Reactions In Supercritical Water Research Articles

Subcritical and supercritical water gasification of humic acid as a model compound of humic substances in sewage sludge

Humic acid is a model compound of sewage sludge that occurs as a result of decomposing organic matter in wastewater. In this study, humic acid gasification was performed at variable subcritical and supercritical water temperatures (325–600°C), feed concentration (10–25wt%) and reaction time (30–90min). High H2 yield of 0.79mol/kg was obtained at 600°C, 15wt% humic acid and 75min without any catalyst. Catalysts such as K2CO3, Ni/Al2O3–SiO2, FeCl3 and ZnCl2 were examined to enhance H2 production and humic acid degradation. While H2 yield increased exponentially with rising FeCl3 loading, Ni/Al2O3–SiO2 and K2CO3 reduced H2 yield due to bimolecular condensation and catalyst deactivation. Maximum yield of H2 (4.09mol/kg) and total gases (6.20mol/kg) were obtained with 15wt% humic acid and 15wt% FeCl3 at 600°C and 75min. Elevated temperatures and high FeCl3loading promoted the degradation of humic acid with higher gas yields and fragmented surface morphology in char residues.

Facile Approach for the Syntheses of Ultrafine TiO2 Nanocrystallites with Defects and C Heterojunction for Photocatalytic Water Splitting

In this paper, a supercritical water (sc-H2O) reaction medium was employed for the syntheses of ultrafine TiO2 nanocrystallites (at ca. 5 nm) that were linked with lactate species at surface. The resulting hybrid material was then subjected to an aging at ca. 300 °C for 2 h under N2 atmosphere. After subjected to spherical aberration corrected STEM and EPR analyses, it was noted that the aged sample was shown with highly distorted crystal lattice with oxygen vacancies at surface and Ti3+ in the bulk. The anoxic aging also caused incomplete combustion for lactate species, leading to the formation of C heterojunction with TiO2. UV–vis, PL and transient photocurrent (TP) measurements revealed that the resulting surface oxygen vacancies and C heterojunction had conferred a combination of advantages in enhancing visible light absorption and promoting electron–hole pair separation for aged sample, which led to significantly promoted hydrogen production efficiency in photocatalytic water splitting under a full-spectrum irradiation (where the aged TiO2 had yielded ca. 4-fold higher hydrogen production rate than the nonaged one and ca. 40–50-fold higher than commercial Degussa P25). We expected that the work conducted herein could provide a facile and controllable approach to produce simultaneously defects and C heterojunction for ultrafine TiO2 nanocrystallites, which might lead to scale-up production of them for industry.

Rapid syntheses of ultrafine LaMnO3 nano-crystallites with superior activity for catalytic oxidation of toluene

Supercritical water (sc-H2O) reaction environment was evidenced with the capability to dramatically facilitate the solid-state reaction between hybrid oxides. This led to one-step and rapid (within several seconds) syntheses of ultrafine LaMnO3 nano-crystallites (at ca. 6nm) with no need of additional heat-treatment (literaturelly need to be heat-treated at 600–800°C for 4–6h in static air). The ultrafine LaMnO3 was shown with remarkable surface area and distinct oxygen mobility and reducibility, which yielded an activation energy (Ea) in toluene oxidation at only ca. 54.3kJ/mol, comparable to some noble metal catalysts.

Solid Oxide Fuel Cell Integrated with Supercritical Water Fueled by Methanol

A solid oxide fuel cell (SOFC) is known as an interesting energy conversion device because of its fuel flexibility and high efficiency. The hydrogen-rich stream is used as fuel carrier converting to generate electrical energy. A non-stoichiometric thermodynamic model based on minimum free energy was performed to predict the amount of hydrogen production via the methanol reforming under supercritical water (SCW) condition. The effects of SCW reaction temperature and water-to-methanol molar ratio on the SOFC power generation integrated with SCW reforming from methanol were investigated. The hydrogen yield, the required heat duty for a feed preheater and a SCW reactor and the SOFC power generation increase with increasing the SCW reaction temperature and the amount of water fed in SCW reactor. Under operating parameters of SCW reformer based on 1 mole/sec of methanol fed at the high temperature of 1273 K and water-to-methanol molar ratio of 5, the SOFC electrical power of 246 kW was produced with the maximum fuel utilization of 0.7.

