Continuous flame oxidation in supercritical water

Abstract

Continuous flames have been observed in Supercritical water oxidation (scWO) of isopropyl alcohol (IPA), using a vertical continuous reactor with sapphire windows and a mixing nozzle. Two types of continuous flame were confirmed: the one was long pale blue colored and the other was red short cone shaped, changing blue to red at around air ratio 2.0. The flame was strongly influenced by IPA concentration, air ratio and design of the mixing nozzle. Results for decomposition of PA are presented for IPA concentrations ranging from 600 up to 28260 ppm as TOC and initial reactor temperatures, were mostly around 490°C, at 25 MPa. Decomposition rate at steady state was over 99.9%. Experimentally measured CO₂ and O₂ concentrations at the flue gas were in good agreement with theoretically calculated values. Even for low air ratio as 1.1, high decomposition rate without CO, NO, NO₂ was achieved.     

Corrosion on continuous supercritical water oxidation for polychlorinated biphenyls

Abstract

The corrosion resistance of Hastelloy C-276 and titanium alloy G5 against C1 anion which was generated by supercritical water oxidation (scWO) of 3-chlorobiphenyl (3-PCB) in a flow reactor was investigated over a period of 336 hours at a constant pressure of 30 MPa. The condition of the scWO reaction zone was fixed at 750 K. The concentration of chlorine in the effluent collected after cooling to ambient temperature was about 850 wppm. The Hastelloy C-276 has a nominal ratio of approximately 3.7 Ni/l Cr. The Ni/Cr ratio in the effluent was the same with the nominal ratio, and the Hastelloy C-276 was proved to be corroded on the surface located between the bottom of the reactor and the cooling parts, which are under sub- and near- critical conditions of water. The corrosion rate was found to be 5 to 34mm/y in the zone where the temperature ranged from 543 K to 650 K and the corrosion spread over the whole surface of the alloy; however, one can hardly observe such a serious corrosion except the zone located in the between the reactor and the cooling part.     

Oxidation of garbage in supercritical water

Abstract

In the present study, 17 kinds of organic materials selected from groups of vegetable, meat, fat and fish as main components of garbage were oxidized with H₂O₂, using a batch reactor system under the condition of supercritical water, in order to treat organic wastes of high water content and low calorific value. It was found that they were easily oxidized, and remarkable influence of the kind of material in each group on the TOC decomposition was not recognized, but meats, fats and fishes, especially beef suet, were more difficult to be oxidized than vegetables.

The residual intermediate product was acetic acid in reactions of one minute at 400°C for all materials. Based on this result, the rate expression for supercritical water oxidation of acetic acid was determined.     

Impact of non-ideal transport modeling on supercritical flow simulation

The simulation of a supercritical fluid flow requires sophisticated models for real gas thermodynamic and non-ideal phenomena. They both are presently addressed through the simulation of a non-reacting and reacting high pressure H₂/O₂ splitter-plate configuration. In particular, the diffusion velocity of species is evaluated through the gradient of chemical potential (${\mathbf{d}}_l^{\text{NI}}=X_l{\left(?{\mu }_l\right)}_T$) expressed with the Peng–Robinson equation of state, or with the classical low-pressure approach ${\mathbf{d}}_l^{\mathrm{I}}=?X_l,$ which only uses the gradient of the lth species molar fraction, X_l. In addition, the high pressure binary diffusion coefficients are estimated by the correction of Kurochkin et al. or with the Takahashi approach. The results for the non-reaction case are consistent with the literature for mean and rms values using ${\mathbf{d}}_l^{\mathrm{I}}$. The use of ${\mathbf{d}}_l^{\text{NI}}$ has a limited impact but the temperature profiles become steeper. In the reactive case, the two approaches lead to a difference of 50 K on the average temperature just downstream of the injector and about 100 K further downstream. A non-ideal transport is then required for the modeling of supercritical flow simulation.     

Ozone oxidation of 4H-SiC and flat-band voltage stability of SiC MOS capacitors

We investigate the effect of ozone (O₃) oxidation of silicon carbide (SiC) on the flat-band voltage (V_fb) stability of SiC metal-oxide-semiconductor (MOS) capacitors. The SiC MOS capacitors are produced by O₃ oxidation, and their V_fb stability under frequency variation, temperature variation, and bias temperature stress are evaluated. Secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) indicate that O₃ oxidation can adjust the element distribution near SiC/SiO₂ interface, improve SiC/SiO₂ interface morphology, and inhibit the formation of near-interface defects, respectively. In addition, we elaborate the underlying mechanism through which O₃ oxidation improves the V_fb stability of SiC MOS capacitors by using the measurement results and O₃ oxidation kinetics.     

