Formation, Chemical Characterization, and Oxidative Dissolution of Amorphous Polymeric Dithiazine (apDTZ) during the Use of the H₂S Scavenger Monoethanolamine-Triazine

Abstract

Hexa-1,3,5-hydrotriazines form the main product class used for chemical hydrogen sulfide (H2S) scavenging. The reaction mechanism between these triazine species and H2S is discussed in detail with the emphasis laid upon the reaction products and their fate. The paper goes on to then describe a novel method of oxidative dissolution of these reaction products, including a full analysis of the resultant species and a mechanistic postulation. The single reaction product of monoethanolamine (MEA)-triazine (1,3,5-tris­(2-hydroxyethyl)­hexahydro-s-triazine) has been repeatedly found by the authors to be initially monomeric 5-(2-hydroxethyl)-hexahydro-1,3,5-dithiazine which, when separated out of aqueous solution, invariably polymerizes to an insoluble, solid polymeric species. This solid product is referred to as amorphous polymeric 5-(2-hydroxyethyl)-hexahydro-1,3,5-dithiazine, abbreviated as apDTZ. The occurrence of this material in oil production systems causes heavy deposits or fouling in pipelines, valves, chokes, and turbine blades. Its removal by any means other than physical intervention is extremely challenging. The current work shows how the presence of a terminal hydroxyl functionality is critical in enabling the dithiazine to polymerize to form the apDTZ. This work goes onto dispel previous misconceptions in the industry and the literature regarding this process which is finally systematically addressed. Specifically, two very important issues are dealt with in this work which were previously unresolved in the literature. An explanation why the thiadiazine reaction product (first sulfur molecule substitution) from tris­(2-hydroxyethyl) triazine (MEA-triazine) is never observed. Following upon the above explanation, why the dithiazine (second sulfur molecule substitution) in all cases never progress to the trithiane (third sulfur molecule substitution). This is probably the greater misconception in the industry and literature regarding triazine and H2S reactions. Despite the widespread occurrence of apDTZ in the oil and gas industry, there are very few studies of effective methods for its removal. This study presents such a process.

Experimental Study of the Aqueous Phase Reaction of Hydrogen Sulfide with MEA-Triazine Using In Situ Raman Spectroscopy

“…It is highly corrosive to metals and poses a serious health and environmental threat because of its high toxicity. Its concentration in export oil and gas streams must be within allowable limits, which often requires its removal from the produced fluids at the oil and gas separation facilities near the wellheads . With regard to natural gas, export specifications typically require a maximum concentration below 4 ppm…”

“…One of the methods available for the removal of H 2 S from natural gas streams is the direct injection of H 2 S scavengers, which are chemicals reacting with H 2 S and transforming it into substantially less toxic and corrosive species . The applicability of H 2 S scavenging is limited to H 2 S concentrations in the gas typically up to approximately 5000 ppmv due to the process economics . The most common H 2 S scavengers used nowadays are 1,3,5-hexahydrotriazines, often simply called triazines, with 1,3,5-tris­(2-hydroxyethyl)-hexahydro- s -triazine (HET) being largely predominant (at least 80% of the oilfield market) due to its high scavenging efficiency, high solubility in water, and high solubility of its byproducts in water. , In particular, the injection of HET into gas streams is the preferred H 2 S scavenging method in topside process units in offshore oil and gas production due to the low footprint and simplicity of implementation.…”

“…TDZ, in turn, can also undergo protonation and reaction with HS – , leading to 5-(2-hydroxyethyl)­hexahydro-1,3,5-dithiazine (dithiazine, DTZ) and another molecule of MEA. The substitution of a third sulfur atom into the triazine ring, which would give trithiane, is instead typically not observed. ,− This is presumably attributed to the absence of a nucleophilic carbon center in DTZ susceptible to an attack by HS – , as opposed to TDZ . Thus, from a stoichiometric standpoint, 2 mol of sulfur from H 2 S can be converted into DTZ using 1 mol of HET, with the liberation of 2 mol of MEA.…”

“…Its concentration in export oil and gas streams must be within allowable limits, which often requires its removal from the produced fluids at the oil and gas separation facilities near the wellheads. 1 With regard to natural gas, export specifications typically require a maximum concentration below 4 ppm. 2 One of the methods available for the removal of H 2 S from natural gas streams is the direct injection of H 2 S scavengers, which are chemicals reacting with H 2 S and transforming it into substantially less toxic and corrosive species.…”

An orthorhombic polymorph of 2-(1,3,5-dithiazinan-5-yl)ethanol or MEA-dithiazine

“…The reaction of hydrogen sulfide with the MEA-triazine (1) scavenger has been described, see Fig. 1 (Bakke et al, 2001;Wylde et al, 2020). Furthermore, while hydrogen sulfide can theoretically react with a third molecule to yield a trithiane molecule, this reaction does not proceed under most conditions.…”