Global Reaction Rate Research Articles

PAH emissions from high-temperature oxidation of vaporized anthracene

The presence of polycyclic aromatic hydrocarbons/compounds (PAH/PAC) in the exhaust of combustion equipment as well as their growth to soot particles depends on the balance of formation and depletion reaction pathways. Detailed understanding of PAH oxidation is necessary for a correct quantitative description of combustion processes. This study focuses on the high-temperature oxidation of anthracene, a 3-ring PAH, by molecular oxygen. By reacting this already formed PAH, under non-sooting conditions, its oxidative depletion processes were isolated from its formation processes, as well as from soot formation pathways. Anthracene powder, fluidized in nitrogen, was introduced into a two-stage electrically heated laminar-flow drop-tube furnace. In the first stage, kept at 900 °C, anthracene was vaporized. Thereafter, upon mixing with preheated oxygen-containing gases in a venturi, gaseous anthracene was oxidized in the second furnace at oxygen mole fractions of 1–5% and gas temperatures of 950–1050 °C. Gas temperature and velocity profiles in the furnaces, as well as anthracene particle vaporization trajectories were obtained with a computational fluid dynamics code. Sampling, followed by chemical analysis, was conducted at three locations within the oxidizing furnace. Trends with residence time show increasing CO and CO 2 yields in the furnace and gradually decreasing O 2. CO increased with higher oxidizing temperatures and oxygen concentrations. Acetylene was the most abundant light hydrocarbon, followed by methane, ethylene, benzene, propylene and ethyl acetylene. Prevalent PAC included fluorene-9-one, 9,10-anthracenodione, biphenylene, dibenzofuran, fluorene, indene, naphthalene, anthrone, xanthone and 2-naphthalenecarboxaldehyde. Most species’ yields increased with rising oxidation temperatures. A global anthracene reaction rate with oxygen was deduced, and an activation energy of 71 kJ/mol was calculated. An anthracene oxidation reaction scheme was proposed.

A Numerical Study Promoting Algebraic Models for the Lewis Number Effect in Atmospheric Turbulent Premixed Bunsen Flames

This numerical investigation carried out on turbulent lean premixed flames accounts for two algebraic – the Lindstedt–Vaos (LV) and the classic Bray–Moss–Libby (BML) – reaction rate models. Computed data from these two models is compared with the experimental data of Kobayashi et al. on 40 different methane, ethylene and propane Bunsen flames at 1 bar, where the mean flame cone angle is used for comparison. Both models gave reasonable qualitative trend for the whole set of data, in overall. In order to characterize quantitatively, firstly, corrections are made by tuning the model parameters fitting to the experimental methane–air (of Le = 1.0) flame data. In case of the LV model, results obtained by adjusting the pre-constant, i.e., reaction rate parameter, CR, from its original value 2.6 to 4.0, has proven to be in good agreement with the experiments. Similarly, for the BML model, with the tuning of the exponent n, in the wrinkling length scale, Ly = Cl⋅ lx(sL/u′)n from value unity to 1.2, the outcome is in accordance with the measured data. The deviation between the measured and calculated data sharply rises from methane to propane, i.e., with increasing Lewis number. It is deduced from the trends that the effect of Lewis number (for ethylene–air mixtures of Le = 1.2 and propane–air mixtures of Le = 1.62) is missing in both the models. The Lewis number of the fuel–air mixture is related to the laminar flame instabilities. Second, in order to quantify for its influence, the Lewis number effect is induced into both the models. It is found that by setting global reaction rate inversely proportional to the Lewis number in both the cases leads to a much better numerical prediction to this set of experimental flame data. Thus, by imparting an important phenomenon (the Lewis number effect) into the reaction rates, the generality of the two models is enhanced. However, functionality of the two models differs in predicting flame brush thickness, giving scope for further analysis.

