Homogeneous kinetics constant-pressure system

CONP Kee, R. J., Rupley, F. and Miller, J. A. Sandia National Laboratories, Livermore, CA. A Fortran program (conp.f) that solves the time-dependent kinetics of a homogeneous, constant pressure, adiabatic system. The program runs in conjunction with CHEMKIN and a stiff ordinary differential equation solver such as LSODE (lsode.f, Hindmarsh, A. C. LSODE and LSODI, Two Initial Value Differential Equation Solvers, ACM SIGNUM Newsletter, 15, 4, (1980)). The simplicity of the code is particularly valuable for those not familiar with CHEMKIN. 

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Another useful technique in kinetic studies is the measurement of the total pressure in an isothermal constant volume system. This method is employed to follow the course of homogeneous gas phase reactions that involve a change in the total number of gaseous molecules present in the reaction system. An example is the hydrogenation of an alkene over a catalyst (e.g., platinum, palladium, or nickel catalyst) to yield an alkane ... 

The dynamics of a chemical reaction system with species is governed by the sytem of conservation equations for mass, momentum, energy, and species masses. For the moment let us restrict to a simple closed adiabatic homogeneous system at constant pressure, where the chemical kinetics is governed by the n rate equations... 

Sikalo et al. (2014) compared several options for the application of genetic algorithms to mechanism reduction, exploring the trade-off between the size and accuracy of the resulting mechanisms. Information on the speed of solution was also taken into account, so that, for example, the least stiff system (Sect. 6.7) could be selected. An automatic method for the reduction of chemical kinetic mechanisms was suggested and tested for the performance of reduced mechanisms used within homogeneous constant pressure reactor and burner-stabilised flame simulations. The flexibility of this type of approach has clear utility when restrictions are placed on the number of variables that can be tolerated within a scheme in the computational sense. However, the development of skeletal mechanisms is rarely the end point of any reductiOTi procedure since the application of lumping or timescale-based methods can be applied subsequently. These methods will be discussed in later sections. 

The kinetics and mechanism of the reaction of methanol with bromine is not well known. The overall rate constant was found to depend either on the chemical composition or the pressure of the system, which may be explained by a change in the nature of the termination reaction involving bromine atoms (hetero- and homogenous recombination with other radicals).77,78 Results of a... 

The decay of fluorine atoms in the other reactions was estimated to be less than 1%.229,241 Pulse radiolysis combined with UV absorption was employed by Jodkowski et al.229 at pressures of 500 - 1000 mbar (M = SF6), while a fast-flow system with a quadruple mass spectrometer was applied at pressures in the range of 0.7 - 5.1 mbar (M = He). The influence of other homogeneous reactions on the reaction kinetics was also analyzed by computer simulation and found to be negligible. The rate constant ki = (1.3 + 0.3) x 10 10 cm3molecule"1s 1 at 298 K has been estimated at pressures of 500 - 1000 mbar of SF5.229 The theoretical analysis of the reaction kinetics was based on the approach developed by Troe et al.17 23 The fall-off curves for the reaction CH3... 

Clearly, regime 1 is the simplest to analyze because it is kinetically controlled and therefore for all practical purposes can be treated as a homogeneous system. Even here, however, the solubility of A in phase 2 (the liquid), its partial pressure, and solvent vapor pressure can exert a significant influence. Thus, although the rate constant increases with increase in temperature, the solubility generally decreases, but the overall effect would still be positive because the activation energy is almost always higher than the heat of solution. On the other hand, the solubility can also increase with temperature (particularly for liquids), for example, chlorobenzene in aqueous sulfuric acid and esters such as ethyl p-nitrobenzoate and dichloroethyl oxalate in water. This serves only to supplement the effect of temperature on the rate constant. 

As illustrated in Figure 4.56, the deposition may be controlled by surface-reaction kinetics e,g, high gas velocity at low temperatures/pressures), or by diffusion/mass transport e,g, low gas velocity at elevated temperatures or pressures (such as atmospheric)).Whereas the deposition rate in the former system is dependent on the concentration/reactivity of the precursor gases, the deposition rate in the latter is dependent on the diffusion rates of reactants and byproducts. Though the substrate is often placed horizontally in the CVD chamber, it is more desirable to tilt the substrate in order to increase the deposition rate and film-thickness homogeneity. For horizontally-positioned substrates, the velocity of the precursor vapor does not remain constant across the substrate surface, but will decrease downstream. Accordingly, the thickness of the boundary layer will increase at downstream substrate positions, giving rise to depressed thicknesses of the deposited film... 

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. 

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