Ind. Eng. Chem. Res. 1993,32, 519-532
519
Reduction, Sulfidation, and Regeneration of Mixed Iron-Aluminum Oxide Sorbents Valerie Patrick+and George R. Gavalas' Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Pramod K. Sharma Jet Propulsion Laboratory, Pasadena, California 91 109
The reduction and sulfidation of Fe203 and Fe203-Al203 sorbents were investigated by thermogravimetric analysis (TGA). Mixed iron-aluminum oxides were reduced more slowly and by a more complex mechanism than pure iron oxide. Several phases that were responsible for this difference were identified by temperature-programmed reduction (TPR),X-ray diffraction (XRD),BET surface area (BET), and scanning electron microscopy (SEM). Sulfidation of reduced sorbents in the TGA a t 600 "C produced two crystalline phases: high-temperature, hexagonal pyrrhotite (Fel,S), and unreacted FeA1204. The reaction of the pure and mixed oxide with a mixture of H2S-H2-H20-N2 was studied in a tubular microreactor to evaluate the performance of these materials as hightemperature H2S sorbents. At 650 "C the mixed oxide yielded considerably lower prebreakthrough outlet H2S levels than the pure iron oxide. Regeneration in pure SO2 and SOZ-air mixtures of sulfided samples resulted in complete conversion of iron sulfide to iron oxide and quantitative recovery of elemental sulfur. The reactions of hydrogen sulfide with various metal oxides have been investigated extensively in connection with the desulfurization of coal gas to be used in advanced power generation cycles. Although hydrogen sulfide removal can be carried out at ambient temperatures by established liquid absorption processes, desulfurization by reaction with metal oxides at high temperatures can improve considerably the economics of power generation. Early research on high-temperature desulfurization concentrated on the evaluation of sulfidation equilibrium and kinetics of various pure oxides. Sflidation equilibrium data have been summarized in a paper by Westmoreland and Harrison (1976)and in a report (Morgantown Energy Research Center, 1978). Among various oxides, zinc oxide has the highest equilibrium constant for sulfidation and has been singled out as the sorbent of choice for desulfurization of coal gas down to a few parts per million of H2S as required, for example, for use in molten carbonate fuel cells. Recent research and development has focused on the binary oxides zinc ferrite (ZnO-FezOs)and zinc titanate (ZnOaTiOz). Zinc ferrite has sflidation equilibriacomparable to that of zinc oxide, but better capacity and regenerability (Grindley and Steinfeld, 1981;Tamhankar et al. 1986). At temperatures above about 650 OC, however, both zinc oxide and zinc ferrite in contact with a reducing gas generate elemental zinc vapors and suffer gradual loss of zinc. Zinc titanate has been found superior to zinc ferrite in terms of zinc loss and regenerability (Lew et al., 1989,1992;Siriwrdane and Poston, 1990). When somewhat higher H2S concentrations can be tolerated in the fuel gas, several other oxides may be considered. Copper oxide has sulfidation equilibrium less favorable than zinc oxide but does not suffer loss by reduction and volatilization as zinc oxide does. Even less favorable sflidation equilibria are afforded by calcium oxide and iron oxide. Because of their low cost, calcium oxide and the related calcium carbonate and dolomite were early favorites (Ruth et al., 1972;Freund, 1984;Borgwardt
* To whom correspondence should be addressed. t
Present address: Monsanto Company, Springfield, MA 01152. 0888-588519312632-0519$04.00/0
and Roache, 1984). These sorbents could be used once through or could be regnerated by H20-CO2 or by 0 2 to the carbonate or oxide forms (Kearns et al., 1976;Chou and Li, 1984;Sommer et al., 1984). The regeneration offgas is a relatively dilute stream containing H2S or SO2 that must be processed further for final sulfur disposal or conversion to elemental sulfur or sulfuric acid. Iron oxide is also relatively inexpensive and has been emphasized in early studies. Previous work with iron oxide includes kinetic studies of reduction, sulfidation, and regeneration by Tamhankar et al. (1981,1985)and Tseng et al. (1981). Because of their cost, all oxides other than calcium oxide must be used as regenerable sorbents. In the case of zinc oxide and copper oxide regeneration requires reaction with steam and air mixtures resulting in an yoffgas" dilute in SO2 and H S . Final disposition of this offgas either by conversion to sulfuric acid or otherwise adds considerably to the cost of desulfurization. For iron oxide, however, regeneration can also be carried out by reaction with S02, producing elemental sulfur and not requiring any additional sulfur recovery or disposal (Schrodt and Best, 1978; Joshi et al., 1979). This alternative route to regeneration and the high rates of sulfidation make iron oxide an interesting sorbent, although its sulfidation equilibrium is not sufficient for the advanced gasification-power generation processes under development. In sflidation of iron oxide, copper oxide, and other transition metal oxides, reduction to lower oxidation states proceeds simultaneously with sulfidation, complicating the kinetics and equilibria of the reaction. Since sulfidation becomes less favorable with decreasing oxidation state, the outlet H2S level initially governed by the higher oxide jumps to a higher level once the higher oxide is reduced to a lower oxide or the element. Tamhankar et al. (1986)found that copper oxide prepared by a complexation technique as a mixed oxide with alumina yields considerably lower outlet H2S concentrations than pure CuO in packed bed operation. The mixed oxide undergoes slower reduction to elemental copper due to interaction with alumina; therefore, H2S removal is governed for a longer period by reaction with surviving Cu+,which has 0 1993 American Chemical Society
520 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 Table I. Composition, Preparation Conditions, Surface Area, and Phase Content of Various Sorbents
phases present4 sorbent code Fe:Al (molar ratio) calcination temp ( O C ) l:o 650 F650 600 1:l FA600 650 1:l FA650 700 1:l FA700a 700 1:l FA700b 800 1:l FA800 600 1:2 F2A600 700 1:2 F2A700 800 1:2 F2A800 600 21 2FA600 700 2: 1 2FA700 800 2: 1 2FA800 4
calcination time (h) BET surface area (m2/g) 4 2.2 8 92 NA 4 57 8 54 16 19 8 8 43 16 89 8 57 10.5 80 8 39 8 15
Fez03 large small large large large large none small large medium major major
Fe304 none large medium large medium medium none medium trace large medium small
FeAlzOl none none small medium small trace none large medium small trace none
Relative amount within the total crystalline material.
