Mechanism of propane cracking reactionCH3-CH2-CH3 →CH3 + CH2-CHChain initiationCH2-CH3→CH-CH2+H 21CH3-CH-CH3 + CH4Chain propagation(3) CH3 + CH3-CH2-CH3CH2-CH2-CH, + CHHydrogen abstractionreactionsCH3-CH-CH3 + H2(4) H·+CH3-CH2-CH3CH2-CH2-CH3 + H2Decomposition reactions(5) CH-CH-CH3 →CH3-CH=CH2 + H.(6) CH2-CH2-CH3 → CH2=CH2 + CH3Chain termination(8) 2CH3 —→C2H6(9) CH3 +CgH7→C4H10(10) C3H+C3H7Etc.C6H141
Chain initiation Chain propagation Chain termination (1) CH3-CH2-CH3 CH3 + CH2-CH3 (2) CH2-CH3 CH2 =CH2 + H (3) CH3 + CH3-CH2-CH3 CH3 -CH-CH3 + CH4 CH2 -CH2 -CH3 + CH4 (4) H + CH3 -CH2 -CH3 CH3-CH-CH3 + H2 CH2-CH2-CH3 + H2 (5) CH3 -CH-CH3 CH3 -CH=CH2 + H (6) CH2 -CH2 -CH3 CH2 =CH2 + CH3 (8) 2CH3 C2H6 (9) CH3 + C3H7 C4H10 (10) C3H7 + C3H7 C6H14 Mechanism of propane cracking reaction Etc. Hydrogen abstraction reactions Decomposition reactions
Therules offree radical decompositionCH2-CH,CH3→ CH2-CH2+CH1CH2-CH2CH3X→ CH2=CHCH3 +H.1Activation energy: (1) < (2)CH3-CHCH3(3CH=CHCH3 +HX→ CH,=CH, +CH(4)CH3-CHCH3 -③Thefree-radicalswithmorecarbonatomsthanC3producedfrom decomposition andhydrogen abstractionreactions canbecontinuously decomposed to generate the alkenes with less carbonatoms
The rules of free radical decomposition： ① Activation energy：(1) < (2) ② ③ The free-radicals with more carbon atoms than C3 produced from decomposition and hydrogen abstraction reactions can be continuously decomposed to generate the alkenes with less carbon atoms. CH3 -CHCH3 CH2 =CHCH3 + H (3) CH3-CHCH3 CH2 =CH2 + CH3 (4) CH2 -CH2CH3 CH2 =CH2 + CH3 (1) CH2-CH2CH3 CH2 =CHCH3 + H (2)
2.Primary reactions and secondary reactionsThe primary reactions are the cracking reactionsproduced firstly in the cracking process of the rawmaterial hydrocarbons, and to produce ethene andpropene.The secondary reactions are the subseguentreactions produced by primary reaction products
⚫ The primary reactions are the cracking reactions produced firstly in the cracking process of the raw material hydrocarbons, and to produce ethene and propene. ⚫ The secondary reactions are the subsequent reactions produced by primary reaction products. 2. Primary reactions and secondary reactions
Chemical process of secondary reactionsSecondary cracking of alkenesHydrogenationC=C+H → C-CCarbonC=C台C=C+H, -Dehydrogenationformation2C=C-C-→C=C-C-C+C=CDisproportionation2C=C→C=C-C=C+H,Polymerization+ 2HC=C+C=C-C=C-CyclizationCokingAromaticcondensation+H(oo)
Chemical process of secondary reactions Secondary cracking of alkenes Aromatic condensation 2 +H2 C = C C C + H2 2C=C−C→C=C−C−C+C=C C C C C C H2 2C = → = − = + C=C+C=C−C=C→ + 2H2 脱氢 岐化 聚合 环化 生碳 结焦 Hydrogenation 加氢 C = C + H2 C C Dehydrogenation Disproportionation Polymerization Cyclization Carbon formation Coking
DehydrogenationC.H16CH14ΛMWPrimarycrackingCrackedproductsCH+CHgC.HC.HeAΛSecondaryCycloadditionscracking?anddehydrogenationsCH + C2H42CHCokeExample of reaction occurring during thermal crackingofhigheralkanes
Example of reaction occurring during thermal cracking of higher alkanes