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In our DHT plant we are preocessing Light gas oil and Heavy gas oil. Diesel product 95% recovery maintain at @355degc. Diesel product density maintain @822. We are optimising kerojet production over diesel in condensate fractionation point. As we draw more kerojet at condensate fractionation point Diesel become little heavy. After reactor outlet in stripper we are facing problem of less reflux and resulting in higher overhead temperature. Our diesel product total sulphur content is maintain in range of 4-6 ppm. How to increase reflux flow in stripper with kerojet maximisation at condensate fractionation unit? Is there any relation between total sulphur of diesel product and stripper reflux flow?
 
Answers
15/03/2018 A: Emmanuel Aunde, Refining Company, lemma012@yahoo.com
Your experience is as expected. Drawing more kero will lead to more hot vapour returning to the fractionator at the kero draw zone. This will increase the temperature of the column from the kero draw off to the column overhead. If the heat duty of your overhead condenser is not increased, you will generate less liquid in the receiver. If you maintain a constant wild naphtha draw off and pressure, the you will observe an increase in the off gas flow rate as well as a reduction in reflux flow. Simply increase the heat duty of your overhead condenser to maintain the same outlet temperature and you will be fine.
09/01/2017 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
You should be able to simply work the column harder by starting with increasing reboiler heat input.
07/01/2017 A: Ganesh Maturu, Self, maturu.ganesh@gmail.com
This is the common problem in hydrotreating units. By adjusting some parameters, we can increase stripper reflux.
1. If there is net naphtha flow from stripper overhead receiver, reduce the net overhead liquid from stripper top. Most of the places, maintaining total reflux will solve the problem if naphtha and lighter make is very less in DHT unit especially during SOR conditions. If you still face the lower reflux flow, follow below steps.
2. If the stripper overhead condenser is air cooled and if it does more cooling like its temperature is less than 55-60 deg C, Optimize the air cooler duty by stopping some fans and maintain air cooler outlet temperature at 55-60 deg C. Too much cooling of stripper overhead vapor ie <50 deg C, reflux cools the stripper vapor and overall circulation flow/reflux flow and hence is advisable to maintain stripper condenser outlet temperature between 55-60 deg C.
3. I assume you are optimizing the stripper pressure by maintaining minimum value by floating to stripper overhead gas battery limit pressure. If there is margin, reduce the stripper top pressure.
4. Increase stripping steam if there is margin available in stripper overhead dewpoint margin. Increasing stripping steam reduces the partial pressure of lighters further and increases separation and hence increases reflux flow. Make sure that you maintain dew point margin of 25 deg F as increasing stripping steam reduces dew point margin.
5. If you can increase stripper feed temperature, increase stripper temperature that will increase vaporization and increases overhead reflux.
6. By doing all the above steps, if it still shows less than minimum reflux, providing external naphtha stream at reflux drum will definitely solve the problem. When you maintain total reflux, you may not need external naphtha. However, providing external naphtha to reflux drum is alternate solution.
05/01/2017 A: Sarath Konda, DuPont India, sharathsatya@gmail.com
You can lower your stripper pressure slowly to improve the reflux rate, but also need to cautiously watch the wild naphtha flow. I also don’t see any relationship between product sulfur and reflux flow, the whole objective of stripper is to remove H2S and improve flashpoint of diesel. You can meet the same objective with less reflux rate also. In typical cases where feed is heavy top tray feeding is one of the best options available, but that requires physical change in the system.
04/01/2017 A: Mohan Shyam Pathak, UOP A Honeywell Company, mohanspathak@gmail.com
I suggest increasing the stripper bottom heat input. Increased heat in reboiler will result into increased vapor traffic and in turn the reflux flow rate will increase. The overhead condenser fan should be adjusted to handle increased vapor flow towards it.
If the stripping is not sufficient H2S will remain dissolved in Diesel liquid and hence the total sulfur content will be higher in the diesel product. Lower reflux flow is not always an indication of improper operation. The Reflux flow should be maintained enough to ensure complete stripping of H2S content.