Chapter 3Thermal Cracking of Hydrocarbons
Thermal Cracking of Hydrocarbons Chapter 3
Thermal Cracking of HydrocarbonsRawmaterials of petroleumhvdrocarbons areheated athightemperatures (750-900C), so that hydrocarbon moleculesproceed the dehydrogenation or chain scission reactions togenerate small molecular weight alkenes, alkanes and otherhydrocarbons with different molecular weight.This process is called thermal cracking of hydrocarbons.Rawmaterials:Natural gas, Refinery gas, Light oil, Diesel, Heavy oil, etc.Lower molecular alkanes:Ethane,PropaneMainproducts:EthenePropene、ButadieneBenzene,Toluene,Xylenes
Raw materials： Natural gas, Refinery gas, Light oil, Diesel, Heavy oil, etc. Lower molecular alkanes: Ethane, Propane Main products： Ethene、Propene、Butadiene Benzene、Toluene、Xylenes Thermal Cracking of Hydrocarbons Raw materials of petroleum hydrocarbons are heated at high temperatures (750-900℃), so that hydrocarbon molecules proceed the dehydrogenation or chain scission reactions to generate small molecular weight alkenes, alkanes and other hydrocarbons with different molecular weight. This process is called thermal cracking of hydrocarbons
3.1 Chemical reactions of thermal cracking process-,Reactionrulesof thermal cracking ofhydrocarbons>The thermal cracking process of hydrocarbons is verycomplex,andproducts arealsovery complex evenif purecomponentis cracked.>Thermal cracking of hydrocarbons contains the reactions ofdehydrogenation, chain scission,isomerization, cyclization,disproportionation,polymerization,coking,diene synthesisetc.1.Cracking reaction ofalkanesThe cracking reactions of normalalkanemainlycontain the dehydrogenationreactionand the chain scissionreaction.The dehydrocyclizationmay occurforthe alkaneswithcarbonatomnumber≥C5
3.1 Chemical reactions of thermal cracking process 一、Reaction rules of thermal cracking of hydrocarbons 1. Cracking reaction of alkanes The cracking reactions of normal alkane mainly contain the dehydrogenation reaction and the chain scission reaction. The dehydrocyclization may occur for the alkanes with carbon atom number ≥ C5. ➢The thermal cracking process of hydrocarbons is very complex, and products are also very complex even if pure component is cracked. ➢Thermal cracking of hydrocarbons contains the reactions of dehydrogenation, chain scission, isomerization, cyclization, disproportionation, polymerization, coking, diene synthesis, etc
Dehydrogenationreaction :C,H2n+2C,H2n+H2(CleavageofCHbond)Chain scission reaction :C,H2n+2CmH2m+ C,H2k+2m+k=nDehydrocyclization reactionCH3CH2CH3CH2CH3+H2CH2CH2CH2CH2CH2CH2
Dehydrogenation reaction ： CnH2n+2 CnH2n+H2 （Cleavage of C—H bond ） Chain scission reaction ： CnH2n+2 CmH2m+ CkH2k+2 m＋k=n Dehydrocyclization reaction
Comparison of various bond energy键能/键能/碳氢键碳碳键(kJ /mol)(k//mol)426.8CH3--CH3346HC--H405.8CH3—CH2--CH3343.1CH,CH2--H397.5CHCH2---CH2CH3338.9CH3CH2CH2--HCH,CH2CH2—CH3(CH)2CH—H384.9341.8CH3CHaCH2CH2CH2--H393.2CH3CH2CH--HHCC-CH3376.6314.6CH3CH3364(CH)3C—H325.1CH,CH2CH2--CH2CH2CH3378.7CH3CH(CH3)CH(CH)CH310.9CH (般)
1.各种键能比较 Comparison of various bond energy
