A simplified kinetic model for isothermal catalytic ignition

The ignition of catalytic combustion of the stoichiometric propane/air mixture on an isothermally heated platinum wire in different experimental conditions of total pressure and wire temperature is studied and discussed on the basis of a simplified kinetic model. The platinum wire is heated electrically with a specially designed power supply, which ensures a quasi-rectangular profile of its temperature. The ignition process is monitored by measuring the input power required to maintain a constant temperature of the wire during an exothermic catalytic reaction. The difference between the input powers recorded in air and in a fuel/air mixture, for the same wire temperature and gas total pressure, allows the elimination of the heat transferred to surroundings and conversion of the results into the catalytic reaction rate r _R versus time curves of S-shaped form, illustrating the transition from kinetic to diffusion regime. The curve can be used to evaluate the ignition delay, as reported previously and also to fit different models to the data. The quasi-exponential increase of the isothermal reaction rate during the early stages of the process can be rationalized on the basis of a simplified kinetic model implying the multiplication of the adsorbed active intermediates. The adopted hypotheses allow the derivation of an analytical solution for the catalytic reaction rate before and during the ignition process, without diffusion limitations.     

Influence of noble metals on thermoacoustic oscillations and boundaries of the region of negative temperature coefficient during combustion of n-pentane – air mixtures

Using the combustion of n-pentane – air mixtures as an example, it was shown that thermoacoustic regimes of ignition disappear in the presence of a platinum surface, which generates catalytic centers propagating into the volume. Thus, the platinum catalyst eliminates a certain stage of kinetic mechanism, and the negative temperature coefficient phenomenon vanishes. In the presence of a palladium catalytic surface that does not generate catalytic centers propagating into the volume, the phenomenon of a negative temperature coefficient occurs.     

Effect of CO<Subscript>2</Subscript> dilution on propane–air isothermal catalytic combustion on platinum

The present work aims to investigate several kinetic aspects of catalytic combustion of the stoichiometric propane–air mixture on a platinum wire in the presence of progressive CO₂ dilution, using a micro-calorimetric method. The method allowed the determination of the induction periods and of the reaction rates during transient and steady-state regimes at different initial pressures P ₀ (10–101 kPa), wire temperatures T _w (470–575 K) and gas-phase composition. The kinetic parameters were evaluated from regression analysis using empirical and mechanistic models. The dilution with CO₂ of the stoichiometric propane–air mixture (4.02% propane) was 10, 20, 30, 40, 50%. For additions equal to or greater than 23%, the gaseous system is outside the flammability limits reported in the literature. The kinetic characteristics of the catalytic combustion remain similar for the whole range, even for 50% CO₂ addition when the propane concentration in mixture diminishes to 2%, a concentration smaller than the lower explosion limit in air in normal conditions.     

Effect of oxygen concentration and temperature on ignition time of polypropylene

The presented article deals with the assessment of combined impact of temperature and flow of oxidising atmosphere, its oxygen concentration and heat flux on the ignition time of isotactic polypropylene (PP). The ignition time was determined in a specially adapted hot air Setchkin furnace at temperatures (450 and 600?°C), density of heat flux (12.4 and?26.4?kW m^?2), flows of oxidation mixture (6 and 8?L?min^?1) and volume oxygen concentrations (3, 9, 15, 21, 27, 33, 39, 45 and 50?%). Obtained data allows us to assume that the temperature influence on PP induction period of ignition increases with decreasing flow rate of oxidising atmosphere. At the flow of oxidising mixture equal to 6?L?min^?1 and temperature of 600?°C, oxygen concentration had only a negligible impact on the the induction period of ignition in the analysed period. From the presented results, the induction period of ignition depends on the temperature and also on the flow rate of oxidising mixture and oxygen concentration in it. In addition, heat flux has a significant influence on the induction period. However, the quantification of the heat flux influence was not possible with the applied experimental device.     

Autoinflammation et stabilité thermique de la poly(vinyl-4-pyridine) quaternisée—IV Methode d'étude des mécanismes d'autoinflammation. Application aux cas du bromure de poly(vinyl-4-pyridinium) et du bromure de poly(N(3-propionyl)4-vinylpyridinium)

We have determined the nature of the combustible gas produced from oxidizing pyrolysis of a polymer (the bromide of poly(vinyl-4-pyridinium) and the bromide of poly(N(3-propionyl)4-vinylpyridinium)). The pyrolysis temperature chosen was just below the self ignition temperature and the pyrolysis was performed in air at atmospheric pressure. We analysed the gaseous products by gas chromatography and mass spectrometry. We found a correlation between the change of the self ignition temperature, the change of the composition of the gas phase and the change of the percentage of residue. We assumed that the gas mixture originates the self ignition of the polymer. The composition of the gas mixture depends on the composition of the polymer.     

