Q & A > Question Details
Our crude vacuum distillation column overflash pump suction strainer gets fequently full of coke.
Overflash pump suction temperature is 374℃~385℃, flash zone pressure 21mmhg, top pressure 5mmhg.
Metal contents of overflash are 154.7ppmw(Ni 36ppm, vanadium 118.7ppm) and Metal contents of Vacuum Residue are 223.8ppmw(Ni 49.2ppm, Vanadium 174.6ppm). How can we prevent this?
16/05/2016 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Not familiar with the word Overflash in vac column. Is it the internal reflux of Slop wax / dirty wash oil (DWO) ???. Coking of slop wax bed is not an unusual phenomenon. Coking is caused by dry spots in the beds (a result of poor bed wetting / vapor channeling thru the packed bed / entrainment due to poor liquid disengagement in flash zone). Good bed wetting can be achieved by the correct selection of spray nozzles for reflux distribution headers that ensures no dry spots in the packed beds. Heavy Crude processing units are being provided with channel distributors (Gutter distributors) similar to those installed in Coker fractionators nowadays and that prevents the spray nozzle choking up with coke particles / prevent coking due to channeling of reflux flows from the spray nozzles caused by the pressure fluctuations etc., Installation of good feed mix distributors (vapor vane / Schoepentoters in flash zone ensure good liquid disengagement.
14/05/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The metals are very high. I assume the flash zone pressure is O.K. That would mean that the coil outlet temperature is not too high. The issue would then be the design or operation of the wash section. Not good contact of fresh wash oil with the vapor or insufficient wash oil flow rate. Poor contact could be caused by poor design or channeling due to coked packing. Did you have this problem after clean startup or after a few months?
12/05/2016 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
I agree with the answer posted by Celso Pajero. Check the residence time of liquid on chimney tray. higher residence time on chimney tray will lead to coke formation.
Metal content in slop wax is quiet high. As metal and asphaltene travels with liquid i.e. entrainment from feed to the chimney tray. there can also be a possibility that there is high entrainment from feed to wash bed but entrained metals and coke are coming to chimney tray becasue you have enough wash oil rate to de-entrain it and then it is going with a slop wax.
check the chimeny tray residence time, try to make the chimeny tray with slop and check the feed nozzle velocity and momentum.

12/05/2016 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Deep cut operations require elevated temp (vac furnace COT) and in turn higher draw off temp for slop wax bed. However, to ensure operational reliability (hassle free run of vac unit for 5 to 7 years between turnaround) it is imperative to restrict slop wax pumping temp and residence time. Modern designs provide external vessel for slop wax collection from vac flash zone and has cold recycle to reduce temp of the drum to less than 340 deg C. Another aspect is to reduce the entrainment of asphaltenes into flash zone vapours by having good VL disengagement system. Take with your process design consultants.
11/05/2016 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
One potential cause is liquid with high residence time in the overflash collector tray. Please verify the collector tray is sloped, second calculate the residence time in the collector and pump suction piping , If the vapors coming from the flash zone are between 400 C, the residence time needs to be below 100 seconds to minimize coke.
Other possibility is the grid or packing in the wash section is already coked and coke pieces are falling into the collector tray and going to the pumps.
11/05/2016 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
One way is to change the design of the draw system. The key is to minimize residence time at temperature. There are things which can be done with both the layout and the control of the system to help with this.
11/05/2016 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
You do not disclose what crudes you are running, but the metal content is quite high. If the overflash temperature is 374-385°C, then your coil outlet is probably north of 400°C. I am not surprised by the coke formation at these temperatures. You could reduce the furnace temperature, although this will reduce your cutpoint and increase resid yield. Simultaneously increasing stripping steam while reducing flash temperature could improve this, but there will be an impact on the overhead system. Finally, you do not provide any details on the wash bed configuration. There may be issues therein with getting good wash distribution. I would advise you consult with a vacuum column design expert.