KINETICS OF THE POLY(OXYMETHYLENE) GLYCOL FORMATION IN AQUEOUS FORMALDEHYDE SOLUTIONS

The absorption of gaseous formaldehyde in water is a widely used process. In aqueous solutions formaldehyde is mainly dissolved chemically. Therefore, designing absorption equipment for formaldehyde gas requires not only the knowledge of the vapor-liquid equilibrium but also information on reaction kinetics. While reliable procedures for describing the vapor-liquid equilibrium are available, very little is known on the kinetics of the formation of formaldehyde reaction products, e.g., poly(oxymethylene) glycols, in aqueous solutions. In this work rate constants of the formation of poly(oxymethylene) glycols are determined by measuring the density changes of formaldehyde solutions after their dilution with water. The measurements cover the temperature range from 275 to 313 K at pH numbers from 1.5 to 6.5. For the interpretation of the data, a simple group contribution model for the density is combined with second-order reaction kinetics. The rate constant k strongly depends on temperature and pH. At room temperature the observed minimal value of 1/k is about 8 min; at 275 K it is 40 min. These numbers are larger than the residence time in typical absorption equipment, which demonstrates that it is crucial to consider reaction kinetics in the design. The data determined in this work allow reasonable estimates of reaction kinetic effects. The results obtained from the density measurements are compared to older, unpublished NMR spectroscopic data. Qualitative agreement is observed.