Kinetic analysis for decomposition of 2,4-dichlorophenol by supercritical water oxidation

2,4-Dichlorophenol (2,4-DCP), as a halogenated model pollutant, was decomposed by using supercritical water oxidation (SCWO) in a batch reactor made of Hastelloy C-276. SCWO experiments for 2,4-DCP decomposition were performed in the range of 380–420 °C, 230–280 bar and 0.074-0.221 mol/L H2O2. The effect of oxidant concentration on decomposition rate and efficiency was significant near the critical temperature of 380 °C. However, the role of the oxidant concentration in the SCWO process decreased with an increase in temperature; also, excess oxidant played a key role in quite significantly decreasing the activation energy of 2,4-DCP oxidation. Variation of the reaction rate by the change of pressure was negligible even at a near critical temperature. The kinetic rate for the decomposition of 2,4-DCP in the SCWO process was well described by a simple first-order kinetic and global reaction rate model. From the SCWO experiments, the various intermediates identified with a GC/MS implied that the first reaction pathway for 2,4-DCP decomposition led to dimers such as dichlorophenoxyphenols, and the second led to single-ring and ring-opening products.

Reaction–Diffusion Processes on Small-World Networks

In the past decades, several papers have been devoted to the theoretical and numerical investigation on the reactant segregation phenomena in anomalous kinetics. An important contribution to this field of research has been given by Ovchinnikov and Zeldovich, who developed the segregation phenomenon. It is well known that the binary reaction has primarily been investigated on the process of ternary reactions and many phenomena in nature. Until now, the segregation of reactants has been investigated a reaction–diffusion process based on assumption that the scaling form for Aþ B ! C has almost been verified by the computer simulation and the experiment. Cornell et al. have argued the diffusion-limited reaction nAþ mB ! C for both homogeneous and inhomogeneous conditions under initially separated reactants. They have discussed that the global reaction rate decreases as t 1=2 in long time limit, independent of n and m, and that the upper critical dimension is dc 1⁄4 2 for the reaction–diffusion process. Recently, Zumofen et al. have studied mainly the particle density and the pair correlation function on the two-particles reaction process. Particularly, they found that the particle density is distributed as PðnÞ n 1 , n > 0, 1 dc, and that the critical dimension dc 1⁄4 2 for Aþ A and 4 for Aþ B. Next the Aþ B ! 0 process on a regular network is introduced for Levy walks using the reaction–diffusion equation,

HEAD-ON QUENCHING OF TRANSIENT LAMINAR FLAME: HEAT FLUX AND QUENCHING DISTANCE MEASUREMENTS

ABSTRACT This work presents results of quenching distance and heat flux measurements during the head-on quenching of transient laminar stoichiometric methane/air flame in the pressure range 0.05–1.7 MPa. The results of direct visualization have been used to measure the quenching distance and to prove the relationship relating the quenching distance and heat flux to the wall over the global combustion reaction rate. It is shown that quenching distance decreases from 0.43 to 0.016 mm with pressure rise from 0.05 to 1.7 MPa. The maximum wall heat flux increases nonlinearly from 0.35 to 2.3 MW/m2 with pressure rise from 0.05 to 1.7 MPa. The dimensionless value of the heat flux depends only slightly on the pressure and decreases from 0.3 to 0.2 with rise of pressure in this pressure range. Flame–wall interaction time is about constant and equal to 0.15–0.155 ms in all ranges of pressure variation.

Removal of chromium(VI) from aqueous solution by activated carbons: Kinetic and equilibrium studies

The objective of this study is to assess the uptake of hexavalent chromium (Cr(VI)) from aqueous solutions onto activated carbons (AC) produced from wood. Two activated carbons are tested, a KOH-activated carbon and a commercial H 3PO 4-activated carbon (Acticarbone CXV). The adsorption of Cr(VI) is maximal at the lowest values of pH (pH 3) and increases with temperature for both adsorbents. The KOH-activated carbon shows higher capacity for adsorption of Cr(VI) than Acticarbone. The sorption isotherms fit the Langmuir model accurately. The adsorption reaction was found to obey a pseudo second-order rate. The activation energy and the pre-exponential factor as well as the thermodynamic functions related to adsorption reaction, Δ S°, Δ H°, Δ G°, were determined. Nevertheless, the global reaction rate is probably controlled by the intra-particular diffusion of Cr(VI) and the mass diffusivity of Cr(VI) was evaluated.

