Miscibility in Systems Containing (Poly(oxymethylene) Ethers (OME) + Hydrocarbons + Water)

Abstract

Poly(oxymethylene) dimethyl ethers (OME) are interesting synthetic fuels: they have better combustion properties than fossil diesel fuel and can be obtained from renewable resources. Furthermore, they can be blended with hydrogenated vegetable oils (HVO) to obtain a new class of environmentally benign fuels. However, it has been observed in technical processes that initially homogeneous mixtures of (OME + HVO) split into two liquid phases after some time, which is unacceptable for use as fuels. It is assumed that this de-mixing is a consequence of the uptake of small amounts of water from the air by the mixtures. However, up to now, only qualitative information on this phenomenon was available. We have, therefore, carried out systematic experimental studies of liquid–liquid equilibria (LLE) and liquid–liquid–liquid equilibria (LLLE) in ternary model systems of the type (OME + n-alkane + water) at temperatures between 265 and 293 K. The n-alkanes were (n-dodecane, n-hexadecane); two different OME were studied, OME2 and OME4, respectively. Additionally, also multicomponent mixtures containing different OME and mixtures containing additionally the aromatic hydrocarbon toluene were studied. The results were modeled using UNIFAC, a group-contribution model of the Gibbs excess energy, and a good correlation of the observed complex phase behavior was obtained. The results from the present work confirm that the uptake of small amounts of water is the reason for the phase split observed in technical (OME + HVO) mixtures. Measures to circumvent this problem are discussed.