Fuel 93 (2012) 238–244

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Fuel

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Regenerability of zeolites as adsorbents for natural gas sweetening: A case-study

Marco Tagliabue a,⇑, Caterina Rizzo b, Nicola B. Onorati b, Enrico F. Gambarotta b, Angela Carati b,Francesca Bazzano b

a Eni S.p.A., Research Center for Unconventional Energies, v. G. Fauser 4, 28100 Novara, Italyb Eni S.p.A., Refining & Marketing Division, v. F. Maritano 26, 20097 San Donato M.se, Italy

a r t i c l e i n f o

Article history:Received 3 August 2010Received in revised form 23 August 2011Accepted 24 August 2011Available online 10 September 2011

Keywords:Carbon dioxideMethaneNatural gasAdsorptionZeolites

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.08.051

⇑ Corresponding author.E-mail address: marco.tagliabue@eni.com (M. Tag

a b s t r a c t

Zeolite adsorbents are widely used for the removal of contaminants from natural gas. Typically, raw nat-ural gas is passed through packed columns, obtaining a methane-rich stream suitable for pipeline trans-mission or further processing. After defined service time, bed regeneration is requested. Adsorbentchemical composition is a key parameter influencing both specific capacity and regenerability. A case-study, concerning Na-FAU zeolites applied for carbon dioxide bulk removal from natural gas, is discussed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Novel gas transportation systems, remarkable reserves foundand environmental sustainability all point to Natural Gas (NG) asthe primary energy source in the near future [1].

NG is generally collected at wellhead as gaseous mixture con-taining a wide range of impurities in addition to methane. Accord-ing to that, gas conditioning operations are requested to meetspecifications imposed by downstream processors and customers(Table 1) [2,3].

Carbon dioxide removal from raw NG is of considerable impor-tance inasmuch as it can be present to a significant extent. In fact,high carbon dioxide levels promote the formation of solids in cryo-genic equipments and steel-pipes corrosion. Furthermore, carbondioxide does not contribute to NG heating value, enhancing itstransportation cost referred to energy unit.

Alkanolamine absorption and solvent adsorption are by far thedominant technologies for carbon dioxide rejection [2–4]. Carba-mate (or carbonate, depending on the nature of the alkanolamine)formation is favoured by contacting raw NG with basic solutionsinto scrubbers. These adducts are then decomposed by hightemperature heating (generally at no more than 400 K) into regen-eration units, making the solution suitable for further absorptionoperations. The methane-rich stream coming from alkanolamineprocesses is water-saturated. Thus, it has to be sent to glycol-based

ll rights reserved.

liabue).

dehydrators before further processing. Carbon dioxide (whenpresent in raw NG at partial pressure of at least 500 kPa) can alsobe adsorbed on selective organic solvents. Low temperatures (till220 K) are generally requested to maximise carbon dioxide adsorp-tion and regeneration is operated by depressurisation by exploitingsequential flash drums. High capital and operative expenses havetill now discouraged the application of traditional technologies totreatment of marginal streams such as gases accompanying petro-leum extraction (oil-associated gas), gases entrapped in coal seams(coalbed methane) [5] and gases deriving from the decompositionof municipal wastes (landfill gas) [6,7].

As a consequence, these gaseous mixtures are frequently aban-doned with overall negative impact on energy efficiency and envi-ronmental protection [8]. In this context, among the emerging gasconditioning technologies, Pressure Swing Adsorption (PSA) pro-cesses have drawn market attention because of their intrinsiceco-compatibility and flexibility.

The concept of PSA for gas separation is relatively simple [9,10].Certain components of a gas mixture are selectively adsorbed on aporous solid (adsorbent) at relatively high pressure by contactingthe gas with the porous solid in packed columns. This operationproduce a gas stream (raffinate) enriched in the less stronglyadsorbed components of the feed, saving most of the inlet streampressure. The adsorbed components are then desorbed from theporous solid by lowering their gas-phase partial pressures. At thispoint, the porous solid is ready for further adsorption–desorptioncycles. The desorbed gas (extract) is enriched in the more stronglyadsorbed components of the feed.

