Mn−Cu and Mn−Cu−V Mixed-Oxide Regenerable Sorbents for Hot

Ind. Eng. Chem. Res. 2005, 44, 5221-5226

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Mn-Cu and Mn-Cu-V Mixed-Oxide Regenerable Sorbents for Hot Gas Desulfurization Dilek Karayilan and Timur Dogu* Department of Chemical Engineering, Middle East Technical University, 06531 Ankara, Turkey

Sena Yasyerli and Gulsen Dogu* Department of Chemical Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey

Porous Mn-Cu and Mn-Cu-V mixed-oxide sorbents prepared by the complexation technique were tested for high-temperature removal of H2S in the presence of hydrogen gas. Sorption experiments carried out at 627 °C in a fixed-bed reactor showed higher reactivity and also higher sulfur retention capacity (over 0.15 g of S/g of sorbent) for the Mn-Cu mixed oxide than for the Mn-Cu-V mixed oxide. Successive sulfidation-regeneration cycles demonstrated that the MnCu mixed oxide was also more stable than Mn-Cu-V, maintaining its activity and about 90% of its sulfur retention capacity at the end of five cycles. Although formation of some MnSO4 was observed during regeneration at 700 °C with a gas mixture containing 6% oxygen in nitrogen, this did not cause significant reduction in the activity of this mixed-oxide sorbent. It was also shown that a previously proposed deactivation model gave good predictions of the experimental breakthrough curves obtained with these sorbents. Introduction High-temperature desulfurization of coal gas and removal of H2S from other hot process gases, from several thousand parts per million to values lower than 100 ppm, is a major concern. Hydrogen sulfide is a toxic gas creating significant environmental problems. Integrated gasification-power generation combined-cycle systems require H2S levels as low as 10 ppm to eliminate turbine blade corrosion. In the case of fuel cell applications, much lower H2S levels (lower than 1 ppm) are required to eliminate poisoning of the electrodes. In recent years, significant research has focused on the development of regenerable metal oxide sorbents for the removal of H2S from process gases in the temperature range of 400-1000 °C. In the pioneering work of Westmoreland and Harrison,1 it was shown that manganese, zinc, iron, copper, vanadium, molybdenum, cobalt, tungsten, and calcium oxides have good potential for high-temperature removal of H2S from coal gas. The decrease of the H2S sorption capacity of Fe2O3 due to reduction to FeO and the loss of zinc by vaporization at high temperatures diverted the attention to copper- and manganese-based oxide sorbents and also to mixed metal oxide sorbents.2-9 In the literature, it was shown that reduction of CuO to Cu in the reducing atmosphere was restarted by the synthesis of copper-based mixed metal oxides and, in this way, both the activity and stability of the sorbent were increased.9,10 In our recent studies,9,11 porous mixed oxides of copper with vanadium and molybdenum were shown to give high activities for the removal of H2S both in the presence and in the absence of hydrogen in the temperature range between 300 and 700 °C. However, some sintering was observed at higher temperatures in mixed oxides containing molybdenum. Following the variations * To whom correspondence should be addressed. E-mail: [email protected] (G.D.), [email protected] (T.D.).

in H2S, SO2, and H2O concentrations in the fixed-bed reactor off-gas and also following the changes in the structure of the oxide sorbent, detailed information was obtained about the reaction sequences. It was also shown that H2S breakthrough curves predicted from the deactivation model agreed well with the experimental results. More recently, our studies showed that Cu-V and Cu-V-Mo mixed oxides acted as good catalysts for the selective oxidation of H2S to elemental sulfur.12 Manganese-based oxide sorbents have attracted the attention of number of researchers, especially in the past decade. It was shown by Slimane and Hepworth13 that manganese-based sorbents can be safely utilized at temperatures over 700 °C in H2S removal. The kinetic study of Westmoreland et al.20 showed that the reactivity of MnO is higher than those of CaO, ZnO, and V2O3 in H2S removal. In the work of Yoon et al.,14 it was reported that the addition of CuO and NiO to natural manganese ore led to an improvement in the sulfidation capacity of the sorbent. In the published literature, sorbents containing both copper and manganese oxides were prepared either by the impregnation of manganese and copper acetates into a silica-rich zeolite15 (supported oxide sorbent) or by the calcination of powder mixtures of manganese and copper oxides.6,16 Atimtay et al.15 showed that regeneration of the supported sorbent with a 50% air-50% steam mixture is preferred over air regeneration to decrease sulfate formation. Sulfate formation was also shown to decrease in the regeneration step with increasing regeneration temperature.17 However, hot spots can form during the regeneration step because of the exothermicity of the regeneration reactions, and this can cause a loss of surface area and a corresponding loss of activity of the sorbent at very high temperatures. In a recent study, regeneration of Mn-Fe-Zn-O mixed-oxide sorbent supported on Al2O318 was achieved in the temperature range of 600-750 °C using different percentages of oxygen in nitrogen, with regeneration at 700 °C giving the best results. In the

