20 replies to this topic

[vvsastry66]

HI

 

We have an existing demethaniser, with a vertical thermosyphon reboiler, heated by hot raw gas at 23 degc. Demethaniser op.pr is 23 barg, and top/bottom temp are -100/-5 degc.

 

If column feed is reduced ( both liquid/vapor), how does it effect reboiler performance.  raw hot gas is on shell side (condensing), and boiling on tube side)

 

Flow through reboiler depends on head available and pressure drop. As hot raw gas flow is maintained same, will it increase column bottom temp? It's once thro circulation.  My colleague says, as flow is reduced on column side, there will be less boiling, and so less thermosyphon affect, and hence column bottom temp falls. and to keep reboiler performing well, she says, column can not be operated below design flow.

 

The Problem is reboiler is not giving design heat duty/UA.

 

My feeling is as long as hot raw gas is flowing, liquid will continue to get vaporised (may be more), and better thermosyphon effect. Of-course, till nucleate boiling.

 

Thank you.

[Bobby Strain]

Well, you say it is a thermosyphon. And you also say it is once-through. It can only be one or the other.

 

Bobby

[vvsastry66]

it's once thro circulation type thermosyphon. not re-circulation type.

[Bobby Strain]

So it is not a thermosyphon reboiler? A sketch would help.

 

Bobby

[Art Montemayor]

vvsastry66:

 

Bobby is correct.  You can't have a thermosyphon reboiler that has the process fluid circulated once-through.  That, within the definition of what a thermosyphon reboiler is, can't happen.

 

Why don't you simply draw a clear and neat sketch of what you are referring to so that everyone can understand what it is that you have?  That would would not only be more efficient than words, but it would also be more accurate.

[vvsastry66]

Dear

 

pls find sketch.

Attached Files

•  Demethaniser- reboiler.pdf   51.08KB   198 downloads

[fallah]

vvsastry66:

 

Bobby is correct.  You can't have a thermosyphon reboiler that has the process fluid circulated once-through.  That, within the definition of what a thermosyphon reboiler is, can't happen.

 

 

As far as i know there is "Once through thermosyphon reboiler" among all types of the reboilers in which outgoing process fluid from the bottom tray of the tower enters to the reboiler and come back, as a two phase flow, to the tower at the space below mentioned tray...

[paulhorth]

Well, as shown in this sketch, all the liquid from the bottom tray is fed to the reboiler, from inside a weir, and the mixed vapour and liquid from the reboiler is returned to the column bottom below the weir. If that is correct then it is indeed a once- through reboiler and I would call it a thermosyphon because the fluid movement is driven by a density difference. For recirculation, the return would have to be back to the feed side of the weir - this is difficult to show on a sketch but this is the important detail which may or may not be correct.

 

I have designed several horizontal thermosyphon reboilers, with a feed weir like this, and recirculation in this way back to the feed side of the weir. The recirculation of liquid enables the design to provide the required vapour flow with a vaporization of about 25% - 30% of the feed by mass. This ratio is recommended to achieve a steady thermosyphon effect. I have also specified a HCV as shown on this sketch, specifically to allow the operator to adjust the recirculation. If there is no recirculation then the HCV is redundant.

In the sketch as drawn, it is not possible to set this vaporization ratio because all the vapour has to come out of the bottom tray liquid. This will indeed make turndown a problem as there will be less liquid to vaporise, so a greater proportion will be vaporised, leading to a coffee percolator effect as the reboiler boils dry then refills.

 

Paul

[breizh]

Consider this lecture about thermosyphon reboilers  with the description of equipment.

Hope this helps

 

Breizh:

[Bobby Strain]

vvsastry66:

 

   Your sketch doesn't show elevations or spatial relationships, and this is important. Also, it is possible that the dividing baffle in the column sump has a big hole near the bottom to allow recirculation. My guess is that the latter applies, otherwise there is no need for the valve to control flow. Unless the designer was pretty dumb.

Bobby

Edited by Bobby Strain, 11 August 2013 - 02:50 PM.

[vvsastry66]

Well, Thanks to all for replying. and for the lecture attached. I have read again this lecture. it's useful.

 

Coming back to my question:

 

If feed to column is reduced, how does it effect reboiler performance, maintaining same raw gas flow. 

 

1. Will it reduce or increase vaporisation?

 

2. Will it decrease or increase reboiler return temp?

 

3. Will it reduce circulation through reboiler

 

As feed is reduced,the level in sump of bottom tray might be lower, which might reduce flow to reboiler, hence there should be more vaporisation, (within system limits). Please confirm or explain any other way.

 

I will put elevations, if required to answer the question. 

