INTRODUCTION TO CATALYTIC COMBUSTION

Transcription

1 INTRODUCTION TO CATALYTIC COMBUSTION R.E. Hayes Professor of Chemical Engineering Department of Chemical and Materials Engineering University of Alberta, Canada and S.T. Kolaczkowski Professor of Chemical Engineering Department of Chemical Engineering University of Bath, United Kingdom GORDON AND BREACH SCIENCE PUBLISHERS Australia Canada China France Germany India Japan Luxembourg Malaysia The Netherlands Russia Singapore Switzerland Thailand United Kingdom

2 Table of Contents Preface Acknowledgements Nomenclature xvii xix xxi CHAPTER 1. Introduction The basics: Terminology and conservation equations Commonly used terms Classification of reactors Momentum balances Energy balances Material balances The coupling of material, energy and momentum balances Catalytic combustion and transport processes Catalytic combustion chemistry Formation of NO x Catalyst support systems Pellets in a packed or fluidized bed Multichannel monoliths Parallel plates Fibre pads and gauzes Sintered metals Palladium based catalysts for methane combustion Influence of temperature on chemical composition of the catalyst Catalyst dispersion Catalyst poisoning and fouling Catalyst/support interactions Catalyst promoters Additives to the catalyst support Preparation of palladium catalyst Example applications of catalytic combustion Examples of primary combustion processes Stationary gas turbines Radiant heaters Process heating Examples of secondary combustion processes Catalytic converters for gasoline (petrol) engines 67

3 X CONTENTS Catalytic converters/traps for diesel engines Catalytic incineration of organic emissions Catalytic monoliths in NOx reduction reactors System design Summary 87 Introductory Note to Chapters 2 to 6 89 Additional Reading on Catalytic Combustion 89 Additional Reading on Catalytic Converters 90 References 90 CHAPTER 2. Thermodynamics, Kinetics and Transport Phenomena Thermodynamics Open and closed systems The thermodynamic state Equations of state The ideal gas law Non-ideal behaviour Multicomponent mixtures Energy balances in open and closed systems Enthalpy changes in systems Enthalpy change resulting from temperature change the heat capacity Enthalpy change with pressure Enthalpy change due to composition change the heat of reaction The first law for an open steady state reacting system Heat of combustion Chemical reaction equilibrium the equilibrium constant Catalyst decomposition pressure Kinetics Rate expressions and mechanisms Homogeneous combustion kinetics Homogeneous combustion of carbon monoxide Homogeneous combustion of hydrocarbons Catalytic combustion kinetics Adsorption Langmuir Hinshelwood Hougen Watson reaction models Rate models for catalytic oxidation reactions Comments on the use of rate equations Final word on catalytic rate models 177

4 CONTENTS XI 2.3 Transport phenomena Newton's law of viscosity Flow regimes Flow in ducts The equation of continuity The friction factor for flow in a duct Flow in circular ducts Flow in rectangular ducts Flow in triangular ducts Other shapes Flow in porous media Fourier's law of conduction and the energy equation Conduction in one dimensional systems Multidimensional systems Conduction and convection in fluid systems with and without reaction Conduction in porous media Boundary layers and the heat transfer coefficient Boundary layer development over a heated flat plate Boundary layer development in the entrance of a circular duct Fick's law of diffusion and the species balance equation Boundary layer development and the mass transfer coefficient The mass and heat transfer analogy External heat and mass transfer resistance in catalysis Diffusion and reaction in porous catalysts The effective diffusivity The effectiveness factor Non-isothermal effectiveness factors Effectiveness factors with complex kinetics Diffusion and reaction in complex catalyst geometries Combined internal and external mass transfer resistance Heat transfer by radiation 269 Further Reading 275 References 276 CHAPTER 3. Modelling of Catalytic Combustion Reactors Basic modelling concepts Types of equations 282

