Modeling Transient Adsorption/Desorption Behavior in a

Gas Phase Photocatalytic Fiber Reactor

Siegfried Denys, Jeroen van Walsem Siegfried.denys@uantwerpen.be

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

2

Photocatalysis: principle Catalyst: increases reaction rate without being consumed

Photo-catalyst: catalyst activated by (UV-)light

Most often titanium dioxide (TiO2)

Po

ten

tia

l (v

s. S

HE

) [V

]

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0Valence band

Conduction band e- (-0.52)

h+ (+2.53)

hn ≥ Eg

H2/H2O (-0.413)

O2/H2O (+0.83)

O2/O2-• (-0.28)

•OH/H2O (+2.27)

TiO2

Red

Red

Ox

Ox

VOC’s

H2O, CO2

3

Photocatalysis: application fields

Water purification/desinfection

Self-cleaning materials

Air purification

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

4

Gas phase photocatalytic fiber reactor

TiO2

Tubular photoreactor

UV source

Inlet: • air • acetaldehyde • controlled C

and Q Outlet: (to FTIR

spectroscope)

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

• To model the transient, dynamic adsorption/desorption of acetaldehyde as contaminated air flows through the reactor

• To estimate the adsorption/desorption rate constants

5

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

Comsol model: goals

6

Comsol model: geometry

Air outlet

Air inlet

FTIR cell (measurement)

Fiber

Fiber

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

Approx . 80,000 tetrahedral cells + refinement

7

Comsol model: Physics

• Laminar flow (Re < 800)

• Darcy equation (single-phase gas flow in a porous medium)

stationary solver

• Species transport in porous media (incl. free flow)

time dependent solver

bulkAcalbulkAcal

bulkAcal CCDt

C,,

,

u

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

8

Comsol results: no adsorption

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

9

Langmuir adsorption/desorption

𝑟𝑎𝑑𝑠 = 𝒌𝒂𝒅𝒔𝐶𝐴𝑐𝑎𝑙,𝑏𝑢𝑙𝑘(1 − 𝜃𝐴𝑐𝑎𝑙)

𝑟𝑑𝑒𝑠 = 𝒌𝒅𝒆𝒔𝜃𝐴𝑐𝑎𝑙

𝑟𝑎𝑙𝑑𝑜𝑙 = 𝒌𝒂𝒍𝒅𝒐𝒍(𝐶𝐴𝑐𝑎𝑙,𝑓𝑖𝑙𝑡𝑒𝑟)2

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

𝜃𝐴𝑐𝑎𝑙 =𝐶𝐴𝑐𝑎𝑙,𝑓𝑖𝑙𝑡𝑒𝑟

𝛤𝑓𝑖𝑙𝑡𝑒𝑟

• Laminar flow (Re < 800)

• Darcy equation (single-phase gas flow in a porous medium)

stationary solver

• Species transport in porous media (incl. free flow)

• Domain ODE

time dependent solver + optimization solver (SNOPT)

10

Comsol physics incl. adsorption/desorption

ttCFDoutAcaltxpeoutAcal

aldoldesadsfilterAcal

desadsbulkAcalbulkAcalbulkAcal

CCObj

rrrt

C

rrCCDt

C

2

,,,,,,

,

,,, u

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

11

Comsol results incl. adsorption/desorption

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

Parameters from analysis (exp) Parameters from Comsol optimization

filter (mol kg-1

) K (m3

mol-1

) filter (mol kg-1

) K (m3

mol-1

)

Sample A 0.0126 255 0.0119 220

Sample B 0.0425 222 0.0382 221

12

Conclusions

Introduction Fiber reactor Comsol model Comsol model incl.

adsorption/desorption Conclusions

• CFD/multiphysics is a versatile tool for parameter estimation:

• Time dependent solution

• Spatial distribution

• Implicit correction for air displacement

• Extension of Langmuir model

1 vs several experiments