Endothermic reactions performed in microreactors are driven using heat from an external source. However, the temperature of the gas stream providing the heat is limited by constraints imposed by the materials of construction. The present study is focused primarily upon the designs and operations of heat integrated reactors for thermochemically producing hydrogen from methanol by steam reforming. A symmetry boundary condition is used to model half of each system where symmetry exists. Computations are performed using grids with varying nodal densities to determine the optimum node spacing and density that would give the desired accuracy and minimize computation time. The final grid density is determined when the centerline profiles of temperature and species concentration do not show obvious difference. The second-order upwind scheme is used to discretize the mathematical model, and the semi-implicit method for pressure-linked equations algorithm is employed to solve for the pressure and velocity fields. The convergence is judged upon the residuals of all governing equations. The present study aims to provide a fundamental understanding of the designs and operations of heat integrated reactors for thermochemically producing hydrogen from methanol by steam reforming. Particular emphasis is placed upon the effect of various factors on the thermochemical steam reforming processes in heat integrated reactors. The results indicate that steam reforming produces hydrogen and carbon monoxide when heat is added to a catalytic reactor containing steam and hydrocarbons. Alternating channel parallel plate designs can be applied to thermally coupling endothermic steam reforming with combustion in neighboring channels. Balancing the heat requirements of an endothermic reaction with heat generated by an exothermic reaction flowing parallel to and on the opposite side of a separating plate is extraordinarily difficult since the endothermic reaction is likely to have a very different dependence upon concentration and temperature than the endothermic reaction. A convenient way to supply heat is to couple the endothermic reaction with an exothermic combustion reaction in the heat exchange channels. The process gas is raised in temperature and this energy can be utilized by the reforming process. The catalyst coating thickness depends upon the process proceeding within the catalyst matrix. The arrangement leads to improved heat transfer and therefore chemical conversion. Heterogeneous combustion aids in spreading the heat generation along the length of the channel and helps prevent hotspot formation.Keywords: Hydrogen production; Endothermic reactions; Discrete channels; Flow arrangements; Chemical conversion; Heterogeneous combustion

In conventional thermal combustion fuel and air in inflammable proportions are contacted with an ignition source to ignite the mixture which will then continue to burn. Flammable mixtures of most fuels are normally burned at relatively high temperatures, which inherently results in the formation of substantial emissions of nitrogen oxides. In purely catalytic combustion systems, there is little or no nitrogen oxides formed in a system which burns the fuel at relatively low temperatures. The present study is focused primarily upon the combustion characteristics of small alkanes on noble metal surfaces in pre-mixed homogeneous-heterogeneous hybrid systems. The homogeneous-heterogeneous combustion characteristics small alkanes on noble metal surfaces are investigated to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into how to design pre-mixed combustors with improved stability and robustness. The essential factors for design considerations are determined with improved combustion characteristics. The primary mechanisms responsible for the loss of flame stability are discussed. The present study aims to explore how to effectively operate catalytically stabilized combustion. Particular emphasis is placed upon the catalytic combustion characteristics of small alkanes in the pre-mixed hybrid systems. The results indicate that the combustion effluent is characterized by high thermal energy and typically by low nitrogen oxides content. Precise tuning of the combustion process is needed to establish a balance between stable combustion and low emissions. Simply changing the combustor geometry to maintain near-stoichiometric ratios will not avoid nitrogen oxides formation. The catalytic reactor oxidizes substantially all of the ingested fuel and produces thermal energy. Adiabatic combustion systems, from a practical standpoint, have relatively low heat losses, thus substantially all of the heat released from the combustion zone of such systems appears in the effluent gases as thermal energy for producing power. Catalytic oxidation has the disadvantage that the physical reaction surface which must be supplied for complete oxidation of the fuel increases exponentially with decreasing inlet temperatures, which greatly increases the cost of the combustor and complicates the overall design. The operating temperature is determined by the theoretical adiabatic flame temperature of the fuel-air admixture passed to the combustor and thus is dependent on the initial temperature of the air as well as the amount of fuel contained therein. The temperature of the catalyst zone is controlled by adjusting the composition and initial temperature of the fuel-air admixture as well as the uniformity of the mixture.Keywords: Combustion; Metals; Designs; Fuels; Alkanes; Oxidation

