E-book materials – Delayed coking

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Delayed Coking – Basic Knowledge of Delayed Coking Units I. Overview of the Unit Carbonization (abbreviated as coking) is a process of deep thermal cracking, and it is also one of the methods used to process residue oil. It is also the only process capable of producing petroleum coke, and it cannot be replaced by any other process. In particular, the specific demand for high-quality petroleum coke in certain industries has ensured that the coking process continues to play an important role in the oil refining industry. Coking is a process in which hydrogen-poor heavy residual oils (such as vacuum residue, cracking residue, and asphalt) are used as raw materials to undergo deep thermal cracking reactions at high temperatures (400–500°C). Through a cracking reaction, a portion of the residue oil is converted into gaseous hydrocarbons and light oils ； Due to the condensation reaction, another portion of the residue is converted into coke. On the one hand, the raw materials are heavy and contain a considerable amount of aromatics; on the other hand, the reaction conditions for coking are more stringent. As a result, the condensation reaction plays a major role, leading to the production of large amounts of coke. The coking methods that have been used in the refining industry include batch coking, open-hearth coking, contact coking, delayed coking, fluidized coking, and flexible coking. Delayed coking is the most widely used method; it is the primary means for refineries to increase the yield of light oils and to produce petroleum coke. It will continue to play an important role in China’s refining industry. Delayed coking is a mature process for processing vacuum residue, and has been used as a method for the in-depth processing of heavy oil for many years. In recent years, as the properties of crude oil have deteriorated (i.e., sulfur content has increased), consumption of heavy fuel oils has declined while demand for light oils has risen, leading to a rapid increase in coking capacity. It is precisely because coking units can process various residual materials from refineries that they are referred to as the \"trash bins\" of refineries; they are also important facilities that enable refineries to achieve zero emissions of residue oil at present. II. Introduction to the coking process: Delayed coking: The feedstock is heated to the reaction temperature using a heating furnace, and under conditions of high flow rate and short residence time, only a minimal amount of reaction occurs; thereafter, the feedstock quickly leaves the heating furnace and enters the coke tower where it undergoes cracking and condensation reactions to form coke. It is currently the main method for the deep processing of residue in the world (see petroleum refining process), accounting for three-quarters of the total processing capacity for petroleum coking. Depending on the different raw materials and operating conditions, the yield of various products can be adjusted: to produce more gasoline or diesel, or more heavy distillates that serve as feedstock for cracking, or more coke. The generated coking gas is one of the sources of refinery gas. Delayed coking is a semi-continuous operation. When the coke in one coke tower accumulates to a certain height (which takes about 24 hours), operation is switched to another coke tower to continue the coking process. After the coke-filled tower is purged with steam and cooled by water, the coke inside the tower is removed using hydraulic coke removal equipment, after which it is switched back to coke production mode. The main operating conditions for delayed coking are: an outlet temperature of 495–505°C in the heater, a pressure of 0.18–0.28 MPa (gauge pressure) in the coke tower, and a bottom temperature of the fractionation tower not exceeding 400°C. Hydraulic coking removal equipment comes in various types, including jacked, semi-jacked, and jackless types. Derrick-type coking removal equipment is generally used. Heating furnaces come in two types: vertical furnaces and flameless combustion furnaces. In a flameless combustion furnace, the tubes of the radiation chamber are arranged in the middle of the furnace chamber, while the burners (also known as nozzles) are located on both sides ； Gas fuel is injected through a flameless burner, creating a very short flameless combustion flame that ensures uniform heating on both sides of the furnace tube, thereby minimizing coking of the residue oil feedstock on the inner wall of the furnace tube due to overheating. The delayed coking unit is currently capable of processing more than 60 types of feedstocks, including straight-run (viscosity-reduced, hydrocracked) residue, pyrolysis tar and recycled oil, tar sand, asphalt, deasphalted tar, clarified oil, as well as coal derivatives, catalytic cracking slurry, and refinery sludge. The Conrad residue of the crude oil used in processing ranges from 3.8% to 45% or higher, with a specific gravity index of 2.20. l Fluidized coking: The feedstock is fed into a fluidized bed reactor, where high-temperature coke particles are in a fluidized state; oil gas and steam are