Alkali and alkaline-earth cation exchanged chabazite zeolites for adsorption based CO2 capture

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Microporous and Mesoporous Materials 111 (2008) 478–487 www.elsevier.com/locate/micromeso

Alkali and alkaline-earth cation exchanged chabazite zeolites for adsorption based CO2 capture Jun Zhang, Ranjeet Singh, Paul A. Webley

*

CRC for Greenhouse Gas Technologies, Department of Chemical Engineering, Monash University, Victoria 3800, Australia Received 27 March 2007; received in revised form 5 August 2007; accepted 16 August 2007 Available online 24 August 2007

Abstract In this study chabazite zeolites were prepared and exchanged with alkali cations – Li, Na, K and alkaline-earth cations – Mg, Ca, Ba and were studied to assess their potential for CO2 capture from flue gas by vacuum swing adsorption for temperatures below 120 C. Isotherm measurements (CO2 and N2) were made for all samples at 273 K, 303 K and 333 K using a volumetric apparatus and represented with the Dual-site Langmuir model for CO2 and N2. Henry’s constants and isosteric heats of adsorption were calculated and qualitative analyses performed for all samples. Adiabatic separation factor (ASF) and capture figure of merit (CFM) were proposed and used as indices for assessing adsorbent performance and compared with a commercial NaX-zeolite sample. It was found that NaCHA and CaCHA hold comparative advantages for high temperature CO2 separation whilst NaX shows superior performance at relatively low temperatures.  2007 Elsevier Inc. All rights reserved. Keywords: Chabazite; Vacuum swing adsorption; Heat of adsorption; Cation; Alkali; Alkaline-earth; CO2 capture

1. Introduction It is widely acknowledged that CO2 emissions make the major contribution to global warming and their reduction is urgently needed [1]. The capture and storage of CO2 emitted from major industries, such as the steel industry and power industry, have been proposed by many governments and major energy agencies. Various technologies for carbon capture and storage (CCS) from flue gas of power plants have been proposed, among which absorption, adsorption, membrane and cryogenic processes are the leading candidates. Because of its relatively low operating and capital costs, pressure/vacuum swing adsorption (PSA or VSA) has attracted much research effort. Research aspects of cyclic adsorption processes include cycle design, process optimization, new adsorbent development, and equipment innovation. Recent research in the field of

*

Corresponding author. Tel.: +61 3 9905 3628; fax: +61 3 9905 5686. E-mail address: [email protected] (P.A. Webley).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.08.022

adsorbents, which are the most important aspect of the adsorption process, has made rapid progress in the last decade and X, Y, A, ZSM-zeolites, chabazites, metal oxides, various carbons etc. have been widely studied [1–5]. Among those adsorbents, chabazite zeolite is a promising but often ignored option. Chabazite (CHA) is of great interest for its ion-exchange and gas separation possibilities. Naturally occurring chabazite is often found in cavities in basalt, andesite, and other igneous rocks, and as an alteration product of volcanic glass. Chabazite from the Bowie deposit has been used successfully at the Oak Ridge, Hanford, and Savannah River Nuclear Facilities to remove radioactive cesium-137 and strontium-90 from process waters [2,3]. Chabazite is also used commercially as a desiccant. It is stable at a pH of 2.5 which makes it suitable for removing water from hydrogen chloride gas streams, and trace gas removal, such as nitrogen removal from argon, oxygen separation from argon [4–6]. The chabazite structure consists of double 6-ring (D6R) building units arranged in layers in the sequence ABCABC and linked by tilted 4-rings. This results in a 3-D

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Nomenclature B D LCO2 LN2 M n PH PL T1 T2 yi

adsorption coefficient (1/kPa) adsorption coefficient (1/kPa) CO2 loading (g mol) N2 loading (g mol) adsorption constant (g mol/kg) adsorption amount (g mol/kg adsorbent) adsorption pressure (kPa) desorption pressure (kPa) adsorption temperature (K) desorption temperature (K) CO2 concentration

