Studies on Selective Adsorption of Biogas Components on Pillared

Oct 23, 2008 - Comparative adsorptions of four gases (natural gas and landfill gas components), viz., CO2, CH4, C2H6, and N2, were studied on four dif...
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Environ. Sci. Technol. 2008, 42, 8727–8732

Studies on Selective Adsorption of Biogas Components on Pillared Clays: Approach for Biogas Improvement JOÃO PIRES,* VIPIN K. SAINI, AND ´ S L. PINTO MOISE Department of Chemistry and Biochemistry and CQB, Faculty of Sciences, Building C8, University of Lisbon, Campo Grande, Lisbon, Portugal

Received May 27, 2008. Revised manuscript received September 10, 2008. Accepted September 25, 2008.

Comparative adsorptions of four gases (natural gas and landfill gas components), viz., CO2, CH4, C2H6, and N2, were studied on four different pillared clays (PILCs) to develop a selective material. Such material could be useful for the separation/ purification process of waste gases. These materials (PILCs) were prepared from two different natural montmorillonite clays, by pillaring with Al2O3 and ZrO2, separately and were characterized by means of nitrogen adsorption and XRD. The adsorption isotherms for pure component gases were determined for each PILC, up to 103 kPa. The isotherms data were explored to calculate the selectivity of PILCs for either gas in any binary mixture. It was observed that the surface area of the clays pillared with Al2O3 was higher than that of the clays pillared with ZrO2. At the highest studied equilibrium pressure, the order of maximum adsorption was found to be CO2 > C2H6 > CH4 > N2 for each material. With the help of adsorption modeling, the selective adsorption from binary mixtures was predicted at different equilibrium pressures and compositions. Among the four PILCs, a ZrO2 PILC was found to be the most suitable material, in terms of separation possibility. To further assess the efficiency of these materials in commercial processes, the adsorption capacity in terms of working capacity was also calculated at two different regeneration pressures, i.e., at 1.0 atm and 1.0 Torr.

Introduction Due to a high methane content, biogas and other digester gases are considered to be the most attractive alternatives of natural gas. In addition, these gases have many socioeconomic advantages (1). However, compared from natural gas, these gases contain a fairly large amount of carbon dioxide (40-45%) (2, 3). The minimum fuel quality for compressed natural gas driven vehicles now corresponds to the G25 reference test fuel (85% methane, 14% nitrogen). Therefore, it has become mandatory that these waste gases must be upgraded to the G25 reference test fuel standard before being used as a vehicle fuel (4). In this situation, enrichment of biogas in methane is a requisite step for its use, which is primarily achieved by carbon dioxide removal (5). * Corresponding author phone: (+351) 217 500 898; fax: (+351) 217 500 088; e-mail: [email protected]. 10.1021/es8014666 CCC: $40.75

Published on Web 10/24/2008

 2008 American Chemical Society

Currently, several methods are in use on the commercial level for the removal of carbon dioxide and other gases from biogas. These methods may include adsorption (5) or chemical absorption (6) on a solid surface, membrane separation (7), cryogenic separation (8), and chemical conversion (9). Among these methods, adsorption processes have become increasingly competitive and already favorable for small- to medium-scale operations. They involve the transfer of a component in the gas stream to the surface of a solid material, where it concentrates mainly as a result of physical or van der Waals forces. Gas purification through adsorption, particularly for CO2 removal, has been worked out on various groups of adsorbents such as aluminas (10), activated carbons (5), zeolites (11), clays (12) and clay-based materials (13, 14). Among these materials, pillared interlayer clays (PILCs) are well-known porous solids due to their moderately high surface area and porous volume whose preparation methods and properties were reviewed elsewhere (15). The literature shows that many works have employed these PILCs for gas separation and purification. Yang et al. (16) reported tailoring of interpillar spacing in ZrO2 pillared clays (Zr-PILCs) and their application to kinetic gas separation. Their study demonstrates the separation of air and xylene isomers. Li et al. (17) studied adsorption of methane on Zr-PILCs, and on the basis of optimized parameters, the adsorption of methane on three different porosity Zr-PILCs was simulated. It was observed that the larger the porosity of the Zr-PILCs materials, the greater the amount of methane adsorbed. Pereira et al. (14) have also used Zr-PILCs for methane and ethane separation. They evaluated binary mixture adsorption with the help of vacancy solution theory (VST), but the experimental data were always obtained below atmospheric pressure. Among various PILCs, Al-PILCs and Zr-PILCs are known for ease of preparation and stability (18). In fact, aluminum/ oxygen oligomeric cations can easily be prepared and are found to be very efficient for the intercalation of clays. Similarly, ZrO2 has been known to be a promising pillaring material owing to its high thermal stability and acidity. In the present study, pillared clays, prepared from natural clays, were used as a separating material to study the selective adsorption of the main components of natural and landfill gases (CO2, CH4, C2H6, and N2) up to 103 kPa. Furthermore, data obtained from adsorption isotherms were then mathematically treated to obtain various useful predictions about the behavior of studied materials toward a binary mixture of the studied gases. To the best of our knowledge, it would be the first study of its own kind which reports a systematic investigation of the main component of natural gas and biogas at high pressure in PILCs. In fact, most of the gaseous adsorption studies reported in the literature are limiting up to 100 kPa (near atmospheric pressure), so the utility of these studies is always limited to only low pressure range processes. However, in the case of gaseous mixture separation (purification), the processes are always operated at relatively high pressures (higher than 100 kPa), and therefore, adsorption results at high pressure are more relevant in the present context.

