The effect of carbon additives on the mesophase induction period of Athabasca bitumen

The ability of solid carbonaceous material to retard the formation of coke during thermal cracking and hydrocracking of heavy hydrocarbons is well kno wn. In this study, we used in-situ microscopy (hot-stage) to obtain additio nal mechanistic information on whether fine coke and fullerene soot particl es retard the growth of mesophase during thermal cracking of Athabasca bitu men, thus reducing the possibility of fouling in preheaters and furnaces. T he findings from this study could also have application in other non-cataly tic thermal processes such as visbreaking and coking. In the absence of add itives in the Athabasca bitumen feed, the formation of mesophase occurred a fter 61 and 67 min (measured from room temperature) at reaction temperature s of 450 degreesC and 440 degreesC, respectively. The addition of solid cok e (ca. 5 wt.%) from a commercial delayed coking operation shortened the mes ophase formation time to almost 45-50 min under similar conditions. The cok e, having surface area of only 1.65 m(2)/g, resulted in enhanced bitumen fl uidity and large-textured mesophase. These observations were rationalized b ased on the ability of delayed coker coke to release hydrocarbons into the bulk fluid during thermal cracking. Light hydrocarbons released from coke m ay have changed the solvating power of the liquid phase in bitumen and prom oted phase separation, resulting in a shorter induction period. In contrast , adding small amounts of fine fullerene soot (ca. 1 and 5 wt.%) delayed th e appearance of mesophase significantly under similar conditions. The abili ty of fullerene soot to physically absorb the mesophase precursors into its pore structure led to an increase in the apparent viscosity of the bulk ph ase, which is known to reduce mesophase size and prolong the induction peri od. Consistent with this, the induction period was prolonged an additional 10 min when the soot surface area was increased from 152 to 208 m(2)/g. The increase in induction period is significant with respect to reaction times and suggests that these fullerene soot materials could be effective in all owing for increased severity and liquid products yield from visbreaking, wi th less likelihood of fouling in the preheater tubes and furnace walls. (C) 2001 Published by Elsevier Science B.V.