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We are designing an LPG sweetening unit. The sour LPG consists of H2S, methyl mercaptans, ethyl mercaptans, propyl and butyl mercaptans , COS as the sulphur impurities. To remove H2S we are using amine absorption tower using MDEA solvent. Then it is followed by caustic wash for mercaptan removal. We observe that butyl mercaptan is not removed effectively from caustic wash. The caustic wash circulation has to be increased to a very high unreleastic values to achieve 10ppmw sulfur at the downstream of caustic wash. Can you please inform on the various options for butyl mercaptan / H2s levels of sour LPG after amine absorption :
(in ppmw) Sour LPG
Methyl Mercaptan : 0.966
Ethyl Mercaptan : 6.877
Propyl Mercaptan: 12.529
Butyl Mercaptan: 108.822
Hydrogen Sulfide: 15.000
Carbonyl Sulfide :36.316
07/06/2019 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
Butyl Mercaptan is very unlikely to be present in LPG streams because it has a boiling point of close to 100 C. This does not match LPG. Butyl mercaptan should be in the gasoline phase.
06/06/2019 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
Best possible way would be to use Zeolite adsorption column.Little experimentation needs to be done to select the best suitable Zeolite for the application. You may then optimize the caustic wash. High strength and high surface area Zeolite > 230 should be used for better results.You may also try GAC followed by Zeolite if Zeolite alone is found inadequate. Zeolite would probably the most practical and economical solution. The treatment is to be used after caustic wash
05/06/2019 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
The butyl mercaptan content is extremely high (~8 times the propyl mercaptan), Is the LPG composition including C5's? It is not typical for C3-C4's LPG to have this high amount of Butyl mercaptan.
One explanation for the high butyl mercaptan (besides wrong lab results) is the debutanizer having flooding issues which allow heavy mercaptans to show with LPG. Have you check the debutanizer performance?
To maximize butyl mercaptan removal you need to adjust caustic concentration and circulation.