10064
Ind. Eng. Chem. Res. 2008, 47, 10064–10070
Breakthrough Characteristics of Reformate Desulfurization Using ZnO Sorbents for Logistic Fuel Cell Power Systems Hongyun Yang,† Ryan Sothen, Donald R. Cahela, and Bruce J. Tatarchuk* Center for Microfibrous Materials Manufacturing, Department of Chemical Engineering, Auburn UniVersity, Auburn, Alabama 36849
Sulfur breakthrough behaviors during reformate desulfurization were investigated using a novel ZnO-based sorbent with minimized mass transfer resistance. The presence of CO, CO2, or water affected the breakthrough characteristics of H2S and carbonyl sulfide (COS). CO and CO2 did not significantly affect the reaction between H2S and ZnO, but they reacted with H2S to form COS, which cannot be efficiently removed by ZnO. The mechanisms of COS formation via two different pathways were also investigated. CO reacted with H2S to form COS homogeneously; CO2 reacted with H2S heterogeneously on the sulfide surface. COS formation by CO and CO2 was suppressed by H2 and water. Water also severely hindered the reaction between ZnO and H2S and significantly decreased H2S breakthrough time. At low water concentrations, sulfur breakthrough was determined by the homogeneous COS formation; at high water concentrations, it was controlled by H2S breakthrough. Capacity loss due to COS formation and adsorption of water was also observed. Novel sorbent and process designs are required to improve the desulfurization performance. 1. Introduction Logistic fuel cell power systems such as mobile power supplies, remote power supplies in a military environment, and auxiliary power units (APUs) for long-haul diesel trucks1 receive increasing attention due to their high energy efficiency. Reformers such as steam reformers, catalytic partial oxidation (CPO) reformers, and autothermal reformers (ATR)1-5 are typically employed in these systems to convert logistic fuels to H2enriched gases (reformates). Reformates mainly consist of H2, CO, CO2, low molecular weight hydrocarbons, water, and trace amounts of sulfur species such as hydrogen sulfide (H2S) and carbonyl sulfide (COS). These sulfur species are poisonous to the downstream catalysts in fuel processing units and the electrolytes in fuel cells that can only tolerate a total sulfur concentration of 1 ppm by volume (ppmv) or less. Thus, it is necessary to develop high efficiency fuel cleanup technologies to remove sulfur compounds from reformates. Current technologies using metal oxide-based sorbents can reduce sulfur concentration from several thousand ppmv to subppmv levels.6-11 Zinc oxide (ZnO) is the most widely used sorbent to remove sulfur species such as H2S from gas streams due to its high sulfur capacity and favorable sulfidation thermodynamics at low temperatures (350-400 °C). Other ZnO-based sorbents such as ZnO stabilized by Fe2O312,13 and TiO214,15 are also widely employed in desulfurization applications at higher temperatures. Therefore, it is important to investigate sulfur breakthrough characteristics during reformate desulfurization using ZnO-based sorbents. The reactions between ZnO and reformates, however, have complications such as the reaction between ZnO and H2S, water gas shift reaction (WGS), COS formation in the presence of CO and CO2, and ZnO reduction. The reactions between ZnO and a reformate stream (H2S-H2-CO-H2O-CO2-N2) at 500 °C were described by Sasaoka et al. in 1994.11 They found that COS is not as active as H2S to react with ZnO.16 They also * To whom correspondence should be addressed. Tel.: (334) 8442023. Fax: (334) 844-2065. E-mail:
[email protected]. † Current address: IntraMicron Inc., 368 Industry Drive, Auburn, AL 36832.
