Dynamic Approach for the Estimation of Olefins in Cracked Fuel Range Products of Variable Nature and Composition by 1H NMR Spectroscopy

Abstract

A dynamic ¹H NMR-based method for the estimation of olefin content in all cracked fuel range products, in general, and in gasoline/naphtha streams, in particular, irrespective of types and composition of olefins and boiling range of samples has been developed. This is in continuation of our earlier works where two methods were described for the determination of hydrocarbon types in straight-run gasoline (no olefins) and cracked full range gasoline/naphtha (with olefins). The average absolute number of unsaturated hydrogen (H) in the olefinic region (4.4–6.5 ppm) was directly estimated with the help of a ¹H NMR spectrum using dynamic variables in terms of differential population of various kinds of olefins. The average alkyl chain length (n) was estimated by various methods including ¹³C NMR and carbon number distribution by a gas chromatography-based detailed hydrocarbon analyzer [DHA, ASTM D6730-01(2016)] and simulated distillation [ASTM D2887-16a] data. The percentage of unsaturated hydrogen (% UH) in an average olefin was then obtained providing a multiplication factor (fₒ) by which the weight percentage of olefin is estimated using a normalized ¹H NMR spectrum. The dynamic estimation of H and n for each sample removes the possibilities of errors in the estimation. The method has efficiently been extended to coker kero and coker diesel range products where there has been no method available for olefin estimation. The method was validated by using DHA following ASTM D6730, by the Reformulyzer-based ASTM D6839 method, and finally by fluorescent indicator adsorption following ASTM D1319. All the methods were compared. Whereas the proposed NMR method is extremely general, free from manual error, the limitations of existing ASTM methods and the old NMR method vis a vis a new NMR method are also discussed.