2 replies to this topic

[Megamass]

Dear all,

In order to follow up quality of prewashing caustic on our LPG Merox unit, we are performing analyses of LPG stream inlet and outlet from prewash vessel. LPG stream is analysed for H2S and RSH content and prewash caustic is analysed for alkalinity and % spent. All of the analyses are performed potentiometrically.

LPG inlet analyses are never showing the presence of H2S. The problem is with the analyses of RSH content that is higher in outlet stream than in LPG inlet, and this is happening sometimes even when the prewash caustic quality is good (high alkalinity, low % spent). In theory, RSH content should rise in outlet stream only when the caustic is high in RSNa content and when the H2S is present in LPG stream and the reaction occures:
RSNa+H2S=RSH+Na2S
(H2S is stronger acid than RSH)

Does anyone face similar problem? Is it possible that some of the H2S still passes unnoticed? Is it possible that entrained caustic can imposse problems with RSH content analyses in LPG?

Our laboratory personnel are also claiming that when the potentiometric analyses of caustic are performed, kind of yellow slug is precipitated on the electrodes and this affects good measurement. Any experience with this?

Thanx in advance!

[Babu Prasad]

[Hello friend!
Presence of H2S in the feed will continuously reduce caustic concentration due to formation of non-regenerate salt (Na2S). This will affect the Mercaptan Extraction in LPG extractor. Therefore a Pre-wash is provided before Extraction Unit. It may be, you are using the extraction rich caustic for the makeup of pre-wash vessel. Extraction rich caustic may have the catalyst which may induce some extraction of RSH from LPG. More over RSH will get attached with LPG ratherthan caustic in pre-wash vessel.
1) Try to dump all pre-wash caustic and make-up fresh caustic and check for RSH content in outlet of pre-wash
2) Do the analysis for catalyst content in pre-wash caustic. it is possible to check but I don’t know the method.
Regards
Babu Prasad

[GAURANG PATEL]

I think Mr.Babu Prasad is correct.
Dont go for Pre-wash caustic change out with Regenerated caustic.
Do change out with fresh caustic.
Check out for COS in your feed LPG as well as in Caustic Pre wash inlet LPG.
COS may be reason behind formation of H2S in LPG even after removal by Amine absorber & caustic Pre wash.