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May-2016

The fastest route to higher octanes

The refinery octane pool is balanced and determined by the aggregate performance of multiple process units.

Rosann Schiller and Christophe Chau
Grace Catalysts Technologies
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Article Summary
The octane making units - naphtha catalytic reforming, light naphtha isomerisation, etherification, and alkylation - provide the potential for octane improvement and optimised blending strategies that allow the refinery to comply with gasoline specifications, mainly octane, aromatic content, RVP, sulphur, and oxygen. However, the fluid catalytic cracking unit (FCCU), by far, provides the most flexibility. Over a wide range of feed stocks, the FCCU operation and catalyst formulation can be adjusted to increase octane and meet other yield objectives.

Operating Conditions and Strategies to Maximise Octane
FCC naphtha octane can be increased by adjusting operating conditions, such as riser outlet temperature (ROT). A general rule is that the gasoline research octane number (RON) increases by 1 number for every 18°F increase in ROT, whereas ROT has less impact on motor octane number (MON). Since further octane gains diminish as ROT is increased, the starting base octane must be considered.

Moreover, unit constraints like LPG handling capability, wet gas compressor limit, or regenerator temperature can limit FCC operating flexibility. From an operational standpoint, increasing mix zone temperature will increase octane, but also will increase conversion, potentially creating a challenging balancing act against an LPG handling constraint.

Depending upon available options, economics, and refinery constraints, it might be beneficial for the refinery to consider changes in feedstock composition to address the gasoline octane shortfall. For example, a more aromatic feedstock would tend to provide a higher octane gasoline component compared to a more paraffinic feedstock. Additional residual feedstock processing could also meet an enhanced octane objective. However, a change in the catalyst composition would likely also be required to enable the processing additional resid feed in the FCCU without exceeding some of the constraints of the operation like coke and gas make.

If the refinery is limited by wet gas compressor capacity, minimising dry gas and hydrogen production will create room to make other operational changes to increase gasoline octane. Reductions in dry gas or hydrogen yield can be achieved catalytically or through optimisation of operating variables. Operating changes that lower the mix zone temperature and modify heat balance can help reduce dry gas. Selection of feed stocks with reduced metal contaminants can impact the hydrogen and gas make by lowering the occurrence of dehydrogenation reactions promoted by Ni, and to a lesser extent V and Fe. Additionally, incremental FCC LPG olefinicity can provide more feedstock for the alkylation unit, which will ultimately increase the refinery octane pool. The option to boost LPG olefinicity may not prove as beneficial if the LPG constraint results from a hydraulic recovery capability, storage, alkylation capacity constraint, etc., downstream of the FCCU.

Operating conditions and strategies are of primary importance, but many refiners do not have the operating window to drive significant increases in octane with operational moves alone. A more dramatic shift can be achieved with catalyst optimisation.

Driving Octane through FCC Catalytic and Additive Strategies
Catalysts provide several possibilities to drive octane (see Figure 1). Optimisation of the FCC catalyst formulation can minimise hydrogen transfer reactions, which produce lower octane gasoline components in favour of isomerisation or branching reactions; these, in turn, produce higher octane gasoline components.

Catalytically, hydrogen transfer reactions can be adjusted through control of zeolite unit cell size (UCS). Catalytic solutions also can help widen the range of unit operating conditions without hitting the FCC unit constraints. An example is a catalyst formulated to reduce hydrogen and dry gas, relieving a wet gas compressor capacity limitation, thereby providing an opportunity for increased operational severity.
In addition to traditional catalyst optimisation for controlling hydrogen transfer activity and low value product selectivities, there are three main improvement routes for gasoline octane: new FCC catalysts, octane boosting FCC additives, and gasoline sulphur reduction additives.

New FCC Catalysts to Maximise Octane
Grace continuously drives innovation to address market changes. In 2014, Grace introduced ACHIEVE® 400 FCC catalyst, which is designed to provide a more olefinic yield slate. ACHIEVE® 400 catalyst is formulated with multiple zeolites and tailored acidity to deliver an optimum level of butylenes, to keep a downstream alkylation unit full and maintain refinery pool octane. Incorporation of isomerisation activity into the catalyst particle itself results in a more desirable yield pattern than would be realised by use of a traditional octane boosting FCC additive. In addition, ACHIEVE® 400 catalyst has been shown to increase the octane of FCC naphtha. In multiple commercial trials, ACHIEVE® 400 catalyst is delivering incremental octane and butylene, worth on average $0.60/bbl.

Figure 2 shows field performance of ACHIEVE® 400 catalyst. LPG was shifted significantly, with a marked improvement toward light olefins, preferentially boosting butylene yield and selectivity, together with gasoline octane. Using ACHIEVE® 400 catalyst provides the advantage of a direct improvement in FCC gasoline octane, as well as operational flexibility to further increase FCC gasoline octanes, while increased butylene yield contributes to increased downstream high octane alkylation gasoline production.

Traditional Octane Boosters
ZSM-5 additives, such as Grace’s OlefinsUltra® additives, are proven solutions for boosting gasoline octane and petrochemical feedstocks. The use of FCC additives has recently surged in popularity due to their ability to increase LPG olefins. Propylene and butylene yields increase at the expense of FCC naphtha with ZSM-5 additive use, while resulting in FCC naphtha with a higher octane value, improving both RON and MON.
As prevailing economics shift, the LPG olefin and octane benefit can be optimised by adjusting additive injection rates without changing the FCC catalyst system. Moreover, neat addition of ZSM-5 additives allows the refiner to increase LPG olefins and gasoline octane and adapt with additional flexibility to evolving market needs. Increasing FCC LPG olefinicity with ZSM-5 additives - and, as mentioned earlier, with more selective FCC catalysts — provides an efficient and flexible route to increase alkylation unit feedstock, improving gasoline pool octane and yields.

“Unconventional” Approaches to Maximise FCC Gasoline
Typical gasoline hydrotreating can significantly reduce the octane of the gasoline pool. Commercially proven gasoline sulphur reduction catalysts and additive solutions can reduce FCC gasoline sulphur, allowing the refiner to decrease gasoline hydrotreating severity, minimising gasoline octane loss.

Some refiners are already minimising the octane loss in gasoline post-treatment units by lowering FCC naphtha sulphur with Grace’s GSR® FCC catalyst and additives. Gasoline sulphur reduction technologies with GSR® catalysts and additives can reduce FCC gasoline sulphur by 20-40%, preserving octane while meeting Tier 3 gasoline specifications. Since 2000, GSR® catalysts and additives have been used continuously around the globe to minimise sulphur in FCC gasoline. Yet, amid today’s regulatory environment, these products are frequently utilised for octane preservation.
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