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New π-Complexation Adsorbents for Propane-Propylene Separation Carlos A. Grande, Jose´ D. P. Araujo, Simone Cavenati, Norberto Firpo,† Elena Basaldella,† and Alı´rio E. Rodrigues* Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal Received December 18, 2003. In Final Form: April 5, 2004 New adsorbents were prepared and tested for the separation of propane-propylene mixtures by adsorption. The ordered mesoporous material SBA-15 was used as the starting material for silver-ion deposition for π-complexation with propylene. Two different loadings of silver were evaluated. Adsorption equilibrium and kinetic measurements of propane and propylene in the matrix (pure SBA-15) and the silver-modified adsorbents were performed at 323, 343, and 373 K. In this temperature range, the selectivity of propylene in one of the materials (Ag/SBA-15 ) 0.5) is in the range 13-22 because the amount of propane adsorbed is very small, 0.095 mmol/g of propane versus 1.12 mmol/g of propylene at 100 kPa and 343 K. The diffusivity of both hydrocarbons is not seriously affected by the introduction of silver into the mesoporous structure.
Introduction Propylene separation from propane-propylene streams is challenging due to the similar size of both hydrocarbon molecules. Polymer-grade propylene requires a purity of >99.5%, actually achieved in large distillation columns with very small separation factors. Adsorption techniques have been explored in the last years to carry out olefinparaffin separations as an alternative to the traditional processes. Adsorption is a suitable alternative because the adsorbent can increase the separation factor several times and also diminish cost demands. The difficulty to overcome in PSA (pressure swing adsorption) units is that propylene has to be recovered in the blow-down step with very high purity (>99.5%), requiring a very selective adsorbent. The search for an adequate adsorbent is still an important primary research activity. Many commercial adsorbents have been tested for this separation.1-4 Some new materials were also reported to adsorb propylene, excluding partially or totally propane.5,6 The group of R. T. Yang followed the research line of π-bond formation between olefins and some transition metal ions and reported many adsorbents applied to olefin-paraffin separation.7-11 In their papers, different matrixes (polymeric resins, silica, Al2O3, Y-zeolites, pillared clays, and * To whom correspondence should be addressed. Phone: +351 22 508 1671. Fax: +351 22 508 1674. E-mail:
[email protected]. † Current address: CINDECA, University Nac. de La Plata. Calle 47 No 257 (1900), La Plata, Argentina. (1) Lewis, W. K.; Gilliland, E.; Chertow, B.; Cadogan, W. Ind. Eng. Chem. 1950, 42, 1326-1332. (2) Ja¨rvelin, H.; Fair, J. R. Ind. Eng. Chem. Res. 1993, 32, 22012207. (3) Da Silva, F. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 1999, 38, 2434-2438. (4) Grande, C. A.; Silva, V. M. T. M.; Gigola, C.; Rodrigues, A. E. Carbon 2003, 41, 2533-2545. (5) Cheng, L. S.; Padin, J.; Rege, S. U.; Wilson, S. T.; Yang, R. T. U.S. Patent 6406521, 2002. (6) Olson, D. H. U.S. Patent 6488741, 2002. (7) Yang, R. T.; Kikkinides, E. S. AIChE J. 1995, 41, 509-517. (8) Cheng, L. S.; Yang, R. T. Adsorption 1995, 1, 61-75. (9) Rege, S. U.; Padin, J.; Yang, R. T. AIChE J. 1998, 44, 799-809. (10) Padin, J.; Rege, S. U.; Yang, R. T.; Cheng, L. S. Chem. Eng. Sci. 2000, 55, 4525-4535. (11) Padin, J.; Yang, R. T. Chem. Eng. Sci. 2000, 55, 2607-2616.
