Establishing the maximum carbon number for reliable quantitative gas chromatographic analysis of heavy ends hydrocarbons

Abstract

This Thesis investigates the two main limitations of high temperature gas chromatography (HTGC) in the analysis of heavy n-alkanes: pyrolysis inside the GC column and incomplete elution. The former is studied by developing and reducing a radical pyrolysis model (7055 reactions) into a molecular pyrolysis model (127 reactions) capable of predicting low conversions of (nC14H30-nC80H162) at temperatures up to 430°C. Validation of predicted conversion with literature data for nC14H30, nC16H34 and nC25H52 yielded an error lower than 5.4%. The latter is addressed by developing an analytical model which solves recursively the diffusion and convection phenomena separately. The model is capable of predicting the position and molar distribution of components, using as main input the analytes’ distribution factors and yielded an error lower than 4.4% in the prediction of retention times. This thesis provides an extension of the data set of distribution factors of (nC12H26– nC98H198) in a SGE HT5 GC capillary column, based on isothermal GC measurements at both constant inlet pressure and flow rate. Finally, the above two models were coupled, yielding a maximum mass lost of 1.3 % in the case of nC80H162 due to pyrolysis and complete elution up to nC70H142, in a 12 m HT5 column.