Adsorptive Desulfurization of Natural Gas Using Lithium-Modified

Oct 18, 2012 - Anton Koriakin, Yo-Han Kim, and Chang-Ha Lee*. Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro ...
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Adsorptive Desulfurization of Natural Gas Using Lithium-Modified Mesoporous Silica Anton Koriakin, Yo-Han Kim, and Chang-Ha Lee* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro Seodaemun-gu, Seoul, 120-749, Korea ABSTRACT: The dynamic adsorption and desorption behaviors of lithium-modified mesoporous silica adsorbents (YSP-Li and MCF-Li) were investigated to remove sulfur compounds from natural gas. A mixture of methane and methyl mercaptan (291 μmol/mol) was used as feed gas. The adsorption capacities were determined from breakthrough experiments conducted at different temperatures and flow rates. The dynamic thermal desorption of both adsorbents was studied by applying stepwise temperature increases using nitrogen or methane. Both adsorbents could easily be regenerated at 100 °C with a purge gas. MCFLi exhibited stronger adsorption affinity and higher adsorption capacity to methyl mercaptan than YSP-Li even though its surface area and the doping amount of Li were smaller. The desorption from MCF-Li was more affected by the applied purge gas (CH4 or N2) than YSP-Li.

1. INTRODUCTION The current demand for clean energy has motivated the generation of syngas from fossil fuels. Gaseous hydrocarbons are considered to be promising to generate hydrogen for fuel cells due to their safety and adaptability.1−4 And it has been proposed that hydrogen can be supplied by steam reforming of municipal gas. However, municipal gas contains several ppm of organic sulfur compounds, which are also present in natural gas.5 The sulfur compounds in natural gas can irreversibly deactivate fuel processing catalysts and fuel cell electrodes by forming metal sulfide species on the surface. The poisoning effect of sulfur components in the catalytic reaction has been demonstrated even at low levels of sulfur compounds. Therefore, the sulfur concentration in the fuel should be reduced to less than 1 ppmw for proton exchange membrane fuel cell (PEMFC) and less than 10 ppmw for solid oxide fuel cell (SOFC) applications.6,7 Additionally, as the operating temperature is decreased in fuel cell development, the poisoning of catalysts can be intensified by the thermodynamically increased adsorption of sulfur compounds.4 Therefore, the removal of sulfur compounds is the very important first step to purify the gaseous hydrocarbon fuel stream before the steam reforming process. The removal of sulfur-containing compounds from natural or municipal gas has traditionally been achieved by the catalytic hydrodesulfurization (HDS) process operated at elevated temperatures and pressures using sulfided Ni−Mo/Al2O3 and Co−Mo/Al2O3 catalysts.8 Due to the production of H2S during HDS, this process is usually combined with zinc oxide adsorbents to remove H2S.7 However, this process requires hydrogen and consistent energy supplies due to the elevated temperatures and pressures that are required. Adsorption technology is a promising alternative to overcome these problems because sulfur compounds can be adsorbed at room temperature and the adsorbents can be regenerated. Therefore, many different types of adsorbents have been developed including metal oxides, metal-containing zeolites, and metal-containing Al2O3 to remove sulfur compounds.9−13 © 2012 American Chemical Society

