Adsorption equilibria and kinetics of CO2, CH4 and N2 in natural zeolites

Separation and Purification Technology 15 (1999) 163–173

Adsorption equilibria and kinetics of CO , CH and N 2 4 2 in natural zeolites Rosario Herna´ndez-Huesca, Lourdes Dı´az, Gelacio Aguilar-Armenta * Centro de Investigacio´n de la Facultad de Ciencias Quimicas de la Beneme´rita Universidad Auto´noma de Puebla, Bvrd. 14 Sur y Av., San Claudio CU Col. San Manuel, C.P. 72570 Puebla, Mexico Received 10 December 1997; received in revised form 13 July 1998; accepted 16 July 1998

Abstract The ability of natural zeolites (ZAPS, ZNT and ZN-19) to adsorb pure CO , CH and N was studied experimentally. 2 4 2 The volume of CO adsorption in the monolayer (Langmuir) was found to be close to the micropore volume estimated 2 by the Dubinin–Astakhov model (N , 77 K ) for all three zeolites. Considerable differences in the adsorption of 2 CO , CH , and N with these zeolites were observed, a factor that can be used for the separation of CO –CH 2 4 2 2 4 and N –CH mixtures. The mechanism of activated diffusion was detected in the adsorption of CH with ZN-19. 2 4 4 © 1999 Elsevier Science B.V. All rights reserved. Keywords: Activated diffusion; Gas adsorption; Isosteric heat

1. Introduction The most significant progress in the field of adsorption and separation of gas and vapor mixtures in the last four decades is doubtlessly represented by the study, development and use of crystalline microporous adsorbents such as synthetic as well as natural zeolites. As a consequence of their defined crystalline structures, these materials have uniform pore sizes in the interval of 3 to ˚ , a property that allows them to separate 10 A molecules by means of the molecular sieve effect. Separation of gases in zeolites can also take place through the mechanism of selective adsorption of those molecules that have relatively large energetic non-saturations (p bonds, dipoles and quadru* Corresponding author.

poles). Generally speaking, separation of gases by these adsorbents depends on three factors: structure and composition of the framework, cationic form, and zeolitic purity. Different cationic forms of a given zeolite may lead to significant differences in the selective adsorption of a given gas, due to both the location and size of the interchangeable cations which affect the local electrostatic field, and the polarization of the adsorbates. The energetic characteristics of the adsorption of different gases in natural and synthetic zeolites have been studied extensively. It has been reported [1], for instance, that the isosteric heat of adsorption of Ar in shabasite does not depend on the amount of adsorption, whereas that of N decreases abruptly initially and later remains 2 constant. The energetic behavior of CO adsorption with 2

1383-5866/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S1 3 8 3 -5 8 6 6 ( 9 8 ) 0 0 09 4 - X

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this natural zeolite is, however, very different. The initial isosteric heat of CO is much greater than 2 that of either Ar or N , and when the adsorbed 2 amount is increased, first a minimum and later a maximum appears. Natural zeolites, primarily shabasite and clinoptilolite, have been used in purifying natural gas [2], with a high content of CO , 2 H S and humidity. For the past 20 years, there 2 has developed a great interest in utilizing energy resources, previously considered not very profitable [3]. For example, in 1975 an experimental module was assembled in the US with the objective of recovering and purifying a gas stream (50% H , 40% CO ) generated by the anaerobic decom4 2 position of organic waste. The gas to be treated was passed through adsorbers in order to remove H O, H S and mercaptans. Subsequently the semi2 2 purified gas was further treated by adsorption using a mixture of natural zeolites (erionite–shabasite) for CO removal. It has also been reported 2 [4] that clinoptilolite can be used for the separation of N –CH mixture, using the PSA method. It was 2 4 found that the efficiency of this separation can be controlled by the cationic surface population, and that separation PSA is economically feasible for small plants. Here we present a study of the adsorption capacity of three natural zeolites, namely erionite (ZAPS ), mordenite ( ZNT ) and clinoptilolite (ZN-19), for the adsorption of CO , CH and 2 4 N , and, based upon the results obtained, assess 2 their possible use in separating mixtures of carbon dioxide–methane and nitrogen–methane. In addition to the study of the equilibrium adsorption capacity (adsorption isotherms), the corresponding adsorption kinetics were also measured in order to evaluate the effect of the diffusion effect in the adsorption of each of the gases.

