Equilibrium Partitioning of Di-ethylene Glycol Monomethyl Ether (DiEGME) between Fuel and Aqueous Phases at Sub-Ambient Temperatures

Improved understanding of the effect of temperature and concentration on the equilibrium partitioning of Fuel System Icing Inhibitor (FSII) additive between fuel and aqueous phases can assist in identifying required dose concentrations for safe aircraft operability. A novel experimental system was designed and used to quantify the equilibrium partitioning of the currently approved FSII, di-ethylene glycol monomethyl ether (DiEGME), under conditions relevant to actual aircraft fuel system operation. This included temperatures from ambient to -47 degrees C, total water contents from 130 to 560 ppmV, and initial FSII concentrations from 100 to 1500 ppmV. The partitioning of DiEGME was a strong function of temperature, exhibiting nonideal solution behavior. For a constant temperature, the resulting phase partitioning was independent of initial FSII and total water concentrations, with a single equilibrium correlation established. FSII partitioning into the aqueous phase increased with both decreasing temperature and initial FSII dose concentration in the fuel. The overall behavior was attributed to hydrophilic interactions between the glycol ether and water, which become more favored at lower temperatures and concentrations. The behavior is consistent with that expected based on the effect of temperature and concentration on the corresponding FSII activity coefficients in each phase, and has previously been observed for analogous glycol ethers. Based on the partitioning behavior, very low concentrations of FSII are expected to be sufficient to prevent water solidification to temperatures below the specification freeze point of the fuel.