Langmuir 1996, 12, 3927-3931
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Optimization of the Textural Characteristics of an Alumina To Capture Contaminants in Natural Gas Christophe Ne´dez,*,† Jean-Paul Boitiaux,‡ Charles J. Cameron,§ and Blaise Didillon§ Rhoˆ ne-Poulenc Recherches, 52 rue de la Haie-Coq, 93308 Aubervilliers Cedex, France, Procatalyse, 212-216 avenue Paul-Doumer, 92500 Rueil-Malmaison, France, and Institut Franc¸ ais du Pe´ trole, 1&4 avenue de Bois-Pre´ au, BP 311, 92506 Rueil-Malmaison, France Received November 14, 1995. In Final Form: February 21, 1996X Capillary condensation of water can seriously impair the performance of an adsorbent intended for use in removing contaminants (mercury, arsenic, sulfur, etc.) in natural gas. Adsorption and desorption isotherms were used to determine how the nature and pore structure distribution of the adsorbent (alumina, active cabon) affect water adsorption. The contribution of the different phenomena involved (chemisorption, physisorption, capillary condensation) have been determined. The threshold of capillary condensation is reached much more rapidly on active carbon and microporous alumina (40% relative humidity) than on a highly mesoporous alumina (75-80% relative humidity).
Introduction Natural gas and hydrocarbon fractions from this source are used both as energy sources and as raw materials in the petrochemical industry, where many steam cracker units are using LPG (liquified petroleum gases) or natural gas condensates as feedstock. Natural gas, however, and its derivatives often contain impurities, such as mercury and arsenic, that must be removed before use. Not only are these contaminants toxic but they can also cause significant damage to downstream units (structural corrosion, poisoning of catalysts). Natural gas is often purified by trapping gasified impurities on an adsorbent made up of an active phase deposited on a carrier agent.1 The choice of carrier is fundamental to the effectiveness of the trapping solid. This choice depends on physical properties (specific surface area, mechanical resistance, pore size distribution, etc.). Pore size distribution is of prime importance because the gas being treated can contain variable quantities of water and heavy hydrocarbons. In industrial use conditions these compounds lead to capillary condensation that fills up the smallest pores of the carrier, making the active phase inaccessible to the contaminants. This will obviously keep the trapping mass from functioning properly.
Table 1. Comparison of the Physical Characteristics of the Four Procatalyse Aluminas and the Active Carbon C characteristics particle size (mm) specific surface area (m2/g) tight-packed density (kg/m3) total pore volume (mL/100 g) V0.1µm (mL/100 g)b V1µm (mL/100 g)c c
A1
A2
A3
B
C
1.4-2.8 1.5-3.35 2.4-4.8 2-5 2-5a 170 119 190 349 270 456
602
600
809
nd
108
74
70.5
44
40
38 20
29 21
29.5 21.5
6 4.5
12 4.5
a Crushed. b Volume of pores with diameter above 0.1 µm. Volume of pores with diameter above 1 µm.
Experimental Section Materials. The four aluminas studied, referred to as A1, A2, A3, and B, were prepared by Procatalyse. C is an active carbon. The main characteristics of these substrates are given in Table 1, and their respective pore size distributions are shown in Figure 1. Porous distributions of the adsorbents have been measured by conventional mercury porosimetry and specific areas have been determined according the BET techniques. The series A aluminas are macroporous oxides whose microporosity has been eliminated; their surfaces, however, as well as their degree of meso- and macroporosity, are different. The influence of the microporosity of the alumina can be established by studying the adsorption properties of alumina B. The effects †
Rhoˆne-Poulenc Recherches. Procatalyse. Institut Franc¸ ais du Pe´trole. X Abstract published in Advance ACS Abstracts, July 1, 1996. ‡ §
(1) Cameron, C. J.; Barthel, Y.; Sarrazin, P. Mercury Removal from Wet Natural Gas, Proceedings of the 72nd GPA Annual Convention, 7-9 March 1994, p 256.