framework which contains large ellipsoidal cavities each ˚ . Each cavity can be accessed by approximately 6.7 · 10 A 6 openings each of which consists of 8-rings of somewhat variable dimension depending on extent and type of ion exchange and adsorption. These 8-ring windows have ˚ (hydrated) or 3.1 · approximate dimension 3.7 · 4.2 A ˚ (dehydrated). A cation-free CHA has a nominal 4.4 A ˚ providing a maxi8-ring window dimension of 3.8 · 3.8 A ˚ . This is considerably changed mum free dimension of 4.3 A when cations are present. Kington’s systematic studies on the crystal structure of chabazites noted that the size of silicate rings and the cations were the influential factors for the adsorption energy of a given adsorbate [7]. Based on the results of structural study and further calorimetric experiments on natural and calcium chabazites, it was found that the heat of adsorption at low coverage is related to the electrostatic field within the intracrystalline voids; also it was noted that the major contribution to energetic heterogeneity of adsorption is the interaction of the adsorbate quadrupole moment with the electrostatic-field gradient in the adsorbent [8]. Regarding the specific research of cation effects on CO2 adsorption in chabazite, there are very few studies. Barrer and Davies [9] conducted a study of adsorption of a range of gases with permanent quadrupole moment, including CO2, on decationated (hydrogen) chabazite and found that the initial isosteric heats of adsorption of the gases correlated well with their polarizability. They concluded that the gases with permanent quadrupole moment were bound by considerable field-gradient quadrupole energy. A study conducted by Khvoshchev et al. indicated that Ca-chabazite had greater adsorption heat than Na-chabazite and partial cation exchange of Ca-Chabazite with Mg2+ lead to the sharp decrease of the heat of adsorption [10]. Inui, Okugawa and Yasuda studied the adsorption behaviour of different zeolites by the PSA process and claimed that chabazite and 13X were most appropriate for CO2 separation among the zeolites studied, which included chabazite, clinoptilolite, mordenite, ferrierite, erionite, MS-5A, MS-4A, MS13X, H-ZSM-5 [11]. Coe and Gaffney found that at around Si:Al = 2, chabazite has the largest adsorption capacity for

Greek letters a adiabatic separation factor / total adsorption energy /D dispersion energy /Fl field–dipole interaction, zero for the case of _ CO2 /FQ field gradient–quadrupole interaction _ /R repulsion energy /P polarization energy /SP adsorbate–adsorbate interaction energy, zero for low coverage

N2. They concluded that cation sitting, Si/Al ratio and activation temperature all played a role in N2 capacity [4]. Although much work has been done in structural and functional analysis, synthetic cation selectivity and siting, and CO, O2, N2, CH4 adsorption, little work has been done on the adsorption of CO2 on chabazite exchanged with different cations in terms of CO2 capture from flue gas. Data for the heat of adsorption are rarely reported and applications of chabazite in P/VSA processes for CO2 capture from flue gas have not been reported. In contrast, cation effects on CO2 adsorption in zeolite X,Y and ZSM have been widely studied [12–15]. Alkali metal cation effects on CO2 adsorption in Y and X zeolites were studied and it was concluded that Li cation exchanged X/Y zeolites have the largest CO2 capacities as a result of greatest ion–quadrupole interactions with CO2, although no CO2/N2 selectivity was reported [14]. In an experimental adsorbent screening study conducted by Harlick et al., using isotherm equilibrium analysis, various zeolites (excluding chabazite) were examined and it was claimed that an adsorbent with a near linear isotherm and a low SiO2/Al2O3 ratio is the most promising option for CO2 capture [16]. However, this analysis used ideal pressure swing cycles and overlooked the important role of the heat of adsorption in altering the adsorbent temperature and hence its working capacity. It is essential to include this effect since operation of a PSA process with strongly adsorbed species such as CO2 lead to large temperature swings. Indeed, temperature swings up to 20 K have been observed in our own pilot scale CO2 PSA process [17]. A good adsorbent must have good adiabatic working capacity, and adiabatic working selectivity which can be determined either by experiments or simulation. These two parameters can be combined into appropriate adsorbent screening parameters: the adiabatic separation factor (ASF) and capture figure of merit (CFM) which are very good predictors of an adsorbents separation performance under actual process conditions. The goal of our study is several-fold: First, we synthesize a range of alkali and alkaline-earth cation exchanged chabazite materials and measure the adsorption isotherms,

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zero-loading Henry’s constants and heats of adsorption of CO2 and N2 over a range in temperature and pressure and explain these effects based on energetics of the CO2–cation interaction. We then determine the CO2 working capacity, selectivity, adiabatic separation factor and capture figure of merit for the CO2/N2 separation on chabazite which allows us to assess the suitability of this adsorbent for CO2 capture from flue gas streams.

