cleers 2013 zeolite hc traps: experiments and …manish sharma cold start nmhc on pzev 150k-mi w/...
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p. 1 Chemical Engineering Department Manish Sharma
Zeolite HC Traps: Experiments and Simulations
April 10, 2013
Presenter - Manish Sharma
Ford Motor Company
CLEERS 2013
p. 2 Chemical Engineering Department Manish Sharma
Outline
-Introduction -Mathematical model development and simulation -Detailed Analysis
-Zeolite Adsorption, HC adsorption and competitive adsorption between HC and water. -Water Condensation. -TWC Layer Optimization (work in progress).
-Optimizing trap design and future work.
p. 3 Chemical Engineering Department Manish Sharma
Introduction
p. 4 Chemical Engineering Department Manish Sharma
1. Adsorption - Zeolite allows hydrocarbon molecules smaller than the pore diameter to enter channels and adsorb
2. Retention - Zeolite pore structure holds hydrocarbon molecules while precious metal catalyst in washcoat warms up to activation temperature
3. Release - Adsorption bonds are cleaved as a function of temperature, bond type and hydrocarbon characteristics
4. Conversion – The released hydrocarbons are oxidized over the TWC layer.
Square channels on front face of HC Trap brick
Catalyst
Zeolite
Monolith
Catalyzed HC trap washcoats on ceramic honeycomb monolith
Square channels on front face of HC Trap brick
Catalyst
Zeolite
Monolith
Catalyzed HC trap washcoats on ceramic honeycomb monolith
Three WayCatalyst WashcoatZeoliteMonolith
HEAT CO2+ H2OCO2+ H2OCxHyOz
CxHyOz
+ O2
2.
3.
4.
1.
Introduction: Traditional Hydrocarbon Traps
p. 5 Chemical Engineering Department Manish Sharma
Cold Start NMHC on PZEV 150K-mi w/ E85 Through Catalyzed HC Traps (C&D)
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0.002
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0 25 50 75 100 125 150 175 200 225 250 275 300Time [s]
NM
HC
Em
issi
ons
[g]
0
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600
700
800
Tem
pera
ture
[C]
Trap Inlet NMHC [g]Trap Outlet NMHC [g]Cat C midbed Temp.Cat D midbed Temp.
Trapping
A B C D
Hydrocarbon (HC) Traps
HC Traps are needed to trap the HCs from engine exhaust during the cold start. The results from this work are also applicable in enabling to meet emission standards for lean burn and diesel engines.
T plateau at about 40 degree C.
p. 6 Chemical Engineering Department Manish Sharma
Background: In-Line HC Trap Systems
Passive In-Line HC Trap PZEVs • 2000MY Nissan Sentra CA*
[SAE 2000-01-1583] • 2003MY Nissan Sentra CA*
[SAE 2003-01-1861] • 2003MY Toyota Camry*
[SAE 2003-01-0817] • 2009 Mitsubishi Lancer*
• Passive In-Line HC Traps are in the exhaust stream all of the time like typical catalytic converters (no exhaust valves or air pumps)
• Zeolite activity for HC storage and retention is not as durable as typical catalytic converter activity so care must be made regarding placement in the exhaust system
• All production passive in-line HC traps are located in the underbody position where maximum mid-brick temperatures should be below 800°C during demanding in-use operation or rapid dynamometer aging cycles.
DC
A
B
HEGO
HEGOHC Trap Catalysts 2.3L PFI
Engine
TWCs
* Production vehicles
p. 7 Chemical Engineering Department Manish Sharma
Challenges for HC Traps with High Temperature Aging
• Retention of Stored HC - A fresh HC trap may retain 70% of stored HC emissions above 150°C in the zeolite layer for potential oxidation, but exposure to high temperature exhaust weakens the retention strength and more HC is released at lower temperature.
• Low Temperature Oxidation – A fresh TWC needs about 140°C to convert 50% of inlet CO emissions (Light-off), but exposure to high temperature exhaust sinters the precious metal and support, and a higher inlet temperature is required for light-off.
• The overlapped area below the curves correlates to the HC trap performance metric of Adsorbed HC Conversion and the vehicle tailpipe emissions benefit over a conventional TWC.
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Sample Inlet Temperature [C]
CO
Con
vers
ion/
Sto
red
HC
COHC
AGEDAging
Effects
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Sample Inlet Temperature [C]
CO
Con
vers
ion/
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red
HC
COHC
FRESH
p. 8 Chemical Engineering Department Manish Sharma
Mathematical Model Development
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Model Development
• 1-D mathematical model, includes mass and energy balances. • Kinetic model considers the following steps: - HC adsorption/desorption on zeolite layer. - Reactions in the TWC layer. - Predicts HC slip and zeolite temperature.
