Evaluation of an Entropy‐Based Combustion Model using Stochastic Reactors

The entropy transport concept (ETC) presented in this paper is a novel approach to describe reaction systems such that the dynamic behavior of a chemical system can be reproduced with a minimum in independent parameters. It is shown that, for adiabatic conditions, the mixture fraction and the reaction entropy are sufficient to describe combustion processes without significant loss of information. Entropy is used as a measure of the reaction progress in this context. In order to evaluate the applicability of the ETC for combustion modeling in turbulent systems, the entropy transport concept was implemented into a stochastic reactor model. For several test cases, the results of this ETC‐based reactor were compared with a reactor that directly integrates the species transport equations.     

Kinetic Modeling of Hydrocarbon Processing and the Effect of Catalyst Deactivation by Coke Formation

Previously derived fundamental rate equations for coke formation and catalyst deactivation are applied to the modeling of a number of commercial processes: steam reforming of natural gas, styrene production from ethylbenzene, catalytic cracking of heavy oil fractions, methanol-to-olefins on SAPO 34, and solid acid alkylation on a Y zeolite. The modeling accounts in great detail for the chemistry of the process, including the formation of the deactivating agent, commonly called coke. It is shown that the deactivating agent is not an inert substance but is involved in reactions, sometimes of the same type as those leading to the main products of the process.     

Modeling of Soot and Polycyclic Aromatic Hydrocarbons in Diesel Diffusion Combustion

By integrating a reduced n‐heptane oxidation chemical kinetic reaction mechanism in a multidimensional computational fluid dynamic solver, simulation of the turbulent diffusion reactive flow inside the direct injection (DI) diesel combustor through compression and expansion stroke were implemented. Coupled with the soot kinetic model by the method of moments, the soot formation is modeled simultaneously with a gas‐phase flame. Under fuel lean flame, orders of magnitude of the multi‐ring polycyclic aromatic hydrocarbon (PAH) intermediate molar fraction are very small. Though soot surface growth is overwhelmed quickly by oxidation under current modeling conditions, the simulated soot number density and average diameter curves are consistent with experimental results. A detailed soot kinetic mechanism is a prospect for further research in computational fluid dynamics (CFD) modeling.     

In Vitro Double Oxidation of n‐Heptane with Direct Cofactor Regeneration

A novel concept for the direct oxidation of cycloalkanes to the corresponding cyclic ketones in a one‐pot synthesis in water with molecular oxygen as sole oxidizing agent was reported recently. Based on this concept we have developed a new strategy for the double oxidation of n‐heptane to enable a biocatalytic resolution for the direct synthesis of heptanone and (R)‐heptanols in a one‐pot reaction. The bicatalytic cascade employs an NADH driven P450 BM3 monooxygenase variant (WT^NADH, 19A12^NADH or CM1^NADH) and an (S)‐enantioselective alcohol dehydrogenase (RE‐ADH). In the initial step n‐heptane is hydroxylated under consumption of NADH to produce (R/S)‐heptanol. In the second oxidation step the (S)‐heptanol enantiomers are transformed to the corresponding ketones, reducing and thereby regenerating the cofactor. Characterization of initial hydroxylation step revealed high turnover frequencies (TOF) of up to 600 min⁻¹, as well as high coupling efficiencies using NADH as cofactor (up to 44%). In the cascade reaction a nearly 2‐fold improved product formation was achieved, compared to the single hydroxylation reaction. The total product concentration reached 1.1 mM, corresponding to a total turnover number (TTN) of 2500. Implementation of an additional cofactor regeneration system (D ‐glucose/glucose dehydrogenase) enabled a further enhancement in product formation with a total product concentration of 1.8 mM and a TTN of 3500.     

Modeling of Kinetic Expressions for the Reduction of NOx by Hydrogen in Oxygen‐Rich Exhausts Using a Gradient‐Free Loop Reactor

The reduction of NO_x by hydrogen under lean conditions is investigated in a gradient‐free loop reactor. Using this computer‐controlled reactor, the reaction rates can be measured under exact isothermal conditions. Systematic variation of the input concentrations of hydrogen, nitric oxide, oxygen as well as reaction temperature provides a complete data set of reaction rates for the given reaction system. A number of kinetic rate expressions were evaluated for their ability to fit the experimental data by using toolboxes of MATLAB. The temperature influence on reaction rate constants and adsorption equilibrium constants were correlated simultaneously using Arrhenius and van’t Hoff equations, respectively. The kinetic rate expression based on a Langmuir‐Hinshelwood‐type model describes the data and the model can be improved by introducing a correction term in square root of hydrogen partial pressure over the range of conditions investigated.     

