Fundamental Determination of the Average Relative Molecular Mass of Distillate Fuels Using the Ideal Gas Law

Abstract The average relative molecular mass is an indicator of properties relevant to the evaluation and regulation of transportation fuels. An accurate and repeatable method of measurement is therefore crucial. Current techniques, such as through size-exclusion chromatography, gas chromatography–mass spectrometry, and vapor pressure osmometry, involve complex measurement systems which necessitate calibration and/or correlating functions. This paper describes and validates a new method using the ideal gas law, providing a simpler measurement system with lower uncertainty. At constant volume and temperature, the pressure of a gas is directly proportional to the number of moles of it present, which is equal to its mass divided by its molecular mass. It follows that an independently measured mass of hydrocarbon mixture can be vaporized when introduced into a chamber of constant volume at a sufficient and constant temperature. A pressure change inside the chamber can be measured with which the number of moles present, and consequently, the average relative molecular mass of the mixture can be determined through the ideal gas law. The accuracy of this method depends on the homogeneity of the conditions and the applicability of the ideal gas law. The method was validated by using it to determine the average relative molecular mass of pure hydrocarbons and known mixtures up to a molecular mass of 130 g mol–1. The average relative molecular mass of six FACE gasolines (Fuels for Advanced Combustion Engines) and Haltermann E10 certification fuel is measured to below 5% uncertainty and compared to determinations by detailed hydrocarbon analysis (DHA) and five empirical correlations.