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Tableofcontents
CHAPTER1INTRODUCTION 4
CHAPTER2LITERATUREOVERVIEW 8
2.1UtilizationofactivatedcarbonsinCCSfield 8
2.1.1CO2adsorptionontoas‐synthesizedactivatedcarbons 8
2.1.2CO2adsorptiononfunctionalizedactivatedcarbons 10
2.2OtherclassesofCO2sorbentsinvestigated 12
2.2.1Zeolites 12
2.2.2Orderedmesoporoussilicas(OMS) 12
2.2.3Calciumoxide 13
2.2.4Metal‐organicFrameworks(MOFs) 14
2.3Ionicliquids(ILs) 15
2.3.1ILsapplicationsforCO2capture 16
2.3.2Functionalizationofsolidswithionicliquids(ILs) 17
CHAPTER3THEORETHICALFRAMEWORKOFTHEADSORPTIONPROCESS 19
3.1Adsorptionequilibria 19
3.2Dynamicsofadsorptioncolumns 21
CHAPTER4MATERIALSANDMETHODS 28
4.1Activatedcarbons,ionicliquidsandimpregnationprotocols 28
4.2Solidscharacterizationtechniques 29
4.2.1CO2/N2porosimetricanalyses 29
4.2.2Thermogravimetricanalyses(TGA) 30
4.3Lab‐scaleplantforCO2adsorption/regenerationtests 31
4.4Experimentalprotocolsforfixedbeddynamicexperiments 33
4.4.1ContinuousCO2adsorptiontests 33
4.4.2Adsorption/desorptioncyclesandregenerationtests 35
CHAPTER5RESULTSANDDISCUSSION 37
5.1Adsorbentscharacterization 37
5.1.1PorosimetricanalysesforF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly] 37
5.1.2TGAanalysesforF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly] 41
5.1.3PorosimetricanalysesforN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly] 43
5.1.4TGAanalysesforN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly] 46
3
5.1.5ComparisonbetweenTGAandporosimetricanalysesfortheinvestigatedsorbents 48
5.2CO2captureperformancesontheinvestigatedsorbents 51
5.2.1CO2adsorptiontestsontorawF600‐900andN.RGC30 51
5.2.2CO2adsorptiontestsontoF600‐900rawandfunctionalizedwith[Hmim][BF4]/[Emim][Gly] 56
5.2.3CO2adsorptiontestsontoN.RGC30rawandfunctionalizedwith[Hmim][BF4]/[Emim][Gly] 63
5.2.4IntertwiningamongsolidspropertiesandCO2captureperformances 69
5.3Adsorption/desorptioncyclesandregenerationexperimentsforF600‐900raw 73
5.4Adsorptionthermodynamicsandkineticsmodelling 79
5.4.1Thermodynamicaspects 79
5.4.2Kineticaspects 86
CHAPTER6CONCLUSIONSANDFUTUREPERSPECTIVES 92
Bibliography 95
4
CHAPTER1
INTRODUCTION
TheincreaseinworldwideCO2emissions,mainlyderivingfromfossilfuels, iscommonly
believedtobeamongthemaincontributorstoglobalwarming(Metzetal.,2005;Figueroaet
al., 2008). Currently, 85% of the global energy demand is supplied by fossil‐fueled power
plants accounting for about 40% of total CO2 emissions (Yang et al., 2008). Nowadays,
different options are available to mitigate CO2 emissions deriving from power sector,
including higher power generation efficiency, use of non‐carbon fuels (hydrogen and
renewableenergy),developmentofnewenergyproductionsystems,suchasoxy‐combustion
andchemical‐loopingcombustion,andtheadoptionofefficienttechnologiesforCO2capture
andstorage(CCS)(Metzetal.,2005;LiandFan,2008).
The CCS approach has the potential to reduce overall mitigation costs and increase
flexibilityinachievingareductioningreenhousegas(GHG)emissions:accordingtotheBLUE
MapScenariooftheInternationalEnergyAgency(IEA),thisroutecouldcontributetoa19%
cut inCO2 emissionsby2050 (International EnergyAgency (IEA), 2010).More specifically,
CCS technologies involve the separation and concentration of CO2 produced in large point
sources, the transportof thegas toasuitablestorage locationand long termisolation from
atmosphere (Metz et al., 2005).Main CO2 sequestration routes include geological injection,
oceandumpandmineralcarbonation(Metzetal.,2005).
Three main technological pathways can be pursued for CO2 capture from fossil‐fueled
power plants: post‐combustion capture, pre‐combustion capture and oxy‐combustion
(Figueroaetal.,2008;LiandFan,2008;Kannicheetal.,2010).Amongthem,post‐combustion
systemhas the greatest near‐termpotential for reducingGHG emissions, because it canbe
retrofittedtoexistingunitsthusprovidingaquickersolutiontomitigateCO2environmental
impacts(Figueroaetal.,2008;Leeetal.,2012). Themainbarrier to the implementationof
thistechnologyonindustrialscaleisrelatedtothelowthermodynamicdrivingforceforCO2
capturefromflue‐gas(gaspartialpressureisusuallylessthan0.15bar).Moreover,applying
current state‐of‐the‐art CO2 separation processes (absorption, adsorption, membrane
purification and cryogenic distillation) to existing coal‐fired power plantwould reduce the
power generation capacity by roughly one‐third (Figueroa et al., 2008). Post‐combustion
chemicalabsorptionofCO2inaqueousaminesolutions(mainlymonoethanolamine,MEA)is
the most widely used purification technology (Strube and Manfrida, 2011; Brúder and
5
Svendsen, 2012). The MEA process suffers many drawbacks related to the considerable
amounts of thermal energy required for absorbent regeneration, the high equipment
corrosion rate causedby contactwithMEAsolutionand the solventdegradationcausedby
oxygenandoxygen‐basedcompoundssuchasSO2andNOxpresentinatypicalflue‐gas(Kittel
et al., 2009; Strube and Manfrida, 2011). As a consequence of the aforementioned issues,
several research groups are making great efforts to develop high‐performance and cost‐
effective CO2 advanced separation processes in order to accelerate the techno‐economic
feasibilityofpost‐combustioncapturesystems.
Inthisscenario,adsorptionseemstobeaverypromisingtechnology,widelyusedforthe
treatment of gaseous and liquid effluents due to its potentially high removal efficiency and
operating flexibility, general low maintenance costs and, if coupled with an effective
regenerationprocess,fortheabsenceofby‐products(Abanadesetal.,2004;Choietal.,2009;
Balsamoetal.,2010;Sayarietal.,2011;Sjostormetal.,2011;Samantaetal.,2012;Balsamoet
al., 2013). Many sorbents can be used on purpose either raw or functionalized. Activated
carbonsshowhighpotentialityforapplicationinCO2capturebecausetheyaregenerallyless
costlythanothermaterials(e.g.,orderedmesoporoussilicas,metalorganicframeworks,etc.)
and have a complex structure characterized by high surface area and tunable porosity and
surface properties (Marsh and Rodrίguez‐Reinoso, 2006; Whaby et al., 2010). In addition,
carbon‐basedsorbentsareeasilyregenerableallowingtheiruseinprocessessuchaspressure
swingadsorption(PSA),temperatureswingadsorption(TSA)andvacuumswingadsorption
(VSA) (Gomes and Yee, 2002; Tlili et al., 2009). Despite these advantages, CO2 removal
performancesand long‐termstabilityof activatedcarbonsunder typical flue‐gas conditions
(CO21‐15%byvol.andatmosphericpressure)havebeenpoorlyinvestigated.
Anotherwidespreadresearchlineinthecontextofpost‐combustionpurificationsystems
concerns the investigationof ionic liquids (ILs) as innovative solvents forCO2 capture.The
ever‐increasing interest for this class of compounds in different fields is justified by their
uniquecharacteristicssuchasextremelylowvaporpressure,highthermo‐chemicalstability
andtunablechemico‐physicalproperties(Zhangetal.,2006a;Bourbigouetal.,2010;Hasib‐
ur‐Rahmanetal.,2010).Inparticular,thepossibilityoffunctionalizingILswithbasicgroups
(like amines) makes them very attractive for CO2 capture processes (Zhang et al., 2011).
Numerous literature studies are now focusing on the use of ILs supported on porous
membranes inorder toovercomethemain limits in the industrial‐scaleapplicationof ionic
liquidsforCO2capture,whicharerelatedtotheirhighcostandviscosity(Hasib‐ur‐Rahmanet
al., 2010; Lemus et al., 2011; Kolding et al., 2012). Notwithstanding a huge number of
6
scientificpapersdealswiththeutilizationofdifferentclassesofILsinCCSfield,thefollowing
criticalaspectscanbehighlighted:
CO2absorptiontestsaregenerallycarriedoutathighpressureandroomtemperature
whichareexperimentalconditionsnotrepresentativeofarealflue‐gas;
There is fragmentary information concerning the effect of confining ILs into
nanoporous substrates, particularly activated carbons, on CO2 capture performances
withrespecttotheirbulksolventproperties.
On the basis of the above‐mentioned analysis, the aim of this work is to provide a
contributioninelucidatingCO2captureperformancesofionicliquidssupportedonactivated
carbons characterized by different porosimetric properties. Specific thermodynamic and
kineticadsorptiontestshavebeencarriedoutonselectedactivatedcarbons,bothasrawand
impregnatedwithILsatdifferentconcentrations.Experimentaltestshavebeenperformedin
alab‐scalereactorandunderrealisticoperatingconditions(e.g.typicalflue‐gascompositions
and temperatures). Preliminary regeneration studies have been conducted on the sorbent
which displayed the highest CO2 capture capacity in order to determine its performances
underconsecutiveadsorption‐desorptioncyclesandassesstheoptimaloperatingconditions
forCO2storageafterdesorption.Theintertwiningamongrawsolidsproperties‐impregnation
conditions‐properties of the functionalized materials‐solids capture capacity has been
investigated by comparing CO2 adsorption results with outcomes obtained from sorbents
CO2/N2 porosimetric and thermogravimetric analyses. Adsorption isotherms have been
interpreted in the lightof theoreticalmodels fora comprehensionof themainmechanisms
involved in the capture of CO2 by the investigated solids. Breakthrough data have been
modelledalsoinordertoidentifytherate‐determiningstepoftheadsorptionprocessandthe
effectofoperatingparametersonmasstransferphenomena.
ThisPhDDissertationisorganizedasfollows.InChapter2aliteraturesurveyisreported
in order to analyse themain classes of sorbents employed for CO2 capturewith particular
emphasis on activated carbons together with themain applications of ionic liquids in CCS
field.Chapter3providesthemaintheoreticalaspectsconcerningboththermodynamicsand
kinetics of the adsorption phenomenon. Chapter 4 describes the experimental protocols
adopted for activated carbons impregnationwith ionic liquids, solids characterizations and
adsorption/regenerationexperiments;adescriptionofthelab‐scaleplantdesigned,builtand
optimizedfortheexecutionofadsorptionexperimentsisalsoprovided.InChapter5themain
results obtained from adsorbents characterizations and CO2 capture/regeneration tests are
7
discussed together with the main aspects derived from thermodynamics and kinetics
modelling.Finally,conclusionsandfuturedevelopmentsarereportedinChapter6.
8
CHAPTER2
LITERATUREOVERVIEW
In this Chapter the main adsorbent materials employed in CCS field are analysed with
particularemphasison theapplicationsofactivatedcarbons forCO2capture,as theyareof
majorinterestforthisPhDproject.Themainapplicationsofionicliquidsinthiscontextare
alsodiscussed.
2.1 UtilizationofactivatedcarbonsinCCSfield
Activated carbons are carbonaceousmaterialswith a common structuremade up of an
assemblyofdefectivegraphenelayersthathavehighpotentialityforCO2capturethankstoa
complex structure characterized by micropores that determine high surface area for
adsorption,butalsomeso‐andmacroporeswhichcanfacilitatethediffusion(fastkinetics)of
theadsorbatetotheinnerporosity(MarshandRodrίguez‐Reinoso,2006;Whabyetal.,2010;
Sayarietal.,2011).ActivatedcarbonsactasphysisorbentstowardsCO2,thustheiradsorption
capacity decreases rapidly as temperature increases (Choi et al., 2009; Sayari et al., 2011).
Moreover, the mild adsorption strength in the low‐pressure regime (<0.5 bar) makes
activated carbons easily regenerable. In the following, main retrieved results for CO2
adsorption on raw activated carbonswill be discussed (Section 2.1.1);moreover, themain
activationtreatmentsaimedattheintroductionofhighlyCO2‐affinefunctionalgroupsonthe
carbonsurfacewillbeanalysed(Section2.1.2).
2.1.1 CO2adsorptionontoas‐synthesizedactivatedcarbons
TheuseofactivatedcarbonsforCO2captureisnowadaysconsideredaviableroutemainly
for storage purposes because they can be efficiently used in pure CO2 streams and at high
pressures(Sayarietal.,2011).Nevertheless,thesynthesisoftailoredmicroporousstructures
canextendtheirusealsoforseparationandpurificationfieldsbydiscriminatingmoleculeson
shapeand/orsizebasis(Whabyetal.,2010).
Table2.1reportsmainliteraturedataconcerningCO2equilibriumadsorptioncapacityeq
onrawactivatedcarbonsandinpureCO2streamsatdifferentpressuresandtemperatures;it
isunderlinedthatthesorbentCO2/N2selectivity(S)isevaluatedastheratiooftheadsorbed
amountsofthetwogasesobtainedinsinglecompoundequilibriumtests.
9
Table2.1CO2equilibriumadsorptioncapacityinpurestreamsonas‐synthesizedactivatedcarbonsatdifferentpressuresandtemperatures
SorbentT[°C]
P[bar]
eq[mmolg‐1]
S(CO2/N2)
Reference
F30/470† 24† 0.16† 0.65† n.a.† (BerlierandFrère,1996)
Ajax† 25† 0.2† 0.75† n.a.† (DoandWang,1998)
Salnchunri†25†
55†
0.1†
0.1†
0.60†
0.25†
1.2†
1.3†(Naetal.,2001)
Filtrasorb400† 25† 0.1† 0.57† n.a.† (Luetal.,2008)
pitch‐basedactivated†carbon
30†
90†
1†
0.2†
1.9†
0.1†
5†
4†(Shenetal.,2010)
pitch‐basedVR‐5‐M†molecularsieve
0†
25†
50†
1†
1†
1†
8.6†
4.2†
2.3†
2.8†
n.a.†
n.a.†
(Whabyetal.,2010)
MaxsorbIII†30†
50†
2.86†
2.57†
5.4†
3.4†
n.a.†
n.a.†(Sahaetal.,2011)
†notavailable
As expected, it can be evidenced a decrease of CO2 adsorption capacity at higher
temperatures. Whaby et al. (2010) compared zeolites 13X and 5A with carbon molecular
sieves and observed that the latter show higher CO2 adsorption capacity at 1 bar and 0°C.
Theyalsoinferredthatthepresenceofnarrowmicropores(diameter<0.7nm)playsamajor
role indeterminingthesolidadsorptioncapacity.Noteworthy,Silvestre‐Alberoetal. (2011)
showedthatCO2adsorptiononcarbonmolecularsievemonolithsishighlyreversible,withno
loss of adsorption capacity under three consecutive adsorption/desorption cycles, making
themexcellentcandidatesforpressureswingadsorptionunits.
AninterestingexperimentalcampaignconcerningCO2captureinCO2/H2/N2(20/70/10%
by vol.) mixtures on raw activated carbon was carried out by García et al. (2011). In
particular,theystudiedtheremovalofCO2inafixedbedapparatusandanalysedtheeffectof
the temperatureandCO2partialpressureson the systemdynamicperformances.Themain
resultsshowedthatateachtemperatureandfixedcarbondioxideconcentration,higherCO2
partial pressures (obtained by increasing the system total gas pressure) determine longer
breakpointtime,andthisbehaviourwasimputedtoaslowerconcentrationfrontinthebed;
moreover, at higher temperatures the process was faster but a parallel reduction in
adsorptioncapacitywasobserved.Finally,Shenetal.(2011)studiedtherecoveryofCO2from
saturatedpitch‐basedactivatedcarbonbymeansofVacuumPressureSwingAdsorptionafter
fixed‐bedadsorptioninaCO2/N2mixture(15/85%byvol.):resultsshowedthatforaN2feed
pressureof2bar,aCO2purityof94%and78%recoverycouldbeobtained.
2.1.2
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11
capacityforbothfunctionalizedsorbents(1.11and0.70mmolg‐1forMEA‐andAMP‐carbon,
respectively)with respect to the rawmaterial (0.41mmol g‐1); the higher performances of
MEA‐carbon was ascribed to a more efficient dispersion of less sterically hindered MEA
moleculesoverthesupportsurfacewithrespecttoAMP,thuscreatingmoreaccessiblesites
forCO2capture.
Impregnationwithalkali/alkalineearthmetals
Impregnationofactivatedcarbonswithcalciumandmagnesiumoxidewasinvestigatedby
Yongetal. (2001), since thesemetalshaveahighbasicnaturewhich favors the interaction
withCO2acidicmolecule.Inparticular,theyobservedthatatlowtemperature(28°C)theraw
materials exhibited higher pure‐CO2 adsorption capacity with respect to metal‐doped
materials: thiswasascribedtothereductioninsurfaceareaoccurringduringtheactivation
process.Conversely,athighertemperature(300°C)theactivatedmaterialsshowedimproved
performancesbecauseoftheprevailingofchemisorptioneffectsoverphysisorption,thelatter
beingsurfacedependentanddominantatlowtemperatures.
Amination
Amination is a treatment usually referred to the reaction of gaseous ammoniawith the
surfaceof activatedcarbons,performedathigh temperatures (ranging from400 to900°C),
aimedatincreasingthesolidnitrogencontent(Plazaetal.,2009;Shafeeyanetal.,2010;Plaza
etal.,2011).Ammoniacanreactwithsurfaceoxidesandactivesitespresentattheedgesof
thegraphenelayerstoformamines,amides,imides,lactams,nitriles,pyridine‐orpyrrole‐like
functionalities. As an example of amination effect, Plaza et al. (2009) observed an
enhancement of pureCO2 adsorption capacities for almond shell‐derived activated carbons
aminated at temperatures greater than600°Cwith respect to theparent carbon;moreover
the authors observed that in the range 400‐900°C the sample aminated at 800°C had the
greatest CO2 adsorption capacity due to a maximum nitrogen content, as confirmed by
ultimateanalyses.Plazaetal.(2011)investigatedtheuseofaminatedbiomass‐basedcarbon
to capture CO2 in a 17% by vol. gaseous stream (balance N2) at 20°C and atmospheric
pressure;inparticular,theyfocusedonthesolidregenerabilityshowingthatThermalSwing
Adsorption carried out at 100°C allows an easy recovery of thepollutant and that after40
cyclestheadsorbentdidnotdisplayrelevantdeactivation.
12
2.2 OtherclassesofCO2sorbentsinvestigated
2.2.1 Zeolites
ZeolitesaremicroporouscrystallinealuminosilicatesbuiltofaperiodicarrayofSiO4and
AlO4tetrahedra(Ruthven,1984).Theiruniformporesizegrantsauniqueabilityasmolecular
sieves. CO2 is captured by zeolites mainly via electrostatic interactions generated by the
exchangeablecationsintheporesandbyhydrogenbondswithsurfacesilanolgroups(Choiet
al., 2009). In particular, zeolites characterized by a low Si/Al ratio have a high content of
extra‐framework cationswhich favourably interactwith CO2molecule (Sayari et al., 2011).
Generally,CO2adsorptiononzeolitesisnegativelyaffectedbyatemperatureincrease(Sayari
etal.,2011).Tlilietal. (2009)observedasix‐timesreductioninCO2adsorptioncapacityon
5Azeolitebyvarying theoperating temperature from25 to200°C. Inaddition, thesesolids
showlowerCO2adsorptioncapacityunderhumidconditions.RegeandYang(2001)showed
bymeans of FTIR analyses that there is a competition betweenwater vapour and CO2 for
adsorptionsitesonNaXzeolitesurface.Ithasbeenhighlightedthatzeoliticadsorbentshavea
strongerphysicalinteractionwithCO2andhigherheatsofadsorptioncomparedtoactivated
carbons, thusrenderingthedesorptionprocessmoreenergyintensive(Whabyetal.,2010).
Moreover, the hydrophobic nature ofmost activated carbonsmakes them less sensitive to
competitiveadsorptioneffectsbetweenCO2andwatervapourwithrespecttozeolites(Choi
etal.,2009).Ingeneral,itisunderlinedthatequilibriumadsorptionexperimentsonzeolites
arecarriedoutinpureCO2inmostofthecasesinvestigatedintheliterature:typicalreported
adsorptioncapacityvariesbetween0.2‐1.6mmolg‐1inthepressurerange0.1‐0.4barandata
temperatureof60°C(Sayarietal.,2011).Finally,CO2adsorptionkineticsonzeolitescanbe
rankedamongthefastestknown,reachinganequilibriumconditionwithinafewminutesin
mostcases(Choietal.,2009).
2.2.2 Orderedmesoporoussilicas(OMS)
Orderedmesoporous silicas (e.g.,MCM‐41, SBA‐15, TUD‐1, HMM‐33 and FSM‐16) are a
classof silicamaterials characterizedbydifferent cagestructures (suchashexagonal, cubic
andlamellar)thathaveattractedattentionincatalysisandseparationduetotheirextremely
highsurfaceareaandprecisetuningofporesizes(Chewetal.,2010).
Generally,fewstudiesareretrievableinthepertinentliteratureforCO2adsorptiononas‐
synthesized ordered mesoporous silicas, but many concern the removal of carbon dioxide
ontoamine‐modifiedOMS (BelmabkhoutandSayari,2009; Jangetal., 2009;Devadaset al.,
13
2010).Asamatteroffact,puresilicasurfacescontainresidualhydroxylgroupsthatarenot
abletointeractstronglywithCO2(Chewetal.,2010).TypicalCO2adsorptioncapacityinflue‐
gas conditions (temperature 75°C, CO2 5‐10% by vol.) reported for PEI‐impregnated OMS
varies in the range 2.1‐3.8 mmol g‐1 (Sayari et al., 2011). Post synthesis grafting is a
functionalizationtechniquewidelyappliedforthemodificationoforderedmesoporoussilicas
andinvolvesareactionbetweensurfacehydroxylgroupsofOMSandthealkoxyligandsofan
aminosilane,determiningalayeroftetheredaminegroupsonthesupportsurface(Changet
al.,2009;Serna‐GuerreroandSayari,2010;Sayarietal.,2011).Theseadsorbentshaveaclear
advantageoveramine‐impregnatedsilicasas theydonotshowanyamines leaching(unless
conditions are strong enough to break covalent bonds), thus determining potentially less
problems of equipment corrosion, usually associated to liquid amines (Choi et al., 2009).
FinallygraftedOMShaveshowngreat stabilityunder thousandsCO2adsorption‐desorption
cycles(Sayarietal.,2011).
2.2.3 Calciumoxide
Calcium minerals are the most abundant in nature among alkaline earth metal oxides,
commonly found in the formof carbonates suchas limestoneordolomite.When treatedat
hightemperatures,calciumcarbonatesliberateCO2andgeneratecalciumoxides.
