The Experts below are selected from a list of 10986 Experts worldwide ranked by ideXlab platform
Timothy P. Weihs - One of the best experts on this subject based on the ideXlab platform.
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studying Exothermic Reactions in the ni al system at rapid heating rates using a nanocalorimeter
Journal of Applied Physics, 2013Co-Authors: P Swaminathan, Michael D. Grapes, K. Woll, David A. Lavan, S C Barron, Timothy P. WeihsAbstract:Heats of reaction and heat capacity changes were measured using scanning nanocalorimetry for a nickel and aluminum bilayer where initial heating rates of 104 K/s were achieved. Multiple exotherms were observed on the initial heating, but the number of intermediate exotherms decreased with increasing heating rate. The final phase was the B2 NiAl intermetallic. Results from the nanocalorimeter were compared with a conventional differential scanning calorimeter (operating at 0.7 K/s) to understand the effect of significant (10 000×) increases in heating rate on the phase transformation sequence. The high heating rate in the nanocalorimeter delays reaction initiation, causes the Exothermic peaks to shift to higher temperatures, and appears to suppress the formation of intermediate, metastable phases. Potential explanations for this apparent suppression are discussed.
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phase transformations during rapid heating of al ni multilayer foils
Applied Physics Letters, 2008Co-Authors: Jonathan C Trenkle, Timothy P. Weihs, Lucas J Koerner, Mark W Tate, Sol M Gruner, T C HufnagelAbstract:We have used self-propagating Exothermic Reactions in Al/Ni multilayers as a means to explore the effect of rapid heating on phase transformations. Using time-resolved synchrotron x-ray microdiffraction with an extremely fast detector, we were able to examine the reaction sequence in detail at heating rates of ∼106 K s−1. We observed that the intermediate phases formed during the self-propagating Reactions are different from those formed at lower heating rates, even though the final phases are the same. In situ characterization is essential, as other means of studying self-propagating Reactions (such as quenching the reaction followed by ex situ analysis) provide different—and potentially misleading—results.
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reactive nanostructured foil used as a heat source for joining titanium
Journal of Applied Physics, 2004Co-Authors: Alan Duckham, Timothy P. Weihs, Michael Reiss, Stephen John Spey, Jiaping Wang, Etienne Besnoin, Omar M KnioAbstract:We have joined titanium alloy (Ti-6Al-4V) specimens at room temperature and in air by using free-standing nanostructured Al∕Ni multilayer foils to melt a silver-based braze. The foils are capable of undergoing self-sustaining Exothermic Reactions and thus act as controllable local heat sources. By systematically controlling the properties of the foils and by numerically modeling the reactive joining process, we are able to conclude that the temperatures reached by the foils during reaction are critical in determining the success of joining when using higher melting temperature braze layers.
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modeling and characterizing the propagation velocity of Exothermic Reactions in multilayer foils
Journal of Applied Physics, 1997Co-Authors: A B Mann, A J Gavens, Michael Reiss, D Van Heerden, G Bao, Timothy P. WeihsAbstract:Combustible multilayer foils can be fabricated by sputter depositing alternate layers of materials which react Exothermically during thermally induced intermixing. Current models for these Reactions consider pure materials which only intermix during the self-propagating stage of the reaction, though in reality during fabrication the materials undergo partial intermixing. An analytical model dealing with the premixing is presented and compared with experimental results for Al/Ni and Al/(Ni:Cu) multilayers. The model and the results indicate that premixing lowers the propagation velocity both by slowing the rate of atomic diffusion between layers and by lowering the temperature of the reaction. The lower temperature can cause solid/liquid phase changes to dominate the reaction path. It is concluded that to use these foils in commercial and engineering applications, the method of fabrication and the phase changes occurring during the reaction must be controlled to give the desired characteristics.
Gerhart Eigenberger - One of the best experts on this subject based on the ideXlab platform.
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autothermal reactor concepts for endothermic fixed bed Reactions
Chemical Engineering Research & Design, 2004Co-Authors: B Glockler, G Kolios, Achim Gritsch, Aristides Morillo, Gerhart EigenbergerAbstract:Autothermal reactor concepts for the heat integrated coupling of endothermic and Exothermic Reactions are required for an efficient on-site production of hydrogen from alcohols or hydrocarbons for use in fuel cells. Existing experience with countercurrent reactors and reverse-flow reactors for weakly Exothermic Reactions can be utilized for their design. However, specific features of coupling endothermic and Exothermic Reactions must be taken into account. The respective considerations are presented and discussed for different modes of operation: the simultaneous coupling of both Reactions; coupling with cocurrent recuperative heat exchange between the two process streams in the reaction zone; and asymmetric operation with counter-current recuperative or regenerative heat exchange. It is shown that asymmetric operation requires spatial distribution of the feed for the Exothermic reaction in order to prevent excess temperatures and to ensure stable operation. Both simulation and experimental results are presented for methanol steam reforming, gasoline reforming and methane steam reforming.