2. Experimental Adsorption of the gases was carried out in a high-vacuum volumetric system, previously calibrated with He. The apparatus was made of Pyrex glass, and equipped with grease-free valves. The equilibrium pressures were measured by two APR 017 and TPR 017 (Balzers) pressure transducers

covering a range from 10−4 to 1000 Torr. The vacuum was created by a turbomolecular pump (Balzers), capable of establishing pressure of less than 1×10−7 Torr. The measurement temperatures (10, 17, 20, 27, 40 and 56°C ) were controlled by a Haake L. ultrathermostat with a precision of ±0.2°C. The 0°C temperature was created by a bath of ice water. All samples were first activated in situ over a temperature of 250–300°C by means of a non-programmable oven up to a residual pressure of less than 6×10−4 Torr. The weight loss of the adsorbents was evaluated by heating samples to 250°C at atmospheric pressure in a conventional oven.

3. Results and discussion 3.1. Characterizations of the adsorbents Prior to the study of gas adsorption, the mineralogy of the samples was determined by powder X-ray diffraction, and the results are presented in Table 1. It was found that ZAPS, ZNT and ZN-19 have different diffraction patterns corresponding to erionite, mordenite and clinoptilolite, respectively. The sample porosity was determined by N adsorption at 77 K (Fig. 1). The textural prop2 erties of the zeolites are summarized in Table 2. Due to the fact that the entire micro- and mesopores are occupied at high relative pressures close to unity, for instance at P/P =0.99, the zeolitic 0 purity ( Z.P.), i.e. the micropore content, can be assessed by the relationship which exists between the micropore volume V and the total pore 0 volume V (V =V +V ), where V is the volume S S 0 me me of mesopores. Fig. 1 shows that ZAPS and ZNT display type I isotherms (BDDT ), although a second increment was found for P/P >0.5, due to multilayer adsorp0 tion and capillary condensation into mesopores of the accompanying material (feldspar, quartz, and others). The abrupt increase of N adsorption with 2 erionite and mordenite at very low relative pressure (P/P <0.1) occurs because N molecules (s= ˚ ) are0 able to penetrate freely2into the micro3.7 A pores of these zeolites. The Langmuir equation was found capable of describing isotherms for up

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R. Herna´ndez-Huesca et al. / Separation and Purification Technology 15 (1999) 163–173 Table 1 X-ray diffraction results of three natural zeolites ZAPS [5]

ZNT [6 ]

˚) d (A

I

11.41 9.50 7.83 6.80 6.32 5.86 5.609 5.471 5.215 5.039 4.671 4.417

100 20 20 71 3 12 3 11 2 4 16 72

rel

˚) d (A

I rel

˚) d (A

I

4.230 4.040 3.867 3.834 3.633 3.453 3.197 2.998 2.949 2.880 2.848 2.714

28 6 42 35 18 15 18 15 15 58 52 10

2.621 2.515 2.368 2.309 2.216 2.132 2.108 1.997 1.937 1.895 1.844 1.767

4 26 3 4 15 16 12 2 3 8 6 11

rel

ZN-19 [7]

˚) d (A

I

14.07 9.301 6.673 6.124 5.901 5.559 4.129 4.036 3.500 3.420 3.240 2.995 2.923

50 90 90 50 50 81 30 90 100 90 100 20 20

rel

˚) d (A

I rel

˚) d (A

I

2.824 2.777 2.708 2.579 2.529 2.466 2.279 2.129 2.100 2.020 1.957 1.929

68 10 30 40 50 20 10 10 5 15 8 3

8.93 7.87 6.75 5.22 5.09 4.63 4.46 3.95 3.89 3.74 3.54 3.45 3.38 3.16 3.11 3.06

68 40 30 30 37 31 27 100 65 37 34 42 41 45 31 29

rel

Since N molecules at 77 K cannot penetrate freely 2 into the micropores of clinoptilolite, water adsorption data at 295 K were used to determine the Z.P. of ZN-19 (~62%), and with a microporous volume of 0.1078 cm3/g (Dubinin–Astakhov). As expected, the characteristic energy E of the adsorption of H O ( Table 2) is greater than that of N , 2 2 because the highly polar H O interacts more 2 strongly with the electric field created by the cations of the micropore structure.

