DETAILED MODELLING OF CATALYTIC CHEMISTRY IN SHORT CONTACT TIME REACTORS

Transcription

1 DETAILED MODELLING OF CATALYTIC CHEMISTRY IN SHORT CONTACT TIME REACTORS RUDDY SERGE VINCENT DIPL. ENG., D.E.A. THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF IMPERIAL COLLEGE LONDON MECHANICAL ENGINEERING DEPARTMENT IMPERIAL COLLEGE LONDON DECEMBER 28

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3 Pour Maman, Papa et Anna... Only when knowledge is thoughtfully adsorbed, can it become your own wisdom.

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5 Abstract The current thesis presents a detailed modelling study of the selective oxidation of ethane over noble metal coated surfaces in short contact time reactors. Computational studies were performed featuring heated gas streams flowing through ceramic-foam catalysts coated with platinum and followed by a long inert section. The detailed chemical kinetic mechanisms, with coupled surface and gas-phase chemical reactions, were explored via extensive reaction path and sensitivity analyses to assess the relative contributions of the homogeneous and heterogeneous chemistries and to establish the key heterogeneous pathways driving the chemical processes. A comprehensively validated detailed chemical mechanism was used for the gas phase. The mechanism initially featured 44 chemical species and 271 reversible reactions and was later extended to 176 reactants with 993 reversible reactions. Heterogeneous models describing the surface chemistry were derived on the basis of classical kinetic collision theory and with energy barriers obtained from Density Functional Theory studies combined with the Unity Bond Index-Quadratic Exponential Potential method. The derived surface mechanisms account for differences in site occupation and surface bonding types and include four reaction classes (direct adsorption, adsorption on an adsorbate, surface reactions with adsorbed reactants and uni-molecular surface reactions including desorption) via 35 adsorbed chemical species and 284 reversible reactions. The complete chemistry was thoroughly evaluated by comparison with multiple sets of existing and new experimental data provided by industrial partners. Key modelling parameters in the process, such as streams velocities, temperature profiles, catalyst loading and pressure were critically examined. The reaction dynamics were validated with C 2 H 6 /O 2 /H 2 mixtures with different initial hydrogen contents and with oxygen to car- 5

6 6 bon weight ratios ranging between.25 and.9. The major chemical pathways for the production of ethylene through the selective dehydrogenation of ethane, combined with the heterogeneous oxidative were identified.

7 Acknowledgments First, I would like to thank Professor R. P. Lindstedt. He has been such a great supervisor, guiding me in this academical venture, understanding both my strengths and needs. Special thanks to collaborators Dr. N.A. Malik and B.E.Messenger who have contributed a lot to the work in this thesis and to Dr. I.A.B. Reid, Dr. V.C. Williams and Dr. I.R. Little of Ineos Technologies Ltd, whose conttribution was exceptionally important throughout this project. My key gratitude goes to Dr. R.K. Robinson who performed many of the quantum mechanical computations. My thanks to Ms. S. Dalrymple and Ms. V. Rice for their continued support. My special thanks to my colleagues in the thermofluids section, Drs. F. Cerru, C. L. Cheung, M. J. Cleary, A. Kempf, T. S. Kuan, D. S. Luff, H.-C. Ozarovsky, M. L. Potter, K.-A. Rizos, A. Wandel and Messrs. J. Floyd, P. Geipel, K. Gkagkas, K. Goh, B. Lad, M. Leaford, V. Markaki, M. Bonnani, M. Montini, L. Nicolaou Fernandez, S.-W. Park, M. Persson, A. Rabhiou, S. Saha, O. Stein, P. Vaishnavi, N. Vaughan and K. Vogiatzaki. Thank you to all my friends for their support all these years. Finally, my indebted heartful thank you are for L.Y.A. Chee who believed in me from the start and through all the difficult times and to A. Manzoor (aka Snowy) who did tag along for the ride. Evidently, I shall forever be thankful to my parents and family who have supported me and to God for giving me strength during my long university studies. 7

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9 Contents Table of Contents 9 List of Tables 12 List of Figures 16 List of Symbols 38 1 Introduction 43 2 Mathematical Model Computational methods Coupling with heterogeneous chemical reactions Heterogeneous Chemistry Heterogeneous reaction schemes The surface coverage The gas phase chemistry Three-body reactions Pressure dependent reactions Thermodynamic considerations Site density considerations Reaction class based estimation of rate constants Reaction class I: Direct adsorption Reaction class II: Adsorption on an adsorbate

10 1 CONTENTS Reaction class III: Surface reactions with adsorbed reactants Reaction class IV: Unimolecular surface reactions Energetics of surface reactions Enthalpies of adsorption and heats of reactions Comments on heats of adsorption Energy barriers for forward and reverse reactions Isomerization reactions Surface diffusion barriers Semi-automatic generation of surface mechanisms Calculation of conversion and selectivity Oxidative Dehydrogenation of Ethane to Ethylene Introduction Experimental setups Numerical simulations vs. experimental results The impact of different O/C ratios Sensitivity to heat losses Sensitivity to heats of adsorption Site density and surface kinetics Heterogeneous and homogeneous reactions Impact of reactor residence time Chemical mechanisms and path analysis Heterogeneous oxidation of hydrogen Ethane oxidative dehydrogenation Heterogeneous depletion of ethylene Heterogeneous oxidative mechanisms Summary Impact of Hydrogen Co-feed on Chemical Pathways Introduction Experimental methods

11 CONTENTS Computational methods Mixtures with short residence times Dynamics and effect of the residence time Heterogeneous oxidation of hydrogen Ethane oxidative dehydrogenation Impact of hydrogen addition in the feed Heterogeneous oxidative pathways Heterogeneous oxidative decomposition of ethylene Summary Heterogeneous Chemistry at Elevated Pressure Introduction Experimental data sets Computed intra-diffusion limitation Extended homogeneous chemistry Elevated pressures Impact of pressure variations Chemical mechanisms Hydrogen feed variations Impact of different H 2 /O 2 ratios Influence of high pressure regime Mixtures with ethylene addition Summary Conclusions and outlook 233 Bibliography 239 Appendix 259 A Gas Phase Reaction Mechanism 259 B Thermodynamic data 293

