Abstract:
This research study determines the potential to increase substantially the anisotropy of a coke from an aliphatic Waxy Oil produced by Sasol Synfuels at Secunda, South Africa. Experimental modifications included filtration, distillation and thermal treatment, followed by distillation with the aim of producing a carbonised product similar to needle coke. The substantial concentration of an iron oxide catalyst (up to 2%) in Waxy Oil is increased by an order of magnitude upon carbonisation and calcination due to low coke yield and reactivity factors. The catalyst also promotes oxidative polymerisation of the residue, acts as a barrier to mesophase formation and promotes multi-phase graphitisation. Filtration of Waxy Oil using a 0.5 ìm sintered metal filter reduces the ash content to 0.006% and increases the anisotropy of the carbonised product to 54% flow domains compared with 22% for the carbonised product of virgin Waxy Oil. Thermal treatment followed by distillation of Waxy Oil reduces the effect of organic reactivity promoters (mainly multi-alkylated aliphatics/aromatics and oxygenates), while increasing the concentration of thermally stable (C18 to C30) normal alkanes to 85% compared with 38% in the filtered Waxy Oil. Compared with the filtered Waxy Oil, thermally stabilised Waxy Oil reduces the amount of the pre-carbonisation residue (from 98.7 to 43.0%), while “static” carbonisation thereof increases the green coke yield (from 19.8 to 36.3%) and increases the anisotropicity (from 54 to 100% flow domains). The carbonisation mechanism of filtered and thermally treated Waxy Oil involves initial cracking of high molecular weight normal alkanes (C18 to C30), thus concentrating the molecular weight of normal alkanes (C18 to C22). This is followed by a slow cyclisation step involving both self condensation and cyclo addition reactions to form two- to six-ring cyclo-alkanes or hydro-aromatics. The hydro-aromatics are dehydrogenated rapidly to form methyl and di-methyl three- to six-ring substituted aromatics. Further thermal degradation dealkylates these molecules to form stable four- to six-ring “pre-mesogens”. The mesospheres are nucleated from the isotropic matrix and grow to more than 0.050 mm in diameter, with a volume of 2.61 x 10-3 mm3. Subsequent coalescence of the mesospheres produces mesospheres with diameters of over 0.200 mm and volumes of 41.82 x 10-3 mm3. The resultant microstructure of the solid carbon is composed of flow domains more than 400 ìm in length. Although needle cokes have historically been produced from aromatic residues, this research is the first to show that a coke with a similar microstructure can be produced from a totally aliphatic residue. The research thus provides potential for the development of a needle coke from a totally unique Waxy Oil residue with negligible sulphur (< 0.008%) and nitrogen (< 0.09%) contents. This is the first academic study of the chemistry of Waxy Oil.
