Zeolite HC Traps: Experiments and Simulations

Transcription

1 CLEERS 2013 Zeolite HC Traps: Experiments and Simulations Presenter - Ford Motor Company April 10, 2013 p. 1 Chemical Engineering Department

2 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. 2 Chemical Engineering Department

3 Introduction p. 3 Chemical Engineering Department

4 Introduction: Traditional Hydrocarbon Traps Catalyzed HC trap washcoats on ceramic honeycomb monolith Square channels on front face of HC Trap brick Zeolite Catalyst Monolith Three Way Catalyst Washcoat Zeolite Monolith C x H y O z 1. HEAT C x H y O z + O 2 CO 2 + H 2 O 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. p. 4 Chemical Engineering Department

5 Hydrocarbon (HC) Traps NMHC Emissions [g] Trapping A B C D Trap Inlet NMHC [g] Trap Outlet NMHC [g] Cat C midbed Temp. Cat D midbed Temp Temperature [C] Time [s] 0 T plateau at about 40 degree C. 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. p. 5 Chemical Engineering Department

6 Background: In-Line HC Trap Systems Passive In-Line HC Trap PZEVs 2000MY Nissan Sentra CA * [SAE ] 2003MY Nissan Sentra CA * [SAE ] 2003MY Toyota Camry * [SAE ] 2009 Mitsubishi Lancer * 2.3L PFI Engine HEGO A TWCs B HEGO * Production vehicles HC Trap Catalysts C D 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 inuse operation or rapid dynamometer aging cycles. p. 6 Chemical Engineering Department

7 Challenges for HC Traps with High Temperature Aging CO Conversion/Stored HC FRESH CO HC CO Conversion/Stored HC Aging Effects AGED CO HC Sample Inlet Temperature [C] Sample Inlet Temperature [C] 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. p. 7 Chemical Engineering Department

8 Mathematical Model Development p. 8 Chemical Engineering Department

9 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. 9 Chemical Engineering Department

10 Model Development 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. p. 10 Chemical Engineering Department

11 Adsorption Model HC adsorption is assumed to follow langmuir kinetics Ø For single species, Rate ad =k ad.p A.Ө V, Rate de =k de.ө A Ø For single species, At equilibrium, Ө A =K A.p A /(1+K A.p A ) Ø For competitive adsorptions of species A and B, at equilibrium Ө A =K A.p A /(1+K A.p A +K B.p B ) Ө B =K B.p B /(1+K A.p A +K B.p B ) Ө A + Ө B + Ө V =1 Ø Similar analytical results can be derived for multiple species competitive adsorption. p. 11 Chemical Engineering Department

12 Energy Balance Equations Model Overview - Gas temperature T g t Tg = u x hingsa ε ρ Cp g in ( Tg Ts g ) - Solid temperature T t s = kcond ρ Cp s s 2 T 2 x s hingsain + (1 ε ) ρ Cp s s ( T g T s ) houtgsaout (1 ε ) ρ Cp s s ( T s T amb ) W loading GSA (1 ε ) ρ Cp s s ( R jδh j ) T g T s Q (Exotherm) T amb p. 12 Chemical Engineering Department

13 Model Overview Mass Balance Equations - Gas phase C t g, i = u C g, i x GSA k ε m ( Cg, i Cs, i ) - Washcoat Equations 2 2 Cs, i Cs, i Cs ε w = De + D 2 e 2 t w x, i ρ wc R j also E ad E = kad exp Cs iθ, RT des Ratedes = kdes exp θi s RT s, Ratead, v Different reactions in the two washcoat layers. Also the washcoat loadings are proportional to the ratio of elements for TWC vs zeolite layer. p. 13 Chemical Engineering Department

14 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. cumu. gms (NMHC) time (s) FG TP (exp) TP (model) p. 14 Chemical Engineering Department

15 Adsorption in Zeolites p. 15 Chemical Engineering Department

16 Diffusion in zeolites - 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 T 1.5 (not activated) Knudsen Diffusion α T 0.5 (not activated) Zeolite Diffusion α T 0.5. exp(-e a /RT) - E a varies from 2-15 Kcal/mol. p. 16 Chemical Engineering Department

