Reactions In Supercritical Water Research Articles

A high-temperature high-pressure small-angle neutron scattering cell for studying hydrothermal reactions in supercritical water

A high-temperature high-pressure cell equipped with a metallic window was developed for small-angle neutron scattering (SANS). The feasibility of a Ti alloy (Ti–6Al–4V) as a candidate window material was assessed, considering its mechanical strength and neutron characteristics. SANS experiments should be conducted using safe and reliable materials without risk of window damage or leakage of the activated samples due to window breakage. SANS profiles measured for supercritical heavy water showed maximum scattering intensity at temperatures and pressures near the critical density. Additionally, the utility of the cell for observations of the decomposition of plastic in supercritical water is presented. The cell facilitates analysis of reaction mechanisms under sub- and supercritical conditions, which could provide detailed information to aid efficient decomposition and recycling, contributing to a sustainable society.

Numerical Analysis of The Temperature Characteristics of a Coal—Supercritical Water-Fluidized Bed Reactor for Hydrogen Production

Supercritical water gasification (SCWG) of coal is a promising clean coal technology, which discards the traditional coal combustion and oxidation reaction to release carbon dioxide and other pollutants and replaces coal with a gasification reduction reaction in supercritical water to finally convert coal into a hydrogen-rich gas product with no net carbon dioxide emissions and no pollutant emissions, and thus has received much attention in recent years. However, the experimental conditions of coal to the hydrogen reactor are harsh, costly, and not easy to visualize and analyze, so numerical calculation and simulation analysis are important for the design, optimization, and industrial scaling-up of the reactor. In order to study the effect of the temperature field on the hydrogen production rate of the coal supercritical water gasification hydrogen production reactor, a numerical simulation calculation model is developed for this reactor in this paper. Comparing the experimental data in the literature, the maximum relative error of the gasification product yield per kg of coal between the two is less than 5%, which verifies the accuracy of the model built and the numerical method adopted in this paper. On this basis, the effects of supercritical water temperature and coal slurry temperature on the reactor’s gasification products and reaction rate were investigated in depth. The results show that increasing the supercritical water temperature is beneficial to improve the reactor hydrogen production efficiency, while the high coal slurry temperature is not conducive to adequate reaction, thus reducing the hydrogen production efficiency. For the laboratory coal supercritical water gasification to hydrogen reactor studied in this paper, the ideal temperature of supercritical water is 850~900 K, and the ideal temperature of coal slurry is 400–450 K. The conclusions of this paper can provide some reference for subsequent industrial scale-up studies of the reactor.

Nucleation Mechanism of Nano-Iron Oxide in a Supercritical Water Reaction

Despite its ubiquitous application in various branches of engineering and materials science, nucleation with reactions from supercritical water remains elusive. In this work, molecular dynamics with a reactive force field was studied to demonstrate the nucleation process of nano-iron oxide crystals synthesized via the supercritical hydrothermal synthesis method using Fe2(SO4)3 as the precursor. It was shown that during the early stages of nucleation, high local density areas first occurred, making these sites primary candidates for nucleation. Furthermore, the amorphous intermediate reorganizes from the aggregation and coalescence of the prenucleation clusters (PNCs) and precedes the emergence of a crystalline nucleus rather than direct one-step nucleation from classical theory. Through calculation, it was found that the average nucleation rates increase generally with the increase in reaction temperature and system density, and the volumes of nano-iron oxide clusters are maintained at 10–12 nm3 when simulating for 3 ns. It was further confirmed that increasing the reaction temperature and density is conducive to enhancing the crystallinity of iron oxide nanocrystals within a certain range of increments, with larger clusters forming faster at higher temperatures and densities. It was demonstrated that the screw dislocation formation mechanism caused nano-iron oxide lattice defects. Although not all of the nano-iron oxide nuclei generated have lattice defects, all of the perfect nano-iron oxide nuclei are generated by adjusting the configuration of nano-iron oxide nuclei with lattice defects.

Understanding sulfonated kraft lignin re-polymerization by ultrafast reactions in supercritical water

Re-polymerization reactions are a commonly reported issue on the way to the higher recovery of monomers from lignin. The reactivity of monomers obtained from lignin depolymerization and their contribution to the re-polymerization in supercritical water (SCW) was investigated. Sulfonated Kraft lignin (SKL) was used along with four monomers: vanillin, vanillic acid, vanillyl alcohol and acetovanillone. Indulin Kraft lignin was also employed as the reference to understand the re-polymerization of SKL in SCW. The formation of diarylmethane structures was detected in HSQC spectra of solid residue after the SCW process. Lignin released fragments with free phenolic β-O-4 structures, as well as the monomeric product vanillyl alcohol are involved with the formation of o-o´ and o-p ´ diarylmethane structures. Chemical structure of Kraft lignin and its polymeric product after the SCW process was remarkably similar, as shown by HSQC, indicating that re-polymerization reactions occur through cross-linking polymerization, mainly in their fractions of low molecular weight products.

