RESPONSABLE DE LA INVESTIGACION:

B. CARBONETTO and P. PECCHI, GE Oil & Gas, Florence, Itlay

Regenerator (1,300°F-1,400°F) Reactor (920°F-1,050°F)

Power recovery Flue gas

Electricity Steam

Spent catalyst

Regenerated catalyst

Air

Fuel gas

LPG Gasoline HCN (gasoline)

LCO (diesel)

Clarified oil (fuel oil) Raw oil: VGO,

resid, etc.

Fractionator

Flow diagram of FCC unit.

FIG. 1 FIG. 1A Hot gas FCC expander—side view.

Select 59 at www.HydrocarbonProcessing.com/RS

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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 profitability.

Hot gas expanders can be used to drive the compressor that provides air to the regenerator—the 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.

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.

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 expander–generator set stands to benefit the customer the most. The key benefits include:³

• Easily added to existing FCC installations

• Small footprint

• Installed remote from the FCC unit or main air blower train

• Does not need to match the air blower operating conditions—speed

• No modifications to the air blower equipment

• Installed during FCC operation and tied in at a scheduled turnaround

• Taken on- or off-line at any time without affecting FCC unit operation

• Has a high efficiency due to equipment optimization.

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.

Although the flue gas has been processed through multiple separation stages, a significant amount of catalyst particles will

Expander generator power recovery system for an FCC Plant.

FIG. 2

Expander generator set.

FIG. 4

Main air blower power recovery train.

FIG. 3 Cross-sectional view of the FCC hot gas expander (flue gas

flow path).

FIG. 5

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remain in the flue gas. The catalyst particles pass through the expander and can potentially cause erosion. The expander flow path’s 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 operating cycles.

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.

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 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 develop-ment of expanders over the decades included:²

• 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 addi-tional 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 pres-sures and temperatures meant lower power recovery opportu-nities 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 custom-ized. Only a few expander manufacturers had the capabilities to design custom solutions. To overcome this issue, pre-standard-ized frames were developed to cover refineries’ production needs (see Fig. 7 for sample chart).

• 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 pollu-tion-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 consumption.¹

Over the last 20 years, design programs have been developed to address the user industry’s 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 dem-onstrated this capability in many applica-tions.

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 tem-perature drop that is experienced through

0 50 100 150 200 250 300 350 400

Flue gas flowrate, lbs/sec

Pressure ratio

FEX-81 FEX-97 FEX-107 FEX-125 FEX-142 22 TBD 40 TBD 55 TBD 75 TBD 110 TBD

Refinery capacity: TBD = thousands barrels per day

Frame selection chart.

FIG. 7

Cross-sectional view of the FCC hot gas expander (major components).

FIG. 6

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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 pro-duction, 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 include:

• Steam generation losses (flow and temperature) may be experienced and must be debited

• Installed cost is approximately $30–$55 million

• 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 refinery’s EII by 7%–10%, thus helping them to reduce their environmental impact and to comply with specific regulations.

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 manufacturer’s FCC hot gas expand-ers, 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 Protec-tion Agency (US-EPA) Emissions Calculator, 4.3 TWh of electric-ity saved per year translates into approximately 3.1 million metric tons of CO2 emissions avoided per year.⁵ In simpler terms, this is equivalent to:

• Annual greenhouse gas emissions from 565,000 passenger vehicles

• 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.

Outlook. As the petroleum refining industry continues to strive for more ways to save energy, reduce costs and improve the

envi-ronment, 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 sig-nificant contribution in the drive for “green” applications in the oil and gas industry. HP

LITERATURE CITED

1 US Environmental Protection Agency Executive Summary, 2008 Sector Strategies Performance Report, Executive summary, http://www.epa.gov/ispd/

pdf/2008/executive_summary.pdf

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. Greenhouse Gas.

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. 1–7, 1999.

5 Equivalencies Calculator, 2009. US Environmental Protection Agency, (http://www.epa.gov/cleanenergy/energy-resources/calculator.html).

TABLE 1. Potential recovered power and savings

Year

Design conditions 1962 2007

Inlet temperature–Basis: the assumption 1,100 1,400 of a 25 degree drop from the regenerator, °F

Inlet pressure–Basis: the assumption of a 13 50 4 psi drop from the regenerator, psig

Discharge pressure, psig 1.5 13

Flow, lbm/hr 200,000 1,750,000

Delivered power, kW 6,340 52,944

Electricity savings–Basis: average electricity $3,332,304 $27,873,366

cost of 6 cents/kW-hr

Ben 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 ser-vices engineering manager (2007). He holds a BS degree in mechanical engineering and mechanics from Drexel University and is a member of ASME.

Paolo Pecchi joined GE Oil & Gas in 1996 as a fluid dynam-ics 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 company’s 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 manager—turbomachinery. Mr. Pec-chi graduated with a degree in mechanical engineering in 1995 from the University of Florence, Italy.