Structural and Mechanistic Studies of Escherichia coli Nitroreductase with the Antibiotic Nitrofurazone: REVERSED BINDING ORIENTATIONS IN DIFFERENT REDOX STATES OF THE ENZYME

The antibiotics nitrofurazone and nitrofurantoin are used in the treatment of genitourinary infections and as topical antibacterial agents. Their action is dependent upon activation by bacterial nitroreductase flavoproteins, including the Escherichia coli nitroreductase (NTR). Here we show that the products of reduction of these antibiotics by NTR are the hydroxylamine derivatives. We show that the reduction of nitrosoaromatics is enzyme-catalyzed, with a specificity constant approximately 10,000-fold greater than that of the starting nitro compounds. This suggests that the reduction of nitro groups proceeds through two successive, enzyme-mediated reactions and explains why the nitroso intermediates are not observed. The global reaction rate for nitrofurazone determined in this study is over 10-fold higher than that previously reported, suggesting that the enzyme is much more active toward nitroaromatics than previously estimated. Surprisingly, in the crystal structure of the oxidized NTR-nitrofurazone complex, nitrofurazone is oriented with its amide group, rather than the nitro group to be reduced, positioned over the reactive N5 of the FMN cofactor. Free acetate, which acts as a competitive inhibitor with respect to NADH, binds in a similar orientation. We infer that the orientation of bound nitrofurazone depends upon the redox state of the enzyme. We propose that the charge distribution on the FMN rings, which alters upon reduction, is an important determinant of substrate binding and reactivity in flavoproteins with broad substrate specificity.

Internal Noise Stochastic Resonance in NO Reduction by CO on Platinum Surfaces

We have studied the roles of internal noise in the oscillatory reaction kinetics during NO reduction by CO on low-index single crystal platinum surfaces using the chemical Langevin equation and the Poisson approximation algorithm. Considering that the surface is divided into small well-mixed cells, we have focused on the dynamical behavior inside a single cell. It is found that internal noise can induce rate oscillations and the performance of the stochastic rate oscillations undergoes a maximum with the variation of internal noise level for a given NO partial pressure, demonstrating the occurrence of internal noise stochastic resonance. Such a phenomenon is robust to the change of external parameters, such as NO pressures. Interestingly, it is also found that internal noise noticeably alters the oscillatory waveform and, hence, the reaction activity and oscillation signal intensity, but it nearly does not change the global reaction rate.

Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion

The kinetics of reduction with CH4, H2, and CO and oxidation with O2 of a Cu-based oxygen carrier prepared by impregnation on alumina to be used in a chemical-looping combustion (CLC) system have been determined in a thermogravimetric analyzer. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5−70 vol % of the fuel gas and 5−21 vol % of O2. The analysis of the sample carried out by scanning electron microscopy using energy-dispersive X-ray and chemisorption showed that the CuO was well dispersed in the porous surface of the alumina matrix and a uniform thin layer on the porous surface was considered. The shrinking-core model for platelike geometry of the reacting surface was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. No effect of the gas products (H2O and CO2) on the reaction rate was detected. The reaction order depended on the fuel gas, and values of 0...

Pilot Scale System for Removal of Phenol in Phenolic Wastewater of Olefin Plant

Elimination of phenol from phenolic wastewater of an olefin plant using ozone is reported. In this work a bench-scale semi-batch ozonation system was set up in the Environmental Engineering Research Center (EERC). Aventuri injection and a circulation system was used for injection of ozone into the ozonation contactor. Samples were taken from buffered aqueous phenol solutions at different pH values with an initial concentration of 80 ± 5 ppm. Experimental results revealed that phenol removal efficiency increased with increasing pH and reached >99.5% after 15min ozonation at apH of 11. With increasing pH, ozone consumption (i.e. mol O3 per mol phenol) decreased from 6.1 to 5.1. The results were compared with Roth's model, which was used to describe the global reaction rate of ozonation of phenol. Using Roth's model and results of bench-scale system a pilot scale ozonation system was designed and set up in a petrochemical complex. From the results of steady-state pilot scale tests, it was shown that ozonation could significantly reduce phenol concentration to the desired level in actual wastewater.

