Catalizzatori strutturati (a monolite) elevata A/V  basse  P/L

Aspetto di un singolo canale

Un’applicazione diffusa: Impianti DeNOx

Un’applicazione interessante: Turbine a gas

Macro- vs micro- (homogeneous) kinetics Macro CH O 2 CO H 2 O with A = 1.3x10 7 or 2.1x10 9 s -1, E a = 125 or 200 kJ/mol 1.Parameters are limited to a range of T and  2.Current detailed models accout for 325 reactions and 53 species (GRI3.0)

Macro- vs micro-(surface) kinetics Macro CH O 2 CO H 2 O 1.C are in the bulk of the gas 2.Rate laws are adaptive and surface/flow specific with A = 8.66x10 5 s -1 cm 5/2 g -0.5, s -1 E a = kJ/mol

Macro- vs micro-(surface) kinetics Micro

Macro- vs micro-(surface) kinetics Micro (part of) Kunz et al. Modeling the Rate of Heterogeneous Reactions, 2011

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

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 SiH 4 (g) + Si(s) → 2H 2 (g) + Si(s) + Si(b) this is close to the surface → add the bulk of the gas!

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  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)

Reactors beyond ideal ones: pseudo-2D PFR Correlations (I) Laminar flow in isothermal walls (Hawthorn, 1974) Reaction is ignored

Reactors beyond ideal ones: pseudo-2D PFR Correlations (II) From evaporation test, through porous monoliths (Votruba, 1975) Valid for short surfaces Much smaller than asymptotic values

Reactors beyond ideal ones: pseudo-2D PFR Correlations (III) Propene oxidation (Bennett et al, 1991) Underestimates asymptotic values (without reaction)

Reactors beyond ideal ones: pseudo-2D PFR Limits of the correlations 1.valid for SS only 2.reaction effect simulated, but not described 3.non-isoT walls 4.Validity of mass/heat analogies

Reactors beyond ideal ones: pseudo-2D PFR Use of mode detailed modles (2-3D) to calibrate correlations But: profiles close to a reactive surface are strongly influenced by the reaction

Reactors pseudo-2D PFR: application CPO Mech R. Schwiedernoch, S. Tischer, C. Correa, O. Deutschmann, Chem. Eng. Sci. 58 (2003) 633.

Reactors CFD+  KIN - Application CH 4 combustion on Pt Honeycomb with inlet CH 4 (1%) O 2 (1%) T=500°C Pt

Methane catalytic partial oxidation Detailed chemistry R. Quiceno, J. Perez-Ramyrez, J. Warnatz, O. Deutschmann, Appl. Catal. A: General (2006)

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 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)