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Delayed coker

A delayed coker, or delayed coking unit (DCU), is a thermal cracking process in petroleum refineries that converts heavy residual oils—such as vacuum residuum from crude oil distillation—into lighter, more valuable hydrocarbon liquids and solid petroleum coke through controlled high-temperature decomposition.^[1]^[2] The process begins with preheating the heavy feedstock, typically asphalt-like bottoms from atmospheric or vacuum distillation, in a fired heater to 485–505°C under low pressure (2–3 bar), where initial thermal cracking initiates but is delayed from completing.^[1]^[3] The hot effluent is then routed into one of two or more large, insulated vertical coke drums (typically 4–9 meters in diameter and up to 40 meters tall), where it resides for 12–24 hours, allowing further cracking, polymerization, and coke deposition on the drum walls while volatile vapors rise and exit for separation.^[1]^[4] While one drum fills and cokes, the other undergoes steam stripping to remove entrained hydrocarbons, quenching with water to cool and solidify the coke, and decoking via high-pressure hydraulic water jets to cut out the solid mass, enabling a semi-batch, continuous operation with cycles lasting about 16 hours on average.^[1]^[5] The overhead vapors from the drums are sent to a fractionator column, where they are cooled and separated into products including fuel gas, liquefied petroleum gas (LPG), naphtha, light gas oil, and heavy gas oil, which can be further refined into gasoline, diesel, and other fuels.^[1]^[2] The solid byproduct, petroleum coke, forms in various structures depending on feedstock composition—such as sponge coke (porous, suitable for fuel or anodes), needle coke (elongated, premium for graphite electrodes in steelmaking), or shot coke (spherical granules from high-asphaltene feeds)—and accounts for 20–30% of the feed by weight, serving as a key raw material for the aluminum, steel, and power generation industries.^[1]^[5] Delayed coking originated in the late 1920s, with the first commercial unit built in 1929 by Standard Oil of Indiana, and has since become essential for maximizing refinery yields from heavier, sour crudes, led by regions like China with over 150 million metric tons of capacity annually as of 2024.^[1]^[6] It enhances refinery economics by rejecting carbon as coke rather than burning it, though challenges include emissions management and handling high-sulfur feeds.^[2]^[7]

Overview

Definition and purpose

A delayed coker is a type of thermal cracking unit employed in petroleum refineries to upgrade heavy residual oils into lighter products and solid petroleum coke. It operates as a semi-batch continuous process, in which high molecular weight petroleum derivatives are heated in a furnace to thermal cracking temperatures of approximately 450–500°C, with the subsequent polymerization and cracking reactions occurring in a series of insulated coke drums.^[8]^[1]^[3] The primary purpose of the delayed coker is to transform low-value heavy residues, such as vacuum residuum from crude oil distillation, which are unsuitable for further conventional processing, into marketable distillate fractions—including naphtha and precursors for diesel fuel—along with solid petroleum coke as a byproduct.^[9]^[3] This conversion enhances the overall yield and economic value of refinery operations by utilizing otherwise underutilized heavy fractions of crude oil.^[5] At its core, the delayed coker exemplifies a carbon rejection process, where heavy, undesirable carbon components in the feedstock are deliberately concentrated and removed as solid coke, avoiding their release as less desirable gaseous byproducts and thereby improving the quality and quantity of the liquid distillates produced.^[1]^[10]

Role in petroleum refining

The delayed coker unit is integrated into petroleum refineries downstream of the atmospheric and vacuum distillation units, where it processes heavy residual fractions such as vacuum residuum that would otherwise yield low-value heavy fuel oil. This placement allows refineries to upgrade these refractory bottoms, transforming them into more valuable streams and thereby optimizing the overall crude oil processing chain.^[11]^[12] Economically, the delayed coker enhances refinery profitability by converting approximately 70-80% of the heavy residue feedstock into liquid distillates, including naphtha, light gas oil, and heavy gas oil, which can be further refined into transportation fuels like gasoline and diesel. This high conversion rate minimizes the production of residual fuel oil, improves refining margins through higher-value product slates, and generates petroleum coke as a marketable byproduct for applications in power generation, aluminum smelting, or graphite electrodes.^[3]^[13]^[14] The unit's role is particularly vital for processing heavier, sour crudes, where it boosts the yield of lighter products from otherwise underutilized residues by rejecting carbon as coke rather than requiring costly hydrogen addition. Global delayed coker capacity has seen major expansions concentrated in high-capacity refining hubs like the US Gulf Coast and Asia to accommodate increasing volumes of heavy oil imports.^[15]