• Input required - FG concentrations of reactants and FG temperature, along with initial
conditions in the catalyst. - Input for trap dimensions
. Cell density with wall thickness . Volume and length . Washcoat loading.
p. 10 Chemical Engineering Department Manish Sharma
• The washcoat is divided into radial elements, the top elements represent TWC layer, the bottom elements are for zeolites. The number of elements is a model parameter. • Diffusivities of gases are different in the two layers (since TWC and zeolite washcoats have different pore sizes/tortuosity etc). • TWC layer is the site for HC/CO oxidation, oxygen storage, NOx reactions.
• Gases have to diffuse through the TWC layer to reach the zeolite layer. Adsorption and storage of HCs occurs in the zeolite layer. • Model accounts for both radial and axial pore diffusion in the washcoat.
• Solid temperature is assumed to be uniform in the washcoat elements. The model does not differentiate between the temperatures in the two washcoat layers.
Model Development
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Adsorption Model
HC adsorption is assumed to follow langmuir kinetics
Ø For single species, Ratead=kad.pA.ӨV, Ratede=kde.ӨA
Ø For single species, At equilibrium, ӨA=KA.pA/(1+KA.pA)
Ø For competitive adsorptions of species A and B, at equilibrium ӨA=KA.pA/(1+KA.pA+KB.pB) ӨB=KB.pB/(1+KA.pA+KB.pB) ӨA+ ӨB+ ӨV=1
Ø Similar analytical results can be derived for multiple
species competitive adsorption.
p. 12 Chemical Engineering Department Manish Sharma
)( sggg
iningg TTCpGSAh
xT
utT
−−∂
∂−=
∂
∂
ρε
( )∑ Δ−
−−−
−−−
+∂∂
=∂∂
jjss
loadingambs
ss
outoutsg
ss
inins
ss
conds HRCpGSAW
TTCp
GSAhTTCp
GSAhxT
Cpk
tT
ρερερερ )1()(
)1()(
)1(2
2
Tamb
(Exotherm)
Tg
Ts Q
- Gas temperature
- Solid temperature
Model Overview
Energy Balance Equations
p. 13 Chemical Engineering Department Manish Sharma
)( ,,,,
isigmigig CCkGSA
xC
utC
−−∂
∂−=
∂
∂
ε
,exp, , viss
adadad C
TREkRatealso θ⎟⎟
⎠
⎞⎜⎜⎝
⎛−= i
s
desdesdes TR
EkRate θ⎟⎟⎠
⎞⎜⎜⎝
⎛−= exp
∑−∂
∂+
∂
∂=
∂
∂jwc
ise
ise
isw R
xC
DwC
DtC
ρε 2,
2
2,
2,
- Gas phase
- Washcoat Equations
Model Overview
Mass Balance Equations
Different reactions in the two washcoat layers. Also the washcoat loadings are proportional to the ratio of elements for TWC vs zeolite layer.
p. 14 Chemical Engineering Department Manish Sharma
Modeling Results The model was calibrated using lab data. HC species were binned into three categories based on Bag1 FTP data. C2-C3 data was represented by propylene, C4-C5 data was represented by Isopentane, and C6+ data was represented by Toluene. The individual HC results from the model are combined to give cumulative gms.
-0.010
0.010.020.030.040.050.060.070.080.09
0.1
0 50 100 150 200 250 300 350time (s)
cum
u. g
ms
(NM
HC)
FG TP (exp) TP (model)
p. 15 Chemical Engineering Department Manish Sharma
Adsorption in Zeolites
p. 16 Chemical Engineering Department Manish Sharma
- The diffusion of molecules in zeolites is intra-crystalline or configurational diffusion. - For most zeolite adsorptions, the shape of the isotherm is adequately represented by the Langmuir equation. - Qualitatively, diffusion in zeolites is bound by knudsen and solid state diffusion. - In ZSM-5, bulky molecules (aromatics and branched paraffins) sit at channel intersections, while small molecules sit in the channels. Polar molecules like water and alcohol, are adsorbed very strongly. -Bulk Diffusion α T1.5 (not activated)
Knudsen Diffusion α T0.5 (not activated)
Zeolite Diffusion α T0.5. exp(-Ea/RT) - Ea varies from 2-15 Kcal/mol.
