Hypersonic aircrafts suffer from significant heat load. The temperature of the aircraft is too high due to aerodynamic heat and heat from the engine combustor. Because the heat load results in structural change of the engine and malfunctioning of the aircrafts, thermal management is important in adv...
Hypersonic aircrafts suffer from significant heat load. The temperature of the aircraft is too high due to aerodynamic heat and heat from the engine combustor. Because the heat load results in structural change of the engine and malfunctioning of the aircrafts, thermal management is important in advanced hypersonic aircraft technologies. Nowadays, the endothermic fuels were the best coolant for the hypersonic aircraft cooling. Endothermic fuels are liquid hydrocarbon fuels that undergo endothermic reaction at high temperature and pressure (hypersonic flight conditions). exo-Tetrahydrodicyclopentadiene (exo-THDCP, C10H16) is a prospective jet fuel with high energy density due to bridged ring structure. Its energy density is greater than those of conventional kerosene jet fuels, which makes exo-THDCP suited for a volume-limited aircraft. In this study, exo-THDCP was selected as an endothermic fuel.
The cooling capacity (heat absorption) of endothermic fuels comes from physical heat (sensible heat and latent heat) and endothermic heat of reactions. As the physical heat is dependent on heat capacities of fuels and temperatures, it cannot be improved when the system conditions are fixed. On the other hand, the endothermic heat of reactions can be improved by catalysts which can control chemical reaction. Although endothermic fuels have attracted considerable attention, there have been few studies associated with improvement of its heat of reactions. The goal of this study was to increase the heat of reaction in the endothermic reaction of exo-THDCP by developing catalysts.
At first, endothermic reactions with commercial zeolites were investigated in this study. To increase the heat of reaction, it is necessary for zeolites to produce high conversion and high yield of low molecular weight hydrocarbons and olefins. HZSM-5 led to a higher heat of reaction compared to other zeolites due to its high conversion and composition of low molecular weight products. The high conversion resulted from high total acid site density, and the high composition of low molecular weight products resulted from its sinusoidal pore structure.
Secondly, Developing catalysts for improvement of endothermic reaction heat was performed. Catalysts with different silica/alumina ratio were synthesized and investigated. It was observed that enhancing the Si/Al molar ratio of HZSM-5 zeolites induces a decrease in the conversion of exo-THDCP. Because acid sites are formed by bonding structure of silica and alumina, the number of total acid sites cannot exceed the total number of aluminum atoms. As a result, large Si/Al led in decrease in the conversion. As the conversion of exo-THDCP’s endothermic reaction was dependent on the total acid site density, the conversion decreased with increase in Si/Al. Therefore, the optimum Si/Al ratio was 20. Pore controlled catalysts were synthesized using structure directing agents, SDA. Pore controlled HZSM-5s were synthesized using tetramethylammonium bromide(TMA-Br) and tetrapropylammonium bromide(TPA-Br) as structure directing agent(SDA) and their mixture. HZSM-5 synthesized with mixed SDA had larger total acid site density and micropore volume. To increase the heat of reaction, it is inevitable for fuel to decompose into low molecular weight and unsaturated hydrocarbons with high conversion. HZSM-5 synthesized with mixed SDA induced higher heat of reaction than other synthetic and commercial HZSM-5 in catalytic cracking reaction. It resulted from high conversion of reactant and composition of low molecular weight products. High total acid site density led to high conversion, and increased micropore volume brought the high composition of low molecular weight products. Metal loaded catalysts were synthesized. (Palladium, Platinum, and Lanthanum was selected) Although the metal loading led decreased in total acid density sites, the conversion of metal loaded zeolite increased. It is supposed that metal sites could be also active sites for endothermic reaction as well as acid sites. Especially, La loading resulted in high conversion compared to other metal loading. La/HZSM-5 zeolites showed that the the higher yield of ethylene and propylene and it is resulted from inhibiting of olefin adsorption.
Finally, for endothermic reaction of exo-THDCP, the large production of olefins was important and the mixing of zeolites technique was applied in this study. HY was selected as an additive to HZSM-5. The ability of HY for C2-C4 production was lower, but its large micropore diameter and its low steric hindrance pore structure can allow the reactant to diffuse fast in the pore. As the HY cracked C10 into smaller products (C6-C9), the ability of HZSM-5 for C2-C4 production would increase due to fast diffusion of C6-C9 products compared to C10. Therefore, the heat of reaction increased with mixed zeolites.
Also, fluorination was applied to HZSM-5 since it could enhance total acid density and micropore structures.Upon modification by F, F/HZSM-5 showed an obvious increase in total acid density compared to parent HZSM-5. It is supposed that F modification was able to improve the acid amount of HZSM-5 and might also introduce some new strong acid sites. The increase in the acid amount could be ascribed to the formation of new acidic hydroxyl groups by the reaction of the zeolite surface with the protons of the remaining (H+ ••• F-) ion pairs. Also, NH4F treatment could create some secondary pore or dredge the channels of the HZSM-5 zeolites, which may result from the induced dissolution of amorphous substances in HZSM-5 pores by HF species generated from the decomposition of NH4F.