Process Description

Feedstock characteristics

The delayed coker primarily processes heavy hydrocarbon feedstocks that are unsuitable for most catalytic upgrading processes due to their high molecular weight and impurity levels. Typical inputs include vacuum residuum, which is the heaviest fraction from crude oil vacuum distillation, and atmospheric residuum from atmospheric distillation units.^[14] Other common feedstocks encompass heavy oils such as bitumen derived from Canadian oil sands or Venezuelan Orinoco heavy crude, as well as decant oil (clarified slurry oil) from fluid catalytic cracking (FCC) units.^[16]^[17] These materials are selected for their refractoriness, allowing thermal conversion without the need for hydrogen or catalysts.^[18] Key characteristics of these feedstocks include a high initial boiling point exceeding 500°C, reflecting their composition of large, complex molecules that remain liquid only under vacuum conditions.^[19] They typically exhibit elevated asphaltene content, often in the range of 10-20% by weight, which consists of polar, aromatic-rich compounds that contribute to coke formation.^[17] The Conradson carbon residue (CCR), a measure of the non-volatile carbonaceous material, ranges from 15-30 wt%, indicating significant potential for solid residue production.^[14]^[18] Sulfur content can reach up to 5 wt%, primarily in organic forms, while metals such as vanadium and nickel are present at concentrations typically of 100–2000 ppm, though can exceed 3000 ppm in heavy crudes like Orinoco, posing challenges for downstream processing if not managed.^[18]^[1]^[20] These properties make the feedstocks viscous and unstable, influencing the severity of thermal cracking required.^[16] Pretreatment of delayed coker feedstocks is minimal compared to hydrotreating processes, focusing on desalting to remove inorganic salts and filtration or centrifugation to eliminate solids and water that could foul equipment.^[16] Unlike catalytic units, hydrotreating is not required, enabling the process to handle contaminated feeds economically, though occasional dewaxing or viscosity adjustment may occur for specific operations.^[14] The inherent heterogeneity of these feeds, such as varying asphaltene-to-CCR ratios, can affect cracking patterns by promoting different coke morphologies.^[17]

Thermal cracking mechanism

The thermal cracking mechanism in delayed coking involves free-radical reactions that decompose heavy hydrocarbon feedstocks into lighter distillates and solid petroleum coke without the use of catalysts or added hydrogen. At elevated temperatures of 450–500°C, carbon-carbon (C-C) bonds undergo homolytic cleavage, generating free radicals that initiate a chain reaction leading to the breakdown of large molecules. This process promotes the polymerization and condensation of asphaltenes and other heavy components into a coke matrix, while lighter fractions volatilize as gases and liquids. The "delayed" aspect refers to the controlled residence time in insulated coke drums, allowing the reactions to proceed to completion over 12–24 hours, which maximizes coke formation and minimizes unreacted residue.^[11]^[21] The key reactions follow a classic free-radical chain mechanism. Initiation occurs through the thermal breaking of weak C-C bonds in aliphatic side chains attached to aromatic cores, producing primary radicals. Propagation involves hydrogen abstraction from surrounding molecules to form more stable radicals, followed by β-scission, which cleaves additional C-C bonds to yield smaller alkyl radicals and olefins; these steps also include radical addition and displacement reactions that contribute to molecular fragmentation. Termination happens via recombination of radicals, forming stable, high-molecular-weight structures that aggregate into the solid coke phase, often through exothermic polymerization of asphaltenes. No catalytic surfaces are involved, distinguishing this from other cracking processes, and the absence of hydrogen prevents hydrogenation side reactions.^[22]^[21]^[11] Operating conditions are precisely controlled to optimize reaction severity. The furnace outlet temperature reaches 480–510°C to initiate cracking rapidly, with the process operating at low pressures of 1–3 atm to favor vaporization and reduce coke deposition in transfer lines. During filling, the coke drum maintains temperatures of 420–450°C, allowing the delayed soaking phase where radicals propagate and terminate without further heating. Feedstock composition, such as asphaltene content, influences reaction severity by altering radical stability and polymerization rates.^[11]

