Page 70 - Fluid Catalytic Cracking Handbook

48 Fluid Catalytic Cracking Handbook

method, but it is used for light products. Therefore, the boiling points
are obtained using either ASTM D-1160 or ASTM D-2887.
D-1160 is run under vacuum (one millimeter of mercury). The
results are converted to atmospheric pressure, using standard cor-
relations. Some newer apparatuses have built-in software that performs
the conversion automatically. D-1160 is limited to a maximum EP
temperature of about 1,000°F (538°C) at atmospheric pressure. Above
this temperature, the sample begins to crack thermally.
D-2887 is a low-temperature simulated distillation (SIMDIS) method
that measures the vol% of true boiling point (TBP) cuts using gas
chromatography (GC). Like D-1160, its use is limited to a maximum
end point of about 1,000°F (538°C). However, new GC~based systems
on the market can measure boiling point temperatures as high as
l,35Q°F (750°C). This development will be useful in determining
boiling points of feedstocks that contain residue and in the char-
acterizing of raw crades. Compared with D-1160, SIMDIS is less labor
intensive, more reproducible, and generally more accurate at the IBP
and at the 5% and 10% points.
Distillation data provides information about the light fraction of feed
boiling at less than 650°F (343°C). Light virgin feed, the fraction that
boils under 650°F (343°C), often results in a greater LCO yield and
lower unit conversion. Sources of these fractions come from atmos-
pheric gas oil, light vacuum gas oil, and light coker gas oil. Lower
conversion of light virgin feed is caused by:

1. Lower molecular weight means the oil is more difficult to crack,
2. Light coker stocks are very aromatic.
3. Light aromatics have fewer crackable side chains.
Economics and unit configuration dictate whether to include 650°F
material in the FCC feed. As a general rule, this fraction should be
minimized. Minor improvements in the operation of the upstream
distillation columns can substantially reduce the amount of light gas
oil in the FCC feed. However, including light gas oil in FCC feed
reduces the amount of coke laid on the catalyst. Less coke means a
lower regenerator temperature. Light gas oil can be used as a "quench"
to decrease the regenerator temperature and to increase the catalyst-
to-oil ratio.
The distillation also provides information about the fractions that
boil over 900°F (482°C). These fractions provide an indication of the