Over the years, petroleum refineries around the world have
researched and invested in ways to reduce the amount of waste
energy and pollutants produced and released into the
environment from their processing operations. The diverse
processes used to produce crude oil based products usually
require large amounts of electricity to run the various
compressors and pumps.
Many oil refineries have projects to recapture the energy as
part of their cost savings plan and are driven by environmental concerns to reduce
pollutants generated by process units. The hot gas expander is
a single-stage power turbine capable of converting the
potential energy of flue (waste) gas into mechanical work. This
article describes how refineries can go green by
implementing fluid catalytic cracking (FCC) hot gas
Highlighting the FCC process.
The FCC process is widely used for manufacturing of gasoline
and petrochemical feedstock. The process uses three
main process vessels: the reactor, fractionator and
regenerator. As shown in Fig. 1, feedstock (crude oil) enters the FCC
reactor; here, a catalyst strips carbon molecules from the larger
hydrocarbon chains in the feed oil. This reaction breaks the
hydrocarbons down into smaller, more useful hydrocarbon
products. The hydrocarbon mixture is sent to the fractionator
where it is vaporized and cooled under controlled conditions to
various levels. The process allows desirable petroleum-oil refinery products to be separated
Fig. 1. Flow diagram
of FCC unit.
Fig. 1A. Hot gas FCC
The final portion of the FCC process is the regenerator. The
used (spent) catalyst from the reactor is sent to the
regenerator to be stripped of carbon and recycled back to the
reactor. Compressed air is pumped into the regenerator to mix
with the used catalyst in a combustion process. Also taking
place in the regenerator is the separation of particles from
the flue gas. There are generally two stages of cyclones in the
regenerator that strip the exhaust gas of its catalyst. The
flue gas exiting the top of the regenerator will typically pass
through an additional vessel called a third-stage separator
(TSS) to further reduce the catalyst amount in the flue
The flue gas from the FCC process exiting the regenerator
has significant pressure, temperature and volume, and it is a
source of useful energy that represents an energy cost-saving
opportunity to a refinery. One method of harvesting the
potential of the flue gas is a heat recovery steam generator
(HRSG). The HRSG uses the heat from the flue gas to create
steam. However, this method ignores the pressure component, a
potential energy source that can be converted to mechanical
A second method is to use an expander to recover energy from
the flue gas. This energy can then be used to drive the
compressor that provides air to the regenerator (the main air
blower) or an electric generator.
In this case, the FCC process is the existing (and primary)
process and it can be useful to think of the power recovery
system as a secondary process (see Figs. 1 and 2). The refinery
operator is primarily interested in the FCC process. It is the
FCC process that generates the refinery primary products
(gasoline, propane, fuels, etc.) and, thus, it yields revenues
for the refinery. The power recovery process increases energy
efficiency for the overall plant and therefore, it increases
Fig. 2. Expander
generator power recovery system for
an FCC Plant.
Hot gas expanders can be used to drive the compressor that
provides air to the regeneratorthe power recovery train
(PRT). Alternatively, it can be used as the driver for
stand-alone expander-generator sets. Fig. 3 shows a main air
blower PRT; Fig. 4 illustrates an expander-generator set. In
both cases, the expander maximizes recovery of available energy
from the flue gas.
Fig. 3. Main air blower
power recovery train.
Main air blower train.
A main air blower PRT consists of a steam turbine,
compressor, motor/generator and expander. The expander in the
PRT is used to drive the compressor, and often supplies
additional power for the generation of electricity. In this
case, the expander cannot provide all of the power needed to
drive the compressor; motor/generator will operate in motor
mode. A steam turbine is used for startup.
Fig. 4. Expander
Electric power generation train.
Expanders in an expander-generator application drive a
generator, thus using the entire power production to generate
electricity for the refinery. In general, the
expandergenerator set stands to benefit the customer the
most. The key benefits include:3
Easily added to existing FCC
Installed remote from the FCC unit or main air
Does not need to match the air blower operating
No modifications to the air blower equipment
Installed during FCC operation and tied in at a
Taken on- or off-line at any time without affecting
FCC unit operation
Has a high efficiency due to equipment
FCC expanders: Description and operation.
