person skilled in the art also knows that if a highly exothermic reaction is performed in a piston reactor the radial heat transfer may become limiting for the hydrogenation of the benzene derivatives which involves the saturation of at least three conjugated double bonds the control of heat transfers is therefore critical the method according to the invention involves a chemical reaction of type in equation 2 u03bd is the stoichiometric coefficient of the gas and u03bd p is the stoichiometric coefficient of the product in accordance with the invention the gas b is hydrogen and the benzene derivative to be hydrogenated is in the form of a pure liquid or a liquid diluted in a liquid solvent or in the form of a solid diluted in a liquid solvent in general the performance of the reactors are given by two characteristic quantities which respectively describe the heat transfer and the mass transfer these characteristic transfer times are defined below by simplified equations the hydrodynamic model is the same whether the reactor is a piston reactor or a fully agitated reactor insofar as the hydrogenation reaction is limited by the mass transfer t therm u03c1 ue89e ue89e c p ue89e v liq k ue89e ue89e s equation ue89e ue89e 3 the characteristic mass transfer time t mat 1 k l a equation 4 in these equations the following parameters are used the density of the liquid u03c1 the heat capacity of the liquid c p the overall transfer coeff i cient u03ba defined below the heat e x change surface s const a nt for a given reactor beca u se it is fixed by the design thereof the product between the gas l iquid m ass transfer coefficient on t he liquid side k l and the.