Supercritical Water Oxidation Conditions Research Articles

Supercritical water oxidation: a breakthrough approach for remediation TNT-contaminated pink water

ABSTRACT This study aimed to optimize the operating conditions of the supercritical water oxidation (SCWO) process for 2,4,6-trinitrotoluene (TNT)-containing pink water treatment. Lab-scale SCWO experiments were performed by varying the temperature (400–600°C), reaction time (45–180 sec), and oxidant ratio (100–300%). The performance of SCWO in terms of pink water treatment was evaluated using TNT and total organic carbon (TOC) removal efficiency, SCWO effluent toxicity, and generated by-products. The results showed that as temperature and residence time increased, the TNT and TOC removal efficiency increased due to solute-solvent reaction acceleration under SCWO conditions and long contact time between organic matter and oxidant. The optimal conditions for SCWO were identified as follows: a temperature of 500°C, a reaction time of 120 seconds, and an oxidant ratio of 150%. These conditions resulted in a TNT removal efficiency of 99.9% and a TOC removal efficiency of 93.5%. The by-products analysis results showed a relationship between operating temperature and the type of by-products produced. Various compounds such as toluene, nitrobenzene, TNT isomers, naphthalene, and simple alkane structures were formed. However, above 500°C, many of these species decomposed, giving rise to octadecanoic acid and 13- Docosenamide. The microbial toxicity test demonstrated that SCWO effluents showed no toxicity for all investigated SCWO conditions, demonstrating the superiority of the SCWO method for the toxicity removal of TNT-containing pink water.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

The Effect of Molten Phosphate on Corrosion of 316 Stainless Steel, Alloy 625, and Titanium TA8 in Supercritical Water Oxidation Conditions

The early formation of phosphate oxide formed on 316 stainless steel (316 SS), nickel-based Alloy 625, and titanium alloy TA8 exposed in supercritical water (400 °C, 25 MPa) containing phosphate, chloride, and oxygen was investigated. Phosphate corrosion products of austenitic stainless steel displayed the severest spallation. Stable phosphates oxide films were inclined to form on Alloy 625. TiO2 and Ti2O3 are the two main components of oxide films on TA8. There is a strong synergistic effect between phosphates, oxygen, and supercritical water, leading to severe corrosion. The corrosion behavior of the three alloys at the top and bottom of the reaction tube was compared. Both at the top of the reaction tube and at the bottom of the reaction tube, TA8 showed an increase in mass. 316 SS and alloy 625 showed mass gain at the top and mass loss at the bottom. The alloys' detailed molten corrosion mechanism after exposure to supercritical water is discussed.

Effects of oxidants on the degradation of tributyl phosphate under supercritical water oxidation conditions

Effects of oxidants on the degradation of tributyl phosphate under supercritical water oxidation conditions

Determining the kinetic constants leading to mineralization of dilute carbamazepine and estradiol-containing solutions under continuous supercritical water oxidation conditions

Continuous subcritical and supercritical water oxidation experiments were conducted on dilute carbamazepine- and estradiol-containing synthetic solutions used to simulate the removal of model emerging pollutants from secondary municipal effluents. The operating conditions comprised 340–500 °C, retention time of 24–453 s and a stoichiometric oxidant ratio (O.C.) between 4 and 64. The transformation of the various species was determined at the outlet and by modeling a segmented non-isothermal reaction system. Four empirical power law kinetic models were established to represent both the pollutants' degradation and TOC removal efficiencies, using nonlinear multiple regression coupled with bootstrapping and K-fold cross-validation. The mineralization and degradation models for both pollutants yielded a R2 of 67–80.5% vs. the experimental results. Discussion on the various model assumptions revealed that attributing full model deviations to the constant oxygen concentration or to the laminar reactor flow, yielded a deviation of 6% and 15% in the removal efficiencies, respectively. However, the expected deviation of the models was lower than 0.32% at conditions leading to (almost) full mineralization (45–60 s, 480–500 °C and O.C.s of 5–10). The methodologies developed in the study are useful for interpreting future results obtained from SCWO of actual secondary effluent solutions.

Decomposition of tributhyl phosphate at supercritical water oxidation conditions: Non-catalytic, catalytic, and kinetic reaction studies

Tributhyl phosphate (TBP) decomposition in supercritical water oxidation (SCWO) was performed with and without catalyst, at different temperatures ranging from 370 to 480°C, and different reaction times. Ag2O, CuO, Fe2O3, MgO, and ZnO synthesized by supercritical water were examined in TBP decomposition as the catalyst. TBP structure decomposed under the non-catalytic reaction; nevertheless, the results indicated that the use of catalysts improved the reaction efficiency, which calculated based on total organic carbon (TOC) removal, by far. However, TOC removal using Fe2O3 was 20% higher than the non-catalytic reaction. Having the ability in completion of the redox cycle, iron oxide catalytic nanoparticles preserve their stability and performance when used in a sequential TBP decomposition experiment. As for the estimation of the reaction rates, the kinetic constants were evaluated. Possessing a high predictive power with the accuracy more than 90%, the proposed kinetic equations can be applicable for further studies dealing with the TBP decomposition adopting SCWO.

