Supercritical Water Oxidation Research Articles

Treatment of pistachio processing industry wastewaters by supercritical water oxidation in a continuous-flow reactor

ABSTRACT The objective of the present study is the treatment of pistachio processing industry wastewaters (PPIW) using the supercritical water oxidation method. The experiments were conducted within a 400–600°C temperature range and a 30–150 s reaction time range, while maintaining a constant pressure of 25 MPa and using an O2/COD ratio of 1:1. To observe the effects of the initial PPIW and O2 concentrations on the treatment efficiency, experiments were also conducted with O2/COD ratios ranging from 0.5 to 3, while maintaining a constant reaction temperature and time of 500°C and 60 s, respectively. The influence of reaction temperature, reaction time and O2/COD ratio on the total organic carbon (TOC) and total nitrogen (TN) contents of the liquid PPIW effluents were investigated. Treatment efficiencies up to 99.75% regarding TOC conversion and 78.72% regarding TN conversion were obtained in very short reaction times. Additionally, the kinetics of oxidation of PPIW was studied, and reaction rate expressions based on TOC and TN were proposed.

Effect analysis on recycling of cathode material from spent ternary lithium-ion batteries via supercritical water oxidation and acid-leaching

This study proposed a recycling method for spent batteries by combining supercritical water oxidation and acid-leaching. In supercritical water oxidation process, the cathode material was separated from the electrode plate, and 98% of lithium in the cathode was leached to achieve the separation of lithium from Ni, Co and Mn. Ni, Co and Mn were then leached during the acid leaching process. The effect of solid-liquid ratio, oxidant mass concentration, reaction time and temperature in supercritical process on metal leaching was analyzed. The leaching rate of Ni, Co and Mn was improved by increasing the mass concentration of oxidant and the reaction temperature. The effect of reaction time on metal leaching is related to the mass concentration of oxidant in the solution. Through the control of paraments, the leaching efficiencies of Ni, Co and Mn were significantly improved from 90.4%, 84.7%, 86.4–99.7%, 96.8% and 98.5%.

Bench-scale supercritical and near-supercritical water oxidation assays to treat Propylene Oxide-Styrene Monomer (POSM) wastewater

Supercritical Water Oxidation is a promising technology to deal with the phenolic-benzoic nature of the wastewater from Propylene Oxide-Styrene Monomer processes. To assess its feasibility to meet discharge regulations for a continuous process, lab-scale assays were conducted with real wastewater. After a screening to maximize Chemical Oxygen Demand (COD) removal efficiency, a supercritical, four-hour duration test was run at 550 °C, 235 bar g, with 50% excess oxygen and residence time of 120 seconds. Though a successful COD degradation of 99.97 ± 0.01% was achieved, salt scaling and corrosion occurred. The key findings are that to redissolve salts and avoid corrosion, focus must be on the supercritical to subcritical transition location. Near-supercritical oxidation was also tested as a cost-effective alternative to avoid scaling. Residence times up to 192 min were required at 300 °C, 165 bar g and 40% excess oxygen to achieve similar degradation obtained to supercritical conditions.

Tracking contaminants in the aqueous effluent during supercritical water gasification and oxidation of swine manure in the flow mode

Biomass waste processing is an area of great interest due to its potential for reducing environmental impact and recovering resources. The present work investigates for the first time the composition of products resulting from supercritical water (SCW) gasification and oxidation of swine manure (SM) continuously fed into a tubular reactor at a temperature gradient along its vertical axis of 390–600 °C and a pressure of 25 MPa. Special attention is paid to how the operating parameters, including SM flow rate (6.5–10.7 g/min), oxidation ratio (OR ≤ 1.78), and the presence of NaOH additive (1 wt%) influenced the composition and concentration of contaminants (phenols, N-bearing aromatics, polyaromatic hydrocarbons (PAHs), and ammonia) in the effluent collected at the reactor outlet. The results of gas chromatography-mass spectrometry indicate that water-soluble products comprise predominantly of phenols, pyridines, pyrroles, indoles, pyrazines, anilines, and quinolines. Increasing the reagent’s residence time during SCW gasification was observed to lower the amount of phenols in the effluent. In addition, the introduction of NaOH increases the yields of syngas and NH3 while lowering the concentration of N-bearing aromatics and PAHs. SCW oxidation of SM not only reduces contaminant levels in the effluent by several orders of magnitude, but also partially compensates for thermal inputs needed for the process implementation. Among N-bearing compounds, ammonia, pyridines, indoles, and quinolines display the highest resistance to oxidation. The paper explores the resource-saving and ecological benefits of SCW gasification and oxidation of SM.

