Catalytic cracking and normal carbocation reaction mechanisms

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The catalytic cracking and normal carbocation reaction mechanisms: 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 yielding 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 radical reaction mechanism; the proportion of ethylene in the gas products resulting from catalytic cracking is higher than that in the gas products of catalytic reforming. 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 speaking, 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, namely carbocationic and radical mechanisms. However, the specific cracking reaction mechanism varies depending on the catalyst used and the cracking process employed. During high-temperature cracking on Ca-Al series catalysts, a radical reaction mechanism plays a dominant role ； 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) The influencing factors of catalytic cracking are similar to those of catalytic cracking itself; the main factors affecting catalytic cracking 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 feedstock oil, the greater the content of saturates and the lower the BMCI value; consequently, the yield of light olefins (such as ethylene, propylene, and butene) obtained through cracking is also 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 yield of low-carbon olefins 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; it must ensure the production of a large amount of lower olefins during the cracking process, while minimizing the yields of hydrogen, methane, and liquid products. Additionally, it should exhibit 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 Effects 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 gasification effects of the feedstock, the higher the conversion rate of the feedstock oil, and the higher the yield of low-carbon olefins ； The higher the reaction temperature and the greater the oil-to-reagent 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 employ high temperatures, 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 increasing ethylene production, while using a riser combined with a fluidized bed reactor is advantageous for increasing propylene production. (4) Introduction to catalytic cracking processes. Research on the catalytic cracking of hydrocarbons has a history of half a century. The research scope encompasses light hydrocarbons, distillates, and heavy oils; various cracking processes have been developed. Below is a brief introduction to them. ① Catalytic cracking process (DCC process). This process was developed by the Research Institute of Petroleum Processing under Sinopec. It utilizes heavy oil as a feedstock and solid acid shape-selective molecular sieve catalysts to carry out cracking reactions under relatively mild reaction conditions, thereby producing low-carbon olefins or iso-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 employs relatively mild operating conditions and carries out the reaction in a riser reactor, thereby maximizing the production of small-molecule olefins such as propylene, isobutylene, and isopentene, while also producing high-octane, premium-quality gasoline. ② Catalytic thermal cracking process (CPP process). This process is a patented technology developed by the Research Institute of Petroleum Processing under Sinopec for the production of ethylene and propylene. Based on conventional catalytic cracking technology, it uses heavy oils such as wax oil, mixtures of wax oil and residue oil, or atmospheric residue oil as raw materials. Utilizing a riser reactor, specially developed catalysts, and a continuous reaction-regeneration cycle involving fluidized catalyst transport, this process produces ethylene and propylene under milder operating conditions than those used in steam cracking. The CPP process was developed based on the catalytic cracking DCC process. Its key technology involves further improvements to the process and catalyst, thereby changing the target products from propylene to ethylene and propylene. ③ Process for producing ethylene via 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 cracking effect of paraffin-based feedstocks is better than that of naphthenic-based feedstocks. Therefore, the vast majority of catalytic cracking processes use paraffin-based 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) The difference 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, butenes, 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, while 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 radical mechanism.