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Control of air emissions from FCCU regenerator flue gas

Reduction of air emissions from FCCU regenerator flue gas has become an increasingly important topic.

Edwin H Weaver
Belco Technologies Corporation (Now BELCO Clean Air Technologies)
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Article Summary
Along with a multitude of issues that could require emissions reductions, refiners must also consider the best approach for complying with the refinery MACT II standard. It is essential that an approach be taken which ensures that present requirements are met while the flexibility remains to accommodate future regulatory requirements or process changes. This needs to be achievable with a minimum economic impact.

This paper examines three specific examples of refineries installing wet scrubbing systems to achieve emissions compliance. The first example is an existing refinery which installed a system to control both particulate and SO2. The next refinery examined installed a system only for the control of particulate. Finally, a new installation for the control of both particulate and SO2 is examined. Each facility is examined with respect to the specific design requirements and how the air pollution control system was designed to accommodate these requirements. Finally, the operation and performance of each system is examined with reliability being a key issue addressed.

The FCCU application
In most refineries, the flue gas from the regenerator in the FCCU represents the greatest single air emission source. It contains significant quantities of catalyst fines and sulphur oxides.

Particulate (catalyst) emissions from this source vary depending on the number of stages of internal and external cyclones. Although cyclones are effective in collecting the greater constituent of catalyst recirculated in the FCCU regenerator, the attrition of catalyst causes a significant amount of finer catalyst to escape the cyclone system with relative ease. Typically, emissions will range from 0.10 to 0.30 grains per dry standard cubic foot of gas (gr/DSCF).

Sulphur emissions in the form of SOx (SO2 and SO3) from the regenerator vary significantly depending on the feed sulphur content and the FCCU design. In the FCCU reactor, 70% to 95% of the incoming feed sulphur is transferred to the acid gas and product side in the form of H2S. The remaining 5% to 30% of the incoming feed sulphur is attached to the coke and is oxidised into SOx which is emitted with the regenerator flue gas. This is illustrated in Figure 1. The sulphur distribution is dependent on the sulphur species contained in the feed, and in particular the amount of thiophenic sulphur. SO2 can range from 200 to 3,000 parts per million dry volume basis (ppmdv), whereas SO3 typically varies from 5% to 15% of the SO2 content.

In 1989, the U.S. New Source Performance Standard (NSPS) set the current national standard for particulate and SO2 from FCCU regenerator flue gas on new units. These values are provided in Figure 2. Additionally, the proposed Maximum Achievable Control Technology (MACT) rule will lower particulate emissions on existing source to the NSPS level. Also, SOx emissions may eventually need to be reduced due to expansion projects that will trigger NSPS, the desire to process higher sulphur feed stocks, and local regulations that limit “bubble” emissions.

Other than the pollutant issues discussed, the FCCU application presents the additional requirement that in order to match the reliability of the FCCU, the air pollution control equipment must also operate on line for 3 to 5 years without interruption. It must be able to tolerate significant fluctuations in operating conditions and withstand the severe abrasion from catalyst fines and system upsets. The operability of the air pollution control system can be no less than that of the FCCU process.

SOx/particulate control with wet scrubbing systems
Wet scrubbing is used successfully on this application to control particulate only and both particulate and SOx. Most installations utilise caustic soda (NaOH) to absorb SOx and discharge it in the form of a soluble sodium sulfate salt, but other alkalis have been used in cases where the economics of final residue disposal and alkali cost justify a different selection.

The wet scrubbing technology detailed in this paper is the EDV technology offered by Belco Technologies Corporation. This technology has been found to be the most cost effective of proven wet scrubbing technologies for this application and thus the best basis for cost analysis.

The EDV system consists of a spray tower, filtering modules, and a droplet separator. The general configuration is shown in Figure 3. The flue gas from the regenerator enters the spray tower where it is immediately quenched to saturation temperature. The spray tower itself is an open tower with multiple levels of spray nozzles. These nozzles remove coarse particulate by impaction on the water droplets. These are the same nozzles which spray the reagent to reduce SO2 emissions should this be required. These nozzles used for the quench and the spray tower are both G nozzles. Their unique design is a key element of the system. They are non-plugging, constructed of abrasion and corrosion resistant material, and capable of handling high concentrated slurries. Also, they produce relatively large water droplets to prevent the formation of mist and the need for a conventional mist eliminator which can be prone to plugage.

Upon leaving the spray tower, the saturated gases are directed to the EDV Filtering Modules for removal of the fine particulate. This is achieved through saturation, condensation, and filtration. Since the gas is already saturated, condensation is the first step in the filtering modules. The gases are accelerated slightly to cause a change in their energy state and a state of super saturation is achieved through adiabatic expansion.

Condensation occurs on the fine particulate and acid mist. This causes a dramatic increase in size of the fine particulate and acid mist, which significantly reduces the required energy and complexity of its removal. An F nozzle located at the bottom of the filtering module and spraying upward provides the mechanism for the collection of the fine particulate and mist. This device is illustrated in Figure 4.

To ensure droplet free gas, the flue gas then goes to a droplet separator. It is an open design with fixed spin vanes to induce a cyclonic flow of the gas. As the gases spiral down the droplet separator, the centrifugal forces drive any free droplets to the wall, separating them from the gas stream. This device is shown in Figure 5.

The primary advantage of the wet scrubbing system is its ability to effectively remove particulate while providing a device which also controls SO2 emissions should this be a requirement of the system. If initially designed for particulate control only it can easily be adapted to control SO2 when a future expansion or regulation dictates that this must be accomplished. It also requires a relatively small plot space, something which can be critical in an existing refinery. In addition, it can be designed to be placed in service while the system is in operation, something that also can be important depending upon the refinery turnaround schedule in relation to the compliance dates for the new standards.
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