Formation of alkanes during hydrous pyrolysis of an oil and n-C24 at high temperatures

A crude oil and pure tetracosane (n-C-24) were separately heated with kaolin, diatomite, and graphite at temperatures of 350-550 degreesC in closed hydrous pyrolysis systems for 72 h. Recoverable chloroform extractable organic matter (EOM) accounted for 20-40% of the initial amount of oil and 50-95% of the initial amount of n-C-24 for different mineral matrixes at 350 degreesC. The amount of EOM decreased to less than 1% of the initial material with an increase of temperature. Mass balance calculation suggests that the initial organic material was nearly quantitatively converted to gaseous compounds at high temperatures. Composition analysis showed that the alkane composition at 350 and 400 degreesC was similar to the composition of the oil, whereas only a minor amount of short-chain alkanes was detected in the pyrolysate of n-C-24. At 450-500 degreesC, the alkane products from both the oil-mineral and n-C-24-mineral pyrolysates were shifted from light hydrocarbons to bimodal distribution of both light and heavy alkanes, with light hydrocarbons being more abundant than long-chain hydrocarbons. At 550 degreesC, n-alkane compositions were dominated by long-chain hydrocarbons of n-C21+ with only a minor amount of n-C20-. The systematic changes of the alkane composition with an increase of temperature suggest that the thermal cracking of alkanes from long-chain hydrocarbons to light compounds was the dominant process at temperatures of 350-400 degreesC, whereas the formation or preferential preservation of long-chain alkanes became the dominant processes at 450-550 degreesC.