a more favorable sulfidation equilibrium than that of elemental copper. The role of alumina in the reduction and sulfidation of copper oxide was subsequently investigated in more detail by Patrick et al. (1989) and Patrick and Gavalas (1990). Iron oxide undergoes a similar reduction-sulfidation sequence. In its interaction with coal gas containing H2 and H20, as well as HzS, the relevant reactions are 3Fe20, + H, = 2Fe304+ H,O
+ H, = 3Fe0 + H,O Fe,03 + 2H,S + H, = 2FeS + 3H20 Fe304+ 3H,S + H, = 2FeS + 4H20 FeO + H,S = FeS + H,O Fe30,
(1)
(2)
(3) (4) (5)
As will be seen later, for a mixture typical of coal gas at 650 "C, the equilibrium H2S concentration is approximately 6 times higher and 10 times higher from reactions 4 and 5 than for reaction 3. The purpose of the present investigation was to examine if association with alumina improves the sulfidation properties of iron oxide, as it does for copper oxide, while preserving the potential for elemental sulfur recovery during regeneration. Addition of a second oxide increases the number of phases possible during reduction, sulfidation, and regeneration. Much of the experimental effort, therefore, was concerned with identification of the phases present at different stages of the reaction cycle, and with the reactivity of these phases toward the reduction and sulfidation. Experimental Section 1. Sorbent Preparation. A complexation technique known as the citrate method (Marcilly et al., 1970) was used to synthesize the mixed-oxide sorbents in a highly dispersed state. The complexing agent (citric acid) was added to an aqueous solution of metal nitrates of the desired stoichiometriccomposition. The amount of citrate in the mixture was equimolar to the total metal cations. An amorphous citrate precursor was prepared by evaporation of this solution. A sufficiently small volume of solution was used for each batch to obtain sufficiently rapid evaporation and obtain a well-dispersed product. Evaporation proceeded first at atmospheric pressure and 70 "C until a marked increase in viscosity of the solution was observed,and then for severalhours (4-16) in a vacuum oven at 70 "C until an amorphous solid foam formed. The
Table 11. Major Phases Detected in the Sorbent Samples
formula Fez03 Fe& Fel-,O FeA1zO4
name hematite magnetite wustite hercynite
formula y-AlzO3 Fe(Fe1_,Al,)204 Fel-,S
name y-alumina spinel solid solution pyrrhotite
foam was carefully broken up and calcined at the desired temperature between 600 and 800 "C in an air stream to produce the final mixed oxide. A total of 11samples were prepared at calcination temperatures of 600-800 "C, calcination times of 4-16 h, and molar ratios of Fe3+ to A13+ of 0.5 to 2. These samples will be referred to below as mixed oxides. A sample without aluminum was also prepared as a reference. The compositionan calcinationtreatment of the various sorbents are given in Table I. The sample code used is such that 2FA700 denotes a sorbent of stoichiometry 2Fe203Al203 prepared by calcination at 700 "C, etc. 2. X-Ray Diffraction. A Norelco intermittent diffractometer employing Ni-filtered Cu(Ka) radiation (1.540 56 A) was used for qualitative chemical analysis of the polycrystalline components present in a sample. The X-ray tube was operated at 45 kV and 20 mA. X-ray powder diffraction patterns were obtained in a 28 range of 25O-70" which enabled detection of various phases the most abundant of which are listed in Table 11. Samples were finely ground and slurried in acetone with a small amount of Ducocementas binder. The slurry was carefully packed into the 11-X 5- X 1-mm depression of a Bakelite sample holder so as to avoid preferred orientations. Spectra were scanned at 0.1" intervals (in 28) for 60 s per interval. 3. Scanning Electron Microscopy. Samples were examined by an ETEC Corporation electron microscope operating at 20 kV with resolution of 70 A. Each sample was carefully ground, sprinkled on a metal stub containing a light coat of silver paste, and coated with a goldpalladium film 100 A thick prior to observation. 4. Thermogravimetric Analysis. A Du Pont 951 thermogravimetricanalyzer interfaced through an analogto-digital converter to a microcomputerserved to measure the sample weight continuously. The quartz housing and flow path of the TGA were modified so that the system could accommodate corrosive gases such as H S (Ruth et al., 1972). In addition, a quartz sample pan was used instead of the standard platinum sample pan to eliminate sulfur chemisorption on platinum. A gas flow of 80 cm3/ min was used for both the protective N2 backflow and the feed. A 5% HZin NZmixture was used for reduction runs, a 4.2% HzS in N2 mixture was used for sulfidation runs, and air was used for regeneration runs. A temperature
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 621 1.1 1 0.9 0.8 0.7
0.6
3z
0.5
0.4 0.3 0.2
0.1 5 5 25
20
0. 0.
70
140
210
280
60
350
29 20 Time ( m i d Figure 1. Normalized weight-loss profiles for TPR of F650 (- - -) and FA650 (-) at 1.7 "C/min from 233 to 912 "C, accompanied by XRD patterns of partially reduced samples after quenching to room temperature during the TPR experiments at the positions indicated (a, b, C, and d).
FA
"0
250
500
Time (min) Time (min) Figure 2. Reaction rate versus time for TPR of Fe-A14 sorbents calcined at 600
programmer enabled either isothermal operation or operation under a linear temperature profile. Typically, a 45-mg sampleof particles, -120/+170 mesh, was employed. It was verified experimentally that for the range of flow rates and sample sizes employed the reactions were free of internal and external mass-transfer effects. 5. Packed-Bed-MicroreactorExperiments. The reactor system, described here briefly, has been described in detail previously (Tamhankar et al., 1986). The reactor consists of a quartz tube of l-cm i.d. and 41-cm length loaded to a bed height of 4-6 cm with a mixture of sorbent granules (-20/+35 mesh) and inert alumina particles of low surfacearea (Alcoa T-64, -28/+48 mesh). The sorbent bed was supported by a fritted quartz disk on one end and packed in with quartz wool on the opposite end. The reactor tube was mounted vertically inside an electric furnace, and the bed temperature was monitored by a K-type thermocouple moving inside a quartz thermowell (0.3-cm i.d.) concentric to the reactor tube. Different gases
-
(- -), 700 (-),
Time (min) and 800 "C (-
- -).
