Published: August 12, 2011

r 2011 American Chemical Society 11343 dx.doi.org/10.1021/ie2005115 | Ind. Eng. Chem. Res. 2011, 50, 11343–11349

ARTICLE

pubs.acs.org/IECR

Supported Monoethanolamine for CO2 SeparationZhuoyan Sun,† Maohong Fan,†,* and Morris Argyle†,‡

†Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States‡Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602, United States

ABSTRACT: An alternative method for using monoethanolamine (MEA) in CO2 separation is developed from the viewpoints ofthe MEA�CO2 reaction environment and the process of spent sorbent regeneration. MEA�TiO2 (MT) CO2 sorbent issynthesized using pureMEA and a support material, TiO2. The performance of theMT sorbent on CO2 separation was investigatedin tubular reactors under various experimental conditions. The sorption capacity of the MT sorbent reached 1.09 mol-CO2/kg-MTat 45 wt %MEA.However, an optimum of 40 wt %MEA loading was chosen for most of the sorption tests. Temperature affected theCO2 sorption capacity considerably, with optimum values of 45 �C for adsorption and 90 �C for regeneration, while humidity had asmall positive effect. TiO(OH)2 appears to be the best support material for MEA, but more evaluation is needed. TheMT sorbent isregenerable, with a multicycle sorption capacity of ∼0.91 mol-CO2/kg-MT under the given experimental conditions.

1. INTRODUCTION

The atmospheric CO2 concentration has increased by almost38% since the beginning of the industrial revolution to a currentlevel of about 386.8 ppm.1 More than 30% of all anthropogenicCO2 emissions are estimated to have resulted from fossil fuel-based electricity generation.2 These fossil fuels, including coal,oil, and natural gas, will be used as major energy sources for theforeseeable future due to their low prices and abundance.However, people are concerned about the increase of CO2

concentration in the atmosphere since CO2 has been implicatedas one of the main greenhouse gases leading to global climatechanges. Accordingly, capture of CO2 from flue gas streams infossil-fuel based power plants has been considered as one of themajor strategies for reduction of anthropogenic CO2 emissionsand thus the potential risks resulting from climate changes.

To date, all commercial CO2 capture processes have beenbased on liquid amine compounds. Amine solutions are basic andcan chemically removemany acid gases, including CO2, from fluegas.3 Among those frequently used amine compounds is mono-ethanolamine (MEA). Aqueous amines along with membraneshave been successfully used for separating CO2 from natural gas;however, they have not been used in fossil fuels based powerplants since the overall costs associated with the current tech-nologies are too high to be acceptable. The high costs are mainlydue to use of large amounts of water in the aqueous aminesolutions required for carbon dioxide separation. Typical aminesolutions used by the natural gas industry for gas cleaning cancontain as much as 70 wt % water.4,5

In recent years, people are increasingly interested in usingsolid sorbents synthesized with amines and solid supports orgrafting materials for CO2 capture in power plants. Differentsupport materials6�8 have been used for immobilization ofamines. Compared to aqueous amines, solid sorbents haveseveral advantages when used for separation of CO2 from fluegases in power plants.9�11 First, solid amine sorbents require lessenergy than aqueous amines for separation of the same amountof CO2 since they avoid energy needed to heat H2O, with its highspecific-heat-capacity and latent heat of vaporization, in aqueous

amine solutions during sorbent regeneration or CO2 strippingprocesses. Second, they are easy to handle and transport. Inaddition, they are less problematic than aqueous amine solutionsfrom an operational viewpoint because they are less corrosive.

Unlike traditionally immobilized amine-based CO2 sorbents,the pure MEA in the sorbent developed in this research isimmobilized during the CO2 sorption phase, but is mobilizedduring CO2 desorption phase. More specifically, immobilizedMEA reacts with CO2 in a sorption reactor, but is transported toanother reactor during the CO2 desorption process due to thedifference in sorption and desorption temperatures. The sup-porting materials for MEA should be inexpensive and widelyavailable. In addition, they should have low densities, which isbeneficial to the reduction in the operation cost of CO2 separa-tion. TiO2, Al2O3, SiO2, FeOOH, and TiO(OH)2 are thecandidates which can meet all these requirements. However,compared to Al2O3

12 and SiO2,13 TiO2, FeOOH, and TiO-

(OH)2 have rarely been tested as the supports of alkalinechemicals especially amines for CO2 separation, therefore, theywere chosen as the support agents of MEA in this research, whichhowever mainly uses TiO2 due to its wider availability and lowerprice compared to FeOOH and TiO(OH)2.