4. Equilibrium adsorption

4.1. CO 2 Fig. 1. Adsorption of N (77 K ) with natural zeolites. 2

to P/P ≤0.35, whereas the BET equation was able 0 to fit this data only over 0.01≤P/P ≤0.10. The 0 low adsorption with ZN-19 is probably due to steric factors; as a result, the micropores of ZN-19 do not participate in the adsorption process. The BET equation was found to fit the isotherm data well (P/P ≤0.40), but not the Langmuir equation. 0

Fig. 2 shows the adsorption isotherms of CO 2 at 17°C of the three natural zeolites. It can be seen that in all three cases there is an abrupt increase of adsorption at low equilibrium pressures. Considering that erionite ( ZAPS), mordenite ( ZNT ) and clinoptilolite (ZN-19) adsorb molecules with a kinetic diameter (s) not exceeding ˚ [8], respectively, and that 4.3, 3.9 and 3.5 A ˚ s(CO )=3.3 A, it is obvious that CO adsorption 2 2 with these adsorbents is not limited by the steric

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Table 2 Textural properties of zeolites, ZAPS, ZNT and ZN-19 S (m2/g)

Pore volume (cm3/g)

Z.P.a (%)

Zeolite

Lang

BET

Vb 0

Vc S

V /V 0 S

ZAPS ZNT ZN-19 (N ) 2 ZN-19 (H O) 2

509 325 — —

426 266 23 —

0.1876 0.1186 — 0.1078

0.2195 0.1900 0.0660 0.1738d

85.5 62.4 — 62.0

Eb (kJ/mol )

23.8 19.5 — 35.0

a Z.P.=zeolite purity; impurities are basically quartz and feldspar. b Dubinin–Astakhov (n=1). c P/P =0.99. 0 d V =0.1078 (H O, 295 K )+0.0660 (N , 77 K )=0.1738 cm3/g. S 2 2

Fig. 2. Adsorption isotherms of CO and CH at 17°C with 2 4 natural zeolites.

factor, i.e. the CO molecules freely penetrate the 2 entrance windows towards the micropores. These results indicate that the capacity of adsorption of ZAPS is greater than those of ZNT and ZN-19, even at low pressure (P<1 Torr) or low degrees of coverage. The relative adsorption capacities (ZAPS )>(ZNT )#(ZN-19) over the entire equilibrium pressure range are related to two factors: (1) the number of cations available per unit mass of the dehydrated zeolites (cationic density); and (2) the limiting volume of the micropore. The

amount of CO adsorbed at low pressures ( low 2 degrees of coverage) is directly proportional to the first factor, whereas the micropore volume plays a decisive role at high pressures (high degrees of coverage). The amount of cations which represent the ‘active specific centers’ for the adsorption of CO molecules depends on the Si/Al ratio of the 2 given zeolite type. It is known [8], for example, that for erionites: 3≤Si/Al≤3.5; for mordenites: 4.17≤Si/Al≤5.0; and for clinoptilolites: 4.25≤Si/Al≤5.25. This means that the adsorption in erionite (ZAPS) at low pressures is greater, because there is a larger amount of Al, and therefore a greater cationic density than in the other two samples. On the other hand, the similarity in the adsorption capacities of mordenite and clinoptilolite can be attributed to the fact that the Si/Al ratio as well as the micropore volume ( Table 2) of these two zeolite types are very close. The isotherms measured at 27°C exhibit a behavior similar to the one displayed by the isotherms obtained at 17°C. Table 3 compares the CO adsorption results 2 obtained in the present study with those of other Table 3 CO adsorption (mmol/g) with natural zeolites 2

27°C 25°C

100 Torr

100 Torr

10 Torr

ZAPS: 2.60 ERa: 2.61

ZN-19: 1.59 CLINa: 1.43

ZNT: 1.10 MORb: 1.70

a ER and CLIN are erionite and clinoptilolite, respectively from the USA (tables 8.11 and 8.13, respectively, of ref. [8]). b MOR is mordenite from Canada (table 8.7 of ref. [8]).

