Supercritical Water Oxidation Process Research Articles

Energy Consumption and Economic Analyses of a Supercritical Water Oxidation System with Oxygen Recovery

Oxygen consumption is one of the factors that contributes to the high treatment cost of a supercritical water oxidation (SCWO) system. In this work, we proposed an oxygen recovery (OR) process for an SCWO system based on the solubility difference between oxygen and CO2 in high-pressure water. A two-stage gas–liquid separation process was established using Aspen Plus software to obtain the optimized separation parameters. Accordingly, energy consumption and economic analyses were conducted for the SCWO process with and without OR. Electricity, depreciation, and oxygen costs contribute to the major cost of the SCWO system without OR, accounting for 46.18, 30.24, and 18.01 $·t−1, respectively. When OR was introduced, the total treatment cost decreased from 56.80 $·t−1 to 46.17 $·t−1, with a reduction of 18.82%. Operating cost can be significantly reduced at higher values of the stoichiometric oxygen excess for the SCWO system with OR. Moreover, the treatment cost for the SCWO system with OR decreases with increasing feed concentration for more reaction heat and oxygen recovery.

Combined processes of ozonation and supercritical water oxidation for landfill leachate degradation

Leachate is a highly variable, heterogeneous and recalcitrant wastewater generated in landfills which may contain high concentrations of many organic and inorganic compounds, hampering the application of a single technique in its treatment. Therefore, this paper assessed leachate degradation through supercritical water oxidation (ScWO) as well as combined processes of ozonation and supercritical water oxidation (O3/ScWO and ScWO/O3), a yet innovative combination. Ozonation was carried out at different reaction times (30–120 min). ScWO was developed at 600 °C, 23 MPa, and spatial time (τ) from 29 to 52 s. A combination of ozonation (30 min) and supercritical water oxidation process (O3-30’/ScWO) was the most efficient technique for the degradation of the leachate assessed. These conditions enabled to remove high values of apparent and true color (92% and 97%, respectively), biochemical oxygen demand (BOD5,20) (95%), chemical oxygen demand (COD) (92%), total organic carbon (TOC) (79%), nitrite (78%), nitrate (84%), total (96%), dissolved (96%) and suspended (94%) solids. In addition, the combined process presented significant decrease in electric conductivity (EC) (68%) and less leachate turbidity removal (43%). Except for ammonia and nitrite, all parameters of the leachate treated by O3-30’/ScWO met the specifications of Brazilian legislation (CONAMA Resolutions No. 357/2005 and No. 430/2011) for the disposal of wastewater in water bodies. Besides, both processes are considered to be clean technologies. This shows the great possibility of applying the O3/ScWO combination to landfills leachates.

Treatment of penicillin with supercritical water oxidation: Experimental study of combined ReaxFF molecular dynamics

Supercritical water oxidation (SCWO) of penicillin (PCN) was investigated under different operating conditions. The chemical oxygen demand (COD) removal rate could reach 99.4% at 400 °C, 24 MPa, 1min and oxidation coefficient (OC) of 2. Experimental results showed that COD removal had no significant dependence on temperature and pressure variations. By contrast, COD removal could be significantly promoted with OC increasing from 0 to 2.0, but the effect was negligible as the OC further increased; similarly, longer residence time than a definite value seemed to contribute little to COD removal. Initial and deeper degradation pathways of penicillin were proposed based on the reactive force field (ReaxFF) molecular dynamics (MD) simulations. By tracing the evolution of intermediates, the migration routes of S and N during the SCWO process were obtained with H2S and NO2 produced as the corresponding products. Simulation results showed that SCW and oxidant not only accelerated the degradation by producing highly reactive radicals or molecules, but also participated in reactions by serving as H and O sources. Moreover, catalysis of water clusters in C-heteroatom bond cleavage was also observed.

