Ignition Characteristics of Zr/BaCrO4 Pyrolant

Zirconium (Zr) and barium chromate (BaCrO₄) is a pyrolant of high energy content. Zr has a high and BaCrO₄ a low sensitivity. The mixture of the two components has been investigated with respect to its ignition characteristics. The ignition delay time of the pyrolant decreases with an increasing concentration of Zr particles. That means, since the concentration of Zr is proportional to its total surface area S, the ignition delay time decreases with increasing S. An ignition delay time, τ_ig, is obtained adding τ_p and τ_c, where τ_p is the physical ignition delay time and τ_c is the chemical ignition delay time. τ_p does not change when the concentration of Zr increases. However, τ_c decreases with an increasing concentration of Zr, resulting in an increase of its surface area S. It is expected that Zr easily reacts with barium chromate.     

Thermal diffusivities of some porous oxides

An unsteady-state method for determination of the thermal diffusivity of a porous sphere, based on direct contact condensation of steam, was developed. A modified formula relating thermal diffusivity to porosity was proposed from theoretical considerations. Experimental measurements on known solids showed good agreement with literature values. Unknown thermal diffusivities of some metallic oxides (in powder form) were determined at 60°C, with the following results: Cr₂O₃: α = 1.23 (10 ⁶) m²/s: CuO: α = 2.63 (10 ⁶) m²/s: Fe₂O₃: α = 1.02 (10 ⁶) m²/s; V₂O₅: α = 1.14 (10 ⁶) m²/s.     

Thermoanalytical Investigation of Some Binary Pyrotechnic Mixtures of Ammonium Azide as Chemical Gas Generating Systems

The conventional NaN₃/oxidants gas generator pyrotechnic mixtures suffer from leaving undesirable solid residues. The presented investigation aimed to overcome this problem through mixing of NH₄N₃ particles with some metallic components, and to introduce some new gas generator pyrotechnic mixtures. In this work, volatile ammonium azide particles were initially stabilized via microencapsulation technique, and blended with Zr, Ti, ZrH₂, and TiH₂ powders to produce different gas generation mixtures. Thermal properties and kinetic parameters of these pyrotechnic mixtures were investigated by using thermal analysis (TG/DTA and DSC) techniques. The apparent activation energy (E), ΔG^#, ΔH^#, ΔS^#, and critical ignition temperature (T_b) of the ignition processes of the mixtures were obtained from the DSC experiments. Among the investigated mixtures, NH₄N₃/ZrH₂ composition was found to show desirable efficiency, and also it can be considered as a safe pyrotechnic composition for gas generation property, due to its moderate ignition temperature.     

Studies on thermal degradation of synthetic polymers. XV. Estimation of the product yield on the basis of intensity function for thermal gasification of isotactic and atactic polypropylenes

The kinetic equation for the pyrolysis gasification reaction of isotactic and atactic polypropylenes has been established. The difference in tacticity of the samples does not much affect the kinetic parameters. The exponent a of the intensity function I_F = Tδ^a (K·sec^a), concerning the severity of decomposition conditions, has been approximated as 0.039 (isotactic) and 0.040 (atactic), respectively from the kinetic parameters in this experiment. The calculated values of the product yield from Arrhenius equations, k = 2.0 × 10¹⁰ exp(?40.9 × 10³/RI_F) (isotactic) and k = 1.2 × 10¹⁰ exp(–41.4 × 10³/RI_F) (atactic), for the I_F standard agree with the experimental values.     

Study of sawdust pyrolysis and its devolatilisation kinetics

Pyrolysis of sawdust was studied using a thermogravimetric analyser (TGA) to understand the devolatilisation process and to obtain its global kinetic parameters. The influences of particle size, initial weight of the sample and heating rate on the devolatilisation of sawdust particles have been studied. Results from proximate analysis show that smaller particle size has more ash content compared to larger particle size. The TG and derivative TG curve for variation in particle size and initial weight of the sample showed significant difference in the third stage of the pyrolysis. In addition, the pyrolysis of sawdust differed significantly for variation in heating rate. As the heating rates increased, the char yield also increased. The devolatilisation kinetics was studied considering different stages of pyrolysis. The kinetic parameters for thermal devolatilisation of the sawdust were determined through a nonlinear optimisation method of two independent parallel nth‐order reaction models. The kinetic parameters such as activation energy, frequency factor and order of the reaction for the two stages considered in the model were: E₂ = 79.53 (kJ/mol), E₃ = 60.71 (kJ/mol); k₀₂ = 1.90 × 10⁶ (1/min), k₀₃ = 1.01 × 10³ (1/min); n₂ = 0.91, n₃ = 1.78, respectively. The results show good agreement between the proposed model and the experimental data of the sawdust pyrolysis.     