TheremovalofCO2fromfluegasbycalciumoxidecanbeaccomplishedintwosteps(see
eqs.(2.1)and(2.2)):
Carbonation:CaO(s)+CO2(g)→CaCO3(s),exothermic(2.1)
Calcination:CaCO3(s)→CaO(s)+CO2(g),endothermic(2.2)
afirstreactionoftheoxidewithCO2toformcalciumcarbonate,performedinacarbonatorat
temperatures in the range 650‐700°C, and a subsequent heating of the carbonate at
temperatures higher than those of the carbonation step (calcination) to regenerate the
calciumoxideandreleaseconcentratedCO2(AbanadesandAlvarez,2003;Choietal.,2009;
Blameyetal.,2010).
Hughesetal.(2005)exploredtheinsituCO2capture,at700°Candatmosphericpressure,
inadual fluidizedbedcombustionsystem.TheadsorptionkineticsofCO2oncalciumoxide
adsorbents is much slower than on physisorbents such as zeolites and activated carbons,
sometimesrequiringseveralhourstoachieveca.70%ofthetotaladsorptioncapacity(Choiet
al.,2009).Moreover,calciumoxide‐basedadsorbentssufferfromarapiddegradationofCO2
capturecapabilityduringtherepetitionofcarbonation/calcinationcycles:thisreductionhas
14
mainlybeenascribedtoporeblockingandadsorbentsintering(AbanadesandAlvarez,2003).
Finally, material loss due to attrition and fragmentation in fluidized bed systems together
withthesorbentdeactivationproducedbythesulphationreactionwithSO2(alwayspresent
in a typical flue‐gas) are important issues related to Ca‐based sorbents (Montagnaro et al.,
2010;Coppolaetal.,2012aand2012b).
2.2.4 Metal‐organicFrameworks(MOFs)
Anemergingnewclassofcrystallinesolidscalledmetal‐organicframeworks(MOFs)has
recentlybeen investigatedassorbents forCO2capture.Thesematerialsgenerallyconsistof
three‐dimensional organic–inorganic hybrid networks formed by multiple metal–ligand
bonds(Eddaoudietal.,2002;Choietal.,2009;AnandRosi,2010;Sahaetal.,2010;Sayariet
al.,2011).ThesesolidsarehighlyversatilebecausetheporespacesofMOFsaretuneableover
asubstantialrangebyusingligandswithdifferentmoleculardimensions(AnandRosi,2010):
with some of the larger ligands the materials even became mesoporous. MOFs have been
developedforuseasCO2physisorbentsorstoragematerials,byoptimizingtheporessizefor
the carbon dioxidemolecule. Even if they show good adsorption capacities towards CO2 at
highpressures(greaterthan10bar),ithasbeenhighlightedthatforlowpressurerange(of
practical interest for post‐combustion capture) MOFs exhibit unfavourable adsorption
isotherms(Sayarietal.,2011).Additionally,MOFsareusuallyunstable inhumidconditions
andhigh temperatures showing lowCO2 selectivitywith respect toN2 (Sayari et al., 2011).
Finally,theirperformancesovermultipleadsorptionanddesorptioncycleshavetobetested
(Choietal.,2009).
InadditiontoMOFs,newtypeofsolidsarecurrentlybeinginvestigatedasCO2adsorbents
but are still at early stage of development: poly(ionic liquid)s (see Section 2.3.1), zeolitic
imidazolateframeworks(ZIFs)andcarbonnanotubes(Choietal.,2009;Herzogetal.,2009).
Fromtheanalysisoftheaforementionedsolids(eitherassynthesizedorfunctionalized),
recently reviewedbyChoi et al. (2009), Sayari et al. (2011) andSamanta et al. (2012), the
following aspects canbehighlighted: i) zeolites andactivated carbons are characterizedby
very fast CO2 adsorption kinetics but their performances decrease at temperatures greater
than100°Candinthepresenceofmoisture(alwayspresent ina fluegas); ii)calciumoxide
provides high CO2 adsorption capacities but requires high temperatures for regeneration,
which determine structural changes with loss of activity during several
carbonation/calcinations cycles; iii) amine‐functionalized solids usually display an
15
enhancementoftheCO2capturecapacityinpresenceofmoisture(relatedtothepossibilityof
forming carbonate/bicarbonate), but their application at high temperatures is limited by
aminesdegradability.
Asafurtherremark,itshouldbenotedthatevenifgreateffortsarecurrentlydevotedto
thedevelopmentofhigh‐performancesorbents forCO2capture, theiruse in typical flue‐gas
conditionsandformanyadsorption/desorptioncyclesisstilllimited.Moreover,thereisalack
of exhaustive information concerning their dynamic performances in different reactor
configurations(e.g., fixedbed, fluidized‐bed,circulating‐bedetc.), thisbeingakeyaspectfor
thedesignofindustrial‐scalepost‐combustionpurificationsystems.
In this scenario, the use of ionic liquids for porous solids functionalization is an
interestingalthough limitedlyexploredresearch topic, inorder todevelophighlyCO2‐affine
sorbents.Inthefollowing,abriefoverviewonionicliquids,theirapplicationandperspectives
inCO2capturetechnologiesispresented.
2.3 Ionicliquids(ILs)
Ionicliquids(ILs)areorganicsaltscomposedentirelybyionswithmeltingpointusually
lower than 100°C; many ILs are liquids at room temperature and, for this reason, are
commonly referred as Room Temperature Ionic Liquids (RTILs) (Zhang et al., 2006a;
Bourbigou et al., 2010; Hasib‐ur‐Rahman et al., 2010). ILs are characterized by negligible
vapourpressureatroomtemperature,abroadtemperaturerangeofliquidstate(depending
on the anionic/cationic couple), excellent thermal and chemical stabilities: these unique
propertiesmake themas optimal candidates as solvents and catalysts (Zhang et al., 2006a;
Boschettietal.,2007;Bourbigouetal.,2010).Theirtypicalviscosityrangesfrom50to1000
cP at room temperature (Figueroa et al., 2008). Some typical ILs cations and anions are
reported in Figure 2.2: cations are usually organics such as imidazolium, pyridinium or
ammoniumwhileanionsincludehalidesandfluoro‐borate/phosphate/sulphonate(Hasib‐ur‐
Rahman,etal.,2010).
F
ILspro
specific p
Liquids(T
2.3.1
Use of
overcome
degradatio
itcouldbe
significant
ManyR
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benzene)
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2011).
Ionicli
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Figure2.2Ty
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roperties:
TSILs)(Zha
ILsappli
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the prob
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epossiblet
tlythecapt
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iquidabso
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anbeadequ
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icationsfo
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blems asso
uipmentcor
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ebeenstud
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esearch pap
in imidaz
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stronglyd
rptionprop
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e is gainin
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peratureco
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r, RTILs p
with tempe
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hancedbyf
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onalgroup
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al.,2010):
itfirst,thu
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ouetal.,20
olvents (e.g
ureofthea
weakLewis
underlined
high pres
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with CO2,
l., 2009;Zh
functionali
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16
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zingtheIL
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6
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2008); thi
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(smaller t
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the use of
CO2solvent
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ingsolidm
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um‐basedp
raetal.(20
containing
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hibit high v
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Figure2.3)
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etal.(2006
senceofsm
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amino‐acid
absorption
tion. Finall
midazolium
mol /mo
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orptionca
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(ILs)
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capacityw
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m cation a
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2005) obs
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thpolargr
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heamount
mic benefit
17
ze a TSIL
cation;this
atreported
2002)
nic liquids,
wasdouble
al. (2011)
nd taurine
Cand1bar
ssibility of
Baraetal.,
erved that
omparedto
oups(such
CO2ofthe
r those ILs
icability to
potentially
ss transfer
onic Liquid
tofILused
s for their
7
L
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)
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,
t
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s
o
y
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d
d
r
18
application(Lemusetal.,2011).Nevertheless,thebehaviourofILsconfinedinnanospacesin
thecontextofCO2captureprocessesisonlyatapreliminarystageofinvestigation(Tanakaet
al.,2011;Koldingetal.,2012).Forexample,Tanakaetal.(2011)observedthatthedispersion
of 1‐hexadecyl‐3‐methylimidazolium chloride ionic liquid into nanoporous silica
microspheres determines an enhancement of its CO2 capture performanceswith respect to
thebulksolventproperties.Theauthorsascribedthisbehaviourtotheformationofordered
moleculardomains,promotedbysilicasurface‐ILinteractions,inwhichCO2occupiesspecific
positions.Zhanget al. (2006b)observed that forCO2adsorptionbyamino‐acidbased ionic
liquids supported on porous silica gel, the equilibrium is reached faster than bubbling CO2
through bulk ILs. In addition, ILs can be supported on porous alumina membranes or
adsorbedonpolymericmembranestoincreasetheirselectivitywithrespecttoCO2(Hasib‐ur‐
Rahman et al., 2010). Finally, immobilization of imidazolium‐type ionic liquids onto silica
supports is gaining great interest also because ILs act as high‐activity catalysts in the
cycloaddition reaction between epoxides and CO2 to produce five‐membered cyclic
carbonates (Shim et al., 2009; Udayakumar et al., 2009 and 2010). Cyclic carbonates are
excellent aprotic polar solvents and intermediates commonly applied in the production of
pharmaceuticalsandfinechemicals.
19
CHAPTER3
THEORETHICALFRAMEWORKOFTHEADSORPTIONPROCESS
InthisChapterthemaintheoreticalaspectsconcerningboththermodynamicsandkinetics
oftheadsorptionphenomenonwillbediscussedinordertoprovideausefulbasisforadeep
comprehensionof themainmechanisms involved in the captureof CO2by the investigated
solids.
3.1 Adsorptionequilibria
Thethermodynamicstudyofadsorptionprocessesallowsobtainingadsorptionisotherms,
i.e.experimentalcurveswhichestablisharelationshipbetweenthesolidspecificadsorption
capacitytowardatargetpollutantandthecontaminantpartialpressureinthegasphaseata
fixed temperature and under equilibrium conditions. The evaluation of equilibrium
adsorption capacity for a given gas‐solid system is of great importancenot only because it
providesinformationabouttheamountofpollutantthatcanbeloadedonthesorbentandthe
natureof interactionsgoverning theprocess: thesystem thermodynamicbehaviour, in fact,
affects thedynamicperformancesofanadsorptionprocessandconsequently thesizeofan
adsorberunit.Theinterpretationofexperimentalequilibriumadsorptiondatabymeansofan
adequatetheoreticalmodelallowstodefinethesolidaffinitytowardaspecificpollutant(by
evaluating the interaction energy between the sorbent and each gaseous species) and the
capturemechanisminvolvedintheprocess.
Asimplemodelusedintheliteraturefortheinterpretationofequilibriumadsorptiondata
isLangmuirmodel,which isbasedonthe followingassumptions: i)eachsitecanguestone
adsorbatemolecule(monolayer); ii)thereisnomobilityofadsorbedspeciesonthesurface;
iii) theheatof adsorption is constantwith loading; iv) all sites are energetically equivalent
(Ruthven,1984).ThegeneralformoftheLangmuirisothermcanbeexpressedas:
ω (3.1)
whereeq[mmolg‐1]andPeq[bar]aretheadsorbentspecificmolaradsorptioncapacityand
theequilibriumgaspartialpressureoftheadsorbaterespectively;KL[bar‐1]andmax[mmol
g‐1] represent the Langmuir equilibrium constant and themaximumadsorption capacity of
theadsorbedspeciesrespectively.
20
Alternatively, for lowadsorbatepartialpressures, the solid‐fluid adsorptionequilibrium
canbeexpressedbytheHenry’slaw:
ω K P (3.2)
inwhichKH[mmolg‐1bar‐1]istheHenryequilibriumconstant.
TheFreundlichisothermcanbeappliedtoaccountforsurfaceheterogeneity:
ω K P (3.3)
In eq. (3.3), KF [mmol g‐1 bar‐1/n] and 1/n [‐] are the Freundlich constant and the
heterogeneityparameterrespectively(bothgenerallytemperature‐dependent)(Do,1998).
Thepotential theoryhasbeendevelopedmainlybyDubininandPolanyi to characterize
the adsorption process in microporous solids (such as activated carbons) (Polanyi, 1932;
Dubinin and Radushkevich, 1947; Ruthven, 1984; Do, 1998). In such solids, the pore
dimensioniscomparabletothatoftheadsorbatemoleculeandtheadsorptionmechanismis
duetofillingbecausetheadsorptionfieldencompassestheentiremicroporevolume. Inthe
microporefillingtheoryanadsorptionpotentialAisdefinedas:
A RTln (3.4)
where P0 and Peq are the liquid sorbate vapour pressure and the pressure of the gas in
equilibriumwiththeadsorbatephaserespectively,atthesametemperatureT.Animportant
featureofthemicroporefillingtheoryisthatforagivenadsorbent‐adsorbatesystemthereis
a unique temperature‐independent relationship between the adsorption potential and the
adsorbatefractionalloadingreferredasthecharacteristiccurve(Ruthven,1984;Do,1998).
Dubinin and Radushkevich suggested the following Gaussian expression to relate the
degreeofmicroporefillingandtheadsorptionpotential(DubininandRadushkevich.,1947):
exp (3.5)
inwhichVisthevolumeofadsorbateinthemicroporesperunitmassofthesolid,V0isthe
maximum specific volume that the adsorbate can occupy (obtainable from porosimetric
analyses), and E is a characteristic energy (related to the adsorption strength between
adsorbateandadsorbent).OnceVandV0areknown, thesolidmolaradsorptioncapacity
21
and the saturation uptake max can be calculated assuming a liquid‐like adsorbed phase
accordingtoGurvitschas(Do,1998):
ω (3.6)
ω (3.7)
whereVmistheliquidmolarvolume.
Itshouldbeunderlinedthatabovetheadsorbatecriticaltemperaturetheconceptofliquid
ceases to exist. In this context, different methods have been proposed to evaluate Vm and
replacethevapourpressurewithapseudo‐vapourpressure;someoftheseexpressionshave
beencollectedin(Do,1998)andarenotreportedhereforthesakeofbrevity.
Finally, the determination of the adsorption isotherms at different temperatures allows
theestimationoftheisostericheatofadsorption(qst),whichisausefulparameterproviding
information on the degree of energetic heterogeneity of gas‐solid interactions. It can be
computedbyapplyingthewell‐knownClausius‐Clapeyronequation(Do,1998):
q RT (3.8)
3.2 Dynamicsofadsorptioncolumns
Mostoftheadsorptionprocessesarecarriedoutinfixedbedadsorbers,typicallytubular
reactors packed with the adsorbent material which is contacted with a gaseous stream
containing thepollutant tobe removed. In suchsystems, thecompositionsofboth the fluid
and solid phases changewith time as well as with the position in the bed (McCabe et al.,
1993). In the commonpracticeof adsorptionexperiments it isdifficult tomeasure internal
compositionprofilesforevaluatingthedynamicperformancesofanadsorptioncolumn;thus,
itismoreconvenienttomonitortheconcentrationoftheadsorbateatthecolumnoutletasa
functionof time, obtaining the so‐calledbreakthrough curve as reported inFigure3.1. It is
highlightedthatthetimeevolutionofthecompositionprofilecanbeconvenientlyexpressed
intermsoftheratiobetweenthevolumetricflowrateoftheadsorbedispeciesatthecolumn
outlet and the correspondent value of the feed (Qiout(t)/Qiin), if percentage volumetric
concentrationscanbeexperimentallyobtained(asinthecaseoftheNDIRanalyzeradoptedin
thisproject,cf.Chapter4).
22
t
Qiou
t (t)/
Qiin
, -
0.0
0.2
0.4
0.6
0.8
1.0
Figure3.1Characteristicbreakthroughcurveobtainedfromfixed‐bedadsorptionexperiments
It can be observed that the curve is S‐shaped: initially the pollutant is completely
adsorbed,thenitsconcentrationattheoutletincreasesuptotheinletlevelwhenthesolidis
completely saturated. In general, for the breakthrough curve it is possible to define a
characteristic breakpoint time tb for which Qiout(t)/Qiin=0.05; in the industrial practice, it
usuallyrepresentsalimitingworkingconditionfortheadsorbercorrespondingtoregulation
emissionlimit(McCabeetal.,1993).
Itcanbedemonstratedthattheareaabovethebreakthroughcurveisproportionaltothe
totalamountofpollutantcapturedbytheadsorbent;asamatteroffact,inordertoevaluate
thesolidadsorptioncapacity,amaterialbalance[mg]onispeciesovertheadsorptioncolumn
isrequiredasreportedineq.(3.9):
Q ρ dt Q t ρ dt mdω(3.9)
in which Qiin and Qiout(t) [L s‐1] represent again the column inlet and outlet i species
volumetric flow rates respectively, i [mg L‐1] is the pollutant density at the operative
temperatureandpressure,m[g]istheadsorbentmassandd[mgg‐1]thedifferentialsolid
adsorption capacity. The mass balance equation (3.9) takes into account that, in the
differentialtimedt,thepollutantmassadsorbedonthesolid(md)equalsthesamequantity
lostbythegaseousphase(LHSineq.(3.9)).
Byrearrangingeq.(3.9)andintegratingbetweenzeroandtimet*forwhichQiout(t)/Qiinis
practically unitary (complete solid saturation), it is possible to evaluate the solid total
tb
0.05
23
adsorptioncapacityeq,asreportedineq.(3.10):
ω
1 dt∗
(3.10)
The correct interpretationof the effectsof the adsorptionkineticsand theextentof the
axialmixinginthecolumnontheadsorbentdynamicperformancesrequiresanappropriate
modelthatgivesatheoreticalbreakthroughcurvematchingtheexperimentaldata.Moreover,
the design of an adsorption column can be realized without the recourse to extensive
experimentationbypredictingapriorithekineticresponsecurvefromequilibriumdataand
byestimatingthemasstransfercoefficientsoncethesolid‐sorbatepropertiesareknownand
thesystemfluiddynamicsisfixed.Therigorousmathematicalmodelrequiresthesolutionof
equationsderiving frommass,momentumandenergybalances.Thehypothesesadopted in
this work to describe the fixed bed dynamics are (Ruthven, 1984; Ding and Alpay, 2000;
Delgadoetal.,2006;Serna‐GuerreroandSayari,2010;Shenetal.,2010):
theflowpatternisdescribedwiththeaxiallydispersedflowmodel;
themasstransferrateisgovernedbyalineardrivingforce(LDF);
thegasphasebehavesasanidealgasmixture;
radialconcentrationandtemperaturegradientsarenegligible;
thesystemisisothermal.
It should be noted that even if a typical industrial‐scale adsorption column is operated
adiabatically, lab‐scale experiments are usually carried out in a temperature‐controlled
environmentwhich leads to the useful approximation of an isothermal fixed‐bed adsorber
that simplifies the system mathematical modelling (Serna‐Guerrero and Sayari, 2010). In
addition,typicallyemployedphysisorbents(suchasactivatedcarbons)showlowadsorption
heat which determines a negligible effect on the gas temperature, thus supporting the
hypothesisofanisothermalprocess.
Massandmomentumbalances
On thebasis of the above‐mentioned assumptions, themassbalance for the adsorbate i
speciesinadifferentialelementofthecolumndz(totallengthL)isgivenby:
εD ε 1 ε ρ 0(3.11)
24
whereCi istheadsorbateconcentrationinthegaseousstream,tthetime, thebedvoidage
fraction,uthegassuperficialvelocity,pistheadsorbentparticledensityandDaxrepresents
the axial dispersion coefficient. The resolution of themass balance equation (3.11) can be
obtainedbyfixingthefollowinginitialandboundaryconditions:
t=0i=0z(3.12a)
t=0Ci=00<z≤L(3.12b)
z=0Ci=Ciint(3.12c)
=0t(3.12d)
inwhichCiinisthepollutantconcentrationinthegaseousphaseatthecolumninlet.Itshould
be noted that in eq. (3.12a) it has been assumed that the adsorbent is initially free of
adsorbate(i=0).
Therateofadsorptionfortheadsorbateisexpressedas:
1 ε ρ 1 ε ρ k , ω∗ ω (3.13)
where ks,i is a lumpedmass transfer coefficient andi* the solid adsorption capacity for i
componentwhichwouldbeinequilibriumwithitsconcentrationinthegaseousphase(Ci).In
this context, it is underlined that the resolution of eq. (3.13) requires an appropriate
equilibrium expression i*=f(Ci,T) which can be obtained from one of the adsorption
isothermsdiscussedinSection3.1.
Therelationshipbetweenthetotalpressuregradientandthegassuperficialvelocitycan
bederivedfromtheErgun’sequation:
u.
u (3.14)
whereandgaretheviscosityanddensityofthegasrespectively,whiledprepresentsthe
meanSauterparticlediameter.
Evaluationofaxialdispersionandmasstransfercoefficients
Theresolutionof theequationsrelativeto therateofadsorptionandmassbalance for i
component of the system expressed in eqs. (3.11 and 3.13), requires an estimation of the
effectoftheaxialdispersion(Dax)andtheglobalmasstransfercoefficientks,i.
25
In general for aporousadsorbentmaterial the adsorptionprocess is characterizedbya
complexmechanismwhichinvolves(Ruthven,1984;PerryandGreen,1997):
externalmasstransferofthepollutantinthefluidfilmlayersurroundingthesolid
particle;
macroporeandmicroporediffusionoftheadsorbatewithintheadsorbent;
adsorptiononthesolidactivesites.
The pseudo‐reaction between the pollutant and the solid sorbent is usually fast for
physical adsorption, thus the evaluation of diffusion resistances allows the identification of
therate‐determiningstepoftheprocess.
Theglobalmasstransferresistanceiscommonlyexpressedasalinearcombinationofthe
film, macropore and micropore diffusion resistances as (Ruthven, 1984; Perry and Green,
1997):
, , , ,(3.15)
where kext,i is the external fluid film mass transfer coefficient for i, Dmacro,i and Dmicro,i its
macropore and micropore diffusivities, p represents the particle porosity, Hi is the
dimensionless Henry constant for i obtained from the slope of the linear part of the
adsorption isotherm (by expressing the solid adsorption capacity in terms of volumetric
concentrationasafunctionoftheconcentrationofiinthegaseousphase).
In order to evaluate the film mass transfer coefficient, it is useful to define the
dimensionlessReynolds(Re),Sherwood(Sh)andSchmidt(Sc)numbersas:
Re ; Sh , ;Sc
in which Dij is the i molecular diffusivity into the gas matrix (i.e. CO2/N2 mixtures in this
project), which can be evaluated according to the Chapman‐Enskog equation (Perry and
Green,1997):
D 1.858 ∗ 10.
.
(3.16)
where Mi and Mj are the molecular weights for i and j species, ij the average collision
26
diameterandDatemperature‐dependentcollisionintegral(tabulated).
Thevalueofkext,icanbeobtainedaccordingtotheWakaoandFunazkricorrelation(Perry
andGreen,1997;Shenetal.,2010):
Sh 2 1.1Re . Sc (3.17)
ThemacroporediffusivityDmacro,icanbeevaluatedas(Shenetal.,2010):
,
τ,(3.18)
wherepistheporetortuosity.TheKnudsendiffusivityDk,iisgivenby(Ruthven,1984):
D , 48.50d.(3.19)
withdpore[m]representingthemeanporediameter.
Themicropore diffusion is an activated process and exhibits an Arrhenius dependence
fromtemperature(Ruthven,1984):
D , D , exp (3.20)
inwhichD0micro,iisthelimitingdiffusivityatinfinitetemperatureandEatheactivationenergy;
Dmicro,i is usually evaluated from chromatographic andNMR studies or from separate batch
adsorptionexperiments(Ruthven,1984).