Analysis of temperature simulation in downhole reaction chamber of hydrothermal jet drilling

Hydrothermal jet is an alternative drilling method for the exploitation of oil and geothermal energy in deep hard formations. For the application of this novel technology, the successful generation of hydrothermal jet is very important. This paper focuses on investigating applications of different reaction, turbulence and radiation models to the supercritical water oxidation process in downhole reaction chamber of hydrothermal jet drilling. The objective is to identify the pros and cons of each model and determine a set of models that are the most appropriate for the reaction. Simulation models are tested and optimized through two different operating conditions. Simulation results are compared with experimental data. Results show that the entire space of the reaction chamber is in a high temperature state using the laminar finite rate model. The finite rate model is suitable for the simulation compared with other reaction models discussed. The Magnussen constant A and B in the finite rate model can be modified to be 7 and 0.5 to further reduce the error. In addition, the high temperature areas in k-omega model and SAS model are more concentrated, while they are more uniform in RNG k-epsilon model and standard k-epsilon model. The RNG k-epsilon model and DO or DTRM are the most appropriate turbulence and radiation models through comparison. Results in this paper can provide implications for the reaction simulation of hydrothermal jet drilling.

Municipal Solid Waste Leachates Destruction and Metals Recovery by Supercritical Water Oxidation

ABSTRACTMunicipal solid waste leachate dissolved with nitrates of Cr, Pb, and Cu was investigated using a supercritical water oxidation process in this study. The destruction efficiency of chemical oxygen demand increased with the higher temperature and longer reaction time. A first-order expression applied for chemical oxygen demand was determined with a pre-exponential factor of 318 s–1 and activation energy of 58 kJ/mol. Recovery efficiency of Cr, Pb, and Cu from initial solution into solid product increased with the higher temperature and longer reaction time. X-ray diffraction of solid products showed that the metal species were found to be Cu2O, HCrO2, and CaCO3 at temperature 400°C, while the structure were identified as CuO, Cr2O3, PbCrO4, and CaCO3 at temperature 500°C.

QSAR study on the removal efficiency of organic pollutants in supercritical water based on degradation temperature

This paper aims to study temperature-dependent quantitative structure activity relationship (QSAR) models of supercritical water oxidation (SCWO) process which were developed based on Arrhenius equation between oxidation reaction rate and temperature. Through exploring SCWO process, each kinetic rate constant was studied for 21 organic substances, including azo dyes, heterocyclic compounds and ionic compounds. We propose the concept of TR95, which is defined as the temperature at removal ratio of 95%, it is a key indicator to evaluate compounds’ complete oxidation. By using Gaussian 09 and Material Studio 7.0, quantum chemical parameters were conducted for each organic compound. The optimum model is TR95 = 654.775 + 1761.910f(+)n − 177.211qH with squared regression coefficient R2 = 0.620 and standard error SE = 35.1. Nearly all the compounds could obtain accurate predictions of their degradation rate. Effective QSAR model exactly reveals three determinant factors, which are directly related to degradation rules. Specifically, the lowest f(+) value of main-chain atoms (f(+)n) indicates the degree of affinity for nucleophilic attack. qH shows the ease or complexity of valence-bond breakage of organic molecules. BOx refers to the stability of a bond. Coincidentally, the degradation mechanism could reasonably be illustrated from each perspective, providing a deeper insight of universal and propagable oxidation rules. Besides, the satisfactory results of internal and external validations suggest the stability, reliability and predictive ability of optimum model.

Aggregation of Na2SO4 Nanocrystals in Supercritical Water

Supercritical water oxidation processes (SCWO) have been developed as an alternative technology to treat toxic and/or complex chemical wastes with very good efficiency. However, one main limitation of SCWO processes comes from the precipitation of inorganic compounds which can lead to clogging and interruption of the continuous process. Unfortunately little information is available in the literature regarding the precipitation mechanism and the salt particle properties in supercritical water (T ≥ 374 °C, P ≥ 22.1 MPa). This work intends to study the formation of salt aggregates from one common salt: disodium sulfate (Na2SO4). A specific and dedicated experimental setup is presented and succeeded in recovering salt powders from supercritical precipitation. Several analyses are performed on the salt samples to obtain aggregate sizes, morphologies, and size distributions. On the basis of the experimental results and a previous work, a numerical modeling of the salt precipitation and aggregation is reported t...

