Optimization of geometric configuration and fuel flow rate in a microchannel reactor for steam reforming of methane

다국어 초록 (Multilingual Abstract)

There are some limitations in a heat exchanger integrated microchannel reactor for steam reforming of methane that hot spot formation is inevitable because of the imbalance between the generated and absorbed heat, and a lot of time and energy are required for reactor start-up. Therefore, it is necessary to optimize geometric configuration and fuel flow rate in a microchannel reactor for steam reforming of methane to minimize hot spot and achieve rapid start-up.
First of all, a stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by a response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 145 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.94 and 2.37, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 6.0% less than that of the uniform stripe catalyst layer.
Secondly, hydrogen assisted catalytic combustion was applied for rapid start-up. Hydrogen assisted catalytic combustion does not require preheating because the catalytic combustion of hydrogen occurs at room temperature. After start-up by hydrogen catalytic combustion, fuels of hydrogen and methane were changed to methane. The geometric configuration of the counter-flow reactor was optimized by the simulation model under steady state condition. The hydrogen flow rate in the counter-flow reactor was also optimized by transient simulations using the response surface methodology. As a result, the counter-flow reactor showed extremely short start-up time because of the optimized configuration and the optimized hydrogen flow rate. Hot spots were avoided because of the hydrogen shut-off after start-up. The operating characteristics of the counter-flow reactor were compared with those of the co-flow reactor.

목차 (Table of Contents)

• Abstract ii
• Contents iv
• List of Figures vii
• List of Tables ix
• Nomenclature x
• Chapter 1. Introduction 1
• 1.1 Background 1
• 1.2 Literature review 4
• 1.2.1 Minimization of hot spot in a steam reforming reactor 4
• 1.2.2 Hydrogen assisted catalytic combustion in a steam reforming reactor 6
• 1.3 Objectives and outline of this Study 8
• Chapter 2. Numerical Analysis 9
• 2.1 Kinetics of steam reforming of methane and catalytic combustion 9
• 2.2 Steady state simulation 13
• 2.2.1 Computational model 13
• 2.2.2 Governing equations 16
• 2.2.3 Boundary conditions 17
• 2.3 Transient simulation 19
• 2.3.1 Computational model 19
• 2.3.2 Governing equations 20
• 2.3.3 Initial and boundary conditions 20
• 2.4 Properties 22
• Chapter 3. Model Validation 24
• 3.1 Grid dependency and time-step test 24
• 3.2 Comparison with experimental data 28
• Chapter 4. Minimization of Hot Spot with the Stripe Combustion Catalyst Layer in a Microchannel Reactor for Steam Reforming of Methane 32
• 4.1 Introduction 32
• 4.2 Operating conditions 34
• 4.3 Flow and species distributions 35
• 4.4 Optimization of stripe configuration 39
• 4.5 Comparision of the uniform and the optimal stripe catalyst layer 45
• 4.6 Effects of design parameters 50
• 4.7 Reduction of catalytic surface area 53
• 4.8 Summary 55
• Chapter 5. Optimization of a Counter-flow Microchannel Reactor using Hydrogen Assisted Catalytic Combustion for Steam Reforming of Methane 56
• 5.1 Introduction 56
• 5.2 Operating conditions 58
• 5.3 Optimization of the geometric configuration of the counter-flow reactor 59
• 5.4 Optimization of hydrogen flow rate 67
• 5.5 Comparison between the co-flow and counter-flow reactors 79
• 5.6 Summary 85
• Chapter 6. Concluding Remarks 86
• 6.1 Conclusions 86
• 6.1.1 Minimization of hot spot with the stripe combustion catalyst layer 86
• 6.1.2 Optimization of a counter-flow microchannel reactor using hydrogen assisted catalytic combustion 87
• 6.2 Future works 89
• References 90
• Appendix A. User Defined Functions 98
• Appendix B. Sintering of Nickel Catalyst 109