Page 78 - Fluid Catalytic Cracking Handbook

56 Fluid Catalytic Cracking Handbook

few others, feeds with high nitrogen also have other impurities.
Therefore, it is difficult to evaluate deleterious effects of nitrogen
alone. Hydrotreating the feed reduces not only the nitrogen content
but also most other contaminants.
Aside from catalyst poisoning, nitrogen is detrimental to the unit
operation in several other areas. In the riser, some of the nitrogen is
converted to ammonia and cyanide (H-CN). Cyanide accelerates the
corrosion rate of the FCC gas plant equipment; it removes the protec-
tive sulfide scale and exposes bare metal to further corrosion. This
corrosion generates atomic hydrogen that ultimately results in hydrogen
blistering. Cyanide formation tends to increase with cracking severity.
In addition, some of the nitrogen compounds end up in light cycle
oil (LCO) as pyrolles and pyridines [5]. These compounds are easily
oxidized and will affect color stability. The amount of nitrogen in the
LCO depends on the conversion. An increase in conversion decreases
the percentage of nitrogen in the LCO and increases the percentage
on the catalyst.
The source and gravity range of raw crude greatly influence the
amount of nitrogen in the FCC feed (Table 2-2). Generally speaking,

Table 2-2
API Gravity, Residue, and Nitrogen Content of Typical Crudes

Vacuum Total Nitrogen* of
°API Bottoms, Heavy Vacuum
Crude Source Gravity vol% Gas Oil, PPM

Maya 21.6 33.5 2498
Alaska North Slope 28.4 20.4 1845
(ANS)
Arabian Medium 28.7 23.4 829
Forcados 29.5 7.6 1746
Cabinda 32.5 23.1 1504
Arabian Light 32.7 17.2 1047
Bonny Light 35.1 5.3 1964
Brent 38.4 11.4 1450
West Texas Intermediate 38.7 10.6 951
Gushing (WTIC)
Forties 39.0 10.1 1407

*Nitrogen level varies with crude source and residue content.