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Hydrodesulfurization

Hydrodesulfurization (HDS) is a catalytic hydrotreating process used in petroleum refining to remove sulfur compounds from hydrocarbon feedstocks such as naphtha, diesel, and gasoline precursors by reacting them with hydrogen to produce hydrogen sulfide, which is subsequently separated.^[1]^[2] The process employs bifunctional catalysts, typically molybdenum or tungsten sulfides promoted by cobalt or nickel and supported on high-surface-area alumina, under elevated temperatures of 300–400 °C and hydrogen partial pressures of 2–6 MPa to achieve high conversion rates of refractory sulfur species like dibenzothiophenes.^[3]^[4]^[5] HDS has become indispensable in refineries to comply with stringent sulfur specifications in transportation fuels—such as the 10–15 ppm limits for ultra-low sulfur diesel—thereby minimizing sulfur dioxide emissions from combustion and safeguarding downstream catalytic reformers and cracking units from poisoning.^[3]^[6] While effective for most thiols and sulfides, challenges persist with sterically hindered polycyclic sulfides, driving ongoing research into advanced catalysts and process intensification for deeper desulfurization without excessive hydrogen consumption.^[7]^[8]

History

Origins and Early Development

The foundations of hydrodesulfurization (HDS) emerged from early 20th-century advancements in catalytic hydrogenation, initially applied to coal liquefaction rather than petroleum. In 1897, French chemist Paul Sabatier demonstrated that finely divided nickel effectively catalyzes hydrogen addition to unsaturated hydrocarbons, such as ethylene and benzene, marking a pivotal discovery in heterogeneous catalysis.^[9] This laid the groundwork for subsequent hydroprocessing techniques, as sulfur-containing compounds in feedstocks required hydrogenation to form removable hydrogen sulfide. High-pressure hydrogenation processes, developed by Friedrich Bergius starting in 1910, advanced these efforts by converting coal and heavy tars into liquid fuels under conditions of 200 atm and 400–500°C, with incidental desulfurization occurring via catalyst-mediated hydrogenolysis.^[10] Bergius established an experimental plant near Mannheim in 1921, supported by Royal Dutch Shell, which demonstrated practical scalability.^[10] By 1926, BASF's Leuna synthetic fuel plant utilized catalyst 3510—a mixture of molybdenum trioxide (MoO₃), zinc oxide (ZnO), and magnesium oxide (MgO)—for hydrogenation stages that targeted sulfur removal alongside cracking.^[10] These German industrial initiatives, driven by resource scarcity and pre-World War II energy demands, shifted focus from oxides to sulfide catalysts; in 1930, BASF introduced catalyst 5058, derived from ammonium sulfotungstate to form tungsten disulfide (WS₂), enhancing activity for refractory sulfur species.^[10] The transition to petroleum-specific HDS accelerated in the late 1930s and early 1940s amid wartime synthetic fuel programs by IG Farben and affiliates, where hydrotreating catalysts were refined for distillate upgrading. In 1941, nickel-molybdenum on alumina (NiMo/Al₂O₃, catalyst 8376) was developed for targeted hydrodesulfurization, exhibiting superior performance in removing sulfur from heavier fractions compared to prior oxide systems.^[10] This era's innovations, building on researchers like Matthias Pier and Alwin Mittasch, established sulfide-based catalysis as central to HDS, with mechanisms involving edge-site activity on layered structures like MoS₂ precursors.^[10] Post-war, these technologies adapted to abundant crude oil refining, setting the stage for commercial HDS units by the mid-1950s, initially for naphtha feeds to produce low-sulfur gasoline.^[9]

Commercialization and Key Milestones

Hydrodesulfurization (HDS) was first commercialized in the late 1940s and early 1950s primarily as a pretreatment step for naphtha feeds to catalytic reforming units, where sulfur poisons noble metal catalysts. The UOP Platforming process, introduced in 1949, necessitated low-sulfur feeds, prompting the development of catalytic HDS to achieve sulfur levels below 10 ppm.^[11] Concurrently, Union Oil patented an HDS method in 1950, marking an early technological foundation for the process.^[1] A significant milestone occurred in 1957 with the construction of the world's first dedicated HDS plant at the Exxon Baytown refinery in Texas, which substantially reduced sulfur content in refined products and improved overall quality amid postwar refinery expansions.^[12] This unit exemplified the shift toward integrated hydrotreating, leveraging byproduct hydrogen from reforming to enable milder conditions for sulfur removal. By the late 1950s, HDS processes saw rapid growth, with multiple refineries adopting cobalt-molybdenum catalysts on alumina supports for naphtha and light distillates.^[13] The 1960s and 1970s marked broader commercialization for heavier feeds like diesel and gas oils, driven by initial environmental regulations and engine performance demands. The U.S. Clean Air Act of 1970 accelerated adoption, requiring sulfur reductions that expanded HDS capacity globally.^[14] Key advancements included fixed-bed reactors operating at 300–400°C and 30–100 bar, achieving 90–99% desulfurization efficiency depending on feedstock.^[10] Subsequent milestones focused on ultra-deep desulfurization to meet stringent standards, such as the U.S. and EU mandates for ultra-low sulfur diesel (10 ppm) implemented in 2006 and 2009, respectively, which necessitated catalyst innovations and process retrofits in over 90% of refineries worldwide.^[15] These developments, supported by high-activity nickel-molybdenum systems, reduced sulfur emissions by converting refractory compounds like dibenzothiophenes, though at higher hydrogen consumption rates of 500–1000 scf/bbl.^[16]

