Catalytic strategies to meet gasoline sulphur limits
Stricter regulations reducing average gasoline sulphur content will require further reduction of FCC gasoline sulphur
PATRICK GRIPKA, OPINDER BHAN, WES WHITECOTTON and JAMES ESTEBAN
Criterion Catalysts & Technologies
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The US Environmental Protection Agency (EPA) has finalised new regulations designed to reduce air pollution from passenger cars and trucks. The regulations (commonly referred to as Tier 3) set new vehicle emission standards and lower the annual average sulphur content of gasoline from 30 ppm to 10 ppm (see Figure 1). Additionally, the regulations maintain the current 80 ppm refinery gate and 95 ppm downstream caps. The implementation date is 1 January 2017. These Tier 3 gasoline sulphur specifications are similar to levels already being achieved in California, Europe, Japan, South Korea and several other countries.
Implications for refinery processing
The gasoline pool is composed of gasoline boiling range hydrocarbons from several sources in the refinery. Typical gasoline pool blending components include butanes, ethanol, light straight run naphtha, isomerate, reformate, alkylate, FCC gasoline and hydrocracker gasoline. In addition, purchased blending components may also be present. Most of these components are very low in sulphur (typically <1 ppm) except for the FCC gasoline. Not only does the FCC gasoline have the highest sulphur content, but it is typically also the largest volume component of the gasoline pool. As a result, FCC gasoline sulphur will have to be reduced to 20-30 ppm in order for a typical refinery to meet the proposed Tier 3 regulations.
At present few, if any, refineries are able to blend significant amounts of FCC gasoline into the gasoline pool without employing hydrotreating to reduce sulphur. Options refiners are currently utilising to meet current Tier 2 regulations include:
• Pretreatment of FCC feed: pretreatment reduces the sulphur of the FCC feed, which in turn lowers the sulphur of the FCC products including FCC gasoline
• Post-treat FCC gasoline: post-treatment directly reduces FCC gasoline sulphur
• Combination of FCC feed pretreatment and FCC gasoline post-treatment.
Current unit constraints and relative economics of the available options will determine the technology selection for meeting Tier 3 regulations.
Catalyst developments in FCC pretreatment
To meet the demand for improved catalysts in FCC pretreatment service to meet Tier 2 regulations, Criterion Catalysts & Technologies L.P. (Criterion) developed and commercialised the Ascent family of catalysts with DN-3551 NiMo and DC-2551 CoMo. Criterion has also developed and commercialised the Centera family of catalysts for FCC pretreatment: DN-3651 NiMo and DC-2650 CoMo.
Figure 2 highlights the continuing evolution of FCC pretreatment NiMo catalyst development by Criterion. Refiners were able to take advantage of the increased activity of DN-3551 to meet Tier 2 regulations and still achieve long catalyst life; similarly, the increased activity of the recently commercialised Centera DN-3651 will assist refiners in meeting the proposed Tier 3 regulations.
Criterion’s newest CoMo FCC pretreatment catalyst, Centera DC-2650, is often used in conjunction with Centera DN-3651, especially in lower pressure units to optimise hydrodesulphurisation (HDS) and hydrodenitrification (HDN) performance.
These new catalytic developments allow current FCC pretreatment units to produce lower product sulphur at the same operating conditions and minimise the investments required to meet Tier 3 requirements.
Capital avoidance from developments in FCC pretreatment
Many refiners have invested heavily in robust FCC pretreatment units to meet Tier 2 regulations as well as MACT standards for FCC emissions. Leveraging advanced catalyst technologies with existing assets can, in many cases, provide attractive solutions to both minimise capital investment as well as improve refinery profitability. The FCC pretreatment unit plays a critical role in optimising FCC performance. Removal of sulphur from FCC feed improves FCC product quality while the removal of nitrogen and contaminant metals improves FCC catalyst performance and reduces catalyst usage. Additionally, hydrogenation of the FCC feed improves conversion by reducing the concentration of polynuclear aromatic species. In many applications, drop-in catalytic solutions for FCC pretreatment units can achieve higher severity with little to no capital investment and minimal change in cycle life.
There are several key factors to consider when evaluating FCC pretreatment units for higher severity operations:
• Hydrogen availability including recycle gas capacity to account for additional consumption
• Heat balance for operation at higher reactor temperatures
• Cycle life targets
• Current and future capacity targets as it relates to reactor space velocity
• Operating constraints such as fractionation limitations.
Table 1 is derived from Criterion’s industry-wide database to illustrate a comparative analysis of the performance improvements expected for FCC pretreatment units using drop-in catalytic solutions with Centera products. In addition to product quality improvements, estimated improvements for FCC conversion are provided.
For a medium pressure unit with average feed properties and a typical 36-month cycle life currently producing 1000 ppm product sulphur, the more severe FCC pretreatment operation to produce FCC gasoline sulphur in the 20-30 ppm range requires FCC pretreatment product sulphur to be in the 300 ppm range and when using Centera catalyst, a cycle life of 24 months or more can be achieved. In addition, the product nitrogen is reduced significantly and hydrogen consumption, FCC pretreatment volume gain and FCC conversion are increased.
The improvements in FCC performance and yields from higher severity operation of the FCC pretreatment unit are linked to the increased saturation of polynuclear aromatics. The saturation of aromatic rings in these complex molecules determines both the product distribution and the relative sulphur distribution in the FCC products. In the FCC, aromatic rings do not crack while functional groups attached to the aromatic rings can be removed. The number of unsaturated rings adjacent to each other is critical in determining the boiling range of the final FCC product. Molecules with one ring end up in the FCC naphtha cut, two- and some three-ring molecules go to the LCO cut while most three-ring and greater molecules are either found in the HCO and clarified oil streams or deposit as coke. Saturation of aromatics results in higher value products and greater conversion in the FCC. Saturation of aromatic rings starts from the centre of the molecule with a decrease in relative reaction rate as polynuclear aromatics are hydrogenated.
The critical operating parameters that influence these reaction rates are hydrogen partial pressure and operating temperature. In order to maximise aromatics saturation for a given unit, it is important to maximise hydrogen purity and hydrogen availability to optimise hydrogen partial pressure, particularly at the reactor outlet. In addition to maximising hydrogen partial pressure, operating temperatures must be increased to maximise saturation. However, saturation of aromatics is equilibrium limited at constant hydrogen partial pressure so there is an optimum temperature range for maximum saturation. This optimum temperature range is often referred to as the kinetic region or the aromatics saturation plateau. Operating in the kinetic region provides the best quality feed for the FCC.
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