Conventional methods of producing a synthesis gas are expensive and complex installations. In order to overcome the complexity and expense of such installations, it is proposed to generate the synthesis gas within autothermal fixed bed reactors that utilize structured catalysts and different surface features to generate the heat necessary to support endothermic heating requirements of the steam reforming reactions. The present study is focused primarily upon the thermal chemical reaction characteristics of autothermal fixed-bed reactors with structured catalysts and different surface features. The heat capacity, thermal conductivity, and viscosity of the mixture are calculated as a mass-weighted average of the values for each constituent. In addition, the material properties of each individual species are functions of the local temperature. When solving the species mass transport equations, binary mass diffusion coefficients are used directly. Only isotropic catalyst structures are considered and the permeability and inertial resistance factor are held constant. The effective thermal conductivity of the catalyst structure system including the porous structure and intervening gas is calculated based upon the porosity of the porous medium, the fluid phase thermal conductivity, and the porous medium effective thermal conductivity as measured in air at ambient conditions. The present study aims to explore how to effectively generate the synthesis gas within autothermal fixed bed reactors that utilize structured catalysts and different surface features. Particular emphasis is placed upon the heat and mass transport phenomena involved in autothermal fixed bed steam reforming reactors. The results indicate that for catalytic thermal chemical reactions, both kinetic impediments are substantially reduced permitting realization of theoretical or near theoretical reaction kinetics. The heat transfer chamber is in thermal contact with the reaction chamber volume, the heat transfer chamber transferring heat at the enhanced heat transfer rate across the wall between the heat transfer chamber and the reaction chamber, thereby obtaining the enhanced production rate per reaction chamber volume for the thermal chemical reaction. Structured catalyst configurations result in high rates of heat transport from the oxidation side to the reforming side. Typically, combustion takes place at low or near-atmospheric pressure, although high pressure combustion is widely practiced. The heat that is generated on the combustion side is quickly transferred on the reforming side. The heat integrated reforming reactor offers several advantages over conventional flame-based reforming reactors. The incorporation of a simultaneous endothermic reaction to provide an improved heat sink may enable a typical heat flux of roughly an order of magnitude above the convective cooling heat flux. The wall can be constructed from any material, but materials that offer low resistance to heat transfer such as metals and metallic alloys are preferred. In this configuration, heat is generated by combustion in the catalytic chamber and is transported very easily and efficiently though the wall to the reforming chamber where the heat demanding reforming reactions take place. The presence of a catalyst and lower temperatures permit significantly higher space velocities to be used compared to flame-based reformers.Keywords: Hydrocarbons; Reactors; Structures; Properties; Reforming; Combustion

Introducing a uniform distribution of carbon nanotubes into a polymer matrix can yield property enhancements that go beyond that of a simple rule of mixtures. The challenge is to take full advantage of the exceptional properties of carbon nanotubes in the composite material. Carbon nanotubes are ideal reinforcing material for polymer matrices dur to their remarkable properties. However, property improvements are not significant due to poor interfacial bonding and severe agglomeration. The present study is focused primarily upon the mechanical properties of fiber-reinforced polymer composites containing graphene-carbon nanotube hybrid materials. The polymer composites utilize nanotechnology enhancements to provide advantageous durability and structural stability improvements over conventional fiber-reinforced polymer composites. The effect of hybrid material weight fraction on the modulus of elasticity and hardness is evaluated. Stress-strain responses of the composite tensile deformation are illustrated and the effect of strain on the bond order parameters is investigated. The present study aims to explore how to effectively improve the mechanical properties of polymers by utilizing graphene-carbon nanotube hybrid materials. Particular emphasis is placed upon the effect of weight fraction on the mechanical properties of polymer composites reinforced with graphene and carbon nanotubes. The results indicate that graphene-carbon nanotube multi-stack three-dimensional architectures can overcome the limitations and restricted performance typically encountered with carbon-based materials by using the combined strategies of three-dimensional architecture and low-dimensional nanomaterial characteristics. Poor dispersibility greatly affects the characteristics of the polymer composites. The modulus of elasticity of the polymer composite is enhanced as compared to the neat polymer. The hybrid material exhibits great improvements in hardness and yield strength and major deteriorations in strain at break. The carbon nanotubes exhibit no preferred orientation and are approximately random. The doping permanently increases the charge concentration in semiconducting carbon nanotubes present in the film, thereby decreasing the sheet resistance of the network. The ability to strengthen polymers is limited by the strength of interfacial bonding. The polymer composite differs from a conventional carbon-fiber composite where there is a much higher interface area between reinforcing carbon and polymer matrix phases.Keywords: Graphene; Carbon; Composites; Polymers; Fibers; Hardness

