Application Data Sheet
Power
Optimizing Catalyst Regeneration
with an In Situ Oxygen Probe
Optimizing the Performance of the
Catalytic Cracking Process Through
Better Gas Analysis
Catalytic crackers have long been utilized to extract additional
gasoline from heavier components resulting from the distillation
process. The distillation process is the physical separation of a
MIXTURE of different molecules, based upon the different boiling
points of these molecules. The catalytic cracking process splits
larger hydrocarbon molecules into lighter and higher value
components such as gasoline by using a catalyst, which aids the
reaction or "cracking" process. The cracking process produces
carbon, or coke, which remains on the catalyst particle, reducing
its effectiveness over time. Fluidized Catalytic Cracking Units
(FCCU) will continuously route coked catalyst into a regenerator
unit where oil remaining on the surface of the catalyst is stripped
off with steam or solvent. The catalyst is then sent into the
regenerator, where air is introduced to burn the coke off of the
hot catalyst, usually in suspension. There are many different
variations in the regeneration process, including the Continuous
Catalyst Regenerator (CCR) process.
The regeneration of catalyst frequently becomes a bottleneck
that limits the throughput of the catalytic cracking process, so
optimizing this process is important.
Complete Burn Regeneration
A complete burn regeneration process burns all of the coke off,
and the resulting flue gases are routed through gas-cleaning
equipment, and then to a smokestack. Oxygen is measured in
the flue gas resulting from the coke burn-off to maximize the
coke removal, and throughput. This may take place at the top
of the regeneration tower, and under pressure, or after a turboexpander that recovers energy and results in lower pressures
closer to the smokestack (see Figure 1). In-situ zirconiumoxide oxygen analyzers can be utilized to measure the flue
gas O2 resulting from regeneration in either location. Pressure
affects the analyzer readings, so a pressure balancing system is
recommended for pressures above 2 PSI.
Figure 1 - Typical Regenerator with Integral Combustor Section
To Fractionation
O2 Probe Mounted
in Duct Work
or Stack
Combustor-style
Regenerator
Open Riser
System
Flue Gas
Combustor
Riser
Hot-Catalyst
Circulation
Feed
Catalyst
Cooler
Air
Lift Media
Partial Burn Regeneration
A partial burn regeneration process endeavors to volatize,
or outgas the coke, and produce CO gas that is then burned
in a separate CO boiler. Some coke is also burned in this
process, and one goal is to control the CO-to-CO2 ratio in the
regeneration off-gases in order to maximize the burn-off, and
hence the throughput. An extractive analytical system utilizing
gas chromatographs or Infra-red process analyzers is typically
used to measure and control the CO and CO2 ratios. Refer to
Application Note PGC_ANO_Refining_Improving_Catalytic_
Cracking_Vapor for a detailed description of these measuring
systems.
Oxygen Enrichment Improves
Throughput (see Figure 2)
Enriching the air used in the regeneration process can increase
the coke burn-off rate. Many refineries will mix pure oxygen with
the air used for regeneration, resulting in a mixture of 21 to 25 %
O2, which increases the efficiency of the regeneration process.
continued...
Power
This oxygen measurement can be used for operator information,
alarming, or automatic control of the oxygen injection valve
(see Figure1). Pressures are normally 35 PSI, so the probe must
be pressure-balanced with reference air (see reverse side).
Figure 3 - CCR Regenerator
Air
Spent
Catalyst
Cooler
Figure 2 - Typical Regenerator with Separate CO Boiler
(Not Shown)
Heater
Flue Gas to CO Boiler
Plenum
Chamber
2 Stage
Cyclone
Separators
Dip
Leg
O2 Probe
Installation
Spent Catalyst
from Reactor
Fluidized
Catalyst Bed
Air
Grid
Liquid Oxygen
Tank
Vaporization
Uunit
Figure 4 - Pressure Balanced in Situ O2 Probe with Optional
Isolation Valving System (Probe Withdrawn)
Spent Catalyst
from Reactor
Oxygen
Analyzer
Probe
Oxygen
Analyzer
Valve
Regenerated Catalyst to
Reactor-rise
+30 in > H2O Pressure
IsolationValve
Air Blower
Oxygen Enrichment Improves Throughput
Enriching the air used in the regeneration process can increase
the coke burn-off rate. Many refineries will mix pure oxygen
with the air used for regeneration, resulting in a mixture of
22–30 % O2, which increases the speed and throughput of the
regeneration process.
Pressure Balancing System
Oxymitter
Oxygen Probe
The rate of oxygen injection can be controlled by an O2 analyzer
just after the mixing valve injection valve (see Figure 1).
Pressures are normally 35 PSI, so the probe must be pressurebalanced with reference air.
HART-Wireless capable
Pressure Balanced O2 Probe
Continuous Catalytic Regenerator (CCR)
Utilized with a "moving bed" process, the CCR employs a
regeneration gas loop to control temperature and O2 levels in the
regenerator "burn zone." This recirculation loop heats or cools
the combustion flue gas resulting from the regeneration process
to control temperature, and air is also controlled to an optimum
O2 concentration (see Figure 3). The O2 measurement is critical
to maintaining optimum rate of regeneration, and for preventing
thermal damage inside the burn zone. Process pressures may
be close to atmospheric, or pressurized to approximately 35 PSI.
Pressure balancing may be required, and an isolation valving
system could be needed if probe insertion or withdrawal is
required with the process on-line. Pressure balancing will adjust
to pressure variations, while traditional O2 systems place a fixed
compensation into the electronics. Chlorine is injected into this
unit to assist in getting an even coating of platinum or other
catalyst materials. A special HCL-resistant sensing cell is required.
Most in situ oxygen analyzers utilize zirconium oxide sensing
technology, which is sensitive to pressure variations in the
process. An output change of approximately 1 % of reading
(not 1 % FS, or 1 % O2) can be expected for every 4'' of H2O
pressure in the process. Special accommodation must be made
to balance the process pressure with the inside of the oxygen
probe. Rosemount Analytical has a special probe design that
balances the inside of the probe to the same pressure as the
process. A sealed probe is used along with a "booster relay" which
duplicates the pressure of the process with the instrument air
being used as a reference gas (see figure 4). For accurate, reliable
oxygen analysis, Rosemount Analytical offers the 6888 O2
Analyzer with integral electronics. The user friendly design of the
probe allows convenient access to internal probe components for
in-house service. In addition, the Rosemount Analytical patented
electronic cell protection automatically protects the sensor cell's
electrodes from harmful corrosive gases. And, the HART® Field
Communications Protocol permits all operator functions to be
performed from the control room.
continued...
Page 2
Power
The Emerson Solution
Emerson Process Management offers a wide array of
instrumentation for improving the operation of the catalyst
regeneration process including physical measurements
(pressure, temperature, and flow), complete analytical solutions,
valves and actuation, and SMART Process advanced control
solutions. Emerson’s technologies have set the standard for
on-line process measurement and control.
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