observed that ZnS had catalytic activity to convert COS to H2S according to the following reaction:17 2COS + H2+ H2O h 2H2S + CO + CO2 (1) However, the effects of CO, CO2, H2, and H2O on the sulfur breakthrough characteristics have not been studied yet. The research efforts in this Article were focused on revealing these effects during reformate desulfurization using ZnO sorbents. 2. Experimental Section The sorbent employed in this study is ZnO/SiO2 developed at the Center for Microfibrous Material Manufacturing (CM3) at Auburn University for Proton Exchange Membrane (PEM) fuel cell applications. It contained 17 wt % of ZnO supported on SiO2 (100-200 µm particles). It was prepared by incipientwetness impregnation at room temperature using Zn(NO3)2 aqueous solutions as a precursor. Following impregnation, the sorbent was dried and calcined. The sorbent had a high surface area of 250 m2/g and a porosity of 0.63. It demonstrated a high sulfur capacity and minimized intraparticle mass transfer resistance.6,18 The detailed sorbent preparation procedure was described by Lu et al.6 If not otherwise stated, all gases were ultrahigh purity (UHP) gases purchased from Airgas Inc. The sources of H2S were pure H2S (>99.5%, Sigma-Aldrich) and 2 vol % H2S-H2 mixture. The challenge gases containing 1.3 vol % H2S were prepared by diluting pure H2S with CO (>99.5 vol %, Sigma-Aldrich) or CO2 and He. Challenge gases containing 4000 ppmv H2S were prepared by mixing the 2 vol % H2S-H2 source with other gases such as CO, CO2, H2, He, and steam. Water was introduced to the gas stream by passing H2/He/CO through a vaporizer with a temperature controller. The water in the vaporizer was predistilled and preheated to 100 °C to remove oxygen. The steam was carried in a 1/8” stainless steel tubing wrapped in a heating tape. The steam-containing stream was mixed with the H2S-containing stream before entering the reactor. Reformates containing H2 (∼40 vol %), CO2 (∼20 vol %), CO (∼10 vol %), and water (∼30 vol %) were prepared by mixing each individual compound together. The detailed experimental setup is shown in Figure 1.
10.1021/ie8008617 CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10065
Figure 2. Effects of CO and H2O on H2S breakthrough curves.
Figure 1. Experimental setup.
The effects of CO, CO2, and water on sulfur breakthrough and COS formation were investigated, while C1-C3 hydrocarbons were considered to be inert. Unless otherwise stated, each test challenged a sorbent bed containing 0.5 g of ZnO/SiO2 in a quartz tubular reactor (1 cm i.d., bed height 1.1 cm) with 4000 ppmv H2S in various gaseous compositions at 400 °C at a face velocity of 9.9 cm/s (GHSV ) 13 900 h-1). The stoichiometric saturation time of the packed beds was calculated to be 31.6 min. After the sorbent bed was loaded, air (breathing quality) was passed through the reactor at 100 mL/min until the reactor temperature reached 400 °C. Helium (100 mL/min) was then flowed through the reactor for 10 min to eliminate oxygen in the reactor to reduce side reactions such as sulfide oxidation. Next, H2 was passed through the reactor for 10 min to stabilize the temperature profile along the reactor. Finally, the challenge gas was passed through the reactor at the same flow rate as H2. The outlet H2S and COS concentrations were analyzed by a Varian GC-3800 equipped with a TCD detector (H2 as carrier gas), which was able to accurately measure H2S concentration down to 200 ppmv. The concentrations of CO2 and COS below 200 ppmv were also detectable at a high TCD bridge current. Gas samples were injected into the GC every 3 min (every 1 min for H2S-H2-He challenge gas) by a programmed 6-portvalve with a sampling loop of 50 µL. For convenience, the breakthrough concentration was defined at 40 ppmv, which corresponds to 1% of inlet H2S concentration. Breakthrough times were read from breakthrough curves. All equilibrium constants were calculated using HSC 3 software. 