This document is applicable to the low-boiling mixed liquid with propionic acid as the main component obtained from the top of the light removal tower in the process of producing propionic acid through oxidation and distillation of propionaldehyde as raw material, which is used as raw material for the production of mixed esters. Molecular formula of propionic acid: C3H6O2 Relative molecular mass of propionic acid: 74.09 (according to the 2016 international relative atomic mass)