Simple Reaction Scheme Research Articles

Studies on the oxidation mechanism of H2S based on direct examination of the key reactions

By conducting an excimer laser photolysis (193 and 248 nm) behind shock waves, three elementary reactions important in the oxidation of H2S have been examined, where, H, O, and S atoms have been monitored by the atomic resonance absorption spectrometry. For HS + O2 → products (1), the rate constants evaluated by numerical simulations are summarized as: k1 = 3.1 × 10−11exp|-75 kJ mol−1/RT| cm3molecule−1s−1 (T = 1400-1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S + O2 → SO + O (2), and SO + O2 → SO2 + O (3) yield k2 = (2.5 ± 0.6) × 10−11 exp|-(15.3 ± 2.5) kJ mol−1/RT| cm3molecule−1s−1 (T = 980-1610 K) and, k3 = (1.7 ± 0.9) × 10−12 exp|-(34 ± 11) kJ mol−1/RT| cm3molecule−1s−1 (T = 1130-1640 K), respectively. By summarizing these data together with the recent experimental results on the H(SINGLE BOND)S(SINGLE BOND)O reaction systems, a new kinetic model for the H2S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO2 formation obtained by Bradley and Dobson. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 57–66, 1997.

Steady-State Waste Combustion and Air Flow Optimization in a Field Scale Rotary Kiln

This study describes a computational modeling study of a full-scale rotary kiln using the commercial CFD-code PHOENICS. A steady-state mode is assumed in which solid waste (a mixture of cellulose and toluene) is volatilized and burned. The focus of the model is on the gas phase fluid dynamics. Buoyancy is accounted for, turbulence is modeled with the κ-∈ model, radiative transfer is in a first step modeled with the composite flux model of radiation and combustion is modeled using the simple chemical reaction scheme (SCRS) model for diffusive, mixing limited reacting flows. The volatilization model makes use of a first-order Arrhenius-type reaction rate that takes the volatilization properties of both toluene and cellulose into account. The goal of this study is to screen various air inlet designs for their mixing characteristics and residence time distribution. This is translated in the destruction efficiency of selected compounds. The actual air inlet configuration in an existing full-scale incinerator is chosen as a reference for comparison. Alternative designs are analysed. The methodology and usefulness of the approach is fully demonstrated and typical design results are reported. Key words: Rotary kiln; Computational Fluid Dynamics; solid waste volatilization; air inlet design

Kinetics of the reaction of 4,4'-methylenedianiline with propylene oxide in ethylene glycol

Abstract The kinetics for the reaction of 4,4'- methylenedianiline, MDA, with propylene oxide in ethylene glycol as a solvent to generate secondary and tertiary aromatic alkanolamines were investigated. This reaction is important in converting glycolyzed, liquefied, recycled polyurethane foam, containing high levels of MDA and primary aromatic amines, into a high-performance, recycled polyol substitute that can be used in the manufacture of new polyurethane foams. The kinetics were obtained using a combination of data from heat flow calorimetry and chemical analysis using HPLC. Reactions were run using a series of programmed propylene oxide feed additions to generate data-rich sets of global thermal rate data. Chemical analysis yielded the distribution of mono-,di-, tri-, and tetra-sustituted species. Using a weighted regression analysis the data sets for several experiments at different temperatures were analyzed simultaneously to obtain the kinetic parameters of a simplified reaction scheme that accurately described this system. The kinetic analysis shows that propylene oxide is a preferred epoxide to maximize the yield of secondary amines.

Si/XeF2 etching: Temperature dependence

The temperature dependence of the Si(100)/XeF2 etch reaction is studied quantitatively in a molecular beam setup. At a sample temperature of 150 K the reaction probability reaches unity initially, after which the XeF2 condenses on the surface and blocks the etching process. For increasing temperatures the XeF2 reaction probability initially decreases from 100% at 150 K down to 20% around 400 K, but for temperatures above 600 K it increases again up to 45% at 900 K. In a simple reaction scheme the high etch rate at low temperatures is explained by a XeF2-precursor, with an activation energy for desorption of 32±4 meV. Furthermore the increased etch rate at high temperatures is explained by the desorption of SiF2 with an activation energy of 260±30 meV. The steady-state fluorine content of the SiFx reaction layer, measured using thermal desorption spectroscopy, reaches a maximum of 5.5 monolayers at 300 K. For increasing temperatures it decreases to a submonolayer coverage above 700 K. The temperature dependence of the formation of the reaction layer is described well by including the XeF2-precursor in a previously developed adsorption model.

