The dynamics of global gasoil (GO) and ultra-low-sulfur diesel (ULSD) demand are driven by regulatory demands for transport diesel and evolving needs for vastly improved marine-bunker quality in specific emission control areas (ECAs), as well as in economic growth. It is expected that this will drive regional variability in ULSD/0.1% sulfur GO differential over time and consequently provide incentives for refiners to maximize flexibility in both hydrocracker distillate yields and ULSD unit performance. Adding in an expectation of gasoline supply becoming long in the Middle East over the next several years, the refiner is further motivated to evaluate and select premier catalyst systems for the production of both diesel products from their hydrocracking and diesel treating units.
The growth in refined products is strongly driven by the demand for clean diesel and new regulations on GO between now and 2020. Notably, cumulative annualized global growth in diesel/GO demand is predicted to be about 2%, outpacing gasoline at 1.3% with the estimated gasoline/diesel-GO ratio dropping from 0.85 in 2012 to 0.81 in 2020.1 In addition to steady growth in Asia and continued recovery elsewhere, new motor vehicle mileage standards, ethanol mandates in North America and emerging regulatory restrictions on marine fuels add further momentum to this trend. Bunker fuel regulations becoming effective in 2015 and requiring 0.1% sulfur limits in ECAs in North America and Northern Europe will underlie a demand shift to diesel/GO products with an expected concurrent boost in diesel price primarily due to quality requirements.
Meanwhile, refining capacity additions will outstrip global demand in the 2015 to 2020 period, thus continuing pressure on refining margins. In short, it will be a period of opportunity for flexible refiners with hydrocracking capabilities, especially those coupled with a robust ULSD hydrotreater that can marry their catalyst system needs and operational responses to changing economic scenarios.
Molecular management and hydroprocessing units
Of all diesel boiling range materials, fluid catalytic cracking (FCC) light cycle oil (LCO) stands out as one of the lowest value feedstock materials. It is usually the most difficult to manage operationally in a hydroprocessing unit, largely due to the combination of olefins and the refractory nature of the LCO. It has the highest demand for hydrogen to produce a clean diesel or even 0.1% sulfur marine GO, and offers heat release management challenges when processed at higher fractions in a hydroprocessing unit feed. Provided there is adequate hydrogen supply, LCO is sometimes best processed in the hydrocracker along with other feeds such as automotive gas oils (AGOs) or vacuum GO (VGOs). Adding to the equation is that all LCOs are not created equal. Depending upon the refinery configuration, the LCO may be produced from an FCC with a feed pretreater and consequently contain fairly modest levels of sulfur and nitrogen. Although they appear to be easier feeds due to their lower levels of contaminants, the remaining impurities are also the toughest to treat.
Coker GO can also be processed either in the hydrocracker or ULSD unit subject to individual unit capacities and infrastructure limitations such as hydrogen availability, pressure, cut point and impurities. Heavy coker GO (boiling well above the diesel range of > 975°F) will prove to be problematic for catalyst life cycles if processed in the ULSD unit and can present significant challenges to processing in significant quantities even in modern, robustly designed hydrocrackers. A hydrocracker originally designed or revamped for VGO service is a more suitable outlet.
This offers more potential to maximize diesel yields, especially in recycle flow configurations and higher pressures. With adequate hydrogen partial pressure and the appropriately tailored catalyst system to remove contaminants and provide sulfur conversion, light coker GO are readily processed to ULSD in the diesel hydrotreater.
Straight-run (SR) GO present the least challenging processing constraints, and can be fed to either the hydrocracker or ULSD unit (although the ULSD unit is typically the preferred outlet). Exceptions include cases where the SR feeds are needed as diluent components to aid in managing hydrogen consumption limitations and heat release issues in hydrocrackers designed for more paraffinic and naphthenic feeds.
Processing tactics are balanced between these feedstock molecular management considerations and the designs, limitations and strategic intent of the unit in the refining scheme. Hydrocrackers have traditionally been designed with the intent to pump hydrogen into the feedstock to convert heavier, higher-boiling materials into more valuable products while capitalizing upon aromatics saturation to increase volume swell, as well as product value parameters. Until recently, ULSD has been a secondary priority and generally not even a consideration in the original design of units operating today. With a robust ULSD in the refinery, this prioritization need not be overridden, but can be augmented by the utilization of the latest catalyst systems for hydrocracking that have been designed for maximum hydrodesulfurization (HDS) activity, as well as for the fundamental hydrodenitrogenation (HDN), hydrocracking and saturation needs.
