18-08-2012, 12:25 PM
Cryogenic CO2 capture usingdynamicallyoperatedpackedbeds
Cryogenic CO2 capture usingdynamicallyoperatedpackedbeds.pdf (Size: 359.87 KB / Downloads: 101)
1. Introduction
Reduction ofanthropogenicCO2 emissions isbecominganurgent
issue asconcernsaboutglobalwarmingareincreasing.Energypro-
duction processesbasedonfossilfuelswillhavetobereplacedby
new processesusingrenewableresources,viz.wind,solar,biomass
and fusionenergy.However,manyofthesetechnologiesstillrequire
much furtherdevelopment,anditisnotrealistictoassumethatour
energy productionwillbeswitchedtowardrenewablesonashort
term. Itisexpectedthattheworldwillremainlargelydependenton
fossil fuelsforthenextdecades(US DepartmentofEnergy,2007)
therefore makingcaptureandstorageofCO2 from fluegasesakey
measure toreduceCO2 emissions totheatmosphere.
Technologies forCO2 capture areoftenclassifiedintooxyfuel,
pre- andpost-combustionprocesses.Inoxyfuelprocessesfossil
fuels arecombustedusingpureoxygen,circumventingdilutionof
CO2 with nitrogen.Inpre-combustionprocessesfossilfuelsare
gasified, CO2 is subsequentlycapturedandhydrogenisfedtothe
combustion chamber.Post-combustionprocessesarebasedoncap-
turing CO2 from fluegasesfromconventionalairfiredpowerplants.
This technologycanthereforeberetrofittedtoalreadyoperating
power plantsandindustries.Forthisreasonpost-combustionis
∗ Corresponding author.Tel.:+31534894478;fax:+31534892882.
E-mail address: m.vansintannaland[at]tnw.utwente.nl (M. vanSintAnnaland).
0009-2509/$ -seefrontmatter © 2009 ElsevierLtd.Allrightsreserved.
doi:10.1016/j.ces.2009.01.055
considered themostrealistictechnologyontheshortterm.Several
capture processesarecurrentlyunderdevelopment,suchasscrub-
bing withamines(Linde, 1985), pressureswingadsorption(PSA)
(Ravikumar andReddy,1999) ormembraneprocesses(Powell and
Qiao, 2006). Allsuggestedoptionswillresultinenergypenalties,
caused byregenerationofCO2 loaded absorbentsorbyrecompres-
sion offluegasstreamsinviewoftheoperationatelevatedpres-
sures. State-of-the-arttechnologyisaminesolventscrubbing.Main
difficulties forthistechnologyarethestabilityofthesolventsand
the energyrequirementstostripCO2 from theloadedsolvent.
A relativelynovelCO2 capture technologyisbasedoncryogenic
removal ofCO2. Expensiverefrigerationcanpossiblybeavoided
when exploitingthecolddutyavailableatliquefiednaturalgas(LNG)
regasification sites.Currently,LNGisbeingregasifiedusingseawater
or byusingwaterbathswhichareheatedbyburningafuelgas(Ertl
et al.,2006). TheglobalLNGmarketisstronglygrowing(John and
Robertson, 2008), thereforeintegrationofLNGregasificationanda
cryogenic CO2 capture technologycouldbebeneficial.Greatadvan-
tages ofcryogenicCO2 capture arethatnochemicalabsorbentsare
required andthattheprocesscanbeoperatedatatmosphericpres-
sures. Clodic andYounes(2002,2005) have developedacryogenic
CO2 capture process,whereCO2 is desublimatedasasolidontosur-
faces ofheatexchangerswhicharecooledbyevaporatingarefrig-
erants blend.Withcalculationsandexperimentalteststheyshowed
that theirprocesscouldcompetewithotherpost-combustionCO2
capture processes.Themaindisadvantageoftheirsystemisthat
the watercontentinthefeedstreamtothecoolingunitsshouldbe
M.J. Tuinieretal./ChemicalEngineeringScience65(2010)114--119 115
minimal inordertopreventpluggingbyiceoranunacceptablyhigh
rise inpressuredropduringoperation.Therefore,severalcostlysteps
are requiredtoremoveallwatertracesfromthefluegas.Inaddi-
tion theincreasinglayerofsolidCO2 onto heatexchangersurfaces
during thecapturecyclewilladverselyaffecttheheattransfer,re-
ducing theprocessefficiency.Moreover,thecostlyheatexchangers
have tobeswitchedtoregenerationcyclesoperatedatadifferent
temperature, whichshouldbecarriedoutwithgreatcaretoavoid
excessive mechanicalstresses.
In thiswork,apromisingnovelcryogenicCO2 removal processis
developed usingdynamicallyoperatedpackedbeds,withwhichthe
before mentioneddrawbackscanbecircumvented.Thepaperisor-
ganized asfollows.First,theworkingprincipleoftheconceptisex-
plained. Subsequentlytheexperimentalsetupandnumericalmodel
are described.Finallysimulationsandexperimentsarecomparedfor
N2/CO2 mixtures.