In addition, a low-temperature adsorption process for fuel desulfurization has been studied using either activated carbon or zeolite.14−17 Zeolites usually have higher adsorption capacity for sulfur compounds than activated carbons, but the regeneration of such materials is extremely costly in practical applications because of the high temperature for complete regeneration, which is between 400 and 500 °C. On the other hand, activated carbon materials exhibit a regeneration temperature close to 200 °C for sulfur compounds.11,18 Removal of odorous CH3SH from air or gas streams has been achieved by catalytic oxidation and chemisorption on activated carbon treated by such species as NaOH, Na2CO3, NH3, and HNO3/H2SO4.14 The adsorption of methyl mercaptan on activated carbon has a close relationship with the pore structure and the properties of the adsorbent surface.19 Therefore, the surface properties and pore characteristics play important roles in the adsorption ability for CH3SH. Compared to activated carbon, only a few studies have been conducted on silica-based adsorbents for desulfurization.20,21 Most of these studies focused on the desulfurization of liquid fuels20 and conversion of gas fuels.21 It was noted from the breakthrough results that equilibrium capacity and rate are important factors in adsorptive desulfurization and regeneration. In previous studies, the desulfurization and denitrogenation of a raw diesel fuel by metal-ion modified silica materials were reported in which the adsorption of sulfur compounds was significantly affected by the competitive adsorption of nitrogen compounds.20,22,23 However, the mesoporous silica adsorbents contributed to minimizing mass transfer resistance in adsorption and regeneration due to the relatively large pore size. The development of an adsorptive desulfurization process to address these concerns requires the development of improved adsorbents with respect to higher adsorption capacity, ease of Received: Revised: Accepted: Published: 14489

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obtained from the solution were isolated by vacuum filtration, dried, and calcined at 500 °C for 8 h. The synthesis steps for YSP-Li and MCF-Li were identical to those of YSP and MCF except for the addition of lithium acetate.23 The physical properties of Li-modified YSP and MCF were obtained from N2 adsorption/desorption isotherms generated with a BET analyzer (Micromeritics, ASAP2020). The structural properties of the developed mesoporous adsorbents were different from those of YSP and MCF. Compared to YSP and MCF, YSP-Li possessed a larger surface area (815.2 m2/g) and a smaller pore size (3.73 nm) while MCF-Li had a smaller surface area (551.6 m2/g) and a larger pore size (19.6 nm). The pore volume of MCF-Li (0.78 cm3/g) was larger than that of YSP-Li (0.53 cm3/g). The lithium content in the synthesized adsorbents was recorded via ICP-AES (ELAN 6100, Perkin-Elmer SCIEX) with an argon plasma (6000 K) source. In the previous study, Si−Zr Cogel formed by Zr4+ contained about 3% of Zr.20 However, the amounts of lithium in the prepared YSP-Li and MCF-Li were 3.846 × 10−4 % (3846.0 ppbw) and 1.912 × 10−5 % (191.2 ppbw), respectively. Even though almost twice the amount of lithium acetate was applied to the synthesis of MCFLi, the amount of Li in MCF-Li was kept to about 20 times less than that found in the YSP-Li. The amount of Li that may be doped into the MCF is limited by the preparation method in the study. TEM images (JEOL model JEM-300 at 300 kV) of YSP-Li (a) and MCF-Li (b) are shown in Figure 1. The particle shape of YSP-Li is different from that of YSP, which exhibits a spherical shape with a micrometer scale particle size.23 MCF-Li consists of large spherical cells that are interconnected by uniform windows (8.6 nm, BJH adsorption), as shown in Figure 1b.24 The structures of the adsorbents were analyzed by small angle X-ray scattering (SAXS with GADDS) using CuKβ tube radiations (40 kV and 45 mA, λ(radiation) = 1.5406) at a range of 0° ≤ 2θ ≤ 10°. Figure 2 illustrates the SAXS results of the adsorbents. The MCF and MCF-Li showed very intense primary peaks at 2θ = 0.48°, indicating a well-ordered mesoporous structure and corresponding to the planar distance d = 184 Å. And the pore size of MCF-Li was in good agreement with the results observed in TEM (Figure 1) and calculated from BET analysis. However, the peak of the MCF-Li showed relatively lower intensity than that of the MCF, indicating that Li-modification led to less ordered structure of MCF. On the other hand, YSP-Li had two intense peaks at 2θ = 0.50° and 2θ = 1.96°, indicating a well ordered structure with planar distance d = 177 Å and d = 45 Å. The distance d = 45 Å was also in good agreement with the pore size observed from BET analysis and TEM (Figure 1). And the secondary (small) peak near 2θ = 3.3° suggests that YSP-Li has a hexagonal conformation, like YSP.23 2.3. Dynamic Experiments of Adsorption and Desorption. Methyl mercaptan in methane was used as an adsorbate to simulate sulfur compounds in natural gas or municipal gas. The desulfurization and regeneration efficiencies of the synthesized adsorbents were evaluated by dynamic breakthrough experiments using the system shown in Figure 3. After packing 0.3 g of adsorbents in a column, the adsorbents were activated to remove water and other impurities. The column was heated to 150 °C with a pure methane flow of 5 mL/min for 1 h. After cooling by pure methane flow, methane containing 291 μmol/mol methyl mercaptan was supplied to