The MEA utilization approach studied in this research isexpected to considerably reduce CO2 separation energy con-sumption and costs. The resultant saving is expected to be muchlarger than that needed for compensation of the added sorbentcost due to the use of TiO2. Several major factors potentiallyaffecting the CO2 sorption capacities of the proposed MEAutilization method were investigated. The results obtained in theresearch could be used for further development or optimizationof the MEA-based CO2 separation technology.

Received: March 15, 2011Accepted: August 12, 2011Revised: August 5, 2011

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2. EXPERIMENTAL SECTION

TiO2 Preparation and Characterization. The supportmaterial (TiO2) used in this research was prepared withTi(OC2H5)4 (99 wt %, Acros) containing 33�35 wt % TiO2.The first preparation step was to add a predetermined amount ofTi(OC2H5)4 to water with a H2O:Ti(OC2H5)4 molar ratio of26.3, followed by stirring for 1 h. The resulting precipitate wasfiltered, washed with deionized water, and then dried at 393 K for1.5 h. TiO2 was obtained by calcining the resultant TiO(OH)2 inair at 1023 K for 3 h.The prepared TiO2 and TiO(OH)2 were characterized with a

Micromeritics TriStar 3000 V6.04 A nitrogen physisorptionanalyzer to determine surface areas by the BET (Brunauer,Emmett, and Teller) method. The FeOOH with 181.6 m2/gBrunauer, Emmet, and Teller (BET) surface area, 74.4 nmaverage pore size, and 3.3 cm3/g volume was provided by KemiraWater Solution, Inc. (Bartow, FL). Powder X-ray diffraction(XRD) of TiO2 was performed on a Philips X’Pert diffractometerusing Cu�Kα radiation under the following operating condi-tions: voltage, 40 kV; current, 40 mA; start angle, 10�; end angle,90�; step size, 0.01�; time per step, 0.05 s; and scan speed, 0.02.The experimental data were digitally collected and recorded.Each MEA�TiO2 (MT) sorbent was prepared by loading a

certain amount of as-purchased MEA (99 wt %, Acros) ontothe prepared TiO2. Five MEA:TiO2 mass ratios or MEA loadingswere used for preparing the MT sorbents tested for this research.The best loading was determined and used for all subsequent tests.Apparatus. The experimental setup used for the CO2 separa-

tion tests is shown in Figure 1. It has three parts: a gas preparationunit, a CO2 sorption/desorption system, and gas-phase CO2

concentration analysis equipment. Dilute CO2 from cylinder 1(1 mol % CO2 in 99 mol % N2) was used for the sorption tests.N2 from cylinder 2 (100 mol %) was used for CO2 desorptiontests and cleaning the apparatus. The flow rates of the inlet gaseswere controlled by two flow meters (Matheson Trigas FM-1050,labeled 30 and 300). An additional flow meter (3000) was used tomeasure the flow rate of the whole system.

Sorption tests were performed in the bottom reactor (110),which has an inner diameter and length of 9 mm and 610 mm,respectively. The sorbent bed (9) was prepared by loading MTsorbent between two bed holders (8) made from quartz wool.The bottom reactor (110) was held in a tube furnace (10, ThermoCorporation, TF55030A-1), where its temperature was con-trolled (7, Yokogawa M&C Corporation, UT150). A syringepump (4) was used to generate the water vapor used in moisture-containing gas streams. Temperature controlled (6, MiniTrol,Glas-Col Inc.) thermo-tapes (5) heated the inlet gas tubes toprevent condensation of water vapor prior to entering the bottomreactor. The effluent gas stream from the bottom reactor passedthrough a sorbent bed (12, consisting of the support material forMT sorbent, which was generally TiO2) in the top reactor (1100)to condense the MEA vaporized from the bottom reactor usingcooling water circulating through a spiral copper pipe (13, innerdiameter: 1.5 mm) and held at 12 �C by a small refrigeration unit(14, MGW Lauda, RC-20 controller). The effluent gas from thetop reactor (1100) entered a water removal unit (15) and then aninfrared gas analyzer (16, ZRE, Fuji Electric System Co. Ltd.).The sorption profiles were collected by a data collection com-puter (17).CO2 Sorption/Desorption. Each CO2 desorption test was

