Process and apparatus for preparing saturated carboxylic acids having one to four carbon atoms

A process for preparing saturated carboxylic acids having from one to four carbon atoms comprising

• gas-phase oxidizing in a reactor at a reaction temperature of from 100° C. to 400° C. and at a pressure of from 1.2×105 to 51×105 Pa of a C4-hydrocarbon selected from the group consisting of a saturated C4-hydrocarbon, an unsaturated C4-hydrocarbon and mixtures thereof, an oxygen-containing gas and water vapor and in the presence of at least one catalyst to produce a reactor outlet gas;
• reducing acid concentration in a recirculated portion of said reactor outlet gas by means of separating crude acid from said reactor outlet gas by using a countercurrent scrub;
• and
• recirculating in a reaction gas circuit part of said reactor outlet gas from said countercurrent scrub.