[Cracking Question Bank] The factors that affect the radiation in cracking furnaces include

1. Properties of raw materials 2. Pyrolysis temperature 3. Hydrocarbon partial pressure 4. Residence time 5. Metal catalyst

Factors that affect the radiation in a pyrolysis furnace include: 1. Properties of the feedstock and degree of pyrolysis; 2. Pyrolysis temperature; 3. Hydrocarbon partial pressure and dilution steam; 4. Residence time; 5. Metal catalysts

Main factors affecting coking in cracking furnaces 1.1 Properties of the feedstock Coking during the cracking of hydrocarbons is primarily caused by aromatic compounds present in the feedstock as well as secondary reaction products of the cracking gases. The higher the content of aromatics and olefins in the raw materials, the faster the coking rate. 　　1.1.1 Aromatic compounds factor: For pyrolysis feedstocks with a high aromatic index, during moderate pyrolysis, the precursors for coking originate mainly from the aromatics present in the feedstock; during deep pyrolysis, these precursors come from the polycyclic aromatic hydrocarbons formed through the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes generated in the radiation zone of the pyrolyzer. For pyrolysis feedstocks with a low aromatic index, during moderate to deep pyrolysis, the precursors for coking are the cycloaromatic and polycyclic aromatic hydrocarbons resulting from the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes produced in the radiation zone of the pyrolyzer. 　　1.1.2 Layering of raw materials Naphtha is a multi-fractionated oil product, and severe layering occurs once it enters the storage tank. Analysis of samples taken from different parts of the same naphtha tank showed that when the naphtha was first taken out of the tank, the feedstock consisted mainly of heavy components, with a high content of aromatics, which made coking in the furnace tubes likely to occur. 　　1.1.3 Olefin factors Olefin cracking can involve reactions such as chain scission, dehydrogenation, diene formation, and aromatization. At high temperatures, these olefins tend to condense into aromatic hydrocarbons, naphthenes, and cycloolefins, with significant amounts of coke being produced; therefore, the lower the content of olefins in the feedstock, the better. When the separation operation is unstable, the olefin content is high, and the coking rate of the furnace tubes accelerates. 　　1.2 Cracking temperature Hydrocarbon cracking primarily involves chain scission and dehydrogenation reactions, both of which are endothermic processes. It is necessary to supply sufficient heat to the system at high temperatures. From the perspective of chemical equilibrium, increasing the reaction temperature raises the equilibrium constant of endothermic reactions, thereby increasing the degree of conversion at chemical equilibrium. From a reaction kinetics standpoint, raising the cracking temperature increases the relative rate of the primary reaction products compared to the secondary reactions. However, from a thermodynamic point of view, an increase in cracking temperature leads to greater cracking depth and faster secondary reactions, as a result of which the coking rate also increases. 　　1.3 Hydrocarbon partial pressure: In the reactions that occur during cracking, whether they are dehydrogenation reactions or chain-scission reactions, the number of gas molecules increases. Reducing the system pressure helps to increase the equilibrium conversion rate of ethylene. From a reaction kinetics analysis, reducing the pressure can increase the rate of the primary reaction relative to the secondary reaction, improve the selectivity for ethylene, suppress the occurrence of the secondary reaction, and thereby reduce coking. 　　1.4 Residence Time When a pyrolysis reaction takes place at a certain temperature, if the residence time of the reactants in the high-temperature zone is too short, the primary reaction of pyrolysis cannot proceed adequately, resulting in a low conversion rate and low ethylene yield. If the residence time is too long, the degree of pyrolysis increases, and both primary and secondary reactions occur, which accelerates coking. When the hydrocarbon partial pressure is low, the effect of residence time on the selectivity of pyrolysis is much greater than that of the hydrocarbon partial pressure. Therefore, determining an appropriate residence time is of great significance for improving the selectivity of ethylene and extending the operating cycle of the cracking furnace. Once the processing volume is determined, the flow rate of the dilution steam can be adjusted to control the residence time inside the tube. 　　1.5 Metal catalysis During the thermal cracking of hydrocarbons, the material composition of the furnace tube surface has a catalytic effect on coke formation. The materials used in the inlet and outlet sections of the cracking furnaces in ethylene plants generally contain metal elements such as Cr and Ni, which exert a certain catalytic effect in promoting coke formation.

