Overcoming clean fuels challenges

Improvements in FCC pretreat catalyst systems enable refiners to meet ultra-low sulphur gasoline regulations and increase diesel output.


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Article Summary

Refiners increasingly need to produce ultra-low sulphur gasoline and increase diesel fuel output, which can be challenging. FCC pretreatment (FCC-PT) units are, therefore, more important than ever, and they require robust and reliable catalyst systems to give them high activity and good stability while meeting specific operating goals.

However, no single FCC-PT unit caters equally to all clean fuels hydrotreating needs. FCC-PT unit operations span a wide range of operating conditions and process many types of heavy, dirty oil feedstocks with differing compositions and contaminants. An individual refiner’s primary operating objectives can also vary from targeting constant product sulphur to maximising hydrodenitrogenation (HDN) and hydrodearomatisation (HDA), and/or increasing conversion to produce more diesel fuel. Given these wide ranges of operating conditions, feed properties and operating objectives, Albemarle has determined that VGO Stax–FCC-PT technology is a preferred solution to enable refiners to meet their FCC-PT challenges.

FCC-PT is an increasingly important refinery process. Unlike processes that produce clean fuel products directly, its economic value is largely derived from how well it improves FCC unit product yields, product qualities and operations.

Depending on the refinery, the key objectives for FCC-PT operations may be to:
a. Maintain a low product sulphur level with high hydrodesulphurisation (HDS) to ultimately meet environmental regulations on gasoline sulphur content and FCC unit SOx emissions;
b. Reduce nitrogen and aromatics levels by maximising HDN and HDA to improve FCC unit product yields, selectivities and operations; or
c. Increase the conversion of vacuum gas oil (VGO) feed to diesel-range products.

In addition to these objectives, controlling FCC-PT catalyst fill costs and achieving target cycle lengths are high priorities for most refiners.

Existing FCC-PT units are designed and operated to meet specific operating strategies and objectives. They can generally be characterised by operating objectives and hydrogen partial pressure (ppH2). ‘Low’ pressure units operate with inlet ppH2 <55 bar (800 psi); ‘moderate’ pressure units operate with inlet ppH2 between 55 and 90 bar (800-1300 psi); and ‘high’ pressure units operate at ppH2 >90 bar (1300 psi).

Units in Europe, the Middle East and India operate mostly in the low to moderate pressure range. These units are typically operated to achieve deep HDS and, in some cases, to increase conversion of VGO feed to diesel product. North American units fall primarily into the moderate to high pressure range. More than 50% of the world’s moderate pressure FCC-PT units and more than 75% of the high pressure units are in North America. While many of these units focus on deep HDS, maximising HDN/HDA is also a major focus. With the US Environmental Protection Agency (EPA) implementing its Tier 3 ULSG regulations in 2017, many of these FCC-PT units will need to operate at even deeper HDS, especially those units in refineries that do not have FCC naphtha post-treat capabilities. Relatively fewer units in the Americas currently strive to increase diesel yield compared with the rest of the world.

Leveraging VGO Stax-FCC-PT technology

Albemarle has a track record in supplying cobalt-molybdenum (CoMo) and nickel-cobalt-molybdenum (NiCoMo) catalysts into low to moderate pressure FCC-PT units where the primary objective is to achieve low product sulphur targets. In recent years, product sulphur targets have become even lower, thus increasing the need for more active and stable catalysts. In addition, for moderate to high pressure FCC-PT units, the need for deeper HDN and greater HDA to improve FCC yields and selectivities has increased.

To better meet these broad market needs, Albemarle has conducted research across the spectrum of operations to assess how to help refiners meet their needs. During this research, it became clear that the wide variations in FCC-PT operating conditions, unit objectives and constraints make it practically impossible for a single catalyst to meet every objective. Therefore, we concluded that Stax systems for FCC-PT are preferable for almost all applications.

Stax is Albemarle’s proprietary process technology for designing a catalyst system to meet unit objectives within given operating conditions and constraints. This technology accounts for the chemical reactions and reaction environments encountered in the different zones of the reactor and matches the catalyst benefits with each zone to optimise the overall catalyst system’s performance. Ultra-low sulphur diesel (ULSD) Stax technology has been successful in applications with three reaction zones for over 10 years. VGO Stax–FCC-PT technology has been successfully applied for the past six years. However, it comprises only two reaction zones, as FCC-PT operations currently do not go deep enough in HDS/HDN to achieve the reaction conditions experienced in Zone 3. Figure 1 illustrates the basic concepts of VGO Stax–FCC-PT technology by showing the types of reactions occurring in each zone and the inhibitors slowing these reactions.

Most low to moderate pressure units have constant HDS as their primary objective. In Zone 1, the top portion of the reactor, where direct desulphurisation (DDS) is the predominant reaction, CoMo and NiCoMo catalysts are ideal for boosting HDS reaction rates. HDN reaction rates tend to be limited by lower pressure, and nitrogen inhibition further slows HDN and HDA rates in Zone 1. When intermediate product sulphur and nitrogen fall to certain levels, Zone 2 is achieved. HDS reactions can occur via a combination of DDS and hydrogenation (HYD) reactions at an overall slower rate than in Zone 1. Due to the lower concentration of nitrogen, HDN and HDA reactions will be faster compared with Zone 1, though still relatively slow. Thus, NiCoMo catalysts, with their balance of HDS and HDN reactions, can be very beneficial in this part of the reactor, especially at moderate ppH2. The conversion of VGO feed to lighter products in these units is essentially due to thermal conversion caused by increased temperature, although different catalysts can also stimulate different amounts of cracking.

Units operating at moderate to high pressures may have any of the three primary operating objectives. As ppH2 increases, achieving increased HDN/HDA tends to become increasingly important. For applications where overall catalyst system HDS activity is the primary objective, using CoMo and/or NiCoMo catalysts in both Zones 1 and 2 can still be effective for meeting unit objectives. To achieve moderate levels of HDN and HDA, using NiCoMo catalyst in Zone 2 with CoMo or NiCoMo catalyst in Zone 1 may be sufficient. Achieving deep levels of HDN/HDA will generally require NiMo catalyst in Zone 2, and possibly in Zone 1 as well. However, for units that are HDS activity-limited in any way, CoMo and/or NiCoMo catalysts are beneficial in Zone 1 and, in some cases, the upper portion of Zone 2 as well.

To bolster VGO Stax–FCC-PT technology applications, Albemarle has a proprietary process model that gives reliable performance estimates and is used to generate and assess catalyst loading design recommendations for refinery units.

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