1 M CaCl2 (5 g zeolite: 300 ml CaCl2). The solution was heated at 353 K for 12 h, decanted and fresh solution was added. This procedure was repeated five times. Finally, the Ca-CHA formed was filtered and washed with copious amount of de-ionized water and dried at 373 K overnight. Barium and magnesium forms of chabazites were prepared from BaCl2 and MgCl2 solutions in a manner similar to that described above for Ca-CHA.

2. Experimental

2.2. Isotherm measurements

2.1. Chabazite preparation

Measurement of CO2 and N2 isotherms for all samples were conducted on a modified Micromeritics ASAP2010 system at temperatures of 273 K, 303 K and 333 K respectively. Temperature control was achieved by forced circulation of water from a water bath with heating and chilling functions. A novel glass Dewar was designed to cater for the purpose of beyond-ambient temperature isotherm measurement. By use of ethylene glycol and de-ionized water mixture as heating fluid and minimizing the length of sample tube exposed to the atmosphere, the maximum error for isotherm measurement was reduced to less than 0.0025 mmol/g. The addition of ethylene glycol was to increase the fluid boiling point and thus to reduce the vapour heating effect of the tube. This low cost innovative modification made the above-ambient temperature isotherm measurement using ASAP2010 possible, convenient and accurate. With this design, isotherms over a pressure range from 0 to 120 kPa and temperature from 273 K to 363 K can be measured very accurately. In addition to these isotherms measurements, the surface areas (BET) of the samples in this study were measured through N2 adsorption at liquid N2 temperature (77 K) which is a standard operation of the ASAP2010. In those experiments, all samples were degassed carefully at low heating rates (1 K/min) under vacuum overnight at 623 K.

The cation exchanged chabazite samples, which include alkali series-Li-chabazite, Na-chabazite, K-chabazite and alkaline-earth series – Ca-chabazite, Mg-chabazite, Bachabazite were prepared as described below. To confirm the ion-exchange level, elemental distributions and Si/Al ratios, elemental analyses were done by inductively coupled plasma-mass spectroscopy (ICP-MS). 2.1.1. Synthesis of chabazite Chabazite was synthesized following the procedure reported by Bourgogne et al. with gel composition 0.17 Na2O:2.0 K2O:5.18 SiO2:Al2O3:224 H2O [18]. A typical procedure involved mixing of the required amounts of potassium hydroxide (45% solution) and water, followed by addition of zeolite Y (H-form). The mixture was shaken and transferred into a polypropylene bottle (500 ml, FEP, Nalgene) and heated in an oven for 8 days at 368 K. The polypropylene bottle was quenched with cold water, the product obtained was filtered, washed with water, and dried in an oven at 373 K. The crystal structure of chabazite and degree of crystallinity was confirmed by powder XRD. 2.1.2. Cation exchange Chabazite obtained as described above was first converted into its sodium form (Na-CHA) by three consecutive ion exchanges with 1 M NaCl. Typically, 300 ml 1 M NaCl at pH 9 (adjusted by addition of 0.01 M NaOH) was added to 5 g of zeolite, and the solution was heated to 368 K and stirred for 12 h. The solution was decanted and fresh solution was added. This procedure was repeated three times. After the final exchange the solution was vacuum-filtered and washed with copious amount of de-ionized water. The resulting sodium chabazite was dried at 373 K overnight. The potassium form of chabazite (KCHA) was prepared from Na-CHA by three consecutive ion exchanges using 1 M KCl (pH 9 was adjusted by addition of 0.01 M KOH) and using a similar procedure as that used for preparation of Na-CHA. Lithium chabazite was prepared from Na-CHA, by five consecutive ion exchanges of Na-CHA with 2 M LiCl (3 g zeolite: 100 ml LiCl) at pH 9 (adjusted by addition of 0.01 M LiOH). The calcium form of chabazite (Ca-CHA) was prepared from NaCHA by five consecutive ion exchanges of Na-CHA with

3. Results and discussion 3.1. Elemental analysis data In order to confirm the extent of ion-exchange, which is also very important for the analysis of cation effect, ICPMS analysis was conducted. The subsequent unit cell formula for each sample is shown in Table 1.