10.1021/ie0492496 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/12/2005

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present study, porous mixed oxides of manganese, copper, and vanadium were prepared by the complexation method and their activities and regenerabilities were tested in a fixed-bed reactor. Sorbent Preparation and Characterization In the complexation method used for the preparation of the mixed-oxide sorbents, manganese acetate tetrahydrate (C4H6MnO4‚H2O), ammonium vanadate (H4NO3V), and copper nitrate trihydrate [Cu(NO3)2‚ 3H2O] were used as the manganase, vanadium, and copper sources, respectively. The details of the complexation technique is available in the literature.9,19 In this method, citric acid was used as the complexation agent. Equimolar amounts of citric acid and the salts of the metals were mixed in solution. In the mixed-oxide sorbents, equimolar quantities of manganese and copper were used. The citric acid-metal solution was evaporated at 65 °C for over 24 h with continuous stirring. Dehydration was then finished in an oven at 70 °C by placing the viscous solution as a thin layer in a glass dish. The porous solid foam produced at the end of the dehydration step was then calcined at 550 °C for 8 h. Caution: Care should be taken with this method to avoid any explosion of the citric acid. The crystalline phases present in the mixed-oxide sorbents prepared in this work were determined with a Philips PW 1840 X-ray diffractometer employing a Cu KR radiation source. The major crystalline phases present in the Mn-Cu mixed oxide were found to be Cu1.5Mn1.5O4 and CuMn2O4. XRD results also indicated the presence of some CuO. The XRD patterns of the Mn-Cu-V mixed oxide indicated a highly amorphous structure. SEM images of these two mixed oxides also indicated the crystalline structure of Mn-Cu and the noncrystalline structure of the Mn-Cu-V mixed-oxide sorbents (Figure 1). The surface areas of the sorbents were measured using a Quantachrome BET sorptometer. Pore size distributions were determined with a Quantachrome Autoscan 60000 psig mercury intrusion porosimeter. The physical properties of the sorbents are summarized in Table 1. The higher values of the BET surface areas as compared to the surface area values measured by mercury porosimetry and also the shapes of surface area distribution curves indicated the presence of some pores smaller than 2 nm. Fixed-Bed H2S Sorption and Regeneration Experiments H2S sorption experiments with Mn-Cu and MnCu-V mixed-oxide sorbents were performed in a fixedbed microreactor with a gas mixture containing 1% H2S and 10% H2 in helium. The experimental setup consists of a quartz microreactor placed into a tubular furnace, an FTIR spectrometer (Perkin-Elmer), and a gas chromatograph (Perkin-Elmer) placed in series after the reactor for continuous analysis of the reactor effluent stream. In all experiments, 0.2 g of sorbent was packed into the 0.6-cm-diameter quartz reactor. Sorbent particles were supported by quartz wool from both ends. H2S sorption experiments were carried out at 627 °C. The temperature of the reaction section was controlled by a temperature controller. All pipes, fittings, and valves were made of stainless steel to minimize corrosion and also heated to eliminate any condensation. A

Figure 1. SEM images of Mn-Cu and Mn-Cu-V mixed-oxide sorbents. Table 1. Physical Properties of the Mixed-Oxide Sorbents surface area (m2/g) sorbent

porosity

BET

porosimetera

Mn-Cu-O Mn-Cu-V-O

0.77 0.70

45 57

9 30

a Porosimeter (mercury) results correspond to pores larger than 2 nm.