The axial dispersion in packed beds usually derives from two main mechanisms:
molecular diffusion and turbulentmixing arising from splitting and recombination of flows
aroundtheadsorbentparticle(Ruthven,1984).Theseeffectscanbeconsideredadditive,thus
theaxialdispersioncoefficientcanbeexpressedas:
D γ D (3.21)
with and constants; the values proposed for 1=0.73 and 20.5 1 by
EdwardsandRichardsonhavebeenusedinthiswork(PerryandGreen,1997).
Finally, theeffectofaxialdispersioncanbeevaluatedbycomputingthefixed‐bedPéclet
numberdefinedas:
27
PeuLεD
Typically, for Pe>100 it is possible to consider an ideal plug‐flow for the system (i.e.
negligibleaxialdispersion)(InglezakisandPoulopoulos,2006).
The numerical resolution ofmass andmomentum balance equations (3.11), (3.13) and
(3.14)wasobtained in thisworkwithAspenAdsimTMmodelling environment adopting the
methodoflines:aTaylor‐basedUpwindDifferencingSchemewasusedforthediscretization
of first‐order spatial derivatives and a second‐order Central Differencing Scheme for the
discretization of the second‐order term (axial dispersion in eq. (3.11)). The aim of the
mathematicalmodellingwastoprovideanestimationofthemicroporediffusivityDmicro,i(for
each investigated gas‐solid adsorption system) which is the only parameter not directly
computable (kext,i and Dmacro,i values can be determined once the adsorbent properties are
knownandthesystemfluiddynamicsisfixed).Tothisend,AspenAdsimTMsoftwareenables
the evaluation of Dmicro,i as a fitting parameter by minimizing the sum of the squared
differences between numerically calculated and experimentally observed values of the
gaseousphasecompositionatthefixed‐bedoutlet(leastsquaresmethod).Thecomparisonof
masstransferresistancesineq.(3.15)allowedthedeterminationoftherate‐limitingstepof
theadsorptionprocess.
In this
with ionic
discussed.
experimen
4.1 A
Two a
work in o
efficiency
size600‐9
1000m,
Two d
(supplied
(molecula
methylimi
ILwhichi
2012).The
Thead
batch stir
differenta
was separ
order to
methanol
[Hmim][B
s Chapter t
c liquids, s
. A descrip
ntsisalsop
Activated
ctivated ca
order to a
forbothra
900m,F6
N.RGC30).
different io
by Sigma
rweight:2
idazoliumg
isabletofo
eILsmolec
Figure4
dsorbentsi
red system
activephas
rated from
completely
for [Emim
F4] impreg
(a)
MA
the experim
solids char
ption of th
provided.
dcarbons
arbonswer
assess the
awandILs
600‐900)a
onic liquids
Aldrich):
254gmol‐1
glycine[Em
ormcarbam
cularstruct
4.1Molecula
mpregnati
m with an
seinitialco
m the liquid
y remove
m][Gly], a
gnated acti
CH
ATERIAL
mental pro
racterizatio
he lab‐scal
s,ionicliq
re selected
effect of
impregnat
andMeadW
s were tes
1‐hexyl‐3‐
)whichisa
mim][Gly](
matewith
turesarere
arstructures
onwascar
IL solution
oncentratio
d solution b
the solven
further e
ivated carb
HAPTER
LSAND
otocols app
ons and a
le plant de
quidsand
d for the ex
different
tedmateria
WestvacoN
sted as im
‐methylimi
atypicalph
(molecular
carbondio
eportedin
sof(a)[Hmi
rriedoutas
n (liquid to
ons(5.6×10
by filtratio
nt, ethyl a
evaporation
bons, the s
R4
METHO
plied for a
adsorption/
esigned fo
dimpreg
xperimenta
porosimetr
als:Calgon
NucharRG
mpregnating
idazolium
hysicalsolv
weight:18
oxide(Zhan
Figure4.1.
im][BF4]and
sfollows:t
o solid rat
0‐3and2.2×
on and ove
acetate in
n step wa
solvent wa
(b)
ODS
ctivated ca
/regenerati
r the exec
gnationp
al campaig
ric structu
CarbonFil
C30(granu
g agents of
tetrafluoro
venttoward
85gmol‐1)
ngetal.,20
d(b)[Emim]
hesubstrat
tio equal to
×10‐2M).A
en dried at
the case
as required
s removed
arbons imp
ion experi
cution of a
protocols
gn carried
ures on CO
ltrasorb40
ulometric r
f the selec
oborate [H
dsCO2and
anamino
006a;Kasa
][Gly]ILs
tewascon
o 5.4 mL g
After1wee
t 100°C ov
of [Hmim]
d. In part
d by feedin
28
pregnation
ments are
adsorption
s
out in this
O2 capture
00(particle
range600‐
cted solids
Hmim][BF4]
d1‐ethyl‐3‐
acid‐based
aharaetal.,
ntactedina
g‐1) at two
ekthesolid
ernight. In
][BF4] and
ticular, for
ng pure N2
8
n
e
n
s
e
e
‐
s
‐
d
,
a
o
d
n
d
r
2
29
(flowrateequal to60NLh‐1, for1h)at180°C intoacolumnpackedwith the impregnated
solid while in the case of [Emim][Gly] functionalized sorbents, methanol removal was
achieved in an oven at 100°C under vacuum for 5 h. It is underlined that the adopted
operating conditions for the evaporation stage were chosen on the basis of the different
thermal stabilities of the two ILs (lower decomposition temperature for [Emim][Gly], cf.
Chapter5).Moreover,theeffectivenessoftheevaporationstepwasvalidatedfromprevious
experiments inwhichthesolidswereimpregnatedonlywiththesolvents:completesolvent
removalwasverifiedbyweighingthesorbentbeforeandafterthermaltreatment.Foraneasy
identification, the functionalized solids were labelled according to the adopted activated
carbon,ILandimpregnationconcentrationas:
F600‐900andN.RGC30forrawactivatedcarbons;
F600‐900[Hmim][BF4]10‐3Mand10‐2M,N.RGC30[Hmim][BF4]10‐3Mand10‐2M
for activated carbons impregnated with [Hmim][BF4] and adopting IL initial
impregnationconcentrationsof5.6×10‐3and2.2×10‐2M;
F600‐900 [Emim][Gly]10‐3Mand10‐2M,N.RGC30[Emim][Gly]10‐3Mand10‐2M
foractivatedcarbonsfunctionalizedwith[Emim][Gly]underILinitialimpregnation
conditionsof5.6×10‐3and2.2×10‐2M.
4.2 Solidscharacterizationtechniques
Thesolids tested in thiswork forCO2captureexperimentswerecharacterizedadopting
the following techniques: i) CO2/N2 porosimetric analyses to determine the solids textural
parameters; ii) thermogravimetric analyses (TGA) to evaluate the amount of ionic liquid
loaded on each sorbent after the impregnation treatment and for assessing the thermal
stability of the active phase confined in the porous substrates. All the analyses have been
carried out at the Laboratorio de Materiales Avanzados (LMA), Department of Inorganic
ChemistryofUniversidaddeAlicante(Spain).
4.2.1 CO2/N2porosimetricanalyses
Porosimetric analyses were carried out in a home‐made fully automated equipment
designed and constructedby theAdvancedMaterials group (LMA), now commercialized as
N2Gsorb‐6(GastoMaterialsTechnology;www.g2mtech.com),workingat‐196and0°CforN2
and CO2 respectively. Adsorption measurements were recorded in the relative pressure
(P/P0)rangeof10‐7‐1fornitrogenand10‐7‐0.03forcarbondioxide.Priortoadsorptionruns
30
each sample was degassed under vacuum at 100°C in order to remove humidity or other
volatileimpurities.Noteworthy,thedegassingproceduredidnotdeterminedesorptionofthe
ionic liquid fromthesubstrate, as confirmedbyTGAanalysescarriedouton functionalized
sorbents before and after vacuum application. Moreover, it is highlighted that CO2
measurementswereconductedonly for rawF600‐900andN.RGC30activatedcarbons.The
reason for this choice is related to the specific interactions establishingbetween theprobe
CO2 gaseousmolecules and the ionic liquid dispersed onto the substrate (with a chemical
conversion into carbamate occurring in the case of [Emim][Gly]), which could modify the
distributionoftheactivephaseinsidethesorbentpores,thusleadingtoapossibleincorrect
evaluationofthenarrowmicroporosityoftheimpregnatedsamples.
The raw N2/CO2 adsorption data were processed according to the common models
retrievablefromliterature inordertoevaluatethesolidmicrostructuralparameters,andin
particular:
thesorbentstotalporevolumewasderivedfromN2adsorptionisothermsbyapplying
theGurvitsch rule for the volumeofnitrogen adsorbed atP/P0=0.97 (Leofanti et al.,
1998);
thetotalmicroporevolumewasevaluatedwithDubinin‐Radushkevich(DR)equation
(appliedintheN2isothermregionP/P0=10‐4‐10‐2)(MorlayandJoly,2010);
the apparent surface area was obtained from N2 adsorption data by means of BET
equationappliedintherelativepressurerangeP/P0=0.01‐0.15,whichbestagreedwith
criteria proposed by Roquerol et al. (2007) for the applicability of BET method to
microporoussorbents;
thevolumeofnarrowmicropores (porediameterup to0.7nm)wasevaluated from
CO2adsorptionisothermat0°CusingtheDRequation;
the absolute pore size distributions were obtained by applying the Quenched‐Solid
Density Functional Theory (QSDFT) to N2 adsorption data (Neimark et al., 2009;
Silvestre‐Alberoetal.,2012).
4.2.2 Thermogravimetricanalyses(TGA)
Thermogravimetric analyses on both raw and IL‐impregnated solids were performed
using a TA Instrument SDT 2960 operated in the temperature range 25‐400°C under a N2
inertatmosphere(flowrate95mLmin‐1)at5°Cmin‐1scanrate.TGAmeasurementsallowed
tofollowthesamplemass%evolutionasa functionofthetemperature.Thecomparisonof
the therm
amount o
adoptedfo
adsorbed
%wt.
in which
wt.%impr
impregnat
ionic liqu
particular
determine
4.3 L
Figure
apparatus
adsorption
F.C.massfMpressuVvalveB.H.bandb
CO2
F
mogramsob
of IL loade
ortheimpr
onthesub
∆wt
%wt.IL‐ads
(dec‐IL) a
ted and ra
id decomp
, the mass
edbyvolati
Lab‐scale
s 4.2 and
s designed
n/regenera
flowcontrolleuregauge
barrelheaters
N2
.C.
F.C.
btained for
d on the a
regnationp
stratewas
t.% ∆
s is the m
and wt.%
awmateria
poses de
s loss of th
ilemattero
plantfor
4.3 show a
d and buil
ationruns.
Figure
er
s
A
r rawand i
activated c
procedure.
derivedac
∆T
mass perc
%raw(dec‐I
ls, respect
c‐IL (that c
he raw ma
orhumidity
rCO2ads
a schemati
lt during
e4.2Layout
BfixedbeT.C.temperTtemperVtvent
Adsorberli
impregnate
carbons, at
Morespec
ccordingto
∆wt.%
centage of
IL) represe
tively, in t
can be rou
aterial is s
ydesorbing
sorption/
ic represen
this PhD
oftheexper
edadsorberraturecontrolratureprobe
B
ine
T
V1
T.C.
T
edmateria
t each init
cifically,aq
thefollowi
% ∆T
f the IL
ent the m
the temper
ughly ident
subtracted
gfromthe
/regener
ntation and
project fo
rimentalapp
llerP.I.D.
By‐passline
T
B.H.B
M
M
T
lsprovided
tial active
quantitative
ingexpress
in the im
mass perce
rature rang
tified from
to elimina
substrated
rationtes
d a picture
or the ex
paratus
FfloCcoIRinPCpe
e
F
Vt
PC
V
d informat
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eestimatio
sion:
mpregnated
entage loss
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m literature
ate any co
duringthe
sts
e of the exp
xecution o
owmeterompressornfraredCO2anersonalcomp
t
IR
V2
F
Vt
31
tionon the
ncentration
onoftheIL
(4.1)
d sorbent,
ses of the
h the pure
e data). In
ontribution
test.
perimental
f dynamic
nalyzeruter
C
M
V3
1
e
n
L
,
e
e
n
n
l
c
The fe
FlowBron
gascompo
CO2ad
bedcolum
45 μm po
operations
arrangedc
bandheat
PMPIDc
a relation
granularb
was defin
interspace
CO2co
by a cont
acquisition
LabView™
monitored
exit of the
acquisition
massflcontroll
Figure4.3
edgas com
nkhorst20
ositions(1‐
dsorption/r
mn(length=
orous sept
s.Thefixed
coaxiallyw
ters,envelo
controllers
nship amo
bedtemper
ned during
etemperatu
oncentratio
tinuous AB
n and elab
™ software.
dbymeans
e adsorptio
n.
owlers
3Pictureof
mposition (
01‐CV),whi
‐15%CO2b
regeneratio
=0.13m;in
tum and c
dbedtemp
withtheads
opedinath
(Watlow).
ng band
ratures.On
adsorption
urebymea
onmeasure
BB NDIR (n
borationw
. Gas volum
sofamass
on column,
fixedbeadsorbe
+heatingsy
thelab‐scale
(N2+CO2)w
ichallow t
byvol.).
ontestson
nerdiamet
composed
peraturew
sorberunit
hermalinsu
Beforedyn
heater sur
ncethethe
n tests by
ansoftype
ementsdur
non‐disper
ere perform
metric flow
s flowcont
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eder
ystem
eplantforC
wasdeterm
togenerate
theinvesti
ter=0.02m
of two u
wascontroll
t:itconsist
ulatinglay
namictests
rface‐band
ermalprofi
setting an
Jthermoco
ringadsorp
rsive infrar
med by in
w rate var
trollerseri
ally interfa
flowme
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minedviam
egaseouss
igatedsorb
m)madeup
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ledbymea
tsofthree
erofceram
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d heater/fi
lewaskno
nd controll
ouples.
ption/regen
red) AO202
terfacing t
iations, oc
iesElFlow
acedwith t
eter
NDIRanaly
on/regenera
mass flowc
streamssim
bentswere
ofPyrexg
dsorbent c
nsofanad
500Wcyli
micfibers,a
tioncurvew
xed bed i
own,thefix
ing the ba
nerationte
20 Uras 26
he analyze
curring in
wBronkhor
the PC uni
yzer
ationtests
controllers
mulating ty
carriedou
glass,equip
charging/d
dhocheati
indricalsh
andconnec
wasbuiltt
interspace‐
xedbedte
and heater
estswerec
6 gas anal
erwith a P
the fixed
rst201‐CV,
it for an on
PC
32
s (seriesEl
ypical flue‐
utinafixed
pedwitha
discharging
ngsystem,
ellWatlow
ctedtoEZ‐
oestablish
‐adsorbent
mperature
/fixed bed
carriedout
lyzer. Data
PC unit via
bed, were
, setat the
n‐line data
2
l
‐
d
a
g
,
w
‐
h
t
e
d
t
a
a
e
e
a
33
Aby‐passlinewasalsoimplementedinthelab‐scaleapparatusinordertoverifythefeed
compositionpriortoadsorptionexperiments:athreewayballvalve(V1)allowstoadequately
switchthegasflowwhiletwoballvalves(V2andV3)aredevotedtoavoidbackflowtowards
theexcludedline.
4.4 Experimentalprotocolsforfixedbeddynamicexperiments
InthisSectionadescriptionoftheexperimentalprotocolsandconditionsadoptedforCO2
adsorption experiments, adsorption/desorption cycles and regeneration tests will be
provided.
4.4.1 ContinuousCO2adsorptiontests
CO2continuousadsorptionrunsrequiredtwodifferentsteps:a)plantpreparation;b)test
execution.Aschematicdescriptionoftheexperimentalprotocolisprovidedinthefollowing.
a)Plantpreparation
Charge of reactor (column) with a known adsorbent amount previously heated
overnightat105°Ctoremovehumidity;
Filloftheremainingpartofthecolumnwithinertglassbeadstouniformthegaseous
flowatthecolumninlet;
CheckofthedefinedCO2concentrationbyNDIRanalyzer,directingthegaseousstream
throughtheby‐passlinewithvalveV3closedtoavoidbackflowintheadsorberline;
Flushofallplantpipelineswithnitrogentoeliminatethepresenceofatmosphericair;
Checkofgasleakageforallpipelinesconnectionsinpresenceofanitrogengasstream;
Check of column leakage by closing the solenoid valve of the mass flow controller
placedattheadsorberoutlet,pressurizingwithN2andmonitoringthepressurewith
pressuregauges.
b)Testexecution
Injection of the defined N2 and CO2 volumetric flow rates viamass flow controllers
throughtheby‐passlinefor30sandventingtotheatmospherewithvalveV2closedto
reachastationaryvalueofconcentration;
SwitchingofthegaseousstreamtowardtheadsorberlineviathreewayvalveV1and
simultaneouslyacquiringtheNDIRconcentrationsignal.
34
The sorbents CO2 capture performances have been investigated under the following
experimentalconditionsadoptedforthefixed‐bedreactor:
Sorbent dose: 15 g for F600‐900 derived sorbents and 13 g for N.RGC30 based
materials;
Totalgasflowrate:2.5×10‐2Ls‐1(evaluatedatT=20°CandP=1bar);
CO2initialconcentration:1‐30%byvol.,balanceN2;
Temperature:30,50and80°C;
Totalgaspressure:1bar.
It isunderlinedthatforN.RGC30sorbents(rawandILs‐impregnated)experimentswere
conductedusingalowersolidamountbecauseofthelowerdensityofthissubstrate(13gof
solid completely filled the column). In addition, CO2 capture tests at initial pollutant
concentrations greater than typical 15% flue‐gas (namely 25 and30%)wereperformed in
ordertobetterinterpretthequalitativetrendoftheadsorptionisotherms.
Thedynamicbehaviorofthegas‐solidadsorptionsystemwasfollowedbymonitoringthe
concentration of the adsorbate at the column outlet as a function of time, obtaining the
breakthrough curves. In particular the time evolution of the composition profile was
expressed in termsof theratioof thevolumetric flowratesofCO2speciesat thebedoutlet
relativetothatinthefeedQ t /Q .
CO2 kinetic adsorption results at 30, 50 and 80°C were processed to obtain the
corresponding adsorption isotherms. The material balance on CO2 over the adsorption
column,leadstothefollowingexpressionfortheequilibriumCO2specificadsorptioncapacity
eq [mmolg‐1](seeSection3.2):
ω
1 dt∗
(4.2)
whereCO2[mgL‐1]representsthepollutantdensity(evaluatedat20°Cand1bar)whileMCO2
[mgmmol‐1]isitsmolecularweight;m[g]isthesorbentdoseandt*[s]representsthetime
requiredforreachingcompletesolidsaturation.ThetimeevolutionoftheCO2volumetricflow
rate at the column outlet Q t was obtained from NDIR analyzer which provides this
measurebyfollowingthetemporalvariationsofthegaseouscompositionasreportedineq.
(4.3):
35
Q t
(4.3)
inwhichC t representstheCO2time‐dependentpercentagevolumetricconcentrationin
thegaseousstreamatthecolumnoutlet.
Theresolutionoftheintegralineq.(4.2)foreachinvestigatedinletCO2concentration,was
obtainedbyapplyingthetrapezoidalruletotheexperimentalkineticdata.
Itshouldbeobservedthatineq.(4.3)itisassumedthattheN2volumetricflowrateQ is
practicallyconstantduringthetest:thismeansthatN2adsorptiononallthetestedsorbentsis
negligible. This hypothesis was experimentally verified, prior to CO2 adsorption tests, by
injectingpureN2(2.5×10‐2Ls‐1)throughtheadsorberchargedwitheachinvestigatedsorbent
andmeasuringthegasflowrateatthecolumnoutletbymeansofamassflowmeter.
4.4.2 Adsorption/desorptioncyclesandregenerationtests
Adsorption/desorptioncyclesandpreliminaryregenerationtestswerecarriedoutonthe
sorbent which displayed the highest CO2 capture capacity among the investigated
experimentalconditions(namelyrawF600‐900,cf.Chapter5).
For adsorption/desorption cycles realization, F600‐900 raw was first saturated with a
15%CO2 inN2 gasmixture under the same conditions of total gas flow rate, pressure and
sorbentdosagepreviouslydescribed(adsorptionstep).Subsequently,thepackedcolumnwas
flushed with pure N2 to remove adsorbed CO2 (desorption step), until its concentration
reachedtheNDIRanalyzerlowdetectionlimit(0.1%byvol.).Thisprocedurewasreiterated
for10consecutiveadsorption/desorptioncyclesat30°C.
Regenerationtestswereconductedinordertodefinetheoptimaloperatingconditionsfor
CO2recoveryfromthespentactivatedcarbon.Tothisend,agaseousstreamcontainingCO2at
15%by vol. (total flow rate=2.5×10‐2 L s‐1, P=1 bar)was continuously fed to the adsorber
(m=15 g, T=30°C) until equilibrium conditions were reached. After, the packed bed was
heateduptothedefinedtemperatureforthedesorptionstep(approximately1hrequiredto
reachthermalequilibriumconditions),afixedamountofpureN2asdesorbingagentwassent
through the columnandCO2 concentrationwasmonitoredbymeansof theNDIRanalyzer.
Thisdesorptionstepwascarriedoutat60and100°Cand,foreachtemperaturelevel,three
N2 flow rateswere tested (6.95×10‐3 L s‐1, 1.11×10‐2 L s‐1 and 1.39×10‐2 L s‐1, evaluated at
T=20°CandP=1bar).
36
The regeneration profiles were elaborated to obtain the total specific amount of CO2
desorbedfromtheactivatedcarbondes[mmolg‐1],throughamaterialbalancesimilartothat
reportedinEq.(4.2):
ω
Q t dt. (4.4)
in which t0.1 is the time required to complete the desorption process, assumed as the one
correspondingtotheNDIRlowdetectionlimit(0.1%CO2byvol.).
CO2 concentration profiles were also quantitatively analysed to define the best
experimental conditions for an efficient regeneration process in terms of CO2 recovery
amount, time required for the desorption process (at a fixed regeneration level) and CO2
concentrationinthedesorbingflow,thelatterbeingacriticalaspectforCO2storagepurposes.
To this aim, four different characteristic desorption times (t50, t70, t80, t90) have been
considered,eachcorrespondingtoadifferentCO2recoverypercentageofthetotaladsorbed
amount (e.g. t50 corresponds toa50%of totalCO2 recoveredbydesorption).Consequently,
themeanCO2concentrationinthedesorbingflow(i
2COC )uptothetimeticanbeexpressed
as:
C
(4.5)
inwhich i2COV and i
2NV represent theCO2 totalvolumedesorbedandthepurgegasvolume
fedtothecolumnuptotimeti,respectively.Thevaluesof i2NV and
i
2COC werecomputedfor
variable CO2 recovery percentage i.e. i=50%, 70%, 80% and 90% for each investigated N2
purgeflowrate( des2NQ )anddesorptiontemperature( des
C60T and desC100T ).