다양한 촉매들을 통한 모델 바이오매스-초임계수 촉매 가스화에서 수소 생산 성능에 대한 연구

>> In this study, the model biomass was used for hydrogen production by supercritical water gasification (SCWG). Model biomasses were glycerol, glycine, lignin and cellulose. The feed concentration was set to 1 wt%. Experiments were conducted in a reactor at 440 °C and above 26.3 MPa for 30 min. The effects of catalysts such as alkali metal salt (K2CO3 and Na2CO3) and transition metal salts (Ni(NO3)2, Fe(NO3)3 and Mn(NO3)2) on the gasification were systematically investigated. No tar or coke was observed in all experiments. The results showed that the gasification efficiency increased with various catalysts. For the cellulose and glycerol, all catalysts were effective for the promoted H2 production compared with no catalyst. The significant decrease of H2 production compared with no catalyst was observed with Na2CO3 and Fe(NO3)3 for glycine and lignin. respectively. The highest H2 production, 1.24 mmol was obtained for glycerol-SCWG with Mn(NO3)2. Conclusively, the addition of Mn(NO3)2 enhanced all model biomass gasification efficiency and increased the hydrogen production promoting the supercritical water reaction.

Rapid determination of products of phenol hydrogenation in a supercritical water system using headspace gas chromatography

AbstractThis paper reports a headspace analysis technique for the determination of products, i.e., cyclohexanone (CE) and cyclohexanol (CL), of phenol hydrogenation in a supercritical water reaction system (SWRS) with water removal by hydrate formation. An addition of anhydrous calcium chloride leads to water absorption resulting in crystal water; thus, the samples can be quantitatively measured without the influence of water. After achieving equilibrium at 150­°C and maintaining it for 5 min, the obtained results showed a relative standard deviation of less than 5.3 % and the recovery ranged from 93 % to 104 %. The presented method is simple and accurate for the analysis of CL, CE and phenol in samples from phenol conversion in SWRS.

Effect of water density on heterogeneous catalytic water gas shift reaction in the presence of Ru/C, Pd/LaCoO3 and Fe3O4 in supercritical water

Water gas shift reaction in the presence of heterogeneous catalysts such as Ru/C, Pd/LaCoO3 and Fe3O4 was examined in supercritical water at 673K from 0.1 to 0.5g/cm3 of water densities. These catalysts catalyzed water gas shift reaction and the main products were CO2 and H2. CH4 was formed from CO and H2 produced by water gas shift reaction in the presence of Ru/C. CO conversion tended to decrease with increasing water density for all catalysts, which means that water essentially suppressed heterogeneous catalytic water gas shift reaction in supercritical water. The kinetics of water gas shift reaction was analyzed with an elementary-like reaction model using fugacity. The model calculation represented the trend of experimental results and revealed that the suppression of water gas shift reaction in high water density region was probably because water would cover on active sites and stabilize the adsorbed species on catalyst to decrease the ratio of vacancy of active sites on catalyst.

Kinetics of the Non-catalytic Water Gas Shift Reaction in Supercritical Water

A kinetic analysis of the noncatalytic water gas shift reaction in a supercritical water medium was investigated using a specially designed 383 mL Haynes Alloy 230 tubular reactor at a constant pressure of 24.12 ± 0.04 MPa, water to carbon monoxide molar feed ratios of 5 to 37 moles of water per mole of carbon monoxide, and at temperatures varying from 768 to 1,048 K. The carbon monoxide concentration in the effluent gas reached a minimum of one mole-percent at 1,048 K, which corresponds to a 98% carbon monoxide to carbon dioxide conversion. Using global first-order kinetics a frequency factor of 105.76 ± 1.42 s−1 and an activation energy of 139.8 ± 24.5 kJ/mol was determined. It was also found that the developed kinetic rate model closely fits the experimental reaction data, with a coefficient of determination of R2 = 0.95, over a wide range of temperatures and reactant concentrations.