2. Concept
When feedingarelativelyhotfluegas(at Tin) containingCO2,
H2O andinertgases(suchasN2, O2, Ar)toaninitiallyuniformlyre-
frigerated packedbed(T0), aneffectiveseparationbetweenCO2, H2O
and thepermanentgasescanbeachievedonthebasisofdifferences
in dewandsublimationpointsasschematicallyillustratedin Fig. 1.
Axial position
Temperature Mass deposition
Tin
Ts1
Ts2
T0
H2O at t1
H2O at t2
CO2 at t1
CO2 at t2
mCO2
mH2O
t1
t2
Fig. 1. Typical evolutionofaxialtemperatureandcondensed/depositedH2O/CO2
mass profilesfrom t1 to t2 when feedingaN2/CO2/H2O gasmixture(at Tin) toa
packed bedrefrigeratedbeforeuniformly(T0).
The packingmaterialwillbeheatedupandthegasmixturewillbe
cooled downuntilH2O startstocondense.Condensationwilltake
place untilthepackingmaterial(andthegasphase)willreachan
equilibrium temperature(TS1). Duetothischangeinphase,afront
of condensingH2O willmovethroughthebedtowardtheoutletof
the bed.However,atthesametime,thepackingmaterialcloserto
the inletofthebedwillagainbeheatedupfromtheequilibrium
temperature TS1 to theinlettemperatureofthegasmixture Tin. This
increase intemperaturewillcausethepreviouslycondensedH2O
to beevaporatedagain.ThereforeanotherfrontofevaporatingH2O
will movethroughthebedtowardtheoutletofthebed.Theveloc-
ity ofthecondensingfrontisinherentlyfasterthanthevelocityof
the evaporatingfront,duetotheopposedenthalpiesinvolvedinthe
condensation/evaporation. Afterthewaterbeingcondensedatthe
packing surface,theremaininggasmixturewillbefurthercooled
until CO2 starts todesublimateandanewequilibriumtemperature
(TS2) isreached.Basedonthesameprinciples,againfrontsofsubli-
mating anddesublimatingCO2 will developandmovethroughthe
bed towardtheoutletofthebed.Interestingly,theamountofH2O
condensed andtheamountofCO2 desublimated perunitvolume
solid packingreachesamaximum,whichisrelatedtothemaximum
amount ofcoldstoredinthesolidpacking.Thus,problemswith
plugging orunacceptablepressuredropincreaseduringthecapture
cycle canbeintrinsicallycircumvented.Thisisoneofthemajorad-
vantages ofthisnovelconceptwithperiodicallyoperatedpacked
beds. Anotherimportantbenefitofthisconceptisthattheoutletgas
temperature isattheveryminimumtemperatureoftherefrigerant
during almosttheentirecapturecycle,sothatthemaximumpos-
sible CO2 capture isactuallyachieved.Whenthethirdtemperature
front reachestheendofthebed,CO2 will starttobreakthrough.At
this point,thebedisswitchedtoaregenerationcycle,whereapure
gaseous CO2 flow isusedtorecoverthefrostedCO2. Theheatstored
during thecapturecycleinthefirstzoneofthebedcannowbeef-
fectively usedtoevaporatecondensedH2O anddesublimatedCO2.
When allCO2 is recovered,thebedisswitchedtoacoolingcycle,
where thebediscooleddownusingarefrigeratedinertgas.
3. Experimentalsetupandprocedure
A packedbedwasconstructed,consistingofaborosilicateglass
tube (OD×ID×L=40×35×300mm)surroundedbyaglassvacuum
jacket (OD × ID = 60 × 55 mm).Thetubeispackedwithspherical
glass particles(dp = 4.04mm,s
= 2547 kg/m3). Aflowsheetofthe
experimental setupisshownin Fig. 2. Duringcoolingcyclesthebed
was fedwithaN2 gas flowwhichwasrefrigeratedinacoilpositioned
in aliquidnitrogenbath.Aftercoolingdownthebed,thefeedwas
switched toaN2/CO2 mixture atambienttemperature.Thegasfeed
flow rateswerecontrolledwithmassflowcontrollers(Bronkhorst
El-flow). Thetemperaturesinthebedweremeasuredalongthebed
length intheradialcenterwith11thermocouples(Thermo-Electric
K-type) atevery3cminaxialdirection.Thepressureattheinlet
of thebedwasmeasuredusingananaloguepressureindicator.The
CO2 content intheoutletstreamwasanalyzedwithanIR-analyzer
(Sick-Maihak, s610,0–3vol%).ThefrontofdesublimatedCO2 could
be visuallyinspectedwithacamera.