Showing papers on "Elementary reaction published in 2004"

DFT Study of Formaldehyde and Methanol Synthesis from CO and H2 on Ni(111)

Abstract: We present a density-functional-theory study of formaldehyde and methanol synthesis from CO and H2 on a Ni catalyst We investigate the intermediate products of the reaction and calculate the reaction enthalpy and energy barrier for each elementary reaction, taking into account several different adsorption geometries and the presence of isomers of the intermediate products Hydrogenation of CO is favored over desorption or dissociation of CO on flat Ni(111), to form the formyl radical (HCO) or its isomer, COH Subsequent hydrogenation leads to formaldehyde (CH2O), methoxy (CH3O), and, finally, methanol (CH3OH) The overall reaction barrier for formaldehyde and methanol formation is 20 eV

Mineralogical approaches to fundamental crystal dissolution kinetics

Abstract: We introduce a general kinetic model for crystal dissolution that explicitly tracks all the various atoms in the crystal structure as part of the reaction mechanism. This model will be used in this and subsequent articles to develop a theory for the treatment of experimental and field water-rock kinetic data. The model is based on a many-body reaction mechanism. It is built from both elementary reactions, i.e., bond-breaking and bond-forming, and basic reactions, i.e., dissolution of surface units, adsorption and incorporation of solution units, and mobility of units at the crystal surface. The full crystal structure is used to calculate the interactions of neighboring atoms as well as possible defects of the crystal lattice in the model. This approach is different from models based on either molecular precursor complexes or adsorption. We analyze several fundamental concepts such as activation energy, surface free energy, the solubility product, inhibition/catalysis, and saturation-state dependence using our approach. In addition, surface features such as nucleation, steps, and defects are presented and put in a quantitative basis in this paper. The resulting kinetic framework can handle explicitly any crystal structure, treating the actual bonding and position of all atoms within a given surface orientation in the structure. Investigation of the properties of such a general kinetic model leads to new relations between the activation energy and the net energy changes in the hydrolyses reactions, between surface free energy and activation energies and between inhibition and the statistical mechanics of kink sites. The kinetic model can actually account for the emergence of a solubility product from a reaction mechanism involving independent kinetics for the different species using steady-state concepts on the behavior of surface sites. The possible Δ G dependence of the overall rate is studied with the general approach. Isotachs are used to exhibit the interplay of Δ G and inhibition within a simple AB mineral structure. The crystal-based reaction mechanism not only leads to a unified explanation of many observed water-rock features but also produce a series of modifications of kinetic results not fully understood before.

Mechanisms of Staudinger reactions within density functional theory.

Abstract: The Staudinger reactions of substituted phosphanes and azides have been investigated by using density functional theory. Four different initial reaction mechanisms have been found. All systems studied go through a cis-transition state rather than a trans-transition state or a one-step transition state. The one-step pathway of the phosphorus atom attacking the substituted nitrogen atom is always unfavorable energetically. Depending on the substituents on the azide and the phosphane, the reaction mechanism with the lowest initial reaction barrier can be classified into three categories: (1). like the parent reaction, PH(3) + N(3)H, the reaction goes through a cis-transition state, approaches a cis-intermediate, overcomes a PN-bond-shifting transition state, reaches a four-membered ring intermediate, dissociates N(2) by overcoming a small barrier, and results in the final products: N(2) and a phosphazene; (2). once reaching the cis-intermediate, the reaction goes through the N(2)-eliminating transition state and produces the final products; (3). the reaction has a concerted initial cis-transition state, in which the phosphorus atom attacks the first and the third nitrogen atoms of the azide simultaneously and reaches an intermediate, and then the reaction goes through similar steps of the first reaction mechanism. In contrast to the previous predictions on the relative stability of the unsubstituted cis-configured phosphazide intermediate and the unsubstituted trans-configured phosphazide intermediate, the total energy of the substituted trans-configured phosphazide intermediate is close to that of the substituted cis-configured phosphazide intermediate. The preference of the initial cis-transition state reaction pathway is thoroughly discussed. The relative stability of the cis- and the trans-intermediates is explored and analyzed with the aid of molecular orbitals. The effects of substituents and solvent effects on the reaction mechanisms of the Staudinger reactions are discussed in detail.

