Reactions of Porous Particles

Strictly speaking, the validity of the shrinking unreacted core model is limited to those fluid-solid reactions where the reactant solid is nonporous and the reaction occurs at a well-defined, sharp reaction interface. Because of the simplicity of the model it is tempting to attempt to apply it to reactions involving porous solids also, but this can lead to incorrect analyses of experimental data. In a porous solid the chemical reaction occurs over a diffuse zone rather than at a sharp interface, and the model can be made use of only in the case of diffusion-controlled reactions. 

If the reactant solid is porous, the reactant fluid would diffuse into it while reacting with it on its path diffusion and chemical reaction would occur in parallel over a diffuse zone. The analysis of such a reaction system is normally more complex as compared to reaction systems involving nonporous solids. Here also it is important to assess the relative importance of chemical reaction kinetics and of mass and heat transport. 

The dissolution of porous minerals, the combustion of porous carbon, the reaction between porous carbon and carbon dioxide, and the formation of nickel carbonyl from pure nickel are some examples of fluid-solid reactions where the reactant solid is porous and where no solid reaction product is formed. A reaction of this type can be represented as 

A reaction in which a solid product layer is formed can be represented as 

The reactant solid B is porous and the reaction occurs in a diffuse zone. If the rate of the chemical reaction is much slower compared to the rate of diffusion in the pores, the concentration of the fluid reactant would be uniform throughout the pellet and the reaction would occur at a uniform rate. On the other hand, if the chemical reaction rate is much faster than the pore diffusion rate, the reaction occurs in a thin layer between the unreacted and the completely reacted regions. The thickness of the completely reacted layer would increase with the progress of the reaction and this layer would grow towards the interior of the pellet). 

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From 1996 to the present, many stationary phases have been used. For the separation of sorbitol from other mono- and disaccharides, stationary phases prepared by the reaction of porous particles of chloromethylated styrene-divinylbezene copolymer with various amines are preferred. Because of subtle differences in the pAa values of hydroxyl groups, the oxyanions of carbohydrates generated under alkaline conditions interact with the positively charged stationary phase and facilitating their separation by the anion-exchange mechanism. 

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