Proof of concept of hydrothermal oxidation for treatment of triazine-based spent and unspent H2S scavengers from offshore oil and gas production

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

“…The actual pH established in the aqueous phase in contact with the sour gas stream in field operational use depends on the balance between the H 2 S absorption, which tends to decrease the pH of the aqueous phase, and the scavenging reactions, which tend to increase it. Technical solutions of HET have a pH of around 11 (see Section ), while spent scavenger samples are seen with a pH of around 9 . The pH of the aqueous phase in contact with the sour gas is expected to be in the range from 8–9 to 11–12.…”

“…Monoethanolamine (MEA) is the main impurity of this solution. The concentrations of HET and MEA were determined by means of GC-FID analysis, according to a method reported elsewhere, 13 and were found to be 2.60 ± 0.06 and 2.06 ± 0.39 M, respectively. The density of the triazine solution was measured by weighing accurately measured volumes of the solution by means of an analytical balance (Sartorius 1702, accuracy 0.1 mg).…”

“…Technical solutions of HET have a pH of around 11 (see Section 2.1), while spent scavenger samples are seen with a pH of around 9. 13 The pH of the aqueous phase in contact with the sour gas is expected to be in the range from 8−9 to 11−12. Based on the rate constants determined by Bakke et al, 5 the expected loss of HET due to hydrolysis in our experiments should not be above 0.22%.…”

“…The production of MEA in the scavenging reaction with HET is the reason for the common name of this scavenger in the oil and gas industry, where it is typically referred to as MEA-triazine. 12−20 kg of the commercial product for the removal of 1 kg of H 2 S. 2 The stoichiometric requirement that can be calculated from the abovementioned stoichiometry gives 6−8 kg of the commercial MEA-triazine-based scavenger per 1 kg of H 2 S. Thus, a large stoichiometric excess of HET is actually used by offshore oil and gas operators, which is detrimental to the environment, due to the discharge of relatively large amounts of unspent scavengers into the sea, and to the operating expenditures since a large part of the injected HET is actually not reacting with H 2 S. 13 The application of a large excess of MEA-triazine is a conservative measure owing to the insufficient fundamental physicochemical knowledge of the absorption−reaction process and consequent lack of predictability of the outcome of the operation as a function of design variables (e.g., injector type, gas velocity, temperature, and partial pressures). 2,14 The lack of experimental quantitative data on the rate of the H 2 S scavenging reaction with HET is one of the factors hindering the development of physicochemical models of the absorption−reaction process, which could allow a rational design of this operation.…”

Reducing the environmental impact of offshore H2S scavenging wastewater via hydrothermal oxidation

“…The H 2 S scavenger is typically based on 1, 3,5-hexahydrotriazines (henceforth "triazines"), that are widely used in the field, and particularly water-soluble variants such as 1,3,5-tri-(2-hydroxyethyl)-hexahydro-S-triazine (HET, also known as MEA-triazine) (Kelland, 2014;Taylor et al, 2019). Since HET is typically injected in stoichiometric excess, with respect to H 2 S, it ends up partly unreacted (unspent scavenger), together with the reaction products (spent scavengers), in the wastewater stream that is separated from the natural gas (Kelland, 2014;Montesantos et al, 2022). In line with the reaction scheme generally accepted in the literature, the main scavenging reaction products that appear in the spent scavenger wastewater are monoethanolamine (MEA) and 5-(2-hydroxyethyl)hexahydro-1,3, 5-dithiazine (DTZ) (Bakke et al, 2001;Romero et al, 2021;Taylor and Matherly, 2010).…”