bypass line allowed flow-rate adjustments and concentration measurements of the feed stream. The FTIR spectrometer equipped with a heated flow cell and the gas chromatograph equipped with a TCD detector were used for on-line analysis (H2S, SO2, H2O) of the composition of the reactor exit stream. A Poropak-T column was used in the chromatograph. Multiple bands observed between 2614 and 2772 cm-1 in the FTIR spectra correspond to H2S. Details of the chemical analysis were reported elsewhere.9 The sulfur balance around the reactor was also checked by the chemical analysis of the solid product for total sulfur, after the sorption experiments. Regeneration experiments were also performed in the same packed-bed microreactor. Sulfided samples were first purged in a flowing nitrogen gas stream for about 30 min at 700 °C. Then, regeneration was achieved with a gas stream containing 6% O2 in nitrogen at 700 °C. Preliminary experiments carried out with a manganesebased oxide sorbent had indicated significant reduction in activity of the sorbent regenerated at temperatures lower than 700 °C. Low temperatures and high oxygen

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The sulfur retention capacity of the Mn-Cu mixed-oxide sorbent was found to be quite high (0.152 and 0.155 g of sulfur/g of sorbent) in the two experiments reported in Figure 2. Also, the H2S breakthrough curves were quite sharp, indicating high reactivity of the sorbent. Results of the six successive sulfidation-regeneration cycles indicated a slight shift of the breakthrough curves to smaller times (Figure 3). The sulfur retention capacity of the sorbent decreased from 0.152 to 0.141 g of S/g of sorbent in the first five cycles of the sulfidationregeneration experiments. The overall reactions expected in the H2S sorption runs are Figure 2. H2S breakthrough curves obtained with two identically prepared Mn-Cu mixed-oxide sorbents (1% H2S + 10% H2 in helium, Q ) 275 mL/min at 627 °C).

concentrations promote sulfate formation. When much higher temperatures (over 800 °C) were used in the regeneration step, loss of activity due to possible sintering effects was observed. The total flow rate of the gas stream was kept at about 100 cm3/min, measured at 298 K. Variations in the SO2 concentration in the exit stream were also analyzed during the regeneration runs. Regeneration using 6% O2 + N2 gas was continued until the concentration of SO2 approached zero in the off-gas. Then, the oxygen was turned off, and the sorbent was treated with pure nitrogen gas at 700 °C for another hour. At the end of this procedure, no further emission of SO2 was observed in the off-gas. Experimental Results and Discussion A. Mn-Cu Mixed-Oxide Sorbent. Results of the two replicate H2S sorption experiments carried out at 627 °C with two fresh Mn-Cu mixed-oxide samples having identical physical and chemical properties (Figure 2) showed that the experimental breakthrough curves obtained for H2S removal are highly reproducible. No SO2 was observed in the reactor exit stream in these experiments. However, formation of SO2 was observed with Cu-V and Cu-Mo mixed oxides in our previous studies.9,11 In these sorption experiments, the number of moles of H2S removed was evaluated by the numerical integration of the experimental concentration versus time curves obtained at the reactor exit stream.

4Cu1.5Mn1.5O4 + 9H2S + 7H2 f 3Cu2S + 6MnS + 16H2O (1) 2CuMn2O4 + 5H2S + 3H2 f Cu2S + 4MnS + 8H2O (2) 2CuO + H2S + H2 f Cu2S + 2H2O

(3)

Considering that the major phase of the sorbent is Cu1.5Mn1.5O4, the theoretical sulfur retention capacity can be estimated from reaction 1 as 0.297 g of S/g of sorbent. The experimental sulfur retention capacities were found to be about one-half of this theoretically estimated value. During the regeneration step carried out at 700 °C, formation of some MnSO4 was expected in addition to the following reactions

Cu2S + 2O2 f 2CuO + SO2

(4)

MnS + 3/2O2 f MnO + SO2

(5)

As discussed by Slimane and Hepworth,17 MnSO4 is rather stable at 700 °C at high oxygen partial pressures

MnO + SO2 + 1/2O2 f MnSO4

(6)

However, CuSO4 is not expected to be stable at the regeneration conditions of this work. To decrease the formation of MnSO4, 6% O2 in nitrogen was used as the regeneration medium instead of air or pure oxygen. MnSO4 is expected to decompose at higher temperatures

Figure 3. H2S breakthrough curves obtained with Mn-Cu mixed-oxide sorbent in six successive sulfidation cycles after successive regenerations (1% H2S + 10% H2 in helium, T ) 627 °C).

