Porous Solids of Unchanging Overall Sizes

In general, one may say that at low temperatures and for small particles the system tends to region I, while at high temperatures mass transfer and pore diffusion become important, causing the system to tend to regime II or III. 

The above classification was developed for isothermal systems. However, nonisothermal behavior would not alter the basic nature of these three mechanisms, although the existence of temperature gradients within the solid could cause additional complications. A discussion of some of these effects is also given in the text. 

In many gas-solid reactions encountered in chemical and metallurgical processes, solid products are formed. Such reactions may be described by the general reaction equation 

The overall volume of solid may increase or decrease depending on the relative density of the solid product compared with that of the solid reactant. In most cases, however, the change is rather small so that the overall size may be regarded as constant. 

Examples of such a system are the regeneration of coked catalysts, the reduction of some metal oxides, and the roasting of sulfide ores. 

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POROUS SOLIDS OF UNCHANGING OVERALL SIZES 135 diffusion into the pellet ... 

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