Fundamental Chemistry

Reaction Mechanisms

Hydrodesulfurization (HDS) reactions proceed via catalytic hydrogenation of organosulfur compounds on transition metal sulfide surfaces, primarily cleaving C-S bonds to produce hydrocarbons and hydrogen sulfide (H₂S). The process typically involves dissociative adsorption of hydrogen on the catalyst, forming surface hydrides that facilitate sulfur removal. Common catalysts, such as molybdenum disulfide (MoS₂) promoted by cobalt or nickel, feature active sites at edge planes where sulfur vacancies (coordinatively unsaturated sites) enable adsorption and reaction.^[17]^[18] Two parallel pathways dominate HDS mechanisms: direct desulfurization (DDS) and hydrogenation-desulfurization (HYD). In the DDS route, the organosulfur molecule adsorbs via its sulfur atom onto a sulfur vacancy, followed by direct C-S bond scission with hydrogen addition, yielding H₂S and an unsaturated hydrocarbon intermediate that undergoes subsequent hydrogenation. This pathway minimizes hydrogen consumption but is kinetically slower for sterically hindered compounds. The HYD route first saturates the aromatic rings via stepwise hydrogenation, forming alicyclic intermediates like tetrahydrothiophenes, which then undergo C-S cleavage more readily due to reduced π-system conjugation. For thiophene, a model compound, HYD predominates under typical conditions, involving intermediates such as 2,5-dihydrothiophene and n-butenethiol.^[17]^[18]^[10] For refractory sulfur species like dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), the HYD pathway is favored due to steric blocking of the sulfur atom in the DDS route by alkyl substituents and fused rings, which hinder adsorption on edge sites. Theoretical density functional theory (DFT) studies indicate that Mo-S edge sites catalyze DDS via σ-bond activation, while rim-edge sites promote HYD through π-orbital interactions. Activation barriers for C-S scission are lower on promoted CoMoS structures, with Co enhancing electron transfer for hydrogen activation. Experimental kinetics confirm pseudo-first-order dependence on sulfur compound partial pressure and square-root dependence on H₂ pressure, supporting a rate-determining hydrogenation step in many feeds.^[17]^[19]^[18] In heavy oil upgrading, HDS mechanisms are complicated by polyaromatic and nitrogen co-reactants, which compete for active sites and inhibit via π-donation or basicity, shifting selectivity toward HYD. Reviews emphasize that deep HDS (>99% removal) requires optimized conditions favoring DDS, such as high H₂ pressure (up to 100 bar) and temperatures (300–400°C), to overcome thermodynamic barriers in C-S bond dissociation energies (around 400–500 kJ/mol). Surface science investigations using model catalysts reveal that the "overflow" mechanism—where reactants spill from edge to basal planes—may contribute under high coverage, though edge-dominated kinetics prevail in industrial reactors.^[17]^[20]

Sulfur Compounds in Feedstocks

Sulfur in petroleum feedstocks occurs predominantly as organic compounds, including aliphatic species such as mercaptans (thiols, R-SH), sulfides (R-S-R'), disulfides (R-S-S-R'), and polysulfides, as well as heterocyclic thiophenic structures like thiophene (C₄H₄S), benzothiophene (C₈H₆S), and dibenzothiophene (C₁₂H₈S).^[3]^[21] These compounds arise from natural maturation processes in source rocks and vary by crude oil origin, with total sulfur content ranging from 0.05% to 14% by weight depending on the feedstock type.^[22] In lighter feedstocks such as naphtha (boiling range C₅–140°C), mercaptans and simple sulfides constitute the majority of sulfur, often comprising short-chain variants like ethanethiol (C₂H₅SH) and propanethiol, which are relatively reactive under hydrodesulfurization conditions.^[3] Heavier fractions, including kerosene (140–240°C), diesel, and gas oil (240–370°C), contain increasing proportions of aromatic sulfur compounds, with benzothiophenes and dibenzothiophenes (including alkylated derivatives like 4,6-dimethyldibenzothiophene) dominating and accounting for up to 70% of residual sulfur after initial processing.^[23]^[24] Thiophenic compounds exhibit lower reactivity in hydrodesulfurization due to the stability of the C-S bond within the aromatic ring and steric hindrance from alkyl substituents, particularly in dibenzothiophenes where methyl groups at positions 4 and 6 block hydrogen access to the sulfur atom.^[25]^[26] This distribution influences process design, as aliphatic sulfurs are removed under milder conditions (e.g., 300–350°C, 30–60 bar), whereas refractory thiophenics require harsher regimes (350–400°C, 60–100 bar) to achieve deep desulfurization levels below 10 ppm for ultra-low-sulfur fuels.^[27]^[5]

Industrial Implementation

Process Description

Hydrodesulfurization (HDS) is conducted in fixed-bed reactors where sulfur-bearing hydrocarbon feedstocks, such as naphtha, diesel, or vacuum gas oil, are contacted with hydrogen gas in the presence of a heterogeneous catalyst. The feedstock is preheated and mixed with recycled hydrogen before entering the reactor, typically in a downflow configuration, at temperatures ranging from 260–400 °C and pressures of 30–100 bar.^[28]^[29] The primary reactions involve the cleavage of carbon-sulfur bonds in organosulfur compounds (e.g., thiophenes, sulfides) to form hydrogen sulfide (H₂S) and corresponding hydrocarbons, with hydrogen consumption varying by feedstock sulfur content and desired removal level.^[2] The catalyst, commonly cobalt- or nickel-promoted molybdenum sulfide (CoMoS or NiMoS) supported on alumina, facilitates hydrogenation and desulfurization pathways, often requiring presulfidation to activate the sulfidic phases.^[3] Reactor effluent is cooled and directed to high- and low-pressure separators to recover unreacted hydrogen for recycle, while the liquid phase undergoes further processing to remove dissolved H₂S and light hydrocarbons.^[30] H₂S is stripped from the liquid product via amine absorption or adsorbed onto zinc oxide beds, with the purified H₂S stream often routed to a Claus process for sulfur recovery.^[31] Process configurations may include single-stage or multi-bed reactors with interbed quenching to manage exothermic heat release, particularly for deep desulfurization targets below 10 ppm sulfur, as required by modern fuel standards. Hydrogen partial pressure and space velocity are adjusted to optimize conversion while minimizing over-hydrogenation of olefins or aromatics.^[7] Tail gas from separators is compressed and treated to remove impurities before hydrogen recycle, ensuring efficient operation and catalyst longevity.^[27]