12 12 CONTENTS C Gas Phase Chemical Structures 37 D Surface phase Reaction Mechanism 317

13 List of Tables 3.1 Sticking coefficients s and molecular properties: VDW is the radius of the projected van Der Waals surface for the compound A in Ångström, M is its molecular weight in g mol Heats of adsorption (Q) of species on a platinum surface in kj mol 1. The indices s σi indicate the number of platinum atoms involved in the adsorption of the compounds. The values retained in the surface mechanism shown in Appendix D are displayed in bold font. The alternative values also listed were used in the generation of the UBI-QEP based mechanism Energy barriers for gas phase isomerization reactions. Units are in kj mol 1. Computations were performed by Dr. R.K. Robinson [1] using Gaussian3 at the G3B3 level [2] Energy barriers used to assess the sensitivity to surface isomerization reactions in kj mol 1. The activation energies were estimated form the gas phase reactions given in Table 3.3 with barriers corrected using the enthalpy of reaction (Method 1) and a transition state (TST) approach (Method 2) according to Eq. (3.67). The heats of adsorption used are consistent with the reaction mechanism given in Appendix D Experimental parameters: L t (mm) total length before sampling point, oxygen to carbon (O/C) weight ratio, hydrogen to oxygen volume ratio H 2 /O 2, superficial inlet velocity U in (m s 1 ), inlet temperature T in (K), wall temperature at the entrance of catalyst T w in (K) and the maximum adiabatic temperature T max (K) used in heat loss calculations

14 14 LIST OF TABLES 4.2 Path analysis for the consumption of O 2 and production of H 2 O at a distance 5 mm inside the pore for Case 4.B.5. The overall percentage contribution of the matching reaction is denoted Abs. (%) Experimental and modelling parameters: L c (mm) total length of catalyst, L t (mm) total length before sampling point, oxygen to carbon (O/C) weight ratio, hydrogen to oxygen H 2 /O 2 volume ratio, pressure P (atm), N 2 concentration per volume (%), W Pt catalyst loading per weigth (%), pore per inch (PPI), superficial initial inlet velocity U in (m s 1 ), inlet temperature T in (K), wall temperature at the entrance of catalyst T w in and the maximum adiabatic temperature T max used in heat loss calculations Path analysis for the consumption of O 2 and production of H 2 O at a distance 5 mm inside the pore with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.57 (Case A1). The overall percentage contribution of the matching reaction in the hybrid mechanism is denoted Abs. (%) and the equivalent contribution in the Zerkle mechanism is denoted Z-Abs. (%) Path analysis for the heterogeneous consumption of O 2 and C 2 H 6 at a distance 1 mm inside the pore with H 2 /O 2 =, O/C =.65 and U in = 1.65 m s 1. The overall percentage contribution of the matching reaction in the hybrid mechanism is denoted Abs. (%) and the equivalent contribution in the Zerkle mechanism is denoted Z-Abs. (%) Experimental parameters: L t (mm) total length before sampling point, oxygen to carbon (O/C) weight ratio, pressure P (atm), hydrogen to oxygen volume ratio (H 2 /O 2 ), ethylene to ethane weight ratio (Eth) in the initial feed, superficial inlet velocity U in (m/s), inlet temperature T in (K), wall temperature at the entrance of catalyst T w in and the maximum adiabatic temperature T max used in heat loss calculations

15 LIST OF TABLES Path analysis for the consumption of O 2 and production of H 2 O at a distance 5 mm inside the pore with hydrogen addition, H 2 /O 2 = 1, O/C =.64 and elevated pressure P = 11 atm (Case 6.B4). The overall percentage contribution of the matching reaction is denoted Abs. (%) Path analysis for the heterogeneous consumption of O 2 and C 2 H 6 at a distance 5 mm inside the pore with no hydrogen addition, H 2 /O 2 =, O/C =.65 and elevated pressure P = 11 atm. The overall percentage contribution of the matching reaction in the hybrid mechanism is denoted Abs. (%) Arrhenius rate constants for the C 1 C 2 gas phase chemistry Modified Arrhenius parameters for the C 2 H 5 + O 2 system used for elevated pressures above 1 atm Arrhenius rate constants completing the C 1 C 6 gas phase chemistry Thermodynamic data The hybrid heterogeneous chemistry: Major heterogeneous reactions involved in the partial oxidation of ethane over platinum. The indices f. and r. stand for forward and reverse rates The hybrid heterogeneous chemistry: Major heterogeneous reactions involved in the partial oxidation of ethane over platinum with the alternative (lower) heats of adsorption of ethylene corresponding to 29.3 kj mol 1 for C 2 H 4 (s) and 46.1 kj mol 1 for C 2 H 4 (s 2 ). The indices f. and r. stand for forward and reverse rates

16 16 LIST OF TABLES

17 List of Figures 3.1 Heat of formation of C 2 H x and CH y species on Pt(111) computed from Table 4.1. All energy levels are referenced to zero kj mol 1 in the gas phase at 298 K Calculated axial profiles of (a) ethane conversion and (b) temperatures at the center of the pore for different O/C ratios:.45 (dashed lines),.55 (dot-dashed lines),.65 (solid lines) and.7 (short dashed lines) (Cases 4.B.1, 4.B.3, 4.B.5 and 4.B.6). The vertical double arrow demarcates the exit of the catalyst. Experimental data available at sampling points for the corresponding cases are shown with symbols. Simulations are presented with κ 2 =.7 and with the mid point value of the site density Calculated axial profiles of (a) ethane conversion and (b) temperatures at the center of the pore for different O/C ratios from.45 to.7 (Cases 4.A.1-4.A.6). Experimental data available at sampling points for the corresponding cases are shown with symbols. Simulations are presented with κ 2 =.7 and with the mid point value of the site density. The vertical arrows direct the profile shift when the severity is incremented sequentially Calculated axial profiles of selectivity to (a) C 2 H 4, (b) CO, (c) CH 4 and (d) CO 2 at different O/C ratios:.45 (dashed lines),.55 (dot-dashed lines),.65 (solid lines) and.7 (short dashed lines) (Cases 4.B.1, 4.B.3, 4.B.5 and 4.B.6). The vertical double arrow demarcates the exit of the catalyst

