Part-stream distillation

A continuous process for preparing propylene oxide proceeds by (a) reacting propene with hydrogen peroxide in a reaction apparatus in the presence acetonitrile as solvent, obtaining a stream S0 containing propylene oxide, acetonitrile, water, at least one further component B; (b) separating propylene oxide from S0, obtaining stream S1 containing acetonitrile, water and B; (c) dividing S1 into streams S2 and S3; (d) subjects S3 to vapor-liquid fractionation in a first fractionation unit, obtaining vapor fraction stream S4a being depleted, relative to S3, of at least one of B and obtaining liquid bottoms stream S4b, and subjecting at least part of vapor fraction stream S4a to vapor-liquid fractionation in a second fractionation unit, obtaining vapor fraction stream S4c and liquid bottoms stream S4 being depleted, relative to S4a, of at least one of B; (e) recycling at least a portion of S4 to (a).