PURPOSE: To improve the thermal recovery of a heat exchanger, and to increase the production per unit reactor, by hydrogenating an aromatic hydrocarbon, if necessary with hydrogen, introduced into a catalyst circulation system before the heat exchanger.
CONSTITUTION: An aromatic hydrocarbon, e.g. benzene, is introduced and hydrogenated in the catalyst circulation system before the heat exchanger (E), be means of a liquid-phase slurry reactor woherein a powder hydrogenation catalyst, e.g. Raney nickel, is suspended, or in a homogeneous liquid-phase reactor, e.g. a metal complex catalyst, if necessary with hydrogen introduced before E, to give a naphthenic hydrocarbon while recovering the heat of hydrogenation with E. The aromatic hydrocarbon may be introduced as a seal oil as well as via a pipeline.