Table 1Typical compositional specifications on feed to LNG plant and on pipeline gas (totalsulphur refers to H2S + carbonyl sulphide, COS + organic sulphur).

Impurity LNG-grade Pipeline-grade

H2O <0.1 ppmv 150 ppmvH2S <4 ppmv 5.7–22.9 mg/Sm3

CO2 <50 ppmv 3–4 vol.%Total sulphur <20 ppmv 115–419 mg/Sm3

N2 <1 vol.% 3 vol.%Hg <0.01 mg/Nm3 –C4 <2 vol.% –C5+ <0.1 vol.% –Aromatics <2 ppmv –

M. Tagliabue et al. / Fuel 93 (2012) 238–244 239

PSA technology main advantages, when applied to NG condi-tioning, are:

� no dangerous chemicals have to be handled (e.g. alkanolamine,organic solvents and wastes coming from their progressivedegradation);� PSA systems able to produce marketable NG without any addi-

tional conditioning treatment (e.g. dehydration) can bedesigned by exploiting adequate adsorbents;� methane-rich stream can be obtained at high pressure as raffi-

nate stream, making unnecessary further re-compression work(e.g. needed for pipeline transmission);� adsorbent regeneration does not require heating. Consequently,

PSA energy intensity is low;� PSA systems can be easily downsized to compact skid-mounted

modules suitable for the treatment of relatively small NG flows(less than 3 � 105 Sm3/d [11]) or for installation in sites whereroom saving is a considerable benefit (e.g. off-shore facilities);

Molecular Gate™ units, based on a proprietary Engelhard (nowpart of BASF) titano-silicate adsorbent [11,12] and XebecAdsorption BGX™ systems, characterised by an innovativemechanical design [13,14], can be cited as successful commercialapplications of NG conditioning by PSA.

Synthetic FAU zeolite adsorbents are widely used for removal ofimpurities from NG [15] and in this context, their application inPSA systems has been studied [16,17]. Presence of aluminium andextra-framework cations makes these adsorbents extremely selec-tive towards molecules exhibiting electric n-poles, such as carbondioxide. This is particularly evident for samples exchanged with cat-ions characterised by small ionic radii, such as lithium and sodium[18–20]. On the other hand, due to such strong interactions, removalof adsorbed molecules during regeneration could require expensiveoperations, affecting overall process performance and complexity(e.g. vacuum application, extensive bed sweeping by recycling partof raffinate stream or even high temperature heating) [21]. Accordingto that, zeolite regenerability has to be optimised by tuning chemicalcomposition. A case-study, focused on sodium-exchanged FAU zeo-lites is reported. Adsorbent key performances were evaluated bothfrom single-gas isotherms and from adsorption–desorption cyclesoperated on a fixed-bed adsorber fed with simulated raw NG.

2. Experimental

2.1. Adsorbent characterisation

Three sodium-exchanged zeolite samples having the same FAUstructure were selected in order to study the effect of SiO2/Al2O3

molar ratio on respective adsorption performances: (a) X zeolite(Fluka); (b) Y zeolite (Tosoh HSZ320NAA™); (c) DeAluminated Yzeolite (Tosoh HSZ360HUA™), the latter from now on brieflyreferred as DAY. All zeolites were supplied as binder-free samples.

X and Y zeolites were provided as sodium form and tested asreceived. Conversely, DAY zeolite was supplied as protonic form.In this case, sodium exchange was performed by treating twice theDAY zeolite with sodium chloride aqueous solution at 333 K for 1 h.

Quantitative elemental analysis was performed on each sample.Specifically, aluminium and sodium were determined by Induc-tively Coupled Plasma – Atomic Emission Spectrometry (ICP–AES,Thermo Intrepid Dual View™) while silica was determined bygravimetric analysis, after sample digestion into mineral acids.

