chemical reaction engineering kinetics -...
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P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Chemical Reaction Engineering Kinetics
Prof. Paolo Canu
University of Padova - Italy
Computer-Aided Chemical Reaction Engineering Course
Graduate School in Chemical Engineering (GSCE) Åbo Akademi - POKE Researchers network
May 2014
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Contents 3. Kinetics
3.1 Power law 3.2 LHHW 3.3 Detailed
4. Kinetic studies
5. Applications
5.1 Tuning of Sh(z) in a monolith (2.3 + 3.1) 5.2 CH4 combustion on Pt in a monolith (2.3 + 3.1) 5.3 CO combustion in an annular reactor (2.2 + 3.3) 5.4 CH4 partial oxidation on Rh foam (2.2 + 3.3) 5.5 CH4 partial oxidation on Pt monolith (2.3 + 3.3) 5.6 H2 oxidation on pure Pt in stagnation flow (2.3 + 3.3)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Macro- vs micro-(surface) kinetics Macro
CH4 + 2 O2 CO2 + 2 H2O
1. C are in the bulk of the gas
2. Rate laws are adaptive and surface/flow specific
→Pt
−=
scmg
CCRTEAR CHa
OCH 25.0hetero 4
24exp
with A = 8.66x105 s-1 cm5/2 g-0.5, s-1 Ea = 60.72 kJ/mol
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Macro- vs micro- (homogeneous) kinetics Macro
CH4 + 2 O2 CO2 + 2 H2O
with A = 1.3x107 or 2.1x109 s-1 , Ea = 125 or 200 kJ/mol
1. Parameters are limited to a range of T and φ
2. Current detailed models accout for 325 reactions and 53 species (GRI3.0)
→
−= −
scmg
CCRTEAR CHa
OCH 33.13.0homo 4
24exp
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Macro- vs micro-(surface) kinetics Micro
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Macro- vs micro-(surface) kinetics Micro (part of)
Kunz et al. Modeling the Rate of Heterogeneous Reactions, 2011
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Macro- vs micro-(surface) kinetics Structure sensitive
CH4 decomposition kinetics on low index planes of Ni single crystal (450 K and 1 Torr methane pressure)
T.P. Beebe Jr., D.W. Goodman, B.D. Kay, J.T. Yates Jr., J.Chem. Phys. 87 (1987) 2305.
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Advantages
1. Fundamental → chemistry and physics are well distinguished
2. Account for surface structure (including heterogeneity)
3. Elementary steps → fundamental rate laws (from physical chemistry)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Disadvantages
1. Many species → many Material Balances (MBs) (reformulating species MBs to balances on single reactions could not be a reduction in eqs. to be solved)
2. Surface ↔ gas phase species
3. Exponential increase of kinetic parameters
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Reformulation of species MBs as balances on single reactions
νAA + νBB → νCC + νDD
• Material Balances (MBs) for A,B,C,D are required
• Thanks to stoichiometry: Ci = Ci° + ε νi
a single ‘MB’ on ε (reaction progress variable) is sufficient (like a single, ‘key species’ MB)
• Reduction from NC → NR equations (ODEs, AEs,..) is achieved
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics how to deal with
1. Ignore (psuedo-homogeneous rate laws)
2. Simplify by assumptions (LHHW)
3. Simplify by sensitivity
4. Use it, properly
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics using it
A. Kinetics
1. Rate expression
2. Kinetic parameters
B. Reactor (flow arrangement)
1. Ideal reactors (0D or 1D)
2. BL models
3. CFD
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Microkinetics Nomenclature
g = in the gas (fluid), in front of the surface s = adsorbed b = in the bulk of the solid
SiH4(g) + Si(s) → 2H2(g) + Si(s) + Si(b)
picture close to the surface → add interaction with the bulk!