Table 1 Alkali and alkaline-earth ionic radius and elemental analysis Sample

Ionic radius (nm)

Unit cell formulaa

Si/Al, atom

LiCHA NaCHA KCHA MgCHA CaCHA BaCHA

0.068 0.095 0.133 0.065 0.099 0.135

Na2.1Li8.5 [Al10.6Si25.4O72] Na10.3 [Al10.3Si25.7O72] K8.3Na2.3[Al10.6Si25.4O72] Na3.3Mg3.8 [Al10.9Si25.1O72] Na0.7Ca4.9 [Al10.6Si25.4O72] Na0.2Ba5.2 [Al10.6Si25.4O72]

2.4 2.5 2.4 2.4 2.4 2.4

a

As derived from ICP-MS data.

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3.2. CO2 and N2 isotherms CO2 and N2 isotherms for all chabazite samples and one NaX pellet sample (for comparison) from UOP were measured over a range of temperatures and pressures (0–120 kPa). Figs. 1 and 2 shows a representative set for CO2 and N2 at 273 K. Additional data were taken at the low pressure range (not shown) to improve estimation of Henry’s Law constants. The figures show both the data and the curve fit according to the Dual-site Langmuir

481

(CO2) or single-site Langmuir (N2) models as described earlier. The isotherms are uncoupled and the Dual-site Langmuir fit was also applied to N2 but was reduced to a single-site as the parameters for the second site were redundant. The use of uncoupled isotherms would cause certain errors in simulation because of competition for adsorption sites between CO2 and N2, however, these errors can be neglected in a preliminary adsorbent screening process. A Virial plot for CO2 on the CHA samples is shown in Fig. 3 and was used to determine Henry’s constants.

Amount adsorbed, gmol/kg adsorbent

6

4

LiCHA NaCHA KCHA MgCHA CaCHA BaCHA NaX

2

0 0

20

40

60

80

100

120

Pressure, kPa Fig. 1. Adsorption isotherms for CO2 on a range of ion-exchanged chabazite zeolites at 273 K.

Amount adsorbed, gmol/kg adsorbent

1.4

BaCHA NaCHA KCHA LiCHA MgCHA CaCHA NaX

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0

20

40

60

80

100

120

Pressure, kPa Fig. 2. Adsorption isotherms for N2 on a range of ion-exchanged chabazite zeolites at 273 K.

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100.00

10.00

P/q

1.00

0.10

Ba-CHA Ca-CHA Mg-CHA K-CHA Na-CHA Li-CHA

0.01

0.00 0.0

1.0

2.0

3.0

4.0

5.0

6.0

q(gmole/kg) Fig. 3. Virial plot of equilibrium isotherms for CO2 on a range of ion-exchanged chabazite zeolites.

The Henry’s constant is directly related to the interaction of the molecules with the surface of the adsorbent since at low pressure, molecule-surface forces predominate. Table 2 shows the isotherm parameters and Henry’s constant for all samples. Fig. 4 shows the dependence of the log of Henry’s constant on the cation charge density (valence/ ionic radius3) and we see two series: one for the alkali metals and one for the alkali-earth metals. At near-zero coverage, the sequence of CO2 adsorption capacity under 273 K is BaCHA > CaCHA > NaCHA > MgCHA > KCHA > LiCHA which changes as adsorbate coverage increases to LiCHA > NaCHA > CaCHA > KCHA > MgCHA > BaCHA Barium chabazite has a very steep adsorption curve at low coverage which is also indicated in Fig. 4 by the high 5

+2

Ba

+2

Ca

4.5

ln (Henry's constant)

Henry’s constant. LiCHA on the other hand shows the lowest adsorption at low pressure but presents the largest capacity at 100 kPa among all samples. This sequence also agrees with the sequence of CO2 adsorption amounts on Li, Na, and K exchanged X zeolites at high coverage [14]. This consistency of results may be explained by the interaction of the CO2 molecular quadrupole moment and the electrostatic field of the chabazite. The intensity of the electrostatic field (in particular the charge density) increases in the sequence of K < Na < Li resulting in enhanced electrostatic field and higher CO2 adsorption energy. In addition, the pore sizes of LiCHA and NaCHA are larger than that of KCHA and correspondently the pore volumes LiCHA and NaCHA are larger than KCHA – this accounts for the larger adsorption capacity in LiCHA and NaCHA at higher pressure. In contrast, the alkaline-earth cation chabazites exhibits different trends and different sequence changes, which may be a result of framework oxygen shielding of

4 3.5 +1

Na

3

Mg

2.5 K

+2

+1

2

+1

Li

1.5 1 0.5 0 0

1

2

3

4

5

6

7

cation charge density (valence/radius3) Fig. 4. Henry’s constant dependence on cation charge density at 273.15 K.