1. Properties of raw materials: Coking during hydrocarbon cracking is primarily caused by aromatic compounds in the raw materials as well as secondary reaction products of the cracking gases. The higher the content of aromatics and olefins in the raw materials, the faster the coking rate. For pyrolysis feedstocks with a high aromatic index, under moderate pyrolysis conditions, the precursors for coking originate mainly from the aromatics present in the feedstock; under deep pyrolysis conditions, these precursors come primarily from the polycyclic aromatic hydrocarbons formed through the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes generated in the radiation zone of the pyrolyzer. For pyrolysis feedstocks with a lower aromatic index, under moderate to deep pyrolysis conditions, the precursors for coking are the cycloaromatic and polycyclic aromatic hydrocarbons resulting from the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes produced in the radiation zone of the pyrolyzer. 3. The raw material “stratified” naphtha is a multi-distillate oil product that exhibits severe stratification upon entering the storage tank. Analysis of samples taken from different parts of the same naphtha tank showed that when the naphtha was first taken out of the tank, the feedstock consisted mainly of heavy components, with a high content of aromatics, which made coking in the furnace tubes likely to occur. 4. Olefin factors: Olefin cracking can involve reactions such as chain scission, dehydrogenation, diene formation, and aromatization. At high temperatures, these olefins tend to condense into aromatic hydrocarbons, naphthenes, and cycloolefins; moreover, a large amount of coke is produced. Therefore, the lower the content of olefins in the feedstock, the better. When the separation operation is unstable, the olefin content is high, and the coking rate of the furnace tubes accelerates. 5. Pyrolysis temperature: In hydrocarbon pyrolysis, the main reactions are chain scission and dehydrogenation, both of which are endothermic reactions. It is necessary to supply sufficient heat to the system at high temperatures. From the perspective of chemical equilibrium, increasing the reaction temperature raises the equilibrium constant of endothermic reactions, thereby increasing the equilibrium conversion rate of the chemical reactions. From a reaction kinetics standpoint, raising the pyrolysis temperature increases the relative rate of the primary reaction products compared to the secondary reactions. However, considering thermodynamics, an increase in pyrolysis temperature leads to greater pyrolysis depth and faster secondary reactions, as a result of which the coking rate also increases. In the pyrolysis process using 6 hydrocarbons as a cracking agent, whether it is a dehydrogenation reaction or a chain-scission reaction, both are reactions that result in an increase in the number of gas molecules; reducing the system pressure helps to increase the equilibrium conversion rate of ethylene. From a reaction kinetics analysis, reducing the pressure can increase the rate of the primary reaction relative to the secondary reaction, improve the selectivity for ethylene, suppress the occurrence of the secondary reaction, and thereby reduce coking.

1 Properties of raw materials: Coking during the cracking of hydrocarbons is primarily caused by aromatic compounds in the raw materials as well as secondary reaction products of the cracking gases. The higher the content of aromatics and olefins in the raw materials, the faster the coking rate. 2. Aromatic factors: For pyrolysis feedstocks with a high aromatic index, during moderate pyrolysis, the precursors for coking originate mainly from the aromatics in the feedstock; during deep pyrolysis, these precursors come primarily from the polycyclic aromatic hydrocarbons formed through the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes generated in the radiation zone of the pyrolyzer. For pyrolysis feedstocks with a low aromatic index, during moderate to deep pyrolysis, the precursors for coking are the cycloaromatic and polycyclic aromatic hydrocarbons resulting from the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes produced in the radiation zone of the pyrolyzer. 3. “Stratification” of raw materials: Naphtha is an oil with multiple fractions, and severe “stratification” occurs once it enters the storage tank. Analysis of samples taken from different parts of the same naphtha tank showed that when the naphtha was first taken out of the tank, the feedstock consisted mainly of heavy components, with a high content of aromatics, which made coking in the furnace tubes likely to occur. 4. Olefin factor: Olefin cracking can involve reactions such as chain scission, dehydrogenation, diene formation, and aromatization. At high temperatures, these olefins tend to condense into aromatic hydrocarbons, cycloalkanes, and cycloolefins, with significant amounts of coke being produced; therefore, the lower the content of olefins in the feedstock, the better. When the separation operation is unstable, the olefin content is high, and the coking rate of the furnace tubes accelerates. 5. Pyrolysis temperature: The pyrolysis of hydrocarbons involves chain scission and dehydrogenation reactions, both of which are endothermic processes. It is necessary to supply sufficient heat to the system at high temperatures. From the perspective of chemical equilibrium, increasing the reaction temperature raises the equilibrium constant of endothermic reactions, thereby increasing the degree of conversion at chemical equilibrium. From a kinetic perspective, raising the pyrolysis temperature increases the relative rate of the desired primary reaction products compared to secondary reactions. However, considering thermodynamics, an increase in pyrolysis temperature leads to greater pyrolysis depth and faster secondary reactions, as a result of which the coking rate also increases. 6. Hydrocarbon partial pressure: In the cracking process, whether it is a dehydrogenation reaction or a chain-scission reaction, both are reactions that result in an increase in the number of gas molecules; reducing the system pressure helps to increase the equilibrium conversion rate of ethylene. From a reaction kinetics analysis, reducing the pressure can increase the rate of the primary reaction relative to the secondary reaction, improve the selectivity for ethylene, suppress the occurrence of the secondary reaction, and thereby reduce coking.

