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Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

CHAPTER 11.2

BELCO EDV WET SCRUBBING SYSTEM: BEST AVAILABLE CONTROL TECHNOLOGY (BACT) FOR FCCU EMISSION CONTROLEdwin H. Weaver and Nicholas ConfuortoBelco Technologies Corporation Parsippany, New Jersey

THE FCCUA UNIQUE PROCESS FOR EMISSIONS CONTROLThe control of particulate and SO2 emissions with wet scrubbing systems is not uncommon. However, the control of these emissions, combined with the special needs and requirements of the fluid catalytic cracking unit (FCCU) process, indeed makes this a special process for wet scrubbing systems. Uncontrolled 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 3.0 to 8.0 lb per 1000 lb of coke burn-off. Sulfur emissions in the form of SOx (SO2 and SO3) from the regenerator vary significantly depending on the feed sulfur content and the FCCU design. In the FCCU reactor, 70 to 95 percent of the incoming feed sulfur is transferred to the acid gas and product side in the form of H2S. The remaining 5 to 30 percent of the incoming feed sulfur is attached to the coke and is oxidized into SOx which is emitted with the regenerator flue gas. The sulfur distribution is dependent on the sulfur species contained in the feed, and in particular the amount of thiophenic sulfur. SO2 can range from 200 to 3000 parts per million dry volume basis (ppmdv), whereas SO3 typically varies from an insignificant value to a maximum of 10 percent of the SO2 content. The FCCU application presents the additional requirement that in order to match the reliability of the FCCU, the air pollution control equipment must operate on-line for 3 to11.15 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

BELCO EDV WET SCRUBBING SYSTEM: BEST AVAILABLE CONTROL TECHNOLOGY (BACT) FOR FCCU EMISSION CONTROL 11.16SULFUR COMPOUND EXTRACTION AND SWEETENING

5 years without interruption. It must be able to tolerate significant fluctuations in operating conditions, withstand the severe abrasion from catalyst fines, and maintain operation through system upsets. The robust design of the wet scrubbing system must tolerate all operations without requiring a shutdown. It is paramount that the operability of the air pollution control system be no less than that of the FCCU process.

CONTROLLED EMISSIONSA TREND TOWARD LOWER LEVELSBy examining the trends of emissions regulations in the United States, a trend for better control of emissions from FCCUs can be established. The United States Environmental Protection Agency (USEPA) established New Source Performance Standards (NSPS) for emissions from FCCUs for new or significantly modified units. A summary of this standard is provided in Table 11.2.1. Additionally, a maximum achievable control technology (MACT) standard is in the final stages of promulgation. This standard, which is intended to regulate the amount of hazardous air pollutants (HAPs) from the FCCU, essentially established the particulate emission level at the same level as NSPS, or 1.0 lb/1000 lb of coke burned. The USEPA also has been aggressive in pursing enforcement actions against refiners who, in its opinion, have significantly modified their facilities but avoided the NSPS regulations. This has resulted in several consent decrees where a refiner has agreed to install pollution controls to mitigate the impact of any past modifications made to its facility. Refiners who have reached consent decrees with the USEPA include Koch Refining, British Petroleum, Motiva/Equilon/Shell, Marathon Ashland LLC, Holly Corporation, Premcor Refining, Conoco, and Murphy Oil. In many cases, the agreed-to emissions levels (25 ppm SO2 and 1. 0 lb/1000 lb of coke burned) are more restrictive than the NSPS regulations. Wet scrubbing systems are mandated for many of the facilities in the consent decrees.