A Model for the Phosphorus Dynamics of VPO Catalysts during the Selective Oxidation of n‐Butane to Maleic Anhydride in a Tubular Reactor

A model for the phosphorus dynamics in vanadium‐phosphorus oxide (VPO) catalysts for the oxidation of n‐butane to maleic anhydride was developed. According to the model, reversible sorption processes determine the phosphorus content of the catalyst. Simulations reveal that several phenomena can be successfully described. If no phosphorus is added to the reactant feed, the catalytic activity increases until runaway occurs. With addition of a proper amount of phosphorus, the loss can be compensated while excessive phosphorus addition results in complete catalyst deactivation. Adjusting the model parameters to experimental data may result in a model that can be used to optimize the performance of maleic anhydride reactors.     

Performance Enhancement by Unsteady‐State Reactor Operation: Theoretical Analysis for Two‐Sites Kinetic Model

Theoretical analysis of the reactor performance under unsteady‐state conditions was carried out. The reactions are described by two kinetic models, which involve the participation in catalytic reaction of two types of active sites. The kinetic model I assumes the blocking of one of the active sites by a reactant, and the kinetic model II suggests a transformation of active sites of one type into another under the influence of the reaction temperature. The unsteady‐state conditions on the catalyst surface are supposed to be created (i) by forced oscillations of temperature and concentration in the reactor inlet (periodic operation of reactor) and (ii) by catalyst circulation between two reactors in a dual‐reactor system (spatial regulation). The influence of various parameters like concentration of reactant, cycle split, length of period of forced oscillations, temperatures and the ratio of catalyst volumes in the dual‐reactor was investigated with respect to the yield of the desired product. It is shown that for both cases of unsteady‐state conditions (periodic reactor operation as well as in a dual‐reactor system), a mean reaction rate predicted by the kinetic model I was up to two times higher than the steady‐state value. The kinetic model II shows a 20 % increase of the selectivity towards the desired product.     

Kinetic Modeling of the Gas Antisolvent Process for Synthesis of 5‐Fluorouracil Nanoparticles

Mathematical modeling for 5‐fluorouracil (5‐FU) nanoparticle synthesis via gas antisolvent (GAS) process was investigated. 5‐FU was precipitated from a dimethyl sulfoxide (DMSO) solution using CO₂ as antisolvent. The particle size was controlled by nucleation and growth rates, therefore, the kinetic modeling study is essential. Thermodynamic modeling was applied to determine optimal operating conditions for experimental 5‐FU synthesis. Kinetic parameters were evaluated by fitting the particle size distribution predicted by the model to experimental data. The experimental and modeling results indicated that the particle size decreased with increasing the antisolvent addition rate.     

Finite Volume Simulation of High Speed Combustion of Premixed Air‐Acetylene Mixtures in a Microchannel

High speed combustion characteristics of premixed stoichiometric air‐acetylene mixtures inside microchannels are numerically studied by solving a Navier‐Stokes (NS) system of equations with a single‐step chemistry model. A two dimensional explicit finite volume solver has been developed using modified advection upwind splitting methods (AUSM+) to predict the complex interactions among hydrodynamic processes, shock structures and combustion in microdimensions. The effects of channel aspect ratio and wall temperature on high speed microcombustion have been studied in this work. The increase in wall temperature due to wall friction in reduced dimensions initiates the chemical reaction of the combustible mixture near the wall region, and the reacted zone reaches the centerline for smaller height‐to‐length ratios of the microchannels. The wall temperature plays an important role in hypersonic combustion at the microscale.     

Combustion Mechanism of Triaminoguanidine Nitrate

The combustion behavior of triaminoguanidine nitrate (TAGN) was investigated over a wide pressure range and a detailed combustion mechanism has been proposed. Temperature profiles in the TAGN combustion wave were measured with thin tungsten‐rhenium microthermocouples. It was shown that the surface temperature in combustion of TAGN as well as for other onium salts is controlled by the process of dissociation. The burning rate of TAGN is governed by processes in the condensed phase.     

