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Energy & Fuels 2006, 20, 778-782
Removal of C2H4 from a CO2 Stream by Adsorption: A Study in Combination of ab Initio Calculation and Experimental Approach Jinxia Zhou, Yongchun Zhang, Xinwen Guo,* Weijie Song, Hongliang Bai, and Anfeng Zhang State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian, China, 116012 ReceiVed June 22, 2005. ReVised Manuscript ReceiVed NoVember 21, 2005
Removal of C2H4 from a CO2 stream is very important in ultrapurification of CO2 for its application and utilization. However, it is difficult to remove C2H4 completely from a CO2 stream via conventional cryogenic distillation because the volatility of C2H4 is similar to that of CO2. Another choice is the catalytic combustion method, but it is an energy-inefficient process, especially when it is coupled with the cryogenic distillation. Selective adsorption for removing C2H4 might be a promising alternative one. A great effort has been made in olefin/paraffin separation and recovery of olefin. There are, however, few reports about the removal of a trace of C2H4 from a CO2 stream. In this study, ab initio calculations have been conducted to screen the potential adsorbents. We found that AgNO3 exhibits a much higher adsorption affinity for C2H4 through a π-complexation interaction, indicating that the AgNO3-modified zeolite might be a promising adsorbent. Furthermore, the AgNO3-modified NaY zeolite was prepared, and its adsorption performance was tested. We showed that the AgNO3/NaY is able to remove C2H4 from a CO2 stream to less than 1 ppmv with an adsorption capacity of 13 mL/g at a concentration level of 1030 ppmv C2H4. The results also imply that the ab initio calculations and the developed adsorption models in the present study are useful for designing the novel adsorbents.
Recovery of CO2 is significant for both reduction in greenhouse gases and best utilization of carbon resources. However, a captured CO2 stream is not easy to be deep purified because some impurities, such as C2H4, in the CO2 stream are difficult to be removed. Ethylene oxide and ethylene glycol processes produce a large amount of CO2 that can be further purified as food-class liquid additive or other useful pure CO2 feed. But C2H4 species presented in the CO2 feed cannot be deeply removed by cryogenic distillation because of the close relative volatilities. Catalytic combustion can ideally oxidize C2H4 to H2O and CO2, but the operational condition (>300 °C, containing sufficient O2)1 is still an energy-intensive process, especially connected with cryogenic distillation. Technoeconomic adsorption-separation approaches might be promising alternatives for such a difficult purification. However, despite the great efforts made in olefin/paraffin separation or in recovery of C2H4,2-6 there are few reports about removal of a trace amount of C2H4 from CO2 feed. Since the property of CO2 is not the same as that of C2H6, it is necessary to provide a fundamental study of the feasibility of deep removal of C2H4 from CO2 feed by adsorption. Recently, rapid progress in molecular orbital (MO) theory calculations based on quantum mechanics has been used to
study the nature of adsorption. Chen and Yang7 reported an ab initio MO study on adsorption of oxygen, nitrogen, and ethylene on silver-zeolite and silver halides and gave a detailed discussion on the bonding nature between the adsorbates and the adsorbents. By comparing the π-complexations of CO/C2H4 adsorbing on CuCl/AgCl, Huang et al.8 pointed out that both CO and C2H4 adsorb more strongly on CuCl than on AgCl and the bonds with CO are stronger than that with C2H4. Huang et al.9 also studied the bonding nature of C2H2 adsorbing on different nickel halides by MO calculations. The adsorptions of a number of aromatic-metal ion systems were also been studied by using MO study, including adsorption of benzene on chlorides of CuCl, PaCl2, AgCl, AuCl2, and PtCl4,10 thiophene/benzene on Cu+ and Ag+,11 benzene/cyclohexane on various Y-zeolites,12 and so on. However, there were few reports on the application of MO to guide the design of the adsorbent for a given separation or purification system, such as designing a suitable adsorbent for deeply removing C2H4 impurity from a CO2 stream. The purpose of this work was to perform a feasibility study of an adsorptive approach to removing C2H4 impurity from a CO2 stream by an ab initio molecular theory study, connected with experiments, which were done to prove the accuracy of calculation results.