Zeolite textural properties were evaluated from nitrogen (nitro-gen purity higher than 99.999 vol.%, Air Liquide) adsorption–desorption isotherms acquired at 77 K by using a MicromeriticsASAP 2010™ instrument based on static-volumetric approach [22].About 0.2 g samples were used for each experiment. Outgassingwas operated at 623 K for 16 h under rotary pump vacuum.

Apparent Specific Surface Area (A-SSA, BET method) and Spe-cific Pore Volume (VP, Gurvitsh’s rule) were evaluated from the ac-quired isotherms.

Skeletal density (q) necessary to correct the aberration inducedby buoyancy effect during high pressure adsorption measurements(as described in Section 2.2) was determined by helium displace-ment (helium purity higher than 99.998 vol.%, Rivoira) on aMicromeritics Accupyc 1330™ pycnometer.

2.2. Single-gas adsorption measurements

Single-gas low pressure (i.e. from vacuum to atmospheric)adsorption experiments were carried out on a Micromeritics ASAP2010™ instrument (Section 2.1).

High pressure (i.e. from 500 kPa to 3500 kPa) single-gas adsorp-tion measurements were run on a Rubotherm Isosorp™ magneticsuspension balance operated according with a semi-batch mode(25 Ncm3/min feed flow to sample cell till equilibration, followedby weight acquisition under static conditions). About 0.5 g sampleswere used for each high pressure experiment.

Amount of adsorbed gas was evaluated by using the followingequation:

WA ¼WM þ ðqG � VSÞ �WV ð1Þ

where, WA is the weight of adsorbed gas; WM is the weight regis-tered by the balance at temperature set (T) and equilibrium pres-sure (PE); qG = f (PE, T) is the gas density evaluated from theSoave-Redlich-Kwong equation of state [23]; VS is the overall vol-ume resulting from the sum of sample volume (considering skeletaldensity) and sample-holder volume (obtained from blank experi-ments); WV is the weight under vacuum of the outgassed sample.

Both low and high pressure instruments were equipped with li-quid-circulation thermostatic baths (Julabo FP50™) for precisecontrol of sample cell temperature.

Outgassing was operated in situ as reported in Section 2.1.Methane (purity higher than 99.5 vol.%, Sapio) and carbon diox-

ide (purity higher than 99.999 vol.%, Air Products) were employed.From single gas adsorption isotherms the following descriptors

were calculated:

� CO2/CH4 selectivity;� carbon dioxide specific capacity;� carbon dioxide working capacity by isothermal

depressurisation;� carbon dioxide isosteric adsorption heat.

2.3. Adsorption–desorption tests with simulated raw NG

Adsorption–desorption tests were carried out on a bench-scalefixed-bed adsorber fed by a ternary gaseous mixture composed by

Fig. 1. Bench-scale apparatus for adsorption–desorption tests with simulated raw NG.

0

10

20

30

40

50

60

70

0.00.00 0.15.00 0.30.00 0.45.00 1.00.00Time on Stream [h.min.s]

X CO

2 [%

]

ADSORPTION DESORPTION

Fig. 2. Typical data acquisition from adsorption–desorption tests with simulatedraw NG. Carbon dioxide content values (XCO2 ) downstream the adsorber arereported for a single cycle. Data refers to X zeolite as adsorbent at 303 K.Dots = adsorption step. Line = desorption.

240 M. Tagliabue et al. / Fuel 93 (2012) 238–244

CO2:N2:CH4 = 20:15:65 vol.% (approximated values) and simulat-ing raw NG.