Ionizing-Radiation Ignition of a Hydrogen–Air Mixture

The behavior of a hydrogen–air mixture under the action of ionizing radiation was studied. A kinetic model for radiation-chemical processes in the H₂–O₂–N₂ system was constructed, and the effects of the main parameters of radiolysis on shifting its flammability limits were analyzed. It was found that, under normal conditions (p = 0.1 MPa, T ₀ = 300 K), the ignition of the stoichiometric mixture began at a radiation intensity of about 0.1 kGy/s. The induction period of the chain H₂ oxidation reaction shortened with increasing radiation dose rate and pressure.     

Experimental and theoretical study of surface tension of n-pentane,n-heptane,and some of their mixtures at different temperatures

The surface tension of n-pentane, n-heptane and some of their mixtures were measured at 91.3 kPa and T = (283.15 to 323.15) K by the maximum bubble pressure method. The surface tension results were correlated by a new model obtained from coupling scaled particle theory (SPT) and the MMM equation of state. The parameters of the model (SPTM3) for n-pentane and n-heptane were calculated and reported. The pure component parameters obtained for the model were used for calculating the surface tension of n-pentane and n-heptane binary mixtures at different temperatures. The comparison between the mixture surface tension results and those calculated by the model indicate that the new proposed model can be satisfactorily used for calculating the surface tension of n-pentane, n-heptane and their mixtures.     

Kinetics of the reactions of OH radicals with n-alkanes at 299 ± 2 K

Relative rate constants for the reaction of OH radicals with a series of n-alkanes have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10^?12 cm³ molecule^?1s^?1, the rate constants obtained are (X10¹² cm³ molecule^?1 s^?1): propane 1.22 ± 0.05, n-pentane 4.13 ± 0.08, n-heptane 7.30 ± 0.17, n-octane 9.01 ± 0.19, n-nonane 10.7 ± 0.4, and n-decane 11.4 ± 0.6. The data for propane, n-pentane, and n-octane are in good agreement with literature values, while those for n-heptane, n-nonane, and n-decane are reported for the first time. These data show that the rate constant per secondary C—H bond is ∽40% higher for —CH₂— groups bonded to two other —CH₂— groups than for those bonded to a —CH₂— group and a —CH₃ group.     

Kinetic model of the H2-O2 surface reaction on platinum at high partial pressure

Heterogeneous ignition temperature of H₂/O₂/N₂ mixture on polycrystalline platinum was measured for a wide range of composition at 101,330 Pa. We propose a new surface kinetic model of H₂ oxidation on platinum by modification of the traditional surface reaction model. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chain ignition of propane-air and pentane-air mixtures in a heated vessel

The spatial propagation of the chain ignition of propane-air and pentane-air mixtures with oxygen at a pressure of 1 atm and T = 600–800 K is studied. It is established that the features of the spatial propagation of chain ignition process are determined by the conditions of the reactor’s surface. It is shown that the site (or sites) of ignition are located on the surface of the reaction vessel; the flame front propagates from the site into the volume at a normal speed corresponding to the reactor temperature and the composition of the combustible mixture.     

Mathematical Modeling of Aluminum Diboride Combustion in an Air Flow

A mathematical model and the results of calculating the ignition and combustion of energetic condensed systems based on mono- and polydispersed aluminum diboride particles in air flows in constant-cross-section channels are reported. The kinetic characteristics of the transformations that separate aluminum diboride particles formed by gasification of energetic condensed systems undergo in a high-temperature oxidizing medium were determined using the dependences of the ignition induction period and combustion time on the air temperature and diameter and initial temperature of the particles. These dependences, in turn, were calculated using the model of parallel chemical reactions. The range of combustion conditions corresponding to the initial air temperatures from 300 to 2000 K and Mach numbers in the channel from 0.1 to 1.5 was considered. The influence of the aluminum diboride particle size and of the rate and initial temperature of the air flow on the combustion efficiency was demonstrated. The relationships between the combustion completeness factor of aluminum diboride particles at various initial parameters of the air flow and gasification products of energetic condensed systems at various fuel mixture equivalence ratios, corresponding to the diffusion and kinetic combustion, were determined. The conditions of the transition between the diffusion and kinetic control modes were found.

