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Preeti Aghalayam OCT 2011
0
2000
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12000
2002-‐‘03 2004-‐‘05 2006-‐‘07 2008-‐‘09
Passenger Vehicles
Two Wheelers
Total
The number of vehicles (thousands) on Indian roads is increasing at a furious pace!
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The number of (gigatons) of gaseous and particulate emissions is high!
Government emission norms are getting increasingly stringent
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More and more focus on controlling automotive emissions!
¡ “Automotive Catalysts” introduced in US in 1975 to detoxify auto exhaust § Adopted in Japan & Europe in 1985 § The scheme an unparalleled success!
¡ Several challenges overcome § Lead-‐based additives in fuel legislated out (to prevent catalyst poisoning)!
§ Recycling and regeneration of the spent catalyst ensured (to ensure economic viability)
§ Compactness, high volumetric Tlow rates & temperatures, and low back pressures achieved (using monolith structures)
§ Expensive Pt & Rh catalysts, at least partially replaced with Pd (“Oxygen storage” components added)
§ etc.
A typical three way catalyst (TWC) consists of • Large number of parallel channels, in honeycomb arrangement
• These small channels provide catalytic surface area • The catalyst is in a 10-‐150 um thick porous, washcoat layer
• The washcoat is sintered alumnia with noble metals (Pt, Rh, Pd)
• Ceria & Zirconia based components are used to provide stability
• The reactions occur in the washcoat • Convective transport in the axial direction is important
• This arrangement lets us have low back pressures, compactness, and is stable at high Tlow velocities and temperatures
• Front & rear oxygen sensors are required to ensure optimal catalyst performance.
¡ Current issues and challenges in automotive catalysis are-‐ § ‘Start-‐up’ situations ▪ Transient operation ▪ Cold-‐start condition
§ ‘lean burning’ condition § Diesel & other engines § Catalyst life § Expensive catalysts! § Small vehicles.. § Fuel quality
In addition to understanding adsorptions, desorptions, and surface reactions, a grasp of mass transport is required in order to model the monolith
Convection in axial direction
Diffusional mass transfer in transverse direction
“Effective diffusion”into the porous washcoat Reaction at the active
centers in the washcoat
¡ In general, we divide things that happen in a porous catalyst into three levels: § External Transport: Bulk-‐>External surface § Internal Transport: Surface-‐>Interior of pore network § Intrinsic Reaction: (Close to) interior surface
¡ We draw a black-‐box around the catalyst § The internal transport and intrinsic reaction occur within the black box § The external transport links the black box to the outside medium
¡ We carefully determine intrinsic reaction features
¡ Next, we modify these values to reTlect a combined effect of internal transport + reaction
¡ Finally, we explicitly write Tlux balance equations at the surface of the black-‐box § Reaction rate Tlux from inside = Mass transport Tlux on outside
¡ Solving this equation will determine all that we need to know § Surface concentrations, average ‘inside’ concentrations, conversions, etc.
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¡ Level 0: Neglect transport, use kinetic equations
¡ Level 1: Incorporate external mass transport
¡ Level 2: Incorporate EMT & IMT
v dCdz
= !krC
v dCdz
= !krCw km (C !Cw ) = krCw
v dCdz
= !!krCw km (C !Cw ) =!krCw
First order reaction is a big assumption!
Neglecting all but one main species!
This ‘η’ – Effectiveness Factor – can be estimated – it’s a function of the Thiele Modulus
¡ The three cases can be generalised thus: v dCdz
= !kOverallC
kOverall =krkmkm + kr
kOverall =!krkm
km +!krkOverall = kr
Reaction Only EMT + Reaction All Three
Level 0 Level 1 Level 2
Φ
η
• The ODE can be solved in each case to predict outlet conversions
• Reation rate constants, mass transfer coefTicients, diffusion coefTicients in the washcoat, need to be known
For this experiment, the calculations reveal that all three processes are important and should be accounted for in any model worth its weight
From (Santos & Costa, 1999)
¡ In a typical catalytic process (such as the automotive catalytic converter), there is a complex interaction between transport and chemical reaction § Transport of reactants/products in axial, transverse directions through bulk diffusion & convection
§ Transport through the pores of the catalyst via bulk and Knudsen diffusion
¡ Mathematical models for catalytic systems have to account for external and internal transport via various means § Dimensionless numbers Thiele Modulus, Peclet Number etc. come in handy
¡ H. Santos & M. Costa, “The relative importance of external and internal transport phenomena in three way catalysts,” Int. J. of Heat and Mass Trans. 51 (2008) 1409–1422
¡ M. V. Twigg, “Progress and future challenges in controlling automotive exhaust gas emissions,” Appl. Catal. B: Env. 70 (2007) 2–15
¡ K. C. Taylor, “Nitric Oxide Catalysis in Automotive Exhaust Systems,” Catal . Rev.-‐Sci. Eng., 35(4). 457-‐481 (1993)
