy components to accumulate at the bottom of the kettle.

    If you use steam instead of gas stove, ever-increasing boiling point of your mixture will eventually equal your steam temperature, causing pinch and ceasing heat transfer.

 

Hope it helps.

 

xavio

[breizh]

Hi Daraj,

Probably good to read .

Hope this helps

Breizh

[daraj]

Thanks for your inputs Bobby, xavio and Breizh.

 

xavio thanks for the points. I was imagining kettle reboiler as some kind of immersion heater with thetubes immersed in the process fluid and tubes carrying steam. So at the surface of the tubes the process fluid is getting vaporized and vapor is being formed. But the vapor blanketing and bubbling issue seems to be tied onl to thermo, which is what I was wondering why

[daraj]

"If you use steam instead of gas stove, ever-increasing boiling point of your mixture will eventually equal your steam temperature, causing pinch and ceasing heat transfer.

"

but even a thermosiphon cannot solve this problem, right?

even in a thermo the light components boil off first. Its just that since we limit vaporization to less than 20%, there is little chance of pinch happening. or am I missing something?

[daraj]

also xavio, I have often seen kettle reboilers with bottom liquid products. so that 100% vaporization that you are talking about is not that common. But vaporization is high as compared to thermo, I agree. But maybe 60-80%

[daraj]

also found a good article that explains somewhat the boiling regimes to keep in mind while designing a reboiler. Useful article. I still did not get the physics behind the transition and how things work in kettle but this should be a practically useful guide for any designer

http://www.cambchemt...s_CCTI_2010.pdf

[PingPong]

In a thermosyphon reboiler part of the liquid from the reboiler return is usually recycled back to the reboiler inlet, so the vaporization limit is not an issue.

 

Although in textbooks and websites reboilers are often shown as kettles, in actual practice they are seldom used as such. (the same sources often show column trays with bubble caps, for which the same applies: bubble caps are seldom used nowadays, only for very specific applications.)

 

In my 30 years of design work on refinery and petrochemical processes I can't remember ever having seen a kettle reboiler being used on hydrocarbons. I only saw a few on amine regenerator and TEG regenerator. Maybe they make sense on atmospheric columns.

 

In my experience kettles are more often used as a condensor or cooler (vaporizing refrigerant or generating LP steam) than as a reboiler.

Edited by PingPong, 15 October 2014 - 08:05 AM.

[xavio]

 I was imagining kettle reboiler as some kind of immersion heater with thetubes immersed in the process fluid and tubes carrying steam. So at the surface of the tubes the process fluid is getting vaporized and vapor is being formed. But the vapor blanketing and bubbling issue seems to be tied onl to thermo, which is what I was wondering why

 

The nucleate, transition, film boiling transitions happen to both vaporizer types.

So, your kettle can also experience vapor blanketing (film boiling) at some heat flux and temp. difference combination.

However, the transitions are not related to vaporization ratio of the kettle because of its non-flowing, pool boiling characteristic.

You can easily pick a "safe" operating point for your kettle and operate at desired boiling regime.

Therefore, it is easier to design a kettle than a thermosyphon.

 

In thermosyphon, vaporization ratio determines flow regime which, in turn, will affect heat transfer mode and efficiency.

Boiling regimes and flow regimes are tied together, so it is more complex to design a thermosyphon.

If you don't design the hydraulics well, e.g. having too high vaporization ratio, you may end up with unstable operation.

Here is one useful link:

http://www4.ncsu.edu...eatTransfer.pdf

 

 

"If you use steam instead of gas stove, ever-increasing boiling point of your mixture will eventually equal your steam temperature, causing pinch and ceasing heat transfer.

"

but even a thermosiphon cannot solve this problem, right?

even in a thermo the light components boil off first. Its just that since we limit vaporization to less than 20%, there is little chance of pinch happening. or am I missing something?

 

You're right, you can limit the vaporization ratio as you want so that pinch can be avoided.

 

Good luck!

 

xavio

 

 

 

 

 

[daraj]

Pingpong and xavio, thanks for the further inputs.

 

In a thermosyphon reboiler part of the liquid from the reboiler return is usually recycled back to the reboiler inlet, so the vaporization limit is not an issue.

 

 

 

I think it depends on the return convention used. In the software that I use there are options for on-stage or above-stage return and whether the columns has baffles at the bottom or not. So depending on he option chosen the liquid from the reboiler return might go to the stage above and might not get mixed up with the liquid coming down to the reboiler, if I understood what you said correctly.

 

xavio, what you said about the link between flow  regime and boiling regime, I think that's the crux of the issue here. I believe that was what I was looking for. I'll read up more on this link to understand things better, thanks.

[PingPong]

I think it depends on the return convention used. In the software that I use there are options for on-stage or above-stage return and whether the columns has baffles at the bottom or not. So depending on he option chosen the liquid from the reboiler return might go to the stage above and might not get mixed up with the liquid coming down to the reboiler, if I understood what you said correctly.

"Return convention used" ? You need to design the column bottom section to suit the chosen reboiler type, not the other way around.

 

Unless you have a very good reason to select a kettle reboiler, you design the column bottom for a thermosyphon reboiler, preferably with proper baffling and recirculation of part of reboiled liquid, to get right % vaporization at reboiler outlet. Designing for once through thermosyphon is often asking for operational trouble.

Edited by PingPong, 16 October 2014 - 03:41 AM.

[Bobby Strain]

Once-through is not a thermosyphon.

 

Bobby

[PingPong]

You may be right if the liquid is taken directly from the downcomer of the last tray, but not if the liquid from that tray first falls in a compartment with a liquid level below the reboiler return nozzle.

 

However I am used that in both cases it is called a once-through thermosyphon. It is so in my company design manual and in all other sources that I have read through the years. Google also gives plenty of hits for once-through thermosyphon.

Edited by PingPong, 16 October 2014 - 10:01 AM.

[Bobby Strain]

My main point is that the system hydraulic design is significantly different. I always used HTRI for thermosyphon reboiler design. Forced-feed- once-through is a much less complicated design.

 

Bobby

[daraj]

Bobby and pingpong, thanks for further clarifications