Raw material properties and pyrolysis depth: Primarily caused by aromatic compounds in the raw materials as well as products of secondary reactions in the pyrolysis gases. The higher the aromatic content in the raw material, the faster the coking rate. During the cracking of light hydrocarbons, the precursors for coking are mainly products of secondary reactions; whereas in the cracking of heavy feedstocks, the aromatics present in those feedstocks are the primary precursors for coking, especially those aromatics with side chains. At the same time, the cracking depth increases, the active components for coking increase, secondary reactions intensify, and coking also worsens. Injection of dilution steam can reduce the hydrocarbon partial pressure and suppress the polycondensation reaction, thereby reducing the degree of coking. It can also shorten the residence time and inhibit secondary reactions, thus lowering the coking rate. Moreover, water vapor has an oxidizing effect on Fe and Ni, which helps to suppress catalytic carbon formation reactions. Reduced residence time and pyrolysis temperature minimize the occurrence of secondary reactions, which helps to slow down coking. However, reducing the residence time often requires increasing the flow rate or decreasing the diameter of the furnace tubes, as well as raising the temperature of these tubes. This increases the mass transfer rate within the coke-forming medium. When the rate of coke formation is lower than the mass transfer rate, the coke formation process is controlled by the reaction rate; conversely, it is controlled by mass transfer. Generally, during high-temperature pyrolysis, the coking rate is controlled by the mass transfer process. Reducing the residence time has both a positive effect – by lowering the concentration of substances that lead to coking and thus reducing the coking rate – and a negative effect – by increasing mass transfer and consequently raising the coking rate. However, the positive effect prevails, which is why pyrolysis technologies have evolved in recent years toward higher temperatures, shorter residence times, and lower hydrocarbon partial pressures. According to the coking mechanism, reducing the residence time can help alleviate coking in the pyrolysis reactor tubes and accelerate the removal of coke or coke precursors during the pyrolysis process. The coking inhibition techniques that have been adopted or are under development in industries both domestically and internationally mainly include three methods: improving the quality of the pyrolysis feedstock, treating the surface of the reactor tubes, and adding coking inhibitors. Improvement of pyrolysis feedstock – high-quality pyrolysis feedstock. Lightweight and high-quality pyrolysis feedstocks not only yield high rates of trienes (ethylene, propylene, and dienes), but also reduce coking, extend the operating cycle, and lower energy and material consumption. Reducing the aromatic content can be achieved through aromatic extraction, or by converting olefins and similar compounds into saturated hydrocarbons through hydrogenation. To reduce coking, it is first necessary to select appropriate hydrocarbons; converting aromatics into naphthenes can also help achieve the goal of reducing coking. Treating the surface of the furnace tube is necessary because its surface contains active centers; by treating this surface, these catalytically active centers can be reduced or covered, thereby reducing or eliminating catalytic coking. Surface treatment techniques include forming glass or ceramic coatings on the inner wall of the furnace tube, or forming alloy coatings. The coating material should possess the following properties: a) it must not contain any substances that are harmful to the pyrolysis reaction; b) it must be able to withstand high temperatures without undergoing any harmful physical or chemical changes, and must remain stable under high-temperature and thermal shock conditions. ) The thermal expansion coefficient should be compatible with that of the furnace tube metal, so as to prevent detachment under temperature changes; d) The processing temperature of the surface coating should be lower than the highest temperature that the metal can withstand, to ensure that the processing does not cause damage to the furnace tube; e) The coating should adhere tightly to the tube wall, evenly without any gaps, covering all the catalytically active sites on the surface of the furnace tube. Adding coking inhibitors is a common method for reducing coking. Early coking inhibitors included sulfur-containing compounds, alkali metal salts, phosphorus-containing compounds, and others. Under pyrolysis conditions, the hydrosulfide generated by the decomposition of sulfides reacts with the metal surface; the resulting ferrous sulfide compounds passivate the surface of the furnace tubes, preventing coke from adhering and also suppressing the catalytic coking reactions of Fe and Ni. Alkali metals can catalyze the water-gas reaction of coke, continuously converting it into CO and CO2, thereby reducing the amount of coke deposition and achieving an effect of suppressing coking. In addition, alkali metal salts also have the function of covering and dulling the furnace tubes as well as shielding them from metal-catalyzed coking reactions. Phosphides decompose under pyrolytic conditions, and the decomposition products form a dense phosphide film on the metal surface. This film can inhibit catalytic coking reactions on the metal surface; moreover, the phosphides can alter the structure of the coke, making it looser and easier to remove. A good inhibitor should possess the following various functions [8]. a) Surface passivation. It reacts with the metal surface of the furnace tube to passivate it, thereby suppressing catalytic coking. In the early stages of operation of the device, the coking reaction is primarily metal-catalyzed coking, and the coking rate is at its highest at this time; the addition of a passivator can effectively suppress the coking reaction; b) anti-polymerization effect. By adding additives to eliminate or suppress free radical compounds, and by altering or terminating the free radical reaction pathways involved in coking, the formation of coke is reduced; c) Gasification. Appropriate additives can accelerate the gasification reaction of coke, thereby reducing the amount of coking; d) Coke modification. Some additives can change the physical structure of coke, making its structure loose and fragile, thereby preventing it from adhering to the walls of the furnace tubes and facilitating its removal; e) Dispersing effect. The dispersed tar particles make it difficult for them to adhere to the walls of the furnace tubes, allowing them to be carried away by the high-speed airflow, thereby reducing coking. Catalyst coking is mainly caused by gold; f) Corrosion prevention function. Improve the corrosion resistance of the furnace tubes, so as to at least prevent corrosion of them; g) Low pollution. It has no impact on the separation and reprocessing of downstream products.

