2810
Ind. Eng. Chem. Res. 1994,33, 2810-2818
Spectroscopic Characterization of Molybdenum-ContainingZinc Titanate Desulfurization Sorbents Ftanjani V. Siriwardane,' James A. Poston, and Grover Evans, Jr. Morgantown Energy Technology Center, U.S. Department of Energy, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virgina 26505
Molybdenum-containing zinc titanate has been identified as a potential regenerable desulfurization Porbent, removing H2S from coal-derived fuel gas up to temperatures of 1035 K Sorbents were tested in a laboratory scale fixed bed reactor using simulated coal-derived fuel gas and then removed for analysis. The analyses consisted of X-ray photoelectron spectroscopy, scanning electron microscopy/energy-dispersive x-ray spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, and X-ray diffraction. In the sulfided sorbent, migration of zinc to the surface and sulfur in the sulfide form were observed. In the regenerated sorbent, residual sulfur in the sulfate form was observed. In the sulfided and regenerated sorbent after multicycle testing, a small decrease in the ZnPTi ratio was detected, indicating some zinc migration. The chemical structure of the sulfided sorbent was different from that of the original sample, as observed by Fourier transform infrared spectroscopy.
Introduction Present and future environmental and economic concerns mandate the removal of various pollutants from the product stream of coal-derived fuel gas. Of these pollutants, the removal of sulfur-containing compounds has been of primary interest, due to the large environmental and economic impact. Several mixed metal oxide sorbents have been shown to be promising regenerable high-temperature sorbents and, subsequently, have been investigated for the removal of hydrogen sulfide, the major sulfur-containing compound, from the fuel gas a t coal gasification temperatures (Grindley et al., 1984; Shamsi et al., 1984; Jha et al., 1986; Lew et al., 1989; Flytzani-Stephanopoulos et al., 1989; Atimtay et al., 1993). Zinc titanate containing molybdenum has been identified as one of these potential regenerable mixed metal oxide sorbents which could be used to remove H2S from fuel gas. The sorbent was tested in a laboratory scale fixed bed reactor. Spalling, the breaking apart, of the sorbent after sulfidation and regeneration was observed. Spalling has been observed in other sorbents that were tested previously (Woods et al., 1989;Ayala et al., 1991, 1993; Everitt et al., 1990). However, the causes of spalling as well as the changes in sorbent composition and structure during sulfidation and regeneration have not been investigated. In this study, research focused toward the surface and bulk analysis of molybdenum-containing zinc titanate sorbents, and was performed after a series of tests carried out on a 2-in.-diameter high-pressure, hightemperature sorbent screening unit. These tests were performed in order to determine the changes in the sorbent during sulfidation and regeneration. The primary analyses performed utilized X-ray photoelectron spectroscopyOZPS),scanning electron microscopylenergydispersive microanalysis (SEM/EDS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Experimental Section Molybdenum-containing zinc titanate was prepared by United Catalyst using high-temperature, solid-state mixing. The formulation used in these series of tests contained ZnO/TiOz molar ratio of 2, with 2.5 w t %
Table 1. Experimental Parameters: Nominal Sulfidation Conditions testno.
t%Ej
OC1-S1 OC2-Sl OC3-S1 OC4-S1 OC5-Sl OC6-S1 MCl-S4
1.034 1.034 1.034 1.034 1.034 1.034 1.034
MC2-S2 MC2-S3
1.034 1.034
temp
velocity
us gas composition 866 0.6096 312 9% HzO, 21% CO, 7% COz, 866 0.3048 156 15%Hz, 41.97% Nz, 866 0.3048 156 800 ppm HzS, 100 pprn 811 0.3048 167 COS 922 0.3048 147 922 0.1524 73 866 0.1524 104 9% HzO, 21% CO, 7% COz, 15% Hz, 41.97% Nz, 800 ppm HzS, 15 ppm HCl 811 0.1524 83 30% Hz0,8% CO, 11%COz, 922 0.6096 293 16%Hz, 34.92% Nz, 800 ppm HzS (K)
m/s