37
CHAPTER5
RESULTSANDDISCUSSION
In this Chapter the main results obtained from solids characterization techniques and
adsorption/regeneration experiments will be discussed with a particular emphasis on the
intertwining among adsorbents microstructural properties and their CO2 capture
performances.Finally,theresultsderivedfromboththermodynamicandkineticmodellingof
adsorptiondatawillbeanalysed.
5.1 Adsorbentscharacterization
5.1.1 Porosimetric analyses for F600‐900 raw and impregnated with
[Hmim][BF4]/[Emim][Gly]
Figures5.1and5.2showtheN2adsorption‐desorptionisothermsat‐196°Cobtainedfor
the sorbents F600‐900 raw and impregnated with [Hmim][BF4]/[Emim][Gly] ILs at initial
activephaseconcentrationsC°=5.6×10‐3and2.2×10‐2M.Asageneralconsideration,itcanbe
observedthattheisothermsforallthesamplesareverysimilarandtheyaretypeIaccording
to IUPAC classification (Patrick, 1995) and the strong adsorption observed at very low
relative pressures (i.e. P/P0<10‐3) testifies the presence of a highly developedmicroporous
structure (Morlay and Joly, 2010). A comparison between the adsorption and desorption
branches testifies the non‐negligible presence of mesopores: the narrow hysteresis loop
observed from isotherms is of typeH4 according to IUPAC, commonly associatedwith the
presenceofslit‐shapedpores(MorlayandJoly,2010).Inaddition,itcanbeobservedthatthe
volumeofN2adsorbedreducesastheILconcentrationincreasesforbothactivephaseswith
respect to the parent material above all in the “knee” region of the adsorption isotherms
(P/P0<0.1): this is a clue of the preferential adsorption of both ILs in the substrate
micropores.
38
P/P0, -
0.0 0.2 0.4 0.6 0.8 1.0
Vad
s ST
P, c
m3
g-1
0
100
200
300
400
F600-900
F600-900 [Hmim][BF4] 10-3 M
F600-900 [Hmim][BF4] 10-2 M
Figure5.1N2adsorption‐desorptionisothermsat‐196°CforF600‐900rawandimpregnatedwith
[Hmim][BF4]atC°=5.6×10‐3and2.2×10‐2M
P/P0, -
0.0 0.2 0.4 0.6 0.8 1.0
Vad
s ST
P, c
m3
g-1
0
100
200
300
400
F600-900
F600-900 [Emim][Gly] 10-3 M
F600-900 [Emim][Gly] 10-2 M
Figure5.2N2adsorption‐desorptionisothermsat‐196°CforF600‐900rawandimpregnatedwith
[Emim][Gly]atC°=5.6×10‐3and2.2×10‐2M
Table5.1reportstheapparentsurfacearea(SBET),thetotalspecificporevolume(Vt),the
specificmicroporevolume(V0)andthespecificmesoporevolume(Vmeso=Vt−V0)determined
for both raw and [Hmim][BF4]/[Emim][Gly] impregnated sorbents by applying themodels
39
indicatedinSection4.2.1toN2adsorptiondataat‐196°C.Thevolumeofnarrowmicropores
(Vn)derivedforrawF600‐900fromCO2adsorptionisothermat0°Cisalsoincluded.
Table5.1TexturalparametersobtainedforF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILs
SampleSBET
[m2g‐1]Vt
[cm3g‐1]V0
[cm3g‐1]Vn
[cm3g‐1]Vmeso
[cm3g‐1]
F600‐900 1076 0.58 0.41 0.32† 0.17
F600‐900[Hmim][BF4]10‐3M 1018 0.55 0.39 n.a.† 0.16
F600‐900[Hmim][BF4]10‐2M 961 0.52 0.36 n.a.† 0.16
F600‐900[Emim][Gly]10‐3M 1029 0.55 0.39 n.a.† 0.16
F600‐900[Emim][Gly]10‐2M 971 0.52 0.36 n.a.† 0.16†notavailable
Results highlight a prevailingmicroporous nature for all the investigated sorbents (for
F600‐900 raw micropores contribute to a nearly 70% of the total pore volume), thus
confirming the observations derived from the analysis of the N2 adsorption isotherms.
Moreover, a comparison between the microstructural parameters of raw and ILs
functionalized F600‐900 shows a decrease of both SBET and V0 values when the initial
concentration of each ionic liquid increases, while the mesopore volume reduction is not
significant.TheseobservationsconfirmthatbothILspartiallyobstructonlythemicroporesof
therawmaterial.Finally,at fixed impregnationcondition [Hmim][BF4]and [Emim][Gly] ILs
determinethesamemicroporevolumereduction.
Figures5.3and5.4depict theabsoluteporesizedistributions (PSD)obtained forF600‐
900 raw and functionalized with [Hmim][BF4]/[Emim][Gly] ionic liquids by applying the
QSDFTmethodtoN2adsorptiondataat‐196°Candassumingaslit‐shapegeometryforpores.
Theplotsareexpressedintermsoftheratiobetweenthedifferentialvariationofthespecific
porevolumerelativetothatoftheporediameter(dVp(d))asafunctionoftheporediameter
(d). Results confirm the highly microporous nature of all the samples analysed with a
prevailing contribution of pores smaller than 10 Å. The pore size distributions for ILs‐
impregnatedF600‐900adsorbentsshowsimilarqualitativepatternswithrespecttotheraw
material except for a slight reduction of dVp(d) values in the pore diameter region smaller
than10Åandduetothealreadydescribedpartialmicroporeclogging.
40
pore diameter, Å
0 10 20 30 40 50
dV
p(d
), c
m3
g-1 Å
-1
0.00
0.02
0.04
0.06
0.08
0.10F600-900
F600-900 [Hmim][BF4] 10-3 M
F600-900 [Hmim][BF4] 10-2 M
Figure5.3AbsoluteporesizedistributionsforF600‐900rawandimpregnatedwith[Hmim][BF4]atC°=5.6×10‐3and2.2×10‐2M
pore diameter, Å
0 10 20 30 40 50
dV
p(d
), c
m3
g-1 Å
-1
0.00
0.02
0.04
0.06
0.08
0.10F600-900
F600-900 [Emim][Gly] 10-3 M
F600-900 [Emim][Gly] 10-2 M
Figure5.4AbsoluteporesizedistributionsforF600‐900rawandimpregnatedwith[Emim][Gly]at
C°=5.6×10‐3and2.2×10‐2M
41
5.1.2 TGA analyses for F600‐900 raw and impregnated with
[Hmim][BF4]/[Emim][Gly]
Figures 5.5 and5.6 show the results obtained from thermogravimetric analyses carried
outonrawandILs‐impregnatedF600‐900intermsofsamplemasspercentageasafunction
of the temperature. Thermograms show for all materials a weight loss (1‐3%) for
temperatureslowerthan100°Cwhichcanbelikelyascribedtothedesorptionofhumidityor
othervolatileimpurities.Moreover,forF600‐900[Hmim][BF4]solidsthemasslossdetectable
in the temperaturerange280‐380°C(clearly identifiable for thesample impregnatedunder
more concentrated conditions, Figure 5.5) can be ascribed to the IL decomposition, in
accordance with published research findings concerning TGA analyses for the pure ionic
liquid (Crosthwaite et al., 2005). Finally, for [Emim][Gly] impregnated adsorbents the ionic
liquiddecompositioncanbeapproximatelylocatedintheT‐range170‐330°C(Muhammadet
al.,2011),asconfirmedbythedifferentslopesof thethermogramsforthesematerialswith
respecttorawF600‐900inthesametemperaturerange.
T, °C
0 100 200 300 400
% m
ass
93
94
95
96
97
98
99
100
F600-900
F600-900 [Hmim][BF4] 10-3 M
F600-900 [Hmim][BF4] 10-2 M
Figure5.5ThermogravimetricanalysesforF600‐900rawandimpregnatedwith[Hmim][BF4]atC°=5.6×10‐3and2.2×10‐2M
42
T, °C
0 100 200 300 400
% m
ass
93
94
95
96
97
98
99
100
F600-900
F600-900 [Emim][Gly] 10-3 M
F600-900 [Emim][Gly] 10-2 M
Figure5.6ThermogravimetricanalysesforF600‐900rawandimpregnatedwith[Emim][Gly]at
C°=5.6×10‐3and2.2×10‐2M
The comparison of themass loss profiles for each impregnated sorbentwith respect to
raw F600‐900 allowed to estimate the amount of ionic liquid adsorbed on the substrate
accordingtothenumericalproceduredescribedinSection4.2.2(eq.(4.1)).Table5.2reports
acomparisonoftheactivephasemasspercentageadsorbedonthesubstrate(%wt.IL‐ads)with
respect to the one initially used for the impregnation procedure (%wt.IL‐load) for both
[Hmim][BF4] and [Emim][Gly] ILs. Quantitative results are also conveniently expressed in
terms of both IL specificmolar amount initially loaded (mmolIL‐load g‐1AC) and the ILmolar
specificadsorptioncapacityofF600‐900(AC)(mmolIL‐adsg‐1AC).
Table5.2QuantitativeparametersderivedfromTGAanalysesforF600‐900impregnatedwith[Hmim][BF4]and[Emim][Gly]ILs
Sample %wt.IL‐load %wt.IL‐ads mmolIL‐loadg‐1AC mmolIL‐adsg‐1AC
F600‐900[Hmim][BF4]10‐3M 0.76 0.38 0.030 0.015
F600‐900[Hmim][BF4]10‐2M 2.98 1.29 0.121 0.052
F600‐900[Emim][Gly]10‐3M 0.56 0.51 0.030 0.028
F600‐900[Emim][Gly]10‐2M 2.19 1.04 0.121 0.057
Resultsevidencethatfor[Hmim][BF4]‐adsorbentsnearly50%and43%oftheinitialamount
ofILusedfortheimpregnationisadsorbedonthesubstrateforC°=5.6×10‐3and2.2×10‐2M,
43
respectively. The estimated adsorption efficiencies are 93% and 47% for F600‐900
[Emim][Gly]10‐3Mand10‐2Mrespectively.Finally,undermoreconcentratedconditionsthe
amountof[Emim][Gly]adsorbedisslightlyhigherthaninthecaseof[Hmim][BF4](0.057vs.
0.052mmolg‐1)whileitisalmostdoubleforC°=5.6×10‐3M(0.028vs.0.015mmolg‐1).
5.1.3 PorosimetricanalysesforN.RGC30 raw and impregnated with
[Hmim][BF4]/[Emim][Gly]
N2porosimetricresultsat ‐196°CforN.RGC30rawandfunctionalizedwith[Hmim][BF4]
and[Emim][Gly]ionicliquidsaredepictedinFigures5.7and5.8.
P/P0, -
0.0 0.2 0.4 0.6 0.8 1.0
Vad
s ST
P, c
m3
g-1
0
200
400
600
800
N.RGC30
N.RGC30 [Hmim][BF44
] 10-3 M
N.RGC30 [Hmim][BF44
] 10-2 M
Figure5.7N2adsorption‐desorptionisothermsat‐196°CforN.RGC30rawandimpregnatedwith
[Hmim][BF4]atC°=5.6×10‐3and2.2×10‐2M
44
P/P0, -
0.0 0.2 0.4 0.6 0.8 1.0
Vad
s ST
P, c
m3
g-1
0
200
400
600
800
N.RGC30
N.RGC30 [Emim][Gly4
] 10-3 M
N.RGC30 [Emim][Gly4
] 10-2 M
Figure5.8N2adsorption‐desorptionisothermsat‐196°CforN.RGC30rawandimpregnatedwith
[Emim][Gly]atC°=5.6×10‐3and2.2×10‐2M
IsothermsaremixedtypeIandIVandinallthecaseswithahighadsorptioncapacityat
low relative pressures which testifies the significant presence of micropores, while the
observed hysteresis loops indicate a well‐developed mesoporosity for all the investigated
samples. Inaddition, forboth [Hmim][BF4]adsorbents it canbeobservedareductionofN2
adsorbedvolumewithrespecttotheparentactivatedcarbon,andtheiradsorptionisotherms
almostoverlap(aslightlyhigheradsorptioncapacityforthesampleimpregnatedundermore
diluted conditions is detected for P/P0<10‐3). In the case of [Emim][Gly]materials, amore
marked reduction of N2 adsorption capacity is observed under more concentrated
impregnationconditions,whileforthesampleobtainedatC°=5.6×10‐3Mdifferenceswithraw
N.RGC30canbedetectedonlyatlowrelativepressures(withaslightdecreaseofN2adsorbed
volumefortheimpregnatedmaterialatP/P0<10‐2).
The main microstructural parameters obtained from CO2 porosimetric analysis at 0°C
(onlyrawN.RGC30)andN2adsorptiondataat ‐196°Caresummarized inTable5.3.Results
underline a prevailing mesoporous nature for N.RGC30 activated carbon with a 57%
contribution to theoverallporosity;moreover, thesignificantdifferencebetweenV0andVn
valuessuggeststhepresenceofabroadmicroporesizedistributionforthissample(Krutyeva
etal.,2009).AcomparisonbetweenV0andVmesovaluesindicatesthepreferentialmicropore
occlusioninducedbyeachIL,thisbeingmoreimportantastheinitialconcentrationofactive
45
phase used for the impregnation process increases. Finally, it can be observed that under
more diluted conditions [Hmim][BF4] IL determines a highermicropore volume reduction,
whilethiseffectiscomparableforbothionicliquidsatC°=2.2x10‐2M.
Table5.3TexturalparametersobtainedforN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILs
SampleSBET
[m2g‐1]Vt
[cm3g‐1]V0
[cm3g‐1]Vn
[cm3g‐1]Vmeso
[cm3g‐1]
N.RGC30 1427 1.15 0.50 0.32 0.65
N.RGC30[Hmim][BF4]10‐3M 1350 1.12 0.47 n.a.† 0.65
N.RGC30[Hmim][BF4]10‐2M 1318 1.11 0.46 n.a.† 0.65
N.RGC30[Emim][Gly]10‐3M 1418 1.14 0.49 n.a.† 0.65
N.RGC30[Emim][Gly]10‐2M 1307 1.11 0.46 n.a.† 0.65†notavailable
Pore size distributions obtained for N.RGC30 raw and functionalized with
[Hmim][BF4]/[Emim][Gly]ionicliquidsarereportedinFigures5.9and5.10.Resultsconfirm
the presence of a broad micropore size distribution and a significant contribution of
mesopores with a mesopores‐peak centred approximately at 35 Å. In addition, both ionic
liquidsdonotaffectremarkablytherawmaterialPSD,apart fromlittledifferences forpore
diameters lower than 10 Å which can be ascribed to the already observed pore occlusion
effect(Table5.3).
pore diameter, Å
0 10 20 30 40 50
dV
p(d
), c
m3
g-1 Å
-1
0.00
0.01
0.02
0.03
0.04
0.05
0.06N.RGC30
N.RGC30 [Hmim][BF44
] 10-3 M
N.RGC30 [Hmim][BF44
] 10-2 M
Figure5.9AbsoluteporesizedistributionsforN.RGC30rawandimpregnatedwith[Hmim][BF4]at
C°=5.6×10‐3and2.2×10‐2M
46
pore diameter, Å
0 10 20 30 40 50
dV
p(d
), c
m3
g-1 Å
-1
0.00
0.01
0.02
0.03
0.04
0.05
0.06N.RGC30
N.RGC30 [Emim][Gly4
] 10-3 M
N.RGC30 [Emim][Gly4
] 10-2 M
Figure5.10AbsoluteporesizedistributionsforN.RGC30rawandimpregnatedwith[Emim][Gly]at
C°=5.6×10‐3and2.2×10‐2M
5.1.4 TGA analyses for N.RGC30 raw and impregnated with
[Hmim][BF4]/[Emim][Gly]
Thermogravimetric patterns derived for N.RGC30 both raw and impregnated with
[Hmim][BF4]and[Emim][Gly]ILsat C°=5.6×10‐3and2.2×10‐2MareshowninFigures5.11
and5.12.Experimentalresultsshowthatapartfromthedesorptionofhumidityandvolatile
matterobservedfortemperaturelowerthan100°C,thethermaldecompositionrangeforeach
ionicliquidpracticallycoincideswiththatobservedforF600‐900functionalizedsorbents(cf.
Figures 5.5 and 5.6): 280‐380°C and 170‐330°C for [Hmim][BF4] and [Emim][Gly]
respectively. This observation leads to the conclusion that the different microstructural
propertiesofF600‐900andN.RGC30activatedcarbonsdonotaffectremarkablythethermal
stabilityoftheionicliquidsinvestigated.
47
T, °C
0 100 200 300 400
% m
ass
96
97
98
99
100
N.RGC30
N.RGC30 [Hmim][BF4] 10-3 M
N.RGC30 [Hmim][BF4] 10-2 M
Figure5.11ThermogravimetricanalysesforN.RGC30rawandimpregnatedwith[Hmim][BF4]at
C°=5.6×10‐3and2.2×10‐2M
T, °C
0 100 200 300 400
% m
ass
96
97
98
99
100
N.RGC30
N.RGC30 [Emim][Gly] 10-3 M
N.RGC30 [Emim][Gly] 10-2 M
Figure5.12ThermogravimetricanalysesforN.RGC30rawandimpregnatedwith[Emim][Gly]at
C°=5.6×10‐3and2.2×10‐2M
Thecomparisonofthemasslossprofilesbetweentherawmaterialandtheimpregnated
ones allowed an estimation of the amounts of each ionic liquid adsorbed on N.RGC30
sorbents;themainderivedparametersareschematicallyreportedinTable5.4.Ingeneral,it
48
canbeobserved that theadsorptionefficienciesarehigher in thecaseof [Emim][Gly] ionic
liquid(90%and47%forC°=5.6×10‐3and2.2×10‐2M,respectively),whilesimilaractivephase
uptakeefficiencieswererecordedforN.RGC30[Hmim][BF4]10‐3and10‐2M(43%and42%
undermoredilutedandconcentratedimpregnationconditions,respectively).
Table5.4QuantitativeparametersderivedfromTGAanalysesforN.RGC30impregnatedwith[Hmim][BF4]and[Emim][Gly]ILs
Sample %wt.IL‐load %wt.IL‐ads mmolIL‐loadg‐1AC mmolIL‐adsg‐1AC
N.RGC30[Hmim][BF4]10‐3M 0.76 0.32 0.030 0.013
N.RGC30[Hmim][BF4]10‐2M 2.98 1.28 0.121 0.051
N.RGC30[Emim][Gly]10‐3M 0.56 0.49 0.030 0.027
N.RGC30[Emim][Gly]10‐2M 2.19 1.04 0.121 0.057
5.1.5 Comparison between TGA and porosimetric analyses for the
investigatedsorbents
The main parameters derived from thermogravimetric and porosimetric analyses for
F600‐900 and N.RGC30 both raw and impregnated with [Hmim][BF4]/[Emim][Gly] ionic
liquidsaresummarizedinTable5.5.Thepercentagereductionofthetotalmicroporevolume
derivedforeachimpregnatedmaterialwithrespecttotheparentone(V0‐raw−V0‐impr)/V0‐raw
(with V0‐raw and V0‐impr representing the total micropore volume of the parent and
functionalizedsorbent,respectively)isalsoreportedforabetterunderstandingoftheeffect
of the impregnation treatment conditions on the microstructural properties of each
investigated sorbent. The most interesting aspects deduced from the data analysis are
highlightedinthefollowing.
RawN.RGC30ischaracterizedbyabroadermicroporesizedistributionwithrespectto
F600‐900,asconfirmedbythehigherdifferencebetweenV0andVnvalues(Krutyeva
et al., 2009), and displays a remarkably higher contribution of mesopores (Vmeso is
nearlyfourfoldthevalueobtainedforF600‐900).
[Hmim][BF4] and [Emim][Gly] ILs preferentially adsorb in micropores for both
substrates and in general the higher the IL loading, the greater the micropore
occlusion.
For F600‐900 impregnated sorbents, even if the specific amount of [Emim][Gly]
adsorbedon the substrate is higher than in the case of [Hmim][BF4] (almost double
undermorediluted impregnationconditions) themicroporevolumereduction is the
49
sameatfixedinitialactivephaseconcentration.AsimilarsituationoccursforN.RGC30
materials functionalized at C°=2.2×10‐2 M, while at lower initial active phase
concentration(5.6×10‐3M)[Emim][Gly]producesamicroporevolumereductionthat
isone‐thirdtheoneobservedinthecaseof[Hmim][BF4],despiteofadoublespecific
amountadsorbed.Thedescribedtrendscanberelatedtothemorebranchedshapeof
[Hmim][BF4] imidazolium ring which is characterized by a C6 side chain (C2 for
[Emim][Gly], cf. Section4.1): the largercationic sizeof this ionic liquiddeterminesa
similar (or even higher) pore occlusion effectwith respect to [Emim][Gly] for lower
adsorbedamount.
A comparison between F600‐900 and N.RGC30 impregnated with [Hmim][BF4]
revealsthatsimilarspecificamountsofionicliquidareadsorbedatfixedinitialactive
phase concentration: under more diluted impregnation conditions the micropore
volume reduction is comparable for both substrates (4.9 and 6% for F600‐900 and
N.RGC30respectively),whiletheporecloggingeffectismoresignificantforF600‐900
athigherILconcentration.Likewisefor[Emim][Gly]adsorbents,asimilarILadsorbed
amount for both substrates at fixed impregnation condition determines a higher
microporevolume reduction forF600‐900activated carbon.These results shouldbe
interpreted in the lightof thedifferent texturalpropertiesofF600‐900andN.RGC30
activated carbons. As a matter of fact, the presence of a narrower micropore size
distributionwithaprevailingcontributionofverysmallporediameters(<10Å,cf.also
Figures5.3and5.4)forF600‐900determinesamorerelevantocclusioneffectforeach
ILandat fixed impregnationcondition than in thecaseofN.RGC30adsorbentwhich
exhibitsagreatercontributionofwiderporesinthemicroporesrange(cf.Figures5.9
and5.10).
The aforementioned analysis is considered to be an important reference for the
subsequent discussion concerning the effects of the different adsorbents microstructural
propertiesontheirCO2captureperformances.
