(117ag) Hydrothermal Oxidation of Spent H2S Scavengers in a Continuous-Flow Reactor: Preliminary Results

The most common process for removing H₂S from natural gas in offshore oil and gas installations consists in: (i) injecting and dispersing an aqueous solution of MEA-triazine into the natural gas stream, allowing H₂S to react with MEA-triazine to form dithiazine (DTZ) and monoethanolamine (MEA), which are by far less harmful than H₂S; (ii) separating the aqueous phase, resulting from the H₂S scavenging process, from the gas. This aqueous phase is a wastewater mainly containing unreacted MEA-triazine (unspent scavenger), DTZ and MEA (spent scavengers). In oil and gas practice this aqueous phase is known as spent H₂S scavengers and it is, in many cases, discharged into the sea without any treatment due to lack of feasible alternatives because of its fouling and scaling potential. The discharge of spent H₂S scavengers into the sea is detrimental to the environment due to the high toxicity of this wastewater [1].

In the research project ZeroH₂S, run at Aalborg University and funded by the Danish Offshore Technology Centre (DOTC) as part of the research program “Produced Water Management”, it was proved that unreacted MEA-triazine can be recovered from the spent scavengers by means of nanofiltration [2], while the ecotoxicity of the spent H₂S scavengers can be drastically reduced by applying hydrothermal oxidation (HTO) prior to discharge [1,3]. The two processes were proved at laboratory scale [1-3].

Based on the obtained proof-of-concept, a new project was funded by DOTC, called “Zero H₂S – DEMO”, which aims at demonstrating the recovery of MEA-triazine and the feasibility of HTO on the spent H₂S scavengers in continuous-flow mode of operation and on a larger scale. This new project is run by AAU in collaboration with Aquarden Technologies ApS.

This work presents preliminary experimental results of the application of the HTO process in continuous-flow mode of operation. The experimental campaign is starting in April 2023.

Materials and Methods

Spent H₂S scavengers collected in the North Sea from one offshore oil and gas installation will be used as feed for the HTO process. Based on analysis carried out in previous projects at AAU on spent H₂S scavengers collected in the period 2014 – 2022, it is known that this wastewater has an extremely high content of water-soluble organics, with COD and TOC in the range (120 - 320) g/L and (40 - 115) g/L, respectively, and pH values in the range 8.9 - 9.6. The application of NMR analysis confirmed the presence of a large amount of unreacted MEA-triazine, together with the reaction products MEA, MEA-thiadiazine, and MEA-dithiazine, as well as some by-products deriving from the hydrolysis of MEA-triazine.

The HTO tests will be carried out in a continuous-flow reactor available in Aquarden. Two feeds will be used: (i) the spent H₂S scavenger wastewater as it is; and (ii) the spent H₂S scavengers downstream of the membrane separation and recovery of MEA-triazine. The feed flow rate of the continuous-flow reactor will be in the order of 1 L/h. The oxidant will be oxygen-enhanced air. The reactor will be operated at approx. 250 bar. Two reaction temperature levels will be investigated: a lower level (around 280 – 300 °C) and a higher level (around 350 - 360 °C). The values of the wastewater flowrates will be varied to achieve different residence times in the reactor. The oxygen-enriched air flowrate will be set to guarantee the process is run under excess of oxygen, with respect to the COD. An important aspect of the experimental campaign will be the evaluation of the maximum COD level of the feed that can be handled safely, considering that this wastewater has a large amount of organics and oxidation reactions are highly exothermic. This is critical information as it will determine whether dilution is required for the HTO feed in the offshore process.

The aqueous phase produced in the HTO process will be characterized by means of COD and TOC analysis, chemical characterization, and ecotoxicity, in line with the procedures developed in a previous work [1].

Extensive analysis of the aqueous phase produced by the HTO reactions will be carried out, including COD, TOC, Total Nitrogen, Total Sulfur, Ion Chromatography, GC-MS, SPME-GC-MS, HPLC, as well as ecotoxicity analysis, according to procedures developed in previous works [1,3]. Moreover, analysis will be carried out on the gas effluent, to verify that the gas is constituted only by oxygen, nitrogen, and carbon dioxide, i.e., not containing any toxic species which could be a showstopper for the industrial implementation of the technology.

Results and Discussion

Results will be available after the experimental campaign scheduled from April 2023 to September 2023.

Implications of the Results.

The results will be used for making the basic design of a prospective HTO unit to be installed offshore.

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] M.N. Fini, N. Montesantos, M. Maschietti, J. Muff, Performance evaluation of membrane filtration for treatment of H2S scavenging wastewater from offshore oil and gas production, Separation and Purification Technology 277 (2021) 119641.

[3] 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.