Ind. Eng. Chem. Res. 1993,32, 548-552
548
Adsorption of a Nitrogen-Oxygen Mixture in NaCaA Zeolites by Elution Chromatographyt Venkateswarlu N. Choudary, Raksh V. Jasra, and Thirumaleswara S. G. Bhat' Research Centre, Indian Petrochemicals Corporation Limited, Vadodara, 391 346, India
Adsorption of nitrogen and oxygen is studied on various NaCaA zeolite samples using elution gas chromatography. The influence of calcium content on the adsorption of nitrogen is very pronounced, but oxygen adsorption is affected only to a negligible extent. The Henry constant and adsorption selectivity of nitrogen show unexpectedly high values for zeolite samples with higher calcium content (290%calcium exchange). This observation has been explained in terms of interaction of nitrogen molecules with Ca2+ions located in site B. The presorption of water and carbon dioxide molecules on zeolite adversely affects the Henry constant, heat of adsorption, and selectivity of nitrogen whereas adsorption of oxygen is unaffected.
Introduction
Table I. Pseudo Unit Cell Compositions and Adsorptive Characteristics of the Molecular Sieve Samples
The pressure swing adsorption (PSA) process for the production of oxygen from air (with maximum 95% 02 purity) has become (Keller and Jones, 1980; Jasra et al., 1991) a viable alternative to the conventional cryogenic separation, at least up to 30 tonstday oxygen production. Zeolite NaCaA is one of the widely used adsorbents for selective adsorption of nitrogen in the PSA process. The feed air usually contains traces of moisture and carbon dioxide which are more selectively adsorbed on zeolite A, and hence adversely affect the performance of the adsorbent. Furthermore, it is reported in literature (Coe and Kuznicki, 1984) that in zeolite X the mode of dehydration of the surface has a profound influence on the nitrogen adsorption selectivity. We have studied various aspects of nitrogen and oxygen adsorption on ion exchanged forms of zeolite A. In the present paper, we report the effect of presorbed water and carbon dioxide as well as the zeolite activation procedure on the nitrogen adsorption capacity and selectivity with respect to the degree of calcium ion exchange.
adsorbent samde NaA NaCaA-24 NaCaA-43 NaCaA-60 NaCaA-75 NaCaA-90 NaCaA-93 NaCaA-97
Materials. Zeolite NaA in the form of l/lc-in. "binderless" extrudates was supplied by the CATAD division of Indian Petrochemicals Corp. Ltd. Ion exchange was carried out by refluxing the zeolite samples with a 10% w/w calcium chloride solution followed by thorough washing with hot distilled water. The samples were dried and calcined in an air oven at 673 K. The degree of calcium exchange in these samples was determined by chemical analysis using a Varian Techtron Model 1200 automatic atomic absorption spectrophotometer. The percent calcium exchange of the samples is given in Table I. Ultrahigh-purity nitrogen, oxygen, hydrogen, and helium were used in all gas chromatographic measurements. Method. The gas chromatographic (GC) method (Conder and Young, 1979; Kiselev and Yashin, 1969) was used to measure the adsorption of nitrogen and oxygen. This method is limited to the low-pressure region of the isotherm and hence yields values of limiting selectivity. The zeolite sample was ground and sieved to obtain 6080-mesh particles. The samplewas packed in a thoroughly cleaned 6- X 600-mm stainless steel column which was subsequently placed in a Varian Vista 6000 gas chromatograph. The zeolite sample in the column was
Theoretical Section The Henry constant, K,limiting nitrogen selectivity, aN2/02, and heats of adsorption, A&, of nitrogen and oxygen at zero coverage were calculated from gas chromatographic data. The net retention volume, VN,is obtained from corrected retention time using the equation (1)
where
i = ( 3 / 2 ) ~ @ , / P ,-) ~ - 111 The retention volume per gram of an adsorbent is given
IPCL Communication No. 206. 0888-5885/9312632-0548$04.0010
equilib sorption capacity, moleculedpseudo unit cell water n-octane 22.96 22.85 22.88 2.27 23.02 2.44 23.14 2.44 23.82 2.52 23.85 2.53 24.25 2.58
activated using one of two methods, viz., activation I and activation 11. In the first method (activation I),the sample was subjected to programmed heating from ambient temperature to 673 K (2 K/min) for 12 h. In the second method (activation 111, the zeolite sample was saturated with water by injecting distilled water in the column followed by stepwise heating for 12 h each at 373,423,473, 573, and finally 673 K. During activation, a ultra-highpurity hydrogen flow of 60 mL/min measured by a bubble gas flowmeter was maintained through the column. After the activation, the column temperature was lowered to 303 K and the carrier gas flow to 30 mL/min. A pulse of (0.5 mL) gas mixture consisting of 0.5,2.0, and 2.0 vol 7% of oxygen, nitrogen, and helium, respectively, in hydrogen was injected into the column using a gas sampling valve, and the retention times of the gases were measured. This procedure was repeated at 313, 323, and 333 K. The column was equilibrated for at least 1 h at every temperature before injecting the gas mixture. The reproducibility in the values of the retention times was checked by several measurements at each temperature, and the values were precise within fO.O1 min. The corrected retention times were obtained by subtracting the helium retention time from those of nitrogen and oxygen.