A novel reutilization method for waste printed circuit boards as flame retardant and smoke suppressant for poly (vinyl chloride)

In this study, a novel reutilization method for waste printed circuit boards (PCBs) as flame retardant and smoke suppressant for poly (vinyl chloride) (PVC) was successfully testified. A supercritical water oxidation (SCWO) process was applied to treat waste PCBs before they could be used as flame retardants of PVC. The results indicated that SCWO conditions had a significant effect on the flame retarding and smoke suppressing properties of waste PCBs for PVC. Cu2O, CuO, and SnO2 were the main active ingredients in waste PCBs-derived flame retardants. A conversion of Cu elements (Cu0→Cu+→Cu2+) during SCWO process with the increase of reaction temperature was found to be the key influence factor for the flame retarding properties of SCWO-treated PCBs. The experiment results also showed that there was a synergistic effect of flame retardancy between Cu+ and Cu2+. After the optimized SCWO treatment, SCWO-treated PCBs significantly improved the flame retardancy and smoke suppression of PVC. Limiting oxygen index (LOI) and char yield (CY) increased with increasing SCWO-treated PCBs content in PVC, while smoke density rating (SDR) and maximum smoke density (MSD) decreased markedly. The mechanical properties of PVC samples were influenced in different degree by adding different content SCWO-treated PCBs.

Supercritical water oxidation of dioxins and furans in waste incinerator fly ash, sewage sludge and industrial soil

Three environmental samples containing dioxins and furans have been oxidized in the presence of hydrogen peroxide under supercritical water oxidation conditions. The samples consisted of a waste incinerator fly ash, sewage sludge and contaminated industrial soil. The reactor system was a batch, autoclave reactor operated at temperatures between 350°C and 450°C, corresponding to pressures of ∼20–33.5 MPa and with hydrogen peroxide concentrations from 0.0 to 11.25 vol%. Hydrogen peroxide concentration and temperature/pressure had a strong positive effect on the oxidation of dioxins and furans. At the highest temperatures and pressure of supercritical water oxidation of 450°C and 33.5 MPa and with 11.25 vol% of hydrogen peroxide, the destruction efficiencies of the individual polychlorinated dibenzo-ρ-dioxins/polychlorinated dibenzofurans (PCDD/PCDF) isomers were between 90% and 99%. There did not appear to be any significant differences in the PCDD/PCDF destruction efficiencies in relation to the different sample matrices of the waste incinerator fly ash, sewage sludge and contaminated industrial soil.

Treatability of Phenol by Supercritical Water Oxidation

This study focused on the treatment of phenol wastewater by supercritical water oxidation(SCWO). Tests were conducted by using a continuous-flow reactor system. Based on COD, organic destruction efficiencies of phenol wastewater were obtained at supercritical water conditions. Temperatures and pressures, respectively, ranged from 400-500°C,and 25-40Mpa. The reaction times varied from 30 to 190 seconds, and hydrogen peroxide was used as oxidant. Under SCWO conditions, destruction efficiencies greater than 99% were achieved.

Corrosion of nickel–chromium alloys, stainless steel and niobium at supercritical water oxidation conditions

Results of immersion tests of UNS N06625 (alloy 625), UNS S31609 (alloy 316 L), Ni–20Cr alloy and Nb coupons exposed to oxygenated ammoniacal sulphate solution at supercritical water oxidation (SCWO) conditions are presented. The corrosion behavior of the alloy 316 L (UNS S31603) SCWO reactor tubing is also presented under the same conditions. Immersion coupons corroded at a rate not exceeding 40 mm yr −1 while the reactor tube itself corroded at a rate of between 160–1500 mm yr −1, depending on which length of the reactor is considered to have corroded. Morphological and chemical analysis of the oxides present on the coupon samples suggest that iron oxides, which had initially precipitated on the corroding alloy 316 tube surface, were removed by heat flux-driven fluid mechanical action and transported through the reactor where they deposited on the coupons. Niobium was resistant to corrosion at the tested conditions.

Degradation Fate of Nitrogenous Compounds in Supercritical Water Oxidation Conditions with Organic Additives

The fate of eight nitrogenous compounds (nitrogen substrates) degraded by supercritical water oxidation in the presence of added methanol was investigated. The effect of methanol addition on the nitrogen removal efficiency was evaluated for each nitrogenous compound. The main (>99%) inorganic nitrogenous compound observed in the treated water containing acetonitrile was nitrate ion, while in the case of the other substrates it was ammonium ion. Moreover, the nitrogen removal efficiency of urea with acetonitrile increased compared with that of urea with methanol, in spite of the higher initial nitrogen load.

