Liquid–Liquid Phase Behavior of Diesel/Biofuel Blends: Ternary Mixtures of n-Hexadecane, 2,2,4,4,6,8,8-Heptamethylnonane, and Ethanol and the Binary Subsystems

Abstract

Mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane are common diesel fuel surrogates. This paper describes a systematic investigation of the liquid–liquid phase behavior of the ternary mixtures of the normal alkane n-hexadecane and its isomer 2,2,4,4,6,8,8-heptamethylnonane (both CnH2n+2; n = 16) with ethanol as a model system of surrogate fuels. By applying the cloud point method in the temperature range of T = 240–330 K at ambient pressure (p = 1013 mbar), the complete liquid–liquid phase diagram is obtained. The binary mixtures of the alcohol with both isomeric alkanes show partial miscibility with upper critical solution points. The behavior of the ternary mixtures is examined by investigating the quasi-binary subsystems of mixtures of the alcohol with mixtures of the two isomers for various fixed mole ratios. By changing the ratio of the different isomers, the region of the phase diagram of the alcohol mixture with the n-alkane is gradually shifted toward lower temperatures when adding the branched isomer toward the phase diagram of the binary mixture of the alcohol with the isomeric compound. The general shape of the phase bodies of the pseudobinaries, predefined by the ratio of the isomeric components, shows only a slight change. For the numerical analysis of the measured phase behavior of the pseudobinary mixtures, concepts are used that are applied for binary solutions presuming Ising criticality. A master curve is obtained when applying the corresponding state principle on the binary and the pseudobinary mixtures. Superposition of the different results allows for a comprehensive description of the liquid–liquid phase behavior, and it estimates the loci of critical line and tie lines. The limits of the range of validity of the descriptions are determined by the appearance of solid phases, which are also reported here. This study is expected to establish a basis for optimizing blends of petrochemical fuels and fuels from renewable sources.