used to maintain the fluidization of these particles. The coking reaction takes place on the surface of the coke particles, with the resulting coke adhering to them. The reaction product gas oil, after the coke particles have been removed via a cyclone separator, is sent to the distillation tower. During the reaction, coke particles are continuously withdrawn and fed into a calciner, where air is used to burn off a portion of the coke particles. These are then recirculated back to the reactor to provide the heat required for the coking reaction. Any excess coke particles are discharged from the system. Flexible coking: It is a process that, based on fluidized coking units, incorporates a set of coke gasification equipment (one-stage or two-stage coke gasification) to convert the coke produced by fluidized coking into fuel gas (and syngas). After being produced in the fluidized coking reactor, the coke enters a heater for heating; then part of it returns to the reactor while another part is sent for gasification. Coke gasification is divided into two stages: gasification and water-gas gasification. In the first stage of gasification, air is used to burn the coke in order to provide the heat required for the heater and the water-gas reaction, while also generating gas with a low calorific value. In the second stage of gasification, steam is used to produce syngas (H2+CO). Flexible coking has a high degree of adaptability; it can process various heavy oils with high sulfur, high metal content, and high residual carbon levels. It is capable of converting approximately 99% of the feedstock into gas, gasoline, intermediate distillates, and heavy distillates, with the remaining 1% becoming petroleum coke. (Since delayed coking is the most widely used method, this article focuses on delayed coking.) III. Process principle of delayed coking: Heavy oil is heated in a tubular furnace by using high flow rates (water is injected into the furnace tubes) and high heat intensity (the temperature at the furnace outlet is 500°C), which allows the oil to reach the temperature required for coking reactions in a short time. The oil then enters the coke tower rapidly, so that the coking reaction takes place there instead of in the heating furnace; hence, it is called delayed coking. IV. Chemical Principles of Coking Reactions 1. Cracking reaction: Under high-temperature conditions (400–550°C), large molecular hydrocarbons crack to form smaller molecular hydrocarbons, thereby converting residue oil into gaseous hydrocarbons and light oils ； 2. Condensation reaction: Hydrocarbons undergo a condensation reaction, converting residue oil into coke. Condensation reaction: refers to the interaction of small hydrocarbon molecules to form larger molecular compounds, while other small molecular compounds are also produced. V. Typical process flow of delayed coking The production process of a delayed coking unit is divided into coking and decoking stages; coking is a continuous operation, while decoking is a batch operation. Since industrial plants generally have two or four coke towers, the entire production process remains a continuous operation. There are different types of process flows for delayed coking units. In terms of production scale, there are processes such as the one furnace with two towers (coke towers) setup and the two furnaces with four towers setup. 1. Crude oil preheating stage: The coking feedstock (vacuum residue) first enters the feedstock buffer tank, and then is pumped into the convection section of the heater to be heated to approximately 340–350 °C. 2. The preheated crude oil enters the bottom of the distillation tower, where it exchanges heat with the oil and gas produced by the coke tower inside the distillation tower (with a bottom temperature not exceeding 400°C). Function: On one hand, it vaporizes the light oils present in the raw material, while simultaneously heating the raw material (to around 390–395 °C). 3. The feed oil and recycled oil are drawn together from the bottom of the distillation tower, pumped into the radiant section of the heating furnace using a hot oil pump, heated to the temperature required for the coking reaction (around 500 °C), and then enter the coke tower from the bottom via a four-way valve to undergo the coking reaction. To prevent oil from reacting and coking inside the furnace tubes, water must be injected into them in order to increase the flow rate within the tubes (generally above 2 m/s), thereby reducing the time that the oil stays inside the tubes. The amount of water injected is approximately 2% of the amount of crude oil. The high-pressure residue entering the coke tower must remain inside the tower for a sufficient amount of time to allow for adequate reaction. 4. The feedstock reacts within the coke tower to produce coke, which accumulates in the tower. The oil and gas emerge from the top of the coke tower and enter the distillation tower, where they exchange heat with the feedstock oil; subsequent distillation yields gas, gasoline, diesel, and wax oil. The bottom circulation oil, along with the raw material, undergoes coking reaction again. The coke produced during coking remains in the coke drum and is removed from the drum via hydraulic decoking. VI. Characteristics of the delayed coking unit