MCM-41) were used to disperse silver and copper (Cu+) by several techniques, such as cation exchange, thermal dispersion, and incipient wetness. Also, a proprietary adsorbent, Olesorb-1, a dispersion of silver nitrate in bentonite clay with some promoters, was employed.12,13 Other works involving the use of supported silver and cuprous ions have been published mainly in the field of membranes.14-17 The point of forming π-complexation bonds between olefins and transition metal ions is to obtain a strong enough bond between the olefin and the adsorbent to provide high selectivity but not too strong of a bond, which allows desorption of the olefin by changing the operating conditions (temperature or pressure). As shown in Figure 1, from the different kinds of adsorption forces, the π-bond formation lies in the optimal range of bond strengths to be used in adsorption processes.18 SBA-15 is a novel ordered mesoporous silica19 that has already been used for many applications, including protein adsorption20 and preparation of nanostructured carbon materials.21 It was also evaluated for the separation of C2-C3 olefin-paraffin systems.22,23 This material has many advantages such as robust synthesis conditions, a large surface area per mass of adsorbent, (12) Cho, S. H.; Han, S. S.; Kim, J. N.; Kumar, P.; Choudary, N. V.; Bhat, S. G. T. U.S. Patent 6315816, 2001. (13) Choudary, N. V.; Kumar, P.; Bhat, S. G. T.; Cho, S. H.; Han, S. S.; Kim, J. N. Ind. Eng. Chem. Res. 2002, 41, 2728-2734. (14) Teramoto, M.; Takeuchi, N.; Maki, T.; Matsuyama, H. Sep. Purif. Technol. 2002, 28, 117-124. (15) Kim, H. S.; Ryu, J. H.; Kim, H.; Ahn, B. S.; Kang, Y. S. Chem. Commun. 2000, 14, 1261-1262. (16) Kim, Y. H.; Ryu, J. H.; Bae, J. Y.; Kang, Y. S.; Kim, H. S. Chem. Commun. 2000, 3, 195-196. (17) Jose, B.; Ryu, J. H.; Lee, B. G.; Lee, H.; Kang, Y. S.; Kim, H. S. Chem. Commun. 2001, 20, 2046-2047. (18) Humphrey, J. L.; Keller, G. E., II. Separation Process Technology; McGraw-Hill: New York, 1997. (19) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (20) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712-10713. (21) Han, Y.-J.; Stucky, G. D.; Butler, A. J. Am. Chem. Soc. 1999, 121, 9897-9898. (22) Newalkar, B. L.; Choudary, N. V.; Kumar, P.; Komarneni, S. Bhat, T. S. G. Chem. Mater. 2002, 14, 304-309.
10.1021/la036400s CCC: $27.50 © 2004 American Chemical Society Published on Web 05/26/2004
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Grande et al. microscopy (TEM). Semiquantitative chemical analysis was carried out by X-ray dispersion (EDAX). XRD patterns were recorded with a Philips APD 1700 diffractometer, using Cu KR radiation, at 40 kV, 30 mA, and 0.02 and 1 s step times. The size and morphology of solid particles were observed by SEM using a Philips 505 microscope. Transmission electron microscopy (TEM) images were recorded in a Carl Zeiss unit. Textural properties were determined by nitrogen isotherms at 77 K measured with a Micromeritics Accusorb 2100E instrument. Pure propane and propylene adsorption equilibrium was measured using a Rubotherm microbalance device operated in closed mode. Activation of the samples was performed at 385 K overnight under vacuum. Pure gas adsorption kinetics was measured by means of breakthrough curves with diluted feed to guarantee that the gas velocity in the column was constant and that the operation was isothermal (experimentally verified by means of a thermocouple located at the outlet of the column). Mixtures of helium/C3 were prepared in a bottle, and both mixtures had a concentration of 1.1% hydrocarbons. Detailed information about the experimental setup for kinetic measurements is given elsewhere.4 The column has a 3.13 cm length with a 0.35 cm internal diameter. The flow rates and the masses of adsorbent used for each experiment are shown in Table 2. Air Liquide provided all the gases used in this report: propane N35, propylene N24, and helium 50 (purity greater than 99.95, 99.4, and 99.999%, respectively).
