Elementary reaction modeling

Reduced elementary reaction model of the propane pyrolysis... 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

Fig. 3. Reduced elementary reaction model of the propane pyrolysis. (1010°C, r= 20 ms)...

By reducing an elementary reaction model taken fi om the database, a comprehensive gas-phase reaction model of propane pyrolysis was derived objectively. The reaction rate constants that were not accurate under the conditions of interest were found and refined by fitting with the experimental results. The obtained reaction model well represented the effects of the gas residence time and temperature on the product gas composition observed in experiments under pyrocarbon CVD conditions. 

Holgate, R.H. and Tester, J.W., Oxidation of hydrogen and carbon monoxide in sub-and supercritical water reaction kinetics, pathways, and water-density effects. 2. Elementary reaction modeling, ]. Phys. Chem. Technol., 98, 810-822, 1994. 

The destruction of hazardous chemical wastes by oxidation in supercritical water is a promising new technology which has several advantages over conventional methods of toxic chemical waste disposal. Although the feasibility of the supercritical water oxidation process has been demonstrated, there is little kinetic information available on the underlying reaction mechanisms. We have recently determined the oxidation kinetics of several model compounds in supercritical water, and now report on our results of the oxidation of methanol, a conunon industrial solvent, in supercritical water. Globd kinetic expressions are presented and our attempts to model the reaction using a free-radical mechanism with 56 elementary reactions are discussed. The inability of the elementary reaction model to represent oxidation in supercritical water is demonstrated and future model modifications are discussed. 

Table II. Elementary Reaction Model for Methanol Oxidation in Supercritical Water...

Several recent publications suggest that new reaction paths are needed to model aromatic oxidation and combustion [156, 154, 175, 176] Zhang et al. [156] published an elementary reaction model for high-temperature benzene combustion under fuel rich conditions - near sooting environment. They report that the flame speeds for benzene could not be matched by their model and suggested that some important reaction paths may be missing. Shandross et al. 

Flere, we shall concentrate on basic approaches which lie at the foundations of the most widely used models. Simplified collision theories for bimolecular reactions are frequently used for the interpretation of experimental gas-phase kinetic data. The general transition state theory of elementary reactions fomis the starting point of many more elaborate versions of quasi-equilibrium theories of chemical reaction kinetics [27, M, 37 and 38]. 

In earlier chapters, we have seen how kinetic phenomena can be interpreted, for example, to provide a reaction scheme consisting of a set of elementary reactions. Over the years, several models have been devised to explain and sometimes to predict the rates of elementary reactions. It is these that we now wish to examine on a more fundamental basis in this chapter, plus Chapters 9 and 10. 

Step 4. A reactant molecule is adsorbed onto the internal surface of the catalyst. The adsorption step is modeled as an elementary reaction, the simplest version of which is... 

Recently the polymeric network (gel) has become a very attractive research area combining at the same time fundamental and applied topics of great interest. Since the physical properties of polymeric networks strongly depend on the polymerization kinetics, an understanding of the kinetics of network formation is indispensable for designing network structure. Various models have been proposed for the kinetics of network formation since the pioneering work of Flory (1 ) and Stockmayer (2), but their predictions are, quite often unsatisfactory, especially for a free radical polymerization system. These systems are of significant conmercial interest. In order to account for the specific reaction scheme of free radical polymerization, it will be necessary to consider all of the important elementary reactions. 

In this paper, we first briefly describe both the single-channel 1-D model and the more comprehensive 3-D model, with particular emphasis on the comparison of the features included and their capabilities/limitations. We then discuss some examples of model applications to illustrate how the monolith models can be used to provide guidance in emission control system design and implementation. This will be followed by brief discussion of future research needs and directions in catalytic converter modeling, including the development of elementary reaction step-based kinetic models. 

The first step in constructing a micro-kinetic model is to identify all the elementary reaction steps that may be involved in the catalytic process we want to describe, in this case the synthesis of ammonia. The overall reaction is... 

Table 10.4 lists the rate parameters for the elementary steps of the CO + NO reaction in the limit of zero coverage. Parameters such as those listed in Tab. 10.4 form the highly desirable input for modeling overall reaction mechanisms. In addition, elementary rate parameters can be compared to calculations on the basis of the theories outlined in Chapters 3 and 6. In this way the kinetic parameters of elementary reaction steps provide, through spectroscopy and computational chemistry, a link between the intramolecular properties of adsorbed reactants and their reactivity Statistical thermodynamics furnishes the theoretical framework to describe how equilibrium constants and reaction rate constants depend on the partition functions of vibration and rotation. Thus, spectroscopy studies of adsorbed reactants and intermediates provide the input for computing equilibrium constants, while calculations on the transition states of reaction pathways, starting from structurally, electronically and vibrationally well-characterized ground states, enable the prediction of kinetic parameters. 

The reaction between hydrogen and oxygen leads to the formation of water. This reaction has extended explosive regimes with respect to the p,T,c-parameters. A mechanistic analysis of the elementary reactions is available and the explosion mechanisms are imderstood in detail. Accordingly, this reaction serves well as a model for other dangerous processes in the explosive regime such as many oxidations with pure oxygen. 

To rationally govern the selectivity of a catalytic process, the elementary reaction steps on real catalyst surfaces must be identified. The use of well-defined organometallic compounds (possible intermediates in surface reactions) can be very useful in the determination of these steps. The use of kinetic modelling techniques combined with statistical analysis of kinetic... 

In the near future probably computer modelling, allowing the analysis of adsorption and elementary reactions at surfaces, will become increasingly helpful in catalyst selection. On the experimental side the field is changing drastically. Parallel testing equipment is now the state of the art. This field is often referred to as Combinatorial Chemistry . It is expected to have a large impact already in the near future. In fact, at present already companies have been formed in this field. 

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