| Summary: | For a gas diffusing through a porous solid and reacting with it isothermally and irreversibly, the mathematical formulation consists of a nonlinear parabolic PDE for the gas concentration coupled with an ODE for the solid concentration. Under the assumption of constant porosity, a fairly complete analysis was provided by Diaz and Stakgold, [3]. Here some of the results are extended to the case when the porosity increases as the solid is consumed. In particular, estimates are given for the time to full conversion of the solid when the reaction rate is proportional to the product of the gas concentration and a fractional power of the solid concentration.
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