Theoretical
Figure 1. Bond energies for adsorption. The ranges marked in black are the optimal ranges for reversible complexation.18 This figure is adapted from Humphrey and Keller.18
and pore size tuning according to preparation and hydrothermal stability. In this work, we have coupled the use of SBA-15 as a large surface area matrix to deposit silver for π-bond formation between propylene and transition metal ions and explored the selectivity toward propylene for propanepropylene separation purposes. The effects of silver loading were assessed by employing two different metal loadings. The silver-containing adsorbents were prepared by incipient wetness impregnation. Adsorption equilibrium and kinetic measurements of propane and propylene in the three adsorbents (pure SBA-15 and two samples with different amounts of silver deposition) were studied at three different temperatures: 323, 343, and 373 K. Experimental Section SBA-15 was prepared using tetraethyl orthosilicate (TEOS, Aldrich) and poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (H(-OCH2CH2)x[-OCHCH3CH2-]y(-OCH2CH2-)zOH, average Mw ) 5800 g/mol, Aldrich) as the organic structure-directing agent. A gel having the appropriate composition19 was stirred overnight at 308 K and submitted to hydrothermal conditions at 353 K for 16 h. The resulting solid phase was filtered and washed, then dried at 383 K, and finally heated at 763 K in air for 6 h. The silver adsorbents were prepared by incipient wetness impregnation on an SBA-15 support with an aqueous solution of AgNO3 (Aldrich Co., 99.9%) using distilled deionized water followed by drying and calcination at 473 K in nitrogen flow for 12 h. In this way, two AgNO3/SBA-15 samples with different silver/substrate ratios were prepared: Ag/SBA-15 ) 0.5 and Ag/ SBA-15 ) 1.0. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron
In the analysis of adsorption equilibrium, we have to consider a model that takes into account physical adsorption and also the enhanced adsorption due to chemisorption. In this work, the Toth model24 was used to describe physical adsorption, while π-complexation was described with a Langmuir model assuming that adsorption sites have a uniform energy distribution.7 The complete model is
q)
qmK1/nP [1 + KPn]
+ 1/n
[
]
qmc 1 + bcPes ln 2s 1 + bcPe-s
(1)
where K is the physical adsorption constant, qm and qmc are the maximum adsorbed phase concentrations of physical adsorption and chemisorption, respectively, bc is the chemisorption constant, and s is the spread of energy distribution. Although the nature of the Toth model is empirical, it offers several advantages: it has a large flexibility for describing experimental data and a direct extension to multicomponent predictions. These two properties are very important in PSA (pressure swing adsorption) modeling. The adsorption constants have Arrhenius dependence in the form
(-∆H RT ) -Ψ b ) b ° exp( RT )
K ) K° exp
c
c
(2)
where K° and bc° are the infinite adsorption constants, -∆H is the enthalpy of adsorption, and Ψ is the mean variance of the uniform energy distribution.7 This description of adsorption equilibrium involves many parameters. The parameters of the Toth equation (qm, K°, -∆H, and n) were determined fitting the Toth (23) Newalkar, B. L.; Choudary, N. V.; Turaga, U. T.; Vijayalakshmi, R. P.; Kumar, P.; Komarneni, S.; Bhat, T. S. G. Chem. Mater. 2003, 15, 1474-1479. (24) Toth, J. Acta Chim. Acad. Sci. Hung. 1971, 69, 311-328.
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Table 1. Adsorption Equilibrium Parameters for Propane and Propylene Adsorption on SBA-15 and Silver-Modified SBA-15 Samples adsorbent
gas
qm (mmol/g)
K° (1/kPa)n
∆H (kJ/mol)
n
SBA-15 pure sample Ag/SBA-15 ) 0.5
C3H8 C3H6 C3H8 C3H6 C3H8 C3H6
2.041 2.062 0.221 0.221 0.189 0.189
3.55 × 10-6 3.55 × 10-6 1.04 × 10-6 1.04 × 10-6 1.04 × 10-6 1.04 × 10-6
-24.240 -23.885 -34.028 -34.028 -36.866 -36.866
0.87 0.91 0.53 0.53 0.36 0.36
Ag/SBA-15 ) 1.0
