Scavenging Alkyl Mercaptans: Elucidation of Reaction Mechanisms and Byproduct Characterization

The removal of alkyl mercaptans via chemical means in the oil and gas industry has received very little attention compared with the much more extensive work on hydrogen sulfide (H2S) scavenging. The removal of H2S from produced liquids, aqueous, hydrocarbon, and gas is a vitally important part of the oil and gas industry. Typically, liquid chemical scavengers are employed for H2S levels up to approximately 5000 ppm. Above this, sequestering-type methods are employed such as amine recycling units from which the H2S can be regenerated and used most typically to produce elemental sulfur. It has been known for some time that mercaptan scavenging is possible but anecdotally reported as being less effective than H2S scavenging. This current study seeks to improve the understanding of the efficacy and mechanism of mercaptan scavenging by the common H2S scavengers in use today. A thorough literature review is provided of the current state of the art in mercaptan scavenging is provided. For the first time, a systematic series of quantified scavenging performance results for a variety of prominent scavengers with volatile alkyl mercaptans have been presented using the well-known autoclave method. These results are directly compared with the corresponding values for H2S under identical reaction conditions; overall, the best product was 1,3,5-trimethylhexahydrotriazine (MMA-triazine). The chemical structures of the reaction byproducts with alkyl mercaptans, to date only suggested but never confirmed, have been proven by the application of rigorous analytical methods. Results for both MMA-triazine and 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (MEA-triazine) reactions with ethyl, propyl, and butyl mercaptan comprehensively show, via the use of GCMS(-EI) analysis, representatives from each step in the sequential reaction pathway, containing each 1, 2, and 3 nitrogen atoms showing definitive evidence for the proposed reaction pathway.