Catalytic cracking is a process in which high-temperature cracking of petroleum hydrocarbons is carried out in the presence of a catalyst to produce low-carbon olefins such as ethylene, propylene, butylene, and at the same time, producing light aromatic hydrocarbons. Due to the presence of the catalyst, catalytic cracking can lower the reaction temperature, increase the yield of light olefins and the yield of light aromatic hydrocarbons, and increase the flexibility of the distribution of the cracked product.
1. Catalytic cracking is the result of the interaction between the carbocation reaction mechanism and the free radical reaction mechanism. The proportion of ethylene in the cracked gas product is greater than the proportion of ethylene in the catalytic cracking gas product.
2. To a certain extent, catalytic cracking can be regarded as high-level catalytic cracking, the gas yield is much greater than catalytic cracking, and the aromatic content of liquid products is high.
3. The reaction temperature of catalytic cracking is very high, and the gas product with larger molecular weight will undergo secondary cracking reaction. In addition, the low-carbon olefin will undergo hydrogen transfer reaction to form alkanes, and polymerization or aromatization reaction will also occur to form gasoline and diesel.
In general, the catalytic cracking process takes place both in the catalytic cracking reaction and in the thermal cracking reaction. It is the result of the reaction mechanism of carbon cation and free radical, but the specific cracking reaction mechanism varies with the catalyst and the cracking process. And there are differences.
In the pyrolysis process on Ca-Al series catalysts, the free radical reaction mechanism dominates; in the low temperature cracking process on the acidic zeolite molecular sieve cracking catalyst, the carbon cation reaction mechanism predominates; while in the double acidic center Both the carbocation mechanism and the free radical mechanism play an important role in the mid-temperature cracking process on zeolite catalysts.
Post time: 2019-10-28