关键词: 碳氢化合物, 分子结构, 热解, 焦化, 热沉

Abstract:

The fuel molecular structure is an important factor affecting the pyrolysis coking process in the active cooling channel. Stripping the effect of turbulent diffusion on pyrolysis coking, the pyrolysis coking patterns of typical aviation kerosene components such as n-heptane (nC₇H₁₆), iso-heptane (iC₇H₁₆), methylcyclohexane (MCH), n-dodecane (nC₁₂H₂₆), and toluene (A₁CH₃) were compared in a pre-oxidized STS304 (Φ3.0 mm×0.5 mm×1.0 m, outer diameter × wall thickness × length) flow reactor tube at 600—800℃, 1.0 MPa, and under a laminar near-chemical kinetic state. The normalized molar fraction distributions of pyrolysis gas-liquid products of the five typical fuels were quantitatively obtained, and the unsteady-state coking characteristics and along-range coking distribution patterns of the fuels were compared by weighing method. The results show that the conversion of chain alkanes is higher at the lower temperature of 650℃, the conversion of MCH as well as A₁CH₃ is lower due to the stabilized cyclic structure, and the conversion of A₁CH₃ increases abruptly above 775℃. The hydrogen (H₂), methane (CH₄) content of iC₇H₁₆ and MCH is significantly higher due to the presence of branched methyl groups. Straight-chain alkanes have higher contents of ethylene (C₂H₄) and ethane (C₂H₆) through β-scission. Propylene (C₃H₆) is a by-product of C—C bond breakage. The content of C₃H₆ is higher at lower temperatures, while the content of C₂H₄ and C₃H₆ tends to decrease due to the secondary reaction of coking at high temperatures. The main products in the liquid phase included benzene, A₁CH₃, naphthalene and other diphenyl aromatic hydrocarbons. The content of polycyclic aromatic hydrocarbons (PAHs) increased at high temperatures, and tetraphenyl aromatic hydrocarbons such as perylene, phenanthrene, and anthracene were detected by GC-MS, but they accounted for a very low molar fraction of the bulk phase. The coking rates of MCH and nC₁₂H₂₆ were low at lower temperatures, and the coking rates of MCH and A₁CH₃ increased significantly at higher temperatures. Branched and long straight-chain alkanes favor high-temperature coking inhibition. Straight-chain alkane coking peaks at the beginning of the thermostatic zone, with coking by addition of unsaturated light hydrocarbons. MCH and A₁CH₃ coking peaks are closer downstream, with coking by deposition of aromatic polymerization. Straight-chain alkanes have the highest content of cracked C₂H₄ and C₃H₆, which is conducive to providing high heat sink. The cleavage of nC₁₂H₂₆ firstly produces 1-olefins such as 1-dodecene, 1-undecene, 1-decene, 1-octene, 1-hexene, 1-pentene etc., and the deep cleavage of these 1-olefins produces C₂H₄ and C₃H₆etc. Since the depth of pyrolysis of long carbon-chain nC₁₂H₂₆ is lower than that of the short-chain nC₇H₁₆, it exhibits the heat sink that is even lower at high temperatures. Branched-chain alkanes are more stable than straight-chain alkanes and have a weaker tendency to form carbon deposition, but the presence of methyl group makes the product alkylated to a higher degree. The coking rates of the five typical hydrocarbon fuels were closely related to the temperature, showing a three-stage unsteady-state growth pattern, with differences in the location of the peak coking along the process. The coking rate of straight-chain alkanes was larger at low temperatures, and the contribution of MCH and A₁CH₃ coking was significantly higher at high temperatures.

Key words: hydrocarbons, molecular structure, pyrolysis, coking, heat sink