Process for obtaining isobutene from C.sub.4 -hydrocarbon mixtures containing isobutene

A process for obtaining isobutene from a C.sub.4 -hydrocarbon mixture containing isobutene, which comprises

• (a) reacting the mixture with a primary C.sub.3 - or C.sub.4 -alcohol in the presence of an ion exchanger in its acid form as a condensing agent to form the corresponding C.sub.3 - or C.sub.4 -alkyl tert-butyl ether by feeding the primary C.sub.3 - or C.sub.4 -alcohol and the C.sub.4 -hydrocarbon mixture, with or without prior mixing, to an etherification reaction zone which contains the ion exchanger;
• (b) distilling the reaction mixture obtained from the etherification reaction zone in a first distillation zone, taking off as the top product without water washing a C.sub.4 -hydrocarbon mixture comprising the unconverted hydrocarbons and not more than 1,000 ppm by weight of the primary C.sub.3 - or C.sub.4 -alcohol and taking off as the bottom product the resulting C.sub.3 - or C.sub.4 -alkyl tert-butyl ether, which may contain therein primary C.sub.3 - or C.sub.4 -alcohol which may have been added in excess;
• (c) feeding the bottom product to a second reaction zone, containing an acid catalyst, in which the C.sub.3 - or C.sub.4 -alkyl tert-butyl ether is decomposed at an elevated temperature to give isobutene and primary C.sub.3 - and C.sub.4 -alcohol;
• (d) feeding the mixture of isobutene and primary C.sub.3 - and C.sub.4 -alcohol produced in step (c) to a second distillation zone, taking off as the top product without a water wash isobutene containing not more than 500 ppm by weight of primary C.sub.3 - or C.sub.4 -alcohol and taking off as the bottom product the remaining primary C.sub.3 - or C.sub.4 - alcohol produced in step (c);
• and
• (e) Recycling the primary C.sub.3 - or C.sub.4 -alcohol which is the bottom product of step (d) to the etherification reaction zone.