8

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483

Table 2 Langmuir isotherm parameters and Henry’s Law constant (273 K) for CO2 and N2 on a range of metal exchanged chabazite zeolites Sample

M1 (g mole/kg)

B10 (1/kPa)

Q1 (J/g mole)

M2 (g mole/kg)

B20 (1/kPa)

Q2 (J/g mole)

KH(273 K) (g mole/kg . kPa)

CO2 LiCHA NaCHA KCHA MgCHA CaCHA BaCHA

3.0774 3.8968 3.8113 2.1131 1.052 1.7465

1.88 · 1006 4.80 · 1007 7.81 · 1008 7.30 · 1008 5.51 · 1008 1.31 · 1004

31,850 37,505 39,851 41,533 47,805 30,201

2.0296 1.2808 1.0349 2.243 3.4242 1.4393

2.13 · 1009 5.50 · 1010 2.20 · 1004 2.84 · 1007 8.23 · 1008 1.87 · 1003

44,177 41,300 9311 29,764 35,660 11,782

8.321 28.11 12.88 13.83 82.16 100.28

N2 LiCHA NaCHA KCHA MgCHA CaCHA BaCHA

2.2226 2.0465 1.6234 1.5545 0.7887 1.2547

9.132 · 106 1.191 · 106 16.33 · 106 1.741 · 106 2.337 · 106 90.31 · 109

16,218 20,848 13,501 19,007 18,612 30,932

– – – – – –

– – – – – –

– – – – – –

the cation electrostatic field and makes the explanation of the interaction of CO2 and the cation much more complicated. At high coverage (100 kPa, 273 K), the adsorption capacity is CaCHA > MgCHA > BaCHA. CaCHA has higher charge density than BaCHA which explains the greater CaCHA capacity. However, MgCHA has higher charge density but less adsorption capacity than CaCHA. One possible reason is that due to its very high charge density, MgCHA was not completely dehydroxylated during activation [19], and subsequently would adsorb less CO2. Although CO2 adsorption capacity is important, the capacity of the zeolite for N2 should not be ignored since it is the CO2/N2 separation selectivity that dominates the eventual recovery of CO2 from the VSA process. At 333 K, the adsorption capacity of N2 at 100 kPa is in the sequence

0.0256 0.0236 0.0101 0.0117 0.0067 0.0932

3.3. Heat of adsorption

and at 273 K,this becomes

The isosteric heats of adsorption of CO2 were calculated by use of Clapeyron equation at different temperatures. The isosteric heat of adsorption for CO2 at 273 K is shown in Fig. 5 as a function of CO2 loading. For low coverage, the sequence is MgCHA > KCHA > NaCHA > CaCHA >LiCHA > BaCHA and all are in the range 30–42 kJ/ g mol. Alkali metal cations and alkaline-earth metal cations present very different behaviour. For alkali metal cation type chabazites, the initial heat of adsorption decreases with the ionic radius. The heats of adsorption for NaCHA and LiCHA are changed very little, if any, with the increase of loading while for KCHA decreases very slowly from around 0–3 mmol/g adsorption amount and has a sharp decrease after the adsorption amount exceeds around 3 mmol/g. Similar curves of heat of adsorption for NaCHA was observed calorimetrically in the literature [10]. The slight increase of heat of adsorption for NaCHA and LiCHA may be explained by the strong adsorbate–adsorbate interactions [20]. The energy contributions to the adsorption energy may be written as

LiCHA > BaCHA > NaCHA > MgCHA > KCHA

/ ¼ /D þ /R þ /P þ /Fl _ þ /FQ _ þ /SP

BaCHA > NaCHA > KCHA > LiCHA > MgCHA > CaCHA > NaX

> CaCHA > NaX The N2 capacity sequence of different cations is different from those of CO2 which may be explained partly by the cation size effects on CO2 and N2 adsorption. As the N2 adsorption capacity for chabazites is much smaller than CO2, this results in comparatively low coverage on the adsorbent for N2 and thus the effect of cation size would not be as important as for CO2 adsorption. This is confirmed by the sequence of adsorption capacity for both series, LiCHA > NaCHA > KCHA and BaCHA > MgCHA > CaCHA. However, it should be noted that over the temperature range studied, all chabazites exhibit higher N2 capacity compared with NaX.

