Ind. Eng. Chem. Res. 2003, 42, 123-129
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Desulfurization of Liquid Fuels by Adsorption via π Complexation with Cu(I)-Y and Ag-Y Zeolites Arturo J. Herna´ ndez-Maldonado and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Fixed-bed adsorption using different π-complexation adsorbents for desulfurization of liquid fuels was investigated. Cu(I)-Y (autoreduced Cu(II)-Y), Ag-Y, H-Y, and Na-Y zeolites were used to separate low-concentration thiophene from mixtures including benzene and/or n-octane, all at room temperature and atmospheric pressure. Sulfur-free (i.e., below the detection limit of 4 ppmw sulfur) fuels were obtained with Cu(I)-Y, Ag-Y, and H-Y but not Na-Y. Breakthrough and saturation adsorption capacities obtained for an influent concentration of 760 ppmw sulfur (or 2000 ppmw thiophene) in n-octane follow the order Cu(I)-Y > Ag-Y > H-Y > Na-Y and Cu(I)-Y > H-Y > Na-Y > Ag-Y, respectively. Cu(I)-Y zeolite adsorbed 5.50 and 7.54 wt % sulfur at breakthrough and saturation, respectively, for an influent concentration of 760 ppmw sulfur in n-octane. For the case of 190 ppmw sulfur in mixtures containing both benzene and n-octane, Cu(I)-Y adsorbed 0.70 and 1.40 wt % sulfur at breakthrough and saturation, respectively. Regeneration of the adsorbent was accomplished by using air at 350 °C, followed by reactivation in helium at 450 °C. The observed adsorption behavior, in general, agrees well with previous studies performed for pure-component vapor-phase adsorption of thiophene and benzene with the same adsorbents. Introduction The federal government mandates a reduction in gasoline and diesel sulfur levels to 30 and 15 ppm, respectively, down from the current levels of 300-500 ppm. This should be attained by the year 2006, making refiners consider eliminating production of on-board transportation fuels because of the high costs that will arise from such regulations.1 For fuel cell applications using gasoline as the feed, the sulfur content is preferably below 1 ppm. Hydrodesulfurization (HDS) is very effective in removing thiols and sulfides, but it is not adequate for the removal of thiophenic compounds. For instance, the H2S produced during reaction of some thiophene derivatives is one of the main inhibitors for deep HDS of unreactive species.2 For HDS to meet the new federal government mandates, reactors with volumes 5-15 times larger (depending on the H2 pressure) than those currently used are needed. This makes HDS an inappropriate solution and, thus, the use of adsorption to selectively remove the sulfur compounds at ambient conditions an excellent option. Recent studies in our laboratory have already shown that π-complexation adsorbents are superior to all other adsorbents for this application.3-5 Weitkamp et al.6 and King et al.7 studied ZSM-5 for removing thiophene from different solutions using fixedbed breakthrough experiments. Weitkamp et al.6 did both vapor- and liquid-phase removal of thiophene from benzene but at concentrations an order of magnitude higher than that present in gasoline and diesel. In general, they obtained sulfur removal capacities of about 1.4 wt % at saturation for a 0.5 wt % thiophene concentration. King et al.7 also performed vapor-phase fixed-bed breakthrough experiments to remove thiophene (1 wt %) but from toluene and p-xylene instead of * To whom correspondence should be addressed. Phone: (734) 936-0771. Fax: (734) 764-7453. E-mail:
[email protected].