The adopted scheme, described in Fig. 1, resembles the one pro-posed for gas separation studies under competitive conditions [24].An AISI 316L stainless-steel fixed-bed adsorber (L = 1000 mm,ID = 12.3 mm) was employed. Temperature control during tests(303 K) was assured by room conditioning. Dry gases were pro-vided by cylinders. Streams were quantified by Mass Flow Metersand Controllers (MFMs, MFCs Brooks Smart Gas Flow™) and ana-lysed on-line by a micro Gas Chromatograph (GC, VarianCP4900™) equipped with a Varian Poraplot U™ 10 m columnand a thermo-conductive detector. Pressure was controlled bytwo Pneumatic Relief Valves (PRVs) and measured by two pressuretransducers (Rosemount) inserted, respectively, up- and down-stream the adsorber. Bed temperature was monitored by a thermo-couple inserted along its axis. Ancillary lines were provided inorder to allow controls, calibrations, outgassing and safety opera-tions. Dead volume was evaluated by gas displacement experi-ments carried out on the adsorber charged with glass beads.

Sample outgassing was operated in situ by helium sweeping for2 h followed by rotary pump vacuum for 3 h, both at 623 K. Hightemperature heating was provided by a toroidal programmableelectric oven.

Moisture content of each sample was quantified ex-situ by ther-mogravimetry by using a Mettler M3™ thermobalance (10 K/minfrom room temperature to 623 K under 200 cm3/min nitrogenflow) just to have an estimation of the amount of charged dryadsorbent.

Adsorption–desorption cycles were run automatically by usinga process control and data recording interface.

Specifically, the following routine was adopted:

(a) pressurisation till PMAX = 3000 kPa by gaseous mixture feed-ing (GHSV = 500 1/h);

(b) adsorption under (a) conditions till fixed bed saturation(1 h);

(c) depressurisation till PMIN = 400 kPa;(d) sweeping with pure methane under (c) conditions

(GHSV = 200 1/h for 5 h);(e) back to (a) conditions.

Experiments were performed by using 50–60 g of granulatedadsorbent (20–40 mesh). Preliminary tests were run at different

GHSVs to find optimal conditions, in order to avoid parasitic phe-nomena due to external diffusion.

Carbon dioxide (purity higher than 99.999 vol.%, Air Products),nitrogen (purity higher than 99.997 vol.%, Air Liquide), methane(purity higher than 99.5 vol.%, Sapio) and helium (purity higherthan 99.998 vol.%, Rivoira) were used.

Breakthrough curves were recorded on line both during adsorp-tion and desorption operations (as exemplified in Fig. 2, concerningthe first adsorption–desorption cycle of the sequence performedwith X zeolite) and then integrated in order to get the followingdescriptors:

� carbon dioxide specific capacity under competitive conditions;� carbon dioxide desorption rate by isothermal depressurisation

and bed sweeping.

Although empirical and semi-quantitative, these descriptorshave provided a picture of the behaviour of the different adsorbentunder PSA-type conditions.

3. Results and discussion

Sample elemental compositions are resumed in Table 2. As ex-pected, sodium content is roughly correlated with aluminium

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1P/P0

Q [m

mol

/g]

X Y DAY

Fig. 3. FAU zeolites textural characterisation: nitrogen adsorption–desorptionisotherms at 77 K.

Table 3FAU zeolites textural characterisation. Column ID, from left to right: (a) ApparentSpecific Surface Area (A-SSA); (b) Specific Pore Volume (VP); (c) skeletal density (q).Pore volume restricted to the micropore region (P/P0 < 0.1) is reported, as well.

Zeolite A-SSA (m2/g) VP (cm3/g) q (g/cm3)

X 672 0.33 (0.31) 2.20Y 810 0.45 (0.35) 2.31DAY 586 0.44 (0.27) 2.35

0

2

4

6

8

10

0 20 40 60 80 100 120P [kPa]

Q [m

mol

/g]

283 K 303 K 323 K

Fig. 4. Carbon dioxide adsorption isotherms on X zeolite from vacuum toatmospheric pressure. Dots = experimental data. Line = fitting by Toth’s equation.

0

2

4

6

8

10

0 20 40 60 80 100 120P [kPa]

Q [m

mol

/g]

283 K 303 K 323 K

Fig. 5. Carbon dioxide adsorption isotherms on Y zeolite from vacuum toatmospheric pressure. Dots = experimental data. Line = fitting by Toth’s equation.

