Supercritical Water Conditions Research Articles

Hydrothermal synthesis, crystal structure, and vibrational and Mössbauer spectra of a new tricationic orthophosphate — KCo3Fe(PO4)3

Crystals of a new potassium cobalt(II) iron(II) triorthophosphate salt, KCo3Fe(PO4)3, as well as amorphous nanoparticles of the same material have been produced by hydrothermal reactions, under supercritical water (SCW) conditions, of equivalent quantities of aq. CoCl2·6H2O (2 mol/L) and K4P2O7(1 mol/L) in concd. HCl in a stainless steel batch reactor at 400–450 °C and 25–32 MPa. The dark-violet crystals of the new salt have been characterized by single-crystal X-ray diffraction and IR, Raman, and Mössbauer spectroscopy. The Fe(II) in the new orthophosphate salt must derive from leaching of iron from the stainless steel walls of the reactor by concd. HCl under the SCW conditions. The salt crystallizes in the orthorhombic space group Pnnm, Z = 4, with unit cell parameters as follows: a = 9.672(1) Å, b = 16.457(2) Å, c = 6.2004(8) Å, V = 986.97 Å3. Microporous KCo3Fe(PO4)3exhibits infinite chains of corner-sharing [PO43–], [FeO6], and [CoO5] units, which form two large rhombus tunnels, occupied by K+ions, running parallel to [100] in the unit cell. The vibrational spectra have been interpreted partly on the basis of factor group effects and are in accord with the centrosymmetric structure of the material. The particle sizes of this new nanomaterial and their distribution have been determined by scanning electron micrography and laser scattering particle size distribution analysis, while the amorphous nanomaterial has also been characterized by X-ray powder diffraction and Raman spectroscopy. Key words: crystal structure, vibrational and Mössbauer spectra, orthophosphate, nanomaterial.

Composition of Products from the Supercritical Water Gasification of Glucose: A Model Biomass Compound

Glucose, as a representative biomass model compound, was gasified under supercritical water conditions in a batch reactor. The influences of temperature in the subcritical and supercritical regimes of water, oxidant concentration, glucose concentration, and residence time were examined in relation to the yield and composition of the product gases and oils. The product gases were analyzed by packed-column gas chromatography, and the oils were extracted using solid-phase extraction and analyzed by coupled gas chromatography/mass spectrometry. The main gases produced were carbon dioxide, carbon monoxide, methane, and hydrogen, and there was significant production of oil and char. As the temperature was increased, there was an increase in the yield of gas; similarly, gas yield also increased as the concentration of the oxidant, hydrogen peroxide, was increased. The influence of residence time on the yield of products was small, with a slight decrease in oil yield and an increase in char yield. The oils were composed of a range of oxygenated compounds, including cyclopentanone, methoxybenzene, acetic acid, furfural, acetophenone, phenol, benzoic acid, and their alkylated derivatives. A reaction mechanism is proposed that describes a possible reaction route for the formation of the characteristic compounds found in the oils.

Surface Chemical Analysis on the Corrosion of Alloys in the Supercritical Water Oxidation of Halogenated Hydrocarbon

A surface chemical analysis on the corrosion of various alloys under supercritical water oxidation (SCWO) conditions with relatively high concentrations of halogenated compounds and hydrogen peroxide was conducted using an Auger electron spectroscopy/scanning Auger-electron spectroscopy. The tested alloys (Inconel 600, Hastelloy C-276, Monel 400, stainless steel (SUS 316), Titanium G2, and Zirconium 702) were exposed to the same conditions: 4000 mg/L of 2,4-DCP at 400 °C and 250 bar, with 700% stoichiometric H2O2 in a Hastelloy C-276 batch reactor. Even under the supercritical water (SCW) condition of 4.8 pH without H2O2, a certain degree of corrosion was observed on the surface of all the alloys, especially SUS 316. Under the severe SCWO condition with excess H2O2, the surface corrosion of all the alloys was significant, but the types of corrosion on the alloy surfaces differed. Chromium in the SCWO process that contained H2O2 for halogenated compounds could potentially lead to the corrosion type and de...