benzene. Although selectivity toward thiophene was accomplished, saturation adsorption capacities were low (∼1 wt %). In addition, the small pore dimensions of ZSM-5 (5.2-5.6 Å) would limit the passage of sulfur compounds with more than one ring (i.e., benzothiophenes and dibenzothiophenes), which are also present in gasoline and diesel. The use of larger poresize molecular sieve adsorbents, such as faujasite, to remove sulfur was also tried.8,9 Mingels et al. used Cu(II)-Y zeolite to remove thiophene from benzene in both batch and fixed-bed breakthrough experiments.9 Nevertheless, their adsorbent capacity was approximately 0.6 wt % sulfur (for an influent concentration of 4320 ppm thiophene). For this case it is clear that π complexation was not intended because of the use of Cu2+ instead of Cu+. Another adsorbent that has been studied was ALCOA Selexsorb, which is an activated alumina. In one specific application,10 this proprietary material was used in a temperature-swing adsorption process in order to continuously adsorb hetereoatoms from hydrocarbon mixtures and produce full boiling range FCC gasoline products with a maximum sulfur content of 30 ppmw. Chemical complexation adsorbents, such as that for π complexation, have barely been utilized in industrial adsorption applications. The π-complexation bonds are stronger than those formed by van der Waals interactions, but they are also weak enough to be broken by traditional engineering means such as increasing temperature and/or decreasing pressure.11 Therefore, this leaves room for tailoring and developing new adsorbents for processes where selective adsorption is needed, such as in the case of sulfur removal from fuels. Our group has already developed π-complexation adsorbents for olefin/paraffin, diene/olefin, and aromatics/aliphatics separations.12-21 Although most of these studies can be applied directly to liquid-phase problems, the purpose of the present study is to use π-complexation adsorbents,
10.1021/ie020728j CCC: $25.00 © 2003 American Chemical Society Published on Web 12/02/2002
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specifically Cu-Y (autoreduced) and Ag-Y zeolites, to separate liquid mixtures of thiophene/benzene, thiophene/ n-octane, and thiophene/benzene/n-octane at room temperature and atmospheric pressure using fixed-bed adsorption/breakthrough techniques. These mixtures were chosen to understand the adsorption behavior of sulfur compounds present in hydrocarbon liquid mixtures and to study the performance of the adsorbents in the desulfurization of transportation fuels. Moreover, a technique for regeneration of the adsorbents was developed in this study. Experimental Section Adsorbent Preparation. The adsorbents used in this study were different cation forms of Y zeolite. These were prepared initially using conventional liquid-phase ion-exchange techniques. The starting material, sodiumtype (Na) Y zeolite (Strem Chemicals, Si/Al ) 2.43), was used as received. H-Y was obtained by calcination of NH4-Y (Strem Chemicals, Si/Al ) 2.43) at 500 °C. All adsorbents were used in powder form. Ag-Y zeolite was prepared by ion-exchanging Na-Y with an excess amount of silver nitrate (AgNO3) in an aqueous solution (0.2 M) at room temperature for 2448 h in the absence of any source of light. The amount of silver content in the solution was equivalent to a 4-fold cation-exchange capacity. It is known that Y zeolite shows higher selectivity toward silver over sodium during ion exchange,22 which makes preparation of 100% Ag-Y relatively easy. After ion exchange, the solid was recovered by filtration, washed with large amounts of deionized water, and then dried at room temperature also in a dark area. Cu(I)-Y (or reduced Cu(II)-Y) was prepared by first ion-exchanging Na-Y with a Cu(NO3)2 aqueous solution (0.5 M) for 48 h, followed by reduction of Cu2+ to Cu+. The amount of copper in the ion-exchange solution was equivalent to a 5-fold cation-exchange capacity. The adsorbent was recovered by filtration and also washed with copious amounts of deionized water, followed by drying at 100 °C for at least 24 h. Autoreduction of cupric ions to cuprous ions in synthetic zeolites has been reported by several groups.23-25 Larsen et al., for instance, reported autoreduction of Cu2+ to Cu+ in ZSM-5 by using a helium atmosphere at temperatures between 423 and 500 °C for 1 h.23 Elemental Analysis. Cu(II)-Y, Ag-Y, and Na-Y adsorbents were characterized using neutron activation analysis (NAA) in the research nuclear reactor of the Phoenix Memorial Laboratory at the University of Michigan. The data were obtained from a 1-min coreface irradiation delivered via a pneumatic tube to a location with an average thermal flux of 2.13 × 1012 neutrons/cm2/s. Following irradiation, two separate counts were made, one after a 13-min decay (for Al, Cu, and Ag) and a second count after a 1 h and 56 min of decay (for Na and K); both were for 500 s. The concentrations of Al, Na, and K were determined based on a comparison with three replicates of the standard reference material NIST1633A (coal fly ash); determination of the Cu content was evaluated relative to a high-purity copper wire, while the Ag content was determined through a comparison with a high-purity Ag foil. Data reductions for NIST1633A were based on NIST certified values. Fixed-Bed Adsorption/Breakthrough Experiments. All adsorption/breakthrough experiments were
Figure 1. GC chromatograms as a function of the fixed-bed adsorption/breakthrough experiment sampling time. The liquid mixture entering the fixed-bed had a concentration of 2000 ppmw thiophene (in n-octane).
