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Fluidised catalytic cracking
Leading-edge technology for enhanced reliability and environmental performance.
Refiners are under immense pressure to improve the performance of their fluidised catalytic cracking (FCC) units. At the same time, there are changing market demands and environmental issues to consider. Shell Global Solutions helps by selecting technology from an extensive line-up to devise process solutions that meet individual customer’s objectives.
About the technology
Since 1943, the Shell Group (“Shell”) has designed 33 FCC units and performed 75 major unit revamps for internal and external customers. Shell operates approximately 1 MMbbl/d of owned capacity and advises customers having a further 1 MMbbl/d of throughput.
Figure 1: Flue gas enters the TSS through a central pipe and moves down to the swirl tubes. A swirl vane generates a high-velocity spin that removes fine catalyst particles; these exit the bottom of the TSS with a small volume of gas. The cleaned flue gas reverses its flow direction and leaves from the top of the TSS.
First-hand operating experience, combined with commercial awareness, proven technology and a dynamic R&D programme, means we can offer practical advice, robust engineering and the most appropriate technical solutions for today’s evolving markets.
FCC technology from Shell Global Solutions includes a range of features, some of which are not available elsewhere.Feed injection system: Proprietary side- and bottom-entry feed nozzles offer good riser coverage and mixing with the catalyst. This can result in higher gasoline yields, less dry gas, lower steam consumption and reduced pressure requirements. Riser internals: Riser internals improve catalyst distribution and reduce spent catalyst reflux, which minimises nonselective thermal cracking. Close-coupled reactor cyclones with coke catcher: In addition to providing high separation efficiency, close-coupled cyclones minimise post-riser cracking and reactor vessel coking. High-efficiency stripper: Proprietary PentaFlow stripper packing removes up to 95% of hydrocarbons. The relatively open design prevents plugging, enhances catalyst flux and facilitates access for maintenance. Catalyst circulation enhancement technology: This improves circulation rates by up to 50% and is applicable to both the stripper and the regenerator standpipes; it improves stability and pressure build-up by optimising the catalyst condition near the inlets. Flue-gas technology: Shell pioneered third-stage separator (TSS) technology (Figure 1 overleaf) and has installed more than 60 units over the past 50 years. A TSS typically reduces flue gas articulate emissions to less than 50 mg/Nm3. It also protects the flue gas system from erosion. Product separation: Excellent separation can be achieved through the use of high-capacity mass transfer internals, including the Shell Calming Section and the Shell HiFi‡ and Shell Grid trays.
Increased profitability: Petróleos de Venezuela has increased margins by $5–6 million at its Cardon refinery using Shell Global Solutions’ advanced nozzles. This is the result of increased gasoline yield, a decrease in lower-value products and improved coke selectivity.
Improved reliability: The 20,000-bbl/d FCC unit at Suncor Energy Inc.’s Commerce City refinery, Colorado, USA, suffered from substantial particulate emissions from its flue gas system, which affected its compliance with MACT-2 regulations. Regenerator afterburn was also leading to poor reliability. The regenerator was fitted with new Shell cyclones, an outlet plenum and catalyst and air distribution systems. A TSS was also added. The results were a 5% capacity increase; a fall in average particulate emissions to 0.376 lb/1,000 lb of coke burn (well within the MACT-2 limit); and improved stability for regenerator operation, even with a 5% higher feed rate.
Reduced emissions: In 2007, a revamp of the flue gas cleaning system of an Australian Shell FCC unit cut particulate emissions to below 40 mg/Nm3. This revamp employed Shell Global Solutions’ swirl tube separator technology in combination with a small underflow filter cleaning system, and did not need an electrostatic precipitator or a wet scrubber.
Shell Global Solutions’ FCC units outperform the industry average in every reliability-related benchmarking measure. They also exceed the industry average for availability, despite running with Conradson carbon levels significantly above average. And, where permitted, they typically run for four to five years between shutdowns.
Gasoline demand is weakening as diesel becomes more widely used. At the same time, propylene demand is soaring and the steam cracking route is unable to keep up. In response, Shell Global Solutions has developed the MILO S (MIddle distillate and Lower Olefins Selective) process.
The hardware is all proven, which makes it ideal for revamping an existing FCC. The key element of the MILO S process is a separate second riser in which the optimal conditions can be created to produce propylene without excessive dry gas, coke and butylene make. The existing riser is used for normal cracking or can be tuned for enhanced light cycle oil yield.
Pilot plant trials indicate that using a MILO S system can more than double the propylene yield or raise the light cycle oil production by more than 20% (relative) with an increase of at least seven in the cetane index.