According to a recent paper by Brian Watkins, David Krenzke and Chuck Olsen with Advanced Refining Technologies (ART), processing increasing volumes of LCO through diesel hydrotreaters requires the ability to improve cetane uplift and ASTM color specifications involving catalyst systems capable of saturating PNAs. The Watkins et al paper discussed several options for upgrading LCO, taking into account feed endpoints, reactor severity and temperature, hydrogen partial pressures and LHSV.
With refiners reporting a significant downturn in refining margins for the second half of 2016, efforts are being accelerated to maximize efficiency and product value from catalyst formulations and related process technology. Gasoline surpluses have compelled refiners to again shift the gasoline-to-diesel ratio towards higher diesel and distillate production in general. In this effort, the onus is on maximizing the upgrading potential of FCC light cycle oil (LCO), which has long been a common component of feed to diesel hydrotreaters.
Increasing the quantity of LCO to the diesel hydrotreater impacts hydrotreater performance and resulting ULSD product properties. Additional LCO has the combined effect of lowering diesel pool cetane as well as limiting end of run (EOR) by making it difficult to maintain diesel ASTM color specifications.
The extent of LCO’s impact depends upon a number of factors including the amount and endpoint of LCO in the feed, and the catalyst used in the ULSD unit. These challenges can be overcome with proper choice of catalyst system and an understanding of the impact LCO has on both unit performance and ULSD product quality.
It is generally accepted that LCO addition to diesel increases feed severity and requires higher reactor temperature (relative to straight run [SR] feeds) in order to meet product sulfur targets (e.g., less than 10 ppm). When comparing pilot plant data demonstrating the effect on reactor temperature (increase) with feed LCO concentration ranging from 0 to 75%, it is clear that even low levels of LCO can impact catalyst activity. For example, at high severity (<50ppm sulfur), about 45 °F higher temperature is required for 15% LCO, and this increases to 65 °F higher temperature for 30% LCO concentration relative to SR feed. The required activity debit reaches 100 °F for the feed containing 50% LCO.
In addition to the amount of LCO, the endpoint also has a significant influence on feed reactivity. Pilot testing has shown that increasing LCO endpoint by 30°F (by D2887) can require nearly 30°F higher temperature for ULSD sulfur levels.
Units processing significant amounts of LCO also need special attention in order to meet product cetane requirements. LCOs have very high concentrations of naphthalene- type aromatic species, which have very low cetane numbers, causing LCO to have relatively low cetane. The increased processing of LCO will also have an impact on the product total and polynuclear aromatic (PNA) concentration, which is very dependent on the catalyst system in place.
In cases where increasing LCO concentration in the feedstock can affect the product total aromatic content, ART’s high activity NDXi catalyst has been used in an effort to provide maximum aromatic conversion. The PNA content of the products can also be a concern, due to a much higher starting concentration and the thermodynamic equilibrium constraint on conversion as the hydrotreater reaches the end-of-run (EOR). With the use of a selective ring opening (SRO) catalyst ART is able to improve the hydrotreater’s ability to meet product total aromatic and PNA targets.
The aromatic/PNA laden feedstocks were also tested using a system containing NDXi and a layer of SRO catalyst at the bottom of the hydrotreater. The catalyst system provided the same HDS and HDN activity, and showed the ability to process additional LCO while achieving higher aromatic saturation conversion compared to the hydrotreating catalyst alone.
The use of a ring opening catalyst provides an additional decrease in total aromatic content, especially with the 50% and 75% LCO feedstocks. This benefit is also apparent at the higher temperatures with reduced product PNA concentrations relative to the hydrotreating catalyst alone. The new SmART Catalyst System® series with SRO catalyst capability is very effective for reducing aromatic rings, which translates to improved cetane and color performance.
- B. Watkins and C.Olsen, 2009 NPRA Annual Meeting, paper AM-09-78
- G.Rosinski and C.Olsen, Catalagram 106, Fall 2009
- X. Ma et. al., Energy and Fuels, 10, pp 91-96 (1996)
- T.Takatsuka et.al., 1991 NPRA Annual Meeting, Paper AM-91-39
- G.Rosinski, B.Watkins and C.Olsen, Catalagrm 105, Spring 2009