Supercritical Fluids as Solvents for the Future

High pressure technologies involving sub and supercritical fluids offer the possibility to obtain new products with special characteristics or to design new processes, which are environmentally friendly and sustainable. Supercritical fluids are flexible tools for processing materials. Supercritical fluids have been applied to mass-transfer processes, phase-transition processes, reactive systems, materials-related processes, and nanostructured materials. In addition to extraction, application areas include impregnation and cleaning, multistage countercurrent separation, particle formation, coating, and reactive systems such as hydrogenation, biomass gasification, and supercritical water oxidation. Polymers are modified with supercritical fluids, and colloids and emulsions as well as nanostructured materials exhibit interesting phenomena when in contact with supercritical fluids that can be industrially exploited.

Integration of hydrothermal liquefaction of Cyanophyta and supercritical water oxidation of its aqueous phase products: Biocrude production and nutrient removal

Cyanophyta has the potential to produce biocrude via hydrothermal liquefaction (HTL). However, aqueous phase products (APs), as by-products of HTL, pose a risk of eutrophication for the high levels of carbon, nitrogen, and phosphorus. Supercritical water oxidation (SCWO) can efficiently convert organics into small molecules, offering a technique for the harmless treatment of APs. Effects of holding time, pressure, and moisture content on the biocrude yields from isothermal HTL (300 °C) and fast HTL (salt bath temperature of 500 °C) were comprehensively investigated. Biocrude properties were characterized by elemental analysis, FT-IR and GC–MS. Subsequently, the APs obtained under the conditions producing the highest biocrude yield were subjected to SCWO at 550 °C with different oxidation coefficients (n) from 0 to 2. Removal rates of chemical oxygen demand (COD), ammonia nitrogen (NH3−N), and total phosphorus (TP) were further explored. The results show that the highest biocrude yields from isothermal HTL and fast HTL were 24.2 wt% (300 °C, 1800 s, 25 MPa, and 80 wt% moisture content) and 21.9 wt% (500 °C, 40 s, 25 MPa, and 80 wt% moisture content), respectively. The biocrude primarily consisted of N-containing heterocyclic compounds, amides, and acids. SCWO effectively degraded the COD and TP in APs, while the NH3-N required further degradation. At n = 2, the highest removal rates of COD, NH3-N and TP were 98.5 %, 22.6 % and 89.1 %, respectively.

Experimental and kinetic modeling study of acetic acid oxidation and hydrolysis in supercritical water

The oxidation and hydrolysis of 0.11–0.14 M acetic acid in supercritical water (SCW) have been investigated at temperatures of 773–873 K and a pressure of 25.0 MPa in a flow reactor. CO and CO2 were the main products at oxygen-rich conditions. For hydrolysis experiments, CH4 and CO2 were produced in nearly equal amounts while the yield of CO was negligibly small. A detailed chemical kinetic model was developed by revising the kinetic parameters of key elementary reactions. The model performance was evaluated by comparison to the present data as well as data from literature. The model predicted the kinetic behavior of acetic acid oxidation fairly well, despite of slight underprediction of CO and overprediction of CH4. It also reproduced satisfactorily the species concentrations during acetic acid hydrolysis at high temperatures. Based on the model, kinetic analyses were further conducted to identify important elementary steps and to infer reaction mechanisms.

Energy yield from wastewater by supercritical water oxidation process: Experimental validation and simulation from the viewpoint of energy system

The degradation of high concentration organic wastewater and the recovery of energy using supercritical water oxidation (SCWO) technology have the potential to become viable in the future. High concentration organic wastewater contains a significant amount of energy. This paper focuses on utilizing high concentrated wastewater as a renewable energy source, from an energy system perspective, an experimental rig was designed to determine the optimal working conditions. The experiments were conducted with an oxidation coefficient of 1.1, a flow rate of high concentration organic wastewater of 9 ml/min, a heating temperature of 400 °C, and a reaction pressure of 25 MPa. Under these conditions, the COD and TOC removal rates of the high concentration organic wastewater were found to be 99.383 % and 99.876 % respectively. Additionally, the phenomenon of “Abnormalities in pressure sensitivity” observed during high concentration organic wastewater the experiment was further explained through additional experiments. Finally, by comparing the experimental energy recovery with the simulation results, an energy recovery efficiency of 78.95 % was achieved. The feasibility of the simulated energy recovery in practice was verified, confirming the potential to convert the previous energy-consuming wastewater treatment process into a productive one. This study provides a theoretical and experimental foundation for further related research.

