Characteristics and reaction mechanism of catalytic cracking

Catalytic cracking is the most important process in the secondary processing of crude oil; it is the main method for producing liquefied petroleum gas, gasoline, kerosene, and diesel, and holds a pivotal role in refineries. Catalytic cracking generally uses vacuum distillate oil and coker wax oil as feedstocks. However, with the increasing trend toward heavier crude oils and the strong market demand for lighter petroleum products, some refineries have begun to use vacuum residue as a feedstock, or even use atmospheric residue directly as the material for cracking. Catalytic cracking will be introduced from seven aspects below. (1) General characteristics of catalytic cracking: ① High yields of light oils (including gasoline, kerosene, and diesel) can be achieved, ranging from 70 to 80 wt%, whereas the yield of light oils from the initial distillation of crude oil is only 10 to 40 wt%. ② Catalytic cracking gasoline has a high octane rating; its research octane number can exceed 85, and it also possesses good stability. ③ Catalytic cracking diesel has a low cetane number, and is often blended with straight-run diesel, or hydrogenated to improve its cetane number. ④ Catalytic cracking gas products account for about 10–20 wt%, of which 90% is liquefied petroleum gas; they also contain large amounts of C3 and C4 olefins, making them excellent raw materials for the petrochemical industry and for the production of high-octane gasoline components. (2) Characteristics of heavy oil catalytic cracking: ① High coke yield. The coke yield in heavy oil catalytic cracking is as high as 8–12 wt%, whereas the coke yield in distillate oil catalytic cracking is usually 5–6 wt%. ② Catalysts contaminated with heavy metals. Compared to distillate oil, heavy oil contains more heavy metals, which accumulate on the surface of the catalyst during catalytic cracking, leading to catalyst contamination or poisoning. ③ The effect of sulfur and nitrogen impurities. The content of impurity atoms such as sulfur and nitrogen in heavy oil is relatively high, which results in higher levels of sulfur and nitrogen in the light oils produced through cracking, thereby affecting the quality of the products ； On the other hand, it also leads to higher sulfur and nitrogen contents in the coke; during the coking of the catalyst, more sulfur and nitrogen oxides are generated, which corrode equipment and pollute the environment. ④ Under catalytic cracking conditions, heavy oil cannot be completely gasified. Under catalytic cracking conditions, heavy oil can only be partially vaporized; the unvaporized liquid droplets adhere to the surface of the catalyst. At this point, the mass transfer resistance cannot be ignored, and the reaction process is a complex gas-liquid-solid three-phase catalytic reaction. (3) Catalytic cracking reaction of monohydrocarbons ① Alkanes mainly undergo decomposition reactions to produce smaller alkanes and alkenes. ② In addition to decomposition reactions, olefins also undergo isomerization, hydrogen transfer, and aromatization reactions. ③ Cycloalkanes can undergo ring-opening reactions to form linear alkenes, or they can undergo hydrogen transfer reactions to form aromatic hydrocarbons. ④ Aromatic hydrocarbons do not undergo ring-opening reactions; instead, only side-chain cleavage occurs, with the cleavage taking place primarily at the bonds connecting the side chains to the aromatic ring. (4) Reaction mechanism and product distribution patterns of hydrocarbon catalytic cracking. The vast majority of scholars believe that the catalytic cracking of hydrocarbons follows a carbocation reaction mechanism. According to the carbocation reaction mechanism, the reaction behavior and product distribution in hydrocarbon catalytic cracking are as follows: ① Among the cracking feedstocks, alkenes crack rapidly, as do aromatic hydrocarbons when their side chains are broken off; naphthenes and isoparaffins react more slowly, while normal paraffins have the slowest decomposition rate. ② Gasoline has a high content of olefins, as well as a relatively high content of aromatic hydrocarbons; it also has a high octane rating. ③ Diesel has a low content of alkanes and a low cetane number. ④ The cracking gas contains high levels of C3 