Scavenging Alkyl Mercaptans: Elucidation of Reaction Mechanisms and Byproduct Characterization

Abstract

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.

Oxazolidine-Based H2S and Mercaptan Scavengers: Uncovering the Myths

“…The gas breakthrough test is routinely used to determine the efficacy of H 2 S scavengers. The principle of this test has been described previously and involves passing H 2 S of suitable concentration in a carrier gas, most often nitrogen for dilute H 2 S mixtures and/or carbon dioxide for concentrated H 2 S mixtures. , The gas enters a column with a specified volume of scavenger and is dispersed by a glass sinter at the inlet of the delivery tube. H 2 S is removed from the gas stream, and the exiting gas passes through the detection device, which records the concentration of H 2 S. During the experiment, the scavenger solution will, at some point in time, no longer be able to remove all of the H 2 S and the detector will start to see H 2 S emerging from the column.…”

“…To further explore the properties of oxazolidine-based scavengers, MBO and US-MBO were additionally evaluated using methyl mercaptan in place of hydrogen sulfide. It is generally regarded that H 2 S scavengers also scavenge alkyl mercaptans but to a lesser degree of efficiency . By using a feed gas that contained 3000 ppm H 2 S and CO 2 as well as a feed gas that contained 3000 ppm methyl mercaptan plus 6000 ppm CO 2 , the capacity of both MBO and US-MBO for both H 2 S and methyl mercaptan was determined.…”

“…Commercial products include alkaline sodium nitrite, chlorine dioxide, hydrogen peroxide, and per­(oxy)­acetic acid. − Nonoxidative scavengers chemically react with H 2 S converting it to an inorganic sulfide or more commonly to an organic sulfide where new, and often irreversible, carbon–sulfur bonds are formed. By far the most common nonoxidative scavenger family used are the triazines with hexahydro-1,3,5-Tris­(hydroxyethyl)-s-triazine or monoethanolamine (MEA) triazine being the most ubiquitously used. − …”

Development of a Raman-Based Test for Rapid Thiol Scavenger Detection in Oil and Gas Applications

“…They can be applied in batches in a liquid or solid form inside bubble towers or packed bed scrubbers or via continuous direct injection into the produced fluids. This scavenger type includes several known H 2 S sequestering agents such as triazines, glyoxal, , oxazolidines, acrolein, and hemiformals . Moreover, less traditional compounds, including hydantoins, ionic liquids, and sulfide quinone reductase (SQR) enzymes, have also been successfully used to remove thiols from hydrocarbon streams.…”

Synthesis, Reaction Byproduct Characterization, and Mechanistic Understanding of Hemiformal Hydrogen Sulfide Scavengers: Part 2─Identification of Byproducts, Reaction Mechanisms, and Suppression Thereof

“…The concept of a hemiformal scavenger is to use a material that has formaldehyde chemically bound in a more latent form that can be released upon demand to the nucleophilic H 2 S molecules. Hemiformals are a major class of formaldehyde releasers and have been widely employed for scavenging H 2 S. ,− The concept for such scavengers is that, while they can deliver the chemically bound formaldehyde for scavenging purposes, the free formaldehyde level is, at any given time, extremely low and under the control of Le Chatelier’s principle, as described in the equilibrium of Figure . A close analogous reaction to hemiformal H 2 S scavenging is that of H 2 S with formaldehyde, which has been studied as early as 1890 when Bauman initially proposed that the first formed species in the subsequent reaction was hydroxymethylmercaptan (or hydrated thioformaldehyde), as shown in structure 1 of Figure .…”