from cylinders passed through purifiers and calibrated flowmeters into a common gas line. The desired gas mixture flowed either upward (sulfidation) or downward (regeneration)through the sorbent bed. The lines leading to the reactor tube were insulated and heated. Nitrogen bubbling through water maintained at a constant temperature in a three-neck flask assembly was used to introduce known amounts of water vapor into the feed gas stream. Temperatures at various locations in the reactor system were monitored by K-type thermocouples connected to a multichannel digital readout. The reactor pressure in all cases was slightly above atmospheric. In a typical sulfidation run, fresh or sulfur-free sorbent was first exposed to a feed gas containing 10-20% Hz, 20-25% HzO,l% H2S, and the balance N2, at a constant temperature of 600 or 650 "C,and then the sulfidedsorbent was regenerated using N2-air or steam-air mixtures at a temperature of 650 or 750 "C. Feed gas rates of about 200 cm3/min (STP) were typically used and the gas hourly
522 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 Table 111. Conversion, Rate, and Temperature at Successive Peaks of TPR first peak second peak -(dW~/dt)lO~ -(d WN/dt)IO3 sample 1 - WN( % ) (min-') T('C) 1-wN(%) (min-1) F650 7 32 371 66 94 21 386 38 16 FA650 9 11 55 344 48 48 2FA600 2FA700 8 55 352 42 44 2FA800 7 42 396 52 39 FA600 15 37 338 56 33 FA700a 11 59 345 43 39 FA800 11 35 396 52 38 F2A600 18 49 371 F2A700 20 49 357 42 23 F2A800 11 62 349 40 25
space velocity was about 2000 h-' (STP) in most tests. In addition, a few independent experiments were performed with regeneration of sulfided sorbents in pure SO:! and SO2-air mixtures at 700 "C. The experiments conducted in the TGA and packedbed microreactor each have distinct advantages and disadvantages and provide complementary information. The packed-bed-reactor experiments are suitable for determining overallsorbent performance under conditions similar to those of an industrial reactor. These experiments can provide information about sulfur removal efficiency and can generate large samples for solid analysis. They are not well suited, however, to intrinsic kinetic studies because of the inherent gradients of gas and solid compositions along the reactor. TGA runs, on the other hand, entail uniform gas and solid compositions and are better suited for kinetic investigations. However,the small sample size that must be used in order to avoid masstransfer limitations is often insufficient for certain analytical procedures.
Results and Discussion 1. XRD and TPR of Sorbents. Table I summarizes the preparation conditions and the crystalline phases identified by XRD for these samples. The XRD patterns of mixed iron-aluminum oxides prepared with different Fe:A1 ratios and different calcination temperatures reveal a complex structure includinga large amount of amorphous phases which could have been formed in the rapid cooling following sample calcination. This amorphous material produces weak X-ray signals providing only partial information about the phases present. Nevertheless, the X-ray patterns indicate that both crystallinity and Fez03 content increase with calcination temperature regardless of the content of alumina. Moreover, the detection of Fe304 and FeAl204in fresh samplessuggeststhe hindering of complete oxidation of iron to the +3 state during calcination. For fixed calcination time and temperature the content of Fe3+decreases with increasing content of alumina. This is immediately evident from a comparison of samplesF650 and FA650. The pure oxide sorbent consists exclusively of hematite while the mixed sorbent contains a significant amount of magnetite and a lesser amount of hercynite, in addition to the principal component,hematite. Assuming that the sorbent preparation steps preceding calcination result in the same mix of oxidation states, the differences after calcination must be attributed to a retardation by alumina of the oxidation of Fe2+to Fe3+. Whether this retardation is due to encapsulation of iron oxidecrystallites by nonporous alumina or to a chemical effect cannot be inferred from these results alone.
T('C) 559 559 537 537 638 616 530 634 507 551
l-WN(%) 80 88 87 81 79 87 85 81 85 74
third peak -(dW~/dt)lO~ (min-l) T ('C) 12 22
28 34 27 28 31 40 37 38
804 768 797 813 767 808 843 859 865
794
Temperature-programmed reduction (TPR) is a useful technique for characterizing the reducible species. Pure iron oxide should exhibit a characteristicTPR profilewhile alumina is inert to reduction. Differences between the TPR profile of a mixed Fe-A1 oxide and pure iron oxide are indicative of interactions between iron oxide and alumina including formation of new phases or formation of interfacial species with modified properties. Figure 1 shows the normalized weight, WN,of samples F650 and FA650 during TPR. The normalized weight is defined by
W,=-
m-mf
mo - mf where m = instantaneous mass, mo = initial mass, and mf = final mass at complete reduction to FeO. We first observe that F650 is reduced more rapidly than FA650. With the two samples prepared under identical calcination conditions, the slower reduction of FA650 must be due to an interaction between iron oxide and aluminum oxide. An interaction between hematite and alumina has been observed in Fe203/A1203 materials prepared by impregnation (Ying-Ru et al., 1981; Lycourghiotis and Vattis, 1981). The TPR curve for F650 in Figure 1 shows an abrupt change in slope at WN = 0.89 which is consistent with a two-stage reduction of Fez03 according to
Fe,04(s) + 4H2(g)= 3Fe(s) + 4H20(g)
(8)
where reaction 7 accounts for weight loss from W N= 1to 0.89 and reaction 8 accounts for weight loss from WN = 0.89 to 0, assuming that the reactions occur consecutively. Samples of F650 partially reduced by TPR and quenched to room temperature were analyzed by XRD, confirming this reaction path with no detection of wustite as an intermediate (Figure 1).This consecutivereaction scheme is also consistent with a recent investigation of hematite reduction by transmission electron microscopy (Rau et al., 1987). The TPR trace for FA650 in Figure 1 indicates three stages of reduction, W N= 1-0.83, W N = 0.83-0.43, and WN = 0.43-0. However, the third stage was too slow to be followed to completion. At point c the FA650 sample contains magnetite (Fe304) and smaller amounts of hercynite (FeA1204) with very little, if any, hematite. Therefore, the first stage of reduction involves primarily reduction of hematite to magnetite (reaction 4). A t point d the content of hercynite has increased at the cost of magnetite but no elemental iron has appeared; therefore, the second stage of reduction involvesthe transformation
Ind. Eng. Chem. Res., Vol. 32,No. 3, 1993 523
Fe,04 + 3A1,0, + H,= 3FeA1,04 + H,O (9) The absence of elemental iron at point d in contrast to its presence at point b suggests that interaction with alumina retards the reduction of FeaO4 aa well as provides for transformation 5 to occur. The final stage in the TPR of FA650 involves reduction of hercynite to elemental iron according to FeA1,04 + H, = Fe + N,O, + H,O (10) A closer examination of the XRD patterns for FA650 in Figure 1 indicates that the phase structure may be somewhat more complicated than described above. The top pattern contains peaks for the twospinel phasesFe304 and FeA1204,but the bottom pattern contains a pattern for a single spinel phase which may be a solid solution of Fez04 and FeA1204. The peaks in the top pattern correspond to lattice parameters of about 8.32 A for Fes04 andabout 7.96AforFeAl2O4,whilethepeaksinthe bottom pattern correspond to a lattice parameter of about 8.15 A, which lies between those of the individual phases. Figure 2 shows the normalized rate of TPR of three mixed oxides (FA, FZA, 2FA) calcined at different tem-
peratures. As noted above in connection with sample FA650, reduction takes place in three stages (with the exception of sample F2A600). Each stage is characterized by a peak, i.e., a local maximum in the reduction rate. Table 111 lists the conversion, temperature, and rate of reduction at the peak maximum of each stage. The TPR profile of FA600 resembles that of FA650, for which a spinel solid solution between magnetite and hercynite was proposed to form during the second stage of reduction. The second and third peaks in the rate profiles of Figure 2 are broad and overlapping, perhaps reflecting the simultaneous reduction of FesO4 and this spinel solid solution. Similarly, the temperature at which the reduction rate is a maximum during the third stage (i.e., third peak) is the same for samples FA600 and 2FA600, implying that a solid solution between Fez04 and FeAlzO4 also forms in sample 2FA600. In contrast, the temperature at which the reduction rate becomes maximum during the second stage (i.e., second peak) is lower for sample 2FA600 than for FA600 and the maximum rate of reduction in the first stage (Le., first peak) is higher for sample 2FA600 than for FA600. Both these observations suggest a more facile
524 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 18
15
12
k
9
6
3
0.
