Popular science on delayed coking – useful knowledge worth saving!

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Delayed coking is a type of thermal cracking process. Its main purpose is to convert residual oil with high carbon residue into light oils. The equipment used can operate in a cyclic manner, that is, the heavier fractions from the coking distillates of heavy oil are used as circulating oil and remain in the equipment for a longer period of time. I. Technical Overview Delayed coking is a secondary petroleum processing technique that involves using hydrogen-poor heavy oils as raw materials to carry out deep thermal cracking and condensation reactions at high temperatures (around 500°C), thereby producing rich gas, crude gasoline, diesel, wax oil, and coke. It is one of the main methods for the deep processing of residual oil worldwide, accounting for one-third of the total capacity for residual oil treatment. The so-called delay refers to the process in which coking oil (feed oil and recycled oil) is heated rapidly to the coking reaction temperature using a heating furnace; no coking occurs inside the reaction tubes, and instead the oil goes into the coking tower where the coking reaction takes place. This delay effect is what gives rise to the term delayed coking technology. Typically, it consists of one heating furnace and two coking towers, or two heating furnaces and four coking towers; the heating furnace feeds material continuously while the coking towers operate in rotation, resulting in a semi-continuous processing process. The feed oil (vacuum residue or other heavy oils such as deoiled asphalt, clarified oil, or even sludge oil) is heated to 495–505°C and fed into the coke tower. The hot feed oil undergoes coking reactions within the tower, and the light products generated emerge from the top and enter the distillation tower, where gas-rich streams, crude gasoline, diesel, and heavy distillates are separated out. The heavy distillates can be sent for further processing (such as as feedstock for catalytic cracking or hydrocracking), or they can be recycled in whole or in part back to the crude oil system. Once the coke towers are gradually filled (leaving some space), the feed material is directed to another coke tower, while the coke remaining in the coke tower is removed using hydraulic coke removal methods. Heat the feed material only after the coke tower has been restored to an empty state. In this process, the yield of coke generally varies with the Conradson carbon residue (CCR) of the feed oil; the yield of rich gas is typically around 10% on a mass basis (gas yield % = 7.8 + 0.144 × CCR). The remaining values vary depending on the recycle ratio, but the fuel/gas ratio is greater than 1. Delayed coking feedstocks can be heavy oil, residue, or even asphalt. Delayed coking products are divided into gas, gasoline, diesel, wax oil, and coke. For domestic residue, the gas yield is 7.0~10%, the crude gasoline yield is 8.2~16.0%, the diesel yield is 22.0~28.66%, the wax oil yield is 23.0~33.0%, the coke yield is 15.0~24.6%, and the unprocessed oil yield is 1~3.0%. Coker gasoline and coker diesel are the main products of delayed coking, but their quality is poor. Coker gasoline has a very low octane rating, typically ranging from 51 to 64 (MON), while diesel fuel has a higher cetane number, usually between 50 and 58. However, both types of oils have high levels of olefins, as well as high contents of impurities such as sulfur, nitrogen, and oxygen; their stability is poor. Therefore, they can only be used as semi-finished products or intermediate materials. Only after refining can they be used as components in the formulation of gasoline and diesel. Due to its high content of sulfur and nitrogen compounds, gums, and char, coker wax oil is a low-quality wax oil unsuitable for further processing; as of 2014, it was typically blended into catalytic or hydrocracking processes as a feedstock. Petcoke is one of the important products of the delayed coking process; depending on its quality, it can be used as electrodes, in metallurgy, and as fuel. After desulfurization, coking gas can be used as a raw material for hydrogen production or fed into the fuel pipeline network as fuel. II. Advantages and Disadvantages: Delayed coking can process low-quality feedstocks with high residual carbon and metal contents, thereby increasing the yield of light oils and the efficiency of decarburization. It offers advantages such as continuous operation, large processing capacity, high flexibility, high decarburization efficiency, as well as low investment and operating costs. The disadvantage is a high yield of low-value coke products and poor properties of the liquid products. III. Delayed Coking Process: Delayed coking is similar to thermal cracking, except that the material is heated to the temperature required for coking reactions over a short period of time. Cracking reactions are prevented from occurring in the furnace tubes, and instead are delayed until a dedicated coke tower, which is why this process is called \"delayed coking\". The delayed coking unit is mainly composed of 8 parts: 1. The coking section, whose main equipment are the heating furnace and the coke tower. There are configurations with one furnace and two towers, two furnaces and four towers, as well as those that are directly integrated with other devices. 