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Theoretical study of naphthenic hydrocarbon pyrolysis kinetics properties and heat sink

Author: ChenLong
Tutor: WangWenLiang
School: Shaanxi Normal University
Course: Physical and chemical
Keywords: methylcyclohexane ReaxGen pyrolysis mechanism rate constant productdistribution heat sink
CLC: O624.1
Type: Master's thesis
Year: 2013
Downloads: 43
Quote: 0
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Abstract


The aerocraft’s cooling problem has already become more and more important along with the hypersonic increase. Hydrocarbon compounds are used as coolant in aeronautic field, these fuels crack into small molecule hydrocarbons by absorbing heat form the high temperature part of aerocraft, which could meet the cooling requirement. These fuels mainly include linear alkanes, branched alkanes and cycloalkanes. Cycloalkanes widely exist in diesel, jet, and gasoline fuels, where cycloalkanes account for as much as10%-35%by volume. The substituted cyclohexane and fused ring compounds are the prominent representatives among cycloalkanes. Consequently, the character of pyrolysis mechanism, product distribution and heat sink has great significance.The main contents and conclusions in our work are listed as follows:In chapter1, we mainly discuss the significance meaning of hydrocarbon pyrolysis form the view of cooling. In recent years, Li’s group has developed the ReaxGen programe. A brief introduction of reducing mechanism and ReaxGen programe based on the fundamental principle and formula.In chapter2, a brief introduction of the quantum chemistry theory and compute methods, intrinsic reaction coordinate, transition state theory, tunneling effect correction and heat sink estimation.In chapter3, the detailed mechanism of cycloalkanes pyrolysis is investigated by using the ReaxGen program, and it also can give thermdynamic and dynamic data for every elementary reaction. On the basis of above information, the physical, chemical and total heat sink are estimated at different temperatures by using ChemkinⅡ program. The result shows that the physical heat sink increases linearly along with temperature rise, the chemical heat sink changes with the number, size and site of substitutes. The pyrolysis of single, double and tertiary substituted cyclohexane takes place mainly at650～800℃,650～750℃,700～780℃, respectively. The range of temperature is narrow gradually with the substituent increase. The total heat sink of single ring is higher than that of fused ring, and they are rapidly pyrolysis at650～800℃and650～750℃, respectively. The range of temperature is narrow gradually with the size of cyclo enlarge. The total heat sink of cycloalkanes is higher than the aliphatic hydrocarbon at the same carbon atoms.In chapter4, methylcyclohexane (MCH) is one of the prominent representatives among cycloalkanes and exists commonly in jet fuels. The thermal decomposition of MCH has been investigated at the CBS-QB3and CCSD level of theory. The pyrolysis of MCH follows a radical chain mechanism, which includes C-C bond scission, H-atom abstraction, secondary and biradical reactions. Thermodynamic data of species involved in this study are computed at the same level. The rate constants for elementary reactions also are calculated with conventional transition state theory (TST), where Eckart method is adopted to correct the quantum mechanical tunneling effect. On the basis of thermodynamic and dynamic data, the total heat sink for the thermal pyrolysis of MCH is estimated at500-1000℃and1-50atm. The results show that the physical heat sink is increase linearly with the temperature rise. The chemical heat sink is equal to zero at500-650℃, and it increases rapidly at650-800℃, and it decreases slightly with the pressure rise at800-1000℃. The reason might be attributed to the effects of exothermic reactions, which the radicals formed begin to recombine. Herein, the pyrolysis of MCH takes place mainly at650-800℃. Furthermore, the main products of MCH pyrolysis are hydrogen (H2), methane (CH4), ethylene (C2H4), ethane (C2H6), propylene (C3H6) and1.3-butadiene (C4H6) at600～1000℃and atmospheric pressure.

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CLC: > Mathematical sciences and chemical > Chemistry > Organic Chemistry > Carbocyclic compounds,alicyclic compounds > Alicyclic hydrocarbons and their derivatives
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