Supercritical water oxidation as a technology for the treatment of model and industrial wastewaters: reaction kinetics and reactor configurations

This work investigates the advantages of a multi-stage supercritical water oxidation (SCWO) reactor over a single-stage configuration in treating dimethylformamide (DMF), a model compound representing nitro-organic wastewaters. Single-stage SCWO of complex wastewaters was also investigated. A PFR rig was designed and constructed to investigate reactor temperature, initial DMF concentration, stoichiometric ratio, residence time and oxidant distribution effects on component yields. Reaction temperature was the critical variable for treatability; T>500$^0$C caused near-complete DMF/TOC removal at relatively short residence times (approx. 6 s). DMF SCWO displayed Arrhenius-type kinetics, and the DMF (1) and O$_2$ (0.36) reaction orders, activation energy (140 kJ mol$^-$$^1$) and pre-exponential factor (1x10$^1$$^2$ M$^0$$^.$$^3$$^6$ s$^-$$^1$) were evaluated. Certain injection configurations resulted in higher TOC removals than single-stage, particularly when the second injection occurred at 0.5L and delivered 50 – 67% of the oxidant, although ammonia yield exceeded those in single-stage SCWO at these points. Single-stage SCWO outperformed a small number of configurations. T>500$^0$C was shown to be suitable to treat complex wastes. Complete conversion of TOC to products likely occurs within 10$^2$ s. It was seen that TOC value alone may not be a suitable input when attempting to determine treatability in SCWO and that composition must also be considered

Supercritical water oxidation for the destruction of hazardous waste:better than incineration

Supercritical water oxidation (SCWO) is an advanced process mainly employed for the treatment of hazardous stable wastes, otherwise treatable by incineration. It is based on the unique properties of water above its critical point ( T c =675 K, P c =22.2 MPa), making it a superior reaction medium for the destruction of all organics in the presence of oxygen. This work presents preliminary laboratory scale studies on SCWO of nitrogen (N)-containing hazardous hydrocarbons, with a view to enhancing the process performance, using available reagents and non-complex reactor design. This article investigates the destruction of dimethylformamide (DMF), carried out in a continuous (plug flow) reactor system. SCWO of DMF was enhanced by (i) a split-oxidant system, where stoichiometric oxidant was divided between two inlet ports at various ratios and (ii) the addition of isopropyl alcohol (IPA) as a co-fuel, premixed with the feedstock. Testing a range of temperatures, initial DMF concentrations, oxidant ratios, IPA ratios and oxidant split ratios, selected results were presented in terms of % total organic carbon and % N removal. Reaction kinetics were studied and showed a dramatic decrease in the activation energy upon adding IPA. Split-oxidant-feeding enhancement depended on the split ratio and secondary feed position. </jats:p