A kinetic model for sulfur accelerated vulcanization of a natural rubber compound

Vulcanization kinetics for a natural rubber compound were studied by a kinetic approach using the cure meter and DSC methods. A simplified but realistic model reaction scheme was used to simulate induction, curing, and overcure periods continuously. Physically significant parameters of the model were extracted from isothermal experimental data using a cure meter. The calculation demonstrated a good correspondence with isothermal cure meter data over the temperature range studied. The length of induction time, variation of maximum modulus with temperature, and the reversion phenomena observed from cure curves can be predicted. DSC data were found to be incompatible with the cure meter test, because the complex vulcanization reaction system is multiexothermal and it is difficult to isolate the heat due to crosslinking. Hence, the cure meter technique is suggested for the study of crosslink formation. The kinetic approach provides a way to incorporate vulcanization kinetics into simulation of reactive processing operations. © 1996 John Wiley & Sons, Inc.

A Study of the Vulcanization Kinetics of an Accelerated-Sulfur SBR Compound

Abstract Vulcanization kinetics for a SBR compound was studied by using both curemeter and DSC methods by a kinetic approach. A simplified but realistic model reaction scheme was used to simulate both induction and curing periods simultaneously. Model parameters were extracted from isothermal curemeter experiments. The model prediction demonstrated a good agreement with isothermal curemeter data over a temperature range of 120°C to 180°C. The variation of equilibrium modulus with temperature, observed from cure curves, can also be predicted. However, DSC experiments showed a different reaction behavior in the curing period as compared to model calculations. This was explained by the assumption that the reaction heat observed in DSC is due to all possible exothermal reactions, and the formation of crosslinks is only a part of these reactions. Hence, the curemeter can provide a good indication of crosslink formation, while DSC displays the entire reaction heat released during the vulcanization process. The kinetic approach allows one to incorporate vulcanization kinetics into the practical simulation of reactive processing operations.

Acid−Base Equilibria Involved in Secondary Reactions Following the 4-Carboxybenzophenone Sensitized Photooxidation of Methionylglycine in Aqueous Solution. Spectral and Time Resolution of the Decaying (S∴N)+ Radical Cation

A radical cation with an intramolecular sulfur−nitrogen bond was formed in the photoinitiated transfer of an electron from the sulfur atom of the dipeptide Met-Gly to 4-carboxybenzophenone in its triplet state. The sulfur−nitrogen coupling involved two-center, three-electron bonds. The kinetics of the reactions of these radical cations, which were initiated by a laser flash, were followed over time. The spectrum of the radical cation had to be extracted from the interference of other absorbing transients. The principal method of implementing the spectral resolutions was accomplished through a multiple linear regression technique. This spectral analysis was repeated for numerous time windows during the lifetime of the transients' decays. The resulting concentrations of the transients were consistent with an independent factor analysis. It was found that the decay of the radical cations was multiexponential and that the decay varied with pH. A simplified reaction scheme was proposed whereby the absorbing radical cations can alternatively decay by an irreversible channel or react reversibly with OH-. Rate constants for the three elementary reactions of this scheme were determined from an analysis of the decay of the concentration of the radical cations. In addition, the equilibrium constant for the reversible reaction was determined by two separate procedures. In one, the equilibrium constant was calculated from the approximate linear [OH-] dependence of the faster of the two rate constants (in the observed two-exponential decay). The second determination of the equilibrium constant was made from the preexponentials of the empirical two-exponential fits. It was shown how these preexponentials were related to the equilibrium concentrations of the species involved in the equilibrium.

An Experimental Study on the Kinetics of the Formation and Decomposition of Sulfanes in the Sulfur/H2S System

The question whether the sulfur transport in sour gases proceeds via physically solved elemental sulfur or the formation of volatile sulfanes is of great practical importance. A gas chromatographic technique has been worked out which allows one to determine the kinetics of the formation and decomposition of sulfanes in the sulfur-hydrogen sulfide system at high temperature. Quantitative kinetic data are of great importance not only for research in that field but also for various practical applications like, for example, the sulfur deposition problem in sour gas reservoirs. Kinetic measurements have been carried out at different constant temperatures in the range of 120--270 C in a chromatographic reactor, containing sulfur-coated glass beads and quartz powder, respectively. In order to determine the kinetic parameters, a numerical model of the gas chromatographic reactor has been developed. Based on the results obtained by fitting the experimental chromatograms to the model, a simplified reaction scheme is proposed. Preliminary results show that sulfane decomposition in the sulfur/H{sub 2}S system is a rather slow reaction.

