Page 76 - Fluid Catalytic Cracking Handbook

54 Fluid Catalytic Cracking Handbook

pentane insolubles are widely used for measuring asphaltenes. Although
they do not provide rigorous definitions of asphaltenes, they provide
practical ways of assessing coke precursors in FCC feedstocks. It
should be noted that the traditional definition of asphaltenes is that
they are heptane insoluble. Pentane insoluble minus heptane insoluble
is the definition of resins. Resins are molecules larger than aromatics
and smaller than asphaltenes,
IMPURITIES

In recent years, refiners have been processing heavier crudes.
Heavier crudes provide a slim economic margin to the refiner, but slim
is better than none. The cat cracker, as the main conversion unit, is
designed to handle a variety of feedstocks. Today's FCC feedstocks
are generally heavier and contain higher levels of impurities like
nitrogen, sulfur, and metals. These impurities have negative effects on
unit performance. Understanding the nature and effects of these
contaminants is essential in feed and catalyst selection as well as
troubleshooting the unit.
Most of the impurities in the FCC feed exist as components of large
organic molecules. The most common contaminants are:
* Nitrogen
* Sulfur
» Nickel
« Vanadium
* Sodium
Except for sulfur, all these contaminants poison the FCC catalyst,
causing it to lose its ability to produce valuable products. Sulfur in
the feed increases operating costs because additional feed and product
treatment facilities are required to meet product specifications and
comply with environmental regulations.

Nitrogen

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 nonbasic nitrogen.
Basic nitrogen is about one fourth to one half of total nitrogen.