Steam cracking of bio-derived normal and branched alkanes: Influence of branching on product distribution and formation of aromatics

The presence of large amounts of oxygen in the molecular structure of triglyceride and fatty acid based feedstocks makes direct use in conventional steam crackers impossible without substantial modifications to the cold section. Full or partial catalytic deoxygenation has potential to resolve this, giving a mixture which consists primarily of normal and branched alkanes. Two of these deoxygenated mixtures have been investigated theoretically and experimentally in a dedicated bench setup (P = 0.17 MPa, T = 1050-1150 K, FHC = 4.17 10−2 g s−1, steam dilution of 0.3 and 0.5 gH2O/gHC). Furthermore, the degree of branching of the hydrocarbon mixtures impacts the product distribution, in particular the alkene selectivity. The newly generated, validated detailed kinetic model shows that small alkenes are formed by hydrogen abstraction and successive CC β-scission reactions. In the studied temperature range mono-aromatics are formed by three competing pathways: a series of recombination reactions of allylic radicals followed by hydrogen abstraction and intramolecular radical additions, additions of allylic and vinyl radicals on dienes followed by intramolecular radical addition, and finally recombination reactions of carbon-centered radicals with 1,3-cyclopentadienyl followed by hydrogen abstraction and ring enlargement.