regeneration, and less byproduct formation. The purpose of this work was to study the adsorption behaviors of synthesized lithium-modified mesoporous silica adsorbents toward sulfur compounds in municipal gas. Considering both equilibrium capacity and rate, the performance of the developed adsorbents was evaluated by a breakthrough test. The adsorption capacities and dynamic behaviors were evaluated by breakthrough experiments using a mixture of methane and methyl mercaptan as a feed gas. Because hydrogen for fuel cells can be supplied by steam reforming of municipal gas, the concentration of methyl mercaptan was fixed at 291 μmol/mol, which is the typical concentration of Korean municipal gas as an odorant. In addition, the adsorption capacities of two mesoporous silica adsorbents were compared at different temperatures and flow rates. Since the cyclic process consisting of adsorption and thermal desorption steps may provide a cost-effective desulfurization technique for fuel cell application, the thermal regeneration behaviors were investigated. The desorption dynamics of both adsorbents were studied by stepwise temperature increases using nitrogen or methane.

2. EXPERIMENTAL SECTION 2.1. Materials. Pluronic P123 (EO20PO70EO20, Kumkamg Chemical Korea Co., South Korea), cetyltrimethylammonium bromide (Sigma-Aldrich, USA, 95%), tetrabutylammonium bromide (Junsei Chemical Co., Ltd., Japan, 98%), petroleum benzine (Kanto Chemical Co., Inc., Japan, fraction 50−90 °C, min 90%), tetraethyl orthosilicate (Sigma-Aldrich, USA, 98%), mesitylene (Sigma-Aldrich, USA, 98%), hydrochloric acid (Sigma-Aldrich, USA, 37%), and lithium acetate (SigmaAldrich, USA, 99.99%) were used as received. Milli-Q deionized water was also used in the synthesis of the silica adsorbents. A standard gas mixture (291 μmol/mol CH3SH, balance CH4) was used as the model natural gas while methane with a 99.95% purity and nitrogen with a 99.99% purity were used as the purge gases during the thermal regeneration step. 2.2. Synthesis of Lithium-Modified Adsorbents. To synthesize YSP-Li, tetrabutylammonium bromide (TBAB: 1.13 g) and cetyltrimethylammonium bromide (CTMAB: 1.275 g) were mixed in an aqueous HCl solution (46.5 mL of HCl and 63 mL of water) at room temperature. Lithium acetate (99 mg, 1.5 mmol) and 6 mL of petroleum benzine were added to the solution, and the mixture was held at 4 °C for 1 h. The fabricated solution was then mixed with 4 mL of tetraethylorthosilicate (TEOS) and stored at 4 °C for 12 h. The synthesized particles were recovered by filtration, dried at 50 °C for 24 h, and calcined at 550 °C for 5 h. The preparation and characteristics of YSP were described in detail in a previous study.22 MCF-Li was prepared in aqueous hydrochloric acid using a dilute solution comprised of a nonionic block copolymer surfactant, Pluronic P123 (EO20PO70EO20), and 1,3,5-trimethylbenzene (TMB) as an organic swelling agent.24 In a typical preparation, P123 (2.0 g, 0.4 mmol) was dissolved in a 2 M solution of HCl (75 mL) at room temperature while being stirred in a beaker. Then, TMB (0.7 mL) was added and the mixture was heated to 45 °C. After stirring at 600 rpm for 1 h, lithium acetate was added to the solution (198 mg, 3 mmol) and stirred for 15 min. Tetraethyl orthosilicate (TEOS, 4.5 mL) was introduced into the mixture under stirring (300 rpm) at 45 °C for 20 h. The reaction mixture was aged at 100 °C under static conditions at atmospheric pressure for 24 h. The mixture was then cooled to room temperature. White precipitates 14490

