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)