[Bobby Strain]

I'm puzzled by your query. This looks like NGL recovery from natural gas where the feed gas flow and column feed, etc. are directly related. So what is the purpose of your query? Are you trying to solve some operating problem?

 

Bobby

[vvsastry66]

We want to raise reboiler return temp, so that methane is driven off and NGL is on spec. though there is enough approach ( 23 degc and -5 degc), and reboiler has enough H.T. area, the temp is not increasing.

 

It's believed that column was originally designed for higher liquid flows. Now column has lower liquid flows. My collegue told me because of lower liquid flows, thermosyphon effect is less, and there is no velocity to push the 2 phase fluid from reboiler to column. I feel this may not be true.

 

Please review.

[paulhorth]

vvsatry,

Thank you for stating your problem clearly.

I would expect the vaporisation in the reboiler to be more, not less, with reduced liquid flow to the column, so if this is your view, then I agree with you. So, I don't have an explanation for your problem of insufficient methane stripping. I can suggest two possible causes:

(1) not enough duty in the side reboiler, which then performs insufficient stripping higher in the column and thus overloads the bottom reboiler. Maybe there is insufficient hot gas flow to the side reboiler.

(2) with low liquid traffic in the column, the trays could be weeping, which will reduce the stripping effect.

 

But neither of these seems to me to be very convincing. Can you reduce the column pressure?

Can you send us a PID?

 

Regarding the flow through the bottom reboiler, if your sketch is correct, there is a gravity head from the inlet to the outlet so even with no heat at all there should always be liquid flow through this reboiler. If there is enough heat input, there will be vaporisation.

Maybe others with better operatinal experience could have better explanations.

 

Paul

[vvsastry66]

Thanks Paul.

 

I will wait for other views.

[Bobby Strain]

And we will wait for a more detailed sketch of the installation and column sump.

 

Bobby

[vvsastry66]

sure i will send it 

[Babu Prasad]

 

 

vvsatry,

Try to reduce side reboiler duty by throttling liquid flow from tray 8 or reduce raw feed gas flow, it will get more lighter in the bottom which will help to get thermo siphon effect in the bottom reboiler in turn it will increase bottom temp and controls C1 slippage.

At lower feed, C1 slippage is inevitable, Hope you are not operating plant with below minimum turndown ratio.

Edited by Babu Prasad, 20 September 2013 - 08:58 AM.

[curious_cat]

Might you be facing vapor lock? I can't exactly explain how but I recall seeing a similar situation before. 

 

I'll double check my notes. I can't find a primary source but my notes had this verbatim snippet that I had borrowed and noted from some book / paper:

 

If the liquid feed entering the reboiler is nearly completely vaporized: In this  situation, the liquid heating zone is small and nucleate boiling occurs almost immediately. In the  reboiler above the nucleate boiling zone, mist flow develops and heat transfer will superheat the gas. This results in a low overall heat transfer coefficient. This explains the poor performance of the reboiler

Edited by curious_cat, 23 September 2013 - 01:11 AM.

[curious_cat]

Is there a chance to throttle the thermosiphon inlet line and see if it improves performance?

[Padmakar Katre]

HI

 

We have an existing demethaniser, with a vertical thermosyphon reboiler, heated by hot raw gas at 23 degc. Demethaniser op.pr is 23 barg, and top/bottom temp are -100/-5 degc.

 

If column feed is reduced ( both liquid/vapor), how does it effect reboiler performance.  raw hot gas is on shell side (condensing), and boiling on tube side)

 

Flow through reboiler depends on head available and pressure drop. As hot raw gas flow is maintained same, will it increase column bottom temp? It's once thro circulation.  My colleague says, as flow is reduced on column side, there will be less boiling, and so less thermosyphon affect, and hence column bottom temp falls. and to keep reboiler performing well, she says, column can not be operated below design flow.

 

The Problem is reboiler is not giving design heat duty/UA.

 

My feeling is as long as hot raw gas is flowing, liquid will continue to get vaporised (may be more), and better thermosyphon effect. Of-course, till nucleate boiling.

 

Thank you.

 

Hello,

Please provide a detailed control system loop for this reboiler (mainly on hot fluid side). As you have mentioned in your post, the condensing fluid flow is maintained same, I have serious doubt about it. At turn-down either you have options like,

1. Variable duty (flow control valve on hot fluid inlet to exchanger) and

2. Variable area, (condensate level in shell thereby reducing the heat transfer area)

If your statement of having constant hot fluid feed rate is right, then you are artificially loading the column to the design vapor-liquid rates rather running column at turn-down conditions. What are the tray deck types in column? Any concerns of lower vapor flow operation e.g. weeping?

I hope this helps you.