Low‐temperature phase behavior of vegetable oil/co‐solvent blends as alternative diesel fuel

Abstract

AbstractVegetable oils (triacylglycerols) have many characteristics that make them attractive candidates as renewable alternative fuels for compression‐ignition (diesel) engines. Unfortunately, vegetable oils are too viscous to be compatible with modern direct‐injection diesel fuel systems and engines. Co‐solvent blending is a simple and flexible technology that reduces viscosity by mixing the oil with low molecular weight alcohol. A co‐solvent (A), consisting, of surfactant plus an amphiphilic compound, is added to solubilize otherwise nearly immiscible oil‐alcohol mixtures into a single‐layer (isotropic) solution. This work examines low‐temperature phase behavior of two soybean oil (SBO)/methanol mixtures solubilized by A=unsaturated long‐chain (C18) fatty alcohol/medium‐chain alkanol (n‐butanol and 2‐octanol), one SBO/methanol mixture solubilized by A=triethylammonium linoleate/2‐octanol, and one SBO/95 wt% ethanol (E95) mixture solubilized by n‐butanol. The E95‐blend was further blended in 1∶1 (vol/vol) mixtures with No. 2 diesel fuel. Two types of anisotropic phase behavior were observed; formation of a cloudy layer of solid crystals suspended in bulk solution (Type 1) and formation of two immiscible liquid layers (Type II). The type of phase separation in a given solution was influenced by phase separation temperature (Tϕ) relative to the crystallization characteristics of compounds in the SBO and fatty alcohol or amine constituents present in solution. Solutions with relatively low Tϕ values experienced crystallization of small solid particles favoring Type 1 separations. Conversely, solutions with Tϕ sufficient to avert crystallization of high melting point compounds favored Type II separations where Tϕ=critical solution temperature (Tcritical). Increasing the A/oil (SBO or No. 2 diesel/SBO mixture) mass ratio decreased Tϕ while increasing the mass fraction of alcohol (methanol or E95) increased Tϕ. This work shows that vegetable oil/A‐based blends can be formulated with cold flow properties superior with respect to cloud point and comparable with respect to kinematic viscosity (v) of methyl soyate (biodiesel), either neat or blended with petroleum middle distillates.