Homogeneous Kinetics Research Articles

A study of nitrous oxide decomposition over calcium oxide in combustion condition

A study of N2O decomposition reaction over a bed of CaO particles in a fixed bed reactor has been conducted. Effects of parameters such as concentration of inlet N2O, reaction temperature and effects of CO, CO2, O2, and NO gas presented in the combustion gas environment have been investigated. The experiment showed that the N2O decomposition reaction was accelerated by the increase of reaction temperature, and the existence of CO, while the reaction was hindered by the existence of CO2, NO. O2 also affected the N2O decomposition. Heterogeneous gas-solid reaction kinetics were proposed for the reaction conditions and compared with homogeneous reaction kinetics.

Homogeneous nucleation of droplets from supersaturated vapor in a closed system.

Kinetic equations describing homogeneous nucleation kinetics within standard model are solved numerically under the condition of a constant number of molecules in the considered system. It has consequences to decrease the supersaturation of the supersaturated vapor during the process of the formation of small droplets of a new phase. The decrease of supersaturation occurs in a short time and reaches some value which remains unchanged for a relatively long time (quasistationary regime), especially at lower initial supersaturations. This time interval decreases with increasing value of the initial supersaturation. In the quasistationary regime the nucleation rate reaches its stationary value. At higher initial supersaturation, the rate of formation of nuclei goes to some maximum value corresponding to the stationary nucleation rate and then decreases with time due to the decrease of supersaturation.

The peculiarities of homogeneous nucleation of reactive Cu 0 colloidal particles in the presence of functional oligoperoxides

A method is proposed that combines the stage of formation of colloidal metal and metal-oxide particles with the stage of their surface modification by functional surface-active oligoperoxides (FSAP), which are sorbed irreversibly. Investigation of copper particle homogeneous nucleation kinetics witnesses the significant influence of supermolecular micelle-like structures formed by FSAP in solution on the reduction rate of Cu 2+ cations caused by a phenomenon analogous to micellar catalysis. The rate constants of copper reduction in different local zones of the process have been determined. Particle homogeneous nucleation kinetics in the presence of surface-active oligoperoxides has been found to correspond to the main regularities of the Michaelis–Menten equation describing micellar catalysis. The carrying out of copper particle formation in distinct zones correlates well with the particle size distribution. The presence of reactive ditertiary peroxidic fragments on the particle surface as a result of FSAP sorption confers reliable protection from oxidation, hydrophobicity, and ability to form free radicals and participate in elementary stages of radical processes.

CATALYTIC COMBUSTION OF HYDROGEN-AIR MIXTURES OVER PLATINUM: VALIDATION OF HETERO/HOMOGENEOUS CHEMICAL REACTION SCHEMES

The validity of various hetero/homogeneous chemical reaction schemes in the catalytically stabilized combustion (CST) of hydrogen/air mixtures over platinum is investigated experimentally and numerically. The work stems from the internationally intensified effort to commercialize power generation turbines operating with the ultra-low NO x CST technology. Crucial to the development of such systems is the understanding of the heterogeneous (catalytic) and the homogeneous (gas-phase) kinetics. Experiments were performed in an optically accessible catalytic channel reactor and involved planar laser induced fluorescence (PLIF) of the OH radical and line-Raman measurements of major species concentrations. The comparisons between measurements and detailed numerical predictions using elementary, reduced, or one-step mechanisms have revealed substantial differences in the performance of the reaction schemes.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 4. The kinetic current regime

The theory for depletive stripping chronopotentiometry at scanned deposition potential (SSCP) has been extended to include the case of kinetic currents. Invoking the Koutecký–Koryta approximation, i.e., assuming a spatially discontinuous transition from labile to non-labile behaviour, allows a rigorous expression to be obtained for the full SSCP wave. Theoretical predictions are supported by experimental data for the quasi-labile Cd–NTA system at a conventional HMDE and a mercury microelectrode. The characteristic features of the SSCP wave for systems influenced by homogeneous kinetics allow dynamic speciation characteristics to be unambiguously identified and discriminated from those due to irreversibility in the electrochemical reaction or heterogeneity in the chemical speciation.

