A method for processing lignin may comprise flowing a lignin composition comprising a lignin polymer and a solvent through a reaction chamber of a continuous flow reactor, the lignin polymer comprising hydroxycinnamic groups bound to a polymeric backbone; flowing ozone through the reaction chamber containing the lignin composition under conditions to maximize oxidative cleavage of the hydroxycinnamic groups to produce one or more types of aromatic monomers while minimizing oxidative cleavage of the polymeric backbone; and collecting the one or more types of aromatic monomers, e.g., by a size-selective membrane separation device.