Experimental Section
+
calcium exchanne. -.% 0 24 43 60 75 90 93 97
(6
1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 549 I II
ACTIVATION
02 I1
ACTIVATION
0 N2
0 N2
5b
ACTIVATION
0
I
ACTIVATION
0
I1
ACTIVATION
4-
20
40
60
80
100
PERCENT ION EXCHANGE
L
I
20
I
I
I
40 60 80 PERCENT ION EXCHANGE
I
100
I
Figure 2. Dependence of nitrogen selectivity on calcium exchange of NaCaA zeolite at 303 K.
Figure 1. Dependence of Henry constants on calcium exchange of NaCaA zeolite at 303 K.
0 N2
I1
02 I
0
ACTIVATION ACTIVATION
where w8is weight of adsorbent present in the column. V , has the units cm3/gor (mol/g)/(mol/cm3). Vm is the initial slope of an adsorption isotherm in which the quantity on the ordinate has the units of mol/g and that of the abscissa has the units of mol/cm3. Using the ideal gas law, V , can be converted to the initial slope of K (Henry constant in the units of mol/(g.atm)) of the isotherm in which the quantity on the abscissa is the (partial) pressure of the adsorbate in units of pressure: lot
K = V,/RT
(3) where R is the gas constant (-82.05 cm3 atm/(mol.K)) and T is the GC column temperature in kelvin. The selectivity of nitrogen over oxygen, aN2/02 is defined
L
1
I
I
I
40 60 80 PERCENT I O N EXCHANGE
20
1
100
Figure 3. Dependence of heats of adsorption on calcium exchange of NaCaA zeolite.
as aNzloz = n t n?/n,Bn,"
=
'm(NZ)/ vm(O,)
(4)
The imsteric heat of adsorption, AHo,is obtained as follows:
Hence AHo can be obtained from the slope of the plot log(VJ2') versus 1/T. The uncertainties in the values of K, aN2p2,and AH,, were calculated using the method of propagation of errors from the known errors in the experimental parameters. The uncertainties in the values of K, a N 2 p 2 , and A H 0 were found to be *l.l, h1.6, and 11.8% , respectively.
