Reactors, chemical flame

Many extensive models of the high-temperature oxidation process of methane have been published [20,20a, 20b, 21]. Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix H permits the development of these models, which can be used to predict flow reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not flrmly established. The purpose here is to point out the dominant reaction steps in these complex models of methane oxidation from a chemical point of view, just as modem sensitivity analysis [20, 20a, 20b] can be used to designate similar steps according to the particular application of the mechanism. The next section will deal with other, higher-order hydrocarbons. 

Reactors, chemical, 567-582 classification, 568 ebbulating bed. 593 fermentacon, 654,659,660 fired heater, 574,575 fixed bed, 572 flame, 573... 

Wegner, K. Pratsinis, S. E. Scale-up of nanoparticle synthesis in diffusion flame reactors. Chemical Engineering Science 2003, 58, 4581-4589. 

Shen L, Wu J, Gao Z, Xiao J. Reacbvity deteriorabon of Ni0/Al203 oxygen carrier for chemical looping combustion of coal in a 10 kWth reactor. Combust Flame 2009 156 1377. 

Validation and Application. VaUdated CFD examples are emerging (30) as are examples of limitations and misappHcations (31). ReaUsm depends on the adequacy of the physical and chemical representations, the scale of resolution for the appHcation, numerical accuracy of the solution algorithms, and skills appHed in execution. Data are available on performance characteristics of industrial furnaces and gas turbines systems operating with turbulent diffusion flames have been studied for simple two-dimensional geometries and selected conditions (32). Turbulent diffusion flames are produced when fuel and air are injected separately into the reactor. Second-order and infinitely fast reactions coupled with mixing have been analyzed with the k—Z model to describe the macromixing process. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Consider Equations (6-10) that represent the CVD reactor problem. This is a boundary value problem in which the dependent variables are velocities (u,V,W), temperature T, and mass fractions Y. The mathematical software is a stand-alone boundary value solver whose first application was to compute the structure of premixed flames.Subsequently, we have applied it to the simulation of well stirred reactors,and now chemical vapor deposition reactors. The user interface to the mathematical software requires that, given an estimate of the dependent variable vector, the user can return the residuals of the governing equations. That is, for arbitrary values of velocity, temperature, and mass fraction, by how much do the left hand sides of Equations (6-10) differ from zero ... 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

Particular attention is to be paid to closure models exploiting various types of PDFs such as beta, presumed, or full PDFs (e.g., Baldyga, 1994 Fox, 1996, 2003 Ranade, 2002). While PDFs have successfully been exploited for describing chemical reactions in turbulent flames, tubular reactors (Baldyga and Henczka, 1997), and a Taylor-Couette reactor (Marchisio and Barresi, 2003), they have never been used successfully in stirred reactors so far. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

Using these methods, the elementary reaction steps that define a fuel s overall combustion can be compiled, generating an overall combustion mechanism. Combustion simulation software, like CHEMKIN, takes as input a fuel s combustion mechanism and other system parameters, along with a reactor model, and simulates a complex combustion environment (Fig. 4). For instance, one of CHEMKIN s applications can simulate the behavior of a flame in a given fuel, providing a wealth of information about flame speed, key intermediates, and dominant reactions. Computational fluid dynamics can be combined with detailed chemical kinetic models to also be able to simulate turbulent flames and macroscopic combustion environments. 

The chemical reactor is the most hazardous unit in any chemical plant because most accidents occur by uncontrolled reaction, either within the reactor or after reactants have escaped the reactor and perhaps reacted with oxygen in air. Obviously no reactor or piping can withstand the temperatures and pressures of total combustion unless designed specifically for these conditions. We will consider the energy balance and temperature variations in continuous reactors in more detail in Chapters 5 and 6, while flames and explosions will be considered in Chapter 10. 

There are many important situations where the velocities are low (compared to sound speed), yet the density variations are large, owing to temperature or species variations. For example, in low-speed flames or chemical-vapor-deposition reactors, where the pressure is essentially uniform, large density variations are the result of temperature or species variations. Even though the density may vary by a factor of five, the pressure variations remain small since they are associated primarily with the velocity field. 

Stagnation flows represent a very important class of flow configurations wherein the steady-state Navier-Stokes equations, together with thermal-energy and species-continuity equations, reduce to systems of ordinary-differential-equation boundary-value problems. Some of these flows have great practical value in applications, such as chemical-vapor-deposition reactors for electronic thin-film growth. They are also widely used in combustion research to study the effects of fluid-mechanical strain on flame behavior. 

The primary objective of this chapter is to develop low-dimensional representations of chemically reacting flow situations. Specifically these include batch reactors (corresponding to homogeneous mass-action kinetics), plug-flow reactors (PFR), perfectly stirred reactors (PSR), and one-dimensional flames. The derivations also serve to illustrate the approach that is taken to derive appropriate systems of equations for other low-dimensional circumstances or flow situations. 

A further consequence of the upstream diffusion to the burner face could be heterogeneous reaction at the burner. Such reaction is likely on metal faces that may have catalytic activity. In this case the mass balance as stated in Eq. 16.99 must be altered by the incorporation of the surface reaction rate. In addition to the burner face in a flame configuration, an analogous situation is encountered in a stagnation-flow chemical-vapor-deposition reactor (as illustrated in Fig. 17.1). Here again, as flow rates are decreased or pressure is lowered, the enhanced diffusion tends to promote species to diffuse upstream toward the inlet manifold. 

The model of the reaction may be hypothesized in the following manner. The total molal flow, m, splits into an infinite number of elemental masses on entering the boundaries of the reactor. A certain fraction, y, of these elemental masses burns to chemical equilibrium composition and attains the adiabatic flame temperature. The remainder (1 — y) remains unburned. It is further assumed that all of the constituents of the burned fraction act only as diluents and can in no way contribute anything further to the reaction. 

The dehydrogenation of iso-butane was carried out in a recirculating batch reactor.12 Reaction products were analyzed by gas chromatography using flame ionization. The vanadium and vanadium carbide powder materials were purchased from Aldrich Chemical Co. Their bulk compositions were confirmed by the X-ray diffraction measurements. Prior to the dehydrogenation reactions, these powder materials were heated for 1 h at 900 K in pure H2 at a flow rate of 200 cm3 per minute. 

The theoretical questions which are posed and solved in these papers by Ya.B. and by Ya.B. with Yu. A. Zysin (articles 17 and 17a) have developed into an extensive separate branch of science—the theory of chemical reactors. Combustion in a reactor with ideal mixing is an example of the simplest thermal and gasdynamic situation, when the analysis requires only algebraic relations. This allows explicit demonstration of the basic features of exothermic chemical reactions in a flow which are also present in more complicated form in other combustion regimes—a laminar flame, diffusive combustion, detonation wave and others. Critical conditions of ignition and extinction and the existence of several regimes whose occurrence depends on the initial conditions—these are the most remarkable effects of combustion which attract the attention even of laymen. The relative ease of recording them makes them a convenient tool for physico-chemical research. 

---