S0743-7463(95)01035-3 CCC: $12.00
Figure 1. Pore distributions (cumulative values) of the five solids studied. of the nature of the support material will be evinced by comparing B and C (alumina versus active carbon). Measurement of Sorption Isotherm of H2O. All the substrates were pretreated at 280 °C (a temperature commonly used in industrial regeneration processes) for 2 h, under helium gas flow. All the experiments were carried out on pretreated beads or granules. Particle size differences in the samples were shown to have no effect on static adsorption or desorption of water (no diffusional aspects). Our adsorption and desorption study was carried out at 30 °C, a realistic application temperature. Weight gain and loss were expressed as percentages of relative humidity (RH); 100% relative humidity corresponds to an atmosphere whose partial water pressure is at saturating vapor tension. Thus: RH ) 100P/P0,
© 1996 American Chemical Society
3928 Langmuir, Vol. 12, No. 16, 1996
Ne´ dez et al. Scheme 1
Figure 2. Water adsorption isotherm on alumina A1 at 30 °C. where P is the partial pressure at time t and P0 is the saturating vapor pressure. The experiments were performed with a Mac Bain balance, according to the following design: A camera monitors, as a function of time, the gain or loss of weight of an adsorbent placed in a laboratory dish and connected to a silica spring. The balance is constantly maintained in a gas flow, at first composed entirely of dry helium and then gradually instilled with water vapor. The equilibrium values obtained after each stage of partial pressure yield an adsorption isotherm. Proceeding in the opposite direction (gradually decreasing the proportion of water vapor in the helium flow) yields a desorption curve. Strenuous tests for reproducibility (particularly for the experiments with A1, B, and the active carbon C) were quite satisfactory; our measurements are precise to within (0.1 g/100 g.
Results and Discussion Adsorption Isotherm of H2O on Alumina A1. The water adsorption isotherm obtained for alumina A1 is plotted in Figure 2. This isotherm is a type II Brunauer model,2 from the classification system still in accepted use today.3,4 We must, however, go beyond this visual description and give the curve a correct physical and even chemical interpretation. Since the first paper published on an isotherm of water adsorption on alumina,5 many other studies have been carried out on this subject, but very often the studies undertaken are unsatisfactorily interpreted. The porous system of alumina,6,7 its very texture, seems, nevertheless, mostly to determine its adsorption characteristics.8 In fact, several of the phenomena involved overlap.9 The clear initial increase in the isotherm corresponds to a chemisorption of water on the alumina. Then comes the formation of growing multilayers of water vapor. In (2) (a) Emmett, P. H.; Brunauer, S. J. Am. Chem. Soc. 1937, 59, 1553. (b) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (c) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. J. Am. Chem. Soc. 1940, 62, 1723. (3) Adamson, A. W. In Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; p 609. (4) Yang, R. T. In Gas Separation by Adsorption Processes; Butterworths: London, 1987; p 27. (5) Munro, L. A.; Johnson, F. M. G. Ind. Eng. Chem. 1925, 17 (1), 88. (6) (a) Lippens, B. C.; Linsen, B. G.; de Boer, J. H. J. Catal. 1964, 3, 32. (b) de Boer, J. H.; Lippens, B. C. J. Catal. 1964, 3, 38. (c) Lippens, B. C.; de Boer, J. H. J. Catal. 1964, 3, 44. (d) de Boer, J. H.; van den Heuvel, A.; Linsen, B. G. J. Catal. 1964, 3, 268. (e) Lippens, B. C.; de Boer, J. H. J. Catal. 1964, 3, 319. (7) Anderson, R. B. J. Catal. 1964, 3, 50. (8) Goodboy, K. P.; Fleming, H. L. Chem. Eng. Prog. 1984 (11), 63. (9) Ruthven, D. M. In Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984; p 55.
