Solid—Gas Reactions

In this chapter a number of reactions are discussed in which the rate-determining step occurs in the solid state, and the solid is chemically changed by the 

A new solid phase is formed therefore, this kind of reaction involves the whole of the processes of surface nucleation and growth (see Chapter 10). The gas phase that constitutes one of the reactants forms obviously a continuum around the grains of solid 1. 

From the experimental point of view, all these reactions are accompanied by a variation of the mass of the condensed phases. The thermogravimetric method will be the best method to follow the extent of the reaction. 

We will illustrate with some examples thus, we will study successively the processes of growth of  

We will generally assume pseudo-steady state modes and usually in the case of a separable rate. The space function is thus given as in Chapter 10, since all the assumptions are retained. For this reason, we will concentrate in this chapter primarily on the mechanisms and reactivities. 

We will distinguish two classes for gas-solid reactions synthesis reactions and double-decomposition reactions that are characterized by a gaseous emission. 

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Permeability Bulk sohd permeability is important in the iron and steel industiy where gas-solid reactions occur in the sinter plant and blast furnace. It also strongly influences compac tion processes where entrapped gas can impede compaction, and solids-handling equipment where restricted gas flow can impede product flowabihty. The permeabihty of a granular bed is inferred from measured pressure drop under controlled gas-flow conditions. 

A. K. Galwey, Reactions in the Sohd State, in Bamford and Tipper, eds.. Comprehensive Chemical Kinetics, vol. 22, Elsevier, 1980. Galwey, A. K., Chemistry of Solids, Chapman and Hall, 1967. Sohn, H. Y, and W. E. Wadsworth, eds.. Rate Frocesses of Extractive Metallurgy, Plenum Press, 1979. Szekely, J., J. W. Evans, and H. Y. Sohn, Gas-Solid Reactions, Academic Press, 1976. Uhmann, ed., Enzyklopaedie der technischen Chemie, Uncatalyzed Reactions with Solids, vol.. 3, 4th ed., Verlag Chemie, 1973, pp. 395-464. 

In most circumstances, it can be assumed diat die gas-solid reaction proceeds more rapidly diaii die gaseous transport, and dierefore diat local equilibrium exists between die solid and gaseous components at die source and sink. This implies diat die extent and direction of die transport reaction at each end of die temperature gradient may be assessed solely from diermodynamic data, and diat die rate of uansport across die interface between die gas and die solid phases, at bodi reactant and product sites, is not rate-determining. Transport of die gaseous species between die source of atoms and die sink where deposition takes place is die rate-determining process. 

The value of tire heat transfer coefficient of die gas is dependent on die rate of flow of the gas, and on whether the gas is in streamline or turbulent flow. This factor depends on the flow rate of tire gas and on physical properties of the gas, namely the density and viscosity. In the application of models of chemical reactors in which gas-solid reactions are caiTied out, it is useful to define a dimensionless number criterion which can be used to determine the state of flow of the gas no matter what the physical dimensions of the reactor and its solid content. Such a criterion which is used is the Reynolds number of the gas. For example, the characteristic length in tire definition of this number when a gas is flowing along a mbe is the diameter of the tube. The value of the Reynolds number when the gas is in streamline, or linear flow, is less than about 2000, and above this number the gas is in mrbulent flow. For the flow... 

Perhaps the most common use for REELS is to monitor gas—solid reactions that produce surface films at a total coverage of less than a few monolayers. When Eq is a few hundred eV, the surface sensitivity of REELS is such that over 90% of the signal originates in the topmost monolayer of the sample. A particularly powerfiil application in this case involves the determination of whether a single phase of variable composition occurs on the top layer or whether islands occur that is, whether... 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

Hydrogen-absorbing alloy can reversible absorb and desorb a large amount of hydrogen. Hydrogen gas is rapidly absorbed in the gas phase, then desorbed on the alloy (gas-solid reaction). In the electrode reaction, the alloy electro-chemically absorbs and desorbs hydrogen in an alkaline solution (electrochemical reaction) ... 

When a reactive atmosphere is present [402,1249], the reaction undergone by a solid may be changed, and perhaps complicated, by the presence of a concurrent or consecutive gas—solid reaction. When one or more of the products of the reaction in vacuum is an oxidizable metal [60] or... 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

A recent second edition of another standard text that emphasizes gas-solid reactions is... 

The rates of gas—solid reactions are surface area dependent, so finely divided metals, coal etc. may be prone to oxidation leading to spontaneous combustion. 

The rates of gas-solid reactions are surface area dependent, so finely- vided metals, coal etc. may be prone to oxidation leading to spontaneous combustion. A combustible dust will burn much more rapidly than the bulk sold, and if dispersed in air cause a dust explosion (refer to Table 6.2). 

The direct bromination of aniline and phenol derivatives in solution results in polybromination to give a complex mixture. But bromination by gas-solid and solid-solid reactions proceeds more efficiently and selectively. Bromination by gas-solid reaction can be accomplished through a very simple procedure. For example, the powdered crystalline aniline 13 and Br2 were placed in... 

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... 

The most widely used reactors for gas-solid reactions in fine chemistry are fixed-bed tubular... 

Reactions among solids occurring through gaseous intermediates can be viewed as coupled gas-solid reactions, and thus can be analyzed in the light of the discussions presented in the preceding sections regarding fluid-solid reactions. 

J. Szekely, J. W. Evans and H. Y. Sohn, Gas-Solid Reactions, Academic Press, New York, 1976. 

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