3. Results and Discussion 3.1. Performance of ZnO/SiO2. The reaction between ZnO and H2S takes place heterogeneously according to reaction 2. Because water is a product of the reaction, water content will influence the equilibrium H2S concentration. ZnO + H2S h ZnS + H2O
(2)
Reaction 2 is thermodynamically favorable and has an equilibrium constant of 3.38 × 105 at 400 °C. The corresponding
equilibrium H2S concentration is 0.59 ppmv in the presence of 20% water. As compared to the challenge H2S concentration of 4000 pmmv, this equilibrium H2S concentration is negligible. Therefore, the equilibrium of reaction 2 should not change the sulfur capacity of ZnO sorbent. In this Article, the breakthrough curve obtained using 4000 ppmv H2S-20 vol % H2-He challenge gas, as shown in Figure 2 (line -0-), is used as a control curve for comparison purpose. The saturation time based on the t1/2 concept (time to reach 50% of the inlet concentration) of the control curve was 31.5 min. This is very close to the calculated stoichiometric saturation time of 31.6 min. The H2S breakthrough time was 28 min. These results suggest the ZnO in ZnO/SiO2 was completely accessible and the breakthrough ZnO utilization was 90%. A further increase in face velocity will have little effects on the shape of breakthrough curves; thus, the breakthrough characteristics described in this study can be considered as close approximations to the intrinsic reaction behaviors. ZnO can be reduced to zinc metal in the presence of CO or H2 at high temperatures.9,14 Because no zinc metallic deposition was observed in this study, zinc loss was not significant for ZnO/SiO2 sorbent at 400 °C. 3.2. CO-H2O-H2 Challenge Gas System. Carbon monoxide has strong influences on the desulfurization performance of ZnO sorbents, and it reacts with H2S to form COS according to reaction 3. CO + H2S h COS + H2
(3)
Considering the similarity between sulfur and oxygen atoms, reaction 3 is analogous to the well-known water gas shift (WGS) reaction outlined in reaction 4. CO + H2O h CO2+ H2
(4)
Reaction 3 is a reversible reaction with an equilibrium constant of 0.0363 at 400 °C. Although it is much smaller than that of the WGS reaction (12.4), it is still crucial for most fuel cell applications. For example, the equilibrium COS concentration was calculated to be 1.4 ppmv at 400 °C in the presence of 100 ppmv H2S, 15 vol % CO, and 40 vol % H2. This equilibrium concentration is higher than the sulfur thresholds of most proton exchange membrane fuel cells (∼0.1 ppmv) and some solid oxide fuel cells (∼1 ppmv). The sharpness of breakthrough curves, which can be characterized using the lumped K concept, is used as an index of the apparent reaction rate.18-21 Given a constant challenge concentration tested with the same sorbent, a sharper breakthrough curve and a longer breakthrough time indicate faster reaction kinetics, or vice versa. If the addition of one gas compound significantly broadens the H2S breakthrough curve, then it must be decelerating the reaction between ZnO and H2S.
10066 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 3. Effects of CO and H2O on COS formation.
Figure 4. Effects of CO and H2O on total sulfur breakthrough curves.