In situ EXAFS study on the oxidation state of Pd/Al 2O 3 and Bi–Pd/Al 2O 3 during the liquid-phase oxidation of 1-phenylethanol

In situ XANES and EXAFS were used to uncover the oxidation state of the metal constituents in a 5 wt% Pd/Al 2O 3 and a 0.75 wt% Bi–5 wt% Pd/Al 2O 3 catalyst during the liquid-phase dehydrogenation and oxidation of 1-phenylethanol. The catalytic reactions were carried out in a continuous-flow fixed-bed reactor serving at the same time as a spectroscopic cell for XANES/EXAFS. A special technique was applied to deposit Bi mainly as adatoms onto the supported Pd particles, as evidenced by XPS and XAS. Due to this technique, most Bi atoms were located on the Pd surface and thus the Bi dispersion was higher than that of Pd. This feature of the bimetallic catalysts led to more pronounced changes in the XANES region at the Bi L 3- than at the Pd K-edge. As a result, the method was more surface sensitive to the minor component Bi than to the major component Pd. The studies revealed that both the active sites (Pd) and the promoter (Bi) were in a metallic state during dehydrogenation in He. After the introduction of molecular oxygen the metals remained in a reduced state as long as the oxygen supply was limiting the global reaction rate, indicating a dehydrogenation mechanism of alcohol oxidation. As soon as the rate of oxygen supply exceeded the rate of alcohol dehydrogenation, both Bi and Pd were successively oxidized, leading to the well-known “overoxidation” (deactivation) phenomenon. An important consequence of the observations to the reaction mechanism is that the promoter effect of Bi cannot be related to its (partially) oxidized state (Bi n+ ), or to Bi leaching and homogeneous catalysis.

A kinetic study of the high temperature NO–char reaction

The heterogeneous reaction between NO and a coal char was studied in the temperature range 1273–1573 K in a drop tube furnace. The char was prepared at high temperature and its surface and porosity was measured. The parameters investigated were the concentration of NO, the temperature and the solid concentration. It was observed that the experimental results support the first-order reaction assumption with respect to NO and surface area. In order to extract an intrinsic kinetic law from these measurements, a complete two-dimensional simulation of the two-phase flow in the drop tube furnace was built using FLUENT software. This model allowed to take into account the group effects and the influence of radial gradients on the global reaction rate. A description of the diffusion limitation in the porous structure of the particle was added to the fluent heterogeneous reaction sub-model. From these simulations, we were able to obtain a good agreement between the measured and computed NO reduction yield with the kinetic constant k=88 exp(−14,400/T) m/ s which correspond to an activation energy of 120 kJ/mol.

Mechanism of the complexation reaction of aluminum chloride with o-hydroxy-benzophenone

A theoretical and experimental study on the formation reaction of the complex between AlCl 3 and 2-hydroxy-benzophenone in primary alcohols was performed. The influence of the temperature, the ionic strength and the solvent on the reaction rate were investigated. Under the adopted experimental conditions the stoichiometric composition of the formed complex is 1:1. In the theoretical study, thermodynamic magnitudes, torsional angles, length bonds, energy and reactivity indices were used. The calculations were performed at the RHF/3-21G and RHF/6-31G(d) levels of theory, using the Onsager's and COSMO methods. To explain the kinetic results obtained we proposed a reaction mechanism that includes the acid–base dissociation of 2-hydroxy-benzophenone, the reaction between AlCl 3 and a solvated proton to form the AlCl 2 + cation, and the formation of the metal complex by an ionic reaction between the 2-hydroxy-benzophenonate anion and the metal cation. This reaction between two ions of opposite charges is the rate-determining step of the complexation reaction analyzed. On the other hand, the theoretical conclusion on the greater planarity of the molecules of the complex with respect to the free molecules of 2-hydroxy-benzophenone is in agreement with the batochromic shift experimentally observed. Also, the proposed mechanism satisfactorily explains the global rate of the second order reaction, and the dependence of the rate constant with the temperature, ionic strength and permittivity of the reaction medium.

