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Wet Gas Scrubber Features and Benefits
While many systems can be used to obtain particulate and sulfur oxide reductions, not all systems
are created equal. The following sections outline a number of key factors, which make the ExxonMobil
Wet Gas Scrubber as supplied by Hamon Research Cottrell the best choice for FCCU emissions control:
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This Wet Gas Scrubbing system has been practiced on FCCU's for almost thirty years. Sixteen systems have been
placed in operation (five at ExxonMobil refineries and eleven licensed), and these represent over 265 years of
operating experience. The system is exceptionally reliable and flexible. A WGS has never been the cause of a
Fluidized Catalytic Cracking Unit shutdown. Furthermore, the system has been able to handle a wide variety of
changes to FCCU operations (e.g. increased FCCU feedrate, change in feed sulfur level, cyclone wear through)
and FCCU upsets without experiencing operational problems or causing upsets for the upstream FCCU.
In addition, WGS HRC can also provide complete facilities to treat the liquid purge stream from the WGS.
Treatment includes solids removal to reduce effluent suspended solids content to below required ppm level
and then solids dewatering to produce a liquid free cake; and oxidation of the liquid effluent to reduce
COD (Chemical Oxygen Demand).
The design philosophy, of which you will become a beneficiary, insures the highest levels of equipment
operability, maintainability and reliability. This allows, you the refiner, to focus on your primary
business, the production of petroleum products.
The ExxonMobil WGS system was the first scrubbing system installed on a Fluid Catalytic Cracking Unit (FCCU).
The first unit began operation in 1974 and is still reliably operating, and exceeding its original performance
targets. The ExxonMobil WGS system is the most proven FCCU scrubbing system having over 260 years of operating
experience (as of July 1, 2002).
Commercial experience has shown that the service factor of the EMHRC WGS is higher than similar systems and/or
alternative control technologies. In fact, the WGS's service factor, in excess of 99.9%, is higher than most
FCCUs. These "standard" units have been in continuous operation and maintained compliance with regulatory
requirements for FCCU runs of four (4) to five (5) years. The current, record run for a WGS system is in
excess of seven (7) years. In 260+ years of operational experience, no FCCU that has an ExxonMobil WGS
system has been shut down due to a scrubber problem.
Commercial experience demonstrates that the Wet Gas Scrubber system provides flexible and forgiving service.
Generally, daily changes in Fluidized Catalytic Cracking Unit operation such as normal rate changes, changes
in catalyst carryover, or changes in feed sulfur content, require no attention. In one instance, to meet timing
of an FCCU turnaround, the WGS unit handled an order of magnitude increase in FCCU catalyst losses for a
six-month period with only minor changes in operations while meeting all environmental requirements.
Major FCCU upsets are not a problem. Upsets that units have handled without any shutdown time include:
- A sudden loss to the WGS of total FCCU inventory (300 tons in five minutes),
- Complete FCCU power failure and immediate restart,
- Reverse flow and major FCCU operator errors.
Wet Gas Scrubber is designed with sufficient liquid inventory to handle most catalyst-upset conditions.
Existing EMHRC Scrubbers have never lost circulation during catalyst loss events. One unit has continued in
operation during 300 Ton catalyst loss events. Material was collected in WGS and max. 70% by Wt. was continuously
circulated until all catalyst was purged from the WGS in normal operation. EMHRC WGS does not require special
provisions OR any emergency dump pit to accommodate this event.
The WGS system is designed to operate with little or no operator intervention, especially during FCCU upsets.
For over a decade, the initial scrubber refractory lined designs were upgraded to alloy or alloy clad materials. The
concept used in these units represents a commercially proven design that has the highest level of reliability and flexibility.
As our experience listing shows, Wet Gas Scrubber systems have been designed to handle flue gas rates ranging between
110,000 ACFM (186,900 m3/hr) and 865,000 ACFM (1,470,000 m3/hr). These rates translate to FCCU
capacities between 30 and 250 kB/SD. In the latter case, a single WGS system handles the flue gas from two adjacent
FCCUs. From a practical standpoint, the WGS units have operated from less than 15% to over 150% of original design.
However, in all cases the WGS is tailored for the individual refinery's project needs.
The WGS system can be applied to any FCCU regenerator configuration. Half of the operating WGS systems are
on FCCUs that operate in the partial burn mode and thus are downstream of CO boilers. The other half are on
FCCUs that operate in the full burn regeneration mode. In addition, a third of the operating WGS systems are
part of regenerator overhead systems that have, or have provisions to include, a flue gas expander. (See the
table on the previous page for details).
Design particulate or catalyst loading for these units ranges from 100 lb/hr (45.4 kg/hr) to 500 lb/hr
(226.8 kg/hr). However, they have successfully handled long term normal (not upset) catalyst losses from
50% to 1,000% of the design values while maintaining or exceeding design performance.
Design flue gas sulfur loading for these units ranges from 300 to 3,000 vppm. However, at one refinery the WGS handles
the flue gases from both the FCC and from the SRU (Sulfur Recovery Unit) incinerator. Long term this WGS has
consistently handled sulfur loading in excess of 6,000 vppm while maintaining or exceeding design performance.
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Within the refining industry, onsite plot space is usually a precious commodity. The ExxonMobil
WGS is frugal with regard to its onsite plot space requirements. As shown in the adjacent photograph,
an ExxonMobil WGS can be located anywhere even over a road. Unlike other refinery scrubbing systems
which require multiple vessels and ground mounted stacks, the ExxonMobil system uses only a single
vessel. Since ExxonMobil employs an upflow vessel, the stack is mounted on and supported atop the
vessel. This saves both plot space requirements and cost associated with additional foundations, ducting
and structural supports. For comparative flue gas rates, the single ExxonMobil vessel is about one-third
smaller than any one of the vessels found in other systems.
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To further save onsite plot space, the PTU does not have to be located adjacent to the WGS. It can be located offsite.
Commercially, PTUs have been located within 100 ft (30 m) of the WGS and as far away as 5 miles (8 km).
Feedback on design innovations and operational procedures are communicated promptly and quickly incorporated into new designs.
It is recognized that all equipment, including air pollution control equipment, installed in a refinery must comply with
refining industry reliability, and operational standards. These features are inherent within the WGS system. Thus, unlike
other scrubbing systems, the overall performance guarantee is provided by ExxonMobil, a company well-known for its
processing excellence. Commensurate with refining industry practice, innovations/enhancements developed up to two
(2) years after startup are available without cost.
Related Technical Articles (pdf):
Current U.S. Air Pollution Control Technologies & Future Trends
Electrostatic Precipitators for Refinery Fluid Cracking Units
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