and C4 products, primarily propylene and butylene; among the C4 products, isomeric hydrocarbons are present in significant amounts. (5) Factors affecting catalytic cracking The factors affecting catalytic cracking mainly include the properties of the feed oil, the properties of the catalyst, operating conditions, and the reaction equipment. ① Impact of the properties of cracking feedstock oil. Generally speaking, the higher the H/C ratio of the crude oil and the higher its saturated fraction content, the higher the yields of gasoline and light oils obtained through cracking. The higher the residual carbon value of the feedstock, as well as the levels of sulfur, nitrogen, and heavy metals, the lower the yields of gasoline and light oils, and the poorer the quality of the products. ② Properties of the catalyst. Catalytic cracking catalysts are divided into aluminum silicate catalysts and molecular sieve catalysts. The activity, selectivity, stability, resistance to heavy metal contamination, fluidization properties, and wear resistance of these catalysts all have varying degrees of influence on catalytic cracking. Generally speaking, the higher the activity of the catalyst, the greater the conversion rate of the raw materials ； The higher the selectivity of the catalyst, the higher the yield of light oil products. The activity and selectivity of molecular sieve catalysts are generally superior to those of aluminum silicate catalysts, enabling an increase in gasoline yield by 15–20%. ③ Effect of operating conditions. Operating conditions include the atomization and gasification effects of the feedstock, reaction temperature, reaction pressure, reaction time, fuel-to-oil ratio, amount of water vapor, and catalyst residence time, among others. The better the atomization and vaporization effects of the feedstock, the higher the conversion rate of the feedstock oil, and the higher the yield of light oil products as well ； The higher the reaction temperature and the greater the oil-to-reagent ratio, the higher the conversion rate of the feed oil and the gasoline yield, but the coke yield also increases ； The residence time of oil and gas should not be too short, nor too long; it is generally between 2 and 4 seconds ； The longer the catalyst residence time, the more reactions occur per unit of catalyst, which leads to a decrease in the average activity of the catalyst and consequently a reduction in the conversion rate of the feed oil ； The effect of reaction pressure is relatively small. ④ Currently, risers are commonly used as reaction vessels in the catalytic cracking units of refineries. The length of the riser has an impact on the cracking process: the longer the riser, the more intense the secondary reactions become, resulting in higher yields of gas and coke. Furthermore, the performance of the feed oil atomization nozzle and the cyclone separator also has a certain impact on the distribution of the cracking products. Catalytic cracking is a process in which petroleum hydrocarbons are subjected to high-temperature cracking in the presence of a catalyst to produce low-carbon olefins such as ethylene, propylene, and butenes, while also generating light aromatic hydrocarbons. Due to the presence of a catalyst, catalytic cracking can reduce the reaction temperature, increase the yield of low-carbon olefins and light aromatic hydrocarbons, and enhance the flexibility of the cracking product distribution. (1) General characteristics of catalytic cracking: ① Catalytic cracking is the result of both the carbocation reaction mechanism and the free radical reaction mechanism; the proportion of ethylene in the gas products resulting from catalytic cracking is higher than that in the gas products of conventional catalytic cracking. ② To a certain extent, catalytic cracking can be regarded as high-deep catalytic cracking, with a much higher gas yield than conventional catalytic cracking, and a high content of aromatics in the liquid products. ③ The reaction temperature in catalytic cracking is very high; gas products with larger molecular weights undergo secondary cracking reactions. Additionally, low-carbon olefins undergo hydrogen transfer reactions to form alkanes, and they may also undergo polymerization or aromatization reactions to produce gasoline and diesel. (2) Reaction