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Conversion of low-value petroleum residues to high-value distillates by catalytic hydrocracking has gained considerable importance in recent years. One of the common operating problems encountered in the process is the formation of cokelike sediments in the products. The carbonaceous sediment materials usually deposit on the reactor and downstream vessels as well as on catalyst surface and cause both operability and rapid catalyst deactivation problems. Prevention or suppression of coke-like sediment deposition is highly desirable alleviate equipment fouling problems and to attain high conversions in residue hydrocracking operations. In the present work, the influence of adding some aromatics-rich gas oil streams, such as heavy cycle gas oil (HCGO) and light cycle oil (LCO), produced in the F-CC units to vacuum residue on various conversions as well as on sediment formation was investigated. The results revealed that mixing aromatics-rich gas oils with vacuum residue feed was very effective in reducing sediment formation even at very high conversion levels (>80%, The diluents, however, did not have any appreciable benefits in improving the yield of distillates and the extent of other hydrotreating reactions. The results are consistent with the asphaltenes phase separation mechanism. The addition of aromatics-rich diluents may improve the solubility of the asphaltene cores in the product oil medium and control the onset of their precipitation. [ABSTRACT FROM AUTHOR]

Influence of catalyst acidity and feedstock quality on the extent of initial coke formation and its role in hydrotreating catalyst deactivation were investigated. A series of Ni–Mo/Al2O3 catalysts with varying acidities were prepared by using sodium and fluoride ions as acidity modifying additives. Acidities of the catalysts decreased in the sequence: Ni-Mo-F(5%) > Ni-Mo-F(2%) > Ni-Mo > Ni-Mo-Na(1%) > Ni-Mo-Na (5%). The amount of coke formed on the catalyst did not show any direct relation to catalyst's acidity but was found to depend strongly on the type of additive used for modifying acidity. In the case of sodium addition, although, the catalyst's strong acidity decreased, coke deposition increased probably due to the suppression of catalyst's hydrogenation function by interaction between sodium and molybdenum. Fluoride addition at low concentration levels (e.g. 2wt%) increased both HDS and HDN activities of the catalyst, but at high concentration levels of fluoride (e.g. 5 wt%), the catalyst showed a sharp decrease in HDS and HDN activity. Sodium addition at all levels decreased catalysts activity for both HDS and HDN. In the feed quality effect studies, a feedstock with a higher concentration of nitrogen and asphaltenes led to rapid coke build up during the initial period. Solid state carbon-13 NMR analysis of the coke on various spent catalyst samples showed that the coke formed from residual oil feed was more aromatic in character than that from VGO, and the initial coke changed its aromatic character with processing time. [ABSTRACT FROM AUTHOR]