Catalysts and Supports

Hydrodesulfurization (HDS) catalysts are predominantly transition metal sulfides, with molybdenum disulfide (MoS₂) as the primary active component, promoted by cobalt (Co) or nickel (Ni) to enhance sulfur removal efficiency from hydrocarbon feedstocks.^[32] These promoted catalysts operate under sulfidic conditions, where the MoS₂ slabs provide edge sites for hydrogen activation and adsorption of sulfur-containing molecules like dibenzothiophene.^[33] Cobalt promotion favors direct desulfurization pathways, making CoMoS catalysts particularly effective for aliphatic thiols and sulfides in lighter feeds such as naphtha and gasoline, while NiMoS excels in hydrogenation routes, suiting heavier feeds like diesel with refractory thiophenic compounds.^[34] Typical metal loadings are 10-15 wt% MoO₃, 3-4 wt% CoO or NiO, optimized for dispersion and slab length to maximize active sites.^[35] The support material is crucial for catalyst stability and activity, with γ-alumina (Al₂O₃) serving as the industry standard due to its high surface area (200-400 m²/g), thermal stability up to 500°C, and ability to anchor the active phase via strong metal-support interactions that prevent sintering under high-pressure hydrogen and H₂S environments.^[36] Alumina's acidic hydroxyl groups facilitate precursor impregnation and sulfidation, forming well-dispersed MoS₂ crystallites with stacking degrees of 2-4 layers.^[37] Modifications such as phosphorus doping (1-2 wt% P) improve promoter dispersion and HDS activity by up to 20-30% through enhanced MoS₂ edge exposure and resistance to coke formation, as demonstrated in commercial NiMo/Al₂O₃ formulations for ultra-low sulfur diesel production.^[38] Alternative supports like activated carbon offer higher H₂S tolerance for heavy oil processing but suffer from lower mechanical strength and hydrothermal instability compared to alumina.^[39] Recent advances focus on optimizing catalyst morphology and composition for deep HDS (below 10 ppm S) required by regulations like Euro 6 standards.^[7] Techniques such as citric acid chelation during impregnation yield highly dispersed NiMo precursors with Type II morphology, boosting activity by 15-25% over conventional Type I structures.^[34] Unsupported or bulk catalysts, including nitride-derived MoS₂, show promise for high-temperature applications but face scalability issues due to poor attrition resistance.^[40] Mesoporous supports like SBA-15 silica provide ordered pores (5-10 nm) for better mass transfer in refractory sulfur removal, achieving 10-20% higher thiophene conversion than alumina in model studies, though industrial adoption lags due to cost and poisoning susceptibility.^[41] Nano-alumina variants with particle sizes below 50 nm further enhance dispersion, enabling HDS efficiencies exceeding 95% for gas oil feeds under mild conditions (300-350°C, 30-50 bar).^[35]

Applications and Co-Processes

Primary Refining Role

Hydrodesulfurization (HDS) constitutes the principal method for sulfur removal in petroleum refining, converting organosulfur compounds into hydrogen sulfide (H₂S) via catalytic hydrogenation, thereby producing compliant fuels and safeguarding subsequent refinery operations. This process targets distillate fractions such as naphtha, kerosene, and gas oil, where sulfur levels in untreated crudes can exceed 1-5 weight percent, reducing them to trace amounts essential for environmental compliance and process integrity.^[27]^[3] In transportation fuel production, HDS ensures diesel and gasoline meet ultra-low sulfur specifications, with global standards mandating less than 15 ppm sulfur in diesel and 10 ppm in gasoline to curb SO₂ emissions from combustion, which contribute to acid rain and air pollution. For instance, U.S. refineries adopted HDS upgrades to achieve 15 ppm diesel by 2006 under EPA highway diesel rules, necessitating hydrogen consumption rates of 200-500 scf per barrel and reactor pressures up to 1,000 psi. Gasoline HDS similarly addresses refractory sulfur species like benzothiophenes in fluid catalytic cracking (FCC) naphtha, enabling tiered reductions to 10 ppm by 2017 in regions enforcing Tier 3 standards.^[42]^[7]^[43] Beyond direct fuel quality, HDS pretreats feeds for downstream units, mitigating catalyst deactivation; sulfur poisons noble metals like platinum in naphtha reforming, where pretreatment reduces sulfur to under 1 ppm to maintain octane-boosting efficiency and extend catalyst life from months to years. In hydrocracking and FCC, residual sulfur similarly deactivates zeolitic catalysts, with HDS integration preventing yield losses and operational downtime across integrated refinery schemes processing heavy sour crudes.^[27]^[2]

Hydrodenitrogenation and Olefin Saturation

Hydrodenitrogenation (HDN) involves the removal of nitrogen from organonitrogen compounds in hydrocarbon feedstocks through hydrogenolysis, producing ammonia and hydrocarbons.^[44] In refinery hydrotreating units designed primarily for hydrodesulfurization (HDS), HDN proceeds concurrently, converting compounds such as pyridines and quinolines via stepwise hydrogenation of aromatic rings followed by C-N bond cleavage.^[44] ^[45] For instance, pyridine (C₅H₅N) reacts with five moles of hydrogen to yield pentane (C₅H₁₂) and NH₃, often involving intermediates like piperidine and amylamine.^[44] HDN demands more severe operating conditions than HDS due to the stability of C-N bonds, typically requiring higher hydrogen partial pressures and temperatures 20–50°C greater, along with increased hydrogen consumption—up to several times that of HDS for refractory nitrogen species.^[45] ^[46] Catalysts such as nickel-promoted molybdenum sulfides (NiMo/Al₂O₃) are favored for HDN owing to their enhanced hydrogenation activity compared to cobalt-promoted variants used primarily for HDS.^[45] Basic nitrogen compounds can inhibit HDS by adsorbing strongly on catalyst sites, necessitating staged processing to remove ammonia and hydrogen sulfide between reactors for deep denitrogenation, often targeting <50 ppm nitrogen in products like diesel.^[45] ^[44] Olefin saturation, or hydrogenation of carbon-carbon double bonds, occurs as a parallel reaction in hydrotreating processes, converting alkenes to alkanes and enhancing fuel stability while consuming hydrogen.^[45] In naphtha hydrodesulfurization, particularly for fluid catalytic cracking (FCC) gasoline, this saturation is unavoidable during deep HDS to meet ultra-low sulfur specifications (<10 ppm), but it leads to octane number loss of 5–10 units due to the conversion of high-octane olefins to lower-octane paraffins, with C₅–C₇ olefins contributing most significantly. ^[47] Efforts to mitigate octane penalty include selective catalysts that prioritize thiophenic sulfur removal over olefin hydrogenation, achieving HDS with <40% olefin saturation.^[48] In diesel hydrotreating, where olefin content is lower, saturation improves cetane number and oxidative stability without major drawbacks, though overall hydrogen use rises with unsaturation levels in the feed.^[49] Co-processing in multi-stage units allows initial mild saturation of diolefins to prevent gum formation, followed by targeted HDS/HDN under optimized conditions.^[45] Ammonia from HDN can further influence saturation kinetics by competing for catalytic sites, underscoring the interconnected nature of these reactions in refining hydrotreaters.^[50]