Equipment and operation

The delayed coker unit features several major components essential for its semi-continuous operation. The fired heater, configured as a multi-tube furnace with horizontal tubes typically 100 mm in internal diameter and 6-12 mm wall thickness made from 9% chrome alloy, heats the residual oil feedstock to cracking temperatures of approximately 485-505°C (905-941°F) at a pressure of about 4 bar (60 psig), ensuring high in-tube velocities and minimal residence time to prevent premature coking.^[11] Multiple gas burners, each rated at around 3 million BTU, provide the necessary heat flux, limited to less than 9000 BTU/hr/ft² for optimal performance.^[11] Paired coke drums serve as the primary reaction vessels, consisting of vertical cylindrical structures typically 4-9 meters (13-30 feet) in diameter and 25 meters (82 feet) in straight-side height, with overall dimensions up to 36 meters (118 feet) including top and bottom flanges; these are constructed from 25 mm thick carbon steel plates with 2.8 mm stainless steel cladding for corrosion resistance and insulated with 10 cm of fiberglass to retain heat.^[11] The drums operate at pressures of 1-5.9 bar (typically 2-3 bar) and alternate in use to enable continuous processing. The fractionator tower, positioned downstream, receives and separates the overhead vapors from the drums into fractions such as gases, gasoline, diesel, heavy coker gas oil, and recycle streams, with bottom temperatures maintained at 343-382°C (650-720°F) and pressure regulated by a gas compressor.^[11] Blowdown and quench systems manage post-reaction cleanup, including venting of stripped vapors and controlled water injection for cooling.^[11] Operation of the delayed coker follows a cyclic batch process in the drums combined with continuous upstream and downstream flows to achieve semi-continuous production. The preheated feedstock from the fired heater is directed via a switching valve—often a motorized three-way ball valve—into one of the two coke drums, where it fills over a period of 12-24 hours (typically 16 hours), allowing thermal cracking to form solid petroleum coke at the bottom while generating vapors that are continuously routed to the fractionator for separation and recovery.^[11]^[12] During this filling phase, the second drum is offline and undergoes decoking, ensuring uninterrupted feed processing and vapor handling. Upon completion of filling in the active drum, the switching valve redirects the hot feed to the empty drum, initiating a new cycle while the filled drum transitions to decoking.^[11]^[12] The decoking sequence begins with a brief steam-out period of about 0.5 hours to strip residual hydrocarbons and facilitate heat transfer from the coke mass, followed by water quenching lasting 4-5 hours at controlled rates to cool the drum without causing structural damage like case hardening.^[11] Hydraulic cutting then removes the solidified coke using high-pressure water jets at 86-275 bar (1250-4000 psig), starting with a pilot hole approximately 1 meter in diameter and proceeding spirally to clear the drum in 3-4 hours, for a total decoking duration of 6-12 hours.^[11] Once decoked, the drum is pressure-tested, warmed, and prepared for the next filling cycle, maintaining the unit's overall efficiency above 95% during normal operation.^[24] Flow control throughout relies on automated valves and monitoring to synchronize drum switching and prevent interruptions in the continuous heating and fractionation steps.^[12]

Products and Byproducts

Petroleum coke properties

Petroleum coke, the primary solid byproduct of the delayed coking process, is a carbon-rich material produced through thermal cracking of heavy petroleum residues. It exists in green form immediately after coking and can be further processed into calcined coke by heating to 1200–1400°C to remove volatile matter and enhance purity. Green coke typically contains 8–14% volatile matter, while calcined coke has less than 5%, resulting in a more stable structure suitable for industrial applications.^[14]^[1] The type of petroleum coke produced depends on feedstock characteristics, such as asphaltene and aromatic content. Sponge coke, the most common form, features a porous, amorphous structure and serves as the primary product for fuel or anode use when impurities are low. Needle coke, formed from low-sulfur feeds like FCC decant oils, exhibits an anisotropic, needle-like crystalline structure with aligned carbon layers, offering superior graphitization properties. Shot coke, resulting from high-aromatic feeds, consists of small spherical particles (2–5 mm in diameter), often appearing as "beebees," and is denser but less desirable for premium applications due to its tendency to form aggregates.^[25]^[1]^[26] In terms of composition, petroleum coke is predominantly carbon, with green coke containing 86–92% carbon on a dry basis, increasing to 99.5% after calcination. Sulfur levels range from 1–6%, with anode-grade coke limited to ≤4% in green form and ≤3.5% calcined; needle coke achieves as low as 0.5%. Hydrogen content is 0.5–5%, higher in green coke due to volatiles. Trace metals like vanadium and nickel are typically <100 ppm in low-metal feeds, though anode specifications allow up to 250 ppm Ni and 400 ppm V in green coke. Volatile matter is 8–14% in green coke, reduced significantly upon calcination. The material has a high heating value of approximately 14,000 BTU/lb, making it an efficient fuel source.^[14]^[25]^[1] Key physical properties include a density of 1.2–1.4 g/cm³ for green coke, with real density reaching 2.05–2.14 g/cm³ in calcined forms; anode-grade coke requires a vibrated bulk density ≥0.87 g/cm³. Porosity is notably high in sponge coke at 20–50%, contributing to its lightweight and absorbent nature, while needle and shot cokes are less porous. Electrical resistivity is low in premium grades, such as 320 × 10⁻⁶ ohm-in for needle coke, enabling its use in conductive applications. Calcination not only lowers volatiles and density variations but also stabilizes the microstructure, reducing hydrogen and sulfur while concentrating carbon.^[27]^[25]^[1]

Distillate products

The distillate products from the delayed coking process consist of gaseous and liquid fractions generated through thermal cracking of heavy residual feedstocks, primarily comprising off-gas, coker naphtha, and coker gas oil. These products are recovered as vapors from the coke drums, which are quenched with water or steam to halt further cracking, then directed to a fractionating column for separation into distinct streams based on boiling points.^[1] Off-gas, the lightest fraction, includes C1-C4 hydrocarbons such as methane, ethane, propane, and butane, along with hydrogen sulfide and other light components, making it suitable for use as refinery fuel gas or petrochemical feedstock after sulfur removal in a gas treating unit. This stream is separated as the overhead product from the fractionator and processed in the refinery's gas plant to meet fuel specifications.^[14]^[1] Coker naphtha, boiling in the range of approximately 30-200°C, is a light liquid distillate rich in olefins, diolefins (around 5% of total olefins), and aromatics, with sulfur content 10-20 times higher than straight-run naphtha, rendering it unstable and prone to gum formation upon exposure to oxygen. Its high olefin content, indicated by a bromine number typically between 50-100, necessitates hydrotreating to stabilize it and reduce sulfur before use as a feedstock for fluid catalytic cracking (FCC) units or incorporation into gasoline blends. In FCC processing, diolefins contribute to coke formation on the catalyst, while olefins crack into lighter products like LPG.^[28]^[29]^[1] Coker gas oil, encompassing light and heavy fractions with a boiling range of 200-565°C, exhibits high sulfur (typically 2-4 wt%), nitrogen, and aromatics content, along with a Conradson carbon residue (CCR) of 5-10%, making it unsuitable for direct blending without further refining. This stream, recovered as side draws from the fractionator with pump-around cooling to enhance separation, requires hydrodesulfurization and hydrocracking to produce low-sulfur diesel or gasoline precursors, often serving as preferred feedstock for hydrotreaters or FCC units due to its cracked nature.^[1]^[14]

Yield and quality factors

In delayed coking, product yields typically range from 22-38% petroleum coke, 14-19% naphtha, 29-52% gas oil, and 7-16% gas and butanes by weight of the feedstock, with variations depending on feed composition and operating conditions.^[21] Higher Conradson carbon residue (CCR) in the feed, often exceeding 20 wt%, substantially increases coke yield, as coke production correlates approximately 1.6 times the CCR value.^[11] Process variables significantly influence product quality. Elevated temperatures, typically 480-515°C in the coke drum, promote greater unsaturation and olefin content in distillate products through enhanced thermal cracking of paraffins.^[21] Drum pressures of 0.1-0.4 MPa inversely affect yields, with lower pressures favoring higher liquid distillate production and reduced coke formation.^[25] Feed asphaltene levels, measured as heptane insolubles, determine coke porosity; higher concentrations (e.g., >15 wt%) lead to denser, less porous structures like shot coke.^[11] Recycle ratios of 5-10% optimize liquid yields by minimizing over-cracking, though higher ratios increase coke and reduce volatiles.^[9] Modern optimization employs advanced controls, including AI-driven systems, to enhance yield balance during coke drum switching. These systems use iterative learning algorithms to analyze real-time data on temperature, pressure, and flow, automating switches to minimize disturbances and stabilize product distribution.^[30] For instance, AI models predict fouling and adjust parameters like heater duty and steam injection, extending run lengths and improving overall liquid recovery.^[31] Feeds with low sulfur content (<4 wt%) enable production of premium anode-grade coke.^[25]

