Modeling of SpaciMS Species Profiles in Diesel Oxidation

Catalyst (DOC) Monolith Cores

Gamma User Group Meeting, November 8, 2010

Acknowledgements

.

William S. Epling and Karishma Irani

Ed Bissett and Jon Brown

Richard Blint

Objectives• Demonstrate that global kinetics can

describe species profiles along the catalyst channel

• Evaluate the spatial effect of the inhibition function on the global kinetics

• Develop a technique to optimize global rate constants using spatial species profiles

• Optimize new rate constants as needed

Thin washcoat (Platinum, Palladium and alumina)

Calculated DOC light-off Profiles

•Based on Sampara-Bissett production catalyst kinetics•CO 50% conversion occurs at approximately ~200 oC•H2 lights off very early and has a long decay profile

PGM

SpaciMs Measurements

Exp Profiles (Constant Flow) ~40 0C Before Light-off

No measured HC consumption

Initial CO and H2 consumption

Exp Profiles (Constant Flow) at Approximately Light-off

Still no measured HC consumption

Exp Profiles (Constant Flow) at ~40 oC After Light-off

•Hydrocarbon decay measured well after CO light-off

Appreciable HC consumption

𝑟𝑟𝑖𝑖 =𝑘𝑘𝑖𝑖𝐶𝐶𝑠𝑠,𝑖𝑖𝐶𝐶𝑠𝑠,𝑂𝑂2

𝐺𝐺

Kinetic equations

Where ki is the rate constant, Cs,i are the species concentrations and G is the inhibition term

Rate (ri) of consumption of H2, CO and hydrocarbons in units of concentration/second

𝑘𝑘𝑖𝑖 = 𝐴𝐴𝑖𝑖𝑒𝑒−𝐸𝐸𝑎𝑎

𝑅𝑅𝑅𝑅�

Where Ai is the prefactor, Ea is the activation energy.

For clarity of modeling, Ai is expressed as a reactive site density times a turn over rate. The turn over rate is characteristic of the catalytic coating. The active site density is characteristic of the catalyst loading and aging.

Example of a Rate Constant including the Inhibition Term

•Representative concentrations are used for the inhibition term•Rate constants cross close to the light-off temperature•Sampara et. al, Ind. Eng. Chem. Res., 47, 311-322, 2008

CO=730 ppmNO=130 ppm

Inhibition equations

Where G is the inhibition term Cs,i are the species concentrations and K is

Where Ai is the prefactor, Ea is the activation energy.

𝑲𝑲𝑖𝑖 = 𝐴𝐴𝑖𝑖𝑒𝑒−𝐸𝐸𝑎𝑎

𝑅𝑅𝑅𝑅�

Spatial Dependence of the Inhibition Term

However, in this kinetic set, the NO inhibition dominates

Constant Flow CO Profiles With Adiabatic Thermal Conditions

•CO published kinetics much faster than experimental measurements

Reaction Parameter Optimization

• 12 possible independent variables and 16 total with NO oxidation reaction

• Inhibition terms common for all reactions, except the NO oxidation

• Objective function introduced based on the spatial concentration measurements

where Tinlet is the gas inlet temperature and α(12) is each of the reaction parameters

• At this time only pair optimization have been completed

𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏�𝑻𝑻𝒊𝒊𝒏𝒏𝒊𝒊𝒊𝒊𝒊𝒊,𝜶𝜶(𝟏𝟏𝟏𝟏)� = ���𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐜𝐧𝐧𝐦𝐦𝐜𝐜𝐜𝐜𝐦𝐦𝐜𝐜𝐦𝐦𝐜𝐜𝐜𝐜𝐧𝐧

� − 𝟏𝟏�𝟐𝟐𝟏𝟏𝟏𝟏

𝒏𝒏=𝟏𝟏

Constant Flow CO Profiles With Adiabatic Thermal Conditions and Optimized Kinetic Parameters

•Optimization of the CO rate constant improves the fits at 128 and 168 oC, but at 200 oC CO calculated profile is still faster than measured

Constant Flow H2 Profiles With Adiabatic Thermal Conditions

•H2 consumption moves toward the front of the catalyst with increasing inlet temperature

Constant Flow H2 Profiles With Adiabatic Thermal Conditions and Optimized Kinetic Parameters

•Description of H2 consumption improves at higher inlet temperature

Constant Flow Dodecane Profiles With Adiabatic Thermal Conditions and Optimized Kinetic Parameters

•Calculated dodecane profiles give good agreement with measured profiles at higher inlet gas temperatures

Catalyst Ea (CO, kJ) Ea (H2, kJ) Source

Pt DOC 22.1 30.3 Sampara, et al., Ind. Eng. Chem. Res., 46, 7993-8003, 2007

Prod DOC 81.3 15.3 Sampara et. al, Ind. Eng. Chem. Res., 47, 311-322, 2008

GMT-800 65.2 18.6 Deakin, et. al (manuscript)

PtPd DOC 79.1 14.4 SpaciMS fit

Activation Energies

•Reasonable similarity between Prod DOC activation energies and GMT-800•Clearly kinetics are catalyst and condition dependent•Prod DOC kinetics selected as base kinetics for this study

Conclusions• Global kinetics does correctly describe the species

profiles along the DOC monolith channel• In these measurements hydrocarbon consumption

occurs only after the CO has been consumed• Global kinetic rate constants have been developed from

SpaciMS measurements• The “fast”, low temperature hydrogen oxidation rates do

not result in complete oxidation of the hydrogen at the inlet of the channel

• The NO inhibition effect dominates the overall inhibition for these fitted rate parameters

• GT-Power provides a flexible program for both predicting these profiles and the tools to optimize the rate constants