Hydrothermal oxidation of MEA-triazine spent and unspent H₂S scavengers in continuous-flow reactor

The removal of hydrogen sulfide by injecting MEA-triazine in produced gas generates a wastewater (spent and unspent scavengers: SUS) that in offshore practice is often discharged untreated into the sea, thereby contributing largely to the environmental footprint of oil and gas fields. In this work, the feasibility of Hydrothermal Oxidation (HTO) of SUS in continuous-flow mode of operation (SUS flow rate 0.48–0.65 L/h) was investigated in a reactor operated at 24 MPa and 325–350 °C, for residence times of 9–23 min. Four SUS samples were used: two from a North Sea installation and two related samples obtained by nanofiltration. The study proved the feasibility of HTO for feeds with chemical oxygen demand (COD) up to 168 g/L, under autothermal conditions. Higher COD values proved problematic for temperature control under near-critical water conditions. The COD and total organic carbon (TOC) of the liquid effluent were reduced by 91 %-99 % and 86 %-100 %, respectively, while the ecotoxicity was reduced by more than 99 % and 94 %-98 % towards marine bacteria and algae, respectively. The effect of the feed COD on the buffer capacity of the reacting system is discussed, highlighting its importance in preventing pH drops within the HTO reactor. Increasing the pH of the feed proved necessary for the SUS permeate feeds, and positive for reducing carbon monoxide in the gas effluent for all feed types. The main reaction products in the liquid effluent were ammonium, sulfate, and acetic acid. An extensive analytical characterization of both the liquid and gas effluent is provided.