Process for upgrading olefinic C.sub.4 cuts

A process for upgrading a dried C.sub.4 olefinic cut, obtained from a cracking unit or a steam-cracking unit, substantially freed of butadiene, and dried, said process comprising the steps of:

• (a) feeding the dried C.sub.4 cut to a selective catalytic polymerization zone in the presence of a polymerization catalyst, said catalyst being a fluorinated alumina, a boron-alumina or a silica-alumina, and converting at least 95% of the isobutene of said olefinic cut, in major part to isobutene dimers and trimers, while limiting the overall conversions of the normal butenes of said starting olefinic cut to not over 3% by weight, the butane and isobutane of said olefinic cut being substantially unconverted;
• (b) feeding the effluent from the polymerization zone to a hydroisomerization zone, in the presence of hydrogen and an isomerization catalyst comprising at least one Group VIII metal deposited on a carrier, and isomerizing the polymerization effluent under such conditions that at least 90% of the 1-butene of said polymerization effluent is isomerized to 2-butenes, the percentages of the other constituents of said effluent remaining substantially unchanged, the proportions of the normal butenes in the hydroisomerization effluent at the end of the isomerization reaction being at least 92% by weight of 2-butenes and less than 8% by weight of 1-butene;
• (c) fractionating the effluent from the isomerization zone, and separately recovering a bottoms poly gasoline fraction comprising, in major part, isobutene dimers and trimers, and an overhead fraction comprising, in major part, isobutane, butane and butenes;
• and
• (d) feeding the overhead fraction to an alkylation zone, fractionating the alkylation effluent, and separately recovering an overhead fraction of high saturated C.sub.4 hydrocarbon content, and an alkylate.