Catalytic Inhibition Of Coking Of Ndecane By Modified HZSM-5 And Removal Of Coke By Plasma

Endothermic hydrocarbon fuel in the active thermal protection is gradually faced with the problems of insufficient heat sink and easy coking,which limits its further use.ZSM-5 is a molecular sieve catalyst based on Si,Al and O elements to form the basic skeleton.The catalytic performance of traditional HZSM-5 is defective.In this paper,the following work has been carried out to face the problems of endothermic hydrocarbon fuel.The catalyst modified with HZSM-5 substrate was used to improve the catalytic capacity.At the same time,hydrogen plasma was used to clean the coking on the metal wall.The work in the following aspects has been carried out:Supercritical n-decane and n-decane with industrial HZSM-5 were used to cool the metal tube by experimental methods.The coking of tube wall and catalyst HZSM-5 were studied.The tube was completely blocked by the coke without the catalyst at the outlet temperature of 728?,which was reduced to 687?after the addition of industrial HZSM-5.The HZSM-5 also rapidly coked and deactivated.In order to study the catalytic cracking and coking process of HZSM-5 and metal walls,molecular dynamics and quantum mechanical density functional methods were used to study the catalytic cracking and adsorption process of HZSM-5.HZSM-5 was found to promote n-decane cracking,while the catalytic activity Br?nsted acid sites and damaged skeleton Si were found to adsorb hydrocarbon products.The density functional method was used to obtain that the Br?nsted acid site could adsorb up to two hydrogens,and the more damaged the skeletal silicon,the more likely it was to adsorb hydrocarbons.The results confirm the partial simulation results of molecular dynamics.Molecular dynamics was also used to study the catalytic cracking and coking process of n-decane on the surface of iron metal and it was found that the metal wall promoted the cracking of n-decane.Finally,a straight chain carbon coking precursor was formed along the surface texture of the metal.According to the calculation results of catalytic cracking and adsorption properties of HZSM-5,when modifying the catalyst,how to change the desorption ability of hydrogen at the Br?nsted acid site can be considered.The key to the modification of the catalyst is to balance the ability of hydrogen desorption at the acid site and stabilization of the skeleton structure of the catalyst.For the problems of insufficient heat sink,easy coking,and poor catalysis of HZSM-5,NiO nanoparticle hybrid thin layers were successfully wrapped on the commercial HZSM-5(denoted as HZSM-5/NiO),and showing significant modulation ability for the acidity properties of HZSM-5 to boost supercritical cracking of n-decane.The as-prepared catalysts make the NiO and HZSM-5 show synergistic effect which dramatically lowers the overall acidity of composite,thus changes the cracking path of n-decane,and results in effective anti-coking properties.HZSM-5/NiO decelerated many additional free radicals and generated more gases products by monomolecular cleavage reaction.Gases products also show positively correlating with the heat sink.When the heating tube is completely blocked by the coking,the catalysts improve the heat sink of n-decane from 3.77 MJ/kg at 728?to 4.59 MJ/kg at 780?.Present work demonstrates that the introduction of nanostructured metal oxides into HZSM-5 will synergistically adjust the acidity of composite,leading to the manipulation of catalytic cracking path and extent,gas products,heat sink,coking behav ior of supercritical n-decane.Modification of NiO directly change the stripping ability of hydrogen on Br?nsted acid sites and protect the skeleton structure.Decoking coupled with fluid is another idea through the formation of loose coke by catalyst.In essence,the acid potential density and capacity of the modified catalyst are still changed,but the catalyst effect and the mechanism are different.The catalysis can be associated with the flow field to inhibit coking.The modification of HZSM-5 was attempted by using Co₃O₄.Incipient-wetness impregnation method was selected to load Co salts in commercial HZSM-5 with the average diameter of 2.4±0.5?m to form Co₃O₄nanosheets with the average thickness of 15 nm.The ratio of Br?nsted/Lewis acidity decreased from 8.00 to 2.79 after modification by Co₃O₄ nanosheets.The synergistically catalytic effect between Co₃O₄ nanosheets and HZSM-5 was beneficial to the generation of larger gas production rate with higher content of alkene,and thus resulting in higher heat sink than benchmarked fuels.Catalytic cracking of n-decane(C10)in the presence of 0.1 wt%Co₃O₄ nanosheets/ZSM-5 could yield heat sink as high as 4.64 MJ/kg at 758?,much higher than those of bare HZSM-5(2.99MJ/kg at 687?)and thermal cracking of C10(3.77 MJ/kg at 728?)when the heating tube is completely blocked by the coking.Meanwhile,the smart combination of Co₃O₄ nanosheets and commercial HZSM-5 could effectively suppress the coke deposition on external surface of composites,the loose coke made it easy to be washed away by the fluid,thus resulting in efficient catalytic cracking at elevated temperature for obtaining higher heat sink.Then the two modified catalysts were compared and evaluated.Even though the use of catalyst extended the heat absorption ability and delayed the coking of the hydrocarbon fuel,it could not completely inhibit coking.For coking on metal surfaces,the hydrogen plasma cleaning was used to remove the coke offline.It was found that the glow discharge plasma has stronger chemical cleaning effect.CH₄ was the main gases products as well as the existence of C₂H₆,C₂H₄,C₃H₈ and C₃H₆.The chemical reactions due to the plasma occur mainly through hydrogenation reactions on graphite defects.The cathode has obvious characteristics of being cleaned and corroded.Many amorphous carbon particles less than 1 micron with fuzzy boundary had been found in the ground electrode.The plasma in the cleaning showed significant emission of light centered at the wavelength of 388 nm and 776nm.The bonding strength of cleaned iron and copper substrates was determined by the material properties of the metal substrates.Finally,the plasma gas reaction process was analyzed by free radical reaction combining with the distribution of gas products.According to the same working condition,the plasma was used to clean the real coke on the metal wall,and a good cleaning effect was obtained.