Pitch-mesophase-coke transformation

The Pitch-Mesophase-Coke Transformation As Studied by Thermal Analytical and Rheological Techniques... 

Figure 6, Schematic diagram describing the pitch-mesophase-coke transformation.

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

It is well known that pitch, solvent-refined coal (SRC), and coking coal produce various kinds of mesophase at the early stages of carbonization (3y 4y 5). The mechanisms of many chemical reactions and physical transformations relating to mesophase formation are studied by quenching techniques. Such research techniques as polarized-light microscopy can be extremely fruitful. On the other hand, observation of phenomena at reaction temperatures may yield more easily interpretable or more relevant results. 

During the course of the pyrolysis of a petroleum-derived residue (pyrolysis tar, cat-cracker residue) or a filtered coal tar pitch it is possible to observe, under the polarisation microscope and at a certain temperature, the formation of anisotropic spherules, which grow as the reaction time lengthens and the temperature increases, coalesce and, at around 500 to 600 °C, are transformed into a semi-coke phase with marked anisotropy. Figure 13.2 shows photomicrographs of a filtered coal tar pitch pyrolyzed at 400 °C with the formation of spherulitic mesophases after reaction times of 2, 6,10 and 16 hours. 

Polymerization. The pitch is heated to approximately 400°C and is transformed from an isotropic to a mesophase (or liquid crysteil) structure consisting of large polyaromatic molecules with oriented layers in parallel stacking. This structure is similar to the needle-coke stage of molded carbons described in Ch. 4, Sec. 2.3. 

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