* To whom correspondence should be addressed. Phone: 86-41188993607. Fax: 86-411-83698170. E-mail:
[email protected] or
[email protected]. (1) Hirayama, H. (Okazaki, JP); Kanazawa, T. (Toyota, JP); Sakurai, K. (Gotenba, JP) U.S. Patent No. 6,147,023, 2000. (2) Padin, J.; Yang, R. T. Chem. Eng. Sci. 2000, 55, 2607-2616. (3) Eldridge, R. B. Ind. Eng. Chem. Res. 1993, 32, 2208-2212. (4) Safarik, D. J.; Eldridge, R. B. Ind. Eng. Chem. Res. 1998, 37, 25712581. (5) Xie, Y. C. CN Patent No. 1,073,422, 1993. (6) Yang, R. T.; Kikkinides, E. S. AIChE J. 1995, 41, 509-517.
(7) Chen, N.; Yang, R. T. Ind. Eng. Chem. Res. 1996, 35, 4020-4027. (8) Huang, H. Y.; Padin, J.; Yang, R. T. Ind. Eng. Chem. Res. 1999, 38, 2720-2725. (9) Huang, H. Y.; Yang, R. T.; Chen, N. Langmuir 1999, 15, 76477652. (10) Takahashi, A.; Yang, F. H.; Yang, R. T. Ind. Eng. Chem. Res. 2000, 39, 3856-3867. (11) Yang, R. T.; Takahashi, A.; Yang, F. H. Ind. Eng. Chem. Res. 2001, 40, 6236-6239. (12) Takahashi, A.; Yang, R. T. AIChE J. 2002, 48, 1457-1468.
1. Introduction
10.1021/ef050182o CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2005
RemoVal of C2H4 from a CO2 Stream by Adsorption
Energy & Fuels, Vol. 20, No. 2, 2006 779
structures were used for the energy calculation and the natural bond orbital (NBO)18 theory analysis at the B3LYP level and the following basis sets: DZVP (DFT orbital) for Ag and Na and 6-311+g** for C, H, Si, Al, N, and O. The basis set superposition error (BSSE)19 was eliminated when calculating these weak-interacting energies. The adsorption energies were calculated according to the following expression:20 Figure 1. Model cluster of zeolite for calculation.
2. Calculation Section Molecular Cluster Selections. The requirements for the adsorbent used for removal of C2H4 from CO2 feed are as following: first, the active sites of the adsorbent should have high adsorption selectivity for C2H4 over the corresponding CO2, even if the sites have been occupied by CO2 molecular, so it is possible to thoroughly remove impurity as well as to get a high adsorptive capacity; second, the adsorbing bonds should be weak enough to be broken by simple engineering operations such as raising the temperature or decreasing the pressure of the system, so the regeneration of the adsorbent is possible; and third, the active sites of the adsorbent should be relatively stable in operating conditions. As for C2H4 adsorbent, Ag+ and Cu+ have been used as the active species most frequently.13 But Cu+ with poor chemical stability can be easily oxided into Cu2+ in the presence of O2 and then loses adsorbability drastically. In addition, compared with active carbon, resin, or carbon fiber, metal ion-modified zeolites can form stronger adsorptions with adsorbate and are usually used for purification.13 Therefore, AgNO3-modified zeolite was selected as the target adsorbent. To obtain a quantitative understanding of the adsorptionpurification mechanism, the Na-zeolite was also selected as a comparison in this study. Selecting a finite structure that not only represents the real structure perfectly but also saves computational cost is very important. A large number of model selections for zeolites have been employed.7,14-17 As for AgNO3-modified zeolite adsorbent, we suppose that the AgNO3 is dispersed on the zeolite surface by wet impregnation, and then the Ag+ is mainly in the form of AgNO3. Thus, the AgNO3 cluster is selected to stand for this kind of adsorbent for computation. The smallest model of a silica-alumina zeolite is composed of silica-oxygen and alumina-oxygen, SiO4 and AlO4, which are joined at the corners of the polyhedra by a shared oxygen. The cations (i.e., Na+) are not part of the framework but are used to offset the negative charge of the aluminum atoms in AlO2- groups. Considering the computational economy as well as the essential structural and chemical information of zeolite, the cluster structure of unmodified zeolite adsorbent was selected as shown in Figure 1, NaSiAlO7H6. It contains Na+, silica-oxygen, and alumina-oxygen, and the oxygen dangling bonds are saturated with hydrogen to terminate the model. Computational Method. The MP2 theory at the DZVP (DFT orbital) basis set for Ag and the 6-311g basis set for Na, C, H, Si, Al, N, and O was used to optimize the geometries of C2H4, CO2, AgNO3, NaSiAlO7H6, C2H4-AgNO3, CO2-AgNO3, C2H4-NaSiAlO7H6, and CO2-NaSiAlO7H6. Then the optimized (13) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley: New York, 2003; pp 192-197. (14) Sauer, J. Chem. ReV. 1989, 89, 199-255. (15) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phy. Chem. 1993, 97, 12773-12782. (16) Hill, J. R.; Sauer J. J. Phys. Chem. 1995, 99, 9536-9550. (17) Treesukol, P.; Limtraku, J.; Truong, T. N. J. Phys. Chem. B 2001, 105, 2421-2428.