CONVERSION AND GLOBAL REACTION RATE COEFFICIENT IN THE ABSORPTION OF SO2 BY DIFFERENT TYPES OF LIMESTONE IN A FLUIDIZED BED REACTOR

This work concerns the study of the effect of limestone type on SO2 absorption in a bench fluidized bed reactor plant. Conversion and global reaction rate coefficients were established for conditions typical to fluidized bed combustion of coal. The bench plant is a bubbling bed reactor 160 mm internal diameter using silica sand as bed material, fluidized by pre-heated air. In order to simulate conditions close to the fluidized bed coal combustion ambience, the fluidizing air is pre-heated at high temperature (850 oC) and SO2 is added to the fluidizing air in a concentration typical of the process (1000 ppm). All the particulate, i.e. silica sand and limestone particles, was fed to the bed in a narrow size distribution between two subsequent ASTM sieves (with 545 μm mean diameter). In transient batch experiments charges of limestone are quickly injected into the bed, while the consequent variations of the exit concentrations of SO2, CO2 and O2 are continuously recorded. Analysis were performed on the effects of the type of limestone in the process, taking into account possible reaction controlling resistances, and considering possible effects of the calcination on the sulfation process.

CONVERSION AND GLOBAL REACTION RATE COEFFICIENT IN THE ABSORPTION OF SO2 BY DIFFERENT TYPES OF LIMESTONE IN A FLUIDIZED BED REACTOR

This work concerns the study of the effect of limestone type on SO2 absorption in a bench fluidized bed reactor plant. Conversion and global reaction rate coefficients were established for conditions typical to fluidized bed combustion of coal. The bench plant is a bubbling bed reactor 160 mm internal diameter using silica sand as bed material, fluidized by pre-heated air. In order to simulate conditions close to the fluidized bed coal combustion ambience, the fluidizing air is pre-heated at high temperature (850 oC) and SO2 is added to the fluidizing air in a concentration typical of the process (1000 ppm). All the particulate, i.e. silica sand and limestone particles, was fed to the bed in a narrow size distribution between two subsequent ASTM sieves (with 545 μm mean diameter). In transient batch experiments charges of limestone are quickly injected into the bed, while the consequent variations of the exit concentrations of SO2, CO2 and O2 are continuously recorded. Analysis were performed on the effects of the type of limestone in the process, taking into account possible reaction controlling resistances, and considering possible effects of the calcination on the sulfation process.

In situ spectroelectrochemical study of the reductive coupling of benzaldehyde catalyzed by Ti(III) complexes in the presence of Mg(II)

The UV–vis spectroelectrochemical technique has been used to visualize for the first time, the formation of a trimetallic intermediate Ti(III)Mg(II)Ti(III) during the reductive coupling of benzaldehyde in the presence of electrogenerated Ti(III) and MgCl 2. This intermediate is supposed to induce the high diastereoselectivity of the reaction. We have built a special thin layer cell adapted from the scheme proposed by A. Neudeck and L. Dunsch [‘J. Electroanal. Chem. 370 (1994) 17; J. Electroanal. Chem. 386 (1995) 135’] which allows the simultaneous recording of the cyclic voltammogram and of a great quantity of UV–vis spectra at a frequency of 1 spectrum per second by means of a UV–vis diode array spectrometer. The intermediate complex has been characterized by its UV–vis spectrum ( λ max=287, 321 and 370 nm). Its formation needs the simultaneous presence of electrogenerated Ti(III), MgCl 2 and benzaldehyde in the spectroelectrochemical cell. The global chemical reaction rate constant of Cp 2TiCl 2 with ArCHO in the presence of MgCl 2 has been determined by cyclic voltammetry and spectroelectrochemistry. Both techniques are in good agreement and provide the same value, k=410±127 M −3/2 s −1