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(over 900 °C) in the presence of oxygen. However, preliminary experiments indicated some sintering and decrease of activity in manganese oxide sorbent regenerated at such high temperatures. As discussed in the previous section, the sorbent was purged with pure nitrogen gas at 700 °C after regeneration with a gas mixture containing 6% O2 in nitrogen. We observed some SO2 in the off-gas during this purging stage of the regeneration cycle. This result indicated some decomposition of the sulfates at this stage. The XRD patterns of the sorbent after the six sulfidation-regeneration cycles showed a more amorphous structure than the original sorbent. A small band observed at a 2θ value of 24.1° together with the increased intensity of the band at a 2θ value of 33° also indicated the formation of some MnSO4 during the regeneration steps. However, this formation of MnSO4 did not cause significant reduction in the H2S sorption capacity of the Mn-Cu mixed metal oxide sorbent at the end of six sorption-regeneration cycles. An interesting observation was the formation of elemental sulfur during the regeneration stages of the sorbent in the sulfur condenser that was placed after the reactor. Formation of elemental sulfur indicated the occurrence of the following reaction during the regeneration stage

MnS + 1/2O2 f MnO + 1/2S2

(7)

The thermodynamics of this reaction is favorable at the regeneration conditions of this work. The low level of oxygen in the regeneration gas mixture might be the reason for the incomplete oxidation of MnS to MnO and SO2. During the sulfidation step, significant variations are expected in the pore structure, active surface area, activity per unit area, and diffusional resistances. Our previous studies9,11 indicated that a deactivation model proposed for these systems gave good predictions of the experimental breakthrough curves. According to this model, the rate of change of the activity of the sorbent with respect to time was expressed as a function of concentration of H2S and activity itself as follows

-

da ) kdCAa dt

(8)

The species conservation equation for the reactant gas (H2S) was then written by making a pseudo-steady-state approximation and neglecting the axial dispersion term in the fixed-bed reactor

-Q

dCA - k0CAa ) 0 dW

(9)

The following approximate expression was then derived for the breakthrough curves9

[(

{

})

k0W [1 - exp(-kdt)] 1 - exp CA Q ) exp CA0 [1 - exp(-kdt)]

]

exp(-kdt)

(10)

In this expression, k0 is the initial rate constant, and kd is the deactivation rate constant of the sorbent. Regression analysis of the experimental breakthrough curves obtained in successive sulfidation cycles gave

Figure 4. Experimental and predicted (eq 10) H2S breakthrough curves obtained with Mn-Cu mixed-oxide sorbent at 627 °C in the first three sulfidation cycles (1% H2S + 10% H2 in helium). Table 2. Rate Parameters of the Deactivation Model for the Sorption of H2S on Mn-Cu Mixed Oxide at 627 °C in Successive Sulfidation Cycles sulfidation cycle

k0W/Qa

Qa (mL/min)

k0 × 10-3 (mL/g‚min)

kd (min-1)

1 2 3

6.35 6.12 7.71

275 272 266

8.8 8.5 10.2

0.30 0.32 0.37

a

Q values are evaluated at 627 °C.

Table 3. Comparison of Initial (First Sulfidation) Rate Parameters of Mn-Cu and Mn-Cu-V Mixed-Oxide Sorbents with Those of Other Copper-Based Oxide Sorbents in the Presence of Hydrogen sorbent

temperature (°C)

k0 × 10-3 (mL/g‚min)

kd (min-1)