started immediately after the bed was saturated with CO2, asdetermined when the outlet CO2 concentration during a sorp-tion step became equal to the inlet CO2 concentration. During adesorption step, pureN2 from cylinder (1) was used as the carriergas to bring the desorbed CO2 from the bottom reactor (110)through the top reactor (1100) and finally to the gas analyzer (16).MEA vapor resulting from the CO2 desorption in the bottomreactor (110) also flowed into the top reactor (1100) and con-densed there. Desorption temperatures were controlled by thebottom temperature controller (7). When CO2 desorption wascompleted, the material in the bottom rector (110) was pureTiO2 because all MEA was transported to the top reactor (1100)and formed MT sorbent with the TiO2 there due to the con-densation of the MEA vapor from the bottom reactor (110) on

Figure 1. Schematic diagram of the carbon dioxide separation setup: (1)N2 cylinder; (2) CO2 cylinder; (30/300/300 0) flowmeters; (4) syringe pump; (5)heat tape; (6) temperature controller for heat tape; (7) temperature controller for furnace; (8) quartz wool; (9) sorbent bed; (10) furnace; (110/1100)bottom reactor/top reactor; (12) sorbent support material (TiO2/TiO(OH)2/FeOOH); (13) cooling water; (14) cooling water temperaturecontroller; (15) water vapor removal unit; (16) multigas analyzer; (17) data collection unit.

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the surface of pure TiO2 originally in the top reactor (1100). Thenthe positions of the top and bottom reactors were switched tostart the next sorption�desorption cycle.

3. RESULTS AND DISCUSSION

3.1. Characterization of TiO2 and TiO(OH)2. The BETsurface area, pore average size, and volume of the sorbent supportmaterial, TiO2, are 5.68 m2/g, 66.4 nm, and 0.11 cm3/g,respectively. Those of TiO(OH)2 correspond to 123.2 m2/g,4 nm, and 0.08 cm3/g. The obtained TiO2 X-ray diffractionpattern is shown in Figure 2. Three major diffraction peaksappear at 2θ values of 27.5�, 36.2�, and 54.4�, corresponding todiffraction from the (110), (101), and (211) crystal planes,respectively, which are consistent with TiO2 in the rutilephase.14,15

3.2. Factors Affecting CO2 Sorption. 3.2.1. MEA Loadingand Distribution on TiO2.The relationship betweenMEA loadingon the surface of TiO2 and CO2 sorption capacity of synthesized

MT sorbent is shown in Figure 3. The CO2 sorption capacity oftheMT sorbent increases with theMEA loading and reaches 1.09mol-CO2/kg-MT when the MEA loading percentage is 45 wt %under the given experimental conditions.MEA is well-known for its reactivity with CO2, which was also

observed in this study. Typically, the MT sorbent could achieveone-half of its total capacity within 10 min under any testconditions used in this research. However, much longer periodsof time were needed to attain the full capacity of an MT sample.The average CO2 adsorption rate of the supported sorbent in thefirst 5 min is about 0.182 mol-CO2/kg-MT/min, indicating thatCO2 is readily able to react with MEA on the surface of thesorbent. However, MEA molecules far away from the surface ofMT sorbent (close to the surface of the support TiO2 particles)or condensed in the TiO2 pores are not easily accessible to CO2

due to diffusion limitations. This explains why the CO2 sorptioncapacity did not improve much when MEA loading on the MTsorbent increases from 40 to 45 wt %, as observed in Figure 3.Therefore, 40 wt % MEA loading was chosen to evaluate theeffect of all the other factors on CO2 sorption.3.2.2. Moisture. The MT sorbent was developed to overcome