this zone of partial pressures, the slope of the isotherm is directly linked to pore size. In small pores, according to Kelvin’s equation, the saturating vapor pressure is lowered, which eventually transforms water vapor into the liquid phase.10 From that point on, the solid becomes more of a receptacle than an adsorbent. When the alumina used is not microporous, but rather strongly mesoporous (as for the alumina A1, cf. Figure 1), the beginning of capillary condensation coincides with a sharp inflection of the adsorption isotherm. For the alumina A1 this threshold is found at around 75% relative humidity, a point that we shall come back to later. The initial chemisorption is a dissociative addition of water on the alumina surface (Scheme 1). This representation is highly simplified. Whereas the surface of a silica is relatively simple, as there is only one kind of silanol and the bonds are mostly covalent,11 surface chemistry of alumina is more complicated. The Al-O bonds are more ionic than covalent, the surface charges vary in intensity, and a great many hydroxyl surface sites coexist.12,13 There is even the possibility of the oxygen atom from the water complexing with an aluminum atom of the alumina surface.14 It should be pointed out that, given the pretreatment temperature of the alumina (280 °C) in this study, the surface is far from being completely dehydroxylated; there should remain about 6-8 OH/nm2.15 As more water is added, hydrogen bonds16 develop immediately between two adjacent surface hydroxyl sites,
and then between a hydroxyl group and a molecule of nondissociated water.
Although the exact nature of these hydrogen bonds is not yet well understood in spite of all the interested parties working on it,17-23 they are generally treated as phenom(10) Ponec, V.; Knor, Z.; Cerny, S. In Adsorption on Solids; Butterworths: London, 1974, p 402. (11) Morrow, B. A. Stud. Surf. Sci. Catal. 1990, 57A, A161. (12) (a) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev. Sci. Eng. 1978, 17, 31. (b) Boehm, H. P.; Kno¨zinger, H. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1983; Vol. 4. (13) Ne´dez, C.; Lefebvre, F.; Choplin, A.; Niccolai, G. P.; Basset, J. M.; Benazzi, E. J. Am. Chem. Soc. 1994, 116, 8638. (14) Rossi, P. F.; Oliveri, G.; Bassoli, M. J. Chem. Soc., Faraday Trans. 1994, 90, 363. (15) Iwasawa, Y. In Tailored Metal Catalyst; Iwasawa, Y., Ed.; Reidel D. Publishing Company, Dordrecht, 1986, p 20. (16) Kotoh, K.; Enoeda, M.; Matsui, T.; Nishikawa, M. J. Chem. Eng. Jpn. 1993, 26, 355. (17) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1994, 116, 909.
Optimization of Adsorbents
Figure 3. Water adsorption and desorption isotherms on alumina A1 at 30 °C. Difference of the two curves.
ena of physisorption. This is the beginning of a veritable “house of cards”, which can be described more scientifically as the generation of first a monolayer and then many multilayers of water, whose growth is stopped by capillary condensation. Desorption Isotherm of H2O on Alumina A1. As the desorption isotherm does not retrace the adsorption isotherm (Figure 3), a hysteresis loop is generated. The phenomena involved are complex, as is shown by the controversy arising from their study.24,25 The importance of the porous walls is, however, well established. One interpretation emphasizes the metastable nature of the water film adsorbed during the adsorption phase,26 where the true equilibrium state would be represented by the condensed liquid in the pores. For others the main thing is the form of the water meniscus at the pore opening, which would be radial during adsorption and hemispheric at desorption.27 We shall now try to identify the components of the phenomena involved (chemisorption, physisorption, condensation) from the isotherm of water adsorption on alumina. The literature contains several models of isotherms of water adsorption on alumina. Attempts have been made, for example, to linearize the isotherm:28 this simple method is, however, worthless when the slopes are as clearly inflected as in our case. A more fruitful method subjects the isotherm, cut up into several segments, to a polynomial mathematical treatment.29 A BET model has also been envisioned.30 Another procedure is based on the study of (18) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290. (19) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063. (20) Plummer, P. L. M.; Chen, T. S. J. Phys. Chem. 1983, 87, 4190. (21) Buffey, I. P.; Brown, W. B.; Gebbie, A. Chem. Phys. Lett. 1988, 148, 281. (22) Fowler, P. W.; Quinn, C. M.; Redmond, D. B. J. Chem. Phys. 1991, 95, 7678. (23) Liu, J. X.; Bowman, T. L.; Elliott, J. R. Ind. Eng. Chem. Res. 1994, 33, 957. (24) Gusev, V. Y. Langmuir 1994, 10, 235. (25) (a) Mayagoitia, V. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2931. (b) Mayagoitia, V.; Rojas, F.; Kornhauser, I. J. Chem. Soc., Faraday Trans. 1 1988, 84, 785. (c) Mayagoitia, V.; Gilot, B.; Rojas, F.; Kornhauser, I. J. Chem. Soc., Faraday Trans. 1 1988, 84, 801. (d) Mayagoitia, V.; Cruz, M. J.; Rojas, F. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2071. (e) Cruz, M. J.; Mayagoitia, V.; Rojas, F. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2079. (26) Foster, A. G. J. Chem. Soc. (London) 1952, 1806. (27) Cohan, L. H. J. Am. Chem. Soc. 1938, 60, 433. (28) Chou, C. L. Chem. Eng. Commun. 1987, 56, 211. (29) Gabez, P. Ind. Chim. Belg. 1965, 30, 917. (30) Baguenne, M.; Bellagi, A. J. Soc. Chim. Tunis. 1992, 3, 183.