Experimental study of the supercritical water oxidation process within the framework of solving the task of chemical production industrial water runoff treatment

The given paper presents results of an experimental study of the industrial water runoff oxidation within the propylene epoxidation process carried out under supercritical fluid conditions (T=653-723 K, P=29-32 MPa) at a continuous operation installation using atmospheric oxygen as the oxidizing agent. Efficiency of the water runoff oxidation depending on the thermodynamic conditions of oxidation reaction was established. Solid residue elemental composition was determined.

Investigation of the precipitation of Na2SO4 in supercritical water

SuperCritical Water Oxidation process (SCWO) is a promising technology for treating toxic and/or complex chemical wastes with very good efficiency. Above its critical point (374°C, 22.1MPa), water exhibits particular properties and organic compounds can be easily dissolved and degraded with the addition of oxidizing agents. But these interesting properties imply a main drawback regarding inorganic compounds. Highly soluble at ambient temperature in water, these inorganics (such as salts) are no longer soluble in supercritical water and precipitate into solids, creating plugs in SCWO processes. Although this precipitation phenomenon is well known asa limiting factor for SCWO process, it is still not well understood. This work intends to investigate the precipitation phenomenon with a new methodology. A common salt, disodium sulfate (Na2SO4), is taken asa reference for the study. Na2SO4 solubility in sub- and supercritical water is determined on a wide temperature range using a continuous set-up. Crystallite sizes formed after precipitation are measured with in situ synchrotron wide angle X-ray scattering (WAXS). Combining these experimental results, a numerical modeling of the precipitation in supercritical conditions is performed by taking into account all the implied physical phenomena: thermodynamic, hydrodynamic and nucleation & growth.

Total nitrogen removal, products and molecular characteristics of 14 N-containing compounds in supercritical water oxidation

Supercritical water oxidation (SCWO) process of 14 N-containing compounds was investigated with residence time of 150 s, at a stable pressure of 24 MPa, temperatures of 350–500 °C and 500% excess oxygen, resulted in total nitrogen (TN) removal from 41 to 96%. The products of N-containing species were mainly N2, nitrate, ammonium, as well as hardly nitrite or NOX. The yield distributions of nitrate and ammonium were different: the main nitrate concentrations were obtained from the compounds containing nitro-group and diazonium, like nitrobenzene, 2-nitrophenol and eriochrome blue black R (EBBR); the predominant ammonium yields were achieved from amino-group and N-heterocyclic compounds, such as aniline, 5-chloro-2-methylaniline, 3,4-dichloroaniline, 1-methylimidazole, 1,10-phenanthroline, cyanuric acid, indole and 2,3-indolinedione. It is interesting that 2-nitroaniline, possessing both nitro- and amino-group, would dominantly decompose into N2. To explore the relationship between TN removal and molecular structural characteristics, density functional theory (DFT) method was used to calculate molecular descriptors of all 14 N-containing compounds. The correlation results showed that among all the fifteen molecular descriptors, q(C)-, q(C-H)+ and F(0)x greatly affects temperature behavior of TN removal.

Na2CO3 and K3PO4 solubility measurements at 30 MPa in near-critical and supercritical water using conductimetry and high pressure calorimetry

Knowledge of the phase equilibria and solubility of inorganic salts is crucial to improving the handling of salt precipitation in supercritical water oxidation (SCWO) processes. However, there is little such data in the literature, especially at 30MPa, the pressure at which the continuous SCWO processes of the CEA (the French atomic energy commission) operate. In this study, the solubility of Na2CO3 and K3PO4 was determined under near-critical and supercritical conditions (320°C<T<500°C, P=30MPa) by high pressure differential scanning calorimetry or by measuring the conductivity of samples taken from a high-pressure vessel. This approach enabled different phase changes to be detected for the two salts: direct solid precipitation from the homogeneous fluid phase for Na2CO3, and the appearance of a gas-liquid equilibrium before precipitation for K3PO4. On the whole, the results obtained are in agreement with Valyashko’s classification of binary water/salt systems.