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efficient reactor concepts for coupling of endothermic and Exothermic Reactions
Chemical Engineering Science, 2002Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:Multifunctional autothermal reactors are a novel concept in process integration and intensification. They can be implemented as a countercurrent or reverse-flow reactor. A promising field of application is the coupling of endothermic and Exothermic Reactions. Methane steam reforming coupled with methane combustion is considered as a particular example. Several novel reactor configurations with co- and countercurrent flow in the reaction zone will be discussed by numerical simulation and an example for experimental verification will be presented.
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a simplified procedure for the optimal design of autothermal reactors for endothermic high temperature Reactions
Chemical Engineering Science, 2001Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:Abstract The occurrence of excessive maximum temperatures is presently the main obstacle in the design of countercurrent reactors for the autothermal coupling of endothermic and Exothermic Reactions. The reasons for the appearance of these high-temperature maxima are elucidated and discussed using a simplified reactor model. Based on it, measures are derived of how the maximum temperature can be reduced and the thermal efficiency of the integrated reactor can be improved. Simple formula and an efficient graphical procedure for a short-cut reactor design are provided. The results of the simplified design procedure have been verified through simulation with a more detailed reactor model.
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autothermal fixed bed reactor concepts
Chemical Engineering Science, 2000Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:The principles, properties and applications of autothermal fixed-bed reactor concepts are presented. First we focus on different reactor types for weakly Exothermic Reactions and discuss their basic behavior, their stability and nonlinear dynamic features. The second part is devoted to the autothermal coupling of endothermic and Exothermic Reactions. A systematic classification is proposed for the process alternatives developed so far and a simplified model is developed from which basic features of an optimal design can be deduced.
G Kolios - One of the best experts on this subject based on the ideXlab platform.
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autothermal reactor concepts for endothermic fixed bed Reactions
Chemical Engineering Research & Design, 2004Co-Authors: B Glockler, G Kolios, Achim Gritsch, Aristides Morillo, Gerhart EigenbergerAbstract:Autothermal reactor concepts for the heat integrated coupling of endothermic and Exothermic Reactions are required for an efficient on-site production of hydrogen from alcohols or hydrocarbons for use in fuel cells. Existing experience with countercurrent reactors and reverse-flow reactors for weakly Exothermic Reactions can be utilized for their design. However, specific features of coupling endothermic and Exothermic Reactions must be taken into account. The respective considerations are presented and discussed for different modes of operation: the simultaneous coupling of both Reactions; coupling with cocurrent recuperative heat exchange between the two process streams in the reaction zone; and asymmetric operation with counter-current recuperative or regenerative heat exchange. It is shown that asymmetric operation requires spatial distribution of the feed for the Exothermic reaction in order to prevent excess temperatures and to ensure stable operation. Both simulation and experimental results are presented for methanol steam reforming, gasoline reforming and methane steam reforming.
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efficient reactor concepts for coupling of endothermic and Exothermic Reactions
Chemical Engineering Science, 2002Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:Multifunctional autothermal reactors are a novel concept in process integration and intensification. They can be implemented as a countercurrent or reverse-flow reactor. A promising field of application is the coupling of endothermic and Exothermic Reactions. Methane steam reforming coupled with methane combustion is considered as a particular example. Several novel reactor configurations with co- and countercurrent flow in the reaction zone will be discussed by numerical simulation and an example for experimental verification will be presented.
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a simplified procedure for the optimal design of autothermal reactors for endothermic high temperature Reactions
Chemical Engineering Science, 2001Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:Abstract The occurrence of excessive maximum temperatures is presently the main obstacle in the design of countercurrent reactors for the autothermal coupling of endothermic and Exothermic Reactions. The reasons for the appearance of these high-temperature maxima are elucidated and discussed using a simplified reactor model. Based on it, measures are derived of how the maximum temperature can be reduced and the thermal efficiency of the integrated reactor can be improved. Simple formula and an efficient graphical procedure for a short-cut reactor design are provided. The results of the simplified design procedure have been verified through simulation with a more detailed reactor model.