18 18 LIST OF FIGURES 4.4 Calculated axial profiles of selectivity to (a) C 2 H 4, (b) CO, (c) CH 4 and (d) CO 2 for different O/C ratios set at (circles).55, (squares).65 and (triangles).75 (Cases 4.A.2, 4.A.4, 4.A.6). Simulations are presented with κ 2 =.7 and with the mid point value of the site density. The vertical arrows indicate the profile shift when the severity is incremented sequentially Calculated axial profiles of oxygen selectivity to (a) H 2 O, (b) CO and (c) CO 2 at different O/C ratios:.45 (dashed lines),.55 (dot-dashed lines),.65 (solid lines) and.7 (short dashed lines) (Cases 4.B.1, 4.B.3, 4.B.5 and 4.B.6). The vertical double arrow indicates the exit of the catalyst Calculated axial profiles of oxygen selectivity to (a) H 2 O, (b) CO and (c) CO 2 for different O/C ratios set at (circles).55, (squares).65 and (triangles).75 (Cases 4.A.2, 4.A.4, 4.A.6). Simulations are presented with κ 2 =.7 and with the mid point value of the site density. The vertical arrows indicate the profile shift when the severity is incremented sequentially Simulation results (lines) compared to experimental data (symbols). (a) Ethane (circles) and oxygen (squares) conversion, (b) H 2 conversion (triangles left) and, selectivity to (c) C 2 H 4 (triangles up), (d) CO (diamonds), (e) methane (triangles down) for different O/C ratios with an inlet velocity of 1.34 m s 1 (Cases 4.A.1 4.A.6). Simulations were performed using the mid point value for the site density and with κ 2 = (dashed lines), κ 2 =.7 (solid lines) and κ 2 =.15 (dot-dashed lines) Simulation results (lines) compared to experimental data (symbols). O- selectivity to (a) H 2 O (triangles up), (b) CO (diamonds) and (c) CO 2 (triangles down) for different O/C ratios with an inlet velocity of 1.34 m s 1 (Cases 4.A.1 4.A.6). Simulations were performed using the mid point value for the site density and with κ 2 = (dashed lines), κ 2 =.7 (solid lines) and κ 2 =.15 (dot-dashed lines)

19 LIST OF FIGURES Simulation results (lines) compared to experimental data (symbols) and with the alternative (lower) heats of adsorption of ethylene, corresponding to 29.3 kj mol 1 for C 2 H 4 (s) and 46.1 kj mol 1 for C 2 H 4 (s 2 ), and other species as listed in Table 3.2. (a) Ethane (circles) and oxygen (squares) conversion, (b) H 2 (triangles left) coversion and selectivity to (c) C 2 H 4 (triangles up), (d) CO (diamonds) and (e) CH 4 (triangles down) for different O/C ratios with an inlet velocity of 1.34 m s 1 (Cases 4.A.1 4.A.6). Simulations were performed using the mid point value for the site density and with κ 2 = (dashed lines), κ 2 =.7 (solid lines) and κ 2 =.15 (dot-dashed lines) Simulation results (lines) compared to experimental data (symbols) with the alternative (lower) heats of adsorption of ethylene, corresponding to 29.3 kj mol 1 for C 2 H 4 (s) and 46.1 kj mol 1 for C 2 H 4 (s 2 ), and other species as listed in Table 3.2. O-selectivity to (a) H 2 O (triangles up), (b) CO (diamonds) and (c) CO 2 (triangles down) for different O/C ratios with an inlet velocity of 1.34 m s 1 (Cases 4.A.1 4.A.6). Simulations were performed using the mid point value for the site density and with κ 2 = (dashed lines), κ 2 =.7 (solid lines) and κ 2 =.15 (dot-dashed lines) Simulation results (lines) compared to experimental data (symbols) of cases 4.A.1 4.A.6. Conversion of fuel against C-selectivity to (a) C 2 H 4, (b) CO, (c) CH 4 and O-selectivity to (d) H 2 O, (e) CO and (f) CO 2. Simulations were performed with the two mechanisms corresponding to the heats of adsorption of selected species set to the (lower) alternative (dashed lines) and standard (solid lines) values listed in Table 3.2 (also see text). Simulations were performed using the mid point value for the site density and with κ 2 =

