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When our VDU column products, vacuum slop and vacuum residue were tested for H2S it showed H2S presence >10 ppm.What could be the possible reasons for high H2S?
We are maintaining VDU bottom level and temperature low to avoid cracking still H2S reported is high.Coil steam and Velocity steam were increased and kept 120% of the PFD values. Chances of exchanger leak was also checked and observed no leak.
 
Answers
21/05/2019 A: Prabhakar Reddy, Suncor Energy Inc, preddy@suncor.com
You do get cracking in the vacuum furnace, which contributes to the H2S in vac slops.
19/02/2019 A: John Bettinger, Qualex Engineering, john.bettinger@qualex.com
Until the temperature drops below the cracking temperature, H2S will continue to evolve. At one refinery we had to temporarily run VR to tankage. The vent was extremely sour. We sampled the cooled VR and tested it in the lab by heating it up and pulling Dragers. When it got to VTB temp, significant H2S evolved. We had to use H2S scavenger on the VR tank to get the level down so that the VR could be barged away.
19/02/2019 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Presence of reactive sulphur (Mercaptans specially) in vac products can create such. It is not mentioned what test method you use to detect H2S in slop and vac residue. Any heating even just above 145deg C can lead to such H2S presence owing to reaction while heating. Check crude diet change and as such crudes like Rasgharib is known to have such occurrence. Also, you may correlate with ejector off gas H2S level to witness such.
18/02/2019 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Many sulfur compounds found in crude oil are less thermally stable than the crude oil hydrocarbons. H2S can start to evolve from crude oil at relatively low temperatures and will continue as the oil is heated. If you did a GC measurement on the vacuum tower off gas, you would find a high % of H2S in it as well. There are tests that can be done to quantify the amount of H2S generated vs. temperature for crude oil. The amount generated is crude specific.