The Effect of Flow Rate of Compressed Hot Water on Xylan, Lignin, and Total Mass Removal from Corn Stover

Flowing hot water through cellulosic biomass offers many promising features for advanced pretreatment, and a better understanding of the mechanisms responsible for flowthrough behavior could allow us to capitalize on its key attributes while overcoming its limitations. In this study, extensive data were developed to show the effect of flow on the fate of hemicellulose, lignin, and total mass for hot-water pretreatment of corn stover in a small tubular flowthrough reactor at 180, 200, and 220 °C. Solubilization of hemicellulose increased with flow, especially at high temperatures; a result that is inconsistent with traditional first-order kinetic models. The dissolved xylan in the hydrolyzate was mostly oligomers over this temperature range, and the fraction as oligomers increased with flow rate. Also of importance, lignin removal increased from less than about 30% for batch reactors to about 75% at high flow rates and was nearly linearly related to hemicellulose release for the flowthrough reactor. These observations suggest that mass transfer or other physical factors, and not strictly first-order homogeneous chemical kinetics, impact hemicellulose hydrolysis. In addition, lignin appears to be released throughout hydrolysis, but its fate may be governed by subsequent precipitation reactions unless it is removed first.

Computational Modeling of CO/CO2 Ratio Inside Single Char Particles during Pulverized Coal Combustion

A recently developed model was used to study the CO/CO2 ratio inside a burning pulverized coal particle, to better understand the effect of bulk gas composition on the equilibrium partial pressure of reduced metal species at the surface of ash inclusions. The motivation was to improve the ability to model submicrometer particle formation by ash vaporization, as a function of furnace conditions. Assumptions for the CO/CO2 ratio that have been made in previous studies are compared to predictions from a psuedo-steady-state model for a single porous particle that considers homogeneous and heterogeneous reaction kinetics and mass transfer both in particle pores and in the boundary layer. This is the first publication of model predictions for the CO/CO2 ratio as a function of radius for a coal char particle in a furnace with a bulk gas CO2 concentration in the range of 0%−79%. A method is proposed for summarizing the effects on the CO/CO2 ratio that are due to changes in the bulk furnace gas O2 and CO2 concentration, furnace temperature, and particle size, using an empirical equation that is suitable for incorporation as a submodel into comprehensive computational fluid dynamics-based codes for combustion simulation. Trends from the model simulations show general agreement with experimental data; however, the accuracy of the predictions is limited by the lack of fuel-specific input data.

Electrochemical studies of the anaesthetic agent isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) in binary gas mixtures with oxygen and nitrous oxide

The electrochemical reduction of the inhalation anaesthetic agent isoflurane was investigated at a variety of microelectrode substrates (Au, Ag and Cu) in DMSO solvent, individually, and also as a component of a simple binary gas mixture with oxygen or nitrous oxide. In a binary gas mixture with oxygen, isoflurane is shown to react with the superoxide anion radical, formed from the electro-reduction of oxygen complicating their simultaneous detection. However, the electro-reduction of oxygen is shown to be dependent of the electrode size and the concentration of the anaesthetic agent, isoflurane. Using microelectrodes with diameters ≤5 μm and low isoflurane concentration ([ISO]<0.2% (v/v)), the reduction is shown to be a one-electron process, since the homogeneous reaction kinetics are essentially “out-run”. Using larger microelectrodes (diameters≥10 μm), or with higher concentrations of isoflurane, (0.5>[ISO]<2.0% (v/v)) the reduction current is augmented by the catalytic regeneration of oxygen, formed by the reaction between superoxide and isoflurane. By contrast, in a binary gas mixture with nitrous oxide, isoflurane reduction is shown to be unaffected by nitrous oxide or it is reduction products. The reduction waves for isoflurane and nitrous oxide are concomitant which complicates their simultaneous detection somewhat. In essence, this work provides scope for the development of a rapid, accurate and inexpensive electrochemical gas sensor for measuring the concentration of anaesthetic agents during surgery.

Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas

Abstract Direct electrochemical oxidation (DECO) solid-oxide fuel cells (SOFCs) offer the potential to generate electrical power from hydrocarbon fuels without the need for upstream fuel processing, such as reforming. However, with pure hydrocarbon fuel entering the flow channels at temperatures typically above 700 °C, fuel pyrolysis can cause molecular-weight growth and the formation of deleterious carbonaceous deposits. This paper, which develops a plug-flow model for fuel (natural gas surrogate) within the anode channels, considers the elementary gas-phase chemical kinetics of fuel pyrolysis and oxidation. It also considers the limiting case of local chemical equilibrium. Formation of cyclic hydrocarbon species is used to indicate deposit propensity. Results show that the likelihood of deposit formation depends strongly on cell temperature, current density, and residence time. Generally speaking, equilibrium favors deposit formation early in the channel whereas, owing to limited residence time, the homogeneous finite-rate kinetics predicts relatively low levels of deposit precursors. In the downstream portions, because of electrochemical oxygen flux though the electrode–electrolyte membrane, chemical equilibrium shifts strongly away from deposit formation to volatile carbon-oxygen species. However, the homogeneous finite-rate kinetics predictions show a continuing increase in coking propensity.

Hydrothermal Denitrogenation of Fuel Oil Derived from Municipal Waste Plastics in a Continuous Packed-Bed Reactor

The hydrothermal processing of fuel oil produced from municipal waste plastics (kerosene fraction: Cl content = 27 ppm, N content = 927 ppm) was investigated under subcritical conditions so as to remove the organic nitrogens contained in the fuel oil. The hydrothermal processing was carried out in a small SUS316 stainless steel continuous packed-bed reactor (packed with Raschig rings) under liquid-phase conditions. Although the denitrogenation took place in pure water, the reaction proceeded much more readily in aqueous solutions of NaOH. That is, the nitrogen content in the product oil decreased to 73 ppm upon processing with 0.05 mol/L NaOH for 4.0 min at 275°C and at the reactant weight feed ratio of unity, whereas it decreased to 16 ppm when 0.10 mol/L NaOH was similarly used at 325 °C. The chlorine content in the product oil also decreased to 3 ppm as a result of the processing at 275°C. The rate of hydrothermal denitrogenation could well be described in terms of the homogeneous first-order reaction kinetics with the equilibrium nitrogen content under the hydrothermal processing conditions employed. Upon hydrothermal processing at 275°C, the density, kinematic viscosity, and ignition point of the fuel oil decreased slightly.

An EFTEM study of compositional variations in Mg–Ni–Nd bulk metallic glasses

A range of Mg–Ni–Nd bulk metallic glasses has been cast. Around the composition Mg 65Ni 20Nd 15 (at.%), but not at neighbouring compositions, energy-filtered transmission electron microscopy provides evidence for strong compositional inhomogeneities in the as-cast glass. In this fully glassy material, an apparent two-phase cellular microstructure is seen; one phase is rich in Ni, the other in Mg and Nd. Annealing at a temperature just above the glass transition, or even holding at room temperature for several months can eliminate the inhomogeneities. The diffusivity values derived from the homogenization kinetics are in good agreement with those known for other bulk metallic glasses. The present results suggest the existence of a closed-loop miscibility gap in the glass-forming liquid.

Comparison of fully coupled approaches for approximating nonlinear transport and reaction problems

The purpose of this work is to evaluate the computational efficiency of fully coupled approaches for approximating a common class of nonlinear, two-phase advective–dispersive–reactive equations. The general problem considered includes homogeneous phase chemical kinetics, equilibrium interphase mass transfer, and rate-controlled interphase mass transfer––all of which may be nonlinear. Aspects of the problem investigated include discrete mass conservative formulations, temporal discretization approaches, and nonlinear equation solution methods. Their effect on computational efficiency is investigated through a series of numerical experiments using a nondimensional model problem. The effect of problem characteristics such as large sorption capacity, strong sorption nonlinearity, fast mass transfer, fast reactions, and strong diffusion is investigated. Comparisons of solution efficiency show that the optimal approach depends upon: (1) the characteristics of the problem considered, which may be described in a nondimensional form; and (2) the accuracy achieved in the solution. Results offer general guidance for selecting solution approaches for the class of problems investigated and introduce some new solution approaches to the water resources field that may be applicable to other problems.