Sol-Gel-Derived Ni3Al Coating on Nickel Alloy for Oxidation Resistance in Supercritical Water Environments.

Although nickel-based alloys are widely used in industries due to their oxidation and corrosion resistance, the pursuit of better performance in harsh environments is still a great challenge. In this work, we developed a sol–gel method to synthesize Ni3Al coating on a nickel alloy, assisted by a post-annealing process, and investigated the oxidation-resistant performance. The coating thickness can be controlled by designing the deposition times, which keep the pure Ni3Al phase stable. In addition, the surface morphologies indicate that the coating is compact without obvious voids or cracks. Furthermore, the oxidation-resistant property of the coating was investigated by carrying out a supercritical water oxidation experiment. The crystalline structure and surface morphology of the samples before and after 72-h oxidation demonstrated the superior oxidation resistance of the coating. This work provides a convenient method to fabricate an oxidation-resistant coating on a nickel-based alloy, which would be significant for prolonging the service life of vessels under oxidation conditions, especially for supercritical water reactions.

Supercritical fluid synthesized Cu2ZnSnS4-Polyaniline nanocomposites for supercapacitor application

In the work, Cu2ZnSnS4 (CZTS)/Polyaniline (PANI) nanocomposites was successfully synthesized by an in-situ supercritical water reaction from the mixture of CZTS-precursors and PANI. The synthesized samples were characterized by X-ray diffraction, TEM, thermogravimetry/differential thermal analysis (TG-DTA), scanning electron microscopy, and elemental mapping of CZTS powder. In order to evaluate the electrochemical performance of CZTS nanoparticles and CZTS/PANI nanocomposites, their supercapacitor electrodes were fabricated. The electrochemical experiment revealed that the specific capacitance of CZTS/PANI in 1 M KOH aqueous electrolyte at a current density of 2.5 A/g was 371 F/g, which was significantly more than the specific capacitance of the pure CZTS sample (271 F/g) after 1000 cycles also, the composite retained 97% of its capacitance. Thus, it can be concluded that the reduced electrolyte charge-transfer resistance and ion-diffusivity can be the cause of enhanced conductivity and a large surface of the material, which synergistically improved the capacitive performance of the electrode material.

Mechanism of H2-O2 reaction in supercritical water

H2-O2 reaction in supercritical water has been widely studied due to its promising application in industry. The global rate of this reaction was found independent of O2 with equivalence ratio ranging from 0.042 to 4.1 in the literature. Aim of current study is to examine intermediates interaction responsible for properties of the O2 independence and others. The study involves experiment re-examining the oxygen independence phenomenon and simulation centering on reduced mechanisms derived by quasi-steady-state approximation. The experiment with equivalence ratio ranging from 3.5 to 7.2 produces similar temperature increases, signifying a global reaction rate independent of oxygen. The phenomenon of oxygen independence was then inspected by a three-step reduced mechanism and is attributed to a special role of H + O2 + M → HO2 + M. This particular reaction dominating H and O2 consumption with a fast rate is responsible for the concurrence of independences of its reaction coefficient, H, and O2 to global reaction rate. Another property that HO2 quantity indicates reactive intensity was investigated by a two-step reduced mechanism and is credited to a small activation energy of reaction HO2 + HO2 → H2O2 + O2 and a thermal system.

Hydrothermal liquefaction of wood chips under supercritical and subcritical water reaction conditions

This work describes batch-type hydrothermal liquefaction (HTL) treatments of conifer wood chips at 180–425 °C, under either air or nitrogen atmosphere. Such experiments allow efficient extraction of 5-hydroxymethyl furfural (HMF) and other valuable chemical substances, such as glycolic acid and acetic acid, from the lignocellulosic biomass. These compounds and their decomposition products present in the samples after HTL are analyzed and quantified using spectroscopic and chromatographic techniques. In general, the relatively higher-pressure nitrogen atmospheric condition is more suitable for obtaining the desired products, relative to the air atmosphere. Based on the quantitative results, the optimal temperatures for producing acetic acid, glycolic acid, and HMF are 300 °C, 250 °C, and 180 °C, respectively. The interesting relationship between HMF yield and temperature is also discussed; as the temperature increases, the yield of HMF first decreases and then increases. This phenomenon is explained by the exothermic nature of the HMF decomposition reaction, which is inhibited by excessively high temperature (in the range from 380 to 425 °C). At moderately high temperatures (optimized conditions; 300 °C), the generation rate of HMF exceeds its decomposition rate, resulting in a high yield of HMF. Based on the results of the experiments conducted in this study, the decomposition mechanism describing HTL treatment of wood chips can be elucidated. This study therefore provides guidance for future work involving HMF extraction from lignocellulosic biomass.

