Control of the viscosity of reaction solutions in hydroformylation processes

A method for controlling the viscosity of reaction solutions in the hydroformylation of olefin-containing mixtures, the method comprising:

• i) contacting a mixture comprising saturated and olefinically unsaturated hydrocarbons, a composition which is catalytically active in hydroformylation, a gas mixture comprising carbon monoxide and hydrogen, and at least one solvent, in at least one reaction zone;
• ii) separating off products, with a gas stream comprising a mixture of saturated and olefinically unsaturated hydrocarbons, carbon monoxide and hydrogen being introduced into the at least one reaction zone, with the proviso that the products are discharged via a gas phase from the reaction zone;
• iii) condensing the products which have been separated off via the gas phase and passing them to further work-up;
• and
• iv) passing a bottom stream from at least one reaction zone to at least one membrane filtration,
• wherein the composition which is catalytically active in hydroformylation is retained in a retentate and recirculated to the at least one reaction zone and degradation products of the composition which are catalytically active in hydroformylation are removed via a permeate.