PH Oscillations in the Bromate−Sulfite−Marble Semibatch and Flow Systems

A self-accelerating oxidation of an unbuffered aqueous sodium sulfite−hydrogen sulfite solution by sodium bromate and a selective removal of the hydrogen ion by solid marble chips from the reaction mixture have been used to construct an oscillatory system. The system exhibits large-amplitude oscillations between pH 3.5 and 7.5 at 25.0 °C in a continuous-flow stirred tank reactor and in a semibatch configuration. The shape, the periodic time (from 10 min to 2 h), and the region of oscillations can be controlled by using different amounts and grade size of marble. A simple reaction scheme, consisting of the protonation equilibria of SO32- and HSO3-, the oxidation of HSO3- and H2SO3 by BrO3-, and a removal of H+ by the CaCO3 in marble in the form of HCO3- has successfully been used to simulate the observed dynamical behavior.

Yield of ions produced in the luminescence quenching of tris (2,2′‐bipyridine)ruthenium(II) by dichlorophenolate ions

AbstractThe electron‐transfer quenching of Ru(bpy) (tris(2,2′‐bipyridine)ruthenium(II) excited state by 2,4‐dichlorophenol (2,4‐DCP), 2,5‐dichlorophenol (2,5‐DCP) and 2,6‐dichlorophenol (2,6‐DCP), in their anionic forms, was studied in de‐aerated solutions of neat methanol and 25, 50 and 75% v/v aqueous methanol at 30°C. The quenching rate constants (kq) were determined and their values range from 1 × 107 to 8 × 109 M−1 s−1. In all the mixtures methanol : water the kq values for 2,5‐DCP were lower than for 2,4‐DCP and 2,6‐DCP. This result was discussed in terms of the effect of chloro atom on the quencher oxidation reaction. The photoredox product Ru(bpy) was observed by laser flash photolysis and its quantum yield (ΦRu(+)) calculated by using a comparative method with the formation of zinctetraphenylporphyrin triplet state. The yields of cage escape (Yce) for the different methanol/water mixtures were calculated from the obtained values of ΦRu(+)). Yce increases from 0·1–0·2 in 25% v/v aqueous methanol to 0·5–0·6 in neat methanol. This change of Yce with the solvent composition was discussed in terms of combined effects of viscosity and solvent cage on the rate constant of cage escape (k−d) and the rate constant for back electron‐transfer to the ground state from geminate pair (kbc), respectively. The constants k−d were calculated with the Eigen equation. The constants kbc were estimated from Yce and k‐d by applying a simplified reaction scheme. The kbc kbc values range from 3·2 × 109 to 1·3 × 1010 s−1.

Stable pattern and standing wave formation in a simple isothermal cubic autocatalytic reaction scheme

The formation of stable patterns is considered in a reaction-diffusion system based on the cubic autocatalator, A+2B → 3B, B → C, with the reaction taking place within a closed region, the reactant A being replenished by the slow decay of precursor P via the reaction P → A. The linear stability of the spatially uniform Steady state % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaamaabmGabaGaamyyaiaacYcacaWGIbaacaGLOaGaayzkaaGaeyyp% a0ZaaeWaceaacqaH8oqBdaahaaWcbeqaaiabgkHiTiaaigdaaaGcca% GGSaGaeqiVd0gacaGLOaGaayzkaaaaaa!48C3!\[\left( {a,b} \right) = \left( {\mu ^{ - 1} ,\mu } \right)\], where a and b are the dimensionless concentrations of reactant A and autocatalyst B and μ is a parameter representing the initial concentration of the precursor P, is discussed first. It is shown that a necessary condition for the bifurcation of this steady state to stable, spatially non-uniform, solutions (patterns) is that the parameter % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaaiaadseacqGH8aapcaaIZaGaeyOeI0IaaGOmamaakaaabaGaaGOm% aaWcbeaaaaa!4139!\[D < 3 - 2\sqrt 2 \] where D=D b/Da (D a and D b are the diffusion coefficients of chemical species A and B respectively). The values of μ at which these bifurcations occur are derived in terms of D and λ (a parameter reflecting the size of the system). Further information about the nature of the spatially non-uniform solutions close to their bifurcation points is obtained from a weakly nonlinear analysis. This reveals that both supercritical and subcritical bifurcations are possible. The bifurcation branches are then followed numerically using a path-following method, with μ as the bifurcation parameter, for representative values of D and λ. It is found that the stable patterns can lose stability through supercritical Hopf bifurcations and these stable, temporally periodic, spatially non-uniform solutions are also followed numerically.