the shortcoming of conventional aqueous MEA-based CO2

separation technologies by eliminating the use of water whilemaintaining its advantage of strong CO2 absorption. However,the effect of water on the CO2 sorption of MT has to beconsidered since flue gas from all combustion processes, includ-ing coal-fired power plants, contain water despite the MTsorbent being made without water. Therefore, a gas containing0 vol %H2O, 1.0 vol %CO2, and 99 vol %N2 and two other gases(0.5 vol % H2O, 1.0 vol % CO2, and 98.5 vol % N2; 1.0 vol %H2O, 1.0 vol % CO2 and 98.0 vol % N2) were compared for theirCO2 sorption profiles (Figure 4).Two major facts are shown in Figure 4. The first one is that the

CO2 sorption capacity of MT are affected to some degree by theconcentration of H2O in gas stream as observed during the initialCO2 sorption periods within which the outlet CO2 concentrations ofthe gas streams are 0 vol%or all theCO2molecules in gas streams arecompletely adsorbed on MT. Second, the three CO2 adsorption

Figure 2. X-ray diffraction pattern of the prepared TiO2.

Figure 3. Effect of MEA loadings on sorption capacity of MT sorbent(CO2, 1.0 vol %; N2, 99.0 vol %; gas flow rate, 0.3 L/min; sorptiontemperature, 45 �C).

Figure 4. Effect of moisture: (A) H2O, 0 vol %; MT, 40 wt % MEAloading; CO2, 1.0 vol %; N2, 99 vol %. (B) H2O, 0.5 vol %; MT, 40 wt %MEA loading; CO2, 1.0 vol %; N2, 98.5 vol %. (C) H2O, 1.0 vol %; MT,40 wt % MEA loading; CO2: 1.0 vol %; N2: 98.0 vol %. For the threegases: gas flow rate = 0.3 L/min; sorption temperature = 45 �C.

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curves intersect at the reaction time of 11min. The phenomena needto be understood from the perspectives of the CO2 sorption reactionpathways and kinetics associated with the technology.The chemical process of CO2 sorption with MT without the

presence of water can be represented with16

2RNH2 þ CO2 rsfkR1, k�R1

RNHCOONH3R ðR1Þwhile the reactions corresponding to CO2 sorption withMTwiththe presence of water can be characterized with

2RNH2 þ CO2 þ H2O rsfkR2, k�R2 ðRNH3Þ2CO3 ðR2Þ

ðRNH3Þ2CO3 þ CO2 þ H2O rsfkR3, k�R3

2RNH3HCO3

ðR3Þor

RNH2 þ CO2 þ H2O rsfkR4, k�R4

RNH3HCO3 ðR4Þwhere ki (i = 1, 2, 3, and 4) and k-i stand for the forward reactionrate constants of reaction Ri, respectively. R1 demonstrates that2 mols of RNH2 are needed for adsorption of 1 mol of CO2,whereas only 1 mol of RNH2 is required when CO2 adsorptionoccurs with the presence of H2O according to R4. In other words,theoretically, the CO2 sorption capacities of MT obtained withthe gas containing 0.5 vol % and with the gas containing 1.0 vol %should be higher, and much higher than that with the bone drygas, respectively. More specifically, according to stoichiometriesof R1 and R4, the sorption capacity of MT achieved with the1.0 vol % H2O gas stream should be 100% higher than that with0 vol % H2O gas stream. However, on the basis of Figure 4, thedifferences of the CO2 sorption capacities of MT achieved underthree different conditions are not so large, which needs to beinvestigated by considering the specific pathways of overallreactions of CO2 sorption with and without the presence ofH2O. According to literature,

3,11,15�17 the pathway of R4 includ-ing various reversible elementary reaction steps and differentnitrogen containing intermediate species is longer than that ofR1. In other words, under either bone dry or moisture condi-tions, not all the amino groups in MEA react with CO2 andgenerate the ending products of R1 and R4, (RNH3)2CO3 andRNH3HCO3, respectively, while more or much more aminogroups (depending on the concentration of water) exist in theform of intermediate species when water is present. This is one ofthe reasons that adding water does not improve the CO2 sorptioncapacity of MEA by the percentage as high as stoichiometricallypredicted, as shown in Figure 4.The differences in the reaction kinetics of CO2 sorption by

