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Turning a Tier 3 profit

In the highly competitive refining market, profitability is not a benefit of successful business, but rather a necessity to sustainable business.

James Esteban, Criterion Catalysts and Technologies
Michael Hartman, Marathon Petroleum Company
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Article Summary
As such, profitable solutions and technologies are inherently key to a refiner’s success, providing the driving force to transition from survival in tough markets to thriving sustainable development. The industry faces threats on many fronts which challenge profit margins and business development. Growth in challenging crude and product markets requires the careful application of best practices for asset utilisation and strong technical solutions to adapt to changing constraints.

Environmental regulations have become increasingly challenging for refiners to adapt profitable solutions while maintaining feedstock and product flexibility. Notably, the continued reduction in refined product sulphur specifications has revolutionised the landscape of the industry over the last decade with the continued rise of importance for hydrotreating solutions within refinery complexes. This saga will continue as the industry moves toward the future Tier 3 regulations where the production of less than 10 ppm ultra-low sulphur gasoline (ULSG) will be required in 2017. However, technology has answered the call and responded with resounding success. Several refiners have already implemented strategies to meet Tier 3 blend stock requirements providing profitable, flexible solutions. This is a testament to the industry’s commitment to enriching the environment and communities in which we provide energy solutions.

Tier 3 Strategies

Many of the blend components of typical gasoline product streams are very low in sulphur and thus achieving Tier 3 specifications for most refiners requires focus on a limited number of blend components in the gasoline pool. Generally these blend components being: untreated light straight run gasoline; straight run naphtha; natural gasoline; purchased blend stocks; and FCC (Fluid Catalytic Cracking Unit) gasoline. Most of these components are small portions of the overall pool and often fit into a simple model for sulphur reduction consisting of: inclusion in existing treating facilities; additional conventional treating; or exclusion from the blend pool.

The primary target stream for reduction in sulphur is FCC gasoline, since in most refineries it is the largest blend component as well as the highest sulphur contributor to the blend. In most applications a target of 20-30 ppm sulphur in FCC gasoline is required to meet the less than 10 ppm specification in the blended gasoline product streams set by Tier 3 regulations. FCC gasoline is also a large contributor of octane barrels to the gasoline pool and retention of superior blend properties is highly important when considering options for stream sulphur reduction. This is where the challenge presents itself in providing profitable solutions for a superior blend component low in sulphur that does not result in the hydrogenation of valuable olefins found in FCC gasoline.

Reduction of FCC gasoline sulphur is achieved today in industry by way of multiple approaches; the pretreatment of FCC feed streams, the post treatment of FCC gasoline, or a combination of the two. The post treatment of FCC gasoline, unlike conventional hydrotreating, targets the selective removal of sulphur from the feed stream while limiting hydrogenation of the feed to minimise the reduction of valuable olefins in the stream. These olefins contribute to the higher octane of FCC gasoline and are inherently essential to the overall value of the stream as a blend component.

Established processes for post treatment have demonstrated successful production of Tier 2 blend components without significant losses of octane. However, there remains some degradation in product value for this method of sulphur reduction. Further reductions in sulphur using typical post treatment methods to achieve Tier 3 levels require increases in operating severity which can result in additional octane loss creating a significant economic penalty.

Alternatively, the pre-treatment of FCC feed streams removes heteroatoms including sulphur and nitrogen resulting in favourable reductions in product sulphur contents for all FCC products. In addition, pretreatment also removes metals, and aromatics from the feed streams reducing FCC catalyst poison effects and improving feed crackability respectively. To achieve additional reductions from existing pre-treatment facilities requires higher severity operation which can result in reduced catalyst life cycles. Recent developments from Criterion for both processes have provided favourable results for the application of potential drop in solutions in existing facilities.

Benefits of FCC Pretreat Strategy
Synergies between an FCC pretreat unit and an FCC may start with but are not limited to reduced sulphur in products. Pre-treatment of FCC feed provides significant upgrades in feed quality for an FCC resulting in improved yields and beneficial distribution of heteroatoms in the product streams. The hydrogenation of FCC feed streams is necessary for deep desulphurisation especially when operating at higher sulphur conversion targets for Tier 3 FCC gasoline production. This hydrogenation of feed results in the removal of metals and nitrogen which are poisons to FCC catalysts as well as the saturation of aromatics, improving the conversion potential of feed streams. More highly hydrogenated feed streams achieve higher conversion in an FCC given constant operating conditions.

It is important to note also that conversion in any FCC is a choice, meaning that the desired product slate is flexible within heat balance constraints with the adjustment of operating parameters. Since an FCC must remain in heat balance, the introduction of upgraded feed streams results in lower coke production and thus higher catalyst to oil ratios. The unit responds by increasing the circulation of catalyst from the regenerator to the riser to generate similar coke make and remain in heat balance. The increased catalyst to oil ratio provides a boost in conversion of feed to saleable liquid products. Typically day to day changes in FCC conversion are made by controlled adjustments in the riser top or reactor temperature. These changes influence the operation of the regenerator slide valve controlling the contact of hot catalyst with feed injected at the bottom of the riser. However, feed preheating is also used to influence conversion by pre-atomisation of feed prior to entering the mixing zone of the riser. Feed preheat also impacts catalyst to oil ratio by playing a role in the amount of hot catalyst required to achieve the target riser top temperature. A higher degree of feed preheat results in a reduced need for hot catalyst from the regenerator and thus a reduction in conversion by indirect effect on catalyst to oil ratio. Additionally, the recycling of products such as heavy cycle oil and slurry oil increases coke deposition on catalyst resulting in conversion control, regenerator heat balancing, and black oil minimisation. Beyond day to day operating parameter control, the very nature of a circulating fluidised bed allows for the adjustment of catalyst formulations and custom control on catalytic activity throughout the operating cycle. It is this dynamic nature which plays well into the synergies of FCC pretreat and FCC operation yielding market trending control for profit maximisation.

The addition of hydrogen to FCC feed results in an increase in feed API gravity due to aromatic saturation and removal of heteroatoms, which results in an increase in total liquid volume yield. This increase in feed gravity is associated with a shift in feed boiling range since the boiling point of the saturated aromatic structures is lower. Figure 1 displays this effect.

Aromatic rings do not crack in the FCC, however, saturated molecules do, creating improved feed crackability. The saturation of rings in polynuclear aromatics increases the available molecules for conversion in the FCC. Additionally, the removal of nitrogen from FCC feed streams reduces the inhibition of cracking mechanisms critical to both FCC performance, as well as distribution of the remaining heteroatoms in FCC product streams. Targeting low FCC feed nitrogen levels results in more favourable distribution of sulphur in FCC product streams with lower FCC gasoline sulphur levels. Which indicates that FCC pretreat hydrodenitrification (HDN) and hydrodearomatisation (HDA) performance, not solely hydrodesulphurisation (HDS), are critical influencing factors in the production of Tier 3 quality FCC gasoline.
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