model to the propane adsorption equilibrium data alone. In the case of propylene, the same parameters were used for physical adsorption, while the chemisorption parameters (qmc, bc°, Ψ, and s) were determined from the best data fitting.7 Even when the simpler PSA modeling can be done using only the equilibrium data,25 it is interesting to know the kinetics of adsorption of the pure gases. Particularly for the cases of propane and propylene, the difference of diffusivity between both gases in zeolite 4A can be as large as 100 and the separations are kinetically controlled and not equilibrium controlled. In this study, we measured breakthrough curves of both hydrocarbons in the linear zone of the adsorption equilibrium. In this case, the amount of material adsorbed is very small compared to the total flux entering the column, which allows us to consider constant velocity. Also, considering ideal gas behavior and isothermal operation, a mass balance in the column can be described as
Dax
∂2CB ∂z2
( )
∂CB ∂CB 1 - c ∂〈q〉 ) + -v Fp ∂z ∂t c ∂t
|
∂CB ∂z
(4)
(t,0)
∂CB ∂z
|
) v(CB - CB0)
(t,L)
(5)
)0
(6)
The mass balance in a volume element of the cylinder particles is represented by
p
∂C ∂C ∂q 1 ∂ + Fp ) DR ∂t ∂t R ∂R p p ∂R
[
]
(7)
with the initial condition
C(R,0) ) 0
(8)
and the boundary conditions
pDp
|
∂C ) kf(CB - C|(Rp,t)) ∂R (Rp,t)
|
∂C )0 ∂R (0,t)
Ψ (kJ/mol)
s
1.870
2.0 × 10-14
-78.352
3.58; 5.52; 6.91
1.475
10-14
-75.893
2.66; 4.84; 6.45
2.0 ×
Table 2. Experimental Conditions (Masses of Adsorbent and Flow Rates) for Breakthrough Curves, Stoichiometric Times, and Adsorbed Phase Concentration adsorbent SBA-15 pure sample mads ) 0.134 g
gas C3H6 C3H8
SBA-15 Ag/SBA-15 ) 0.5 mads ) 0.311 g
C3H6 C3H8
SBA-15 Ag/SiO2 ) 1.0 mads ) 0.422 g
C3H6 C3H8
a
and Danckwert’s boundary conditions
Dax
bc° (1/kPa)
(3)
with the initial condition
CB(0,z) ) 0
qmc (mmol/g)
(9) (10)
where Dp is the pore diffusion coefficient, p is the pellet porosity, P is the gas pressure in the macropores, q is the adsorbed phase concentration, Rp is the SBA-15 particle (25) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994.
T (K)
flowa (cm3/s)
ht (s)
q (mmol/g)
323 343 373 323 343 373 323 343 373 323 343 373 323 343 373 323 343 373
0.85 0.91 0.85 0.99 0.88 0.69 0.39 0.39 0.37 0.43 0.44 0.43 0.88 0.88 0.85 0.64 0.56 0.51
24.3 14.4 9.1 17.2 12.8 9.3 822.6 591.6 351.1 21.9 15.9 12.8 198.1 141.3 85.8 21.6 16.4 12.3
0.059 0.037 0.021 0.049 0.034 0.020 0.420 0.302 0.167 0.013 0.009 0.006 0.166 0.118 0.069 0.013 0.008 0.005
The flow rates were measured at 298 K.
radius, and kf is the film mass transfer resistance in the external surface of the pellet. The particle-averaged adsorbed phase concentration (〈q〉) is defined by
〈q〉 )
∫0R qR dR
2 Rp2
p
(11)
Adsorbent Characterization To determine relevant properties of the adsorbent, many observations were performed. The chemical composition analysis carried out with the EDAX electronic microprobe indicated that the impregnated samples Ag/SBA15 ) 1.0 and Ag/SBA-15 ) 0.5 contained a Ag/Si ratio of ∼1.09 (w/w) and ∼0.54 (w/w), respectively. Initially, nitrogen adsorption experiments were measured on the different samples to determine the area of the base material (SBA-15) and its reduction with the introduction of different amounts of silver. The area of SBA-15 is 996 m2/g, while that of Ag/SBA-15 ) 0.5 is 594 m2/g and that of Ag/SBA-15 ) 1.0 is 297 m2/g. Clearly, the introduction of higher amounts of silver produced a significant decrease in the adsorbent area. The reduction in the area can be due to a reduction of the diameter of the mesopores due to silver-ion deposition or to partial (or total) blocking of the mesopores either by silver ions or even metallic silver. XRD (range, 5° < 2θ < 60°) of starting SBA-15 and Ag/ SBA-15 samples (see Figure 2) showed reflections corresponding to the AgNO3 used for impregnation. These reflections are very weak in the pattern of Ag/SBA-15 ) 0.5. Transmission electron microscopy (TEM) analyses were performed on all of the samples. Images with the results for the three samples are shown in Figure 3. The typical ordered structure reported for these mesoporous materials
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Figure 4. Two different magnifications of SEM images of the pure SBA-15 sample.
