Thermodynamics And Kinetics Of Radical Dehydrogenation From Polycyclic Aromatic Hydrocarbons And Heterocyclic Aromatic Hydrocarbons

To better understand the condensation during the thermal processing of heavy oil,the formation of aromatic carbon radicals by the H-abstraction of methyl radicals(CH3)or ethyl radicals(·C2H5)from polycyclic aromatic hydrocarbons(PAHs)or heterocyclic aromatic hydrocarbons(hetero-PAHs)was studied by density functional calculation.The calculation involved 53 different sites of 1～7 rings PAHs and hetero-PAHs.The H-abstraction from different PAHs all has a positive standard state Gibbs free energy change.Only by simultaneously increasing reaction temperature and alkyl radical concentration can the reaction become a spontaneous process.Limited by thermodynamics and reaction kinetics,CH3 instead of ·C2H5 contributes to the formation of aromatic carbon radicals.The particularity of the H-abstraction from hetero-PAHs is mainly reflected at the α-site of heterocycles.The H-ion from N-containing hetero-PAHs occurs preferentially at the α-site of heterocycles,and both ·CH3 and ·C2H5 could participate in abstracting the α-H.The H-abstraction from S-containing hetero-PAHs occurs on the coupled aromatic rings with priority,showing the same reaction behavior with the H-abstraction on PAHs.A simulation model for the pyrolysis of heavy oil was established based on the ReaxFF molecular dynamics,and the dynamic pyrolysis process and the generation of small molecular free radicals were investigated using Lammps software at 1500～3500 K,and then the structural information of pyrolysis products was identified.The results show that the temperature has an effect on the molecular species distribution of thermal cracking products,and the change of temperature above 2500 K has little effect on the molecular species.The yield of H2 and hydrocarbon C1～5 increased with the increase of temperature,while the yield of C6～20 products decreased.The hydrogen abstraction from PAHs or hetero-PAHs is the first step in the process of condensation coking,and the β-scission of alkyl side chain to produce olefins is also an important part of the thermal cracking condensation.