performed in vertical custom-made quartz adsorbers equipped with a supporting glass frit. The setup consisted of a low-flow liquid pump equipped with a ceramic piston and cylinder liner, Kynar compression fittings, two Pyrex feed tanks, and a heating element. Initially, the adsorbents were loaded inside the adsorber (between 1 and 2 g) and heated in situ (250-450 °C) while flowing either helium or nitrogen upward. Temperatures were maintained at steady values using a proportionalintegral-derivative temperature controller. The treatment gases were pretreated inline prior to contacting the sorbent using a 3A-type zeolite. The latter allows removal of trace water from the gases, which may adsorb in the adsorbents that are being tested. After activation treatment, the zeolite adsorbent under study was allowed to cool to room temperature under helium and then tapped to ensure proper packing. Next, a sulfur-free octane or benzene solution was allowed to flow downward through the sorbent at a rate of 0.5 cm3/ min. After wetting the adsorbent for about 30 min, the feed was changed to a mixture of C8H18 (n-octane) and/ or C6H6 (benzene) containing different concentrations of C4H4S (thiophene) also at a 0.5 cm3/min rate. The experiments were performed for C4H4S concentrations of 500 and 2000 ppmw or 190 and 760 ppmw sulfur basis, respectively. Samples were collected at regular intervals until saturation was achieved, which depended on the adsorption dynamics and the amount of adsorbent. Gas Chromatography (GC) Analysis. All of the samples collected during the breakthrough experiments were analyzed using a Shimadzu GC unit equipped with a polar column, an automatic multisampler, and a flame ionization detector. The column temperature was set to 65 °C, and a split ratio of 20/1 was used for the analysis. The minimum thiophene concentration detection was around 10 or 4 ppmw on a sulfur basis. Figure 1 shows a chromatogram depicting the “evolution” of the thiophene peak as a function of both GC retention time and fixed-bed adsorption sampling time. The tailing observed at approximately 4 min corresponded to n-octane, which accounted for about 99.8 wt % of the bulk liquid mixture in that experimental run. The n-octane solvent peak is not shown for clarity. Results and Discussion Adsorbent Characterization. NAA was used to characterize the adsorbents used in this study. All of
Ind. Eng. Chem. Res., Vol. 42, No. 1, 2003 125 Table 1. Composition in Terms of Molar Ratios for Different Ion-Exchanged and Na-Y Zeolites molar ratio adsorbent
Na/Al
Na-Y H-Y Ag-Y Cu(II)-Y
0.94 0.04 0.28
Ag/Al
Cu/Al
Si/Al
0.36
2.43 2.43 2.43 2.43
1.15
the zeolites were in hydrated conditions before testing. As seen in Table 1, more than 100% ion exchange was achieved for the Ag-Y case. If it is assumed that one silver cation compensates for one aluminum tetrahedron, then there are excess ions probably occupying extraframework positions. For the Cu(II)-Y case, the results showed incomplete ion exchange. As a first approach to understanding the results, one can assume that if one Cu2+ cation compensates for two aluminum tetrahedra, then for our case the ion exchange resulted in only 70% substitution of the original sodium ions (i.e., 2Cu/Al ) 0.72). The remaining sodium species were then compensating for the other aluminum tetrahedra; in other words, the (2Cu + Na)/Al ratio should be unity, which is observed here. This scheme will be the simplest way to describe the ion-exchange behavior, but it is wellknown that Cu2+ hydrolyzes in aqueous solutions.26 The hydrolysis should proceed as follows:
2Cu2+ + 2H2O T Cu2(OH)22+ + 2H+
(1)
Figure 2. Breakthrough of thiophene in a fixed-bed adsorber with Ag-Y (0), Na-Y (]) or Cu(I)-Y (O) adsorbents, with a liquid feed containing 2000 ppmw (Ci) thiophene in benzene, at room temperature. Table 2. Breakthrough and Saturation Loadings for Thiophene in Different Zeolites and in the Presence of Benzene and/or n-Octane
adsorbent
C4H4S concn (ppmw)
Na-Y
2000
H-Y Ag-Y
2000 2000
Cu(I)-Y
2000 500