Experimental Section Pillared Clays. Two clays from different origins were used as starting materials: a Wyoming clay (Volclay SPV-200) from the American Colloid Co. and the other from a soil deposit in Benavila, Portugal. The structural formula of both starting materials is given as SI1 in the Supporting Information. Both VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Adsorption isotherms of the studied gases on PILCs (AlW, ZrW, AlB and ZrB) at 298 K. clays were pillared with aluminum oxide and zirconium oxide, individually, to get a total of four pillared clays. The Alpillaring procedure was taken from the literature (18) (see SI1 in the Supporting Information for details). It is noteworthy here that this methodology of preparing Al-PILCs consumes less water and is faster than other procedures described in the literature (15) and therefore is more appropriate for scaleup. Similarly, pillaring of zirconium oxide was achieved by following the procedure which has been already been optimized and discussed in our previous work (14). In the following text, pillared clay samples are referred to as AlW, ZrW, AlB, and ZrB, where “Al” and “Zr” represent the pillars and “W” and “B” in the subscript denote the starting clay (Wyoming and Benavila, respectively). Methods. Nitrogen (Air Liquide, 99.999%) physisorption experiments were performed at 77 K using a volumetric apparatus (NOVA 2200e, from Quantachrome). The samples, about 0.2 g each, were previously degassed for 24 h at 473 K at a pressure lower than 0.133 Pa. XRD data of the PILCs (oriented mounts) were determined between 3° and 10° by using a Philips PX 1820 instrument using Cu KR radiation. Pure gas isotherms, viz., those for carbon dioxide, ethane (Air Liquide, 99.995%), methane (Matheson, 99.995%), and nitrogen (99.99%), were measured on each PILC. The adsorption experiments were carried out in a conventional volumetric apparatus, with a pressure transducer (Pfeiffer Vacuum, APR 266) equipped with a vacuum system which allowed a vacuum better than 10-2 Pa. The adsorption temperature was maintained with a stirred thermostatic water bath (Grant Instrument, GD-120) at 298 K. Each sample was degassed for 2.5 h at 573 K before the experiments. In the calculation of the adsorbed amounts, the nonideality of the gas phase was taken into account by using the second virial coefficients. (see SI4 in the Supporting Information). The uncertainties of the experimental method, estimated by the repeatability of the experiments, were below (2%. The effect of this maximum value on the calculated selectivity is below (8% and does not have a significant influence on the interpretation of the results. 8728