History and Development

Invention and early adoption

The delayed coking process was developed in the late 1920s by Standard Oil of Indiana as an evolution of earlier thermal cracking techniques, aimed at upgrading heavy petroleum residues into lighter distillates and solid petroleum coke through controlled thermal decomposition. This innovation built on the Burton thermal cracking process, originally patented in 1913, by introducing a "delayed" residence time in large drums to enhance cracking while minimizing unwanted side reactions in the heating stage. The first commercial delayed coker unit was constructed and started up in 1929 at Standard Oil's Whiting refinery in Indiana, marking the practical implementation of the technology for residue conversion.^[11]^[32] Early adoption of delayed coking remained limited during the 1930s, confined largely to U.S. refineries handling Midwestern heavy crudes, as the process was still refining its operational parameters and competing with simpler visbreaking methods. Initial units operated with coke yields typically ranging from 18% to 30% by weight of the feedstock, depending on residue composition, though early configurations often achieved lower efficiencies due to suboptimal temperature control and drum design. The technology saw expanded use during World War II, when U.S. refineries increasingly relied on thermal cracking processes like delayed coking to process heavier crudes into critical fuels such as gasoline and diesel for military needs, helping to boost overall refinery output amid wartime shortages.^[21]^[33] Significant challenges plagued early delayed cokers, particularly the labor-intensive and hazardous manual decoking procedures, which required workers to enter hot drums to break and remove solidified coke using tools or rudimentary mechanical aids, resulting in frequent equipment wear, high maintenance costs, and safety incidents. These issues restricted widespread implementation until hydraulic decoking innovations emerged in the late 1930s, pioneered by Shell Oil at its Wood River refinery in Illinois through patented high-pressure water jets. Despite these hurdles, the process demonstrated economic viability for bottom-of-the-barrel upgrading, laying the foundation for its role in modern refining.^[11]^[34]

Technological advancements

Following World War II, significant advancements in the delayed coker process focused on improving operational efficiency and safety. In the 1950s, the widespread adoption of hydraulic decoking systems, pioneered by Shell Oil Company, replaced labor-intensive manual methods, substantially reducing downtime associated with coke removal from the drums. This innovation allowed for faster turnaround times between cycles, enabling refineries to process heavier residues more reliably without the hazards of manual labor inside the vessels.^[32] During the 1970s, engineering improvements led to the construction of larger coke drums, with diameters expanding to up to 30 feet (approximately 9 meters), which increased unit capacity by roughly fivefold compared to earlier designs from the 1950s. These larger drums facilitated higher throughput of heavy feedstocks, supporting the growing demand for residue upgrading amid increasing production of high-sulfur crudes. By the 1990s, further refinements included the introduction of antifoam additives, such as silicone-based agents, to mitigate excessive foaming during the coking phase and prevent carryover into downstream equipment. Concurrently, advancements in metallurgy, including structural weld overlays and corrosion-resistant alloys, enhanced the durability of coke drums when processing high-sulfur feeds, extending equipment life and reducing maintenance frequency.^[11]^[35]^[36]^[35] In the 2020s, modern innovations have integrated digital technologies and process hybrids to boost sustainability and performance. Digital twins, virtual replicas of coker units powered by AI algorithms, enable predictive maintenance by simulating drum stresses and operational conditions in real time, minimizing unplanned outages and optimizing cycle management. Hybrid configurations combining partial hydrotreating upstream of delayed coking have emerged to produce lower-sulfur petroleum coke, with studies showing sulfur content reductions to below 3% while increasing light distillate yields by up to 29%. Capacity expansions in Asia, exemplified by upgrades at Reliance Industries' Jamnagar refinery complex—which supports over 1 million barrels per day of processing through enhanced coker integration—demonstrate scalable applications of these technologies. Efficiency gains include shortened overall cycle times to 10-12 hours, achieved through advanced automated cutting tools that accelerate decoking while maintaining safety. Additionally, integration with carbon capture systems, such as post-combustion amine absorption on flue gases from the coker furnace, has been piloted to reduce CO2 emissions, aligning with broader refinery decarbonization goals.^[37]^[38]^[39]^[35]