20 2 LIST OF FIGURES 4.12 Simulation results (lines) compared with experimental data (symbols). (a) C 2 H 6 (circles) and O 2 (squares) conversion, (b) H 2 (triangles left) conversion and selectivity to (c) C 2 H 4 (triangles up), (d) CO (diamonds), (e) CH 4 (triangles down) for different O/C ratios with an inlet velocity of 2.1 m s 1 (Cases 4.B.1 4.B.6). Simulations are presented for κ 2 =.7 and with Γ (solid lines), Γ max (dashed lines) and Γ min (dot dashed lines) Simulation results (lines) compared with experimental data (symbols). (a) O 2 molar fraction along the centre line of the pore for O/C ratio equal to.45 (dashed lines),.55 (dot-dashed line) and.65 (solid line). O- selectivity to (b) H 2 O (triangles up), (c) CO (diamonds) and (d) CO 2 (triangles down) for different O/C ratios with an inlet velocity of 2.1 m s 1 (Cases 4.B.1 4.B.6). Simulations are presented for κ 2 =.7 and with Γ (solid lines), Γ max (dashed lines) and Γ min (dot dashed lines). The vertical double arrow indicates the exit of the catalyst Simulation results (lines) compared to experimental data (symbols) of cases 4.A.1-4.A.6. Conversion of fuel against C-selectivity to (a) C 2 H 4, (b) CO, (c) CH 4 and O-selectivity to (d) H 2 O, (e) CO and (f) CO 2. Simulations were performed with κ 2 =.7 and Γ mid (solid lines), Γ max (dashed lines) and Γ min (dot dashed lines) Calculated rates of consumption of (a) C 2 H 6, (b) H 2, (c) O 2 and (d) H 2 O from heterogeneous reactions (solid lines) and gas phase processes (dashed lines) above the catalytic section of the pore for Cases 4.B.1, 4.B.3 and 4.B.5 with κ 2 =.7. Negative values indicate that the compound is produced. The arrows indicate the profile shift when the O/C ratio is progressively increased as.55,.65 and Integrated relative contribution of heterogeneous reactions (solid lines) and gas phase reactions (dashed lines) in the production of (a) C 2 H 4, (b) CO, (c) CH 4 and (d) CO 2 when the O?C ratio is.65. The arrows direct the profile shifts when the site density was increased from Γ = 5x1 5 mol m 2 to Γ max = 7.5x1 5 mol m

21 LIST OF FIGURES Simulation results (lines) compared with experimental data (symbols). (a) C 2 H 6 (circles) and (b) O 2 (squares) conversions. Selectivity to main products: (c) C 2 H 4 (triangles up), (d) CO (diamonds), (e) CH 4 (triangles down) at different inlet velocities (Cases 4.B11, 4.C.1 4.C.4). Simulations with κ 2 =.7, the mid point value of Γ (solid lines) and Γ max (dashed line) Simulation results (lines) compared to experimental data (symbols). (a) O 2 molar fraction along the pore for 2.1 m s 1 (solid line), 4.18 m s 1 (dashed line), 5.86 m s 1 (dot-dashed line) inlet velocities. O-selectivity to (b) H 2 O (triangles up), (c) CO (diamonds), (d) CO 2 (triangles down) at different inlet velocities and with O/C equal to.65 (Cases 4.B.5, 4.C.1 4.C.4). Simulations with κ 2 =.7 and the mid point value of Γ (solid lines) and Γ max (dashed line) Calculated axial profiles of (a) C 2 H 6 and (b) O 2 conversion and (c) C 2 H 4, (d) CO,at different inlet velocities: 2.62 m s 1 (solid lines), 4.18 m s 1 (dashed lines) and 5.86 m s 1 (dot dashed lines). All computations used κ 2 =.7 and the mid point value of Γ. Experimental conditions correspond to Cases 4.B.5, 4.C.2 and 4.C Major channels in the heterogeneous reaction mechanism of ethane and ethylene on the surface at a distance of 5 mm from the entrance of the pore. Figures correspond to the cumulative contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Major channels in the heterogeneous reaction mechanism of ethane and ethylene on the surface at a distance of 29 mm from the entrance of the pore. Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound

22 22 LIST OF FIGURES 4.22 Major channels in the heterogeneous oxidative mechanism producing carbon monoxide and carbon dioxide at a distance of 29 mm from the entrance of the pore. Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Simulation results (lines) compared with experimental data (symbols) from set 5.A with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.57: (a) Conversion of C 2 H 6 (circles) and O 2 (squares), selectivity to (b) H 2 (triangles left), (c) C 2 H 4 (triangles up), (d) CO (diamonds) and (e) CH 4 (triangles down) for different superficial inlet velocities. Solid lines are predictions from this work. Dashed lines are predictions obtained using the Zerkle et al. [3] mechanism. The vertical arrows indicate the profile shift when the site density is increased from Γ min to Γ mid Simulation results (lines) compared with experimental data (symbols) from set 5.A with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.57: (a) O 2 mole fraction along the center line of the pore when the superficial inlet velocity U in is increased from 3.81 m s 1 to 9.35 m s 1 (cases 5.A1 and 5.A7) and oxygen distribution to (b) H 2 O (triangles up), (c) CO (diamonds) and (d) CO 2 (triangles down) for different superficial inlet velocities. Solid lines are predictions from this work. Dashed lines are predictions obtained using the Zerkle et al. [3] mechanism. The arrow in (a) directs the profile change associated with the increasing velocity whereas, the vertical arrows in (b,c,d) indicate the profile shift when the site density is increased from Γ min to Γ mid Integrated axial profiles of (a) ethane conversion and (b) temperatures at the center of the pore when the superficial inlet velocity U in is increased from 3.81 m s 1 to 9.35 m s 1 (cases 5.A1 and 5.A7) Solid lines are predictions with the hybrid mechanism and dashed lines are predictions obtained using the Zerkle et al. [3] mechanism. The arrows indicate the change associated with increasing velocity

