Technologies for meeting MACT II - controlling FCCU regenerator emissions
For many refineries, an FCCU (Fluid Catalytic Cracking Unit) represents the single largest air emission source of particulate and sulphur emissions. Regulatory pressure to reduce overall refinery emissions is increasing and bringing much more attention to FCCUs.
Kevin Gilman, Scott Eagleson and Ed Weaver
Belco Technologies Corporation (Now BELCO Clean Air Technologies)
Viewed : 4151
The recently proposed MACT II standards in the USA require reductions in particulate emissions for many existing FCCUs. MACT II does not address reductions in sulphur emissions, so compliance may simply mean the installation of a new emission control system designed only for particulate control.
Faced with this investment of capital, refiners may want to consider what other benefits can be gained besides simply meeting the MACT II requirements. Emissions control systems currently in use on FCCUs are examined in consideration of MACT II compliance. Processing heavier/sour feed stocks, pushing capacity, increasing the time between scheduled turnarounds, and meeting future regulatory requirements are also considered. Can the installation of emission controls for MACT II compliance increase profitability? This discussion will help answer this question.
Regulations effecting FCCUs
Much of the recent attention on the control of FCCU emissions is the newly proposed National Emission Standards for Hazardous Air Pollutants from Petroleum Refineries1 also referred to as the refinery MACT II standards. Proposed in September of 1998 by the United States Environmental Protection Agency (USEPA), the standards limit the emission of HAPs (Hazardous Air Pollutants) from refineries by setting maximum emission limit levels for pollutants emitted from a number of refinery processes that includes fluid catalytic cracking units (FCCUs).
It is expected that all FCCUs in operation in the U.S.A. will be required to demonstrate compliance with the MACT II standards within 36 months from the date the standards are finalised and promulgated. With the current schedule of comments and revisions, promulgation is estimated for year 2000 with compliance in year 2003.
Monitoring the control of each individual HAP would be costly and not effective with today’s monitoring technologies. To simplify this, MACT II identifies specific pollutants that can be readily monitored to ensure the control of all the HAPs emissions. For FCCUs, limits are set for CO (carbon monoxide) as an indication of the organic HAPs and for particulate (or nickel) as an indication of the inorganic HAPs. The control of NOx and sulphur emissions are not addressed by MACT II.
A maximum CO limit of 500 ppmdv (parts per million dry volume) is specified for controlling organic emissions. MACT II provides two options for controlling inorganic HAPs. In the first, particulate emissions must be controlled to below 1.0 pound per 1,000 pounds of the FCCU regenerator coke burn-off rate with particulate being a surrogate for nickel and the other HAP metals. The second option allows a maximum emission of nickel at 0.029 pounds per hour as a surrogate for all the HAP metals, regardless of the FCCU capacity and process feed rates.
It appears that most refiners are looking at the 1 pound per 1,000 pounds of coke burn-off rate option for compliance. Some refiners with smaller units processing sweet/light feed stocks may not need to reduce particulate emissions to below 1 pound per 1,000 pounds of coke burn-off rate if they can demonstrate compliance with the 0.029 pound per hour nickel limit.
The New Source Performance Standard (NSPS)2, in effect in the USA since January 1984, also regulates FCCU emissions for both CO and particulate. Like MACT II, CO is limited to 500 ppmdv and particulate is limited to 1 pound per 1,000 pounds of regenerator coke burn-off rate. Unlike MACT II, NSPS also regulates sulphur emissions. Figures 1 and 2 summarise the FCCU emission requirements of MACT II and NSPS.
Units that have demonstrated compliance with NSPS should also be in compliance with the MACT II requirements. Generally this includes units installed after 1984 and any older units that have been re-permitted as new units. NSPS includes provisions to re-permit older units when/if they: significantly rebuild/revamp the unit, increase the amount of feed sulphur, or increase the processing capacity. Units not re-permitted under NSPS have been considered “grandfathered.” Many of these may need to reduce particulate emissions as MACT II takes away the “grandfathered” status for particulate. However, MACT II does nothing to require reductions in sulphur emissions.
An FCCU can represent the single largest air emission source of CO, NOx, particulate, and sulphur emissions in a refinery. This discussion focuses on particulate and sulphur emissions as pertaining to MACT II and NSPS. Each is characterised to provide a better understanding of the sources prior to discussing control. Generally, CO and NOx emissions are not closely associated with particulate and sulphur emissions.
Particulate emissions result from catalyst escaping the FCCU regenerator in the flue gas. Cyclones, incorporated within the FCCU process, remove a large portion of catalyst from the flue gas and return it to the process. While effective in collecting catalyst for re-circulation, cyclones allow a significant amount of fine catalyst to escape. Typically uncontrolled catalyst emissions exiting cyclones used in the FCCU process range from 5 to 10 pounds of catalyst per 1,000 pounds of regenerator coke burn-off.
Sulphur emissions result from a portion of the feed stock sulphur being carried through the FCCU and emitted as SOx (SO2 and SO3, typically expressed as SO2) in the FCCU regenerator flue gas. Levels vary significantly. Sulphur distribution within the FCCU, as depicted in Figure 3, depends on many factors. In an FCCU reactor, typically 70% to 98% of incoming feed sulphur is transferred to product and fuel gas. The remaining 2% to 30% is deposited with coke on catalyst. During catalyst regeneration, the coke and sulphur are burned off. The sulphur is oxidised to SOx and emitted in the flue gas. SOx emissions from 200 to 3,000 parts per million-volume-dry basis (ppmdv) are not uncommon.
Controlling emissions for MACT II and beyond
The use of add-on control technologies is the likely solution for FCCUs that need to reduce particulate emissions for MACT II compliance. The use of electrostatic precipitators and wet scrubbers is specifically addressed in MACT II. Both technologies are in widespread use on FCCUs for controlling particulate emissions below the levels proposed under MACT II. The use of third stage separators (high efficiency cyclonic separators) may also be a consideration, but their use has generally been limited to protecting turbo-expansion systems and not for meeting regulatory emission requirements. The advantages and disadvantages of each have been discussed in some detail previously4 and are summarised in Figure 4 for electrostatic precipitator and wet scrubbing systems for the focus of this discussion.
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