Catalyst system flexibility
While several hydrocracker configurations are in usage, two dominate the landscape, especially when addressing clean fuels production: single-stage, once-through (SSOT) configurations and two-stage, recycle (TSREC) configurations.2 A perspective over a decade of application is provided in Fig. 1.
| Fig. 1. Hydrocracker licenses by type on capacity basis. |
Hydrocracking conversion spans a range from approximately 40% to 100%. The SSOT configuration is both simple and versatile, and it represents the simplest configuration when unconverted oil (UCO) has high value as either a lube plant feed or an FCC feed. This configuration dominates the low conversion market (> 70%). The SSOT process configuration is shown in Fig. 2.
| Fig. 2. SSOT process configuration.|
Catalyst system optimization for the SSOT is often influenced strongly by the desired outlet for the UCO it produces: lube plant feed will favor higher viscosity index (VI), aromatics saturation and HDS, while FCC feed will favor HDN, poly-nuclear aromatics removal and HDS. Balanced with these needs are the light product drivers: ULSD or the less-demanding 0.1% sulfur marine fuel. If ULSD production is a target and cannot be produced within the SSOT unit constraints, it is critical to factor in and model the effect of this pre-processed component as feed to the ULSD unit. It will clearly include more difficult, sterically hindered sulfur compounds for HDS in the ULSD unit.
Catalyst system selection and optimization are controlled by many constraints that must be accommodated in a single stage:
- Hydrodemetalization (HDM) needs driven by heavy VGO (HVGO) and/or deasphalted oil (DAO) components, as well as by crude source (arsenic and other contaminants) and coker products in the feed (silicon contaminations)
- HDN requirements for the hydrocracking function in the lower catalyst system
- Hydroconversion targets to remove of heavy components
- HDS needs for products such as ULSD, marine GO and UCO
- Aromatics removal (lube or FCC applications)
- Isomerization (for lube needs).
SSOT catalyst system optimization is further challenged when the application involves a unit converted from a former service such as FCC pretreatment or, in less common cases, diesel treating. In such cases, heat release and hydrogen consumption come into play, as these units typically contain only a few deep beds. Semantics can sometimes obscure the proper application of catalyst technology. A mild hydrocracker is a low-conversion SSOT (< 40%) and is most effectively evaluated as a part of the SSOT catalyst system continuum.
SSOT systems typically demand the highest activity catalyst components to meet HDN and HDS needs.
Lacking the flexibility to recycle and adjust the recycle cut point (RCP), product selectivity in the SSOT is controlled by the catalyst system choice. This choice relies mainly on hydrocracking catalyst component selection, along with setting the operating temperature regime and span.
Hydrocracking catalysts typically exhibit a trade-off between selectivity to distillates and activity (temperature required for a target conversion level). Premier catalyst performance is defined by innovations that increase both selectivity and activity. Fig. 3 shows the progression of such performance for the hydrocracking catalysts provided by one such supplier.
| Fig. 3. Selectivity and activity improvements |
for hydrocracking catalysts by a
hydroprocessing catalyst supplier.
Catalyst system design in an SSOT can involve more than a single solution. While a single hydrocracking catalyst from the B range might seem an obvious solution for a refiner desiring A selectivity but lacking the infrastructure to compensate for the lesser activity, synergies in multi-catalyst combinations might instead point to a system of A and C catalysts and can actually achieve a better result than pure B alone.
TSREC configurations offers a high level of flexibility in addition to providing the more favorable means to achieve conversion levels of 90% plus. TSREC configurations also are the preferred means to achieve full naphtha/gasoline selectivity. The configuration is shown in Fig. 4.
| Fig. 4. TSREC process configuration.|
TSREC units offer the refiner the ability to operate the two stages differently to simultaneously meet separate goals for each stage. This configuration also offers the flexibility to balance the stages to optimize the desired product selectivity and qualities. Note that, although this unit is shown with two reactors, they are often built with multiple reactors, generally as part of the first stage providing even greater ability to process poorer value stocks.
As an example, the first stage could be targeted to both provide a diesel draw suitable for marine fuel blending, as well as pretreatment for the second stage, which could be targeted to produce ULSD. Contrasted to the SSOT, the TSREC has added operational flexibility provided by the ability to adjust RCP and per-pass conversion in each stage plus a second catalyst system that allows optimization of an additional catalytic component. In addition, feedstocks can be shifted within the ULSD unit to further add operating space. This integration and flexibility permits the refinery to take full advantage of seasonal or frequent economics.
Catalyst system selection and optimization in the first stage is often influenced by feed quality and contaminants, with nitrogen, sulfur, metals, silicon and arsenic being the typical suspects. This is especially challenging for older, existing units but can be equally daunting even for new units.
