Hydrous pyrolysis of alkanes, alkenes, alcohols and ethers

Abstract

The decomposition reactions of alkanes, alkenes and related compounds have been systematically studied under conditions of hydrous pyrolysis: 3–5 days at 317°C, in the presence of simulated oil field brine and clay. n-Hexadecane slowly cracks to give n-alkanes and n-alkenes C-2 through C-14. Ethers and alcohols rapidly hydrolyze and dehydrate producing alkenes. Alkenes add to hexadecane to give branched alkanes. Tetrahydronaphthalene inhibits the cracking and alkylation of hexadecane both. n-Hexadecene cracks like n-hexadecane, but a large fraction of it dimerizes. Most though not all reactions observed may be explained in terms of free radical chain mechanisms. For example, there is little evidence for the rearrangement of carbon skeletons, and the distribution of cracking products obtained from squalane is also consistent with a free radiical chain reaction. However, the dehydration of alcohols may involve carbonium ions, and extensive migration of double bonds in alkenes was observed. These results demonstrate why alkenes are insignificant in natural oils, while they are commonly present in artificial shale oil. Even at 317°C, the rate of alkylation is comparable to the rate of cracking. Because the activation energy for alkylation is smaller than for cracking, the probability of alkylation increases relative to cracking at lower temperature. Therefore, alkylation will be much faster than cracking in petroleum source beds and reservoirs, and the steady state concentration of alkenes will be very small. In the low pressure, high temperature pyrolysis of oil shale of fraction of the alkenes produced boil off together with the alkanes before they are able to react, and appear in the product distillate.