Pyrolysis of petroleum residues II. Chemistry of pyrolysis

Pyrolysis of two petroleum residues, R1 and R2 of different aromaticities, were studied at 0.1 and 1.0 MPa, 420-480 degrees 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 monitor ed by polarised light optical microscopy. The extraction and microscopy met hods are critically compared in monitoring pyrolysis pr ogress. Differences between measured growths of the extraction fractions and anisotropy are ex plained. Growth in aromaticity in the HS and TS was followed using H-1-NMR and FTIR (CHal/CHar), and in NMPS and NMPI fractions by FTIR. Vapour pressu re osmometry estimated average molecular weights of four fractions from R1. Aromaticities in the final semicokes from R1 and R2, a ere very similar, d espite initial differences in the parent feedstocks, Growth in constituent molecular size probably occurs via a consecutive reaction mechanism. The ea rly stages of pyrolysis involve dealkylation reactions. Early mesophase sti ll 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 R 1 and R2 are compared. The paper reports in considerable detail aspects of the chemistry of these processes. (C) 2000 Elsevier Science Ltd, All rights reserved.