23 LIST OF FIGURES Integrated axial profiles of carbon distribution to (a,e) C 2 H 4, (b,f) CO, (c,g) CO 2, (d,h) CH 4 with hydrogen co-feed at H 2 /O 2 = 2 and with O/C =.57. Predictions with this work (a,b,c,d) and with the Zerkle et al. [3] mechanism (e,f,g,h) correspond to an superficial inlet velocity U in set at 3.81 m s 1 (solid lines) and 9.35 m s 1 (dashed lines) (Cases 5.A1 and 5.A7). The vertical double arrow indicates the exit of the catalyst Integrated axial profiles of (a) the conversion of ethane, selectivity to (b) ethylene and (d) methane versus residence time with hydrogen co-feed at H 2 /O 2 = 2 and with O/C =.57. The corresponding axial distance is displayed in (c). The inlet velocities were set to 3.81, 5.7, 6.6, 7.5 and 9.3 m s 1 (Cases 5.A1, 5.A3-7) Calculated rates of heterogeneous (a,b,c,d) and gas phase (e,f,g,h) consumption of (a,e) C 2 H 6 and (b,f) O 2 and production of (c,g) C 2 H 4 and (d,h) H 2 O with hydrogen co-feed at H 2 /O 2 = 2 and with O/C =.57. Predictions with the hybrid (solid lines) and Zerkle et al. [3] (dashed lines) mechanism corresponding to inlet velocities U in at 3.81 m s 1 and 9.35 m s 1 (Cases 5.A1 and 5.A7). Negative values indicate that the compound is either produced (C 2 H 6 ) or consumed (C 2 H 4 ) depending on if it is a reactant or a product. The arrows indicate the change associated with increasing velocity Integrated relative contribution of heterogeneous reactions (solid lines) and gas phase reactions (dashed lines) in the production of (a) C 2 H 4, (b) CO, (c) CH 4 and (d) CO 2. The horizontal arrows indicate the profile shift when the inlet velocity U in was increased from 3.81 m s 1 to 9.35 m s 1 (Cases 5.A1 and 5.A7). Negative values for ethylene indicate net consumption on the surface

24 24 LIST OF FIGURES 5.8 Major channels in the heterogeneous reaction mechanism of C 2 H 6 and C 2 H 4 on the surface at a distance of 5 mm from the entrance of the pore obtained with the hybrid surface mechanism with O/C =.57 and H 2 /O 2 = 2 (Set 5.A). Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Major channels in the heterogeneous reaction mechanism of C 2 H 6 and C 2 H 4 on the surface at a distance of 5 mm from the entrance of the pore obtained with the Zerkle et al. [3] mechanism with O/C =.57 and H 2 /O 2 = 2 (Set 5.A). Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Major channels in the heterogeneous reaction mechanism of C 2 H 6 and C 2 H 4 on the surface at a distance of 5 mm from the entrance of the pore obtained with the Zerkle et al. [3] surface mechanism with O/C =.57 and H 2 /O 2 = 2 (Set 5.A). Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Major channels in the heterogeneous reaction mechanism of C 2 H 6 and C 2 H 4 on the surface at a distance of 5 mm from the entrance of the pore obtained with the hybrid mechanism with O/C =.57 and H 2 /O 2 = 2 (Set 5.A). Figures correspond to the cumulative absolute contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Conversion of (a) ethane and the carbon distribution to (b) C 2 H 4, (c) CO, (d) CO 2, (e) CH 4 and (f) C 2 H 2 when the H 2 /O 2 ratio is varied between and 3. Solid lines are predictions from this work. Dashed lines are predictions obtained using the Zerkle et al. [3] mechanism (Set 5.B)

25 LIST OF FIGURES Simulation results (lines) compared with experimental data (symbols) from Set 5.C with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.66: Integrated axial profiles of the conversion of (a) C 2 H 6 (circles) and O 2 (squares) and the selectivity to (c) C 2 H 4 (triangles up), (d) CH 4 (triangles down), (e) CO (diamonds) and (f) CO 2 (triangles left). Axial temperature profiles inside the pore. (b) Predictions correspond to Tin w w = 11 K (solid lines) and Tin = 12 K (dashed lines) Simulation results (lines) compared with experimental data (symbols) from Set 5.C with no hydrogen in the feed, H 2 /O 2 = and O/C =.66: Integrated axial profiles of the conversion of (a) C 2 H 6 (circles) and O 2 (squares) and the selectivity to (c) C 2 H 4 (triangles up), (d) CH 4 (triangles down), (e) CO (diamonds) and (f) CO 2 (triangles left). (b) Axial temperature profiles inside the pore. Predictions correspond to T w in = 11 K (solid lines), T w in = 115 K (dot-dashed lines) and T w in = 12 K (dashed lines) Calculated rates of heterogeneous (solid lines) and gas phase (dashed lines) production of (a) C 2 H 6, (b) O 2, (c) C 2 H 4, (d) CH 4, (e) CO, (f) CO 2, (g) H 2 and (h) H 2 O with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.66 and with T w in=11k. (Cases 5.C1). Negative values correspond to consumption Calculated rates of heterogeneous (solid lines) and gas phase (dashed lines) production of (a) C 2 H 6, (b) O 2, (c) C 2 H 4, (d) CH 4, (e) CO, (f) CO 2, (g) H 2 and (h) H 2 O without hydrogen co-feed, H 2 /O 2 = and O/C =.66 and with T w in=11k. (Cases 5.C2). Negative values correspond to consumption

26 26 LIST OF FIGURES 5.17 Simulated results (lines) compared with experimental data (symbols) from sets 5.E and 5.F: (a,b) Temperature profiles and (c,d) conversion of C 2 H 6 (circles) and O 2 (squares) without (a,c) hydrogen in the feed, H 2 /O 2 = and (b,d) with hydrogen co-feed set at H 2 /O 2 = 2 and an inlet velocity of 1.65 m s 1. (Sets 5.D, 5.E). Simulations were performed with the hybrid mechanism Simulation results (lines) compared with experimental data (symbols) from sets 5.D and 5.E, without hydrogen (H 2 /O 2 = ) (a,c,e) and with hydrogen co-feed (H 2 /O 2 = 2) (b,d,f): Selectivity to (a,b) C 2 H 4 (triangles up), (c,d) CO (diamonds) and (e,f) CH 4 (triangles down) for different O/C ratios at an inlet velocity of 1.65 m s 1. Simulations were performed with the hybrid mechanism Simulation results (lines) compared with experimental data (symbols) from sets 5.D and 5.E, without hydrogen (H 2 /O 2 = ) (a,c,e) and with hydrogen co-feed (H 2 /O 2 = 2) (b,d,f): Oxygen distribution to (a,b) H 2 O (triangle up), (c,d) CO (diamond) and (e,f) CO 2 (triangle down) for different O/C ratios at an inlet velocity of 1.65 m s 1. Simulations were performed with the hybrid mechanism Integrated axial conversion profiles of (a,b) oxygen and (c,d) ethane for different O/C ratios:.6 (dashed lines),.7 (dot-dashed lines)and.8 (solid lines) when (a,c) no hydrogen is in the initial feed (H 2 /O 2 = ) and (b,d) with hydrogen co-feed (H 2 /O 2 = 2). Experimental data available at sampling point for the corresponding cases (5.D3, 5.D4, 5.E2, 5.E3, 5.E4) are shown with symbols Integrated axial selectivity profiles for (a) C 2 H 4 (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios:.6 (dashed lines),.7 (dot-dashed lines)and.8 (solid lines) with hydrogen co-feed (H 2 /O 2 = 2). Experimental data available at sampling point for the corresponding cases (5.E2, 5.E3, 5.E4) are shown with symbols