1 Properties of raw materials: Coking during the cracking of hydrocarbons is primarily caused by aromatic compounds in the raw materials as well as secondary reaction products of the cracking gases. The higher the content of aromatics and olefins in the raw materials, the faster the coking rate. 2. Aromatic factors: For pyrolysis feedstocks with a high aromatic index, during moderate pyrolysis, the precursors for coking originate mainly from the aromatics in the feedstock; during deep pyrolysis, these precursors come primarily from the polycyclic aromatic hydrocarbons formed through the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes generated in the radiation zone of the pyrolyzer. For pyrolysis feedstocks with a low aromatic index, during moderate to deep pyrolysis, the precursors for coking are the cycloaromatic and polycyclic aromatic hydrocarbons resulting from the polymerization, cyclization, dehydrogenation, and condensation of olefins and dienes produced in the radiation zone of the pyrolyzer. 3. “Stratification” of raw materials: Naphtha is an oil with multiple fractions, and severe “stratification” occurs once it enters the storage tank. Analysis of samples taken from different parts of the same naphtha tank showed that when the naphtha was first taken out of the tank, the feedstock consisted mainly of heavy components, with a high content of aromatics, which made coking in the furnace tubes likely to occur. 4. Olefin factor: Olefin cracking can involve reactions such as chain scission, dehydrogenation, diene formation, and aromatization. At high temperatures, these olefins tend to condense into aromatic hydrocarbons, cycloalkanes, and cycloolefins, with significant amounts of coke being produced; therefore, the lower the content of olefins in the feedstock, the better. When the separation operation is unstable, the olefin content is high, and the coking rate of the furnace tubes accelerates. 5. Pyrolysis temperature: The pyrolysis of hydrocarbons involves chain scission and dehydrogenation reactions, both of which are endothermic processes. It is necessary to supply sufficient heat to the system at high temperatures. From the perspective of chemical equilibrium, increasing the reaction temperature raises the equilibrium constant of endothermic reactions, thereby increasing the degree of conversion at chemical equilibrium. From a kinetic perspective, raising the pyrolysis temperature increases the relative rate of the desired primary reaction products compared to secondary reactions. However, considering thermodynamics, an increase in pyrolysis temperature leads to greater pyrolysis depth and faster secondary reactions, as a result of which the coking rate also increases. 6. Hydrocarbon partial pressure: In the cracking process, whether it is a dehydrogenation reaction or a chain-scission reaction, both are reactions that result in an increase in the number of gas molecules; reducing the system pressure helps to increase the equilibrium conversion rate of ethylene. From a reaction kinetics analysis, reducing the pressure can increase the rate of the primary reaction relative to the secondary reaction, improve the selectivity for ethylene, suppress the occurrence of the secondary reaction, and thereby reduce coking.

1. Properties of raw materials 2. Pyrolysis temperature 3. Hydrocarbon partial pressure 4. Residence time 5. Metal catalyst

Guys, what’s the difference between coking and carbonization? How can one tell whether the pyrolyzer is coking or producing carbon? ? ? Someone knows that! ! ! ! I beg! !