chemical reaction engineering kinetics -...

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


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)


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


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


Macro- vs micro-(surface) kinetics Micro


Macro- vs micro-(surface) kinetics Micro (part of)

Kunz et al. Modeling the Rate of Heterogeneous Reactions, 2011


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.


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)


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


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


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


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


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!


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


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!)


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:


Micro-(surface) kinetics example of forward rate parameters


Micro-(surface) kinetics Reaction rate

for surface reactions (e.g. moles/cm2 s)

[X] = concentrations; may have different units (gas/bulk or surface phase)


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 (-)


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)


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


Micro-(surface) kinetics example of forward rate parameters

Surface-coverage dependent activation energy!


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)


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)


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


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


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


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)

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