Abstract: Apparatus for the cracking of hydrocarbons in the presence of steam for the production of olefins and diolefins which includes a thermal radiation enclosure enclosing at least one cracking tube having an inlet to which the hydrocarbons and steam are supplied and having an outlet for the cracking products. The internal diameter of the tube diminishes continuously or discontinuously from the inlet to the outlet and the ratio between the internal diameters of the tube at the inlet and the outlet is between 1.2 and 3. The ratio of the length of the tube within the enclosure to the mean internal diameter of the tube is between 200 and 600. Heat is supplied to the tube by burners within the enclosure so that the thermal power applied along the length of the tube increases in the direction from the inlet to the outlet and so that the ratio between the thermal power applied to the half of the tube extending from the inlet to the thermal power applied to the remaining half of the tube is between 40/60 and 15/85.

Abstract: The present invention relates to a process for the preparation of olefins and diolefins by the cracking of hydrocarbons in the presence of steam, consisting in passing a mixture of hydrocarbons and steam flowing in a cracking tube disposed inside a radiation zone of a furnace. The process is characterized in that the mean dwell time of the mixture of flowing in the cracking tube between the inlet and the outlet of the radiation zone is from 300 to 1800 milliseconds, and the reaction volume is greater in the first half of the tube length than in the second one. The present invention relates also to a cracking furnace in which the ratio between the length and the mean diameter of the cracking tube is from 200 to 600, and the tube diameter decreases from the inlet to the outlet of the radiation zone.