Showing papers on "Reaction rate published in 2000"

Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water

Abstract: Decomposition experiments of microcrystalline cellulose were conducted in subcritical and supercritical water (25 MPa, 320−400 °C, and 0.05−10.0 s). At 400 °C hydrolysis products were mainly obtained, while in 320−350 °C water, aqueous decomposition products of glucose were the main products. Kinetic studies of cellulose, cellobiose, and glucose at these conditions showed that below 350 °C the cellulose decomposition rate was slower than the glucose and cellobiose decomposition rates, while above 350 °C, the cellulose hydrolysis rate drastically increased and became higher than the glucose and cellobiose decomposition rates. Direct observation of the cellulose reaction in high-temperature water at high-pressure conditions by using a diamond anvil cell (DAC) showed that, below 280 °C, cellulose particles became gradually smaller with increasing reaction time but, at high temperatures (300−320 °C), cellulose particles disappeared with increasing transparency and much more rapidly than expected from the lowe...

Detailed chemical kinetic mechanisms for combustion of oxygenated fuels

Abstract: Thermodynamic properties and detailed chemical kinetic models have been developed for the combustion of two oxygenates: methyl butanoate, a model compound for biodiesel fuels, and methyl formate, a related simpler molecule. Bond additivity methods and rules for estimating kinetic parameters were adopted from hydrocarbon combustion and extended. The resulting mechanisms have been tested against the limited combustion data available in the literature, which was obtained at low temperature, subatmospheric conditions in closed vessels, using pressure measurements as the main diagnostic. Some qualitative agreement was obtained, but the experimental data consistently indicated lower overall reactivities than the model, differing by factors of 10 to 50. This discrepancy, which occurs for species with wellestablished kinetic mechanisms as well as for methyl esters, is tentatively ascribed to the presence of wall reactions in the experiments. The model predicts a region of weak or negative dependence of overall reaction rate on temperature for each methyl ester. Examination of the reaction fluxes provides an explanation of this behavior, involving a temperature-dependent competition between chain-propagating unimolecular decomposition processes and chain-branching processes, similar to that accepted for hydrocarbons. There is an urgent need to obtain more complete experimental data under well-characterized conditions for thorough testing of the model.

The Sonochemical Degradation of Azobenzene and Related Azo Dyes: Rate Enhancements via Fenton's Reactions

Abstract: The sonochemical degradation of aqueous solutions of azobenzene and related azo dyes (methyl orange, o-methyl red, and p-methyl red) was performed at 500 kHz and 50 W, under air, O2, or Ar saturation at 288 K. Reaction products and intermediates were identified by HPLC-ES-MS. Total organic carbon (TOC) was also determined as a function of reaction time. We propose a reaction mechanism based on the observed species and the extent and rate of TOC depletion. The addition of OH radicals to the azo double bond is considered to be the first step of the sequence of oxidative bond cleavages leading to the production of carboxylic acids, quinones, carbon dioxide, and nitrate ions as the main degradation products. The effects of the dye structures and of the background gas on the sonochemical bleaching rates were also investigated. The reaction rates for o-methyl red were approximately 30−40% faster than those for the other compounds. This appears to be a strong influence by a carboxylic group ortho to the azo grou...

Reductive Dechlorination of Trichloroethene and Carbon Tetrachloride Using Iron and Palladized-Iron Cathodes

Abstract: This research investigated the effectiveness of electrochemical reduction for removing trichloroethylene (TCE) and carbon tetrachloride (CT) from dilute aqueous solutions. The kinetics, reaction mechanisms, and current efficiencies for TCE and CT reduction were investigated using flow-through, iron electrode reactors and with amperometric measurements of reduction rates. The electrode reactors were operated over a range of flow rates, pH, ionic strength, dissolved oxygen concentration, and working electrode potentials. Typical reduction half-lives for TCE and CT in the iron reactor were 9.4 and 3.7 min, respectively. The addition of palladium as an electrocatalyst at a level of 1 mg of Pd per m2 of electrode surface area increased the reaction rates by a factor of 3. When operated continuously, reaction rates in the palladized-iron reactor were stable over a 9-month period of operation, indicating that there was no loss of palladium from the electrode. In both the iron and Pd−iron reactors, TCE was reduce...

Reaction of Plutonium Dioxide with Water: Formation and Properties of PuO2+x

Abstract: Results show that PuO(2+x), a high-composition (x

Char Gasification with O2, CO2, and H2O: Effects of Pressure on Intrinsic Reaction Kinetics

Abstract: Measurements of the intrinsic reaction rates of two Australian coal chars (made under laboratory conditions) with O2, CO2, and H2O at increased pressures (up to 30 atm) have been made using a pressurized thermogravimetric analyzer (TGA). It was found that the reaction order in CO2 and H2O was not constant over the pressure range investigatedvarying from 0.5 to 0.8 at atmospheric pressure and decreasing at pressures above approximately 10 atm. The apparent reaction order in oxygen was less affected by pressure over the range 1 to 16 atm. Char surface area after reaction at higher pressures was generally greater than that after reaction at lower pressures. This resulted in a reduced effect of pressure on the intrinsic rates at 10% conversion. Activation energies for all three reactions were not significantly affected by an increase in reaction pressure. The intrinsic rate data obtained in this work were used to estimate the high-temperature reactivity of the chars using a basic knowledge of the pore structu...

Intrinsic kinetics of the gas-solid Fischer-Tropsch and water gas shift reactions over a precipitated iron catalyst

Abstract: The kinetics of the gas–solid Fischer–Tropsch synthesis over a commercial Fe–Cu–K–SiO 2 catalyst was studied in a continuous spinning basket reactor. Experimental conditions were varied as follows: reactor pressure of 0.8–4.0 MPa, H 2 /CO feed ratio of 0.25–4.0, and space velocity of 0.5–2.0×10 −3 Nm 3 kg cat −1 s −1 at a constant temperature of 523 K. A number of Langmuir–Hinshelwood–Hougen–Watson type rate equations were derived on the basis of a detailed set of possible reaction mechanisms originating from the carbide mechanism for the hydrocarbon formation and the formate mechanism for the water gas shift reaction, respectively. 14 models for the Fischer–Tropsch reaction rate and two water gas shift reaction rate models were fitted to the experimental reaction rates. Bartlett’s test was used to reduce the set of Fischer–Tropsch rate equations to 3 models, which were statistically indistinguishable. It could be concluded that the reaction rate of the Fischer–Tropsch synthesis is controlled by the formation of the monomer species (methylene) by hydrogenation of associatively adsorbed CO, whereas the carbon dioxide formation rate (water gas shift) is determined by the formation of a formate intermediate species from adsorbed CO and dissociated hydrogen. Simulations using the optimal kinetic models derived showed good agreement both with experimental data and with some kinetic models from literature.

Reactions of ethanol over metal oxides

Abstract: The reaction of ethanol over a series of oxides (Fe2O3, Fe2O3/CaO, Fe3O4, TiO2, CaO, and SiO2) has been investigated. The main reaction product in all cases is acetaldehyde, with secondary products acetone and ethyl acetate. At 473 K, the rate constant k decreases according to the series Fe2O3>Fe3O4>CaO>TiO2≫SiO2. Titration of basic sites by CO2 adsorption at room temperature shows that the reaction rate can be successfully normalised by basic site density for oxides that adsorb CO2 except for CaO. Acetone production, presumably via acetate ketonization, is highest over TiO2 while ethyl acetate formation, by Tishchenko reaction, is highest over Fe2O3.