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natural zeolites previously reported [8], at equal equilibrium pressures and very similar temperatures. Although the comparisons are between natural zeolites of the same type, their CO adsorp2 tion capacities may indeed vary, since both the zeolitic purities and the cationic composition of each of the zeolites may be different. In Fig. 2 the isotherm data of a commercial activated carbon CG with a wide micropore size distribution [9] are also included in order to demonstrate the influence of the chemical nature of the surface on the adsorption of CO . Unlike zeolites, the surface of 2 activated carbon is practically non-polar, which accounts for the fact that its capacity to adsorb CO is inferior to that of the zeolites, especially at 2 low equilibrium pressures, even though the activated carbon is also a microporous adsorbent with a specific surface area of S=688 m2/g (Langmuir, N , 77 K ) and a micropore volume of V = 2 0 0.2484 cm3/g, superior to those of the zeolites ( Table 2). The CO adsorption isotherms with zeolites can 2 be fitted with the Langmuir equation, with a correlation coefficient r≥0.999 ( Table 4). The volumes of CO adsorbed in the monolayer 2 at 17°C for ZAPS, ZNT and ZN-19 are ~10% less than the micropore volumes determined by the adsorption of N ( Table 2) and H O ( ZN-19). 2 2 These results show that the adsorption of CO at 2 temperatures close to room temperature can be used as a means that is much cheaper than the adsorption of N at 77 K for the purpose of 2 evaluating the microporous volume of natural zeolites. In addition, the adsorption of N at 77 K 2 is not applicable to ultramicroporous zeolites, as in the case of clinoptilolite (ZN-19). The Sips [10] equation and the Langmuir equa-

Table 4 Amount of CO adsorption at 17°C with natural zeolites 2 Zeolite

a (mmol/g) m

V (cm3/g) m

K ( Torr−1)

ZAPS ZNT ZN-19

3.0447 1.9132 1.7770

0.1673 0.1050 0.0977

0.13 0.14 0.21

V =a V ;V (CO )=54.96 cm3/mol. m m molar molar 2 a is the adsorbed amount in the monolayer. m

tion are: log(q/1−q)=log A+C log P

(1)

log(q/1−q)=log K+log P

(2)

where q is the degree of coverage, P is the equilibrium pressure, A, C and K are empirical constants. It can be observed that Eq. (1) reduces to Eq. (2) if C=1. Therefore, the value of C obtained by plotting log(q/1−q) vs. log P, where q=a/a , m may serve as a criterion for evaluating the validity of the Langmuir model [10–12] in describing CO adsorption. In other words, if C has a value 2 which is distinct from unity, it may be assumed that factors which the Langmuir model does not take into account, such as adsorbate–adsorbate interactions, the energetic heterogeneity of the adsorption centers, irreversible adsorption, or others, may be important in the adsorption process. The Sips constants of the three zeolites are give in Table 5. As shown in Table 5, the values of C for the three samples are distinctly different from unity, especially for ZAPS. C is also found to decrease with the increase of temperature, specially for ZAPS. This probably indicates that there are strong adsorbent–adsorbate and adsorbate–adsorbate interactions in ZAPS, even at low coverage. From Table 5 the relative magnitude of C for the three zeolites is found to be (ZAPS ) <( ZNT )#(ZN-19), which is similar to the relative Si/Al ratio. One may therefore conclude that the higher the ‘cationic density’, the greater the deviation from the Langmuir model. In other words, one or several factors may not play such an important role in the ZAPS sample as in the other two. To ascertain the extent of irreversible Table 5 Sips constants for the adsorption of CO with natural zeolites 2 17°C

ZAPS ZNT ZN-19

27°C

C

A

Ka

C

A

Ka

0.829 0.886 0.888

0.16 0.17 0.24

0.13 0.14 0.21

0.778 0.873 0.885

0.15 0.16 0.21

0.10 0.14 0.19

a K is Langmuir’s constant.

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adsorption, experiments were performed in the following manner. Adsorbent samples saturated with a given adsorbate (namely similar at the end of the adsorption experiment described above) were subjected to vacuum at the same temperature as the prior experiment until a residual pressure of P<6×10−4 Torr was reached. Subsequently these samples were used for the adsorption isotherm measurements. The difference between the first (total ) and the second (reversible) isotherms represents the irreversible adsorption, i.e. the amount of adsorbate which cannot be desorbed, at the given experimental temperature. These results are shown in Figs. 3–5. It can be seen from these figures that irreversible CO adsorption was 2 present in all cases. The relative amount of irreversibly adsorbed CO at 50 Torr decreases in the 2 order ZN-19 (17%)>ZNT (9%)>ZAPS (5.8%). At P>50 Torr the proportion of irreversibly adsorbed CO remains constant. Similarly, the 2 order of the relative irreversible adsorption at a pressure of 0.4 Torr was found as: ZN-19 (31%)>ZAPS (14%)&ZNT (#0%). The proportion of irreversible adsorption increases as the pressure decreases for ZN-19 and ZAPS, but prac-

Fig. 3. Adsorption of CO and CH with virgin and regenerated 2 4 ZAPS at 27°C.