Non-thermal pasteurization of lipid emulsions by combined supercritical carbon dioxide and high-power ultrasound treatment

Supercritical carbon dioxide (SC-CO₂) is a novel method for food pasteurization, but there is still room for improvement in terms of the process shortening and its use in products with high oil content. This study addressed the effect of high power ultrasound (HPU) on the intensification of the SC-CO₂ inactivation of E. coli and B. diminuta in soybean oil-in-water emulsions. Inactivation kinetics were obtained at different pressures (100 and 350 bar), temperatures (35 and 50 °C) and oil contents (0, 10, 20 and 30%) and were satisfactorily described using the Weibull model. The experimental results showed that for SC-CO₂ treatments, the higher the pressure or the temperature, the higher the level of inactivation. Ultrasound greatly intensified the inactivation capacity of SC-CO₂, shortening the process time by approximately 1 order of magnitude (from 50 to 90 min to 5–10 min depending on the microorganism and process conditions). Pressure and temperature also had a significant (p < 0.05) effect on SC-CO₂ + HPU inactivation for both bacteria, although the effect was less intense than in the SC-CO₂ treatments. E. coli was found to be more resistant than B. diminuta in SC-CO₂ treatments, while no differences were found when HPU was applied. HPU decreased the protective effect of oil in the inactivation and similar microbial reductions were obtained regardless of the oil content in the emulsion. Therefore, HPU intensification of SC-CO₂ treatments is a promising alternative to the thermal pasteurization of lipid emulsions with heat sensitive compounds.     

Collective dynamics of supercritical water

Inelastic X-ray scattering experiments on sub- and supercritical water were performed to investigate collective dynamics of this unique solvent. Analysis within a generalized Langevin formalism shows that the positive dispersion of the sound velocity, as compared to the hydrodynamic value, first decreases (1.0<ρ<0.8 g cm⁻³) at all measured momentum transfers (1.3-10.7 nm⁻¹), and then increases (0.7<ρ<0.26 g cm⁻³) again only at higher momentum transfers. We suggest the initial decrease is due to approaching the percolation limit in the number of hydrogen bonds, and the subsequent increase is due to the formation of rigid dimers in sub- and supercritical water.     

Direct synthesis of luminescent SiC quantum dots in water by laser ablation

Direct synthesis of luminescent SiC quantum dots in pure deionized water by laser ablation is reported. A zeta‐potential of –49 mV, higher than for nanoparticles produced by electrochemical etching, suggests a higher colloidal stability of the laser‐produced nanoparticles. IR spectroscopy shows that surface charges related to carboxylate anions ensure long‐term stability of the colloidal solutions based on the as‐prepared SiC quantum dots. Main radiative channel at room temperature corresponds to the recombination of quantumly confined photogenerated charge carriers. Emission from the nanoparticles is mainly centred at 2.75 eV regardless of laser power used for ablation. According to photoluminescence spectra, a partial transformation of the SiC quantum dots from cubic to hexagonal crystalline arrangement is supposed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diffusion of ions in supercritical water: Dependence on ion size and solvent density and roles of voids and necks

We have performed molecular dynamics simulations of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and halide (F⁻, Cl⁻, Br⁻, I⁻) ions in supercritical water at 673 K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35 g cm⁻³ to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7 g cm⁻³). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35 g cm⁻³) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed.     

Determination of the thickness and density of the ion bombardment induced altered layer in SiC by means of reflection electron energy loss study

The steady state surfaces of ion bombarded 3C-, 4H- and 6H-SiC samples were studied by means of reflected electron energy loss spectroscopy (REELS). The REELS exhibit a well-defined loss peak in the region of about 20 eV. The position of the maximum of the loss peak depends on the bombarding ion energy (decreasing with increasing ion energy), and on the primary electron beam energy (increasing with increasing primary energy). This behavior can be explained if we suppose that the plasmon energy in the altered layer (produced by ion bombardment) is different from that of the unaltered bulk. In this case the measured loss peak is the sum of two overlapping plasmon peaks. With modeling the system as a homogeneous altered layer and a homogeneous unaltered substrate the plasmon energy in the altered layer was derived to be 19.8 eV. The large change of the plasmon energy with respect to the bulk value of 23 eV is explained by a thin low density overlayer on the surface of the sample produced by the ion bombardment.     