Flameless incineration of pyrene under sub-critical and supercritical water conditions

Pyrene was used as a typical four-ring polycyclic aromatic hydrocarbon (PAH), to investigate the mechanisms and incineration behaviour of large organic molecules in a batch supercritical water oxidation reactor using hydrogen peroxide as oxidant. The distribution of carbon as gaseous species and organic species in relation to the temperature and pressure, and reaction time was monitored. The results showed that at 200 °C, pyrene was only slightly decomposed but as the temperature increased to 250 °C and then to 280 °C, carbonisation and thermal cracking became prevalent leading to char formation and decomposition of pyrene to phenanthrene, and later naphthalene. Rapid dissolution and oxidation of the char and organic species started occurring from 300 °C. Increasing reaction time resulted in increased formation of carbon dioxide and carbon monoxide. Initially high product formation of phenanthrene at short reaction times was followed by high decomposition of the organic products in solution as the reaction conditions became progressively more severe. Oxygenated organic species such as aldehydes, ketones, phenols, xanthone, and benzoic acid were identified as the temperature and reaction times were increased between 300 and 380 °C. From the analytical results obtained, carbon mass balances were calculated for each experiment. A proposed mechanism for the observed oxidative decomposition of pyrene is also reported.

Effect of water density on polymorph of BaTiO 3 nanoparticles synthesized under sub and supercritical water conditions

The effects of water density on polymorph of BaTiO 3 particles synthesized hydrothermally under sub and supercritical conditions have been studied. Experiments were performed within the temperature range of 300 to 420 °C and the pressure of 20 to 40 MPa using a flow reaction system. These conditions correspond to the water density of 0.15 to 0.70 g/cm 3. Aqueous TiO 2 sols and Ba(OH) 2 solution were used as starting materials. Characterization of products was performed by powder X-ray diffraction, transmission electron microscopy and Raman spectroscopy. From the results, it has been found that the crystal phase of products depends not only on the temperature but also the pressure. The BaTiO 3 particles with tetragonal phase were obtained under the supercritical conditions, when the density of water was smaller than 0.5 g/cm 3.

Conversion of Organic Streams in Supercritical Water

AbstractThermal treatment of aqueous streams loaded with organics can be efficiently performed at pressures and temperatures above the critical data for water (Pc = 22.1 MPa, Tc = 374 °C). Two applications are under investigation using supercritical water (SCW) as solvent and reactant: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).SCWO is typically operated at 25-35 MPa and 600-900 °C, because water, oxygen (or air), CO2 and most of the organics form a single fluid phase with rapid oxidation kinetics. Thus, SCWO can be processed with high space-time yield and in some cases self-sustaining.Moreover, expensive off-gas treatment is prevented because NOx formation is suppressed. Other heteroatoms form acids like HCl, H2SO4 and H3PO4 or their corresponding salts. However, acids may lead to corrosion, formation or presence of salts to plugs.To avoid these problems a transpiring wall reactor (TWR) has been developed and installed. Results of SCWO of different industrial effluents are very promising. Destruction of the organic waste compounds was close to 100 %, even for effluents containing solids and salts up to 5%wt., each.In accompanied studies material tests have been performed. Long time runs clearly indicate that alloy 625 is most suited to withstand the aggressive environment at temperatures higher than about 500 °C. A corrosion mechanism has been proposed.The SCWG process of biomass is performed under SCW conditions. The aim of this work is to study the conversion of biomass (e.g.) to fuel gas with high energetic value. R&D is focused on process optimization particularly with respect to energy efficiency as well as applicability to extended feedstocks even with high amounts of solids. At SCW conditions organic matter reacts with water to form a hydrogen containing gas. The feed carbon is converted preferentially to CO2, which can be separated by e.g. stripping, and to methane. While the organic carbon is oxidized to CO2, water is reduced to from hydrogen, e.g. for glucose:C6H12O6 + 6 H2O → 6 CO2 + 12 H2In basic studies of the biomass gasification, the main reaction pathways were identified. In addition the influence of ingredients of biomass and additives / catalysts was investigated. Here the influence of salts is of special interest, because biomass with an high water content, which is most suitable for the SCWG process, usually contain high salt contents. The changes in selectivity of different reaction pathways observed, opens the possibility to manipulate the chemistry of biomass gasification in supercritical water. Therefore high gas yields with different feedstock of various compositions can be achieved.SCWG process is performed using efficient heat exchangers at SCW conditions.Compression work is low since non compressible water slurry is pressurized. The reaction of the organic substances with water proceeds fast and completely, thus with high space-time yield. At about 600 °C and 25 MPa high gasification yield can be achieved. Advantageously, formation of CO, tar and char is low enhancing the efficiency of the SCWG process.Consequently, based on lab scale plants, a 100 kg/h SCWG plant called Verena has been installed and is operated since few years. A high thermal efficiency of about 80% for diluted educt streams (only 5 wt% OM) has been measured. Experimental results with the Verena plant confirmed the production of a hydrogen rich gas and the high thermal efficiency of the process.

