Page 67 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization 45

comparison with cracking paraffins, cracking aromatic stocks results
in lower conversion, lower gasoline yield, and less liquid volume gain,
but with higher gasoline octane.

Characterizing an FCC feedstock involves determining both its
chemical and physical properties. Because sophisticated analytical
techniques, such as mass spectrometry, are not practical on a daily
basis, physical properties are used. They provide qualitative measure-
ment of the feed's composition. The refinery laboratory is usually
equipped to carry out these physical property tests on a routine basis.
The most widely used properties are:

* °API Gravity
» Distillation
* Aniline Point
* Refractive Index (RI)
* Bromine Number (BN) and Bromine Index (BI)
* Viscosity
* Conradson, Ramsbottom, Microcarbon, and Heptane Insoluble

°API Gravity

The °API gravity measures the density of a hydrocarbon liquid.
Specific gravity (SG) is another common measurement of density. The
liquid SG is the relative weight of a volume of sample to the weight
of the same volume of water at 60°F (15.5°C).
Compared with specific gravity, °API gravity magnifies small
changes in the feed density. For example, going from 24°API to
26°API changes the specific gravity by 0.011 and the density by 0.72
3
3
lb/ft (0.0115 gram/cm ). Neither is very significant, but a two-number
shift in °API gravity can have significant effects on yields.
The SG relates to °API gravity by the following equations:

141.5
J
^ (@60°F)
131.5+ °
3
API Gravitv = 1 3 1.5
SG(at 60°F)