New roles for FCCU: carbon capture unit and coke gasifier
Environmental and energy security concerns may evoke new roles for the FCCU in addition to the production of liquid fuels and light olefins. Gasification and carbon capture are two such possibilities
Erich J Mace, Adrienne M Blume and Thomas W Yeung
Hydrocarbon Publishing Company
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Fluid catalytic cracking has been evolving since the first unit was commissioned at Esso’s Baton Rouge refinery in 1942.1 Today, it continues to be a unit in transformation, adapting to increasingly heavier feeds, strict transportation fuel specifications, product demand changes and unit emissions standards.
The FCCU is traditionally configured to produce gasoline for maximum yield, maximum octane or maximum octane barrels as needed, but its role is changing. For example, gasoline demand, although still growing worldwide, is on shakier ground due to biofuels mandates that require certain percentages of gasoline to be replaced with ethanol and the recent economic turmoil that has dampened consumption. Refiners are finding value in other FCC products, such as propylene and LCO, and units running on lower-quality feeds, such as resid, are becoming increasingly common. Environmental regulations have also shaped the current state-of-the-art, with flue gas systems and catalyst additives now standard to mitigate NOx, SOx, PM and CO emissions and meet reformulated gasoline requirements.
The aforementioned impetuses are certainly enough to continue to foster technological development in fluid catalytic cracking. However, the current geopolitical climate suggests a new wave of FCC developments is on the horizon, potentially transforming the unit into a polygen operation including fuels production, petrochemical production, syngas production, power generation, and carbon capture. Figure 1 compares the current roles of the FCCU to its future roles.
In the US, President Barack Obama’s environmental platform seeks to cut CO2 emissions to 1990 levels by 2020, curb emissions to 80% below 1990 levels by 2050, and require fuel producers to decrease the carbon content of their fuels by 10% by 2020. Obama has repeatedly warned, “Global warming is not a ‘someday’ problem — it is now,” and supports a carbon cap-and-trade scheme that would require greenhouse gas (GHG) emitters to either buy pollution permits or reduce emissions directly from their operations. Refiners will initially spend their extra capital on emissions permits rather than plant expansions, but as CO2 credit prices rise they will need to consider making energy efficiency improvements and capturing CO2 from major emissions point sources. Analysts assert that the introduction of a US cap-and-trade scheme would foster the development of such low-carbon technologies. Outside the US, Canada’s Conservative government expressed interest in Q4 2008 in forming a climate change pact with the US and, in the EU, 2008 saw the adoption of a plan to implement CO2 permit auctions starting in 2013. EU refiners will begin purchasing 20% of their permits by auction, transitioning to 100% by 2020.2 Such moves by Western nations may also put pressure on countries like China and India to invest in similar technologies.
According to the US Energy Information Administration (EIA), the US emits approximately 6.0 billion mt/y of CO2. The manufacturing sector produces 1.4 billion mt/y of that total, of which petroleum refining claims 19.8% (277.6MM mt/y in 2002) of the emissions. Excluding the combustion of refinery fuel, the FCCU is the largest emitter of CO2 in the refinery.3 Regenerator flue gas is almost exclusively the source and can comprise anywhere from 15–50% of refinery CO2 emissions.4 Since it is the largest non-fuel-derived source in the refinery, it is in refiners’ interest to investigate FCCU CO2 reduction methods. A study released by Petrobras in 2003, shown in Figure 2, details the distribution of refinery CO2 emissions.5
As the coked FCC catalyst is regenerated via combustion, the following important model reactions occur (reactions 1–10). Coke and CO combustion (reactions 3 and 6) are the chief contributors to flue gas CO2. Thus, if the combustion of catalyst coke can be reduced (and carbon leakage within the refinery does not occur), refiners can decrease CO2 emissions. Also, notice that the standard heats of reaction for reactions 1, 3 and 6 are large and exothermic, supplying the bulk of the heat for the FCCU:
1. 2C + O2 → 2CO, ΔHºrxn = -221.1 kJ/mol
2. C + CO2 → 2CO, ΔHºrxn = 172.4 kJ/mol (reverse Boudouard reaction or CO2 reforming)
3. C + O2 → CO2, ΔHºrxn = -393.5 kJ/mol
4. C + H2O → CO + H2, ΔHºrxn = 131.3 kJ/mol
5. C + 2H2O → CO2 + 2H2, ΔHºrxn = 90.2 kJ/mol
6. CO + 1/2O2 → CO2, ΔHºrxn = -283.0 kJ/mol
7. 2H2 + O2 → 2H2O, ΔHºrxn = -483.7 kJ/mol
8. S + O2 → SO2, ΔHºrxn = -296.8 kJ/mol
9. 2SO2 + O2 → 2SO3, ΔHºrxn = -197.7 kJ/mol
10. N + xO → NOx, ΔHºrxn (dependent on x and minor in unit heat balance calculations)
Gasification of FCC catalyst coke
Several efforts, including work by BP and UOP, suggest that one method to shift selectivity away from combustion reactions is to operate the regenerator in gasification mode.6,7,8,9,10 Operating conditions for an FCCU regenerator are typically 1250–1500ºF (675–815ºC), with pressures near atmospheric pressure. For comparison, coal gasification typically requires temperatures ranging from 1400–2800ºF (760–1537ºC) at pressures anywhere from atmospheric up to 1000 psig (7 MPa).11 The defining difference between gasification and combustion is that significantly less air/oxygen is supplied during gasification. Thus, a reduction in oxygen concentration along with pressure and temperature modifications can shift an FCCU regenerator from a combustor into a gasifier, producing syngas (CO and H2) instead of CO2 and H2O. One way to achieve gasification conditions is to reduce oxygen content by increasing the CO2/O2 ratio or the H2O/O2 ratio in the regenerator air feed. This action improves the rates of the two key gasification reactions: the water-gas reaction (reaction 11) and the reverse Boudouard reaction (reaction 2). A heat supply is also required to shift the thermodynamic equilibrium of these endothermic reactions to products. Typically, this heat is supplied when the exothermic combustion and partial combustion reactions also occur. The following reactions, along with reactions 1, 2, 3, and 6, are those central to gasification:12
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