Challenges and Limitations

Technical Difficulties

One major technical difficulty in hydrodesulfurization (HDS) arises from refractory sulfur compounds, particularly in heavy oil feedstocks, where species such as dibenzothiophenes with alkyl substituents or fully conjugated aromatic systems resist conversion under standard conditions. These compounds, often featuring fewer and longer alkyl side chains attached to polycyclic structures, exhibit low reactivity due to steric hindrance and electronic effects that stabilize the sulfur-carbon bonds, necessitating higher temperatures (above 350–400°C) and hydrogen partial pressures (up to 100 bar) for effective removal, which increases process severity and energy demands.^[51]^[52] Catalyst deactivation poses another critical challenge, primarily through mechanisms like coke deposition, metal sulfides (e.g., vanadium and nickel from organometallic precursors), and pore-mouth plugging, which reduce active site availability and diffusion pathways in supported CoMo or NiMo catalysts. Nitrogen-containing compounds can further poison sites by forming stable nitride species, while sintering at elevated temperatures leads to agglomeration of active phases, shortening catalyst lifespan to as little as 1–3 years in residue processing units.^[53]^[54] Inhibition by co-existing heteroatoms and polyaromatics exacerbates these issues, as competitive adsorption on catalyst surfaces slows sulfur hydrogenation kinetics, often requiring staged reactor designs or guard beds to mitigate upstream contaminants. Achieving ultra-low sulfur levels (<10 ppm) demands precise control of hydrogen-to-feed ratios (typically 500–2000 scf/bbl) and recycle streams, yet thermodynamic limitations and mass transfer constraints in trickle-bed reactors limit efficiency for complex feeds.^[42]^[54]

Economic and Operational Constraints

Hydrodesulfurization entails substantial economic costs driven by hydrogen consumption, catalyst procurement, and energy requirements. Chemical hydrogen usage in hydrotreating processes typically ranges from 50 to 250 standard cubic feet per barrel of feedstock, with total consumption often higher due to inefficiencies and recycle needs, elevating operational expenses in refineries where hydrogen can represent a major variable cost.^[55] Catalyst costs further compound this, as high-activity materials for deep desulfurization demand frequent replacement amid deactivation, with global HDS catalyst market valuations exceeding USD 3.3 billion annually as of 2025.^[56] For ultra-low sulfur diesel production, capital expenditures to achieve sulfur levels below 50 ppm can total $3,000–4,300 million per refinery unit, alongside manufacturing costs of $12–18 per metric ton of product and $6,000–9,000 per metric ton of sulfur removed.^[57] Operational constraints arise from the process's severity, including temperatures of 300–400°C and pressures up to several hundred atmospheres, which accelerate equipment corrosion, fouling, and energy demands—often requiring high-pressure hydrogen partial pressures that strain reactor design and safety protocols.^[58] Refractory sulfur compounds in heavy feedstocks resist removal, necessitating longer residence times or higher severities, which exacerbate catalyst poisoning by metals and coke deposition, leading to reduced activity and mandatory shutdowns for regeneration every 1–3 years depending on feedstock quality.^[42] Smaller or less advanced refineries face amplified limitations from these factors, as high capital and maintenance outlays hinder adoption of deep HDS without scale economies, while over-hydrogenation risks unintended saturation of valuable olefins, diminishing product yields and economics.^[59]^[60]

Regulatory and Economic Context

Sulfur Emission Regulations

Sulfur emission regulations primarily target the reduction of sulfur oxides (SOx) from fuel combustion, which contribute to acid rain, respiratory illnesses, and particulate matter formation, thereby necessitating advanced hydrodesulfurization (HDS) processes in refineries to produce low-sulfur fuels.^[61]^[62] These standards have progressively tightened worldwide since the 1990s, driven by environmental legislation linking fuel sulfur content to vehicle and industrial exhaust limits, with non-compliance penalties including fines and operational restrictions.^[63] Refineries must achieve ultra-low sulfur levels—often below 10-15 parts per million (ppm)—to meet these mandates, compelling investments in HDS catalysts, hydrogen supply, and process intensification. In the United States, the Environmental Protection Agency (EPA) established the Tier 2 Gasoline Sulfur Control program in 2000, effective from 2004, capping average sulfur at 30 ppm and individual batches at 80 ppm, a 90% reduction from prior levels exceeding 300 ppm, to enable advanced catalytic converters in vehicles.^[61] For diesel, the 2001 Highway Diesel rule mandated a 97% sulfur reduction to 15 ppm ultra-low sulfur diesel (ULSD) by 2006 for on-road use, extending to non-road applications by 2010-2012, transforming refinery operations from high-sulfur straight-run fuels to deeply hydrotreated products.^[64] Further refinements in 2017 lowered gasoline sulfur to a 10 ppm annual average, aligning with Tier 3 emission standards.^[63] European Union directives have similarly enforced stringent limits, with Directive 1999/32/EC and successors requiring sulfur content below 10 ppm in gasoline and diesel since 2009 for road fuels and 2011 fully, as part of Euro emission standards progression from Euro 1 (1992) onward.^[65]^[66] For marine fuels, EU rules under Directive (EU) 2016/802 align with global norms but impose 0.10% sulfur in sulfur emission control areas (SECAs) like the Baltic and North Sea since 2015.^[67] Internationally, the International Maritime Organization's (IMO) MARPOL Annex VI amendments, effective January 1, 2020 (IMO 2020), capped global marine fuel oil sulfur at 0.50% mass/mass (from 3.50%), with 0.10% in designated emission control areas, spurring refinery production of compliant very low sulfur fuel oil (VLSFO) via HDS or alternative desulfurization.^[62] These marine regulations alone are projected to reduce global SOx emissions by 77%, but they strain refinery capacity, as high-sulfur residues previously blended into bunker fuels now require treatment or diversion.^[68] Developing nations increasingly adopt similar thresholds, such as India's BS-VI standards (10 ppm sulfur since 2020), amplifying global HDS demand.^[66]

