To develop a new technology for the purification of formaldehyde from
formalin by a distillation method, the vapor-liquid equilibria of form
aldehyde water binary systems and the vapor-liquid equilibria of forma
ldehyde water systems in the presence of diluent were investigated. It
was shown by calculation that in some cases, when the concentration o
f diluent is high, the volatility of formaldehyde is always higher tha
n that of water and that these calculations are in good agreement with
experimental data. Based on this principle, the purification of forma
ldehyde by an extractive distillation method was established.