hydrogenation - in refining, the chemical addition of hydrogen to a hydrocarbon in the presence of a catalyst; a severe form of hydrogen treating. Hydrogenation may be either destructive or non-destructive. In the former case, hydrocarbon chains are ruptured (cracked) and hydrogen is added where the breaks have occurred. In the latter, hydrogen is added to a molecule that is unsaturated (see unsaturated hydrocarbon) with respect to hydrogen. In either case, the resulting molecules are highly stable. Temperatures and pressures in the hydrogenation process are usually greater than in Hydrofining. See distillation.