Reduced Kinetic Mechanism of n‐Heptane Oxidation in Modeling Polycyclic Aromatic Hydrocarbon Formation in Diesel Combustion

A reduced chemical mechanism was developed for the chemical kinetics of n‐heptane oxidation in modeling polycyclic aromatic hydrocarbon formation in diesel combustion. The complete kinetic mechanism, which comprises five hundred and seventy‐two reactions and one hundred and eight species, was reduced to a minor mechanism that includes only seventy‐six reactions and forty‐eight species by using net reaction rate analysis and sensitivity analysis, yet the model based on this reduced mechanism predicted the temperature profile and concentrations of C₇H₁₆, O₂, H₂O, CO₂, benzene, naphthalene, phenanthrene, biphenyl, and pyrene that are essentially indistinguishable from those of the complete mechanism under the range of reaction conditions of interest.     

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.     

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.     

Numerical Simulation of Open‐Circuit Continuous Mills Using a Non‐Linear Population Balance Framework: Incorporation of Non‐First‐Order Effects

Population balance modeling has been used as a tool for simulating, optimizing, and designing various particulate processes, including milling. A fundamental tenet of the traditional models for milling processes is the first‐order breakage kinetics. Ample data obtained from batch milling studies show that this assumption is not necessarily valid for certain milling systems. In the present theoretical investigation, an attempt has been made to incorporate these experimentally observed non‐first‐order effects into continuous mill models within the context of a novel non‐linear population balance framework. In view of two idealized flow regimes, i.e., perfect mixing and plug‐flow, continuous mills operating in the open‐circuit mode are numerically simulated. The simulations indicate that not only does the product size distribution depend on the degree of mixedness in a continuous mill, but also on the non‐first‐order effects arising from multi‐particle interactions.     

Simulation of Barium Sulfate Precipitation using CFD and FM‐PDF Modeling in a Continuous Stirred Tank

A mixing‐precipitation model combining computational fluid dynamics (CFD), finite‐mode PDF (probability density function) model, population balance and kinetic modeling has been proposed to simulate the barium sulfate precipitation process in a continuous stirred tank agitated by a Rushton turbine. The effect of various operating conditions such as impeller speed, feed concentration, feed position and mean residence time on the barium sulfate precipitation process is clearly demonstrated. It is shown that the mean crystal size increases by increasing the impeller speed and mean residence time. However, when the feed concentration is increased, the mean crystal size decreases. The predictions are in reasonable agreement with the experimental data in the literature.     

Arrested Reactive Milling Synthesis and Characterization of Sodium‐Nitrate Based Reactive Composites

Arrested reactive milling was used to synthesize three composite powders using sodium nitrate as an oxidizer, and magnesium, aluminum, and mechanically alloyed aluminum‐magnesium (Al_0.5Mg_0.5) as respective fuels. Both magnesium and aluminum powders formed flakes with varying thickness from hundreds of nm to several μm sandwiched between sodium nitrate particles. Three‐dimensional composite and nanocomposite particles were formed with Al Mg mechanically alloyed powder. No change in the crystallinity of any components was observed from X‐ray diffraction patterns of the composite materials. Materials were characterized using differential thermal analysis (DTA) and simultaneous thermogravimetry (TG), carried out in argon. In composite materials, the decomposition of sodium nitrate starts at lower temperatures than for pure sodium nitrate. Weight loss is observed to start at relatively low temperatures. The most significant exothermic events occur at substantially higher temperatures, and therefore in a material that may have been significantly altered from its initial state. The results of thermal analysis indicate that the composite with mechanically alloyed Al Mg powder is most stable at low temperatures. Ignition of the prepared composites was studied using a thin layer of powder coated on an electrically heated filament. These experiments showed that the composite with mechanically alloyed Al Mg powder ignites at lowest temperatures and thus is expected to have the shortest ignition delays in practical applications. The emission spectra of the prepared composites burning in air are presented.     

One‐Dimensional Modeling and Simulation of Bubble Column Reactors

Reactor models that feature a practical way to design bubble columns on the semi‐industrial or even industrial scale have been published only rarely in the usual scientific literature. Creating a one‐dimensional model in the equation‐oriented simulation software ASPEN Custom Modeler? (ACM), one can reach a compromise between model precision and modeling – i.e. computational power – based on correlations selected specifically for the application in question. The model quantitatively describes, with sufficient accuracy, the processes in a bubble column reactor. The paper discusses investigations for designing a pilot plant reactor for hydrogenating 2‐ethylhexanal as an example of its application. Geometry and operating conditions were optimized, and the results are shown in the form of spatially resolved reaction and temperature profiles.     