Many chemical processes utilize catalysts to enhance chemical conversion behavior. A catalyst promotes the rate of chemical conversion but does not affect the energy transformations which occur during the reaction. The present study is focused primarily upon the performance and efficiency analysis of steam-methanol reforming processes in combined parallel plate heat exchanger-reactors. The steady-state continuity, momentum, energy, and species conservation equations are solved in the fluid phase and the heat equation is solved in the solid phase using a finite volume approach. An adaptive meshing scheme is used for the discretization of the differential equations. Computational fluid dynamics simulations are carried out over a wide range of material conductivities. Continuity in temperature and heat flux is applied at the fluid-solid interfaces. Neither heat-transfer nor mass-transfer correlations are employed. Parallel processing employing a message passing interface is used to speed up the most demanding calculations. The present study aims to explore how to effectively enhance chemical conversion behavior by utilizing catalysts. Particular emphasis is placed upon the effect of wall thermal conductivity on the performance and efficiency of steam-methanol reforming processes in combined parallel plate heat exchanger-reactors. The results indicate that the arrangement leads to improved heat transfer and therefore chemical conversion. The honeycomb structure imparts strength to the overall system permitting the walls to be very thin and thereby being responsible for the low weight and rapid thermal response of the reactor configuration. The heat transfer from the process catalyst to the dividing wall is highly efficient, however, the uptake of the energy by the heat transfer fluid will suffer from all of the limitations of traditional heat transfer operations. The process provides more efficient utilization and uniform usage of the heat generated by the exothermic reaction, thus allowing the endothermic reaction to be carried out at a somewhat higher temperature. The temperature control possible with this system is extremely efficient because all the catalytic material in the reactor channels is on the surface of the walls that can transmit heat through the walls directly to the thermal control channels. Adiabatic conditions prevail in the autothermal reactor because the catalytic oxidation reaction is exothermic in nature and the heat generated in the course of such a reaction is usually sufficient to initiate and sustain the endothermic reforming reaction. The thickness of the catalyst coating depends upon the process proceeding within the catalyst matrix. The thin walls and small internal diameters of the channels result in an extremely difficult system for carrying out reactions and transferring heat by giving rise to a high ratio of wall surface area.Keywords: Chemical processes; Autothermal reactors; Heat transfer; Reaction chambers; Heterogeneous oxidation; Transport phenomena

Flammable mixtures of most fuels are normally burned at relatively high temperatures, which inherently results in the formation of substantial emissions of nitrogen oxides. In the case of gas turbine combustors, the formation of nitrogen oxides can be greatly reduced by limiting the residence time of the combustion products in the combustion zone. However, even under these circumstances, undesirable quantities of nitrogen oxides are nevertheless produced. Additionally, limiting such residence time makes it difficult to maintain stable combustion even after ignition. The present study relates to the design of gas turbine combustors for the reduction of nitrogen oxides emissions by heterogeneous catalysis. Steady-steady simulations are performed using computational fluid dynamics. The fluid viscosity, specific heat, and thermal conductivity are calculated from a mass fraction weighted average of species properties, and the specific heat of each species is calculated using a piecewise polynomial fit of temperature. Natural parameter continuation is performed by moving from one stationary solution to another. Particular emphasis is placed upon the sustained combustion of at least a portion of fuel under essentially adiabatic conditions at a rate which surmounts the mass transfer limitation. The results indicate that it is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. Flammable mixtures of carbonaceous fuels normally burn at relatively high temperatures, and substantial amounts of nitrogen oxides inevitably form if nitrogen is present. Complete catalytic combustion of a target species can only occur when oxygen gas is found in molar stoichiometric excess; a condition which is easily met when the target species is present in trace quantity in air. In combustion systems utilizing a catalyst, there is little or no nitrogen oxides formed in a system which burns the fuel at relatively low temperatures. In the mass transfer limited zone, the reaction rate cannot be increased by increasing the activity of the catalyst because catalytic activity is not determinative of the reaction rate. Among the unique advantages of the catalytically supported thermal combustion in the presence of a catalyst is the fact that mixtures of fuel and air which are too fuel-lean for ordinary thermal combustion can be burned efficiently.Keywords: Heterogeneous catalysis; Nitrogen oxides; Gas turbines; Flammable mixtures; Thermodynamic properties; Combustion phenomena