Such a process can produce also mononuclear species such as CuOH+ in addition to the ones shown in eq 1. Thus, copper ion exchange in Y zeolite could result in the substitution of sodium by Cu2+, H+, and Cu2(OH)22+. Parrillo et al. reported the presence of proton sites in CuZSM-5 (exchanged with Cu2+) by doing isopropylamine decomposition temperature-programmed desorption-thermogravimetric analysis studies.24 Isopropylamine decomposition occurs in the presence of Brønsted acid sites, which makes studies such as the one previously mentioned suitable to identify H+ species. In general, it appears that the Cu ion-exchange process is rather complicated and should require further analysis in order to identify and quantify all of the species substituted in the framework. Adsorbent Activation and Copper Autoreduction. Because Y zeolite is known to be highly hydrophilic (uptake > 20 wt % water at ambient conditions), all of the gases used for activation of the adsorbents were pretreated with 3A-type zeolites prior to entering the fixed-bed unit. Na-Y and Ag-Y zeolites were activated in situ (as described above) using helium at 350 °C for at least 12 h. This temperature is high enough to remove the physisorbed water molecules, leaving the internal voids and cations of the zeolites available for adsorption. For the Cu(II)-Y case, the activation was performed at 450 °C in helium to promote autoreduction of Cu2+ species to Cu+, which is desired for π complexation (Cu(I)-Y). Larsen et al. reported that about 40% of the cupric ions in CuZSM-5 are reduced under helium at 410 °C.23 Also, electron paramagnetic resonance studies done by Takahashi et al. showed that 50% of Cu2+ in Cu(II)-Y zeolite is reduced under vacuum or helium at 450 °C,21 which is in good agreement with the findings of Larsen et al. It should be mentioned that, after 18 h of auto reduction, the color of Cu(I)-Y was pale green, compared to a bluish green
a
solvent
breakthrough loading (mmol/g)
saturation loading (mmol/g)
C6H6 C8H18 C8H18 C6H6 C8H18 C6H6 C8H18 C6H6/C8H18a C8H18
0.02 0.07 0.46 0.04 0.54 0.19 1.82 0.22 0.68
0.10 1.05 1.15 0.17 0.90 0.54 2.55 0.44 1.28
20 wt % C6H6.
typically observed in Cu(II)-Y. This is evidence of autoreduction because Cu+ should result in a white color (like in CuCl). Further evidence of reduction of Cu2+ should come from the fixed-bed adsorption tests, which are discussed next. Fixed-Bed Adsorption Tests. After the adsorbents were activated and prepared, a solution containing thiophene as a sulfur source was passed through a fixedbed column and the outlet concentration monitored as a function of time. Figure 2 shows breakthrough curves for a benzene/thiophene mixture. The influent concentration was 2000 ppmw thiophene (760 ppmw sulfur basis). Both Na-Y and Ag-Y adsorbents show little selectivity toward thiophene molecules, having saturation adsorption capacities of 0.10 and 0.17 mmol/g, respectively (see Table 2). Despite the low capacity (at both breakthrough and saturation), the silver-based zeolite adsorbed almost twice as much as the sodiumbased one, showing evidence of interactions that were not present in Na-Y. Takahashi et al. reported purecomponent vapor-phase equilibrium adsorption capacities for benzene and thiophene at different concentrations,4 and they found that Ag-Y zeolites adsorb both types of molecules in similar quantities. Thus, it is no surprise that when using mixtures containing an excess of C6H6 (i.e., 99.8 wt %), there will be little room for thiophene adsorption on Ag-Y. Nevertheless, the amount of aromatics in commercial fuels is well below the amount in this experiment, and the effect of this will be discussed later. Also, another variable that plays an important role here is solvent-adsorbate interactions. Thiophene should have a great affinity toward benzene because both are similar species and, as a result, will compete against the zeolite adsorbents for interactions.
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Figure 3. Breakthrough of thiophene in a fixed-bed adsorber with Ag-Y (0), H-Y (4), or Na-Y (]) adsorbents, with a liquid feed containing 2000 ppmw (Ci) thiophene in n-octane, at room temperature.
Figure 4. Breakthrough of thiophene in a fixed-bed adsorber with Cu(I)-Y (O) or H-Y (4) adsorbents, with a liquid feed containing 2000 ppmw (Ci) thiophene in n-octane, at room temperature.