Progressive and invasive functionalization of carbon nanotube sidewalls by diluted nitric acid under supercritical conditions

Distinctive direct sidewall functionalization of multi-wall carbon nanotubes (MWCNTs) has been carried out using dilute nitric acid (HNO3) under supercritical water (SCW) conditions. The functionalization proceeded invasively from the outer to the inner graphitic layers of the MWCNTs as the reaction progressed. The resulting nanotube-derived material was comprised of a functionalized amorphous carbon sheath and the remaining inner nanotube covered with the sheath. The functional groups induced in the sheath region included alcoholic hydroxyl groups, which endowed the nanotube-derived material with hygroscopicity. The reaction pathways for the functional group formation involved hydration and dehydration processes as well as HNO3 oxidation in the SCW medium. The results of this work have demonstrated the effectiveness of the SCW regime on the direct chemical modification of CNT sidewalls.

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.

Hydrothermal synthesis of potassium niobate photocatalysts under subcritical and supercritical water conditions

Hydrothermal synthesis of potassium niobate powders was carried out under various subcritical and supercritical water conditions using crystalline Nb2O5 powder as a starting material. A single phase of K4Nb6O17 was formed under subcritical water conditions, while mixed phases of K4Nb6O17 and KNbO3 were obtained under supercritical water conditions where KNbO3 was predominant over K4Nb6O17 as the heating duration was increased. Characterization of these hydrothermally synthesized potassium niobates by XRD, SEM, and TG-DTA analyses revealed that fine hydrated powders can be obtained under subcritical and supercritical water conditions. The hydrothermally synthesized potassium niobate powders were used for photocatalytic hydrogen evolution from water decomposition. The crystallinity is responsible for the high photocatalytic performance of the hydrothermally synthesized potassium niobate powders. The maximal hydrogen evolution rate was achieved for the potassium niobate hydrothermally synthesized at 400 °C for 4 hours. Besides, the hydrogen evolution rate was enhanced more than 10-fold by Ni loading for the hydrothermally synthesized potassium niobate powder which was much higher in comparison with the Ni loaded solid-state synthesized photocatalyst.

Biodegradability Improvement and Structural Conversion of Polyvinyl Alcohol (PVA) by Sub- and Supercritical Water Reaction

The feasibility of improving the biodegradability of refractory pollutants in sub- and supercritical water was investigated. Poly vinyl alcohol (PVA) was chosen as a test material to represent high-molecular-weight refractory pollutants. At a temperature of 350°C, BOD increased from 100 mg/L to 3,350 mg/L within 17.4 min, while TOC reduction did not exceed 18%. BOD improvement was calculated based on the change of molecular weight distribution to the range lower than 380 and the decrease of UV absorbance to the range lower than 2.0 × 104 μV in the same molecular weight range. The structural conversion from PVA to biodegradable substances was mainly obtained from hydrolysis and thermal decomposition under sub- and supercritical water conditions. The suitable values of biodegradability (BOD/TOC) for conventional biological treatment methods, 0.78 and 0.79, were obtained in 17.4 min at 350°C and in 7.4 min at 400°C, respectively. The production of biodegradable substances from refractory pollutants by sub- and supercritical water reactions was the most important factor on the viewpoint of the pretreatment of refractory pollutants.

The Partial Oxidation of Isobutene in Sub‐ and Supercritical Water

AbstractThe task of this work was to investigate which kind of reaction pathway takes place under supercritical water conditions in the oxidation of isobutene with molecular oxygen. The reaction was examined at temperatures of 350–20 °C, pressures of 25–35 MPa, uncatalyzed and under platinum‐catalyses. Isobutyraldehyde was the main product. Tert‐butanol was used as a source for isobutene, which dehydrates to isobutene quantitatively under the conditions used.

