A study of the liquid-vapor equilibrium of the formaldehyde-water-methanol ternary systems in the 22-92 °C range - analysis of components with absorption and Raman spectroscopy.

Formaldehyde (CH2O) is an important intermediate in the low-temperature combustion chemistry of hydrocarbon fuels and an undesired pollutant that is highly reactive even at room temperature. Due to its reactivity, it is often stored in solvents and chemical models have been developed to predict equilibrium liquid and vapor-phase compositions between CH2O and the surrounding liquids. Further understanding of such systems and development of models would require a more extensive experimental data set than currently available. The present work employed a combination of consecutive Absorption and Raman Spectroscopy measurements to obtain a CH2O Raman cross-section of 2.5 ∙ 10-30 cm2/sr of the CH-stretching vibration at 2780 cm-1 for a laser wavelength of 532 nm. This has been employed to acquire new data on vapor-phase compositions of various CH2O/CH3OH/H2O systems at temperatures up to 92 °C. Revisiting experimental conditions used for model development reported in the literature validated the present methodology. However, vapor compositions for new experimental conditions with higher CH2O concentrations in the liquid phase could not be predicted by the model. Raman spectra confirm the presence of CH2O polymers in the vapor phase, which the model does not account for. The new data thus provide input for further model development.