[PDF] An experimental investigation of evaporation rates for different

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An ExperimentalInvestigation ofEvaporation Rates

for DifferentVolatile OrganicCompounds

Fr?ed?eric Heymes,

a

Laurent Aprin,

a

Aur?elia Bony,

a

Serge Forestier,

a

Stefano Cirocchi,

b and

Gilles Dusserre

a a

Institut desSciences desRisques, Ecoledes Minesd'Ale `s, 6A venuedeClavie`res, 30319Ale `s Cedex,France; frederic.heymes@mines-ales.fr(forcorrespondence)

b

Department

performedinordertomeasureevapora- tion rates

2-propanol,

tion

Different

correlationsweretestedversus theexperimental.

Exponents

tal VOC experimentation(ethanol)and the accuracyofthecorre- lation investi-gated versus temperatureandwindvelocity.

Keywords:

evaporation;

VOC;volatileorganiccompound;

spill

INTRODUCTION

Liquid

volatileorganic compounds(VOC)arefrequently used coatings, solvents,fuels,additives,etc.

Thistypeofliquid

evaporates quicklyand formsvaporswithnegativeeffectson people andthe environment.Onceinavaporstate,these compounds dispersed overtheindustrialareas.Numerous workswhere achieved in thefieldofliquidevaporation,withafocuson three liquefied recently, interestinglobalsafetyinvestigated unconventional scenarios. are pumped gas increasingly used.Global safetyscenariosmayconsistinthe impact ofaprojectile onliquidstorages.Aleakwilloccur entailing a pool of liquid and therefore evaporation [6,7]. The aim quantify the conditions.

Differentwind

velocitiesandfivedifferentliquids were recorded anddiscussed.

THEORY

When aliquid isspilledontheground,theopensurfaceof the thermodynamicequilibrium. Thepool ischaracterized by geometrical parameterssuch asthickness andsurface area but alsoby itstemperature andchemical composition.Due to evaporation,the pooltemperature andcomposition will vary dependingon heatand masstransfers withthe environ- ment. Theevaporation ofthe puddleis afunction ofdiffer- ent assessmentsmade betweenthe puddleand the environment. Incase ofpure fluids,it mustbe notedthat evaporation willbe regulatedby boundary-layertype mass transfer,thus windis relevant.No masstransfer resistance has tobe consideredin theliquid sinceno massgradient can happen.Brighton [8]investigated theseboundary layer effects onpool evaporation.However ,the mostcommon theory usedto modelthe masstransfer betweenthe liquid phase andgas phaseis theWhitman theory,known alsoas the “DoubleFilm Model."This theoryis describedand discussed inprevious works[9]. Inthis work,the masstrans- fer isonly regulatedby thefilm inthe gaseousphase. The evaporation flowrate canbe writtenas: J5D g d g ?ðC ?g 2C g

Þ(1)

whereJis themass flowrate (kgs 21
m 22
),D g is thediffusiv- ity ofthe compoundin air(m 2 s 21
),C g is theconcentration of thecompound inthe air(kg m 23
),C ?g is thegas phase equilibrium concentrationwith theliquid, dg is thethickness of thefilm onthe gasside (m).Using themass transfernota- tions, theflow Jcan bewritten: J5K g ?ðC ?g 2C g

Þ(2)

whereK g is themass transfercoefficient inthe gasand in the liquidphase, respectively(m s 21
). Byconsidering the gas phaseas ideal,the followingequation canbe written: C ?g 5 P v

TðÞ?M

R?T(3)

whereP v (T) isth evapor pressureof thecompound(Pa) ,Tis theli quidtemperat ure(K),Misth emolecu larweightofthe compound(kgmol 21
),Ris theuniv ersalgasconstant58.31441 (J mol 21
K 21
). Byco uplingEq.2and3 ,itcan bewr itten: J5K G P v

TðÞ?M

R?T(4)

The evaporationof thespecies willprovoke adecrease of the liquidtemperature sincevaporization isendotherm icand will consumevaporization enthalpyDH vap . Theenthalpy bal- ance willresult fromendotherm icphase changeandheat transfers byconvection withair ,conduction withgroundand radiative fluxes(sun, atmosphere).The massbalance equa- tion hasto becompleted bya heatbalance: J52q L dðhÞ dt5K g P v T L

ðÞ?M

R?T h?q L dðC P;L T L dt5u ground 1u air 1u atm 1u sun 1J?DH vap 8>< (5) wherehis theliquid thickness(m), q L is theliquid density (kg m 23
),DH vap the vaporizationenthalpy (negative)(J kg 21
),C p,L is theheat capacityof theliquid (Jkg 21
K 21
u ground ,u air ,u atm ,u sun are theheat fluxeswith theground (W m 22
), theair ,theatmosphere,and thesun. Themain work wasachieved inorder toevaluate K g in regardwith the airvelocity andthe VOCidentity. Themass transfercoef- ficient inair K g depends onthe velocityof thewind u air , the air densityq air , theair dynamicviscosity l air , thediameter of the evaporationpool d, andthe compounddiffusivity D g .A dimensional analysisallows correlatingK g withu air ,q air ,m air d, andD g by theclassic equationSh5fðRe;ScÞ, whereShis the Sherwoodnumber ,Scis theSchmidt number, Reis the

Reynolds number.

Sc5 l air q air :D g Re5 q air ?u air ?d l air Sh 5 K m :d D g (6)

A keypoint indeter miningthe Reynoldsnumberisthe

definition ofthe windvelocity u air . Indeed,the velocitypro-

file ofincoming winddepends stronglyon theatmospheric boundary layer,atmosphericstability, groundroughness, andlocal configurationsuch asthe presenceof obstaclesor

bunds. Therefore,different definitionsof thewind velocity can beused: at10 metershigh, atthe edgeof thepool or can beaveraged overa height. The functionf(Re,Sc) wasevaluated fromexperimental data byRaj andMorris [10],Mackay [4],and Green[11]. Itis interesting tocompare thevalue ofexponents. Forthat, the equations wererewritten as: K G

5fðu

a ;d b ;D cg ;m d

Thus, theproposed relationshipevolves differently

according toeach ofthe differentparameters (Table 1).The formulafrom Greenand Maloneytakes intoaccount wind velocity lessthan others.The diameterof thepool istaken into accountin verydissimilar frommodel tomodel. The gap iseven morenotable fordiffusivity. Indeed,according to Ref.4, increaseddiffusivity promotesthe coefficientofquotesdbs_dbs17.pdfusesText_23

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