     

Effect of Thermal Activation of Supported Catalysts Pt/MeOx,Where MOx = Al2O3, CeO2, La2O3, and ZrO2, for Complete Oxidation

A sharp increase in the atomic catalytic activity (ACA) of supported platinum catalysts in the model reaction of n-pentane complete oxidation is found on going from the preliminary calcination temperature of 500–600°C to a temperature of 700°C. ACA increases by an order of magnitude for the Pt/-Al₂O₃ system, 3 times for Pt/ZrO₂, and 1.5 times for Pt/CeO₂. The per-gram activities of all catalysts decrease because of a decrease in the dispersion of supported platinum with an increase in the temperature of preliminary calcination.     

Co60 γ-radiation-induced copolymerization of ethylene and carbon monoxide

The γ-radiation-induced free-radical copolymerization of ethylene and CO has been investigated over a wide range of pressure, initial gas composition, radiation intensity, and temperature. At 20°C., concentrations of CO up to 1% retard the polymerization of ethylene. Above this concentration the rate reaches a maximum between 27.5 and 39.2% CO and then decreases. The copolymer composition increases only from 40 to 50% CO when the gas mixture is varied from 5 to 90% CO. A relatively constant reactivity ratio is obtained at 20°C., indicating that CO adds 23.6 times as fast as an ethylene monomer to an ethylene free-radical chain end. For a 50% CO gas mixture, the above value of 23.6 and the copolymerization rate decrease with increasing temperature to 200°C. The kinetic data indicate a temperature-dependent depropagation reaction. Infrared examination of copolymers indicates a polyketone structure containing ? CH₂? CH₂? and ? CO? units. The crystalline melting point increases rapidly from 111 to 242°C., as the CO concentration in the copolymer increases from 27 to 50%. Molecular weight of copolymer formed at 20°C. increased with increasing CO, indicating M?_n values >20,000. Increasing reaction temperature results in decreasing molecular weight. Onset of decomposition for a 50% CO copolymer was measured at ≈250°C.     

Surface modes of catalytic ignition of flammable gases over noble metals

The features of catalytic ignition of 2H₂ + O₂ and (70% H₂ + 30% C₂H₆) + air mixtures over Pd wire are established. It is shown that the catalytic wire or foil is heated unevenly before ignition: initial ignition centers always appear. With sequential ignitions, a clearly observed ignition center changes its location on the wire with every succeeding ignition.     

Nonoxidative conversion of methane and <Emphasis Type="Italic">n</Emphasis>-pentane over a platinum/alumina catalyst

Methane adsorption on the Pt–H/Al₂O₃ and Pt/Al₂O₃ catalysts begins at Т = 475°C and is accompanied by the appearance of hydrogen in the reaction medium. At a higher temperature is raised to 550°C, the amount of adsorbed hydrogen increases to 1.1 and 0.8 mol/(mol Pt), respectively. According to the calculated degree of methane dehydrogenation on platinum sites at Т = 550°C, the Н/C ratio is 1.3 (at/at) for the Pt–H/Al₂O₃ catalyst and 1.5 (at/at) for the Pt/Al₂O₃ catalyst. The introduction of n-pentane into the reaction medium increases the yield of aromatic hydrocarbons (benzene and toluene) by a factor of 8.8 over the arene yield observed in individual n-pentane conversion. A mass spectrometric analysis of the arenes obtained with the Pt/Al₂O₃ catalyst has demonstrated that 37.5% of the adsorbed methane is involved in the methane–n-pentane coaromatization yielding benzene and toluene.     

Mechanism of phthalic anhydride formation in the oxidation of n-pentane on a vanadium-phosphorus oxide catalyst

The reaction paths of product formation in the partial oxidation of n-pentane on vanadium-phosphorus oxide (VPO) and VPO-Bi catalysts are considered. The condensed products of n-pentane oxidation were analyzed by chromatography-mass spectrometry, and the presence of C₄ rather than C₅ unsaturated hydrocarbons was detected. It was found that the concentration of phthalic anhydride in the products increased upon the addition of C₄ olefins and butadiene to the n-pentane-air reaction mixture. With the use of a system with two in-series reactors, it was found that the addition of butadiene to a flow of n-butane oxidation products (maleic anhydride, CO, and CO₂) resulted in the formation of phthalic anhydride. The oxidation of 1-butanol was studied, and butene and butadiene were found to be the primary products of reaction; at a higher temperature, maleic anhydride and then phthalic anhydride were formed. The experimental results supported the reaction scheme according to which the activation of n-pentane occurred with the elimination of a methyl group and the formation of C₄ unsaturated hydrocarbons. The oxidation of these latter led to the formation of maleic anhydride. The Diels-Alder reaction between maleic anhydride and C₄ unsaturated hydrocarbons is the main path of phthalic anhydride formation.     