A 1-D non-isothermal dynamic model for the thermal decomposition of a gibbsite particle

A 1-D mathematical model describing the thermal decomposition, or calcination, of a single gibbsite particle to alumina has been developed and validated against literature data. A dynamic, spatially distributed, mass and energy balance model enables the prediction of the evolution of chemical composition and temperature as a function of radial position inside a particle. In the thermal decomposition of gibbsite, water vapour is formed and the internal water vapour pressure plays a significant role in determining the rate of gibbsite dehydration. A thermal decomposition rate equation, developed by closely matching experimental data reported previously in the literature, assumes a reaction order of 1 with respect to gibbsite concentration, and an order of −1 with respect to water vapour pressure. Estimated values of the transformation kinetic parameters were k₀ = 2.5 × 10¹³ mol/(m³ s) for the pre-exponential factor, and E_a = 131 kJ/mol for the activation energy. Using these kinetic parameters, the gibbsite particle model is solved numerically to predict the evolution of the internal water vapour pressure, temperature and gibbsite concentration. The model prediction was shown to be very sensitive to the values of heat transfer coefficient, effective diffusivity, particle size and external pressure, but relatively less sensitive to the mass transfer coefficient and particle thermal conductivity. The predicted profile of the water vapour pressure inside the particle helps explain some phenomena observed in practice, including particle breakage and formation of a boehmite phase.     

Static Electric Sensitivity Characteristics of Zr/BaCrO4 Pyrolants

Zirconium (Zr) easily ignites in air atmosphere and generates high‐energy output. Therefore, it is used as a fuel for pyrolants. Barium chromate (BaCrO₄) dose not ignite in air, so it is one of safety materials and it is used as oxidizer. These materials are mixed with viton as coating material/agent, since viton is easily dissolved in acetone. The static electric sensitivity of only‐BaCrO₄‐coated material is lower than that of Zr‐coated one. The static electric energy, E, influences the chemical and the physical ignition delay times and they decrease with increasing E. The chemical ignition delay time decreases with increasing concentration of Zr. The chemical and physical ignition delay times of only‐BaCrO₄‐coated pyrolant are longer than those of both‐coated material and only‐Zr‐coated, so the pyrolant of only‐BaCrO₄‐coated pyrolant is the safer material.     

An experimental study of flash evaporation from liquid pools

The objective of this study is, based on experiments, to improve the understanding of flash evaporation from pools, and to develop empirical correlations of the total quantity of vapor released and the rate of vapor generation (both in non-dimensional form) as a function of the primary nondimensional parameters which govern the process: the Jakob number (Ja_p), Prandtl number (Pr), a dimensionless hydrostatic head (Δp/H), and the salt concentration (C). The work was done in the range of parameters characteristic to desalination plants and to open-cycle ocean-thermal energy conversion. Experiments were performed with fresh water and with saline water (3.5% NaCl concentration), for initial temperatures from 25°C – 80°C, flash-down temperature differences from 0.5°C – 10°C, and pool depths of 6.5″ (165 mm), 12″ (305 mm) and 18″ (457 mm). The prediction of flashed mass as a function of the nondimensional parameters is accurate to within 7.5%. A new time scale τ = (σ/Δp)²/α₁, (where σ = surface tension, α₁ = thermal diffusivity of the liquid, and Δp = pressure difference between the liquid and the vapor space) was employed. The expressions for the rate of flashed steam using this time scale show a scatter of ± 68%. The asymptotic value of the flashed mass increases with Jakob number, increases slightly with pool depth, and decreases with increasing liquid Prandtl number. The comparisons show very good agreement with the experimental values, but underpredict the data obtained from other experiments.     

Study of the Oxygen Diffusion on Three-Way Catalysts: A Kinetic Model

A computer model was developed to take into account all the phenomena that can occur in ¹⁸O/¹⁶O isotopic exchange over Pt/CeZrO _x materials: adsorption/desorption on the metal, surface and bulk O diffusion. Discriminating each step of the exchange process is no longer necessary: kinetic parameters and O diffusivity can be calculated in a single experiment.     

Analytical solution using a pore diffusion model for a pseudoirreversible isotherm for the adsorption of basic dye on silica

An analytical solution for a two resistance mass transfer model explaining the adsorption of Astrazone Blue dye (Basic Blue 69) onto Sorbsil silica has been developed. The model includes a film mass transfer coefficient, k_f1 = 80 × 10⁻⁶cm·s−1, and an internal effective diffusivity, D_eff = 18×10⁻⁹cm²·s⁻¹ which controls the internal mass transport processes based on a pore diffusion mechanism.     