50
Table5.5MainparametersderivedfromporosimetricandTGAanalysesforF600‐900andN.RGC30rawandimpregnatedwith[Hmim][BF4]and[Emim][Gly]ILs
SampleSBET
[m2g‐1]V0
[cm3g‐1]Vn
[cm3g‐1]Vmeso
[cm3g‐1](V0‐raw‐V0‐impr)/V0‐raw
%mmolIL‐adsg‐1AC
(TGA)
F600‐900 1076 0.41 0.32 0.17
F600‐900[Hmim][BF4]10‐3M 1018 0.39 n.a.† 0.16 4.9 0.015
F600‐900[Hmim][BF4]10‐2M 961 0.36 n.a.† 0.16 12.2 0.052
F600‐900[Emim][Gly]10‐3M 1029 0.39 n.a.† 0.16 4.9 0.028
F600‐900[Emim][Gly]10‐2M 971 0.36 n.a.† 0.16 12.2 0.057
N.RGC30 1427 0.50 0.32 0.65
N.RGC30[Hmim][BF4]10‐3M 1350 0.47 n.a.† 0.65 6 0.013
N.RGC30[Hmim][BF4]10‐2M 1318 0.46 n.a.† 0.65 8 0.051
N.RGC30[Emim][Gly]10‐3M 1418 0.49 n.a.† 0.65 2 0.027
N.RGC30[Emim][Gly]10‐2M 1307 0.46 n.a.† 0.65 8 0.057
†notavailable
51
5.2 CO2captureperformancesontheinvestigatedsorbents
5.2.1 CO2adsorptiontestsontorawF600‐900andN.RGC30
Figure5.13depictsCO2adsorptionisothermsobtainedforrawF600‐900andN.RGC30at
(a)30,(b)50and(c)80°Cintermsofthesolidmolaradsorptioncapacityeqasafunctionof
thepollutantequilibriumpartialpressure inthegaseousphase(Peq). It ishererecalledthat
eqvalueswerederivedbyintegrationofthebreakthroughcurvesobtainedfromkineticruns
at different CO2 concentrations (1‐30% by vol.) according to the procedure described in
Section4.4.1.ResultsevidenceareductionofCO2adsorptioncapacitywhenthetemperature
increases for both activated carbons, as a consequence of the exothermic character of
physisorption (Plaza et al., 2007). In particular, under typical flue gas conditions (Peq=0.15
bar) eq is 0.575 mmol g‐1 at 30°C for F600‐900 which is 1.8‐ and 4.2‐times the values
obtainedat50and80°C,respectively.Similarly,eqforN.RGC30atPeq=0.15barreducesfrom
0.499mmolg‐1at30°Cto0.276and0.139mmolg‐1at50and80°C,respectively.Asageneral
considerationCO2adsorptionperformancesobtainedinthisworkforF600‐900andN.RGC30
arecomparablewiththosereportedintheliteratureforrawactivatedcarbonstestedunder
similarexperimentalconditions(cf.Table2.1inSection2.1.1).
AcomparisonofF600‐900andN.RGC30adsorption isothermsat thedifferentoperating
temperatures reveals that at 30°C the former sorbent displays higher CO2 removal
performances for all investigated pollutant equilibrium partial pressures, while differences
reduceat50°Ctobecomepracticallynegligibleat80°C.Theseresultsshouldbe interpreted
considering that at lower temperatures physisorption plays a major role in the pollutant
capture and its contribution decreases at higher temperatures (Plaza et al., 2007). In this
context, on the basis of the sorbents textural properties reported in Section 5.1 (with
particularreferencetoTable5.5)onewouldhaveexpectedhigherCO2capturecapacitiesfor
N.RGC30samplebecauseof itshigher surfaceareaandmicroporevolume.Nevertheless, as
recentlyhighlightedbyWhabyet al. (2010and2012), thepresenceof anarrowmicropore
sizedistributionwithwell‐definedporesizeentrances(mainlyporediameters<5Å)seemsto
beakeyfactorindeterminingCO2adsorption,becauseinnarrowmicroporestheoverlapping
potentialproducesamoreeffectivepackingofCO2molecules.Asaconsequence,thenarrower
microporesizedistributionwithaprevailingcontributionofverysmallporediameters(<10
Å)observedforF600‐900solidshouldberesponsibleforitshighercaptureperformancesat
30°C. At intermediate temperature (50°C) physisorption contribution is less important and
52
consequentlydifferencesinCO2sorptivepropertiesbetweenthetwoactivatedcarbonstend
toreduceandbecomepracticallynegligibleat80°C.
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.0
0.2
0.4
0.6
0.8
1.0F600-900N.RGC30
(a)
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.0
0.2
0.4
0.6
0.8
1.0F600-900N.RGC30
(b)
53
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.0
0.2
0.4
0.6
0.8
1.0F600-900N.RGC30
(c)
Figure5.13CO2adsorptionisothermsobtainedforrawF600‐900andN.RGC30adsorbentsat(a)30,(b)
50and(c)80°C
TherawadsorbentsdynamicadsorptionperformancesarecomparedinFigures5.14and
5.15whichreportthebreakthroughcurvesofCO2onF600‐900andN.RGC30,respectively,at
30,50and80°Cadoptinga15%byvol.CO2‐concentratedgaseousstream,representativeofa
typicalflue‐gascomposition.
t, s0 20 40 60 80 100 120
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
T=30°CT=50°CT=80°C
Figure5.14BreakthroughcurvesofCO2onrawF600‐900at30,50and80°Cfora15%byvol.CO2
gaseousstream
54
t, s0 20 40 60 80 100 120
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
T=30°CT=50°CT=80°C
Figure5.15BreakthroughcurvesofCO2onrawN.RGC30at30,50and80°Cfora15%byvol.CO2gaseous
stream
As a general consideration, it can be observed that for each activated carbon the
breakthroughcurveshowsamonotonicshift towards lowertimesasthetemperaturerises.
Forinstance,thebreakpointtimetb(Q t /Q =0.05)is43sat30°CforF600‐900,avalue
4.8‐timesgreaterthantheoneobtainedat80°C(9s).InthecaseofN.RGC30tb=32,18and8s
for adsorption temperaturesof 30, 50 and80°C.Moreover, higher temperaturesdetermine
fasteradsorptionkineticsastestifiedbytheincreasingslopeofthesigmoidandthereduction
of the equilibrium time t* (for which CO2 concentrations at the bed inlet and outlet are
practicallyequal).Asamatterof fact, t* isapproximately2.7‐and2.2‐timesgreaterat30°C
withrespectto80°CforN.RGC30(195vs.71s)andF600‐900(293vs.131s),respectively.
These results can be interpreted considering that two factors determine faster adsorption
kineticsathighertemperatures:i)thereductionoftheadsorptioncapacity(lowernumberof
active sites to be occupied)which allows higher diffusion rates; ii) an intrinsic increase in
intraparticle diffusivitywith temperature (mainly inmicropores, cf. Sections 3.2 and 5.4.2)
(Ruthven,1984).
ConsideringthatdifferentamountsofF600‐900andN.RGC30wereusedforadsorption
experiments (15 and 13 g for F600‐900 and N.RGC30 respectively, cf. Section 4.4.1), a
comparisonofthedynamicperformancesofthetwosorbentsonthebasisoftbisnotfeasible
because breakthrough curves translate along time axis by varying the sorbent dose under
55
constantpatternconditions(Ruthven,1984;McCabeetal.,1993).Inthiscontext,differences
in mass transfer rates can be better evaluated by introducing a time parametert0.9−tb
(with t0.9 being the time for which Q t /Q =0.9) which is related to the slope of the
linearpartofthesigmoid:thesmallerthisparameterthesteeperthebreakthroughcurveand,
consequently, the faster adsorption kinetics. The values of derived from the kinetic
patterns forF600‐900andN.RGC30rawat30,50and80°Care listed inTable5.6 together
withthesorbentsmainmicrostructuralparameters.
Table5.6Comparisonbetweenvaluesobtainedfromkineticadsorptiontestsat30,50and80°CandthemaintexturalparametersderivedforF600‐900andN.RGC30activatedcarbons; =15%byvol.
Sample
[s]
Microstructuralparameters
T=30°C T=50°C T=80°C V0[cm3g‐1]
Vn[cm3g‐1]
Vmeso[cm3g‐1]
F600‐900 22 10 5 0.41 0.32 0.17
N.RGC30 15 8 5 0.50 0.32 0.65
Results show that for each adsorbentvalue decreases from 30 to 80°C confirming
fasteradsorptionkineticsathighertemperature;moreover,isgenerallyhigherforF600‐
900sorbentwithrespecttoN.RGC30anddifferencesreduceastheadsorptiontemperature
rises (at80°C thevaluespractically coincide).Theseoutcomes suggest fastermass transfer
rates for N.RGC30 sorbent which can be ascribed to the larger contribution of mesopores
(Vmeso=0.65and0.17cm3g‐1forN.RGC30andF600‐900,respectively)andtothepresenceof
widermicropores(cf.V0andVnvalues)withrespecttoF600‐900allowingaquickerdiffusion
ofCO2moleculesinsidetheadsorbentpores.Theeffectofthedifferenttexturalparameterson
the adsorbents dynamic adsorptionperformances becomes less important from30 to 80°C
because of the already‐mentioned increase in intraparticle diffusion rates at higher
temperatures.
56
5.2.2 CO2 adsorption tests onto F600‐900 raw and functionalized with
[Hmim][BF4]/[Emim][Gly]
Figure 5.16 reports CO2 adsorption isotherms on F600‐900 raw and impregnated with
[Hmim][BF4]/[Emim][Gly] ILs atdifferent activephase concentrations andat (a)30, (b)50
and(c)80°C.
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.2
0.4
0.6
0.8
1.0
F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(a)
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(b)
57
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.1
0.2
0.3
0.4 F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(c)
Figure5.16CO2adsorptionisothermsonF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]
ILsatC°=5.6×10‐3and2.2×10‐2Mandata)30,(b)50and(c)80°C
Aqualitativeanalysisoftheadsorptionisothermsdenotesthefollowingaspects:
At30°CF600‐900rawshowsthehighestCO2captureperformancesamongthetested
adsorbents in the whole Peq range investigated. Moreover, [Emim][Gly] sorbents
display higher eq values with respect to [Hmim][BF4] materials and adsorption
isothermsatdifferentimpregnationconditionsoverlapforeachionicliquidexamined.
At50°C[Emim][Gly] functionalizedsorbentsshowsimilaradsorptioncapacitieswith
respect to the raw material up to Peq=0.15 bar, while an improving of the parent
adsorbent capture performances are observed for higher pollutant concentrations;
once again, [Hmim][BF4] materials are characterized by worse adsorption
performanceswhichismoreevidentforthesampleimpregnatedatC°=2.2×10‐2M.
At 80°C the eq ranking is F600‐900 [Emim][Gly] 10‐3 M>F600‐900 raw≈F600‐900
[Hmim][BF4]10‐3M>F600‐900[Emim][Gly]10‐2M>F600‐900[Hmim][BF4]10‐2M.
Before providing a quantitative correlation between the sorbents microstructural
properties and their sorption capacities, it is fundamental to underline that many factors
contribute in determining a complex phenomenology in CO2 removal for the investigated
systems.Inparticular,inthecaseoftherawactivatedcarbonphysisorptionisthemainactive
mechanism for CO2 capture and its contribution decreases with temperature (Plaza et al.,
58
2007).Differently,dealingwith impregnatedsamples,theCO2capturedeterminedbytheIL
and the occlusion of the substrate pores act in opposite directions, the latter leading to a
reduction in the contribution of the rawmaterial adsorption. In addition, for both ILs the
increaseintemperatureisdetrimentalforthecaptureprocess:[Hmim][BF4]actsasaphysical
solvent towardsCO2 (solubility is typically equal to 0.05mol /mol at roomT andnear
atmosphericpressure),while[Emim][Gly] includesanamino‐groupinitsstructurewhichis
potentiallyabletoformcarbamatewithcarbondioxideviaareversibleexothermicreaction
following the same scheme described for commonly employed amine‐based solvents
(theoreticalcapturecapacityunderdryconditions0.5mol /mol )(Kimetal.,2005;Plaza
etal.,2007;Krumdiecketal.,2008;Zhangetal.2011;Kasaharaetal.,2012).Moreover, the
less sterically‐hindered [Emim][Gly] molecule makes the access of CO2 to its active site
(amino‐group) and in the sorbentporespotentially easier than in the caseof [Hmim][BF4].
Finally,itshouldbealsoconsideredthatthedispersionoftheactivephaseintothesubstrate
porescouldbealteredbothbytemperaturevariations(influencingforexampletheILsurface
tension and viscosity) and by interactions establishingwith CO2molecules (with a further
dependenceontheamountofpollutantadsorbed).
In light of the aforementioned observations, Table 5.7 reports a comparison of the
adsorptioncapacitiesobtainedfortheinvestigatedsorbentsundertypicalflue‐gasconditions
(CO215%byvol.,ω %)andthemainquantitativeparametersderivedfromporosimetricand
TGAanalyses(porevolumereductionandthespecificamountofionicliquidadsorbedonthe
substrate,seealsoTable5.5).
Table5.7Comparisonamong %valuesandthemainparametersderivedfromporosimetricandTGA
analysesforF600‐900rawandimpregnatedwith[Hmim][BF4]and[Emim][Gly]ILs
Sample
%[mmolg‐1] (V0‐raw‐V0‐impr)/V0‐raw
%mmolIL‐adsg‐1AC
(TGA)T=30°C T=50°C T=80°C
F600‐900 0.575 0.315 0.137
F600‐900[Hmim][BF4]10‐3M
0.437 0.264 0.139 4.9 0.015
F600‐900[Hmim][BF4]10‐2M
0.443 0.230 0.101 12.2 0.052
F600‐900[Emim][Gly]10‐3M 0.502 0.324 0.157 4.9 0.028
F600‐900[Emim][Gly]10‐2M 0.502 0.317 0.125 12.2 0.057
59
Results suggest that at T=30°C the pores blocking induced by the presence of the ILs
prevailsontheircontributiontoCO2capture,thusdeterminingareductionoftheadsorption
performancesof the functionalized sorbentswith respect to theparentmaterial.Moreover,
for each IL, the higher pore volume reduction experienced under more concentrated
impregnationconditioniscounterbalancedbythecontributiontoCO2capturedeterminedby
ahigheramountofILloaded.Thehigherω %valuesobtainedat30°Cfor[Emim][Gly]solids
with respect to [Hmim][BF4] ones could be explained considering that, even if at fixed
impregnation condition thepore clogging is the same forboth ILs, a greateramountof the
aminoacid‐basedionicliquidisadsorbedonthesubstrate(almostdoubleatC°=5.6×10‐3M)
and its capture capacity towards CO2 is one order magnitude higher than in the case of
[Hmim][BF4] (0.05and0.5mol /mol for [Hmim][BF4]and[Emim][Gly]respectively).At
50°Ctheporeblockingeffectonthefunctionalizedsorbentsadsorptioncapacitiesisgenerally
reduced(duetothedecreasingphysisorptionof therawmaterial)and it isbalancedbythe
active phase CO2‐capture contribution in the case of [Emim][Gly], while it persists for less
CO2‐affine[Hmim][BF4](toagreaterextentforthesampleobtainedundermoreconcentrated
condition for which the pore volume reduction is higher). Finally, at 80°C pore clogging
becomesnegligibleforF600‐900[Hmim][BF4]10‐3Msamplewhileitisstillimportantforthe
sample obtained at C°=2.2×10‐2 M. Differences between [Hmim][BF4] adsorbents could be
ascribedtoavery lowCO2solubility in the ILat this temperature(even if solubilitydataat
this temperaturearenotavailable in the literature), thusundermorediluted impregnation
conditiontheloweramountofILchargedandthehigherspreadingoftheILonthesupport
surface(inducedbyareductionofitssurfacetension)wouldmakeeasiertheaccesstoF600‐
900 active siteswith respect to F600‐900 [Hmim][BF4] 10‐2M sample. On the other hand,
[Emim][Gly]isabletoproduceanincreaseinF600‐900CO2capturecapacityonlyundermore
diluted impregnation condition (ω %is nearly 15% greater with respect to the parent
material), thanks to a good compromise between IL loading, CO2 affinity and pore
accessibility, while the pore volume reduction prevails for the sorbent impregnated at
C°=2.2×10‐2M.
The breakthrough curves obtained for F600‐900 raw and impregnated with
[Hmim][BF4]/[Emim][Gly] ILs at different temperatures and for a 15%CO2 gas stream are
reportedinFigure5.17((a)30°C,(b)50°Cand(c)80°C).
60
As a general consideration, it can be observed that, as expected, the increase in the
operatingtemperaturedeterminesfasteradsorptionkineticsforalltheimpregnatedsamples
with a shift of the breakthrough curves towards lower times as a consequence of higher
diffusion rates of CO2molecules in the sorbent pores.Moreover, the equilibrium times are
approximately 10‐ and 3‐times lower at 80°C with respect to 30°C for [Hmim][BF4] and
[Emim][Gly]sorbents,respectively.
t, s0 20 40 60 80 100 120
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(a)
t, s0 20 40 60 80
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(b)
61
t, s0 10 20 30 40
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
F600-900
F600-900 [Hmim][BF4] 10-3
M
F600-900 [Hmim][BF4] 10-2
M
F600-900 [Emim][Gly] 10-3
M
F600-900 [Emim][Gly] 10-2
M
(c)
Figure5.17BreakthroughcurvesofCO2onF600‐900rawandimpregnatedwith
[Hmim][BF4]/[Emim][Gly]fora15%byvol.CO2gaseousstreamat(a)30,(b)50and(c)80°C
AcomparisonoftheadsorbentsdynamicperformancesisprovidedinTable5.8whichlists
breakpointtimestbandvalues(cf.Section5.2.1)obtainedfrom15%CO2adsorptiontests
at30,50and80°C.
Table5.8Comparisonbetweenandtbvaluesobtainedfromkineticadsorptiontestsat30,50and80°CforF600‐900rawand[Hmim][BF4]/[Emim][Gly]‐impregnated; =15%byvol.
Sample
[s]
tb[s]
T=30°C T=50°C T=80°C T=30°C T=50°C T=80°C
F600‐900 22 10 5 43 21 9
F600‐900[Hmim][BF4]10‐3M 16 9 6 31 20 10
F600‐900[Hmim][BF4]10‐2M 17 9 5 32 16 7
F600‐900[Emim][Gly]10‐3M 17 10 6 38 24 11
F600‐900[Emim][Gly]10‐2M 17 10 6 39 23 10
Results show that the ranking observed for breakpoint times at each temperature
generally coincides with the one shown in Table 5.7 in terms of equilibrium adsorption
capacities (1 s differences fall within the experimental error). In the case of F600‐900
62
[Emim][Gly]10‐2and10‐3M,tbvaluesat50°Caregreaterwithrespecttotheoneobtainedfor
F600‐900raw(21,24and23sforF600‐900raw,[Emim][Gly]10‐3and10‐2Mrespectively),
eveniftheadsorptioncapacitiesarepracticallyequivalent:obviously,eqvaluealsodepends
on the sorbent capture contribution for times greater than tb. The analysis of data
highlights that the only significant difference in dynamic performances can be observed at
30°CforwhichthisparameterisgreaterforF600‐900raw(sloweradsorptionkinetic)with
respect to the impregnated materials (which display similar values discrepancies are
practicallynegligibleathighertemperatures.Thispatterncouldbeexplainedconsideringthat
at lower temperaturedifferences in adsorption capacities of the impregnated sampleswith
respecttotheparentonearegreater,thusthehigherthenumberofactivesitestobeoccupied
(forF600‐900raw)theslowerthekinetics.Asthetemperaturerises,eqdifferencesreduce
andintraparticlediffusivityishigher,determiningsimilarCO2capturekinetics.Moreover,itis
hererecalledthattheporesizedistributionofthesubstrateisnotappreciablyinfluencedby
the impregnationprocess(cf.Section5.1)andtheonlyobservedeffect isareduction inthe
totalmicroporevolume(Vmesoispracticallyconstant):thisparameter(Vmicro)itselfisnotable
to explain kinetic differences between the raw and impregnated materials. Vice versa the
distributionofbothILsinthesorbentmicropores(widthoftheILfilm,positioningatthepore
mouth,completeorpartialporefillingetc.),whichcanbealsoinfluencedbythetemperature
andthecaptureprocessitself, islikelyafurtherfactoraffectingthesorptionkineticsforthe
impregnatedsamples.
63
5.2.3 CO2 adsorption tests onto N.RGC30 raw and functionalized with
[Hmim][BF4]/[Emim][Gly]
The thermodynamic behaviour of N.RGC30 raw and impregnated with
[Hmim][BF4]/[Emim][Gly] ILs in the CO2 capture process is illustrated in Figure 5.18 for
adsorptiontemperaturesof(a)30,(b)50and(c)80°C.
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.2
0.4
0.6
0.8
1.0
N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(a)
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(b)
64
Peq, bar
0.0 0.1 0.2 0.3 0.4
eq
, mm
ol g
-1
0.0
0.1
0.2
0.3
0.4N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(c)
Figure5.18CO2adsorptionisothermsonN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]
ILsatC°=5.6×10‐3and2.2×10‐2Mandata)30,(b)50and(c)80°C
Theanalysisoftheadsorptionisothermsrevealsthecomplexeffectofthetemperatureon
CO2 adsorption performances of the investigated adsorbents and the main aspects can be
summarizedasfollows:
For an adsorption temperature of 30°C the eq ranking is N.RGC30 raw>N.RGC30
[Emim][Gly] 10‐3 M>N.RGC30 [Emim][Gly] 10‐2 M≈N.RGC30 [Hmim][BF4] 10‐3 M≈
N.RGC30[Hmim][BF4]10‐2M.
At 50°C N.RGC30 raw and impregnated with [Emim][Gly] under more diluted
impregnationconditionsexhibitthehighesteqvaluesandtheirisothermspractically
overlap up to Peq=0.15 bar, while for higher CO2 concentrations the active phase
determines an improvement of the adsorption capacity with respect to the parent
sorbent. The other functionalized materials show generally similar adsorption
performancesexceptN.RGC30impregnatedwith[Hmim][BF4]atC°=2.2×10‐2Mwhich
ischaracterizedbyworsecapturecapacitiesforPeq>0.15bar.
Finallyat80°C, [Emim][Gly] ionic liquid iseffective inamelioratingN.RGC30capture
capacities for all the investigated Peq‐range under more diluted impregnation
conditions,whileeqisslightlygreaterforN.RGC30[Emim][Gly]10‐2Mwithrespectto
therawactivatedcarbononlyforCO2equilibriumpartialpressuresgreaterthanthose
of a typical‐flue gas.Moreover, N.RGC30 [Hmim][BF4] 10‐2M is again theworst CO2
65
adsorbent, while the sample impregnated with the same IL at C°=5.6×10‐3 M is
equivalenttotherawmaterial.
Inordertoshedlightontheintertwiningamongtheimpregnationconditions,thesorbent
properties and their CO2 capture capacities, Table 5.9 reports a comparison of the main
parametersderivedfromTGAandporosimetricanalysesforN.RGC30rawandimpregnated
with [Hmim][BF4]/[Emim][Gly] ILs and their adsorption capacities obtained under typical
flue‐gasconditionsandforoperatingtemperaturesof30,50and80°C.
Table5.9Comparisonamong %valuesandthemainparametersderivedfromporosimetricandTGA
analysesforN.RGC30rawandimpregnatedwith[Hmim][BF4]and[Emim][Gly]ILs
Sample
%[mmolg‐1] (V0‐raw‐V0‐impr)/V0‐raw
%mmolIL‐adsg‐1AC
(TGA)T=30°C T=50°C T=80°C
N.RGC30 0.499 0.276 0.139
N.RGC30[Hmim][BF4]10‐3M
0.391 0.246 0.141 6 0.013
N.RGC30[Hmim][BF4]10‐2M
0.386 0.240 0.122 8 0.051
N.RGC30[Emim][Gly]10‐3M
0.426 0.275 0.163 2 0.027
N.RGC30[Emim][Gly]10‐2M
0.381 0.238 0.137 8 0.057
Experimentaldatahighlightthatat30°Cporeblockingistherulingfactorindetermininga
reductionofthe impregnatedmaterialsadsorptionperformanceswithrespecttotheparent
one. In particular, the comparable micropore volume reduction observed for N.RGC30
[Hmim][BF4] 10‐2M and 10‐3M andN.RGC30 [Emim][Gly] 10‐2Mdetermines similarω %
values forthesesamples.Conversely, the lowermicroporevolumereductiondeterminedby
[Emim][Gly]at10‐3M(2%)producesalessmarkedω %decreasewithrespecttotheparent
materialalsoforthepossibleaccessofCO2moleculestotheILactivesites,whichseemstobe
hindered in the case of higher IL loading. At 50°C the contribution of [Emim][Gly] active
phaseinCO2capturebalancestheporecloggingeffectonlyundermoredilutedimpregnation
conditionsmakingω %values similar for the functionalizedand rawactivated carbon; for
the other impregnated adsorbents the considerations expressed for an adsorption
temperature of 30°C still hold at 50°C, even if differences in capture performances with
respecttotheparentmaterialreduceforthelowercontributionofphysisorptionofthelatter
66
as the temperature rises. Finally, at 80°C the pore blocking is negligible for N.RGC30
[Hmim][BF4] 10‐3M andN.RGC30 [Emim][Gly] 10‐2M determining equivalentω % values
with respect to the parent material, while this effect is still important for N.RGC30
[Hmim][BF4]10‐2M.InthecaseofN.RGC30functionalizedwith[Emim][Gly]atC°=5.6×10‐3M,
theactivephaseisabletodetermineanincreaseinCO2adsorptioncapacitywhencompared
to the raw substrate (ω % is approximately 16% greater) thanks to stronger chemical
interactionsbetweenacidicCO2moleculesandbasicaminegroupsandtheeasieraccessibility
inthesorbentporesdeterminedbythelowILloading.