Catalytic hydrothermal treatment of pharmaceutical wastewater using sub- and supercritical water reactions

Application of subcritical and supercritical water technology for destruction of pharmaceutical compounds (carbamazepine, metoprolol and sulfamethaxazole) was investigated. The experiments were conducted inside batch reactor at a temperature ranging from 473 to 773K and with different residence times of 5 to 50min. The results show that carbamazepine, metoprolol and sulfamethaxazole are destructed by 90.27%, 99.99% and 98.84% after a 20min exposure to 623K, 673K and 573K, respectively. In comparison with the conventional methods of pharmaceutical waste treatment, the current technology provides a higher destruction efficiency (approximately 90–100%) which is achievable in shorter durations. NaOH and CuSO4·5H2O were also applied as catalysts in the temperature range of 473K to 723K. Comparing these catalysts, CuSO4·5H2O demonstrates a higher destruction efficiency, especially at lower temperatures. Based on the proposed pathway, the products of destructioncan be classified as environmentally-friendly compounds. The results show that this technology can be used as a green alternative for efficient removal of pharmaceutical compounds from wastewater streams.

Further evidence for the stabilization of U(V) within a tetraoxo core.

Two complex layered uranyl borates, K10[(UO2)16(B2O5)2(BO3)6O8]·7H2O (1) and K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O (2), were isolated from supercritical water reactions. Within these compounds, borate exists only as BO3 units and is found as either isolated BO3 triangles or B2O5 dimers, the latter being formed from corner sharing of two BO3 units. These anions, along with oxide and hydroxide, bridge between uranyl centers to create the complex layers in these compounds. U(VI) cations are found within uranyl, UO2(2+) units, that are bound by four or five oxygen atoms to create tetragonal and pentagonal bipyramidal environments. The most striking feature in this system is found in 2, where a [UO4(OH)2] unit exists that contains U(V) within a tetraoxo core with trans hydroxide anions; therefore, this compound is a mixed-valent U(VI)/U(V) borate. The presence of a 5f(1) uranium site within 2 leads to unusual optical properties.

Upgrading of Heavy Oil by Hydrogenation through Partial Oxidation and Water-gas Shift Reaction in Supercritical Water

Upgrading of Heavy Oil by Hydrogenation through Partial Oxidation and Water-gas Shift Reaction in Supercritical Water

Simulations of Acid Dissociation Constants of Polyprotic Acids in Near-Critical and Supercritical Water

This paper reports a molecular dynamics study on the dissociation of sulfuric acid and phosphoric acid in near-critical and supercritical water. pK(a) is known to vary as the temperature and pressure vary, and this variation has important implications for corrosion in supercritical water reaction vessels. This work uses the SPC/E water model and solutes based upon DFT calculations to examine both structural and thermodynamic properties of the dissociation processes. An increase in solute-solvent ordering is observed for larger charges, and this also corresponds to a lower rate of diffusion. All dissociation reactions become less favored with increasing temperature except pK(a1) for sulfuric acid which becomes significantly more favored until 748 K.

Mechanistic Roles of Hydroxyl Group in the Supercritical Water Reaction of Polyhydric Alcohols

Mechanistic Roles of Hydroxyl Group in the Supercritical Water Reaction of Polyhydric Alcohols

In-situ Raman spectroscopy of BaTiO3 particles for tetragonal–cubic transformation

Tetragonality of the hydrothermally synthesized BaTiO3 particles were analysed by X-ray diffractometry (XRD) and Raman spectroscopy in comparison with the commercial BaTiO3 particles. The BaTiO3 particles were synthesized by hydrothermal reaction in supercritical water with the variation of reaction time for 8ms, 2s and 2h. The reactants of TiO2 sol and Ba(OH)2 solution were used as starting materials and that was heated up to 400°C under the pressure of 30MPa. The XRD patterns and Raman spectra revealed that the crystal phase of the obtained particles for 2s and 2h was tetragonal BaTiO3, while that for 8ms was cubic BaTiO3. The relation between crystal phase and particle size was discussed. Primarily particle size of the BaTiO3 particles determined by means of BET surface area increased with an increase in the reaction time from 10nm to over 100nm. Intensity of Raman band at 305cm−1 increased with the particle size, indicating that the tetragonality is prominent for the highly crystalline BaTiO3 particles. Tetragonal to cubic phase transition was observed in-situ Raman spectra under air conditions where Curie temperature was between 100 and 150°C.

