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Jan-2003

Role of advances in FCC catalyst technology

Sulphur-reducing chemistry that preserves gasoline yield and minimises octane loss while avoiding cut point penalties can help meet clean fuel specifications where post treatment and additive systems may not be adequate.

J B McLean, D M Stockwell, W A Weber, W S Winkler, M R Greczek and T E Masak, BASF Catalysts

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Article Summary

In the next five years refiners in the USA and Europe will be faced with achieving significant reductions in their overall gasoline pool sulphur level. The US regulations call for a level of 30ppm by 2005, which represents an order of magnitude reduction from current average levels of around 300ppm. European regulations are even more stringent, calling for availability of 10ppm gasoline by 2005. The vast majority of the pool sulphur is contributed by FCC naphtha, since other gasoline blending streams, such as reformate and alkylate, tend to be virtually sulphur-free.

The challenge faced by refiners, then, is to significantly reduce the FCC naphtha sulphur content. Options under consideration include feed hydrotreating, operation of the FCCU itself (including process and catalytic technology options) and post-treating the FCC naphtha. Process and capital planning decisions are being made today for implementation by 2005 and operation for decades beyond.

In Europe, an intermediate target of 150ppm applies in the pre-2005 time frame. The US Environment Protection  Agency (EPA) has allowed refiners to generate “sulphur credits” which may be used to offset or delay the 2005 implementation or may be sold to other refiners. These credits can be earned by producing pool sulphur levels at least 10% below 1997–98 baseline levels. This translates to a reduction in FCC naphtha sulphur of 20–30%, depending on the refiner’s pool mix.

One option being considered is the use of a catalytic sulphur reduction additive, with reductions of up to 30% reported. Another option is to reduce the end point of the FCC naphtha. Since most of the sulphur is concentrated in the “back end” of the gasoline, this can effectively reduce the sulphur level, but with the penalty of reduced gasoline yield.

For the post-2005 time frame it is expected that some form of gasoline post-treating will be required by most FCC operators. A number of processes are under development and demonstration based on selective hydrotreating, catalytic distillation and adsorption of sulphur species.
FCC technology advances, both process and catalyst, have historically worked together to achieve significant improvements in FCC performance. There is every reason to expect that this will continue to be the case in the future. These advances will have an impact on the volume and quality of the FCC gasoline produced and will offer additional operating flexibility. This should be recognised in refiners’ decision processes today.

Technology benefits
Engelhard has recently commercialised a new FCC catalyst technology, NaphthaMax, which has demonstrated significant improvements in conversion and gasoline yield in multiple commercial applications. The technology features Engelhard’s Pyrochem-Plus zeolite with a new matrix technology known as distributed matrix structures (DMS) to achieve optimised porosity along with the high activity needed for today’s short contact time (SCT) cracking applications.

The technology is currently in use in 15 commercial FCC units, replacing previous 
catalysts from both Engelhard and other suppliers. Typical performance improvements noted include:
• Conversion + 3-5 lv%
• Total C3+ liquid yield + 2-4 lv%
• Gasoline (C5-221°C) yield + 2-4 lv%
• Slurry (343°C+) yield –(1–3) lv%
• No increase in coke or C2- gas

Gasoline sulphur distribution
Figure 1 shows the distribution of sulphur species in a typical FCC gasoline. While there is a tendency to assume a smooth and increasing trend of sulphur content with boiling range, the true distribution is quite discontinuous owing to the specific sulphur species involved.

The final 10% of the full range gasoline contains 35–40% of the total gasoline sulphur due to the presence of benzothiophene. By reducing the FCC naphtha end point, the benzothiophenes can be dropped into the LCO fraction and a corresponding gasoline sulphur reduction can be achieved. The penalty for doing this in most cases is a significant gasoline yield loss. For example, a 10% reduction in relative gasoline yield can translate into over 5% loss on feed. This heavy naphtha would then drop into the LCO product. Since its sulphur level is lower than the full range LCO, it has the side benefit of reducing the overall LCO sulphur content, as well as increasing its yield. Careful control of gasoline end point is used by most refiners to fine-tune the yields and qualities of the products, but in a gasoline oriented operation the large yield penalty incurred by a major cut point reduction is not desirable.

Yield benefits
Refiners have achieved documented gasoline yield increases with NaphthaMax. This is true regardless of the gasoline cut points employed. In addition, a reduction in gasoline sulphur has been observed in one trial, as indicated in Figure 2. As the catalyst inventory changed out to higher concentration of NaphthaMax, gasoline yield increased while gasoline sulphur was reduced.

The steady trends shown in each plot are verification that the changes are due to catalyst effects and not due to feed or operating condition changes, which would be more of a step change response. These improvements were noted without any significant change in feed quality, operating conditions, or product cut points.

With NaphthaMax-LSG (LSG: low sulphur gasoline) technology, the refiner has additional flexibility to reduce gasoline sulphur content via cut point control while preserving or even increasing gasoline yield due to the improvement in gasoline selectivity achieved with the catalyst. The additional benefits of lower coke and gas selectivities, improved bottoms cracking, and higher light olefins yields (when desired) are also achieved with this approach.


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