Hypersonic aircrafts suffer from significant heat load. The temperature of the aircraft is too high due to aerodynamic heat and heat from the engine combustor. Because the heat load results in structural change of the engine and malfunctioning of the aircrafts, thermal management is important in advanced hypersonic aircraft technologies. Nowadays, the endothermic fuels were the best coolant for the hypersonic aircraft cooling. Endothermic fuels are liquid hydrocarbon fuels that undergo endothermic reaction at high temperature and pressure (hypersonic flight conditions). exo-Tetrahydrodicyclopentadiene (exo-THDCP, C10H16) is a prospective jet fuel with high energy density due to bridged ring structure. Its energy density is greater than those of conventional kerosene jet fuels, which makes exo-THDCP suited for a volume-limited aircraft. In this study, exo-THDCP was selected as an endothermic fuel.
The cooling capacity (heat absorption) of endothermic fuels comes from physical heat (sensible heat and latent heat) and endothermic heat of reactions. As the physical heat is dependent on heat capacities of fuels and temperatures, it cannot be improved when the system conditions are fixed. On the other hand, the endothermic heat of reactions can be improved by catalysts which can control chemical reaction. Although endothermic fuels have attracted considerable attention, there have been few studies associated with improvement of its heat of reactions. The goal of this study was to increase the heat of reaction in the endothermic reaction of exo-THDCP by developing catalysts.
At first, endothermic reactions with commercial zeolites were investigated in this study. To increase the heat of reaction, it is necessary for zeolites to produce high conversion and high yield of low molecular weight hydrocarbons and olefins. HZSM-5 led to a higher heat of reaction compared to other zeolites due to its high conversion and composition of low molecular weight products. The high conversion resulted from high total acid site density, and the high composition of low molecular weight products resulted from its sinusoidal pore structure.
Secondly, Developing catalysts for improvement of endothermic reaction heat was performed. Catalysts with different silica/alumina ratio were synthesized and investigated. It was observed that enhancing the Si/Al molar ratio of HZSM-5 zeolites induces a decrease in the conversion of exo-THDCP. Because acid sites are formed by bonding structure of silica and alumina, the number of total acid sites cannot exceed the total number of aluminum atoms. As a result, large Si/Al led in decrease in the conversion. As the conversion of exo-THDCP’s endothermic reaction was dependent on the total acid site density, the conversion decreased with increase in Si/Al. Therefore, the optimum Si/Al ratio was 20. Pore controlled catalysts were synthesized using structure directing agents, SDA. Pore controlled HZSM-5s were synthesized using tetramethylammonium bromide(TMA-Br) and tetrapropylammonium bromide(TPA-Br) as structure directing agent(SDA) and their mixture. HZSM-5 synthesized with mixed SDA had larger total acid site density and micropore volume. To increase the heat of reaction, it is inevitable for fuel to decompose into low molecular weight and unsaturated hydrocarbons with high conversion. HZSM-5 synthesized with mixed SDA induced higher heat of reaction than other synthetic and commercial HZSM-5 in catalytic cracking reaction. It resulted from high conversion of reactant and composition of low molecular weight products. High total acid site density led to high conversion, and increased micropore volume brought the high composition of low molecular weight products. Metal loaded catalysts were synthesized. (Palladium, Platinum, and Lanthanum was selected) Although the metal loading led decreased in total acid density sites, the conversion of metal loaded zeolite increased. It is supposed that metal sites could be also active sites for endothermic reaction as well as acid sites. Especially, La loading resulted in high conversion compared to other metal loading. La/HZSM-5 zeolites showed that the the higher yield of ethylene and propylene and it is resulted from inhibiting of olefin adsorption.
Finally, for endothermic reaction of exo-THDCP, the large production of olefins was important and the mixing of zeolites technique was applied in this study. HY was selected as an additive to HZSM-5. The ability of HY for C2-C4 production was lower, but its large micropore diameter and its low steric hindrance pore structure can allow the reactant to diffuse fast in the pore. As the HY cracked C10 into smaller products (C6-C9), the ability of HZSM-5 for C2-C4 production would increase due to fast diffusion of C6-C9 products compared to C10. Therefore, the heat of reaction increased with mixed zeolites.
Also, fluorination was applied to HZSM-5 since it could enhance total acid density and micropore structures.Upon modification by F, F/HZSM-5 showed an obvious increase in total acid density compared to parent HZSM-5. It is supposed that F modification was able to improve the acid amount of HZSM-5 and might also introduce some new strong acid sites. The increase in the acid amount could be ascribed to the formation of new acidic hydroxyl groups by the reaction of the zeolite surface with the protons of the remaining (H+ ••• F-) ion pairs. Also, NH4F treatment could create some secondary pore or dredge the channels of the HZSM-5 zeolites, which may result from the induced dissolution of amorphous substances in HZSM-5 pores by HF species generated from the decomposition of NH4F.
주제어
#exo-tetrahydrodicyclopentadiene
#jet fuel
#endothermic reaction
#endothermic fuel
#zeolite
#HZSM-5
#structure directing agents
#metal ion exchange
#fluorination
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