Use of ethanol is attracting increasing attention both as primary feedstock and as an alternative to increase the feedstock flexibility in a given unit. However, steam reforming of ethanol is not a straight forward process. Equilibrium of the reaction is shifted towards the production of hydrogen even at low temperature. However, in practice, ethanol is also converted to ethylene. The present study is focused primarily upon the production of hydrogen in fixed-bed reactors by steam-ethanol reforming under different temperature conditions. Computational fluid dynamics is used to model fluid flow, heat and mass transfer, and chemical reactions. The governing integral equations are solved for the conservation of mass, momentum, and energy and other scalars such as laminar flow and chemical species. Steady-state analyses are performed using computational fluid dynamics. The reaction rates are computed by the laminar finite-rate model. The present study aims to explore how to effectively produce hydrogen in fixed-bed reactors by steam-ethanol reforming at different temperatures. Particular emphasis is placed upon the effect of temperature on the transport and reaction characteristics of fixed-bed reactors for polymer electrolyte membrane fuel cell applications. The results indicate that under a thermodynamic point of view, high temperatures and steam-ethanol molar ratios promote hydrogen yield. Low ethylene content is obtained at high pressure and low temperature. At high temperatures the contribution of steam reforming reactions results in a marked increasing of overall enthalpy, enhancing process endothermicity, whereas the exothermic contribution of water-gas shift and methanation reactions reduces the external heat supply and the overall energy penalty at lower temperature. Although the equilibrium of the water-gas shift reaction favors the products formation at lower temperatures, reaction kinetics are faster at higher temperatures. The typical products distribution of ethanol steam reforming reaction, according to thermodynamic evaluations, results in considerable hydrogen production rates at higher temperatures and high methane yields at lower temperatures. The majority of supported metals as catalysts expresses better performance at high temperatures, and the production of oxygenated products increases and the formation of coke is thermodynamically favored at low temperatures. Low reaction temperatures generally favor the Boudouard reaction mechanism while methane decomposition is the main route at high temperatures.Keywords: Hydrogen; Ethanol; Reformers; Burners; Reforming; Combustion

The present study is focused primarily upon the electrical and thermal properties of epoxy matrix composite materials reinforced with multi-walled carbon nanotubes under different weight fraction conditions. Stable suspensions of carbon nanotubes are achieved in water with the use of surfactants, and non-covalent and covalent attachment of polymers. Scanning electron microscopy characterization is performed and electrical resistance is measured. Mechanical properties are studied and the loading rate is continuously adjusted to keep a constant representative strain rate. The Oliver-Pharr method is used to analyze partial load-unload data in order to calculate the indentation elastic modulus as a function of the indenter penetration. The present study aims to provide an improved method for the preparation of epoxy matrix composite materials reinforced with multi-walled carbon nanotubes with reduced volume resistivity and enhanced thermal conductivity. Particular emphasis is placed upon the effect of carbon nanotube weight fraction on the volume resistivity and thermal conductivity of the epoxy matrix composite materials reinforced with multi-walled carbon nanotubes. The results indicate that single-walled carbon nanotube structures can have smaller effective pore size than multi-walled carbon nanotube structures. Single-walled carbon nanotubes are harder to disperse and more difficult to functionalize than multi-walled carbon nanotubes. Heat resistance of carbon nanotubes varies depending on the diameter of carbon nanotubes and the quality of a graphene sheet constituting the wall of carbon nanotubes. As a G to D ratio of the carbon nanotube becomes higher, a degree of graphitization becomes higher. The single-walled carbon nanotube-reinforced fracture surfaces express substantial increases in the micron-level surface roughness. The multi-walled carbon nanotubes interact with the crack path and result in crack deflection and a more torturous fracture path. The percolation threshold for conductive particles embedded in an insulating polymer matrix is sensitive to the structure of the reinforcement, and the decrease in electrical resistivity with an increase in reinforcement content is attributed to the probability of reinforcement contact. Unlike electrical conductivity, where a sharp percolation threshold is achieved, the increase in thermal conductivity with increasing carbon nanotube concentration is nearly linear.Keywords: Electrical properties; Thermal properties; Carbon nanotubes; Electrical conductivity; Thermal conductivity; Thermogravimetric analysis