Reaction Route Graphs. I. Theory and Algorithm

Abstract: A theory and algorithm for reaction route (RR) network analysis is developed in analogy with electrical networks and is based on the combined use of RR theory, graph theory, and Kirchhoff's laws. The result is a powerful new approach of “RR graphs” that is useful in not only topological representation of complex reactions and mechanisms but, when combined with techniques of electrical network analysis, is able to provide revealing insights into the mechanism as well as the kinetics of the overall reactions involving multiple elementary reaction steps including the effect of topological constraints. Unlike existing graph theory approaches of reaction networks, the approach developed here is suitable for linear as well as nonlinear kinetic mechanisms and for single and multiple overall reactions. The theoretical approach for the case of a single overall reaction involving minimal kinetic mechanisms (unit stoichiometric numbers) is developed in Part I of this series followed by its application to examples of...

Adapting the nudged elastic band method for determining minimum-energy paths of chemical reactions in enzymes.

Abstract: Optimization of reaction paths for enzymatic systems is a challenging problem because such systems have a very large number of degrees of freedom and many of these degrees are flexible. To meet this challenge, an efficient, robust and general approach is presented based on the well-known nudged elastic band reaction path optimization method with the following extensions: (1) soft spectator degrees of freedom are excluded from path definitions by using only inter-atomic distances corresponding to forming/breaking bonds in a reaction; (2) a general transformation of the distances is defined to treat multistep reactions without knowing the partitioning of steps in advance; (3) a multistage strategy, in which path optimizations are carried out for reference systems with gradually decreasing rigidity, is developed to maximize the opportunity of obtaining continuously changing environments along the path. We demonstrate the applicability of the approach using the acylation reaction of type A β-lactamase as an example. The reaction mechanism investigated involves four elementary reaction steps, eight forming/breaking bonds. We obtained a continuous minimum energy path without any assumption on reaction coordinates, or on the possible sequence or the concertedness of chemical events. We expect our approach to have general applicability in the modeling of enzymatic reactions with quantum mechanical/molecular mechanical models.

A wave packet based statistical approach to complex-forming reactions.

Abstract: A wave packet based statistical model is suggested for complex-forming reactions. This model assumes statistical formation and decay of the long-lived reaction complex and computes reaction cross sections and their energy dependence from capture probabilities. This model is very efficient and reasonably accurate for reactions dominated by long-lived resonances, as confirmed by its application to the C(1D)+H2 reaction.

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

Abstract: First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation....

Calculation of Free Energy Profiles for Elementary Bimolecular Reactions by ab Initio Molecular Dynamics: Sampling Methods and Thermostat Considerations

Abstract: A study designed to refine the procedure for performing ab initio molecular dynamics calculations (AIMD) on chemical reactions is presented. Of key interest is the calculation of changes in free energy along the entire reaction path. Several simple elementary reactions are studied with the Car−Parrinello projector augmented-wave (CP−PAW) density-functional theory (DFT) methodology. The illustrative gas-phase bimolecular addition reactions are (i) a σ complexation of BH3 + H2O → H2O·BH3, (ii) the Diels−Alder reactions of butadiene with ethene, C4H6 + C2H4 → cyclohexene, 1,3-cyclopentadiene (CP) and ethene, CP + C2H4→ norbornene, and the stereoselective reaction of 5-amino-CP with ethene, amino-CP + C2H4 → amino-norbornene, (iii) the carbene cyclopropanation Cl2C + C2H4→ Cl2C3H4, and (iv) the dimerization of ketene. These reactions were used to test both the slow-growth and point-wise thermodynamic integration (STI and PTI) methods of phase-space sampling as well as the Nose−Hoover and Andersen thermostats....

Hot compressed water: a suitable and sustainable solvent and reaction medium?