A PROVEN WET SCRUBBER DESIGN FOR THE FCCU PROCESSThe worldwide leading technology to control emissions from this process is Belco Technologies Corporations EDV wet scrubbing system. This wet scrubbing system controls particulate (catalyst dust), SO2 (sulfur dioxide), and SO3 (sulfuric acid mist) all in one system. Removal of relatively coarse particulate, which constitutes the majority of the par-

TABLE 11.2.1 Pollutant Particulate SO2

New Source Performance Standards for FCCU Regenerator Emissions FCCUs affected Emission regulation 1.0 lb/1000 lb Coke burn-off and 30% opacity 50 ppm SO2 or 90% reduction, whichever is least stringent 9.8 lb SO2/1000 lb coke burn-off Or 0.3% Sulfur in feed (% by weight)

All With add-on SO2 control device Without add-on SO2 control device

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BELCO EDV WET SCRUBBING SYSTEM: BEST AVAILABLE CONTROL TECHNOLOGY (BACT) FOR FCCU EMISSION CONTROLBELCO EDV WET SCRUBBING SYSTEM: BACT FOR FCCU EMISSION CONTROL

11.17

ticulate from the FCCU, is accomplished in the absorber vessel where caustic soda (NaOH) or other reagents are utilized to absorb SO2 and discharge it in the form of a soluble salt. Fine particulate control and significant reduction of SO3 in the form of sulfuric acid mist are accomplished in devices known as filtering modules. Excess water droplets are removed in highly efficient droplet separators. An EDV wet scrubbing installation in Texas is shown in Fig. 11.2.1. Another U.S. Gulf Coast refinery EDV wet scrubber is illustrated in Fig. 11.2.2. The flue gas from the FCCU enters the spray tower, where it is immediately quenched to saturation temperature. Although the flue gas normally enters the wet scrubber after passing through a heat recovery device, the system is designed so that it can accept flue gas directly from the FCCU regenerator at the temperature at which it exits the FCCU regenerator. The spray tower itself is an open tower with multiple levels of spray nozzles. Each level of nozzles can have one or multiple nozzles depending on the diameter of the absorber vessel. Since it is an open tower, there is nothing to clog or plug in the event of a process upset. In fact, this design has handled numerous process upsets where more than 100 tons of catalyst has been sent to the wet scrubber in a very short period of time. An illustration of this spray tower is provided in Fig. 11.2.3.

FIGURE 11.2.1

EDV wet scrubbing system in Texas.

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BELCO EDV WET SCRUBBING SYSTEM: BEST AVAILABLE CONTROL TECHNOLOGY (BACT) FOR FCCU EMISSION CONTROL 11.18SULFUR COMPOUND EXTRACTION AND SWEETENING

FIGURE 11.2.2 EDV wet scrubber at U.S. Gulf Coast refinery.

In the spray tower, coarse particulate is removed through the simple process of liquid from the spray nozzles impacting on the particulate. Reduction of SO2 is accomplished by adding reagent, usually caustic, to the liquid being circulated in the absorber vessel. Assuming caustic is used, the SO2 reacts with caustic to form sodium sulfites, some of which oxidizes to sodium sulfates. Both of these are dissolved solids. These nozzles, used for both the quench and the spray tower, are LAB-G nozzles. They are a unique design and a key element of the system. They are nonplugging, constructed of abrasion-corrosion-resistant material, and capable of handling high concentrated slurries. Unlike in most nozzle designs, this nozzle has a large opening that cannot plug and is designed to operate at low liquid pressure, both important factors in long-term life. As previously noted, these nozzles remove coarse particulate by impacting on the liquid droplets. They also spray the reagent solution to reduce SO2 emissions. By design, they produce relatively large water droplets, which prevent the formation of mist and the need for a conventional mist eliminator that will be prone to plugging. This is unique in wet scrubbing system designs as any other design that uses a nozzle which produces mist size water droplets will require a mist eliminator to eliminate these droplets. Mist eliminators have

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BELCO EDV WET SCRUBBING SYSTEM: BEST AVAILABLE CONTROL TECHNOLOGY (BACT) FOR FCCU EMISSION CONTROLBELCO EDV WET SCRUBBING SYSTEM: BACT FOR FCCU EMISSION CONTROL

11.19

FIGURE 11.2.3 EDV absorber vessel/spray tower.

plugged in the presence of catalyst. This nozzle is illustrated in Fig. 11.2.4 and is shown spraying liquid in Fig. 11.2.5. 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 supersaturation is achieved through adiabatic expansion. Condensation

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