Page 109 - Fluid Catalytic Cracking Handbook

86 Fluid Catalytic Cracking Handbook

meters per gram, do not readily admit hydrocarbon molecules that have
a molecular diameter greater than 8.0°A to 10°A.
The elementary building block of the zeolite crystal is a unit cell.
The unit cell size (UCS) is the distance between the repeating cells
in the zeolite structure. One unit cell in a typical fresh Y-zeolite lattice
contains 192 framework atomic positions: 55 atoms of aluminum and
137 atoms of silicon. This corresponds to a silica (SitX,) to alumina
(A1 2O 3) molal ratio (SAR) of 5. The UCS is an important parameter
in characterizing the zeolite structure.

Zeolite Chemistry
As stated above, a typical zeolite consists of silicon and aluminum
atoms that are tetrahedrally joined by four oxygen atoms. Silicon is
in a +4 oxidation state; therefore, a tetrahedron containing silicon is
neutral in charge. In contrast, aluminum is in a +3 oxidation state. This
indicates that each tetrahedron containing aluminum has a net charge
of -1, which must be balanced by a positive ion.
Solutions containing sodium hydroxide are commonly used in
synthesizing the zeolite. The sodium serves as the positive ion to
balance the negative charge of aluminum tetrahedron. This zeolite is
called soda Y or NaY. The NaY zeolite is not hydrothermally stable
because of the high sodium content. The ammonium ion is frequently
used to displace sodium. Upon drying the zeolite, ammonia is vaporized.
The resulting acid sites are both the Bronsted and Lewis types. The
Bronsted acid sites can be further exchanged with rare earth material,
such as cerium and lanthanum to enhance their strengths. The zeolite
activity comes from these acid sites.

Zeolite Types
Zeolites employed in the manufacture of the FCC catalyst are
synthetic versions of naturally occurring zeolites called faujasites.
There are about 40 known natural zeolites and over 150 zeolites that
have been synthesized. Of this number, only a few have found commercial
applications. Table 3-1 shows properties of the major synthetic zeolites.
The zeolites with applications to FCC are Type X, Type Y, and
ZSM-5. Both X and Y zeolites have essentially the same crystalline
structure. The X zeolite has a lower silica-alumina ratio than the Y
zeolite. The X zeolite also has a lower thermal and hydrothermal