Cracking rules of normal alkanes(1)Thebond energy of C-H bond is higher thanthat of C-Cbond forthealkanewith samecarbon atomnumber.So the chain scissionis easierthanthe dehydrogenation.(2)Thelongerthe carbonchainis,theeasierthecrackingreactionsare.(3)Thecrackingreactionofalkaneisstrongendothermicreaction,andtheheatvalueofdehydrogenationreactionishigherthanthatofthechainscissionreaction.butthe(4)Thechainscissionreactionisirreversibleprocess,dehydrogenationreactionis reversible process.(5)ThescissionofC-Cbondattheterminalpositionofthemoleculeiseasierthanthatatthecentralpositionofthemolecule(6)Theethanecannotproducethechainscissionreaction,onlyproducedehydrogenationreactionto form ethene,whereas methaneis notchangedatgeneralcrackingtemperature
(1) The bond energy of C-H bond is higher than that of C-C bond for the alkane with same carbon atom number. So the chain scission is easier than the dehydrogenation. (2) The longer the carbon chain is, the easier the cracking reactions are. (3) The cracking reaction of alkane is strong endothermic reaction, and the heat value of dehydrogenation reaction is higher than that of the chain scission reaction. (4) The chain scission reaction is irreversible process, but the dehydrogenation reaction is reversible process. (5) The scission of C-C bond at the terminal position of the molecule is easier than that at the central position of the molecule. (6) The ethane can not produce the chain scission reaction, only produce dehydrogenation reaction to form ethene, whereas methane is not changed at general cracking temperature. Cracking rules of normal alkanes
Cracking rules of isoalkanes(1) Thecracking reactions ofthe isoalkane are easierthan thatof the normalalkane.(2)Thedehydrogenation abilityis related to the molecular structure,and thesequenceistertiaryhydrogen>secondaryhydrogen>primaryhydrogen伯HPrimaryhydrogenHCSecondary hydrogen仲HTertiaryhydrogen叔HCCHHH
（1）The cracking reactions of the isoalkane are easier than that of the normal alkane. （2）The dehydrogenation ability is related to the molecular structure, and the sequence is tertiary hydrogen > secondary hydrogen > primary hydrogen. Cracking rules of isoalkanes C C H C C C C C C 键的可能断裂方式 H H C H H C H H 叔H 仲H Primary hydrogen 伯H Secondary hydrogen Tertiary hydrogen
(3)Theyieldofethene,propeneislowerbytheisoalkane cracking than thatby normal alkane cracking,while theyield of hydrogen, methane, C4 and aboveC4alkenesishigher.(4)Theyield difference of the ethene andpropene obtained bythe normal alkane andisoalkane cracking decreases withincreasing carbon atomnumber
（3）The yield of ethene, propene is lower by the isoalkane cracking than that by normal alkane cracking, while the yield of hydrogen, methane, C4 and above C4 alkenes is higher. （4）The yield difference of the ethene and propene obtained by the normal alkane and isoalkane cracking decreases with increasing carbon atom number
2.Crackingreactionsofalkenes(1)ChainscissionreactionsThe chain scission of the alkene mainly occurs in theβ C-Cbond,because the dissociation energy ofβ C-C bond is lowerthan that of a C-C bond.Cn + m H2(n + m)-→ C, H2n +Cm H2m3例如 CH2-CHCH2--CH2CH3-—-→-CH2--CH--CH3 + CH2CH(2)Dehydrogenationreactions-→ C4H6 + H204H&-C2H4 → CHz + H2
2. Cracking reactions of alkenes (1) Chain scission reactions The chain scission of the alkene mainly occurs in the β C-C bond, because the dissociation energy of β C-C bond is lower than that of α C-C bond. (2)脱氢反应(dehydrogenation reaction) (2) Dehydrogenation reactions
(3)Disproportionationreactions2C3H6→ C2H4 + C4Hg2C3H6 → CHe + C4H2C,H6 -→ CsH + CH4(4)Dienesynthesisreactions(Diels-Alderreactions)2HCH2CH2CHCH2CH32H21+CH2CHCH2CH2CH2CH2
(3) Disproportionation reactions (4) Diene synthesis reactions (Diels-Alder reactions)