Pyrolysis of petroleum residues: II. Chemistry of pyrolysis Original Research Article

Pyrolysis of two petroleum residues, R1 and R2 of different aromaticities, were studied at 0.1 and 1.0 MPa, 420–480°C, for soak times up to 12 h. Pyrolysis was followed by extracting resultant semicokes, sequentially, with heptane, toluene and N-methyl-pyrrolidinone to obtain four fractions HS, TS, NMPS and NMPI. Development of the anisotropic mesophase was monitored by polarised light optical microscopy. The extraction and microscopy methods are critically compared in monitoring pyrolysis progress. Differences between measured growths of the extraction fractions and anisotropy are explained. Growth in aromaticity in the HS and TS was followed using 1H-NMR and FTIR (CHal/CHar), and in NMPS and NMPI fractions by FTIR. Vapour pressure osmometry estimated average molecular weights of four fractions from R1. Aromaticities in the final semicokes from R1 and R2, were very similar, despite initial differences in the parent feedstocks. Growth in constituent molecular size probably occurs via a consecutive reaction mechanism. The early stages of pyrolysis involve dealkylation reactions. Early mesophase still contains significant amounts of aliphatic side-chains, which are reduced as pyrolysis proceeds. Rates of transfer of HS to TS to NMPS to NMPI for R1 and R2 are compared. The paper reports in considerable detail aspects of the chemistry of these processes.