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Low tray efficiency and large overlap in crude still

Low tray efficiency and large overlap in crude still

Low tray efficiency and large overlap in crude still


I recently started a new job at an oil refinery and quickly noticed that the 90%-10% overlap between kero to diesel and diesel to gas oil are 100 deg F and 150 deg F respectively. My background is mostly in FCC's not crude stills but I am used to overlap in main Frac's being more in the range of 20 deg F.

The tower only has an overall dP no sectional dP's and no bleeders to get readings. The overall dP is typically 1.5 psig. The tower has 34 trays between these pressure taps and the tray spacing is 3 feet. The trays are all original cast iron bubble caps. The tower is running at design rates and there is an identical tower running at the same rate with the same overlap.

Initially I would have thought that the problem was caused by weeping, but as long as the risers in the bubble caps were designed properly, my understanding is that weeping is not possible. I don't think it could be mechanical damage since this poor fractionation has always existed at this refinery and is duplicated in the identical crude unit 2. Do you think it could be caused solely by low liquid and vapor loading. Pumparounds are cut back to a minimum to try to increase internal reflux but I have only seen minimal improvement. The pumparounds return a couple trays below the draw trays. The only clean cut in the tower is between the kero and the overhead, which I suspect may be due to the high top pump around as there is no reflux used in the tower.

Any suggestions? The only other idea I can come up with is to adjust weir heights to increase liquid levels on the trays, currently suspect 3 in level on tray with 3 foot tray spacing so I believe there is plenty of room before hitting flood point.

Have I missed anything? Any suggestions would be appreciated.

RE: Low tray efficiency and large overlap in crude still

If you have been experiencing identical problems in both CDU's, perhaps it could be a design issue?
Do you have any other data - temperature profiles? Do the side-strippers perform as they should?

Finally, how much different is the current crude oil when compared against the design crude assay?


RE: Low tray efficiency and large overlap in crude still


The problem exists in both CDU's, it always has. I'm 100% sure its a design issue. The question is how to improve it.

The side strippers are fine. The only temps on the tower are draw temps. The temperature profile responds to changes in P/A as they should. For instance, Mid pump around returns just below the diesel draw and there is no pumparound between the diesel and kero, so reduced mid pumparound improves the fractionation between the Diesel and Kero as it allows more loading in this section of the column. The problem is, the improvement is very minor and the pumparounds are typically minimized to keep the top pumparound maxed out.

RE: Low tray efficiency and large overlap in crude still

Did you try to increase stripping steam (or reboiler duty) in Diesel and AGO strippers, and if yes - how much?

How many trays are between Kero/Diesel and Diesel/AGO?


RE: Low tray efficiency and large overlap in crude still

Stripping steam in the side strippers is typically at a minimum to push flash spec, trying to maximize kero and diesel yield. I have push stripping steams in the past with out seeing any change.

I'm not quite sure what you are getting at with the side strippers. Are you thinking stripping more here will increase the load going back to the CRude tower through the vapor lines and help increase load in the tower? Because the side strippers themselves will not affect the fractionation, they are only there to control the front end not the back end.

In one tower there are,10 trays between each cut. The other was modified to have 20 trays between the kero and diesel and 10 between the ago and diesel.

RE: Low tray efficiency and large overlap in crude still

Side strippers will remove the front cut of a distillate, thus reducing the D86 overlap between the stripped product and the adjacent product drawn from upper section of the column above that product. For example, stripping more lighter fractions from Diesel will reduce the Kero/Diesel overlap because the 5% D86 point for Diesel will be higher than the initial 5% point - all assuming that strippers are an issue. I would pull the samples of stripper feed and stripper bottoms and see how much the IBP and 5% D86 points increase across the stripper. If there is no significant increase, then there is a problem with stripper(s).

Strippers are very intolerant to subcooled feed so check how close to the bubble point is the feed stream of your stripper(s). There shouldn't be any heat sinks in the line from the column draw-off nozzle to the stripper inlet nozzle, and the line itself should be insulated.

I have operated a similar CDU tower in my past, with around 10-12 trays between Kero and Diesel and 15 trays between Diesel and AGO. If I remember well we had a gap (not overlap) between overhead Naphtha and Kero (close to 15C), zero overlap between Kero and Diesel, and 20C overlap between Diesel and AGO.


RE: Low tray efficiency and large overlap in crude still

I understand your theory but I am 99% sure the strippers are not a problem. I understand strippers can remove the light ends form the front of the distillation, however they are there to control the flash by removing a very small amount of material which may change the 5% point or less as you mentioned. However, I don't believe side stripper should be used to push 40% of the AGO back into the diesel.

I've operated many columns and I have only had to use side strippers to slightly adjust the front end for flash. IF you have to use them to adjust your cuts that much, then as far as I'm concerned the problem is in the crude tower itself. Additionally the side strippers operation is up to par. There are absolutely no heat sinks in the draw line. The product draw temp and the stripper inlet temp are the same, and we see increased temperature drop across the stripper as we raise steam rates, showing good vaporization of light ends. The flash also increases as we adjust stripping steam.

I still believe the problem is in the tower. The only clean cut is between Naphtha and kero which I believe is due to the large top pumparound rate that returns on the top tray. As I reduce mid pumparound which comes in at the diesel tray I see improved fractionation between the kero and diesel, due to shifting the heat balance up the tower and increasing the loading between the diesel and kero.

RE: Low tray efficiency and large overlap in crude still

Can you provide us with a sketch of the tower showing visible process parameters you have information about, or/and a DCS snapshot of the column?

What happens if you change the tower top temperature significantly, and increase the internal reflux rates?
And, how far is the current throughput from the column's nameplate capacity? How different is the crude oil you are currently processing from the design blend?

Senior Process Engineer

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