Role of Methanol in Supercritical Water Oxidation of Ammonia

The role of methanol (MeOH) in the supercritical water oxidation (SCWO) of ammonia (NH3) was investigated near the typical operating conditions of a commercial plant at 560−620 °C and 25 MPa using a tubular reactor. It was found that the presence of MeOH strongly affected the reaction behavior of nitrogen. With the addition of MeOH, at concentrations more than twice the initial concentrations of NH3 (1.4−5.7 mmol/mol of the feed mixture), the conversion of NH3 to N2O increased by about four times to 0.5−0.6 compared to that with no MeOH addition. The production of NO3- from NH3 increased in the liquid effluent with a conversion of up to 0.2. The productions of NO2-, NO, and NO2 from NH3 had similar behavior under the presence of MeOH at the tested conditions, and significant effects were observed at MeOH concentrations more than 2−6 times of the initial concentrations of NH3. The experimental results made it clear that NO and NO2 were produced under the SCWO conditions. Methods for the reduction of N2O and NO3- were also investigated. Results showed that SCWO is not suitable for the decomposition of N2O. Other post-treatment methods such as catalytic decomposition should be applied to reduce N2O in the gas effluent. NO3- reduction methods (e.g., two-step reaction) proposed in this study should also be considered to prevent corrosion of the reactor material.

Reactor Corrosion in Ceria Production by Hydrothermal Synthesis under Supercritical Conditions

Abstract Type 316 (UNS S31600) stainless steel (SS) is used for reactor tubes in a hydrothermal process under supercritical water oxidation (SCWO) conditions for the production of fine ceria particles in a large scale. Reliability of this reactor material depends upon corrosion resistance under SCWO. To assess this, a case study was analyzed and simulated tests on Type 316 SS in 3% cerium(III) nitrate (Ce[NO3]3·6H2O) at 400°C and 300 atm were carried out. Case study analysis and characterization of the exposed specimen were performed using x-ray diffraction (XRD), scanning electron microscopy (SEM)/energy-dispersive x-ray (EDX), and x-ray photoelectron spectroscopy (XPS) measurements. Dissolution of cations was analyzed by induction-coupled plasma spectroscopy (ICP-MS). Type 316 SS was well protected under SCWO conditions despite the absence of a protective Cr-oxide/hydroxide layer on the alloy surface. Corrosion protection was mainly due to the formation of a Ce oxide layer on the surface of the tube.

Corrosion Protection of Reactor Metals in Supercritical Water Oxidation Conditions

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Hydrothermal corrosion behavior of iron aluminide under SCWO conditions

Fe3Al based iron aluminide was tested un- der supercritical water oxidation (SCWO) conditions to explore its possible use as a reactor material for hydrothermal processes. Degradation resistance of the material were evaluated in distilled water at 450 � C and 300 atm in a closed reactor. The results of the kinetics studies, metallographic observations as well as phase and chemical analysis of the corrosion products indicated limited applicability of iron aluminide in SCWO conditions.

Destruction of Environmental Organic Pollutants by Supercritical Water Oxidation

The supercritical water oxidation of two environmental pollutants in the form of diesel fuel and two waste landfill leachate samples have been examined in relation to sub-critical and supercritical water oxidation conditions. Supercritical water oxidation involves reactions in an aqueous fluid phase under conditions below and around the critical point of water (Tc = 374 °C, Pc = 22.1 MPa). Experiments were carried out using a batch autoclave reactor at temperatures between 300 °C and 380 °C corresponding to pressures between 10.5 MPa and 22.8 MPa. The oxidative decomposition of the diesel fuel, which was loaded onto a sand matrix to simulate a contaminated land sample, was determined in relation to process conditions. The results showed that almost complete destruction of the components of the diesel fuel could be achieved. In addition, intermediate oxygenated compounds were formed and identified. The level of loading of the diesel fuel onto the sand matrix influenced the level of compound decomposition and the amount of intermediate oxygenated compounds formed. The characteristics of the waste landfill leachates were determined and were shown to be highly chemically complex. The high chloride concentration of the leachate resulted in significant corrosion of the reactor. However, almost complete oxidation of the organic components of the leachate could be achieved under supercritical water oxidation conditions.