Polyaniline: Forms and Formation

The forms of polyaniline, viz. pernigraniline, emeraldine and leucoemeraldine, their mutual transformations and their roles in the chemical polymerization of aniline, are discussed. Absorption spectra of its various protonated forms and corresponding bases are reported. A simple reaction scheme is proposed to account for the colour and acidity changes during polymerization.

Transalkylation of m-diethylbenzene over large-pore zeolites

The transalkylation of m-diethylbenzene with benzene to ethylbenzene has been studied over some β- and Y-zeolites at 210–270dgC and 48 bar total pressure. A rather complex mechanism has been found for the reaction, which is accompanied by the formation of several undesired byproducts, especially over Y-zeolites. No direct formation of ethylbenzene from m-diethylbenzene was observed. Only o- and p-isomers can be transformed into the desired product. Furthermore, the latter isomers do not isomerise directly to the other form. Naphthalene forms very likely from o-diethylbenzene only, while diphenylethanes and methylethylbenzenes seem to form preferentially from m-diethyl-benzene. A simplified general reaction scheme is proposed and the kinetic parameters for such reactions were estimated, together with their Arrhenius parameters.

Photochemical Formation of Colloidal Silver in the Presence of Benzophenone

Abstract An aqueous silver perchlorate–sodium dodecylsulfate (SDS)–benzophenone (BP) solution was photolyzed with either UV or near-UV light. Irradiation with 253.7 nm light, which was absorbed by both BP and silver ions, brought about the formation of collidal silver and the photobleach of BP. Peptization of colloidal silver agglomerates after prolonged irradiation was ascribed to electron injection from the benzophenone ketyl radical (BPK). The formation of colloidal silver and the photobleach of BP were also caused by irradiation with 365 nm light, which was absorbed solely by BP (sensitized reaction). A simple reaction scheme was introduced and the initial rates for the sensitized photoreduction of silver ions and the photobleach of BP were expressed in closed forms. Suitable combinations of the kinetics parameters and the extinction coefficient of colloidal silver were proposed in order to interpret the dependences of the initial rates on the reactant concentrations.

A numerical study of the vorticity field generated by the baroclinic effect due to the propagation of a planar pressure wave through a cylindrical premixed laminar flame

The importance of vorticity production in combustion systems has been highlighted previously by several authors (Markstein 1964; Picone et al. 1984). The consequent distortion and enlargement of flame surfaces can lead to substantial enhancement of the burning rate which may be beneficial or disastrous depending on the physical context. We describe the results of numerical simulations of an experimental configuration similar to that described by Scarinci &amp; Thomas (1992), who examined the effect of initially planar pressure signals on two-dimensional flame balls. The flame ball is here set-up from ignition using a code, based on the second-order Godunov scheme described by Falle (1991). A simple Arrhenius reaction scheme is adopted in modelling a unimolecular decomposition. As in previous papers (Batley et al. 1993 a, b) the thermal conductivity is assumed to vary linearly with temperature, and the Lewis and Prandtl numbers are taken as unity. A short time after ignition, when the flame ball has reached a radius of approximately 2 cm, a very short-lengthscale pressure step disturbance is introduced, propagating towards the combustion region. As the signal crosses the flame, the interaction of the sharp, misaligned pressure and density gradients, creates a strong vorticity field. The resulting roll-up of the flame eventually divides it into two smaller rotating reacting regions. In order to gauge the effect of the chemical reaction and in particular the viscous diffusion on the evolution of the vorticity field, the results are compared with analogous solutions of the Euler equations.

Kinetics of peptide synthesis studied by fluorescence of fluorophenyl esters

The kinetics of the reaction of Boc-alanine-trifluorophenyl, Boc-alanine-tetrafluorophenyl, Boc-alanine-pentafluorophenyl, and Boc-alanine-p-chlorotetrafluorophenyl esters (BocAlaOTrf, BocAlaOTfp, BocAlaOPfp, and BocAlaTfc, respectively) with leucine amide and with valine methyl ester have been measured using changes in fluorophenyl chromophore emission at 375 nm. The kinetic data cannot be well fit with a simple second-order reaction scheme. Measurements of the reaction kinetics at different concentrations of the reagents showed that the expression for the reaction rate is V = kC(N)0.5C(AE)1.5, in which k is the reaction rate constant, CN is the concentration of either LeuNH2 or ValOCH3, and CAE is the concentration of the fluorophenyl ester. This reaction equation indicates a complex, probably chain-like, reaction mechanism. The order of reactivity for these active esters with ValOCH3 is BocAlaOTfc > BocAlaOPfp > BocAlaOTfp > BocAlaTrf. The apparent rate constant, k, for the reaction with LeuNH2 is higher than that for the reaction with ValOCH3.