0.8 0.
20
60
40
0.
25
50
Time (min)
Time ( m i d
Figure 4. Normalized weight-lossand reaction rate for the reaction of FA650 (-) and F650 (- - -) at 700 O C with a mixture of 13% Hz, 4% HzS-Nz, followed by regeneration in air.
a.
..-..----
sorbent F650 FA600 FA700a FA700b FA800 F2A600 F2A700 F2A800 2FA600 2FA700 2FA800
1.4
1.2
5"
1
Feo.& 88 87 89 85 52 75 75 100 92 85
case 1 FeAl204 12 13 11 15 48 25 25 0 8 15
case 2 FeomS Fez03 93 81 80 82 76 20 58 59 100 87 76
7 9 10 8 24 80 42 41 0 13 24
% molar.
0.8
0.6 0
Table IV. Estimated Yields. to Different Products following Consecutive Reduction and Sulfidation at 600 OC
10
20
30
40
J
50
Time (min)
Figure 5. Normalized weight-loss for reduction followed by sulfidation at 600 "Cof 2FA600 (a),F650 (b),FA600 (c),and F2A600 (d).
reducibility of iron-containing phases in the early stages of reduction for 2FA600 compared to FA600. In contrast to the TPR behavior of sample FA650, the reaction rate profile of F2A600 has no discernible second peak and has a third peak which is distinct and isolated. On the basis of SEM analysis and BET measurements, F2A600 has been postulated to contain a large fraction of FeAl204. In fact, assuming that the fresh F2A600contains only Fe203, FeAl204, and A1203 and that the first peak corresponds to reduction of Fez03 while the second peak corresponds to reduction of FeA1204, then a conversion of 0.51 at the onset of the second peak (Table 111)translates into a content of 95 mol ?6 FeA1204 in fresh F2A600. Consistent with the assumption that the third peak characterizes the reduction of FeA1204is the observation
that the temperature at which the reduction rate is a maximum for this peak is notably higher than for other samples prepared by calcination at 600 OC. Specifically, the temperature for which the reduction rate is a maximum during the final stage for F2A600 is 860 versus 770 "C for 2FA600 and FA600. This difference in temperatures suggests the greater reducibility of a magnetite-hercynite solid solution as compared to pure hercynite. The presence of Fe3Or and FeAl204in fresh sampleshas been attributed earlier to incomplete oxidation of iron to +3 during calcination. For example, while 650 "C is a high enough temperature for complete oxidation of iron to the +3 state in the absence of alumina, the presence of only a 1:2 molar ratio of aluminum to iron results in incomplete oxidation of iron to +3 at 800 OC. It has been found, however, that oxidizing samples FA600, FA700a, FA700b, 2FA600, and 2FA700 in air at 912 "C following TPR produces Fez03 as the only crystalline phase according to XRD analysis. Even under these harsh oxidation conditions, F2A700 with a 2:l molar ratio of aluminum to iron contained a small amount of FeAl204 in addition to Fe2O3. Therefore, the presence of alumina has a profound effect on oxidation as well as on reduction reactions.
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 626 coarsely textured surface for F650 at one extreme and a smooth, untextured surface for F2A600 at the other extreme. The surface of samples FA600 and 2FA600 displays a granular texture, which could be construed as a combination of the two extreme textures, smooth and coarse. In fact, the coarseness of the surface increases with increasing iron content. These SEM results are analogous to those for the Cu-Al-0 system, in which a granular surface was detected in scanning electron micrographs when copper was present as a separate phase from alumina (e.g., as CuO or Cu) but a smooth surface was detected when copper was present as a compound withAl203 (e.g., CuAl204) (Patricket al., 1989). Similarly, the SEM results indicate that F2A600 contains predominantly FeAl204, while FA600 and 2FA600 contain mostly a mixture of oxides (e.g., Fe203, Fe304, and Al203). The presence of FeAl204 in F2A600 was not confiied by XRD, suggesting that this phase is microcrystalline, or amorphous, or a combination of the two. 3. Isothermal Reduction and Sulfidation of Sor-
bents in the TGA. a. SimultaneousReduction and Sulfidation. In the foregoing sections we have pointed
0.5
0.6
0.7
0.0
0.9
W N (reduction) Figure 6. Sulfur uptake versus extent of reduction for different Fe-Al-O samples. Fe:Al = 0.5 (0); 1 (A); 2 temperature is 600 (a), 700 (b), and 800 (c).
