Preeti Aghalayam OCT 2011

Transcription

1 Preeti Aghalayam OCT 2011

2 The number of vehicles (thousands) on Indian roads is increasing at a furious pace! Passenger Vehicles Two Wheelers Total More and more focus on controlling automotive emissions!!"#$%&'($)*!+,* -+.* /0* 123!* 24)$)* "#!$%#! &#'! ()! *"!!"')*"56*7$$8)* "(!+')! ""! (! "#! 9".(* "(&$! &! )! )! 9'4:;)*"56* <&''($)* 9'"(=$')*"56* 9'":#&')*!!"#$%&'()*+,- The number of (gigatons) of gaseous and particulate emissions is high! $'!"#+! #*+! (#! ))+! %&!*&%! ))&! )#! )(! Government emission norms are getting increasingly stringent $" (#$" (" '#$" '" &#$" &" %#$" %"!#$"!" %))%" %))(" %))*" &!!!" &!!'" &!!+" &!!)" &!%&"!"#$%

3 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.

4 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 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.

5 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

6 Diffusional mass transfer in transverse direction Convection in axial direction Effective diffusion into the porous washcoat Reaction at the active centers in the washcoat In addition to understanding adsorptions, desorptions, and surface reactions, a grasp of mass transport is required in order to model the monolith

7 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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9 Level 0: Neglect transport, use kinetic equations v dc dz =!k rc Level 1: Incorporate external mass transport First order reaction is a big assumption! v dc dz =!k r C w k m (C! C w ) = k r C w Neglecting all but one main species! Level 2: Incorporate EMT & IMT v dc dz =!!k r C w k m (C! C w ) =!k r C w This η Effectiveness Factor can be estimated it s a function of the Thiele Modulus

10 The three cases can be generalised thus: v dc dz =!k Overall C Level 0 k Overall = k r Reaction Only Level 1 k Overall = k rk m k m + k r k Overall =! k rk m k m +!k r EMT + Reaction Level 2 All Three 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 η Φ

11 From (Santos & Costa, 1999) For this experiment, the calculations reveal that all three processes are important and should be accounted for in any model worth its weight

12 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

13 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) 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) (1993)