MoO3, 10.0% organic binder, and 3.0% bentonite. The above mixture was calcined at 1033 K (760 "C) for 2 h. X-ray photoelectron spectroscopy ( X P S ) spectra were recorded with a Physical Electronics Model SAM-590 equipped with a cylindrical mirror analyzer and a 15kV X-ray source from the Physical Electronics Division of Perkin-Elmer. The system was routinely operated within a pressure range of 1.3 x to 1.3 x Pa (10-9-10-8 Torr). The instrument was calibrated using the photoemission lines EB(CU2P3/2) = 932.4 eV and EB(Au 4f7/2) = 83.8 eV (Wagner et al., 1979). The binding energies obtained were referenced to the C(ls) level at 284.6 eV for adventitious carbon. All intensities reported are experimentally determined peak areas divided by the instrumental sensitivity factors. Energydispersive-spectroscopy(EDS) spectra were obtained by a Noran Instruments Micro-Z Energy Dispersive Spectrometer mounted on a JEOL-84OA scanning electron miroscope (SEM). The detector and the SEM are interfaced to a Noran Instruments TN-8502s analysis system. The detector resolution, with respect t o the manganese Ka spectra line, was 151 eV. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was carried out using a Perkin-Elmer 1750 infrared Fourier transform spectrometer equipped with a Specta-Tech diffuse reflectance attachment. Molybdenum-containing zinc titanate spherical pellets (General Electric L-3787M) were obtained during a series of tests conducted on a 2 in.-diameter, variableheight, high-temperature-high-pressure, laboratory scale fixed bed reactor. Desulfurization sorbents were ex-
This article not subject to U S . Copyright. Published 1994 by the American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2811 Table 2. Experimental Parameters: Nominal Regeneration Conditions testno. MC7-R1
press. temp (MPa) (K) 0.345 922-1033
m/s
Us
gas composition
0.1524
24
MC7-R3 MCP-Rl
1.034 0.345
922-1033 922-1033
0.1006 0.1524
48 24
MC2-R3
1.034
922-1033
0.1006
48
2.5% 0 ~ , 9 0 % HzO, 7.5% Nz 1.5%02,98.5%Nz 2.5% 0 ~ , 9 0 % HzO, 7.5% Nz 1.5%Oz,98.5%Nz
posed to simulated coal process gases (Table 1). Sulhrcontaining gases entered the sorbent bed from the top of the reactor, while regeneration of the sorbent took place with regeneration gases (Table 2) entering the reactor from the bottom of the bed. For XPS and SEM analysis, pellets were cross sectioned. In XPS,both the outer surface and the surface of the pellet interior were analyzed, while for EDS analysis, a series of 12 analysis points were obtained across the radius of the pellet (line scan), with the analysis points always going from the pellet surface to the pellet interior. The pellets for the analyses were obtained from the top 1 in. and bottom 12-16 in. of the reactor bed. A n accelerating voltage of 25 kV was used for the EDS analysis of the sorbent pellets. All samples were carbon coated in order to reduce sample charging during EDS analysis. In order to ascertain reproducibility,typically two to three pellets from the top and bottom of the reactor bed were analyzed for the single sulfidation, while for the multicycle tests on which more emphasis was placed, typically five or more sorbent pellets were analyzed from the top and bottom region of the reactor bed. For DRIFTS analysis, the sorbent was first crushed using a mortar and pestle with further reduction of size achieved using a wiggle bug. A sonic sifier was used to separate the particles into the desired 35-75-pm range. The sorbent was then mixed in a wiggle bug (without the metallic balls to avoid further reduction in size) with KC1 (35-75 pm) in a ratio of 1 wt % to 99 w t % of sorbent to KC1, providing a homogeneous mixture. Laboratory-prepared samples representing the three major phases of zinc titanate usually found, ZnTiOa, Zn2Ti3O8, and ZnzTiO4, were made utilizing wet chemistry techniques. The samples were prepared from 60 mol % titanium methoxide and 40 mol % zinc acetate. The samples were calcined at 873, 1083, and 1273 K, respectively, to obtain the desired phases (Swisher, 1992). Standards used in the analyses consisted of ZnO, TiO2, MoOz,ZnTiO3, and pure bentonite. All the above standards were of high purity (’99.9%) and were obtained from Alfa except for the bentonite which was provided by Research Triangle Institute (RTI). Sorbent test parameters were given in Tables 1 and 2. In the test identification label, the letters OC stand for one cycle while the letters MC stand for multiple
cycle. The number following either OC or MC stands for the test identification number. The letters S and R at the end of the identification label stand for sulfidation and regeneration respectively and indicate the experimental condition on which a test terminated. The number following the leter at the end of the identification label indicates the number of cycles. The term “cycle” refers to the number of times the sorbent was exposed to those experimental conditions. A regeneration cycle always follows a sulfidation cycle.