The exergy release mechanism and exergy analysis for coal oxidation in supercritical water atmosphere and a power generation system based on the new technology

The oxidation environment has important influence on the transformation of the energy contained in fuel and generation of pollutants. To the problem of nearly 50% exergy losses in coal oxidation at air atmosphere, this research intends to change oxidation atmosphere from air to supercritical water/oxidant and achieve efficient release of exergy in coal at about 650°C with the aid of a high solubility and unique performance of heat and mass transfer of supercritical water. Therefore, firstly, based on the exergy analysis theory and the energy-utilization diagrams, the release mechanism of exergy of coal in supercritical water oxidation process is revealed. It is pointed out that supercritical water oxidation has changed the release pathways of chemical exergy, and decreased the level difference between chemical exergy and thermal energy, and more exergy is released. Meanwhile, there is also no exergy loss of physical heat transfer. As a result, supercritical water oxidation has higher exergy efficiency than conventional oxidation. Secondly, the exergy losses, level difference between chemical exergy and thermal energy as well as exergy efficiency, are quantitatively investigated. Results show that the exergy efficiency of supercritical water oxidation reactor may be as high as 80.1% and has increased by 27% relative to conventional boilers. Thirdly, based on supercritical water oxidation of coal, a concept power generation system is constructed. Exergy efficiency is calculated and exergy analysis is provided for the supercritical water oxidation power plant. Compared with conventional power plant, exergy efficiency in supercritical water oxidation plant reaches as high as 61.3% and 21% higher than that in conventional power plant. Finally, from the results obtained, it is believed that the commercial breakthrough of the supercritical water oxidation process will be possible when the corrosion and salt deposition in the reactor are solved.

Simulation of supercritical water oxidation reactor in transitory state: Application to time-dependent processes

Supercritical Water Oxidation (SCWO) is a powerful process for the treatment of organic wastewater. It has already been studied for decades and, nowadays there are several demonstration and industrial plants under construction worldwide. Simulation tools play an important role, both to predict and optimize the operating conditions during the process and to evaluate its economic viability. In the case of SCWO, transitory state models are sparse in the literature. In order to exhaustively know the behavior of the SCWO process during time-dependent applications, a transitory tubular reactor model has been built. That model has been developed by the finite difference method in two dimensions. It takes into account the reaction of wastewater along the reactor and the heat loss from the fluid to the atmosphere in a radial way, both along the pipe and through the isolation material.In order to validate the model, a set of experiments under different operating conditions were carried out in a pilot plant. Their aim was to characterize the heat loss in the system to fit both simulated and experimental data in steady state. Then, the transitory model is tested and typically time-dependent applications at industrial scale are analysed under different operating conditions (varying feed concentrations, reactor inlet temperature or amount of cooling water injection). In each case, the model results are collected and they show the time variation according to the changes in operating parameters such as temperature, feed concentration or heat loss along the reactor. The model developed can also be used for other technologies based on supercritical water, such as supercritical water gasification.

Advanced degradation of brominated epoxy resin and simultaneous transformation of glass fiber from waste printed circuit boards by improved supercritical water oxidation processes