l Worst feedstocks: residue oil, slurry oil, deoiled asphalt, contaminated oil, sludge
l Most diverse products: dry gas, liquefied gas, gasoline, diesel, wax oil, recycled oil, coke
l Longest process flow: heater – coke tower – distillation tower – compressor – absorption and stabilization → coke removal → coke discharge → by rail/road → venting
l Highest furnace temperature: 490–505 °C
l Highest pressure: 13.0–15.0–30.0 MPa
l Most equipment: furnaces, towers, machines, pumps, tanks, high-pressure water pumps, coke cutters, elevators, grabs, cranes
l Most job types: process crew, coke removal crew
l Production mode: continuous/intermittent, 8 hours to 36 hours per coke production cycle

VI. Production cycle of the coke tower in delayed coking
The coke towers operate in pairs. Each set of delayed coking units consists of either one set (two units) or two sets (four units) of coking towers. The two sets of towers can be operated independently or in parallel. In each set, while one tower is undergoing coke removal due to reaction, the other is in the coking phase. That is, when the coke accumulates to a certain height inside the tower (usually at about 2/3 of the tower’s height), a switchover is performed. After the switchover, steam is introduced to remove light hydrocarbons, water is added for cooling, and then coke removal takes place. The switching cycle for each tower is generally 48 hours, of which 24 hours are spent on coking, and 24 hours on decoking and other auxiliary operations. The coke tower is a batch-operated device; while one tower is receiving feed, another tower is in the process of treatment. Taking two-column coking as an example, the main operating steps in the coke tower are as follows: ① At 19:00, a small amount of steam is introduced into the coke tower for 2 hours to blow the oil and gas into the distillation tower ； ②At 21:00, a large amount of steam was blown into the coke tower for 2 hours; this steam was used to strip the oil and gas, which were then sent to the steam vent tower. The contaminated oil was recovered through condensation and cooling in the venting system ； ③At 23:30, a small amount of water is fed into the coke drum (for 1–2 hours); the steam generated from the water evaporation along with oil vapors are directed to the flare tower ； ④A large amount of water is fed into the coke drum (for 3–4 hours); the water then overflows into the hot water tank of the quenching water system ； ⑤6:30 Drain water from the coke drum into the hot quenching water tank (2–4 hours) ； ⑥8:00 Open the flanges at the top and bottom of the tower for coke discharge (0.5~1 hour) ； ⑦High-pressure water is used to remove coke inside the coke drum (2–4 hours) ； ⑧Install top and bottom tower flanges (0.5~1 hour) ； ⑨At 12:30, a steam pressure test is conducted on the coke tower to check the sealing of its top and bottom covers (0.5–1 hour) ； ⑩At 13:30, the oil and gas from another coke tower is used to preheat this tower (for 4–6 hours); the oil and gas enter the tower from the top, flow to the bottom of the tower, and then return to the distillation tower via an oil separation tank. Once the temperature of the coke drum reaches the required level, switch the four-way valve. This valve then feeds material normally, while the other drum undergoes the aforementioned process. VII. Sources of Feedstock and Destinations of Products for Delayed Coking
Feedstock sources: vacuum residue, crude oil, catalytic slurry, dissolved asphalt, contaminated oil, ethylene cracking tar
Product destinations: Delayed coking produces about 70% liquid products, of which gasoline accounts for 10%~20% ； After hydrogenation treatment, it can be used as a feedstock for ethylene cracking or as a reforming feedstock. l Diesel: 25%~35% ； After hydrogenation treatment, it is sold as hydrogenated diesel. l Cracking feedstock (wax oil) 25%~35% ； After hydrotreating, it is used as feedstock for catalytic cracking units to be further cracked into products such as gasoline and diesel. l Liquefied petroleum gas: 6%~8% ； The liquefied gas is recovered after compression; the dry gas is desulfurized and then used as a raw material for hydrogen production or fed into the fuel gas pipeline network. After desulfurization and demethanization, the liquefied gas is exported or sent to a gas separation unit. c Coke (also known as petroleum coke) 15%~20%. After calcination, it is used as electrodes for aluminum production; needle coke is used as electrodes for steel manufacturing. The gas produced serves as fuel. High-sulfur coke and limestone are crushed and screened respectively before being fed into CFB boilers to generate high-pressure steam, enabling combined steam-and-electric power generation. High-sulfur coke is also used as a raw material for silicon production. 　