R. Singh and X. E. R. MALDONADO, Topsoe Inc., Houston, Texas; and F. B. JOHANSSON, Topsoe A/S, Lyngby, Denmark
Hydroprocessing units in refineries process crude oil feedstocks in the presence of hydrogen (H2) and catalysts at high temperatures and pressures to produce clean fuels (i.e., gasoline, diesel, jet) while meeting stringent environmental standards dictated by U.S. and European Union (EU) frameworks. Lately, bio-based feedstocks are also being processed in these units to produce renewable diesel and sustainable aviation fuel (SAF). The reaction pathway for fossil fuels production primarily involves the use of heterogeneous catalysts to remove sulfur, nitrogen, metals and aromatics species in fossil crude oil, resulting in clean and upgraded refined products. The catalysts employed predominantly contain a base metal [i.e., cobalt (Co), nickel (Ni), molybdenum (Mo)] on an alumina (γ-Al2O3) or silica (SiO2) as a support and are widely used in the refining industry for hydrocracker pretreat and hydrotreating applications.
The primary objective of a hydrocracker pretreat unit is to achieve a deep hydrogenation of the organic sulfur, nitrogen and aromatic molecules present in the feed to facilitate an enhanced activity performance of the hydrocracking catalysts downstream; a hydrotreater’s objective is, as mentioned, to reduce sulfur, nitrogen and aromatics in naphtha, jet, diesel and gasoil in their respective units. The performance of these units and the ability to reach the desired product specifications and cycle length depend on multiple variables, such as the physical properties of the feed (density, viscosity, boiling point), the chemical properties of the feed (sulfur, nitrogen, aromatics, oxygen, contaminant levels), the operating conditions (H2 partial pressure, temperature, space velocity, make-up and recycle gas flow, operational upsets) and the right choice of catalyst.
One of the most unpredictable variables are the operational upsets that can strongly influence the cycle length of the catalyst and the ability to achieve the unit’s overall process and profitability objectives. Therefore, the catalyst choice becomes particularly important to ensure optimal performance of the unit throughout the cycle. A robust catalyst system with an ability to provide stable performance during various upset scenarios is particularly desirable for the refining industry.
Alumina-supported catalysts as the preferred choice for hydroprocessing applications. Alumina as the main component of hydroprocessing catalysts has been continuously developed and optimized by the author's company for decades, and it forms an integral part of the company's corporate research and development programs. It is well understood that the physical strength of the catalyst system originates from an alumina support providing desirable textural properties (i.e., surface area, pore volume, optimal average pore diameter, pore size distribution) to ensure fast reactant diffusion and accessibility to the highly dispersed active metal sites inside the catalyst extrudates (FIG. 1).
Combining these textural properties with the desired microstructure, attrition resistance and phase compositions of the alumina provides strength, hardness, inertness, and chemical and hydrothermal stability to the catalyst support and to the final catalyst.
The ways these characteristics of alumina-supported catalysts enhance the performance of hydroprocessing units are discussed in the following sections.
Fixed-bed catalysts loading and unloading. Optimal loading is one of the key parameters to achieve full catalysts utilization by the feed and maximum saturation activity. The mechanical strength and hardness of the alumina support allow for proper loading of the catalyst into the reactors. This minimizes the risk of attrition or uneven loading that might otherwise lead to the premature development of pressure drop, poor distribution, high radial temperature spread and shortened cycle lengths—all of these factors can eventually decrease the profitability of the unit. Additionally, the controlled surface acidity of the support minimizes uncontrolled coke formation. A low coke formation rate also helps to reduce the risk of agglomeration and fused catalyst beds, which would require a substantial amount of work and time to dislodge during unloading, reducing the operational time of the unit.
Supported alumina catalysts reduce bulk densities, resulting in lower mechanical stress on support grids, beams, outlet collectors, foundations and associated mechanical fixtures. Alumina-supported catalysts can be loaded into the unit’s reactors without specialized mechanical considerations.
Thermal stability. The thermal stability of the catalyst is a key operational characteristic that is strongly dependent on a catalyst’s ability to maintain its surface area, pore size and pore volume during any temperature upset scenario, such as sudden increases in bed exotherms, the cooling process, and temperature excursions during operation or sulfiding and/or runaways. Alumina-supported catalysts inherently provide this quality and structural integrity during such upsets, helping to maintain/minimize the risk of loss in activity and mitigate potential pressure drop build-up.
Stability in a H2-deficient environment. Make-up and recycle compressor failures can occur during a cycle and cause coke formation on the catalyst from a lack of H2 in the system. As mentioned, an alumina-supported catalyst provides balanced acid/base surface characteristics that positively impact the ability of the catalysts system to tolerate carbon formation during H2 depletion scenarios, particularly during compressor failure events.
Avoiding pressure drop build-up issues. During operational upsets involving quench flow increase, or depressurizing events, the high crush strength of the alumina support protects the catalyst from attrition damage so that a stable void space and a stable pressure drop are maintained throughout the cycle. This helps to avoid costly premature outages and/or process intended charge rates.
Feedstock reactivity. Refiners are now processing various feed diets during the cycle—ranging from typical to opportunistic feeds—or facing sudden feed changes due to another unit’s turnaround. This requires operational flexibility from the hydroprocessing catalyst system in terms of bed exotherms, quench requirements and pressure drop limitations in the reactor beds. Highly aromatic feeds will release a large amount of heat due to saturation reactions on such catalysts, but it is also highly profitable to upgrade highly aromatic feeds from cokers or feeds containing cracked stocks.
