8 replies to this topic

[Shichiroji_mano]

Ladies, Gents,

 

I have an inquiry regarding thermosiphon failure. I have read in certain literatures that fouling due to polymerization of diolefins is quite common in FCC debutanizer reboiler. However, I cannot conclude that it is the current cause of thermosiphon failure in our reboiler. Note that we have 2 other debut. reboiler using steam which is working well, while the one with the problem is heat integrated with HCO pumparound.

 

Per checking in historical data, we have experienced it in the past and corrected upon start-up (from unit shutdown). One foreseen cause is the decrease in HCO inlet temperature. Consequently, inlet and outlet temperatures of reboiled side (naphtha) becomes equal; 2 other debutanizer reboiler is not turned-off.

 

Upon restart and stabilization at short periods of time, it seems that the reboiler have not regained the thermosiphon circulation: naphtha inlet temperature is greater than outlet which means bottom of the tower is not reboiler. Duty of the reboiler also decreased with respect to HCO inlet and outlet temperatures.

 

Have anyone experienced that start-up of thermosiphon reboilers affects its circulation? and what should be the right sequence. 

 

I have attached sample trend for reference.

 

Thank you!!! 

 

 

Attached Files

•  Capture.JPG   96.65KB   3 downloads

[Bobby Strain]

You need to present information that is easy for us to understand. We're not going to do a lot of work for you to negotiate your DCS screen capture.

 

Bobby

[Shichiroji_mano]

Apologies for that. I thought I have highlighted my question:

 

Have anyone experienced that start-up of thermosiphon reboilers affects its circulation?

 

The DCS screen capture is just for reference in case anyone needs it =)

[breizh]

Hi ,

Yes sure . Probably good to use the earch engine in this forum . Sometimes you will find orifice installed to help at the start up .

Definitely not easy task . Rely on skilled operators .

Note : make sure no air trapped .

 

Good luck.

Breizh

Edited by breizh, 19 April 2016 - 08:42 PM.

[Shichiroji_mano]

Thank you! 

 

Can you elaborate more on how orifice helps on thermosiphon reboiler start-ups?

 

If I can rephrase my question: if reboiler S/S inlet temperature is hotter than outlet before fully commissioning the thermosiphon reboiler (due to other reboilers in the tower already commissioned), will it hinder establishing thermosiphon circulation upon introduction of flow in the T/S?

 

Note that S/S is the cold stream (debutanizer bottom) and T/S is the hot stream (HCO)

[breizh]

Hi ,

Consider the documents attached to support your investigation .

 

The valve is to calibrate the flow at the start to avoid dry out .

For similar set up it tooks sometime more than 12 hours to get stady state .

 

Good luck ,

Breizh

Edited by breizh, 20 April 2016 - 01:17 AM.

[cea]

Dear Shichiroji_mano,

 

It appears that you seek for first cut opinion about your query "whether startup of another thermo siphon reboiler affects circulation of parallel reboiler & what shall be startup sequence" Hope, I am right in my understanding.

 

Since your question is general in nature & hence my reply.

 

Firstly, please note that % vaporization in thermo siphon reboiler is in range of 10- max 30%. Thus there is huge circulation flow always through thermo siphon reboiler. This flow is established due to density difference due to heating. Thus, there is no external source available to establish circulation flow.

 

Knowing above facts, startup of one exchanger will not hamper performance of another exchanger as long as liquid head in column is maintained to same value. Further, during startup, you cannot expect both, a running exchanger & new exchanger, will start performing equally with respect to circulation flow (piping configuration is assumed to be same for both exchanger). However, over the period, it will establish by itself, once utility flow to both exchanger becomes nearly same.

 

An operator is required to ensure following during startup

1) Liquid head in column pot shall be properly maintained throughout startup

2) Keep the new exchanger little bit hot before opening the valves

3) Donot increase utility flow abruptly to get circulation flow same as running exchanger.

 

Hope, this meets your expectation.

[Shichiroji_mano]

Thank you very much!

 

Will consider this. Anyhoo. I shall update this topic with the actual cause of the problem. 

[manojkaila]

Dear Mano,

 

I run the debut column but never face such issue. What is you exact problem to maintain the main column HCO tray temp or the steam consumption in debut re boiler. If you met all the process condition then may be steam flow is little higher. May be you can try to slowly reduce steam ( carefully) then all heat load to be shift to HCO.

 

Good Luck

 

Manoj