Electrochemical studies of the anaesthetic agent enflurane (2-chloro-1,1,2-trifluoroethyl difluoromethyl ether) in the presence of oxygen: reaction with electrogenerated superoxide

The electrochemical reduction of the anaesthetic agent enflurane was investigated at gold disc microelectrodes in the presence of oxygen in DMSO solvent. The superoxide anion radical, formed from the electro-reduction of dissolved oxygen, is shown to react with enflurane, complicating their simultaneous detection. The kinetics of the enflurane/superoxide reaction are found to be first order with respect to both superoxide and enflurane with a rate constant of 0.25 M −1 s −1 determined by three independent methods: steady-state voltammetry, digital simulation of cyclic voltammetric data and UV–vis spectroscopic analysis. The likely reaction scheme is outlined below (where R=CHFCF 2OCHF 2). R Cl+ O 2 −→ R O 2 + Cl − R O 2 + O 2 −→ R O 2 −+ O 2 R O 2 −+ R Cl→ R OO R+ Cl − or R O 2 + R O 2 →[ R OOOO R]→ R OO R+ O 2 The electro-reduction of oxygen in the presence of enflurane is found to be dependent on electrode size and enflurane concentration. Using small electrode sizes, the reduction is shown to be a single one-electron process, since the homogeneous reaction kinetics are effectively ‘out-run’. At larger electrode sizes, or with higher concentrations of enflurane, the reduction current is enhanced via the catalytic formation of oxygen, formed from the reaction between superoxide and enflurane.

Hydrothermal Refining of Fuel Oil Derived from Municipal Waste Plastics in a Continuous Packed-Bed Reactor

The hydrothermal dechlorination and denitrogenation of fuel oil derived from municipal waste plastics (kerosene fraction; Cl content = 62 ppm, N content = 1150 ppm) was investigated under subcritical liquid-phase conditions using a small SUS316 stainless steel continuous, packed-bed reactor. Although these two reactions took place in pure water, they proceeded much more readily in aqueous solutions of NaOH. For example, the nitrogen content in the oil decreased to 73 ppm upon processing with 0.10 mol/L NaOH for 5.50 min at 300°C and at the reactant weight feed ratio of unity. The chlorine content in the oil decreased to 2 ppm as a result of the alkaline hydrothermal processing. Both rates of these two reactions could well be described in terms of a homogeneous first-order reaction kinetics with a tentative equilibrium heteroatom content under the hydrothermal processing conditions used.

Using trimethylamine dehydrogenase in an enzyme linked amperometric electrode

A rational design for the site-specific immobilization of the protein trimethylamine dehydrogenase (TMADH) to facilitate charge transfer between enzyme and an electrode is described. Protein engineering and site-specific chemical modification have been used to extend the electron pathway from the protein surface to redox mediators. The kinetics of TMADH mutants (V344C and Y442C) modified with the iodoacetamide and 4-iodoacetamido 1-naphthole (IAN) showed that modification at position 344 has a more profound influence on intra- and inter-molecular electron transport, and the catalytic parameters kcat and KMapp became a function of chemical modification. Ferricenium ion was shown to act as an electron acceptor for both mutants, but as its site of interaction is the residue 344, it was rejected for wiring in favour of [Fe(5-NH2-phen)3]2+, the latter showing similar very fast homogeneous electron exchange kinetics, ideal for ‘wire’ construction. The Y442C mutant was successfully immobilised on to an electrode surface which had been chemically modified with the redox polymer, poly-[Fe(5-NH2-phen)3]2+. This design enabled direct electrical communication between the enzyme and electrode. Using a partly oxidized polymer to limit the supply of oxidised electron acceptor, gave evidence for transition from the fast “0/2- cycle” to the “1/3-cycle” for the TMADH.

A study on the homogeneous nucleation kinetics of model charged spheresuspensions

We investigated the kinetics of homogeneous nucleation from a shear melt for amodel system of deionized aqueous suspensions of charged spheres. With the aidof nearly index-matched perfluorinated particles of low polydispersity,we extended previous studies into the regime of large metastability andnucleation-dominated solidification. With increasing particle number densityn,the solidification time determined by the appearance of a finite shearmodulus decreased from minutes to milliseconds. Nucleation rate densitiesJ werederived from the width of the principal peak in the static structure factoras measured by means of light scattering after complete solidification.Jwas observed to increase approximately exponentially withn asexpected from classical nucleation theory in the absence of a kinetic glasstransition. Additional measurements of the elastic and dynamic behaviour,however, show that for the largest concentrations, the sample properties areglass-like.

