In the C3 to merox stream, is it possible to convert some Methyl Mecaptans into DiMethyl Disulfide in the Amine absorbor before the Prewash or the Extractor?
I doubt that it can be done. In any event, why would you want to do that? The disulfide would contaminate the amine solution and end up fouling the regenerator. Or worse yet, if any of the disulfide somehow managed to get into the regenerator overhead, it would mess up the Claus sulfur plant product.
Milton Beychok
(Visit me at www.air-dispersion.com)
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I don't want this to happen in the Amine Absorber. I want this to happen in the Oxidizer, but our Deprop Overhead has no Dimethyl Disulfide in it but between the Amine Absorber and the Prewash the Methyl Mercaptans decrease and we gain Methyl Disulfides, (MM=250 & DMDS=0 before Absorber and after MM=200 and DMDS=50. They then end up in the C3 after the dryer sample going to stg.
That is strange to say the least. I know that you have probably checked everything, so excuse me for asking some quesions:
(1) I assume that your liquid C3 is sent through an amine absorber (presumably to remove H2S), then through a caustic prewash column (for further removal of H2S), then through the Merox extractor, then through a caustic settler (to remove any entrained Merox caustic from the extractor), and finally through a water wash (to remove any residual Merox caustic) and a dryer of some sort (maybe a salt bed) to remove water. Am I correct in this? If not, please spell out your flow scheme.
(2) Is there any other stream that joins the C3 stream between the absorber and the caustic prewash?
(3) Have you checked to see whether the lean caustic (from the Merox unit disulfide-caustic separator) is perhaps recycling some entrained disulfides into the top of the Merox extractor?
(4) Where is the sample point that shows gain in disulfides? Is it truly between the amine absorber and the Merox extractor? Or is it somewhere else?
(5) Are you positively sure that the laboratory analyses are correct?
Again, excuse me for asking questions that you may already have looked into.
Milton Beychok
(Visit me at www.air-dispersion.com)
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1) The Disulfides are showing up between the Amine Absorbor and the Caustic Prewash. There is no increase of disulfides after the extractor either, (no re-entry).
2) I'm running the samples and I have standards that tell me that these peaks times are correct and identifed correct also. We have also have used new sample bombs and taken the whole stream, (Deprop ovhd to Amine Absorb, after Amine Absorbor but before Prewash, after Prewash but before Extracrtor, after Extractor before Sand Filter and C3 to Stg.) all at the same time 3 or 4 times now. All bombs are Teflon lined. The C3 feed to the C3 Amine Absorber results show H2S and Methly Mercaptan only, no trace of Dimethly Disulfide. After the Amine Absorbor but before the Caustic Prewash sample results confirm that ther is no H2s, the amount of Methly Mercaptan decreases by about 1/4 parts and now we are seeing appox. 1/4 parts Dimethly Mercaptan.
3) I'm just a lab tech. just for the record not a engineer.
4) I was an operator years ago in the Crude and Sats gas units and if my memory serves me correctly, your assumtion on the flow of the unit sounds correct to me.
5) There are no tie-in lines between the Abs. & the Prew. The only thing that feeds this C3 merox is the ovhd of the depropanizer????????(Could this deprop ovhd stream have too much methly mercaptan? Maybe the deprop tower is not correct? Maybe they should allow more Methly Mercatans to fall to the btm of the deprop and be fed to the depent. At this point, in the Depentizer the Methly mecaptans would go overhead with the C4s and then to the C4 Merox Train. The C4 merox train has no Amine absorbor because there is no H2S at this point, just a prewash and a extractor. We never had this problem before. Just thinking out the box here. Maybe we never allowed this much Methly mercaptan to go to the C3 merox stream?
6) You don't have to excuse yourself, I'm the one asking for help here.
7) Lets just say samples before the extractor and after the extractor confirmed, that the extractor is extracting all of whats left of the methly mercatan in the C3 stream. The problem is there is no way to remove the disulfides if they are in the C3 stream feeding the merox or if some how this reaction from mercaptans to disulfides happen before the extractor removes the mercatans and brings them to the oxidizer to get reacted and then removed in the diseperator. The disulfides will tend to ride through with the propane and not go with the caustic in the extractor if they already exist before the extractor.
