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Our FCC plant Downstream debutanizer tower having pressure increase issue due to exchanger perforamance and cooling water temperature issue,
If only reboiler steam is reduced, Debutanizer OVHD C3’s composition is started increasing,Though Debutanizer’s purpose is to separate C3&C4 LPG (to Top) and Naphtha (To bottom),in case of sudden reducing reboiler steam, C4 component starts accumulation in Debutanizer column and C3 concentration at OVHD starts increasing,In fact, after only reducing reboiler steam, Debutanizer OVHD temp temperature started decreasing and Debutanizer OVHD pressure started increasing simultaneously.OVHD pressure may increase tentatively due to composition profile change in the column, then it was come down.
Above Phenomena little tough to understand. Anyone kindly explain?

 
Answers
02/09/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Without knowing instrumentation, the following can be said: Reducing reboiler duty drops butane into the bottoms product . With less butane in the overhead product, the percent propane increases but the quantity does not. Knowing the instrumentation could lead to more comments.
02/09/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
At least say how the column pressure is controlled and I will guess at the rest of the instrumentation. Then, I can analyze your situation.
01/09/2018 A: Sanjib Kumar Rabha, IndianOil Corporation Limited, rabhasanjib@gmail.com
In Debutanizer column generally if you decrease reboiling temperature than the design temp, lesser LPG components will get separated from naphtha components resulting in higher RVP in the Naphtha run down, you will experience pressure surge in downstream Naphtha Splitter column. And by the same action Debutanizer over head vapor composition will have higher concentration of lighter components which have higher vapor pressure than the naphtha components resulting in pressure surge in Debutanizer column (top pressure). You can sustain Debutanizer operation by putting lesser reflux but you will lose LPG in naphtha stream at bottom. Naphtha Splitter operation if sustained, the Light Cut Naphtha will have higher RVP which will not meet required specification.
Thumb rule is to try to stick to design parameters and of course lab result of top and bottom streams of the column.
31/08/2018 A: James Chacko, Suez Water Tecnologies & Solutions, james.chacko@suez.com
Distillation impacts due to volatility differences of C3 & C4 are your final No-Go situation
Start increasing the reboiler steam steadily until overhead composition is achieved. May have to send some off gases out and start reflux to achieve proper temperature profile of column. If you can achieve this stability, send bottom sample for RVP and confirm all OK
Still if you cannot achieve stability, look for fouling at reboiler or Overhead cooling, or even reboiler leaks
Which steam are you using? HP, MP etc
30/08/2018 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
You haven't mentioned reflux rates or how the pressure or tower top temperature is being controlled. To operate a tower with limited heat removal at the top you must limit the amount of steam to the reboiler. To achieve good separation you must maintain the highest differential temperature (tower bottom/tower top). this normally done by increased reflux but it sounds like you are limited.... Reflux indirectly comes from the reboiler. If you are trying to cool the top of your tower with more reflux (because of cooling restrictions) than the reboiler duty is producing then you are past the incipient flood point and are flooding the top tray of the tower and the overhead.
You can run with lower reboiler duty and lower reflux rates if you lower the tower overhead pressure....
30/08/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Would need a sketch showing column instrumentation to analyze your question. E-mail sketch and I will try.