To develop a new technology for the purification of formaldehyde from formalin by a distillation method, the vapor-liquid equilibrium of formaldehyde—water binary systems was investigated. Based on the studies of Thompson et al. and Iliceto et al., the vaporliquid equilibrium of formaldehyde and water are calculated and these calculations are in good agreement with the reported data. It was found that the vapor pressure of formaldehyde is determined by the free formaldehyde concentration (mol/L), whereas the vapor pressure of water is determined by the molar ratio of free water. The vapor-liquid equilibrium of formaldehyde-water in the presence of a diluent was also investigated. It was shown by calculation that in some cases, when the concentration of the diluent is high, the volatility of formaldehyde is always higher than that of water and these calculations are in good agreement with experimental data. Based on this principle, the purification of formaldehyde by an extraction distillation method was established. © 1993 John Wiley & Sons, Inc.
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