0

2

4

6

8

10

0 20 40 60 80 100 120P [kPa]

Q [m

mol

/g]

283 K 303 K 323 K

Fig. 6. Carbon dioxide adsorption isotherms on DAY zeolite from vacuum toatmospheric pressure. Dots = experimental data. Line = fitting by Toth’s equation.

Table 2FAU zeolites elemental composition.

Zeolite SiO2

(wt.%)Al2O3

(wt.%)Na(wt.%)

SiO2/Al2O3

(mol/mol)Na/Al(mol/mol)

X 39.3 27.2 12.1 2.5 1.0Y 51.1 15.5 6.9 5.6 1.0DAY 81.5 10.2 1.6 13.6 0.4

M. Tagliabue et al. / Fuel 93 (2012) 238–244 241

content. In particular, the Na/Al molar ratio is 1.0 for both X and Yzeolites. By contrast, the lower value observed for DAY zeolite canbe attributed to the presence of both protonic sites and octahedralaluminium, the latter not requiring counter-ions.

Evidences from textural characterisation are reported in Fig. 3and Table 3.

Langmuir-type isotherms (typical of zeolites) have been ob-served for all samples (Fig. 3).

A-SSA values following the order Y > X > DAY have beenobtained. Conversely, the sequence Y > DAY > X has been recordedconsidering VP values. In particular, Y zeolite has shown both A-SSAand VP values higher than X zeolite ones. This is consistent with therelatively low aluminium and sodium content of Y zeolite (e.g. lesscounter-ions occupying internal void space). On the other hand,DAY zeolite A-SSA value markedly lower than X and Y zeolite oneshas to be considered as a consequence of partial crystal structurecollapse due to dealumination [25].

Correlations among FAU zeolite characteristics (both composi-tional and textural) and the respective carbon dioxide and meth-ane adsorption properties have been evidenced.

Carbon dioxide and methane adsorption isotherms acquired at303 K have shown that specific adsorption capacities for both gases(Q) are higher on the aluminium-rich X and Y zeolites (Figs. 4–9).These adsorbents are also characterised by the highest A-SSA andVP values in the micropore region, among the considered samples.

Furthermore, adsorption isotherms taken in the low pressurerange with carbon dioxide vary from Langmuir-type (X, Y zeolites)to virtually Henry-type (DAY zeolite) pointing to a strong correla-tion between the strength of adsorbent–adsorbate interactions and

the chemical composition of each sample (specifically, the alumin-ium content).

In any case, carbon dioxide adsorption is strongly favoured, evenat high pressures: e.g. at 303 K and 500 kPa the amount of adsorbedcarbon dioxide is at least two-and-a-half times higher than theamount of adsorbed methane under the same conditions (Table 4).

Tests with simulated raw NG, have provided specific adsorptioncapacities under competitive conditions (C) close to the ones ob-tained with pure carbon dioxide (Tables 4 and 5). These results point

0

0.2

0.4

0.6

0.8

1

0 25 50 75 100 125P [kPa]

Q [m

mol

/g]

X Y DAY

Fig. 7. Methane adsorption isotherms on FAU zeolites at 303 K, from vacuum toatmospheric pressure.

0

2

4

6

8

10

0 700 1400 2100 2800 3500P [kPa]

Q [m

mol

/g]

X Y DAY

Fig. 8. Carbon dioxide adsorption isotherms on FAU zeolites at 303 K from 500 kPato 3500 kPa.

0

1

2

3

4

5

0 700 1400 2100 2800 3500P [kPa]

Q [m

mol

/g]

X Y DAY

Fig. 9. Methane adsorption isotherms on FAU zeolites at 303 K from 500 kPa to3500 kPa.