Figure 2. XRD spectra of the samples: (1) Ag/SBA-15 ) 0.5; (2) Ag/SBA-15 ) 1.0. The inset corresponds to the detection of ordered mesoporous material formation in the case of SBA15.19
Figure 5. Adsorption equilibrium of propylene and propane on SBA-15 at 323, 343, and 373 K. The solid lines are the Toth model fittings.
samples, but the ordered shape of the pores remains constant when compared to that of the original SBA-15 sample. The pore diameters of the samples are 8 (SBA15), 6 (Ag/SBA-15 ) 0.5), and 4 nm (Ag/SBA-15 ) 1.0). Finally, in Figure 4, the morphology of SBA-15 determined by scanning electron microscopy (SEM) is shown in order to use it in the kinetic model. Cylinders 1.1 µm in length with a 0.4 µm diameter were found. The size distribution of these cylinders is also very narrow. Adsorption Equilibrium
Figure 3. TEM images: (a) pure SBA-15; (b) Ag/SBA-15 ) 0.5; (c) Ag/SBA-15 ) 1.0.
The adsorption equilibrium isotherms of propane and propylene measured at 323, 343, and 373 K on the SBA15 sample are shown in Figure 5. The amount adsorbed of both hydrocarbons is very similar, indicating that the adsorbent has a very small selectivity. The adsorption
was found.25-28 Two general conclusions can be taken from these pictures. The first one is that the amount of metallic silver is very small in Ag/SBA-15 ) 1.0 and is not detectable in Ag/SBA-15 ) 0.5. The other observation is that the diameter of the mesopores decreases in the modified
(26) Choi, D.-G.; Yang, S.-M. J. Colloid Interface Sci. 2003, 261, 127132. (27) Janssen, A. H.; Van der Voort, P.; Koster, A. J.; de Jong, K. P. Chem. Commun. 2002, 15, 1632-1633. (28) Zhu, J.; Ko´nia, Z.; Puntes, V. F.; Kiricsi, I.; Miao, C. X.; Ager, J. W.; Alivisatos, A. P.; Somorjai, G. A. Langmuir 2003, 19, 4396-4401.
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Figure 6. Adsorption equilibrium of (a) propylene and (b) propane on Ag/SBA-15 ) 0.5 at 323, 343, and 373 K. The solid lines are the Toth model fittings.
Figure 7. Adsorption equilibrium of (a) propylene and (b) propane on Ag/SBA-15 ) 1.0 at 323, 343, and 373 K. The solid lines are the Toth model fittings.
and desorption points were coincident, indicating that the isotherms are completely reversible. The propylene data are in very good agreement with other published data, although propane was slightly more adsorbed in this sample.22-23 The propane isotherms are in good agreement with the ones measured in silica gel with large surface areas and narrow pores.30 The adsorption isotherms of propane and propylene measured at 323, 343, and 373 K in SBA-15 with Ag/SBA-15 ) 0.5 are shown in Figure 6. The amount of propylene adsorbed is much larger than that adsorbed in the pure sample, particularly at lower pressures. On the other hand, the amount of propane adsorbed has decreased significantly and now the ratio of the adsorbed concentration of propylene and propane is ∼15. The corresponding isotherms measured in the sample Ag/SBA-15 ) 1.0 are shown in Figure 7. In this case, the amount of propane adsorbed is quite similar to that of the other modified SBA (Ag/SBA-15 ) 0.5), but the amount of propylene adsorbed has decreased. Although, in this sample, the ratio of propylene to propane adsorbed is still high (∼13). We have seen that the dispersion of silver ions (Ag/SBA-15 ) 0.5 and 1) reduces the area available by factors of 2 and 4, approximately. In the case of the Ag/SBA-15 ) 0.5 sample, the area reduction is counterbalanced by an increase in the amount of propylene adsorbed. On the other hand, the amount of propane adsorbed decreases by ∼10 times. When the proportion of silver ions is increased (Ag/SBA-15 ) 1.0), the amount of propane remains essentially constant, although propylene adsorbs a little less. This indicates that for propane there may be a limiting amount adsorbed (the one observed