∆E. ) Eadsorbate-adsorbent - (Eadsorbate + Eadsorbent)
(1)
where Eadsorbate and Eadsorbent are the BSSE eliminated energies of adsorbate and adsorbent, respectively, and Eadsorbate -adsorbent is the total energy of the complex. The calculations are performed using the Gaussian98 program.21 3. Experimental Section Preparation and Characterization of Adsorbents. The NaY powder Si/Al ) 2.7, from Wenzhou Zeolite Co., Zhejiang, China, used in the present study was shaped into cylindrical pellets (1.2 mm × 3 mm) by mixing with colloidal silica, extruding, drying, and calcining. AgNO3 (0.15 g) was dispersed on 1-g support by the incipient wetness technique, termed as the AgNO3/NaY adsorbent. The Si/Al ratio of the zeolite powder was measured by the Sequential X-ray Spectrometer System (SRS-3400, Bruker Co., Rheinstetten, Germany). The decomposing temperature of AgNO3 dispersed on the support was measured by TG/DTA analysis (TGA/ SDTA851e, Mettler-Toledo Co., Greifensee, Switzerland). The X-ray powder diffraction analysis (D/Max 2400 Automatic X-ray diffractometer, Rigaku Denki Co. Ltd., Tokyo, Japan) was used to determine the dispersion degree of the AgNO3 on the support. The BET surfaces of the AgNO3/NaY and the NaY were 473.6 m2/g and 645.5 m2/g, respectively, measured by Physical Adsorption Apparatus (AUTOSORB-1. Quantachrome Co., Boynton Beach, FL) using N2 adsorption at 77 K. The heats of adsorption were measured in a DSC-910 modulus (TA Instrument Co., New Castle, DE) coupled to a TGA thermal analyzer (TGA/SDTA851e, MettlerToledo). Experimental Equipment and Conditions. The adsorption isotherms of C2H4 on the AgNO3/NaY and the NaY adsorbents at 30 °C and atmospheric pressure were determined with a frontal chromatographic method.22 The frontal chromatographic adsorption system was a modified gas chromatograph that had the chromatographic column replaced with the sample column (made in our laboratory) and six switch valves and a flow control system were installed. The gases (offered by Guangming Institute, Dalian, China) used in the experiment were ethylene (CP grade, g99.5%), CO2 (CP grade, g99.95%), and N2 (CP grade, g99.99%). The sample column was filled with 1.0 g of adsorbent, and the C2H4 was diluted with N2 and CO2 at different concentration levels. N2 was used as regeneration/blow gas. (18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (19) van Duijneveldt, F. B.; van Duijneveldt-van de Rijdt, J. G. C. M.; van Lenthe, J. H. Chem. ReV. 1994, 94, 1873-1885. (20) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley: New York, 2003. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (22) Chen, Y. Y.; Sun, Y. H.; Ding, Y. J.; Zhou, R. X.; Luo, M. F. Adsorption and Catalysis; Henan Science and Technology Press: Zhengzhou, China, 2001.
