The preparation of pitches from anthracene oil

Abstract

Eskom has embarked on the development of the Pebble Bed Modular Reactor to generate electricity at low cost compared to that of coal with no greenhouse gas emissions. The proposed PBMR requires about 600 tons of nuclear-grade graphite blocks in its construction. A considerable amount of pitch is required as impregnants for the manufacture of this graphite. Anthracene oil is a high-boiling coal-tar distillate fraction consisting of a complex mixture of mainly unsubstituted polyaromatic hydrocarbons, with some methyl substitution. Conversion to pitch may be effected by dehydrogenation, i.e. volatile oil molecules condensing together to yield material of high molecular mass. Two dehydrogenation reagents, elemental sulfur and a novel reagent, hexachlorocyclohexane (HCH), an industrial waste product, were used to give three series of pitches with differing softening points. These products were characterised by standard pitch analyses, elemental analyses, thermogravimetric analysis (TGA), proton nuclear magnetic resonance spectroscopy (1HNMR), gas chromatography (GC), gas chromatography–mass spectrometry (GC-MS), Maldi-TOF mass and mass probe spectrometry and then compared with commercial pitch grades from Mittal Coke and Chemicals. The results obtained showed that the reaction products reacted with 15 and 20 wt % of sulfur, with the softening points of 61.3 and 91.4 °C, would be suitable as commercial products. All pitches showed very little, if any, ash, making them suitable for use in the manufacture of high-purity graphite. Pitches prepared with 15-20 wt % of sulfur and HCH pitches may have more applications in impregnation because of their low QI content. Total sulfur content detected by elemental analysis is high. This is undesirable because sulfur may cause puffing which will result in poor quality of graphite. All pitches show very little, if any, ash, making them suitable for use in the manufacture of high-purity graphite. TGA results showed that pitches prepared with higher concentrations of sulfur can be suitably used as impregnants because of their high coking yields. The coking yields of HCH-derived pitches are lower than those of Mittal Coke and Chemicals commercial pitches. Results from the (1HNMR showed that cross-linking on reaction with sulfur is through aliphatic carbon atoms; the percentages of aliphatic protons decrease with the increase in the sulfur concentration, and aromatic percentages increase. There is a slight increase in the proportion of aromatic protons of the HCH pitches; therefore cross linking is through aromatic-aromatic carbon atoms. Model reactions were prepared by reacting pure aromatic hydrocarbons with HCH, demonstrating the incorporation of the dehydrogenation reagent. These reactions were used to determine the reactivity of the main components of anthracene oil. The reaction products were characterised by Maldi-TOF and GC-MS. The results obtained from HCH pitches showed the formation of chlorobenzenes. This is undesirable because chlorine may cause corrosion of the equipment during coking.