Solid–Liquid Equilibria and Kinetics of the Solid Formation in Binary and Ternary Mixtures Containing (Formaldehyde + Water + Methanol)

Abstract

Solid precipitation from aqueous formaldehyde solutions is a major technical problem, leading to troubles such as plugging and fouling. While these problems are ubiquitous in formaldehyde technology, it is hard to predict them, so their mitigation is difficult. Data on the formation of a solid phase in formaldehyde-containing systems are scarce and basically only available for the system (formaldehyde + water); even these few data are contradictory. We have tackled these problems from different sides in the present work. First, a new method for measuring the solid–liquid equilibria and kinetics of solid formation in formaldehyde-containing systems was developed, and it was shown that, for technically relevant conditions, long-term effects are essential: it may take over 1000 days until equilibrium is reached. Using this new technique, reliable data for the solid–liquid equilibrium of the system (formaldehyde + water) were obtained. The liquidus line in the phase diagram of that system has two branches meeting in a eutectic point: one where the solid is water, and one where it is formaldehyde-rich. The formaldehyde solubility is found to be much lower than in most previous works. As methanol is often used for stabilizing aqueous formaldehyde solutions, we also investigated solid–liquid equilibria in the systems (formaldehyde + methanol) and (formaldehyde + water + methanol). Furthermore, data on the kinetics of the formation of formaldehyde-rich solids were acquired for the studied systems. Based on the new data and extensive previous work on the chemical equilibria and reaction kinetics, a physico-chemical model was developed, which describes both the solid–liquid equilibria as well as the kinetics of the solid formation. It is shown that the very slow kinetic effects in the studied systems are mainly caused by the interaction of the slow liquid phase reaction kinetics and the precipitation of a single oligomer of formaldehyde with water for which the solubility limit is reached.