Industry Impacts and Costs

Hydrodesulfurization imposes substantial capital expenditures on refineries, with upgrades to achieve ultra-low sulfur levels, such as 0.05% in diesel, requiring investments of $3,000 to $4,300 million across affected facilities, alongside manufacturing costs of $12 to $18 per metric ton of product.^[57] Operating expenses for HDS processes typically range from 0.08 to 0.24 USD per barrel, encompassing hydrogen consumption, catalyst replacement, and energy demands, which escalate with heavier feedstocks due to increased hydrogen needs and reduced catalyst longevity.^[69]^[70] The cost of sulfur removal itself can reach $6,000 to $9,000 per metric ton removed, reflecting the process's intensity and the economic trade-offs in treating sour crudes.^[57] Regulatory mandates amplify these costs, as seen in the European Union's sulfur limits, which have elevated refinery compliance expenses by approximately 15% through mandated HDS enhancements.^[71] In response to the International Maritime Organization's 2020 sulfur cap reducing marine fuel sulfur to 0.5%, refineries invested hundreds of millions per plant in desulfurization upgrades to produce very low sulfur fuel oil, reshaping global refining capacity toward compliant products and straining margins for facilities processing high-sulfur residues.^[72] Smaller or less complex refineries face disproportionate burdens, often deeming advanced HDS uneconomical without scale, leading to closures or shifts to lighter crudes.^[59] Despite the expenses, HDS enables premium pricing for low-sulfur fuels, with U.S. diesel refining margins projected to rise due to regulatory premiums, though volatile oil prices and hydrogen supply constraints challenge overall profitability.^[73] For heavy oil processing, HDS integration boosts product quality and market access but elevates unit costs by 10-20% compared to lighter crude refining, influencing industry consolidation toward integrated complexes capable of absorbing such investments.^[74] These dynamics underscore HDS's role in sustaining refinery viability amid tightening emissions standards, albeit at the expense of higher operational complexity and energy intensity.^[27]

Recent Advances

Catalyst Innovations

Recent innovations in hydrodesulfurization (HDS) catalysts have focused on enhancing activity for refractory sulfur compounds, improving stability under severe conditions, and reducing reliance on expensive supports to enable ultra-low sulfur levels in fuels. Traditional catalysts, such as cobalt- or nickel-promoted molybdenum sulfides (CoMoS or NiMoS) on alumina, have been augmented with nanostructuring and bimetallic or trimetallic formulations to increase edge site density and hydrogen activation. For instance, unsupported catalysts, which avoid diffusion limitations from porous supports, have shown markedly higher turnover frequencies for dibenzothiophene HDS compared to supported analogs, attributed to greater exposure of active MoS₂ edges.^[75]^[76] A key advancement involves trimetallic unsupported sulfides, such as Ni-Mo-W or Co-Mo-W systems, prepared via thermal decomposition of organometallic precursors or hydrothermal methods, which synergistically boost direct desulfurization pathways over hydrogenation routes. Studies demonstrate that optimizing Ni/Mo/W atomic ratios in granular unsupported Ni-Mo-W catalysts yields HDS activities up to 20-30% higher than bimetallic NiMo for heavy gas oil feeds, due to enhanced molybdenum dispersion and tungsten's promotion of sulfur vacancy formation.^[77]^[78] Similarly, Zn-promoted unsupported catalysts synthesized in 2024 exhibit superior selectivity for hydrogenolysis over isomerization in model diesel HDS, outperforming industrial benchmarks in cycle stability.^[79] Engineering of MoS₂ phases has yielded intercalated structures formed in-situ during sulfidation, dramatically increasing catalytic efficiency; for example, a 2024 study reported near-complete desulfurization of thiophene derivatives at mild conditions (300°C, 3 MPa) with recyclability over multiple runs, linked to expanded interlayer spacing facilitating hydrogen spillover.^[8] Nanostructured variants, including Co-promoted MoS₂ nanoclusters, further enhance methanethiol HDS rates by optimizing slab length and stacking, achieving activities rivaling noble metal alternatives while maintaining sulfidic phase integrity.^[80] Additionally, low-cost supports derived from activated kaolin or bentonite enable effective HDS at reduced pressures (below 4 MPa), removing over 95% sulfur from heavy gas oil with minimal metal leaching, offering economic viability for grassroots refineries.^[81] These developments prioritize scalability and resistance to deactivation by coke or nitrogen compounds, though industrial adoption lags due to validation needs for long-term operation.^[82]