Diffusion in zeolites
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Diffusion in zeolites
- Since organic molecules can rotate and vibrate, and zeolite (thermal) lattice vibrations on the order of tenths of angstroms can occur, effective pore sizes for adsorption or catalytic reaction may appear somewhat larger than crystallographic dimensions. - Thus even structurally complex molecules such as the triglyceride C57, which might appear excessively bulky, can enter into the pores of ZSM-5, since, given adequate energy of activation, conformational/configurational adaptations can be made. - The vertical (configurational) segment of the plot
spans about ten orders of magnitude in diffusion coefficient and reflects only the modest range of pore sizes shown in 4-8 A. - Interaction of adsorbate polar functional groups with lattice polarity near the pore mouth can be the dominant factor, rather than steric effects .
p. 18 Chemical Engineering Department Manish Sharma
Lab Results (High Zeolite Loading)
1min HC adsorption followed by TPD on beta zeolite. Propylene converts into other HCs (next slide)
HC 1 min adsorption, N2 desorption
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ppm
2-Methybutane 150c 0 Propylene 150C Toluene 150C T_out [ーC] T_in [ーC]
p. 19 Chemical Engineering Department Manish Sharma
Propylene converts to Methylbutane and 2-Methylpropene
Lab Results (High Zeolite Loading)
Propylene 1 min adsorption, N2 desorption
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time (s)
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Propylene 150C Ethylene 150C CO ppm 150C T_out [ーC]T_in [ーC] Toluene 150C 2-Methybutane 150c 2-Methylpropene 150C
p. 20 Chemical Engineering Department Manish Sharma
Lab experiments were conducted on Beta and ZSM5 catalysts to understand the effect of water on HC adsorption. The results showed that-
Experiment Beta ZSM5 Beta ZSM5 Beta ZSM5 Beta ZSM5
Water and HC simul. 0.196 0.198 0.006 0.013 0.0028 0.0042 0.007 0.0064
Water First (1min) 0.3 0.31 0.003 0.01 0.004 0.0047 0.0078 0.0072
HC First (1min) 0.147 0.174 0.01 0.02 0.0072 0.0051 0.01 0.011
Water First (1min), cooling on 0.284 0.3 0.002 0.01 0.0067 0.005 0.0072 0.0077
HC First (1min), cooling on 0.188 0.2 0.008 0.021 0.0088 0.0055 0.01 0.011
Water First (5min) 1.65 1.32 0.002 0.011 0.017 0.0122 0.019 0.017
HC First (5min) 0.62 0.66 0.022 0.04 0.027 0.017 0.033 0.027
Water and HC Storage
Water Stored (g) Propylene Stored (g) 2-M, butane Stored (g) Toluene Stored (g)
-Water adsorption remains same for Beta and ZSM5. In experiments where water is fed first, more water gets adsorbed. -ZSM5 leads to more propylene adsorption, HC first experiments result in more propylene adsorption. -M-butane and toluene have similar adsorptions on ZSM5 and Beta zeolite. HC first experiments lead to more M-butane and toluene adsorption. -Water and propylene may be competing for bronsted acid sites.
Lab Results (High Zeolite Loading)
p. 21 Chemical Engineering Department Manish Sharma
Water Condensation on Zeolites
p. 22 Chemical Engineering Department Manish Sharma
0
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time (s)
T (d
eg C
)
TempFG TempTP (exp)
High Zeolite Loading Traps High Zeolite loading monoliths with TWC layer on it. These monoliths have more HC Storage capacity.
HC Trap on Vehicle
When high zeolite loading traps are used on the vehicle, the water condensation plateau is observed at close to 100 degree C. This was modeled by raising the Dew Point (plateau temperature) to 100 degree C. We do not understand the phenomena clearly right now.
T plateau at 100 deg C.
p. 23 Chemical Engineering Department Manish Sharma
Effect of water condensation - HC release shifts (delayed) because of the T plateau, and there may be a small improvement in the trap conversion efficiency.