Limiting the number of barrels of such feeds processed due to constraints on high heat release in the beds will lower the profitability of the unit over its entire cycle. This becomes even more important if hydrocracking catalysts are loaded downstream to avoid runaways. Alumina-supported catalysts provide a moderate axial and radial exotherm that allows the refinery to use the existing quench capacities to facilitate smooth operation and avoid substantially high demand of quench flow to cool down a particular bed, which can lead to pressure drop limitation and hurdle operation. Additionally, avoiding uneven exotherm across the beds allows the quenches to be properly distributed for better gas-to-oil ratio and H2 availability.
Alumina-supported catalysts help provide a balanced H2 consumption within the available supply limitations to support the hydrogenation reactions. Good gas flow distribution in the reactor reduces the propensity towards coke formation, enhancing the cycle length of the unit. This advantage allows for the processing of feeds with high amounts of cracked/aromatic content (as well as opportunity feeds), which leads to enhanced unit profitability. Large axial and radial exotherms can be generated from a sudden increase in saturation reactions, and a substantial amount of time can be spent on balancing the heat release and managing operational hurdles related to the maximum amount of cracked stocks that can be processed. Generating a controlled and manageable exotherm on these types of catalysts while processing highly aromatic feedstocks facilitates a rather quick transition period from low-aromatic feedstocks to much desired highly aromatic feedstocks, avoiding onstream time loss.
Sulfiding requirements. Catalytic metals are initially in an oxidic state and must be converted to sulfides to form the active catalyst. This is accomplished via a sulfiding process using sulfur-donating agents in the commercial unit (in-situ) or by a third-party (ex-situ) sulfiding company. Another advantage of alumina-supported catalysts is the flexibility to perform the sulfiding using either an in-situ or ex-situ process. Alumina-supported catalysts also allow easy diffusion of hydrogen sulfide (H2S) into the pores of the support containing the active metals at a wide range of pressures, providing maximum utilization of the metals for hydrogenation reactions. This benefit also reduces the amount of sulfur-donating agents required to ensure maximum activity per metal content—therefore, savings on turnaround costs can be realized for the refinery.
Moreover, controlled H2 consumption and the resulting heat release from the sulfiding of optimized metal loading allows flexibility in selecting feedstocks to be utilized during the start-up procedure. This feed flexibility allows refiners to avoid adjusting their distillation fractionator operation and/or purchasing special feedstock with low polyaromatics or density to use during sulfiding. The ex-situ sulfiding options available for the supported catalyst offers several benefits:
These advantages can lower turnaround costs and increase profits for the refinery.
Sub-optimal sulfidation increases the risk that not all metal in the catalyst converts to the most catalytically active phase. In the 1970s, researchers from the authors’ company, led by Dr. Henrik Topsøe, identified and characterized these active sites (Type I or II) as nanocrystals of CoMo/NiMo sulfided slabs. With advances in scanning tunneling electron microscopy, these structures could be further investigated. In the early 2000s, researchers from the authors’ company managed to image the active sites, identifying high electron-density areas on the catalyst particles that promote the hydrogenation route critical for sulfur and nitrogen removal from refractive compounds found in crude feeds (FIG. 2).
These sitesa formed the basis for the company’s seriesa of hydrotreating catalysts. Using this knowledge, it has been the continued focus of the company’s research and development team to optimize the alumina and its interaction with the metals to develop catalyst technologiesb,c that ensure high metal dispersion and effective access to the catalyst’s active metal sites. The advanced textural property of alumina allows for an optimized loading and dispersion of the metals (Ni, Mo, Co) to produce high activity per pound of the catalyst at a reasonable price.
Regeneration capability. An additional advantage is that alumina-supported catalysts can be regenerated and refreshed, extending their applicability through reuse in other hydroprocessing units. The ability to regenerate a catalyst for reuse is an attractive option when considering cost-effective and sustainable solutions for refineries.
Takeaway. Top-tier NiMo alumina-supported catalystsb,c have distinct operational advantages for hydrocracker pretreat and conventional hydroprocessing applications where high hydrogenation and stable volume swell throughout the cycle are desired.
The choice of catalysts can significantly impact a unit’s profitability; therefore, catalyst options should be investigated broadly from an operational standpoint to determine the best fit with the unit. A holistic comparison including catalyst activity, H2 consumption, temperature control, pressure drop limitation, yields, turnaround concerns, cycle objectives, operational feasibility, process constraints and catalyst price should all be considered to determine the optimal catalyst system for the unit. HP
NOTES
Rahul Singh is Technical Service Manager for the hydrocracking catalyst segment for the U.S. and Canada at Topsoe. He has been responsible for the overall technical service, sales and support to the hydrocracking clients in North America for more than 10 yr. Dr. Singh began his career at Topsoe as a Research Engineer in Denmark developing and testing catalysts, processes and new products for commercial applications in the energy sector. He received his MS degree and PhD in chemical engineering with a focus on heterogenous catalysis from the University of Akron, Ohio (U.S.).
Xavier E. Ruiz Maldonado is Technical Service Manager within Topsoe North America and has more than 17 yr of experience in technical support in hydrotreating and hydrocracking industrial operations for both the Americas and European business/refinery market. He holds an MS degree in chemical engineering from Simon Bolivar University and a petroleum studies degree from the French Institute of Petroleum.
Frank Bartnik Johansson is the R&D Director for Clean Fuel and Chemicals catalyst development at Topsoe’s headquarter in Denmark. He has worked within Topsoe R&D for the past 18 yr focusing on catalyst development and understanding, up-scaling for production and commercial performance. Dr. Johansson earned a PhD in inorganic chemistry from the University of Southern Denmark.