Below, we derive the governing equations for solid-bound reactive sites and liquid phase species absorbed in the film, respectively, starting from Eq. 13. Assuming reactive site concentrations within a control volume are homogeneous, we can integrate Eq. 13 for the solid part to obtain the governing equation for reactive sites in control volume i of swollen film on tray j (Eq. 14). We defne the control volume with respect to the solid-bound reactive sites and masses, such that all changes in solid-bound reactive sites and masses are due to liquid+solid reactions, as illustrated in Fig. .