Mn-Cu Mn-Cu-V Cu-Va Cu-Moa Cu-V-Moa CuOa

627 627 700 700 700 700

8.8 3.4 6.1 5.9 5.7 7.7

0.30 0.19 0.13 0.15 0.16 0.11

a

Taken from Yasyerli et al.9,11

good agreement with eq 10. Model predictions (from eq 10) and the experimental breakthrough data obtained in the first three cycles are presented in Figure 4, and the values of the rate constants k0 and kd obtained at 627 °C are reported in Table 2. As reported in this table, the value of the initial rate constant k0 did not change much in the three successive sulfidation cycles. The rate parameters of the Mn-Cu mixed-oxide sorbent prepared in this work were also compared with the corresponding parameters reported for CuO sorbent and Cu-V and Cu-Mo mixed-oxide sorbents reported in the literature9,11 (Table 3). As can be seen in Table 3, the initial sorption rate constant (k0) of H2S on the Mn-Cu sorbent prepared in this work is higher than the corresponding rate parameters reported for other Cu-based sorbents. Higher values of k0 and kd are consistent with the observation that experimental breakthrough curves with the Mn-Cu mixed-oxide sorbent were sharper. Higher reactivity is, of course a desirable property for the sorbent to achieve much lower H2S levels in the gas stream leaving the adsorption column. As discussed in the literature, the reduction of CuO to Cu causes a significant reduction in the activity of the copper-based oxide sorbents during the removal of H2S in the presence of hydrogen. The results of this work indicate that such a reduction in the H2S sorption activity is much less for the Mn-Cu mixed-oxide sorbent. Manganese itself is quite active toward H2S, and it seems that Mn

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Figure 5. Experimental H2S breakthrough curves obtained with Mn-Cu-V mixed-oxide sorbent in six successive sulfidation runs at 627 °C (after successive regenerations) (1% H2S+ 10% H2 in helium).

Figure 6. SO2 concentration curves at the reactor exit with MnCu-V sorbent during regeneration at 700 °C.

also stabilizes CuO to decrease its reduction in the presence of hydrogen. B. Mn-Cu-V Mixed-Oxide Sorbent. Experimental breakthrough curves obtained for H2S removal at 627 °C with the Mn-Cu-V mixed-oxide sorbent for the six sufidation-regeneration cycles (Figure 5) showed that this sorbent was less stable than the Mn-Cu mixedoxide sorbent. The sulfur retention capacity of this sorbent was found to decrease from 0.11 to 0.054 g of S/g of sorbent in the first six sulfidation-regeneration cycles. In parallel with this decrease of sulfur retention capacity, the number of moles of SO2 formed during the regeneration stage also decreased as the number of sulfidation-regeneration cycles increased (Figure 6). A comparison of the two sorbents (Mn-Cu and Mn-CuV) in terms of sulfur retention capacity is presented in Figure 7. These results indicate that the Mn-Cu mixed oxide is more reactive and more stable than the MnCu-V mixed oxide in H2S removal. Vanadium oxides are known as good oxidation catalysts. In fact, formation of some SO2 was reported in our previous publications9,11 during the sorption stage of H2S on Cu-V and Cu-V-Mo mixed oxides. Our recent work12 also showed that Cu-V mixed oxide was a very good catalyst for the selective oxidation of H2S to elemental sulfur. However, no SO2 formation was observed with the Mn-Cu-V mixed-oxide sorbent prepared in this work. This is an advantage in the H2S removal stage. Predictions of the breakthrough curves from the deactivation model (eq 10) also showed good agreement with the experimental data obtained with Mn-Cu-V mixed oxide. The initial reaction rate constant (k0) evaluated by a nonlinear regression analysis of the experimental data obtained with this sorbent was found to be much less than the reaction rate constant found for Mn-Cu sorbent. An interesting result obtained with

Figure 7. Comparison of sulfur retention capacities of Mn-Cu and Mn-Cu-V mixed-oxide sorbents in six successive sulfidation runs at 627 °C. Table 4. Rate Constants of the Deactivation Model for the Sorption of H2S on Mn-Cu-V Mixed-Oxide Sorbent at 627 °C in Successive Sulfidation Cycles sulfidation cycle

k0W/Q

Q (mL/min)

k0 × 10-3 (mL/min g)

kd (min-1)