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Figure 3. Scheme of the system for gas adsorption. Figure 1. TEM images of (a) YSP-Li and (b) MCF-Li.

Figure 2. Small-angle X-ray scattering of MCF, MCF-Li, and YSP-Li.

50 mL/min increased to 1.4 atm owing to the dense packing of adsorbent particles. After an adsorption experiment, the adsorbents were regenerated by hot methane or nitrogen gas. A stepwise temperature increase (25, 50, 100, and 150 °C) was applied for the regeneration to elucidate the desorption dynamics. The regeneration procedure was complete when methyl mercaptan was not detected in the effluent gas. Two resistance temperature detectors (RTDs, Pt 100Ω) were inserted into the heater and the adsorption column to accurately measure the temperatures. The gas flow was controlled by two mass flow controllers (MFC), and the system pressure was measured by an electrical pressure gauge. The amount of sulfur compounds in the effluent gas from the column packed with YSP-Li and MCF-Li was measured by an online gas chromatograph (GC) with a flame photometric detector (FPD). The allometric equation was used for the calibration curve for the GC because the emission intensity is approximately proportional to the square of the sulfur atom concentration.25

the column. The desulfurization efficiencies of the adsorbent were evaluated at 25 and 50 °C with the flow rates of 5 and 50 mL/min. The breakthrough experiments at 5 mL/min were performed at an atmospheric pressure while the bed pressure at

3. RESULTS AND DISCUSSION 3.1. Adsorption of Methyl Mercaptan from the Gas Mixture. Figure 4 shows the comparison of the adsorption breakthrough curves of methyl mercaptan obtained from the YSP-Li and MCF-Li beds with the natural gas mixture. The 14491

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adsorption affinity of methyl mercaptan on YSP-Li was stronger than that on MCF-Li even though the adsorption capacity of YSP-Li was smaller, possibly due to the higher amount of Li in YSP-Li than in MCF-Li. As a result, the pore volume (MCF-Li, 0.78 cm3/g; YSP-Li, 0.53 cm3/g) played a key role in improving the adsorption capacity of methyl mercaptan instead of the surface area and Li amount. To evaluate the effect of temperature on the adsorption of methyl mercaptan, the breakthrough experiments were performed at 50 °C with the same feed flow rate (Figure 5).

Figure 4. Comparison of breakthrough curves of methyl mercaptan between YSP-Li and MCF-Li beds at 25 °C, 5 mL/min, and 1 atm.

experiments were performed at 25 °C, a feed flow rate of 5 mL/ min, and atmospheric pressure. The adsorption capacity of YSP-Li was clearly lower than that of MCF-Li. The breakthrough time of methyl mercaptan in the YSP-Li bed was 80 min and the breakthrough curve to reach C/C0 = 0.95 was 390 min. In contrast, the breakthrough time and the time for C/C0 = 0.95 in the MCF-Li bed were significantly longer at 170 and 670 min, respectively. The adsorption capacity of methyl mercaptan was calculated from the time at C/C0 = 0.5 (205 min for YSP-Li and 395 min for MCF-Li) and the results are listed in Table 1. Although YSP-Li possessed a larger surface area

Figure 5. Comparison of breakthrough curves of methyl mercaptan between YSP-Li and MCF-Li beds at 50 °C, 5 mL/min, and 1 atm.