The experimental results tested with the challenge gases containing H2S, CO, and H2 are shown in Figures 2-4. As shown in Figure 2, the addition of CO shifted the breakthrough curves to the right and decreased the plateau H2S concentration at bed saturation. This shift of breakthrough curves to the right side does not mean an increase in sulfur capacity because of COS formation in the presence of CO as shown in Figure 3. The total sulfur breakthrough curves (as shown in Figure 4) actually demonstrated a shift to the left, indicating a drop in sulfur capacity. Figure 2 also shows that the addition of 10-20 vol % CO to the H2S-H2-He system slightly broadened the H2S breakthrough curves. The result suggests CO slightly decelerated reaction 2 at 400 °C. This deceleration is possibly attributed to the formation of COS leading to the effective H2S concentration being slightly reduced. Figures 2 and 3 indicate that COS breakthrough took place earlier than that of H2S in the presence of CO. For example, H2S broke at 27 min in the presence of 10% CO and no water (Figure 2, line -]-); COS broke at 14 min and reached a peak concentration of 1000 ppmv (Figure 3, line -]-). In terms of H2S saturation time, the sulfur capacity of the packed bed was 31.5 min under test conditions. Therefore, approximately 50% of the ZnO was unutilized at COS breakthrough. This amount of ZnO was still able to remove H2S but not COS from the gas stream. These results suggest that COS is generated homogeneously, or it is hardly captured by the ZnO sorbent bed as described by Sasaoka et al.16 According to reaction 3, it is obvious that a high CO concentration favors COS formation as shown in Figure 3 (comparing lines -]- with 10 vol % CO and -∆- with 20 vol % CO). The outlet COS concentrations were higher than the calculated equilibrium values of 142 and 71 ppmv in the presence of 20 and 10 vol % CO, respectively. After reaching the peak values, the COS concentration dropped gradually. Figure 3 suggests that the change in CO concentration affects the COS breakthrough time. For instance, COS breakthrough time decreased by 1.7 min when CO concentration increased
from 10 to 20 vol % in the absence of water. Figure 3 also demonstrates that H2 inhibited the formation of COS. The COS breakthrough in the presence of 40 vol % H2 (line -O-) started 5 min later and achieved a lower concentration plateau as compared to the COS breakthrough curve in the presence of 20 vol % H2 (line -∆-). Water has dominant influences on H2S breakthrough curves. As shown in Figure 2, the H2S breakthrough curves can be classified into two groups according to their water content. The breakthrough curves of all tests performed in the absence of water (marked by open symbols) are located in one position with a steep slope. The breakthrough curves carried out in the presence of 20 vol % water (marked by solid symbols) are broadened and displaced to the left side, indicating a drop in sulfur capacities. This result suggests water significantly hindered the reaction between H2S and ZnO. A possible reason is that the adsorption of water on ZnO blocks the access of H2S molecules. In the presence of 20 vol % water and no CO, the H2S breakthrough curve (line -9-) was shifted to the left of the control curve (line -0-) by 1 min, indicating a loss of sulfur capacity (3% of stoichiometric capacity). Moreover, water drastically decreases the H2S breakthrough time. In the presence of 10 vol % CO, the addition of 20 vol % water reduced the H2S breakthrough time from 27 min (Figure 2, line -]-) to 18.5 min (Figure 2, line -[-). This H2S breakthrough time was very close, albeit larger than that of COS of 17 min (Figure 3, line -[-). As shown in Figure 3, the addition of water also drastically reduced the plateau COS concentrations from 400-600 ppmv in the absence of water to less than 100 ppmv in the presence of 20 vol % water. Because water is not involved in reaction 3 for COS formation, a reasonable hypothesis to this phenomenon is that the plateau COS concentration is primarily controlled by reaction 5. CO2+ H2S h COS + H2O
(5)
Both water and CO2 are involved in reaction 5. The hypothesis above is supported by the detection of CO2 during the experiments, as shown in Figure 3. The presence of CO2, however, complicates the system. Additional research efforts are required for a better understanding of the reaction mechanisms between CO/CO2 and H2S. Figure 4 shows the breakthrough curves of total sulfur including both H2S and COS. The addition of CO dramatically broadened the total sulfur breakthrough curves; therefore, it decelerated the reactions between ZnO and sulfur species. In this case, it was due to the formation of COS, which is not very active to ZnO. The total sulfur breakthrough time is determined by the lesser of the COS breakthrough time and H2S breakthrough time. At higher water and lower CO concentrations, H2S breakthrough takes place earlier than COS. In these cases, the breakthrough time of total sulfur is determined by H2S breakthrough, which is in turn determined by the water concentration. At a low water content, the COS breakthrough time determines the total sulfur breakthrough time. The presence of CO also decreases the breakthrough ZnO utilization. In the absence of water, the total sulfur broke at 14 min in the presence of 20 vol % CO (line -∆-) and at 28 min without CO (line -0-). The breakthrough sulfur capacity and bed utilization dropped by 50% due to the presence of CO. The breakthrough ZnO utilization in the presence of 20% CO and 20% water (line -2-) is less than 40% due to the severe hindering effects of both water and CO. 3.3. CO2-H2O-H2 Challenge Gas Systems. Like CO, CO2 also affects the desulfurization performance because it reacts
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10067
Figure 5. Effects of CO2 and H2O on H2S breakthrough curves. Figure 8. Effects of CO and CO2 on H2S and COS breakthrough.