mechanism of catalytic cracking. Generally, the catalytic cracking process involves both catalytic cracking reactions and thermal cracking reactions; it is the result of the combined action of two reaction mechanisms: carbocationic and radicalic mechanisms. However, the specific cracking reaction mechanism varies depending on the catalyst used and the cracking process employed. During high-temperature pyrolysis on Ca-Al series catalysts, a radical reaction mechanism dominates ； During low-temperature cracking on acidic zeolite molecular sieve cracking catalysts, the carbocation reaction mechanism plays a dominant role ； In the medium-temperature cracking process on zeolite catalysts with dual acidic centers, both the cationic mechanism and the radical mechanism play important roles. (3) Factors affecting catalytic cracking Similar to catalytic cracking, the factors that influence catalytic cracking mainly include the following four aspects: feedstock composition, catalyst properties, operating conditions, and reaction equipment. ① Impact of crude oil properties. Generally speaking, the higher the H/C ratio and characteristic factor K of the feed oil, the higher the content of saturated components and the lower the BMCI value; consequently, the yield of low-carbon olefins (ethylene, propylene, butenes, etc.) obtained through cracking is higher ； The higher the char value of the raw material, as well as the levels of sulfur, nitrogen, and heavy metals, the lower the yield of low-carbon olefins. When hydrocarbons of various types are used as feedstocks for cracking, the order of low-carbon olefin yields is generally: alkanes > cycloalkanes > isoparaffins > aromatics. ② Properties of the catalyst. Catalytic cracking catalysts are divided into two types: metal oxide-type cracking catalysts and zeolite molecular sieve-type cracking catalysts. Catalysts are an important factor affecting the product distribution in catalytic cracking processes. The cracking catalyst should possess high activity and selectivity, ensuring the production of a large amount of low-carbon olefins during the cracking process while minimizing the yields of hydrogen, methane, and liquid products; it should also have high stability and mechanical strength. For zeolite molecular sieve-type cracking catalysts, the pore structure, acidity, and particle size of the molecular sieve are the three most important factors affecting catalytic activity ； For metal oxide-type pyrolysis catalysts, the active component, carrier, and additives of the catalyst are the most important factors affecting its catalytic activity. ③ Effect of operating conditions. The effect of operating conditions on catalytic cracking is similar to their effect on fluid catalytic cracking. The better the atomization and vaporization effects of the feedstock, the higher the conversion rate of the feedstock oil, and the higher the yield of low-carbon olefins as well ； The higher the reaction temperature and the greater the reagent-to-oil ratio, the higher the conversion rate of the feed oil and the yield of low-carbon olefins, but the yield of coke also increases ； Due to the high reaction temperature in catalytic cracking, to prevent excessive secondary reactions, the residence time of the oil and gas should not be too long ； The effect of reaction pressure is relatively small. Theoretically, catalytic cracking should operate at high temperatures, with short residence times, large amounts of steam, and a high oil-to-vapor ratio in order to achieve the highest yield of low-carbon olefins. ④ The reactor is an important factor affecting the distribution of catalytic cracking products. The main types of reactors include fixed-bed, moving-bed, fluidized-bed, riser, and downward-conveying bed reactors. For the CPP process, using a pure riser reactor is beneficial for higher ethylene production, while using a riser combined with a fluidized bed reactor is advantageous for higher propylene production. (4) Introduction to catalytic cracking processes. Research on the catalytic cracking of hydrocarbons has a history of half a century; its scope of study includes light hydrocarbons, distillates, and heavy oils, and various cracking processes have been developed. A brief introduction to these is provided below. ① Catalytic cracking