Semicokes from pitch pyrolysis:: mechanisms and kinetics

The literature of the kinetics and mechanisms of formation of anisotropic semicokes from petroleum residues is reviewed. The concepts of mesophase, of self-assembly of mesogens, and of liquid-crystals are described. Values are reported of reaction order, rate constants and activation energies (98-345 kJ mol(-1)). Limitations of analytical methods are critically discussed. Stages in the transformation of pre-mesogen molecules to mesogen, to mesophase, and the growth of mesophase are presented. Conclusions are: (a) methodologies all appear to have limitations, (b) sub-micron mesophase is present in the isotropic phase (c) mesogen molecules are never in 'solution' in the isotropic phase, (d) mesogen molecules once formed, are transferred directly to liquid-crystal (mesophase systems by the process of self-assembly, (e) solvent extractions of anisotropic material indicate solubility, (f) formation of mesogens is by consecutive reactions, (g) as many as three or four dehydrogenative condensations lead to the formation of mesogens, (h) the molecular weight of the mesogen at 400 degrees C, is >2,000 u., and (i) recent activation energies, using solvent extraction and image analysis data of the anisotropic phase are similar for all the petroleum residues, with values of 250+/-25 kJ mol(-1). (C) 1999 Elsevier Science Ltd. All rights reserved.