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Analytical schemes are presented for characterizing saturate, aromatic, and olefinic fractions derived from petroleum, based upon high resolution nuclear magnetic resonance (NMR) measurements and generally, with the exception of the saturates, elemental analyses plus molecular weight. Saturates are characterized by a “branchiness index” (BI), which is defined as the ratio of the heights of NMR absorption bands due to methyl and methylene groups, respectively. The branchiness index correlates linearly with the naphthene ring carbon content in gas oil saturates from a variety of crude sources, which enables the latter to be determined from a NMR measurement. The scheme for aromatics analysis provides determinations of alkyl and aromatic ring carbon, the extent of alkyl group substitution on aromatic rings, the average number of aromatic rings per nucleus or per molecule, the number of carbons per alkyl group, and the structural group weight. To make these calculations it is necessary to estimate the carbonhydrogen weight ratio of the alkyl groups; this is assisted by a branchiness index measurement of the corresponding saturate fraction. The procedure also depends upon whether one assumes (for very high molecular weights) that aromatic nuclei in a molecule are directly joined or are separated by aliphatic chains. For the latter, an empirical expression depending upon probable average structures (obtained by correlation with mass spectrometer (MS) data) is used in the calculations. The olefin scheme depends in general upon similar types of measurements, but does not require correlative information. Analytical results on typical petroleum stocks are presented, and there is a discussion of sources of errors.
Williams, R. B.
Humble Oil and Refining Co., Baytown, Tex.