The gaseous hydrocarbons and liquid oils obtained from coking contain a high amount of olefins, resulting in poor stability; therefore, they are often used as raw materials for other processes or processed through hydrogenation and refining to become finished products. Types of petroleum coke: Petroleum coke can be roughly divided into three categories. Most of the coke produced in industrial facilities appears as black-brown, porous solid irregular lumps; this type of coke is also known as sponge coke. The second type, which is of higher quality, is called needle coke. Due to its lower electrical resistance and thermal expansion coefficient, it is more suitable for use as electrodes. The third type of hard petroleum coke is called spherical coke; this type of coke has a pellet-like shape, a low surface area, and is less prone to coking. VIII. Main equipment for delayed coking: The coking heater is the core equipment of this unit; its function is to heat the rapidly flowing residue oil inside the furnace to a high temperature of around 500°C. Therefore, a high heat transfer rate within the furnace is required to supply sufficient heat to the oil in a short time, while a uniform heat distribution is also necessary to prevent local overheating that could lead to coking of the furnace tubes. For this reason, delayed coking typically uses flameless furnaces. The coke tower is an empty cylinder made of thick boiler steel plates, and it is the place where the coking reaction takes place. Generally, the height of a coke tower should be below 30 meters. If it is too high, vibration may occur during operation or the tower wall may be damaged, in addition to wasting steel. The top of the tower is equipped with a coking removal port and an oil-gas outlet ； There are level indicators on the side of the tower: The chemical reactions in delayed coking take place mainly within the coke tower, and the coke produced is also stored in this tower. As more oil is introduced, the coking layer gradually rises ； To prevent the foam layer from escaping from the top of the tower and causing coking in the oil and gas pipelines as well as the distillation tower, level gauges capable of monitoring the coke level are installed at various heights within the coke tower. The bottom of the tower is conical, with the base of the cone serving as the coke discharge opening. During normal operation, this opening is sealed with a flange cover, while it is opened when discharging coke. The four-way valve is an important component in delayed coking units; imported ball valves are typically used for this purpose. Its function is to enable switching between the new and old coke towers without causing significant changes in flow rate. Simply put, it consists of a tee structure with an opening created vertically, which serves as the feed direction for the coke tower. The angles between the three outlets are all 120°. Imported valves are used here because the operating conditions are extremely harsh, and coking tends to occur easily; therefore, steam needs to be injected into the ball surface, usually at four points. The steam from the note enters the tower together with the steam injected via the backfeed valve. In hydraulic coking units, the coke towers are used in rotation; that is, when the coke in one tower accumulates to a certain height, the feed is diverted to another coke tower via a four-way valve. The coke tower containing the accumulated coke is first cooled with steam, and then subjected to hydraulic coking. Current desulfurization equipment all employs high-pressure hydraulic desulfurization. In addition to hydraulic coking, high-pressure water at 11.8 MPa is used for coking removal. The remaining coke falls into the coke pit, while it is transported to another location for storage or loaded onto vehicles for shipment using a bridge crane grab. IX. Main reaction conditions for delayed coking The thermal conversion reaction in coking is a free, non-selective thermal cracking reaction, different from the selective cracking reactions that occur under the action of catalysts in catalytic cracking and hydrocracking; therefore, the product distribution and product quality of the thermal conversion reaction in coking depend only on the properties of the feedstock and the operating conditions. 1. Properties of raw materials: Coking feedstock oil is a mixture of hydrocarbons that consist mainly of carbon, hydrogen, sulfur, nitrogen, and oxygen as their main elements, with a molecular weight of approximately 500–1000. The thermal conversion of residue oil involves a very complex sequence of cracking and condensation reactions that occur in equilibrium, making it difficult to express these processes using chemical equations. The product distribution in the coking heat conversion reaction is closely related to the contents of the four components (saturated hydrocarbons, aromatics, resins, and asphalts). Generally, the carbon residue value of the residue oil and the contents of these four components are used to assess the quality of the feedstock and the pattern of product formation. Residue oils with high asphalt content or high carbon residue values tend to coker more easily, resulting in a higher coking rate and a lower yield of light oils. l Salt content, which directly determines the properties of the petroleum coke product as well as the rate of coking in the reaction system (coking in the reaction reactor tubes). It is generally required that the salt content in residue oil be below 4 mg/l ； The sulfur content determines the grade