Figure 6. Effects of CO2 and H2O on COS formation.
Figure 7. Effects of CO2 and H2O on total sulfur breakthrough curves.
with H2S to form COS according to reaction 5. Reaction 5 is reversible with an equilibrium constant of 0.00293 at 400 °C. The equilibrium COS concentration was calculated to be 0.15 ppm in the presence of 100 ppm H2S, 15 vol % CO2, and 30 vol % H2O. This value is higher than the sulfur thresholds of most PEM fuel cells. The effects of CO2 on the sulfur breakthrough characteristics were investigated, and the experimental results tested in the challenge gases containing 4000 ppmv H2S-20 vol % H2-He with various CO2 concentrations are shown in Figures 5- 7. As shown in Figure 5, CO2 barely changed the shape of H2S breakthrough curves, while slightly shifting the breakthrough curves to the right. In the absence of water, the breakthrough curves (marked by open symbols) demonstrated an average H2S breakthrough time of about 28 min. COS was also detected, and COS breakthrough curves are shown in Figure 6. In this figure, the COS breakthrough took place at ∼21 min. This is 7 min earlier than the H2S breakthrough. The COS breakthrough times varied slightly with a change in concentration of CO2 and H2 in the challenge gas. Similar to CO, CO2 also slowed the desulfurization kinetics as shown in Figure 7. The total sulfur breakthrough time was decreased at a higher CO2 concentration;
however, CO2 is not as active as CO in COS formation. For instance, the total sulfur breakthrough took place at 22 min in the presence of 20 vol % CO2 (Figure 7, line -∆-) and at 14 min in 20 vol % CO (Figure 4, line -∆-) in the absence of water. There are two possible pathways for COS formation in the presence of CO2. The first one is direct COS formation by reaction 5. The second is a two-step reaction where CO2 is converted to CO intermediate by the WGS reaction before forming COS by reaction 3. Because H2 is involved in reaction 3 but not in reactions 5 or 6, an increase in H2 concentration would cause CO and subsequent H2S breakthrough time to increase. Figure 6 suggests the direct COS formation via reaction 5 is dominant in the presence of CO2 because the increase in H2 concentration from 20% (Figure 6, line -∆-) to 40% (Figure 6, line -O-) did not demonstrate a significant influence on the breakthrough of H2S and the plateau COS concentration Moreover, COS plateau concentration was almost proportional to CO2 concentration in Figure 6. This also supports the first pathway. When the sorbent bed was close to saturation, CO plateau concentration was detected at 3500 ppmv in the presence of 20 vol % H2 due to the WGS reaction. This is far below the equilibrium CO concentration of the WGS reaction under the experimental conditions. The presence of CO also implies that an equal amount of H2O (3500 ppmv) was generated via the WGS reaction. A further calculation for the gaseous composition of 20 vol % CO2, 20 vol % H2, 4000 ppmv H2S, and 3500 ppmv H2O indicated an equilibrium COS concentration of 502 ppmv. The experimental value was 480 ppmv (Figure 6, line -∆-). In the presence of 20 vol % water, the equilibrium COS concentration for the challenge gas (4000 ppmv H2S-20 vol % CO2-20 vol % H2-20 vol % H2O-He) at 400 °C was calculated to be 12 ppmv. The experimental value was approximately 20 ppmv (Figure 6, line -2-). The experimental COS concentrations are very close to the calculated equilibrium ones. This implies that COS formation via CO2 is a fast reaction, and the plateau COS concentration is mainly controlled by the equilibrium of reaction 5 when the sorbent bed was saturated. 3.4. CO-CO2 Challenge Gas Systems. ZnO/SiO2 sorbent was tested in challenge gas systems containing both CO and CO2 at 400 °C. As shown in Figure 8, CO-CO2 gas systems did not demonstrate a significant influence on the reaction between ZnO and H2S. COS breakthrough curves in Figure 8 suggest that CO concentration determined the breakthrough patterns of COS and total sulfur, while CO2 did not have any significant effects on the COS breakthrough time. This result implies that CO is more active than CO2 in COS formation, which is consistent with the earlier observations. Because the
10068 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 9. Effects of H2 and H2O on H2S breakthrough curves in the presence of CO and CO2.