Results Effect of Calcium Ion Exchange. The values of K, aN2/qand A H 0 were computed from the correded retention times using eqs 3-5. Figures 1-3 show the influence of percent cation exchange on K, aN2/02,and AHo, respectively. It is observed that KN? (Henry constant for nitrogen) increases with calcium exchange. A steep rise in&, is observed for zeolites with more than 90% calcium exchange. KO,(Henry constant for oxygen) values are
lower, 0.11-0.17 mmol/(g.atm), and are little affected by the change in percent calcium exchange. The difference between nitrogen and oxygen adsorption behavior becomes more prominent when Na+ cations are replaced with Ca2+cations. The heat of adsorption of nitrogen increases from 18 to 25 kJ/mol (Figure 3) as the percent calcium exchange increases from 0 to 97 whereas the heat of adsorption of oxygen remains almost constant (14-15 kJ/mol). The A H 0 values of nitrogen are in close agreement with the reported data (Sircar, 1988;Ruthven, 1984;Peterson, 1980). aN2p2also shows a similar dependence on calcium exchange levels in the zeolite (Figure 2) with a sharp rise at higher calcium levels. Effect of Activation Procedure. Figures 1-3 also show the influence of activation procedure on the adsorption behavior of various zeolite A samples. Stepwise activation (procedure 11) results in higher (3-57%1 K N ~ values for samples having Ca2+ions less than four per pseudo unit cell whereas procedure I gives higher ( 3 4 %) nitrogen adsorption for samples having more than four Ca2+ions. On the other hand, Ko2values obtained in the case of procedure I are higher for all the zeolite samples. Influence of Presorbed Water. The zeolite samples with different amounts of presorbed water were obtained by controlled heating of the samples previously saturated with water. The amounts of water retained by the samples
660 Ind. Eng. Chem. Res., Vol. O5
NaCaA-24
a
E
m
E E. 0 2 -
X 0
I,
-
01
-
I
43
-L
NITROGEN a OXYGEN 0
t
0
p 03a
NaCaA
(a) NITROGEN OXYGEN
0
04”
32,No. 3,1993
o
N o C a A - 2L N o C o A - 43
0
I 0
4 NOA
,:, 1
2
3
4
PRESORBED WATER, mmol/g
051
NaCaA-W 0 0
1%
E m
04-
0
NITREEN OXYGEN
I1
0
(d) 0 0
0
0
+
01
NaCaA - 7 5
(C)
I
25
I
1
3
35
0
K.
E02-
0
Table 11. Effect of Presorbed Water on A& of Nitrogen and Oxygen in Different Adsorbent Samples
0
~
~~~
X
a
I
a
a
.
gas
0
N2 PRESORBED WATER, mmol/g
0 2
Nz 0 2 o NITROGEN
0
N2
m
0 2
Nz 02
N2 0 2
u 10
20
AHo, kJ/mol, for preeorbed water, mmol/g of zeolite 0.0 0.9 1.6 3.5 19.6 13.8 20.0 14.3
NITROGEN
E
0
~
I
I
0 - 0
1
2
PRESORBED WATER, mmol/g
E
-
I
15
Figure 5. Effect of presorbed water on nitrogen selectivity at 303
-
1’
1
1
NlrROGEN OXYGEN
y03-
01
I
0.5
30
40
N2
?RESORBED W A T E R , ~ ~ ~ I / ~
02
Figure 4. Effect of presorbed water on Henry constants at 303 K: (a) NaCaA-24, (b) NaCaA-43, (c) NaCaA-60, (d) NaCaA-75, (e) NaCaA-90, and (0 NaCaA-97.
N2
were determined from weight loss measurements in a separate experiment similarly carried out in TGA System 113 incorporating Cahn 2000 Electrobalance. The influence of preeorbed water in the range of 0.9-3.5 mmol/g (1.5-6 molecules per unit cell) at 303 K on K and ffN2/02 are shown in Figures 4 and 5. From these figures, it can be seen that the K N ?values decrease with increase in presorbed water in the zeolite. However, this effect is more pronounced beyond 1 mmol/g of presorbed water. On the other hand, the Ko2values are affected only to a negligible extent. AHovalues given in Table 11also follow asimilar trend. The presorption of water shows significant influence on nitrogen selectivity as seen from Figure 5. Zeolite samples with 24-75% calcium exchange are significantly influenced only when more than 1mmol/g of water is presorbed. On the other hand, highly exchanged zeolite samples show drastic suppression of nitrogen selectivity even with 1 mmol/g of presorbed water. In the case NaA the ffNs/02 decreases from 2.73 to 1.00 when 1.5 mmol/g of water is presorbed. In fact, in the latter case the adsorption of both nitrogen and oxygen molecules is completely suppressed by the presorbed water molecules as indicated by their retention times which were comparable with that of helium.
