Advances in catalytic cracking technologies are improving efficiency, product selectivity and environmental emissions. Optimum performance, reliability and flexibility of fluid catalytic cracking (FCC) units is essential for the continued competitiveness of refineries and their ability to meet market demands for refined products. FCC traditionally converts gas oil and heavier residual oil to mainly gasoline and diesel blending components.
Global-market trends in petroleum refining include continued growing demand for transportation fuels, generally decreasing or slowly growing regional demand for heavy fuel oils and increasing demand for propylene as petrochemical feedstock. Increasing production of heavy and extra-heavy crude oils and greater emphasis on reducing environmental emissions from refineries as well as vehicles are worldwide trends in the refining industry.
Gasoline and diesel fuels
Global demand for transportation fuels over the next ten years or longer will continue to grow and be predominantly met by gasoline and diesel fuels. Growth is forecast to be greatest in Asia (particularly China and India), Russia, Central Europe and South and Central America as these developing economies grow. In the developed economies, the projected growth rates are less, but the market for refined petroleum products is changing nonetheless. Western European refineries are seeing demand for diesel fuel growing faster than the demand for gasoline, some of which is currently exported to North America. North American refineries are experiencing solid demand growth for gasoline and running near capacity limitations.
Specifications for gasoline are set by each country, or in Europe, the European Commission. Voluntary requirements are also imposed, such as the Distillation Index in the U.S. (set at 1,250°F, 677°C in 2001, measured at the refinery gate). In the United States, the mandated formulations also change in vapor pressure and oxygenate requirements between summer and winter. The required octane rating varies with altitude. Oxygenated gasoline reduces the emission of CO. Reformulated gasoline reduces the formation of ozone by reducing volatile organic compounds (VOCs) such as isopentenes and by reducing the nitrogen oxides. Isopentenes are undesirable in the air due to their high atmospheric reactivity and their role in the formation of nitrogen oxides and ozone. Reformulated gasoline in the U.S. is blended to either federal or California formulations. Benzene and total aromatics have a high effect on air toxics. The volatility (Reid vapor pressure) and boiling range are also controlled, and have high and moderate effects respectively on the emission of VOCs. Limiting olefins slightly affects VOCs, air toxics and NOx. Detergent additives to maintain engine fuel system cleanliness are required. Reformulated gasoline composition and properties are specified for the U.S. in Section 211(k) of the Clean Air Act and its amendments. Excluding the octane grades, there are over 15 different gasoline blends produced; the top four summertime blends representing approximately 83% of the market.
Four leading gasoline specifications are summarized in Table 1. The Worldwide Fuel Charter Category 4 is the most stringent category proposed by the automobile and engine manufacturers for future engines. The U.S. Tier 2 and the European Euro 4 specifications come closest to the Worldwide Fuel Charter.
Gasoline specifications change according to an implementation schedule and with new information. Future regulations are expected to continue to tighten, especially in urban areas and in developing countries. In the developed and much of the developing world, all gasoline must be lead free and below a maximum sulfur content specified by the country (see Table 2 for examples). Sulfur content has a high effect on the emission of air toxics and NOx. The U.S. gasoline sulfur limit may be reduced to 10 ppm in the future. Sulfur reduction to 10 ppm is being phased into parts of Europe from 2005 to 2009 and 15 ppm sulfur is going into parts of California. The trend towards low-sulfur gasoline is gaining momentum worldwide. Ultimately, most gasoline will need to meet low-sulfur specifications.
Increasing environmental demands will continue not only for reduced- and near-zero sulfur transportation fuels, but also from initiatives to reduce the amount of carbon dioxide released into the atmosphere. Diesel engines offer great potential for mid-term CO2 reductions according to DaimlerChrysler. Furthermore, current conventional diesels have a lower well-to-wheel CO2 rate per mile driven than conventional gasoline cars. Diesel-electric hybrid vehicles offer still lower well-to-wheel CO2 emissions per mile driven and at a fuel cost less than half that of fuel cells running on hydrogen obtained from natural gas, and lower still than water-electrolysis hydrogen based on a mix of U.S. electrical-generation sources according to an ExxonMobil study. Clean diesel fuel and new, low-emitting diesel engines are more cost effective than alternate fuels conversions.
Two automotive trends are also affecting FCC: lean-burn engines and increasing popularity of “fun-to-drive” diesel engines, especially in Europe. Lean-burn engines are more fuel efficient but require ultra-clean fuels. While sulfur received first emphasis, reduction of aromatics in diesel fuel may be next. Reducing aromatics increases the ratio of hydrogen-to-carbon in the fuel resulting in lower specific CO2 emissions per vehicle mile.
SRI Consulting forecasts that world demand for motor fuels (gasoline and diesel fuel) will increase about 24% from 41.4 million barrels per calendar day (bpcd) in 2004 to 51.4 million bpcd by 2014. The portion of demand for gasoline as compared to diesel fuel is expected to change as outlined in Table 3.