Compared to Figure 4, the adsorption capacity on both adsorbents was significantly affected by the high temperature. The breakthrough time of each adsorbent was less than 10 min for the YSP-Li bed and about 40 min for the MCF-Li bed. From the viewpoint of the breakthrough time and adsorption capacity, the relative decreasing levels of MCF-Li were smaller than those of YSP-Li. In addition, it took a long time to reach saturation after C/C0 = 0.8 in the MCF-Li bed because of deeper pore penetration and weak adsorption affinity. The results also imply that the adsorbed methyl mercaptan can easily be desorbed by thermal regeneration. Figure 6 shows the breakthrough curves in the MCF, MCFLi, and YSP-Li beds at 25 °C and the MCF-Li bed at 50 °C. Even at the high feed flow rate condition (50 mL/min), the differences in the adsorption capacity and breakthrough shape of the YSP-Li and MCF-Li beds are clear.

Table 1. Comparison of Adsorption Capacities of Adsorbents at 25 °C adsorption capacity (mmol/g) adsorbent YSP-Li MCF-Li MCF

flow rate 5 mL/min; pressure 1 atm

flow rate 50 mL/min; pressure 1.4 atm

0.045 0.086

0.13 0.25 0.10

(815.2 m2/g) and a higher amount of Li than MCF-Li (surface area = 551.6 m2/g), the sulfur adsorption capacity of MCF-Li was 0.086 mmol/g, which was almost twice that of YSP-Li (0.045 mmol/g). In the breakthrough curve of the YSP-Li bed, the slope between the breakthrough point and C/C0 = 0.5 was steeper than that between C/C0 = 0.5 and 0.95. Methyl mercaptan molecules with kinetic diameters of 8 Å are expected to penetrate into the pores of YSP-Li (3.73 nm) and to be adsorbed on the surfaces of the pores.26 Therefore, the difference in the breakthrough curve between the front slope and rear slope may be due to the increased mass transfer resistance by the multilayer adsorption of methyl mercaptan in the pores. However, such phenomena in the breakthrough curve of the MCF-Li bed were relatively minor in effect because the pore size of MCF-Li (19.6 nm) is large enough to overcome the mass transfer resistance. In addition, the slope of the breakthrough curve from the YSP-Li bed was steeper than that with the MCF-Li bed. These results imply that the

Figure 6. Breakthrough curves of methyl mercaptan in MCF, MCF-Li, and YSP-Li beds at feed flow rate of 50 mL/min and 1 atm. 14492

