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.

Abstract: This invention is addressed to catalysts for use in the polymerization of olefins comprising compounds of magnesium and one or more transition metals of groups IVa, Va and VIa, which are at least partially reduced, with the catalysts being prepared by reaction of magnesium, at least one mono-halogenated hydrocarbon and a compound of a transition metal having a valence of at least 4. The catalysts can be used for the polymerization of olefins, such as ethylene, in the low-pressure process.