Solubility of formaldehyde in mixtures of water + methanol + poly(oxymethylene) dimethyl ethers

Poly(oxymethylene)dimethyl ethers (OME) are a highly interesting group of oxygenates that could be used in the future as a sustainable and clean alternative to fossil diesel. Processes to produce them from formaldehyde and methanol have been developed. One remaining problem that may occur in these processes is the formation of a formaldehyde-rich solid, which has to be avoided. Nothing is known about the conditions under which this solid is formed. This gap of knowledge is closed in the present work. Based on previous fundamental work of our group on the solid-liquid equilibrium (SLE) in the system (formaldehyde + water + methanol), we have experimentally studied the SLE in the systems (formaldehyde + water + OME) and (formaldehyde + water + methanol + OME) at temperatures between 273 K and 303 K, for two distinct species: OME₂ and OME₄. The solid phase consisted basically of pure oligomeric formaldehyde. The system (formaldehyde + water + OME) shows a complex phase behavior due to the superposition of liquid-liquid equilibria (LLE) and SLE, resulting in a solid-liquid-liquid equilibrium (SLLE). As in our previous work, the formation of the solid was described with a physico-chemical model, which considers the reactions between formaldehyde, water, and methanol. Both the equilibrium and the slow kinetics of the process are described well by the model.