Page 69 - Fluid Catalytic Cracking Handbook

FCC Feed Characterization 47

The simple °API gravity test provides valuable information about
the quality of a feed. But the shift in °API usually signals changes in
other feed properties, such as carbon residue and aniline point. Addi-
tional tests are needed to fully characterize the feed.

Distillation

Boiling point distillation data also provides information about the
quality and composition of a feed. The significance is discussed later
in this chapter. Distillation indicates molecular weight and carbon
number. It indicates whether the feed contains any "clean" products
that could be sold "as is." Before discussing the data, the different
testing methods and their limitations should be reviewed.
The feed to the cat cracker in a typical refinery is a blend of gas oils
from such operating units as the crude, vacuum, solvent deasphalting,
and coker. Some refiners purchase outside FCC feedstocks to keep the
FCC feed rate maximized. Other refiners process atmospheric or
vacuum residue in their cat crackers. In recent years, the trend has
been toward heavier gas oils and residue. Residue is most commonly
defined as the fraction of feed that boils above 1,050°F (565°C). Each
FCC feed stream has different distillation characteristics.
The frequency and method of testing feed streams varies from one
refiner to another. Some refiners analyze daily, others two or three
times a week, and some once a week. The frequency depends on how
the distillation results are applied, the variation in crude slates, and
the availability of lab personnel.
The fractional distillation test conducted in the laboratory involves
measuring the temperature of the distilled vapor at the initial boiling
point (IBP), as volume percent fractions 5, 10, 20, 30, 40, 50, 60, 70,
80, 90, and 95 are collected, and at the end point (EP). Three ASTM
methods are currently used to measure boiling points: D-86, D-1160,
and D-2887.
D-86 is the most common method used in refineries. The distillation
is done at atmospheric pressure. It is used for samples with an EP less
than 750°F (400°C). Above this temperature, the sample begins to
crack. Thermal cracking is identified by a drop in the temperature of
distilled vapor, the presence of brown smoke, and a rise in the system
pressure. Above 750°F liquid temperature, the distilling flask begins
to deform. All of today's FCC feeds are too heavy to use the D-86