Influence of inserting AlN between AlSiON and 4H-SiC interface for the MIS structure

A control of interface between gate insulating film and semiconductor is required to achieve high-power field effect transistors (FET) using SiC. To improve the interface between the high-k layer and SiC, we propose inserting an AlN layer as an interfacial layer. The reason for selecting AlN film is that it has a wide bandgap, as well as almost the same lattice constant as that of 4H-SiC. The insertion of AlN film between 4H-SiC and the insulating film effectively reduces the interfacial roughness. The roughness of the interface between AlN and SiC can be suppressed compared with that of the thermal oxidized SiC. Moreover, the AlSiON film was deposited on the AlN layer as a high dielectric gate insulating film with low leakage current at high temperature and low space charge. The C-V characteristics of the AlSiON/AlN/SiC MIS structure with an AlN buffer layer are improved by increasing the deposition temperature of the AlN film. This demonstrates that AlSiON/AlN/SiC is one of attractive MIS structures for SiC devices.     

Formation of atomically smooth SiO2/SiC interfaces at ∼120 °C by use of nitric acid oxidation method

Conventional thermal oxidation of SiC requires heating at ∼1100 °C. In the present study, we have developed a method of oxidizing SiC at low temperatures (i.e., ∼120 °C) to form relatively thick silicon dioxide (SiO₂) layers by use of nitric acid. When 4H-SiC(0 0 0 1) wafers are immersed in 40 wt% HNO₃ at the boiling temperature of 108 °C and the boiling is kept for 5 h after reaching the azeotropic point (i.e., 68 wt% HNO₃ at 121 °C), 8.1 nm thick SiO₂ layers are formed on the SiC substrates. High resolution transmission electron microscopy measurements show that the SiO₂/SiC interface is atomically flat and the SiO₂ layer is uniform without bunching. When SiC is immersed in an azeotropic mixture of HNO₃ with water from the first, the SiO₂ thickness is less than 0.3 nm. The metal-oxide-semiconductor (MOS) diodes with the SiO₂ layer formed by the nitric acid oxidation method possess a considerably low leakage current density.     

Measurements of saturated basic and acidic rock electrical conductivities at lower crustal temperatures and high pressures

Abstract

Laboratory measurements of the electrical conductivity of brine saturated acidic and metabasic rocks were done at confining pressures up to 0.2 GPa, high pore pressures and temperatures of 900°C. Acidic rocks showed conductivities insufficient to explain the lower crustal high conductivity layer. Basic rocks, however, showed conductivities consistent with the high conductivity layer at those temperatures that the layer is thought to possess.     

Synthesis of Cu-ZnO and C-ZnO nanoneedle arrays on zinc foil by low temperature oxidation route: Effect of buffer layers on growth, optical and field emission properties

Different densities of ZnO nanoneedle films have been prepared by pre-coated zinc foils with thin layer of copper and carbon followed by thermal oxidation at 400 °C in air. The X-ray diffraction patterns show well defined peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. The scanning electron microscope images clearly reveal formation of ZnO needles on the entire substrate surface. The X-ray photoelectron spectroscopy studies indicate that Cu and C ions are incorporated into the ZnO lattice. Photoluminescence studies evaluate different emission bands originated from different defect mechanism. From the field emission studies, the threshold field, required to draw emission current density of ∼100 μA/cm², is observed to be 2.25 V/μm and 1.57 V/μm for annealed zinc foil pre-coated with copper and carbon, respectively. The annealed film with copper layer exhibits good emission current stability at the pre-set value of ∼100 μA over a duration of 4 h. The results show that buffer layer is an important factor to control the growth rate, resulting in different density of ZnO needles, which leads to field emission properties. This method may have potential in fabrication of electron sources for high current density applications.     

Ab initio molecular dynamics investigations of structural, electronic and dynamical properties of water in supercritical carbon dioxide

Ab initio molecular dynamics simulations of a solitary perdeuterated water molecule solvated in supercritical carbon dioxide have been performed along an isotherm at three different densities. Electron donor-acceptor interactions between the oxygen atom of water and the carbon atom of CO₂ as well as hydrogen bonded interactions between the two molecules have been shown to play a dominant role in the solvation. The mean dipole moment of the water molecule increases with the density of the solution, from a value of 1.85 D at low density to around 2.15 D at the highest density. The increase in the solvent density causes the water molecule to exhibit a range of behavior, from a free molecule to one that interacts strongly with CO₂. A blue shift in the bending mode of water has been observed with increasing solvent density. The carbon dioxide molecules which are present in the first neighbor shell of water are found to exhibit larger propensity to deviate from a linear geometry in their instantaneous configurations.      

Supercritical water oxidation (SCWO): a process for the treatment of industrial waste effluents

Abstract

SuperCtical Water Oxidation (SCWO) process is investigated at the Institut of Technical Chemistry, ITC-CPV. The objectives were to determine destruction efficiencies of model compounds and industrial waste effluents and to study the feasability of the SCWO process.

Two continuous SCWO bench scale plants are operated a pipe reactor and a transpiring wall reactor system (design data: T= 630°C, P = 32MPa, feed rate waste water =10 and 50kg/h, air feed rate = 20kglh, transpiring and quench water feed rate = 50 kg/h). Suspensions containing up to 5%wt solid material can be fed to the reactor using a membrane pump.

With the pipe reactor, efficiencies of up to 99.99% were achieved for the oxidation of model compounds (ethanol, toluene, phenol) as well as real waste effluents (paper, chemical, pharmaceutical industry, sewage works). The use of the pipe reactor is limited to feeds without salt to avoid plugging.

Salty feeds are processed using the transpiring wall reactor, which is consisting of a pressure bearing tube outside and a porous tube as reactor inside. Water is steadily running through the porous reactor preventing the formation of deposits on the wall.

SCWO has a high potential at least for the destruction of halogented organic compounds using the transpiring wall reactor system and is seen to be competitive to other processes for waste destruction.     

Investigation of formation mechanism of Ti3SiC2 by high pressure and high-temperature synthesis

ABSTRACT

In this paper, synthesis of titanium silicon carbide (Ti₃SiC₂) under high pressure and high-temperature condition has been investigated by using the reactant systems Ti/Si/C, Ti/SiC/TiC, Ti/SiC/C and Ti/TiC/Si. Results reveal that Ti/TiC/Si is unsuited to the synthesis of Ti₃SiC₂ under a high pressure of 2.0?GPa, while an elemental mixture of Ti/Si/C is applicable. By the addition of Al, Ti₃SiC₂ with 95.8?wt% purity was obtained from elemental mixture with a large excess of silicon. The optimum experimental parameters were determined as Ti/Si/Al/C having the molar ratio of 3:1.5:0.5:1.9, holding at 2.0?GPa and 1300?°C for 60?min.     

Removal of water adsorbed on flouride glass surfaces by NF3 plasma processing

A new method to remove water adsorbed on flouride glass preforms is reported. This method, using NF₃ plasma, can remove surface water on fluoride glasses     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Determination of nitrites and nitrates in plasma-activated deionized water by microchip capillary electrophoresis

In this work, we present the application of a fast and sensitive microanalytical method, microchip capillary electrophoresis (MCE), for the determination of NO₂⁻ and NO₃⁻ ions in deionized water treated by atmospheric pressure plasma jet (APPJ). The MCE technique consisted of an online combination of isotachophoresis with zone electrophoresis, both performed on the microchip. The argon plasma has been characterized by optical emission spectroscopy (OES) and Fourier transform infrared spectroscopy (FTIR). OES confirmed the presence of argon excited species (Ar I) emission (4p → 4s) lines, N₂ emission bands (second positive system C³Π_u → B³Π_g), and OH band (A²∑⁺ → X²Π Δv = 0), as well as the presence of NO and excited NO₂. The presence of NO₂ molecules was also confirmed by FTIR absorption spectroscopy. The performance of the developed MCE method was evaluated for linearity, limit of detection, limit of quantitation, precision, and accuracy, and the concentration of NO₂⁻ and NO₃⁻ in the water as a function of the water treatment time was monitored.