0 2
20.5 14.3 21.0 14.0 21.8 14.0
23.7 14.0 25.1 14.1
NaA 11.7 8.5 NaCaA-24 19.7 14.0 NaCaA-43 20.0 13.5 NaCaA-60 20.6 14.4 NaCaA-75 21.5 13.8 NaCaA-90 21.9 13.7 NaCaA-97 23.7 14.8
19.5 13.4
16.5 13.1
19.7 13.4
16.6 13.0
20.4 13.5
19.8 14.1
21.1 13.8
20.0 14.5
21.7 14.1
19.3 14.3
21.8 13.9
21.3 14.6
Influence of Presorbed Carbon Dioxide. The effect of presorbed carbon dioxide on the adsorption of nitrogen/ oxygen was studied at 303 K. Carbon dioxide was presorbed by injecting a known volume of the gas through a six-port Shimadzu sampling valve. This was followed by the adsorption of nitrogen and oxygen as done in the earlier experiments. It was confirmed experimentallythat the nitrogen and oxygen being adsorbed did not displace the presorbed carbon dioxide. KN*and ~ N ~ are / oplotted ~ as a function of the amount of presorbed carbon dioxide in Figures 6 and 7,respectively. It is seen that both these properties decrease with an increase in the presorbed carbon dioxide. Discussion The following observations are made from the above results: (a)The influence of calcium exchange on the adsorption of nitrogen is very pronounced, but oxygen adsorption is affected only to a negligible extent. (b) The presorption of water and carbon dioxide molecules suppresses K N ~AH,I(N~), , and ‘ Y N ~ / o ~This . effect is
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 551 0 NaA
0 NaCaA 0 NaCaA
A
A
-
2L
63 60 90 97
NaCoA NaCaA Nata A -
Figure 8. Pseudo unit cell of zeolite A (LTA)and the cation positions denoted as per Mortier's (1982) classification.
O.1 0.1
I
1
I
I
I
I
01
02
03
04
05
I
06
I
07
1
0.8
'
coz, rnrnoVg Figure 6. Effect of presorbed carbon dioxide on Henry constants at 303 K. PRESORBED
0
NaA N a C a A - 26
0
N a C a A - 63
A A
NaCaA - 6 0
0
Table 111. Interaction Energy between Isolated Cation and a Gas Molecule, kJ/mol cation Na+ Na+ Ca2+ Ca2+
01
0.2
0.3
NaCaA - 9 0
0.4
0.5
0 2
Nz 0 2
AH = EF-Q+ El
0.6
0.7
PRESORBED C02 , mmol/g
Figure 7. Effect of presorbed carbon dioxide on nitrogen selectivity at 303 K.
too large to be accounted for by the decrease in pore volume by the presorbed molecules. Adsorption of oxygen is not affected. (c) Heating of zeolite A in the presence of water renders the adsorption sites less effective toward nitrogen adsorption but more favorable to oxygen adsorption. Zeolitic cations are the principal sites of interaction between adsorbed molecules and the zeolite surface. The major interactions between a zeolite and an adsorbed molecule such as nitrogen involve electrostatic, induction, dispersion, and short-range repulsive forces. The electrostatic interaction arises between adsorbed molecules possessing dipole or quadrupole momenta and the permanent electric field of the zeolite. The permanent electric field depends on the nature of cations and their location in the zeolite. The electric field also gives rise to induced electric momenta in the adsorbed molecules which in turn interact with the field. The calculated interaction energy between an isolated cation (for Na+ and Ca2+)and the adsorbate molecule given in Table I11 provides an indication of the relative magnitude of different types of interactions between zeolite and the adsorbate molecule. From these data it is evident that the electrostatic interaction tends to enrich nitrogen in the adsorbed phase due to ita higher quadrupole moment (0.545 X C.m2) compared to that of oxygen (0.085 X 10-*0 Om2). It is also clear that the electrostatic interaction energy of Ca2+is higher due to ita charge density, which is higher than that
EF-Q -13.2 -1.9 -25.4 -4.2
Et -18.6 -21.0 -12.5 -81.6
ED
ER
-0.9 -0.9 -2.2 -2.2
+2.1 +1.6 +6.9 +5.8
M H O -30.6 -22.2 -93.2 -82.2
+ ED+ ER.