Applications and Uses

Utilization of petroleum coke

Petroleum coke produced by delayed cokers finds primary application as a raw material in the production of carbon anodes for aluminum smelting, where low-sulfur variants are calcined to form the bulk of anode composition, comprising over 67% of the calcined petroleum coke market share.^[40] This use leverages the coke's low ash and metal content to ensure efficient electrolytic reduction in aluminum production. High-sulfur shot coke, characterized by its spherical morphology and elevated sulfur levels, serves mainly as a fuel in cement kilns and power plants, offering a high heating value ranging from 14,000 to 15,000 BTU/lb that substitutes for coal or natural gas in high-temperature processes.^[41] Additionally, specialty needle coke derived from petroleum sources is essential for manufacturing graphite electrodes used in electric arc furnaces for steel production, prized for its low coefficient of thermal expansion and high graphitizability.^[42] The global market for petroleum coke features substantial international trade, with U.S. exports playing a key role and directing significant volumes to major importers like China and India; for instance, in 2025, U.S. exports have remained strong to these markets.^[43] Overall trade volumes are driven by refinery outputs exceeding domestic consumption in producing regions.^[44] Calcined petroleum coke, suitable for anode and electrode applications, trades at a premium of $300-500 per ton, reflecting its purity requirements, while fuel-grade high-sulfur coke is valued lower at $50-100 per ton due to its broader availability and simpler uses.^[45]^[46] Market challenges include potential declining demand in the aluminum sector amid calls from advocacy groups for the industry's transition to renewable energy sources and carbon-neutral technologies, which seek to reduce reliance on carbon-intensive feedstocks like petroleum coke by 2035.^[47] Overproduction from expanding refinery capacities has led to substantial stockpiling, exacerbating price volatility and storage concerns, with U.S. power sector consumption of petcoke halving over the past decade to below 2 million tons annually.^[44]^[48] Regulatory scrutiny on high-sulfur petcoke imports has increased in regions like India and China as of 2025, affecting fuel-grade applications.^[44]

Integration in modern refineries

In modern refineries, delayed cokers are frequently integrated with hydrocrackers and fluid catalytic cracking (FCC) units to optimize the upgrading of heavy residues into higher-value products. The heavy coker gas oil (HCGO) produced by the delayed coker serves as a key feedstock for these downstream units, where it undergoes further cracking or hydrotreating to yield gasoline, diesel, and other distillates. This synergy allows refineries to manage feedstock surpluses, improve product yields, and maintain operational flexibility; for instance, at Turkey's TUPRAS Izmit refinery, the FCC unit processes HCGO alongside hydrocracker bottoms to prevent capacity reductions in the coker and hydrocracker while enhancing overall profitability.^[49]^[50] Residue upgrade complexes represent another critical integration strategy, particularly for handling extra-heavy crudes, where delayed coking is paired with hydrotreating to achieve comprehensive conversion. In Venezuela's Orinoco Belt, the Petrocedeño upgrader (formerly Sincor) exemplifies this approach, processing 200,000 barrels per day of extra-heavy crude oil (8.3° API) through initial hydrotreating followed by delayed coking of the vacuum residue to produce distillates and petroleum coke, with the distillates routed back for hydrotreating to meet syncrude specifications (32° API). This configuration enables the economic upgrading of otherwise low-value heavy oils by maximizing liquid yields and removing contaminants like metals.^[51]^[52] Contemporary adaptations of delayed cokers emphasize sustainability and efficiency, including pilot-scale co-processing of waste materials to support circular economy goals. As of 2025, Indian Oil Corporation Limited (IOCL) has implemented the INDEcoP2F process, which integrates waste plastics into the delayed coker feedstock to produce fuel-grade liquids, reducing plastic waste while leveraging existing infrastructure.^[53] Similarly, biomass oils can be co-processed in delayed cokers to incorporate renewable components, as demonstrated in patented methods that blend such feeds with conventional residues for thermal cracking without major unit modifications.^[54]^[55] Economically, delayed cokers are indispensable for refineries processing heavy crudes with API gravity below 20°, as they convert high-residue feeds into marketable liquids, thereby enhancing overall refinery margins through bottom-of-the-barrel upgrading. Delayed coking accounts for approximately 88% of global petroleum coke production capacity, with a substantial share dedicated to heavy oil streams that would otherwise yield low-value fuel oil.^[56]^[57]^[21]

