Temperature- and composition-dependent mid-infrared absorption spectrum of gas-phase gasoline: Model and measurements

A spectroscopic model is developed to predict the composition- and temperature-dependent absorption spectrum of gasoline vapor in the mid-IR region of the C–H stretching vibrations near 3.5 μm. Data from a detailed compositional analysis of a representative gasoline sample are used to select and weight the absorption spectra of individual hydrocarbon species, providing class-averaged spectra for five hydrocarbon classes: the normal and branched alkanes, olefins, aromatics, and oxygenates. The absorption spectra for 21 other samples of gasoline are then predicted by weighting these class-averaged spectra according to the volume fractions of olefins and aromatics, and the mass fraction of oxygenates in the gasoline, determined by standard chemical analysis (ASTM D1319 for olefins and aromatics, ASTM D4815 for oxygenates), and the volume fraction contributions from normal and branched alkanes estimated according to fuel grade. The absorption spectra predicted by the model are compared to FTIR-measured absorption spectra of these 21 gasoline samples at 50 and 450 °C. While the measured cross-section at selected wavelengths varies by a factor of 2 for this set of gasoline samples, the data agree well with the model predictions, exhibiting only a 6.8% RMS deviation. This deviation is reduced to 5.5% by excluding four samples of aged gasoline. The good agreement between model and measurements over a large temperature range illustrates the utility of this model to predict the temperature-dependent absorption cross-sections of gasoline in the C–H vibrational absorption band using two commonly available class analyses of gasoline samples.