of Na+. Hence, when Na+ ions are replaced by Ca2+ions in the zeolite cavity, the contribution of the fieldquadrupole interactions increases leading to higher &, LYN2/02, and AHo(N~) values. Zeolite samples with over 90% calcium exchange show unexpectedly high KN?and C ~ N ~ I Ovalues ? (Figures 1 and 2). Far-IR spectroscopic studies (Baker et al., 1988;Jasra et al., 1992) have shown that in this range of calcium exchange a major part of the Ca2+ions are located in site B, (i.e., in the six-ring but displaced into the a-cage) and hence are easily accessible for adsorption. The remaining cations are located a t the inaccessible site C, (i.e., in the six-ring displaced into the @-cage). At lower exchange levels Le., below 7576, Ca2+ions are located at site A, i.e., in the six-ring on the plane of the ring. Figure 8 shows the distribution of Ca2+ions in zeolite A. The present data on nitrogen adsorption also show that calcium ions at exchange levels above 90% are probably more accessible to the adsorbate molecules. When water molecules are adsorbed, cationic centers are hydrated and are effectively "screened" from the quadrupoles of the adsorbate molecules. However, the dispersion interaction between zeolite surface and adsorbed molecules is unaffected. This behavior is reflected , and AHo(N~) with little effect in the decreasing K N ~(YN2/Oe, on oxygen adsorption as seen in Figures 4 and 5 particularly at presorption levels above 1 mmol/g. From desorption studies of water from zeolite A having monovalent and divalent cations (Vucelic et al., 1976;Dondur et al., 19761, three desorption peaks were reported. The first two peaks corresponding to low energy (AH = 56.5 and 60.6 kJ/mol) have been attributed to water desorption from the a-cage whereas the high-energy (AH = 125.5 kJ/mol) third peak has been ascribed to water desorption from the @-cageof the zeolite. Therefore, it follows that presorption of water in zeolite A occurs first in the @-cagefollowed by the a-cage. As @-cageis not accessible to nitrogen molecules,the initial presorption of water is not expected to affect the adsorption of nitrogen. From our studies it appears that presorbed water up to about 1mmol/g largely occupies @-cage.The complete suppression of N2 and 0 2 adsorption on NaA by presorbed water molecules above 1mmol/g is evidently a molecular sievingeffect caused by the blocking of the eightring windows. This effect is some what similar to the
*
0
gasmolecule N2
552 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993
observation made by us (Choudary et al., 1992) in the case of n-alkane adsorption on NaCaA with presorbed pyridine molecules. The influence of the activation procedure on nitrogen adsorption can also be explained on similar lines. When zeolite A with high calcium content is heated in the presence of adsorbed water, calcium ions hydrolyze to give CaOH+ sites, as follows: Ca2++ 2Hz0
-
Ca(OH)++ H+ + H,O
Hydroxylated cations such as Ca(OH)+with diminished charge density are less effective than even Na+ ions in interacting with nitrogen quadrupoles, but display higher nonspecific interaction with oxygen molecules, leading to lower heat of nitrogen adsorption and nitrogen selectivity. A similar observation on adsorption selectivity and capacity for X-type zeolite having polyvalent cations was reported in the literature (Coe and Kuzincki, 1984). It is interesting to note that the effect of presorbed carbon dioxide on nitogen adsorption is more pronounced than presorbed water especially in samples with more than 90 5% calcium exchange (five Ca2+ions per unit cell). Thus / o ~is brought about by 0.24 a 20% reduction in ~ N ~ value mmoVg of presorbed carbon dioxide but needed 0.9 mmoVg of presorbed water. Carbon dioxide may be physisorbed or chemisorbed in NaA and NaCaA zeolites. NaCaA with presorbed COZshows the presence of C0a2- as confirmed by an infrared peak at 1450 cm-l (Delaval et al., 1986). Furthermore, it was observed that during repeated injection of nitrogedoxygen mixtures into higher calcium zeolites containing presorbed carbon dioxide there was a gradual drift in the values of the retention time with each pulse of the mixture. Presumably the available sites for nitrogen adsorption decreased after each pulse. This may be due to a redistribution of the carbon dioxide molecules on the cationic sites within the zeolite cavity.
