PROCESS FOR PREPARING 1,3-BUTADIENE FROM N-BUTENES BY OXIDATIVE DEHYDROGENATION

The invention relates to a process for preparing butadiene from n-butenes, comprising the steps of: A) providing an input gas stream a comprising n-butenes; B) feeding the input gas stream a comprising n-butenes and a gas containing at least oxygen into at least one oxidative dehydrogenation zone and oxidatively dehydrogenating n-butenes to butadiene, giving a product gas stream b comprising butadiene, unconverted n-butenes, water vapor, oxygen, low-boiling hydrocarbons and high-boiling secondary components, with or without carbon oxides and with or without inert gases; Ca) cooling the product gas stream b by contacting with a circulating cooling medium in at least one cooling zone, the cooling medium being at least partly recycled and having an aqueous phase and an organic phase comprising an organic solvent; Cb) compressing the cooled product gas stream b which may have been depleted of high-boiling secondary components in at least one compression stage, giving at least one aqueous condensate stream c1 and one gas stream c2 comprising butadiene, n-butenes, water vapor, oxygen and low-boiling hydrocarbons, with or without carbon oxides and with or without inert gases;D) removing uncondensable and low-boiling gas constituents comprising oxygen and low-boiling hydrocarbons, with or without carbon oxides and with or without inert gases, as gas stream d2 from the gas stream c2 by absorbing the C4 hydrocarbons comprising butadiene and n-butenes in an absorbent, giving an absorbent stream laden with C4 hydrocarbons and the gas stream d2, and then desorbing the C4 hydrocarbons from the laden absorbent stream, giving a C4 product gas stream d1; E) separating the C4 product stream d1 by extractive distillation with a butadiene- selective solvent into a stream e1 comprising butadiene and the selective solvent and a stream e2 comprising n-butenes; F) distilling the stream e1 comprising butadiene and the selective solvent into a stream f1 consisting essentially of the selective solvent and a stream f2 comprising butadiene; which comprises G) removing a portion of the aqueous phase of the cooling medium which circulates in step Ca) and has an aqueous phase and an organic phase as aqueous purge stream g; H) distillatively separating the aqueous purge stream g into a fraction h1 enriched in organic constituents and a fraction h2 depleted of organic constituents.