TheconsiderationsconcerningthecorrelationsbetweensorbentpropertiesandtheirCO2
capturecapacitiesunder typical flue‐gasconditionscanbegenerallyextended to thewhole
Peq‐range,evenifat50and80°CandforCO2equilibriumpartialpressuresgreaterthan0.15
bar, theprocessdrivingforceexertsa furtherrole.Forexample,at50°Cthecontributionin
CO2captureexertedby[Emim][Gly]undermoredilutedconditionsisprevailingwithrespect
toporeblockingforPeq>0.15bar(producinganincreaseinadsorptioncapacitieswithrespect
toN.RGC30raw),while theybalance for lowerprocessdriving force;similarargumentsare
alsovalidforN.RGC30[Emim][Gly]10‐2Mtestedatanadsorptiontemperatureof80°C.
The comparison of the dynamic performances in CO2 capture process for both raw and
ILs‐functionalizedN.RGC30 sorbents under typical flue‐gas conditions (CO2 15%by vol.) is
reportedinFigure5.19foroperatingtemperaturesof(a)30,(b)50and(c)80°C.Table5.10
liststhevaluesofbreakpointtimetbandofthecharacteristicparameterderivedfromthe
kineticprofiles.
Resultsevidence,onceagain,thatathighertemperaturestheadsorptionkineticsisfaster
foralltheinvestigatedsorbentswithamonotonicdecreaseofcharacteristicbreakpointand
saturation times. For example, N.RGC30 [Hmim][BF4] 10‐3 M experiences a saturation 3.5‐
timesfasterat80°Cwithrespectto30°C(t*=165and47sat30and80°C),whileinthecaseof
N.RGC30 [Emim][Gly] 10‐3 M, t* is 1.6‐times lower for the same temperature increase.
Moreovertherankingof tbvaluesobtainedfor thedifferent investigatedsamples is ingood
agreement with the one derived in terms of CO2 adsorption capacities (cf. Tables 5.9 and
5.10). The comparison ofdata reveals thatmain kinetic differences can be identified at
30°C forwhich thisparameters ishigher for rawN.RGC30with respect to the impregnated
onesindicatingslowercapturekineticsforthissample.Moreover,N.RGC30[Emim][Gly]10‐3
Mdisplaysaslightlyhighervalueofwithrespecttotheotherfunctionalizedmaterialsand
thiscanbeascribedtoitslowerdifferenceinω %whencomparedtorawmaterial(cf.Table
67
5.9).Athighertemperaturestheintrinsicincreaseinintraparticlediffusivityandthereduced
differences in adsorption capacity contribute in determining similar capture kinetics for all
samples.Finally, theway inwhich the IL isdispersed in the substratemicropores (without
affectingremarkablythesubstratePSD,cf.Section5.1)shouldalsocontributeindetermining
different adsorptionkinetics for the impregnated sampleswith respect to the rawmaterial
especiallyat30°C.
t, s0 20 40 60 80 100 120
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(a)
t, s0 20 40 60 80
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(b)
68
t, s0 10 20 30 40
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
N.RGC30
N.RGC30 [Hmim][BF4] 10-3
M
N.RGC30 [Hmim][BF4] 10-2
M
N.RGC30 [Emim][Gly] 10-3
M
N.RGC30 [Emim][Gly] 10-2
M
(c)
Figure5.19BreakthroughcurvesofCO2onN.RGC30rawandimpregnatedwith
[Hmim][BF4]/[Emim][Gly]fora15%byvol.CO2gaseousstreamat(a)30,(b)50and(c)80°C
Table5.10Comparisonbetweenandtbvaluesobtainedfromkineticadsorptiontestsat30,50and80°CforN.RGC30rawand[Hmim][BF4]/[Emim][Gly]‐impregnated; =15%byvol.
Sample
[s]
tb[s]
T=30°C T=50°C T=80°C T=30°C T=50°C T=80°C
N.RGC30 15 8 5 32 18 8
N.RGC30[Hmim][BF4]10‐3M 11 8 6 25 15 8
N.RGC30[Hmim][BF4]10‐2M 11 8 5 25 15 7
N.RGC30[Emim][Gly]10‐3M 13 8 5 27 17 10
N.RGC30[Emim][Gly]10‐2M 11 8 6 25 14 7
69
5.2.4 IntertwiningamongsolidspropertiesandCO2captureperformances
Table5.11schematicallysummarizes themainresultsobtained in thiswork in termsof
bothsorbentsCO2captureperformancesundertypicalflue‐gasconditions(CO215%byvol.)
andmainparametersderivedfromTGAandporosimetricanalyses.
Asageneralcomment, itcanbeobservedthatforbothactivatedcarbonsporesblocking
induced by the presence of the ILs is the prevailing effect at 30°C which determines a
worseningofCO2 captureperformancesof each functionalizedmaterialwith respect to the
parentone,duetothemaincontributionofphysisorptionforthelatteratlowertemperatures.
Moreover, the impregnation of F600‐900 and N.RGC30 with [Hmim][BF4] IL (a physical
solvent for CO2) appears to be not suitable (at least under the investigated experimental
conditions) for flue‐gas treatment, because the IL contribution to CO2 capture does not
counterbalancethereductionofthesubstrateadsorptionperformancesinducedbythepores
obstruction,alsoathighertemperatures.Ontheotherhand,theuseofmore‐chemicallyCO2
affine [Emim][Gly] IL can be potentially apt to ameliorate the parent carbon sorptive
performancestowardsCO2at80°C,evenifatthecurrentstageoftheresearchtheobtained
adsorptioncapacitiesarestilltoolowforatechnicalapplicabilityofthesematerialsinlarge
scaleCO2captureprocesses.
The main aspects derived from a deeper analysis of the effects of the impregnation
conditions for each IL on the activated carbons capture performances are discussed in the
following.
For [Hmim][BF4], at 30°C both the impregnation conditions determine a similar
percentagereductionoftherawmaterialsω %values(nearly23%)despiteaslightlyhigher
pore volume reduction for F600‐900 at C°=2.2×10‐2 M. Moreover, for both F600‐900 and
N.RGC30 functionalized with [Hmim][BF4] under more diluted impregnation conditions
differencesinadsorptioncapacitywithrespecttorawsubstratesreduceasthetemperature
risesandtheporescloggingeffectbecomesnegligibleat80°C.TheimpregnationofF600‐900
with[Hmim][BF4]atC°=2.2×10‐2Mdeterminesapracticallyconstantpercentagereductionof
ω %valueswithrespecttotherawmaterialasthetemperatureincreases,whileinthecase
of N.RGC30 impregnatedwith the same IL and adopting the same initial concentration the
poreblockingeffectontheadsorptionperformancesislessrelevantat80°C.Inthiscontext,
differences between F600‐900 and N.RGC30 impregnated at a higher [Hmim][BF4]
concentration could be related to the combined effects of a larger contribution of wider
microporesforN.RGC30andtemperatureonamorefavourableILdispersioninthesorbent
70
pores.ThismeansthatforN.RGC30,asthetemperatureincreases,abetterdistributionofthe
ILonthesubstrateporessurfacemakestheaccessofCO2moleculestotheadsorbentactive
siteseasierthaninthecaseofF600‐900.
For both F600‐900 and N.RGC30 impregnated with [Emim][Gly] at C°=5.6×10‐3 M
comparableamountsofILadsorbedproducesimilareffectsontheadsorptioncapacitywhen
the temperature increases. In particular, each functionalized material displays similar
adsorptioncapacitywithrespecttotherawsubstrateat50°C,whileat80°Ctheactivephase
is able to ameliorate the parent activated carbons capture performances thanks to a good
compromisebetweenILloadingandporeaccessibility(i.e.alowporevolumereduction).In
the case of higher [Emim][Gly] loading, N.RGC30 functionalized material displays similar
adsorptioncapacitywithrespecttotherawoneat80°C,whileF600‐900[Emim][Gly]10‐2M
isequivalent to therawmaterialat50°Candslightlyworseat80°C.Thispatterncannotbe
directlyjustifiedonthebasisofthedifferenttexturalpropertiesoftheinvestigatedactivated
carbons,butshouldbesomehowrelatedtoaverycomplexeffectof thetemperatureonthe
distribution of this IL into the sorbents pores (hardly explicable with the information
currentlyavailable).
As an additional remark, it should be considered that it is a really hard task trying to
discriminatethecontributiontoCO2capturedeterminedbyeachILwithrespect to theone
exertedbytheparentmaterial:abetterunderstandingoftheeffectofthetemperatureonthe
ILdistributiononthesubstrateporeswillbehelpfulinthisdirection.
Tothebestofourknowledge,similarfindingsconcerningtheeffectsoftheimpregnation
conditionsonCO2sorptivepropertiesofmicroporousactivatedcarbons functionalizedwith
ILshavenotyetbeenreportedintheliterature.Nevertheless,itisworthyofmentioningthe
workofPlazaetal.(2007)inwhichanactivatedcarbon(NoritCGPSuper)impregnatedwith
differentamine‐based compoundswasemployed forCO2 capture.Authorsobserved that at
room temperature the activephases arenot able to exert their peculiar captureproperties
becauseofaprevailingporeblockingeffect;athighertemperature,thisphenomenonisstill
prevailing but to a less extent because of the reduced contribution of physisorption of the
parentmaterial.Finally,theyevidencedanincreaseoftherawmaterialcaptureperformances
onlyfortheactivatedcarbonimpregnatedwithdiethylentriamine(withthehighestnitrogen
content)andtestedattemperaturesgreaterthan60°C.
71
Table5.11Comparisonamong %valuesandthemainparameters
derivedfromporosimetricandTGAanalysesforthesorbentsinvestigatedinthiswork
Sample
%[mmolg‐1] V0
[cm3g‐1]Vn
[cm3g‐1](V0‐raw‐V0‐impr)/V0‐raw
%mmolIL‐adsg‐1AC
(TGA)T=30°C T=50°C T=80°C
F600‐900 0.575 0.315 0.137 0.41 0.32
F600‐900[Hmim][BF4]10‐3M 0.437 0.264 0.139 0.39 n.a.† 4.9 0.015
F600‐900[Hmim][BF4]10‐2M 0.443 0.230 0.101 0.36 n.a.† 12.2 0.052
F600‐900[Emim][Gly]10‐3M 0.502 0.324 0.157 0.39 n.a.† 4.9 0.028
F600‐900[Emim][Gly]10‐2M 0.502 0.317 0.125 0.36 n.a.† 12.2 0.057
N.RGC30 0.499 0.276 0.139 0.50 0.32
N.RGC30[Hmim][BF4]10‐3M 0.391 0.246 0.141 0.47 n.a.† 6 0.013
N.RGC30[Hmim][BF4]10‐2M 0.386 0.240 0.122 0.46 n.a.† 8 0.051
N.RGC30[Emim][Gly]10‐3M 0.426 0.275 0.163 0.49 n.a.† 2 0.027
N.RGC30[Emim][Gly]10‐2M 0.381 0.238 0.137 0.46 n.a.† 8 0.057
†notavailable
72
Table 5.12 reports a comparison of the adsorbents dynamic performances at different
temperatures(fora15%CO2gasstream)intermsofanalysistogetherwiththesolidsmain
microstructuralparameters.Itisonceagainrecalledthatthedifferentsorbentdosesusedfor
adsorptiontests(15and13gforF600‐900andN.RGC30sorbentsseries,respectively)donot
allowtodefinekineticsdifferencesbetweenN.RGC30andF600‐900materialsonthebasisof
breakpoint times. Results underline thatmain differences can be observed at 30°C, where
generally higher values obtained for F600‐900 functionalized solids suggest slower
kinetics for these sorbents, while differences reduce at higher temperatures to become
practicallynegligibleat80°C.Onceagain,theoccurrenceofwidermicroporesandthegreater
contribution of mesopores for N.RGC30 adsorbents should determine faster CO2 diffusion
rates with respect to F600‐900 impregnated materials. Moreover, the generally lower
adsorptioncapacityobservedforN.RGC30impregnatedmaterialsat30°C(cf.Table5.11)also
contributes in determining faster kinetics. At higher temperatures, intraparticle diffusivity
increases (see also Section5.4.2) anddifferences in adsorption capacity reducemaking the
sorbentskineticallyequivalentinthecaptureprocessat80°C.
Table5.12Comparisonbetweenvaluesobtainedfromkineticadsorptiontestsat30,50and80°Candthemaintexturalparametersderivedforthesorbentsinvestigatedinthiswork; =15%byvol.
Sample
[s]
Microstructuralparameters
T=30°C T=50°C T=80°C V0[cm3g‐1]
Vn[cm3g‐1]
Vmeso[cm3g‐1]
F600‐900 22 10 5 0.41 0.32 0.17
F600‐900[Hmim][BF4]10‐3M 16 9 6 0.39 n.a.† 0.16
F600‐900[Hmim][BF4]10‐2M 17 9 5 0.36 n.a.† 0.16
F600‐900[Emim][Gly]10‐3M 17 10 6 0.39 n.a.† 0.16
F600‐900[Emim][Gly]10‐2M 17 10 6 0.36 n.a.† 0.16
N.RGC30 15 8 5 0.50 0.32 0.65
N.RGC30[Hmim][BF4]10‐3M 11 8 6 0.47 n.a.† 0.65
N.RGC30[Hmim][BF4]10‐2M 11 8 5 0.46 n.a.† 0.65
N.RGC30[Emim][Gly]10‐3M 13 8 5 0.49 n.a.† 0.65
N.RGC30[Emim][Gly]10‐2M 11 8 6 0.46 n.a.† 0.65
†notavailable
73
5.3 Adsorption/desorption cycles and regeneration experiments for
F600‐900raw
InthisSectionpreliminaryadsorption/desorptionandregenerationexperimentscarried
outonthesorbentwhichdisplayedthehighestadsorptioncapacityamongalltheinvestigated
experimentalconditions,namelyrawF600‐900testedat30°C(cf.Table5.11),areshown.
It is worthy to mention that exploratory CO2 adsorption/desorption tests over 3
consecutive cycles at 30°C have been performed on [Hmim][BF4] and [Emim][Gly]
functionalizedsorbents(notshownhereforthesakeofbrevity)andresultsshowedthatthe
processisreversibleforthesematerials.
InFigure5.20CO2equilibriumadsorptioncapacityofrawF600‐900isreportedover10
consecutiveadsorption/desorptioncyclesat30°Candfora15%CO2gasstream.
Cycle
0 1 2 3 4 5 6 7 8 9 10 11
eq
, mm
ol g
-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Figure5.20EquilibriumadsorptioncapacityofrawF600‐900obtainedover10adsorption/desorption
cyclesat30°C; =15%byvol.
Resultsindicatethatthevalueofeqispracticallyconstantuponthenumberofcyclesand
consequently, F600‐900 can be completely regenerated (reversible adsorption). This
behaviourhasbeencommonlyobservedintheliteratureforactivatedcarbonsandisascribed
to the establishment of weak interactions between CO2molecules and the sorbent surface
active sites (physisorption) (Choi et al., 2009; Sayari et al., 2011). In comparison, sorbents
suchasthosecalciumoxide‐basedsufferfromarapiddegradationofCO2capturecapability
74
during multiple carbonation/calcination cycles caused by pore blocking and adsorbent
sintering, thus requiring a continuous make‐up of fresh sorbent (Abanades and Alvarez,
2003).
Figure5.21reportsthetime‐dependentCO2outletconcentrationprofilesobtainedduring
theregenerationofF600‐900activatedcarbon(previouslysaturatedwitha15%CO2gaseous
stream at 30°C) carried out with desorption temperatures of (a) 60 and (b) 100°C and
adoptingthreedifferentN2purgeflowratelevels(6.95×10‐3Ls‐1,1.11×10‐2Ls‐1,1.39×10‐2L
s‐1,evaluatedatT=20°CandP=1bar).
t, s
0 20 40 60 80 100 120 140 160
CC
O2ou
t (t),
% v
ol.
0
10
20
30
40QN2
des=1.39x10-2 L s
-1
QN2
des=1.11x10-2 L s
-1
QN2
des=6.95x10-3 L s
-1
(a)
75
t, s
0 20 40 60 80
CC
O2ou
t (t),
% v
ol.
0
10
20
30
40
50
60QN2
des=1.39x10-2 L s
-1
QN2
des=1.11x10-2 L s
-1
QN2
des=6.95x10-3 L s
-1
(b)
Figure5.21CO2concentrationprofilesobtainedfromregenerationexperimentsofrawF600‐900asafunctionofN2purgeflowrateat(a)60and(b)100°C(adsorptionstep: =15%byvol.,T=30°C)
Fromaqualitativepointofview,experimentalresultsevidencethatforbothregeneration
temperaturesalltheconcentrationprofilesreachamaximuminCO2outletconcentrationfor
relatively low desorption times, indicating that most of the adsorbed pollutant is quickly
removed from the solid. The regeneration curve becomes narrower as the N2 flow rate
increases, thus testifying a faster desorption process associated to an increase in the gas
velocitythroughthepacked‐bed(higherstrippingrate).However,theconcentrationprofiles
show quite long tails indicating that residual CO2 desorption takes place slowly when the
driving force decreases. Finally, for each investigated purge flow rate, an increment in the
desorptiontemperaturedeterminesapositiveeffectontheregenerationkinetic,whichcanbe
ascribed to a decrease of CO2 adsorption capacity (thermodynamic factor) coupled to an
increaseinthepollutantintraparticlediffusivity(kineticfactor).
Themainquantitativeparametersobtainedfrompost‐processingofregenerationprofiles
at different N2 purge flow rates ( des2NQ ) and desorption temperatures ( des
C60T and desC100T ),
according to the procedure described in Section 4.4.2, are listed in Table 5.13. It is here
recalled thati
2COC ,i
2COV and i2NV represent themean CO2 concentration in the desorbing
flow,thetotalCO2volumedesorbedandthepurgegasvolumefedtothecolumnuptotimeti,
respectively(computedforrecoverypercentagesi=50,70,80and90%).
76
Table5.13MainparametersobtainedfromregenerationexperimentsofF600‐900rawactivatedcarbon
des2NQ
[Ls‐1]
eq
[mmolg‐1]
des
[mmolg‐1]
t50
[s]
t70
[s]
t80
[s]
t90
[s]
502NV
[L]
702NV
[L]
802NV
[L]
902NV
[L]
50
2COC
[%vol.]
70
2COC
[%vol.]
80
2COC
[%vol.]
90
2COC
[%vol.]
desC60T
6.95x10‐3 0.559 0.569 33 50 62 80 0.229 0.348 0.431 0.556 29.5 28.6 27.0 24.5
1.11x10‐2 0.565 0.561 25 37 46 61 0.278 0.411 0.511 0.677 25.4 24.4 22.8 19.91.39x10‐2 0.566 0.554 17 25 32 44 0.236 0.348 0.445 0.612 30.4 28.6 26.8 23.1
desC100T
6.95x10‐3 0.567 0.557 15 22 28 37 0.104 0.153 0.195 0.257 47.0 46.4 43.7 401.11x10‐2 0.562 0.559 9 13 16 21 0.100 0.144 0.178 0.233 49.4 48.2 46.2 42.31.39x10‐2 0.560 0.555 7 11 13 17 0.097 0.153 0.181 0.236 47.7 47.0 45.7 42.2
77
It isworthy toobserve that thecomparisonbetweeneqanddesvalues,obtained from
the integration of the adsorption and desorption kinetic profiles respectively, allows the
verificationoftheCO2massbalanceforeachtest.Resultsconfirmthepositiveeffectsonthe
desorption kinetics induced by an increase of both des2NQ and desorption temperature. As a
matterof fact, ata fixeddesorption temperature, the timerequired toobtainadefinedCO2
recovery percentagemonotonically decreaseswith the N2 purge flow rate: for example, at
bothtemperatures,t50andt90approximatelydoublewhen des2NQ decreasesfrom1.39×10‐2Ls‐1
to 6.95×10‐3 L s‐1. Moreover, a similar trend for ti is observed when the desorption
temperatureincreases:for des2NQ =1.39×10‐2Ls‐1,t50andt90at des
C100T arenearly0.4‐timesthe
correspondingvaluesdeterminedat desC60T .Theresultsobtainedintermsof
i
2COC revealeven
more interesting features. For each des2NQ and for each temperature, when the desired CO2
recoverylevelisincreasedthemeanCO2concentrationinthedesorbingstreamdecreases.In
fact, as the recovery percentage increases the time required for desorption is higher, the
desorptionratedecreaseswithtimeduetoaloweringinthedrivingforceand,consequently,
agreaterpurgevolumeisrequiredtoremoveresidualadsorbedCO2,determiningadilution
effect.Differently,foreachtemperatureandforeachfixedCO2recoverylevel,i
2COC doesnot
substantiallyvarywhencomparingthevaluesobtainedat1.39×10‐2Ls‐1and6.95×10‐3Ls‐1.
In fact, from eq. (4.5) (Section 4.4.2), at fixed regeneration leveli
2COV is constant but i2NV
depends on both desorption time and purge flow rate ( ides
2N tQ , in eq. (4.5)): this product is
comparable at the highest and lowest purge flow rate investigated determining equivalent
concentrationlevels.FromdatareportedinTable5.13,itcanbealsohighlightedthatfor desC60T
arecoverylevelof90%adoptingthelowestN2flowrateallowstoobtainadesorbinggaswith
i
2COC ≈25%, while the mean CO2 concentration only slightly increases for a 50% solid
regeneration(i
2COC ≈30%),but thiswouldeventuallyproduceasignificantreduction in the
sorbentutilizationtime,ifasubsequentadsorptioncyclehastobeperformed.Aregeneration
temperature of 100°C appears to be the best operating condition to obtain higheri
2COC
values: amore concentrated gas is obtained by recovering 90% of the adsorbed CO2 with
respectto desC60T atthesameregenerationlevelandfor des
2NQ =6.95×10‐3Ls‐1andwithahalved
desorption time (t90=80 and 37 s for desC60T and
desC100T , respectively). Finally, regeneration
78
levelsof70%and80%,obtainedat100°Candfor des2NQ =1.11×10‐2Ls‐1,canbe identifiedas
theoptimalsolutionsamongthoseinvestigated,asacompromisebetweentheamountofCO2
recovered, its concentration in the desorbed gas (46‐48% by vol.) and time required to
performthatrecovery.