γ-Radiation-Initiated Polymerization of Vinylidene Fluoride in Dense Carbon Dioxide

The γ-ray-initiated batch polymerization of vinylidene fluoride (VF2) has been investigated in dense carbon dioxide under relatively mild operative conditions (T ≤ 40 °C and P < 25 MPa). When the initial VF2 molar concentration was increased from 3.4 to 6.4 mol/L, monomer conversion increased from 20 to 73%; a similar trend was observed for the number-average molecular weight and the molecular complexity of synthesized poly(vinylidene fluoride) (PVDF) as determined from rheological measurements. Under all adopted experimental conditions, a synthesized PVDF polymer was collected in the form of a white powder. Despite the inherent heterogeneous character of the polymerization process, a homogeneous free-radical chain kinetics model can be used to fit kinetic data collected at 30 °C and 3.4 mol/L VF2 loading, with evidence of heterogeneous-phase polymerization being constituted by the onset of an acceleration period.

Miniature free-piston homogeneous charge compression ignition engine-compressor concept—Part II: modeling HCCI combustion in small scales with detailed homogeneous gas phase chemical kinetics

Operational maps for crankshaft-equipped miniature homogeneous charge compression ignition engines are established using performance estimation, detailed chemical kinetics, and diffusion models for heat transfer and radical loss. In this study, radical loss was found to be insignificant. In contrast, heat transfer was found to be increasingly significant for 10, 1, and 0.1 W engines, respectively. Also, temperature–pressure trajectories and ignition delay time maps are used to explore relationships between engine operational parameters and HCCI. Lastly, effects of engine operating conditions and design on the indicated fuel conversion efficiency are investigated.

To study the direct transformation of methane into methanol in the lower temperature range

A considerable industrial interest has always existed in the catalytic as well as non-catalytic transformation of methane (CH 4) gas into methanol (CH 3OH) as an intermediate product due to its wide applications. The purpose of the present computational study was to study the homogeneous reaction kinetics of methane and to analyze the effect of operating parameters, viz. temperature, residence time and CH 4/O 2 input ratio on direct oxidation of CH 4 into CH 3OH in the lower temperature range suitable to catalytic conversion. It is found that a higher conversion of methane into methanol can be achieved by controlling the operating parameters to their optimum values. A combination of input values as CH 4/O 2 ratio=0.005/0.995, OH=5 ppm and T=100 °C was concluded to be optimum for direct oxidation of methane into methanol. The combination of reactions R 1 (CH 3OH=CH 3+OH) and R 15 (CH 4+OH=CH 3+H 2O) was mainly found responsible for the formation of methanol under the present operating conditions. Though the contribution of direct oxidation of CH 4 into CH 3OH is very small (≈1% at the maximum) with suitable operating parameters, this advantage can be coupled with other optimum catalytic conversion conditions to enhance the production of methanol.

A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation

Alkanes exhibit intermediate plastic–crystalline rotator phases between the liquid and the low-temperature fully ordered crystal. Here we report measurements of the liquid-to-rotator and rotator-to-crystal homogeneous nucleation kinetics using x-ray scattering on emulsified samples. These are the first reported studies of rotator-to-crystal homogeneous nucleation, and they show an anomalous temperature dependence of nucleation. Through use of mixed alkanes, we probe this over a wide temperature range. For three compositions along the C19H40–C20H42 binary, we find that the liquid-to-rotator interfacial free energy, calculated from the nucleation barrier in classical nucleation theory, is the same for all samples. However, the rotator-to-crystal interfacial free energy is both considerably smaller and strongly dependent on composition and temperature. As the temperature range of the rotator phase widens due to chain mixing, both the thermodynamic transition temperature as well as the interfacial energy for the rotator-to-crystal transition decrease. The interfacial energy reaches half its initial value at the lowest point measured, and it extrapolates to zero at a temperature about 5 K lower. Surprisingly, this corresponds to a situation where the latent heat itself is still finite. Typically, first-order phase transitions exhibit hysteresis due to a kinetic barrier. Our data suggests the unusual case of zero hysteresis for a first-order transition due to the disappearance of the homogeneous nucleation barrier.