MT without and with the presence of H2O are also the factorsleading to the phenomena shown in Figure 4. According tothe studies of Rochelle and his research team,17 the rate laws ofthe MT-based CO2 separation technology without and with thepresence of H2O can be expressed as

rMTwithoutH2O ¼ kR1aMEA, R12aCO2 ðE1Þ

rMTwithH2O ¼ kR4aMEA, R42aCO2 ðE2Þ

where rMTwithout H2O and rMTwithH2O are the corresponding CO2

sorption rates of MT under the two different conditions, kR1 andkR4 are the rate constants for R1 and R4, respectively, aMEA,R1,

aMEA,R4 and aCO2are the activities of the specified chemicals, and

all their values are less than 1 in the MT based CO2 separationtechnology. On one hand, kR1 should be higher than kR4 since theactivation energy of the forward reaction of R1 is lower than thatof R4.17 On the other hand, aMEA,R1 should be lower than aMEA,R4

due to the use of H2O, and both should decrease with time duringthe processes of CO2 sorption with MT. The overall effect of kand a determine the CO2 sorption rates of MT, rMTwithoutH2O

and rMTwithH2O. In the first 11�12 min of reaction period shownin Figure 4, the activity of MEA, aMEA, should play a larger rolethan k. Otherwise, according to E1 and E2, it could not bepossible that rMTwithH2O was larger than rMTwithout H2O and thusmore CO2was adsorbed byMTwith the presence of H2O duringthat time period. With the CO2 sorption continuously proceed-ing, the outlet CO2 concentrations of themoisturized gas streamsbecome higher than that of the dry gas stream as observed inFigure 4, indicating that kR1 and kR4 start to dominate the CO2

sorption rates of MT under the given test conditions.3.2.3. Sorption Temperature. Effects of sorption temperature

on the total CO2 sorption capacity of MT sorbents wereevaluated in the temperature range of 25�65 �C. Figure 5 showsthe CO2 breakthrough curves for each of these conditions. The

Figure 5. Effect of temperature onCO2 sorption profile (A) and capacity(B) (MT, 40 wt %MEA loading; CO2, 1.0 vol %; N2, 99.0 vol %; gas flowrate, 0.3 L/min).

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CO2 sorption capacity increases with temperature in the range of25�45 �C, but decreases with the further increases of tempera-ture from 45 to 65 �C.The relationship between T and CO2 sorption capacity of MT

can be understood from the thermodynamic and kinetic char-acteristics of R1. Studies on the thermodynamics and kinetics ofR1 are still lacking, even though those of R2 are well-researchedand many progresses have been made. R1 and R2 have differentreactants and products. Therefore, the thermodynamic andkinetic study results reported in the literature for R2 cannot beused for R1. Actually, even for R2, some disagreements existamong the published papers regarding its thermodynamic andkinetic properties under the same CO2 sorption conditions.For example, the enthalpy change of R2 during CO2 sorptionat 320 K is reported by Palmeri et al.18 as∼57 kJ/mol-CO2, whileMathonat et al.19 report the value as ∼80 kJ/mol-CO2.R1 is an exothermic reaction18,20�22 or its enthalpy change

(ΔHR1 < 0) is negative under the experimental conditions. Basedon the van’t Hoff relationship,23 temperature increases do notfavor R1 since equilibrium CO2 sorption capacity (determined byKR1, the equilibrium constant of R1) decreases due to thenegative ΔHR1

d ln KR1

dT¼ ΔHR1

RT2ðE3Þ

KR1 for MT-based CO2 sorption in a dry environment is afunction of temperature, T. It can be calculated based on thethermodynamic properties of MEA, CO2, and RNHCOONH3Rin R1 using

ΔGoR1¼ � RT ln KR1

¼ ΔHo0, R1 �

TT0ðΔHo

0, R1 �ΔGo0, R1Þ

þΔCoPðT � T0Þ � TΔCo

P lnTT0

ðE4Þ

where T0 is reference temperature, ΔH0o and ΔG0

o are thestandard enthalpy and free Gibbs energy changes of R1 at thereference temperature, and