Abstract: Hot compressed water in the sub-?and supercritical state exhibits exciting physical and chemical properties, which can be varied continuously from gas-like to liquid-like behaviour. Correspondingly, the solvent properties can change from non-polar behaviour as present, for example, in organic solvents to highly ionic characteristics like in salt melts. This opens up several promising opportunities for separation processes and chemical reactions. Under supercritical conditions, substantial amounts of gases and organic substances can homogeneously be mixed with water, which then can be separated by adjusting the subcritical conditions by forming additional phases. This can beneficially be combined with chemical reactions occurring in the homogeneous state leading to integrated processes, which are more effective and competitive. Three approaches to the technical application of hot compressed water are presented to show and discuss the technology, potential, technical hurdles and future research demand in this area of research and development. In supercritical water oxidation (SCWO) water is used as a medium in which organic pollutants are completely degraded under the addition of oxygen, which is completely miscible with water under the process conditions of up to 650??C and pressures around 25?MPa. Thus, high space?time yields in compact reactor designs can be realized. Hydrogen is produced from biomass by hydrothermal gasification. Here, in an excess of water, the reaction at temperatures up to 700??C and pressures around 30?MPa directly leads to valuable hydrogen instead of synthetic gas, as in conventional gasification processes, or methane at subcritical conditions in water. After reaction, pressurized hydrogen is obtained and can easily be enriched due to the different partition coefficients of hydrogen and carbon dioxide between the aqueous and gas phase. Even homogeneous catalysis is possible in supercritical water. This has been demonstrated with the cobalt-catalysed cyclotrimerization of acetylenes to form benzene derivatives or hydroformylation to produce aldehydes from olefins. There, only the addition of CO is necessary, the H2 required being formed by the equilibrium of the water?gas-shift reaction. After a homogeneous reaction in the supercritical state, the reaction mixture can be separated at subcritical conditions. In support of the chemical and technical developments and to principally understand the experimental findings fundamental aspects have to be investigated as well. Intensive studies have been devoted to chemical kinetics including the modelling with elementary reaction steps, e.g.?to separate ionic and radical reaction pathways. Depending on the reaction conditions, ionic or radical reaction pathways can be favoured or suppressed, allowing for control selectivity. Furthermore, corrosion of relevant reactor materials has been investigated.

Thermochemical Properties, Pathway, and Kinetic Analysis on the Reactions of Benzene with OH: An Elementary Reaction Mechanism

Abstract: Kinetics for the chemical activation reaction of the OH radical with benzene and unimolecular dissociation of the adduct are analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) and master equation analysis for pressure falloff. Thermochemical properties and reaction path parameters are determined by ab initio and density functional calculations. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) and MP2(full)/6-31G(d) levels, with single point calculations for the energy at the B3LYP/6-311++G(2df,p)//B3LYP/6-31G(d,p), composite methods of CBS-Q, CBS-QB3 and G3(MP2) and the G3 methods. The OH addition to benzene forms a chemically activated prereactive complex with a shallow well (ca. 3 kcal mol-1), which predominantly dissociates back to reactants. Additional reactions of the energized precomplex include stabilization, or forward reaction to form hydroxycyclohexadienyl radical, C•HDOH, which has a well depth of 16 kcal mol-1. This C•HDOH adduct can eithe...

Reaction Route Graphs. II. Examples of Enzyme- and Surface-Catalyzed Single Overall Reactions

Abstract: The utility of the new reaction route (RR) graph theory developed in the preceding paper (Part I) is illustrated here, with the help of two examples. In the first example, the kinetics of the conversion of 7,8-dihydrofolate and NADPH to 5,6,7,8-tetrahydrofolate and NADP, as catalyzed by dihydrofolate reductase (DHFR), is considered. This system is described by a linear kinetic mechanism that includes 13 elementary reaction steps. The second example is a microkinetic model of the water-gas shift reaction on a copper catalyst, which is highly nonlinear and includes 15 elementary surface reaction steps. For both mechanisms, the RR graphs have been constructed and used to determine the overall rates. The RR graphs and the overall rate equations are further simplified and reduced, using the RR network approach.

A two-step reaction mechanism of oxygen release from Pd/CeO2: mathematical modelling based on step gas concentration experiments

Abstract: A mathematical model has been developed to study the transient release of oxygen from a 1 wt% Pd/CeO2 catalyst in the 450–550 °C range based on alternate step concentration switches between CO and O2. A two-step reaction mechanism that involves the reaction of gaseous CO with the oxygen species of PdO and of the back-spillover of oxygen from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the CO and CO2 transient response curves. The proposed mathematical model allows the estimation of the transient rates of the CO oxidation reaction and of the back-spillover of oxygen process. It also allows the calculation of the intrinsic rate constant k 1 (s−1) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 10.1 kJ/mol was estimated for this elementary reaction step. In addition, an apparent rate constant $$k_2^{app} (s^{-1})$$ was estimated for the process of back-spillover of oxygen.

