Hydrogenation of unsaturated organic compounds represents one of the fundamental processes in organic technology, petrochemistry and in the area of special chemical syntheses. This process replaces formerly employed reduction by chemical reagents, generally accompanied by the formation of waste products. The replacement of reagents with hydrogen gas represents an important contribution to environmental protection. The reaction between hydrogen and an unsaturated substrate necessitates the presence of an active metallic catalyst to enable and accelerate the reaction in the desired way. Where the substrate features either a high boiling point or thermal lability the reaction is conveniently conducted in the liquid phase. In the hydrogenation reactor there are three phases present at the same time, i.e. the gaseous phase (hydrogen), the liquid phase (reaction substrate) and the solid catalyst. For conducting the process on an industrial scale, a trickle bed reactor featuring a very low axial intermixing of the flowing reaction mixture is recommended as this design increases process selectivity and the product need not be separated from the catalyst.