Column reboiler low boilup rate

[louwb]

On our facility we operate a distillation column to remove light components from our oil product before exporting the oil to the refinery. The function of the column is to stabilize the oil.

The reboiler is a vertical type and there are no valves between the column and the reboiler. After turn around it has been very difficult to maintain the bottom spec (<0.8% light components in the bottoms stream). The best way to counter this in the mean time is to increase the reboiler outlet temperature.

I have found that the temperature difference between the reboiler inlet and the outlet has increased by about a factor of 4. Previously the temperature difference was 5°C, now it is 20°C. I suspect that there is a great deal of superheating.

The steam usage is about half of what it used to be. My initial thought was that the level reading in the column is incorrect and that the tubes of the reboiler are only 10-20% filled with liquid which would explain the superheating. We are in the process of checking the levels by re-instating level glasses on the column.

To install the level glasses will take some time, so my question is: Are there any other causes of this problem? Can the problem be on the steam side of the reboiler? The condensate pot level controller is standing less than 25% open, but this is expected with the low steam flows to the reboiler. Can non condensables on the steam side cause this issue?

Any suggestions would be much appreciated.

 

[cheute79]

I'm no expert, having operated and modifed thermosiphon reboilers, but never designing one from scratch, but you stated that after a turn-around, you get higher delta-T wiht less energy input, right? Is there a chance that something happened during the turn-around that caused or induced fouling agents to be cleared away, so that now the heat exchange is up dramatically? This would also lead to the tubes being only 10-20% liquid filled, because the liquid is boiling off faster. If this is correct, then the profile of the column is much different. Since it's just a stripper, basically who cares, as long as the light ends are stripped sufficiently. Seems to me that you should use even less steam, and that maybe the steam control valve might need to be modified to give you control at lower flows.

Alternatively, is there a chance that you have a leak? Is some of the steam now leaking into the oil? A steam leak can really improve the heat exchange, but play havoc with the product.

 

[louwb]

Dear cheute79,
Thank you for the reply. We have found the problem with the column on Tuesday.

Basically what happened was that when the system was commissioned the feed temperature was set to 100°C and not 75°C as per normal operation. The cold feed is heated by the bottoms product before it is exported. The operator wanted to keep the effluent temperature down for another part of the plant, not realizing that the feed temperature has such a big effect on the column operation.
So the feed was way hotter than it should have been, which caused a low boil-up ratio since less steam is required to heat up the already hot feed. Once we set the feed temperature to 75°C the reboiler came back to life out of its own. We went from using about 80kg of steam per m3 of oil to roughly 110kg/m3.

Now we are on spec and running smoothly.

 

[cheute79]

Louwb:

I can't tell you how often that sort of thing has been the cause of major upsets! Thorough prestartup reviews help, but even they miss things like that - because who would have considered it?

As Rosanne Rosannadanna used, to say, "well, it's always something!"