Fig. 4. Adsorption of CO and CH with virgin and regenerated 2 4 ZNT at 27°C.

tically diminishes for ZNT ( Fig. 4). The fact that irreversible adsorption is relatively pronounced ( Figs. 3 and 5) at low pressures, i.e. at low coverage, may be due to the fact that CO molecules 2 tend to occupy the most active centers of adsorption (determined cations), and are thus more difficult to desorb. Taking into account that, besides adsorbent–adsorbate interaction, adsorbate–adsorbate interaction may also be present, the shape of the isotherms at low pressures is to be determined by the relative importance of these two factors. For instance, the results of Figs. 3 and 5, and even more clearly those of Fig. 4, at low pressures are more similar to Henry’s isotherms than to the isotherms of either type I or II [13]. However, this does not mean that the surface of the zeolites is necessarily energetically homogeneous. It is more logical to assume that these two factors may have compensating effects. It is obvious that for a more precise analysis it would be necessary to carry out additional studies, which may include, for example, the influence of the cationic modifications of zeolites on CO adsorp2 tion. The irreversibly adsorbed CO itself desorbs 2 only at temperatures higher than 120°C. This result

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169

may be of importance for the regeneration of these adsorbents. The isosteric heats of CO adsorption were 2 computed from the equilibrium data. At constant adsorbate loading, the Clausius–Clapeyron relationship gives: −DH=Q =R[(T T )/(T −T )] ln(P /P ) (3) st 2 1 2 1 2 1 According to the results obtained (Fig. 6), one common feature is that the isosteric heat of CO 2 adsorption decreases with the increase of the amount of adsorption, a, for a<0.5 mmol/g, due to adsorbate–adsorbent interactions, i.e. through the interactions of the quadrupole with the most active adsorption centers. Subsequently, Q was st found to increase with the increase of a, caused by the presence of adsorbate–adsorbate interactions. The relationship of Q vs. a for ZN-19 st and ZNT is similar. On the other hand, the Q st vs. a curve for ZAPS has multiple (two) maxima and minima. These results are probably due to the higher energetic heterogeneity of ZAPS than of the other two zeolites, a fact that is reflected by the lower C value of the Sips equation of ZAPS (see Table 5).

Fig. 5. Adsorption of CO and CH with virgin and regenerated 2 4 ZN-19 at 27°C.

Fig. 6. Isosteric heats of adsorption of CO with zeolites. 2

4.2. CO and CH 2 4 By comparing the adsorption isotherms data of CO and CH at 17°C shown in Fig. 2, it is clear 2 4 that for all three zeolites their CO adsorption is 2 markedly greater than their CH adsorption. It is 4 very probable that the results of Fig. 2 are due to the specific interactions of the quadrupole of the CO molecule with the electric field created by the 2 cations existing in the structure of the zeolites. It is known that CO is always the more strongly 2 adsorbed component compared to CH for all 4 three zeolites. On this basis, it can be assumed that if a CO –CH mixture is put into contact 2 4 with the three zeolites, CO would be adsorbed 2 preferentially, leading to an enrichment of CH in 4 the gas phase. In Fig. 7, we present the phase diagram Y =f(X ), where Y and X are the mole 2 2 2 2 fractions of CH in the gas phase and in the 4 adsorbed phase, of the adsorption of CO –CH 2 4 mixtures. The binary adsorption equilibrium data were calculated by the method of Lewis et al. [14]. All three equilibrium curves are situated above the diagonal Y =X , for the separation factor. The 2 2 relative efficiency of separation of the three samples is in the order: ZAPS
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Fig. 7. Estimated X–Y diagram for adsorption of CH , 4 CO (1)–CH (2) mixtures at 17°C. 2 4 Fig. 8. Adsorption isotherms of N and CH with ZN-19 at 0 2 4 and 27°C.