Water Condensation Modeling k
• A logistic function is used to approximate step changes in rate constants at the dew point:
( )01 TTmc ekk
−+=
( )01 TTme ekk
−−+=
ck ek
T0T
where T and T0 are the gas and dew point temperatures, and m controls the sharpness of the transition.
k
0TT
k
0TT
k
0T
k
0TT
0T0T0T
(SAE-2010-01-0885)
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time (s)T_
out (
oC)
T, FG (measured) T, TP (measured)Model without Condensation Model with Condensation
p. 24 Chemical Engineering Department Manish Sharma
Lab Experiments (Water Condensation)
Beta Zeolite Substrate
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Time (s)
T (d
eg C
)
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wat
er (%
)
T_out [°C] 2% T_in [°C] 2% T_out [°C] 4% T_in [°C] 4% T_out [°C] 10%T_in [°C] 10% H2O (%) 2% H2O% 4% H2O% 10%
p. 25 Chemical Engineering Department Manish Sharma
Lab Experiments (Water Condensation)
Higher water fraction leads to Tout reaching the plateau temperature faster, but the plateau temperature itself remains unaffected.
Beta Zeolite Substrate
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Time (s)
T (d
eg C
)
T_out [°C] 2% T_in [°C] 2% T_out [°C] 4% T_in [°C] 4% T_out [°C] 10% T_in [°C] 10%
p. 26 Chemical Engineering Department Manish Sharma
Amount of Heat Stored and Water Stored increases with water fraction. At the maximum points, Heat/g 2% = 1580J/g, Heat/g 4% = 1920J/g, and Heat/g 10% = 1190J/g. Latent Heat of Water (Vaporization) = 2260 J/g
Lab Experiments (Water Condensation)
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time (s)
Heat
(J)
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wat
er s
tore
d (g
)
Cumulative (Heat Out-Heat In) (J) 2% Cumulative (Heat Out-Heat In) (J) 4% Cumulative (Heat Out-Heat In) (J) 10%gms of water 2% gms of water 4% gms of water 10%
p. 27 Chemical Engineering Department Manish Sharma
TWC Layer Optimization
p. 28 Chemical Engineering Department Manish Sharma
-0.010
0.010.020.030.040.050.060.070.080.09
0 50 100 150 200 250 300 350 400
time (s)
cum
u. g
ms
(NM
HC)
FG TP (exp) TP (model) TP (model, Double TWC layer thickness).
-0.010
0.010.020.030.040.050.060.070.080.09
0 50 100 150 200 250 300 350 400
time (s)
cum
u. g
ms
(NM
HC)
FG TP (exp) TP (model) TP (model, 1.5 times TWC layer thickness).
1.5x TWC Layer Thickness Base = 22.1%, New Conversion = 30.86%
2x TWC Layer Thickness Base = 22.1%, New Conversion = 34.2%
p. 29 Chemical Engineering Department Manish Sharma
Design Optimization and Future Work
p. 30 Chemical Engineering Department Manish Sharma
Design of HC Trap • 2 key monolith design parameters affect performance:
1. Cell Density 2. Open Frontal Area [OFA]
(inversely related to wall thickness)
• These parameters produce both positive and negative effects on performance. • Higher loading & higher cell density increase zeolite capacity (+), but also increase thermal mass and diffusion length (-)
- The example above shows model-predicted effects of cell density and OFA on trap performance
Modeling Results (Design)
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100 200 300 400 500 600 700
Cell Density
Trap
Con
vers
ion
42% OFA 47% OFA 52% OFA 62% OFA
p. 31 Chemical Engineering Department Manish Sharma
- Trap performance improves with increasing cell density for a given OFA. This is because the pore diffusional resistance decreases with decreasing zeolite layer thickness (increasing cell density). -A trap with higher OFA gives better performance for a fixed cell density. Thermal mass and pore diffusional resistance both increase with increasing Wall (and zeolite layer) thickness (decreasing OFA, for a fixed cell density). -A 200 cpsi 62%OFA performs better than a 600cpsi 42% OFA trap. - All traps assume a 2 gci layer of TWC washcoat on the zeolite samples.
- Results can be repeated for other zeolite-binder ratios.
Modeling Results
p. 32 Chemical Engineering Department Manish Sharma
Future Work
-Expand and Calibrate the model for other reactions in the zeolite/TWC Layer (Ethanol work).
- The goal is to continue the model development so that it may also be used to guide the chemical characteristics needed to improve trap performance (Si/Al ratio, system architecture).
- Since the model is calibrated using individual HCs, it can be used for simulating active traps, passive traps, CC traps and UB traps. -Calculate Effective Diffusivities in the zeolite layer by performing diffusion experiments on the zeolite samples.
p. 33 Chemical Engineering Department Manish Sharma
Acknowledgements- - Jason Lupescu
- Paul Laing
- Jeff Hepburn
Contact Info- Manish Sharma Email - [email protected], [email protected] Phone-3133224673, 8328654900.
p. 34 Chemical Engineering Department Manish Sharma
?????
Questions?