Exploration of the mechanism of coke formation in the petroleum processing process!

http://bbs.hcbbs.com/viewthread.php?tid=237418&highlight=%BD%B9%CC%BF I. Definition of coke Bituminous coal is heated to 950–1050°C in an oxygen-free environment; through stages such as drying, pyrolysis, melting, bonding, curing, and shrinkage, it is ultimately transformed into coke. This process is known as high-temperature coking. Coke produced by high-temperature coking is used in blast furnace smelting, casting, and gasification. Coke oven gas, which is recovered and purified during the coking process, serves as both a fuel with high calorific value and an important raw material for the organic synthesis industry. 　　Metallurgical coke is a general term for blast furnace coke, foundry coke, ferroalloy coke, and coke used in non-ferrous metal smelting. Since over 90% of metallurgical coke is used in blast furnace ironmaking, blast furnace coke is often referred to as metallurgical coke. 　　Casting coke is coke specifically used for melting iron in cupola furnaces. Casting coke is the main fuel for melting iron in blast furnaces. Its function is to melt the charge and superheat the molten iron, as well as to support the charge column and maintain its good air permeability. Therefore, cast coke should possess large lump size, low reactivity, low porosity, sufficient impact resistance, as well as low ash and sulfur content. II. Distribution of Coke 　　Looking at the distribution of coke production in China, it can be seen that coking enterprises are unevenly distributed across the country, with the majority located in North China, East China, and Northeast China. III. Uses of Coke Coke is primarily used in blast furnaces for iron production, as well as in blast furnaces for the smelting of non-ferrous metals such as copper, lead, zinc, titanium, antimony, and mercury. It serves as a reducing agent, a heat source, and a structural element for the charge pile. The use of coke instead of charcoal in ironmaking blast furnaces laid the foundation for the enlargement of modern blast furnaces, marking a significant milestone in the history of metallurgy. To achieve favorable technical and economic parameters in blast furnace operation, coking coal used for smelting (metallurgical coke) must possess appropriate chemical and physical properties, including its properties under hot conditions during the smelting process. In addition to being widely used in iron production and the smelting of non-ferrous metals (as metallurgical coke), coke is also used in foundry industry, the chemical industry, calcium carbide production, and ferroalloys, with varying quality requirements for each application. For casting coke, it is generally required to have a large particle size, low porosity, high fixed carbon content, and low sulfur content ； Coke used for chemical gasification does not require high strength, but it needs to have good reactivity and a high ash melting point ； The coke used in calcium carbide production is required to have as high a fixed carbon content as possible. IV. Physical Properties of Coke The physical properties of coke include the sieve analysis composition of coke, the bulk density of coke, the **apparent density of coke, the apparent relative density of coke, the porosity of coke, the specific heat capacity of coke, the thermal conductivity of coke, the thermal stress of coke, the ignition temperature of coke, the coefficient of thermal expansion of coke, the shrinkage rate of coke, the electrical resistivity of coke, and the air permeability of coke. 　　The physical properties of coke are closely related to its mechanical strength and thermal strength at room temperature, as well as its chemical properties. The main physical properties of coke are as follows: the true density is 1.8–1.95 g/cm3 ； 　　Viscosity density is 0.88–1.08 g/cm3 ； 　　The porosity is 35-55% ； 　　The bulk density is 400-500 kg/m3 ； 　　The average specific heat capacity is 0.808 kj/(kg·K) at 100°C, and 1.465 kj/(kg·K) at 1000°C ； 　　The thermal conductivity is 2.64 kj/(mh·K) at room temperature, and 6.91 kg/(mh·K) at 900°C ； 　　The ignition temperature (in air) is 450-650℃ ； 　　The low calorific value of the dry, ash-free base is 30–32 KJ/g ； The specific surface area is 0.6–0.8 m2/g. V. Reactivity of coke and its strength after reaction The reactivity of coke refers to its ability to undergo chemical reactions with substances such as carbon dioxide, oxygen, and water vapor. The strength of coke after reaction denotes its capacity to resist fragmentation and wear under mechanical and thermal stresses. In blast furnace ironmaking, foundry ironmaking, and fixed-bed gasification, coke undergoes chemical reactions with carbon dioxide, oxygen, and water vapor. Since the reaction of coke with oxygen and water vapor follows patterns similar to those of its reaction with carbon dioxide, most cases **use the reaction properties of coke with carbon dioxide to assess its reactivity. 　　 　　 　　The Chinese standard (GB/T4000-1996) specifies the test methods for coke reactivity and strength after reaction. The method involves reacting coke with carbon dioxide at high temperatures, and then measuring the weight loss of the coke after the reaction as well as its mechanical strength. The repeatability r for coke reactivity CRI and post-reaction strength CSR shall not exceed the following values: CRIr ≤ 2.4%, CSR ≤ 3.2%. The test results for coke reactivity and post-reaction strength are based on the arithmetic average of the results from duplicate tests. VI. Quality specifications of coke Coke is a solid product obtained through high-temperature carbonization; its main component is carbon, and it has a cracked and irregular pore structure (or a porous structure with pores). The number of cracks directly affects the strength and crush resistance of coke, with this property generally being measured by the crack density (referring to the length of cracks per unit volume of coke). The indicator for measuring the pore structure is primarily the porosity (the percentage of the volume of pores in coke relative to its total volume), which affects the reactivity and strength of the coke. Coke intended for different purposes has varying requirements regarding the porosity index; generally, metallurgical coke requires a porosity of 40–45%, casting coke requires 35–40%, while coke intended for export needs a porosity of around 30%. The degree of cracking and porosity in coke are directly related to the type of coal used in coking; for example, coke produced from bituminous coal tends to have many cracks, a high porosity, and low strength ； Coke produced from coking coal as the base coal has fewer cracks, a lower porosity, and higher strength. Coke strength is usually expressed by two indicators: crush resistance and wear resistance. The crushing resistance of coke refers to its ability to withstand external impacts without breaking along the cracks or defects in its structure, and it is expressed by the M40 value ； The wear resistance of coke refers to its ability to resist external friction forces without the formation of surface glass particles or powder, and it is expressed by the M10 value. The crackness of coke affects its crush resistance value M40, while the pore structure of coke influences its wear resistance value M10. There are many methods for determining the M40 and M10 values; in China, the German Migon drum test method is commonly used. VII. Evaluation of coke quality 1. Sulfur content in coke: Sulfur is one of the harmful impurities in the smelting of pig iron; it reduces the quality of pig iron. In steel-making pig iron, a sulfur content of more than 0.07% renders it scrap. 11% of the sulfur brought into the furnace from the blast furnace charge comes from the ore ； 3.5% comes from limestone ； 82.5% comes from coke, so coke is the main source of sulfur in the furnace charge. The sulfur content in coke has a direct impact on blast furnace iron production. When the sulfur content in coke is greater than 1.6%, for every 0.1% increase in sulfur content, the amount of coke required increases by 1.8%, the amount of limestone added increases by 3.7%, and the amount of ore added increases by 0.3%; meanwhile, the blast furnace production decreases by 1.5–2.0%. The specified sulfur content for metallurgical coke is no more than 1%, while the sulfur content in metallurgical coke used in large and medium-sized blast furnaces should be less than 0.4–0.7%. 　　2. Phosphorus content in coke: The phosphorus content in metallurgical coke used for iron production should be below 0.02–0.03%. 　　3. Ash content in coke: The ash content in coke has a significant impact on blast furnace smelting. An increase of 1% in coke ash content leads to a 2–2.5% increase in the amount of coke required. Therefore, it is very necessary to reduce coke ash content. 　　4. Volatiles in coke: The maturity of coke can be determined based on its volatile content. If the volatile content is greater than 1.5%, it indicates green coke ； If the volatile content is less than 0.5–0.7%, it indicates over-cooking; generally, the volatile content of mature metallurgical coke is around 1%. 　　5. Moisture in coke: Fluctuations in moisture can lead to inaccurate measurement of coke, thereby causing fluctuations in furnace operation. Furthermore, an increase in coke moisture leads to higher M04 values and lower M10 values, causing errors in the drum test results. 　　6. Screening composition of coke: The particle size of coke is also very important in blast furnace smelting. In the past, the requirements for coke particle size in our country were as follows: for large coke ovens (1300–2000 square meters), the coke particle size had to be greater than 40 millimeters ； The coke particle size in medium and small blast furnaces is greater than 25 millimeters. However, tests conducted by some steel mills at present show that a coke particle size of 40–25 millimeters is optimal. Coke larger than 80 millimeters should be in whole particles to maintain a relatively constant particle size range. In this way, the coke has uniform size, large pores, low resistance, and the furnace operates well.