Extracting Biochemical Reaction Kinetics from Time Series Data

Abstract: We consider the problem of inferring kinetic mechanisms for biochemical reactions from time series data. Using a priori knowledge about the structure of chemical reaction kinetics we develop global nonlinear models which use elementary reactions as a basis set, and discuss model construction using top-down and bottom-up approaches.

Ab initio modeling of organophosphorus combustion chemistry

Abstract: An ab initio study has been conducted to calculate the thermochemistry of several organophosphorus intermediates, to assess the accuracy of estimated rate constants in current organophosphorus combustion mechanisms and to explore additional reaction pathways for methylphosphonic acid (MPA or PO(OH)2CH3) The organophosphorus reaction rates and thermochemistry were calculated for an elementary reaction rate model to predict MPA oxidation kinetics and reaction pathways in supercritical water The thermochemistry for 17 phosphorus-containing species was calculated using the CBS-Q method, including species such as PO(OH)2CH2˙, PO(O˙)OHCH3, and PO(OH)2CH2O˙, whose thermochemical values are unknown Rate constants for fourteen reaction pathways involving MPA and its intermediates were calculated using conventional transition state theory Unimolecular decomposition rates of MPA, bimolecular hydrogen abstraction rates between MPA and OH˙, and decomposition and bimolecular hydrogen abstraction rates of phosphoric acid and P˙O(OH)2 were calculated and compared to estimated literature rate constants, when available Agreement ranged from within one order of magnitude to more than six orders of magnitude difference The most significant discrepancies occurred for reactions involving P–O bonds, indicating that estimates of these reaction rates based on analogous C–O bond chemistry are unreliable Several types of free-radical organophosphorus reaction pathways were studied for the first time

Elementary reaction rate model for MPA oxidation in supercritical water

Abstract: An organophosphorus elementary reaction rate model has been developed to characterize rates and mechanistic pathways for methylphosphonic acid (MPA or PO(OH)2CH3) oxidation in supercritical water (SCW). The reaction network contains 242 reactions and 41 species, including 92 new phosphorus-containing reaction rates and 13 new phosphorus-containing species. Critical hydrogen abstraction and β-scission pathways omitted from existing organophosphorus combustion models have been added to the present model. With these additions, the MPA SCWO model accurately predicts the experimental concentration profiles of MPA and its measured intermediates and products, H3PO4, CO, CO2, and CH4 at a pressure of 246 bar, [MPA]0 = 1 mM, and stoichiometric conditions at a residence time range of 3 to 10 s and a temperature range of 751 to 845 K in supercritical water (P. A. Sullivan and J. W. Tester, AIChE J., 2004, 50, 673). Modeling results qualitatively agree with the experimental observation that the MPA oxidation rate increases with increasing oxygen concentration. The model does not correctly reproduce the observed experimental trend of increasing MPA oxidation rates with increasing water density; possible reasons for this discrepancy are discussed. The model correctly predicts product selectivity for the carbon-containing products, CO, CO2, and CH4 at varying τ, T, Φ, and P, indicating that the newly proposed reactions are important to properly predict the branching between the MPA reaction pathways.