Results and Discussion (a) Characterization. X-ray diffraction analysis of the samples showed that the structure was primarily Znz-Ti04, with some possible ZnzTi308. Krypton surface area (SA) of the unreacted sorbent was 0.85 f 0.02 m2/ g. Total nitrogen pore volume was 0.0023 m u g with total mercury pore volume being 0.21 mug. Particle density was 2.42 g/mL. In general, there was an increase in the surface area for the sulfided samples (1-2 m2/g,krypton SA). This increase could be a result of increased surface roughness created by the formation of nonuniform zinc sulfide layers on the surface of the grains during reaction. In the regenerated samples, the krypton surface areas were similar to that of the unreacted samples. There was a decrease in the mercury pore volume after the first sulfidation with a corresponding increase in the nitrogen pore volume compared to the unreacted samples. The decrease in the mercury pore volume indicates that some of the macropores have been occupied by either sulfide layers formed during the sulfidation and/or molybdenum, while the increase in nitrogen pore volume indicates the additional formation of meso- and micropores in the uneven sulfide layer. The increase could ge due to the roughness of the uneven sulfide layer. In the regenerated samples, both the nitrogen pore volume and the mercury pore volume were only slightly higher than those of the unreacted samples, indicating only a slight increase in the micro-, meso-, and macropore volume during the total sulfidation and regeneration reaction. (b)XPS Analysis. XPS analysis was performed on both the exterior surface and the cross section of the sulfided and regenerated sorbent pellets. The relative elemental composition and the oxidation states of the elements on the surface of the grains were determined. The decrepitation of the sorbent material observed during sulfidation and regeneration is believed to be due to the failure of the bond between the grain surfaces. Analysis of the unreacted (fresh) sample showed a Z d Ti ratio close to 2.0, both at the exterior pellet surface and the pellet interior. A small amount of chlorine was also detected. Molybdenum was found to be in the 6+ state. Detectable quantities of aluminum and silicon
Table 3. XPS Data on Sulfidation top of the bed test no.
oc1-s1
oc2-s1 OC3-S1 OC4-S1 OC5-S1 OC6-S1 MCl-S4 MC2-S3 a
temD (K) 1373 1373 1373 1273 1473 1473 1373 1473 ,
High > 3.0. High > 5.0.
S/Zn 0.8 0.8 1.1 1.1
0.7 0.9 0.9 0.7
outside Zn/Ti higha high very highb high hizh high very high (no Ti) high
-
SiZn 0.7 0.6 0.7 0.8 0.6 0.7 0.6 0.6
inside Zn/Ti 2.1 2.4 1.9 2.2 2.0 2.5 2.1 2.6
bottom of the bed outside inside SIZn Zfli SIZn Zn/Ti 0.6 2.2 0.7 high 0.8 2.5 0.6 2.9 1.2 2.7 0.7 2.3 0.9 high 0.6 1.8 0.8 2.2 0.3 2.8 0.8 2.0 0.6 2.5 0.9 high 0.6 2.3 0.7 high 0.7 2.2
2812 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
Table 4. XPS Data on Regeneration top outside test no. MC7-R1 MC7-R3 MC2-R3
temp (K) 1473-1673 1473-1573 1473-1673
SO42-IZn
Z m
0.5 0.4,0.2" 0.4
2.4 2.0 3.0
bottom inside ZwTi S042-IZn 0.1, 0.9 0.4. 0.2a 0.4
1.6 2.4 2.5
outside ZOi S042-IZn 0.4 0.4 0.3
1.9 2.5 2.6
inside Zn/Ti S042-/Zn 0.5 0.3 0.4
1.9 2.6 2.2 ~~
Sulfide.
were not observed on the surface in the unreacted sorbent pellets even though the binder, bentonite, was an aluminosilicate. Typical binding energies for Zn, Ti, and Mo were 1022.5, 458.5, and 232.5 eV, repectively. The surface elemental compositions of the sorbent material after sulfidation are shown (Table 3). All samples collected from the top of the reactor bed had a high Zn/Ti ratio, indicating a rearrangement of the zinc and titanium elemental species, with zinc migrating to the grain surface during sulfidation. This rearrangement effect was also observed with SEM/EDS in the multicycle test data as well as in previous work (Siriwardane et al., 1990, 1993). Since zinc is both more volatile and more reactive with H2S compared to titanium in a reducing gas environment, the migration of zinc to the grain surface seems to be a reasonable observation. In the MC1-SI test, samples from the top of the reactor bed showed no presence of titanium on the grain surface. Samples obtained from both the bottom and top of the reactor bed in test series MCl-S4 and MC2-S3 also had high Zn/Ti ratios detected on the outer pellet surface. The sulfur was found to be in the sulfide form on all sulfided samples. The amount of sulfide on the sorbent pellet interior was slightly less than the amount observed on the sorbent pellet exterior, indicating a small concentration gradient due to diffusional resistance of H2S. The XPS data of the regenerated samples are shown (Table 4). The presence of sulfate was observed in both exterior and interior pellet surfaces in all the regenerated samples. It is believed that the sulfate formation at the grain surfaces creates stress between neighboring grains, subsequently resulting in stresses within the sorbent pellet. This stress, a result of the larger size of the sulfate molecule as compared to sulfide, is believed to be one of the causes of the spalling of the sorbents observed during the testing. Thus, the sulfate formation at the grain surfaces could