DECOMPOSITION KINETICS AND MECHANISM OF N-HEXADECANE-1,2-C-13(2) AND DODEC-1-ENE-1,2-C-13(2) DOPED IN PETROLEUM AND N-HEXADECANE

Isotopically labeled n-hexadecane doped at the percent level in three crude oils is used to determine the intrinsic decomposition kinetics a nd mechanism of n-alkanes in petroleum. Adjacent C-13 labels at the en d of the hexadecane and dodecene give a mass fragment sufficiently uni que that its disappearance and many of its products can be followed by ordinary gas chromatography-mass spectr---- ometry. Additional struct ural details of the labeled reaction products are measurable by the NM R INADEQUATE technique, which detects only adjacent C-13 atoms. Sample s were heated at temperatures ranging from 310 to 360 degrees C in cap illary glass tubes and Dickson autoclaves. At temperatures around 350 degrees C, n-alkane decomposition in dissimilar oil matrices forms pri marily normal alkanes smaller than the starting alkane at a rate about 60% as fast as the decomposition of the neat alkane. Unlike in neat h exadecane, no significant branched alkanes are formed from the labeled hexadecane in crude oil by alkylation of alkene intermediates. Doping the oils and n-hexadecane with labeled dodecene confirms that alkenes in two of the three oils are rapidly converted primarily to the corre sponding alkanes, while reaction of alkenes in hexadecane forms primar ily branched alkanes. Reaction of alkenes in the high paraffin oil was intermediate in characteristics. One autoclave experiment included wa ter to assess the importance of water during pyrolysis, with the resul t that the alkane decomposition rate is affected very little. However, coking of aromatics is inhibited, and there is a significant increase in the production of both H-2 and CO2 gas with water present, indicat ing that water is chemically reactive under these conditions. At tempe ratures around 310 degrees C, the decomposition rate of neat hexadecan e is roughly equal to that in a high paraffin oil and substantially sl ower than in North Sea and high sulfur oil, suggesting that the effect of the oil matrix has switched from suppression of propagation reacti ons to enhancement of initiation reactions. The activation energy for doped hexadecane cracking in sealed glass capillaries ranges from abou t 53 kcal/mol in a North Sea oil to about 62 kcal/mol in high paraffin and high sulfur oils. These values are both lower than for neat hexad ecane over the same temperature range but still imply substantial subs urface stability for crude oil. Copyright (C) 1997 Elsevier Science Lt d.