FT-IR in the Quantitative Analysis of Gaseous Hydrocarbon Mixtures

Abstract

An FT-IR spectrometer provided with software for multi component analysis of FR spectra was used for simultaneous determination of up to 12 hydrocarbons in a gaseous mixture. The hydrocarbons, ranging from methane to n-butane, were selected to simulate reaction product mixtures obtained in the dehydrogenation of n-butane. The concentrations of the hydrocarbons in the sample were determined by fitting to the sample spectrum a set of pure calibration spectra of the gases present in the sample. In principle, the entire wavenumber range of the low-resolution IR spectrum could be utilized in the fit. The reliability of the results was checked from the residual spectrum, obtained by subtracting the fitted from the measured spectrum. The residual spetrum will be pure noise when the set of calibration spectra chosen for the analysis fits exactly with the sample spectrum. Precision of the results, expressed as average relative standard deviation, was ±4%. Concentrations of most of the hydrocarbons were determined with an error less than ±10%. The concentrations of isobutane, trans-2-butene, propane, and ethane deviated by more than 10% from the reference concentrations. The sum concentration of ethane and propane, however, deviated less than ±1% from the reference. Difficulties in determining ethane and propane accurately were due to the broad nature and small number of peaks in their IR spectra. IR spectrometry with multicomponent analysis of spectra provides a rapid and powerful method with potential for the study of catalytic reactions at a time resolution of seconds