Chemical Engineering Science 58 (2003) 2455–2459 www.elsevier.com/locate/ces ShorterCommunication Masstransfertimeinadeepbubblebed StoyanNedeltchev 1 ,ShinichiOokawara,KoheiOgawa * Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-Ku, Tokyo 152-8552, Japan Received 11 October 2001; received in revised form 26 November 2002; accepted 7 December 2002 1. Introduction Bubble column reactors (BCRs) are widely adopted as gas–liquidreactorsinindustryforchemical,petrochemical, biochemical,andenvironmentalapplications.BCRsareused formasstransferaswellasforchemicalreactionsbecause oftheirsimpleconstructionandexcellentperformance. From a practical viewpoint, it is essential to know the operationaltimerequiredtoattainthemaximumachievable spatialconcentrationuniformityorequilibriumstateforsuch BCRoperationsasstart-up,exchangeoffeedinggasses,etc. DeckwerandSchumpe(1993) aswellas Letzel,Schouten, Krishna,andVandenBleek(1999) havedenedthemass transfer(saturation)time  MT asfollows:  MT = 1 k L a ; (1) where k L is the liquid-phase mass transfer coecient and a is the interfacial area (=6 G =d s ). According to Deckwer andSchumpe(1993) themasstransfertimereectsthetime neededforabout60%bulkphasesaturation. Inourpreviousworks(Nedeltchev,Ookawara,&Ogawa, 1999, 2000), the spatial concentration uniformity in both lowerandupperzoneswasquantiedlocallyineachzone aswellasgloballyintheoverallbubblebed(BB)bymeans ofnewlydenedqualityofmixedness(QM)indexbasedon theinformationentropytheory.TheQMvariesfrom0(no mixing)to1(completemixing)basedonthespatialliquid concentrationdistribution. Nedeltchevetal.(2000) demon- stratedformediumBBs(2:1 6 LD -1 c 6 4:4)thatoncethe correctedmasstransfertime  exp MT dueto Miller’s(1974) cor- rection method is elapsed, both lower and upper zones as wellastheoverallBBpracticallyreachunityvalueofQM. FortheexamineddeepBB(LD -1 c =7:3),however,thein- troductionofMiller’scorrection k ∗ L =683d 1:376 s to Higbie’s (1935) penetrationtheoryledonlytoanoverlappingofQM ∗ Corresponding author. Tel.: +81-3-5734-3035; fax: +81-3-5734-2882. E-mail address: kogawa@chemeng.titech.ac.jp (K. Ogawa). 1 On leave of absence from the Institute of Chemical Engineering, Bulgarian Academy of Sciences, 1113 Soa, Bulgaria. proles for both zones but the column should continue its operation for some additional time until the QM proles forbothzonesaswellastheoverallBBreachthehighest (practicallyunity)value.Thegas–liquidcontacttimeinthe penetrationtheorywasdenedasaratiobetweentheSauter meanbubblediameter d s (see Akita&Yoshida,1974)and bubble rise velocity u b (supercial gas velocity u g divided bytheoverallgasholdup  G ).TheSautermeanbubbledi- ameters d s calculated by Akita and Yoshida’s (1974) cor- relationcorrespondtoellipsoidalbubbleshapeaccordingto theclassicationmapsprovidedby Clift,Grace,andWeber (1978) and FanandTsuchiya(1990). Inthisstudy,anewmodiedmasstransfertime  mod MT due to FanandTsuchiya(1990) isfoundapplicableforthedeep BBexaminedandphysicalinterpretationispresentedforthe dierencebetweencharacteristictimes  exp MT and  mod MT . 2. Experimental The time-dependent liquid concentration proles were measured in a semi-batch transparent cylindrical BCR of 0:289 m in ID operated at ambient pressure. The reactor wasequippedwithaperforatedplatedistributorof0:005m thickness consisting of 31 holes of diameter 0:002 m ar- rangedonanequilateraltriangularpitchof0:05m.Thegas enteredthechamberbelowthedistributorthrougha0:005m orice.TheexamineddeepBB(LD -1 c =7:3)wasoperated at u g =0:038ms -1 .Suchanoperatingconditionisconsid- eredtofallwithinthechurn-turbulentowregimebasedon theregimetransitioncorrelationof Reilly,Scott,DeBruijn, andMacIntyre(1994) whichpredicts U trans =0:034ms -1 . The clear liquid height L 0 was set equal to 1:84m.Asa result of the adjusted u g value the aerated bed height L was as high as 2:1 m, viz., gas holdup  G was 0.124 (vi- suallyestimated).Deionizedwaterconstitutedthecontinu- ous phase and carbon dioxide supplied from gas cylinders wasusedasatracer.Beforeintroducingthecarbondioxide tracer,thedeionizedwaterwasthoroughlypurgedbynitro- gen.Thetracerconcentrationsinthedierentregionsofthe deep BB were measured three times by a single electrical 0009-2509/03/$-see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0009-2509(03)00094-0