It is mainly a condensation process; polycyclic aromatic hydrocarbons with olefinic chains have the greatest tendency to coking. Metal iron acts as a catalyst for coking, and in most cases it is the passivates of iron on the metal surface, such as sulfur and phosphorus, along with dispersants

In catalytic devices, coke is generally formed from bicyclic aromatics and polycyclic aromatics. Aromatic hydrocarbons with more than five rings all condense into coke; if such aromatic hydrocarbons have side chains, those side chains as well will break off, and not all polycyclic aromatic hydrocarbons turn into coke. Catalytic slurry and reprocessed oil contain a certain amount of pentacyclic aromatics and gums, with a higher number of aromatic rings as well as higher residual values. Therefore, carrying out aromatic extraction on the reprocessed oil is a good method to reduce the residue and coking in the mixed feedstock. Vacuum residue contains high levels of pentacyclic aromatic resins and asphaltenes, as well as a large number of aromatic hydrocarbons; its carbon residue value is also high. Therefore, as the proportion of vacuum residue blended into catalytic cracking increases, its carbon residue value rises rapidly, the coke production increases exponentially, and there is an excess of heat in the plant, necessitating heat removal systems to maintain thermal balance.

Catalytic cracking coke can be divided into four types: 1. Catalytic carbon, which is a highly condensed product of carbocation reactions. 2. Strippable carbon, which is formed from the hydrocarbons carried by the catalyst in the reactor to the regenerator; its amount increases as the catalyst circulation rate and the catalyst-to-oil ratio increase. If the catalyst-to-oil ratio is high, the amount of stripping steam used can be increased appropriately. 3. Additional carbon is formed by the deposition on the catalyst at high temperatures of those substances in the raw material that are prone to condensation and capable of forming König residue. 4. Polluted carbon is formed by the catalytic dehydrogenation of heavy metals on the catalyst. Adding a metal passivator to the raw material can reduce this carbon