OXIDATION REACTIONS OF SOLID CARBON IN SUPERCRITICAL WATER

ABSTRACT Compared to the research for homogeneous phase supercritical water oxidation (SCWO), kinetic understanding of SCWO progress on solid materials is less developed. We studied the reaction progress of carbon particles under SCWO conditions, where mass transfer processes and heterogeneous reactions are involved with mutual interactions. Reaction progress under mass-transfer limited conditions was observed for solid activated carbon at 400–500°C. The overall reaction of synthetic graphite observed at 500–600°C and that of activated carbon below 400°C were controlled by the internal surface heterogeneous reaction. The reaction rate at a unit internal surface area was proportional to the oxygen concentration and the specific reaction rate was described as 1.7 × exp (− 117 kJ mol−1/RT) m s−1. The reaction product was exclusively CO2 with a small amount of CO in the case of activated carbon.

Titanium Dissolution-Passivation in Highly Chloridic and Oxygenated Aqueous Solutions: Reaction Mechanism Extended to Supercritical Water Conditions

The principal objective of this paper is to show that we developed an experimental setup and a dissolution-passivation monitoring technique for use in super critical water oxidation (SCWO) conditions MPa). The principal barrier to this technology development is the corrosion of the reactor materials. High corrosion rates were experienced in industrial plants. A major goal is to develop effective tools to determine and control the electrochemical and corrosion parameters. Until now, electrochemical data concerning metallic materials submitted to aqueous solutions at pressures and temperatures beyond the critical point are scarce. This research, in the context of organic wastes SCWO, first proposes, for both sub and supercritical conditions, technical solutions for an electrochemical cell able to give access to quantitative electrochemical information in aggressive solutions and chloride ions content of 1 g/L). Its second purpose is to obtain information on a titanium alloy (T60 grade) behavior in such conditions. The present study leads to a model establishing a reaction mechanism, which operates under normal conditions of pressure and temperature and can be extended to supercritical ones. The limits of use of this material as a plant reactor material for supercritical water oxidation (SCWO) are evaluated. © 2004 The Electrochemical Society. All rights reserved.

Comparison of the effects of the addition of NaOH on the decomposition of 2-chlorophenol and phenol in supercritical water and under supercritical water oxidation conditions

The effects of the addition of NaOH on the decomposition of 2-chlorophenol (2CP) and phenol in supercritical water (SCW) and under supercritical water oxidation (SCWO) conditions were investigated. The experiments were conducted in a plug-flow reactor at 713 K and 26 MPa. The reactor residence times ranged from 0.13 to 0.51 s. The initial concentrations of 2CP and phenol were 3.89×10 −3 and 5.3×10 −3 mol/l, respectively. Experiments were conducted in the presence and absence of H 2O 2 or NaOH in SCW. Under the experimental conditions, the addition of NaOH accelerated the decomposition of 2CP in SCW, but had little effect on that of phenol. On the other hand, the addition of NaOH accelerated the decomposition of both compounds under SCWO conditions. From 2CP and phenol SCWO, many dimers were identified with a GC/MS. The addition of NaOH to 2CP and phenol SCWO reduced the generation of these dimers, and promoted the dechlorination of 2CP in SCW and under SCWO conditions. These results show that the effects of NaOH on the decomposition of 2CP and phenol SCWO are not negligible, and the effects of NaOH on the decomposition of other organic compounds under SCWO conditions should be considered for determining optimum operating conditions and reactor designs.

Structural transformation of an alumina-supported MnO2–CuO oxidation catalyst by hydrothermal impact of sub- and supercritical water

The structural transformation of a MnO2–CuO-containing catalyst supported on X-ray amorphous Al2O3 used in oxidative conversion of ethyl acetate in water under sub-critical (p = 200 bar, T = 633 K) and supercritical reaction conditions (p = 240 bar, T = 673 K) has been studied by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). TEM investigations revealed very small primary crystallites of the parent sample (ca. 5–20 nm), in contrast to a catalyst specimen exposed to supercritical water for ca. 100–200 hours which exhibited large micron-sized crystallites. This crystallisation process is accompanied by a drastic decline in the catalyst BET surface area. XRD studies support this observation: the basic material is nearly X-ray amorphous whereas the used catalyst shows several crystalline phases (e.g. Mn2O3, Al2O3, AlO(OH)). The patterns of spent catalysts taken from the reactor after different running times also gave evidence for the existence of mixed manganese copper oxides. Furthermore, XRD analysis of the used catalysts revealed a reduction of the basic MnO2 to Mn3O4 and Mn2O3; this was also clearly proven by XPS analysis. Hydrolysis runs at supercritical water oxidation conditions showed that ethyl acetate is only hydrolysed into ethanol and acetic acid but, in contrast to these non-catalysed runs, the further oxidation of the hydrolysis products to carbon dioxide and water occurs in the presence of the oxidation catalyst. It may be stated that during time-on-stream up to ca. 200 h neither activity nor selectivity was found to significantly change, apart from the found structural alterations.