A Moving Boundary Problem The Shrinking Core Model

Illustration 4.10 A Moving Boundary Problem The Shrinking Core Model 

The reader had previously been introduced to the concepts that must be applied in modeling the progress of a gas-solid reaction. They involve separating the system into a core, which reacts and continuously shrinks in size, and an external layer, which grows with time but is assumed to be at a quasisteady state. This configuration is referred to as the shrinking core model and has associated with it the assumption of a quasi- or pseudo-steady state. 

We illustrate these concepts with the following example Suppose a solid particle xmdergoes a reaction according to the scheme 

We start, as recommended, with an imsteady core balance and write 

This is followed by a quasi-steady-state mass balance in the product layer, which is composed as follows  

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