Fig. 5 represents an FCC hot gas expander; it shows the path
of the hot flue gas flow passing through it. Major components
can be seen assembled in Fig. 6. The FCC hot gas expander is a
single-stage axial-flow turbine. The pressurized,
high-temperature flue (exhaust) gas coming from the FCC process
enters the inlet opening of the expander and is accelerated
through the stationary and rotating blades. In the expander,
the pressure and temperature are reduced, and energy is
extracted and converted into mechanical work.
Fig. 5. Cross-sectional
view of the FCC hot
gas expander (flue gas flow
6. Cross-sectional view of the FCC
gas expander (major
Although the flue gas has been processed through multiple
separation stages, a significant amount of catalyst particles
will remain in the flue gas. The catalyst particles pass
through the expander and can potentially cause erosion. The
expander flow paths stationary and rotating components
are optimized to efficiently extract the pressure energy from
the flue gas and to minimize catalyst erosion.4
Rotor disc cooling and seals are used to increase the service
life. The latest developments in material alloys and coatings
can be used to mitigate damaging effects from handling
catalyst-laden flue gas. All components are designed for reliability, including casings,
bearings and supports. All of these factors allow the expanders
to regularly withstand four to five years of continuous
Based on process conditions (pressure, temperature and
flow), a customized flow path has to be selected to meet the
process requirements. Selecting the right expander frame size
is very important, and standardized frames that can cover a
large range of pressures and flows have been developed, as
shown in Fig. 7.
Fig. 7. Frame
History of expanders for FCC applications.
Initial expander development took place between the late
1950s and the mid-1970s. In all, hundreds of units were
installed, each with unique designs for specific installations.
Second-generation expanders were developed from the mid-1970s
through the mid-1980s, and this era experienced significant
expander production. Hot gas expanders became available with
increasing frame size options. However, there were also
increased industry concerns regarding equipment reliability as
well as the desire to increase the time between shutdowns.
Some of the initial problems/issues faced during the
development of expanders over the decades
Lack of proper catalyst separation
mechanism. The expander flow path components would
wear quickly due to the catalyst content in the flue gas. The
introduction of an additional catalyst separator (external to
the regenerator) commonly called the TSS was crucial to
extending the life of expander components.
Lower refinery throughput factors such as
lower pressures and temperatures meant lower power recovery
opportunities for refineries
Difficulties of designing customized
expanders for every FCC application. As every refinery
designs and operates their FCC unit differently, each expander
needed to be customized. Only a few expander manufacturers had
the capabilities to design custom solutions. To overcome this
issue, pre-standardized frames were developed to cover
refineries production needs (see Fig. 7 for sample
Validation of investment cost vs.
benefits over the years. Initially, during the earlier
developmental years (1960s and 1970s), it was not essential to
install an expander since pollution-control measures were not
given as much priority as they are today. As more government
laws have targeted pollution control and green initiatives,
more refineries are investing in ways to reduce their power
Over the last 20 years, design programs have been developed
to address the user industrys increased emphasis on
expander reliability and extended time
between shutdowns. These programs focused on material upgrades,
CFD-designed flow paths, more efficient catalyst removal and
robust control systems. Modern FCCU hot gas expanders are
designed to run for four to five years to coincide with FCC
unit maintenance intervals and have
demonstrated this capability in many applications.
Economic benefits of power recovery.
Energy costs are a major part of the total costs of
operating an oil refinery, and electricity usage is a large
part of these energy costs (steam is an alternate energy
source), with energy requirements ranging from 50 MW to 180 MW.
FCC expanders can help reduce these costs even if the
temperature drop that is experienced through the FCC expander
will result in decreased steam production for the refinery. Due
to the variability of refinery steam requirements and HRSGs,
the reduction in steam temperature and production is not easily
quantified. Additionally, a loss in the quantity or temperature
of the steam does not easily translate into a monetary figure.
These values must be evaluated based on the specifics of the
application. However, a very rough approximation can be made
based on actual cases.
The expander reduces the flue gas temperature entering the
HRSG by approximately 300°F. For a typical HRSG, the exit
flue-gas temperature will be the same with or without an
expander. Thus, the flue-gas temperature reduction through the
HRSG will be 300°F. To maintain superheated steam
production, the amount of steam produced will be less when an
expander is used.
Table 1 shows the estimated savings in electricity per year
attributable to a power recovery unit installation. The notable
savings due to higher power recovery are evident in comparing
current process conditions (2007) to those of the 1960s.