Reaction of Amino Acids in a Supercritical Water-Flow Reactor Simulating Submarine Hydrothermal Systems

Abstract A novel supercritical water flow-reactor was constructed in order to simulate submarine hydrothermal systems. The temperature of fluid inside the reaction tube could be monitored with thermocouples, which was proved to be different from the temperature outside the reaction tube. Oligomers of glycine up to tetraglycine were formed when a 100 mM glycine solution was heated at 200–350 °C for 2 minutes. None of glycine peptides were produced at 400 °C. It was suggested, however, that the formation of glycine condensates at higher temperature, including supercritical conditions of water. The stability of some amino acids under hydrothermal conditions was examined. ω-Amino acids and glutamic acid, which can form intramolecular condensates, showed higher stability than other α-amino acids at higher temperature, including supercritical conditions.

Supercritical water oxidation of sulfide.

Supercritical water oxidation (SCWO) of sulfide wastewater with oxygen as the oxidant was investigated using ammonium sulfide solution as the model waste. The experiments, which were conducted in a bench-scale continuous SCWO installation, indicated that S2- could be completely oxidized to SO4(2-) within the residence time of less than 10 s under the supercritical water conditions at 698.2-773.2 K and 22.0 MPa-30.0 MPa. The reaction pathway of S2- in SCWO was determined to be S2- --> S2O3(2-) --> SO3(2-) --> SO4(2-). The effects of temperature, pressure, and oxygen content on SCWO of S2- were also studied. The kinetic model based on this pathway was established with the reaction rate constants and orders found by fitting the experimental data. The dependence of the rate constants on the temperature can be described with Arrhenius' law. The rate constants are dependent on the density of SCW, and the dependence is logarithmically linear.

Hydrothermal Synthesis of Potassium Hexatitanate Photocatalyst under Supercritical Water Conditions

Hydrothermal synthesis of potassium hexatitanate (KTO) was carried out under supercritical water conditions. Characterization of the hydrothermally synthesized (ht-) KTO samples by XRD and TEM revealed that fibrous materials can be obtained with the crystal growth in the direction of (010) . To determine the optimum Ru loading for highly active photocatalyst, Ru ion exchange on the ht-KTO was examined with the variation of RuCl3 concentration. The maximum photocatalytic activity of RuO2 loaded ht-KTO was achieved when the Ru loading was 1.0wt% which was much higher in comparison with the solid-state synthesized KTO.

E301 INNOVATIVE COAL GASIFICATION IN SUPERCRITICAL WATER

Novel coal gasification in supercritical water concept has been studied and experimentally demonstrated. Hydrocarbon including coal is partially oxidized using oxygen under supercritical water condition (above 374℃ and 22MPa) to generate a gas composed of H_2,CO_2,CH_4 and CO. Hydrogen and carbon dioxide are main constituents. Impurities sulfur and nitrogen are transformed into H_2S and N_2 with small amount of NH_3 that is easily separated from the gas. Carbonyl sulfide (COS) is scarcely generated, so that gas purification is simplified. The preliminary evaluation of supercritical water gasification system shows a possibility of CO_2 separation in the form of Liquid. These results indicate supercritical water gasification is one of promising technologies in future environmental aspects.

Permeability Enhancement by Microfracturing in Granite under Supercritical Water Conditions.

This paper discusses the generation of micro-fractures in a granite under super critical water environment. In order to create an artificial pathway of water in geothermal reservoir with limited permeability, hydraulic stimulation technology is commonly employed. In this study, simulated hydraulic stimulation tests were performed using thick-walled cylindrical specimens of 45mm outer diameter, under temperatures up to 600°C and confining pressures up to 100MPa. The experimental results of simulated hydraulic stimulations showed that no macroscopic fracturing took place and predominant fluid flow occurred at high temperature regime. The permeability of the granite was also measured using the same cylindrical specimen configuration as used in the simulated hydraulic stimulation tests. The permeability test results showed that the permeability of the granite was enhanced drastically when the temperature exceeded the critical point of water, whilst no significant increase in the permeability was observed under the subcritical water condition. Optical microscopy of the micro-structural change revealed that the enhanced permeability was due to the formation of micro-fractures under the supercritical water environment. This laboratory-scale test result suggests that it may be possible to generate a micro-fracture network by injecting water into a high temperature rock mass whose conditions exceed the critical point of water and to extract the heat energy through the generated fracture network from the supercritical rock mass.