In modern industrial practice, a variety of highly exothermic reactions are promoted by contacting of the reaction mixture in the gaseous or vapor phase with a heterogeneous catalyst. A need exists for improved catalytic structures employing integral heat exchange which will substantially widen the window or range of operating conditions under which such catalytic structures can be employed in highly exothermic processes like catalytic combustion or partial combustion. The highly exothermic process characteristics of catalytic reactors are investigated with integral heat exchange structures. Ethane mole fraction and gas-phase reaction rate profiles in catalytic reactors are presented, and ethane mole fraction, flow velocity, gas-phase reaction rate, and temperature contour plots are illustrated for catalytically supported thermal combustion systems. The present study aims to provide an improved reaction system and process for combustion of a fuel wherein catalytic combustion using a catalyst structure employing integral heat exchange affords a partially-combusted, gaseous product which is passed to a homogeneous combustion zone where complete combustion is promoted by means of a flame holder. Particular emphasis is placed upon the catalytic reactor configuration that allows the oxidation catalyst to be backside cooled by any fluid passing through the cooling conduits. The results indicate that the percentage of reaction completed in the exothermic catalytic reaction channel depends both upon the flow rate of the fuel-oxidant mixture through the exothermic catalytic reaction channel and upon the physical characteristics of the catalytic reactor. The tortuosity of the catalytic channels is increased by changing their cross-sectional area at a multiplicity of points along their longitudinal axes. The gas flow velocity entering the exothermic catalytic reaction channel should exceed the minimum required to prevent flashback into the fuel-oxidant stream upstream of the reactor if the fuel-oxidant mixture entering the exothermic catalytic reaction channel is within the limits of flammability. Catalytically-supported thermal combustion in the catalytic reactor is achieved by contacting at least a portion of the carbonaceous fuel intimately admixed with air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of the fuel-air admixture. The film heat transfer coefficient provides useful means of characterizing the different flow geometries provided by the various flow channel configurations which distinguish the catalyst-coated channels from the catalyst-free channels of the catalyst structure. The total residence time in the combustion system should be sufficient to provide essentially complete combustion of the fuel, but not so long as to result in the formation of oxides of nitrogen.Keywords: Catalytic reactors; Physical characteristics; Exothermic reactions; Heterogeneous catalysts; High temperatures; Thermal combustion

The thermal interface material technologies used for electronic packages encompass several classes of materials. However, there is still a need for thermal interface materials and methods for making thermal interface materials having improved thermal conductivity property by maximizing the anisotropic benefit of exfoliated graphite platelets to the fullest extent. The effect of filler volume fraction on the thermal resistivity of the thermal contact and the thermal conductivity of the thermal interface material is investigated for graphite platelets and carbon black. The effect of pressure on the bond line thickness of the thermal interface material is evaluated for smooth and rough surfaces. The present study aims to provide a thermal interface material with aligned graphite nanofibers in the thermal interface material to enhance the material performance. Particular emphasis is placed upon the heat conduction properties of thermally conductive interface materials with exfoliated graphite platelets. The results indicate that polymeric elastomer materials offer both high thermal performance and reasonable gap filling capability to enable good contact between a semiconductor component and a heat sink. Under mechanical pressure, the soft thermal interface material conforms to the microscopic surface contours of the adjacent solid surfaces and increases the microscopic area of contact between the thermal solution surface and the silicon die surface and therefore reduces the temperature drop across this contact. The heat dissipating component should advantageously be relatively anisotropic, as compared to a metal and exhibit a relatively high ratio of thermal conductivity to weight. Thermal interface materials provide a limited heat-conduction path and may include flexible heat-spreading materials and one or more layers of soft thermal interface material. Reducing the strain on the thermal interface material may reduce the potential for pump-out and the associated increase in thermal resistance due to loss of material from the interface. Thermal conductivity is driven primarily by the nature of the filler, which is randomly and homogeneously distributed throughout the matrix. Pump-out of the thermal interface material results in increased thermal resistance due to loss of material from the interface. The thermal interface material can migrate out of the interface volume between the thermal conducting members and onto the power input pads, resulting in excessive heating and part failure at the power interconnect.Keywords: Interface materials; Thermoplastic materials; Smooth surfaces; Rough surfaces; Graphite platelets; Carbon black