Table 4Adsorption data from single-gas isotherms on FAU zeolites at 303 K. Column ID, fromleft to right: a) carbon dioxide specific capacity at 500 kPa (Q CO2

); b) methane specificcapacity at 500 kPa (Q CH4

); (c and d) working capacity evaluated as the amount ofcarbon dioxide desorbable by isothermal depressurisation from 500 kPa to 50 kPaexpressed both as absolute value (DQ A

CO2) and as percentage (DQ%

CO2).

Zeolite QCO2(mmol/g) QCH4

(mmol/g) DQACO2

(mmol/g) DQ%CO2

(%)

X 6.51 2.40 1.56 23.96Y 6.64 1.51 2.05 30.87DAY 3.17 0.80 2.28 71.92

Table 5Adsorption data from experiments performed with simulated NG on FAU zeolites at303 K. Column ID, from left to right: (a) carbon dioxide specific capacity undercompetitive conditions (CCO2 ); (b) desorption time of half the carbon dioxide gotduring the first adsorption step (t1/2).

Zeolite CCO2 (mmol/g) t1/2 (min)

X 6.40 91Y 6.89 22DAY 3.85 9

242 M. Tagliabue et al. / Fuel 93 (2012) 238–244

to a mere dilution effect of nitrogen and methane. In other words,these gases, even when present in relevant amount, are not able tocompete with carbon dioxide during adsorption on FAU zeolites.

Adsorbent regenerability was quantified as working capacity(DQ). Specifically, it was evaluated from adsorption isotherms asthe amount of carbon dioxide desorbable by isothermal depressu-risation from 500 kPa to 50 kPa. The highest pressure was chosenassuming a PSA process operated at 3000 kPa and fed with rawNG containing about 20 vol.% of carbon dioxide. The lowest pres-sure was set considering vacuum equipments compatible withlow capital investments and reasonable process complexity.

The trend observed for working capacity was DAY > Y > X, i.e. in-versely correlated with zeolite aluminium content.

In addition to that, X zeolite partial regenerability was observedduring bench-scale tests (Fig. 10): about 30% of the carbon dioxidecaptured by X zeolite during the first adsorption step was not re-leased by isothermal depressurisation till 400 kPa followed bysweeping with pure methane. Differently to that, Y and DAY zeo-lites showed a complete regenerability under the same conditions.Specific capacity of those zeolites was virtually invariant over suc-cessive cycles. Taking advantage from desorption curves recordedduring bench-scale tests, carbon dioxide release rate was quanti-fied for each sample. In particular, time requested for the desorp-tion of half the carbon dioxide got during the correspondingadsorption step (t1/2) was evaluated (Fig. 11). An opposite trendwith respect to the one recorded for working capacity was ob-served (i.e. X > Y > DAY), referring to the first adsorption–desorp-tion cycle (Table 5). Published infra-red spectroscopy studiesexplain difficult carbon dioxide desorption from X zeolite withthe presence of strong adsorption sites while the effect of carbon-ate formation is considered non-relevant [26].

Thermodynamic data (Table 6) are in agreement with experi-mental evidences. Both Henry’s constant values (K, evaluated fromisotherm slope at 303 K and very low pressure) and Henry’s sepa-ration factor values (aCO2 ;CH4 ) quantify the high selectivity of FAUzeolites for carbon dioxide over methane. On the other hand, ithas to be pointed out that separation factor values around 3[27,28] and in any case, not higher than 104 [29] are generally rec-ommended in order to get both effective gas separation and easyadsorbent regeneration under PSA-type conditions.

According to that, the rather high aCO2 ;CH4 value of X zeolite isconsistent with its partial regenerability.

Zeolite–carbon dioxide interaction strength has been assessedfor each sample by measuring the respective isosteric adsorptionheat (DH). The latter represents the energy released during gasadsorption and consequently, the energy needed for gas desorption(i.e. the energy to be provided for adsorbent regeneration, consid-ering a process perspective). Specifically, isosteric adsorption heathas been determined from isotherm sets acquired between 283 Kand 323 K (Fig. 4–6).

