Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540

Contents lists available at ScienceDirect

Process Safety and Environmental Protection

r .com/ locate /psep

CFD m r,photo e

Guillaum aireLaboratoire 1 ru

a

T whic

a ergla

ion e

concentrations. Neither external mass transfer nor internal diffusion limitations are observed, and the annular

reactor is rst assimilated to aplugowreactor. The LangmuirHinshelwoodmodel gives a gooddescriptionof acetone

degradation, and kinetic and adsorption parameters can be analytically deduced of the performed experiments.

AComputational FluidDynamics description of the reactor is then proposed. The classical NavierStokes equations

describe the ow in the free zone, whereas the Brinkman equation is used for the description of the ow in the porous

z

D

a

b

o

o

p

K

1. Int

Photocatalya promisinPhotocatalyusually TiOto near UVtoinduced rsufcient eof the catacharge septhe valenceduction bathe electrontron can reathe generaoxidized pr

CorresponE-mail aReceived

0957-5820/$doi:10.1016/one. Fromanhydrodynamic point of view, a very good agreement between theoretical andmeasured Residence Time

istribution or upward velocities is obtained, testifying for the good description of the photocatalytic reactor. For

given illumination, the variations of acetone concentration inside the reactor can then be modelled. Comparison

etween theoretical and experimental outlet concentrations allows nally a better precision in the determination

f the Langmuir Hinshelwood kinetic parameters. A relative difference of about 15% appears between the two sets

f kinetic parameters (obtained assuming a plug ow or deduced from the CFD modelling). CFD simulations of

hotocatalytic reactor can thus address a better design of such air treatment devices.

2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

eywords: CFD simulation; Photocatalytic reactor; Acetone degradation; Indoor air treatment

roduction

tic degradation of organic compounds appears asg process for remediation of air polluted by VOCs.tic processes use a semi-conductor photocatalyst,

2, as a slurry or deposited on a support, exposedlight to induce the degradation reactions. The pho-eaction is activated by absorption of a photon withnergy, i.e. equal or higher than the band-gap energylyst (Carp et al., 2004). The adsorption leads to aaration due to the promotion of an electron fromband of the semi-conductor catalyst to the con-

nd, thus generating a hole in the valence band. Ifhole recombination is limited, the activated elec-ct with an oxidant yielding a reduced product, and

ted hole can react with a reductant to produce anoduct.

ding author. Tel.: +33 383 17 53 04; fax: +33 383 17 53 19.ddress: Eric.Schaer@ensic.inpl-nancy.fr (E. Schaer).30 June2009; Received in revised form8 March 2010;Accepted23August 2010

The photocatalytic degradation kinetics is generally rep-resented by the Langmuir Hinshelwood model. Consideringthat the adsorptionof the reactionproducts and intermediatesremains low, the degradation rate, proportional to the numberof adsorbed reactive species, takes the form:

r = k bc1 + bc (1)

where k is a (degradation) kinetic constant that may dependon temperature, incident light, humidity. . . and b is an adsorp-tion constant which depends on the chemical afnity of thepollutant with the catalyst.

This work deals with the study of photocatalytic degrada-tion of acetone, which is characteristic of indoor air pollution.Acetone is naturally produced by oxidation of humic sub-stances and it appears as a metabolic product of plants and

see front matter 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.j.psep.2010.08.004journa l homepage: www.e lsev ie

odelling of an annular reactocatalytic degradation of aceton

e Vincent, Eric Schaer , Paul-Marie MarquRactions et Gnie des Procds, UPR CNRS 3349, Nancy Universit,

b s t r a c t

his study deals with the photocatalytic degradation of acetone,

nnular photoreactor. The TiO2 photocatalyst is deposited on a b

uorescent tube placed at the center of the device. Acetone conversapplication to the

, Orfan Zahraae Grandville, BP 20451, 54001 Nancy Cedex, France

h is a typical pollutant of indoor air, in an

ss support and irradiated by a commercial

xtents up to 90% are obtained for low initial

36 Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540

Nomenc

bCDIkruQVX

Greek let

Subscript0

animals (Rasources, actry, varnishaverage valpean Uniopotential aair of work

Experimare then pmined usinobtained u

2. Ex

The experiow is splitmass ow csaturatorsstatic bath,the seconding systemair, acetonereactor.

This (anilluminatiovery gooddeposed onP25. It is irrthe center olighting cosupport aretion) that calso limitsacting as a66.4 cm3. AFig. 1.

