Production of high-purity isobutene and propylene from hydrocarbon fractions with four carbon atoms

A process for converting an olefinic C4 fraction into high-purity isobutene and into propylene, where said fraction contains diolefins, butene-1, butenes-2, isobutene and acetylenic impurities, and where said process comprises the following successive stages:

• (1) subjecting the olefinic C4 fraction to selective hydrogenation, in the presence of a catalyst, to selectively hydrogenate diolefins and acetylenic impurities and isomerize butene-1 into butenes-2, to obtain a hydrogenation effluent that contains n-butenes in a ratio corresponding to thermodynamic equilibrium, and isobutene, and that contains virtually no diolefins or acetylenic compounds;
• (2) separating by distillation the effluent from stage (1) into a top fraction that contains isobutene and a bottom fraction that contains essentially butenes-2 and butane;
• (3) subjecting the bottom fraction from stage (2) to metathesis with ethylene, in the presence of a catalyst, to obtain a metathesis effluent containing propylene;
• and
• (4) separating propylene from the metathesis effluent;
• wherein stage (2) is conducted in a distillation column that integrates hydroisomerization of butene-1, not already isomerized in stage (1), to butenes-2 and which provides a top fraction of substantially pure isobutene.