Modeling of thermal explosion in constrained dies for B4C–Ti and BN–Ti powder blends

The process of reactive in-situ synthesis of dense particulate reinforced TiB₂/TiC and TiB₂/TiN ceramic matrix composites from B₄C–Ti and BN–Ti powder blends with and without the addition of Ni has been modeled. The objective of modeling was the determination of optimal thermal conditions preferable for production of fully dense ceramic matrix composites. Towards this goal heat transfer and combustion in dense and porous ceramic blends were investigated during heating at a constant rate. This process was modeled using a heat transfer–combustion model with kinetic parameters determined from the differential thermal analysis of the experimental data. The kinetic burning parameters and the model developed were further used to describe the process of combustion synthesis in a constrained die under pressure. It has been shown that heat removal from the reaction zone affects the ignition temperature of thermal explosion.     

A model for the formation of calcium carbide in solid pellets

Experiments have been conducted on the formation of calcium carbide in the solid state from 2.22-cm diameter cylindrical pellets of a 3/1 mole mixture of carbon and lime between 1650°C and 1720°C at 50 mm Hg CO pressure. The reaction was modelled assuming that heat transfer across a growing product layer controls the rate of movement of a reaction front where the overall reaction to form CaC₂ is assumed to occur instantaneously to a fractional conversion f. The model depends upon a knowledge of the CaC₂ decomposition rate in the product layer ko₂e^?ΔE/RgT and the effective thermal conductivity k₀. These may be estimated from independent data but require adjustment for particular pellet conditions. Good agreement with the data shows that the model is adequate for preliminary design and optimization studies.     

Evaluation of kinetic parameters of coal ignition

A method is presented of evaluating the kinetic parameters for the ignition reaction of coal based on the Semenov's thermal ignition theory adopted to coal particle ignition. The evaluated apparent activation energy, E, for bituminous coal, anthracite and bituminous coal char are in the range 46–103 kJ mol^?1. A verification was made by comparing the measured induction periods for the coal particles with those calculated numerically. The oxidation rate is controlled by diffusion into pores.     

Kinetic role of C/Zr ratio in the reaction process,combustion behavior,and synthetic product of 70 wt.% (xC–Zr)–30 wt.% Cu

The kinetic role of C/Zr ratio in the reaction processes, combustion behaviors, and synthesized products of 70 wt.% (xC–Zr)–30 wt.% Cu was investigated. Results indicated that ZrC particles were produced by the replacement reaction between carbon atoms and Zr–Cu melt. With an increase in C/Zr ratio, more carbon atoms combined with the zirconium atoms in Zr–Cu liquid. As a result, the formation rate of massive ZrC enhanced, which shortened the ignition time of combustion reaction. On the other hand, the quantity, the lattice parameter, and the x value of synthetic ZrC_x increased, while the byproduct Cu_yZr_x compounds decreased. These effects contributed to an increase in the burning temperature and ZrC_x particle size. Moreover, it is also revealed that the formation of ZrC_x is a multistep process, which leads to an inhomogeneous distribution of the particle size. Results from this work offer a theoretical reference for the kinetic research of combustion synthesis and related techniques, and provide a valuable guide to the in situ synthesis of composite materials containing ZrC.     

An Evaluation of the Thermophysical Properties of Stoichiometric CeO2 in Comparison to UO2 and PuO2

The thermal conductivity of stoichiometric CeO₂ was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)^?1 relationship with A = 6.776×10^?2 m·K·W^?1 and B = 2.793 × 10^?4 m·W^?1. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO₂. This is attributed to the tendency of CeO₂ to rapidly reduce at elevated temperatures given the available partial pressure of O₂ in air at ambient pressure. The CeO₂ data are compared to literature values for UO₂ and PuO₂ to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance     

The use of DTA to study spontanceous ignition of cellulose

The use of differential thermal analysis has enabled spontaneous ignition behaviour of cotton cellulose to be investigate. The temperature. T_i, at which the onset of spontaneous ignition occurs is recorded as a function of the oxygen concentration of the flowing oxygen-nitrogen atmosphere to which the cellulose sample is exposed in the DTA furnace, when heated at a defined heating rate. The dependence of T_i, on heating rate has enabled the activation energy, E_p, of the rate-determining flammable pyrolysis product reaction to the determined using both a previously derived simple kinetic model and the technique of Ozawa. E_p, increases from a lower limiting value of 112 kJ mol^?1 at zero oxygen concentration to an asymptote value of 169 kJ mol^?1 at oxygen volume concentrations above 30%. This effect is described in terms of oxygen catalysis of competing pyrolysis routes. At a given heating rate, increased oxygen concentration reduces T_i. A plot of 1/T_i versus In [O₂] gives two liner regions which intersect at an oxygen concentration of about 20%, suggesting that two combustion mechanisms exist, one above and the other below this value. Below this concentration, which is similar to the conventional limiting oxygen for cellulose, significant char remains, suggesting that ignition of gaseous products only occurs. If the difference in slopes is sttributed to the variations in E_p with oxygen concentration, then a value for the activation energy of gaseous product oxidation, E_ox = 215 kJ mol^?1 is derived.     

