Optimized thermal separation by prior gas exchange

A process for preparing an aldehyde, the process comprising at least:

• a) reacting, in a hydroformylation, at least one C2 to C20 olefin with synthesis gas in the presence of a homogeneously dissolved catalyst system comprising at least one metal from the group consisting of Co and Rh, and a phosphorus ligand;
• wherein the hydroformylation is conducted in at least one reaction zone and at a pressure of 10 to 350 bar absolute, which affords a first liquid reaction output containing at least the aldehyde and unconverted olefins, and wherein a partial pressure of CO or H2 accounts for at most 70% of a total gas pressure of all gaseous substances present;
• b) performing a gas exchange with the first liquid reaction output, which affords a second liquid reaction output in which the partial pressure of CO or H2 accounts for more than 90% of the total gas pressure;
• c) expanding the second liquid reaction output obtained from b) in a single-stage or multistage expansion, to a pressure of less than 1 bar absolute, which affords a third liquid reaction output;
• and
• d) feeding the third liquid reaction output obtained from c) to a thermal separation in order to separate the third liquid reaction output into a low-boiling fraction and a high-boiling fraction, with the low-boiling fraction containing at least a portion of the aldehyde.