Polymerization Kinetics of Fischer‐Tropsch Reaction on Iron Based Catalysts and Product Grade Optimization

The polymerization kinetics of Fischer‐Tropsch reactions on a K‐promoted Fe catalyst was studied. To represent the product distribution, a kinetic model was developed based on alkyl and alkenyl mechanisms for hydrocarbon chain propagation, which were assumed to occur simultaneously in the Fischer‐Tropsch synthesis. The conclusion was drawn that superimposed Anderson‐Schulz‐Flory (ASF) distributions with different chain growth probabilities, on iron catalysts, can be the result of different chain growth mechanisms. The polymerization mechanism was used to obtain the product distribution for several conditions, and the optimum conditions for the production of transportation fuels were found.     

Comprehensive Polymerization Model for Fischer‐Tropsch Synthesis

The polymerization kinetics of Fischer‐Tropsch reactions on a cobalt‐based catalyst was studied. A kinetic model was developed based on the alkyl and alkenyl mechanisms for hydrocarbon chain propagation that may occur simultaneously in the Fischer‐Tropsch synthesis. The kinetic model comprised initiation of hydrocarbon chains, propagation, termination to paraffin and olefin and readsorption of olefin. The proposed model was validated by F‐test and proved valid at a confidence level of 95 %.     

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).     

Global interinsic kinetics of wood oxidation

Weight loss curves of two wood species (beech and Douglas fir) have been measured in air for four heating rates between 5-40 K/min and a final temperature of 873 K. A four-step series mechanism describes well the details of the thermogravimetric curves of the devolatilization/combustion process. The rates of the first three reactions are linear in the solid mass fraction and the activation energies are 106, 226 and 114 kJ/mol, respectively. The fourth step presents a power-law dependence (n=1.54) on the solid mass fraction and an activation energy of 183 kJ/mol. Pre-exponential factors and stoichiometric coefficients take into account the differences between the two wood species.     

Thermal and Combustion Properties of 3,4‐Bis(3‐nitrofurazan‐4‐yl)furoxan (DNTF)

Thermal decomposition of melted 3,4‐bis(3‐nitrofurazan‐4‐yl)furoxan (DNTF) in isothermal conditions was studied. The burning rates of DNTF were measured in the pressure interval of 0.1–15 MPa. The thermal stability of DNTF was found to be close to the stability of HMX, while the burning rate of DNTF was close to the burning rate of CL‐20. The thermocouple measurements in the combustion wave of DNTF showed that combustion of DNTF was controlled by the gas‐phase mechanism. The DNTF vapor pressure was determined from thermocouple measurements and agreed well with data obtained at low temperatures under isothermal conditions.     

Analysis of TBA‐Based ETBE Production by Means of an Optimization‐Based Process‐Synthesis Approach

The current article presents the extension of a recently developed optimization‐based approach to process synthesis for process intensification. It generates phenomena‐based flowsheet options using superstructure optimization and provides a dedicated translation into equipment‐based flowsheets. The considered case‐study illustrates the application of the method for the analysis of ethyl tert‐butyl ether production, based on the conversion of tert‐butyl alcohol, under consideration of variable configurations of a rector network and a newly introduced pervaporation‐based membrane‐reactor block.     

Model of a pH‐Based Potentiometric Biosensor Immobilizing Organophosphorus Hydrolase

A model for predicting the steady‐state response of a pH‐based potentiometric biosensor immobilizing organophosphorus hydrolase (OPH) is developed. The model combines the processes of diffusion and enzymatic reaction in the membrane. Key factors influencing the system response, such as enzyme reaction order, Thiele modulus, and buffer concentration are discussed in detail. It is possible to obtain a good linear response by designing an enzyme electrode with a high enough Thiele modulus and by appropriately controlling added buffer concentrations. It is found that the model is in reasonable agreement with the experiment results.     

Combustion synthesis of high-temperature ZrB2-SiC ceramics

The work is dedicated to researching into combustion kinetics and mechanism as well as the stages of the chemical transformations during self-propagating high-temperature synthesis of ZrB₂-SiC based ceramics. Dependences of the combustion temperature and rate on the initial temperature (T₀) have been studied. It has been shown that the stages of the chemical reactions of ZrB₂ diboride and SiC carbide formation do not change within the range of T_0?=?298–700?К. The effective activation energy of the combustion process amounted to 170–270?kJ/mol, from which it has been concluded that chemical interaction through the melt plays a leading role. The stages of the chemical transformations in the combustion wave have been studied by dynamic X-ray diffraction. First, ZrB₂ phase forms from Zr-Si melt saturated with boron, and SiC phase is registered later. The SHS method has successfully been used in order to obtain ZrB₂-SiC composite powders and compact ceramics with a silicon carbide content of 25–75%. The ceramics are characterized by a residual porosity of 1.5%, hardness up to 25?GPa, the elastic modulus of 318?±?21?GPa, elastic recovery of 36% and thermal conductivity of 54.9?W/(m?×?K) at T_room.     

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.     

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.