ð1Þ

Non-specific contribution /D: dispersion energy; /R: repulsion energy /P: polarization energy Specific contribution /F_ l : field–dipole interaction, zero for the case of CO2 /F_ Q : field gradient–quadrupole interaction /SP: adsorbate–adsorbate interaction energy, zero for low coverage For CO2 the dominant terms are / ¼ /non-specific þ /FQ _ þ /SP

ð2Þ

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Heat of adsorpiton, kJ/gmole

484

42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8

NaCHA LiCHA CaCHA NaX MgCHA

BaCHA 0

KCHA 4

2

6

Adsorption amount, gmol/kg Fig. 5. Dependence of isosteric heat of adsorption on CO2 loading.

As CO2 adsorption increases, /non-specific þ /FQ _ would drop gradually according to the property of the cation which yields different electrostatic field gradients but /SP would increase substantially for certain cation-type chabazites. For NaCHA and LiCHA, the capacities at high coverage are higher than KCHA, which would provide more interaction energy to offset the drop of total energy and eventually lead to an increase of heat of adsorption as a result of greater adsorption capacity. In the case of KCHA, /nonspecific þ /FQ also decreases (/FQ would even drop _ _ quicker as KCHA has lower charge density) and /SP would increase, but beyond a certain point, the increase of /SP is limited by the adsorption capacity (determined by the charge density and possible cavity blockage) and not large enough to offset the decrease of total energy caused by the drop of /non-specific þ /FQ _ and finally results in a sharp drop in heat of adsorption. For alkaline-earth metal cation-type chabazites, as the capacity of those samples are even smaller than KCHA at high coverage as shown in isotherm figures, the sharp energy drop occurs at an even smaller loading. The heat of adsorption for CaCHA does not decrease rapidly because of higher adsorption capacity at high coverage (due to smaller Ca cation), and results in high adsorbate interaction energy which compensates the total energy decrease. 3.4. Adiabatic separation factor (ASF) To assess the performance of the adsorbents in a P/VSA process, several metrics are available. Based on an earlier study [21], we choose two in this study as being relevant to the operation of an adiabatic P/VSA process: adiabatic separation factor (ASF) (also known as adiabatic working selectivity) and capture figure of merit (CFM). ASF is a

very important index of the separation capability of an adsorbent, which takes into account the CO2 working capacity (difference between CO2 loading at high and low pressure conditions) and nitrogen working capacity (difference between N2 loading at high and low pressure conditions) of the adsorbent under specified pressure swing conditions during adiabatic operation. Since adsorption processes are invariably close to adiabatic (due to large bed sizes), isothermal analysis is unrealistic. Adiabats (not isotherms) must be used for analysis. Therefore, in this work, the working capacity and working selectivity calculations were conducted using adiabatic simulation calculations. If the temperature rise due to adsorption is entirely absorbed by the solid adsorbent, then the temperature rise may be calculated as dT Q ¼ dn C s

ð3Þ

The adiabat is calculated from the total differential of n     on on dn ¼ dðyP Þ þ dT ð4Þ oyP T oT yP Combining these two expression gives the equations for the adiabat dn ðon=oyP ÞT ¼ dðyP Þ 1  ðon=oT ÞyP ðQ=C s Þ     dn on  dðypÞ oðyP Þ dT T
¼ on dðyP Þ oT yP

ð5Þ

ð6Þ

The partial derivatives are obtained from the isotherm equation. In addition to these calculations, it is important to account for the change in mole fraction of CO2 as the

J. Zhang et al. / Microporous and Mesoporous Materials 111 (2008) 478–487

485

7.0

Li 6.0

Na

Adiabatic Working Selectiviy,CO2/N2

K 5.0

Mg Ca

4.0

Ba NaX

3.0

2.0

1.0

0.0 270

290

310

3 30

350

370

390

Temperature, K Fig. 6. Adiabatic separation factor variation with temperature at 12% CO2 feed gas.

bed is pumped down. This procedure to accomplish this is detailed in Kayser and Knaebel and was implemented in this study [22]. Once these quantities have been calculated, the adiabatic separation factor (ASF) may be calculated as DCO2 DN2 LCO2 ðy i ; P H ; T 1 Þadsorption  LCO2 ðy i ; P L ; T 2 Þdesorption ¼ LN2 ðy i ; P H ; T 1 Þadsorption  LN2 ðy i ; P L ; T 2 Þdesorption

