The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the performance of the thermally integrated reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.Keywords: Hydrogen production; Steam reforming; Transport phenomena; Reactor design; Microchannel reactors; Computational fluid dynamics

Little research has been conducted to determine the thermal properties and phenomena of graphane and fluorographene. A clear understanding of the thermal problems involved is needed, which may provide a basis for further research on other material properties. In the present study, molecular dynamics simulations were performed to investigate the thermal properties of graphane and fluorographene and especially the phenomena involved, including thermal fluctuations and bending rigidities. Furthermore, comparisons of thermal properties and the phenomena involved were made computationally between pristine and functionalised graphene. The thermal fluctuations and bending rigidities were determined at different temperatures. The present study aims to provide a clear understanding of the thermal problems involved in hydrogenated and fluorinated graphene. The results indicated that while thermally excited ripples spontaneously appear in graphene, fully hydrogenated or fluorinated graphene is substantially unrippled due to their very high bending rigidities. There is no significant effect of thermal rippling throughout graphane and fluorographene due to their very high bending rigidities. However, partially hydrogenated or fluorinated graphene exhibits strong thermal fluctuations. Graphene behaves differently from graphane and fluorographene with regard to the dependence of bending rigidity on temperature. Furthermore, significant out-of-plane fluctuations may occur in partially fluorinated graphene. Thermal fluctuations of graphene are more sensitive to temperature than those of graphane and fluorographene.Keywords: Thermal properties; Molecular dynamics; Thermal rippling; Thermal fluctuations; Bending rigidities; Thermal phenomena

In spite of significant efforts to investigate the ability of hydrogenated and fluorinated graphene to conduct heat, little research has focused particularly upon their other thermal properties, such as thermal contraction and heat capacity, which have implications for the development of thermal nanotechnology. In an attempt to determine these thermal properties, a few experiments have been carried out, with rather conflicting results. In the present study, calculations were performed using molecular dynamics to investigate the thermal properties of graphane and fluorographene and especially the phenomena involved. The thermal expansion coefficients and heat capacities of the two-dimensional materials were determined at different temperatures. The results indicated that graphane is thermally contracted more significantly than graphene. The calculated molar heat capacity at constant volume is about 25.00 J/(mol·K) for graphene and about 29.26 J/(mol·K) for graphane. The specific heat capacity of fluorographene is always lower than that of graphane. A negative relationship does exist between the binding energy and the temperature.Keywords: Carbon materials; Material properties; Thermal contraction; Structural configurations; Molecular dynamics; Thermal phenomena

The present study focuses upon the physics of heat and mass transfer processes in a protruded millisecond microchannel reactor, wherein a steam reforming reaction is proceeding and protrusions are used to improve the transport processes involved. Parametric analysis of the reactor system is carried out using a three-dimensional numerical model that is sufficiently detailed to delineate the role of geometric features and operation conditions in reactor performance. Computational fluid dynamics analyses are performed under different sets of circumstances. In analysing the mechanisms involved in the intensified processes, account is taken of the factors that may influence the reactor performance. New insights into the physics of the processes are presented, with recommendations on how to optimize reactor design for better performance. The results indicate that the flow rates and feed compositions must be adjusted as needed to maximize production of hydrogen and minimize pressure drops. Protrusions are very effective in improving the transport processes involved without greatly impairing hydraulic performance. Methanol can be converted effectively to hydrogen due to the successive continuous interruptions in the presence of hemispherical protrusions. Necessary adjustments to the molar ratio of steam to methanol can be made to control the maximum reactor temperature within certain needed limits. Protrusions can be used to improve the conversion and productivity due to enhanced heat and mass transfer, as they behave as a baffle to direct flow of the reforming process flow stream.Keywords: Operation conditions; Periodic wakes; Methanol processors; Protruded reactors; Process intensification; Efficient operation

Flow of a fluid past a body is a very complicated phenomenon. Computational fluid dynamics is used for studying the characteristics of flow past an array of hemispherical protrusions that is disposed on the wall surfaces of a millisecond microchannel reactor. Protrusions can be used to improve the transport processes involved, but the causes of the phenomena are still incompletely understood. Parametric analyses are performed under different sets of circumstances to delineate the role of geometric features and operation conditions in reactor performance. Dimensionless quantities are used to simplify the characterization of the reactor system with multiple interacting transport phenomena. The mechanisms involved in the intensified processes are analysed, and performance improvement recommendations are presented. The results indicate that the protruded reactor behaves effectively and good yields can be obtained with only milliseconds residence of the mixtures within the channels. The reactor offers the unique advantage for hydrogen production from methanol in that process intensification is realized while preserving the energy balance between the exothermic and endothermic processes. However, the flow rates must be adjusted as needed to maximize production of hydrogen and minimize pressure drops. The momentum diffusivity is more dominant around the protrusion regions than in the other regions. The thermal diffusivity is more dominant in the protruded channels than in the flat channels. The results have implications for hydrogen production and beyond for the study of transport phenomena in microchemical systems.Keywords: Hemispherical protrusions; Fuel cells; Heat transfer; Microchannel reactors; Endothermic reactions; Mass transfer