2. Fractionation section: The main equipment is the fractionation tower; in delayed coking, chemicals such as defoamers, coke inhibitors, and liquid yield enhancers are generally added. 3. Coking gas recovery and desulfurization: The main equipment includes absorption towers, desorption towers, stabilization towers, reabsorption towers, etc. 4. Hydrodeticing section: Given the high hydrocarbon ratio of heavy residual oil, which makes it highly prone to coking, the feed oil is rapidly heated to a relatively high temperature (480–500°C). This allows the heavy residual oil to be sent to a hollow container (known as a coke tower) in the tubular heater, where it undergoes cracking and condensation reactions before it has time to react. Since heating and coking occur at different times, this process is referred to as delayed coking. The high-temperature oil and gas at 415°C, generated by the pyrolysis reaction in the coke tower, escapes from the top of the coke tower and enters the feed section at the lower part of the main distillation column. After being washed of the coke particles it contains by the wash plates (diagonal baffles), it rises to the evaporation section where it is separated through distillation, yielding rich gas, gasoline, diesel, and light and heavy wax oil fractions. The coke produced by the condensation reaction in the coke tower remains inside the tower. When enough coke has accumulated in the tower, operation is switched to another coke tower to continue the coking process, while the original tower is subjected to coke cleaning operations. Typically, 2 to 4 coke ovens are used in a coking plant. Coking slag removal is carried out using the hydraulic slag removal method: first, a hole is drilled in the center of the coke layer using a drill, from the top down; then high-pressure water with a pressure of 12–30 MPa is injected from below. The impact force of this water is used to dislodge the coke, which is then discharged from the bottom. The high-pressure water delivered by the high-pressure water pump passes through the water supply line, hose, and drill pipe to reach the nozzle of the hydraulic coking device. The high-pressure water ejected from this nozzle forms a high-pressure jet, and it is the powerful impact force of this jet that is used to cut the petroleum coke apart. The drill pipe keeps moving up and down and rotating until all the coke is removed. 5. Main equipment for hydraulic coking removal: (high-pressure water pump), (coking removal control valve), lubricating oil system, pneumatic valves, winch and pulley block, (new type of coking removal hose), (water turbine reducer), (automatic switching combined drilling and coking removal device), (automatic loader/unloader for the tower top cover), (loader/unloader for the tower bottom cover), (elevator), (drill pipe assembly) and (crane with grab). 6. Dewatering, storage, and transportation of coke. Dewatering, storage, and transportation of coke. 7. Blowing and venting system: Blowing and venting system. 8. Steam generation section: The heat source for generating steam is typically diesel from the side stream of the distillation tower, heavy paraffin oil, and bottom circulation oil. 9. Coke baking section: The section for baking coke. The selected outlet temperature of the furnace in China is 495–500°C, and the pressure at the top of the coke tower is 0.15–0.2 Mpa. IV. Rapid Development: It is precisely due to the aforementioned advantages of delayed coking that it has seen rapid development in China. This is mainly because: (1) Delayed coking is an effective means of addressing the imbalance between the supply and demand of fuel oil and gasoline. This is because Chinese crude oil is generally heavy and has a high wax content, resulting in a low yield of diesel; the yield of diesel from domestic crude oil is 5 to 7 percentage points lower on average than that from foreign crude oil. Thus, up to 2014, China imported around 80×10t of diesel per year, while having to export 30×10t of gasoline in order to maintain balance between supply and demand domestically. Secondly, since Chinese refining companies rely primarily on catalytic cracking for secondary processing, the diesel-to-gasoline ratio is low (1.94 for delayed coking and 0.56 for catalytic cracking). Therefore, developing delayed coking is an effective way to address the imbalance between supply and demand of diesel and gasoline, as well as to increase diesel production. ⑵Compared with hydrocracking, although delayed coking has poor stability of light oil products, its low operating costs (the processing cost is about 1/2 to 1/3 of that in hydrocracking) give it strong competitiveness. Due to its advantages of low investment, low operating costs, and high conversion efficiency, delayed coking has become one of the main processing methods for lightening residue oil. Therefore, by 2014, China faced capital shortages, and with a prominent supply-demand imbalance for light oil products, especially diesel and gasoline, delayed coking was one of the more ideal solutions to this issue. The materials in this article are sourced from the Internet; any inaccuracies are welcome to be corrected (compiled by Shihua Yuan; please indicate the source when reproducing)