ΔCoP ¼ Co

P, RNHCOONH3R � 2CoP, RNH2

� CoP, CO2

ðE5Þwhere CP,RNHCOONH3R

o , CP,RNH2

o , and CP,CO2

o represent the heatcapacities of the three reactants and products at constantpressure.The forward reaction rate constant, kR1, increases with T

according to the Arrhenius equation24 while KR1 in E3 and E4decreases with T. Therefore, an optimal CO2 sorption tempera-ture exists that is a compromise between these kinetic andthermodynamic factors to obtain a reasonably high rate of R1and yet large CO2 sorption. In other words, the optimal sorptiontemperature for the MT-based CO2 sorption technology isdefined as that which maximizes the CO2 sorption capacitywithin a given reaction time period. The optimal temperatureat which the maximum total CO2 adsorption capacity wasachieved under the given experimental conditions is 45 �C,although its exact value can not be calculated at the present timeusing E3, E4, E5, and the Arrhenius equation of the forwardreaction of R1 because not all the parameters necessary for thecalculation can be obtained from literature.3.2.4. Desorption Temperature. CO2 desorption tests were

performed at 80, 90, 100, and 110 �C to evaluate the effect of

temperature on CO2 sorption capacity of the MT sorbent regen-erated for next cycle of sorption and desorption. The results areshown in Figure 6. The intermediate temperatures, 90 and 100 �C,are better based on the sorption capacities obtained in the nextsorption�desorption cycle. However, due to the higher energyconsumption at 100 �C, 90 �C was chosen as the CO2 desorptiontemperature for all other MT evaluation tests. Actually, CO2

desorption at a lower temperature not only reduces the energyneeded for overall CO2 separation process but also is favorable tothe structure stability of MEA and environmental protection dueto the reduction of oxidative decomposition of MEA.3.2.5. Alternative Support Materials for MEA. An alternative

Ti-based support material is TiO(OH)2, which can be easilyprepared at low temperatures compared to TiO2. It is stable evenat 400 �C.25 Its performance as a support for MEA is better than

Figure 6. Effect of desorption temperature: MT, 40 wt %MEA loading;CO2, 1.0 vol %; N2, 99 vol %; gas flow rate, 0.3 L/min; sorptiontemperature, 45 �C.

Figure 7. Comparison of different support materials, (A) TiO2, (B)TiO(OH)2, (C) FeOOH, for their effects on CO2 sorption (MEAloading in each sorbent, 40 wt %; CO2, 1.0 vol %;N2, 99.0 vol %; gas flowrate, 0.3 L/min; sorption temperature, 45 �C).

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TiO2 to some degree during most of the sorption period, as shownin CO2 breakthrough curves in Figure 7. The fact might beexplained with the kinetic model obtained by Ramachandranet al.15 They found that the OH� increases the reaction rate bet-ween MEA and CO2. Therefore, TiO(OH)2 can probably accel-erate CO2 sorption to some degree due to theOH

� in its structure.Among many other possible highly porous and inexpensive

MEA support materials is FeOOH. FeOOH starts to dehydrateat 213 �C or 490 K.26 Therefore, it is thermally stable underthe operation conditions used in this research. It also has OH� inits structure and is less expensive than TiO2 and TiO(OH)2. Thesorption results with the pure MEA supported with FeOOH isalso shown in Figure 7. FeOOH is better than TiO2, but not asgood as TiO(OH)2. When choosing support materials for MEA,other factors should also be considered. For example, acidiccompounds in the flue gas, SOx, andNOxmay affect the life spansof the support materials due to their potential reactions with theacidic compounds. Ti-based compounds are better than FeOOHfrom the perspective of their corrosion-resistance abilities.Therefore, comprehensive comparisons should be made whena support material is selected for synthesis of future MEA-basedCO2 separation sorbents.

3.3. Sorbent Regeneration. Industrial chemisorbents arerequired not only to be highly active and selective, but alsoregenerable. Therefore, five-cycle CO2 sorption�desorptiontests with MT sorbents were run under conditions with andwithout moisture. The results are presented in Figure 8A,B. Theaverage adsorption capacities for five-cycle tests at 45 �C underdry and humid (1 vol % H2O) sorption conditions are 1.04 and1.09 mol-CO2/kg-MT, respectively, indicating MT can be usedin both dry and wet environments for effective CO2 separation.The capacities of MT under the two different environments

are higher than that of aqueousMEA, which can absorb 0.82mol-CO2/kg-aqueous-MEA.27 In addition, they are also higher thanthe CO2 sorption capacities of 21 sorbents among 24 evaluatedby Sjostrom and Krutka in 2010.28,29 Most of those 24 sorbentscontain 40�50 wt % amines, which is equal to or higher than theMEA percentage (40 wt %) of the MT sorbent used in thisresearch. The regeneration temperatures of those sorbents variedfrom 80 to 120 �C and increased by 10 �C with each subsequentsorption�desorption cycle compared to the constant 90 �C usedfor the spent MT regeneration. The quantities of CO2 immobi-lized on MT during the sorption period and CO2 desorbed fromspent MT during the desorption process, determined by inte-grating CO2 concentration change profiles in each sorp-tion�desorption cycle, are very close. In other words, theworking capacity, as defined by Sjostrom and Krutka,29 is almostequal to sorption capacity for the MT sorbent. This is the reasonthat the CO2 sorption capacities do not fluctuate considerablyfrom one sorption�desorption cycle to another, as shown inFigure 8A,B.

4. CONCLUSION

A new MEA utilization approach to CO2 separation has beendeveloped by using MT sorbent. MT can be simply synthesizedin an environmentally benign manner since no additionalchemicals, such as organic solvents, are needed. The factorsaffecting the CO2 sorption capacity of MT are the loadingof MEA on TiO2, moisture, CO2 sorption temperature, anddesorption temperature. The CO2 sorption capacity of MTincreases with MEA concentration of the sorbent, while itincreases and then decreases with moisture of the gas stream,CO2 sorption, and desorption temperature to various degrees.The alternative supporting materials for MEA are existent,although more evaluations on them need to be conducted. Theoperation conditions, especially CO2 desorption temperatureneeded for the new CO2 separation method, are not as strict asthose for other absorbents and sorbents. This indicates that theequipment requirements for separation of CO2 with the MT-based technology should not be as demanding as those associatedwith the majority of other CO2 separation technologies.

Development of MT-based CO2 separation technology isbeneficial to both economy and environment. CO2 emissioncontrol is expensive and its implementation in coal-fired powerplants will have considerable impacts on energy industry. Themajor method for significant reduction of the overall cost of CO2

emission control is to cut the energy demand required for theCO2 separation process, specifically CO2 desorption step. ThisnewCO2 separation technology not only avoids the use of a largeamount of water with high specific-heat-capacity and latent heatof vaporization but also appreciably decreases CO2 desorptiontemperature, and thus could substantially reduce the overall CO2

separation energy consumption and thus the total cost. Lowering

Figure 8. CO2 sorption capacities of MT during five sorption�desorp-tion cycles: A (sorption gas, CO2, 1.0 vol %; N2, 99 vol %), B (sorptiongas, H2O, 1.0 vol %;CO2, 1.0 vol %;N2, 98 vol %), sorption (MT, 40wt%MEA loading; gas flow rate, 0.3 L/min; sorption temperature, 45 �C),desorption (N2, 100 vol %; gas flow rate, 0.3 L/min; desorption tem-perature, 90 �C).

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CO2 desorption or spent MT regeneration temperature canlargely decrease the degree of thermal/oxidative degradation ofMEA as observed in aqueous MEA-based CO2 separationprocesses. The major products resulting from the fragmentationof MEA, ammonia, hydroxyacetaldehyde, and formaldehyde, arehazardous, and not only corrode CO2 separation equipment andpipelines but also lead to serious pollution issues. In summary,MT is a not only potentially cost-effective but also an envi-ronmentally benign CO2 sorbent, thus holding a promisingfuture.

’AUTHOR INFORMATION

Corresponding Author*E-mail: mfan@uwyo.edu.

’ACKNOWLEDGMENT

This researchwas supported by the School of Energy Resourcesat the University of Wyoming and the Department of Energy.

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