5 хп CONTENTS Constants and variables Linear algebraic equations Non4inear algebraic equations Differential equations Types of models Choosing the model Model validation Overview/summary of the modelling process Commercial software packages Modelling of a single monolith channel Basic model selection criteria Model dimensionality One dimensional steady state plug flow model laminar/turbulent flow ID pseudo4iomogeneous plug flow model steady state ID heterogeneous plug flow model steady state Heat and mass transfer coefficients One dimensional steady state dispersion model laminar/turbulent flow Transient one dimensional models laminar/turbulent flow Two dimensional model in cylindrical coordinates laminar flow Momentum balance equations Mass balance equation The energy balance equation Radiation modelling in a monolith channel Integral approach to internal radiation transfer Network of finite surfaces method for modelling internal radiation exchange Estimating radiation loss from a monolith reactor Modelling diffusion in the washcoat steady state ID approximation steady state Equations for solution in 2D steady state Evaluating the effectiveness factor in a monolith reactor simulation Effectiveness factors for washcoats with multiple reactions steady state Transient diffusion reaction problems Multiple channel honeycomb reactor models 383

6 CONTENTS хш Continuum model Discrete method with honeycomb reconfiguration 388 Packed bed reactor models One dimensional plug flow model of a packed bed reactor ID pseudo-homogeneous PFR model for an adiabatic packed bed steady state ID heterogeneous PFR model for an adiabatic packed bed steady state ID PFR models for a non-adiabatic packed bed steady state One dimensional dispersion model for a packed bed reactor Pseudo-homogeneous ID axial dispersion model for a packed bed steady state Heterogeneous ID axial dispersion model for a packed bed steady state Two dimensional model of a packed bed reactor Pseudo-homogeneous 2D model for a packed bed steady state Heterogeneous 2D model for a packed bed steady state Transport properties in packed beds Fluid/solid mass and heat transfer coefficients in packed beds Dispersion coefficients in packed beds Thermal conductivities in packed beds Bed to wall heat transfer coefficients in packed beds Effectiveness factors in packed beds 422 Consolidated porous media models 423 Numerical methods Methods for initial value problems Introduction to boundary value problems The basis of the finite difference method Finite difference solution of a one dimensional dispersion model The basis of the finite element method Finite element discretization D discretization of a monolith reactor channel D discretization of the washcoat Discretization error Interpolation polynomials 442

7 XIV CONTENTS The mathematical basis of the Galerkin finite element method Integrating the weak form of the differential equation: The reference element The matrix form of the elementary equations, assembling the global matrix Finite element solution of a diffusion/reaction problem Solution algorithms 459 Further Reading 464 References 464 CHAPTER 4. Homogeneous Gas Phase Reactions General combustion characteristics Combustion chemistry Auto-ignition Burning velocity Combustion models Diffusion flame (laminar/turbulent) model Premixed flame model Shock tube model Well stirred reactor model Plug flow model Structure of rigorous schemes Inclusion of catalytic/surface terms Flame stabilization Summary 501 References 502 CHAPTER 5. Experimental Studies Pre-ageing of combustion catalysts Acquisition and analysis of catalytic rate data Laboratory reactors Tubular reactor Temperature measurement Stirred tank/spinning basket/carberry reactor Internal recycle stirred tank/berty reactor Recycle tubular reactor Error estimation Errors evaluated using mathematical analysis Errors evaluated using sensitivity analysis Finding the kinetic expression 525

8 CONTENTS xv Inter- and intraphase mass and heat transfer Performance of pilot-scale reactor experiments Supply of air Preheating of inlet stream Fuel and air mixing Temperature measurement Flow measurement Gas analysis Data acquisition Pressure drop measurement Monoliths Packed beds Acquisition of transport property data Measurements to characterize the catalyst system The recipe The support system The catalyst coated/impregnated system Pore size, volume and distribution Hydraulic diameter of monolith Transport distance in catalyst phase Measurement of effective diffusion coefficients Gravimetric analysis Catalyst distribution in the support Catalyst loss from the reactor 584 Further Reading on Catalyst Preparation 586 References 586 CHAPTER 6. Combustion Applications: Examples of Modelling Studies Modelling a single monolith channel in 2D Investigating the Nusselt and Sherwood numbers Mass transfer limitation and the 'light-off point Oxidation of CO with LHHW kinetics Multiple steady states Radiation losses from a monolith channel Diffusion in a monolith washcoat use of diffusion barrier to reduce entrance temperature gradients Understanding and interpreting literature results Influence of intrusive measuring devices Catalytic radiant heaters Catalytic incineration of organic emissions Summary 647 References 648

9 XVI CONTENTS Appendix A Appendix В Appendix С Appendix D Appendix E Appendix F Useful conversions Physical properties of ceramic and metal supports Physical properties of gases Summary of dimensionless groups Useful mathematical transformations A note on symbols INDEX 675