Gas-solid catalytic reaction

Fluidized bed noncatalytic reactors. Fluidized heds are also suited to gas-solid noncatalytic reactions. All the advantages described earlier for gas-solid catalytic reactions apply. As an example. 

Heterogeneous combustion, 7 449-454 Heterogeneous copolymerization of acrylonitrile, 11 203—204 with VDC, 25 698-699 Heterogeneous enzyme systems, 10 255-256 Heterogeneous gas-solid catalytic reactions, 21 340-341 Heterogeneous Ideal Adsorbed Solution Theory (HIAST), gas separation under, 1 628, 629... 

It has to be noted that for gas-solid catalytic reactions, the above equations can be also written in terms of pressure instead of concentration. 

Scott, R. P. and Watts, P. (1981). Kinetic considerations of mass transport in heterogeneous, gas-solid catalytic reactions. J. Phys. E Sci. Instrum., 14, 1009-13. 

For most gas-solid catalytic reactions, usually a rate equation corresponding to one form or another of the Hougen and Watson type described above can be found to fit the experimental data by a suitable choice of the constants that appear in the adsorption and driving force terms. The following examples have been chosen to illustrate this type of rate equation. However, there are some industrially important reactions for which rate equations of other forms have been found to be more appropriate, of particular importance being ammonia synthesis and sulphur dioxide oxidation 42 . 

All of the previously mentioned nonlinearities are actually monotonic. Nonmonotonic functions are very common in gas-solid catalytic reactions due to competition between two reactants for the same active sites, and also in biological systems, such as in substrate inhibited reactions for enzyme catalyzed reactions and some reactions catalyzed by microorganisms. The microorganism problem is further complicated in a nonlinear manner due to the growth of the microorganisms themselves. 

We will follow the first path and build on the following physically justified details regarding the relation between Ca and Cas- We consider the chemisorption step of the gas-solid catalytic reaction A —> D as follows ... 

In gas-solid catalytic reactions, the question of whether or not all of the catalyst is uniformly involved in the reaction in a rotating basket reactor was addressed by Smith (1970). He showed that when the reactor had no baffles,... 

In heterogeneous reaction systems, the rate of reaction is usually expressed in measures other than volume, such as reaction surface area or catalyst weight. Thus for a gas-solid catalytic reaction, the dimensions of this rate, are the number of moles of reacted per unit time per unit mass of catalyst (mol/s-g catalyst). Most of the introductory discussions on chemical reaction engineering in this book focus on homogeneous systems. 

Dispersion in Packed Beds. For the case of gas-solid catalytic reactions which take place in packed-bed reactors, the dispersion coefficient, can be esti-... 

A general transient model of diffusion-reaction that uses the effective diffusivity concept described for gas-solid catalytic reactions can be derived here as well, e.g., for a spherical particle ... 

This is a different kind of heterogeneous reaction—a gas-solid noncatalytic one. Let us examine the process at initial conditions (t 0), so that there has been no opportunity for a layer of UF4.(5) to be formed around the UO pellet The process is much like that for gas-solid catalytic reactions. Hydrogen fluoride gas is transferred from the bulk gas to the surface of the UO2 pellets and reacts at the pellet-gas interface, and H2O diffuses out into the bulk gas. If the pellet is nonporous, all the reaction occurs at the outer surface of the UO2 pellet, and only an external transport process is possible. Costa studied this system by suspending spherical pellets 2 cm in diameter in a stirred-tank reactor. In one run, at a bulk-gas temperature of 377°C, the surface temperature was 462°C and the observed rate was — Tuo = 6.9 x 10 mole U02/(sec) (cm reaction surface). At these conditions the concentrations of... 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

A gas-solid catalytic reaction of the type A + B Cis believed to occur by a molecule of B in the gas phase reacting with a molecule of A adsorbed on the catalyst surface. Preliminary experimental studies indicate that both external and internal diffusional resistances are negligible. Also, the equilibrium constant for the homogeneous reaction K = (pjpAP g is very large. 

Two gas-solid catalytic reactions, (1) and (2), are studied in fixed-bed reactors. Rates of reaction per unit mass of catalyst, at constant composition and total pressure, indicate the variations with mass velocity and temperature shown in the figure. The interior pore surface in each case is fully effective. What do the results shown suggest about the two reactions ... 

In a study of the kinetics of two different gas-solid catalytic reactions it is found that all diffusional resistances are negligible. Also, both reactions are irreversible. As an aid in establishing the mechanism of the reactions the rate is measured at a constant composition over a wide range of temperature. For the first reaction, (1), the rate increases exponentially over the complete temperature range. For the second reaction, (2), the rate first increases and then decreases as the temperature continues to rise. What does this information mean with regard to the controlling step in each of the reactions ... 

Case Study 11.3 Gas-Solid (Catalytic) Reaction Modeling of a Complex... 

Many industrially important reactions are characterized by an interface across which heat/mass transfer occurs. They are called heterogeneous reactions. They include fluid-fluid reactions, gas-solid catalytic reactions, gas-solid noncatalytic reactions, and solid-solid reactions. 

Gas-solid (catalytic) reactions. Mass transfer is likely to be more important within the pellet than in the external film, and heat transfer more important in the film than within the pellet. In other words, intraphase mass transfer and interphase heat transfer would normally be the dominant transport processes. Thus the pellet can reasonably be assumed to be isothermal. 

Criteria for the Absence of Mass and Heat Transfer Effects in Gas-Solid Catalytic Reactions (Doraiswamy, 2001) ... 

FIGURE 11.7 Some useful laboratory reactors for obtaining precise kinetic data for gas-solid catalytic reactions. 

Laboratory Reactors for Gas-Solid Catalytic Reactions Their Principal Features and Ratings... 

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