obtained at 27°C were very similar to those recorded at 17°C. 4.3. CH and N in ZN-19 4 2 Because of the slowness in reaching equilibrium ˚ ) with clinoptiin the adsorption of CH (s=3.8 A 4 lolite (ZN-19), it was decided to measure the adsorption isotherms of this gas at different temperatures. For the purpose of comparison, the isotherms of N (which has a kinetic diameter of ˚ , very2 similar to that of CH ) were also s=3.64 A 4 determined. These results are shown in Fig. 8. The results indicate that, over the range of pressures studied, at 27°C there is no significant difference in the amount of adsorption of both gases. However, at low pressures (P ≤130 Torr), N is eq 2 adsorbed in quantities slightly larger than CH , 4 and the reverse is observed at higher pressures. For N adsorption at 0°C, a relatively sharp 2 increase at low pressure is observed and can be attributed to the free penetration of N molecules 2 into the micropores of the zeolite. However, the concavity of the CH isotherm curve near the 4 origin of the coordinate system is very likely due to the penetration difficulties of this gas into the micropores of clinoptilolite. This hypothesis can be supported by the fact that in the pressure range

20–70 Torr, adsorption was extremely slow. For instance, the time required to establish the equilibrium for each point in this range was approximately 20 h. According to these results, this zeolite can represent the property of separating mixtures of these gases, especially at low temperatures (0°C ).

5. Adsorption kinetics Adsorbate uptake curves a=f(t) of the three test gases were measured until equilibrium was reached (t2, a=constant). 5.1. CO and CH 2 4 According to the results shown in Fig. 9, adsorption of both gases occurs quickly in erionite, achieving during the first 20 s more than 70% of the total capacity at both temperatures (0 and 20°C ). Similar results were obtained in the case of mordenite, not presented here. It was established that CO adsorption also occurs quickly in clinop2 tilolite. In case of CH , however, a very distinct 4 behavior was observed: when the temperature is

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171

this zeolite, which coincides with the results of Fig. 8. During the initial period of adsorption in a constant volume, the fractional uptake can be found from the following equation [15]:

A BA B

increased, the amount of adsorbed CH increases 4 in the interval 0–40°C (Fig. 10). Therefore, the rate of the adsorption process of this gas in clinoptilolite is determined by the rate of activated diffusion of the molecules into the micropores of

2S 1+K Dt 1/2 a −a 0 = ext (4) R= t a −a V K p 2 0 where K=[(a ) −(a −a )]/a is the ratio of the 0 g 2 0 2 adsorbate in the gas phase to that in the adsorbed phase at equilibrium; a , a and a are the amount t 0 2 of gas adsorbed at time t, t=0 and t=2 (equilibrium), respectively (in our experiments a =0); 0 S and V are the specific external surface area of ext the particles (cm2/g) and the volume of the crystals (cm3/g), respectively; (a ) is the amount of gas 0g initially available for adsorption; D is the diffusion coefficient (cm2/s). Fig. 11 shows that good agreement between experimental data and Eq. (4) was obtained, with S (BET )=23 m2/g, V=0.48 cm3/g (pycnometry ext with He). Due to the fast adsorption on the surface of major accessibility, i.e. on the external surface of the zeolite, immediately (t0) a leap of a /a t 2 was recorded. Table 6 lists the values of the diffusion coefficients at different temperatures, which

Fig. 10. CH 4 temperatures.

Fig. 11. Presentation of experimental data of CH uptake with 4 ZN-19 according to Eq. (4).

Fig. 9. CO and CH uptake curves with erionite. 2 4

uptake

curves

with

ZN-19

at

different

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Table 6 Diffusion coefficients of CH in ZN-19 at different temperatures 4 T (K) D×1016 (cm2/s)

273 3.6

283 5.2

293 9.5

313 21.5

E =32.5 kJ/mol [Eq. (5)]. a

were found by fitting the experimental data to Eq. (4). The results can be represented as:

C

E D=D exp − a 0 RT

D

(5)

For the purpose of comparison, Fig. 12 shows the CH uptake curves with erionite (ZAPS ) and 4 clinoptilolite ( ZN-19). The results reveal a great difference in the kinetic behavior of CH adsorp4 tion. The uptake rate for ZAPS at t<5 is more than 10 times greater than that of ZN-19. Since CO is instantly adsorbed with these three zeolites, 2 it can be assumed that ZN-19 should be, primarily at low contact times, a more effective adsorbent than the other two in purifying CH , contaminated 4 with CO . 2

Fig. 12. CH uptake curves with erionite and clinoptilolite at 4 0°C.

5.2. CH and N adsorption with ZN-19 4 2 The results of Fig. 13 reveal that the temperature increase from 0 to 56°C caused the uptake curves to approach each other more, probably reducing the efficiency of separation of mixtures CH –N . 4 2 At t=50 s, for instance, the separation coefficient (S.C.), expressed by the relation of the adsorbed amounts of both gases at both temperatures, falls approximately three times when the temperature is increased from 0 to 56°C: (S.C.) = 0 0.38/0.06=6.3; (S.C.) =0.21/0.10=2. This result 58 indicates that separation of the mixture CH –N 4 2 could be carried out more efficiently at low contact times and low temperatures. It was attempted to reduce the degree of penetration of CH in the microporosity by preadsorbing 4 small quantities (0.15 and 0.30 mmol/g) of H O. 2 However, the increase in amount of preadsorbed H O lead to a more drastic decrease in N adsorp2 2 tion rate than that of CH . This may be due to 4 the penetration of the molecules of H O into the 2 micropores, reducing the number of adsorption centers primarily for N adsorption, whereas the 2 adsorption of CH was not affected in the same 4 manner by the difficulties of penetration described above.

Fig. 13. CH and N uptake curves with ZN-19. 4 2

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6. Conclusions Adsorption of CO with the three naturally 2 occurring zeolites used in this study was found to be unaffected by difficulties of CO molecule penet2 ration into the micropores of the zeolites. The capacity of these samples for the adsorption of CO coincides with the type of zeolite (Si/Al ) as 2 well as with their respective purities. Accordingly, CO adsorption at room temperature may serve 2 as a method to evaluate the presence of a certain type of zeolite in a given natural adsorbent. Due to the fact that methane is adsorbed in much smaller quantities than CO , a fact that is attrib2 uted to the specific interactions of the quadrupole with the electric field created by the structural cations, the three zeolites can be used as adsorbents for the purification of methane mixed with CO . 2 Based upon the fact that N and CO are capable 2 2 of diffusing into the micropores of the three zeolites, and that the diffusion of methane is only activated in clinoptilolite, it can be assumed that the clinoptilolite can be applied in the nitrogen–methane separation, and is a better adsorbent than the other two zeolites for purifying CH , 4 contaminated with CO . 2 Acknowledgment The authors wish to express their grateful appreciation to Consejo Nacional de Ciencia y

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Tecnologia (CONACyT ), Mexico, for financial support (Master Scholarship, Reg. No. 93793 and 93792).

References [1] G.L. Kington, A.C. Macleod, Trans. Faraday Soc. 55 (1959) 1799. [2] F.A. Mumpton, Miner. Geol. Natur. Zeol. 4 (1977) 177. [3] J. Goeppner, D.E. Masselmann, Water Wastes Eng. 11 (1974) 30. [4] Frankiewicz, R.G. Donnelly, in: Industrial Gas Separations, American Chemical Society, 1983, pp. 213–233. [5] R.A. Sheppard, GudeA.J., III, Am. Miner. 54 (1969) 875. [6 ] E.S. Dana, Systems of Mineralogy, Wiley, New York, 6th edn., 1942, p. 570. [7] W.S. Wise, W.S. Nokleberg, M. Kokinos, Am. Miner. 54 (1969) 887. [8] D.W. Breck, Zeolite Molecular Sieves. Wiley-Interscience, New York, 1974, pp. 139, 143, 162, 618, 622, 623. [9] F. Rodrı´guez-Reinoso, J. Garrido, J.M. Martı´n-Martı´nez, M. Molina-Sabio, R. Torregrosa, Carbon 27 (1989) 23. [10] R. Sips, J. Chem. Phys. 16 (1948) 491. [11] P.N. Joshi, V.P. Shiralkar, J. Phys. Chem. 97 (1993) 619. [12] M.J. Reddy, Eapen, H.S. Soni, V.P. Shiralkar, J. Phys. Chem. 96 (1992) 7923. [13] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [14] W.K. Lewis, E.R. Gilliland, B. Chertow, W.P. Cadogan, J. Ind. Eng. Chem. 42 (7) (1950) 1319. [15] R.M. Barrer, in: Molecular Sieves Zeolites, Adv. Chem. Ser. 102, American Chemical Society, Washington, DC, 1971.