Residue catalysts contaminant coke

Consequently, there are significant differences in FCC unit operation when residue is added to normal feed. Conversion falls and less gasoline is produced, as shown in Table 5.2, and the catalyst-to-oil ratio must rise as coke yields increase. The coke also has a different composition relative to that produced from normal feed not only because of the higher Conradson carbon levels and high-boiling compounds, which are absoibed by the catalyst particles, but also from the dehydrogenation activity of the metal impurities, which leads to polymerization reactions and contaminant coke formation. 

About 1.2-1.4% Acoke forms on residue catalysts compared with about 0.7- 0.8% on gasoline catalysts. The distribution of Acoke for both types of feed is shown in Table 5.12. Most of the increase is associated with contaminant and feed coke. 

Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

The residual portion of feedstocks contains a large concentration of contaminants. The major contaminants, mostly organic in nature, include nickel, vanadium, nitrogen, and sulfur. Nickel, vanadium, and sodium are deposited quantitatively on the catalyst. This deposition poisons the catalyst permanently, accelerating production of coke and light gases. 

Dehydrogenation is an undesirable catalytic reaction, which is promoted by metal contamination on equilibrium catalyst. The dehydrogenation reaction extracts hydrogen molecules from hydrocarbon molecules, thus increasing the production of highly aromatic products and coke. Nickel contamination is known to promote dehydrogenation. Other metals, such as copper and iron, are also known to increase hydrogen production. Metal contamination comes mainly from the heavy ends of the hydrocarbon feedstock, particularly the residues, as discussed previously. 

Spent catalysts, after hydrotreating of residues, contain combinations of Co, Ni, Mo and W, supported on alumina and contaminated by coke, V, Ni and Fe. Regeneration is generally not performed, and these spent catalysts are discarded. Some studies have shown that a partial rejuvenation of these catalysts is possible by leaching of fouling metals [Trimm et al, 1990]. 

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