A Qualitative Discussion of Gas-Solid Reactions

Gas-solid reactions have several aspects in conunon with reactions catalyzed by a porous solid, discussed already in Chapter 3. In the present case too, transport eflects and reaction have to be considered simultaneously. Again it depends on the relative magnitudes of the rate of transport and the rate of reaction whether or not important gradients inside and around the particle are built up or not. There is one essential difference, however with gas-solid reactions the conditions inside the particle change with time, since the solid itself is involved in the reaction. The process is therefore essentially of a non-steady-state nature. 

In this chapter rate equations are set up for fluid-solid reactions. In Chapter 11 an example of such a reaction carried out in a fixed bed of particles is worked out and illustrates the diflererice in behavior of the reactor as compared with a fixed bed catalytic reactor. 

Concentrating now on the phenomena inside a particle, an easily visualized situation is that of a gas reacting with a solid of low porosity to yield a porous reacted layer, often called ash layer. The reaction then takes place in a narrow zone that moves progressively from the outer surface to the center of the particle. Such a situation is described by the so-called heterogeneous shrinking-core model heterogeneous because there are two distinct layers inside the particle, with clearly 

2 A General Model with Interfacial and Intraparticle Gradients  

When the particle is assumed to be isothermal only differential material balances have to be written. The differential balance on the reacting gaseous component A— the continuity equation for. 4—contains an accumulation term accounting for the transient nature of the process, a term arising from the transit by effective diffusion and a reaction term  

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For a basic understanding, start with Section 4.6.1, where the spectrum of factors influencing the effective rate of gas-solid reactions is discussed. Then study the border cases of non-porous and porous sohds (Sections 4.6.2.1 and 4.6.3.1) in both sections the systems will be analyzed only qualitatively without any mathematics and equations. 

Table 14.1-1 classifies various types of equipment for gas-liquid reactions. It is based on geometric aspects, the presence or absence a solid catalyst, and the flow directions. Before discussing model equations and specific design aspects the various types of reactors of Table 14.1-1 will be briefly characterized in a qualitative way. 

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