780 Energy & Fuels, Vol. 20, No. 2, 2006
Zhou et al. Table 1. Summary of Adsorption Energies (in kJ‚mol-1) C2H4AgNO3 calculated ∆E measured ∆E
-77.5 -73.2
C2H4NaSiAlO7H6 -19.5 -17.7
CO2AgNO3 -23.5 -19.4
CO2NaSiAlO7H6 -18.0 -20.1
Table 2. Summary of Orbital Electron Occupancy Changes of Ag+ and Na+ after Adsorption
Figure 2. Optimized geometries of the complexes. (a) C2H4-AgNO3. (b) C2H4-Na SiAlO7H6. (c) CO2-AgNO3. (d) CO2-NaSiAlO7H6.
The breakthrough curves of the removal of C2H4 from the CO2 stream were measured in a fixed-bed adsorption column with 14 mm internal diameter. The adsorption column and the inlet gas line were equipped with a heat controlling device including a temperature programmer to monitor the operation temperature. The inlet and outlet gas concentrations to and from the adsorbent column were analyzed by a GC-14C gas chromatograph (Shimadzu Co., Kyoto, Japan) equipped with an FID detector and an accessory for reducing CO2 to CH4. The experiments were done under the following conditions: the adsorbents were heated at 250 °C for 3 h in air to remove water before adsorption. For each test, 4 g of adsorbent was loaded into the fixed-bed adsorption column. Breakthrough measurements were performed at four different concentration levels of C2H4 (i.e., 1030, 2340, 5400, and 10 330 ppmv) and at 30 °C and atmospheric pressure. The flow rate was maintained at 100 mL/min (NTP) during the adsorption process. When the effluent concentration of C2H4 reached the specified influent concentration (100% breakthrough), the experiment was stopped. The detailed temperature-programmed desorption (TPD) studies on C2H4 saturated adsorptions have been performed for the analysis of regeneration. A 1-g sample of 20-40 meshes was first saturated at atmospheric pressure by using a stream of C2H4 in N2 or in CO2 at 40 °C. After each saturation step, the TPD analysis was performed under 40 mL/min N2 by ramping the temperature of furnace at 4 °C/min from 40 °C to the temperature when no C2H4 was detected, and GC was used to analyze the content of C2H4.
4. Results and Discussion Calculation Results. Optimized Geometries. The optimized structures for all the complexes are included in Figure 2. The optimized structures of the C2H4-AgNO3 and C2H4-NaSiAlO7H6 clusters involve the perpendicular approach of Ag+ or Na+ to the midpoint of the C-C bond and the four C-H bonds bend away slightly from the metal. The stable geometry optimizations of CO2 on Ag+ or Na+ are of linear structure with one O atom interacting with the metal ion, as shown in Figure 2. The optimized geometry parameters for free C2H4 are 134.6 pm of C-C bond length, 108.6 pm of C-H bond length, and 116.4° of H-C-H angle, which are much closer to the experimental values23 of 134 pm, 107.6 pm, and 116.6°, respectively. The optimized C-C bonds of the C2H4-AgNO3 and the C2H4NaSiAlO7H6 clusters are 136.4 and 135.0 pm, respectively, and after adsorption the C-C bond lengths are both increased compared with the free one. Moreover, the increased amount of the C-C bond length in the C2H4-AgNO3 cluster is larger than that in the C2H4-NaSiAlO7H6 cluster. Since a stronger deformation of the molecule means a stronger adsorption of C2H4 on adsorbent, a longer C-C bond indicates a stronger bonding of adsorption. Therefore, the optimized geometry (23) Huan, W. Y.; Chang, L. W.; Ming, S. X. Organic Chemistry; University of Science and Technology of China Press: Beijing, 1994.