A gas cdetector isduring pho

Ex

UV lation

OCH

erimtioncmd fe

s rant resrate

tingby t

of haationsionidene ofand

ctivitnulantaryccepeactlug an i

)) alloeterseed,

bc

+ bc

inte

X=lature

adsorption constant (m3 kg1)concentration (kgm3)diffusivity (m2 s1)incident light (Wm2)kinetic constant (kgmin1 m3)kinetic rate (kgmin1 m3)velocity (ms1)volumetric ow rate (m3 min1)annular reactor volume (66.4106 m3)conversion extent ()

tersvoid fraction (0.947 here)permeability (m2)viscosity (Pa s)density (kgm3)

sinitial

ppert and Mller, 2005). Regarding anthropogenicetone is widely used as a solvent in the paint indus-es, inks and adhesives (INRS, 2003). Time weightedues are established for acetone (1.210 g/m3 in Euro-n) and its photocatalytic degradation could ndpplication for reducing emissions of VOCs in theing space or for indoor air treatment devices.ental setup is rst described, experimental resultsresented, and kinetic parameters are rst deter-g a plug ow model and then compared to those

sing Computational Fluid Dynamics.

perimental setup

mental setup is presented in Fig. 1. The initial air

3.

Underdegrad

CH3C

Expcentra1.7mW40% anextent

Firsdationindicalimitedteriondegradof diffu

Resabsencstreamconduthe anelemeerally aa real rtor, a p

Fortion (3param

Ind

r = k1

and its

VQC0into threeways, each one controlled by a separatedontroller. Air is continuously bubbling through twomaintained under strong agitation in a thermo-one containing the volatile organic compound andone containing water. After owing through a mix-, the gas stream, composed of dened quantities of

and humidity, enters the photocatalytic annular

nular) axial symmetry conguration offers the bestn conditions for the catalyst, and also ensures acontact between gas and catalyst. The catalyst isa berglass support impregnated of TiO2 Degussa

adiated by a commercial uorescent tube placed atf the device, which offers dened and reproducible

nditions. The uorescent tube and photocatalyticseparated by a liquid solution (a nigrosine solu-

ontrols the temperature inside the reactor and thatillumination during the photocatalytic reaction,liquid lter. The annular volume of the reactor istop view sketch of the reactor is also presented in

hromatograph equipped with a ame ionizationused to follow acetone concentration variationstocatalytic oxidation.

where is ttion, X is thand V is th

The linepresentedmodel is wedegradationare respectobtained vk=2.63mgtion and kiin the reactvariationsFig. 3a. Thea functionAt low conbecomes cocan be seen

The redconcentratinuence odation of aindependenperimental results

ight and in presence of oxygen, the photocatalyticof acetone can be expressed as:

3 +4O2 3CO2 +3H2O (2)

ents have been performed for initial acetone con-between 0.29 and 1.73mgL1, incident light of

2, relative humidities varying between 10% anded ow rate of 300mLmin1. Acetone conversionge between 20% and 90%.ults have ensured that the acetone photodegra-does not depend on the volumetric ow rate,

that the reaction (if it is not a zeroth order) is nothe external mass transfer. Similarly, the Weisz cri-s been estimated at 6105 for the photocatalyticof acetone (Vincent, 2008), reecting the absence

al resistance within the catalyst pellets.ce Time Distributions have been measured inreaction using a pulse of hydrogen in the feeddetected at the photoreactor exit by a thermal

y detector (TCD). The analysis of RTD revealed thatr reactor could be assimilated to a cascade of 26continuously stirred tank reactors. Since it is gen-ted that above a number of 20 elementary reactors,

or can be considered practically as a plug ow reac-ow was rst assumed in the annular reactor.

deal plug ow, integration of mass balance (rela-ws thedetermination of the kinetic andadsorptionk and b.

the masse balance for a plug ow reactor is:

= dcd(V/Q)

(3)

gration leads to:

1kb

[ln(1 x)

C0X

] 1

k(4)

he void fraction, C0 is the acetone initial concentra-e conversion extent, Q is the volumetric ow rate

e annular reactor volume.ar variation of V/(QC0X) against ln(1X)/(C0X)in Fig. 2 shows that the Langmuir Hinshelwoodll adapted for the description of the photocatalyticof acetone. The adsorption and kinetic constants

ively deduced of the slope and intercept, and thealues are b=8.16 Lmg1 (8.16103 m3 kg1) andL1 min1 (2.63103 kgm3 min1). The adsorp-netic constant thus obtained assuming a plug owor give a good description of the conversion extentagainst initial concentration, as can be seen incurrent reaction rate can nally be expressed as

of acetone concentration, according to relation (1).centration the kinetics is rst-order in acetone. Itnstant (zeroth order) for larger concentrations, asin Fig. 3b.