in these two samples), while for propylene there is an optimum amount of silver loading and this optimum is near 0.54 and below 1.09. The solid lines in Figure 4 represent the fitting of the data using the Toth model. In the case of pure SBA, independent parameters were obtained for propane and propylene. They are reported in Table 1. The parameters of both gases are very similar, and thus, the propane parameters will be assumed valid for physical adsorption of both gases. We can see that the heat of adsorption of both gases is ∼24 kJ/mol, indicating that SBA-15 has the same kind of interactions with both gases. This value is larger than the one previously reported for this material,22 but they are in good agreement with the values obtained for silica gel.30 The fitting of the adsorption equilibria on Ag/SBA-15 ) 0.5 and Ag/SBA-15 ) 1.0 was performed in the same way: physical adsorption parameters were estimated fitting propane data and used in the propylene data to fit only the chemisorption parameters (see eqs 1 and 2). The parameters obtained with this routine are also reported in Table 1. We have to note that, in the case of physical adsorption, the heat of adsorption increased when compared with the case of the pure matrix but remained almost constant for both salt-dispersed materials. Also, the degree of heterogeneity (given by the decrease in parameter n) increased when increasing the silver content. In the case of chemisorption, the mean variance of the energy distribution is almost the same in both samples, with a value of ∼76 kJ/mol. Clearly, from the results shown in Figures 5-7, the best adsorbent for propane-propylene separation is Ag/SBA-15 ) 0.5. The equilibrium selectivity of the adsorbents, that is, the qC3H6/qC3H8 ratio at 343 K, is plotted in Figure 8. Note that, in the case of a PSA operating between 10 and 100 kPa, the selectivity of Ag/SBA-15 ) 0.5 is between 22 and 13.
(29) Luan, Z.; Hartmann, M.; Zhao, D.; Zhou, W.; Kevan, L. Chem. Mater. 1999, 11, 1621-1627. (30) Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 1686-1693.
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Figure 8. Propylene-propane ratio of the amount adsorbed at 343 K in the different adsorbents.
Figure 10. Adsorption breakthrough curves of (a) propylene and (b) propane in Ag/SBA-15 ) 0.5 at 323, 343, and 373 K. Hydrocarbon concentration: 1.1% diluted in helium.
Figure 9. Adsorption breakthrough curves of (a) propylene and (b) propane in SBA-15 at 323, 343, and 373 K. Hydrocarbon concentration: 1.1% diluted in helium.
The results of these adsorbents have to be compared with previously published results to see whether the performance of the adsorbent compensates for its high cost. Other silver-based π-complexation adsorbents were used for that purpose.11,13 A silver-modified clay13 has a much lower adsorption capacity than both modified samples reported here due to a smaller superficial area available for adsorption (165 m2/g). Although, propane adsorption is higher than it is in our samples, indicating a low selectivity of this adsorbent with respect to the ones reported in this work. When comparing with silica substrate, the propylene adsorption capacity of Ag/SBA-15 ) 0.5 at 343 K is very similar to that previously reported,11 although again, the amount of propane adsorbed in our adsorbents is 3 times smaller. It is interesting to notice that, in the preparation of silvermodified SiO2, MCM-41,11 and SBA-15 (for Ag/SBA-15 ) 0.5), the area reduction with respect to the original material is 38-40%.
Figure 11. Adsorption breakthrough curves of (a) propylene and (b) propane in Ag/SBA-15 ) 1.0 at 323, 343, and 373 K. Hydrocarbon concentration: 1.1% diluted in helium.
