The oxidative coupling of methane over La203/CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions （973〈T〈 1103 K, 55〈 Ptotal 〈220 kPa, and 3.7〈 mcat/VTp 〈50 kg.s/m^3）....The oxidative coupling of methane over La203/CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions （973〈T〈 1103 K, 55〈 Ptotal 〈220 kPa, and 3.7〈 mcat/VTp 〈50 kg.s/m^3）. A ten-step kinetic model incorporating all main products was used to predict the behavior of the system. Methane conversions and C2 selectivities were calculated by varying methane to oxygen ratios in the feed under different operating conditions which were also compared with the rule of 100. The results show that deviation from this rule depends on the operating conditions. Within the range studied, an increase in pressure, temperature and contact time results in smaller deviation from the rule. This rule is best approximated when the catalyst operates near its optimal performance. For negative deviations, common to the most catalysts, it is found that the optimal performance should occur at methane conversion levels lower than 50%. A plot of selectivity versus conversion for high-yield reported performance data of a large variety of catalysts shows that data points concentrated roughly in 20%-50% methane conversion region, confirming the generality and prediction of modeling.展开更多
Aiming at developing novel microwave-transparent ceramics with low dielectric loss,high thermal conductivity and high strength,Si3N4-AlN(30%,mass fraction) composite ceramics with La2O3 as sintering additive were prep...Aiming at developing novel microwave-transparent ceramics with low dielectric loss,high thermal conductivity and high strength,Si3N4-AlN(30%,mass fraction) composite ceramics with La2O3 as sintering additive were prepared by hot-pressing at 1 800 °C and subsequently annealed at 1 450 °C and 1 850 °C for 2 h and 4 h,respectively.The materials were characterized by XRD and SEM.The effect of annealing process on the phase composition,sintering performance,microstructure,bending strength,dielectric loss and thermal conductivity of the materials was investigated.The results showed that both annealing at 1 850 °C and 1 450 °C promoted the phase transformation of α-Si3N4 to β-Si3N4.After annealing at 1 850 °C,grain growth to a certain extent occurred in the materials.Especially,the elongated β-Si3N4 grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio.As a result,as the annealing time increased to 4 h,the bending strength declined from 456 MPa to 390 MPa,whereas the dielectric loss decreased to 2.15×10-3 and the thermal conductivity increased to 16.3 W/(m·K) gradually.When annealed at 1 450 °C,increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La2Si6N8O3 phase in the materials,which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/(m·K),respectively,and simultaneously the decrease in dielectric loss to 1.53×10-3.展开更多
Pure tungsten （PW） and W-1 wt% La203 （WL10） were prepared by powder metallurgical route fol- lowed by the swaging ＋ rolling process. The logarithmic strains are 0, 0.37, 0.58, and 0.98 for WL10 and 0, 0.58 for PW...Pure tungsten （PW） and W-1 wt% La203 （WL10） were prepared by powder metallurgical route fol- lowed by the swaging ＋ rolling process. The logarithmic strains are 0, 0.37, 0.58, and 0.98 for WL10 and 0, 0.58 for PW. Heat treatments were performed at temperatures var- ied from 1,573 to 2,173 K to determine the recrystalliza- tion temperature. Recrystallization temperatures are 1,973 and 2,173 K for WL1 （logarithmic strain of 0.37） and WL3 （logarithmic strain of 0.98）, respectively. But in the case of WL2 （logarithmic strain of 0.58）, full recrystallization is not achieved at temperature of above 2,173 K. Further- more, the recrystallization temperature of PW with loga- rithmic strain of 0.58 is at least 300 K lower than that of the equivalent WLI0 sample. Moreover, the increase of recrystallization temperature inhibits the strength degra- dation of WL2： samples lose 4 % and 22 % strength when annealed at 1,573 and 1,973 K compared with room tem- perature （RT） sample. Finally, the texture evolution for the swaged ＋ rolled WL10 is significantly related to the deformation degree： the dominated orientation is （001） for WL2 while （110） for WL3.展开更多
文摘The oxidative coupling of methane over La203/CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions （973〈T〈 1103 K, 55〈 Ptotal 〈220 kPa, and 3.7〈 mcat/VTp 〈50 kg.s/m^3）. A ten-step kinetic model incorporating all main products was used to predict the behavior of the system. Methane conversions and C2 selectivities were calculated by varying methane to oxygen ratios in the feed under different operating conditions which were also compared with the rule of 100. The results show that deviation from this rule depends on the operating conditions. Within the range studied, an increase in pressure, temperature and contact time results in smaller deviation from the rule. This rule is best approximated when the catalyst operates near its optimal performance. For negative deviations, common to the most catalysts, it is found that the optimal performance should occur at methane conversion levels lower than 50%. A plot of selectivity versus conversion for high-yield reported performance data of a large variety of catalysts shows that data points concentrated roughly in 20%-50% methane conversion region, confirming the generality and prediction of modeling.
基金Project(50872052) supported by the National Natural Science Foundation of ChinaProject(2009AA05Z313) supported by the National High Technology Research and Development Program of ChinaProject supported by the Commission of Science,Technology and Industry for National Defence,China
文摘Aiming at developing novel microwave-transparent ceramics with low dielectric loss,high thermal conductivity and high strength,Si3N4-AlN(30%,mass fraction) composite ceramics with La2O3 as sintering additive were prepared by hot-pressing at 1 800 °C and subsequently annealed at 1 450 °C and 1 850 °C for 2 h and 4 h,respectively.The materials were characterized by XRD and SEM.The effect of annealing process on the phase composition,sintering performance,microstructure,bending strength,dielectric loss and thermal conductivity of the materials was investigated.The results showed that both annealing at 1 850 °C and 1 450 °C promoted the phase transformation of α-Si3N4 to β-Si3N4.After annealing at 1 850 °C,grain growth to a certain extent occurred in the materials.Especially,the elongated β-Si3N4 grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio.As a result,as the annealing time increased to 4 h,the bending strength declined from 456 MPa to 390 MPa,whereas the dielectric loss decreased to 2.15×10-3 and the thermal conductivity increased to 16.3 W/(m·K) gradually.When annealed at 1 450 °C,increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La2Si6N8O3 phase in the materials,which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/(m·K),respectively,and simultaneously the decrease in dielectric loss to 1.53×10-3.
基金financially supported by the International Thermonuclear Experimental Reactor(ITER) Project of China(No.2014GB123000)
文摘Pure tungsten （PW） and W-1 wt% La203 （WL10） were prepared by powder metallurgical route fol- lowed by the swaging ＋ rolling process. The logarithmic strains are 0, 0.37, 0.58, and 0.98 for WL10 and 0, 0.58 for PW. Heat treatments were performed at temperatures var- ied from 1,573 to 2,173 K to determine the recrystalliza- tion temperature. Recrystallization temperatures are 1,973 and 2,173 K for WL1 （logarithmic strain of 0.37） and WL3 （logarithmic strain of 0.98）, respectively. But in the case of WL2 （logarithmic strain of 0.58）, full recrystallization is not achieved at temperature of above 2,173 K. Further- more, the recrystallization temperature of PW with loga- rithmic strain of 0.58 is at least 300 K lower than that of the equivalent WLI0 sample. Moreover, the increase of recrystallization temperature inhibits the strength degra- dation of WL2： samples lose 4 % and 22 % strength when annealed at 1,573 and 1,973 K compared with room tem- perature （RT） sample. Finally, the texture evolution for the swaged ＋ rolled WL10 is significantly related to the deformation degree： the dominated orientation is （001） for WL2 while （110） for WL3.