Environmental and Safety Considerations

Emissions and environmental impacts

Delayed coking operations produce several key air emissions, primarily sulfur oxides (SOx) derived from sulfur-rich feeds where sulfur content typically ranges from 1% to 5% and concentrates in the petroleum coke byproduct. Nitrogen oxides (NOx) arise from combustion processes in the coker heaters, while volatile organic compounds (VOCs) and particulate matter are released during decoking activities such as drum venting, draining, and coke cutting, with VOC emission factors estimated at 0.12–0.24 lb per ton of coke produced. Greenhouse gas emissions, particularly CO2, occur at rates of approximately 0.5–1.0 lb per ton of coke from process vents, though total emissions including energy use can reach 0.5–1 ton CO2 equivalent per ton of coke. Methane (CH4) emissions from decoking are quantified at 0.01–0.02 lb per ton of coke or 7.9 lb per 1,000 lb of steam used in stripping.^[58]^[59] These emissions contribute to environmental impacts, including air pollution from coke dust generated during handling and storage, which carries heavy metals like vanadium and nickel, leading to elevated particulate matter (PM) levels in nearby communities and potential respiratory health risks. Wastewater generated during quenching contains high chemical oxygen demand (COD) from emulsified oils and dissolved organic matter, along with metals and heteroatom compounds (sulfur, nitrogen, oxygen), complicating biological treatment and risking contamination of surface waters if inadequately managed. Land use for coke storage piles poses leachate risks, as rainwater can mobilize pollutants like heavy metals and hydrocarbons into soil and groundwater, exacerbating local ecological degradation.^[60]^[61]^[58] Mitigation strategies have advanced significantly, with wet scrubbers applied to heater flue gases achieving up to 90% SOx reduction by capturing sulfur compounds before atmospheric release. For VOCs, enclosed decoking systems and work practice standards—such as depressurizing drums to below 2 psig before venting to flares or fuel gas systems—can reduce emissions by 81–92%, alongside flare gas recovery to minimize uncontrolled releases. Regulatory frameworks, including the U.S. EPA's New Source Performance Standards under 40 CFR Part 60 Subpart Ja and National Emission Standards for Hazardous Air Pollutants under Subpart CC, enforce limits on benzene and other hazardous air pollutants (HAPs) from delayed cokers, mandating fenceline monitoring and corrective actions if benzene concentrations exceed 9 µg/m³.^[62]^[63]

Operational safety and regulations

Delayed coker operations present several significant hazards, primarily due to the high temperatures, pressures, and thermal cycling involved in the process. One critical risk is coke drum bulging or cracking, which can occur during the heating phase when thermal stresses cause deformation in the drum's cylindrical shell, potentially leading to structural failure or explosions if not detected early. For instance, in 2018, a major refinery in the Gulf Cooperation Council (GCC) region identified extensive cladding detachment in a coke drum through monitoring, averting a potential fire or explosion that could have resulted from bulging-induced leaks.^[64] Another hazard arises from hydrogen sulfide (H2S) release, particularly when processing sour feeds containing high sulfur content, as the thermal cracking can liberate toxic H2S gas, leading to acute respiratory irritation, poisoning, or fatalities at concentrations above 100 ppm.^[65] Additionally, decoking operations expose workers to ergonomic and physical risks, including musculoskeletal strains from handling heavy equipment, high-pressure water jet injuries during coke cutting, and falls from elevated platforms over 120 feet high.^[65] To mitigate these hazards, industry standards and technologies are employed to ensure drum integrity and operational safety. API 510, the Pressure Vessel Inspection Code, mandates regular in-service inspections of coke drums, including external visual checks, ultrasonic thickness measurements, and internal assessments during shutdowns to detect bulging, cracking, or corrosion before they escalate.^[66] Acoustic emission monitoring uses sensors to detect micro-cracks and stress waves in real-time during operation, allowing for early intervention; in the 2018 GCC case, this technology reduced acoustic pulse energy by 52% through process adjustments, preventing unplanned shutdowns and potential ruptures.^[64] Automated interlocks, integrated with programmable logic controllers (PLCs), prevent erroneous valve operations that could introduce water into hot drums or release hydrocarbons, reducing human error probability from over 1 in 10,000 to less than 1 in 10,000 operations.^[67] For cutting crews, personal protective equipment (PPE) such as flame-resistant clothing, hard hats, safety harnesses, and respiratory protection is required, complemented by comprehensive training on emergency evacuation, heat stress management, and safe water jet handling to minimize injury risks.^[65] Regulatory frameworks enforce these safety practices to protect workers and prevent accidents in delayed coker units. In the United States, the Occupational Safety and Health Administration (OSHA) Standard 1910.146 governs permit-required confined spaces, such as coke drums, requiring atmospheric testing for H2S and other toxics, ventilation, and rescue plans before entry during decoking or maintenance.^[68] OSHA's Process Safety Management (PSM) standard (29 CFR 1910.119) applies to delayed cokers handling hazardous chemicals like H2S, mandating process hazard analyses, mechanical integrity programs, and operating procedures to address explosion and release risks.^[65] Internationally, the European Union's ATEX Directive 1999/92/EC addresses explosion risks from flammable atmospheres in coker units by requiring risk assessments, zoning of hazardous areas, and explosion-proof equipment to safeguard against ignition sources during heating or venting.^[69] As of 2025, updates under the EU's NIS2 Directive emphasize cybersecurity for digital controls in critical infrastructure like refineries, including delayed cokers, by imposing risk management measures for industrial control systems to prevent cyber-induced operational failures that could exacerbate physical hazards.^[70]