1 2 3 4 5 6

2.43 3.48 2.98 3.67 4.34 4.43

278 281 272 263 266 275

3.4 4.8 4.1 4.8 5.8 6.1

0.19 0.29 0.33 0.42 0.44 0.57

this sorbent was the significant increase of the deactivation rate constant with an accompanying increase in the initial sorption rate constant as the sulfidationregeneration cycle number was increased (Table 4). This shows that the H2S sorption rate and also the deactivation rate increased with increasing sufidation cycle, resulting in sharper breakthrough curves. Also, a significant reduction was observed in the sulfur retention capacity with increasing number of sulfidationregeneration cycles (Figure 7). The BET surface area of this sorbent after the sulfidation-regeneration cycles was found to be less than the BET surface area of the fresh sorbent. It decreased from 57 to 20 m2/g after the sulfidation-regeneration cycles. This reduction in surface area of the Mn-Cu-V mixed oxide is the major reason for the decrease in sulfur retention capacity and also the increase in the deactivation rate constant (kd) after successive sulfidation-regeneration cycles. It was interesting to note that the breakthrough curves also became sharper, and the initial sorption rate constant k0 also increased as the number of regeneration cycles increased. Comparison of the XRD patterns of the fresh

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and regenerated (after six sulfidation-regeneration cycles) Mn-Cu-V mixed-oxide sorbents showed that the fresh sorbent was highly amorphous. However, the regenerated sorbent was more crystalline. XRD bands observed at 2θ values of 28.3°, 34.1°, and 37.9° indicated the presence of β-Cu2V2O7 phase. The presence of some CuMn2O4 and CuO was also observed from the XRD patterns. In our previous work,9 the reactivity of R-Cu2V2O7 was shown to be very high for the hightemperature sorption of H2S. This observation showing a more crystalline structure after the regeneration might be the reason for the increase in the value of the initial sorption rate constant after the successive sulfidation-regeneration cycles. These results indicate that this sorbent is less attractive than the Mn-Cu mixed oxide for the high-temperature removal of H2S. Concluding Remarks Between the two manganese-copper based mixed metal oxide sorbents prepared by the complexation method in this work, Mn-Cu mixed oxide showed higher activity and also higher stability than Mn-Cu-V mixed oxide in the successive sulfidation-regeneration runs carried out for H2S removal in a fixed-bed reactor. The sulfur retention capacity of the Mn-Cu mixed oxide was over 0.15 g of S/g of sorbent at 627 °C, and about 90% of this capacity was maintained after six successive sulfidation-regeneration cycles. Regeneration of the sulfided samples was achieved at 700 °C with a gas mixture containing 6% oxygen in nitrogen. The formation of some MnSO4 during the regeneration step did not cause a significant reduction in the activity of this sorbent in the successive cycles of H2S removal. The formation of some SO2 during the purging of the regenerated sorbent at 700 °C with pure nitrogen gas indicated some decomposition of sulfates formed during regeneration. The formation of some elemental sulfur during the regeneration step indicated partial oxidation of MnS at low oxygen partial pressures. Activity of the Mn-Cu mixed oxide for H2S removal in the presence of hydrogen was found to be higher than the activities of Cu-V and Cu-Mo mixed oxides and also pure CuO sorbent reported in the literature. A deactivation model was shown to give good predictions of the breakthrough curves obtained with these mixed-oxide sorbents prepared for high-temperature removal of H2S. Acknowledgment A TUBITAK-MISAG-201 grant is gratefully acknowledged. The contributions of Dr. Irfan Ar and Pinar Caglayan to the experimental work are also acknowledged. Nomenclature a ) activity of the sorbent CA ) concentration of reactant gas, mol/L CA0 ) inlet concentration of reactant gas, mol/L k0 ) initial rate constant, mL/g‚min kd ) deactivation rate constant, min-1 Q ) gas flow rate, mL/min t ) time, min W ) weight of the sorbent