Hydrothermal synthesis of titania photocatalyst under subcritical and supercritical water conditions

Hydrothermal synthesis of titania powders was carried out under various subcritical and supercritical water conditions using titanium(IV) tetraisopropoxide as a starting material. Characterization of these hydrothermally synthesized titania powders by XRD, TEM, and N2 adsorption measurements revealed that mesoporous anatase powders can be obtained by simple hydrothermal processing. The average pore size of the titania powders increased from 4 to 25 nm with an increase in the crystallite size of the titania nanoparticles that aggregate to form the mesoporous structure. Based on TG-DTA and FTIR spectra, surface hydroxyl groups decreased with the hydrothermal temperature, indicating that the condensation of Ti–OH accelerates to form a strong network of Ti–O–Ti at higher hydrothermal temperature. The hydrothermally synthesized titania powders were used for photocatalytic hydrogen evolution in an aqueous methanol suspension. Photocatalytic activity of hydrothermally synthesized titania powders was found to be much higher in comparison with some commercial titania photocatalysts. Activities per unit surface area of more than 10 were obtained for the photocatalysts synthesized under subcritical water conditions and between 39 and 68 for those synthesized under supercritical water conditions. The crystallinity is responsible for the highly photocatalytic performance of the titania powders hydrothermally synthesized under supercritical water conditions.

PCB destruction in subcritical and supercritical water--evaluation of PCDF formation and initial steps of degradation mechanisms.

The reduction of polychlorinated biphenyl (PCB) emissions to the environment are contemporary issues of global efforts, and possible destruction technologies have to be selected and evaluated for PCB remediation. In this study PCB destruction in subcritical and supercritical water were assessed under oxidative conditions and nonoxidative, alkaline conditions. In both cases PCBs could be destroyed by more than 99%. The formation of polychlorinated dibenzofurans (PCDFs) during PCB destruction was evaluated. Under both oxidative and nonoxidative treatments of sub- and supercritical conditions, the formation of PCDFs was observed. The PCDFs formed in the early stages of PCB destruction resulted in up to 47-fold increase in terms of toxic equivalency (TEQ) compared to the initial PCB mixture. However, the PCDFs were destroyed together with the PCBs under more severe conditions i.e., at higher temperature or prolonged residence time. The mechanism of PCDF formation and the initial step of PCB degradation was evaluated. Our laboratory-scale investigation indicates that PCB destruction under supercritical water conditions is feasible, but because of the PCDF formation potential, in particular the high ratio of toxic 2,3,7,8-substituted congeners, conditions have to be carefully selected.

The Effects of Mixing and Oxidant Choice on Laboratory-Scale Measurements of Supercritical Water Oxidation Kinetics

The use of laboratory-scale equipment to measure intrinsic oxidation kinetics in supercritical water environments was evaluated in this study. The objectives were two-fold: (1) to compare the use of hydrogen peroxide with dissolved oxygen as an oxidant and (2) to characterize the dynamics and intensity of mixing organic reactant and oxidant streams. Methanol was used as the model organic as the oxidation rate exhibits a first-order dependence according to extensive earlier studies. No statistically significant difference was observed in the reaction rates or product distributions for the use of either dissolved oxygen gas or hydrogen peroxide that was preheated and fully decomposed before mixing with methanol at supercritical water conditions (500 °C, 246 bar). The intensity of mixing was shown to be an important factor in determining effective mixing times for the reactant and oxidant. Although hydrodynamic effects are certainly dependent on the design and geometry of the mixing tee in the reactor system, fully turbulent (Re > 10 000) cross-flow between entering oxidant and organic streams was found to reduce mixing times to 1 s or less.

218 超臨界水環境下における花こう岩の破壊特性と水圧破砕挙動に関する研究

218 超臨界水環境下における花こう岩の破壊特性と水圧破砕挙動に関する研究