Effect of ionic strength and hydrogen peroxide on the photocatalytic degradation of 4-chlorobenzoic acid in water

Abstract: Assisted-photocatalytic degradation of the pesticide precursor 4-chlorobenzoic acid (4-CBA) in water was investigated using different TiO 2 powders and near-UV radiation. Experiments were performed at pH=3.0 and at different TiO 2 loadings, solution ionic strengths, and concentrations of hydrogen peroxide as an oxidant additive. Dark adsorption equilibrium studies revealed that surface area, ionic strength, and catalyst heterogeneity influenced the adsorption capacity and adsorption mechanism. This was attributed to the type and concentration of adsorption sites and the electrostatic interactions in the solution-semiconductor interface. However, the rates of the photocatalytic reactions were not significantly affected by an increase in the ionic strength by a factor of 50. On the other hand, the reaction rate was a strong function of catalyst crystallinity and loading. This suggested that the reactions were mostly controlled by the rate of formation of the oxidizing species rather than the extent of electric double layer (EDL) compression. Addition of hydrogen peroxide up to 248 mg/l resulted in an increase of the reaction rates with a corresponding increase in photonic efficiency by ≈20%. Above this concentration, hydrogen peroxide either did not enhance or caused a significant inhibition of the mineralization rates.

Full dimensional quantum calculations of the CH4+H→CH3+H2 reaction rate

Abstract: Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted.

An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor

Abstract: The interactions between the local flame structure produced by a premixed swirl-stabilized injector with combustion instabilities were experimentally studied for a model gas turbine combustor operating at high pressure and temperature. The model gas turbine combustor studied utilizes a sudden-expansion dump combustor with a single swirler and bluff body for enhancing mixing rate and flame stabilization, respectively. Laser-based measurements were made for both stable and unstable operating conditions. The local flame front structure was visualized using planar laser-induced fluorescence (PLIF) from the OH⊙, and the global heat release zone was interpreted from flame emission measurements. For stable combustion conditions, the mean reaction rate estimated independently from both OH-PLIF and OH * chemiluminescence measurements showed good agreement, thereby indicating confidence in the use of OH-PLIF measurements for extracting the local mean reaction rate. For unstable combustion conditions, the flamefront characteristics, including flame surface density and mean reaction rate, were evaluated together with the information from the OH * chemiluminescence measurements to identify the boundary of the heat release region at discrete phases of the unstable flame. Analysis of the flame structures during combustion instability indicated significant variations during different phases of the instability. The heat release flow field, particularly in the recirculation regions appearing at the corner and inner face of the dump plane, varied substantially. Rayleigh index information indicated that the recirculation zones play an important part in driving the instability. In contrast, the high shear layer formed along the interface between reactants and hot products produced a region where the instability was damped due to a lowering of the heat release.

Photocatalytic oxygen evolution on α-Fe2O3 films using Fe3+ ion as a sacrificial oxidizing agent

Abstract: Photocatalytic oxygen evolution on α-Fe2O3 films was studied using the Fe3+ ion as a sacrificial oxidizing agent. The reaction conditions affecting the oxygen evolution rate, i.e. anion, Fe3+ concentration, pH, Fe3+/Fe2+ equilibrium concentration and dependence of irradiation wavelength, were investigated. The reaction rate increased with increase of both the Fe3+ ion concentration and the solution pH. Oxygen evolution ceased at an Fe3+ :Fe2+ ion concentration ratio of 3:7–4: 6. The reaction rate decreased rapidly for longer-wavelength irradiation (above around 400 nm), which is considerably shorter than the α-Fe2O3 bandgap of ca. 600 nm (2.1 eV).

Photocatalytic decomposition of NO by TiO2 particles

Abstract: The effects of initial NO concentration, gas-residence time, reaction temperature and ultraviolet (UV) light intensity on the photocatalytic decomposition of NO have been determined in an annular flow-type and a modified two-dimensional fluidized-bed photoreactors. The decomposition of NO by photocatalysis increases with decreasing initial NO concentration and increasing gas-residence time. The reaction rate increases with increasing UV light intensity. The light transmission increases exponentially with the bed voidage at superficial gas velocity above 1.3 times the minimum fluidizing velocity ( U mf ) in the two-dimensional fluidized-bed photoreactor. In the two-dimensional fluidized-bed photoreactor, NO decomposition reaches >70% at the gas velocity of 2.5 U mf . A two-dimensional fluidized-bed photoreactor is an effective tool for high NO decomposition with efficient utilization of photon energy.