Propylene demand is rising
The global and some regional demands for propylene-derived products, especially polypropylene, are growing faster than the demand for ethylene-derived products such as polyethylene. Steam cracking for ethylene supplies about two-thirds of the world’s propylene production; propylene recovered from refinery FCC units supplies most of the balance (about 30%) with just a small portion obtained by dehydrogenation or other sources such as olefin metathesis. In steam cracking plants, the ratio of propylene to ethylene is typically about 0.65 from naphtha feedstock and cannot be adjusted very much. Recovery of propylene from ethane steam cracking is generally uneconomical since very little propylene is produced. The ratio of propylene to ethylene supplied from steam crackers is forecast to decline from 0.39 to 0.35 over the next ten years as new ethane cracking plants startup while the demand ratio increases slightly from 0.59 to 0.60. From 2005 to 2010, the worldwide percentage of ethane feedstock is forecast to increase from about 32% to 36% as the percentage of naphtha feedstock cracked declines from about 52% to 47.5%.
Refineries in North America (almost all on the U.S. Gulf Coast), Western Europe and Northeast Asia that are in close proximity to petrochemical plants recover propylene from FCC units. In the U.S. where steam cracker feed is rather light, FCC propylene surpasses steam cracker propylene as the largest source of petrochemical propylene. As further forecast, the gap between propylene demand and supply increases from steam cracking and FCC production and recovery could increase over the next ten years (Figure 1). This situation creates demand for on-purpose propylene production and an opportunity for the additional production and recovery of propylene from FCC units.
Spotlight on refinery emissions
With the start of the implementation of specifications requiring near-zero-sulfur diesel and gasoline contents and reduced-sulfur limits for off-road diesel fuel and even for residual-fuel oils, attention is growing on the emissions of SOx and NOx from refineries. The FCC unit is the biggest single source of air emissions in the refinery. Emissions from the flue gas are particulates, SOx, NOx, and in some units, CO.
In the U.S., the federal Clean Air Act Amendments and various state and local regulations require industry to reduce air pollutants from both new and existing sources. Federal New Source Performance Standards (NSPS) for FCC units regulates particulate matter, CO and SO2 emissions for units built after January 17, 1984 and existing units that undergo major reconstruction or modifications in equipment or operations.
The Maximum Achievable Control Technology (MACT) standards for petroleum refineries were issued in 1995. In 2002, the National Emission Standards for Hazardous Air Pollutants (NESHAP), commonly called MACT II regulations, established the allowable levels for FCCU particulates and CO emissions. MACT II uses nickel as a surrogate for other metal hazardous air pollutants (includes compounds of antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, and selenium). Metals emissions are limited by controlling the particulates emission at 1 lb particulates/1,000 lb coke burned. An alternative limit of 0.029 lb/hr of Ni from the FCC regenerator stack is provided. Plants located in designated “non-attainment areas” are subject to more stringent requirements.
From 2000-2001 to January 2004, 42 U.S. refineries owned by 11 companies have entered into binding consent decrees with the U.S. Environmental Protection Agency (EPA) and local agencies to, among other things, reduce future air emissions. A number of these refineries will decrease SOx and NOx air pollution from FCC units. The SO2 emission limitations on FCC units in the decrees (targeted at 25 ppm SO2 and 20 ppm NOx) are significantly lower than NSPS levels. Some decrees also include CO and particulate emissions. When additives are involved, the decrees now require EPA approval of the additives, testing protocols and emissions models. Comparative performance testing of multiple additives to determine the best additive and specific data collection and reporting are also required. Final emissions limits are set in post testing negotiations. Part of the basis for these decrees is perceived violations of New Source Review permitting regulations that could have triggered NSPS or more stringent Best Available Control Technology (BACT) requirements. Since consent decrees seem to be an effective tool, consent decrees will in all likelihood continue to be sought for SOx, NOx and CO emissions on a case by case basis.
Canada’s standard for VOCs is more vigorous than the U.S. standard. Canada is also aiming to cap NOx levels through new source standards. In Europe, the EU Commission adopted the National Emission Ceiling (NEC) Directive that requires a 77% reduction in SO2 and a 48% reduction in NOx by 2010 based on 1995 emissions. Compliance review is scheduled for 2006.
Currently, there are less rigorous standards in Asia than in the U.S. Environmental regulatory agencies in several regions of the world including India, the Middle East, China and Taiwan, are, however, addressing the reduction in SO2 from new refinery sources. Japan is actively addressing NOx.
Acknowledgement
This article has been excerpted, by Dorothy Lozowski, from a 366-page report by Richard Nielsen of SRI Consulting. For details on how to order the full report, contact the author using the contact information on p. 29.