定义Definition
裂化是在加热或催化剂作用下，把相对分子质量大、沸点高的烃断裂成相对分子质量小、沸点低的烃的过程。它是炼油厂提高汽油、柴油产量和生产乙烯、丙烯等基础化工原料的关键工艺。温度更高、以生产烯烃为目的的过程通常称为裂解。
Cracking breaks large, high-boiling hydrocarbon molecules into smaller, lower-boiling ones by heat or with a catalyst. It is key to raising refinery yields of gasoline and diesel and to producing basic chemicals such as ethylene and propylene; the higher-temperature, olefin-oriented version is usually called steam cracking or pyrolysis.
机理Mechanism
热裂化按自由基机理进行：C–C 键均裂生成自由基，经 β-断裂、夺氢等链式步骤生成小分子烷烃和烯烃。催化裂化在酸性分子筛（Y 型、ZSM-5）上按碳正离子机理进行：烃被质子化生成碳正离子，经异构化和 β-断裂得到更多支链烷烃和芳烃，因此汽油辛烷值更高。
Thermal cracking runs by a free-radical mechanism: C–C bonds break homolytically, and chain steps such as β-scission and hydrogen abstraction give smaller alkanes and alkenes. Catalytic cracking on acidic zeolites (Y and ZSM-5) follows a carbocation mechanism: protonation gives carbocations that isomerise and undergo β-scission, producing more branched alkanes and aromatics and hence higher-octane gasoline.
发展Development
1891 年舒霍夫在俄国获得热裂化专利；1913 年美国伯顿实现工业化。1937 年胡德利用黏土催化剂实现固定床催化裂化，1942 年流化床催化裂化问世，催化剂在反应器和再生器之间循环烧焦再生。1960 年代莫比尔公司用沸石催化剂取代无定形硅铝，大幅提高了活性和选择性。
Shukhov patented thermal cracking in Russia in 1891, and Burton commercialised it in the United States in 1913. Houdry's fixed-bed catalytic cracking over clay catalysts followed in 1937, and fluid catalytic cracking in 1942, with the catalyst circulating between reactor and regenerator where its coke is burnt off. In the 1960s Mobil replaced amorphous silica–alumina with zeolite catalysts, greatly raising activity and selectivity.
主要类型Main types
催化裂化（FCC，约 500 °C）主产汽油；加氢裂化在氢气和双功能催化剂下进行，主产优质柴油和航煤；蒸汽裂解（750–900 °C，加入水蒸气降低烃分压、停留时间不到 1 秒）主产乙烯和丙烯；延迟焦化把渣油转化为轻油和石油焦。
Fluid catalytic cracking (about 500 °C) mainly makes gasoline; hydrocracking, under hydrogen with bifunctional catalysts, gives high-quality diesel and jet fuel; steam cracking (750–900 °C, with steam to lower the hydrocarbon partial pressure and residence times under a second) mainly makes ethylene and propylene; delayed coking converts residue into light oils and petroleum coke.
反应Reactions
热裂化 450–750 °C；催化裂化约 500 °C，酸性分子筛；蒸汽裂解 750–900 °C。
Thermal cracking 450–750 °C; catalytic cracking about 500 °C over acidic zeolites; steam cracking 750–900 °C.
引发：C–C 键断裂（热裂化生成自由基；催化裂化生成碳正离子）。
Initiation: C–C bonds break, giving radicals (thermal) or carbocations (catalytic).
增长：活性中间体经 β-断裂生成烯烃和更小的活性中间体，并从其他分子夺取氢或氢负离子。
Propagation: β-scission gives an alkene and a smaller intermediate, which abstracts H or hydride from another molecule.
终止：自由基偶合或碳正离子失去质子。
Termination: radicals combine or carbocations lose a proton.
癸烷裂化为辛烷和乙烯
Decane cracks to octane and ethylene
乙烷蒸汽裂解制乙烯
Steam cracking of ethane to ethylene
年表Timeline
- 1891
舒霍夫获热裂化专利
Shukhov patents thermal cracking
- 1913
伯顿热裂化工业化
Burton's process is commercialised
- 1937
胡德利催化裂化投产
Houdry catalytic cracking starts up
- 1942
流化床催化裂化问世
Fluid catalytic cracking is introduced
- 1964
沸石裂化催化剂开始应用
Zeolite cracking catalysts come into use