ASF ¼

where LCO2 and LN2 are the loadings of CO2 and N2 respectively at the (T,P,y) conditions prevailing at adsorption and desorption conditions. The CO2 isotherms were fit with Dual-site Langmuir isotherm model (Eq. (7)). The N2 isotherms were fit with the single-site Langmuir isotherm model (first term in Eq. (7)) since the fit with the Dual- site model yielded redundant parameters for the second site. n ðgmol=kgÞ ¼

M 1  B1  P ðkPaÞ M 2  B2  P ðkPaÞ þ 1 þ B1  P ðkPaÞ 1 þ B2  P ðkPaÞ

ð7Þ

where Bi = Bi0 exp(Qi/RT). ASF calculations were done for different feed gas compositions and different temperatures with fixed adsorption pressure (PH = 1.20 bar) and desorption pressure (PL = 0.03 bar) which were chosen based on our experience with our pilot scale VSA plant. An earlier study [21] has demonstrated that ASF determines system recovery, product CO2 purity and specific power consumption. It should be noted that our analysis above assumes uncoupled isotherms – all isotherms measured were pure component isotherms. It is

known that mixtures of gases compete for adsorption on zeolites – however, for an initial screening, the current analysis suffices. For more accurate and process modelling, mixture isotherms should be determined and the appropriate isotherm models used. Fig. 6 shows the relation of ASF and initial bed temperature for a feed gas condition of 12% CO2 with a fixed PH/ PL ratio (120 kPa/3 kPa). Typical flue gas streams from post-combustion capture contain 10–15% CO2. We see that the ASF increases with the increase of initial bed temperature as a result of relative capacity ratio change, which also implies that the decrease of CO2 working capacity with the increase of temperature is comparatively smaller than the drop of N2 working capacity. In both cases, the reference material NaX has a peak in ASF when the temperature was around 330–360 K while the chabazite samples show continuous increases of ASF with the temperature increase. From those trends, it is seen that chabazites are more suitable for higher temperature CO2 capture compared with NaX. Therefore, according to the calculations of adiabatic separation factor, CaCHA and BaCHA presents potential for high temperature (>370 K) CO2 separation while in low temperature (<370 K) NaX is preferable. 3.5. Capture figure of merit From our previous work [21], we have shown that capture figure of merit (CFM), is better related to process performance (and ultimately capture cost) than other existing

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J. Zhang et al. / Microporous and Mesoporous Materials 111 (2008) 478–487

metrics. The CFM comprises two factors, adiabatic CO2 working capacity and ASF. The former relates to throughput and the latter correlates to purity and recovery. We define CFM as CFM ¼ CO2 Adiabatic working capacity  ASF ð8Þ

Calculations of CO2 adiabatic working capacity and CFM were made for the adsorbents studied at the feed concentration of 12% CO2, 1.2 bar.a feed pressure and 0.03 bar.a blow-down pressure. The results are shown in Figs. 7 and 8 respectively.

0.90

Li

adiabatic working capacity,gmole/kg

0.80

Na K

0.70

Mg Ca

0.60

Ba 0.50

NaX 0.40

0.30

0.20

0.10

0.00 270

290

310

330

350

370

390

Temperature, K Fig. 7. CO2 adiabatic working capacities as a function of temperature.

5.00 4.50

Li

4.00

Na K

Capture Figure of Merit

3.50

Mg 3.00

Ca

2.50

Ba NaX

2.00 1.50 1.00 0.50 0.00 270

290

310

330

350

370

Temperature, K Fig. 8. Dependence of capture figure of merit (CFM) on temperature.

