Kinetics, high-temperature gases

A. Kantrowitz in the paper entitled "Shock Tubes for High Temperature Gas Kinetics , given on pp 241-88 of that book stated that ... 

The computer codes described above are able to simulate spatially homogeneous reaction kinetics systems, which are either characterised by spatially and temporally constant rate coefficients or utilise user-defined functions for the rate parameters (e.g. in the case of KPP). For the simulation of high-temperature gas kinetic systems, such as combustion, pyrolytic and other chemical engineering problems, the rate coefficients may change substantially as a function of temperature and pressure and maybe also as a function of gas composition. Typically, the temperature and pressure is not constant during such simulations due to heat release, and their change has to be calculated during the course of the reaction. Several computer codes are available for such types of simulations. 

Our data can be used to estimate the effective temperatures reached in each site through comparative rate thermometry, a technique developed for similar use in shock tube chemistry (32). Using the sonochemical kinetic data in combination with the activation parameters recently determined by high temperature gas phase laser pyrolysis (33), the effective temperature of each site can then be calculated (8),(34) the gas phase reaction zone effective temperature is 5200 650°K, and the liquid phase effective temperature is 1900°K. Using a simple thermal conduction model, the liquid reaction zone is estimated to be 200 nm thick and to have a lifetime of less than 2 usee, as shown in Figure 3. 

Apparatus for high-temperature gas-phase kinetics experiment. 

Global kinetic models for high-temperature gas-phase reactions were reviewed by Abdalla et al. [193] and Babushok and Dakdancha [194]. Empirical global kinetic schemes for the low-temperature oxidation of hydrocarbons have been recently reviewed by Griffiths [11]. Here, only some characteristic models are mentioned and for a full account the reader is referred to the review by Griffiths. 

V.I. Babushok and A.N. Dakdancha, Global Kinetic-Parameters for High-Temperature Gas-Phase Reactions, Combust. Expl. Shock Waves 29 (1993) 464-489. 

One can assume that most (if not all) high-temperature gas-phase catalytic reactions proceeding at temperatures above 800 K are substantially heterogeneous-homogeneous (in the meaning defined above). Consequently, their adequate kinetic description and modeling require a development of corresponding approaches and procedures. 

This compression gives rise to high temperatures and kinetic energies 1 gas molecules, which through molecular collisions generate active intemiediau cause chemical reactions to occur in the bubble. 

At sufficiently high temperatures, the kinetic energy will exceed the maximum attraction energy between molecules, no matter how high the pressure. The substance will then never become liquid. This temperature limit is called the critical temperature. It represents the upper limit for condensation, beyond which there is no distinct transition between gas and liquid. Other properties at this temperature are also referred to as critical, such as critical volume and critical pressure. Above its critical temperature, a gas cannot liquefy and a liquid cannot continue to exist. (See Fig. 4-10.)... 

In highly vibrationally excited molecules the quantum mechanical aspects of particle motion are less significant and the details of the interparticle potential also less important. Just as with high-temperature gas-phase transport, discussed in Chapter 2, the repulsive part of the potential has the most significant effect on reaction dynamics. A study of the recombination kinetics of Brg with the buffer gases He, Ne, Ar, and Kr shows the steady trend predicted by (10.8)—the more massive the rare gas atom, the more effective it is at stabilizing... 

GuBbransen EA, Jansson SA. Thermochemical considerations of high temperature gas-soKd reactions. In Belton GR, Worrell WF, eds. Heterogeneous Kinetics at Eleoated Temperatures. New York, N.Y Plenum Press, 1970. 

Gulbransen, E. A., and Jansson, S. A., Thermochemical Considerations of High Temperature Gas-SoUd Reactions, in Belton, G. R., and Worrell, W. F. (eds.). Heterogeneous Kinetics at Elevated Temperatures, New York, Plenum Press, 1970, pp. 34 6. 

In high-temperature gas-phase kinetic systems, such as combustion and pyrolytic systems, the temperature dependence of the rate coefficient is usually described by the modified Arrhenius equation ... 

At low pressures can be set equal to 1, and the fact that most gas-phase reactions take place at relatively high temperatures for kinetic purposes, provides additional support for this assumption. 

At the high temperatures found in MHD combustors, nitrogen oxides, NO, are formed primarily by gas-phase reactions, rather than from fuel-bound nitrogen. The principal constituent is nitric oxide [10102-43-9] NO, and the amount formed is generally limited by kinetics. Equilibrium values are reached only at very high temperatures. NO decomposes as the gas cools, at a rate which decreases with temperature. If the combustion gas cools too rapidly after the MHD channel the NO has insufficient time to decompose and excessive amounts can be released to the atmosphere. Below about 1800 K there is essentially no thermal decomposition of NO. 

Because this design has relatively low power density, recent work has focused on a monolithic SOFC, since this could have faster cell chemistry kinetics. The very high temperatures do, however, present sealing and cracking problems between the electrochemically active area and the gas manifolds. 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

A plot of the Maxwell distribution for the same gas at several different temperatures shows that the average speed increases as the temperature is raised (Fig 4.27). We knew that already (Section 4.9) but the curves also show that the spread of speeds widens as the temperature increases. At low temperatures, most molecules of a gas have speeds close to the average speed. At high temperatures, a high proportion have speeds widely different from their average speed. Because the kinetic energy of a molecule in a gas is proportional to the square of its speed, the distribution of molecular kinetic energies follows the same trends. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

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