C2H4-AgNO3 C2H4-NaSiAlO7H6 CO2-AgNO3 CO2-NaSiAlO7H6
∆5s
∆4d
0.125
-0.079
0.031
-0.024
∆3s
∆3p
0.021
-0.000
0.010
-0.000
total changes 0.204 0.021 0.055 0.010
parameters predict the strength of the interaction: C2H4-AgNO3 > C2H4-NaSiAlO7H6. Energies of Adsorption. The energies of adsorption calculated using eq 1 are shown in Table 1. The adsorption energy of the C2H4-AgNO3 is much higher than that of the CO2-AgNO3. It indicates that the Ag+ shows higher adsorption strength with C2H4 than that with CO2, which is an indispensable condition for the adsorbent to deep purify a CO2 stream containing a C2H4 impurity. The calculated results also show that the adsorption energy of the C2H4-AgNO3 complex is much higher than that of the C2H4-NaSiAlO7H6, which indicates that Ag+ can form stronger bonds with C2H4 than Na+ does. However, the adsorbing energies of both the C2H4-NaSiAlO7H6 and the CO2-NaSiAlO7H6 are not high, and the two values are very close. It means that Na+ has little adsorption selectivity for C2H4 over the corresponding CO2. NBO Results. A good summary of orbital electron occupancy changes for Ag+ and Na+ after adsorption is shown in Table 2. After interacting with C2H4 or CO2, the 5s orbital population of Ag+ increases and the 4d orbital population of Ag+ decreases, and the orbital electron occupancy changes also occur in the 3s and 3p orbitals of Na+. However, among these changes, only that of the C2H4-AgNO3 complex is remarkable. We think that the more electrons involved in the total orbital electron occupancy changes, the stronger the interaction of the system. Therefore, the data of the total changes in Table 2 indicate that the interaction of the C2H4-AgNO3 complex is much stronger than the others, and this conclusion is quite in line with that from the energy calculation. Further analysis of the NBO results explains the reason that the Ag+ forms strong adsorption with C2H4. From the detailed description of the orbital electron occupancies of Ag+ and C2H4 shown in Table 3, it can be seen that before adsorption, the Ag+ has a nearly “vacant” 5s orbital and nearly “fully filled” 4d orbitals, and the C2H4 has a nearly “fully filled” π orbital and nearly “vacant” π* orbital. After adsorption, the 5s orbital population of the Ag+ increases and the 4d orbital population of the Ag+ (especially the 4dxy orbital) decreases; simultaneously, the π orbital population of C2H4 decreases and the π* orbital population of C2H4 increases. A detailed summary of orbital electron occupancy changes after adsorption is shown in Table 4. The analysis results indicate that Ag+ forms π complexation with C2H4, which is generally in line with the traditional picture of Dewer-Chatt for olefin-metal complexation.4 The nature of π complexation bonds is as σ-donation of electrons from the filled π orbital of C2H4 to the vacant s orbital of Ag+ and, simultaneously, d-π* back-donation of electrons from the filled d orbitals of Ag+ to the vacant π* antibonding orbital of C2H4. The 3s and 3p orbitals of Na+ can also form symmetrymatched orbitals with the π and π* orbital of C2H4, respectively, but the energy gap between the 3s orbital of Na+ and the π
RemoVal of C2H4 from a CO2 Stream by Adsorption
Energy & Fuels, Vol. 20, No. 2, 2006 781
Table 3. Orbital Electron Occupancies of Ag+ and C2H4 C2H4 AgNO3 C2H4-AgNO3
5s
4dxy
4dxz
4dyz
4dx2y2
4dz2
0.11053 0.23560
1.99455 1.93925
1.99739 1.99563
1.99964 1.99961
1.99520 1.98362
1.99346 1.98366
π
π*
1.99939
0.00000
1.92640
0.06724
Table 4. Orbital Electron Occupancy Changes of C2H4 and Ag+ After Adsorption C2H4-AgNO3
∆5s
∆4dxy
∆4dxz
∆4dyz
∆4dx2y2
∆4dz2
∆π
∆π*
0.125
-0.055
-0.002
-0.000
-0.012
-0.010
-0.073
0.067
