7 replies to this topic

[GSaikrishn]

hello friends,

at our Platforming facility we have a column designed to recoevr benzene rich streams in the overhead section. however due to certain economic benefits, the same column is being operated to recover toluene (C7aromatics) in the overhead section. obviously there has been penalty on the reboiler and condensers. the reboiler is a vertical thermosyphon type with medium pressure (17 kg and 220 deg c) steam as the heating medium on the shell side and process fluid entering at 190 deg on the tube side. Recently we experienced an elbow failure on the mp condensate side. in order to rectify the same, we isolated the reboiler and arrested the leak. Soon after taking the reboiler back in line, we were unable to increase the steam load. original load before isolating the reboiler was 35 tph of mp steam for two reboilers (17.5 tph in each), the steam laod of the reboiler in question is not exceeding 10 tph.

Any suggestions, recommendations for solving the problem are appreciated.

[GSaikrishn]

hello friends,

at our Platforming facility we have a column designed to recoevr benzene rich streams in the overhead section. however due to certain economic benefits, the same column is being operated to recover toluene (C7aromatics) in the overhead section. obviously there has been penalty on the reboiler and condensers. the reboiler is a vertical thermosyphon type with medium pressure (17 kg and 220 deg c) steam as the heating medium on the shell side and process fluid entering at 190 deg on the tube side. Recently we experienced an elbow failure on the mp condensate side. in order to rectify the same, we isolated the reboiler and arrested the leak. Soon after taking the reboiler back in line, we were unable to increase the steam load. original load before isolating the reboiler was 35 tph of mp steam for two reboilers (17.5 tph in each), the steam laod of the reboiler in question is not exceeding 10 tph.

Any suggestions, recommendations for solving the problem are appreciated.
to add further, the column is operating at 1 kg/cm2g. the bottom temperature is 190 degc. the steam entering the reboiler has an xv in line which is full open, the reboiler outlet is mp condensate with a control valve in line (usually 50% opening) and the flow is maintained at 17.5 tph. the bottom product is c8 aromatics rich. The reboiler process side outlet enters the column above the blind tray and flows into the column trough.The trough is always filled and the overflow liquid is withdrawn as the bottom product. Process fluid enters the reboiler from the trough.

[paulhorth]

GSaikrishn,

Since no one else has replied, I will offer a suggestion, although I do not have operating experience with reboilers.
It sounds as though the reboiler is operating partly full of condensate on the shell side, thus covering up part of the surface area and preventing the full heat duty from being achieved. This could be due to a restriction in the condensate outlet, which you say has just been modified and reinstalled. Could this line or the control valve be partly blocked? Can you manually open the condensate control valve to 100%?

When designing a reboiler, I would always choose a horizontal thermosyphon. A vertical thermosyphon is difficult to design correctly, as its function depends on its elevation, the length of the tubes, the arrangement of the steam side outlet, etc etc with no real advantage over the horizontal type.

Paul

[breizh]

Hi ,

Nothing wrong with the quality of the steam ,the opening of the steam valve ( already experienced dirt inside the cage of the valve) or the level of liquid inside the column or the release of condensate ?
No uncondensable material inside the thermosyphon ? what about dirt on tubes which affect the heat transfer coefficient ?

You should use the search button many times this topic has been discussed over the years.

Hope this helps

Breizh

Edited by breizh, 26 September 2011 - 07:47 AM.

[GSaikrishn]

thanks for ur reply.

normally the leakage observed in the mp condensate line is frequent (once in 3 months). during the shutdown of the reboiler, the elbow is replaced with a spare one. now once the reboiler is taken back in line, we ensure to drain the process liquid to a close drain system. this we do to ensure that proper heat transfer is attained as there may be chances of heaviers getting accumulated and affecting the heat transfer in the reboiler and this is directly indicated by the mp condensate control valve which starts to fluctuate as live steam passe through it rather than the mp condensate . While taking the reboiler out of service,we ensure that the mp condensate is fully drained. After this whole exercise we were usually able to achieve the desired heat duty. However this time around we were unable to get the same result.

Thanks for the articles briezh.

[sheiko]

Is it a once-through (with or without vertical baffle?) or a circulating thermosyphon reboiler?
Also, what is the reboiler outlet temperature (process side)?

Edited by sheiko, 27 September 2011 - 11:58 PM.

[npp]

Low steam usage is a sign for low heat transfer, which can be a result of:
1) Lower heat transfer area: check if condensate is backed up in the heat excganger, it reduces HT area
2) Lower temperature difference: check if chess pressure in steam side is reduced or higher temperature inprocess side.
1) Lower heat transfer coefficient: fouling or inert gas blocked in steam side can reduce HT coefficient

[Babu Prasad]

We experienced same problem the Deethaniser column reboiler while starting the plant after a shutdown. The reason found that the heavier hydrocarbon liquid was occupying the columns bottoms & rebolier, which was inhibiting lighter to get vaporize. So heat transfer was poor and steam was not condensing at full rate. Normal steam flow achieved after draining continuously the bottom liquid. The same problem may be experienced in your side as heavier is occupying in columns bottoms (As Toluene is heavier when compare to benzene)

Edited by Babu Prasad, 06 October 2011 - 07:29 PM.