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The adsorption capacity of MCF without Li modification was 0.10 mmol/g (47 min at C/C0 = 0.5) which is 2.5 times smaller than that of MCF-Li at the same experimental conditions. However, the differences in the adsorption capacity and breakthrough shape between MCF and YSP-Li are relatively small. The results clearly demonstrate that lithium modification on mesoporous silica contributes to the improved adsorption capacity of methyl mercaptan. However, when the temperature increased from 25 to 50 °C, the adsorption capacity of MCF-Li decreased significantly. From Figures 4 and 5, methyl mercaptan molecules can only adsorb on strong adsorption sites at a high temperature. Comparing Figure 4 to Figure 5, increased feed flow rate led to a steeper breakthrough shape and shorter adsorption time. The breakthrough times of all of the adsorbents were almost 2 times shorter than those obtained at 5 mL/min. However, the adsorption capacity of MCF-Li at 25 °C was 0.25 mmol/g, which is 2.9 times higher than that obtained at 5 mL/min. Also, the corresponding adsorption capacity of YSP-Li was 0.13 mmol/g, which is also 2.8 times higher than that observed at 5 mL/min. At 50 mL/min, the bed pressure increased to 1.4 atm due to dense packing of adsorbent particles in the column while the breakthrough experiments at 5 mL/min were performed at an atmospheric pressure. Therefore, the improved adsorption capacities of both adsorbents may stem from the increased adsorption pressure. In addition, after C/C0 = 0.8, tailing of the breakthrough curve was also observed, as seen in Figure 5. 3.2. Thermal Regeneration of Methyl Mercaptan. Cost-effective desulfurization of a large amount of natural gas can be efficiently performed by applying a cyclic process that consists of adsorption and desorption steps. As demonstrated in the temperature effect on the breakthrough experiments, the temperature swing adsorption (TSA) process is a promising cycle because the adsorbed methyl mercaptan can be desorbed easily.27,28 The desorption temperature in the cyclic process is a key factor in the evaluation of the regeneration feasibility and efficiency.21 In this study, the desorption dynamics of methyl mercaptan were performed by stepwise thermal regeneration at 25, 50, 100, and 150 °C. Methane and nitrogen were used as the purge gas. Since the usage of methane is a product purge, the effluent gas consists of the same components as the feed gas. On the other hand, nitrogen is typically used as a heat carrier in VOC regeneration processes, but the effluent nitrogen contains concentrated methyl mercaptan. Since each desorption experiment was performed after performing the breakthrough experiment at 25 °C and 5 mL/min, the amount of methyl mercaptan at each bed in the desorption experiment corresponded to the value in Table 1. Figure 7 shows the stepwise thermal desorption curves of YSP-Li and MCF-Li by nitrogen and methane. However, the calibration curve for GC showed a highly nonliner shape showing a relatively large deviation at high concentration. Since the concentrated sulfur was desorbed during the thermal desorption, the concentration calculated at the high concentration condition might contain a certain level of error. Therefore, in the study, the shape and desorption time of desorption curve were qualitatively analyzed at each desorption condition instead of analyzing the desorption amount quantitatively. As shown in Figure 7a, the purge at 25 °C led to desorption of methyl mercaptan from YSP-Li, but it took a long time and was accompanied by a desorption tail. Moreover, the methyl mercaptan concentration during the initial period of time in the

Figure 7. Stepwise thermal desorption of methyl mercaptan from YSPLi (a) and MCF-Li (b) by methane and nitrogen at 5 mL/min and 1 atm: adsorbent mass was 0.3g. The loadings at YSP-Li and MCF-Li were 0.045 and 0.086 mmol/g, respectively.

nitrogen regeneration step was higher than the feed concentration while the methyl mercaptan concentration in the methane regeneration step did not reach the feed concentration. Since the desorption temperature was the same as the adsorption temperature, the difference in the desorption dynamics stems from the properties of the purge gas. Nitrogen with a kinetic diameter of 3.64 Å has a linear molecular structure while methane with a kinetic diameter of 3.8 Å has a tetrahedral structure.29 Even though nitrogen molecules can diffuse into the pores more easily than methane molecules, the contribution of the molecular size and structure to the desorption may be minute due to the large pore size of the adsorbent.29,30 However, at a standard condition, the gas density of nitrogen is almost twice that of methane. And its heat capacity is lower than that of methane. Because nitrogen can work as a better hear carrier, a higher amount of methyl mercaptan can be desorbed by nitrogen over the same period of time. At 50 °C, since more strongly adsorbed methyl mercaptan molecules were desorbed by the purge gas, the desorbed concentration decreased steeply with time after showing a higher desorption concentration than the feed gas. In addition, the difference in the desorption dynamics between nitrogen and methane became negligible. At 100 °C, the difference in the desorption rate between the two purge gases could clearly be observed again. However, no significant amount of methyl mercaptan was detected after subsequent heating to 150 °C under nitrogen regeneration while a small amount of methyl mercaptan was still detected with methane regeneration. In MCF-Li (Figure 7b), it took a long time to regenerate MCF-Li because the adsorption amount was much larger than 14493

