Refractoriness of asphaltenes

When sulfur conversion levels are pushed above 90%, the unique refractoriness of asphaltenes becomes dominant. The tendency of a fine pore catalyst to partially exclude asphaltenes and the complete steric hindrance or "burying" of sulfur in asphaltenes contribute to this refractoriness. 

Asphaltenic sulfur is the most refractory specie in re-sids and the removal of metals, particularly nickel, correlates well with removal of asphaltenic sulfur. 

These heavy oils already contain large percentages of refractory materials such as asphaltenes, and it would be highly undesirable to increase the amount or degrade the quality of these components. We have, therefore, investigated the effect of heat treatment during distillation on the quantity and physical and chemical properties of asphaltenes. Cold Lake crude was chosen for this study since it is known to be a thermally sensitive material. 

The thermal chemistry of heavy oils and bitumen is extremely complicated because of wide variations in chemical compositions. The most refractory components in petroleum feedstocks are asphaltenes, which contribute the most to coke formation dining thermal cracking. Next to asphaltenes, resins and large aromatics also contribute to coke. To investigate the effect of these three heavy oil components on the mesophase induction period, Athabasca bitumen fractions containing varying amounts of asphaltenes (obtained by supercritical fluid extraction) and Venezuelan heavy... 

Table II compares two atmospheric resids, West Coast and Kuwait, in a traditional manner. The obvious differences include sulfur, nitrogen, asphaltenes, total metals and mid-boiling point. Apart from sulfur content, one might surmise a greater catalyst demand by the West Coast feedstock in that the boxed values suggest heavy coke laydown and metals deposition. Neither of the sulfur values is boxed because there is no indication as to (1) what fraction of the sulfur is refractory or "hard" sulfur, nor (2) the degree of desulfurization to be achieved.

Table III extends the comparison of these resids with an emphasis on reactivity, asphaltene characteristics, compound types and the refractory forms of sulfur, such as benzothiophenes and asphaltenic sulfur.

For lower conversions levels, where asphaltenic sulfur removal is not deep, refractoriness appears to be largely influenced by a large proportion of sterically hindered aromatic sulfur compounds. Substituted thiophenes, benzothiophines and di-benzothiophenes are representative compounds. 

In fixed-bed hydrocrackers designed to process VGO, residual oils in the feed can reduce catalyst cycle life if they contain even trace amounts of salts, asphaltenes, refractory carbon, trace metals (Fe, Ni, V), or particulate matter. As mentioned in Section 3.4.2, fixed-bed units designed to process residue remove metals and other contaminants with upstream guard beds or onstream catalyst replacement technology. In contrast, ebullated bed hydrocrackers can and do process significant amounts of residual oils. This is because fresh... 

Increasing the feed endpoint and/or density tends to increase the amount of coke precursors in the feed. The precursors include asphaltenes, refractory carbon, and PAH. 

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