December 16, 2021 at 6:06 pm #2065Nasir HussainKeymaster
Nitrogen in the FCC feed refers to organic nitrogen compounds. The nitrogen content of
FCC feed is often reported as basic and total nitrogen. Total nitrogen is the sum of basic
and non-basic nitrogen. Basic nitrogen is about one-fourth to one-half of total nitrogen. Catalyst poisoning from the presence of basic nitrogen in the FCC feedstock is significant. To compensate for the Nitrogen poisoning reactor temperatures have to increase.
The basic Nitrogen molecules react with the catalysts and neutralize acid sites on the catalyst. This causes a temporary loss of the FCC catalyst activity and a drop in unit conversion. However, nitrogen is a temporary poison. During regeneration, the burning of nitrogen in the regenerator restores the activity of the catalyst. In the regenerator, about 95% of the nitrogen in the coke is converted to elemental nitrogen. The remaining nitrogen is converted to nitrogen oxides (NOx) and leaves the unit with the flue gases. Virtually all the basic nitrogen ends up in coke. The FCC catalyst with a high zeolite and active-matrix content can be used to minimize the negative effects of organic nitrogen.
Hydrotreating is the best approach to remove the Nitrogen from the FCC feed. Hydrotreating not only removes the Nitrogen content but most of the other contaminants which affect the FCC catalyst activity.
Aside from catalyst poisoning, in the riser, some of the nitrogen is converted to ammonia and cyanide (HaCN). Cyanide accelerates the corrosion rate of the FCC gas plant equipment; it removes the
protective sulfide scale and exposes the 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 LCO as pyrroles and pyridines.
These compounds are easily oxidized and will affect colour 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. Generally speaking, heavier crudes contain more nitrogen than
lighter crudes. In addition, nitrogen tends to concentrate in the residue portion of the crude.
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