Effect of pressure on counterflow H2-air partially premixed flames

Abstract: A computational investigation of high-pressure hydrogen–air partially premixed flames (PPFs) is reported to characterize the effect of pressure on the flame structure, and the relevance of reaction limits for these flames. The flames are computed using the Mueller mechanism consisting of 19 elementary reactions and 9 species. Although the mechanism has been validated during previous investigations, additional validations are provided at high pressure. The PPF structure is characterized by two spatially distinct reaction zones, namely a rich premixed zone on the fuel side and a nonpremixed zone on the air side. In both reaction zones, consumption of reactants occurs primarily through reactions H + O2 ↔ OH+ O( R1), H 2 + O ↔ OH+ H( R2), H 2 + OH ↔ H2O + H( R3), and H + O2 + M ↔ HO2 + M (R9). As pressure increases, it decreases the physical separation between the two reaction zones. This can be attributed to the effects of pressure on (i) flame speed associated with the rich premixed zone, which moves this zone further downstream and (ii) mass diffusivity which moves the nonpremixed zone further upstream (toward the fuel nozzle). At higher pressures, however, these effects are significantly reduced, and the flame maintains its twin-flame structure even at very high pressures. Three reaction limits are identified for these flames. While the chemical structure of the nonpremixed zone is characterized by the first reaction limit in the range of pressure investigated (p = 1 to 40 atm), that of the rich premixed zone is characterized by transition from first to second limit, and then from second to third limit, as pressure is increased. This implies that H 2–air PPFs can exploit the advantages of the two reaction zones; each dominated by different reaction limits or chain reactions. Thermal radiation is found to have a negligible effect on the flame structure, while the Soret effect is found to cause transition between the reaction limits at lower pressure.

Electron transfer between quinones in photosynthetic reaction centers

Abstract: The mechanism of electron transfer (ET) from the primary to the secondary quinone of bacterial photosynthetic reaction centers is discussed on the basis of theoretical computations of the minimum energy nuclear configurations and ET coupling elements, and quantum dynamic simulations of elementary reaction steps. For ET to occur via tunneling, unreasonably high values of the electronic coupling elements or very stringent energy conditions, i.e., tight degeneracy (within a few cm-1) between the initial and final vibronic states, are necessary, both for the direct and through-bridge (superexchange) routes. The assumption of tight degeneracy significantly slows down the process so that other competitive processes, such as proton transfer from the H-bonded HisM219 to the primary quinone, can take place. All these results suggest that the iron−histidine bridge can play an important role in the ET mechanism.

Kinetics of OH Chemiluminescence in the Presence of Hydrocarbons

Abstract: Shock-tube experiments and chemical kinetics simulations have been used to optimize a kinetics model for the electronically excited OH radical (OH*) in a hydrocarbon environment at high temperatures (1420 < T < 2335 K) and at pressures between 0.8 and 3.0 atm. Ultraviolet emission near 307 nm was recorded from the shock-tube sidewall, and absolute concentrations were used to obtain a rate expression for the elementary reaction primarily responsible for formation of OH*, i.e. CH + O2 ⇆ OH* + CO. Absolute concentrations were deduced from a previously established calibration employing the relatively well-known kinetics of the H2/O2 system. To isolate the phenomena of interest, experiments were performed with mixtures of CH4 and H2 with O2, highly diluted in Argon, and elementary reactions accounting for formation and quenching of OH* were added to the GRI-Mech 3.0 detailed mechanism. Sensitivity analyses indicate that the maximum OH* concentration is most strongly sensitive to the formation path through CH + O2 so that this reaction rate could be found by varying its coefficient to match the experimental concentration. In this manner, the rate coefficient was found to be k1 = (4 ± 3)×10 cm mol s, independent of temperature and pressure. Uncertainties in this rate due to the calibration reaction and to scatter in the data are considered, and results are compared to values previously available in the literature. The end result is an optimized diagnostic accurate over a broad range of conditions encountered in combustion and propulsion applications.

A quantum chemistry study of the formation of pah and soot precursors through butadiene reactions

Abstract: Density functional theory was used to investigate the kinetics of the reaction of addition of C2H3 and C6H5 to 1,3-C4H6. Kinetic constants for each elementary reaction involved in the reactive processes were calculated at the B3LYP/6-31g(d,p) level with conventional transition state theory, while overall rate constants for the formation of the different products were determined with Quantum Rice Ramsperger Kassel theory. The main result of this study is that, once provided a significant amount of 1,3-C4H6 in the flame environment (such as in butadiene flames), a significant contribution to the rate of formation of benzene, cyclopentadiene, and naphthalene can be ascribed to the investigated reactions.

