(117ad) Hydrothermal Oxidation of H2S Scavenging Wastewater

The removal of hydrogen sulfide (H₂S) from natural gas streams produced offshore is typically carried out by injecting chemicals (H₂S scavengers), which react with H₂S and convert it into by far less harmful species. The most common H₂S scavenger is 1,3,5-tri-(2-hydroxyethyl)-hexahydro-S-triazine, which is a water-soluble species known in oil and gas industry as MEA-triazine. It is used as a basic aqueous solution, which is dispersed into the gas stream where it leads to the absorption of H₂S and the consequent aqueous phase scavenging reactions. Downstream of the injection point, the gas is separated from a wastewater, which is called spent H₂S scavengers and contains the scavenging products, as well as any unreacted MEA-triazine. This is a basic wastewater containing a high amount of carbon, nitrogen, and sulfur, which in some cases cannot be disposed by re-injection into subsea geological formations due to its fouling and scaling propensity. In these cases, it is discharged into the sea without any treatment because of lack of feasible alternatives. This is however not unproblematic for the environment as spent H₂S scavengers exhibit a relatively high toxicity. As a matter of fact, it has been reported that the discharge of spent H₂S scavengers in the North Sea accounts for up to 20% of the total Environmental Impact Factor (EIF) associated to the entire water discharge into the sea, even though spent H₂S scavengers are typically less than 0.1% of the total discharge of produced water [1]. Therefore, the development of an efficient and low-footprint process allowing the treatment of this wastewater on topside offshore installations can provide oil and gas operators with a concrete option for further reducing their EIF.

To reach this goal, the project ZeroH2S was started at Aalborg University (AAU) in 2020 (funded by the Danish Offshore Technology Centre), with one of the objectives being the demonstration of the feasibility of hydrothermal oxidation (HTO) as a means for topside offshore treatment of spent H₂S scavengers. This presentation summarizes the results obtained so far at laboratory scale as proof-of-concept [1,2] and discusses the perspectives for the process scale up, which will be executed in 2023-2024.

Materials and Methods

Several samples of North Sea spent H₂S scavengers, collected offshore in the period 2014-2022, were characterized in AAU laboratories. The COD and TOC of the samples were in the range (120 – 320) g/L and (40 – 115) g/L, respectively, with pH in the range 8.9 – 9.6. NMR analysis showed the presence of unreacted MEA-triazine, as well as the scavenging products MEA-dithiazine and monoethanolamine (MEA). In addition, the presence of the intermediate scavenging product MEA-thiadiazine and of two hydrolysis products of MEA-triazine was also evidenced. The concentrations of MEA-triazine, MEA and MEA-dithiazine were in the range (80 – 150) g/L, (20 – 50) g/L, and (10 – 40) g/L. As can be observed, this wastewater is characterized by a high concentration of water-soluble organics, including a large amount of unreacted MEA-triazine. In addition, the ecotoxicity of this wastewater was determined on marine bacteria (Aliivibrio fischeri) and algae (Skleletonema pseudocostatum). The ecotoxicity analysis revealed that the toxicity of spent H₂S scavengers is dramatically higher than the toxicity of offshore produced water from the same installations. For example, to obtain the same level of inhibition (50% inhibition) of bacterial and algae growth in produced water and spent H₂S scavengers, the former was diluted 9 times for bacteria and 14 times for algae, while the corresponding values for the latter were >4000 and >23000.

The HTO process was tested in a custom-made batch injection 99 mL reactor allowing: (i) water and oxygen to be heated and pressurized to the desired reaction conditions; (ii) the injection of the wastewater into the hot and pressurized water-oxygen system; and (iii) the rapid ejection and quenching of the reaction products. In this way, an accurate control of the reaction time at the desired pressure and temperature conditions can be achieved. The reactor was charged with diluted spent H₂S scavengers, with initial COD in the range 23 to 65 g/kg. The process was studied at three severity levels (200 °C and 70-90 bar; 280 °C and 120-130 bar; 350 °C and 210-250 bar), for reaction times in the range 1 to 360 minutes and two levels of excess of oxygen (106-145% and 190-250% of the COD). Extensive analysis of the aqueous phase produced by the HTO reactions was carried out, including COD, TOC, Total Nitrogen, Total Sulfur, Ion Chromatography, GC-MS, SPME-GC-MS, HPLC, as well as ecotoxicity analysis.

