Oct-1999

Controlling FCCU emissions: EDV Wet Scrubbing

For many refiners, the FCCU (Fluid Catalytic Cracking Unit) represents the single largest air emission source of particulate and SOx emissions in their refinery. Regulatory pressure to control these emissions is increasing.

Scott T Eagleson and Edwin H Weaver
Belco Technologies Corporation (Now BELCO Clean Air Technologies)

Article Summary

At the same time refiners are trying to maximise profitability by choosing to process heavy/sour feed stocks and increase the time between FCCU turnarounds.  Meeting these demands requires the right emission control approach.  More and more refiners are choosing to use wet scrubbing.

This paper examines a number of FCCU projects where the installation of a wet scrubbing system was the right solution. Details on projects in North America, Asia and the Middle East are used to illustrate the many common and very distinct requirements that wet scrubbing can address beyond only controlling FCCU emissions. The information is presented for the consideration of refiners that are today faced with controlling FCCU emissions on both new and existing units  Seeing what others have done regarding the use of wet scrubbing can be of great benefit.

FCCU emissions
Wet Scrubbing provides control for the particulate and SOx emissions generated by FCCUs. 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 recirculation, cyclones allow a significant amount of fine catalyst to escape.

Typically uncontrolled catalyst emissions exiting cyclones used in the FCCU process range from 5 to10 pounds of catalyst per 1,000 pounds of regenerator coke burn-off (300 to 750 mg/Nm3).

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 1, depends on many factors.  In a FCCU reactor, typically 70% to 95% of incoming feed sulphur is transferred to product and fuel gas. The remaining 5% 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. SO2 emissions from 200 to 5000 parts per million volume basis (ppmv) (575 to 14,300 mg/Nm3) are not uncommon. A small percentage of the sulphur reported as SO2 will be found as SO3.

Emission regulations
Regulations covering FCCU emissions vary from location to location.  They may be directed specifically at emissions from the FCCU or reflect overall emissions from all refinery processes. FCCU emission requirements for a number of countries are outlined in Figure 2. These values represent the minimum control requirements for new FCCUs. Emissions for individual units may be controlled to l ower levels to minimise overall refinery emissions or meet more stringent local standards. Requirements for existing FCCUs may vary. Outlet emissions of below 1.0 pound of catalyst per 1,000 pound of regenerator coke burn-off (75 mg/Nm3) and 50 ppmv (143 mg/Nm3) of SO2 are common requirements.

Wet scrubbing systems
Wet scrubbing systems have been used with great success to control particulate and SOx emissions from FCCU regenerator flue gas.  Since 1994, EDV Wet Scrubbing, as offered by Belco Technologies Corporation (Belco), has been selected for use on over a dozen new and existing FCCUs. The projects discussed later in this paper all reflect the use of Belco EDV Wet Scrubbing technology.

A brief introduction will provide an understanding of this technology and wet scrubbing. An EDV Wet Scrubbing System consists of a spray tower along with a set of filtering modules and set of droplet separators. The general configuration is shown in Figure 3. FCCU regenerator flue gas enters the spray tower and is immediately quenched to saturation. Multiple levels of spray nozzles within the open spray tower design remove coarse particulate by impaction with sprayed water droplets. These sprayed water droplets also provide absorption to reduce SO2 emissions when used with a scrubbing reagent. 

 Gas leaving the spray tower is directed to a set of filtering modules for removal of fine particulate. This is achieved through a unique process of forced condensation and filtration with water sprays. A set of simple cyclonic droplet separators removes any free water droplets from the gas prior to discharge in a stack.

Various reagents can be used when controlling SO2 emission. Caustic soda is the most commonly used. Refiners are generally comfortable with caustic soda storage/handling and the sulfate salts produced as a by-product are easily handled. Soda ash and/or magnesium hydroxide are also common.

Lime is being applied for projects where there is a desire to minimise reagent costs, while also producing gypsum solids as a by-product. Belco also provides a regenerative reagent process, Labsorb™, that can dramatically reduce reagent consumption costs while producing a concentrated stream of SO2 (90+% SO2) that can be used as feed for a sulfuric acid plant or directed to a sulphur recovery unit. Figure 4 provides a summary of some wet scrubbing reagent options.

Water is used in the wet scrubbing process with more water being consumed than is discharged. Depending on the scrubbing reagent used, a purge water treatment system is used to treat water discharged from the wet scrubbing process to meet required standards. For a typical caustic soda based design this involves minimising TSS (Total Suspended Solids, catalyst fines removed from the flue gas) and COD (Chemical Oxygen Demands from SO2 removed from the flue gas as sodium sulphite). Water effluent discharge requirements can vary greatly from site to site.

FCCU installations
Examining projects where wet scrubbing is used to control FCCU flue gas emissions can be of great benefit to refiners that are today faced with controlling FCCU emissions. The cases that follow represent a number of FCCU projects employing EDV Wet Scrubbing Systems for the control particulate and SOx emissions. Although the wet scrubbing technology applied in each case is the same, there are unique aspects associated with each project that refiners may want to consider when applying wet scrubbing to their own FCCUs.