Future Prospects in Heavy Oil Processing

Emerging technologies in hydrodesulfurization (HDS) for heavy oil processing emphasize advanced catalysts and alternative methods to overcome limitations such as catalyst deactivation by metals and asphaltenes, high energy demands, and refractory sulfur compounds like dibenzothiophenes. Slurry-phase processes, such as Eni Slurry Technology (EST), utilize nano-sized molybdenum catalysts in ebullated-bed reactors operating at 410–420°C and 16 MPa, achieving greater than 85% desulfurization while minimizing sedimentation and enabling higher throughput for vacuum residues.^[83] Similarly, the Genoil Hydroconversion Upgrader (GHU) integrates fixed-bed HDS stages at 343–510°C, delivering 75–97% sulfur removal through staged hydroprocessing that precedes cracking.^[83] In-situ catalytic approaches, like the Headwaters Catalytic (HCAT) process, employ colloidal catalysts in ebullated or fixed-bed reactors at 430–450°C and 13.79 MPa, yielding 60–98% conversion with integrated sulfur reduction via hydrogen addition, which supports partial upgrading without full refinery integration.^[83] Ultrasound-assisted desulfurization, as in the Sulph-Ultrasound process, oxidizes sulfur to sulfones under ambient conditions for facile separation, attaining over 99% removal efficiency and offering potential for low-energy retrofits to existing HDS units.^[83] Two-stage processes, exemplified by the Mexican Petroleum Institute (IMP) method, sequence hydrodemetallization followed by HDS at 538°C, effectively targeting high asphaltene and sulfur loads in extra-heavy crudes using hybrid fixed/ebullated beds.^[83] Future developments prioritize sustainability through catalyst innovations, such as dispersed-phase nano-catalysts (e.g., US Patent 9994779 B2) and mild hydrocracking formulations that enhance HDS selectivity while reducing greenhouse gas emissions and energy intensity compared to thermal methods like visbreaking.^[83] ^[84] Integration with digital tools, including Industrial Internet of Things (IIoT) and computational fluid dynamics (CFD), is expected to optimize reactor performance and catalyst longevity, facilitating economical processing of heavier feedstocks amid depleting light oil reserves.^[85] Non-catalytic alternatives, such as microwave-assisted or ionic liquid-based deasphalting, show early promise for pre-HDS sulfur preconcentration by targeting asphaltene-bound impurities, potentially lowering overall HDS severity.^[83] These advancements aim to align heavy oil upgrading with stringent environmental regulations while improving yield economics.^[85]