Roles of inhibitors in global gas-phase combustion kinetics

Roles of inhibitors in the global kinetics of methane combustion were investigated computationally, by employing the Arrhenius equation to express the global rate constant. When the concentrations ofthe fuel and the oxidizer in a combustible mixture are kept constant, the global reaction rate is controlled by the flame temperature Ti, the global frequency factor A, and the global activation energy E,. Changes in A and E, with the additions of various inhibitors were estimated using the relationship between these parameters and the laminar burning velocity. Inhibitors investigated are He, CO,, CF,, CHF,, C,HF,, CF,Br, CFJ, NaOH, and Fe(CO),. The global kinetic parameters were found to vary according to the inhibitors' actions, physical, non-catalytic scavenging, and catalytic scavenging. That is, while He and CO, caused negligible changes in both A and E,, CHF, and C,HF, caused significant reduction in A without remarkable changes in E,. In contrast, CF,Br, CFJ, NaOH, and Fe(CO), caused noticeable increase in E,. Although an increase in E, seems to be a general characteristic of catalytic scavengers, its magnitude showed complicated variation due to many factors including the combustion promotion at higher temperatures and the saturation effect, which is speculated to be dependent on the thermochemistry of key inhibitor species. The maximum increase in E, with CF,Br at T,=2000 K was found to be approximately twice as much as that with Fe(CO),, which can be attributed to the coexistence of all the key inhibitor species in the CF,Br-doped flames more equally than those in the Fe(CO),-doped flames at lower temperatures.

Effect of wind velocity on flame spread in microgravity

A three-dimensional, time-dependent model is developed describing ignition and subsequent transition to flame spread over a thermally thin cellulosic sheet heated by external radiation in a microgravity environment. A low Mach number approximation to the Navier-Stokes equations with global reaction rate equations describing combustion in the gas phase and the condensed phase is numerically solved. The effects of a slow external wind (1–20 cm/s) on flame transition are studied in an atmosphere of 35% oxygen concentration. The ignition is initiated at the center part of the sample by generating a line-shape flame along the width of the sample. The calculated results are compared with data obtained in the 10 s drop tower. Numerical results exhibit flame quenching at a wind speed of 1.ccm/s, two localized flames propagating upstream along the sample edges at 1.5 cm/s, a single line-shape flame front at 5.0 cm/s, and three flames structure observed at 10.0 cm/s (consisting of a single line-shape flame propagating upstream and two localized flames propagating downstream along sample edges), followed by two line-shape flames (one propagating upstream and another propagating downstream) at 20.0 cm/s. These observations qualitatively compare with experimental data. Three-dimensional visualization of the observed flame complex, fuel concentration contours, oxygen and reaction rate isosurfaces, and convective and diffusive mass flux are used to obtain a detailed understanding of the controlling mechanism. Physical arguments based on the lateral diffusive flux of oxygen, fuel depletion, the oxygen shadow of the flame, and the heat release rate are constructed to explain the various observed flame shapes.

Oxidation and reduction characteristics of oxygen carrier particles and reaction kinetics by unreacted core model

The reaction kinetics of the oxygen carrier particles, which are used as bed material for a fluidized bed chemical looping combustor (CLC), has been studied experimentally by a conventional thermal gravimetrical analysis technique. The weight percent of nickel and nickel oxide in oxygen carrier particles and reaction temperature were considered as experimental variables. After oxidation reaction, the pure nickel particle was sintered and unsuitable to use as fluidizing particles. The oxidation reaction rate increased with increasing weight percent of nickel in oxygen carrier particles and reaction temperature. The rate of reduction shows maximum point with weight percent of nickel oxide (57.8%) and reaction temperature (750 or 800 °C) increased. In this work, the reaction between air and Ni/ bentonite particle was described by a special case of unreacted core model in which the global reaction rate is controlled by product layer diffusion resistance. However, the reaction between CH4 and NiO/bentonite particle was described by unreacted core model in which the global reaction rate is controlled by chemical reaction resistance. The temperature dependence of the effective diffusivity of oxidation reaction and reaction rate constant of reduction reaction could be calculated from experimental data and fitted to the Arrhenius equation.

Chaos and synchronisation in heterogeneous catalytic systems: CO oxidation over Pd zeolite catalysts

The influence of experimental parameters on the structure of global reaction rate oscillations and the coupling of local oscillators on a catalyst bed in a continuous stirred tank reactor is studied for the oxidation of CO on zeolite supported palladium catalysts. Global coupling can be achieved via mass transfer through the gas phase or via heat transfer in the case of a support of high heat conductivity. Characteristic differences in the activity of catalysts as well as in the period and the amplitude of the oscillations are related to the size of the palladium clusters and can be simulated by adding the state of the oxidation of the metal surface as a parameter to a common kinetic model. The analysis of observed chaotic behaviour leads to the conclusion that diffusional chaos characteristic of a distributed system is observed on the level of the zeolite crystallite that supports the palladium clusters.