17 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 C 57, 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. 17 Chemical Engineering Department

18 Lab Results (High Zeolite Loading) HC 1 min adsorption, N2 desorption ppm time (s) 2-Methybutane 150c 0 Propylene 150C Toluene 150C T_out [ ー C] T_in [ ー C] 1min HC adsorption followed by TPD on beta zeolite. Propylene converts into other HCs (next slide) p. 18 Chemical Engineering Department

19 Lab Results (High Zeolite Loading) Propylene 1 min adsorption, N2 desorption ppm time (s) Propylene 150C Ethylene 150C CO ppm 150C T_out [ ー C] T_in [ ー C] Toluene 150C 2-Methybutane 150c 2-Methylpropene 150C Propylene converts to Methylbutane and 2-Methylpropene p. 19 Chemical Engineering Department

20 Lab Results (High Zeolite Loading) Lab experiments were conducted on Beta and ZSM5 catalysts to understand the effect of water on HC adsorption. The results showed that- Water and HC Storage Water Stored (g) Propylene Stored (g) 2-M, butane Stored (g) Toluene Stored (g) Experiment Beta ZSM5 Beta ZSM5 Beta ZSM5 Beta ZSM5 Water and HC simul Water First (1min) HC First (1min) Water First (1min), cooling on HC First (1min), cooling on Water First (5min) HC First (5min) 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. p. 20 Chemical Engineering Department

21 Water Condensation on Zeolites p. 21 Chemical Engineering Department

22 High Zeolite Loading Traps High Zeolite loading monoliths with TWC layer on it. These monoliths have more HC Storage capacity. 600 HC Trap on Vehicle 500 T (deg C) time (s) T plateau at 100 deg C. TempFG TempTP (exp) 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. p. 22 Chemical Engineering Department

23 Water Condensation Modeling A logistic function is used to approximate step changes in rate constants at the dew point: k k kc = m( T T0 ) ke = m 1+ e ( T T0 ) 1+ e where T and T 0 are the gas and dew point temperatures, and m controls the sharpness of the transition (SAE ) k kc ke T_out (oc) time (s) T 0 T T, FG (measured) T, TP (measured) Model without Condensation Model with Condensation 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. p. 23 Chemical Engineering Department

24 Lab Experiments (Water Condensation) Beta Zeolite Substrate T (deg C) Time (s) water (%) 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. 24 Chemical Engineering Department

25 Lab Experiments (Water Condensation) Beta Zeolite Substrate T (deg C) Time (s) T_out [ C] 2% T_in [ C] 2% T_out [ C] 4% T_in [ C] 4% T_out [ C] 10% T_in [ C] 10% Higher water fraction leads to T out reaching the plateau temperature faster, but the plateau temperature itself remains unaffected. p. 25 Chemical Engineering Department

26 Lab Experiments (Water Condensation) Heat (J) water stored (g) time (s) 0 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% 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 p. 26 Chemical Engineering Department

27 TWC Layer Optimization p. 27 Chemical Engineering Department

28 1.5x TWC Layer Thickness Base = 22.1%, New Conversion = 30.86% cumu. gms (NMHC) time (s) FG TP (exp) TP (model) TP (model, 1.5 times TWC layer thickness). 2x TWC Layer Thickness Base = 22.1%, New Conversion = 34.2% cumu. gms (NMHC) time (s) FG TP (exp) TP (model) TP (model, Double TWC layer thickness). p. 28 Chemical Engineering Department

29 Design Optimization and Future Work p. 29 Chemical Engineering Department

30 Modeling Results (Design) 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 (-) Trap Conversion Cell Density 42% OFA 47% OFA 52% OFA 62% OFA - The example above shows model-predicted effects of cell density and OFA on trap performance p. 30 Chemical Engineering Department

31 Modeling Results - 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. p. 31 Chemical Engineering Department

32 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. 32 Chemical Engineering Department

33 Contact Info- - msharm13@ford.com, manis7ul@gmail.com Phone , Acknowledgements- - Jason Lupescu - Paul Laing - Jeff Hepburn p. 33 Chemical Engineering Department

34 ????? Questions? p. 34 Chemical Engineering Department