Factors taken into consideration for Table 1 values
Steam generation losses (flow and temperature) may
be experienced and must be debited
Installed cost is approximately $30$55
Typical payback is less than three years.
Environmental benefits of power recovery.
In a power recovery system installation, there are environmental benefits associated
with the economic benefits to the refinery. The positive impact
on the environment comes from the fact that the need to install
sources of electricity to run machinery is reduced, with the
consequent elimination of the emission of carbon dioxide (CO2),
nitrogen oxides (NOx) and other pollutants
associated with the combustion of fossils fuels.
Often, the energy usage and efficiency of a refinery is
measured through the Energy Intensity Index (EII). The
installation of a power recovery system can reduce a
refinerys EII by 7%10%, thus helping them to reduce
their environmental impact and to comply with specific
A few numbers can give a better understanding of the huge
benefits of this solution. For example, if we consider the
total installed fleet of one leading manufacturers FCC
hot gas expanders, the estimate is that it produces around 500
mega watts (500 MW) of power, which corresponds to 4.3 billion
kWh (4.3 TWh) of electricity saved per year. Using the US
Environmental Protection Agency (US-EPA) Emissions Calculator, 4.3 TWh of
electricity saved per year translates into approximately 3.1
million metric tons of CO2 emissions avoided per
year.5 In simpler terms, this is equivalent
Annual greenhouse gas emissions from 565,000 passenger
CO2 emissions from 350 million gallons
of gasoline consumed
CO2 emissions from the electricity used
by 428,000 homes in one year
Carbon sequestered annually by 702,000 acres of
pine or fir forests.
As can be seen, the installation of FCC expanders in
refineries has a noticeable environmental impact.
As the petroleum refining industry continues to
strive for more ways to save energy, reduce costs and improve
the environment, more innovative ways will be pursued to
deliver more benefits to customers. FCC hot gas expander technology has grown significantly
from the 1960s to the present, and it now offers state of the
art machinery that can be incorporated into the refinery process without impacting
plant reliability or efficiency. The huge
economic and environmental benefit of this energy recovery
solution proves that FCC hot gas expanders are a significant
contribution in the drive for green applications in
the oil and gas industry. HP
1 US Environmental Protection Agency Executive
Summary, 2008 Sector Strategies Performance Report,
Executive summary, http://www.epa.gov/ispd/
2 Bloch, H. and C. Soares, Turboexpanders and
process applications, First Ed., Gulf Publishing Company,
2001, ISBN 0-88415-509-9.
3 Conroy, C.F and D. H. Linden, Successful
Application of Stand-alone FCCU Expander / Generator
Sets, International Symposium on Turbomachinery,
combined-Cycle Technologies and Cogeneration IGTI-Vol.1.
4 Carbonetto, B. and G. L. Hoch, Advances in
Erosion Prediction of Axial Flow Expanders, Proceedings
of the 28th Turbo machinery Symposium, Turbomachinery
Laboratory, Texas A&M University, College Station, Texas,
pp. 17, 1999.
5 Equivalencies Calculator, 2009. US Environmental Protection Agency, (
Carbonetto is hot gas expander product leader,
GE Oil & Gas. His career spans 15 years in the design
and operation of turbomachinery, starting in 1995 as an
axial compressor / hot gas expander design engineer. In
2001, he was promoted to Sr. (lead) design engineer, a
role which expanded to include responsibility for orders,
engineering and execution as expander product
supervisor/manager. Mr. Carbonetto has also served as
engineering manager (2005) and North America services
engineering manager (2007). He holds a BS degree in
mechanical engineering and mechanics from Drexel
University and is a member of ASME.
Pecchi joined GE Oil & Gas in 1996 as a
fluid dynamics engineer with the R&D team. In 1998,
he joined the technical leadership program (TLP) covering
different technical assignments in the gas turbine
department, following LM2500+ HSPT product introduction
and field installation. Upon TLP graduation, he held the
role of project/system engineer in the
new product Introduction organization, and in 2004, he
was appointed NPI-NTI programs management leader, being
responsible for the management of the companys
technical development programs. In 2008, he relocated to
Bethlehem, Pennsylvania, as manager of the engineering
team. Mr. Pecchi is now the global technical fleet
support managerturbomachinery. Mr. Pecchi graduated
with a degree in mechanical engineering in 1995 from the
University of Florence, Italy.