27 LIST OF FIGURES Integrated axial selectivity profiles for (a) C 2 H 4 (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios:.6 (dashed lines),.7 (dot-dashed lines)and.8 (solid lines) without hydrogen co-feed (H 2 /O 2 = ). Experimental data available at sampling point for the corresponding cases (5.D3, 5.D4, 5.D5) are shown with symbols Integrated axial oxygen- selectivity profiles to (a) H 2 O (b) CO and (c) CO 2 for different O/C ratios:.6 (dashed lines),.7 (dotdashed lines)and.8 (solid lines) without (a,c,e) hydrogen co-feed (H 2 /O 2 = ) and with (b,d,f) hydrogen co-feed (H 2 /O 2 = 2). Experimental data available at sampling point for the corresponding cases (5.D3, 5.D4, 5.D5, 5.E2, 5.E3, 5.E4) are shown with symbols Calculated rates of heterogeneous (solid lines) and gas phase (dashed lines) consumption of (a,b) C 2 H 6 and (c,d) O 2 and production of (e,f) C 2 H 4, (g,h) CO, (i,j) H 2 and (k,l) H 2 O (left) without hydrogen co-feed (H 2 /O 2 = ) (a,c,e,g,i,k) and (right) with hydrogen co-feed (H 2 /O 2 = 2) (b,d,f,h,j,l). Cases featured O/C =.65 and an inlet velocity of 1.65 m s 1. (Sets 5.D and 5.E). Negative values for H 2 indicate a net consumption The temperature dependent effectiveness factors η e computed by Wanker et al. [4] (dotted lines-(1)) and by Hayes et al. [5] for a cylindric shape configuration (solid-dashed line-(2)) and a fillet shape geometry in a square channel (solid line-(3)). The numerical value of η e above 9 K for (2) and (3) are extrapolations suggested in this work

28 28 LIST OF FIGURES 6.2 Simulation results (lines) compared with experimental data (symbols) from Set 4.A with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.57: (a) Conversion of C 2 H 6 (circles) and O 2 (squares), selectivity to (b) H 2 (triangles left), (c) C 2 H 4 (triangles up), (d) CO (diamonds) and (e) CH 4 (triangles down) for different superficial inlet velocities. Dashed lines are predictions with the C 1 C 2 gas phase mechanism and solid lines are predictions obtained using the extended C 1 C 6 mechanism. The vertical arrows indicate the profile shift when the site density is increased from Γ min to Γ mid Simulation results (lines) compared with experimental data (symbols) from Set 6.B. (a) Ethane conversion and (b) selectivity to ethylene for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Dashed lines are predictions with the C 1 C 2 gas phase mechanism and solid lines are predictions obtained using the extended C 1 C 6 mechanism. The vertical arrows indicate the profile shift when the site density is increased from Γ min to Γ mid Simulation results (lines) compared with experimental data (symbols) from Set 6.B. Selectivity to (a) C 2 H 4, (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Dashed lines are predictions with the C 1 C 2 gas phase mechanism and solid lines are predictions obtained using the extended C 1 C 6 mechanism. The vertical arrows indicate the profile shift when the site density is increased from Γ min to Γ mid Simulation results (lines) compared with experimental data (symbols) from Set 6.B. (a) Conversion of O 2 and O-selectivity to (b) H 2 O, (c) CO, (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Dashed lines are predictions with the C 1 C 2 gas phase mechanism and solid lines are predictions obtained using the extended C 1 C 6 mechanism. The vertical arrows indicate the profile shift when the site density is increased from Γ min to Γ mid

29 LIST OF FIGURES Calculated axial profiles of (a) ethane conversion and (b) oxygen mole fraction at the center of the pore for different O/C ratios:.45 and.65 with the C 1 C 2 chemistry (dashed lines) and the extended C 1 C 6 mechanism (solid line). The initial hydrogen content is H 2 /O 2 = 1 and corresponds to Set 6.B. The arrows indicate the profile shift when O/C is increased from.45 to Calculated axial profiles of selectivity to (a) C 2 H 4, (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios:.45 and.65 with the C 1 C 2 chemistry (dashed lines) and the extended C 1 C 6 mechanism (solid line). The initial hydrogen content is H 2 /O 2 = 1, corresponding to Set 6.B. The arrows indicate the profile shift when O/C is increased from.45 to Simulation results (lines) with the extended C 1 C 6 gas phase mechanism compared with experimental data (symbols) from Set 4.A and with hydrogen co-feed at H 2 /O 2 = 2 and O/C =.57: Selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (e) 1, 3-C 4 H 6 and (f) C 6 H 6 for different superficial inlet velocities at 1 atm Simulation results (lines) compared with experimental data (symbols) from Set 6.B. Selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (e) 1, 3-C 4 H 6 and (f) C 6 H 6 for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm Calculated axial profiles of selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (e) 1, 3-C 4 H 6 and (f) C 6 H 6 for different O/C ratios:.45 (dashed lines) and.65 (solid line) with the extended C 1 C 6 mechanism. The initial hydrogen content is H 2 /O 2 = 1, corresponding to Set 6.B