Clausius–Clapeyron’s equation (Eq. (2)), considering derivatesat constant carbon dioxide uptake (Q = const.), has been exploited[22,30]:

DH ¼ R � T2 � @ðln PÞ@T

� �Q¼const:

� R � � @ðln PÞ@ð 1T= Þ

� �Q¼const:

ð2Þ

0

2

4

6

8

10

1 2 3 4 5Cycle Sequence

C [m

mol

/g]

X Y DAY

Fig. 10. Carbon dioxide specific capacities under competitive conditions (CCO2 )acquired on different FAU zeolites during experimental runs with simulated raw NGat 303 K.

0

20

40

60

80

100

1 2 3 4 5Cycle Sequence

t1/2 [m

in]

X Y DAY

Fig. 11. Desorption time of half the carbon dioxide got during the adsorption step(t1/2) acquired on different FAU zeolites during experimental runs with simulatedraw NG at 303 K.

Table 7Toth’s equation parameter values obtained from the interpolation of the carbondioxide adsorption isotherms collected in the range 283–323 K. Correlation coeffi-cients R2 > 99% have been obtained for all models, in accordance with excellent fitting.

Zeolite T (K) m (mmol/g) b (kPat) t (ads.)

X 283 9.16 0.42 0.31303 8.96 0.65 0.33323 9.27 0.93 0.33

Y 283 7.30 5.86 0.82303 7.25 12.72 0.82323 11.14 9.98 0.57

DAY 283 723284.47 1.24 0.050303 89347.36 1.39 0.060323 12855.75 1.81 0.080

0

10

20

30

40

50

0 1 2 3 4 5Q [mmol/g]X Y DAY

Fig. 12. Isosteric adsorption heat (DH) of carbon dioxide on FAU zeolites.

Table 6Thermodynamic data, respectively, for carbon dioxide and methane adsorption onFAU zeolites at 303 K. Column ID, from left to right: (a and b) Henry’s constants (K);(c) Henry’s separation factor (aCO2 ;CH4 ¼ KCO2 =KCH4 ); (d and e) carbon dioxide isostericadsorption heat evaluated both at low uptake (DHL

CO2) and as average value over the

considered carbon dioxide uptake range (DHACO2

).

KCO2 KCH4 CO2; CH4 HLCO2 HACO2

Zeolite [mmol/g�kPa] [mmol/g�kPa] [ads.] [kJ/mol] [kJ/mol]

X 6.64 0.00710 935 46 43Y 0.350 0.00345 101 29 31DAY 0.0670 0.0210 3 32 28

M. Tagliabue et al. / Fuel 93 (2012) 238–244 243

Toth’s equation (Eq. (3)) [31] has been employed for raw iso-therm data interpolation (by StatSoft Statistica™ release 6.00 soft-ware package) allowing sampling at defined carbon dioxide uptakefor each zeolite sample:

Q ¼ m � Pðbþ PtÞ1=t ð3Þ

It has to be pointed out that no physical meaning has beenattributed to m, b, t adjustable parameters (whose values are re-ported in Table 7) although it has been confirmed the ability ofthe model to describe heterogeneities affecting the adsorption pro-cesses, such as those induced by the presence of preferentialadsorption sites in zeolites [32–35].

Isosteric adsorption heat trends are depicted in Fig. 12 and re-ported in Table 6 both as values at low carbon dioxide uptake(DHL) and as average values over the considered carbon dioxideuptake range (DHA). In both cases, the highest values (46 kJ/moland 43 kJ/mol, respectively) have been registered for X zeolite. Inparticular, the decreasing trend from low to high carbon dioxideuptake points to the presence of adsorbing sites markedly strongerthan the others.

Differently to that, the energetics of carbon dioxide adsorptionon Y zeolite is almost invariant over a wide uptake range andaround 30 kJ/mol. This behaviour of Y zeolite has been already ob-served and explained assuming a balance between increasingadsorbate–adsorbate interactions in addition to the energetic het-erogeneity of the adsorbent [17,35].