of petroleum coke products, as well as the degree of corrosion it causes to the system. The level of sulfur content is determined by the desired quality of the petroleum coke products and by the design capabilities of the system to prevent sulfur-induced corrosion. Among the four components, the main requirement is regarding the asphaltenes content, which should generally be kept at no more than 7%; otherwise, the furnace tubes are prone to coking and the formation of shot coke. Of course, the higher the saturation level, the better, as it can increase the yield of coked liquid. However, it would be ideal if atmospheric and vacuum distillation could separate out this fraction; otherwise, it is not conducive to reducing the overall energy consumption of the process or increasing the liquid yield. 2. Circulation ratio: The circulation ratio has a significant impact on the processing capacity of coking units, as well as on the product distribution and product properties. Under the same raw material conditions, generally: as the recycle ratio increases, the yields of gasoline and diesel increase, while the yield of wax oil decreases; the yields of coke and gases also show little increase. Generally, the selection of the recycle ratio is based on the requirements of the downstream processing units. For example, when coker wax oil is used as a feedstock for catalytic cracking, adjusting the recycle ratio serves to control parameters such as the carbon residue value and final boiling point of the wax oil. Generally, if the product quality and plant operation permit it, operation at the lowest possible recycle ratio is adopted. The circulation ratio decreases, the coking rate generally drops, while the yield of coker wax oil increases; the yields of gas, gasoline, and diesel decrease. To increase the liquid yield of the unit, it is generally done by reducing the recycle ratio (0.15–0.25) or operating with a zero recycle ratio ； When it is necessary to produce large amounts of coker naphtha and diesel, a larger circulation ratio (0.25–0.45) is generally used in operation ； When there is no outlet for coker gas oil or when it is necessary to produce the maximum possible amount of naphtha, operations with a high recycle ratio (0.4–1.0) are generally employed. After adopting pressure-reduced deep distillation, the quality of coking feedstock deteriorates, with increases in carbon residue and asphalt content. To prolong the operating cycle and prevent the formation of shot coke, a higher recycle ratio is generally employed. 3. Reaction temperature: The reaction temperature generally refers to the outlet temperature of the radiant coils in the heater. Variations in this temperature directly affect the reaction temperature and reaction depth within the coker drum, thereby influencing the product distribution and product quality. Low temperature results in insufficient depth of coking reaction, low product yield, and high volatile matter content in the coke. Excessively high temperatures lead to over-deep reactions, causing further cracking of gasoline and diesel. This reduces their yields while increasing the yield of gases; moreover, the coke becomes harder, making its removal difficult. The determination of the reaction temperature generally depends on the properties of the raw materials. Raising the coking temperature can increase the yield of liquid products, but due to the characteristics of the coking reaction, the range within which the reaction temperature (controlled by the furnace outlet temperature) can be adjusted is quite limited. Excessively high temperatures can lead to premature coking, which blocks the furnace tubes and oil transfer lines, affecting the operational cycle; moreover, it facilitates the formation of hard petroleum coke, making it difficult to remove the coking deposits ； Too low a temperature results in insufficient heat, leading to inadequate reaction depth, a reduced yield of light oil, increased volatiles in the coke, or the formation of tar. 4. Reaction pressure: Reaction pressure generally refers to the pressure at the top of the coke tower. It has an impact on the distribution of coking products – as pressure increases, the depth of reaction increases, resulting in higher yields of gas and coke and lower yields of liquid; meanwhile, the volatil content of coke increases. Conversely, when pressure decreases, the depth of reaction decreases, leading to lower yields of gas and coke and higher yields of liquid, with a decrease in the volatil content of coke. To improve the economic efficiency of the plant, low-pressure design and operation are typically employed. Low-pressure operation helps to improve the distribution of coking products, and this approach is widely accepted both domestically and internationally. In China, the operating pressure at the top of coke towers is generally between 0.15 and 0.20 MPa, while abroad it can be as low as 0.1 to 0.15 MPa. Reducing pressure generally increases the yield of wax oil. However, if the pressure is too low, the foam layer in the coke tower increases; coke powder is more likely to be carried away, and pellet coke is more likely to form. Additionally, it increases the gas volume flow rate in