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autothermal fixed bed reactor concepts
Chemical Engineering Science, 2000Co-Authors: G Kolios, J Frauhammer, Gerhart EigenbergerAbstract:The principles, properties and applications of autothermal fixed-bed reactor concepts are presented. First we focus on different reactor types for weakly Exothermic Reactions and discuss their basic behavior, their stability and nonlinear dynamic features. The second part is devoted to the autothermal coupling of endothermic and Exothermic Reactions. A systematic classification is proposed for the process alternatives developed so far and a simplified model is developed from which basic features of an optimal design can be deduced.
Gianpiero Groppi - One of the best experts on this subject based on the ideXlab platform.
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a study on the thermal behavior of structured plate type catalysts with metallic supports for gas solid Exothermic Reactions
Chemical Engineering Science, 2000Co-Authors: Enrico Tronconi, Gianpiero GroppiAbstract:The heat transfer characteristics of seven di!erent samples of coated plate-type structured catalysts with highly conductive metallic supports were investigated using the model reaction of CO oxidation over Pd/c-Al 2 O 3 . In the case of supports made of aluminum hot spot temperatures were moderate, and the thermal behavior of the structured catalysts was solely controlled by the heat transfer resistance at the interface between catalyst and reactor wall. Temperature gradients were markedly more signi"cant in the case of a support with identical geometry but made of steel, due to a tenfold reduction of the intrinsic material conductivity. They were still greater in the case of a steel support with thinner plates. For aluminum supports, experiments with a fourfold more active catalytic washcoat and with a modi"ed con"guration of the structured support con"rmed that an isothermal behavior is approached even for conditions corresponding to an adiabatic temperature rise of about 8003C, and that such results can be scaled up to di!erent geometries of the structured systems if the washcoat-to-support volume ratio is conserved. Finally, the wall heat transfer coe$cient was enhanced by a design of the aluminum support with improved thermal contact at the wall. A simple 1D analysis, based on independent intrinsic kinetics, yielded estimates of the overall wall heat transfer coe$cient in the range 80}120 W/(m2 K). ( 2000 Elsevier Science Ltd. All rights reserved.
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Development of novel structured catalytic reactors for highly Exothermic Reactions
Studies in Surface Science and Catalysis, 2000Co-Authors: Gianpiero Groppi, Cinzia Cristiani, M. Valentini, Enrico TronconiAbstract:The potential of annular and plate-type structured reactors for kinetic studies of Exothermic catalytic Reactions is explored for CH 4 and CO combustion. Critical aspects associated with the deposition of adherent and stable catalytic coatings, and with the reduction of temperature gradients are addressed. We show that nearly isothermal conditions can be achieved in a plate-type reactor configuration using highly conductive metallic supports.
Enrico Tronconi - One of the best experts on this subject based on the ideXlab platform.
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a study on the thermal behavior of structured plate type catalysts with metallic supports for gas solid Exothermic Reactions
Chemical Engineering Science, 2000Co-Authors: Enrico Tronconi, Gianpiero GroppiAbstract:The heat transfer characteristics of seven di!erent samples of coated plate-type structured catalysts with highly conductive metallic supports were investigated using the model reaction of CO oxidation over Pd/c-Al 2 O 3 . In the case of supports made of aluminum hot spot temperatures were moderate, and the thermal behavior of the structured catalysts was solely controlled by the heat transfer resistance at the interface between catalyst and reactor wall. Temperature gradients were markedly more signi"cant in the case of a support with identical geometry but made of steel, due to a tenfold reduction of the intrinsic material conductivity. They were still greater in the case of a steel support with thinner plates. For aluminum supports, experiments with a fourfold more active catalytic washcoat and with a modi"ed con"guration of the structured support con"rmed that an isothermal behavior is approached even for conditions corresponding to an adiabatic temperature rise of about 8003C, and that such results can be scaled up to di!erent geometries of the structured systems if the washcoat-to-support volume ratio is conserved. Finally, the wall heat transfer coe$cient was enhanced by a design of the aluminum support with improved thermal contact at the wall. A simple 1D analysis, based on independent intrinsic kinetics, yielded estimates of the overall wall heat transfer coe$cient in the range 80}120 W/(m2 K). ( 2000 Elsevier Science Ltd. All rights reserved.
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Development of novel structured catalytic reactors for highly Exothermic Reactions
Studies in Surface Science and Catalysis, 2000Co-Authors: Gianpiero Groppi, Cinzia Cristiani, M. Valentini, Enrico TronconiAbstract:The potential of annular and plate-type structured reactors for kinetic studies of Exothermic catalytic Reactions is explored for CH 4 and CO combustion. Critical aspects associated with the deposition of adherent and stable catalytic coatings, and with the reduction of temperature gradients are addressed. We show that nearly isothermal conditions can be achieved in a plate-type reactor configuration using highly conductive metallic supports.