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In the 1950s, hydrodesulfurization and mild hydrogenation processes experienced tremendous growth, mostly because large quantities of by-product hydrogen ...
[15]
[PDF] Design Parameters for a Hydro desulfurization (HDS) Unit for ...
1950's: First catalytic reforming process was commercialized. At the same time, the catalytic Hydrodesulphurization of the naphtha feed to such reformers ...
[16]
The HDS Process: Origin, Process Evolution, Reaction Mechanisms ...
This paper presents a literature review of works examining the hydrodesulfurization process of liquid fuels, beginning with searching for the origin of the ...
[17]
Hydrodesulphurisation - an overview | ScienceDirect Topics
Hydrodesulfurization is defined as a chemical process used to remove sulfur from petroleum products, where the hydrodesulfurization plant's operation is ...
[18]
A Review on the Reaction Mechanism of Hydrodesulfurization and ...
Jun 24, 2021 · Review June 24, 2021. A Review on the Reaction Mechanism of Hydrodesulfurization and Hydrodenitrogenation in Heavy Oil Upgrading. Click to ...Overall Status of Heavy Oil... · Hydrodesulfurization of Heavy... · Biographies
[19]
A Theoretical Study on Reaction Mechanisms and Kinetics of ...
Two parallel HDS routes, i.e., direct desulfurization (DDS) and hydrogenation (HYD) were taken into account. It has been found that the major reaction route of ...
[20]
[PDF] A theoretical study on reaction mechanisms and kinetics of ...
The reaction mechanisms including both DDS and HYD routes and reaction ... hydrodesulfurization over the Mo-edge, S-edge and Mo-S connection edge.<|separator|>
[21]
Study on the mechanism of hydrodesulfurization of ...
Hydrodesulfurization (HDS) reaction can significantly reduce the viscosity and sulfur content of heavy oil, while the HDS reaction mechanism of ...
[22]
CRUDE OIL - Occupational Exposures in Petroleum Refining - NCBI
The following types of sulfur compounds have been identified in crude oils: thiols (mercaptans), sulfides, disulfides and thiophenes (Costantinides & Arich, ...
[23]
Review on Sulfur Compounds in Petroleum and Its Products
Sulfur is present in crude oil and petroleum products in the form of various compounds: mercaptans, thiophenes, sulfides (acyclic and cyclic), polysulfides ( ...
[24]
The effect of heavy aromatic sulfur compounds on sulfur in cracked ...
Sep 30, 2007 · The main sulfur compounds in gas oils are benzothiophenes, dibenzothiophenes and their alkylated derivatives, which represent the most ...<|separator|>
[25]
Sulfur compounds in the fuel range fractions from different crude oils
Sep 9, 2016 · The crude oil fractions characterized were light naphtha (C5-90°C), heavy naphtha (90–140°C), kerosene (140–240°C), and gas oil (240–370°C) ...Missing: common | Show results with:common
[26]
Hydrodesulfurization reactivities of various sulfur compounds in ...
Uncovering the Critical Factors that Enable Extractive Desulfurization of Fuels in Ionic Liquids and Deep Eutectic Solvents from Simulations.
[27]
Refractory sulfur compounds in gas oils - ScienceDirect.com
In hydrotreating of gas oils, most of the benzothiophenic sulfur had already been removed at the 50% conversion level of the dibenzothiophenes. Families of ...
[28]
Hydrodesulfurization - an overview | ScienceDirect Topics
Hydrodesulfurization (HDS) is extensively used for the exclusion of carcinogenic compounds from liquid fuels. Although less reactive refractory compounds ...
[29]
In situ observations of an active MoS2 model hydrodesulfurization ...
To accomplish the removal of sulfur, the oil is mixed with hydrogen at a pressure between 5 bar and 160 bar, with the temperature between 260 °C and 380 °C, ...
[30]
Challenges and opportunities to design a highly active ...
Challenges and opportunities to design a highly active hydrodesulfurization catalyst: A comprehensive review ... reaction mechanism for the design of ...
[31]
Hydrotreatment Processes | FSC 432: Petroleum Refining
Catalysts have shorter lives in residue treatment processes, as well, because of harsh reaction conditions and more excessive deposition on catalysts during ...
[32]
Hydrodesulfurization (HDS) - Corrosionpedia
The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting hydrogen sulfide gas. In petroleum refineries, the ...
[33]
Alumina-Supported NiMo Hydrotreating Catalysts Aspects of 3D ...
We prepared two NiMoS/Al 2 O 3 catalyst samples with the same target composition using different chemical sources and characterizing the oxidic NiMo precursors.Introduction · Experimental Section · Results · Conclusions
[34]
Recent Insights in Transition Metal Sulfide Hydrodesulfurization ...
We conclude from this review that hydrodesulfurization performance of TMS catalysts supported or unsupported may be improved by using new methodologies ...
[35]
Recent developments in alumina supported hydrodesulfurization ...
Jun 23, 2020 · This prospective review briefs synthesis factors influencing the performance of an efficient catalyst aiming for the removal of sulfur from transportation ...
[36]
Design and Evaluation of a Co–Mo-Supported Nano Alumina ...
May 18, 2020 · Conventional Al2O3-supported CoMo and NiMo catalysts were quite active for HDS, hydrogenation and hydrogenolysis, but they could not meet ...
[37]
Alumina-Supported NiMo Hydrotreating Catalysts—Aspects of 3D ...
Oct 24, 2022 · Typically, HDS catalysts consist of sulfides of Mo and Ni or Mo and Co, supported on a high-surface-area alumina (γ-Al2O3) carrier. These ...
[38]
Hydrodesulfurization activity of CoMo and NiMo catalysts supported ...
The addition of phosphorus in CoMo or NiMo on alumina was reported to enhance the HDS activity [5]and improve the durability of the catalyst. In both cases ...
[39]
Highly active P-doped sulfided NiMo/alumina HDS catalysts from Mo ...
Dec 5, 2018 · The corresponding CoMo-supported catalysts had improved performance for hydrotreating reactions including aromatics saturation, ...
[40]
Effect of Silica, Activated Carbon, and Alumina Supports on NiMo ...
In general, commercial catalysts consists of NiMo or CoMo metals supported on alumina. ... The NiMo/CoMo HDS catalysts supported on activated carbon (AC) were ...
[41]
Review article Recent advances in the unsupported catalysts for the ...
This review provides the research progress of unsupported catalysts for the HDS of fuel. Firstly, the overview of the HDS process was introduced.
[42]
SBA-15 Mesoporous Silica as Catalytic Support for ... - NIH
In this review, we are going to show the possibilities of the use of SBA-15 silica as support for hydrodesulfurization catalysts.
[43]
Advancing hydrodesulfurization in heavy Oil: Recent developments ...
Sep 15, 2024 · This study focuses on hydrodesulfurization (HDS), a critical technology for upgrading heavy oil. HDS directly impacts the marketability of fuels ...
[44]
Hydrodesulfurization Catalysts Market | Industry Report, 2030
In addition, the Environmental Protection Agency's stringent standards require refineries to produce ultra-low sulfur diesel and gasoline, driving investments ...<|separator|>
[45]
Hydrodenitrogenation | FSC 432: Petroleum Refining
This is similar to HDS being an important pretreatment process for feedstocks that would be exposed to noble metal catalysts (such as Pt in a catalytic ...
[46]
An Overview of Hydrodesulfurization and Hydrodenitrogenation
The hydrogenation processes include three major classes, hydrotreating, hydrocracking, and hydrofinishing. Hydrofinishing is really another form of ...
[47]
Hydrodenitrogenation Reaction - an overview | ScienceDirect Topics
In practice, a hydrocracker operating at 95 vol% conversion of + 343 °C into -343 °C consumes 15 times more hydrogen than low-severity HDS.
[48]
Quantitative relationship between olefin saturation and octane loss ...
Dec 14, 2018 · C5-C7 olefins have a larger negative effect on octane number loss during HDS, and their saturation is the main contributing factor. A formula ...
[49]
US5358633A - Hydrodesulfurization of cracked naphtha with low ...
The incremental octane improvement is small (˜1.6 octane numbers). Again, for both catalysts, olefin saturation is fairly low (<˜40 wt %) and octane penalties ...
[50]