uction of transmitted light for different Nigrosineions within the optical lter enabled studying thef the incident light on the photocatalytic degra-cetone. It appears that the adsorption constant ist of the incident light and that the kinetic constant

Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540 37

Fig. 1 Experimental setup and top view ske

y = 0.047x-0.38

-1

-0.8

-0.6

-0.4

-0.2

0

-10 -8 -6 -4 -2 0ln(1-X)/(C0X)

-V/(QC0X)

Fig. 2 Langmuir Hinshelwood parameters determination.

is proportioal., 1998) a

k = k0I00.38

It is freqrange betwhas been shand 1/2 coucals under(equal to 0electron/hotocatalytic

Finally,kinetics oftch of the reactor.

nal to the incident light according to (as Wang etpower law:

(5)

uently reported that the values of the exponenteen 0 and 1 for most photocatalytic reactions. Itown (Vincent, 2008) that an exponent between 1/4ld be due to a recombination between OH radi-

strong illumination. Here, the measured exponent.38) can be attributed to mixed recombinations ofle pairs and hydroxyl radicals OH during the pho-oxidation of acetone.relative humidity seems to have no effect on thereaction, for the studied range.

38 Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540

0

0.2

0.4

0.6

0.8

1(a)

(b)

0 0.5 1 1.5 2

C0 (mg.L-1)

X

Experimental

Plug Flow Reactor

0

0.2

0.4

0.6

0.8

1

1.2

0

r (mg.L-1.min-1)

Fig. 3 (a)for a plug against con

4. CF

4.1. Mo

4.1.1. HyThe ow inFig. 5), andter accuracbe obtainedtational Fluthen been d

The cldescribe thwhereas thow in the

t+ (u)

ut

+ u

0.E+00

2.E-03

4.E-03

6.E-03

8.E-03

1.E-02

2.25E-02

velo

Fig. 4 Parannular zo

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0

E (

u is

u +

beetermeeds

Readynathetion

+ u

Res

imulComred0.5 1 1.5 2

C (mg.L-1)

Conversion extent against initial concentrationow reactor. (b) Photocatalytic degradation ratecentration.

D simulation

del development

drodynamicsthe reactor is not perfectly plug (see the RTD ina better characterisation of the reactor as a bet-

where

[

(

Thetally dgas sp

4.1.2.Hydrowithinconvec

D2C

4.2.

CFD susingcompay in the kinetic and adsorption parameters couldwith a better description of the ow. A Compu-

id Dynamics modelling of the annular reactor haseveloped.

assical NavierStokes equations (relation (6))e ow in the free zone (in the absence of support)e Brinkman equation was used to describe theporous breglass support (relation (7)):

= 0

u = [(u + (u)T)](6)

2.50E-022.45E-022.40E-022.35E-022.30E-02radius (m)

city distribution (m.s-1)

NumericalTheoretical

abolic axial velocity eld against radius in thene.

kinetic par

4.2.1. HyA full 3D mlaminar oevolution arelatively uBrinkman ethe berglagas owanin the phot

In the alaminar oal., 2002):

uz(r) = 1

where uz(r)are respect given by:

= r0R

, 2

Numeriarea of thetions, and20 40 60 80 100

ts (s)

ts) experimental simulation

Fig. 5 Simulated and measured RTD.

the velocity, is the density and is the viscosity.

(u)T)]

= ku (7)

rglass support permeability has been experimen-ined by head losses measurements for different

. The estimated value is 1.11010 m2.

ctionmic conditions being validated, chemical reactionreactor can be taken into account according to theand diffusion equation:

C = r (8)

ults

ations of such a reactor have been performedsol Multiphysics. Hydrodynamic results are rstwith experimental ones and acetone degradationameters are nally determined.

drodynamicsodel of the reactor is rst solved for a stationary

w in absence of any reaction. The 3D velocity eldlong the reactor axis helps ensuring that the ow isniform in the photocatalytic area governed by thequation (see Fig. 6b). The random arrangement ofss support allows for a fairly good distribution ofd consequently a relatively homogeneous oweldoactive part of the reactor.nnular area, integration of momentum balance forw gives the velocity distribution as follows (Bird etumax2(1 ln 2)

[1

(r

R

)2 1

2

ln(1/)

(R

r

)](9)

is the axial (upward) velocity component, r0 and Rively the internal and external cylinder radii, and

