Page 34 - Fluid Catalytic Cracking Handbook

18 Fluid Catalytic Cracking Handbook

fines. The third-stage separator, which is external to the regenerator,
contains a large number of swirl tubes designed to separate 70% to
95% of the incoming particles from the flue gas.
A power recovery train (Figure 1-12) employing a turbo expander
usually consists of four parts: the expander, a motor/generator, an air
blower, and a steam turbine. The steam turbine is primarily used for
start-up and, often, to supplement the expander to generate electricity.
The motor/generator works as a speed controller and flywheel; it can
produce or consume power. In some FCC units, the expander horsepower
exceeds the power needed to drive the air blower and the excess power
is output to the refinery electrical system. If the expander generates less
power than what is required by the blower, the motor/generator provides
the power to hold the power train at the desired speed.
From the expander, the flue gas goes through a steam generator to
recover thermal energy. Depending on local environmental regulations,
an electrostatic precipitator (ESP) or a wet gas scrubber may be placed
downstream of the waste heat generator prior to release of the flue
gas to the atmosphere. Some units use an ESP to remove catalyst fines
in the range of 5|i-20ji from the flue gas. Some units employ a wet
gas scrubber to remove both catalyst fines and sulfur compounds from
the flue gas stream.

Partial versus Complete Combustion

Catalyst can be regenerated over a range of temperatures and flue
gas composition with inherent limitations. Two distinctly different
modes of regeneration are practiced: partial combustion and complete
combustion. Complete combustion generates more energy when coke
yield is increased; partial combustion generates less energy when the
coke yield is increased. In complete combustion, the excess reaction
component is oxygen, so more carbon generates more combustion. In
partial combustion, the excess reaction component is carbon, all the
oxygen is consumed, and an increase in coke yield means a shift from
CO 2 to CO.
FCC regeneration can be further subdivided into low, intermediate,
and high temperature regeneration. In low temperature regeneration
(about 1,190°F or 640°C), complete combustion is impossible. One
of the characteristics of low temperature regeneration is that at 1,190°F,
all three components (O 2, CO, and CO 2) are present in the flue gas at