Reactivity of olefins in the hydrodesulfurization of FCC gasoline over ...
Aug 5, 2025 · The hydrogenation of C7-C9 olefins, which lead to severe loss of octane number in hydrodesulfurization (HDS) process of gasoline, was ...
[51]
Hydrotreating of Atmospheric Gas Oil and Co-Processing with ...
Mar 12, 2021 · Co-processing implies not only the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of middle distillates but also the deoxygenation ( ...
[52]
Molecular Structures of Refractory Sulfur Compounds in Heavy Oil ...
Jan 21, 2022 · The most refractory sulfur compounds in the heavy oil were those with fully conjugated aromatic rings attached with fewer and longer side chains.Introduction · Experimental Section · Results and Discussion · Conclusion
[53]
Towards a deep understanding of the evolution and molecular ...
Jun 15, 2022 · The molecular structures of the most refractory sulfur compounds during residue hydrotreating process were studied via controlling the HDS rate ...
[54]
Deactivation of hydrodesulfurization catalysts by metals deposition ...
Primary causes of deactivation of hydrodesulfurization catalysts are partial poisoning of the interior pore surface and pore-mouth plugging by deposition of ...
[55]
A comprehensive review of catalyst deactivation and regeneration in ...
Catalyst deactivation in heavy oil hydroprocessing can be caused by various factors, including coke and metal deposition, nitrogen poisoning, and structural ...
[56]
[PDF] Hydroprocessing: Hydrotreating & Hydrocracking
Hydrotreating removes hetero atoms and saturates, while hydrocracking breaks carbon-carbon bonds and reduces molecular weight. Hydrotreating is just enough ...
[57]
Hydrodesulfurization Catalyst Market Trends & Forecast, 2025-2035
Jun 30, 2025 · The global hydrodesulfurization (HDS) catalyst market is estimated to grow from USD 3,345.5 million in 2025 to USD 5,295.8 million by 2035, ...
[58]
[PDF] costs to reduce the sulphur content of diesel fuel - Concawe
Reducing diesel sulphur to 0.05% requires $3000-4300M capital, $12-18/t manufacturing costs, and $6000-9000/t per sulphur removed.
[59]
Hydrodesulfurization (HDS) in Petroleum Refining - Atomfair
The HDS process is indispensable for reducing sulfur dioxide (SO₂) emissions from combustion engines, a major contributor to air pollution and acid rain.
[60]
Hydrodesulfurization Catalysts Market Share Growth Report
Smaller refineries and developing markets face significant obstacles due to the high capital and operational costs of advanced hydrodesulfurization catalysts.<|separator|>
[61]
Advances in Gasoline Hydrodesulfurization Catalysts
Jul 11, 2025 · This section provides an overview of the main sulfur-containing molecules in gasoline, the HDS reaction mechanisms, and the conventional ...
[62]
Gasoline Sulfur | US EPA
Jan 17, 2025 · The Tier 2 Gasoline Sulfur program, finalized in 2000, reduced the sulfur content of gasoline by up to 90 percent, enabling the use of new emission control ...
[63]
IMO 2020 – cutting sulphur oxide emissions
IMO 2020 limits ship fuel oil sulfur to 0.50% m/m, reducing SOx emissions by 77% and improving air quality.
[64]
US: Fuels: Diesel and Gasoline | Transport Policy - TransportPolicy.net
Starting January 1, 2017, EPA requires that federal gasoline will contain an average of 10 ppm sulfur on an annual basis, with the maximum sulfur allowed per ...
[65]
Diesel fuel and the environment - U.S. Energy Information ... - EIA
EPA fuel standards require a major reduction in the sulfur content of diesel fuels. To meet the EPA standards, the petroleum industry is producing Ultra-Low ...Missing: regulations | Show results with:regulations
[66]
Sulphur content of certain liquid fuels - EUR-Lex - European Union
Directive 1999/32/EC extends the legislation on the reduction of sulphur dioxide emissions to cover certain liquid fuels derived from petroleum and used by ...
[67]
EU: Fuels: Diesel and Gasoline | Transport Policy - TransportPolicy.net
EU diesel and gasoline have a 10 ppm sulfur limit since 2009/2011, with mandatory specifications for fuel parameters. EU fuel quality improvements have led to ...
[68]
[PDF] DIRECTIVE (EU) 2016/ 802 OF THE EUROPEAN PARLIAMENT ...
May 21, 2016 · Directive 2016/802 relates to a reduction in the sulphur content of certain liquid fuels and is a codification of previous directives.
[69]
Sulphur 2020 implementation – IMO issues additional guidance
May 20, 2019 · The Sulphur 2020 implementation sets a 0.50% limit on sulphur in ship fuel oil, effective January 1, 2020, with a 0.10% limit in ECAs. Most ...
[70]
[PDF] Techno-Economic Analysis of Heavy Fuel Oil Hydrodesulfurization ...
The economic evaluation and analysis of the process, the net production costs, the total investment cost, and the return of investment are estimated.Missing: constraints | Show results with:constraints
[71]
Process development and techno-economic analysis of microwave ...
Total capital costs (CAPEX) and total annual operating costs (OPEX) were estimated at 6.77 MUSD and 4.23 MUSD (0.24 $/bbl), respectively. The results indicated ...
[72]
Hydrodesulfurization (HDS) Catalysts At a Glance - Reports and Data
$$3,800.00For instance, the European Union's sulfur content regulations have increased compliance costs for refineries by approximately 15%, according to a study by the ...
[73]
Refiners Struggle As Low Sulphur Upgrade Costs Approach $1 ...
Aug 28, 2018 · Analysts say upgrading a refinery with capacity of around 88,000 barrels per day, like Gunvor's Rotterdam plant, could cost hundreds of millions ...Missing: sulfur | Show results with:sulfur
[74]
Regulation Has Wide-Ranging Repercussions
Refining Margins. Because of an increased premium on low-sulfur fuels, EIA forecasts that diesel fuel refining margins will increase from an average of $0.43 ...
[75]
Economics of Residual Fuel Oil Desulfurization - epa nepis
The objective of this study is to establish costs for reducing the sul- fu'r content of fuel oils manufactured from high sulfur crudes.<|separator|>
[76]
Review article Recent advances in the unsupported catalysts for the ...
This review provides the research progress of unsupported catalysts for the HDS of fuel. Firstly, the overview of the HDS process was introduced.
[77]
Recent advances in the unsupported catalysts for ... - ResearchGate
Aug 6, 2025 · This review provides the research progress of unsupported catalysts for the HDS of fuel. Firstly, the overview of the HDS process was introduced ...<|separator|>
[78]
Unsupported Ni—Mo—W Hydrotreating Catalyst: Influence of ... - MDPI
the present work describes a method for the preparation of granular Ni—Mo—W unsupported hydrotreating catalysts and studies the influence of the Ni/Mo/W atomic ...
[79]
Unsupported trimetallic CoMoW sulfide HDS catalysts ... - Bohrium
[3] Recent advances in the unsupported catalysts for the hydrodesulfurization of fuel. With the increasingly prominent concerns on the environmental ...
[80]
Synthesis, characterization, and performance evaluation of ...
Jan 15, 2025 · The results show that the Zn-based unsupported catalysts synthesized here exhibited higher selectivity for the HDS reaction than an industrial one.
[81]
Hydrodesulfurization of methanethiol over Co-promoted MoS2 ...
Aug 21, 2024 · In this work, we present the first direct observations of hydrodesulfurization of methanethiol (CH3SH) gas over CoMoS model catalysts at near- ...
[82]
Low-Pressure Hydrodesulfurization Catalysts of Heavy Gas Oil ...
Aug 12, 2025 · This study introduces a cost-effective and sustainable strategy for hydrodesulfurization (HDS) by employing activated kaolin and bentonite ...
[83]
Towards improvement of hydroprocessing catalysts - understanding ...
Oct 30, 2023 · Improved catalysts can reduce refinery costs, improve selectivity for certain products, allow non-conventional feedstock, such as creosotes, to ...
[84]
[PDF] Advancement In Heavy Oil Upgrading And Sustainable Exploration ...
Jan 9, 2024 · Upgrading heavy oil is a subject of high importance for fossil fuel industry due to the rapid depletion of light oil.
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https://doi.org/10.1016/j.arabjc.2024.105610
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https://doi.org/10.1016/j.arabjc.2024.105610