Vapor-pressure measurement of complex hydrocarbon mixtures by headspace gas chromatography

Abstract

Headspace gas chromatography was used to measure the vapor pressures of single n-alkane hydrocarbons and their binary, ternary and quaternary mixtures at various temperatures in the range 38–107°C. In addition, diesel fuel, gasoline and crude oil vapor pressures were measured at 38 and 70°C. The microprocessor-controlled headspace gas chromatograph automatically thermostated the samples contained in septum-sealed glass vials to equilibrate the vapor and liquid for a pre-set period of time (usually 60 min), pressurized the vials to a pre-set pressure (at least 138 kPa gauge) by inserting a hollow needle through the septum, and sampled the vapor for a pre-set period of time (0.01 min) by allowing the pressure in the vial to drive an aliquot of the vapor ( ca. 9 μl) through the needle and onto the analytical column. Using this technique, single n-alkane hydrocarbon vapor pressures, measured by totally vaporizing μl aliquots in the vials, agreed, on the average, to within 1% with calculated values based on hydrocarbon properties. Likewise, measured total vapor pressures of the binary, ternary, and quaternary mixtures compared well with calculated values, assuming ideal behavior for the mixtures, and the measured vapor pressures of diesel fuel, gasoline, and several crude oils showed good precision and accuracy.