A novel scalable synthesis process of PPTA by coupling n-pentane evaporation for polymerization heat removal

In this study,we compared the effect of n-pentane and ice-water bath on removing the thermal effect in the poly（p-phenylene terephthalamide）（PPTA） polymerization process.The results indicate that the n-pentane can help to transfer the reaction heat faster and better.Adding suitable amount of n-pentanes into the PPTA preparation process not only improve the heat transfer,but also reduce the motor power in the polymerization process.Moreover,the introduction of n-pentane properly does not result in decrease of the inherent viscosity(η_inh) of polymer.Instead,it leads to increased viscosity of polymer during the PPTA preparation process.The results indicate that n-pentane can effectively transfer the reaction heat and avoid overheating during the polymerization of PPTA.     

Phase‐equilibrium behavior of vinyl chloride/n‐butane and its application in determination of vinyl chloride heterogeneous polymerization kinetics

Studies of the phase‐equilibrium behavior of vinyl chloride (VCM)/n‐butane mixtures and the kinetics of VCM heterogeneous polymerization, using n‐butane as a reaction medium, were carried out using a 1‐L glass autoclave. The vapor composition was measured by gas chromatography, showing that the vapor pressure of the VCM/n‐butane mixture was located above the line connecting the points for pure VCM and n‐butane. The concentration of VCM in the vapor phase was greater than that in the corresponding liquid phase. It was confirmed that the presence of poly(vinyl chloride) (PVC) resin had no significant influences on the phase equilibrium of VCM/n‐butane mixtures. Thus, the phase‐equilibrium equations were applied to determine the conversion of VCM during heterogeneous polymerization. The conversions calculated from the variations of vapor pressure or composition agreed with those determined by the weighing method. The conversion–time and polymerization rate–time curves obtained for VCM heterogeneous polymerization showed that the polymerization accelerated at low initiator concentration, but the polymerization rate decreased with an increase of conversion at relatively high initiator concentrations. The chain‐transfer reaction to n‐butane was confirmed by a decrease of the molecular weight and broadening of the molecular weight distribution of PVC. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2179–2188, 2001     

Effect of Bismuth Additives on the Properties of Vanadium–Phosphorus Oxide Catalyst in the Partial Oxidation of n-Pentane

The selective oxidation of n-pentane on vanadium–phosphorus oxide (VPO) catalysts with bismuth additives (Bi/V = 0–0.30) is studied. The catalysts are characterized by XRD, XPS, and specific surface area measurements using nitrogen adsorption. Their acidic properties are studied (using ammonia TPD and the 2-methyl-3-butyn-2-ol reaction). It was found that the introduction of bismuth insignificantly affects the specific surface area but increases the surface concentration of phosphorus and changes the acidic properties of the catalysts. The specific catalytic activity of samples in n-pentane oxidation correlates with the effective charge of surface oxygen (E _b of O1s electrons). The selectivity to citraconic anhydride increases with an increase in the general surface acidity. The selectivity to maleic anhydride increases with an increase in the Brønsted acidity of the surface. The selectivity to phthalic anhydride increases with an increase in the Lewis acidity. The pathways of product formation in the partial oxidation of n-pentane are proposed.     

Ring opening of methylcyclopentane over Al2O3 and SiO2 supported Rh catalysts

Hydrogenolytic ring opening of methylcyclopentane (MCP) was investigated on Rh/Al₂O₃ and Rh/SiO₂ catalysts, prepared by the incipient wetness method. Strong dependence can be seen in the yield and distribution of ring opening products as a function of temperature and hydrogen pressure. They depended also on the support used. The ring opening reaction required high hydrogen coverage, and was not random (hindered in the vicinity of the methyl group), thus, mainly 2-methylpentane (2MP) and 3-methylpentane (3MP) were formed. The fragments consisted of C₁–C₅ alkanes, with methane andi-pentane as main fragments. This means the possibility of breaking two C−C bonds during one sojourn of the reactant on the catalyst, both taking place far from the substituent. The loose positive correlation between the ratios ofi-pentane/n-pentane and 3MP/n-hexane seems to support this conclusion. Dedicated to Professor Pál Tétényi on the occasion of his 70th birthday