Solid catalytic

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. 

The ordinary burning of sulfur produces SO2. This is the hrst step in the manufacture of sulfuric acid. The second step oxidizes SO2 to SO3 in a gas-solid catalytic reactor. The catalyst increases the reaction rate but does not change the equilibrium compositions in the gas phase. 

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

In the vast majority of gas-solid reactions, gaseous or evaporated compounds react at the surface of a solid catalyst. These catalytic processes are very frequently used in the manufacture of bulk chemicals. They are much less popular in processing of the large molecules typical of fine chemistry. These molecules are usually thermally sensitive and as such they will at least partially decompose upon evaporation. Only thermally stable compounds can be dealt with in gas-solid catalytic processes. Examples in fine chemicals manufacture are gas-phase catalytic aminations of volatile aldehydes, alcohols, and ketones with ammonia, with hydrogen as... 

Tubular reactors are widely used in studying single-phase processes and gas-solid catalytic... 

Such processes assume that molecules from a fluid phase in contact with a solid catalytic surface combine chemically with catalyst surface molecules and reaction subsequently proceeds between chemisorbed molecules followed by desorption of the products. A large number of different rate equations with varying numbers of constants can be derived by making various auxiliary assumptions and tested against experimental rate data. Since a more or less plausible mechanism is postulated, the feeling is that a chosen rate equation is somewhat extrapolatable outside an experimental range with greater... 

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... 

The combination of base and alumina, in essence, represents a triphasic solid-liquid-solid catalytic system particularly when highly insoluble bases are used. While the results suggest that... 

On an industrial scale, chemical reactions involving reactants in the gaseous or vaporised state which come into contact with a solid, which is either a catalyst or another reactant, necessarily involve several physical and chemical steps. Some of these physical and chemical steps are coupled and it is this complexity, together with the problem of efficient gas—solid contact, which dictates that gas—solid catalytic and non-catcilytic reactions should be considered as a particular class of problem. 

As described previously, the reactant has to first reach the external surface of the catalyst. For simplicity, we take into consideration the case of a gas reacting on a solid catalytic surface. Owing to gas film resistance, the concentration of the reactant at the catalytic surface (Cs) is lower than that in the bulk of the fluid (Cb). This difference depends on... 

Figure 5.2 Concentration and temperature profiles for fluid —solid catalytic reactions.

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. 

The countercurrent-flow fixed-bed operation is often used for physical absorption or for gas-liquid reactions rather than gas-liquid-solid processes. Shah [1] gives a comparison between a gas-liquid-solid (catalytic) fixed bed reactor and a gas-liquid-solid (inert) fixed-bed reactor. The major difference between these two types of reactors are the nature and the size of the packing used and the conditions of gas and liquid flow-rates. 

G. Wild, F. Larachi and A. Laurent, The hydrodynamics characteristics of cocurrent downflow and cocurrent upflow gas-liquid-solid catalytic fixed bed reactors the effect of pressure, Revue de l lnstitut Franfais du Petrole, 46 (1991) 467-490. 

As the gas flow rate increases beyond that at minimum fluidization, the bed may continue to expand and remain homogeneous for a time. At a fairly definite velocity, however, bubbles begin to form. Further increases in flow rate distribute themselves between the dense and bubble phases in some ways that are not well correlated. Extensive bubbling is undesirable when intimate contading between phases is desired, as in drying processes or solid catalytic reactions. In order to permit bubble formation, the... 

Fig. 4. Substrates used with miscellaneous solid catalytic systems. The number of the modified system (see Table 4) and the best optical yield are given as (Nr/ee).

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 . 

Show that the effect of axial dispersion on the conversion obtained in a typical packed bed gas-solid catalytic reactor is small. As the starting point consider the following relationship (see Chapter 2, equation 2.30). 

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 ... 

Gas solid catalytic systems are two-phase systems involving a solid catalyst with reactants and products that occur in the gas phase. Most gas-solid catalytic systems are continuous rather than multistage systems. 

Notice that the units of r and V depend upon the type of the process. For example, if we are dealing with a gas-solid catalytic system, we will usually define r as per unit mass of the catalyst and replace V with Ws which is the weight of the catalyst. 

The book by Elnashaie and Elshishni on the chaotic behavior of gas-solid catalytic systems [101]. 

S. Elnashaie, S. Elshishini, Dynamic Modeling, Bifurcation and Chaotic Behaviour of Gas-Solid Catalytic Reactors, Gordon and Breach, 1996, 646 p... 

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