Tube explosion during continuous supercritical water oxidation of sludge: An incident investigation

Supercritical water oxidation (SCWO) is well known as a promising end-of-pipe technology for treatment of hazardous wastes. Safety is undoubtedly the first concern as the SCWO should be operated under a harsh chemical and physical environment, including high temperature, high pressure, and high corrosive medium and plugging risk. This paper is the first report about a tube explosion incident that encountered during continuous SCWO treatment of industrial sludge. 3D imaging, Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS) and 3D finite element analysis are used to analyze the incident tube comprehensively. The results indicate that the incident tube was working at a hash corrosive environment of subcritical water and chloride, which leads to all of general, pitting and intergranular corrosions. Of which, the pitting is the most severe one that finally results to perforation. Then, leakage took place. Soon afterwards, the hard inorganic particle from sludge plugged the leaking pore for a second, but immediately split it as a big hole due to the local high pressure around the leaking pore. Finally, explosion occurred. Three lessons are drawn for the future safe design and operation of SCWO, i.e., selecting suitable pressure bearing material, checking regularly, and executing safety regulations.

Continuous Treatment of Refractory Wastewater from Research and Teaching Laboratories via Supercritical Water Oxidation–Experimental Results and Modeling

Teaching and research laboratories generate wastes of various compositions and volumes, ranging from diluted aqueous solutions to concentrated ones, which, due to milder self-regulation waste-management policies, are carelessly discarded, with little attention given to the consequences for the environment and human health. In this sense, the current study proposes the application of the supercritical water oxidation (SCWO) process for the treatment of complex refractory wastewater generated in research and teaching laboratories of universities. The SCWO, which uses water in conditions above its critical point (T > 647.1 K, p > 22.1 MPa), is regarded as an environmentally neutral process, uniquely adequate for the degradation of highly toxic and bio-refractory organic compounds. Initially, the wastewater samples were characterized via headspace gas chromatography coupled with mass spectrometry. Then, using a continuous tubular reactor, the selected operational parameters were optimized by a Taguchi L9 experimental design, aiming to maximize the total organic carbon reduction. Under optimized conditions—that is, temperature of 823.15 K, feed flow rate of 10 mL min−1, oxidizing ratio of 1.5 (50% excess over the oxygen stoichiometric ratio), and sample concentration of 30%—TOC, COD, and BOD reductions of 99.9%. 91.5% and 99.2% were achieved, respectively. During the treatment process, only CO2, methane, and hydrogen were identified in the gaseous phase. Furthermore, the developed methodology was applied for the treatment of wastewater samples generated in another research laboratory and a TOC reduction of 99.5% was achieved, reinforcing the process’s robustness. A thermodynamic analysis of SCWO treatment of laboratory wastewater under isothermal conditions was performed, using the Gibbs energy minimization methodology with the aid of the GAMS® 23.9.5. (General Algebraic Modeling System) software and the CONOPT 4 solver. Therefore, the results showed that SCWO could be efficiently applied for the treatment of wastewater generated by different teaching and research laboratories without the production of harmful gases and the addition of hazardous chemicals.

Methanol oxy-combustion and supercritical water oxidation: A ReaxFF molecular dynamics study

Energy and environmental concerns are motivating the use of renewable fuels such as methanol. Furthermore, the implementation of the oxy-combustion and hydrothermal combustion technologies can help to improve the performance of power generation and reduce NOx emissions. These aspects can contribute to achieve the transition to cleaner sources of energy that is being sought worldwide, and thus we carried out the first molecular dynamics study of the oxidation of methanol at 2700 K and 3000 K in four supercritical environments with compositions CH3OH + O2, CH3OH + O2+CO2, CH3OH + O2+H2O, and CH3OH + O2+CO2+H2O. Reaction mechanisms were obtained and revealed that the initiation reaction is CH3OH unimolecular dissociation in all cases. The CH3OH oxidation chemistry changes when O2 is replaced by supercritical CO2 (sCO2) and/or H2O (sH2O), and a new route for the important oxidation sequence CH3OH→CH2OH→H2CO→CHO→CO→CO2 is reported. The rate constants for the CH3OH unimolecular dissociation were calculated, indicating a positive effect of sH2O. Furthermore, the collisions of CH3OH molecules with those of H2O and CO2 were analyzed with molecular dynamics simulations and quantum chemistry calculations, suggesting that collisions with H2O can activate more efficiently CH3OH for a prospective dissociation event. This study is aimed to help in the development of kinetic models for CH3OH oxidation/pyrolysis in sCO2 and sH2O, and thus in the implementation of the oxy-combustion and hydrothermal combustion techniques for this alternative fuel.