This work investigated various supercritical water oxidation (SCWO) systems, i.e. SCWO1 (only water), SCWO2 (water+H2O2) and SCWO3 (water+H2O2/NaOH), for waste printed circuit boards (PCBs) detoxification and recycling. Response surface methodology (RSM) was applied to optimize the operating conditions of the optimal SCWO3 systems. The optimal reaction conditions for debromination were found to be the NaOH of 0.21g, the H2O2 volume of 9.04mL, the time of 39.7min, maximum debromination efficiency of 95.14%. Variance analysis indicated that the factors influencing debromination efficiency was in the sequence of NaOH>H2O2>time. Mechanism studies indicated that the dissociated ions from NaOH in supercritical water promoted the debromination of brominated epoxy resins (BERs) through an elimination reaction and nucleophilic substitution. HO2, produced by H2O2 could induce the oxidation of phenol ring to open (intermediates of BERs), which were thoroughly degraded to form hydrocarbons, CO2, H2O and NaBr. In addition, the alkali-silica reaction between OH− and SiO2 induced the phase transformation of glass fibers, which were simultaneously converted into anorthite and albite. Waste PCBs in H2O2/NaOH improved SCWO system were fully degraded into useful products and simultaneously transformed into functional materials. These findings are helpful for efficient recycling of waste PCBs.

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.

Corrosion Properties of Candidate Materials in Supercritical Water Oxidation Process

AbstractCorrosion of constructional materials in supercritical water oxidation (SCWO) is one of the main obstacles to commercializing the SCWO process. Statistical analysis of corrosion research results indicate nickel-based alloys are the most widely applied materials in SCWO system among the candidate metals. The present paper reviews the corrosion characteristics of iron-base alloy, nickel-based alloy, titanium-base alloy, ceramics, niobium, tantalum, thallium and gold under the condition of SCWO. The selection scheme of materials is presented in SCWO system flow diagram in detail. The effects of alloying elements on the corrosion behavior of materials in the water environment with high temperature and high pressure water are summarized in this paper. Moreover, some further researches for material corrosion in SCWO process are also proposed. All work was contributed to selecting the appropriate constructional materials and reducing the corrosion damage in SCWO system.

Kinetics study on hydrothermal combustion of methanol in supercritical water

Supercritical hydrothermal combustion has received a great deal of attention as an innovative and potential treatment technology wherein the corrosion and salt deposition problems during supercritical water oxidation processes can be avoided effectively. A detailed chemical kinetics model for methanol was employed and validated to understand the hydrothermal combustion process mechanism in supercritical water. Based on this elementary reaction model, how the two key indicators (ignition and extinction temperatures) worked during combustion reaction was studied. Moreover, the influences of operational parameters (fuel concentration, oxidation coefficient and reactor type) and corresponding reaction mechanism were investigated. It reveals that H2O2 was identified as one key intermediate product in the combustion kinetics of methanol. Initial concentration and injection flow rate of aqueous fuel were two significant factors that determined the extinction temperature, which decreased as the concentration increased or the injection flow rate reduced. Moreover, a minimum limit for the initial fuel concentration existed above which a stable hydrothermal flame could form. The oxidation coefficient affected combustion temperature in a manner that depended on the coefficient range. The combustion temperature elevated with the oxidation coefficient in the fuel-rich area while dropped in the oxidant-rich area. This phenomenon was interpreted from the point of view of thermodynamics and mechanism. In order to maintain stable flames at as a low injection temperature as possible, vessel reactors were more desirable to be applied for hydrothermal combustion reaction, deriving from the discrepancy in order of magnitude for flow velocity between tubular and vessel reactors. Finally, it is found that the enhancement of auxiliary fuel methanol in decomposition of organic pollutants stemmed from two reasons: high reaction heat release and co-oxidative effect, whereas the refractory compounds could suppress the ignition of methanol in supercritical water.

Calculation of critical and engineering parameters for a supercritical water oxidation reaction system as exemplified by water-aromatic hydrocarbon binary mixtures

Approaches to the calculation of critical and engineering process parameters of the supercritical water oxidation (SCWO) of water-aromatic hydrocarbon (benzene, toluene, and phenol, which are the most hazardous chemicals contained in industrial wastewater) binary mixtures are discussed. The calculations were performed based on an example of the oxidation of 10% aqueous solutions of benzene, toluene, and phenol with the use of the Redlich-Kwong two-parameter equation of state. The critical parameters of the reaction mixture components, the parameters of the equation of state, the minimum oxygen amount required for complete oxidation, the fuel content of the reactor, the maximum values of the reaction temperature and pressure, the composition and critical parameters of the expected reaction products, etc., were calculated. The calculated critical reaction parameters can be used as control signals for an automatic control system in the development of a SCWO process for sewage destruction. The high ecological efficiency of the process was demonstrated using the bioassay testing of starting solutions and reaction products.