Figure 10. Effects of H2 and H2O on COS breakthrough curves in the presence of CO and CO2.
Figure 11. Effects of H2 and H2O on total sulfur breakthrough curves in the presence of CO and CO2.
H2S breakthrough times in these tests were larger than those of COS, the total sulfur breakthrough time was determined by COS breakthrough that in turn was determined by CO concentration. ZnO/SiO2 sorbent was challenged with 10 vol % CO-20 vol % CO2 challenge gases with various concentrations of water and H2 at 400 °C. The results are shown in Figures 9-11. The addition of H2 to the CO-CO2 system in the absence of water significantly inhibited the COS formation and extended the total sulfur breakthrough time. The addition of 20% H2 to the challenge gas of 10 vol % CO-20 vol % CO2-20 vol % H2-He shifted the H2S breakthrough curve left by 4 min (comparing lines -]- and -O- in Figure 9) while increasing the COS and total sulfur breakthrough times by 4 min (comparing lines -]- and -O- in Figures 10 and 11). Another comparison between the breakthrough curve of the challenge gas containing 20 vol % H2-20 vol % H2O (lines -b-) and the one containing 40 vol % H2-30 vol % H2O
(lines -[-) suggests that the addition of H2 to the challenge gas containing water did not extend the COS and total sulfur breakthrough time. The addition of H2O to the CO-CO2 system significantly changed the shape of H2S breakthrough curves and plateau COS concentrations as shown in Figures 9-11. Water also reduced the COS concentration. In Figure 10, COS concentration was reduced from 700 ppmv present in H2S-CO-CO2-H2-He systems (lines with open symbols) to below 100 ppmv in the presence of 20-30 vol % water (lines with solid symbols). The addition of H2O also demonstrated a 3 min increase in COS breakthrough time. A possible reason is that the addition of water reduced CO concentration by the WGS reaction and suppressed the COS formation by reaction 5. Because water severely decelerates the reaction between ZnO and H2S and hydrolyzes COS to H2S, H2S breakthrough time decreased with increasing water content as shown in Figure 9. In the presence of 30 vol % water, the H2S breakthrough (Figure 9, line -[-) took place earlier than COS breakthrough (Figure 10, line -[-). As a result, the total sulfur breakthrough time was determined by H2S breakthrough. It is should be noted that the typical reformate composition is similar to the challenge gas of 10 vol % CO-20 vol % CO2-40 vol % H2-30 vol % H2O (Figure 11, line -[-). Figure 11 suggests a low ZnO utilization of 30% at this gas composition. Figure 11 also demonstrates a sulfur capacity loss of approximately 10% of saturation capacity in the test with this challenge gas (comparing line -0- to line -[-). The adsorption of water accounted for 3% capacity loss (comparing line 0 to line -9-) with COS formation accounting for the remaining 7% due to low activity of COS to ZnO (comparing line -9- to line -[-). The sulfur capacity of ZnO sorbent can be increased by converting COS to H2S. 3.5. Mechanism of COS Formation. COS is generated in the presence of CO and CO2 in reformates; however, the COS formation mechanism is not well understood. The earlier discussion suggests CO is more active than CO2 in COS formation. In addition, CO and H2 have a strong influence on COS breakthrough, while water is the controlling factor for the plateau COS concentration when the packed bed is saturated. These phenomena require research efforts to obtain better understanding. In this Article, several experiments were designed to investigate the pathways for COS formation and to understand the COS breakthrough characteristics. 3.5.1. Homogeneous Tests. Two homogeneous experiments were conducted in a clean empty quartz reactor without any sorbent particles or packing materials loaded. Before the experiments, helium was passed through the reactor at a face velocity of 9.9 cm/s at 400 °C. In the first experiment, the challenge gas containing H2S-He and CO was passed through the reactor at 10 min, and COS concentrations around 240 ppmv were detected as shown in Figure 12. The COS formation suggests that a homogeneous reaction between CO and H2S occurred in the tubular reactor; however, COS was generated at a concentration much lower than the equilibrium concentration, indicating a slow, homogeneous reaction. Because COS formation by the homogeneous reaction does not require the presence of ZnS, it can take place anywhere in the reactor tube, sorbent bed, or empty space. The equivalent COS concentration for the test containing 4000 ppmv H2S was estimated to be 70-100 ppmv. This value is close to the plateau COS concentration observed at high water contents where COS formation via CO2 was actually negligible.
