Increased flexibility and profitability from hydroprocessing catalyst
Refiners have faced, and will continue to face, numerous external challenges from multiple fronts to maintain a profitable operation.
KEITH WILSON, LOUIS BURNS and PADMINI LINGARAJU, ExxonMobil
RINUS CERFONTAIN, BOB LELIVELD and BARBARA SLETTENHAAR, Albemarle
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While there are several options, the following are the four key focus areas for maintaining competitiveness: maximise high value products; reduce feed costs; optimise capacity; and meet more stringent regulations. Performing a balancing act of these key factors is critical to achieving profitability. Using catalyst technology enables these key focus areas and achieving profitability.
ExxonMobil and Albemarle, through a longstanding catalyst development partnership, have co-developed a breakthrough in hydrotreating catalysts. Built upon the success of Nebula catalyst, the partnership has commercialised Celestia, a bulk metal catalyst that has demonstrated another step change in hydrotreating activity. Celestia’s activity boost allows profitable processing of difficult feeds with higher end point, nitrogen and sulphur content, enables an increase in capacity, and results in volume gain via aromatic saturation activity. An added advantage of the new catalyst is creation of value beyond the hydrotreating unit. For example, the increased activity allows processing of heavier straight run feed that pulls vacuum gasoil (VGO) into diesel, allowing resid imports to the FCC.
Over the last three years, ExxonMobil and Albemarle have deployed Celestia alongside Nebula and conventional catalysts at ExxonMobil’s sites around the world. Carefully designed combinations of Celestia, which has activity two to three times that of the conventional catalysts, were deployed in stacked configurations with Nebula. The combination lead to significant increase in the total activity in the reactor. Application of these technologies in distillate hydrotreaters, as well as light cycle oil (LCO) and VGO hydrocracker pretreaters, has yielded exceptional returns with a payback period of about four months. However, the key to extracting value from higher activity is to understand how the catalyst can be applied to unlock value through integration. This article will present case studies to illustrate the properties and performance of Celestia, as well as show how ExxonMobil maximised margin improvement in a challenging environment.
Celestia and Nebula are versatile catalyst solutions applicable to many hydroprocessing platforms, from naphtha to VGO. ExxonMobil and Albemarle have successfully commercialised Celestia in distillate hydrotreating, light feed hydrocracking pretreat, and heavy feed hydrocracking pretreat (see Figure 1). Each of its deployments to date have been based on carefully designed stacked catalyst load configurations with supported catalysts and/or Nebula at Exxon Mobil’s operating refineries located in Europe, Asia-Pacific, and the Americas. The key to successful deployment is to combine detailed process chemistry, kinetic, and engineering knowledge with a detailed understanding of the economic needs of the refinery.
For process chemistry and kinetic understanding, Albemarle’s Stax technology for kinetic modelling relies on detailed understanding of the science behind hydroprocessing. Stax applies this knowledge to match catalyst functionality to the location in the hydroprocessing loading system. This technology enables detailed and optimised catalyst loads to be developed on a functional zonal basis, tailored to meet the refiner’s objectives and feed types, and is applicable to flexible hydroprocessing platforms.
From a kinetic perspective, Celestia is most effective when used in high pressure applications. As such, initial commercialisation priorities have focused on hydrocracking pretreat and distillate hydroprocessing applications that benefit from operating at high hydrogen partial pressure. Given the very high volumetric activity, Celestia has been subject to extensive pilot plant testing to support expanding the technology’s applications. Testing has confirmed that its improved performance also extends into the medium pressure range of hydrotreating applications.
Celestia has demonstrated significant base economic improvement, arising from upgrading more severe feedstocks, increased feed rate, increased conversion, improved product quality, and/or allowing new processing opportunities in both fuels and lube base stock service. The value has also come from how the high activity can impact adjacent units. More detailed discussions are listed in the following commercial applications. Further applications of the technology in stack configuration are planned for additional hydroprocessing platforms, including distillate hydrotreaters and hydrocrackers.
Case study: light cycle oil hydrocracking
Diesel fuel production is under re-evaluation because of increasingly stringent environmental regulations. The low quality of LCO with high sulphur and nitrogen, and a high percentage of diaromatic hydrocarbons, limits the possible upgrading possibilities. Celestia has been commercialised in a North American LCO hydrocracker to provide an efficient, alternative way to upgrade LCO to on-spec transportation fuel. The key processing objective for using Celestia was to increase/debottleneck the unit feed rate, improve feed flexibility, and improve distillate yield and quality.
Both Celestia and Nebula were loaded in the unit, with Celestia loading accounting for approximately 26% of the pretreat reactor catalyst load. The unit processes a blend of cycle oil and heating oil, with feed properties shown in Table 1.
Figures 2 and 3 illustrate the Celestia and Nebula stack loading plan, and the progressive increase in using high activity bulk metal catalysts for hydrocracking pretreat over subsequent cycles. The success of bulk metal catalyst technology in this service has justified a steady increase in vol% of load from 13% Nebula to almost 30% Celestia/Nebula over five successive cycles and 18 years’ profitable operation. Initially, this was done to mitigate the risk of adding a bulk metal catalyst given that Nebula was new to the market. However, ExxonMobil and Albemarle now have over 15 years of commercial experience on deploying and operating bulk metal catalysts. This led to ExxonMobil deploying the optimal amount of bulk metal catalyst in the unit.
The relative activity advantage of Celestia versus Nebula catalysts in LCO hydrocracking service is illustrated in Figure 4, showing breakdown of aromatic saturation (arosat), hydrodesulphurisation (HDS), and hydrodenitrogenation (HDN). This activity advantage is also apparent in th›e commercial performance, where the catalyst has delivered outstanding results in terms of challenged feed penetration, product yields, and product quality.
The step-out activity of the Celestia catalyst delivers value in a number of ways. Chief among them is processing greater quantities of more difficult feeds. The hydrocracking unit now processes heavier cycle oil from the FCC, achieved by increased LCO cut point to increase yield while decreasing FCC bottoms yield. The hydrocracker is able to produce on-spec ultra low sulphur diesel (ULSD) while processing a greater volume of deeper cut LCO from the FCC. In this particular case, the hydrocracker also processed more straight run VGO along with the LCO. Figure 5 shows thousands of reactor volumes of cycle oil processed per run length. Figure 6 shows the increase in the amount of reactor volumes of processed LCO per day. Figure 7 shows that the total feed rate (LCO and VGO) is also increased in the current cycle.
The concept of Nitrogen-BBL is the vol% concentration of nitrogen containing molecules in the feed multiplied by the volume of the feed. Figure 8 uses this basis to compare the performance in the various runs and indicated that Celestia enabled the processing of more of the nitrogen containing molecules per day to enhance the hydrocracking functionality of the process unit.
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