(v). Calcination
2. BET Surface Area and SEM of Sorbents. Returning to Table I we note that the BET surface area decreases with decreasing AkFe ratio and increasing temperature. In sampleswith Fe:Al molar ratios of 1and 2, the BET surface area decreases with increasing calcination temperature and increasing Fe:Al ratio (Table I). Analogous to Cu-A1-0 materials (Patrick and Gavalas, 1990), the high values of overall surface area reflect the dominant contribution from unassociated alumina. The surface area of F2A600, however, is lower than that of samples prepared with either the same Fe:Alratio at higher calcination temperatures, or with higher Fe:Al ratio at the same calcination temperature. The implication is that F2A600 contains a higher fraction of FeAl204having lower surface area than a simple mixture of Fez03 and A l 2 0 3 . The formation of this compound oxide is expected to be lees extensive in samples containing higher Fe:Al ratios because of the departure from the stoichiometric composition (1:2 molar ratio of Fe:Al). In addition, according to XRD, the formation of the compound oxide appears to be lese extensive at higher temperatures. This temperature dependency suggests that at temperatures greater than about 600 "C there is a trade-off between enhanced kinetics from the temperature dependenceof the reaction rate and hindered kinetics from diffusional limitations caused by the temperature dependence of sintering. In the analogous Cu-Al oxide system, this trade-off was not realized for temperaturea up to 900 OC. The difference between mixed Fe-Al and mixed Cu-AI oxides might be due to differences in the sintering behavior of iron versus copper. SEM analysis also indicates a pronounced difference between F2A600 and the other samples. For example, the scanning electron micrographs in Figure 3 reveal a
out that mixed iron-aluminum oxides have lower rates of reduction than pure iron oxide and have identified possible mechanisms for the retarded rates of reduction. In this section, we shall consider the kinetics of simultaneous reduction and sflidation of iron oxide and mixed ironaluminum oxides in light of the previous results. Simultaneousreduction-sulfidation of iron oxide, F650, and a mixed iron-aluminum oxide, FA650, was carried out for about 35 min with a mixture of 13% H2 and 3.6% H2S in N2 at 700 OC with subsequent regeneration in air. Figure 4 shows the normalized weight, W Ndefined by eq 6, during this treatment. The weight first undergoes a decrease because, initially, reduction is faster than sulfidation, but as the rate of reduction slows down, the rate of sulfidation dominates and the weight increases. Comparison of the curves for FA650 and F650 shows that the mixed oxide FA650 reacts more rapidly. Recalling from the previous section that in sample FA650 Fe2+species survive longer during reduction, it is possible that the faster sulfidation of FA650 is due to a faster sulfidation of Fe2+ species than FeO, either due to higher reactivity or due to better dispersion. Dispersion in FA650 is expected to be higher both due to initial sample preparation and due to the subsequentslower sintering of oxide phases, compared to the elemental iron phase. XRD analysis of sulfided FA650, obtained from a separate run in which the sample was quenched to room temperature under the reactant gas without exposure to air, identified monoclinic 4C-type pyrrhotite as the only crystalline phase. This phase is a superstructure of an NiAs-type subcell with alternating rows of ordered iron vacancies (Morimotoet al., 1970,1974,1975;Desborough and Carpenter, 1965; Mukherjee, 1969; Nakazawa et al., 1975)and is one of the many reported phases of pyrrhotite, Fel-,S, stable at low temperatures. Naturally occurring monoclinic 4C-type pyrrhotite is often described as having a composition corresponding to x = 0.125 (Le., Fe&; however, synthetic 4C-type pyrrhotites have been reported to have compositions with 0.115 < r < 0.134 (Kissin and Scott, 1982; Carpenter and Desborough, 1964; Putnis, 1975). It is well-known that monoclinic4C-type pyrrhotite arises from cooling high-temperature, hexagonal, 1C-type pyrrhotite, which contains a disordered array of iron vacancies and has a large composition range. The value W N= 1.35 followingsflidation of FA650 shows incomplete sflidation of iron to Fe1,S ( x 2 0).
626 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993
Figure 1. Scanning electron micmnaphs at 8500X (markers are 1 g m ) of 2FA600 (a), F A 6 N (b), and F2AW (c) each reduced at 600 "Cfor 15 min.
b. Consecutive Reduction-Sulfidation. In any application of these sorbents to the removal of H2S from a fuel gas, reduction and sulfidation would occur simultaneously as in the experiments described in the previous section. For the purpose of estimating the reactivity of different phases and measuring reaction rates, however, it is useful to separate the two reactions. Therefore, the remaining sorbents were exposed to 5% HZin N Pfor about 8 min, then to 4.2% H2S in Nz for about 35 min, and finally to 4.2 % HzSin Nz for the remaining time necessary to cool the sample. Typical weight curves for isothermal reduction and sulfidation of mixed iron-aluminum oxides at 600 O C are showninFigure5. Resultsfor 10ofthealumina-containing samples are summarized in Figure 6 by plotting WN followinga 50-minsulfdationversus WNfollowingan&l& min reduction. A good correlation is observed between extent of reduction (1- WN) and sulfur uptake with the exceptionof samplegF2.48lXl,FA800, 2FA700,and 2FA800, which display lower sulfur uptakes. The lower sulfur uptake in samples F2A800, FA800, 2FA700, and 2FA800 is probably due to the presence of
sintered phases either formed during samplepreparation, as the result of poor dispersion of iron-containing phases, or generated during reduction. During sample preparation,aluminaislikelytoactasadispersingagenthindering the sintering of iron. A similar role for alumina was encounteredin Cu@AlzO3 samplesin which the presence of A1203 decreased the sinteringof copper during oxidation (Patrick and Gavalas, 1990). Evidently, calcination at 800 "C results in extensive sintering during preparation of all alumina-containing samples studied (Fe:Al ratios of between 0.5 and 2), calcination at 700 O C promotes sintering only in samples with an Fe:A1 ratio of 2, and calcination at 600 "C does not promote sintering of any of the samples studied here. It is likely that both iron-containing and aluminum-containing phases sinter during calcination of samples when the two components are poorly dispersed. As mentioned earlier, XRD analysis of quenched sulfided samples revealed monoclinic 4C-type pyrrhotite which resulted from quenching the high-temperature 1Ctype pyrrhotite phase. Low-intensity broad peaks were also detected for an FeA1204 phase in most cases. This observation suggests that FeAlzO, is resistant to sulfida-
Ind. Eng. Chem. Res., Vol. 32,No. 3,1993 527
Figure 8. Scanning electron micrographs at 8500X (markers are 1 p m ) of 2FA600 (a), FA600 (b), and F2A600 (0each sulfided at 600 O C for 35 min.
