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Fluctuating Distillation Tower Bed Tempeatures

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MikeInSWLa

Chemical
Jan 11, 2007
10
I have a distillation column with structured packing where I have recently relocated two thermocuples. They were moved from the top of the tower, where there is little temperature profile, to the bed below the feed, where there were existing thermowells but no elements.

Since moving them, the data I heve been getting is quite puzzling. The TI's are about 5 feet apart. The difference between the two temperatures fluctuates quite a bit. When I first put them in the read about 7 deg F apart. However, sometimes the difference is almost zero, and at other times the difference is 30-40 deg F. I also notice that there are timed when the delta T changes abruptly- the upper one suddenly decreases while the lower one suddenly increases.

I am wondering what the cause for this is- TI's usually indicate composition, but in this case, especially when they change so suddenly, could it instead be an indication of loquid holdup or flooding?

I wonder if anyone has ever seen a similar phenomenon and been able to determine the cause.

I am developing a project to put higher capacity packing in the tower and I'd like to undertand what is going on.

Any help would be appreciated.

Thanks,

Mike
 
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Could indicate flooding or distribution problem. Could also be an impurity that gets "trapped" in the tower and moves up and down.

Have you seen pressure swings along with the temperature swings?
 
That is what is puzzling; I see no other barupt canges to the tower operation that happens at the same time. Usually, flooding in structured packing is indicated by a spike in pressure drop or a sudden change in product purity. I don't see either of these.
 
How far into the tower do they extend? Could it be a wall effect?

Any new impurities? or higher levels?

Maybe a distributor problem that has always been there?
 

Not that this is a significant factor, but are the TI's on the same vertical line or are they rotated on the vessel's circumference ?
 
Do you have a simulation of the existing tower? Based on that, you can estimate the expected temperatures at the new thermocouple locations. If the temperature readings are spiking up beyond the expected saturation temperatures, perhaps, your tower is channeling. Have you checked the current tower hydraulic profiles against the the minimum turndown the current packing is capable of?
 
Thanks all for your suggestions.
To answer your questions, the TI's are located about 180 degrees apart on the tower- so maldistribution could be a factor, but due to the fluctuations it would be something that comes and goes- perhaps as a function of loading?
I am checking on the length of the thermowells; I don't have any drawings but I know the instrument tech can tell me.

Also, i am currently working on some simulations of the tower to match plant data. I have some old simulations but I don't believe they have really been matched to the plant data. Once I complete those I will be able to tell better what the expected temperature profile should be compared to what I have been seeing.

 
Curious why you moved the sensors below the feed point and not lower them.

there is usually an optimal location in the tower to get the splits you want. Being lower than the feed point invites a lot of problems.

Are you controlling boilup?


 
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