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(2) Elseviers, W. F.; Verelst, H. Transition Metal Oxides for Hot Gas Desulphurisation. Fuel 1999, 78, 601. (3) Gasper-Galvin, L. D.; Atimtay, A. T.; Gupta, R. P. Zeolite Supported Metal Oxide Sorbents for Hot Gas Desulfurization. Ind. Eng. Chem. Res. 1998, 37, 415. (4) Abbasian, J.; Slimane, R. B. A Regenerable Copper Based Sorbent for H2S Removal from Coal Gases. Ind. Eng. Chem. Res. 1998, 37, 2775. (5) Slimane, R. B.; Abbasian J. Copper-based Sorbents for Coal Gas Desulfurization at Moderate Temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338. (6) Garcia, E.; Palacios, J. M.; Alonso, L.; Molnier, R. Performance of Mn and Cu Mixed Oxides as Regenerable Sorbents for Hot Coal Gas Desulfurization. Energy Fuels 2000, 14, 1296. (7) Garcia, E.; Cilleruelo, C.; Ibarra, J. V.; Pineda, M.; Palacios, J. M. Kinetic Study of High-Temperature Removal of H2S by Novel Metal Oxide Sorbents. Ind. Eng. Chem. Res. 1997, 36, 846. (8) Tamhankar, S. S.; Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos, M. Mixed Oxide Sorbents for HighTemperature Removal of Hydrogen Sulfide. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 429. (9) Yasyerli, S.; Dogu, G.; Ar, I.; Dogu, T. Activities of Copper Oxide and Cu-V and Cu-Mo Mixed Oxides for H2S Removal in the Presence and Absence of Hydrogen and Predictions of a Deactivation Model. Ind. Eng. Chem. Res. 2001, 40, 5206. (10) Li, Z.; Flytzani-Stephanopoulos, M. Cu-Cr-O and CuCe-O Regenerable Oxide Sorbents for Hot Gas Desulfurization. Ind. Eng. Chem. Res. 1997, 36, 187. (11) Yasyerli, S.; Dogu, G.; Ar, I.; Dogu, T. Breakthrough Analysis of H2S Removal on Cu-V-Mo, Cu-V and Cu-Mo Mixed Oxides. Chem. Eng. Commun. 2003, 190, 1055. (12) Yasyerli, S.; Dogu, G.; Ar, I.; Dogu, T. Dynamic Analysis of Removal and Selective Oxidation of H2S to Elemental Sulfur over Cu-V and Cu-V-Mo Mixed Oxides in a Fixed Bed Reactor. Chem. Eng. Sci. 2004, 59, 4001. (13) Slimane, R. B.; Hepworth, M. T. Desulfurization of Hot Coal-Derived Fuel Gases with Manganese-Based Sorbents. 1. Loading (Sulfidation) Tests. Energy and Fuels 1994, 8, 1175. (14) Yoon, Y. I.; Kim, M. W.; Yoon, Y. S.; Kim, S. H. A Kinetic Study on Medium Temperature Desulfurization Using a Natural Manganese Ore. Chem. Eng. Sci. 2003, 58, 2079. (15) Atimtay, A.; Gasper-Galvin, L. D.; Poston, J. A. Novel Supported Sorbent for Hot Gas Desulfurization. Environ. Sci. Technol. 1993, 7, 1295. (16) Alonso, L.; Palacios, J. M. A TEM and XRD Study of the Structural Changes Involved in Manganese Based Regenerable Sorbents for Hot Coal Gas Desulfurization. Chem. Mater. 2002, 14, 225. (17) Slimane, R. B.; Hepworth, M. T. Desulfurization of Hot Coal-Derived Fuel Gases with Manganase-Based Regenerable Sorbents. 2. Regeneration and Multicycle Tests. Energy Fuels 1994, 8, 1184. (18) Jinchang, Z.; Yanhui, W.; Runyu, Ma.; Diyong, Wu. A Study on Regeneration of Mn-Fe-Zn-O Supported upon Al2O3 Sorbents for Hot Gas Desulfurization. Fuel Process. Technol. 2003, 84, 217. (19) Marcilly, C.; Courty, P.; Delmon, B. Preparation of Highly Dispersed Mixed Oxides and Oxide Solid Solutions by Pyrolysis of Amorphous Organic Precursors. J. Am. Ceram. Soc. 1970, 53, 56. (20) Westmoreland, P. R.; Gibson, J. B.; Harrison, P. Comparative Kinetics of High-Temperature Reaction Between H2S and Selected Metal Oxides. Environ. Sci. Technol. 1977, 11, 488.

Received for review August 18, 2004 Revised manuscript received November 9, 2004 Accepted November 12, 2004 IE0492496