Results and Discussion

The results show a fast degradation of MEA-triazine, MEA-dithiazine, and MEA, since these species were never found in the reaction products. The rate of COD reduction is strongly dependent on the reaction temperature. For example, at 200 °C (higher excess of oxygen and initial COD 30-32 g/kg) the COD reduction was 29% in 3 minutes, 50% in 40 minutes, while a plateau was observed around 84% at long times indicating some persistent species that could not be oxidized. For other things being basically the same, at 350 °C the COD reduction was 71% in 1 minute, 85% in approx. 10 minutes, with the residual COD after 2 hours being only 2% of the initial value. Since the implementation of low footprint solutions is critical in topside offshore operations, the results strongly suggest high severity conditions to minimize the volume of the continuous-flow HTO reactor to be installed offshore.

The analytical results show the presence of pyrazines, pyridines and short chain carboxylic acids as intermediate oxidation products. In most cases, the concentration of these species increases over time, reaches a maximum, and then decreases. The reaction times corresponding to the maximum concentrations depend on the reaction severity and on the species under consideration. For example, focusing on a species of concern due to high toxicity, at 350 °C the mass fraction of pyridine reached a maximum of 333 mg/kg after 10 minutes, while decreasing to 12 mg/kg after 120 minutes. In all cases, at high severity all identified intermediate oxidation products exhibited a maximum concentration within 20 minutes, with a trend toward full carbon conversion to CO₂, nitrogen conversion to ammonium and, to small extent, nitrate, and sulfur conversion to sulfate. Therefore, the analytical results also suggest high severity conditions as favorable to reduce the discharge of water-soluble organics.

The ecotoxicity measurements show higher toxicity reductions at higher temperatures, albeit the reduction of ecotoxicity is in general somewhat lower than the corresponding reduction of COD, the trends of COD and ecotoxicity do not always show similar patterns, and the ecotoxicity data cannot be directly inferred from the concentration trends of individual reaction intermediates.

Overall, the results indicate that substantial ecotoxicity reductions can be obtained by HTO of spent H₂S scavengers (above 90% reduction on marine bacteria at all conditions, up to above 80% reduction on marine algae at 280 °C and 350 °C). The results also indicate that the HTO reactor design should not be based on COD measurements only, but both chemical and ecotoxicity analysis should be considered.

Further Development

In 2023-2024 the process is going to be tested in continuous-flow mode of operation on a scale in the order of 1 L/h, with the aim of confirming the results obtained in batch-mode. In addition, the continuous-flow mode of operation will allow to determine the maximum COD of the feed that can be treated due to potential thermal problems in the reactor, since oxidation reactions are exothermic, which will determine if feed dilution is needed. Moreover, analysis will be carried out on the gas effluent, to verify its suitability for discharge. In case of positive results, the process will be tested onshore at a scale of 10-20 m³/day, which is the same scale as needed for an offshore installation for treating spent H₂S scavengers in the North Sea.

Literature Cited

[1] N. Montesantos, L.M. Skjolding, A. Baun, J. Muff, M. Maschietti, Reducing the environmental impact of offshore H₂S scavenging wastewater via hydrothermal oxidation, Water Research 230 (2023) 119507.

[2] N. Montesantos, M.N. Fini, J. Muff, M. Maschietti, Proof of concept of hydrothermal oxidation for treatment of triazine-based spent and unspent H₂S scavengers from offshore oil and gas production, Chemical Engineering Journal 427 (2022) 131020.