We use the consumption of the liquid-phase species by liquid+solid reactions to track how much mass has been added to the solid part of the film. The corresponding governing equation for the mass of solid in control volume i of tray j (m j,i ) is dm j,i dt = -M T w,liq S liq+solid,liq R liq+solid (15) where S liq+solid,liq is the stoichiometric matrix for liquid-phase species absorbed in the film (with a shape of the number of liquid-phase species by the number of liquid+solid reactions), and M w,liq is the molecular weight of the liquid-phase species. The negative sign is because when a liquid-phase species gets consumed by a liquid+solid reaction to form a covalent bond with the solid, the mass of the solid increases.