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 expanders.
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 out.1,2
| Fig. 1. Flow diagram of FCC unit. |
| Fig. 1A. Hot gas FCC expanderside view. |
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 gas.
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 work.
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 profitability.
| 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 generator set. |
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 installations
Installed remote from the FCC unit or main air blower train
Does not need to match the air blower operating conditionsspeed
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.
| Fig. 5. Cross-sectional view of the FCC hot|
gas expander (flue gas flow path).
| Fig. 6. Cross-sectional view of the FCC hot|
gas expander (major components).
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 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.
| Fig. 7. Frame selection chart. |
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 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 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 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 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 consumption.1
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 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 refinerys 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 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 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.
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. 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. 17, 1999.
5 Equivalencies Calculator, 2009. US Environmental Protection Agency, (http://www.epa.gov/cleanenergy/energy-resources/calculator.html).
|The authors |
||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 services 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 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. |