tion, Smiting the uptake of sulfur. In corroboration of this eonclusion.sampleF2A600,reported earliertocontain a significantfraction of FeAlzO4 according to TPR, SEM, and BET data, exhibited the lowest sulfur uptake and the highest crystalline FeAlP04content according to XRD analysis of sulfiided samples. A similar phenomenon was observed in the sulfiidation of CuO-Alz03 sorbents where the compound CuA1204 was found to have the lowest sulfidation rates (Patrick et al., 1989). Furthermore, in adifferentstudy, CoAlz04was reported to be unsulfidable at 400 O C (Chung and Massoth, 1980). There appearsto be a correlation between sulfur uptake and sample stoichiometry. For example,samples with an Fe:Al ratio of 2 exhibit the highest sulfur uptake because this stoichiometry yields an insignificant amount of FeAl204. Samples with an Fe:A1 ratio of 1 contain a moderate amount of FeAlzOaand exhibit moderate sulfur uptake. Finally, samples containing an Fe:A1 ratio of 0.5 exhibit the lowest sulfur uptakes in view of the stoichiometric Fe:Al ratio favoring FeAl204 formation. Figure 5 shows the isothermal reduction at 600 "C followed by sulfidation at the same temperature of alumina-containingsamples prepared by calcination at
600 OC (FA600, F2A600, and 2FA600) and that of iron oxide prepared using calcination at 650 OC (F650). Sulfidation is slowest for F650, probably because of the presence of sinteredFez03orFe~Oainthereducedsorbent. Assuming sulfidation of all iron present in 2FA600 to Fel-3, the sulfur uptake corresponds to x = 0.120. This x-value is within the range 0 < x < 0.145 reported for high-temperature, hexagonal, IC-type pyrrhotite at 600 o C ( T o u l m i n a n d B ~ n1964)andwithintherange0.115 , < x < 0.134 reported for synthesized low-temperature, monoclinic, 4C-type pyrrhotite. Iron sulfide conversions following reduction and sulfiidation at a totalelapsed time of 50 min at 600 OC are estimated for all samples studied in Table IV, assuming that the products are Fen03 and the Feo.aS in one case or FeAlzO4 and Feo.aS in the other case. Actual iron sulfide yields are probably closer to the latter figure for sorbents with Fe:Al ratio of 0.5, closer to the former figure for sorbents with Fe:Al ratio of 2, and intermediate for sorbents witb Fe:Al ratio of 1. C. SEM of Reduced and Sulfided Samples. Scanning electron micrographs of samples reduced at 600 O C (15 min by 5 % HPin NP)showed a number of interesting features. For example, reduced samples of 2FA600 and
528 Ind. Eng. Chem. Res., Vol. 32, No.3, 1993
Figurn 9. Scanning electron micrographs of F2A600 (markers are 1rm) as prepared (a), after reduction at 600 "C (b), and after ntepwine reductionaulfidatian at 600 "C (4.Micrographs a and b are at 170OX; micrograph c is at 4250X.
FA600 looked very similar (Figure 7), both displaying a bimodal distribution of oddly shaped surface clumps. The particles at either extreme of the bimodal size distribution were notably different; those of sample FA600 were far less dispersed and had a broader particle size distribution than those of 2FA600. On the other hand, the reduced sampleof F2A600 displayed faceted crystals of hexagonal shape, which were presumably elemental iron. This same crystal morphology has been reported for the reduction of FeA1204with H Pat 911 "C (Gaballah et al., 1976). When reduced samples (8 min at 600 "C under 5% HZ in Nz)were sulfided (35 min by 4.2% HB in NP)at 600 OC, SEM revealed irregularly-shaped faceted crystals for both 2FA600and FA600 (Figure8). In the case of FZA600, however, theappearanceof thesurfacewasquitedifferent. Instead of smooth, faceted crystals, sulfided F2A600 had the appearance of a smooth surface with superimposed granular textured particles. The smooth surface could be that of unreacted FeAI2O4 with the textured particles belonging to a mixture of FeAI104and pyrrhotite. XRD identified a significant amount of both of these components, which together comprised the entire crystalline
phase. It should be noted that these specimens were obtained by quenching to room temperature; therefore, the observed surface morphology may be different from that of the material formed at high temperatures. The succession of scanning electron micrographs nummarized in each of Figures 9 and 10 show the different structural changes that occur upon reduction and sulfidation of an iron-rich sorbent (2FA600, Figure 10)versus an alumina-rich sorbent (F2A600, Figure 9). While the alumina-rich sample progresses from a smooth surface in ita fresh state to textured, faceted crystals in its sulfided state, the iron-rich sample progresses from a textured surface in ita fresh state to smooth, faceted crystals in ita sulfided state. The difference in surface moruholoeies accompanying reduction and sulfidation of chese 'iwo samplesis not surprising becausedifferentreactivespeciea are involved FeA1204in sample F2A600 and Fez03 and Fe,04 in sample 2FA600. 4. Isothermal Reduction and Sulfidation of Sorbents in the Microreactor. The overall performance of the sorbents with respect to HzS removal and regenerability was tested in a packed bed microreactor. A series
Ind. Eng. Chem. Res., Vol. 32, No. 3,1993 629
Figure 10. Scanning electron micrographs of 2FA600 at 4250X (markersare 1 pm) as prepared (a), after reduction at 600 O C (b), and after stepwise reductionaulfidation at 600 "C (e).
of sflidation-regeneration cycles was conducted using sorbent FA650. Sflidation was carried out with a gas mixture containing 1% HzS, 20% H20, and balance Nz at 600 or 650 OC. After each sulfidation run regeneration was carried out using either air-Nz or air-Hz0 mixtures at 650 or 750 OC. The results of these tests are shown in Figure 11 as a plot of outlet HzS concentration in parts per million (pprn), versus normalized time t/t*,where t* is the time that would be required for complete conversion of iron to FeS. To establish a point of reference for these results, equilibrium data on the sulfidation of iron oxides are compiled in Table V. For sulfidation cycles C-I, C-2, C-4, and C-5 (Figure ll),which were carried out at 600 OC, the outlet HzS levels were below 80 ppm before breakthrough which occurred at sorbent conversions of 4&60%. The relatively low sorbent conversion at breakthrough is evidently due to the slow sulfidation of FeAlzO4 which is the intermediate product in the reduction of Fe2O3. Thus breakthrough can be associated with the disappearanceof Fez03. Before breakthrough, the outlet HzSlevel is controlled by Fez03 (eq 3 of Table V), after breakthroughby FeAlzO4 or Fe304.
OutletHzSlevels below 100ppm up to sorbent conversions of at least 40% were also observed in cycles C-6 and C-7 conducted at 650 O C . These levels also correspond to the equilibrium of reaction 3. Figure 12 presents similar results obtained using a different batchof sorbent (FA700)prepared with the same Fe:A1 ratio but higher calcination temperature. The sulfidation curves show an unusual behavior before breakthrough. The differencesbetweenthe breakthrough curves of Figures 11and 12 are most likely the result of unintended variations in the vacuum evaporation step in the citrate technique, as well as the different calcination temperature. As an additional verification of the role of alumina in slowing down the reduction of FezOs, a control sulfidation run was performed using pure FezOs. The conditions were those of cycle C-1 of Figure 12. Very soon after the beginning of the run the HzSconcentration rose to the level approximatelycorresponding to the equilibrium of reaction 4. 5. Regeneration of Sdflded Sorbents. Regeneration of sulfided sorbents to the oxides was not studied in detail byTGA. The highratioofamorphoustocrystallinephases
530 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 320 -3 B 280
240 n
E a
200
P
W
v)
2
160
c Q) I Y
120
80
t/t* Figure 11. HZS breakthrough in successive sulfidations of sorbent FA650 at 1 atm, 600 "C, for cycles C-1 to C-5 and 650 "C for cycles C-6 and C-7. The sulfidation gas for cycles C-1, C-2, and C-4 to C-7 was 1%HzS, 20% Hz, 25% HzO, balance Nz, and for cycle C-3 was 1%HoS, 27% H2, balance Nz.Regeneration was for cycles C-1 to C-3 at 650 OC with 20% air-Nz and for cycles C-4 to C-7 at 650 "C with 16% air-stream. Table V. Sulfidation Equilibrium Constants and Outlet Equilibrium HzS Content. for Various Iron Oxides
+ 2H2S(g)+ H,(g) = 2FeS(s) + 3H20(g) Fe,O,(s) + 3H,S(g) + H2(g) = 3FeS(s) + 4HzO(g) FeO(s) + H,S(g) = FeS(s) + H,O(g) Fe,O,(s)
T("C) 600 650 700 750
(3) 7.20 6.99 6.81 6.46
log(K) (4) 8.50 8.19 7.90 7.38
(3) (4)
(5)
(H2S),qb (5) 2.55 2.40 2.28 2.06
(3) 70 89 110
165
(4)
395 502 627 934
(5) 705 995 1312 2177
RFor inlet gas with 1% H2S, 20% Hz, 25% H20, l-atm total pressure. Equilibrium level in ppm.
