The formation of mesophase microstructures during the pyrolysis of selected coker feedstocks

Five coker feedstocks ranging from a gilsonite to a decant oil were pyrolyzed at increasing temperatures. The mesophase microstructures in the pyrolysis residues from successive temperature intervals were examined. Observations by polarized-light microscopy revealed that a common pattern of mesophase precipitation, coalescence, and deformation leads to a series of microconstituents whose structures are dependent primarily on the temperature of precipitation, but whose relative proportions depend on the feedstock. For a heating rate of 5°C/hr, the initial mesophase to precipitate during pyrolysis (at 400°C to 430°C) forms a fine-textured Isotropie microstructure which is frozen-in by hardening soon after coalescence of the spherules. Above 430°C, the freshly precipitated mesophase forms a coarse morphology which is deformable by bubble percolation to produce the fine fibrous microstructures of needle coke. The results indicate that the temperature range of the mesophase transformation is one indication of the quality of a feedstock for production of an isotropic or needle coke.