Michlmayr used Ag-Y also for removal of sulfur from gasoline but obtained capacities of 0.07-0.15 wt % (0.02-0.05 mmol/g),27 which is quite low considering their mixture should have contained a much lower concentration of aromatics than that in our case. The Ag-Y used by Michlmayr was intended for hightemperature sorption/reaction with thiophene, because his “preferred temperature” was 200-350 °C, and the zeolite was not calcined or activated prior to use. Cu(I)-Y, however, shows little more selectivity toward thiophene when compared to the other adsorbents for the C6H6/C4H4S mixture at the same conditions (see Figure 2). As is seen also in Table 2, the adsorption capacity (at saturation) for Cu(I)-Y is more than 0.5 mmol/g or about 1.7 wt % sulfur. As mentioned already, Mingels et al. also did fixed-bed adsorption studies for C6H6/C4H4S but with nonreduced Cu(II)-Y.9 For an influent concentration of 4320 ppmw thiophene, they obtained a saturated adsorption capacity of about 0.2 mmol/g (based on their data for 725 g of adsorbent and a flow rate of 500 cm3/h), which is low compared to ours. Their adsorbent was activated in air, which did not reduce Cu2+, and, therefore, was not capable of π complexation. It should be mentioned that both Ag-Y and Cu(I)-Y turned black after exposure to pure benzene, which may well indicate interaction and complex formation between the aromatics and the solids. This was not observed for Na-Y. Benzene was passed through the adsorbents prior to the adsorption analysis in order to remove any trapped gases left from the activation process. Not doing so resulted in little or no adsorption of thiophene. Because C6H6 is evidently strongly adsorbed in both Ag-Y and Cu(I)-Y, it was necessary to use a “nonadsorbing” solvent. n-Octane (C8H18) was selected because it is present in large quantities in commercial gasoline and because this “large” hydrocarbon should not adsorb strongly in zeolites compared to molecules such as thiophene. After activation of the adsorbents, pure C8H18 was allowed to pass through the adsorbents to eliminate any entrapped gas. None of the adsorbents changed color after exposing them to n-octane only. Figures 3 and 4 show breakthrough curves obtained for 2000 ppmw thiophene (760 ppmw sulfur basis) but this time with n-octane as the solvent (the breakthrough curve for Cu(I)-Y is shown in a separate figure for clarity because the abscissas are quite different). Now all adsorbents showed remarkable selectivity toward C4H4S, indicating that C8H18 adsorption is not competitive. Saturation adsorption capacities calculated from
the breakthrough curves were 1.15, 1.05, and 0.90 mmol/g for H-Y, Na-Y, and Ag-Y, respectively (summarized in Table 2). However, for Na-Y and H-Y, the breakthrough of thiophene molecules occurred earlier, at about 2.84 and 12.00 cm3/g, respectively, compared to 22.50 cm3/g in Ag-Y. Thiophene breakthrough behavior in H-Y was probably due to oligomerization processes. Pereira28 studied this phenomenon in several zeolites, including H-Y, on which he observed a distinct dark orange color upon interaction with thiophene molecules. The same color change was observed in our H-Y adsorbent. For Na-Y, the results show evidence of weak adsorbate-adsorbent interactions on the adsorbent, which did not have the ability for π complexation as in the case of Ag-Y. This agrees very well with the pure vapor-phase adsorption data reported by Takahashi et al.4 The saturation adsorption amount in Na-Y was higher than that in Ag-Y because of porevolume differences and differences in the densities of zeolites. Because Ag ions are larger than Na ions, they should occupy more volume in the framework, resulting in a smaller void space. This should be observed particularly in the supercages of Y zeolite because Ag is known to prefer sites II as reported by Hutson et al.29 On the basis of the molecular weights of the aforementioned adsorbents [Na0.292Al0.292Si0.708O2 (MW ) 66.42) vs Ag0.335Al0.292Si0.708O2 (MW ) 95.84)], a considerable amount of pore-volume loss should be observed in Na-Y after ion-exchanging with silver. Takahashi et al. measured cumulative pore volumes using nitrogen at 77 K and reported a 25% reduction in the pore volume in Ag-Y.4,21 The adsorption behavior of Na-Y and Ag-Y here could be visualized in terms of the liquid thiophene volume. The molar volume of thiophene at 25 °C is about 79.47 cm3/mol. For the case of Na-Y, at saturation, 0.083 cm3 of C4H4S is adsorbed per gram of dehydrated adsorbent and, for Ag-Y, 0.072 cm3/g. Because Na-Y has a pore volume of about 0.34 cm3/g, the thiophene liquid volume accounts for 24 and 28% of the pore volumes in Na-Y and Ag-Y, respectively, assuming that the results obtained by Takahashi et al. apply. For the same feed conditions described above, Cu(I)-Y showed again the highest selectivity and capacities among the adsorbents studied. The saturation capacity was 2.55 mmol/g, which was more than twice the amount found for the other adsorbents, indicating superior interaction with the thiophene molecules. For about 2 g of Cu(I)-Y, it took more than 300 min for the thiophene molecules to break through the adsorbent at