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Results and Discussion Characterization of PILC. Nitrogen adsorption isotherms at 77 K were prepared (Figure SI-S1 in the Supporting Information). Though hysteresis loops were observed in each isotherm, for the sake of clarity, desorption points are not shown in the figure. It is clear from the figure that the adsorption capacity of four clays decreases in the order AlB > AlW > ZrB > ZrW. The isotherm data were further explored to compare these PILCs in Table SI-S1 (Supporting Information) for their specific surface area (ABET) and mesoporous (Vmeso) and microporous (Vµp) volumes (see SI2 in the Supporting Information for calculation details). As denoted from the shape of the nitrogen isotherms, which shows an abrupt knee, and from the t-plot analysis, these solids are essentially microporous materials. Table SI-S1 in the Supporting Information shows that Al-PILCs have more surface area and microporous volume compared to Zr-PILCs; however, the reverse is true in terms of mesoporous volume. In terms of starting clays, Benavila PILCs have higher a area than Wyoming PILCs. On the other hand, XRD results of PILCs show the basal spacings (d001) of Al-PILCs are in the range of 17.8-17.9 Å and those of ZrPILCs in the range of 16.4-16.8 Å. Therefore, after subtraction of the thickness of basic clay sheets, the heights of the micropores in Al-PILCs would be in the range of 8.2-8.3 Å and those of of Zr-PILCs in the range of 6.8-7.2 Å. Adsorption of CO2, CH4, C2H6, and N2. Isotherms of all four gases in each PILC (Figure 1) show that the order of adsorption at the highest studied equilibrium pressure is the same, i.e., CO2 > C2H6 > CH4 > N2, for each adsorbent. C2H6 shows saturation behavior with all the PILCs beyond an equibrium pressure of 500 kPa, where a flat horizontal plateau in the isotherm is noticed. Up to certain pressures AlW, AlB, and ZrB show an interesting behavior, as they adsorb more C2H6 than CO2; however, this behavior is absent in ZrW (Figure 1). As a general comment it should be emphasized that the isotherm in Figure 1 even in the case of CO2 has a moderate curvature. This fact has important practical implications, namely, at the step of adsorbent regeneration, which is

TABLE 1. Adsorption Characteristics of the Studied PILCs toward Different Gases at 298 K adsorption capacity at the highest studied equilibrium pressure PILC

gas

mmol · g-1

(mmol · m-2) × 10-3

AlW

CO2 C2H6 CH4 N2 CO2 C2H6 CH4 N2 CO2 C2H6 CH4 N2 CO2 C2H6 CH4 N2

1.65 1.30 0.36 0.15 1.26 0.84 0.14 0.05 1.30 1.22 0.39 0.17 1.29 0.89 0.31 0.21

6.64 5.26 1.44 0.59 7.00 4.64 0.77 0.28 5.11 4.79 1.52 0.67 5.96 4.12 1.43 0.97

ZrW

AlB

ZrB

favored when the isotherm is not steep (19). It was noticed that, at maximum equilibrium pressure, the amount adsorbed by each gas on each PILC is not in direct order of the PILC surface areas. Therefore, the data of Table SI-S1 (Supporting

FIGURE 3. x g vs xa diagrams of binary mixtures involving N2 separation by different PILCs at 100 kPa. Information) and Figure 1 were further explored to calculate the adsorption capacities of each PILC for each gas at the highest studied equilibrium pressure in units of mmol · g-1 and mmol · m-2, which are given in Table 1. The latter parameter gives an idea about the affinity of the adsorbent toward a particular gas and helps us to understand the difference in affinity of any PILC for two or more gases. The results from Table 1 are very informative in terms of separation possibility. However, it is clear that the order of affinity for gases in each PILC is CO2 > C2H6 > CH4 > N2, but the actual difference in affinities for gases is the key factor which makes one PILC better than another for separation purposes. This table has been further discussed with more findings on the separation of binary mixtures. (Affinities for CO2 and C2H6 have been discussed in SI3 and Figure SI-S2 in the Supporting Information.) Separation of Binary Mixtures. To obtain information on the binary mixtures from single-component data, a detailed methodology, described elsewhere (20), was used. In brief, the method involves the determination of the equation of state for the Gibbs free energy of desorption of the solid adsorbent from the pure gas adsorption isotherms at a given temperature and pressure. Analytical expressions of the adsorption isotherm are needed to apply the method, and in the present work the virial equation was used in the following form: p)

nads exp[C1nads + C2(nads)2 + C3(nads)3] K

(1)

(Constants used in virial equation are given in Table SI-S2 in the Supporting Information.) Equation 1 may be analytically integrated to obtain the free Gibbs energy of desorption G ) RT

∫

p

o

1 2 nads dp ) RT nads + C1(nads)2 + C2(nads)3 + p 2 3 3 C (nads)4 (2) 4 3

[

]

FIGURE 2. x g vs x a diagrams of binary mixtures involving CH4 separation by different PILCs at 100 kPa.

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FIGURE 4. Selectivity (S1,2) diagrams for competitive adsorption of gases from equimolar binary mixtures by four PILCs.