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that on YSP-Li. The desorption concentration of methyl mercaptan even in the methane purge at 25 °C was slightly higher than the feed. Roll-up of the desorption curve with the nitrogen purge was observed while it was not observed in the desorption curve with the methane purge. In addition, the desorption rate under nitrogen at 25 °C was much faster than that obtained under methane. The difference in the desorption rate between the two purge gases was also monitored at 50 °C, but the difference became minute at 100 °C with respect to the duration of the desorption peak. As a result, the desorption rate under nitrogen was faster than that obtained under methane with low temperature regeneration probably due to the physical properties of nitrogen. On the other hand, with a regeneration temperature above 100 °C, the differences in the desorption rate and regeneration level between the two purge gases became trivial. In this study, both adsorbent beds saturated by methyl mercaptan at the same conditions were regenerated by methane at 100 °C and 50 mL/min for the cyclic process with a thermal product purge. Figure 8 shows the comparison of the

Figure 9. Adsorption capacities of regenerated YSP-Li and MCF-Li beds at 25 °C , 5 mL/min, and 1 atm.

4. CONCLUSIONS The adsorption and desorption performances toward sulfur components in a methyl mercaptan/methane gas mixture were assessed on mesoporous silica adsorbents modified with lithium. Both Li-modified adsorbents (YSP-Li and MCF-Li) showed much higher adsorption capacities than the mesoporous silica adsorbents without Li. MCF-Li demonstrated significantly higher adsorption capacity of methyl mercaptan than YSP-Li even though its surface area and Li-doped amount were smaller. However, based on the steepness of the adsorption slope in the breakthrough curve obtained at 25 °C, YSP-Li has a slightly higher affinity for sulfur compounds, which may stem from the higher concentration of Li. At a higher temperature (50 °C), the breakthrough time of MCF-Li was much longer than that of YSP-Li. Even at a higher temperature and flow rate, MCF-Li adsorbed a certain amount of methyl mercaptan, resulting in a steep breakthrough curve. The results imply that the surface adsorption affinity of MCF-Li is stronger than that of YSP-Li. In the thermal regeneration step, nitrogen was more efficient than methane. In addition, the purge gas effect was more significant in the MCF-Li regeneration because of steric hindrance in the pores. The regeneration time for MCF-Li was slightly longer due to deeper penetration of sulfur species into pores and its higher adsorption capacity. However, both adsorbents could be regenerated by methane or nitrogen purging at a temperature of 100 °C and, therefore, a thermal cyclic process is possible for bulk treatment.

Figure 8. Desorption curves of methyl mercaptan from YSP-Li and MCF-Li. Desorption curve of methyl mercaptan was done by methane at 5 mL/min, 100 °C, and 1 atm.

desorption dynamics between YSP-Li and MCF-Li. The initial desorption concentration was more than two times the feed concentration. It required 50 min for full regeneration of YSPLi and about 80 min for full regeneration of MCF-Li because the adsorption capacity of MCF-Li was much higher than that of YSP-Li. Considering the difference in the adsorption capacity, the difference in the regeneration times was relatively small. The adsorption capacities of the adsorbents regenerated by methane at 100 °C are presented in Figure 9. The adsorption breakthrough curves of the adsorbents were almost identical to those of the fresh adsorbents. Therefore, the developed mesoporous silica adsorbents doped by Li ions can efficiently remove a trace amount of sulfur compounds from the gas mixture such as natural gas or municipal gas. In addition, since the adsorbents can easily be regenerated by a thermal purge step, the adsorbents can be subjected to a cyclic temperature swing adsorption process for mass production. However, for the packed bed application of the prepared adsorbents, the adsorbents should be pelletized to reduce the pressure drop through the bed. Then, further study is needed for practical applications such as packed bed, ceramic filter, and silica membrane, etc.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 82-02-2123-2762. Fax: + 82-02-312-6401. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by DAPA/ ADD of Korea and Korean Institute of Energy Technology Evaluation and Planning (2011T100100425). 14494

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dx.doi.org/10.1021/ie301066n | Ind. Eng. Chem. Res. 2012, 51, 14489−14495