Comparison with Other Processes

Fluid and flexicoking

Fluid coking represents a continuous variant of the delayed coking process, utilizing fluidized bed reactors to thermally crack heavy petroleum residues into lighter liquids, gases, and coke. Developed by Exxon in the 1950s, this technology operates at temperatures of 500–550°C, where the residuum feed is introduced into a reactor bed of hot coke particles that provide the necessary heat for cracking.^[71] The process features short residence times of mere minutes in the reactor, contrasting with the hours-long soaking in delayed coking drums, which promotes faster cracking and results in a finer coke powder product rather than the larger chunks typical of delayed coking.^[72] Like delayed coking, fluid coking yields naphtha, gas oils, and petroleum coke, but the continuous operation eliminates batch cycles, enabling steady throughput for processing heavier feeds.^[17] Flexicoking extends the fluid coking process by incorporating a gasification step to enhance energy efficiency and minimize coke accumulation, particularly suited for high-residue feeds. Introduced by Exxon as a modification to fluid coking, the first commercial flexicoking unit commenced operations in 1976 at the Toa Oil Company refinery in Kawasaki, Japan.^[73] In this setup, excess coke from the fluid coker reactor is transferred to a heater and then to a gasifier, where it undergoes partial combustion or steam/air gasification at around 900–1000°C to produce a low-sulfur fuel gas rich in carbon monoxide and hydrogen, which supplies process energy and can generate additional hydrogen for refinery use.^[74] This integration reduces the net coke yield by up to 80–95% compared to delayed coking, leaving only a small purge coke (typically 1–2 wt% of feed) while converting the majority into usable gas products.^[74]^[17] Compared to delayed coking, both fluid and flexicoking offer continuous processing that avoids the downtime associated with drum switching and decoking, though fluid coking requires significantly higher energy inputs (approximately 55% more total energy) due to fluidization, while flexicoking's integrated gasification results in comparable or slightly lower net energy consumption.^[75] Liquid yields are typically 5–10% higher in these processes, with lower coke production (e.g., 20–25% vs. 25–30% in delayed coking), attributed to the elevated temperatures and shorter contact times that favor vaporization over condensation.^[72] Capital costs for fluid coking units are comparable to those for delayed coking, while flexicoking units are 30–50% higher, owing to the added complexity of fluidized beds and gasification equipment, limiting their adoption to refineries handling very heavy or contaminated residues where the efficiency gains justify the investment.^[76]

Alternative residue conversion technologies

Hydrocracking represents a primary catalytic alternative to delayed coking for upgrading heavy petroleum residues, employing hydrogenation under elevated temperatures of 350-450°C and pressures of 100-200 bar to achieve 80-95% conversion to valuable liquid products without generating solid byproducts.^[77]^[78] This process, exemplified by Chevron's LC-Fining ebullated-bed technology, produces higher-quality distillates such as diesel with lower sulfur content compared to coking outputs, though it incurs a slight capital cost premium over delayed coking due to the need for high-pressure equipment and hydrogen infrastructure.^[79]^[80] Solvent deasphalting (SDA), often denoted as supercritical deasphalting when using solvents near their critical point, serves as a non-catalytic pretreatment or standalone method to separate deasphalted oil (DAO) from asphaltene-rich residues using light hydrocarbons like propane as solvents.^[81] This extraction typically yields 50-70% DAO, which can be further processed in downstream units, while the remaining asphalt is directed to coking or other disposal, enhancing overall refinery flexibility when integrated before a coker.^[82] SDA's lower energy demands make it cost-effective for partial upgrading, though it does not fully eliminate the need for residue handling.^[83] Among emerging technologies as of 2025, pyrolysis variants and supercritical water processes offer pathways toward zero-coke residue conversion, focusing on thermal or hydrothermal decomposition to maximize liquid and gas yields.^[84] For instance, ebullated-bed hydrocracking like the H-Oil process achieves around 70% conversion of heavy crudes by recirculating unconverted oil, providing higher throughput for opportunity crudes while minimizing sediment formation.^[85] These innovations prioritize complete residue utilization but often require advanced catalysts or conditions to suppress coke.^[86] In trade-offs, alternatives like hydrocracking and SDA reduce or eliminate coke production, enhancing product value and environmental compliance, yet they demand significantly more hydrogen—up to 2-3 times that of coking—and higher energy inputs for compression and heating.^[79] Consequently, delayed coking remains favored for its low capital expenditure and straightforward carbon rejection strategy in cost-sensitive operations.^[80]

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