Indirect Synthesis of Bis(2‐PhInd)ZrCl2 Metallocene Catalyst,Kinetic Study and Modeling of Ethylene Polymerization

Bis(2‐phenylindenyl)zirconium dichloride (bis(2‐PhInd)ZrCl₂) catalyst was synthesized via the preparation of bis(2‐phenylindenyl)zirconium dimethyl (bis(2‐PhInd)ZrMe₂) followed by chlorination to obtain the catalyst. Performance of the catalyst for ethylene polymerization and its kinetic behavior were investigated. Activity of the catalyst increased as the [Al]:[Zr] molar ratio increased to 2333:1, followed by reduction at higher ratios. The maximum activity of the catalyst was obtained at a polymerization temperature of 60 °C. The rate‐time profile of the reaction was of a decay type under all conditions. A general kinetic scheme was modified by considering a reversible reaction of latent site formation, and used to predict dynamic polymerization rate and viscosity average molecular weight of the resulting polymer. Kinetic constants were estimated by the Nelder‐Mead numerical optimization algorithm. It was shown that any deviation from the general kinetic behavior can be captured by the addition of the reversible reaction of latent site formation. Simulation results were in satisfactory agreement with experimental data.     

Kinetics of Sulfite Oxidation in the Simultaneous Desulfurization and Denitrification of the Oxidation‐Absorption Process

The kinetics of sulfite oxidation in the simultaneous desulfurization and denitrification of the oxidation‐absorption process was investigated with a bubbling apparatus under the conditions of varying pH, sulfite concentration, nitrite concentration, temperature, and air flow. The results indicate that the oxidation of sulfite is promoted dramatically by mixing with nitrite, due to the decline of the apparent energy of activation and the initiation of nitric dioxide decomposition by nitrite. An increase of the nitrite concentration, the air flow, the reaction temperature, or the pH supports the decomposition of nitrite and the production of nitric dioxide, thus leading to an enhancement of the sulfite oxidation rate. A kinetic model is established according to the experimental results. A satisfactory agreement between the calculated and the experimental values is obtained.     

Kinetic Analysis of Hydroxide-Catalyzed,Aerobic Oxidation of 2-Mercaptoethanol

Experiments and mechanistic kinetic analyses are conducted for the hydroxide-catalyzed, aerobic oxidation of the thiol 2-mercaptoethanol (2-ME) to water and the corresponding disulfide at 22°C and 1 atm total pressure in tetrahydrofuran. The experiments are performed in an isothermal, bench-scale reactor with a maintained O₂ partial pressure. The 2-ME concentrations are measured using gas chromatograph/flame ionization detector (GC/FID). The mixture includes a small amount of aqueous NaOH required to activate the thiol (RSH) to thiolate (RS^?). The observed 2-ME conversion is first order in 2-ME concentration. A multistep mechanism is proposed consistent with the observed kinetics. The rate-determining step is the coupling of RS^? to dissolved O₂, with an estimated rate constant k = 1.93 × 10^?6 liter/mmol h.     

Modeling of Particle Layer Detachment under Consideration of Transient Kinetic Effects

Particle layers tend to build up on walls in many filtration and separation processes, calling for periodic removal in order to keep the device running. Important factors are the adhesion of the layer on the substrate and the cohesion of the particles in the layer. Models describing such layer detachment generally assume constant and homogeneous conditions for the forces acting on the layer. But in reality detachment is extremely nonstationary concerning place and time, primarily due to changing conditions for the forces on the one hand and changes in the particle layer morphology on the other. This paper describes a model and a simulation considering such transient kinetic effects on filter cake detachment. Diverse computing results are presented and discussed.     

Combustion Characteristics of Silicon‐Based Nanoenergetic Formulations with Reduced Electrostatic Discharge Sensitivity

This paper details the synthesis and combustion characteristics of silicon‐based nanoenergetic formulations. Silicon nanostructured powder (with a wide variety of morphologies such as nanoparticles, nanowires, and nanotubes) were produced by DC plasma arc discharge route. These nanostructures were passivated with oxygen and hydrogen post‐synthesis. Their structural, morphological, and vibrational properties were investigated using X‐ray diffractometry, transmission electron microscopy (TEM), nitrogen adsorption‐desorption analysis, Fourier transform infrared (FTIR) spectrometry and Raman spectroscopy. The silicon nanostructured powder (fuel) was mixed with varying amounts of sodium perchlorate (NaClO₄) nanoparticles (oxidizer) to form nanoenergetic mixtures. The NaClO₄ nanoparticles with a size distribution in the range of 5–40 nm were prepared using surfactant in a mixed solvent system. The combustion characteristics, namely (i) the combustion wave speed and (ii) the pressure‐time characteristics, were measured. The observed correlation between the basic material properties and the measured combustion characteristics is presented. These silicon‐based nanoenergetic formulations exhibit reduced sensitivity to electrostatic discharge (ESD).