Ligand-induced accelerated dissociation of (+)-cis-diltiazem from L-type Ca2+ channels is simply explained by competition for individual attachment points.

We have studied the kinetics of (+)-[3H]cis-diltiazem binding to L-type Ca2+ channels. The association reaction was of the second order with a rate constant of k1 = 5.3 x 10(-5) nM-1 min-1. A dissociation rate of the first order (k-1 = 0.0022 min-1) and the monophasic equilibrium binding curve (KD = 42 nM) seemed to reflect the simple case of one ligand binding to one site. However, when dissociation was monitored after isotopic rather than volume dilution, the dissociation rate constant increased with the concentration of added non-radioactive (+)-cis-diltiazem. Half-logarithmic plots were linear with rate constants of 0.0058, 0.011, 0.015, and 0.020 min-1 when 3, 10, 30, and 50 microM (+)-cis-diltiazem was added, respectively. We quantitatively analyzed our experimental data in terms of three different binding models, which can account for ligand-induced accelerated dissociation: 1) the classical allosteric model of pre-existing conformational states, 2) a general reaction scheme for two sites with negative cooperativity, and 3) competition of ligands for single attachment points within one binding site. Computer fitting of the experimental data to simplified reaction schemes derived from these models revealed that all three can be used to fit our data. However, only the multipoint attachment model does not require additional structural assumptions, not even an additional binding site, and is supported by structure-activity relationship analysis of diltiazem analogues. We conclude that ligand-induced accelerated dissociation should be a widely observed phenomenon that does not prove a non-competitive binding mechanism. Instead, it can be explained with the thermodynamics of non-covalent binding, where high affinity results from the cooperative interaction of a ligand molecule with multiple attachment points.

Initial steps of ?and ?-d-glucose binding to intact red cell membrane

The kinetics of the initial phases of D-glucose binding to the glucose transport protein (GLUT1) of the human red cell can be followed by stopped-flow measurements of the time course of tryptophan (trp) fluorescence enhancement. A number of control experiments have shown that the trp fluorescence kinetics are the result of conformational changes in GLUT1. One shows that nontransportable L-glucose has no kinetic response, in contrast to D-glucose kinetics. Other controls show that D-glucose binding is inhibited by cytochalasin B and by extracellular D-maltose. A typical time course for a transportable sugar, such as D-glucose, consists of a zero-time displacement, too fast for us to measure, followed by three rapid reactions whose exponential time courses have rate constants of 0.5-100 sec-1 at 20 degrees C. It is suggested that the zero-time displacement represents the initial bimolecular ligand/GLUT1 association. Exponential 1 appears to be located at, or near, the external membrane face where it is involved in discriminating among the sugars. Exponential 3 is apparently controlled by events at the cytosolic face. Trp kinetics distinguish the Kd of the epimer, D-galactose, from the Kd for D-glucose, with results in agreement with determinations by other methods. Trp kinetics distinguish between the binding of the alpha- and beta-D-glucose anomers. The exponential 1 activation energy of the beta-anomer, 13.6 +/- 1.4 kcal mol-1, is less than that of alpha-D-glucose, 18.4 +/- 0.8 kcal mol-1, and the two Arrhenius lines cross at approximately 23.5 degrees C. The temperature dependence of the kinetic response following alpha-D-glucose binding illustrates the interplay among the exponentials and the increasing dominance of exponential 2 as the temperature increases from 22.3 to 36.6 degrees C. The existence of these interrelations means that previously acceptable approximations in simplified reaction schemes for sugar transport will now have to be justified on a point-to-point basis.

Control of defects in optical fibers-a study using cathodoluminescence spectroscopy

Cathodoluminescence spectra of germanosilicate and silicate glass in modified chemical vapor deposition (MCVD) and vapor-phase axial deposition (VAD) preforms are presented, and the influence of co-doping with phosphorus, boron, and fluorine on the peaks associated with the E' and the so-called drawing-induced defects are studied. A simple reaction scheme is developed which illustrates that these defects can be controlled by the electron donor (P) and scavenger (B,F) properties of the co-dopant. >

Diffusion approximations for some simple chemical reaction schemes

We shall establish functional limit laws for the concentration of the various species in simple chemical reactions. These results allow us to conclude that, under quite general conditions, the concentration has an approximate normal distribution. We provide estimates for the mean and the variance which are valid at all stages of the reaction, in particular, the non-equilibrium phase. We also provide a detailed comparison of our results with the earlier work of Dunstan and Reynolds ([7], [8]).