The shrinking process of the packed layer has been investigated. Filmy plastic and woody biomass are selected as the packing material. The volume of the layer reduces with time and then approaches an equilibrium value. For filmy mixed plastic layer, the shrinking rate constant can be estimated by the non-liner combination of the component of the pure plastic. On the other hand, for the woody biomass, the shrinking rate constant varies dramatically with the component of the lignin. It is found that the shrinking process of the plastic layer can be expressed by the similar analysis of first-order chemical reaction with the equilibrium in this study.