process (DCC process). This process was developed by the Sinopec Research Institute of Petrochemical Technology. It uses heavy oil as raw material, and solid acid selective molecular sieve catalysts to carry out cracking reactions under relatively mild conditions, thereby producing low-carbon olefins or isomerized olefins as well as high-octane gasoline. This process draws on fluidized catalytic cracking technology, utilizing catalyst fluidization, continuous reaction, and regeneration techniques, and has been brought to industrial application. The DCC process has two operating modes—DCC-Ⅰ and DCC-Ⅱ. DCC-Ⅰ operates under relatively stringent conditions, carrying out the reaction in a riser-type dense-phase fluidized bed reactor to produce large quantities of gaseous olefins primarily composed of propylene ； DCC-Ⅱ operates under milder conditions in a riser reactor to maximize the production of small molecular olefins such as propylene, isobutylene, and isopentene, while simultaneously producing high-octane premium gasoline. ② Catalytic thermal cracking process (CPP process). This process is a patented technology developed by the Sinopec Research Institute of Petrochemical Science for the production of ethylene and propylene. Based on traditional catalytic cracking technology, it uses heavy oils such as wax oil, wax oil blended with residue oil, or atmospheric residue oil as raw materials. A riser reactor, specially developed catalysts, and a continuous reaction-regeneration cycle system for the fluidized transport of catalysts are employed to produce ethylene and propylene under milder operating conditions compared to steam cracking. The CPP process was developed on the basis of the catalytic cracking DCC process, and its key technology lies in further improving the process and catalysts to change the target product from propylene to ethylene and propylene. ③ Process for producing ethylene by direct cracking of heavy oil (HCC process). This process was developed by the Refining Research Institute of Luoyang Petrochemical Engineering Company; it is a catalytic cracking process that produces ethylene directly from heavy oil, while also generating propylene, butenes, and light aromatics. It draws on mature heavy oil catalytic cracking processes, employs fluidized \"reaction-regeneration\" technology, and uses riser reactors or downflow reactors to meet the requirements of high temperature and short contact time in the process. ④ Other catalytic cracking processes. Such as the catalytic-steam thermal cracking process (with reaction temperatures typically very high, around 800°C), the THR process (a catalytic conversion and cracking process for heavy oils developed by the Japanese company Toyo Engineering Corporation), and rapid cracking technology (an olefin production process via catalytic cracking developed jointly by Stone & Webster Company and Chevron Company). ⑤ The pyrolysis efficiency of paraffin-based feedstocks is better than that of naphthenic-based feedstocks. Therefore, the vast majority of catalytic cracking processes use paraffinic distillate oils or heavy oils as cracking feedstocks. For naphthenic feedstocks, particularly the distillate oils and hydrogenated distillate oils obtained from Canadian oil sands asphalt, Professor Shen Baojian from the Key Laboratory of Heavy Oil has developed specialized cracking catalysts. Preliminary evaluation results show that the total yield of ethylene and propylene is close to 30 wt%. (5) Differences between catalytic cracking and catalytic pyrolysis To some extent, catalytic pyrolysis evolved from catalytic cracking, but there are clear differences between the two, as follows: ① Different objectives. Catalytic cracking is aimed at producing light hydrocarbons such as gasoline, kerosene, and diesel, whereas catalytic pyrolysis is intended to produce basic chemical feedstocks such as ethylene, propylene, butylenes, and butadiene. ② The raw materials are different. The feedstocks for catalytic cracking are generally vacuum distillate oil, coker wax oil, atmospheric residue, and a mixture of vacuum distillate oil and vacuum residue ； The range of feedstocks for catalytic cracking is relatively wide; it can include materials used in catalytic cracking, as well as naphtha, diesel, and light hydrocarbons such as C4 and