Chemical reactions, kinetics pressure effects

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. 

Several investigators have suggested that chemical-reaction kinetics control the performance of both ramjet and turbojet combustors (4, 96, 139). Second-order reaction equations were assumed to be the over-all rate determining step, and the influence of combustor inlet-air pressure, temperature, and velocity on combustion efficiency could be explained in terms of their effects on these second-order reactions. Combustion efficiency has been shown to vary inversely with a reaction-rate parameter of the form... 

All these factors are functions of the concentration of the chemical species, temperature and pressure of the system. At constant diffu-sionai resistance, the increase in the rate of chemical reaction decreases the effectiveness factor while al a constant intrinsic rate of reaction, the increase of the diffusional resistances decreases the effectiveness factor. Elnashaie et al. (1989a) showed that the effect of the diffusional resistances and the intrinsic rate of reactions are not sufficient to explain the behaviour of the effectiveness factor for reversible reactions and that the effect of the equilibrium constant should be introduced. They found that the effectiveness factor increases with the increase of the equilibrium constants and hence the behaviour of the effectiveness factor should be explained by the interaction of the effective diffusivities, intrinsic rates of reaction as well as the equilibrium constants. The equations of the dusty gas model for the steam reforming of methane in the porous catalyst pellet, are solved accurately using the global orthogonal collocation technique given in Appendix B. Kinetics and other physico-chemical parameters for the steam reforming case are summarized in Appendix A. 

What Is Chemical Kinetics Chemical kinetics is a study of the rates of chemical reaction and the effect that process conditions have on these rates. These process conditions are temperature, pressure, and reactant concentration. The application of kinetics in the selection and design of commercial reactors is of specific interest in the study of unit processes. ... 

According to [14], the decrease in concentration limit resulting from the pressure rise is characteristic of mixtures with high water steam content. The observed phenomena are the result of the aggregate effects of the chemical reaction kinetics, water molecules/water steam heat radiation and self-absorption of the heat radiation... 

The differential material balances contain a large number of physical parameters describing the structure of the porous medium, the physical properties of the gaseous mixture diffusing through it, the kinetics of the chemical reaction and the composition and pressure of the reactant mixture outside the pellet. In such circumstances it Is always valuable to assemble the physical parameters into a smaller number of Independent dimensionless groups, and this Is best done by writing the balance equations themselves in dimensionless form. The relevant equations are (11.20), (11.21), (11.22), (11.23), (11.16) and the expression (11.27) for the effectiveness factor. 

During the nineteenth century the growth of thermodynamics and the development of the kinetic theory marked the beginning of an era in which the physical sciences were given a quantitative foundation. In the laboratory, extensive researches were carried out to determine the effects of pressure and temperature on the rates of chemical reactions and to measure the physical properties of matter. Work on the critical properties of carbon dioxide and on the continuity of state by van der Waals provided the stimulus for accurate measurements on the compressibiUty of gases and Hquids at what, in 1885, was a surprisingly high pressure of 300 MPa (- 3,000 atmor 43,500 psi). This pressure was not exceeded until about 1912. 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

In chemical equilibria, the energy relations between the reactants and the products are governed by thermodynamics without concerning the intermediate states or time. In chemical kinetics, the time variable is introduced and rate of change of concentration of reactants or products with respect to time is followed. The chemical kinetics is thus, concerned with the quantitative determination of rate of chemical reactions and of the factors upon which the rates depend. With the knowledge of effect of various factors, such as concentration, pressure, temperature, medium, effect of catalyst etc., on reaction rate, one can consider an interpretation of the empirical laws in terms of reaction mechanism. Let us first define the terms such as rate, rate constant, order, molecularity etc. before going into detail. 

As for all trace-level analyses, sample preparation and handling are of crucial importance. In addition to all the usual problems of GC-MS, measurements of isotope ratios must ensure that none of these steps introduce any isotope discrimination. Any chemical reactions, including conversion of the organic sample molecules to the simple gases which are those actually analyzed, must be quantitative (100% conversion) to avoid kinetic isotope effects [627]. Until relatively recently, all gas IRMS experiments employed a dual-inlet system to permit switching between sample and standard C02 contained in two bellows containers. The pressures in the two bellows are adjusted to be equal and,... 