Enhanced denitrification of aniline and its derivatives by catalysts, anions, and co-fuels during supercritical water oxidation process

We investigated the denitrification performance of aniline and its derivatives (N-methylaniline and N, N-dimethylaniline) in supercritical water oxidation (SCWO) under varying conditions (temperature: 643–743 K, reaction time: 0.5–4 min, pressure: 24.0 MPa, oxidation coefficient: 300%). Cu2 + proved to be an efficient homogeneous catalyst, generating radicals via a Fenton-like reaction. Isopropyl alcohol (IPA) outperformed methanol and ethanol as a co-fuel, enhancing TN removal due to its higher heating value. Nitrogen-containing product distribution revealed that N2 generation correlated with org-N depletion, especially with Cu2 + and IPA. A denitrification mechanism was elucidated using DFT, with Fukui indices, EHOMO, and ELUMO highlighting aniline's greater susceptibility compared to MA and DMA. Our findings provide insights into optimizing SCWO denitrification using aniline derivatives, shedding light on sustainable nitrogen removal processes.

Supercritical water oxidation for the destruction of spent media wastes generated from PFAS treatment

Granular activated carbon (GAC) and anion exchange resin (AIX) have been successfully demonstrated to remove per- and polyfluoroalkyl substances (PFAS) from contaminated water and wastewater. These treatment technologies, when applied for PFAS removal, generate spent media loaded with a high mass of PFAS requiring further treatment and disposal. This project is the first study on the use of supercritical water oxidation (SCWO) to destroy both the spent media and the PFAS adsorbed onto it. One sample of spent GAC and one sample of spent AIX were collected from full-scale groundwater remediation systems treating PFAS. A second spent AIX sample was collected from a mobile PFAS treatment unit. The total PFAS concentrations reported in the GAC, AIX and second AIX feedstock slurries were 0.21 mg/kg, 1.3 mg/kg and 0.9 mg/kg, respectively. Each feedstock was processed separately in a one (1) wet metric ton per day tubular reactor SCWO system. The study demonstrated that SCWO is a very effective PFAS destruction technology for spent GAC and AIX, derived from water remediation systems treating PFAS. The spent media were completely mineralized to water, carbon dioxide (CO2) and a negligible amount of residual minerals. Total target PFAS compound concentrations in the SCWO system effluents after treating spent GAC, AIX and second AIX feedstocks were 548, 77 and 796 ng/L, respectively. The results indicated that the percentage elimination of perfluorocarboxylic acids (PFCAs) was better than that of perfluosulfonic acids (PFSAs) and long-chain PFAS elimination was better than short-chain PFAS.

Simultaneous recycling of waste solar panels and treatment of persistent organic compounds via supercritical water technology

The study addresses the application of the supercritical water technology in the simultaneous recycling of obsolete solar panels and treatment of persistent organic compounds. The obsolete solar panels samples were characterized by TEM-EDS, SEM, TG-DTA, XRD, WDXRF, MP-AES and elemental analysis. Initially, the optimized parameters for the degradation of solid organic polymers present in residual solar panels via oxidation in supercritical water were defined by an experimental design. Under optimized conditions, 550 °C, reaction time of 60 min, volumetric flow rate of 10 mL min−1 and hydrogen peroxide as oxidant agent, real laboratory liquid wastewater was used as feed solution to achieve 99.6% of polymers degradation. After the reaction, the solid product free of organic matter was recovered and characterized. On average, a metal recovery efficiency of 76% was observed. Metals such as aluminum, magnesium, copper, and silver, that make up most of the metallic fraction, were identified. Only H2, N2 and CO2 were observed in the gaseous fraction. Then, initial data on the treatment of the liquid decomposition by-products, generated during ScW processing, were reported. A total organic carbon reduction of 99.9% was achieved after the subsequential treatment via supercritical water oxidation using the same experimental apparatus. Finally, insights on the scale-up, energy integration and implementation costs of a ScW solid processing industrial unit were presented using the Aspen Plus V9 software.

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.

Highly-efficient process for recovering gold from discarded mobile phones using pretreatment and mild extraction

