Page 111 - Fluid Catalytic Cracking Handbook

88 Fluid Catalytic Cracking Handbook

cerium, were used to replace sodium in the crystal. The rare earth
elements, being trivalent, simply form "bridges" between two to three
acid sites in the zeolite framework. Bridging protects acid sites from
being ejected and stabilizes the zeolite structure. Rare earth exchange
adds to the zeolite activity and thermal and hydrothermal stability.
The reduction of lead in motor gasoline in 1986 created the need
for a higher FCC gasoline octane. Catalyst manufacturers responded
by adjusting the zeolite formulations, an alteration that involved
expelling a number of aluminum atoms from the zeolite framework.
The removal of aluminum increased SAR, reduced UCS, and in the
process, lowered the sodium level of the zeolite. These changes
increased the gasoline octane by raising its olefinicity. This aluminum-
deficient zeolite was called ultrastable Y, or simply USY, because of
its higher stability than the conventional Y.

Zeolite Properties
The properties of the zeolite play a significant role in the overall
performance of the catalyst. Understanding these properties increases
our ability to predict catalyst response to changes in unit operation.
From its inception in the catalyst plant, the zeolite must retain its
catalytic properties under the hostile conditions of the FCC operation.
The reactor/regenerator environment can cause significant changes in
chemical and structural composition of the zeolite. In the regenerator,
for instance, the zeolite is subjected to thermal and hydrothermal
treatments. In the reactor, it is exposed to feedstock contaminants such
as vanadium and sodium.
Various analytical tests determine zeolite properties. These tests
supply information about the strength, type, number, and distribution
of acid sites. Additional tests can also provide information about
surface area and pore size distribution. The three most common
parameters governing zeolite behavior are as follows:

• Unit Cell Size
• Rare Earth Level
« Sodium Content

Unit Cell Size (UCS). The UCS is a measure of aluminum sites or
the total potential acidity per unit cell. The negatively-charged aluminum
atoms are sources of active sites in the zeolite. Silicon atoms do not