79
5.4 Adsorptionthermodynamicsandkineticsmodelling
In this Section, modelling analysis (using equations described in Chapter 3) of both
adsorptionisothermsandbreakthroughcurves,experimentallydeterminedforallthesolids
investigatedinthiswork,ispresented.
5.4.1 Thermodynamicaspects
Tables 5.14 and 5.15 list the main parameters obtained from the application of
Henry/Langmuir/Freundlich/Dubinin‐Radushkevich models to the adsorption isotherms
obtainedat30,50and80°CforF600‐900andN.RGC30bothrawandILs‐functionalized.
It ishighlighted that: i) for theLangmuirmodelasimultaneous fittingof theadsorption
isotherms at different temperatures was performed by imposing a unique max value
(temperature‐independent) according to the recommendation provided inRuthven (1984);
ii) for Dubinin‐Radushkevich model, the characteristic energy E was obtained by
simultaneously fitting equilibrium data at different temperatures according to eq. (3.5) (cf.
Chapter3),andforeachsolidthemicroporevolumedeterminedfromN2porosimetricdataat
‐196°C (cf. Chapter 5) was imposed as a constant parameter. Moreover for DRmodel, the
molar volume of the liquid adsorbate and the pseudo‐vapor pressure were computed
accordingtotheexpressionsreportedinDo(1998)andSahaetal.(2011).
Asageneralconsideration,itcanbeobservedthattheHenrymodelprovidesthepoorest
qualityofdata fitting forall sorbentsas testifiedby the lowestvaluesof thedetermination
coefficient R2 above all at 30°C; at 80°C the model provides a better fitting of adsorption
isothermsduetoamore lineartrendofadsorptiondata(cf.alsoFigure5.22).Similarly, the
DRmodelisnotadequatetointerpretsatisfactorilyadsorptionisothermsaboveallathigher
temperaturesandthisispossiblyrelatedtotheabsenceofadsorptiondataathighpressures
for a proper fitting of the characteristic curve (eq. (3.5)). Freundlich and Langmuirmodels
determine the best fitting of equilibrium data for all the investigated materials and
experimentalconditionsastestifiedbythehighestR2values(practicallyunitaryforboth).In
particular, for each sorbent KL and KF values decrease with temperature, due to the
exothermicnatureof the adsorptionprocess (Ruthven, 1984;Do, 1998). It is interesting to
observethatinallthecases,thevaluesoftheFreundlichheterogeneityparameter1/ndonot
differ toomuch fromunity,whichclearly indicates that thesorbent surfacesarepractically
energetically homogeneous in the CO2 capture process, also in the case of functionalized
sorbents(videinfra)(Do,1998).
80
Finally, in thecaseofFreundlichmodel it ispossible toobserveageneralbetteragreement
betweentherankingderived intermsof theaffinityparameter(KF)andthoseobservedfor
the adsorption capacities of the investigated sorbentswith respect to a comparison on the
basisoftheLangmuirconstantKL.Forinstance,whencomparingthevaluesofthisparameter
fortherawactivatedcarbons,weobtainKF=2.45and2.03at30°Cand0.80and0.81at80°C
for F600‐900 and N.RGC30 respectively, which agrees with the general trend of the
adsorption isotherms with temperature described in Section 5.2.1 (higher CO2 capture
capacitiesforF600‐900at30°Cwhileequivalentadsorptionperformancesat80°C).
As an example, Figure 5.22 reports the comparison between experimental adsorption
isothermsandthermodynamicmodelspredictionsforF600‐900rawat(a)30,(b)50and(c)
80°C.
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.0
0.2
0.4
0.6
0.8
1.0
exp.HenryLangmuirFreundlichDubinin-Radushkevich
(a)
81
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
exp.HenryLangmuirFreundlichDubinin-Radushkevich
(b)
Peq, bar
0.0 0.1 0.2 0.3 0.4
eqm
mol
g-1
0.00
0.05
0.10
0.15
0.20
0.25
0.30
exp.HenryLangmuirFreundlichDubinin-Radushkevich
(c)
Figure5.22Comparisonbetweenexperimentaladsorptionisotherms(symbols)andHenry,Langmuir,FreundlichandDubinin‐Radushkevichmodels(lines)forF600‐900rawat(a)30,(b)50and(c)80°C
82
Table5.14MainparametersofHenry,Langmuir,FreundlichandDubinin‐RadushkevichmodelsforCO2adsorptionontoF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILsat30,50and80°C
F600‐900 F600‐900[Hmim][BF4]10‐3M
F600‐900[Hmim][BF4]10‐2M
F600‐900[Emim][Gly]10‐3M
F600‐900[Emim][Gly]10‐2M
Henry
KH[mmolg‐1bar‐1]
T=30°C 3.18 2.38 2.35 2.95 2.87T=50°C 1.76 1.63 1.38 1.91 1.89T=80°C 0.88 0.92 0.66 1.01 0.83
R2T=30°C 0.922 0.969 0.970 0.966 0.951T=50°C 0.965 0.989 0.982 0.977 0.983T=80°C 0.990 0.994 0.994 0.999 0.996
Langmuir
KL[bar‐1]
T=30°C 2.07 1.31 1.8 1.28 1.49T=50°C 0.91 0.79 0.87 0.75 0.84T=80°C 0.40 0.41 0.37 0.34 0.32
max[mmolg‐1]
2.51 2.54 2.01 3.19 2.81
R2T=30°C 0.998 0.998 0.999 0.998 0.999T=50°C 0.997 0.999 0.999 0.999 0.999T=80°C 0.998 0.997 0.999 0.999 0.998
Freundlich
KF[mmolg‐1bar‐1/n]
T=30°C 2.45 1.95 1.85 2.45 2.32T=50°C 1.46 1.46 1.20 1.62 1.65T=80°C 0.80 0.88 0.61 1.00 0.78
1/nT=30°C 0.76 0.82 0.78 0.83 0.81T=50°C 0.83 0.90 0.88 0.86 0.88T=80°C 0.91 0.95 0.93 0.99 0.95
R2T=30°C 0.999 0.999 0.998 0.996 0.999T=50°C 0.999 0.999 0.999 0.999 0.999T=80°C 0.999 0.997 0.999 0.997 0.998
DubininRadushkevich
max[mmolg‐1]
T=30°C 8.85 8.42 7.77 8.42 7.77T=50°C 8.42 8.01 7.39 8.01 7.39T=80°C 7.81 7.43 6.86 7.43 6.86
E[kJmol‐1]
9.36 9.05 9.09 9.36 9.41
R2T=30°C 0.978 0.976 0.972 0.987 0.990T=50°C 0.975 0.967 0.977 0.975 0.980T=80°C 0.975 0.686 0.898 0.895 0.945
83
Table5.15MainparametersofHenry,Langmuir,FreundlichandDubinin‐RadushkevichmodelsforCO2adsorptionontoN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILsat30,50and80°C
N.RGC30N.RGC30
[Hmim][BF4]10‐3MN.RGC30
[Hmim][BF4]10‐2MN.RGC30
[Emim][Gly]10‐3MN.RGC30
[Emim][Gly]10‐2M
Henry
KH[mmolg‐1bar‐1]
T=30°C 2.64 2.20 2.12 2.48 2.18T=50°C 1.68 1.56 1.48 1.78 1.57T=80°C 0.88 0.89 0.75 1.05 0.95
R2T=30°C 0.921 0.964 0.944 0.967 0.970T=50°C 0.981 0.995 0.991 0.996 0.998T=80°C 0.990 0.997 0.992 0.998 0.996
Langmuir
KL[bar‐1]
T=30°C 2.04 1.20 1.60 1.06 0.89T=50°C 1.04 0.74 0.94 0.67 0.57T=80°C 0.48 0.39 0.43 0.37 0.32
max[mmolg‐1]
2.10 2.52 1.98 3.14 3.15
R2T=30°C 0.999 0.999 0.999 0.999 0.997T=50°C 0.999 0.999 0.999 0.997 0.995T=80°C 0.998 0.999 0.999 0.999 0.992
Freundlich
KF[mmolg‐1bar‐1/n]
T=30°C 2.03 1.82 1.68 2.07 1.83T=50°C 1.45 1.44 1.33 1.70 1.53T=80°C 0.81 0.84 0.68 1.01 1.03
1/nT=30°C 0.76 0.83 0.79 0.83 0.84T=50°C 0.87 0.93 0.91 0.95 0.98T=80°C 0.92 0.95 0.91 0.96 0.98
R2T=30°C 0.998 0.999 0.999 0.999 0.998T=50°C 0.999 0.999 0.999 0.999 0.998T=80°C 0.999 0.999 0.999 0.999 0.992
DubininRadushkevich
max[mmolg‐1]
T=30°C 10.78 10.15 9.93 10.58 9.93T=50°C 10.27 9.65 9.45 10.06 9.45T=80°C 9.53 8.96 8.76 9.34 8.76
E[kJmol‐1] 8.84 8.67 8.64 8.80 8.67
R2T=30°C 0.931 0.917 0.910 0.925 0.918T=50°C 0.928 0.942 0.933 0.955 0.955T=80°C 0.730 0.501 0.616 0.508 0.534
84
The energetic aspects of the adsorption phenomena for the investigated systems were
evaluatedbycomputingtheisostericheatofadsorptionqstasafunctionofthespecificloading
bymeans of theClausius‐Clapeyron equation applied to experimental adsorptiondata at
differenttemperatures(cf.eq.(3.8)inSection3.1).Theqstvs.trendsarereportedinFigure
5.23forboth(a)F600‐900basedand(b)N.RGC30materials.
, mmol g-1
0.00 0.05 0.10 0.15 0.20 0.25 0.30
q st
kJ
mol
-1
0
5
10
15
20
25
30
35
F600-900
F600-900 [Hmim][BF4] 10-3 M
F600-900 [Hmim][BF4] 10-2 M
F600-900 [Emim][Gly] 10-3 M
F600-900 [Emim][Gly] 10-2 M
(a)
, mmol g-1
0.00 0.05 0.10 0.15 0.20 0.25 0.30
qstk
J m
ol-1
0
5
10
15
20
25
30
35
N.RGC30
N.RGC30 [Hmim][BF4] 10-3 M
N.RGC30 [Hmim][BF4] 10-2 M
N.RGC30 [Emim][Gly] 10-3 M
N.RGC30 [Emim][Gly] 10-2 M
(b)
Figure5.23Isostericheatofadsorptionasafunctionofthespecificloadingfor(a)F600‐900and(b)
N.RGC30adsorbentsbothrawandfunctionalizedwith[Hmim][BF4]/[Emim][Gly]ILs
85
Asitcanbeobserved,theisostericheatofadsorptionispracticallyconstantwithloading
foralltheinvestigatedadsorbents,indicatingthatthesurfacesareenergeticallyhomogeneous
towardsCO2capture(Chakrabortyetal.,2006), inagreementwiththeobservationsderived
from thermodynamic modelling analysis. For impregnated materials, the low amounts of
activephasechargeddonotappreciablymodifytheoverallsurfaceenergetichomogeneityof
theparentsubstratethusdeterminingaconstantqstvaluewith loading. Inthiscontext, it is
hardatthecurrentstageoftheresearchtounderstandtheinterplayofthecontributionsof
the substrate active sites and the ones belonging to the ionic liquid to CO2 capture, and
consequently to explain the trends observed for the functionalized materials in terms of
interactionenergies.On theotherhand, themeanvaluesof the isostericheatofadsorption
obtainedfortherawactivatedcarbonsare28.8and25.5kJmol‐1forF600‐900andN.RGC30
respectively:thisconfirmsthatthepresenceofanarrowermicroporesizedistributionwitha
prevailingcontributionofverysmallporediameters(<10Å)observedforF600‐900produces
stronger interactionswithCO2moleculeswith respect toN.RGC30andconsequentlyhigher
adsorptionperformancesaboveallatlowertemperatures(cf.Section5.2.1).
86
5.4.2 Kineticaspects
The main kinetic parameters determined from the modelling analysis of breakthrough
curves (according to the theoretical frameworks described in Section 3.2) obtained at
differentoperatingtemperaturesandfora15%CO2gasstreamarereported inTables5.16
and5.17forF600‐900andN.RGC30sorbentsseries,respectively.Itishererecalledthatmass
and momentum balance equations were numerically solved with Aspen AdsimTM software
adoptingalineardrivingforceapproximation(LDF)forthemasstransferrate;moreoverthe
onlyfittingparameterofthemodelwasthemicroporediffusivityDmicro,whereastheexternal
and macropore (Knudsen) mass transfer coefficients were calculated from gas/solids
properties (cf. Section 3.2). Figure 5.24 reports, as an example, the comparison of the
experimental and theoretical breakthrough curves obtained for raw F600‐900 for a typical
flue‐gascompositionandat(a)30,(b)50and(c)80°C.
Asageneralconsideration,itishighlightedthattheFreundlichthermodynamicmodelwas
adopted as equilibrium isotherm in the rate of adsorption equation (3.13) (cf. Section 3.2)
becauseitsuppliedslightlybetternumericalsolutionswithrespecttotheLangmuirisotherm.
Noteworthy,thecomputedfixed‐bedPécletnumberwashigherthan100inallcases,thusit
was possible to consider a plug‐flow for all the investigated systems (Inglezakis and
Poulopoulos, 2006); in addition, the pressure drops across the fixed bed, calculated from
Ergun’sequation(3.14),werepracticallynegligible(orderofmagnitude10‐3bar).
Thegeneralfeaturesinferablefromacomparisonamongfilm,macroporeandmicropore
diffusionresistancesaresummarizedinthefollowing.
Microporediffusionmechanismrepresentstherate‐determiningstepoftheadsorption
process in almost all the investigated systems. As the temperature increases,
differences betweenmicropore andmacropore diffusion resistances tend to reduce,
andinanycasetheCO2transportthroughtheexternalfluidfilmisveryfast(negligible
film‐diffusion resistance). Dmicro is generally 2 or 3 orders of magnitude lower than
Dmacroat30°C,andwhiletheformerhasastrongdependenceontemperaturethelatter
onlyslightlyincreasesfrom30to80°C.
When comparing each raw activated carbon with the corresponding functionalized
materials, it can be observed that micropore diffusion resistance is higher for the
formeranddifferencesdiminishwithtemperaturetobecomepracticallynegligibleat
80°C.Thisbehaviourshouldbeonceagainimputedtothealreadydescribedtrendsin
adsorptioncapacityandmicroporediffusivity togetherwith thepossible influenceof
87
the temperature on the IL distribution inside the sorbent pores in the case of
impregnated adsorbents (cf. also Sections 5.2.2 and 5.2.3). Finally, for N.RGC30
sorbents,thediffusionresistancesaregenerally lowerthanthoseobservedforF600‐
900 materials and differences reduce with temperature: this testifies again the
important role exerted by both the occurrence ofwidermicropores and the greater
contributionofmesoporesforN.RGC30adsorbentswithrespecttoF600‐900ones in
determining faster kinetics above all at lower temperatures. The observations here
providedconfirmtheresultsdiscussedintermsofanalysisinthepreviousSections.
88
Table5.16MainkineticparametersderivedfrommathematicalmodellingofbreakthroughcurvesforCO2adsorptionontoF600‐900rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILsat30,50and80°C; =15%byvol.
F600‐900
F600‐900[Hmim][BF4]10‐3M
F600‐900[Hmim][BF4]10‐2M
F600‐900[Emim][Gly]10‐3M
F600‐900[Emim][Gly]10‐2M
Filmdiffusion
[s]
T=30°C 1.37x10‐3 1.37x10‐3 1.37x10‐3 1.37x10‐3 1.37x10‐3
T=50°C 1.23x10‐3 1.23x10‐3 1.23x10‐3 1.23x10‐3 1.23x10‐3
T=80°C 1.08x10‐3 1.08x10‐3 1.08x10‐3 1.08x10‐3 1.08x10‐3
Macroporediffusion
[s]
T=30°C 0.15 0.16 0.16 0.16 0.16
T=50°C 0.15 0.15 0.15 0.15 0.15
T=80°C 0.14 0.14 0.15 0.14 0.15
Dmacro
[m2s‐1]
T=30°C 1.13x10‐7 1.13x10‐7 1.13x10‐7 1.13x10‐7 1.13x10‐7
T=50°C 1.17x10‐7 1.17x10‐7 1.17x10‐7 1.17x10‐7 1.17x10‐7
T=80°C 1.22x10‐7 1.22x10‐7 1.22x10‐7 1.22x10‐7 1.22x10‐7
Microporediffusion
[s]
T=30°C 3.82 1.10 1.21 1.22 1.72
T=50°C 1.08 0.24 0.60 1.20 0.65
T=80°C 0.34 0.22 0.31 0.05 0.40
Dmicro
[m2s‐1]
T=30°C 1.60x10‐10 8.52x10‐10 7.32x10‐10 6.19x10‐10 4.24x10‐10
T=50°C 1.15x10‐9 6.26x10‐9 2.75x10‐9 9.76x10‐10 1.79x10‐9
T=80°C 7.13x10‐9 1.15x10‐8 1.12x10‐8 5.26x10‐8 7.06x10‐9
89
Table5.17MainkineticparametersderivedfrommathematicalmodellingofbreakthroughcurvesforCO2adsorptionontoN.RGC30rawandimpregnatedwith[Hmim][BF4]/[Emim][Gly]ILsat30,50and80°C; =15%byvol.
N.RGC30 N.RGC30[Hmim][BF4]10‐3M
N.RGC30[Hmim][BF4]10‐2M
N.RGC30[Emim][Gly]10‐3M
N.RGC30[Emim][Gly]10‐2M
Filmdiffusion
[s]
T=30°C 1.60x10‐3 1.60x10‐3 1.60x10‐3 1.60x10‐3 1.60x10‐3
T=50°C 1.44x10‐3 1.44x10‐3 1.44x10‐3 1.44x10‐3 1.44x10‐3
T=80°C 1.26x10‐3 1.26x10‐3 1.26x10‐3 1.26x10‐3 1.26x10‐3
Macroporediffusion
[s]
T=30°C 0.09 0.10 0.10 0.09 0.10
T=50°C 0.09 0.10 0.10 0.09 0.10
T=80°C 0.09 0.10 0.10 0.10 0.10
Dmacro
[m2s‐1]
T=30°C 2.10x10‐7 2.10x10‐7 2.10x10‐7 2.10x10‐7 2.10x10‐7
T=50°C 2.11x10‐7 2.11x10‐7 2.11x10‐7 2.11x10‐7 2.11x10‐7
T=80°C 2.12x10‐7 2.12x10‐7 2.12x10‐7 2.12x10‐7 2.12x10‐7
Microporediffusion
[s]
T=30°C 1.66 0.54 0.51 0.63 0.61
T=50°C 0.30 0.15 0.15 0.14 0.18
T=80°C 0.15 0.15 0.08 0.07 0.08
Dmicro
[m2s‐1]
T=30°C 5.89x10‐10 9.34x10‐10 9.58x10‐10 6.95x10‐10 8.33x10‐10
T=50°C 2.41x10‐9 5.89x10‐9 5.57x10‐9 5.23x10‐9 4.46x10‐9
T=80°C 9.57x10‐9 2.15x10‐8 2.02x10‐8 1.75x10‐8 2.03x10‐8
90
t, s0 50 100 150 200 250 300
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
exp.LDF model
(a)
t, s0 20 40 60 80 100 120 140 160
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
exp.LDF model
(b)
91
t, s0 10 20 30 40 50 60 70 80
QC
O2ou
t (t)/
QC
O2in
, -
0.0
0.2
0.4
0.6
0.8
1.0
exp.LDF model
(c)
Figure5.24Comparisonbetweenexperimental(symbols)andtheoretical(lines)breakthroughcurves
obtainedforF600‐900rawat(a)30,(b)50and(c)80°C;CinletCO2=15%byvol.
92
CHAPTER6
CONCLUSIONSANDFUTUREPERSPECTIVES
Theneedofdevelopinghighperformanceandcost‐effectivepost‐combustionpurification
systems for CO2 capture has recently stimulated the research of new adsorbentsmaterials
with tailored microstructural and chemical properties. Contextually, the use of supported
ionic liquid phasematerials (SILP) is a very attractive but limitedly explored investigation
area. In this PhD project the effect of confining ionic liquids (ILs) into activated carbons
characterizedbydifferentporosimetricstructuresontheirCO2captureperformancesunder
typicalflue‐gasconditionshasbeeninvestigated.Theobtainedresultshaveprovidedadeeper
understandinginthisfieldwithrespecttotheavailablescientificliterature.
CO2 adsorption tests havebeen carried inmodel flue‐gas streamsonto two commercial
activated carbons, namely Filtrasorb 400 and Nuchar RGC30, both raw and functionalized
with either 1‐hexyl‐3‐methylimidazolium tetrafluoroborate [Hmim][BF4] or 1‐ethyl‐3‐
methylimidazolium glycine [Emim][Gly] ILs adopting different impregnation conditions
(5.6×10‐3and2.2×10‐2M).
Resultsobtainedforrawactivatedcarbonshaveconfirmed,inagreementwithpreviously
literature findings, that the presence of a narrower micropore size distribution with a
prevailingcontributionofverysmallporediameters(<10Å)observedforFiltrasorb400isa
keyfactorindetermininghigherCO2capturecapacitiesaboveallatlowtemperature(30°C);
ontheotherhand,thesorbentsareequivalentinthepurificationprocessat80°Cbecauseof
thereducedcontributionofphysisorptionathighertemperatures,asexpectedinexothermal
processes.Theseexperimentalevidenceshavebeenalsocorroboratedbythehighervalueof
the isosteric heat derived for Filtrasorb400 solid, testifying stronger interactionswithCO2
moleculeswithrespecttoNucharRGC30activatedcarbon.
Thermodynamic adsorption results onto [Hmim][BF4]‐functionalized sorbents suggest
that the impregnation of micro and micro‐mesoporous activated carbons with this IL (a
physical solvent for CO2) is not suitable for CO2 removal from flue‐gas (at least under the
investigatedimpregnationconditions),becausetheactivephasecontributioninthepollutant
captureisnotabletobalancethereductionofthesubstrateadsorptionperformancesinduced
by the pores obstruction, also at higher temperatures. Conversely, the functionalization of
both activated carbonswith amore CO2 chemically‐affine IL (i.e. [Emim][Gly]) undermore
diluted impregnation condition can ameliorate the parent carbons CO2 adsorption
93
performances at 80°C and for a typical 15% CO2 flue‐gas stream (while pores blocking is
dominant at 30°C) thanks to a good compromisebetween IL loading andpore accessibility
(low pore volume reduction). Nevertheless, the improvement in CO2 adsorption capacity
determined by [Emim][Gly] is still not adequate for the potential application of these
materialsinlarge‐scaleflue‐gaspurificationsystems.
Dynamic adsorption results on the investigated sorbents highlighted the important role
playedbybothagreatercontributionofmesoporesandthepresenceofwidermicroporesfor
Nuchar RGC30‐based materials in determining faster capture kinetics with respect to
Filtrasorb400sorbents, inparticularat low temperature.As the temperature increases the
reduced differences in adsorption capacity coupled with the increase in intraparticle
diffusivitymakethetwoclassesofsorbentskineticallyequivalentinthepurificationprocess.