Experimental verification of the Smoluchowski theory for a bimolecular diffusion-controlled reaction in liquid phase

Abstract: We report experimental verification of the Smoluchowski theory for diffusion-controlled reactions in solution at the steady-state limit. We have determined both the diffusion coefficients and the self-termination reaction rates of the diphenylmethyl radical simultaneously. Smoluchowski theory is insufficient to discuss the reaction rate for the self-termination reaction of the diphenylmethyl radical, so the reaction rate of an encounter complex based on the Collins–Kimball treatment is estimated.

Mutual Influence of Ligands and Reactivity of Gd and Dy Acidophthalocyaninate Complexes

Abstract: The results of the kinetic study of dissociation of Gd(III) and Dy(III) complexes with phthalocyanine of the composition (X)LnPc (X is single-charged acido ligand) with isolation of macrocyclic ligand depending on the temperature, composition of mixed ethanol–acetic acid solvent, and the nature of acido ligand are presented. The total kinetic equations, the rate constants, and activation parameters of dissociation reaction are determined. The stoichiomeric mechanism is suggested for the complex dissociation involving the limiting elementary reaction between acetic acid molecule and the complex that occurs as the chelate salt (X)LnPc or the outer-sphere complex [(HOAc)LnPс]+X–. The state of metal phthalocyaninate at the reaction slow stage is shown to be determined by the electronic structure of the metal cation, the strength of binding of the axial ligand, and by its cis-effect on the metal bonds with macrocycle.

8-oxoguanine DNA glycosylase触媒メカニズムの量子化学的研究: N-glycoside結合の開裂反応における基質支援機構

Abstract: Human 8-oxoguanine DNA glycosylase 1(hOGG1) plays a significant role of repairing oxidized genomic DNAs. The repair process includes the cleavage reaction of N-glycosidic linkage between 8-oxoguanine (8-oxoG) and the deoxyribose. To clarify the atomic-scale reaction mechanism of the N-glycosidic linkage cleavage, quantum chemical (QM) calculation at B3LYP/6-31G(d,p) was performed with a reaction model that consists of the catalytic Lys249 and guanosine that includes the 8-oxoG. It has been found from the QM calculation that the cleavage mechanism proceeds via three elementary reactions. In the first elementary reaction, a proton in the ammonium group of Lys249 is removed by the oxygen atom of 8-oxoG (8O) in concert with the generation of a new hydrogen bond between 8O and O4' of deoxyribose. In the second elementary reaction, N atom in Lys249 side chain (Nζ) nucleophilically attacks on C1' of the deoxyribose in concert with the spontaneous proton migration from 8O to O4'. The proton migration induces the dissociation of ether bond between O4' and C1'. Finally, a proton migrates from Nζ to N9 in 8-oxoG via 8O. N-glycosidic linkage between C1' and N9 is completely cloven in the final elementary reaction. It was confirmed that Schiff base appeared in products of the final reaction. According to the obtained base excision mechanism, 8O participates in the first and second elementary reactions. Therefore, this enzymatic reaction is a substrate-assisted catalysis. It was also found that the reaction path requires large activation energy (<42kcal/mol). This result finely reflects the experimental finding that the enzymatic activity of hOGG1 is not so high.

Numerical computation of reacting flow in porous burners with an extended ch4-air reaction mechanism

Abstract: The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p’( pressure correction), enthalpy and species mass fractions, utilizing a grid of (60X40) nodes. The sixty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The porous burner uses a premixed CH4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH4, CH3, CH2, CH, CH2O, CHO, CO, CO2, O2, O, OH, H2, H, H2O, HO2, H2O2, involving 49 chemical reaction equations. It was found that 900 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H2O, CO2, H2, keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, first increase in the preheating region of the reaction zone, they are then consumed fast in the postreaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products..

A Kinetic Model of Degradation of Phenol in Water by Direct Contact of Gas Nonpulsed Corona Discharge

Abstract: Reaction byproducts from degradation of aqueous phenol by contact of gas d.c. corona discharge with treated water were analyzed by liquid chromatography. According to the retention time, three byproducts, pyrocatechol, hydroquinone, 1,4-benzoquinone, were identified. Also, acetic acid was detected as a final relatively stable byproduct by gas chromatography. A simple reaction model in which relevant elementary reactions are assumed to be first order was proposed to correlate the practical behavior of degradation processes.