Microkinetic Modeling of Catalytic Reactions · PDF filemodeling ! Experimental kinetic study...
Transcript of Microkinetic Modeling of Catalytic Reactions · PDF filemodeling ! Experimental kinetic study...
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Microkinetic Modeling of Catalytic Reactions
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Modern history of catalysis
1910’s 1940’s 1990’s
Michaelis-Menten
Langmuir-Hinschelwood
Hougen-Watson
Dumesic-Rudd Froment
ki is extracted from fitting particular kinetic equations
ki is either calculated or measured
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Catalytic Reactors
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Development of Important Industrial Catalytic Processes
Production of Liquid Fuels!!!
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Development of Important Industrial Catalytic Processes
NO CO CxHy
N2 CO2 H2O
O2
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Transport and Reaction Processes in a Catalytic Reaction
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Heterogeneous Catalysis
A (g) B (g)
• Minimize ΔP • Minimize Mass Transport
Resistances • Maximize Activity • Minimize Poisoning and
Fouling
Support (Al2O3)
Active Metals (Pt, Co, MoO2)
support
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Surface reaction
Metal
1. Adsorption 2. (Diffusion on the surface) 3. Surface reaction 4. Desorption
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Composition of conventional model
CH4
CO CO2
2 H2O
4 H2
H2 H2O
H2O
H2
Syngas production
I. II.
III.
rCO = -r1+r3 rCH4 = r1-r2 rCO2 = r2-r3 rH2 = r1+4r2-r3 rH2O = -r1-2r2+r3
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Why do we need microkinetic modeling n Experimental kinetic study used to determine
details in the mechanism - Problem: Different models may fit data equally well
n Deduction of kinetics from a proposed reaction mechanism
n Historically macroscopic descriptions of the reaction kinetics were used
n Today, detailed scientific information available n Guidance for catalytic reaction synthesis at
various levels of detail
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What is microkinetics about?
Definition of microkinetic analysis examination of catalytic reactions in terms of elementary chemical reactions that occur on the catalytic surface and their relation with each other and with the surface during a catalytic cycle
J. A. Dumesic et al., Ind. Eng. Chem. Res. 1987, 26 (1399)
It means that the subject of investigation is not the overall reaction but each particular elementary reaction.
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Use of Microkinetic Modeling
n Use of kinetic model for description of – Reaction kinetic data
n Spectroscopic observations n Microcalorimetry and TPD n Reduction of large kinetic mechanisms
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Composition of MK model
Overall reactions A+B ↔ AB A+2C ↔ AC2
Elementary reactions A + * ↔ A* B + * ↔ B* C + * ↔ C* A* + B* ↔ C* + * A* + C* ↔ AC* + * AC* + C* ↔ AC2* + * AB* ↔ AB + * AC2* ↔ AC2 + *
2 Independent reactions 12 Species 5 gaseous A, B, AB, C, AC2 G 7 surface *, A*, B*, C*, AC*, AB*, AC2* Sur 12 Equations (G-1) reactor equations
depends whether reactor is CSTR or plug flow
(Sur-1) Steady state dΘ/dt=0
1 mass conversion Σyi=1
1 site conversion ΣΘi=1
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Parameters for Microkinetic Analysis
n Sticking coefficients n Surface bond energies n Pre-exponential factors for surface
reactions n Activation energies for surface reactions n Surface bonding geometries n Active site densities and ensemble sizes
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Elementary Reaction It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equation
A + B à C If it is an elementary reaction,
A B C
-rA = k [A]1 [B]1
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Elementary reactions
n elementary reaction is such a reaction in which one or more of the species react directly to form products
n molecularity is the number of colliding molecules in a single reaction step
n different types Dissociation AB = A + B Combination A + B = AB Disproportionation AB + C = A + BC
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Kinetic variables in MK
n Preexponential factor n Transition State theory n Collision theory
n Activation energy n Unity Bond Index – Quadratic Exponential
Potential (UBI-QEP) n DFT
n Energy barrier n UBI-QEP
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Collision theory
is used to determine rate constants for adsorption processes in terms of number of gas-phase molecules colliding with a surface per unit are per unit time
demanded inputs
n sticking coefficient (as a function of temperature) n pressure
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Composition of MK model
Overall reactions A+B ↔ AB A+2C ↔ AC2
Elementary reactions A + * ↔ A* B + * ↔ B* C + * ↔ C* A* + B* ↔ C* + * A* + C* ↔ AC* + * AC* + C* ↔ AC2* + * AB* ↔ AB + * AC2* ↔ AC2 + *
2 Independent reactions 12 Species 5 gaseous A, B, AB, C, AC2 G 7 surface *, A*, B*, C*, AC*, AB*, AC2* Sur 12 Equations (G-1) reactor equations
depends whether reactor is CSTR or plug flow
(Sur-1) Steady state dΘ/dt=0
1 mass conversion Σyi=1
1 site conversion ΣΘi=1
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Example of Microkinetic Model – NH3 Decomposition over a Ruthenium Catalyst
Deshmukh et al, Microreactor Modeling for Hydrogen Production from Ammonia Decomposition on Ruthenium, Ind. Eng. Chem. Res. 2004, 43, 2986-2999
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Axial Lenth, (m)
Mas
s Fr
actio
n of
Sur
face
Spe
cies
Surface Phase Concentration Profiles in Modeling of NH3 Decomposition using Microkinetic Analysis
H(s)N(s)NH(s)NH2(s)NH3(s)Ru(s)
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Axial Lenth, (m)
Mas
s Fr
actio
n of
gas
pha
se s
peci
esGas Phase Concentration Profiles in Modeling of NH3 Decomposition using Microkinetic Analysis
NH3H2N2Ar
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MK model - example
3 CO + 2 H2O = 2 CO2 + CH3OH
rCO* = 3r1-r5-2r11 r* = -3r1-r2+2r3+r4+r5+r6
+r7+r8-2r9-2r10+2r11 rH2O* = 2r2-2r9 rCO2* = 2r3-2r11 rCH3OH* = -r4+r8 rH* = -r5-r6-r7-r8+2r9+2r10 rHCO* = r5-r6 rH2CO* = r6-r7 rH3CO* = r7-r8 rOH* = 2r9-2r10 rO* = 2r10-2r11
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CO H2 CO2 H2O
a) T = 623K ptot = 2,1 MPa H2/CO = 3
b) T = 573K ptot = 1,1 MPa H2/CO = 3 c) T = 553K ptot = 2,1 MPa H2/CO = 3 d) T = 523K ptot = 2,1 MPa H2/CO = 3
Examples
G. Lozano-Blanco et al., Ind. Eng. Chem. Res. 2008, 47 (5879)
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Compare of micro and conventional models
Micro Conventional
Detail
Very complex
Limited to the terminal species and
r.d.s.
Information
Trends, what’s going on on the surface
Accurate information the selectivity,
temperature profile
Interval of application
Wide
Narrow, determined by experimental
conditions
Application Catalyst and process design
Reactor and process design