Conclusions Henry constant values, adsorption selectivity, and heat of adsorption for nitrogen increase with increasing calcium content of zeolite NaCaA. At higher calcium content (above 90%calcium exchange) the increase is exponential. This behavior may be due to better interaction of nitrogen with Caz+ionslocated at site B, Le., in the six-ring displaced into the a-cage. Ca2+ions occupy this site only when the calcium exchange in the zeolite is above 90%. Oxygen adsorption is marginally affected with increase in calcium content of the zeolite. Presorbed water and carbon dioxide as well as the activation procedure show substantial effect on nitrogen adsorption. The effect of presorbed carbon dioxide on nitrogen adsorption is more pronounced compared to presorbed water. On the other hand, oxygen adsorption is unaffected by these factors.
Acknowledgment We sincerely thank Mr. Rajender D. Parte for his unfailing assistance in carrying out experimental work.
We are grateful to Dr. I. S. Bharadwaj, Director (R&D), for his keen interest in this work and for according permission to publish this work.
Nomenclature ED = dispersion energy EF-Q= field-quadrupole interaction energy E1 = induction energy E R = repulsion energy F = carrier gas flow rate, mL/min A H 0 = initial heat of adsorption, kJ/mol j = compressibility correction K = Henry constant, mol/(gatm) pi = column inlet pressure p o = column outlet pressure pw= water vapor pressure at room temperature T = column temperature, K t~ = corrected retention time, min TR= room temperature, K LYN~IO =~nitrogen adsorption selectivity
Literature Cited Baker, M. D.; Godber, J.; Helwing, K.; Ozin, G. A. Probing extraframework cations in alkali- and alkaline-earth-metal Linde Type A Zeolites by Fourier Transfrom Far-Infrared Spectroscopy. J. Phys. Chem. 1988,92,6017. Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Influence of presorbed moleculeson the sorption of n-alkanes in zeolite NaCaA. J. Chem. SOC.,Faraday Trans. 1992,88 (20), 3111. Coe, C. G.; Kuznicki, S. M. An improved polyvalent ion exchanged adsorbent for air separation. U.S.Patent 4,481,018, 1984. Conder, J. R.; Young, C. L. Physicochemical measurements by gas chromatography; Wiley: New York, 1979. Delaval, Y.; Selondoux, R.; Cohen de lara, E. Determination of isotherms and initial heats of adsorption of COz and N20 in 4A zeolites from infrared measurements. J. Chem. SOC.,Faraday Trans. I 1986,82, 365. Dondur, V.; Vucelic, V.; Vucelic, D.; Susic, M. An analysis of elementary process of water desorption from zeolites of Type A, Part 11. Zeolites with bivalent counterions. Thermochim. Acta 1976, 14, 349. Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Separation of gases by pressure swing adsorption. Sep. Sci. Technol. 1991,26 (70), 885. Jasra, R. V.; Choudary, N. V.; Rao,K. V.; Pandey, G. C.; Bhat, S. G. T. Far IR spectroscopic studies on zeolite NaCaA. Submitted for publication in Chem. Phys. Lett., 1992. Keller 11, G. E.; Jones, R. L. A new process for adsorption separation of gas streams. In Adsorption and ion exchange with synthetic zeolites;Flank, W. H., Ed.; ACS Symposium Series 135;American Chemical Society: Washington, DC, 1980; pp 275-286. Kiselev,A. V.; Yashin, Y. I. Gas-adsorptionchromatography;Plenum Press: New York, 1969. Mortier, W. J. In Compilation of extra framework sites in zeolites; Butterworths: Surrey, 1982. Peterson, D. In Adsorption and ion exchange with synthetic zeolite; Flank, W. H.,Ed.;ACSSympceiumSeries 135;AmericanChemical Society: Washington, DC, 1980; p 107. Ruthven, D. M. InPrincipZes of adsorption and adsorptionprocesses; Wiley: New York, 1984. Sircar, S. A pressure swing adsorption process for production of 2350% oxygen-enriched air. Sep. Sci. Technol. 1988, 23, 437. Vucelic, V.; Dondur, V.; Djurdjevic, P.; Vucelic, D. An analpie of elementary process of water desorption from zeolites of Type A, Part I. Zeoliteswith monovalent counterions. Thermochim.Acta 1976, 14, 341.
Received for review July 2, 1992 Revised manuscript received November 16, 1992 Accepted November 24, 1992
