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Mar-2001

Wet scrubbing for fccus: a case study examining site specific design considerations

Wet scrubbing is a well-proven and reliable method of reducing flue gas emissions from Fluid Catalytic Cracking Units (FCCUs).

Jon A Herlevich, Jr, Marathon Ashland Petroleum LLC
Scott T Eagleson, Alfred H Roth and Edwin H Weaver, Belco Technologies Corporation
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Article Summary
There are, however, many project specific issues affecting the design and configuration of these systems. This paper addresses specific issues related to the EDV Wet Scrubbing system recently installed at the Marathon Ashland Petroleum LLC refinery in Robinson, Illinois. Project specific issues addressed include the minimisation of pressure drop due to pressure constraints on upstream equipment and the design of the reagent system to minimise both chlorides and the quantity of purge water discharged from the facility.

In addition to the issues previously noted, the design of the entire wet scrubbing system is discussed, including the treatment of water purged from the scrubbing system. Also addressed is the project coordination and integration of the scrubbing system into the refinery’s operation. Finally, commissioning of the system is examined.

Background
Since 1992, refiners worldwide have decided to use wet scrubbing on more than 20 FCCUs. Wet scrubbing addresses the control of multiple pollutants in a single system. Both particulate and sulphur emissions in FCCU flue gas are simultaneously controlled. The technology is well proven in providing flexibility to handle added capacity that comes with FCCU expansions and in providing uninterrupted operation/performance exceeding that of FCCUs. Each refiner’s specific reasons for choosing wet scrubbing differ, but these have generally been related to environmental compliance as well as relative costs, reliability and flexibility of wet scrubbing compared to other emission control options.

Marathon Ashland Petroleum LLC (MAPLLC) decided to install a wet scrubber on the FCCU at the Robinson, Illinois refinery. This work was done as part of a major revamp of the FCCU during the fall 2000 turn around. The FCCU revamp involved incorporation of Stone & Webster’s LD2 Riser Termination Device technology, rotating equipment upgrades, and installation of a wet scrubbing system.

Belco Technologies Corporation’s (Belco) EDV Wet Scrubbing technology was selected by MAPLLC after the evaluation of a number of emission control options for particulate and SO2. Combinations of separate particulate and SO2 technologies were considered; electrostatic precipitators and high efficiency third stage separators for particulate control, and SOx transfer catalysts and feed hydrotreating for SO2 control. Wet scrubbing was considered for simultaneous particulate and SO2 control. MAPLLC evaluated wet scrubbing technologies offered by Belco and another licensor. Belco’s EDV Wet Scrubbing was selected for its overall cost effectiveness, proven high on-stream reliability, simplicity, and low-pressure drop.

The basis of design for the EDV Wet Scrubbing system at Robinson evolved during project development to prove compliance with New Source Performance Standards (NSPS as shown in Figure 1) over a range of operating cases. This required control of particulate emissions below 1.0 pound per 1,000 pound of coke burn and SO2 removal in excess of 90%. Control of CO emissions was addressed prior to the wet scrubber. In addition to controlling flue gas emissions, provisions for treating water purged from the wet scrubbing process were required. This water needed to be free of catalyst fines, have minimal Chemical Oxygen Demand (COD resulting from sulphites formed during SO2 removal), be a neutral pH, and be low in chlorides. Typical design criteria for wet scrubbing systems on FCCUs are shown in Figure 2. The ranges illustrate some of the variability seen from a number of different projects. The basis for Robinson is included within these ranges.

Basic EDV Wet Scrubbing System
A basic understanding of EDV Wet Scrubbing is helpful in discussing what makes the Robinson project unique. A typical system is shown in Figures 3 and 4.

The system treats hot flue gas containing catalyst fines and SO2 from the FCCU and discharges cleaned gas to the atmosphere through an integral stack. At the scrubber inlet FCCU flue gas is quenched and saturated by means of multiple water sprays in the Spray Tower’s horizontal quench section. Specialised spray nozzles produce high-density water curtains through which the gas must pass. Each nozzle sprays water droplets that move in a cross-flow pattern relative to flue gas. These cover the entire gas stream and uniformly flush the vessel’s walls clean. The spray nozzles are non-clogging and able to handle highly concentrated slurries.

SO2 absorption and particulate removal takes place immediately after quenching as flue gas rises up through the main Spray Tower where gas is again contacted with high-density water curtains produced by additional spray nozzles. The scrubbing liquid is water controlled to a neutral pH with reagent addition to drive SO2 absorption. Caustic soda (NaOH) is typically used. Multiple levels of spray nozzles provide sufficient stages of gas/liquid contact to remove both particulate and SO2.

In some cases flue gas leaving the Spray Tower is distributed to a bank of parallel Filtering Modules. Within each module flue gas accelerates (compresses) and then decelerates (expands). This action causes water to condense from the flue gas. The water uniformly washes the module’s walls. More importantly, water condenses on fine particle and mists present in the flue gas, increasing them in both size and mass. Some agglomeration also takes place. A high-density water spray curtain at the bottom of each module captures the now enlarged and agglomerated particulate. Filtering Modules are not required on all EDV Scrubbing systems, and were not required at Robinson.

Prior to being discharged to the atmosphere through a stack, the flue gas enters a bank of parallel droplet separators. Each treats a portion of gas flow and separates/collects free water droplets. Gas in each separator passes through a fixed spin vane where centrifugal acceleration causes free water droplets to impinge on the separator’s walls. Collected water droplets flush walls uniformly clean and drain to the bottom. Collected water is recycled for flue gas cleaning in the Filtering Modules or Spray Tower.

Makeup water is added to the system, replacing water lost to evaporation in the quench zone and for water purged from the system. Captured pollutants, i.e. suspended catalyst fines and dissolved sulphites/sulphates (NaHSO3, Na2SO3 and Na2SO4) from sulphur removal are purged from the Spray Tower recycle loop. The purge rate is routinely adjusted. Particulate and chlorides concentrations are minimised for preventing erosion and corrosion, while the concentration of dissolved salts is maximised to minimise the quantity of water purged.
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