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10069
Figure 12. Homogeneous COS formation via the reaction between CO and H2S at 400 °C. Tested with 1.3 vol % H2S-25 vol % CO-He challenge gas at a face velocity of 9.9 cm/s.
Figure 13. Homogeneous COS formation via the reaction between CO2 and H2S at 400 °C. Tested with 1.3 vol % H2S-25 vol % CO2-He challenge gas at a face velocity of 9.9 cm/s.
The second experiment was conducted to investigate the homogeneous pathway for COS formation in the presence of CO2 using the same methodology. The experimental results are shown in Figure 13. The challenge gas of CO2-H2S-He was switched to pass through the tube reactor at t ) 10 min. Unlike the first experiment, no COS formation was detected during this test, even though the calculated equilibrium COS concentration was 3000 ppmv under the test conditions. These results show COS formation via a homogeneous reaction between CO2 and H2S was negligible in all of the tests in this Article. 3.5.2. Heterogeneous Tests. Two experiments were conducted to investigate the possible heterogeneous pathways for COS formation. In each experiment, a packed bed of ZnO/SiO2 sorbent (0.5 g) was presaturated by 1.3 vol % H2S-He challenge gas at a face velocity of 9.9 cm/s at 400 °C for 30 min. The theoretical saturation time for the packed bed under these conditions is 7 min. In the first experiment, the H2S-He mixture was changed to a 1.3 vol % H2S-25 vol % CO-He challenge gas. The experimental results are shown in Figure 14. The spent sorbents bed yielded a stable COS concentration around 190 ppmv, which closely resembled the COS concentration generated via homogeneous reaction. At the 46th minute, H2S was removed from the gas stream, and only CO and He were fed to the reactor. As shown in Figure 14, the COS concentration dramatically dropped below the detection limit. The phenomenon indicates the heterogeneous reaction between ZnS and CO (reaction 6) was negligible. ZnS + CO h Zn + COS (6) The results of the homogeneous and heterogeneous tests on CO suggest that COS formation in the presence of CO is
Figure 14. COS formation in the spent sorbent beds. Tested with 1.3 vol % H2S-25 vol % CO or CO2-He challenge gas at 400 °C.