Isosteric adsorption heat of carbon dioxide on DAY zeolite startsfrom 32 kJ/mol and after preferential sites prompt saturation, itconverges towards 25 kJ/mol. The latter value is close to ones re-ported for siliceous zeolites [18,27,33]. It is noticeable, that DHL va-lue registered on DAY zeolite is slightly higher than the one herereported for Y zeolite and close to the one published for alumina(33 kJ/mol) [36].

According to that, it can be assumed that carbon dioxide prefer-ential adsorption sites on DAY zeolite correspond to extra-frame-work aluminium-rich species whose formation, as a consequenceof dealumination treatments, is largely described [25] althoughtheir precise nature is still under debate [37].

4. Conclusion

Behaviour of sodium-exchanged FAU zeolites having differentSiO2/Al2O3 molar ratio as adsorbents for the removal of carbondioxide from NG has been studied. Single-gas adsorption isotherms

244 M. Tagliabue et al. / Fuel 93 (2012) 238–244

and adsorption–desorption cycles performed on a bench-scaleapparatus fed with simulated NG have been considered.

Carbon dioxide adsorption results strongly favoured over meth-ane in all considered FAU zeolites. On the other hand, specificadsorption capacities for both carbon dioxide and methane arehigher on aluminium-rich X and Y zeolites (SiO2/Al2O3 = 2.5 mol/mol and SiO2/Al2O3 = 5.6 mol/mol, respectively) and lower onDAY zeolite (SiO2/Al2O3 = 13.6 mol/mol) showing also a direct cor-relation with A-SSA and micropore volume.

On the contrary, an inverse correlation has been recorded be-tween zeolite aluminium content and regenerability. Specifically,X zeolite has shown exceptionally high aCO2 ;CH4 value in agreementwith its high affinity for carbon dioxide over methane and onlypartial regenerability by depressurisation and sweeping with puremethane. Complete regenerability under the same operational con-ditions have been recorded both for Y and DAY zeolites, both show-ing aCO2 ;CH4 values within the limits recommended for applicationin PSA processes. In particular, Y zeolite exhibited complete rege-nerability over several adsorption–desorption cycles without anydecrease in carbon dioxide specific capacity with respect to Xzeolite.

Isosteric carbon dioxide adsorption heat consistent with FAUzeolite regenerability have been recorded. In particular, averagedheat values over the considered uptake range followed the orderX > Y � DAY, in excellent agreement with previously publishedones.

Globally, correlation among FAU zeolite chemical compositionand respective carbon dioxide adsorption features resembles thoserecently witnessed in [33] for LTA zeolites.

Recorded evidencies suggest the use of Y and DAY zeolites inPSA processes aimed to the conditioning of NG streams containingrelevant quantities of carbon dioxide (bulk removal processes).These processes need frequent regeneration steps in order to man-age fast bed saturation due to high carbon dioxide content in feed.According to that, adsorbents able to promptly release carbondioxide under mild regeneration conditions are to be preferred.

On the other hand, X zeolite is more suitable for polishing oper-ations where extremely high selectivity is requested for the re-moval of small quantities of polar impurities from NG (e.g. alongLNG chain). These processes require less frequent regenerationoperations due to low carbon dioxide content in feed. In this case,carbon dioxide desorption by high temperature heating (around573 K in Temperature Swing Adsorption processes, TSA) can be aviable option, even if energy intensive.

Acknowledgements

Part of the activity was supported by the European Union (pro-ject: ‘‘Towards Optimised Chemical Processes and New Materialsby Combinatorial Science (TOPCOMBI)’’, NMP2-CT2005-515792)and by the Italian Government (project: ‘‘Integrated Systems forHydrogen Production and Use in Distributed Generation’’).

The colleagues Ms. Monica Catrullo, Ms. Nadia Sommariva, Mr.Carlo Barabino and Mr. Mauro Magni are acknowleged for theireffective technical assistance.

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