the coke tower, which necessarily leads to an increase in the diameter of the coke tower as well as that of the distillation column. This in turn increases the load on the compressors and the overhead condensation system, raising the investment cost for the facility. X. Regarding pellet coking 1. Conditions for the formation of pellet coking: deterioration of raw materials; the raw materials that give rise to pellet coking are those with high residual carbon and high asphaltenes after deep drawing, or deteriorated heavy oil. Asphalt plays a significant role in the production of raw materials for shot peening; typically, the content of asphaltenes in these four components is above 13%. The deterioration of the raw material leads to the earlier formation of carbonaceous asphaltenes; prior to entering the coke tower from the heating furnace, condensation reactions occur, resulting in the formation of tiny particles, which are the precursors of pellet coke. Upon entering the coke tower, influenced by factors such as gas velocity, these small particles gradually coalesce around each other to form pellets. Generally, to achieve better economic benefits, relatively inexpensive heavy crude oil and low-quality crude oil are usually chosen. Meanwhile, in order to produce more high-value semi-finished products, atmospheric and vacuum distillation units employ various measures to increase the recovery rate of crude oil, resulting in the vacuum residue becoming increasingly heavy. The operating conditions are quite stringent. Stringent operating conditions such as a higher furnace exit temperature, a lower circulation ratio, and a lower coke tower pressure were employed to create a production environment for the formation of pellet coke. 2. Effects of pellets coking: Various abnormalities or failures can occur during the production process, such as shaking of the coke tower, blockages in the pipes caused by pellets coking during water supply and discharge, and issues like collapses or stuck drill bits during coking removal. These problems pose a serious threat to the safe production and normal operation of the units; therefore, it is common practice in coking units to adopt measures such as reducing the furnace exit temperature, increasing the circulation ratio, raising the pressure in the coke drum, blending catalytic cracking slurry, and adjusting the raw material composition in order to avoid or suppress the formation of pellet coke and thereby reduce the risks associated with safe production in coking units. To date, there is no delayed coking unit in China that is capable of producing pellet coke safely and steadily, while foreign reports indicate that the United States has coking units designed to maximize the production of pellet coke and liquid products. However, preventing shot coke by adjusting control parameters is, on one hand, quite uneconomical, as it may result in a 2–3% loss in yield; on the other hand, it is rather difficult, since it’s hard to determine the form and quality of coke from operating parameters during normal production, making it impossible to make timely adjustments. Therefore, controlling shot coke by adjusting the blending ratio of raw materials is more economical and practical. 3. Prediction method for the properties of pellet coke feedstock in coking units: One of the methods is based on the API value of the coking feedstock (API = 141.5/d – 131.5) ； When API is less than 7, the device produces a high pellet coking ratio ； It is uncertain whether the device produces pellet coking when the API is between 7 and 8 ； When API is greater than 9, the likelihood of the device producing shot coke is low. The second method involves predicting the formation of pellet coke based on the mechanism of its formation, by using the ratio of residue to asphaltenes in the coking feedstock: when this ratio is less than 1.4, the facility is prone to the formation of pellet coke ； When the ratio of residual carbon to asphaltenes is between 1.4 and 1.6, the plant may produce pellet coke ； When the ratio of residual carbon to asphaltenes is greater than 1.6, the likelihood of the plant producing pellet coke is low. In addition, many devices have proposed the following prediction methods, which have been tested in practical production: the content of raw asphaltene is high, and the ratio of asphaltene to resin is greater than 0.35. When the ratio of raw material gel content to residue carbon is greater than 0.5 and close to 1.0, there is a greater tendency to produce pellet coke ； When the micro residue value (which is the same as the König residue value) is greater than 21%, there is a higher likelihood that coking reactions in the oil sample will result in the formation of pellet coke. When the ratio of residue to asphaltenes in the coking feed is less than 1.4, the plant is more likely to produce pellet coke. Devices with a viscosity greater than 500 are more likely to produce projectile coking. Impurities (especially metal content): Devices with mass fractions of nickel, vanadium, and calcium exceeding 244 μg/g, 185 μg/g, and 200 μg/g respectively are more likely to produce pellet coking, and a calcium content above 200 μg/g guarantees pellet coking.