30 3 LIST OF FIGURES 6.11 Simulation results (lines) compared with experimental data (symbols) from Set 6.B. (a.c) Ethane conversion and (b.d) selectivity to ethylene for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Simulations were performed with (a,b) Γ mid and (c,d) Γ min with the extended C 1 C 6 gas phase mechanism with a constant effectiveness factor η e =.1 (dashed lines), η e =.5 (dot-dashed lines) and temperature dependent η e (solid lines) derived from the circular wall configuration described in Fig Simulation results (lines) compared with experimental data (symbols) from Set 6.B. Selectivity to (a) C 2 H 4, (b) CH 4, (c) CO and for (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Simulations with Γ mid were performed with the extended C 1 C 6 gas phase mechanism with a constant effectiveness factor η e =.1 (dashed lines), η e =.5 (dot-dashed lines) and the temperature dependent η e (solid lines) derived from the circular wall configuration described in Fig Simulation results (lines) compared with experimental data (symbols) from Set 6.B. Selectivity to (a) C 2 H 4, (b) CH 4, (c) CO and for (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and a pressure of 11 atm. Simulations with Γ min were performed with the extended C 1 C 6 gas phase mechanism with a constant effectiveness factor η e =.1 (dashed lines), η e =.5 (dot-dashed lines) and the temperature dependent η e (solid lines) derived from the circular wall configuration described in Fig Simulation results (lines) compared with experimental data (symbols) from Sets 6.B and 6.D. (a) Ethane conversion and (b) selectivity to ethylene for different O/C ratios with H 2 /O 2 = 1 and with a pressure of 11 atm (circles) or a pressure of 21 atm (squares). Lines are predictions with the extended C 1 C 6 gas phase mechanism at 11 atm (solid lines) and at 21 atm (dashed lines)

31 LIST OF FIGURES Simulation results (lines) compared with experimental data (symbols) from Sets 6.B and 6.D. Selectivity to (a) C 2 H 4, (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and with a pressure of 11 atm (circles) or a pressure of 21 atm (squares). Lines are predictions with the extended C 1 C 6 gas phase mechanism at 11 atm (solid lines) and at 21 atm (dashed lines) Simulation results (lines) compared with experimental data (symbols) from Sets 6.B and 6.D. (a) Oxygen conversion and O-selectivity to (b) H 2 O, (c) CO and (d) CO 2 for different O/C ratios with H 2 /O 2 = 1 and with a pressure of 11 atm (circles) or a pressure of 21 atm (squares). Lines are predictions with the extended C 1 C 6 gas phase mechanism at 11 atm (solid lines) and at 21 atm (dashed lines) Simulation results (lines) compared with experimental data (symbols) from Sets 6.B and 6.D. Selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (d) 1, 3-C 4 H 6 and (e) C 6 H 6 for different O/C ratios with H 2 /O 2 = 1 and with a pressure of 11 atm (circles) or a pressure of 21 atm (squares). Solid lines are predictions with the extended C 1 C 6 gas phase mechanism at 11 atm (solid lines) at 21 atm (dashed lines) Calculated rates of consumption of (a,b) C 2 H 6, (c,d) O 2 and (d,e) C 2 H 4 from heterogeneous reactions (b,d,f) and gas phase processes (a,c,e) above the catalytic section of the pore with H 2 /O 2 = 1, O/C =.45 (dashed lines) and O/C =.65 (solid lines) at a pressure of 11 atm. Negative values indicate that the compound is reversely produced Calculated rates of consumption of (a,b) C 2 H 6, (c,d) O 2 and (d,e) C 2 H 4 from heterogeneous reactions (b,d,f) and gas phase processes (a,c,e) above the catalytic section of the pore with H 2 /O 2 = 1 with O/C =.45 (dashed lines) and O/C =.65 (solid lines) at a pressure of 21 atm. Negative values indicate that the compound is produced

32 32 LIST OF FIGURES 6.2 Calculated rates of production of (a,b) CO, (c,d) CO 2, (e,f) CH 4 and (g,h) H 2 O from heterogeneous reactions (solid lines) gas phase processes (dashed lines) above the catalytic section of the pore with H 2 /O 2 = 1, (a,c,e,g) O/C =.65 and (b,d,f,h) O/C =.45 at a pressure of 11 atm. Negative values indicate that the compound is consumed Predicted radial mass fractions of C 2 H 6 at different axial positions at atmospheric pressure, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions correspond to Case 5.A1 of Chapter Predicted radial mass fractions of C 2 H 6 at different axial positions at 11 atm, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions match the data Set 6.B Predicted radial mass fractions of O 2 at different axial positions at atmospheric pressure, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions correspond to Case 5.A1 of Chapter Predicted radial mass fractions of O 2 at different axial positions at 11 atm, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions correspond to Set 6.B Predicted radial mass fraction of C 2 H 4 at different axial positions at atmospheric pressure, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions correspond to Case 5.A1 of Chapter Predicted radial mass fraction of C 2 H 4 at different axial positions at 11 atm, O/C=.65, H 2 /O 2 = 1 and with the extended C 1 C 6 gas phase mechanism. The experimental conditions correspond to Set 6.B