Computational modelling for microchannel reactor design was conducted in the attempt to fully understand autothermal reforming phenomena in a continuous flow reactor. The effects of pressure and flow rate on the efficiency and performance of the reactor were evaluated by performing computational fluid dynamics under different design conditions. The reactor efficiency and performance were assessed by means of the reactant conversion, product yield, reaction rate, hydrogen productivity, and output power. Recommendations for designing an autothermal reforming system were made and strategies for performing efficient operation were set forth. The results indicated that the pressure and flow rate set strict limits on operation, and there is a trade-off between high productivity and low conversion. The pressure and flow rate play competing roles in the reactor efficiency and performance. Lower pressures and flow rates can increase the conversion and yield, but higher pressures and flow rates can significantly improve the hydrogen productivity and output power, which is essential to the success of start-up and acceleration of the downstream equipment. There may exist an optimum pressure and flow rate in terms of both efficiency and performance. The calculated output power is of the order of thousands of kilowatts per cubic meter.Keywords: Chemical processes; Reactor design; Efficiency criteria; Performance criteria; Autothermal reforming; Transport phenomena

Protrusions can be used to improve the transport processes involved, but the causes of the phenomena are still incompletely understood. Computational fluid dynamics analyses are performed under different sets of circumstances to gain insights into the physics of heat and mass transfer processes in a protruded millisecond microchannel reactor, wherein a steam reforming reaction is proceeding and protrusions are used to improve the transport processes involved. Recommendations are made on how to optimize design for better reactor performance. Particular emphasis is placed on delineating the role of methanol-air equivalence ratio and channel length in reactor performance. The results indicate that the equivalence ratio and channel length must be adjusted as needed to minimize pressure drops and maximize production of hydrogen. Necessary adjustments to the equivalence ratio of methanol to air can be made to control the maximum reactor temperature within certain needed limits. The short-channel design may be preferred over the long-channel design in order to simultaneously achieve low pressure drops and sufficiently high conversions in the reactor. Expectable compromises have to be made between hydrogen productivity and pressure drop.Keywords: Flow configurations; Heat fluxes; Fuel cells; Equivalence ratios; Channel lengths; Heat exchange

Numerical methods and algorithms were used to solve and analyse problems that involve fluid flows in a millisecond autothermal reforming reactor. Computers were used to perform the calculations required to simulate the interaction of gasses with catalyst surfaces defined by boundary conditions. Computational modelling for the reactor design was performed to investigate the effects of various factors on the efficiency and performance of the system and clarify the relationship between heat, temperature, and energy. The yield and productivity from the chemical process were determined by performing computational fluid dynamics analysis. Additionally, the efficiency difference between feed compositions was determined by thermodynamic analysis. Strength and weakness were assessed under different reaction conditions. Design recommendations were provided and operation strategies were mapped out. The results indicated that the calculated output power of the system is of the order of thousands of kilowatts per cubic meter. Operation at millisecond contact times is feasible, but optimisation of reaction conditions is necessary to balance efficiency and performance. The conversion to hydrogen is influenced greatly by the feed composition, which must be controlled precisely within certain needed limits to maximize the yield and productivity from the chemical process while avoiding the problems of combustion or explosion. Keywords: Millisecond reactors; Thermodynamic analysis; Autothermal reforming; Output power; Reactant conversions; Reaction phenomena

Physical processes that transfer mass and energy by diffusion or convection occur in gaseous combustion. The management of internal features of a cavity structure for channels offers the possibility of inherently effective operation within the flammable limits of a combustible fluid stream while preserving high stability for the flame. However, the precise mechanism by which the cavity method generally provides increased flame stability for millimeter-scale systems remains unclear. The combustion characteristics of methane-air mixtures in millimeter-scale systems with a cavity structure are investigated experimentally and numerically to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into the characteristics of combustion within spaces with extremely small dimensions. Stable temperature profiles are obtained from thermographic measurements using infrared radiation. The measurements are compared with the model predictions. Subsequent model calculations demonstrate the effects of variations in wall thermal conductivity, heat losses, and equivalence ratio. Methods of applying a cavity structure to channel walls are developed, which may be utilized with presently existing designs of micro-combustion systems. The factors affecting flame stability and combustion characteristics are determined for the systems. The results indicate that the thermal conductivity of the burner walls plays a vital role in flame stability. The design with anisotropic thermal conductivity has significant performance advantages. Improvements in flame stability are achievable by using walls with anisotropic thermal conductivity. Heat-insulating materials are favored to minimize external heat losses. There are issues of efficiency loss for fuel-rich cases. Burners with large dimensions lead to a delay in flame ignition and may cause blowout. The combustion is stabilized by recirculation of hot combustion products induced by the cavity structure.Keywords: Thermographic measurements; Infrared radiation; Internal features; Flammable limits; Fluid streams; Effective operation

