Analytical study of heavy crude oil fractions by coupling of the transalkylation reaction with supercritical fluid chromatography

Abstract

Previous work involved the utilization of the transalkylation reaction (retro-Friedel-Crafts reaction to transfer alkyl-substituting chains from the asphaltenes aromatic matrix to a light aromatic acceptor) in the analysis of heavy crude oil fractions by coupling it with capillary gas chromatography (GC). In such conditions, a mapping of alkyl chains substituting the complex aromatic matrix of heavy crude oil fractions can be set up. Hoewever, it appears from NMR data and quantitative measurements of alkyl chains detected in GC that only about 50% of the transferable chains are effectively involved. This result poses the questions of whether heavy fractions of crude oil contain alkyl chains longer than those evidenced by GC and whether the partial transalkylation is due to an equilibrium. In order to answer these questions, supercritical fluid chromatography (SFC) was used instead of GC. An efficient transfer from model molecules ( n-alkylnaphthalenes) substituted by alkyl chains up to C 31 was observed, whereas GC did not show evidence for chains longer than C 24. However, in with various petroleum asphaltenes (Boscan, Arabian Light, Maya) no transfer of chains longer than C 21–C 28 was observed. Moreover, better quantitative results were obtained by SFC, as the analysis limit is well above C 21. For instance, the global transfer yield of alkyl chains from Boscan asphaltenes was 5.2% versus 4.6% in GC. The transalkylation yields were significantly increased when successive transalkylations were performed.