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a feed rate of 0.5 cm3/min (refer to Figure 4). Saturation was reached after 600 min, which was remarkable for such a small amount of adsorbent. Some notes may be made from these results on Cu(I)-Y. First, a large amount of Cu2+ ions must have been reduced to Cu+. As mentioned earlier, Takahashi et al.21 and others23-25 have reported 50% autoreduction of copper under helium/ vacuum atmospheres after just 1-2 h. The adsorbent used in this work was exposed to helium at 450 °C for no less than 18 h. Possibly longer activation time increased the amount of reduced copper ions, while the adsorption behavior already indicates that the autoreduction process yields promising results. Second, ions (i.e., Cu+) must occupy exposed sites in the Y zeolite framework in order to interact with the thiophene molecules. Recent studies by Fowkes et al. have shown that, upon reduction of Cu(II)-Y, there was a redistribution of cation positions and most of the reduced species (Cu+) occupied sites I* and II.30 Lamberti et al. showed similar results for both reduced Cu(II)-Y and Cu(I)-Y prepared by ion exchange and gas-phase reaction with CuCl, respectively.31 It should be mentioned that site II is exposed to interactions with guest molecules inside the supercage. In addition, some Cu+ ions could be induced to migrate to more exposed sites under the presence of guess molecules. Turnes Palomino et al. found, based on IR data, that this happens when CO molecules are adsorbed in Cu(I)-Y.32 Their zeolite was also prepared by vapor-phase exchange of H-Y with CuCl. Thus, on the basis of Turnes Palomino’s findings, there could also be some synergy that contributes to the observed adsorption capacity in our case. Thus, if one assumes that at least 50% of the cupric ions (about 9.21 wt % Cu2+ in dehydrated adsorbent) present in Cu(II)-Y was reduced,21 then there was approximately 0.75 mmol of Cu+/g of dehydrated zeolite. Assuming that there is 1 thiophene molecule adsorbed/ Cu+ cation and knowing that Cu(I)-Y adsorbed 2.55 mmol of thiophene (sulfur)/g of adsorbent, then 1.83 mmol/g should have been adsorbed by means other than π complexation. As stated before, thiophene seems to adsorb in H-Y (1.15 mmol/g capacity) via an oligomerization process and in Na-Y (1.05 mmol/g capacity) via a pore-filling mechanism in addition to weak interactions when in both cases C8H18 is used as the solvent. Thus, Cu(I)-Y could also adsorb thiophene by interactions with proton sites formed during ion exchange and/or nonreduced cupric ions, pore filling, and adsorbate-adsorbate interactions. It was also possible that more than 1 molecule of thiophene was adsorbed on Cu+ by π complexation. During adsorption of thiophene from n-octane, the Cu(I)-Y adsorbent color changed to a very dark one (nearly black). This color change developed slowly and along the length of the bed, indicating the location of the thiophene concentration front during the experiments. In fact, saturation was reached when the adsorbent was completely black, as was verified by the effluent concentration of thiophene and the GC analysis. It is known that Cu2S crystals are black colored,33 which could indicate formation of thiophene/copper complexes in our case. Again, n-octane alone did not induce a color change in any of the adsorbents studied. Figure 5 shows breakthrough curves for Cu(I)-Y for an influent containing 500 ppmw thiophene (190 ppmw sulfur) in n-octane. The saturation capacity was reduced to 1.28 mmol/g, which was about 50% of the amount
Figure 5. Breakthrough of thiophene in a fixed-bed adsorber with fresh (O) and regenerated (4, 0) Cu(I)-Y adsorbent, with a liquid feed containing 500 ppmw (Ci) thiophene in n-octane, at room temperature: (4) adsorbent regenerated in nitrogen at 350 °C, followed by reactivation in helium at 450 °C; (0) adsorbent regenerated in air at 350 °C, followed by reactivation in helium at 450 °C.