Following feed contaminant removal, the remaining catalyst volume can be used to achieve the conversion and selectivity goals. Often, the first stage is required to achieve 50%60% conversion after removing feed contaminants for both stages. Depending upon unit objectives, the first stage cracking catalyst can be chosen from any of the A, B or C groups.
Second-stage catalyst selection will largely be driven by the performance of the first stage to achieve the desired overall unit goals. Second stage catalyst will contribute significantly to product quality improvement and to the ability to achieve high levels of conversion to the desired product, typically diesel or total distillate.
Catalyst system optimization
ULSD units must also address molecular management. However, a properly designed catalyst system and optimized processing scheme can help the refiner maximize his profit goals. To help refiners deal with the severe demands of ULSD, a stage catalyst system in 2001. This system uses catalyst technology that is staged in the proper proportions to provide the best performance while also meeting individual refiner requirements. Catalyst staging is designed to take advantage of different reaction mechanisms for sulfur removal; a high activity cobalt and molybdenum (CoMo) catalyst efficiently removes the unhindered, easy sulfur via the direct abstraction route, and a high activity nickel and molybdenum (NiMo) catalyst then attacks the remaining sterically hindered, hard sulfur.
An important aspect for a staged catalyst system is designing the optimum proportions of the CoMo and NiMo catalysts that will deliver the best performance. This is dependent upon a number of factors, including the unit objectives, feed and operating constraints.3,4
A key advantage for this system is the efficient use of hydrogen. Fig. 5 illustrates how the system can be tailored to provide the best balance of high HDS activity while minimizing H2 consumption. The figure shows that, as NiMo catalyst is added to the system, there is a significant increase in HDS activity relative to the all CoMo reference, and eventually, a minimum in the product sulfur curve is reached.
| Fig. 5. Balancing high HDS activity |
while minimizing H2 consumption.
The figure also shows the relative H2 consumption, and, as the percentage of the NiMo component increases, the H2 consumption relative to the base CoMo system increases. In the region where the system shows the best activity, the hydrogen consumption is only slightly greater than that for all the CoMo system, and well below that for the NiMo catalyst. This is a direct result of the different kinetics for sulfur and aromatics removal, and it is a critical consideration when customizing the staged catalyst system.
For units that have a hydrogen constraint, the key to designing the proper catalyst system is increasing the hydrogenation selectivity to provide the highest HDS activity while minimizing hydrogen consumption. Fig. 6 shows a rapid decrease in polyaromatics concentration and a corresponding increase in mono-ringed aromatics for both catalysts as the residence time is increased. The NiMo catalyst is much more efficient at hydrogenating the final aromatic ring, as evidenced by the lower mono-ringed aromatic concentration with increasing residence time compared to the CoMo catalyst. At the longest residence time on the chart, the NiMo catalyst has about 15 absolute numbers with a lower mono-ringed aromatics concentration than the CoMo catalyst, and that corresponds to about 300 standard cubic ft per barrel in higher hydrogen consumption for the NiMo catalyst.
| Fig. 6. Effects of increasing residence time.|
The data demonstrates that the systems hydrogenation activity can be tuned by adjusting the relative volumes in the CoMo and NiMo beds within the reactor. Of course, not all units have an H2 constraint, and, in those cases, the incremental increase in aromatics saturation and the correspondingly higher hydrogen consumption obtained by the NiMo catalyst offers benefits such as cetane improvement and the ability to process more cracked stocks.
Experience has demonstrated that a properly designed ULSD unit combined with the right catalyst system can process up to 100% cracked stocks to produce < 10 ppm sulfur and provide significant cetane uplift and volume swell.
In applications where there is sufficient H2 availability and partial pressure, a NiMo catalyst is likely the most active system for HDS. However, it will consume significantly more hydrogen due to its efficiency at catalyzing hydrogenation reactions. If the incremental hydrogen consumption cannot be tolerated, a system can be designed that will deliver high HDS activity and minimize hydrogen consumption. In cases where the hydrogen pressure is lower, the staged catalyst system is often more active than either component alone. HP
1 IHS CERA, Refining and Product Markets Annual Strategic Workbook. 2013.
2 Torchia, D, Arora, A. and L. Vo, Clean, green, hydrocracking machine, Hydrocarbon Engineering, June 2012.
3 Olsen, C. and L. D. Krenzke, Custom Catalyst Systems for Meeting ULSD Regulations, 2005 NPRA annual meeting, Paper AM-05-17.
4 Olsen, C. and G. DAngelo, No Need to Trade ULSD Catalyst Performance for Hydrogen Limits, 2006 NPRA annual meeting, Paper AM-60-06.