Preparation of olefins

A process for preparing olefins from steam cracker or refinery C streams which comprises:

• A) optionally extractively distilling butadiene from the C.sub.4 stream using a butadiene-selective solvent to obtain a substantially butadiene-depleted C.sub.4 stream,
• B) selectively hydrogenating butadienes and acetylenic impurities in the steam cracker or refinery C.sub.4 stream or the stream from step A) with simultaneous or subsequent, at least partial isomerization of 1-butene to 2-butene in order to obtain a C.sub.4 stream which comprises n-butenes and i-butene and is essentially free of butadienes and acetylenic impurities,
• C) removing i-butene from the C.sub.4 stream obtained in step B) by reaction with an alcohol to form an ether which is separated off and, optionally, redissociated to give pure isobutene, resulting in a C.sub.4 stream which comprises n-butenes and, optionally, removing compounds which are lower boiling than the C.sub.4 stream and/or compounds which are higher boiling than the C.sub.4 stream by distillation during the removal of the ether,
• D) separating oxygen-containing impurities from the C.sub.4 stream obtained in step C) Using adsorber materials,
• E) performing a two-stage metathesis of the butenes in the C.sub.4 stream obtained in step D) by
• a) conversion of the 1-butene and 2-butene present in the C.sub.4 stream into propene and 2-pentene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group Vib, VIIb or VIII of the Periodic Table of the Elements,
• b) subsequent separation of the propene and 2-pentene formed and the unreacted butenes and, optionally, at least partial discharge of one or more of these compounds,
• c) subsequent reaction of the 2-pentene with ethene to form propene and 1-butene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group Vib, VIIb or VIII of the Periodic Table of the Elements,
• d) subsequent separation of the propene and 1-butene formed,
• e) optionally, at least partial discharge of a pure 1-butene stream and/or at least partial isomerization of the 1-butene formed to 2-butene in the present of an isomerization catalyst and subsequent recirculation of the 1-butene which has not been discharged and any 2-butene which was formed together with part of the C.sub.4 fraction which has not been converted in step a) back to step a),
• f) isolation of the propene separated off in steps b) and d).