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Fig. 7 shows that the adiabatic working capacities of NaX and CaCHA peak at around 320 K while those of LiCHA, KCHA and MgCHA peak at around 360 K. BaCHA and NaCHA have not reached the maximum adiabatic working capacities yet, in the temperature range studied. As the adiabatic working capacity ranking sequence is quite different from the ASF sequence, the product of the two factors (CFM), represents a different ranking of chabazite adsorbents as shown in Fig. 8. It is seen that in terms of CFM, NaX is still preferable up to 360 K, though the NaX CFM peaks at around 330 K. The CFM of NaCHA increases with temperature and exceeds the CFM of CaCHA and NaX at 380 K. Overall, Fig. 8 suggests that at low temperatures, NaX is preferred to all of the chabazites, except BaCHA. However, we have found that BaCHA was characterised by relatively long adsorption and desorption times and this kinetic limitation (which may be attributed to the Ba cation size and pore size constriction) may reduce the attractiveness of BaCHA. Below 380 K, CaCHA shows the best performance and beyond 380 K, NaCHA is superior to other adsorbents. In summary, NaCHA and CaCHA show some promise for high temperature CO2 separation while in low temperature NaX is preferable. 4. Conclusion In this study, chabazite zeolites were prepared and exchanged with alkali cations – Li, Na, K – and alkalineearth cations – Mg, Ca, Ba. Equilibrium capacities of the samples were examine by measuring isotherms for CO2 and N2 under three different temperatures. The heat of adsorption for CO2 was calculated and it is noted that NaCHA and LiCHA had slight increases of heat of adsorption with the coverage as a result of strong adsorbate– adsorbate interaction at higher loading, while KCHA, BaCHA, MgCHA showed a sharp drop in heat of adsorption drop beyond a certain coverage. In addition, the heat of adsorption of CaCHA was fairly insensitive to loading (although we do not expect the surface to be homogenous). This may be a result of the balance between the increase of adsorbate–adsorbate interaction energy and the decrease of cation–quadrupole interaction energy with the CO2 coverage. The isotherms were fit with the Dual-site Langmuir model (CO2) and single-site Langmuir model (N2) and

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employed in the calculation of adiabatic separation factor (ASF) and capture figure of merit (CFM). These indices, which correlate well with VSA performance, show that NaCHA and CaCHA present some potential for high temperature CO2 separation (>370 K) while at low temperature (<370 K) NaX is preferred. Acknowledgment The authors acknowledge the financial support of CO2CRC (Corporate Research Centre for Greenhouse Gas Technologies, Australia). References [1] N. Stern, in: U.H.M. Treasury (Ed.), Stern Review on the Economics of Climate Change, Cambridge University Press, 2006. [2] D.T. Bostick, W.D. Arnold, B. Guo, M.W. Burgess, Separ. Sci. Technol. 32 (1997) 793. [3] J.J. Perona, AIChE J. 39 (1993) 1716. [4] C.G. Coe, T.R. Gaffney, Process for the Purification of Bulk Gases Using Chabazite Adsorbents,US4943304, 1990, p. 8. [5] M.W. Ackley, H. Saxena, G.W. Henzler, J.J. Nowobilski, Method and Apparatus for Gas Purification,WO03053546, 2003, p. 49. [6] S.U. Rege, R.T. Yang, K. Qian, M.A. Buzanowski, Chem. Eng. Sci. 56 (2001) 2745. [7] G.L. Kington, W. Laing, Trans. Faraday Soc. 51 (1955) 287. [8] G.L. Kington, A.C. Macleod, Trans. Faraday Soc. 55 (1959) 1799. [9] R.M. Barrer, J.A. Davies, Proc. Roy. Soc. Lond. A. 320 (1970) 289. [10] S.S. Khvoshchev, A.V. Zverev, Zeolites 11 (1991) 742. [11] T. Inui, Y. Okugawa, M. Yasuda, Ind. Eng. Chem. Res. 27 (1988) 1103. [12] R.M. Barrer, R.M. Gibbons, Trans. Faraday Soc. 61 (1965) 948. [13] S.E. Siporin, B.C. McClaine, R.J. Davis, Langmuir 19 (2003) 4707. [14] K.S. Walton, M.B. Abney, M.D. Levan, Micropor. Mesopor. Mater. 91 (2006) 78. [15] J. Pires, M.B.D. Carvalho, F.R. Ribeiro, E.G. Derouane, J. Mol. Catal. 85 (1993) 295. [16] P.J.E. Harlick, F.H. Tezel, Micropor. Mesopor. Mater. 76 (2004) 71. [17] J. Zhang, P. Xiao, P.A. Webley. In: Experimental Pilot-Scale Study of Carbon Dioxide Recovery from Flue Gas Streams by Vacuum Swing Adsorption, AIChE Annual Conference, Cincinnati, 2005. [18] M. Bourgogne, J.L. Guth, R. Wey, Process for the Preparation of Synthetic Zeolites, and Zeolites Obtained by Said Process, US4503024, 1985, p. 35. [19] S.K. Zhang, O. Talu, D.T. Hayhurst, J. Phys. Chem. 95 (1991) 1722. [20] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press Inc. (London) Ltd., New York, 1978, p. 497. [21] J. Zhang, P.A. Webley, Ind. Eng. Chem. Res. (submitted for publication). [22] J.C. Kayser, K.S. Knaebel, Chem. Eng. Sci. 44 (1989) 1.