associatedwith fresh and oxidized samples severelylimited the utility of XRD analysis which was needed to interpret the TGA data. The resulta of regeneration runs in the microreactor will be presented in this section. Regeneration of iron sulfide with pure SO2 and SO2-air mixtures has been previously demonstrated (Joshi et al., 1979;Tseng et al., 1981). In regeneration with pure S02, elemental sulfur is the only possible sulfur-containing product. For example, FeS may be regenerated according to 4FeS + 350, = 2Fe20, + 7/2S2 (11) In an industrial process, the SO2 required for the above reaction may be obtained either by burning a part of the elementalsulfur recovered or by carrying out regeneration with a gas mixture of SO2 and air.
A few regeneration tests for the sulfided sorbent FA650 were carried out in the packed-bed microreactor using pure SO2 or SO2-air mixtures. Regeneration with pure SO2 was conducted for 2 h a t 700 "C with 20 cm3/minflow rate. The major portion of the elemental sulfur that formed during regeneration condensed on the quartz wool packed at the exit of the reactor, with the remainder condensing in a glass, ice-cooled,trap in the exit line from the reactor. The sulfur collected in these two locations was determined by iodometric titration and was within about h 5 % of the theoretical value. In addition, a sample of the regenerated sorbent was analyzed by XRD. The analysis on the crystalline portion of the sample (about 60% of the total) gave approximately 85% Fe203, 7% FeA1204, and 8% o-Al203. Regeneration of sulfided FA650 was also conducted at 700 O C with a gas composed of 94% S02,1% 0 2 , and 5% N2 at a 96 cm3/min flow rate. Elemental sulfur recovery in this case was about 80% of that obtained with regeneration in pure S02. This difference in the sulfur recovery can be explained by the fact that the reaction of FeS with SO2 is much slower than the reaction with 0 2 , the latter reaction producing very little elemental sulfur. XRD analysis of the regenerated sorbent showed 75% crystalline phases consisting of 68%Fe2O3,16% FeAl204, and 16% ~pA1203.Apparently,the presence of only a small amount of 02 (1%)in the regenerating gas has a large effect on the amount of Fez03 formed. Conclusions Mixed iron oxide-aluminum oxide sorbents prepared with different ratios of the two metals and different
Ind. Eng. Chem. Res., Vol. 32,No. 3, 1993 631 32C
I
I
I
I
I
I
c-5
'
I
I
I
0.8
0.9
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t/t* Figure 12. H S breakthrough in successive sulfidations of sorbent FA700 at 1 atm, 600 OC, for cycles C-1 to C-3 and 650 OC for cycles C-4 to (2-8. The sulfidation gas for cycles C-1 to C-5 was 1%HzS, 20% Hz, 25% HzO, balance Nz, and for cycles C-6 to C-8 was 1%HzS, 13% Hz 15% CO, 10% COz, 19% HzO, balance Nz. Regeneration was for cycles C-1 to C-5 at 650 OC with 20% air-Nz, for cycle C-6 at 750 "C with 20% air-Nz, and for cycle C-7 at 750 OC with 16% ai-tream.
calcination conditions exhibited markedly different physical and chemical properties. TPR of pure iron oxide consisted of two stages of reduction, corresponding to conversion of Fe3+to Fez+followed by conversion of Fez+ to FeO. By contrast, TPR of the mixed oxides comprised three stages with no clear correlation to the iron oxidation state. However, XRD identified Fe304, FeA1204, and a solid solution of those two components as reaction intermediates. In addition to these differences in the TPR curves, the time to complete reduction to FeO was much longer for the mixed iron-aluminum oxides than for iron oxide. These differences suggest that the alumina in the sample hinders the reduction of Fe203. Analogous observationshave been made in the reduction of Fe203/A1203 and CuO/A1203 materials prepared by impregnation and in the reduction of mixed copper-aluminum oxides prepared by the citrate process. Sulfidation of the various sorbents at 600 and 700 "C in the TGA yielded low-temperature,monoclinic,4C-type pyrrhotite (Fel-,S) as the only crystalline sulfide phase. In TGA experiments with reduction and sulfidation occurring consecutively a t 600 "C, there was good correlation between extent of reduction and sulfur uptake with some exceptions. Low sulfur uptake also appeared to be correlated to high FeA1204 content in the fresh sample. Sflidation of mixed iron-aluminum oxide sorbents at 600 and 650 "C in the packed-bed microreactor yielded sorbent conversions at breakthrough of 40-60% and prebreakthrough H2S levels closer to equilibrium corresponding to Fez03 than equilibrium corresponding to Fe3Or. Reaction with SO2 or with SO2 containing 1?& 02 at 700 OC regenerated the sorbent removing sulfur in its elemental form.
