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Reducing FCC 
regenerator emissions

There are several ways of meeting emission limits, but SOx and NOx reduction additives are very attractive as they require little or no capital investment and have reasonable operating costs, resulting in an overall lower cost than a WGS or SCR

Chris Kuehler, Pieter van de Gender and Subramani Ramachandran
Albemarle Corporation
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Article Summary
Increasingly severe regulation of sulphur oxide (SOx) and nitrogen oxide (NOx) emissions from FCCUs is being implemented around the world. Although there are a variety of approaches for meeting emission limits, the use of SOx and NOx reduction additives appears to be very attractive technically and economically.

SOx and NOx emissions from many industrial sources have long been regulated worldwide. In the US, in addition to possible state and local regulations, SOx and NOx emissions from FCCUs have been most recently impacted by an Environmental Protection Agency (EPA) programme of civil enforcement of the Clean Air Act (CAA). Citing alleged violations of the CAA, the EPA has entered into settlements (consent decrees), which provide for both possible fines and mandatory implementation of pollution control technology.

Since March 2000, the EPA’s national Petroleum Refining Priority Program has resulted in 21 settlements with US refiners, accounting for approximately 86% of the nation’s refining capacity.1 These settlements, covering 95 refineries, will result in an estimated $5 billion investment in control technology with an annual reduction in SOx emissions of 245 000 tonnes. The consent decrees cover a variety of emission sources, but there is a heavy focus on the FCCU.

The consent decrees generally prescribe the implementation of emission reduction technology, but in some cases refiners accept hard limits for the FCCU flue gas, which they meet by methods of their choice. The ultimate SOx emission target for all refiners is 
25 ppmv. The corresponding target for NOx emissions is 20 ppmv.

In the 27 countries of the European Community, the emissions situation is addressed by the Integrated Pollution Prevention and Control (IPPC) Directive adopted in 1996 by the EU and subsequently transposed into the national legislation of EU Member States.2 Integrated pollution prevention and control is based on a permit system for installations. The Directive does not set standards or thresholds for the prevention and control of emissions, or for other environmental aspects, but leaves this responsibility to the Member States. The IPPC Directive applies to both new and existing installations (ie, those built before 2000), where the operators intend to carry out changes that may have significant negative effects on human health and the environment. Member States were given a transitional period until October 2007 to ensure all other existing installations fully comply with the Directive.

In contrast to the US, where the consent decrees set limits on individual FCCUs, most countries in Europe look at emissions from a “bubble” point-of-view. Local authorities issue permits based on Best Available Technique (BAT). BATs, documented in Best Available Technology Reference documents (BREFs), have been developed for individual industrial processes by the exchange of information among experts from industry, Member State authorities, research institutes and non-governmental organisations (NGOs). The original BREFs are currently being updated.

In the rest of the world, a variety of local, regional and national systems regulate gaseous emissions from FCCUs. In South America, each country has different regulations. For example, in Brazil, SOx from the FCCU is limited to 1200 mg/Nm3, while in the industrial regions near Buenos Aires, Argentina, there is a 500 mg/Nm3 regulation.

SOx emissions
SOx emissions from industrial sources have been a problem since the Industrial Revolution, with the most visible manifestation being acid rain caused from burning coal. In recent years, more focus has been put on other industrial sources such as petroleum refineries, where a major source of SOx is typically the FCCU.3

Coke formed in the FCCU cracking process is subsequently burned to both regenerate the catalyst and supply the heat required for vapourisation and cracking. Depending on the character of the feed, 5–10% of the feed sulphur reports to the coke and is quantitatively oxidised to SO2 and SO3 (SOx). Left uncontrolled, SOx concentrations in the regenerator flue gas can reach into 
the thousands of ppm, depending on feed quality.

FCCU SOx reduction technology
The means for reducing SOx emissions from the FCCU regenerator fall into three categories:
—  Lowering the sulphur in the feed
—  Treating the flue gas downstream from the FCCU
—  Use of a SOx reduction additive within the FCCU.

Lowering the feed sulphur can be achieved through the purchase of lower sulphur feeds and by hydrotreating. Both of these approaches can be expensive, although they bring other benefits, such as improved yield structure and lower sulphur levels in liquid products, and they must be judged on the overall economics. If the refiner is faced with a low SOx emission limit, it may also be difficult to meet with these approaches alone.
There have been a variety of processes developed for the removal of SOx from flue gas, but the dominant technology has been the wet gas scrubber (WGS) using caustic as a reagent to regenerate the scrubbing liquid. This technology has been commonly mandated under the consent decrees and is included under BATs in the EU. In a later portion of this article, the economics of this technology relative to SOx reduction additives are discussed.

Background to SOx 
reduction additives
An SOx reduction additive (or SOx transfer additive) is designed to pick up SOx in the FCCU regenerator and release it as H2S in the reactor. This is illustrated schematically in Figure 1. In the late 1970s, Akzo Nobel Catalysts (now Albemarle) developed and patented the use of hydrotalcite (HTC) as a SOx reduction additive4 and has continued to develop this technology.5 This is a layered magnesia-alumina system with the advantage of being rich in magnesium, which results in improved SOx pickup compared to competitive technology.

Since this development, Akzo Nobel licensed the HTC technology to the industry. In 2002, the company began marketing its own additives (KDSOx/SOxDown) with improvements in SOx pickup efficiency and physical properties that ensured retention of the additive in the unit. In subsequent years, HTC technology has accounted for approximately 70% of the SOx reduction technology used in the refining industry. SOxDown is a subsequent improvement of the KDSOx additive.
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