Reaction modeling on the phosphorous-treated Ru/Co/Zr/SiO2 Fischer-Tropsch catalyst with the estimation of kinetic parameters and hydrocarbon distribution

The catalytic performances and the kinetics model of the cobalt-based Fischer-Tropsch (FT) catalyst were investigated on Ru/Co/Zr/SiO₂ with different amounts of phosphorous. The preliminary analysis of the CO conversion, the hydrocarbon selectivity and the activation energy revealed that the 0.5 wt.% phosphorous-treated Ru/Co/Zr/SiO₂ catalyst was found to have the highest catalytic activity because of the good dispersion of the cobalt species and the suppressed aggregation during the FT reaction. Kinetics model was concomitantly developed on the selected catalyst. The model that was reported in the literature was modified in order to explain the characteristics that were observed in the experimental studies, and the kinetic parameters were estimated to fit the experimental data under various reaction conditions. The simulation results along with the estimated kinetic parameters satisfactorily predicted the effects of the operating conditions on both the CO conversion and the entire hydrocarbon distribution.     

Kinetics of the dissolution of silica in aqueous sodium hydroxide solutions at high pressure and temperature

The kinetic of the reaction of sand with aqueous NaOH corresponding to the ratio SiO₂/Na₂O = 2 was studied in a pressure vessel at 220°C and 2.7 MPa. Since the kinetic curves could not be obtained directly from the experimental data, a new method is proposed to plot the entire kinetic graph from experimental data. An analytical expression of the type α = A [1 -exp(-Bt)] describes the system perfectly. The constants A and B were calculated for a silica sample having a narrow granulometric distribution (range i.e. 300–315 μm). The value of A is found to be almost constant, between 0.95 to 0.99 and B ranges from 0.03 to 0.14 when [OH-] increases from 0.5 to 12.5 mol/L. The kinetic order with respect to OH^? is equal to 0.470 + 0.013 and the kinetic constant at 220°C is 3.933 × 10^?6 g/m².s.     

Synthesis and thermal cure of diphenyl ethers terminated with acetylene and phenylacetylene

Two kinds of novel compounds, diphenylacetylene diphenyl ether (DPADPE) and diacetylene diphenyl ether (DADPE), were prepared and polymerized under heating. Raman, DSC and ¹³C CP/MAS NMR analyses were used for studying the polymerization reaction. DPADPE and DADPE have melting points at 190 and 79 °C, with exothermic peaks of the DSC curves at 375 and 215 °C for curing, respectively. Raman and ¹³C CP/MAS NMR spectra show that DPADPE could be cured at a temperature higher than 300 °C and DADPE at a lower temperature of higher than 150 °C. The kinetic parameters for the thermal crosslinking reactions were obtained by the Ozawa method and the results show that the apparent activation energy is 152 kJ mol^?1 for DPADPE and 109 kJ mol^?1 for DADPE. An ene–yne Straus product appears in the cured DADPE, whereas this product has not been identified in the cured DPADPE. The cured DPADPE and DADPE demonstrate good thermal and thermo‐oxidative stability. Copyright © 2006 Society of Chemical Industry     

Reactive-extraction of 2,3-butanediol from fermentation broth by propionaldehyde: Equilibrium and kinetic study

An effective process was developed to separate 2,3-butanediol (2,3-BD) from fermentation broth (FB) by reactive-extraction. Propionaldehyde (PRA) was used as reactant and reaction product 2-ethyl-4,5-dimethyl-1,3-dioxolane (EDD) acted as extractant. HCl was selected as catalyst. Appropriate conditions were obtained by experiment as follows: 10 °C, C _HCl =0.2mol·L^?1, two-stage cross-current extraction, reactant volume ratio (V _PRA : V _FB ) for first stage and second stage is 0.10 and 0.05, respectively. The yield rate of 2,3-butanediol for the whole process can reach 90% w/w, and 2,3-butanediol in the final product can be more than 99% w/w. The novel process required less solution and especially had advantages in treating dilute fermentation broth. Furthermore, equilibrium and kinetic study were investigated on the reaction of propionaldehyde and 2,3-butanediol to provide basic data for process development. The results reveal that reaction enthalpy and activation energy of the reaction were ?21.84±2.38 KJ·mol^?1 and 51.97±2.84 KJ·mol^?1, respectively. Kinetics was well described by pseudo-homogeneous model.