Debutanizer Reboiler Fouling

[Falcon07]

Hello Everybody:

We are suffering from Reboiler fouling of our Debutanizer tower, sever coke on the shell side have been observed during our recent shutdown. I'm not sure if you guys have experienced similar problem in the past and how you mitigate the fouling mechanism in your system.

Do you guys know the exact mechanism(s),?

 

[sshep]

Falcon,

One mitigation that we use is a system of spared reboilers. Although this requires some extra safety considerations because the reboilers need isolation valving and an independent relief system from that of the tower, the solution is a pretty sure mitigation against the need to shutdown the whole train due to a fouled reboiler.

best wishes,
sshep

 

[AndreChE]

Falcon

We have a debutanizer tower in a steam cracker plant and during a 5 years run, we also suffer from severe fouling in the reboilers and loss of heat duty. A steam pressure increase is usual along time.

To overcome that problem we will install a new reboiler, a spare one in parallel with the existing one (already taught by the licensor - tie-in from plant startup).

As sshep said you need isolation valves and for this you also need to change the supporting system in the column for the two reboilers.

AndreChE

 

[SmartEngineer]

Most of the DeC4 columns I have seen in about 4 ethylene plants do have reboiler with spare one.
The reason is that the base temperature causes the conversion of dienes to form polymerization material.
You can do the following:
- Add a spare reboiler if you are concerned about down time and do not want to cut the products to users for some days
- Add inhibitor (antipolymerization chemical in the DeC4 feed or DeC4 bottom)to prevent polymers formation and prolong the life of the reboiler.
- Desuperheat the steam to the rebolier to reduce shell skin temperature that will help reducing polymerization problems.


Hope this would help you

Cheers
SmartEngineer

 

[sshep]

My experience is as well in ethylene plants where reboiler fouling tends toward polymerization rather than hard coke. Inhibitor treatments are used, but are obviously not 100% effective. These are vertical recirculating thermosyphon reboilers.

If you are making any new design, I would focus on getting the tubewall temperatures down by not only desuperheating, but designing for higher process side circulation (lower % vaporization). Not sure what you have downstream but if you have another tower (deC5 tower) recieving the bottoms, a possible mitigation is to recycle some C5 distillate back to your deC4 bottoms to reduce your tower bottoms temperature. Incidently, what sort of temperatures are you getting across your reboiler (tower bottoms vs reboiler outlet)?

best wishes,
sshep

 

[Falcon07]

thank you all for your input.

yes we are thinking to re-design the current exchanger. one of the proposed modification is to go for counter current to co-current operation, some people tried this and it did work.

We do have isolation valves across the DeC4 reboilers, when ever we notice a problem with one of them such as tube leak, we will isolate the bad one and run with one reboiler. Feed to the tower must be reduced to maintain product specs if we run with a single reboiler.

 

[AndreChE]

Do you feel film boiling problems with your deC4 column? Which steam do you use and what is your bottom temperature?

We had film boiling problems in the deC2 and deC3 (I work in a deC2 front end SC plant). We have installed desuperheaters to LP steam (2.8 barg) and it worked.

 

[Falcon07]

we use Heavey oil from FCC fractinator tower as a heating meida, we don't use steam. The heavey oil temp is in the rnage of 350 °C to 380. We are thinking to re-tube the bundle to SS material which can stand higher temp.

 

[AndreChE]

Have you rated your reboiler?

You have a deltaT between the cold and the hot side in the reboiler very high. In those conditions fouling is very severe.

Is it possible to reduce that oil temperature? The best option should be to install a spare reboiler with isolation valves.

AndreChE

 

[Longinthetooth]

We use forced vertical circulation reboilers for this service with process downflow through the tubes. It is an old idea and it helps a lot. Having said that, your 350-380C heating medium is a real showstopper. I'm not surprised you have coke. The polymers probably sit on the tube wall and slowly coke.

Regards,

 

[Falcon07]

We are working on resolving the issue abd I hope we will be bale to do the rigt choice. So far changing to co-current flow is the best option to go with, since others have tried it and it did work.