While burners designed in a backflow way enable operation with very low noxious emissions, they frequently operate very close to the extinction limit of the flame. Cavity structures have been designed for the purpose of improving flame stability. However, the precise mechanism by which the cavity method provides increased flame stability remains unclear. Computational fluid dynamics simulations are conducted to gain insights into burner performance such as reaction rates, species concentrations, temperatures, and flames. Factors affecting combustion characteristics and flame stability are determined. The results indicate that burner dimensions greatly affect flame stability. Burners with large dimensions lead to a delay in flame ignition and may cause blowout. The combustion is stabilized by recirculation of hot combustion products induced by the cavity structure. The inlet velocity of the mixture is a critical factor in assuring flame stability within the cavity-stabilized burner. There is a narrow range of inlet velocities that permit sustained combustion within the cavity-stabilized burner. Design recommendations are provided.Keywords: Burners; Flames; Temperatures; Species concentrations; Reaction rates; Combustion

Hydrogen and methanol are the primary fuels available for current fuel cells. The potential of methanol reforming systems to greatly improve productivity in chemical reactors has been limited, due in part, to the effect of mass transfer limitations on the production of hydrogen. There is a need to determine whether or not a microchannel reforming reactor system is operated in a mass transfer-controlled regime, and provide the necessary criteria so that mass transfer limitations can be effectively eliminated in the reactor. Three-dimensional numerical simulations were carried out using computational fluid dynamics to investigate the essential characteristics of mass transport processes in a microchannel reforming reactor and to develop criteria for determining mass transfer limitations. The reactor was designed for thermochemically producing hydrogen from methanol by steam reforming. The mass transfer effects involved in the reforming process were evaluated, and the role of various design parameters was determined for the thermally integrated reactor. In order to simplify the mathematics of mass transport phenomena, use was made of dimensionless numbers or ratios of parameters that numerically describe the physical properties in the reactor without units. The results indicated that the rate of the reforming reaction is limited by mass transfer near the entrance of the reactor and by kinetics further downstream, when the heat transfer in the autothermal system is efficient. There is not an effective method to reduce channel dimensions if the flow rate remains constant, or to reduce fluid velocities if the residence time is kept constant. The performance of the reactor can be greatly improved by means of proper design of catalyst layer thickness and through adjusting feed composition to minimize or reduce mass transfer limitations in the reactor. Finally, the criteria that can be used to distinguish between different mass transport and kinetics regimes in the reactor with a first-order reforming reaction were presented.Keywords: Fuel cells; Reactor design; Autothermal systems; Hydrogen production; Microchannel reactors; Steam reforming

The steam reforming of hydrocarbon fuels for the production of synthesis gas or hydrogen is a well-established technology. A common process is steam-reforming, where a suitable reforming catalyst facilitates the reaction between the hydrocarbon feed and steam to generate carbon monoxide and hydrogen. This study relates to a thermochemical process for producing hydrogen by the catalytic endothermic reaction of methanol with steam in a thermally integrated microchannel reforming reactor. Computational fluid dynamics simulations are conducted to better understand the consumption, generation, and exchange of thermal energy between endothermic and exothermic processes in the reactor. The effects of wall heat conduction properties and channel dimensions on heat transfer characteristics and reactor performance are investigated. Thermodynamic analysis is performed based on specific enthalpy to better understand the evolution of thermal energy in the reactor. Design recommendations are made to improve thermal performance for the reactor. The results indicate that the peak reaction heat flux increases with the channel dimensions while maintaining the flow rates. Reaction heat flux profiles are considerably affected by channel dimensions. The thermal conductivity of the channel walls is fundamentally important. Materials with high thermal conductivity are preferred for the channel walls. Thermally conductive ceramics and metals are well-suited. Wall materials with poor heat conduction properties degrade the reactor performance. The change in specific enthalpy is positive for the exothermic reaction and negative for the endothermic reaction. The change in specific sensible enthalpy is always positive.Keywords: Efficiencies; Performances; Enthalpy; Hydrocarbons; Conductivities; Hydrogen