C5. ③ The catalysts are different. Catalysts for catalytic cracking are generally zeolite molecular sieve catalysts and aluminum silicate catalysts, whereas catalysts for catalytic pyrolysis are generally zeolite molecular sieve catalysts and metal oxide catalysts. ④ The operating conditions are different. Compared to catalytic cracking, catalytic pyrolysis operates at higher reaction temperatures, requires a larger oil-to-catalyst ratio, uses more steam, has a shorter residence time for the oil and gas, and suffers from more severe secondary reactions. ⑤ The reaction mechanisms are different. The reaction mechanism of catalytic cracking is generally considered to be the carbocation mechanism, whereas the reaction mechanism of catalytic pyrolysis includes both the carbocation mechanism and the free radical mechanism. Catalytic cracking process (DCC-Ⅰ, DCC-Ⅱ) The catalytic cracking process (DCC) is a new technology that uses heavy oil as raw material to produce gaseous olefins through selective catalytic reactions. Among them, catalytic cracking type I (DCC-Ⅰ) aims primarily at producing the maximum amount of propylene, while catalytic cracking type II (DCC-Ⅱ) aims at producing the maximum amount of isobutylene and isopentylene, along with propylene and high-octane high-quality gasoline. The raw materials they process can be wax oil, wax oil mixed with residue oil or reprocessed oil, as well as atmospheric residue oil; this enables the extension of refining processes into the petrochemical industry and opens up a new pathway for directly producing low-carbon olefins from heavy oils, reaching international advanced levels. 　　Due to the different target products, DCC-Ⅰ and DCC-Ⅱ employ different reactor types, catalyst types, and process operating conditions; the differences are listed in Table 1. As can be seen from Table 1, the reaction time, reaction temperature, oil-to-agent ratio, and water injection volume of DCC-Ⅱ are all lower than those of DCC-Ⅰ. 　　Catalytic cracking utilizes the principle of selective catalytic reactions to selectively crack heavy feedstock oils into low-carbon gaseous olefins, with an propylene yield that is more than three times that of conventional FCC. The yields of isobutylene and isopentylene also exceeded 3 times those of FCC. The catalytic cracking process has opened up a new route for producing low-carbon hydrocarbons. 　　The DCC-Ⅰ process has been industrially applied at Jinan Refinery, Anqing Petrochemical Complex, and Daqing Oilfield Chemical Additives Factory. 　　The DCC-Ⅱ process has been industrially applied at Jinan Refinery, Jingmen Petrochemical Complex, Shenyang Paraffin Chemical Complex, as well as at Thailand’s TPI Company. 　　The DCC-Ⅰ and DCC-Ⅱ process technologies have been granted several **patents both domestically and internationally. These two technologies have been adopted by numerous refineries in China, with some plants already in operation, under design, or under construction. Foreign companies are also very interested in these technologies; the DCC-Ⅰ technology has been exported to Thailand, and there is active discussion regarding other transfer arrangements as well. The American company Stone＆Webster Engineering has become the exclusive agent for these two technologies abroad, indicating that catalytic cracking technology holds broad application prospects both domestically and internationally, and is expected to play an important role as the oil refining industry evolves into the petrochemical sector. Table 1: Main differences between DCC-Ⅰ and DCC-Ⅱ processes　　　　　　　　　　　　　 　　　　　　　 DCC-Ⅰ　　　　DCC-Ⅱ Reactor type 　 Lift tube + bed 　 Lift tube Catalyst　　　　 CRP　　　　　CIP Reaction temperature, °C　 540-580　　　 500-530 Oil-to-catalyst ratio　　　　 9-15　　　　　6-9 Water injection rate, %m　　15-25　　　　6-10 Product distribution, %m H2–C2　　　　　11.91　　　　5.59 C3–C4　　　　　42.22　　　　34.49 C5+ gasoline　　 　　26.60　　　　39.00 Diesel　　　　　　6.60　　　　　9.77 Heavy oil　　　　　6.07　　　　　5.84 Coke　　　　　　6.00　　　　　4.31 Losses　　　　　0.60　　　　　1.00 Total　　　　　　100.00　　　　100.00 Propylene　　　　　21.03　　　　 14.29 Total butenes　　　14.03　　　　14.65 Isobutylene　　　5.13　　　　　6.13 Total pentenes　　　--　　　　　　9.77 Isopentylene　　　--　　　　　　6.77 Isobutylene/total butenes 0.36　　　　　0.42 Isopentylene/total pentenes --　　　　　0.69 Gasoline properties RONC　　　　　99.3　　　　　96.4 MONC　　　　　84.7　　　　　82.5