orbital of C2H4 is too large (the energy gap is 8.1 eV, whereas the energy gap between the 4s of Ag+ and the π orbital of C2H4 is only 4.2 eV) and both the 3p orbitals of Na+ and the π* orbital of C2H4 are vacant, and thus few electrons are transferred and a weak interaction occurs between the Na+ and C2H4. For a similar reason, the CO2-AgNO3 and the CO2-NaSiAlO7H6 systems form weak interactions, too. Experimental Results. Characterization. The sharp peak of the DTA curve of the AgNO3/NaY sample in the higher temperature range indicates that AgNO3 decomposes near 470 °C and the peak around 130 °C is caused by water evaporation, as shown in Figure 3. By analyzing the TG curve, it can be concluded that silver salt after supported on the NaY is mainly
Table 5. Equilibrium Isotherm Parameters
AgNO3/NaY-C2H4/N2 AgNO3/NaY-C2H4/CO2 NaY-C2H4/N2 NaY-C2H4/ CO2
qmp (mL/g)
bp (kPa-1)
qmc (mL/g)
bc (kPa-1)
s
60.7 60.6 73.1 70.0
0.037 0.012 0.059 0.012
23.7 23.7
7.749 8.459
7 7
in the form of AgNO3 and the operation temperature in our study is no more than 300 °C; this further demonstrates that, for the AgNO3/NaY sample, the salt on the support is in the form of AgNO3. Figure 4 shows the XRD patterns of the AgNO3, NaY, and the AgNO3/NaYsample. As expected, no sharp characteristic peaks of the salt crystallites are detected on the adsorbent samples, which suggests that the metal salts are probably dispersed in (or near) a monolayer on the support surfaces.2 The heats of adsorption obtained from TG/DSC analysis are shown in Table 1. The adsorption heat of C2H4 on the AgNO3/NaY is much higher than the others. This trend is in agreement with the calculated result, although experimental and calculated data are not identical for a given adsorbateadsorbent system. Equilibrium Isotherms. The adsorption isotherm data of C2H4 on NaY are fitted with the Langmuir isotherm shown as eq 2:
q) Figure 3. DTA and TG curves of the AgNO3/NaY sample.
Figure 5. Equilibrium isotherms of C2H4 on AgNO3/NaY and NaY in N2 and CO2 at 30 °C.
(2)
1 + bp‚p
The adsorption of C2H4 on AgNO3/NaY accounts for both physical adsorption and chemisorption; therefore, a different model2 is used as shown below:
q)
Figure 4. XRD patterns of AgNO3, NaY, and AgNO3/NaY.
qmp‚bp‚p
qmp‚bp‚p 1 + bp‚p
+
qmc 1 + bc‚p‚es ln 2s 1 + b ‚p‚e-s
(3)
c
where qm is the saturated amount adsorbed, b is the Langmuir constant, and the subscripts p and c stand for the physical adsorption and chemisorption, respectively. The parameter s is a heterogeneity parameter indicating the spread of the energy, and the empirical values for s are available from the literature.24,25 Equilibrium isotherms of C2H4 diluted with N2 and CO2, respectively, on the AgNO3/NaY and the NaY are shown in Figure 5, and the fitting parameters are shown in Table 5. Figure 5 shows that the physical adsorbability of C2H4 on the NaY decreases drastically when replacing the N2 dilution gas with CO2, from which it can be seen that the presence of CO2 makes it difficult to selectively adsorb C2H4. For the AgNO3/NaY adsorbent, however, the low-pressure region of the isotherms are steep, and the adsorption capacity of C2H4 in CO2 does not decrease more drastically than that in the N2, especially in the low-pressure region. Compared with the NaY, the AgNO3/NaY (24) Valenzuela, D. P.; Myers, A. L. Adsorption equilibrium data handbook; Prentice-Hall: Englewood Cliffs, NJ, 1989. (25) Kapoor, A.; Yang, R. T. Chem. Eng. Sci. 1990, 45, 3261-3270.
782 Energy & Fuels, Vol. 20, No. 2, 2006
Zhou et al.
Figure 6. Breakthrough curves and fitted plots of C2H4 adsorption on AgNO3/NaY at 30 °C. Table 6. Values of k, τ, Adsorption Capacity, and Purification Degree of C2H4 on AgNO3/NaY Ci (ppmv)
k (min-1)
τ (min)
adsorption capacity (mL/g)
purification degree (ppmv)
1030 2340 5400 10330
0.009 0.016 0.033 0.109
632.2 301.3 140.7 77.4
13.0 14.1 15.1 16.0
e1 e1 e1 e1
shows a substantial superiority because it can form stronger π complexation bonds with C2H4. Breakthrough CurVes. The breakthrough data were fitted with the model of Yoon and Nelson26,27 with the two parameters (shown as eq 4).
