Simulator for Automotive Evaporative Emissions
Restraint Systems
Presentation
Dr.-Ing. Stefan Schlüter, Fraunhofer UMSICHT, Germany
COMSOL Conference 2016 Munich
Introduction
Automotive Evaporative Emissions
• Evaporative emissions from the fuel tanks of parked cars = 11,4 kg/year per car
• Vapor restraint systems = active carbon canisters, used in every car worldwide
• Parking car: fuel gases are adsorbed by the adsorptive system
• Driving car: fuel gases are desorped from the adsorber by fresh combustion air
EU Goal
• 10% of all fossil fuels replaced by biofuels until 2020
Project Goals
• Research on adsorption of Bioethanol/Fuel blends on automotive active carbons
• Development of a simulation model for the adsorption/desorption process
No. 2
Simulation Model
Model development
• 2-d axisymmetric multicomponent adsorption model for COMSOL Multiphysics 5.2
• Adsorption/desorption of gaseous fuel components on active carbon
• Gas components: Pentane (as fuel replacement), Ethanol, Water, Nitrogen
• Adsorption isothermes for specific active carbon types from experiments
• Adsorption and desorption process models
COMSOL implementation
• 2-d axisymmetric geometry
• Equation based modeling
• Porous Media Models of COMSOL (Heat Transfer, Transport of Diluted Species)
• BDF Time Stepping, Fully Coupled, Jacobian update on every iteration
No. 3
Model Equations I
Darcy flow equation
Mass conservation equation
No. 4
23
2
1
150 1
P D
G G
d KP P
u
2 2
2
1,
1
*1
1 1 1, ,
ni GG D
G G G i G
i G
eff
S i P i i ii
N N
G G i i N
G G G G
cT KPP
t t t
a Y Y M
M Mwith M M
P M T M
laminar part of Ergun equation
(Re < 1)
Model Equations II
Component conservation equation, gas phase
Component conservation equation, solid phase
Linear driving force coefficient
No. 5
, 1effi iDi G i i S
G
c YKc c P
t t
*effii P i i
Ya Y Y
t
*, ,
*
10 ( ) 1,
5(1 ) ( ) 2 ( )1
i P S GS i Peff S S ii i P
P i P i S i P i S
withT dRT dY
Bid P T d T
Bi
adsorption isotherme
gas-solid mass transfer
linear driving force (LDF)
adsorptive mass transfer
Model Equations III
Energy equation, gas phase
Energy equation, solid phase
Energy equation, tube wall
No. 6
, ,
GGG P G eff G G P G G GS GS G S
T Pc T c T a T T
t t
u
, ,
,
(1 ) (1 )
(1 )G
S
ads effSS P S i i P i S S
i
T
ads GiS i P i i GS GS G S
i T
Tc Y M c T
t
Yh c dT M a T T
t
0WW W W W
Tc T
t
Experimental Installation – Break Through Experiments
No. 7
temperature
probes
active carbon
2. adsorber
nitrogen
liquid adsorbents
cryostate
wall cooling
flue
adsorbergas preparation unit
effluent weighting
Example: Pentane Adsorption Isotherme
No. 8
0
1
2
3
4
5
6
7
8
9
0,00001 0,0001 0,001 0,01 0,1 1
Lo
ad
ing
[m
ol/
kg
]
relative Saturation = Pi/Psat [1]
10°C Exp
24°C Exp
24°C Exp Wdh.
35°C Exp
10°C
24°C
35°C
50°C
75°C
100°C
125°C
Pentane on Active Carbon (1)
Data adaption with Langmuir-Freundlich
Simulation Runs
Adsorption Experiments
• Adsorption of Pentane 100% on dry Active Carbon (1), 20 l/h N2
• Adsorption of Pentane/Ethanol 78%/22% (E22) on dry Active Carbon (1), 20 l/h N2
• Adsorption of Pentane/Ethanol 78%/22% (E22) on wetted Active Carbon (1), 20 l/h N2
Desorption Experiments
• Desorption directly after adsorption cycle with Nitrogen at ca. 20°C from top
Probes
• Temperature Probes T2-T7 at same positions as in experiment
Simulation
• Time dependent simulation with experimental conditions
No. 9
Temperature / Pentane-Loading in the Adsorber
No. 10
temperature
Pentane loading
Temperature / Loadings / Gas Mole Fraction
No. 11
temperature Pentane loading
Ethanol loadingPentane mole fraction
Model Valididation – Pentane 100% on Active Carbon (1), 20 l/h N2
Adsorption Pentan 100% 20 l/h N2
20
30
40
50
60
70
80
90
100
110
120
130
140
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Zeit [s]
Tem
pe
ratu
r [°
C]
Kanal 2 (Exp.)
Kanal 2 (Sim.)
Kanal 3 (Exp.)
Kanal 3 (Sim.)
Kanal 4 (Exp.)
Kanal 4 (Sim.)
Kanal 7 (Exp.)
Kanal 7 (Sim.)
Time [s]
Tem
pera
ture
[°C
]
Simulation
Experiment
No. 12
Model Validation – Pentane/Ethanol E22 on Active Carbon (1), 20 l/h N2
Adsorption: (EtOH+H2O) 22% + Pentan 78% 20 l/h N2 bis Durchbruch (2g)
20
30
40
50
60
70
80
90
100
110
120
130
140
0 200 400 600 800 1000 1200 1400
Zeit [s]
Tem
pera
tur
[°C
]
Kanal 2 (Exp.)
Kanal 2 (Sim.)
Kanal 3 (Exp.)
Kanal 3 (Sim.)
Kanal 4 (Exp.)
Kanal 4 (Sim.)
Kanal 7 (Exp.)
Kanal 7 (Sim.)
Tem
pera
ture
[°C
]
Time [s]
Simulation
Experiment
No. 13
Model Valididation – Pentane/Ethanol E22 on wetted AC (1), 20 l/h N2
No. 14
20
30
40
50
60
70
80
90
100
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Tem
pe
ratu
re [
°C]
Time [s]
Mixture 78% Pentane / 22% Ethanol on wetted Active Carbon (1), 20 l/h N2
T2_exp
T3_exp
T4_exp
T5_exp
T6_exp
T7_exp
T2_sim
T3_sim
T4_sim
T5_sim
T6_sim
T7_sim
Experiment
Simulation
Adsorption / Desorption – Cycle 3
No. 15
Adsorption 3
Adsorption 1
Adsorption 2
Conclusions I
Adsorption runs
• Good reproduction of temperature probe signals in simulation:
– time of probe temperature increase
– rate of probe temperature increase
– maximum probe temperature
– radial temperature distribution
Adsorption / desorption cycle runs
• Pentane loading decrease from cycle to cycle (as seen in experiments)
• Reason: unsufficient desorption, expecially in the lower part of the adsorber
No. 16
Conclusions II
Simulator
• Good reproduction of the physical system
• Small timesteps (1e-6 s to 5 s) necessary due to steep adsorption front
• Robust numerical time stepping with „fussy“ solver parametrization
• Simulation times in the range of 15-30 minutes for 30-60 minutes process time
• Use of the simulator in further project work planned
No. 17
No. 18
Many thanks for your attention!
Thanks to public funding:
German Federal Ministry of Economics and Technology (BMWi, IGF 16211 N)
German Federal Ministry of Education and Reserarch (BMBF, FKZ 22017308)