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Figure 4. Breakdown of the water adsorption isotherm on alumina A1 at 30 °C into three curves characterizing respectively the chemisorption, the physisorption, and the capillary condensation.
isosteres,31 using one of Antoine’s equations modified from the first appearance of condensation;32 however, in this case the transitions between successive phases seem imprecise. Closer to what we are after are the models based on adsorption potentials that use a Freundlichtype polynomial to get an empirical description.33 We have preferred to draw inspiration from work on the comparison of adsorption and desorption curves,34 as the difference between them clearly evinces the presence of a double hysteresis loop (Figure 3): the first one, located in a zone of low partial water pressures (e5% relative humidity) is due to the initial chemisorption of water on the alumina surface; the second one, found in a region of high partial pressures, is characteristic of capillary condensation. By subtracting one isotherm from the other, we are better able to decipher the components of the phenomena involved: the constant ordinate over a large range of partial pressures (between 5 and 75% relative humidity) represents the importance of chemisorption; the inflection of the slope observed at high partial pressure (>75%) marks the appearance of condensation. We can then bring out precisely the consequences of chemisorption, physisorption, and capillary condensation on the alumina A1 (Figure 4). To sum up, the isotherms of adsorption and desorption of water were determined at 30 °C on alumina A1. The difference between the two curves made it possible to identify the impact of chemisorption, physisorption, and capillary condensation. Thus was the exact threshold of capillary condensation located, that is, the partial water pressure at which condensation appears. This demonstration was particularly crucial to our study. As a result, as long as the relative humidity in the gas flow is held to under 75-80%, no formation of liquid water is observed. In these conditions the trapping solid will perform normally to trap the contaminants (mercury, arsenic, etc.) in the natural gas. We shall now build on our experience to study, in the same way, aluminas A2, A3, and B, as well as active carbon C. (31) Carniglia, S. C.; Ping, W. L. Ind. Eng. Chem. Res. 1989, 28, 1025. (32) Hacskaylo, J. J.; LeVan, M. D. Langmuir 1985, 1, 97. (33) Kotoh, K.; Enoeda, M.; Matsui, T.; Nishikawa, M. J. Chem. Eng. Jpn. 1993, 26, 570. (34) Desai, R.; Hussain, M.; Ruthven, D. M. Can. J. Chem. Eng. 1992, 70, 699.
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Figure 5. Water adsorption isotherms on aluminas A1, A2, A3, and B at 30 °C.
Figure 7. Breakdown of the water adsorption phenomena on alumina B at 30 °C.
Figure 6. Breakdown of the water adsorption phenomena on alumina A2 at 30 °C.
Figure 8. Study of the water adsorption phenomena observed on the active carbon C at 30 °C.
Sorption Isotherms of H2O on Aluminas A2, A3, and B. The same methodology was used to determine and analyze the isotherms for aluminas A2, A3, and B (cf. above). The adsorption isotherms are given in Figure 5, and the decomposition of the phenomena involved is shown for alumina A2 (Figure 6) and alumina B (Figure 7). The commentary on A1 can also be applied to the aluminas A2 and A3. The capillary condensation threshold observed is virtually the same, between 75 and 80% relative humidity. This confirms the fact that the distribution of micropores and mesopores is essential. The deviations between the three adsorption curves of the A series aluminas can be reasonably attributed to the surface differences of the three oxides (Table 1). In contrast, the alumina B, whose microporosity has been described above (Figure 1), behaves quite differently: the inflection point of the adsorption isotherms (marking the capillary condensation threshold) is located at a relative humidity (≈40%) much lower than that for the A series. The much gentler slope beyond this threshold can be explained by the fact that the pore network (volume, distribution) of this kind of alumina is distinctly less well developed (Table 1). These observations are well confirmed when the desorption isotherm is used to decompose the adsorption isotherm (Figure 7). Sorption Isotherms of H2O on Active Carbon C. Active carbon acts very differently from alumina toward water (Figure 8).