dominated by the homogeneous reaction between CO and H2S (reaction 3), and ZnS is not a catalyst for this reaction. In the second experiment shown in Figure 14, the spent sorbent experiment was conducted in the presence of CO2 in an analogous fashion. The results differed from those recorded in the presence of CO. In the CO2 sorbent test, the COS concentration dropped as it did in the CO test after H2S was removed from gas stream; yet the COS concentration remained at 30-80 ppmv. After H2S was removed from challenge gas stream, the only remaining sulfur source under the test conditions was ZnS. The continued presence of COS suggests that the reaction between ZnS and CO2 took place heterogeneously according to reaction 7. Because the homogeneous reaction between CO2 and H2S is negligible, the heterogeneous test results hint that ZnS behaves as a catalyst in COS formation via the heterogeneous reaction between H2S and CO2 (reaction 5). Reaction 5 actually consists of two steps: ZnS + CO2(g) h ZnO + COS(g)
(7)
ZnO + H2S(g) h ZnS + H2O(g)
(8)
The COS is generated via the homogeneous reaction between H2S and CO and the heterogeneous reaction between H2S and CO2 on the surface of ZnS. The heterogeneous reaction is fast and the homogeneous one is slow at a low temperature (ca. 400 °C); therefore, the COS and H2S concentrations in the packed beds are controlled by equilibrium of reaction 5 under test conditions. At a higher temperature (ca. 500 °C), where the homogeneous reaction may become fast, COS formation will be controlled by the equilibria of both homogeneous and heterogeneous reactions as described by reaction 1. These two reaction pathways explain most COS-related phenomena observed in this study. For example, the homogeneous reaction between CO and H2S does not require ZnS. COS is generated everywhere in the presence of CO and H2S. In addition, COS is not active to ZnO sorbent. Thus, COS breakthrough takes place much earlier than H2S especially in the absence of high water contents. COS breakthrough time strongly depends on the concentration of CO and H2. When enough ZnS is generated, the heterogeneous reaction between CO2 and H2S (reaction 5) gradually becomes the dominant factor for COS formation. A high water content suppresses the formation of COS and keeps its concentration at very low levels. The heterogeneous nature of reaction 5 hints that it only takes place in the packed bed of ZnO sorbent. Homogeneous COS formation takes places downstream off the packed bed and yields a plateau COS concentration of about 70-100 ppmv in the presence of high water contents where the heterogeneous COS formation is negligible (Figures 3 and 10).
10070 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Although the COS generation via the reaction between H2S and COS significantly reduces the total sulfur breakthrough time, its homogeneous characteristic can be utilized to mitigate the homogeneous COS formation by employing low reaction temperatures. This is used because homogeneous reactions usually have high activation energies and they are more sensitive to temperature changes than are the heterogeneous reactions. Desulfurization at this low temperature range is a challenge to sorbent designs. It should be noted that desulfurization, unlike most catalytic processes, does not have an unsteady state. It can be described by shrinking core model. Reaction becomes more difficult due to the increasing thickness of product layer. Because of the severe intraparticle mass transfer resistance at low temperatures, only sorbents on the outside layer of sorbent particles are accessible to H2S.22 As a result, sorbents at low temperatures usually demonstrate very low sulfur capacity. 4. Conclusions This basic study outlined the intrinsic breakthrough characteristics of desulfurization using ZnO-based sorbents. The experimental results suggest the sorbent utilization at sulfur breakthrough during reformate desulfurization was 30%. This is much lower than that for H2S-H2 or H2S-He challenge gases. The low efficiency was intrinsic; it resulted from the influences of CO, CO2, H2, and water. Among them, water is the dominant factor for desulfurization performance. It determines H2S breakthrough characteristics and suppresses COS formation. Water at high concentrations determines all sulfur breakthrough characteristics. If a high water content is not available, CO and CO2 determine the COS breakthrough and total sulfur breakthrough. In this case, H2 decelerates COS formation and extends sulfur breakthrough time. Because of severe mass transfer resistance encountered in desulfurization using ZnO extrudates, their desulfurization performances will be worse than these described in this study. The mass transfer effects on sulfur breakthrough behaviors will be investigated in the future. Because the low efficiency is intrinsic, novel sorbent design and process design are highly favored to improve the desulfurization performance. Other metal oxide based sorbents that are highly active to COS will increase the total sulfur capacity and reduce the reactor size. These sorbents can be used alone, and they can also be mixed with ZnO sorbent for a balanced performance. In addition, the reduction of desulfurization temperature may be a viable alternative. At lower temperatures (