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics kinetics
net production rate for surface reactions (e.g. moles/cm2 s)
heterogeneous prod. rate
net production rate for fluid phase reactions (e.g. moles/cm3 s)
homogeneous prod. rate
K = all species (any phase) Fluid species: both
Surface species: only sk
NR
NR
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Reaction rate
for fluid phase reactions (e.g. moles/cm3 s): mass-law
f,r = forward, reverse reaction
[X] = concentration
ν’ , ν’’ = forward, reverse stoich. coeff. (always positive!)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics rate constants
Fluid phase: units depends on molecularity; e.g. 1/[(moles/cm3)n s]
Required for forward reactions; for reverse ones:
From thermodynamics:
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics example of forward rate parameters
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Reaction rate
for surface reactions (e.g. moles/cm2 s)
[X] = concentrations; may have different units (gas/bulk or surface phase)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics rate constants – Arrhenius type
Surface phase: units depends on molecularity and type of species
From surface termodynamics:
Γn = Initial site density for surface phase n (moles/cm2)
σk = number of surface sites occupied by species k (-)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics rate constants – sticking coefficients type
Probability of successful collision with the surface:
ai , bi ci = reaction-specific constants
γi is usually <<1 (experimental)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics surface-coverage modification of rate constants
Zk = surface coverage (θ) by species k
ηki µki εki = surface coverage-dependent parameters for species k in reaction i
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics example of forward rate parameters
Surface-coverage dependent activation energy!
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics rate parameters – sources
Theoretical
(semi)-theoretical methods:
• Density Functional Theory (DFT)
• Molecular Dynamics (MD)
• Monte Carlo (MC)
Surface thermochemistry is required to check thermodynamic consistency
Note: often a pure-crystal Γ is used! It dramatically affects all the calculations (→ surface sites count and their variation)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics rate parameters – sources
Experimental
• AES/XPS:
Auger electron spectroscopy and X-ray photoelectron spectroscopy (surface composition and quantitative information on surface species)
• LEED : low energy electron diffraction (determination of the structure of the single crystal and ordered adsorbate layers)
• STM: scanning tunneling microscopy (imaging the local surface topography with atomic resolution)
• HREELS: high resolution electron energy loss spectroscopy (adsorbed species identification)
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Application
A. Mechanism identification: Quite a number already available (literature/web). Surface/chemistry specific!
• CHEMKIN format
• DETCHEM format
• Different formats in specific research group, often oriented to specific applications
Surface kinetics can use different conventions
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Micro-(surface) kinetics Application
B. Use of a mechanism (requires a reactor model) • Open source routines and programs
CHEMKIN-III (discontinued), Cantera, DETCHEM
• Research codes (owned by specific research group)
• Commercial codes (Reaction Design - CHEMKIN 4)
All require some understanding of the surface chemistry potential and limitations
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
Conclusions
1. Microkinetics represents the modern approach to surface chemistry Account for different surfaces, intermediate, chemistries
2. Exploiting its potential requires equally detailed reactor models → CFD
3. Coupling CFD and detailed surface chemistry can be obtain with chemkin/cantera use
P. Canu – CRE with µKin CACRE, Åbo Akademi, 2014
References 1. L. Kunz, L. Maier, S. Tischer, O. Deutschmann Modeling the Rate of Heterogeneous
Reactions in “Modeling of Heterogeneous Catalytic Reactions: From the molecular process to the technical system” O. Deutschmann (Ed.), Wiley-VCH, Weinheim 2011
2. R. J. Kee, F. M. Rupley, J. A. Miller, M. E. Coltrin, J. F. Grcar, E. Meeks, H. K. Moffat, A. E. Lutz, G. Dixon-Lewis, M. D. Smooke, J. Warnatz, G. H. Evans, R. S. Larson, R. E. Mitchell, L. R. Petzold, W. C. Reynolds, M. Caracotsios, W. E. Stewart, P. Glarborg, C. Wang, O. Adigun, W. G. Houf, C. P. Chou, S. F. Miller, P. Ho, and D. J. Young, CHEMKIN Release 4.0, Reaction Design, Inc., San Diego, CA (2004)