Page 161 - Fluid Catalytic Cracking Handbook

136 Fluid Catalytic Cracking Handbook

about 950°F (510°C). Consider two riser temperatures, 850°F and
1,050°F (454°C and 566°C), at the extreme limits of operation. At
850°F, a large amount of coke is formed because the carbenium ions
do not desorb at this low temperature. At 1,050°F (566°C), a large
amount of coke is formed, largely due to olefin polymerization. The
minimum coking temperature is within this range.

THERMODYNAMIC ASPECTS

As stated earlier, catalytic cracking involves a series of simultaneous
reactions. Some of these reactions are endothermic and some are
exothermic. Each reaction has a heat of reaction associated with it
(Table 4-4). The overall heat of reaction refers to the net or combined
heat of reaction. Although there are a number of exothermic reactions,
the net reaction is still endothermic.
The regenerated catalyst supplies enough energy to heat the feed
to the riser outlet temperature, to heat the combustion air to the flue
gas temperature, to provide the endothermic heat of reaction, and to
compensate for any heat losses to atmosphere. The source of this
energy is the burning of coke produced from the reaction.
It is apparent that the type and magnitude of these reactions have an
impact on the heat balance of the unit. For example, a catalyst with less
hydrogen transfer characteristics will cause the net heat of reaction to be
more endothennic. Consequently this will require a higher catalyst circu-
lation and, possibly, a higher coke yield to maintain the heat balance.

SUMMARY

Although cat cracking reactions are predominantly catalytic, some
nonselective thermal cracking reactions do take place. The two pro-
cesses proceed via different chemistry. The distribution of products
clearly confirms that both reactions take place, but that catalytic
reactions predominate.
The introduction of zeolites into the FCC catalyst in the early 1960s
drastically improved the performance of the cat cracker reaction
products. The catalyst acid sites, their nature, and strength have a
major influence on the reaction chemistry.
Catalytic cracking proceeds mainly via carbenium ion intermediates.
The three dominant reactions are cracking, isomerization, and hydrogen