Adsorption Kinetics The kinetics of adsorption of propane and propylene in the three samples analyzed above was determined in the same temperature range (323, 343, and 373 K). Results
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Table 3. Adsorption Kinetic Parameters Used in the Simulation of Breakthrough Curves adsorbent SBA-15 pure sample
gas
T (K)
C3H6 323 343 373 C3H8 323 343 373 Ag/SBA-15 ) 0.5 C3H6 323 343 373 C3H8 323 343 373 Ag/SBA-15 ) 1.0 C3H6 323 343 373 C3H8 323 343 373
DK Dp Dax Dm (cm2/s) (cm2/s) (cm2/s) (cm2/s) 0.563 0.624 0.718 0.480 0.532 0.612 0.563 0.624 0.718 0.480 0.532 0.612 0.563 0.624 0.718 0.480 0.532 0.612
0.0107 0.0111 0.0116 0.0105 0.0108 0.0113 0.0081 0.0083 0.0087 0.0079 0.0081 0.0085 0.0054 0.0055 0.0058 0.0053 0.0054 0.0056
0.0106 0.0109 0.0114 0.0103 0.0106 0.0111 0.0080 0.0082 0.0086 0.0078 0.0080 0.0084 0.0053 0.0055 0.0058 0.0053 0.0054 0.0056
0.63 0.70 0.80 0.53 0.59 0.69 0.57 0.64 0.73 0.49 0.54 0.63 0.54 0.59 0.69 0.46 0.51 0.59
obtained for the base material (SBA-15) for propane and propylene are shown in Figure 9. As can be seen, the diffusion of both gases is very fast. Similar curves for Ag/ SBA-15 ) 0.5 and Ag/SBA-15 ) 1.0 are reported in Figures10 and 11, respectively. All the curves shown here were measured under isothermal conditions. The adsorbed phase concentration determined from breakthrough curves was in all the cases consistent with the gravimetric data and is also plotted in Figures 4-6 (although it is hard to see because the hydrocarbon partial pressure is 1.1 kPa). The amounts adsorbed determined by breakthrough experiments together with the stoichiometric times are reported in Table 2. The diffusivities used in the simulation model of breakthrough curves with eqs 3-11 (solid lines in Figures 9-11) are reported in Table 3. For the calculation of the pore diffusivity, the Bosanquet equation has been used:
τp τp 1 ) + Dp Dm DK
(12)
where Dm is the molecular diffusivity and DK is the Knudsen diffusivity, and the pore diffusivities are also reported in Table 3. Molecular diffusivity was calculated with the Chapman-Enskog equation, DK ) 9700rpxT/M, where rp (in centimeters) is the pore radius and M is the molecular weight of the gas. In this case, as verified by
TEM, the channels are straight, which means that the tortuosity factor (τp) is 1. In Table 3, we are also reporting the axial dispersion (Dax) calculated with3,4
Dax ) (0.65 + 0.55c)Dm + γ22Rpv
(13)
where c is the porosity of the column, Rp is the particle radius, γ2 ) 0.5, and ν is the interstitial gas velocity. In all the cases, the control of the breakthrough curves was shared by pore diffusion (≈10%) and axial dispersion (≈90%). In the simulation of these curves, no fitting parameters were employed and the predicted curves are in very good agreement with the experimental ones. In the case of propylene, also a self-sharpening of the concentration wave was observed due to the strong nonlinearity of the adsorption isotherms, even at this very low partial pressure of propylene. Conclusions SBA-15 has been used as a substrate to prepare two silver-modified adsorbents for propane-propylene separation. The interesting physical properties of SBA-15 (particularly its large surface area and hydrothermal resistance) make it a suitable adsorbent for metal deposition to produce preferential adsorption of propylene via π-complexation bonds. The modified materials do not show metallic silver aggregates. Adsorption equilibrium and kinetic measurements of propane and propylene have been performed at 323, 343, and 373 K. Adsorption of both gases was fully reversible. The starting material (SBA-15) has a very poor selectivity for propane-propylene separation. Both modified materials have a very good selectivity toward propylene. The material having a ratio of Ag/SBA-15 ) 0.5 presented the best selectivity (∼10). When the amount of silver is doubled, the decrease in surface area is detrimental to the adsorption of propylene, while the adsorption of propane remains constant, giving a selectivity factor toward propylene of ∼8. Acknowledgment. The authors greatfully acknowledge the financial support from the Foundation for Science and Technology (FCT), project POCTI/1999/EQU/32654. C.A.G acknowledges FCT Grant SFRH/BD/11398/2002. The work is also part of the CYTED project V.8 “Clean Technology for the Separation of Light Olefins”. C.A.G. also acknowledges Dr. Rui Fernandes (IBMC, Portugal) for helping in the TEM measurements. LA036400S
