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Jan-1999

FCCU particulate emissions control for the refinery MACT II standard system

With the anticipated promulgation of the refinery MACT II standard, many refiners are faced with a significant capital expenditure to achieve compliance with FCCU regenerator emissions.

Edwin H Weaver and Scott T Eagleson
Belco Technologies Corporation (Now BELCO Clean Air Technologies)
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Article Summary
One of the most critical issues that should be examined is how the selected control equipment will meet the needs of MACT II while also having significant flexibility to meet any future needs without extensive capital intensive modifications.

To illustrate the key issues involved in this decision, two of the most relevant equipment choices, wet scrubbing systems and electrostatic precipitators, are examined in detail. The two systems are compared by both capital and operating costs in a configuration that will achieve the MACT II standard. Each system is then examined with respect to its ability to accommodate process increases, feedstock changes, and additional emissions requirements. The systems are also examined with respect to economics, performance, flexibility, and reliability.

To illustrate the analysis, a case history from a refinery which evaluated these factors is presented to show how this important decision can affect the refinery’s future profitability and flexibility.

Background
In September 1998, the Environmental Protection Agency (EPA) proposed standards for what is commonly known as the refinery MACT II standard. Although this proposed regulation encompasses several pollutants and sources, the focus of this paper will concentrate on one major portion of the proposed regulation, the inorganic pollutants from the FCCU regenerator. The MACT II standard is being promulgated to limit the emissions of hazardous air pollutants (HAPs) from source within the refinery. In order to simplify this regulation, the EPA selected a pollutant that can be readily monitored and limited its emission rate to a level that would ensure that all HAPs would be controlled to an acceptable level. The EPA selected total particulate (catalyst fines) as one option for compliance. The level selected is identical to the level in the New Source Performance Standards (NSPS) which is 1.0 lb per 1,000 lbs of coke burn-off. The EPA also includes a second option. This allows a refiner to limit the emissions of nickel to 0.029 lb/hr, regardless of process rate. It appears that most refiners will look at achieving the 1.0 lb per 1,000 lbs of coke burn-off, so this level will be the one that is addressed in more detail as equipment options are investigated.

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 to 8 lbs per 1,000 lbs of coke burn-off.

In addition to the MACT II regulations, a refinery may also be subjected to NSPS regulations for particulate and SO2 from the FCCU regenerator flue gas. As previously noted, under NSPS particulate emissions are limited to 1.0 lb per 1,000 lbs of coke burn-off. NSPS allows several options for SO2 control. These options include limiting the sulphur content of the feedstock to 0.3% by weight, limiting the SO2 to 9.8 lbs per 1,000 lbs of coke burn-off if the is no add-on control device, or 90% reduction of SO2 if an add-on SO2 control device utilized. A summary of the requirements is provided in Figure 1. Also, SOx emissions may need to be reduced due to expansion projects that will trigger NSPS, the desire to process higher sulphur feed stocks, or local regulations that limit “bubble” emissions. 

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 oxidized into SOx which is emitted with the regenerator flue gas. 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 3000 parts per million dry volume basis (ppmdv), whereas SO3 typically varies from an insignificant value to a maximum of 10% of the SO2 content.

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.

Cyclonic separators
Although the primary focus of this paper addresses electrostatic precipitators and wet scrubbing systems, it is important to look at the issues involved with the use of cyclonic separators. Also simply called cyclones, they have been used for many years to remove catalyst fines from the regenerator flue gas and continue to be an integral and essential part of the FCCU process for catalyst recirculation. Their performance in removing large size catalyst (greater than 10 microns) and controlling emissions to a range of 5-10 lbs per 1,000 lbs of coke burn-off is very well demonstrated.

Many of the FCCUs incorporate two stages of cyclones as an integral part of their regenerator. The catalyst collected is re-circulated in the process. Additionally, a third stage of cyclones have traditionally been utilised to remove additional catalyst to protect downstream equipment, such as power recovery turbo-expanders. A number of suppliers have recently developed improvements to these cyclones to enhance their collection efficiency to a level where they could potentially be used to comply with the MACT II standard. In fact, there has been recently presented data1 from a single facility outside of the United States which indicates that a third stage separator can reduce emissions to a level below the 1.0 lb per 1,000 lbs of coke burn-off.

The relatively low capital cost of this equipment is attractive. However, some very important issues must be addressed before considering this as a viable option. Most of these issues centre around the Compliance Assurance Monitoring (CAM) requirement that all refiners, and other industries, will have to comply with. Simply stated, CAM is a mechanism to ensure that an emission source is in continuous compliance. Before CAM, a facility would pass an emission test on a regular basis, usually once per year. Between emission tests, the facility would only have to comply with other requirements in its permit, such as opacity, to ensure compliance. CAM has changed this. In order to meet the requirements of CAM, a facility must identify and monitor during compliance testing items which are continuously monitored in the facility and demonstrate that the emission control device is maintaining its performance. These items are chosen by the facility, but they must be defendable and acceptable to the regulatory agency. For example, if opacity where chosen and the opacity during the compliance test was 3%, any opacities in excess of 3% would represent a potential emissions violation and would have to be reported as such.
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