Study of Vapor Pressures of Gasoline-Alcohol Blends Finds That Dual-Alcohol Blends Can Result in RVPs Identical to That of Gasoline

RVPs (predicted DVPE) of single- and dual-alcohol blends in gasoline with different relative proportions of ethanol and 1-butanol. Credit: ACS, Andersen et al. Click to enlarge.

A research team from Ford Motor Company and the University of Copenhagen has performed a systematic study of the vapor pressures of single and dual-blends of different alcohols with potential to be used as biofuels, including methanol; ethanol; 1- and 2-propanol; and 1-, 2-, i-, and t-butanol.

As part of the study, they demonstrated a simple method to prepare dual-alcohol-gasoline blends with Reid vapor pressures (RVPs) “indistinguishable” from that of the base gasoline. Their paper was published online 21 May in the ACS journal Energy & Fuels.

Vapor pressure—i.e., the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container—is used as a measure of volatility and is an important property of automotive gasoline fuels. (Higher vapor pressure indicating higher volatility.) Vapor pressure can affect proper cold starting of the engine; vapor lock tendency in older engines without fuel injection; and quality of starting in engines with fuel injection. It is also a critical factor in meeting evaporative emission requirements.

Evaporative emissions from gasoline—volatile organic compounds (VOCs)—are precursors to the formation of tropospheric ozone and contribute to ground-level ozone. Gasoline standards in the US and globally
specify allowable vapor pressures depending upon the type of fuel and ethanol content, geographic location, and season.

While the vapor pressure is independent of the vapor/
liquid ratio for a pure compound, this is not true for mixtures,
such as gasoline. Even highly volatile compounds that are present at small concentrations can contribute greatly to the vapor pressure, but their impact on the measured vapor pressure is progressively reduced with an increasing vapor/
liquid ratio.

…When blending alcohols with gasoline, especially the shorter
chain alcohols, methanol and ethanol, the blend exhibits reductions in distillation temperatures and does not behave like an ideal mixture, because of the formation of a near azeotropic mixture. These non-ideal mixtures also have higher vapor pressures than would be predicted by Raoult’s law