Q & A > Question Details
My question is related with high sulphur content in LPG from crude distillation.
In one of our refineries we have detected high sulphur content in LPG from crude distillation. The scheme is as follows: Crude distillation - Gas concentration unit - Debutanizer - Amine absorber - Merox extractive. The high S content is mainly due to high dimethylsulphide (DMS) and dimethyldisulphide (DMDS). We measure high DMS and DMDS both at the entry and outlet of the amine absorber and LPG Merox. We have also seen some unexpected behaviour with these species: DMDS increase through the amine plant (DMDS higher in the outlet than in the inlet) and decrease in the Merox unit. The same with DMS. But sometimes we have also observe that DMDS decrease in the amine plant (??)
My questions are:
- What could be the origin of DMS and DMDS (synthetic / heavy crudes, slops processing...)?.
DMDS could be re-entry sulphur in Merox, but we have observe it in the inlet of amines and Merox (It seems that both compounds come with the crude)
- Could be DMDS come from oxidation of methylmercaptan in the topping, Gascon or amines (where there is not Merox catalyst) if oxygen is present in the LPG?
- If DMDS is in the crude, according to its boiling point it should end in the heavy light / heavy naphtha. Has anyone observe high DMDS in LPG in his refinery?
- Could DMS and DMDS increase or decrease in the amine plant or Merox? (I do not think so). Are these compunds partially soluble in NaOH or could be removed in the sand filter?
- DMDS could also come from the circulating NaOH in Merox plant, if quality is not good (high concentratrion of disulphide in NaOH)? What is the normal or recomended concentration of disulphides in regenerated NaOH?
- What could be the alternatives to remove these compounds? I expect that nothing can be done in amine / Merox, these compounds are not reactive.
 
Answers
22/12/2011 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
If you are getting carry-under of disulphide oil with regenerated caustic from extractive Merox unit settler, you might want to consider a high efficiency PhaseSep LL coalescer to remove gasolene rinse / disulphide oil from the Merox solution.
It's is a fairly tough separation to make as the InterFacial Tension ( IFT) is very low from our experience - say 5 to 12 dynes/cm - so your settler may well be struggling to make a good separation of the gaslene rinse/ disulphide oil from the Merox Solution.
Are you seeing Disulphide Oil carrying under back to absorber where the disulphide oil is then absorbed back into LPG sort of like a sponge absorber? If that happens then your LPG may be off spec for residue test (and maybe corrosion test also).
21/12/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Regardless of the origin of disulfides, I would expect them to join with the disulfides produced in the Merox section and go to some hydrotreater feed tank. Disulfide re-intry could be happening, as you suggest. Some applications do need a naphtha wash. Does your unit have one?
21/12/2011 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
Our experience is that this is a problem which occurs more often, but is not always identified correctly.
DMDS is normally associated with separation problems and re-entry sulphur in the Merox unit.
We have also observed cases where it seems to be formed in the amine unit.
High levels of DMDS in the feed of the amine extractor are difficult to understand. Could there be an analytical problem?
Does this refinery have an FCC unit or is it just LPG from the topping unit?
Normally extractive Merox units are installed on FCC LPG streams.
The DMDS or DMS could be formed in the topping unit, but this is unlikely. An oxygen source is required. A catalyst should not be the problem. Several metals will catalyse this formation.
DMDS or DMS cannot be removed from the LPG. It is just a hydrocarbon.