FIGURE 5. Phase diagram for expected separation of CO2 and CH4 by four PILCs. composition of the adsorbed phase, were obtained by numerically solving eq 2 to determine the standard-state loadings (ni0) for each of the two pure gases (1 and 2) at a given value of G (in this case G1 ) G2). Then, the two-equation system was simultaneously solved for the phase equilibrium to determine the component molar fractions in the gas (x g) and adsorbed phases (x a) (20):

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pxg1 ) p1(n01)xa1

(3a)

pxg2 ) p2(n02)xa2

(3b)

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The selectivity values were calculated with the following relation: S1,2 )

(xa1/xg1) (xa2/xg2)

(4)

The IAST theory assumes that the mixing of the adsorbed phases of the two components is ideal. Activity coefficients could be estimated from adsorption isotherms of mixtures to account for nonideal behavior. However, these nonidealities arise mainly due to adsorbate-adsorbate interactions

FIGURE 6. Expected working capacity for CO2 adsorption by different PILCs at a regeneration pressure of (a) 1.00 atm (100 kPa) and (b) 1.00 Torr (0.133 kPa). between the two gases and are often less dominant than adsorbate-adsorbent interactions already accounted for by IAST. In the case of the studied gases in the studied pressure range, it is expected that interactions of gas molecules with the surface are much stronger than interactions between different molecules, and therefore, the IAST assumptions are considered valid in the studied systems. Further information on the calculation and model is given in the Supporting Information. The evolutions of the composition of the gas phase as a function of the composition of the adsorbed phase for six possible binary mixtures are shown in Figures 2 and 3. It was noticed that a binary mixture of C2H6/CO2 was not separated significantly by any PILC, so the results were omitted from the figures. In Figures 2 and 3, each plot shows the variation in x a (mole fraction of the component gas in the adsorbed phase) with the variation in x g (mole fraction of the component gas in the gaseous phase at equibrium conditions) at 100 kPa. These plots give an idea about the selective behavior of any PILC for a particular component gas in a given binary mixture. For example, with a binary mixture of CO2 and CH4 (see Figure 2) at a mole fraction of about 0.5 (composition of a typical biogas sample from a landfill) in the gas phase, the mole fraction of CO2 in the adsorbed phase, x a, decreases in the order ZrW > AlW > AlB > ZrB. This means ZrW is the most selective adsorbent for separating this binary mixture, and this is true for the whole composition range. Similarly, for a binary mixture of C2H6 and CH4, at a mole fraction of 0.5 (in the gas phase), ZrB adsorbed more C2H6 than CH4; other PILCs have comparable separation. In the remaining binary mixtures the separation efficiencies follow the order ZrW > AlW ≈ AlB > ZrB. From these findings it is apparent that ZrW shows a higher separation ability than the other studied materials. It is evident that all four PILCs are different from each other, in termes of either the basic clay or the type of pillar. Therefore, the selective adsorption from a binary mixture is caused by the difference in interactions of gas molecules with different PILCs. For example, Zr pillared clays have a more acidic surface than Al pillared clays; because of this, ZrW offers more selective adsorption for all binary mixtures, except C2H6/CH4, where ZrB is succeeded by ZrW in selectivity. Moreover, among the four gases, CO2 is associated with a quadropole moment. Acidic site-quadropole interaction of CO2 causes more selective adsorption of CO2 over other gases. Variation of Selectivity with Pressure. The results in Figures 2 and 3 are given for a fixed pressure (100 kPa); however, the separation efficiency of the adsorbent may vary with the input pressure. Therefore, the selectivity of these PILCs for different binary mixtures was calculated from Figures 2 and 3. This calculation was made for a broad pressure range at an equimolar gas composition (x g ) 0.5) with the help of MathCAD (see SI5 in the Supporting Information for details). Plots of selectivity, defined as the ratio versus input pressure, are given in Figure 4 (the basic

