Page 112 - Fluid Catalytic Cracking Handbook

FCC Catalysts 89

possess any activity. The UCS is related to the number of aluminum
atoms per cell (N Af) by [1]:

N A, + 111 x (UCS - 24.215)

The number of silicon atoms (N si) is;

N si = 192 - N A,

The SAR of the zeolite can be determined either from the above two
equations or from a correlation such as the one shown in Figure 3-3.
The UCS is also an indicator of zeolite acidity. Because the alumi-
num ion is larger than the silicon ion, as the UCS decreases, the acid
sites become farther apart. The strength of the acid sites is determined
by the extent of their isolation from the neighboring acid sites. The
close proximity of these acid sites causes destabilization of the zeolite
structure. Acid distribution of the zeolite is a fundamental factor
affecting zeolite activity and selectivity. Additionally, the UCS
measurement can be used to indicate octane potential of the zeolite.
A lower UCS presents fewer active sites per unit cell. The fewer acid
sites are farther apart and, therefore, inhibit hydrogen transfer reactions,
which in turn increase gasoline octane as well as the production of
C 3 and lighter components (Figure 3-4). The octane increase is due
to a higher concentration of olefins in the gasoline.
Zeolites with lower UCS are initially less active than the con-
ventional rare earth exchanged zeolites (Figure 3-5). However, the
lower UCS zeolites tend to retain a greater fraction of their activity
under severe thermal and hydrothermal treatments, hence the name
ultrastable Y.
A freshly manufactured zeolite has a relatively high UCS in the
range of 24,50°A to 24.75°A. The thermal and hydrothermal environ-
ment of the regenerator extracts alumina from the zeolite structure and,
therefore, reduces its UCS. The final UCS level depends on the rare
earth and sodium level of the zeolite. The lower the sodium and rare
earth content of the fresh zeolite, the lower UCS of the equilibrium
catalyst (E-cat).

Rare Earth Level. Rare earth (RE) elements serve as a "bridge"
to stabilize aluminum atoms in the zeolite structure. They prevent the