Since the earliest work by Dubinin,35 many other studies have been carried out on this subject. The almost total absence of weight gain at low values of relative humidity can be explained by the hydrophobic nature of the active carbon surface.36 The few water molecules that do seem to be retained could be adsorbed onto surface sites containing oxygenated compounds that often remain on the surface of an active carbon.37-39 The overall curve obtained is consonant with the shape reported in the literature.40-43 (35) Dubinin, M. M.; Zaverina, E. D.; Serpinski, V. V. J. Chem. Soc. 1955, 1760. (36) Naono, H.; Hakuman, M. J. Colloid Interface Sci. 1991, 145, 405. (37) Kraehenbuehl, F.; Quellet, C.; Schmitter, B.; Stoeckli, H. F. J. Chem. Soc., Faraday Trans. 1986, 82, 3439. (38) (a) Barton, S. S.; Koresh, J. E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1147. (b) Barton, S. S.; Koresh, J. E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1157. (c) Barton, S. S.; Koresh, J. E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1165. (d) Barton, S. S.; Koresh, J. E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1173. (39) (a) Miura, K.; Morimoto, T. Langmuir 1991, 7, 374. (b) Miura, K.; Morimoto, T. Langmuir 1994, 10, 807. (40) Dubinin, M. M. J. Coll. Interf. Sci. 1967, 23, 487. (41) (a) Stoeckli, F.; Jabukov, T.; Lavanchy, A. J. Chem. Soc., Faraday Trans. 1994, 90, 783. (b) Stoeckli, F.; Huguenin, D.; Rebstein, P. J. Chem. Soc., Faraday Trans. 1991, 87, 1233. (42) Dubinin, M. M.; Serpinski, V. V. Carbon 1981, 19, 402. (43) Yang, R. T. In Gas Separation by Adsorption Processes; Butterworths: London, 1987, p 12.
Optimization of Adsorbents
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We have seen that water adsorption corresponds to the sum of the chemisorption, the physisorption, and the capillary condensation. Since it appears that only this last phenomenon occurs with active carbon, it can be deduced that porosity is the sole decisive factor. This idea is reinforced when the desorption phase is considered: a single hysteresis loop is observed,44 as opposed to two for alumina. All of this indicates that the capillary condensation threshold of the active carbon C will be found at around 40% relative humidity. Conclusion The interaction of water with the surfaces of several aluminas and on active carbon were studied. The adsorption of water on alumina is in fact the sum of three quite different phenomena: chemisorption, which takes place at low partial water pressures; physisorption, due to the formation by hydrogen bonding of multilayers of water vapor in the alumina pores; capillary condensation, at dew point, where water goes from gaseous to liquid state. Because of the hydrophobic surface of active carbon, only the last phenomenon is observed for that carrier. Pore size has a critical effect on the appearance of condensation. Highly microporous solids (such as alumina B, active carbon C) have a capillary condensation threshold (40% relative humidity) that is much lower than for the macroporous aluminas A1, A2, and A3 (75-80% relative humidity) (Figure 9). (44) Stoeckli, F.; Huguenin, D. J. Chem. Soc., Faraday Trans. 1992, 88, 737.
Figure 9. Contribution of capillary condensation to water adsorption on aluminas A1, A2, and B and on the active carbon C at 30 °C.
This means that if these adsorbents are impregnated with an active phase, a macroporous alumina will trap contaminants (mercury, arsenic, etc.) in a natural gas highly saturated with water or other condensable matter (such as heavy hydrocarbons etc.) more efficiently than an active carbon would. In this study we have evinced all the phenomena in play that are relevant to the given industrial application. It is precisely this approach that has made it possible for us to optimize the adsorption trapping masses. LA951035T
