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Sour natural gases are usually contaminated with mercaptans. Light mercaptans (R1SH-R4SH) usually tend to concentrate in NGL, LPG or condensate streams due to similar boiling points. Contamination with mercaptans is also found in refineries when sour crude oils are processed.
The typical market specification is the Copper Strip Test 1A corresponding to about 10 ppm wt of light mercaptans. This low concentration can be achieved by means of caustic wash processes or, for lower mercaptans concentrations, by in situ conversion processes.
In caustic wash processes, light mercaptans (R1SH-R4SH) are first extracted from the liquid hydrocarbon phase by contacting the stream with a concentrated caustic solution which is then regenerated by means of low temperature catalytic oxidation, achieved by the injection of air and converting the mercaptans into disulphide oils that are further separated from the caustic solution.
At lower concentration levels, light mercaptans can be converted directly into disulphides by means of in-situ oxidation, with similar catalyzed oxidation reactions.