Kinetic modeling of carbon monoxide oxidation and water gas shift reaction in supercritical water

Carbon monoxide is a primary intermediate in supercritical water oxidation (SCWO) processes. A validated chemical kinetic model for hydrogen oxidation in supercritical H2O/CO2 mixtures was expanded to describe carbon monoxide oxidation and water gas shift reaction in supercritical water (SCW). The model performance was evaluated against experimental data in literature. Data for carbon monoxide oxidation were well reproduced by the model. Underprediction of hydrogen formation was attributed mostly to hydrolysis reactions occurring on the reactor surface. The rate of water gas shift reaction observed experimentally under SCW conditions strongly exceeds that predicted by the model. Analysis indicates that the observed WGS reaction is largely due to catalytic effects on the reactor surface, even though trace impurities of oxygen present in the experiments may also enhance reaction. Pressure corrections on thermodynamic properties and inclusion of HOCHO subset were found to have negligible influence on modeling results. CO+OH=CO2+H, HO2+H2O=OH+H2O2 and OH+HO2=O2+H2O were identified as the most important elementary reactions by sensitivity analysis.

Molecular dynamics simulation study used in systems with supercritical water

Abstract Supercritical water (SCW) is a green solvent. The supercritical fluids have been increasingly concerned and studied in many areas such as SCW gasification, biofuel production, SCW hydrothermal conversion, organic wastes treatment and utilization, nanotechnology, etc. Because of the severe circumstances and rapid reactions in supercritical water, it is difficult for experimental researchers to disentangle various fundamental reaction steps from the intermediate and product distributions. From this perspective, molecular dynamics (MD) simulation based on quantum chemistry is an efficient tool for studying and exploring complex molecular systems. In recent years, molecular simulations and quantum chemical calculations have become powerful for illustrating the possible internal mechanism of a complex system. However, now there is no literature about the overview of MD simulation study of the system with SCW. Therefore, in this paper, an overview of MD simulation investigation applied in various systems with SCW is presented. In the current review we explore diverse research areas. Namely, the applications of MD simulation on investigating the properties of SCW, pyrolysis/gasification systems with SCW, dissolution systems and oxidation systems with SCW were summarized. And the corresponding problems in diverse systems were discussed. Furthermore, the advances and problems in MD simulation study were also discussed. Finally, possible directions for future research were outlined. This work is expected to be one reference for the further theoretical and molecular simulation investigations of systems involving SCW.

Phenanthrene catalytic cracking in supercritical water: effect of the reaction medium on NiMo/SiO2 catalysts

A series of NiMo/SiO2 catalysts was synthesized by sol-gel method for heavy oil upgrading in supercritical water (SCW). Phenanthrene was used as substrate as it represents polyaromatic structures present in asphaltenes. No phenanthrene conversion was observed in a blank (non‐catalytic) experiment. However, phenanthrene conversions up to 24% after 1 h of reaction in SCW at 425 °C and 230 bar were observed in the presence of NiMo/SiO2, underlining the role of the catalysts in the process. Conversion was found to be dependent mainly on Ni content and the Ni/Mo ratio in the catalysts. The liquid products obtained are thought to be the result of both oxidation and hydrogenation processes. Characterization of the fresh and spent catalysts using X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) was performed. It was revealed that catalysts are not completely stable in SCW, showing that NiMo intermetallic compounds formed the initial catalysts were decomposed, Mo0 and Ni0 were oxidised and the latter formed Ni2SiO4. In addition, MoO2 phase domain size in the catalysts increased and the surface of the spent catalysts appeared to be enriched with Ni and depleted with Mo.

