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I am currently working in diesel hydrotreater plant. The end products are naptha, kerosene and diesel.
According to the lab reports the sulphur content in naptha is 2.7ppm and that in kerosene is 0.5ppm. Kerosene being a higher molecular weight fraction should have higher sulphur content. What is the correct explanation for this?
 
Answers
08/10/2014 A: Wang Tzu Hong, Formosa Petrochemical Corporation, 424606@rffpcc.com.tw
I suggest you can analyze sulfur species by GC-SCD or GC-PFPD and you will find the answer, H2S shall be major sulfur species in naphtha.
02/02/2014 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Diesel Hydrotreater Units operate with 'trickle bed' hydraulics in the reactor and with substantial mole percent of the feed vaporized, particularly when reactor temperatures are higher.
Some of organic sulphur in naphtha can reflect stronger partition and shorter residence times of naphtha versus kerosene and heavy diesel fractions. As noted by other, some of synthetic naphtha may be cracked when forming and thus target for recombination of H2S.
29/01/2014 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Wild naphtha is produced during DHDT. The recombination reactions of light olefins with H2S forming mercaptans are known to occur in hydrotreaters. The reactions may take place even in exchangers. The olefins formed in the process quickly combine with hydrogen sulfide depending on concns. Recombination reactions are favorable at higher temperatures and have been observed often with light olefins present in naphtha cut. So we get more of sulfur in wild naphtha than in kerosine. It is true that H2S if present due to poor stripping of H2S would cause higher sulfur that could be also checked by proper tests. The type of sulfur compound whether Mercaptans or ring sulfur could be detected by sulfur specific GC detectors
27/01/2014 A: Keng Chung, Well Resources Inc., kengchung@hotmail.com
See Sok Yui's publications on hydrotreating. Very likely you are getting close to equilibrium reactions of aromatic compounds. Surprisingly the mono-aromatic compounds are more difficult to remove than multi-aromatics.
27/01/2014 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You can get mercaptan recombination reactions that occur in HDS units operating at higher temperatures. There is a tiny amount of cracking that occurs generating some olefins, which can react with an H2S molecule instead of H2, and the result is the formation of a mercaptan. This is not unusual in diesel hydrotreaters that also treat some naphtha. As others have indicated you might want to double check your naphtha stripper operation to make sure it is working properly. A copper strip corrosion test can also give you a good idea whether your naphtha S is coming from H2S or something else. H2S will cause copper strip corrosion but mercaptans won't, at least not at low levels.
27/01/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Another possibility: Unstripped H2S would be dissolved in the lighter product.
27/01/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The sulfur remaining after hydrotreating depends on the reaction rates of the sulfur containing molecules. The naphtha fraction may be contained in aromatics or other more 'refractory' molecules while those in the kero fraction are simple mercaptans.