Furthermore, modelling analysis of breakthrough curves allowed identifying micropore
diffusionastherate‐determiningstepofCO2adsorption,foralmostalltheanalysedsystems.
Preliminary regeneration studies on raw Filtrasorb 400, which displayed the highest
capture performances among all the adsorbents investigated, showed a complete
regenerability of this sorbent under multiple adsorption/desorption cycles. Moreover,
desorptionexperimentscarriedoutonFiltrasorb400(afterasolidsaturationwitha15%CO2
gasstreamat30°C)atdifferenttemperatures(60°Cand100°C)andN2flowrates(6.95×10‐3
L s‐1, 1.11×10‐2 L s‐1, 1.39×10‐2 L s‐1) evidenced that regeneration levels of 70 and 80%
obtained at 100°C and adopting a 1.11×10‐2 L s‐1 purge flow rate can be considered as the
optimal solutions among those investigated, being a fair compromise between CO2
concentrationinthedesorbingflow(46‐48%)andtimenecessarytoperformtherecovery.
As a general consideration, the results obtained in thiswork encourage future research
efforts in the field of porous activated carbons functionalizationwith amine‐based ILs as a
potentialroutetoimprovetheirCO2captureperformances,inparticularathightemperatures
atwhichtheparentmaterialcontributionisquitelowtoallowacost‐effectivetreatmentofa
real flue‐gas. In this context, the choice of activated carbons characterized by largermean
pore diameters (i.e.mainlymesoporous) could be apt to favour a higher dispersion of the
ionic liquid over the substrate surface avoiding the undesired pore clogging effect. The
synergistic collaborationwith research groups belonging to different fields of investigation
(ChemicalEngineering, InorganicandOrganicChemistry)couldbean importantstrategy to
fosterthedevelopmentofhighlyCO2‐affineSILPmaterialsandacceleratetheirapplicabilityin
post‐combustion systems, aiming at the following goals: i) synthesis of amine‐based ILs
characterized by reduced molecular sizes to minimize potential pore blocking; ii)
94
development of new functionalization protocols, e.g. covalent tethering of the IL to the
support surface which could be an interesting option for both minimizing the amount of
active phase used, with consequent economic benefits, and avoiding the continuous
redistributionoftheliquidphaseinsidethesorbentporesinducedbytemperaturevariations;
iii)adsorptiontestsinmulticomponentsystemsincludingthepresenceofNOx,SO2andwater
vapour; iv) dedicated tests using different reactor configurations (fluidized‐bed, circulating
fluidized beds etc.); v) regeneration studies aimed atmaximizing CO2 concentration in the
desorbing flow (for subsequent storage of the pollutant) while minimizing energy
requirements.
95
Bibliography
Abanades J.C. and Alvarez D., Conversion limits in the reaction of CO2with lime. Energy&Fuels17(2003)308‐315.
Abanades J.C., Rubin E.S. and Anthony E.J., Sorbent cost and performance in CO2 capturesystems.Industrial&EngineeringChemistryResearch43(2004)3462‐3466.
AnJ.andRosiN.,TuningMOFCO2adsorptionpropertiesviacationexchange.JournaloftheAmericanChemicalSociety132(2010)5578‐5579.
ArouaM.K.,DaudW.M.A.W.,YinC.Y.andAdinataD.,Adsorptioncapacitiesofcarbondioxide,oxygen,nitrogenandmethaneoncarbonmolecularbasketderived frompolyethyleneimineimpregnation on microporous palm shell activated carbon. Separation and PurificationTechnology62(2008)609‐613.
BalsamoM.,DiNataleF.,ErtoA.,LanciaA.,MontagnaroF.andSantoroL.,Arsenateremovalfromsyntheticwastewaterbyadsorptionontoflyash.Desalination263(2010)58–63.
BalsamoM.,DiNataleF.,ErtoA.,LanciaA.,MontagnaroF.andSantoroL.,Gasificationofcoalcombustionashforitsreuseasadsorbent.Fuel106(2013)147‐151.
Bara J.E., Gabriel C.J., Hatakeyama E.S., Carlisle T.K., Lessmann S., Noble R.D. and Gin D.L.,Improving CO2 selectivity in polymerized room‐temperature ionic liquid gas separationmembranes through incorporation of polar substituents. Journal ofMembrane Science 321(2008)3‐7.
BatesE.D.,MaytonR.D.,NtaiI.andDavisJr. J.H.,CO2capturebyaTask‐SpecificIonicLiquid.JournaloftheAmericanChemicalSociety124(2002)926‐927.
BelmabkhoutY.andSayariA.,AdsorptionofCO2fromdrygasesonMCM‐41silicaatambienttemperature and high pressure. 2:Adsorption of CO2/N2, CO2/CH4 and CO2/H2 binarymixtures.ChemicalEngineeringScience64(2009)3729‐3735.
BerlierK.andFrèreM.,AdsorptionofCO2onactivatedcarbon:simultaneousdeterminationofintegralheatandisothermofadsorption. JournalofChemical&EngineeringData41(1996)1144‐1148.
BlameyJ.,AnthonyE.J.,WangJ.andFennelP.S.,ThecalciumloopingcycleforlargescaleCO2capture.ProgressinEnergyandCombustionScience36(2010)260‐279.
BoschettiA.,MontagnaroF.,RienzoC.andSantoroL.,Apreliminaryinvestigationontheuseoforganicionicliquidsasgreensolventsforacylationandoxidationreations.JournalofCleanerProduction15(2007)1797‐1805.
BourbigouH.O.,Magna L. andMorvanD., Ionic liquids and catalysis: Recent progress fromknowledgetoapplications.AppliedCatalysisA:General373(2010)1‐56.
96
Brúder P. and Svendsen H.F., Capacity and kinetics of solvents for post‐combustion CO2capture.EnergyProcedia23(2012)45‐54.
Chakraborty A., Saha B.B., Koyama S. and Ng K.C., On the thermodynamicmodeling of theisosteric heat of adsorption and comparisonwith experiments. Applied Physics Letters 89(2006)171901.
Chang F.Y., Chao K.J., Cheng H.H. and Tan C.S., Adsorption of CO2 onto amine‐graftedmesoporoussilicas.SeparationandPurificationTechnology70(2009)87‐95.
ChewT.L.,AhmadA.L.andBathiaS.,Orderedmesoporoussilica (OMS)asanadsorbentandmembraneforseparationofcarbondioxide(CO2).AdvancesinColloidandInterfaceScience153(2010)43‐57.
ChoiS.,DreseJ.H.andJonesC.W.,Adsorbentmaterialsforcarbondioxidecapturefromlargeanthropogenicpointsources.ChemSusChem2(2009)796‐854.
CoppolaA.,MontagnaroF.,SalatinoP.andScalaF.,AttritionoflimestoneduringfluidizedbedcalciumloopingcyclesforCO2capture.CombustionScienceandTechnology184(2012a)929‐941.
CoppolaA.MontagnaroF.,SalatinoP.andScalaF.,Fluidizedbedcalciumlooping:TheeffectofSO2 on sorbent attrition and CO2 capture capacity. Chemical Engineering Journal207‐208(2012b)445‐449.
CrosthwaiteJ.M.,MuldoonM.J.,DixonJ.N.K.,AndersonJ.LandBrenneckeJ.F.,Phasetransitionanddecompositiontemperatures,heatcapacitiesandviscositiesofpyridiniumionic liquids.JournalofChemicalThermodynamics37(2005)559‐568.
Delgado J.A., Uguina M.A., Sotelo J.L. and Ruíz B., Fixed‐bed adsorption of carbon dioxide‐helium, nitrogen‐helium and carbon dioxide‐nitrogen mixtures onto silicalite pellets.SeparationandPurificationTechnology49(2006)91‐100.
DevadasM.,RamalingamB.,RameshK.,TasrifM.andSharratP.,CO2capturebyadsorptiononmesoporous MCM‐68 solid sorbent materials. 21st International Symposium on ChemicalReactionEngineering,Philadelphia,USA(2010).
Ding Y. and Alpay E., Equilibria and kinetics of CO2 adsorption on hydrotalcite adsorbent.ChemicalEngineeringScience55(2000)3461‐3474.
DoD.D.,Adsorptionanalysis:equilibriaandkinetics.SeriesonChemicalEngineeringsvol.2,ImperialCollegePress(1998).
DoD.D.andWangK.,Anewmodelforthedescriptionofadsorptionkineticsinheterogeneousactivatedcarbon.Carbon36(1998)1539‐1554.
Dubinin M.M. and Radushkevich L.V., Equation of the characteristic curve of activatedcharcoal. Proceedings of the Academy of Sciences, Physical Chemistry Section, U.S.S.R55(1947)331‐333.
97
EddaoudiM.,KimJ.,RosiN.,VodakD.,WachterJ.,O’KeefeM.andYaghiO.,Systematicdesignof pore size and functionality in isoreticularMOFs an their application inmethane storage.Science295(2002)469‐472.
FigueroaJ.D.,FoutT.,PlasynskiS.,McIlvriedH.andSrivastavaR.D.,AdvancesinCO2capturetechnology‐The U.S. Department of Energy's Carbon Sequestration Program. InternationalJournalofGreenhouseGasControl2(2008)9‐20.
GarcíaS.,GilM.V.,MartinC.F.,PisJ.J.,RubieraF.andPevidaC.,Breakthroughadsorptionstudyof a commercial activated carbon for pre‐combustion CO2 capture. Chemical EngineeringJournal171(2011)549‐556.
GomesV.G.andYeeK.W.K.,Pressureswingadsorptionforcarbondioxidesequestrationfromexhaustgases.SeparationandPurificationTechnology28(2002)161‐171.
GrayM.L,SoongY.,ChampagneK.J.,PennlineH.,BaltrusJ.P.,StevensJr.R.W.,KhatriR.,ChuangS.S.C.andFilburnT.,Improvedimmobilizedcarbondioxidecapturesorbents.FuelProcessingTechnology86(2005)1449‐1455.
Hasib‐ur‐RahmanM., SiajM. and Larachi F., Ionic liquids forCO2 capture‐Development andprogress.ChemicalEngineeringandProcessing49(2010)313‐322.
HerzogH.,MeldonJ.andHattonA.,Advancedpost‐combustionCO2capture.ReportpreparedfortheCleanAirTaskForce(2009).
HughesR.W.,LuD.Y.,AnthonyE.J.andMacchiA.,Design,processsimulationandconstructionofanatmosphericdual fluidizedbedcombustionsystem for insituCO2captureusinghigh‐temperaturesorbents.FuelProcessingTechnology86(2005)1523‐1531.
Inglezakis V.J. and Poulopoulos S.G., Adsorption, Ion Exchange and Catalysis. Design ofoperationsandenvironmentalapplication.Elsevier(2006).
International Energy Agency (IEA), Energy Technology Perspectives(2010).http://www.iea.org/techno/etp/etp10/English.pdf.
JangH.T.,ParkY.K.,KoY.S.,LeeJ.Y.andMargandanB.,HighlysiliceousMCM‐48fromricehuskashforCO2adsorption.InternationalJournalofGreenhouseGasControl3(2009)545‐549.
KannicheM., Gros‐Bonnivard R., Jaud P., Valle‐Marcos J., Amann J.M. and Bouallou C., Pre‐combustion, post‐combustion and oxy‐combustion in thermal power plant for CO2 capture.AppliedThermalEngineering30(2010)53‐62.
KasaharaS.,IshigamiTandMatsuyamaH.,Aminoacidionicliquid‐basedfacilitatedtransportmembranesforCO2separation.ChemicalCommunications48(2012)6903‐6905.
Khalil S.H., Aroua M.K. and Daud W.M.A.W., Study on the improvement of the capacity ofamine‐impregnated commercial activated carbon beds for CO2 adsorbing. ChemicalEngineeringJournal183(2012)15‐20.
98
KimY.S.,ChoiW.Y.,JangJ.H.,YooK.P.andLeeC.S.,Solubilitymeasurementandpredictionofcarbondioxideinionicliquids.FluidPhaseEquilibria228‐229(2005)439‐445.
KittelJ.,IdemR.,GelowitzD.,TontiwachwuthikulP.,ParrainG.andBonneauA.,CorrosioninMEAunitsforCO2capture:pilotplantstudies.EnergyProcedia1(2009)791‐797.
KoldingH.,FehrmannR. andRiisagerA.,CO2Capture technologies:Current statusandnewdirectionsusing supported ionic liquidphase (SILP)absorbers. ScienceChinaChemistry55(2012)1648‐1656.
KrumdieckS.,WallaceJ.andCurnowO.,Compact, lowenergyCO2managementusingaminesolutioninapackedbubblecolumn.ChemicalEngineeringJournal135(2008)3‐9.
Krutyeva M., Grinberg F., Furtado F., Galvosas P., Kärger J., Silvestre‐Albero A., Sepulveda‐Escribano A., Silvestre‐Albero J. and Rodríguez‐Reinoso F., Characterization of carbonmaterialswiththehelpofNMRmethods.MicroporousandMesoporousMaterials120(2009)91‐97.
Lee Z.H., Lee T.K., Bhatia S. and Mohamed A.R., Post‐combustion carbon dioxide capture:Evolution towardsutilizationofnanomaterials.RenewableandSustainableEnergyReviews16(2012)2599‐2609.
Lemus J., Palomar J., Gilarranz M.A. and Rodriguez J.J., Characterization of supported ionicliquidphase (SILP)materials prepared fromdifferent supports.Adsorption17 (2011) 561‐571.
Leofanti G., Padovan M., Tozzola G. and Venturelli B., Surface area and pore texture ofcatalysts.CatalysisToday41(1998)207‐219.
Li F. and Fan L.S., Clean coal conversion processes–progress and challenges. Energy &EnvironmentalScience1(2008)248‐267.
Lu C., Bai H., Wu B., Su F. and Hwang J.F., Comparative study of CO2 capture by carbonnanotubes,activatedcarbons,andzeolites.Energy&Fuels22(2008)3050‐3056.
MarshH.andRodrίguez‐ReinosoF.,ActivatedCarbon.ElsevierScience&TechnologyBooks(2006).
McCabeW.L.,SmithJ.C.andHarriotP.,UnitOperationsofChemicalEngineering.McGraw‐Hill5thedition(1993).
Metz B., Davidson O., de Coninck H., LoosM. andMeyer L., International Panel on ClimateChange(IPCC)Carbondioxidecaptureandstorage.CambridgeUniversityPress(2005).
MontagnaroF.,SalatinoP.andScalaF.,The influenceof temperatureon limestonesulfationand attrition under fluidized bed combustion conditions. Experimental Thermal and FluidScience34(2010)352‐358.
99
Morlay C. and Joly J.P., Contribution to the textural characterization of Filtrasorb 400 andother commercial activatedcarbons commonlyused forwater treatment. JournalofPorousMaterials17(2010)535‐543.
MuhammadN.,ManZ.B.,BustamM.A.,MutalibM.I.A,WilfredC.D.andRafiqS.,Synthesisandthermophysical properties of low viscosity amino acid‐based ionic liquids. Journal ofChemical&EngineeringData56(2011)3157‐3162.
NaB.K.,KooK.K.,EumH.M.,LeeH.andSongH.K.,CO2recoveryfromfluegasbyPSAprocessusingactivatedcarbon.KoreanJournalofChemicalEngineering18(2001)220‐227.
Neimark A.V., Lin Y., Ravikovitch P.I and Thommes M., Quenched solid density functionaltheoryandporesizeanalysisofmicro‐mesoporouscarbons.Carbon47(2009)1617‐1628.
PatrickJ.W.,PorosityinCarbons:CharacterizationandApplications.HalstedPress(1995).
PerryR.H. andGreenD.W.,Perry'sChemicalEngineers'Handbook.McGrawHill, 7th edition(1997).
Plaza M.G., Pevida C., Arenillas A., Rubiera F. and Pis J.J., CO2 capture by adsorption withnitrogenenrichedcarbons.Fuel86(2007)2204‐2212.
Plaza M.G., Pevida C., Arias B., Fermoso J., CasaL M.D., Martin C.F., Rubiera F. and Pis J.J.,Developmentoflow‐costbiomass‐basedadsorbentsforpostcombustionCO2capture.Fuel88(2009)2442‐2447.
Plaza M.G., Pevida C., Pis J.J. and Rubiera F., Evaluation of the cyclic capacity of low‐costadsorbentforpost‐combustionCO2capture.EnergyProcedia4(2011)1228‐1234.
Polanyi M., Theories of the adsorption of gases. A general survey and some additionalremarks.TransactionoftheFaradaySociety28(1932)316‐333.
Rege S.U. and Yang R.T., A novel FTIR method for studying mixed gas adsorption at lowconcentrations:H2OandCO2onNaXzeolitesandγ‐alumina.ChemicalEngineeringScience56(2001)3781‐3796.
Roquerol J., Llewellyn P. and Rouquerol F., Is the BET equation applicable tomicroporousadsorbents?.StudiesinSurfaceScienceandCatalysis160(2007)49‐56.
RuthvenD.M.,Principlesofadsorptionandadsorptionprocesses.JohnWiley&Sons(1984).
Saha B.B., Jribi S., Koyama S. and El‐Sharkawy I.I., Carbon dioxide adsorption isotherms onactivatedcarbons.JournalofChemical&EngineeringData56(2011)1974‐1981.
SahaD.,BaoZ.,JiaF.andDengS.,AdsorptionofCO2,CH4,N2OandN2onMOF‐5,MOF‐177,andzeolite5A.EnvironmentalScience&Technology44(2010)1820‐1826.
Samanta A., Zhao A., Shimizu G.K.H., Sarkar P. and Gupta R., Post‐combustion CO2 captureusingsolidsorbents:Areview.Industrial&EngineeringChemistryResearch51(2012)1438‐1463.
100
Sayari A., Belmabkhout Y. and Serna‐Guerrero R., Flue gas treatment via CO2 adsorption.ChemicalEngineeringJournal171(2011)760‐774.
Serna‐Guerrero R. and Sayari A., Modeling adsorption of CO2 on amine‐functionalizedmesoporous silica. 2: Kinetics and breakthrough curves. Chemical Engineering Journal 161(2010)182‐190.
Shafeeyan M.S., Daud W.M.A.W., Houshmand A. and Shamiri A., A review on surfacemodification of activated carbon for carbon dioxide adsorption. Journal of Analytical andAppliedPyrolisis89(2010)143‐151.
ShenC.,GrandeC.A.,LiP.,YuJ.andRodriguesA.E.,AdsorptionequilibriaandkineticsofCO2andN2onactivatedcarbonbeads.ChemicalEngineeringJournal160(2010)398‐407.
ShenC.,YuJ.,LiP.,GrandeC.A.andRodriguesA.E.,CaptureofCO2fromfluegasbyvacuumpressureswingadsorptionusingactivatedcarbonbeads.Adsorption17(2011)179‐188.
Shim H.L., Udayakumar S., Yu J.I and Park D.W., Synthesis of cyclic carbonate from allylglycidyletherandcarbondioxideusingionicliquid‐functionalizedamorphoussilica.CatalysisToday148(2009)350‐354.
Silvestre‐Albero J., Silvestre‐Albero A., Rodrίguez‐Reinoso F. and Thommes M., Physicalcharacterizationofactivatedcarbonswithnarrowmicroporositybynitrogen(77.4K),carbondioxide(273K)andargon(87.3K)adsorption incombinationwith immersioncalorimetry.Carbon50(2012)3128‐3133.
Silvestre‐Albero J.,WhabyA., Sepúlveda‐EscribanoA.,Martίnez‐EscandellM.,KanekoK.andRodrίguez‐ReinosoF.,UltrahighCO2adsorptioncapacityoncarbonmolecularsievesatroomtemperature.ChemicalCommunications47(2011)6840‐6842.
Sjostorm S., KrutkaH., Starns T. and Campbell T., Pilot test results of post‐combustion CO2captureusingsolidsorbents.EnergyProcedia4(2011)1584‐1592.
Soriano A.N., Doma Jr. B.T. and Li M.H., Carbon dioxide solubility in some ionic liquids atmoderate pressures. Journal of the Taiwan Institute of Chemical Engineers 40(2009) 387‐393.
Strube R. and Manfrida G., CO2 capture in coal‐fired power plants‐Impact on plantperformance.InternationalJournalofGreenhouseGasControl5(2011)710‐726.
TanakaS.,KidaK.,FujimotoH.,MakinoTandMiyakeY.,Self‐assembling imidazolium‐basedionicliquidsinrigidnanoporesinducesanomalousCO2adsorptionatlowpressure.Langmuir27(2011)7991‐7995.
TangJ.,TangH.,SunW.,RadoszM.andShenY.,Poly(ionicliquid)sasnewmaterialsforCO2absorption.JournalofPolymerScience:PartA:PolymerChemistry43(2005)5477‐5489.
TliliN., GrévillotG. andVallièresC., Carbondioxide capture and recoverybymeansofTSAand/orVSA.InternationalJournalofGreenhouseGasControl3(2009)519‐527.
101
Udayakumar S., Raman V., Shim H.L. and Park D.W., Cycloaddition of carbon dioxide forcommercially‐imperative cyclic carbonates using ionic liquid‐functionalized porousamorphoussilica.AppliedCatalysisA:General368(2009)97‐104.
UdayakumarS.,ShimH.L.,RamanV.andParkD.W.,Thecompleteoptimizationofionicliquid‐functionalizedporousamorphoussilicaunderone‐potsynthesisconditions.MicroporousandMesoporousMaterials129(2010)149‐155.
Wahby A., Silvestre‐Albero J., Sepúlveda‐Escribano A. and Rodrίguez‐Reinoso F., CO2adsorptiononcarbonmolecular sieves.MicroporousandMesoporousMaterials164(2012)280‐287.
Whaby A., Ramos‐Fernández J.M., Martίnez‐Escandell M., Sepúlveda‐Escribano A., SilvestreAlbero J. andRodrίguez‐ReinosoF.,High‐surface‐area carbonmolecular sieves for selectiveCO2adsorption.ChemSusChem3(2010)974‐981.
XueZ.,ZhangZ.,HanJ.,ChenY.andMuT.,Carbondioxidecapturebyadualaminoionicliquidwith amino‐functionalized imidazolium cation and taurine anion. International Journal ofGreenhouseGasControl5(2011)628–633.
Yang H., Xu Z., Fan M., Gupta R., Slimane R.B. and Bland A.E., Progress in carbon dioxideseparationandcapture:Areview.JournalofEnvironmentalSciences20(2008)14‐27.
YongZ.,MataV.G.andRodriguesA.E.,Adsorptionofcarbondioxideonchemicallymodifiedhighsurfaceareacarbon‐basedadsorbentsathightemperature.Adsorption7(2001)41‐50.
Zhang J., Zhang S., Dong K., Zhang Y., Shen Y. and Lv X., Supported absorption of CO2 bytetrabutylphosphoniumaminoacid ionic liquids.ChemistryAEuropean Journal12 (2006b)4021‐4026.
Zhang J., Sun J.,ZhangX.,ZhaoY.andZhangS.,TherecentdevelopmentofCO2 fixationandconversionbyionicliquid.GreenhouseGasesScienceandTechnology1(2011)142‐159.
ZhangS.,ChenY.,LiF.,LuX.,DaiW.AndMoriR.,FixationandconversionofCO2usingionicliquids.CatalysisToday115(2006a)61‐69.
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