Data di Pubblicazione:
2013
Abstract:
The problemofmasstransferbetweenactivesolidparticlesandafluidinmultiparticlesystemsis
examinedwithafocusonthestagnantandthelowReynoldsnumbercases.Thisproblemhasattracted
significantattentionwithregardtooperationoffixedandfluidizedbeds.Itisrecognizedthatdifferent
Sherwoodnumberscanbedefineddependingonthechoiceofthereferenceconcentrationdifference
(drivingforce).Aneffective Sh has oftenbeenusedtoanalyzeexperimentaldatabasedonanoverall
concentrationdifferenceacrossthebed.Alocal Sh can alsobeintroducedbasedonaconcentration
differenceclosetotheactiveparticle.However,theuseofthesetwodifferentSherwoodnumbers
impliesadifferentimplementationofthemassbalanceequations.
The massbalanceequationsarehereanalyticallysolvedbothunderstagnantandnon-stagnant
conditionsinamultiparticlesystemundersuitablesimplifyingassumptions.Equationsfortheeffective
and localSherwoodnumbersarederivedforthegeneralcaseandfortheasymptoticlimits.Allowance
is givenforthevariationofbedvoidageandvolumefractionofactiveparticlesinthebed.Itisshown
that thelocal Sh only dependsongeometricalandfluid-dynamicsconsiderationsandaccordinglyhasa
generalvalidity.Onthecontrary,theeffective Sh also dependsontheassumptionsmadeinderivingthe
mass balanceequationsacrossthebed(e.g.,fluidplugflow).Theuseofthelocal Sh is therefore
suggested.
Resultsshowthatfor Re-0 thelimitingvalueofthelocal Sh is alwaysafinitenumber,whilethe
effective Sh tendstozerolinearlywith Re. Itisshownthatthisresultissimplyaconsequenceofthe
plug flowassumptionmadeinthebedmassbalance.Thegeneralexpressionsderivedherecompare
very welltoexperimentaltrendsforthecasesoflargeReynoldsnumbersandoffewisolatedactive
spheres immersedinabedofinertparticles,wheremostofthereportedexperimentaldatagather.
Unfortunately,forthemostcontroversialcaseofverylow Re in bedsmadeentirelyofactiveparticlesno
reliabledataappearstobeavailabletochecktheaccuracyoftheexpressions.
examinedwithafocusonthestagnantandthelowReynoldsnumbercases.Thisproblemhasattracted
significantattentionwithregardtooperationoffixedandfluidizedbeds.Itisrecognizedthatdifferent
Sherwoodnumberscanbedefineddependingonthechoiceofthereferenceconcentrationdifference
(drivingforce).Aneffective Sh has oftenbeenusedtoanalyzeexperimentaldatabasedonanoverall
concentrationdifferenceacrossthebed.Alocal Sh can alsobeintroducedbasedonaconcentration
differenceclosetotheactiveparticle.However,theuseofthesetwodifferentSherwoodnumbers
impliesadifferentimplementationofthemassbalanceequations.
The massbalanceequationsarehereanalyticallysolvedbothunderstagnantandnon-stagnant
conditionsinamultiparticlesystemundersuitablesimplifyingassumptions.Equationsfortheeffective
and localSherwoodnumbersarederivedforthegeneralcaseandfortheasymptoticlimits.Allowance
is givenforthevariationofbedvoidageandvolumefractionofactiveparticlesinthebed.Itisshown
that thelocal Sh only dependsongeometricalandfluid-dynamicsconsiderationsandaccordinglyhasa
generalvalidity.Onthecontrary,theeffective Sh also dependsontheassumptionsmadeinderivingthe
mass balanceequationsacrossthebed(e.g.,fluidplugflow).Theuseofthelocal Sh is therefore
suggested.
Resultsshowthatfor Re-0 thelimitingvalueofthelocal Sh is alwaysafinitenumber,whilethe
effective Sh tendstozerolinearlywith Re. Itisshownthatthisresultissimplyaconsequenceofthe
plug flowassumptionmadeinthebedmassbalance.Thegeneralexpressionsderivedherecompare
very welltoexperimentaltrendsforthecasesoflargeReynoldsnumbersandoffewisolatedactive
spheres immersedinabedofinertparticles,wheremostofthereportedexperimentaldatagather.
Unfortunately,forthemostcontroversialcaseofverylow Re in bedsmadeentirelyofactiveparticlesno
reliabledataappearstobeavailabletochecktheaccuracyoftheexpressions.
Tipologia CRIS:
01.01 Articolo in rivista
Keywords:
Mass transfer; Sherwood number; Multiphase flow; Diffusion; Packed bed
Elenco autori:
Scala, Fabrizio
Link alla scheda completa:
Pubblicato in: