Separation and Purification Technology 49 (2006) 27–35

Fixed-bed performance for production of oxygen-enriched carbondioxide stream by perovskite-type ceramic sorbent

Qing Yang a,b, Jerry Y.S. Lin a,b,∗a Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA

b Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006, USA

Received 7 May 2005; received in revised form 2 August 2005; accepted 8 August 2005

Abstract

This paper reports on an experimental study on production of oxygen-enriched carbon dioxide stream by a fixed-bed packed with a perovskite-type ceramic sorbent La0.1Sr0.9Co0.5Fe0.5O3−δ, which can adsorb oxygen from air. Oxygen-enriched carbon dioxide stream is produced by passingcarbon dioxide into the fixed-bed saturated with oxygen. Fixed-bed experiments were performed to study the oxygen separation performance ofthis sorbent. The separation process was experimentally confirmed to be a reversible chemical sorption and desorption process. Experimentalresults are also reported to show the effects of operation conditions, including adsorption time, flow rates of feed gases, and fixed-bed temperaturesipod2©

K

1

eidipafar[n

ga

1d

n the adsorption and desorption steps, on average oxygen concentration of the product stream, oxygen recovery, carbon dioxide recovery androductivity. Experimental results suggest that adsorption and desorption temperatures are the most critical operation parameters. Under theptimum conditions (850 and 900 ◦C adsorption and desorption temperatures) the fixed-bed adsorber can produce an oxygen-enriched carbonioxide stream with average oxygen concentration, oxygen recovery, carbon dioxide recovery and productivity, respectively, of 58.8%, 18.7%,6.6% and 0.272 ml/min. This oxygen-enriched carbon dioxide stream may be used as an oxidant in the oxyfuel combustion process.

2005 Elsevier B.V. All rights reserved.

eywords: Perovskite; Carbon dioxide; Adsorption; Desorption; Fixed-bed; Breakthrough curves

. Introduction

The world is increasingly depending on fossil fuels for itsnergy supply. During combustion of fossil fuels, carbon diox-de is emitted to the atmosphere from power plants and is aominating contributor to global warming. Conventionally, airs fed for fuel combustion, and the flue gas contains major com-onents of CO2, N2, O2 and H2O, with minor contents of SOx

nd NOx. Removal of SOx, NOx, and particulate matter (PM)rom the flue gas is usually required before its emission to thetmosphere in order to prevent environmental pollution. Sepa-ation and capture of CO2 may be achieved by amine scrubbing1–3], which is expensive. Development of efficient and eco-omic technologies for CO2 separation is then in order.

Oxyfuel combustion is one of the recently proposed technolo-ies that can simplify flue gas treatment resulting potentially inzero CO2 emission to the atmosphere. Oxyfuel combustion

∗ Corresponding author.E-mail address: Jerry.Lin@ASU.edu (J.Y.S. Lin).

is commonly known as the O2/CO2 recycle combustion pro-cess, in which oxygen (purity of 95% or higher) is fed to theboiler for combustion, and a major part (70–80%) of the CO2-rich exhaust gas is recycled back to the boiler to maintain thecombustion temperature. The remaining flue gas consists mainlyof CO2 and water vapor and small quantities of NOx and SOx.Compared with traditional air combustion, the flue gas treatmentin oxyfuel process is much easier: water can be separated sim-ply by condensing the flue gas. Moreover, the absence of bulknitrogen in the flue gas means that the equipment for flue-gasdesulphurization and nitrogen/nitrogen oxide removal will havea smaller volume, and thus be cheaper, than the correspondingequipment for air-fired power plants [4]. Oxyfuel has recentlyattracted increasing research interests for its applications in coal,gas, and oil combustion in many power plants [5–9].

To obtain high purity oxygen in large scale, cryogenic air-separation is the only commercially available method, whichis known for its major drawback of high-energy consumption.The replacements of the cryogenic air-separation with someother methods of less energy consuming oxygen separation havebeen explored for many years. The possible methods include:

383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2005.08.004

28 Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35

(1) separation through ceramic oxygen-ion transfer membranes[10,11], (2) ceramic auto-thermal recovery [12], (3) chemicallooping combustion [13], and (4) gas adsorption by zeolite X[14,15] and carbon molecular sieve [16,17]. Most of these meth-ods still face their own technological challenges and have notbeen applied in practical applications for large-scale productionof oxygen.

A new high-temperature oxygen sorption process on aperovskite-type ceramic sorbent was first proposed by Lin andcoworkers [18,19]. Based on the property of perovskite-typematerials that the oxygen content in its structure varies withoxygen partial pressure of surrounding gas and temperature,these materials can be applied in pressure swing or temperatureswing adsorption processes [21,22] for oxygen separation fromair. Although the adsorption rate in this process is high, relativeslowness of the desorption step represents a major drawback ofthis type of sorbents [20]. This issue may cause challenges withregard to achieving a high O2-product purity even in pressure-swing adsorption process and also to ensuring a sufficiently highefficiency of sorbent regeneration.

Another important application of a perovskite ceramicsorbent-based production of an O2-enriched CO2 stream couldbe envisioned if CO2 is used to replace O2 from the sorbentcolumn, and it was first illustrated in our laboratory in a recentpublication [23]. The whole process could be described as fol-lows: during the O -adsorption process, air is used as feed gas tositgagCabtmmtceis

Tat

tost

cesses. Four parameters, i.e., oxygen concentration, the oxygenrecovery, carbon dioxide recovery and productivity, are definedto evaluate the efficiency of the separation process. The resultswill help identify the optimal operation conditions for practicaldesign of a separation process based on this type of sorbents forproduction of oxygen-enriched carbon dioxide stream.

2. Experimental

2.1. Syntheses and characterization of sorbent

The liquid citrate method followed by high temperaturesintering was used to prepare these perovskite-type ceram-ics (La0.1Sr0.9Co0.5Fe0.5O3−δ). This method has advantages inpreparing samples with precise stoichiometries [19]. In syn-thesis, metal nitrate precursors of La(NO3)3·6H2O, Sr(NO3)2,Co(NO3)2·6H2O and Fe(NO3)3·9H2O were dissolved in de-ionized water by molar ratio of 1:9:5:5, according to the sto-ichiometry of the final product. The system was under heatingand stirring during the polymerization and condensation reac-tions. Water was gradually evaporated during the condensationprocess to facilitate gelation. Self-ignition occurred at 400 ◦C toburn the organics from the products after they had been dried at110 ◦C for 24 h. Finally, as-prepared powders were sintered at900 ◦C for 20 h with a ramping rate of 60 ◦C/h. The as-preparedsamples were ground into fine powders for characterization andTrtp4a

2

mtotsMflsdcpoLqo

bflttb

2aturate the perovskite-type ceramic sorbent with O2; while dur-ng the O2-desorption process, CO2 is swept through the columno desorb O2 from the sorbent to produce an O2-enriched CO2as. From the preliminary fixed-bed studies reported by Yangnd Lin [23], this sorption process can efficiently separate oxy-en at a high temperature and allows productions of O2-enrichedO2 stream with an average oxygen concentration around 45%nd a maximum oxygen concentration up to 80%, which coulde directly fed into the boiler for fuel combustion. Furthermore,he high temperatures during sorption processes can be easily

aintained due to the hot CO2 gas, which can be obtained fromany other industry processes, and the energy consumption can

hus be reduced. This process is a promising alternative to theonventional cryogenic air separation for production of oxygen-nriched streams. A fundamental study on this sorption processndicates that production of oxygen-enriched carbon dioxidetream is based on the following reversible reaction [23]:

La0.1Sr0.9Co0.5Fe0.5O2.6 + 0.9CO2

⇔ 0.9SrCO3 + 0.05La2O3 + 0.5CoO

+ 0.25Fe2O3 + 0.15O2 (A)

he kinetics of reaction (A) and its reverse reaction both exhibithigh reaction rate in the initial stage followed by a low rate in

he second stage [23].Efficiency of the sorption process should depend on opera-

ion conditions. The purpose of this work is to study effects ofperation conditions including the flow rate of feed gas duringorption and desorption steps, adsorption time, and the operationemperatures on both oxygen adsorption and desorption pro-

GA measurements. XRD analysis (Siemens D-50, Cu KR1adiation) was performed to examine the crystalline structure ofhe as-prepared perovskite-type ceramics [23]. The shape andarticle size of the samples were observed by SEM (Hitachi S-000). The average aggregate diameter for these sorbents wasbout 10 �m.

.2. Experimental setup and procedure

Fixed-bed experiments were conducted to study the perfor-ance of adsorption and desorption processes of the perovskite-

ype material La0.1Sr0.9Co0.5Fe0.5O3−δ (LSCF) to producexygen-enriched carbon dioxide. Fig. 1 shows the experimen-al setup of the fixed-bed system, which includes a gas deliveryystem, an adsorber column, an oxygen analyzer (Ceramatec,

odel 1100) with a data acquisition system, and a thermal massow rate meter (McMillan, 50D-2) with another data acquisitionystem. Oxygen breakthrough curves and flow rate of effluenturing adsorption and desorption were recorded to study theharacteristics of these processes. As the sorbent, the LSCFowder of 3.60 g was packed in the middle of a dense aluminumxide ceramic tube (6 mm i.d. and 9 mm o.d., 100 cm long). TheSCF packing length was about 10 cm, and it was supported byuartz particles (about 0.5 mm in diameter, with packing lengthf about 45 cm on each ends).

To study the effects of operation conditions on the fixed-ed separation efficiency, five parameters, i.e., adsorption time,ow rates of the feed gases and fixed-bed temperatures during

he adsorption and desorption steps, were investigated, respec-ively. Fig. 2 shows typical oxygen adsorption and desorptionreakthrough curves of the fixed-bed separation process. Prior

Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35 29

Fig. 1. Fixed-bed experiment setup.

to oxygen adsorption, the sorbent was exposed to CO2 with5 ml/min flow rate at 800 ◦C for a time period till the O2 con-centration in the effluent from the fixed-bed dropped to 30%.The conditions for this prior process were fixed the same for allthe experiments. In the adsorption step, the O2 concentration ofthe effluent stream was determined by the O2 sensor after thefeed gas was switched from CO2 (as desorption feed gas) to air(as the adsorption feed gas). As shown in Fig. 2, the curve fromt0 to ta corresponds to the adsorption breakthrough curve duringthe adsorption step. The adsorption step was followed by thedesorption step with a switch of the feed gas from air to CO2,and O2 concentration of the effluent stream was also measuredto give oxygen desorption breakthrough curve, as indicated inFig. 2 by the curve from ta to td.

3. Results and discussion

3.1. Effects of adsorption conditions and reversibility

Effects of the oxygen adsorption time and feed gas flow rateon efficiency of the oxygen separation process were studied in

this section. Fig. 3 is the comparison of the five desorption break-through curves at different adsorption time. As seen in Fig. 3,a longer adsorption time leads to a larger amount of oxygenadsorbed, and therefore a larger desorption area in the desorp-tion breakthrough curve (the desorption area is defined as thearea under desorption breakthrough curve during the time whenthe oxygen concentration is above 20%). However, differentfrom physical adsorption [24], the oxygen adsorption processis relatively slow due to the extreme slow reaction kinetics [23].Therefore, it is necessary to search for a proper adsorption time,with a compromise between the amount of oxygen adsorbed andthe duration of the adsorption process.

Since the flow rates of desorption feed gas are the same(5 ml/min) for all the five desorption runs shown in Fig. 3, thedesorption area is roughly proportional to the amount of oxy-gen produced. The desorption area divided by the correspondingadsorption time can serve as an indicator for oxygen separationefficiency, and the results are tabulated in Table 1. As seen inTable 1, the experimental runs with the adsorption time of 0.5and 1.5 h are more efficient than those with longer adsorptiontime. The adsorption time at 1.5 h is used to study the effects ofother operation conditions on the separation results.

Fig. 2. One cycle of sorption and desorption breakthrough curves.

Fig. 3. Desorption breakthrough curves at different adsorption time.

30 Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35

Table 1Effect of adsorption time on the adsorption and desorption processes

Adsorption time (h) O2 concentration in effluentat the end of adsorption (%)

Time range for collecting thedesorption product (t1–t2) (s)

Desorption area (% s) Ratio of desorption area toadsorption time (% s/h)

0.5 17.6 302–418 38.3 76.61.5 18.4 425–750 107.6 71.73.5 18.6 575–1012 168.6 48.26.5 19.4 575–1362 328.9 50.6

11 19.6 712–1862 471.7 42.9

Adsorption and desorption are all at 800 ◦C.

Fig. 4. Adsorption breakthrough curves at different flow rates of adsorption feedgas (air).

Fig. 4 compares the oxygen adsorption breakthrough curvesat different feed gas (air) flow rates, 5 and 10 ml/min. The break-through curves are characterized by a very long tail, due to themuch slower adsorption rate than the feed gas flow rate. Thetwo breakthrough curves are similar in shape, except that theone with slower flow rate gives longer breakthrough time. Thisindicates variation of the flow rate in this range does not have asignificant effect on the breakthrough curve shape. Fig. 5 is thecomparison of the desorption breakthrough curves with a pre-ceding adsorption step for 1.5 h at different feed gas (air) flowrates. As seen in Fig. 5, a higher flow rate of feed gas during

Fg

adsorption leads to a larger desorption area and higher maxi-mum oxygen concentration in desorption curves. This is dueto the larger amount of oxygen adsorbed during the adsorptionprocess at a higher air flow rate for the same adsorption time(1.5 h).

Efficiency of the adsorption separation process at differentflow rates of air requires further consideration. Four parameters,i.e., average oxygen concentration of desorption effluent, oxygenrecovery, CO2 recovery and productivity, are defined to evaluatethe separation process. Average oxygen concentration (χO2 ) inthe desorption effluent is defined as

χO2 =∫ t2t1

Fd,outχO2 dt∫ t2t1

Fd,out dt(1)

where Fd,out is the flow rate of desorption effluent, χO2 the oxy-gen concentration in the desorption effluent, t1 the starting timefor collecting the desorption product when the oxygen concen-tration in the effluent reaches 20%, and t2 is the ending timefor the desorption process when the oxygen concentration in theeffluent drops to 30%.

Oxygen recovery (RO2 ) is defined as ratio of oxygen obtainedin the desorption effluent to the amount of oxygen fed in theadsorption step:

RO =∫ t2t1

Fd,outχO2 dt(2)

wa

Cd

R

ws

da

P

a

ig. 5. Desorption breakthrough curves at different flow rates of adsorption feedas (air) and the same adsorption time (0.5 h).

2 χO2,inFair,inta

here χO2,in is the oxygen concentration in adsorption feed gas,ir, Fair,in the flow rate of air, and ta is the adsorption time.

CO2 recovery (RCO2 ) is defined as the ratio of amount ofO2 collected in the effluent to the amount of CO2 fed duringesorption:

CO2 =∫ t2t1

Fd,out(1 − χO2 ) dt

FCO2,intd(3)

here FCO2,in is the flow rate of CO2 fed during the desorptiontep and td is the desorption time.

Productivity (P) is defined as the ratio of the amount of theesorption effluent, as separation product, to the total time ofdsorption and desorption processes, and is given as

=∫ t2t1

Fd,out dt

tcycle(4)

These four parameters for separation process at differentdsorption feed gas flow rates are compared in Table 2. It shows

Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35 31

Table 2The effect of flow rate of adsorption feed gas (air) on the separation results

Air flow rate (ml/min) Time range to collect the productin desorption (t1–t2) (s)

Average O2 concentration ofproduct gas (%)

O2 recovery (%) CO2 recovery (%) Productivity(ml/min)

5 420–603 39.4 6.62 11.4 0.1138 469–716 44.0 5.02 11.7 0.121

10 501–779 47.4 4.50 13.6 0.149

Adsorption and desorption are all at 800 ◦C.

that increasing the flow rate of the adsorption feed gas within acertain range would increase the average oxygen concentration,CO2 recovery and productivity of the separation process, butreduce oxygen recovery. In other words, the average oxygen con-centration, CO2 recovery and productivity of whole separationprocess could be increased at the expense of oxygen recovery.This is because a higher air flow rate would result in more oxy-gen adsorbed during the adsorption step and a higher efficiencyof the desorption step that follows.

To investigate reversibility of the sorption process, five cyclesof adsorption and desorption were continuously monitored. Theflow rate of feed gas and the temperature for both adsorption anddesorption process were fixed at 5 ml/min and 800 ◦C, respec-tively. The adsorption time is 11 h for each cycle to ensure suf-ficient oxygen adsorbed. Essentially identical desorption break-through curves were obtained for each cycle. The desorptionarea, which indicates the amount of oxygen produced, is cal-culated for each cycle and compared in Table 3. The areas offive curves are almost the same with only 2.7% decrease afterfive cycles, which is within the range of experimental error. Thisresult seems to suggest a good reversibility of the sorption anddesorption processes. However, morphology of sorbent appearssomehow changed to a more dense structure after cycles of sorp-tion process, which might vary the packing efficiency of sorbentsand affect the reversibility in the even long-term operation.

3

ebtoFmpn

TC

C

12345

A

Fig. 6. Desorption breakthrough curves at different flow rates of desorption feedgas (CO2).

physical desorption, the effluent flow rate are usually keepsconstant till the end of desorption step. This difference can beexplained by a two-step reaction mechanism, similar for SO2oxidation–sorption on metal oxides [25], for reaction (A), asdescribed by [23]:

CO2 + A ⇔ A∗ (B)

A∗ + A → A + B + O2 (C)

During the desorption process, CO2 first reacts with the per-ovskite crystalline A to form an intermediate complex A*. Thisis followed by the second step, reaction (C), in which A* reactswith the neighboring crystalline A to form solid product B andO2 at the same time. Therefore, adsorption of CO2 in reaction

Ff

.2. Effect of desorption flow rate

This section focuses on the effect of the CO2 flow rate onfficiency of the separation process. Fig. 6 shows desorptionreakthrough curves at different CO2 feed flow rates duringhe desorption step. The corresponding flow rates of the des-rption effluent at different CO2 feed flow rates are given inig. 7. The flow rate curves are different from those in nor-al physical desorption. The flow rate curves of this separation

rocess are characterized by an immediate decrease at the begin-ing followed by a gradual increase with time. While in normal

able 3omparison of five cycles of desorption area

ycle Desorption area (% s)

613.8612.0605.0605.3597.5

dsorption and desorption are all at 800 ◦C.

ig. 7. Flow rate curves of desorption effluent at different flow rates of adsorptioneed gas (air).

32 Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35

(B) during the initial stage of desorption process results in a verylow flow rate of the effluent (0–0.1 ml/min), as seen in Fig. 7. Astime goes, CO2 reaction with perovskite A becomes slower somore CO2 not reacting with the sorbent come out of the adsor-ber column together with O2 produced by reaction (C), whichresults in a slow increase in the flow rate.

On the other hand, the oxygen desorption breakthroughcurves are different from those in normal physical desorptionas well. As seen in Fig. 6, the desorption breakthrough curveshave a maximum oxygen concentration, which usually do notappear in normal physical adsorption. Such a difference is due tothe specific kinetics of the desorption reaction: according to theTGA study on carbonation kinetics [23], the CO2 reaction withLSCF sorbent, reaction (A), exhibits a very fast rate followedby a gradual transition to a slower rate. The highest reactionrate at the beginning leads to fastest release of oxygen and thusthe maximum oxygen concentration in the effluent. The gradualdecrease of the oxygen concentration followed is consistent withthe decrease of the reaction rate with time.

As shown in Figs. 6 and 7, a higher flow rate of the desorptionfeed gas leads to the desorption breakthrough curve and effluentflow rate curve with a shorter time. To better illustrate the effectsof the desorption feed gas flow rate, Table 4 compares the sepa-ration parameters, as defined in Eqs. (1)–(4), for the desorptionruns shown in Figs. 6 and 7. In the range of the CO2 feed flow rateof 5–10 ml/min, the higher flow rate leads to the higher averageordgma

ivwC(eiInTflmo

As shown in Table 4, in the whole range of the CO2 feedflow rate from 5 to 15 ml/min, the faster flow rate results in thelower oxygen recovery. As previously explained, the desorp-tion process consists of two stages with different reaction rates,and the transition point depends on the concentration of activesites available in the perovskite-type sorbent. In the fast reac-tion stage, the amount of oxygen produced should be the samein all the five desorption runs due to the same amount of activesites available for reaction. In the second stage with low reactionrate, the feed gas flow rate has no effect on the rates of the sor-bent reaction and oxygen desorption due to the excess of CO2fed. The oxygen produced is proportional to the duration of thisstage. Since CO2 dilutes the produced oxygen stream, a fasterCO2 flow rate leads to a faster decrease of the oxygen concen-tration to 30%, the end point of the desorption process. As seenin Fig. 7, the duration for the slow reaction stage is obviouslyshorter for the higher CO2 feed flow rate. This explains whyoxygen recovery decreases with increase of the CO2 feed gasflow rate.

The effect of the CO2 flow rate on CO2 recovery is oppositeto that on oxygen recovery. As shown in Table 4, the higherthe CO2 feed flow rate, the larger the CO2 recovery. This isalso due to the slow reaction rate of the second stage (reaction(C)). In the first stage, all CO2 reacts with the sorbent. In thesecond stage, CO2 fed is more than what is needed for reaction.Therefore, a higher flow rate of CO feed flow rate leads tohott

3

dsffihooraaor

TE

C(

2

ion (%

11

A

xygen concentration in the effluent. However, as the CO2 flowate increases to 15 ml/min, the average oxygen concentrationecreases. As seen in Fig. 6, the corresponding maximum oxy-en concentration in the effluent changes in a similar way. Theaximum oxygen concentration is the highest at ∼10 ml/min

nd starts to decrease afterwards.This phenomenon could also be explained by the TGA kinet-

cs of reaction (A): The CO2 reaction with LSCF sorbent exhibitsery fast kinetics in the first stage, followed by a second stageith much lower rate. In this fixed-bed desorption process, theO2 flow rate is the rate-limiting factor within a certain range

<10 ml/min). To increase the CO2 flow rate would help accel-rate the reaction in the first stage. However, a further increasen the feed flow rate does not affect the reaction rate any more.nstead, it results in dilution of oxygen in the effluent. This expla-ation could be confirmed from the flow rate curves in Fig. 7.he lowest level in the effluent flow rate curves for the CO2 feedow rates smaller than 10 ml/min is in the range of 0–0.1 ml/min,uch smaller than that (0.6 ml/min) for the CO2 feed flow rate

f 15 ml/min.

able 4ffect of flow rate of desorption feed gas (CO2) on the separation process

O2 flow rateml/min)

Time range to collect the productin desorption (t1–t2) (s)

Average Oconcentrat

5 420–603 39.66.5 272–398 40.47.7 261–390 42.10 188–279 43.45 107–158 39.7

dsorption and desorption are all at 800 ◦C.

2igher percentage of CO2 exiting from the fixed-bed. The effectf CO2 feed flow rate on the productivity is not so significant inhis separation process due to the much shorter desorption timehan the adsorption time.

.3. Temperature effects

The effects of fixed-bed temperatures in the adsorption andesorption steps on the separation process are discussed in thisection. Fig. 8 shows the desorption breakthrough curves at dif-erent adsorption temperatures of 800, 850 and 900 ◦C (withxed desorption temperature at 800 ◦C). As seen in Fig. 8, aigher adsorption temperature leads to a longer delay of thexygen desorption breakthrough time and a larger amount ofxygen produced in the desorption process. The effluent flowates in the desorption step at two different adsorption temper-tures are shown in Fig. 9. Consistent with Fig. 8, increasingdsorption temperature delays and slows the increase in the des-rption effluent flow rate, suggesting a longer first stage of fasteaction in the desorption process. The oxygen adsorption at

)Oxygenrecovery (%)

CO2 recovery(%)

Productivity(ml/min)

6.62 11.4 0.1136.55 14.8 0.1366.45 15.3 0.1296.10 15.9 0.1325.16 18.1 0.116

Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35 33

Fig. 8. Desorption breakthrough curves at different adsorption temperatures(desorption temperatures fixed at 800 ◦C).

higher temperature is faster, resulting in more oxygen adsorbedwith the fixed adsorption time (1.5 h). This also leads to moreoxygen recovered in the desorption step with higher efficiency.

Table 5 shows the separation parameters defined in Eqs.(1)–(4) for experiments at different adsorption temperatures.The oxygen recovery, CO2 recovery, average oxygen concen-tration in the desorption effluent, and productivity increaseswith increasing adsorption temperature. Again, this is because oflarger amount of oxygen adsorbed due to faster sorption kineticat a higher adsorption temperature (for fixed adsorption time).The four separation parameters for adsorption at 850 ◦C are veryclose to those at 900 ◦C. If the energy cost in oxygen separationis considered, 850 ◦C might be the optimal adsorption tempera-ture.

Fig. 10 compares the desorption breakthrough curves at dif-ferent desorption temperatures at fixed adsorption temperatureof 800 ◦C. As shown in Fig. 10, a higher desorption tempera-ture leads to a shorter oxygen desorption breakthrough time anda larger amount of oxygen produced. The separation param-eters with desorption step at 800 and 900 ◦C are summarizedin Table 6. As seen in Table 6, the average oxygen concen-tration, oxygen recovery, CO2 recovery and productivity are allimproved with increasing desorption temperature. The more effi-

F

Fig. 10. Desorption breakthrough curves at different desorption temperatures(adsorption temperatures fixed at 800 ◦C).

cient desorption process at the higher desorption temperature isdue to the faster kinetics of reaction (A). Moreover, at highertemperatures, the perovskite-type sorbent may also undergo thefollowing defect reaction in the initial stage of exposure to CO2[26]:

Oo× + 2h• = (1/2)O2(g) + Vo

•• (D)

where Oo× is the lattice oxygen, Vo

•• the oxygen vacancy andh• is the electronic-hole. Thus, extra oxygen is generated duringthe desorption step at high temperatures.

The effluent flow rates in the desorption step at differentadsorption temperatures are shown in Fig. 11. The flow ratecurves at 900 ◦C desorption temperature are different from theone at 800 ◦C. The flow rate curves at 900 ◦C exhibit a firstpeak possibly due to desorption reaction (D), followed by a slowincrease in flow rate due to reaction (A). At 800 ◦C, the effluentflow rate curve only reassembles the slow increase portion for thecurves at 900 ◦C, indicating the negligible contribution of reac-tion (D) to the desorption process at 800 ◦C. Fig. 12 comparesoxygen desorption breakthrough curves at 900 ◦C for the pre-ceding adsorption step run at 800 and 850 ◦C. Since 850 ◦C was

Fo

ig. 9. Flow rates of desorption effluent at different adsorption temperatures.

ig. 11. Flow rate curves of desorption effluent at different adsorption and des-rption temperatures.

34 Q. Yang, J.Y.S. Lin / Separation and Purification Technology 49 (2006) 27–35

Table 5Effect of adsorption temperature on the separation process

Adorptiontemperature (◦C)

Time range to collect theproduct in desorption (t1–t2) (s)

Average oxygenconcentration (%)

Air recovery(%)

CO2

recovery (%)Productivity(ml/min)

800 420–603 39.6 6.62 11.4 0.113850 495–805 48.6 8.18 12.8 0.132900 568–906 48.9 9.80 12.9 0.157

Desorption temperature fixed at 800 ◦C.

Table 6Effect of desorption temperature on the separation process

Adsorptiontemperature (◦C)

Desorptiontemperature (◦C)

Time range to collect the productin desorption (t1–t2) (s)

Average oxygenconcentration (%)

Air recovery(%)

CO2 recovery(%)

Productivity(ml/min)

800 800 420–603 39.6 6.62 11.4 0.113800 900 210–510 50.1 15.3 34.0 0.260850 900 220–670 58.8 18.7 26.6 0.272

Fig. 12. Desorption breakthrough curves at different adsorption and desorptiontemperatures.

identified as the optimum adsorption temperature, a much largerdesorption area is observed for the curve obtained at 900 ◦Cfor the preceding adsorption step run at 850 ◦C. As indicated inTable 6, the separation experiment run at desorption and adsorp-tion temperatures of 900 and 850 ◦C gives improved averageoxygen concentration and oxygen recovery.

4. Conclusions

The adsorption and desorption of oxygen using theperovskite-type ceramic LSCF as sorbent were studied by thefixed-bed experiments. Effects of operation conditions, includ-ing adsorption time, flow rates of adsorption and desorption feedgases, and adsorption and desorption temperatures, on separa-tion efficiency were investigated in terms of oxygen concen-tration of the product stream, oxygen recovery, carbon dioxiderecovery and productivity. Experimental results show that ashorter adsorption time gives lower desorption efficiency buthigher productivity. A proper adsorption time exists for the bestperformance. Increasing the air feed flow rate in a certain range(<10 ml/min) during adsorption can improve the oxygen concen-

tration in the product stream and CO2 recovery at the expenseof oxygen recovery. Increasing CO2 feed flow rate during des-orption within 10 ml/min could enhance the reaction kinetics inthe initial desorption stage, resulting in an increase of the aver-age oxygen concentration in the effluent. However, it decreasesthe oxygen recovery due to the shorter desorption duration, andincreases CO2 recovery due to slower CO2 reaction rate in thesecond stage as compared to the CO2 feed rate. Furthermore,increasing either adsorption temperature or desorption temper-ature helps improve greatly the oxygen concentration, oxygenrecovery, CO2 recovery and productivity. It is found that 850 and900 ◦C are the optimal adsorption and desorption temperatures.A combination of the two optimal temperatures in the adsorptionand desorption processes makes a remarkable improvement foroxygen separation, with average oxygen concentration, oxygenrecovery, carbon dioxide recovery and productivity, respectively,of 58.8%, 18.7%, 26.6% and 0.272 ml/min.

Acknowledgements

The work was supported by National Science Foundation(CTS-0132694), Department of Energy (DE-FG26-00NT4081)and the BOC Group.

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