Reactions

OPINION Reactions ShareShare onFacebookTwitterWechatLinked InRedditEmail C&EN, 2018, 96 (38), p 5September 24, 2018Cite this:C&EN 96, 38, 5Supercritical water to recycle plastics The importance of the need to chemically revamp the recycling of waste plastics was appropriately featured in C&EN’s June 18 issue (page 24). Strangely, the successful use of water alone to cleave the bonds in these and other giant molecules in a matter of seconds was not mentioned.Of course, the water must be in a supercritical state, when it has remarkable solvent and heat transport properties, as collected by Yizhak Marcus in “Supercritical Water: A Green Solvent; Properties and Uses.” A more recent book is “Hydrothermal and Supercritical Water Processes” by Gerd Brunner.In addition to the extreme rapidity of the supercritical water reactions, simultaneously hydrogen atoms are provided from the water that stabilize the free radicals in the small molecules created by bond cleavage in the substrate and thereby reduce or eliminate the objectionable char formation of conventional, purely thermal pyrolysis processes.To take advantage of these unique characteristics, a continuous reactor system is desirable, and a demonstration-sized unit has been constructed and patented (US Patent No. 9932285).Several industrial companies (Licella, Renmatix, and Xtrudx) are actively involved with high-temperature water, and Massachusetts Institute of Technology has a substantial program under the leadership of professor William H. Green of the chemical engineering department.Graham Allan SeattleEditor’s note: Allan is an inventor on the patent mentioned in the letter and is involved in the company Xtrudx. Corrections Aug. 13/20, page 22: A feature story about atropisomers incorrectly stated that a particular drug candidate, BMS-986142, was separated and purified with high-performance liquid chromatography at low temperature and that the column was chilled with ice or cooling equipment. This was done for atropisomeric compounds early in the drug development process rather than for BMS-986142 itself.Aug. 27, page 42: An article about the ACS Summer School on Green Chemistry & Sustainable Energy stated that students from “across the U.S.” were in attendance, but students came from institutions in the Americas outside the U.S. as well.Sept. 17, page 21: A photo of a researcher examining flasks had an incorrect credit and caption. The photo was provided by Indigo Ag, not Pivot Bio, and it depicts an Indigo Ag researcher monitoring growth of symbiotic microbes.

Catalyst Oxidation and Dissolution in Supercritical Water

We use thermodynamic models to predict catalyst oxidation and dissolution in supercritical water (SCW) and use experiments to assess the viability of the models for practical SCW reaction systems and provide relative rates for these mechanisms. We examined the oxidation and dissolution of noble and transition metals, metal oxide catalyst supports, and transition metal carbides and nitrides under SCW conditions. The materials were tested in batch reactors at 400 °C for 60 min, and the SCW density was varied from 0 to 0.5 g/mL to observe the influence of the solvent properties on stability. Oxidation and dissolution were determined by comparing the initial catalyst composition and structure with those of the catalysts recovered from the reactors after exposure to the SCW environment. The gas-phase recovered from the reactors was analyzed for H2 produced from oxidation. The aqueous phase was analyzed for metals from dissolution. The ΔGrxn for oxidation and the solubility of the catalysts in SCW at the experi...

DFT study of the enhancement on hydrogen production by alkaline catalyzed water gas shift reaction in supercritical water

Supercritical water gasification (SCWG) is hopefully to be an acceptable choice for hydrogen production, the hydroxide ion assisted water gas shift reaction (WGSR) has been regarded as the most important reaction to generate hydrogen during the process. However, the principle of practical OH− catalyzed reaction is not possible to acquire by experiments. Thus, density functional theory (DFT) is utilized to investigate the reaction mechanism theoretically in this work. Through first principle calculations, every species and energy barrier for elementary steps are achieved, and formate ion is determined as the important intermediate. Besides, HCOO− + H2O → HCO3− + H2 is the dominant path to generate hydrogen, as well as the rate-determining step with 47.94 kcal/mol energy barrier. Furthermore, the reaction rate constant is calculated to be kcatalytic(s−1) = 2.34 × 1012exp(−1.80 × 105/RT) using transition state theory with Wigner transmission coefficient (TST/w). Lastly, supercritical water condition is demonstrated to be a favored media for WGSR, because it may dissociate, dissolve or hydrolyze more hydroxide anion than conventional steam. The results are expected to benefit the control of reaction process and the design of SCWG reactor.

15.超臨界水中でのWater-Gas-Shift反応を介したナフタレンの水素化反応

15.超臨界水中でのWater-Gas-Shift反応を介したナフタレンの水素化反応

O209 超臨界水中反応の反応熱測定装置の開発(セッション4:技術〜基礎〜,口頭発表)

O209 超臨界水中反応の反応熱測定装置の開発(セッション4:技術〜基礎〜,口頭発表)

3-32 超臨界水中の反応の反応熱測定と反応機構を考慮した考察((7)超臨界ガス化,Session 3 バイオマス等)

3-32 超臨界水中の反応の反応熱測定と反応機構を考慮した考察((7)超臨界ガス化,Session 3 バイオマス等)

Thermodynamic modeling of the water-gas shift reaction in supercritical water for hydrogen production

In this study, a computational model is developed for the estimation of the equilibrium composition of WGSR (water gas shift reaction) based on the stoichiometric and nonstoichiometric thermodynamic approaches. The model employs the Peng–Robinson equation of state (PR-EoS) formulation and the Gibbs free energy minimization. A Matlab computer program is developed for the numerical solution of a highly nonlinear equation of systems satisfying thermodynamic constraints. Molar fractions of each species are determined for different physical conditions using the proposed computational model and the computer code. Comparisons of model predictions with previous results show that molar fractions can be computed accurately using the present computational algorithm. On the basis of the numerical tests, a novel empirical expression is proposed to determine the hydrogen yield for energy considerations.

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.