Existing methods for recovering gold from electronic waste are based on mechanical and hydrometallurgy approaches. However, these methods usually use toxic chemicals and demonstrate low gold extraction efficiency (Ge) due to the negative influence of organic materials and other industrial metals also present in the waste. In this work, a more efficient and more environmentally friendly process is proposed that can achieve a high Ge from electronic waste by pre-treatment and pre-extraction before gold extraction. The powdered waste was first pre-treated by a supercritical water oxidation (SWO) process to remove organic materials and then pre-extracted using a mixture of FeCl3 and HCl to remove industrial metals. The solid powder thus produced was then re-extracted by a DMF-CuCl2-NaCl mild extraction system to complete the recovery of the gold content. Experimental results indicated that pre-treatment temperature, pre-treatment time, and the concentration of the FeCl3 and HCl solutions all significantly influenced the amount of gold extracted. The optimal parameters for achieving the highest Ge extraction (99.6%) using the DMF-CuCl2-NaCl extraction system were determined as follows: an extraction temperature of 65 °C, an extraction time of 70 min, a stirring speed of 420 rpm, a sample size of ≤ 850 µm, a solid/liquid (S/L) ratio of 1:12 g·mL−1, a CuCl2 concentration of 0.8 mol·L−1, and a NaCl concentration of 1.6 mol·L−1. After gold extraction, water was then used to precipitate the gold in the form of nanoparticles. Under optimal conditions (70%v/v water, 80 min), over 98.4% of the gold could be recovered in this way.

Assessment of machine learning algorithms for predicting autoignition and ignition delay time in microscale supercritical water oxidation process

With recent advancements in space technology, there is a need to develop technologies to ensure a sustainable environment for human survival. Among these, treatment of human and organic waste aboard manned space missions is a challenging task for which supercritical water oxidation using hydrothermal flames has been proposed as a possible solution. The critical step in readily adopting this technology from established ground-based setups is scaling the process to microscale. In addition to the challenge of physical realization of the microreactors at these high pressure and temperature (P>22MPa,T>350C) conditions, the need to explicitly analyze the process dynamics at microscale is inevitable owed to the size of the reactors under consideration, the physics being significantly different from meso/mini scale systems. One of the primary objectives is to identify the operating physical parameters for which formation of hydrothermal flames can be obtained. Before proceeding with an expensive computational or experimental approach to determine the exact ignition map, an initial estimate based on physical arguments can help in providing insights into the process. We address this problem using homogeneous ignition calculations to develop machine learning models to predict autoignition as well as ignition delay time. The ingenuity of the work lies in defining autoignition criteria in relation to flow time scales expected at microscale. Various classification models were trained and tested for predicting autoignition and regression models were demonstrated to predict the ignition delay time. While predicting autoignition is a straightforward process, a two-step approach is proposed for ignition delay time. Finally, how machine learning can be used more explicitly, particularly for understanding and designing efficient microreactors, is presented which highlights that machine learning approach is not merely restricted to prediction but can also have real implications on improving the process as a whole.

Development of a detailed kinetic model for high-pressure ethanol oxidation in gas phase and supercritical water

The present study aims to develop a detailed chemical kinetic model for high-pressure ethanol oxidation in the gas phase and supercritical water (SCW). Kinetic parameters for key elementary reactions involving HO2 and CH3OO radicals were updated, deriving from a careful analysis of ignition delay data or analogy. The model performance is validated comprehensively by comparison to a wide variety of recent experimental data from both gas-phase and SCW conditions. The model predicted ignition delay times and speciation measurements (15–60 bar) in the gas phase fairly well. Additionally, the model showed satisfactory performance in reproducing the species concentration profiles at SCW conditions. A comparative kinetic analysis was further conducted to investigate the characteristic differences and similarities of ethanol oxidation at the two conditions. The results indicated SCW lowered the onset temperature for ethanol ignition and facilitated the ethanol oxidation rate, but did not introduce additional oxidation pathways.

Early corrosion properties of titanium alloys, Ni-based alloys and stainless steel in supercritical water oxidation of sewage sludge

Supercritical water oxidation (SCWO) can efficiently and harmlessly dispose sewage sludge, while corrosion phenomena can lead to rapid shutdown and/or failure of expensive process equipments in a supercritical water system. The corrosion behavior of titanium alloys, Ni-based alloys and stainless steel exposed in supercritical water oxidation of sewage sludge at 450 °C, 26 MPa for 60 h was investigated by means of gravimetry, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. 316 SS (stainless steel) showed the highest corrosion rate, while TA10 had the lowest corrosion rate. The oxide scale on the surface of 316 SS contained magnetite and FeCr2O4, while Cr2O3 and NiCr2O4 were the main components of oxide layer on Inconel 600. Among them, magnetite and NiCr2O4 mainly existed in the inner oxide layers of SS 316 and Inconel 600, respectively. The lack of Cr2O3 on the 316 SS surface resulted in severe pitting corrosion and poor corrosion resistance.

Application of Supercritical Water Oxidation to Effectively Destroy Per- and Polyfluoroalkyl Substances in Aqueous Matrices

Application of Supercritical Water Oxidation to Effectively Destroy Per- and Polyfluoroalkyl Substances in Aqueous Matrices