obtained previously with the 2000 ppmw thiophene feed. This indicates that the equilibrium adsorption isotherm was not “rectangular” in shape at low concentrations and rather showed a noticeable decrease in the adsorbed amount as one decreased the concentration. Despite this, the observed saturated amount was not low, when taking into account that the thiophene concentration was 75% less than the case discussed previously (i.e., 2000 ppmw). Another feature or behavior observed in Figure 5 is that the mass-transfer zone or the concentration wave front was more dispersed for the 500 ppm feed as compared with that for the 2000 ppmw feed. This could be seen also from the numbers shown in Table 2. These values showed that, for the 2000 and 500 ppmw thiophene cases, 29% and 47% of Cu(I)-Y will be unused, respectively, if the process is stopped at the breakthrough point. This phenomenon was due to thermodynamic equilibrium effects and possibly finite transport rates.34 Chen and Yang35 studied the concentration dependence for both surface and zeolitic diffusion and developed the following model:
D ) D0
λ λ 1 - θ + θ(2 - θ) + H(1 - λ)(1 - λ) θ2 2 2 (2) λ 2 1-θ+ θ 2
(
)
where D is Fickian diffusivity, D0 is the Fickian diffusivity at zero surface coverage, θ is the fractional surface coverage, λ is a blockage parameter, and H is the Heaviside function. For the simplest case where there is no blockage (i.e., λ ) 0), which is true for large void spaces, eq 2 reduces to
D 1 ) D0 1 - θ
(3)
Equation 3 shows that diffusion increases as the surface coverage is increased. Therefore, if the isotherm is of the Langmuir type, at lower concentrations there will be a decrease in diffusion and this, as a consequence, should increase the mass-transfer zone length. Dispersion or compression (i.e., sharpening) of the concentration wave front can also be caused by the shape of the equilibrium isotherm. DeVault showed this
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Figure 6. Adsorption isotherm for thiophene on Cu(I)-Y at room temperature (solvent: n-octane).
in terms of the concentration wave-front velocity36
uc )
u 1 - dq* 1+ dC
(4)
where u is the interstitial velocity, is the void fraction in the bed, and dq*/dC is the slope of the equilibrium isotherm. For the concave-shaped isotherms, or the “favorable” type, the wave front will self-sharpen as it propagates. Figure 6 shows an isotherm obtained with the saturation adsorption amounts of thiophene in Cu(I)-Y. Segments AB and ABC correspond to the concentration regions covered by the 500 and 2000 ppmw thiophene influent concentrations, respectively, during the propagation of the concentration wave front. For the 2000 ppmw feed, the entire wave front was covered by the ABC segment of the isotherm. Hence, significant sharpening of the wave front occurred. For the 500 ppmw feed, the wave front was covered by the smaller AB segment, which caused less sharpening as compared with the case with 2000 ppmw feed. Despite the dispersed wave front, the autoreduced adsorbent showed a high selectivity and capacity for thiophene even at low concentrations. Figure 5 also shows breakthrough curves after Cu(I)-Y adsorbent regeneration (second cycles). Under an atmosphere of nitrogen at 350 °C, the regenerated adsorbent did not recover the original capacity. The new capacity for the adsorbent at saturation was 0.80 mmol/ g, which was more than a 30% reduction from the original capacity. In fact, the color of the adsorbent remained black, which indicated the presence of copper thiophene complexes. Meanwhile, regeneration under air at 350 °C, followed by reactivation under helium at 450 °C, recovered almost all of the original capacity. For this case, the observed saturation capacity was about 1.20 mmol/g, which was only a 5% reduction from the original capacity. Regeneration under air returned the adsorbent color to the original bluish green color, indicating that the adsorbed thiophene molecules were eliminated through combustion. For the final part of this study, it was desired to use mixtures with compositions similar to those of transportation fuels. Gasoline contains about 20-30 wt % aromatics, many thiophenic compounds, and 70-80% alkanes such as n-hexane and n-octane. The aromatic contents in diesel and jet fuels are