2 = 1 2

ln(1/)(10)

cal axial velocity distributions in the photoactivereactor can be easily deduced from CFD simula-

as can be seen in Fig. 4, are almost superimposed

Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540 39

ann

with theorenamic desc

Finally,sient tracerdomain. Ththe scalar tor tracer cothe RTD deFig. 6 Transient tracer concentration evolutions in thetical axial ones, testifying for the good hydrody-ription of the reactor.a numerical RTD can be performed solving a tran-mass transport on top of the ow eld in the timee assumption to decouple the velocity eld andransport equation is accurate because the acetonencentrations remain low. As can be seen in Fig. 5,duced from the numerical simulation leads to a

good agreemspecic pho

The plutocatalyticthe Navierequation incentrationnumerical

Fig. 7 Acetone concentration variations inside the annuular reactor at different moments.entwith the experimental one, obtainedwith thistocatalytic support.

g ow with axial dispersion observed in the pho-reactor is thus perfectly described by CDF usingStokes equations in the free zone and Brinkmanthe photoactive area. The transient tracer con-

evolutions inside the annular reactor during theRTD are shown in Fig. 6.

lar reactor, C0 =5104 kgm3.

40 Process Safety and Environmental Protection 8 9 ( 2 0 1 1 ) 3540

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

C0 (mg/L)

Conversion (X)

CFD, k = 2.63 mg/min/L, b = 8.16 L/mg

Experimental

CFD k' = 2.23 mg/min/L, b' = 6.94 L/mg

Fig. 8 CFD simulations with two sets of kineticparameters.

4.2.2. Photocatalytic degradationThe coupled simulation (hydrodynamics and chemical reac-tion) of acetone degradation in the annular photocatalyticreactor is nally performed using the hydrodynamic modeland the previously determined kinetic parameters.

It rst helps ensuring that no external diffusion effectsaffect the transfer, since the acetone concentration remainsuniform fomer kineticconversionparametersof the comconcentratevolution inbe seen in

The comfrom the ppresented ibetween boplug ow avery small.(NavierStozone) givedescriptiontone in the

the plug ow deduced parameters induces an overestimationof the conversion extent.

5. Conclusion

The photocatalytic degradation of acetone has been imple-mented in an annular reactor where the TiO2 photocatalyst isdeposited on a berglass support. The reaction is not limitedby the external mass transfer nor internal diffusion, and forrelatively low initial concentrations conversion extents varyfrom 20 to 90%.

The Langmuir Hinshelwood model can be applied for thedescription of the degradation process, and it has been shownthat a CFD modelling gives a better description of the plug owdispersion in the reactor and a better accuracy in the determi-nation of the kinetic parameters. Indeed, application of theNavierStokes, Brinkman and convectiondiffusion equationsgive a pretty good match between simulation and experimen-tal RTD and conversion variations.

CFD simulations of photocatalytic reactor can thus addressa better design or extrapolation of such air treatment devicesand should also allow a better understanding of the physico-chemical phenomena involved in photocatalytic processes.

ence

.B., Stnom., Huitaniu177.003.t, S.,ssionste Mt, G.,traitL, NaK.-H.tocar TiOironmr a given axial position. However utilisation of for-parameters gives a rather poor description of theand more accurate Langmuir Hinshelwood kineticof acetone degradation can nally be deduced

parison between theoretical and measured outletions (Vincent, 2008). The pollutant concentrationside the reactor is then perfectly described, as can

Fig. 7.parison between the kinetic parameters derived

lug ow hypothesis and those obtained by CFD isn Fig. 8. A relative difference of about 15% appearsth kinetic parameters (k and b obtained assuming and k and b deduced from CFD) but the gap remainsThe models used in the two areas of the reactorkes in the free zone and Brinkman in the porousa better representation of the plug ow with axialto predict the photocatalytic conversion of ace-annular reactor. Results show that utilisation of

Refer

Bird, RPhe

Carp, Oof t33

INRS, 2Rapper

emiWa

VincenparINP

Wang,phooveEnvs

ewart, W.E., Lightfoot, E.N., 2002. Transportena, Second ed. Wiley, New York.isman, C.L., Reller, A., 2004. Photoinduced reactivitym dioxide. Progress in Solid State Chemistry 32,

Actone. Fiche Toxicologique 3, 8386.Mller, R., 2005. Odor compounds in waste gass from agricultural operations and food industries.anagement 25, 887.2008. Procd dlimination de la pollution de lairement photocatalytique: application aux COVs. PhD.ncy, France., Tsai, H.-H., Hsieh, Y.-H., 1998. The kinetics oftalytic degradation of trichloroethylene in gas phase2 supported on glass bead. Applied Catalysis B:ental 17, 313320.

CFD modelling of an annular reactor, application to the photocatalytic degradation of acetoneIntroductionExperimental setupExperimental resultsCFD simulationModel developmentHydrodynamicsReaction

ResultsHydrodynamicsPhotocatalytic degradation

ConclusionReferences