Acknowledgment This project was funded by agrant from GeneralElectric Company. Literature Cited Borgwardt, R. H.; Roache, N. F. Reaction of HzS and Sulfur with Limestone Particles. Ind. Eng. Chem. Process. Des. Dev. 1984, 23,742-748. Carpenter, R. H.; Desborough, G. A. Range in Solid Solution and Structure of Naturally Occurring Troilite and Pyrrhotite. Am. Mineral. 1964,49, 1350-1365. Chou, C.-L.; Li, K. Kinetic and Structural Studies of Regeneration of Sulfided Dolomite in Carbon Dioxide. 11. The Cyclic Regeneration. Chem. Eng. Commun. 1984,29, 181-200. Chung,K. S.; Massoth,F. E. Studieson Molybdena-AluminaCatalyats VI11 Effect of Cobalt on Catalyst Sulfiding. J. Catal. 1980, 64, 332-345. Desborough, G. A.; Carpenter, R. H. Phase Relations of Pyrrhotite. Econ. Geol. 1965,60,1431-1450. Freund, H. Intrinsic Global Rate Constant for the High-Temperature Reaction of CaO with HzS. Ind. Eng. Chem. Fundarn. 1984,23, 338-341. Gaballah, I.; Gleitzer, C.; Emeraux, J. P. Kinetics of Reduction of the Iron Aluminate FeAlzOl (hercynite) by Hydrogen. MBm. Sci. Rev. M6tall. 1976 73 (61, 425-433. Grindley, T.; Steinfeld,G. 'Development and Testingof Regenerable Hot Coal Gas Desulfurization Sorbents"; DOE Report DOE/MC/ 16545-1125; Morgantown Energy Technology Center: Morgantown, WV, October 1991. Kearns, D. L.; Newby, R. A,; O'Neill, E. P.; Archer, D. H. High Temperature Sulfur Removal System Development for the WestinghouseFluidized Bed Coal GasificationProces. Prepr. Pap-Am. Chern. Soc., Diu. Fuel Chem. 1976,21 (4), 91-113. Kissin, S. A.; Scott, S. D. Phase Relations InvolvingPyrrhotite Below 35OOC. Econ. Geol. 1982, 77, 1739-1754.
532 Ind. Eng. Chem. Res., Vol. 32, No.3, 1993 Leasing,P. A. Mixed-Cation Oxide Powders via Polymeric Precursors. Am. Ceram. Sac. Bull. 1989, 68 (5), 1002-1007. Lew, S.; Jothimuragesan, K.; Flytzani-Strephanopoulos,M. HighTemperature HzS Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Dioxide Sorbents. Znd. Eng. Chem. Res. 1989, 28, 535-541. Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. The Reduction of Zinc Titanate and Zinc Oxide Solids. Chem. Eng. Sci. 1992, in press. Lycourghiotis, A.; Vattis, D. Temperature Programmed Reduction Study of Alumina Supported Fe+3Catalysts. React. Kinet. Catal. Lett. 1981, 18 (3-4), 337-380. 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 (l),5657. Morgantown Energy Research Center. "Chemistry of Hot Gas Cleanup in Coal Gasification and Combustion"; Morgantown Energy Technology Center Report MERC/SP-78/2;February 1978. Morimoto, N.; Nakazawa, H.; Nishiguchi, K.; Tokonami, M. Pyrrotites: Stoichiometric Compounds with Composition Fel-,S, (1128). Science 1970,168, 964-966. Morimoto, N.; Nakazawa, H.; Watanabe, E. Direct Observation of Metal Vacancies by High-Resolution Electron Microscopy. Part I: 4C Type Pyrrhotite (Fe&). R o c . Jpn. Acad. 1974,50,756759. Morimoto, N.; Gyobu, A,; Mukaiyama, H.; Izawa, E. Crystallography and Stability of Pyrrhotites. Econ. Geol. 1975, 70, 824-833. Mukherjee, B. Crystallography of Pyrrhotite. Acta Crystallogr. 1969, B25,673-676. Nakazawa, H.; Motimoto, N.; Watanabe, E. Direct Observation of Metal Vacancies by High Resolution Electron Microscopy. I. The 4C Type Pyrrhotite (Fe&). Am. Mineral. 1975,60,359-366. Patrick, V.; Gavalas, G. R. Structureand Reduction of Mixed CopperAluminum Oxide. J. Am. Ceram. SOC. 1990, 73 (2), 358-369. Patrick, V.; Gavalas, G. R.; Flytzani-Stephanopoulos,M.; Jothimurugesan, K. High-Temperature Sulfidation-Regeneration of CuOA1203 Sorbents. Ind. Eng. Chem. Res. 1989,28,931-940. Putnis, A. Observations on Coexisting Pyrrhotite Phases by Transmission Electron Microscopy. Contrib. Mineral Petrol. 1975,52, 307-313.
Rau, M.-F.; Rieck, D.; Evans, J. W. Investigation of Iron Oxide Reduction by TEM. Metall. Trans. B. 1987, 18B,257-278. Ruth, L. A.; Squires, A. M.; Graff, R. A. Desulfurization of Fuels with Half-Calcined Dolomite: First Kinetic Data. Enuiron. Sci. Technol. 1972, 6 (12), 1009-1014. Schrodt, J. T.; Best, J. E. Sulfur Recovery from Fuel Gas Desulfurization Sorbents. AZChE Symp. Ser. 1978, 74 (No. 175), 184189. Siriwardane, R. V.; Poston, J. A. Interaction of HzS with ZincTitanate in the Presence of HZand CO. Appl. Surf. Sci. 1990,45,131-139. Sommer, R. E., Werner, A. S.; Kowszun, Z. "Evaluation and Demonstration of the Chemically Active Fluid Bed";GCA Corp. Final Report to EPA, No. EPA-68-02-3168;February 1978. Tamhankar, S.S.; Hasatani, M.; Wen, C. Y. Kinetic Studies on the Reactions Involved in the Hot Gas Desulfurization Using a Regenerable Iron Oxide Sorbent-I. Reduction and Sulfidation of Iron Oxide. Chem. Eng. Sci. 1981,36,1181-1191. Tamhankar, S. S.; Garimella, S.; Wen, C. Y. Kinetic Studies on the Reactions Involved in the Hot Gas Desulfurization Using a Regenerable Iron Oxide Sorbent-111. Reactions of the Sulfided Sorbent with Steam and Steam-Air Mixtures. Chem. Eng. Sci. 1985,40, 1019-1025. Toulmin, P.; Barton, P. B. Thermodynamic Study of Pyrite and Pyrrhotite. Geochim. Cosmochim. Acta 1964, 2.8 (5), 641-671. Turnock, A. C.; Eugster, H. P. Fe-A1 Oxides: Phase Relationships Below loo0 OC. J. Petrol. 1962, 3 (3), 533-565. Westmoreland, P. R.; Harrison, D. P. Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-BTU Gases. Enuiron. Sci. Technol. 1976, 10, 659-661. Ying-Ru, D.; Qi-Jie, Y.; Yuan-Fu, H.; Yong-Shu, J.; Jin-Jeng, Q. A Mbsbauer Investigation of a-FenOs Microcrystals Supported on p4l203. In Mbsbauer Spectroscopy and Its Chemical Applications; Stevens, J. G., Shenoy, G. K., Eds.; American Chemical Society: Washington, DC, 1981; p 609. Received for review July 2, 1992 Revised manuscript received November 5, 1992 Accepted November 23,1992
