(80g) CFD Simulations of Reactions Significant in Three Way Catalytic Converters Using Reduced Order Models Developed By Reaction Route Analysis

Ravikiran
Mandapaka, Vismayie Vandanapu, Preeti Aghalayam^*

Department of Chemical Engineering,
Indian Institute of Technology Madras, Chennai 600036, India

*Corresponding Author; Email: preeti@iitm.ac.in

Key words:  Reaction route, CFD
simulations, Water gas shift reaction.

Abstract:

Several
micro kinetic reaction mechanisms have been developed to study the activity for
metal-catalysed systems in the recent past. Incorporating such details into
computational fluid dynamics based reactor simulations is difficult. However,
the global reaction steps obtained via empirical data fitting are rarely
satisfactory for this purpose since they are not applicable over a wide range
of operating conditions, and are often not physically meaningful. Hence, there
is a need for coherently developed reduced rate expressions, which originate
from validated micro kinetic mechanisms and fit well into CFD models.
Drawing motivation from the successful application of the Reaction Route (RR) analysis
to various reacting systems, this methodology was adopted to study the activity
of the Water Gas Shift Reaction (WGS). For this purpose, the micro kinetic
scheme proposed for WGS by Mhadeshwar and Vlachos¹ on Platinum was
selected from the literature. Using the procedure described by Vilekar et al.²,
reaction route analysis was performed for the above micro kinetic scheme and
reduced rate expressions were obtained. In order to study the effect of mass
transfer, a 2-D reactor model was setup in FLUENT^TM with the rate
parameters and reaction rate expressions obtained using RR analysis.

Reaction
route (RR) analysis proposes the treatment of a micro kinetic scheme as
analogous to an electrical circuit - with the thermodynamic driving force
analogous to voltage of the circuit and current being analogous to the rate of
the reaction. Each elementary reaction step is akin to the ?resistance' in an
electric circuit as per the methodology ? the step with the highest
?resistance' value is the Rate determining Step. Detailed procedures for
setting up the ?electric circuit' and evaluating individual resistance values
for a given micro kinetic scheme are available in literature (Vilekar et al.,²).
From the resistance analysis of the micro kinetic scheme presented for WGS it
was inferred that Resistance 9 is the rate determining step (RDS) in the
mechanism. Furthermore, the reduced rate expression was obtained using the RDS,
which is given in eq. (1).

Overall
rate -----------------------------------
(Eqn 1)

 Where,

,,

 
is the equilibrium constant  and  is
forward rate constant.

This
reduced rate expression (Eqn.1) developed using the RR analysis has been used
to carry CFD simulations for an isothermal monolithic catalytic reactor as per
standard procedures, and the results were validated against experimental data.
Figure 1 shows CO conversion versus reactor temperature. The predictions using
both the ideal PFR model and the CFD simulation (incorporating the same rate
expression (Eqn. 1) for both) are shown. It can be interpreted from figure 1
that the mass transfer resistance also plays an important role along with the
kinetic resistance in determining conversion.

From
this work it can be concluded that the Reaction Route
method provides a handy means of incorporating features of a microkinetic
scheme into CFD simulations, without significant increase in computational
expense. In future work, other reactions of importance to three way catalytic
converters, like the N₂O-CO reduction reaction, will be analysed
using the same methodology of Reaction Route analysis followed by CFD
simulations. This work can have significant impact on predictive model
development and simulations for automotive catalytic converters.

Figure
1: Comparison of CFD simulation and PFR
simulation results with the experimental results of (Xue et al.³).
Experimental data is shown in symbols while the CFD simulation and ideal PFR
simulation results are in lines.

References:

1.      Mhadeshwar,
A.B.; Vlachos, D.G. (2005) .Is the water?gas shift reaction on Pt
simple? Computer-aided microkinetic model reduction, lumped rate expression,
and rate-determining step. Catalysis Today. 105, 162?172

2.     
 Vilekar,S. A; Fishtik,I; Datta,R.( 2012)
The peculiar catalytic sequence of the ammonia decomposition reaction and its
steady-state kinetics. Chemical Engineering Science. 71, 333?344

3.     
Xue ?,E; O'Keeffe ,M; Ross, J.R.H.
(1996).Water-gas  shift  conversion  using  a feed  with  a  low  steam  to
carbon  monoxide  ratio  and  containing  sulphur. Catalysis Today., 30
107-118