33 LIST OF FIGURES Major channels in the homogeneous formation of heavier hydrocarbons at a distance of 4 mm from the entrance of the pore. Figures correspond to the cumulative contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound. (The compound in brackets assists the major reaction) Major channels in the heterogeneous oxidation of ethylene at a distance of 4 mm from the entrance of the pore, (Case 6.B4). Figures correspond to the cumulative contribution (in percentage) of all reactions involved in the path consuming the involved species and producing the target compound Simulation results (lines) compared with experimental data (symbols) from Sets 6.A, 6.B and 6.C. (a) Ethane conversion and (b) selectivity to ethylene for different O/C ratios with H 2 /O 2 = (circles), H 2 /O 2 = 1 (squares), H 2 /O 2 = 2 (triangles) and with a pressure of 11 atm. Lines are predictions with the extended C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines), H 2 /O 2 = 1 (solid lines), H 2 /O 2 = 2 (dotdashed lines) Predicted conversion of ethane (lines) compared with experimental data (symbols) at 11 atm when the O/C ratio is increased from.25 to.8. The H 2 /O 2 ratio is set to (circles) and 1 (squares). Lines are predictions with the extended C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines) and H 2 /O 2 = 1 (solid lines). The operating conditions correspond to Sets 6.J and 6.K Predicted selectivity to ethylene (lines) compared with experimental data (symbols) as the conversion increases at 11 atm. The O/C ratio was increased from.25 to.8. The H 2 /O 2 ratio is set to (circles) and 1 (squares). Lines are predictions with the extended C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines) and H 2 /O 2 = 1 (solid lines). The operating conditions correspond to Sets 6.J and 6.K

34 34 LIST OF FIGURES 6.32 Simulation results (lines) compared with experimental data (symbols) from Sets 6.A, 6.B and 6.C. Selectivity to (a) ethylene, (b) CH 4, (c) CO and (d) CO 2 for different O/C ratios with H 2 /O 2 = (circles), H 2 /O 2 = 1 (squares), H 2 /O 2 = 2 (triangles) at a pressure of 11 atm. Lines are predictions with the extended C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines), H 2 /O 2 = 1 (solid lines), H 2 /O 2 = 2 (dotdashed lines) Simulation results (lines) compared with experimental data (symbols) from Sets 6.A, 6.B and 6.C. (a) Oxygen conversion and (b) O-selectivity to (b) H 2 O, (c) CO and (d) CO 2 for different O/C ratios with H 2 /O 2 = (circles), H 2 /O 2 = 1 (squares), H 2 /O 2 = 2 (triangles) at a pressure of 11 atm. Lines are predictions with the extended C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines), H 2 /O 2 = 1 (solid lines), H 2 /O 2 = 2 (dot-dashed lines) Simulation results (lines) compared with experimental data (symbols) from Sets 6.A, 6.B and 6.C. Selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (d) 1, 3-C 4 H 6 and (e) C 6 H 6 for different O/C ratios with H 2 /O 2 = (circles), H 2 /O 2 = 1 (squares), H 2 /O 2 = 2 (triangle) at a pressure of 11 atm. Lines are predictions with the detailed C 1 C 6 gas phase mechanism with H 2 /O 2 = (dashed lines), H 2 /O 2 = 1 (solid lines), H 2 /O 2 = 2 (dot-dashed lines) Calculated axial profiles of the conversion of (a) C 2 H 6 and (b) O 2 at a pressure of 11 atm with different H 2 /O 2 ratios: (dashed lines), 1 (solid lines) and 2 (dot-dashed lines). The O/C ratio was fixed at.65. The computations correspond to Sets 6.A, 6.B and 6.C Calculated axial profiles of selectivity to (a) C 2 H 4, (c) CH 4, (b) CO and (d) CO 2 at 11 atm with different H 2 /O 2 ratios: (dashed lines), 1 (solid lines) and 2 (dot-dashed lines). The O/C ratio was fixed at.65. The computations correspond to Sets 6.A, 6.B and 6.C

35 LIST OF FIGURES Calculated rates of heterogeneous consumption (a) C 2 H 6 and (c) O 2, production of (b) C 2 H 4, (d) H 2 O, (e) CO and (f) CO 2 when hydrogen is added to the initial feed from H 2 /O 2 = (solid lines) to H 2 /O 2 = 1 (dashed lines) and H 2 /O 2 = 2 (dot-dashed lines). The O/C was fixed to.65 (solid lines) at a pressure of 11 atm. Negative values indicate that C 2 H 6 is produced and C 2 H 4 is consumed Simulation results (lines) compared with experimental data (symbols) from Sets 6.B, 6.C, 6.E, 6.F, 6.G and 6.H. (a,b) Ethane conversion and (c,d) selectivity to ethylene for different O/C ratios at a pressure of 11 atm, with (a,c) H 2 /O 2 = 1 and with (b,d) H 2 /O 2 = 2. Ethylene (Eth) was progressively added to the initial feed with Eth = % (circles), Eth = 5% (squares) and Eth = 1% (triangles). Solid lines are the respective predictions with the extended C 1 C 6 gas phase mechanism. The vertical arrows indicate the profile shift with ethylene introduction Simulation results (lines) compared with experimental data (symbols) from Sets 6.B, 6.C, 6.E, 6.F, 6.G and 6.H. Selectivity to (a,b) CO and (c,d) CO 2 and (e,f) CH 4, for different O/C ratios at a pressure of 11 atm, with (a,c,d) H 2 /O 2 = 1 and (b,d,f) H 2 /O 2 = 2.Ethylene (Eth) was progressively added to the initial feed with Eth = % (circles), Eth = 5% (squares) and Eth = 1% (triangles). Solid lines are the respective predictions with the extended C 1 C 6 gas phase mechanism. The vertical arrows indicate the profile shift with ethylene introduction Simulation results (lines) compared with experimental data (symbols) from Sets 6.B, 6.E and 6.G. Selectivity to (a) C 3 H 6, (b) C 3 H 8, (c) C 4 H 1, (d) C 4 H 8 (N), (d) 1, 3-C 4 H 6 and (e) C 6 H 6 for different O/C ratios at a pressure of 11 atm and with H 2 /O 2 = 1. Ethylene (Eth) was progressively added to the initial feed with Eth = % (circles), Eth = 5% (squares) and Eth = 1% (triangles). Solid lines are the respective predictions with the extended C 1 C 6 gas phase mechanism. The vertical arrows indicate the profile shift with ethylene introduction