Page 92 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization §9

However, physical properties such as °API gravity and distillation are
easy to measure. As a result, empirical correlations have been devel-
oped by the industry to determine chemical properties from these
physical analyses.
Characterizing FCC feed provides quantitative and qualitative esti-
mates of the FCC unit's performance. Process modeling uses the feed
properties to predict FCC yields and product qualities. The process
model should be used in daily unit monitoring, catalyst evaluations,
optimization, and process studies.
There are no standard correlations. Some companies have proprietary
correlations, but this does not mean that these correlations do a better
job at predicting yields. Nonetheless, they all incorporate most or some
of the same physical properties. The most widely published correla-
tions in use today are:

• K Factor
• TOTAL
• n-d-M Method
• API Method

K Factor

The K factor is a very useful indication of feed crackability. The K
factor relates to the hydrogen content of the feed. It is normally
calculated using feed distillation and gravity data, and measures
aromaticity relative to paraffinicity. Higher K values indicate increased
paraffinicity and more crackability. A K value above 12.0 indicates a
paraffinic feed; a K value below 11.0, aromatic.
Like aniline point, the K factor differentiates between the highly
paraffinic and aromatic stocks. However, within the narrow range
(K = 11.5-12.0), the K factor does not correlate between aromatics
and naphthenes. Instead, it relates fairly well to the paraffin content
(Figure 2-11). The K factor does not provide information as to the ratio
of naphthene and paraffin contents. The ratio of naphthenes to paraffins
can vary considerably with the same K values (Table 2-8).
K value is the ratio of the cube root of a boiling temperature to
gravity. There are two widely used methods to calculate the K factor:
K w and the K uop. The equations used for calculating both factors are
as follows: