Feb-2009

Increasing olefins yield

Plant data show how an FCCU’s propylene yield is maximised while minimising its dry gas and slurry yield at a CEPSA refinery

Jose Maria Aguilar and J M Leon Gil, CEPSA Gibraltar-San Roque refinery
Fernando Sanchez Arandilla, Grace Davison

Article Summary

The Spanish oil group CEPSA operates three refineries in Spain — Gibraltar-San Roque, La Rábida and Tenerife — which in total can distil 430 000 bpd (or 21.5 million tpy) of crude oil, representing more than one-third of Spain’s distillation capacity. Two of these refineries, including the 240 000 bpd (or 12 million tpy) Gibraltar-San Roque facility, operate FCCUs that are currently utilising FCC catalysts from Grace Davison’s ProtAgon family of catalysts. The main FCCU objective at the Gibraltar-San Roque refinery is to maximise its propylene production, as well as to minimise its dry gas yield while using high nickel-containing feedstocks, minimise its slurry yield and increase LPG olefinicity.

The Gibraltar-San Roque FCCU is a UOP side-by-side (SBS) design operating at 538°C and typically processing feedstock with a UOP K-factor (K) of 12.0 and specific gravity (SG) of 0.915. The facility is distinguished by its petrochemicals-producing capabilities and its lubricant manufacturing units. Consequently, its range of products is more diverse than those produced by a typical refinery.

FCC operating data
In December 2003, the Gibraltar-San Roque FCCU was modified to maximise propylene production, and by the summer of 2005 was operating successfully. However, when the unit began processing a feedstock that contained twice as much nickel, there was a huge increase in its dry gas yield, which started to limit the light ends area. This resulted in the need to operate at a lower reactor temperature, leading to a lower propylene and butylenes yield and an increase in slurry; in other words, a reduction in total throughput.

CEPSA’s R&D facilities in Madrid carried out an extensive catalyst evaluation in 2007 to look at the options for maximising propylene, while taking into consideration the constraints of the commercial FCCU. The base catalyst Brilliant (preblended with 8–9% OlefinsUltra) was compared to ProtAgon-17C, a catalyst already used in another CEPSA refinery. Grace Davison developed this first-generation ProtAgon catalyst to maintain or increase the propylene make and allow CEPSA to operate a different optimisation scenario. The data obtained in the Davison circulating riser (DCR) pilot plant established that ProtAgon-17C offered all the benefits required to meet the FCCU’s objectives, so CEPSA chose to use it at its Gibraltar-San Roque refinery.

During the first six months, a blend of 50% ProtAgon-17C and 50% Brilliant (with 8–9% OlefinsUltra), the previous catalyst, was used. ProtAgon-17C had a quick impact on the FCCU and continued to perform well. As a result, CEPSA decided to use 100% ProtAgon-17C.

As well as maximising its propylene yield, an important objective for the refinery was to reduce its dry gas production. Figure 1 shows this objective was clearly achieved using ProtAgon-17C. Also, Figure 2 shows that propylene production started to increase as soon as ProtAgon-17C was added to the FCCU, despite the fact that the unit was very close to its hydraulic limit. In addition, when propylene production is compared at constant fresh throughput, it is clear that ProtAgon-17C has a beneficial effect compared to the Brilliant catalyst, as shown in Figure 3. ProtAgon-17C offered CEPSA the opportunity to open its operating window, allowing an increase in LPG yield at constant feed quality, maintaining LPG yield while using a more aromatic feed (Figure 4) or reducing unit severity. Figure 5 shows the evolution of the slurry yield, which has decreased steadily over time following the switch to ProtAgon-17C.

Conclusion
The CEPSA Gibraltar-San Roque refinery switched from a technology that utilised an additive for propylene maximisation to an integral catalyst system. The new technology uses ProtAgon-17C, a propylene maximisation catalyst from Grace Davison. This catalyst led to a clear increase in propylene yield, while providing a lower hydrogen and dry gas make. It also allows CEPSA to process a higher throughput or higher nickel-containing feeds while avoiding limitations in the fuel gas system. Further benefits include a higher conversion and a clear improvement in bottoms cracking, which has resulted in historically low slurry yield values, achieved at slightly lower catalyst additions.