features of these figures are given in SI6, Supporting Information). As can be seen, for a mixture of C2H6 and N2, the selectivity increases with pressure in AlW, whereas it decreases in the rest of the PILCs, the selectivity values varying between 40 and 200. For a mixture of CO2 and N2, the selectivity was found to increase with pressure in all the PILCs, but for ZrB the values were never higher than 30. It may be noted here that the rate of increase in selectivity is greater with Wyoming clays than Benavila clays. The selectivity for the separation of C2H6 and CH4 was found to decrease with an increase in pressure in AlW, AlB, and ZrB; however, the opposite is again true with ZrW. For a mixture of CO2 and CH4, the selectivity does not vary with pressure in the case of AlW, AlB, and ZrB, presenting values of 14, 10, and 5, respectively, but it shows a continuous increase in ZrW between 20 and 70. This particular finding has more significance over the others, since separation of these particular gases is of great scientific interest in the present scenario, as discussed earlier. The selectivity for a mixture of CH4 and N2 does not change much and is found to be uniform over the entire pressure range with every PILC. Phase Diagrams. Phase diagrams were prepared from binary mixture adsorption data to follow the variation in separation efficiency (of the PILC) with a variation in the composition of a binary mixture at constant pressure (100 kPa). Since separation of CO2 and CH4 is of main interest in the present study, the phase diagrams for this binary mixture are given (Figure 5). The diagrams show a variation in the adsorption amount of the individual component gas and the total with a variation in the composition, i.e., the mole fraction of either gas (for more details, see SI5 in the Supporting Information). It is evident that, for the most favorable case, i.e., ZrW, the amount adsorbed at 0.5 mole fraction is comparable to that of AlW and AlB. For the sake of comparison, we consider a binary mixture of 0.5 mole fraction (as already said, the approximated composition of biogas, for instance, from a landfill). At this composition AlW, ZrW, AlB, and ZrB adsorb 93%, 97%, 90%, and 85% CO2 out of the total adsorbed amount. These results can be easily understood by considering the difference in adsorption capacities (mmol · m-2) for these two gases by different PILCs (see again Table 1). It shows that ZrW has the highest capacity for CO2 and the least for CH4 when compared to the other PILCs. This comparison again supports that, among the four PILCs, ZrW has the highest separation efficiency for this particular binary mixture. It should be emphasized that, if the difference in the adsorbed amount (Table 1) for CO2 and CH4 would have been considered in terms of the mass of material only (and not also per unit area), the comparison would not be so evident. Therefore, the favorable results obtained for ZrW most probably result from the chemistry of ZrO2 pillars in the Wyoming clay host. Working Capacity. These PILCs were further studied for their potential use as a selective adsorbent. Before implementation of any material for such separation techniques VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(such as pressure swing adsorption or vacuum swing adsorption 21, 22), a familiarity with the working capacity of the adsorbents at different regeneration pressures is mandatory. For this purpose the PILCs were compared for their working capacities. The expected working capacity was calculated by subtracting the amount of individual gas adsorbed at the regeneration pressure from each of the successive amounts adsorbed at different pressures (for more details, see SI7, Supporting Information). Two common regeneration pressures (for cyclic processes), i.e., 100 kPa (1.0 atm) and 0.133 kPa (1.0 Torr), were considered. The plots obtained are shown as Figure 6. As can be seen in this figure, the regeneration at pressures below atmospheric pressure would improve the expected working capacity by 20% (roughly), and in that case, the initial adsorption capacities are restored. Additionally, in the case of regeneration at 1 atm (100 k Pa), the sequence of the expected working capacities sequentially follows the trends of the initial isotherms. In terms of practical application, these PILCs are suitable materials. A small discussion on the commercial potential of these PILCs is given in the Supporting Information as “Significance for commercial applications” (SI8). Moreover, these are relatively low cost materials, prepared from natural clays. Discussions made in previous sections also prove that these materials could be used as separating materials for separation of the main components of natural gas and biogas through a cyclic process (23) (adsorption/regeneration).

Acknowledgments Financial help from Fundac¸a˜o para a Cieˆncia e Technologia (FCT; Portugal) to CQB is acknowledged. V.K.S. and M.L.P. acknowledge FCT for postdoctoral Grants SFRH/BPD/34872/ 2007 and BPD/26559/2006.

Appendix A Glossary C1, C2, C3 G nads n01, n02 p R S1,2 T xa xb

constants of virial-series expansion Gibbs energy (J · mol-1) number of gas moles adsorbed (mmol · g-1) standard state loading (mmol · g-1) pressure (kPa) universal gas constant selectivity temperature (K) mole fraction of gas in the adsorbed phase mole fraction of gas in the gaseous phase

Supporting Information Available Starting materials and synthesis of PILCs, microporous and mesoporous volume of materials, adsorption isotherms on PILCs as a function of the relative pressure (p/p0), calculation of the adsorbed amounts, adsorption modeling, basic feature of x g vs x a plots, calculation of working capacity, significance for commercial applications, isotherms of nitrogen adsorption on PILCs, adsorption isotherms of CO2 and C2H6 on PILCs as a function of p/p0, surface parameters of studied PILCs measured by nitrogen isotherm data and XRD spectra, and values of the constants used in the virial equations. This

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material is available free of charge via the Internet at http:// pubs.acs.org.

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