k(τ - t) ) ln
Cb Ci - Cb
(4)
where Cb and Ci are the breakthrough (effluent) concentration and influent concentration of C2H4, respectively, t is the breakthrough time, k is the rate constant, and τ is the time required for 50% C2H4 breakthrough (Cb/Ci ) 0.5). The breakthrough curves and the fitted plots of C2H4 adsorption on the AgNO3/NaY at various concentrations are depicted in Figure 6. The values of k, τ, saturate adsorption capacity, and purification degree are shown in Table 6. It was found that k and τ are significantly influenced by the inlet concentrations (i.e., the value of k increases when the value of Ci is increased, while the value of τ decreases). The fitted theoretical curves are compared with the corresponding experimental data in Figure 6. The AgNO3/NaY breakthrough curves are s-shaped, and the curve for 10 330 ppmv C2H4 rises sharply at the breakthrough point, whereas the one for 1030 ppm C2H4 has a gentle slope at the breakpoint. As shown in Table 6, the AgNO3/NaY adsorbent can effectively remove the C2H4 from CO2 feed with high purification efficiency as well as a relatively high adsorption capacity. But for the NaY adsorbent, a large amount of C2H4 breaks out during the initial adsorbing period, and the purification effect of this adsorbent is less significant. As expected, the experimental results are in agreement with the calculated results. TPD Studies and Regeneration. Detailed TPD profiles on C2H4 saturated AgNO3/NaY and NaY are shown in Figure 7, which indicates that the TPD profile for the NaY has a single peak at about 50 °C, which was conformed as physical adsorption peak. However, the AgNO3/NaY showed a larger (26) Yoon, Y. H.; Nelson, J. H. Am. Ind. Hyg. Assoc. J. 1984, 45, 509516. (27) Yoon, Y. H.; Nelson, J. H. Am. Ind. Hyg. Assoc. J. 1984, 45, 517524.
Figure 7. TPD profiles of C2H4 from AgNO3/NaY and NaY at 40 °C. The samples were saturated with 5.6% C2H4/N2 or 2.95% C2H4/CO2.
total amount of C2H4 adsorption than the NaY at higher temperature range, and there are three groups of desorption peaks in the TPD curves of the AgNO3/NaY. The position of the peak at low temperature is similar to that of the NaY, and it may be ascribed to the presence of physical adsorption peak. The shoulder at 80 °C represents the weak chemical adsorption, and the other between 100 and 250 °C is a broad peak, which will be attributed to stronger chemical adsorption sites. This might be due to the different active sites of the AgNO3/NaY sorbent. Weakly bonded C2H4 can be desorbed at temperature below 100 °C, and the C2H4 adsorbed on the stronger active sites needs high desorption temperatures. The low-temperature peak of the AgNO3/NaY saturated with C2H4/CO2 was much lower because CO2 competes with C2H4 for the weak adsorption sites. Therefore, the role of the AgNO3/NaY for removal of C2H4 from the CO2 stream is favored by its stronger chemical adsorption sites. In this TSA purifying process, regeneration performance of the AgNO3/NaY adsorbent is very important. The repeated TPD profile of the AgNO3/NaY saturated with C2H4/CO2 is almost the same as the original one, as shown in Figure 7. The readsorption amount of the regenerated AgNO3/NaY for the same condition is 17.7 mL/g, which is very close to that of the original one (which is 17.8 mL/g). Further study shows that the saturated adsorbent can be effectively regenerated in air or CO2 at 240-260 °C. 5. Conclusion In this article, an ab initio molecular orbital study, combined with experiment, has been performed to investigate the feasibility of deep removal of C2H4 from a CO2 stream by an adsorption method. The calculated results showed that the C2H4-AgNO3 cluster forms a much stronger interaction than the CO2-AgNO3, C2H4-NaSiAlO7H6, and CO2-NaSiAlO7H6, and the detailed analysis of the NBO results gave a theoretical explanation of the interaction mechanism. The experimental results supported the calculated results. AgNO3-modified NaY, through Ag+ active sites forming stronger π complexation with C2H4, can deeply remove C2H4 species from the CO2 stream as well as get a relatively high adsorption capacity. Acknowledgment. We acknowledge Dr. Ma Xiaoliang of Pennsylvania Sate University for his helpful comments and Dr. Song Hui of Dalian Institute of Chemical Physics for her good advice on how to select the chemistry models. EF050182O