The plan of this chapter is as follows. In Section 11 the basics of high-pressure technology and equipment are covered with particular reference to (a) the types of equipment that have actually been used to smdy chemical reactions and (b) the techniques in use for in situ and on-the-fly monitoring of chemical equilibria, products structure, reaction kinetics, and mechanism. Section III deals with fundamental concepts to treat the effect of high pressure on chemical reactions with several examples of applications, but with no claim of extensive covering of the available hterature. In Section IV the results obtained in the study of molecular systems at very high pressures will be discussed, and some conclusive remarks will be presented in Section V. 

The important effect of increasing pressure on the kinetics of chemical reactions has been noted since the hrst chemical experiments at high pressure. The simplest expectation derives from the observation that in liquids the viscosity rapidly increases with pressure. As a result, in strongly compressed liquids, and hnally in glasses, diffusion-controlled processes can be retarded. In contrast, however, other reaction pathways can be substantially accelerated. In general, the evolution of a reaction at high pressure can be heavily controlled by kinetic aspects, and these deeply involve intermolecular effects. 

When very high pressures (> 1 GPa) are applied to liquid phases, glasses, or molecular crystals, mobility is reduced and steric effects become more important both in equilibrium and in kinetic aspects. Equations (9) and (14) are still valid, but equilibria and kinetics of chemical reactions must take into account the energetic, structural, and dynamic properties of the environment as well. 

Petersen [12] points out that this criterion is invalid for more complex chemical reactions whose rate is retarded by products. In such cases, the observed kinetic rate expression should be substituted into the material balance equation for the particular geometry of particle concerned. An asymptotic solution to the material balance equation then gives the correct form of the effectiveness factor. The results indicate that the inequality (23) is applicable only at high partial pressures of product. For low partial pressures of product (often the condition in an experimental differential tubular reactor), the criterion will depend on the magnitude of the constants in the kinetic rate equation. 

Chemical reactions at supercritical conditions are good examples of solvation effects on rate constants. While the most compelling reason to carry out reactions at (near) supercritical conditions is the abihty to tune the solvation conditions of the medium (chemical potentials) and attenuate transport limitations by adjustment of the system pressure and/or temperature, there has been considerable speculation on explanations for the unusual behavior (occasionally referred to as anomalies) in reaction kinetics at near and supercritical conditions. True near-critical anomalies in reaction equilibrium, if any, will only appear within an extremely small neighborhood of the system s critical point, which is unattainable for all practical purposes. This is because the near-critical anomaly in the equilibrium extent of the reaction has the same near-critical behavior as the internal energy. However, it is not as clear that the kinetics of reactions should be free of anomalies in the near-critical region. Therefore, a more accurate description of solvent effect on the kinetic rate constant of reactions conducted in or near supercritical media is desirable (Chialvo et al., 1998). 

The problem of calculating reaction rate is as yet unsolved for almost all chemical reactions. The problem is harder for heterogeneous reactions, where so little is known of the structures and energies of intermediates. Advances in this area will come slowly, but at least the partial knowledge that exists is of value. Rates, if free from diffusion or adsorption effects, are governed by the Arrhenius equation. Rates for a particular catalyst composition are proportional to surface area. Empirical kinetic equations often describe effects of concentrations, pressure, and conversion level in a manner which is valuable for technical applications. 

The treatment given in this section is analogous to the Lindemann theory of unimolecu-lar reactions. It provides a general explanation of pressure effects in bimolecular chemical activation reactions. A more sound theoretical treatment of chemical activation kinetics is given in Section 10.5. 

Equation (305) is unique in chemical kinetics in that it holds true at pressures varying by a factor of 2000 viz., from 1/4 atm (116) to 500 atm (115) (with high pressure effects taken into account). As far as I know, no other reaction has been studied in such a broad pressure range. 

Chemical Reaction Mechanisms and Kinetics. CVD chemistry is complex, involving both gas-phase and surface reactions. The role of gas-phase reactions expands with increasing temperature and partial pressure of the reactants. At high reactant concentrations, gas-phase reactions may eventually lead to gas-phase nucleation that is detrimental to thin-film growth. The initial steps of gas-phase nucleation are not understood for CVD systems, not even for the nucleation of Si from silane, which has a potential application in bulk Si production (97). In addition to producing film precursors, gas-phase reactions can have adverse effects by forming species that are potential impurity sources. 

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