A theoretical study of factors affecting corrosion in supercritical water reaction vessels

This paper reports a molecular dynamics study of the pKW of water, pKa of HCl and pKb of NaOH in supercritical water at a pressure of 1000atm. The extent of acid/base dissociation is known to be an important factor in the corrosion of supercritical water reaction vessels. This work describes classical molecular dynamics simulations using the SPC/e water model to gain insight into the controlling factors. The results indicate that the dissociation of water has a maximum at 448K, whereas both the dissociation of HCl and NaOH become less favourable with increasing temperature due to a decrease in entropy. The simulation results are compared to values calculated from the generalised Born theory, and there is a qualitative agreement in the trend.

Utilization of Sub and Supercritical Water Reactions in Resource Recovery of Biomass Wastes

Recently, the utilization of sub and supercritical water has been proposed to recover waste substances from biomass. This is important not only for prevention of environmental issues, but also for rational utilization of natural resources. Sub and supercritical water treatment is one of the most effective methods for this, because water at high temperature and high pressure behaves as a reaction medium with remarkable properties. This sub and supercritical water treatment process can promote various reactions such as oxidation, hydrolysis, and dehydration. Therefore, sub and supercritical water can be used for the conversion of organic wastes to useful chemical compounds, as well as for oxidizing hazardous waste into CO 2 or harmless compounds. This paper presents the concepts of sub and supercritical water and their application in organic waste recycling at temperatures of 423 - 673 K for substances such as lignin and its derived compounds.

ChemInform Abstract: Organic Reactions in Subcritical and Supercritical Water

AbstractReview: 85 refs.

Hydrothermal synthesis of Eu3+ doped yttria nanoparticles using a supercritical flow reaction system

Highly crystalline Eu3+ doped yttria nanoparticle was synthesized by hydrothermal reaction in supercritical water using a continuous flow reaction system (FHT). The reactants of Y(NO3)3/Eu(NO3)3 mixed solution and KOH solution were used as starting materials and that was heated quickly up to 350–450°C under the pressure of 30MPa for 0.1–15s as reaction time. The XRD results revealed that the crystal phase of as-prepared particles was YOOH and converted into cubic-phase Y2O3 after annealing above 550°C. Primarily particle size of the YOOH was as small as less than 50nm, keeping after annealing at 800°C. Effects of reaction time, annealing temperature and Eu doping amount on photoluminescence were examined. The as-prepared particles exhibited red emission without annealing at high temperatures whereas photoluminescent intensity at 612nm was increased with an increase in the annealing temperature. Photoluminescent intensity was increased with an increase in the Eu doping amount until 4mol% and saturated at 8mol%. The photoluminescent property was compared with reference samples via conventional co-precipitation (CP) and batchwise hydrothermal (BHT) methods. The photoluminescent intensity for annealed samples increased in the order: FHT<BHT<CP owing to the increased particle size.

Carbon formation in supercritical water reaction medium

Supercritical water (SCW) has various physicochemical properties tunable with its temperature and pressure conditions. In recent years, SCW has been used as a new reaction medium for the preparation of carbon. Particularly, the presence of a radical initiator in SCW has provided carbon particles even at the low temperature of 400°C. Furthermore, the selection of adequate starting materials in the corresponding system enables the formation of carbon-metal oxide composites. Here we review research on the application of SCW media to carbon formation.

Waste Treatment by Supercritical Water Oxidation

Supercritical water oxidation (SCWO) is a new oxidation technology that utilizes supercritical water reaction to destroy the configuration of matter. Organic waste is converted to harmless gaseous or liquid substances such as CO2 and H2O. Thanks to its unique technology advantage, it has a rapid development in the recent years, and becoming a potential and green technology of wastewater and organic waste treatment. This paper focused on SCWO technology and apparatus, and reviewed achievements on patents of wastewater and organic waste treatment by SCWO. Eventually, this article prospected the development trend of this new oxidation technology.