Analysis of degradation mechanism of disperse orange 25 in supercritical water oxidation using molecular dynamic simulations based on the reactive force field.

4-[N-(2-cyanoethyl)-N-ethylamino]-4'-nitroazo-benzene (disperse orange 25, DO25) is one of the main components in dyeing wastewater. In this work, supercritical water oxidation (SCWO) process of DO25 has been investigated using the molecular dynamic simulations based on the reactive force field (ReaxFF). For the SCWO system, the effects of temperature, the molecular ratio of DO25, O2 and H2O as well as the reaction time have been analyzed. The simulated results showed that the aromatic rings in DO25 could be attacked by hydroxyl radical, oxygen molecule, and hydroxyl radical together with oxygen molecule, respectively, which caused the aromatic ring-opening reaction to happen mainly through three different pathways. The hydroxyl radicals were mainly from water clusters and H2O2 (which was produced from oxygen molecules reacting with water clusters). However, for the SCW system as comparison, the aromatic rings in DO25 could be attacked by hydroxyl radical only, and the OH radicals just come from water clusters. During the DO25 SCWO degradation process, we also found that N elements in one DO25 molecule were difficult to be converted into environmentally friendly N2 molecules because of steric hindrance, but increasing the number of DO25 molecules could improve the possibility for the connection of N elements, thus promoting N element converting into N2. Extending reaction time could also improve N elements in DO25 to transform into N2 rather than carbonitride.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE WELDING JOINT OF A NEW CORROSION- RESISTING NICKEL-BASED ALLOY AND 304 AUSTENITIC STAINLESS STEEL

With the fast development of industry, a serious global problem, pollution, becomes more apparent. A large number of wastewater is discharged, causing the environment pollution. Supercritical water oxidation(SCWO) becomes the most effective method to treat the wastewater within recent years, but the material used in the equipment plays a key role in restricting the application of the SCWO process. Currently, during the SCWO wastewater treatment process, 304 austenitic stainless steel, alloy 625, P91 and P92 steels are the mainly preheater and reactor materials. In order to reduce the serious corrosion and improve economic efficiency of the materials for this process, a new corrosion resistant Ni-based alloy(called X-2# alloy) has been developed with an aim of replacing the previous ones. In particular, it is highly important to the related behavior of this new alloy welding with the original SCWO. Therefore, the microstructure and mechanical properties of the welding joint of the new alloy and304 austenitic stainless steel with manual argon arc welding were investigated. The microstructure and fracture morphologies of the welding joint were analyzed through OM, SEM and EDS, and the detailed analysis of the micro- hardness, tensile strength and other mechanical properties were performed. The results demonstrated that the parent material with the typical 40~65 mm grains size is helpful for dissimilar steel welding, and the microstructurein fusion zone of X-2# side does not show welding defects. However, some ferrites are further formed near the fusion zone of 304 stainless steel sides. There are Cr-rich and Ni-poor distributions in the ferrites. The grain grows seriously in both the areas near the remelt zone and 304 stainless steel side of heat affected zones(HAZs), which affect heavily the performance of welding joint. In addition, the results also uncover that the Vickers-hardness is the minimum in the HAZ. At room temperature, the fracture location of the tensile tests of X-2#/304 is in the welding seam, whereas at 500 ℃ the corresponding position is in the 304 matrix. Due to the strengthening effects of Al, W and Mo elements, the high temperature mechanical properties of X-2# alloy have been found to be even better than those of the 304 austenitic stainless steel.