Strategies for increasing production of light cycle oil
Refineries should consider several strategies, including recycle and enhanced catalyst activity, to increase LCO output from the FCCU while minimising bottoms production
David Hunt, Ruizhong Hu, Hongbo Ma, Larry Langan and Wu-Cheng Cheng,
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Maximising the FCC light cycle oil (LCO) yield to take advantage of high diesel prices relative to gasoline requires re-optimisation of the product cut point, operating conditions and catalyst technology. It is well known that the LCO-to-gasoline ratio can be increased through lowering conversion by adjusting FCCU operating conditions and decreasing catalyst activity.1,11 The drawback of this approach is the increase in bottoms yield. Recycle is often required to fully maximise LCO, while maintaining the bottoms yield consistent with a traditional maximum gasoline operation.
This article is a general discussion of strategies for maximum production of LCO in the FCCU. We will present laboratory results that quantify the effects of various recycle streams. A resid feedstock was cracked over a low zeolite/matrix (Z/M) Midas catalyst in Grace Davison’s circulating riser (DCR) pilot plant. The product bottoms was distilled to five recycle fractions (650–750°F, 650–800°F, 650–850°F, 650°F+ and 750°F+), blended back at various levels with the original feedstock, and cracked over a Midas catalyst in the ACE unit. Laboratory testing results were used to model a commercial operation to demonstrate the yield advantage of selecting the appropriate recycle stream, recycle ratio and catalyst technology.
The Energy Information Admini-stration (EIA) expects higher diesel prices relative to gasoline to continue through 2010. An average diesel price of $2.27/gallon was expected in 2009, increasing to an average $2.54/gallon in 2010. Gasoline was expected to be an average $1.87/gallon in 2009, reaching an average $2.18/gallon in 2010.2
New Corporate Average Fuel Economy (CAFE) standards are also expected to increase diesel demand relative to gasoline in the coming years. This new standard requires car manufacturers to boost fuel mileage to 35 mpg by 2020 and applies to all passenger vehicles, including light trucks. To meet this challenging, new standard, more efficient vehicles powered by hybrid and diesel engines are expected.
Refiners are increasing the production of LCO from their FCCUs to take advantage of the significantly higher value of diesel relative to gasoline. Figure 1 shows how LCO and bottoms shift versus conversion for a high and low Z/M ratio catalyst. LCO, such as gasoline, is an intermediate product increasing with conversion at very low conversion levels, eventually reaching an over-cracking point. Past this point, the LCO yield declines with increasing conversion. This high conversion regime represents the traditional FCCU operating point. A low Z/M catalyst generally produces higher LCO at the expense of bottoms for a given conversion level, as suggested by Figure 1.
Refiners tend to focus on the following strategies for maximum FCC LCO production:
• Reduced gasoline end point
• Operating conditions:
• Lower reactor temperature
• Higher feed temperature
• Lower equilibrium catalyst activity
• Removal of diesel-range material from the FCC feedstock
• FCC feed hydrotreating severity optimisation
• Residual feedstock optimisation
• Catalyst optimisation:
• Increasing bottoms conversion
• Lower Z/M surface area
• Maintaining C3+ liquid yield and gasoline octane.
Recycle streams: heavy cycle oil or bottoms
Reducing the gasoline end point is a simple and effective way to increase the production of LCO. The LCO flash point specification and main fractionator salting often determine how low a refiner can reduce the LCO initial boiling point (reduce the gasoline end point). The LCO end point should be increased within the maximum constraints of the main fractionator bottoms temperature, slurry exchanger fouling and diesel hydrotreater constraints.
Maximising LCO in the FCCU at reduced conversion without produc-ing incremental bottoms oil is the true challenge. Shifts in operating conditions to reduce conversion increase LCO, but also increase the bottoms oil yield, as seen in Figure 1. In order to produce increased LCO without producing incremental bot-toms, refiners will often reformulate their FCC catalyst to a lower activity with lower zeolite to matrix for improved bottoms conversion and to minimise LCO over-cracking.
Recycle is eventually required to minimise bottoms production as the refinery reduces conversion to reach an optimal LCO yield, as suggested by Figure 1. Almost all FCCUs operated with large recycle streams prior to the introduction of zeolite catalyst in the 1960s and 1970s. However, since the 1970s, recycle was generally removed from FCCUs as the catalyst and equipment technology improved and the FCCU operated at high conversion and feed rates to produce gasoline, C4s and C3s. As a result, our industry has limited recent experience with these recycle streams and their effect on FCC products and coke.
The better to understand these recycle effects with the maximum bottoms cracking catalyst Midas, Grace has conducted a series of riser pilot plant (DCR), ACE and computer simulation studies.
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