enhance the sorbent failure. The amount of sulfate present in the exterior of the pellet surface was similar to that of the interior of the pellet surface for regenerated samples collected a t the bottom of the reactor bed as shown (Table 4). There was a substantial amount of sulfide present in the interior surface of samples collected from the top of the reactor bed in test series MC7-R1 sample (regenerated once). As regeneration is conducted from the bottom of the bed, it is possible that the oxygen had not sufficiently diffused inside the pellets at the top of the bed resulting in the incomplete conversion and removal of sulfide. A very small amount of sulfide was present in a sample obtained from the top of the bed test series MC7-R3. Unlike the sulfided samples from the top of the bed which had a very high Z f l i ratio, the ZnfI'i ratios of the regenerated samples were only slightly higher than that of the fresh sample (except for the sample from top of the bed in test series MC2-W). Thus it seems that the surface elemental composition has rearranged again or excess Zn may have evaporated from the surface during the regeneration. It is possible
that the original zinc titanate structure could not be restored completely during regeneration due to this elemental rearrangement during sulfidation and regeneration. There was no change in the MoEn ratio on the surface of the regenerated samples on either the inside or the outside of the sorbent pellets as compared to the fresh samples. In the sulfided samples, there was a decrease in the Mo/Zn ratio at the outside of the pellets obtained from both the top and the bottom of the reactor bed, while there was an increase in this ratio at the pellet interior. Thus, it appears that there is a migration of molybdenum within the grains during sulfidation and regeneration reactions. ( c ) SEM/EDS Analysis. SEM/EDS analysis was performed in order to obtain the elemental profiles across the cross-sectioned sorbent pellet. SEM/EDS analysis is more of a bulk technique as compared to XPS, whereas the analysis volume in the former is on the order of cubic microns. As a result of resolution limitations in energydispersive spectroscopy, strong peak overlaps can occur, with the degree of overlap such that peak subtraction cannot be used to resolve the individual peaks (Goldstein et al., 1981). Of these overlaps, the most wellknown and applicable to this analysis occurs between the sulfur Ka and the molybdenum L a spectral lines. This peak overlap precludes determination of the concentrations of molybdenum and sulfur in the sorbents using EDS. A multipoint (12 point) energy-dispersive-spectroscopic analysis on both unreacted sorbents (Figure 1) and sorbents reacted without sulfur present (Figure 2) revealed a fairly homogeneous composition with respect to zinc and titanium, but heterogeneous with respect to molybdenum. As a result of this heterogeneity, combined with the spectral overlap problem of the molybdenum La and the Sulfur Ka peaks, the amount of sulfur present could not be determined. Analysis of the Z f l i elemental profiles after singlecycle sulfidation showed, with the exception of tests OC3-S1 and OC5-S1, that the Zn/Ti ratio obtained was comparable to that of the unreacted samples. Sorbent pellets OC3-S1 and OC5-S1 showed a slight decrease in the ZnfI'i value as compared to that of the unreacted sorbent pellets. A typical representative plot of the sulfided samples is given by test sorbent OC6-S1 (Figure 3). As can be seen, there is some scattering of the data when comparing different pellets from the same test series and bed height. Single regeneration of the sulfided sorbent produced mixed results, with the Zn/ Ti ratio being comparable to that of the unreacted sorbents in some cases and slightly lower in others. Primary emphasis was placed on multicycle testing. After a test, sorbent pellets were removed and Zn/Ti elemental profiles obtained. Typical plots are shown (Figure 4). Typically, the Zn/Ti ratio is slightly lower for sorbents that have undergone multiple cycle (sulfi-
Ind. Eng. Chem. Res., Vol. 33,No. 11,1994 2813 2.5
I
I
I
I
I
Top of Bed
Unreacted
0
Sorbent
A BonomofBed
1
i
N
-
0
2.5
2
4
6 6 1 0 Pellet Numbers (a)
1
0
2
1
2
3 4 5 6 Pellet Numbers
Unreacted
0
Sorbent
0
2
4
6 6 1 0 Pellet Numbers (b)
1
datiodregeneration) testing than for those that were unreacted or have undergone only a single cycle test. The lower ZwTi ratio of the reacted sorbent obtained by EDS is in agreement with the data obtained by XPS as well as with previous data (Siriwardane, 1990,1993) and indicates that some zinc migration from the bulk to the surface of the sorbent has occurred. This migration or elemental rearrangement is believed, at least in part, to contribute to the incomplete reformation of the titanate structure during successive regeneration. Photomicrographs of both an unreacted and a multicycle tested sorbent as observed by secondary electron imaging are shown (Figure 5). The effect of spalling is readily seen in the multicycle tested sorbent. The spalling effect observed in this test series has also been observed in previous testing (Everitt et al., 1990;Ayala et al., 1993) conducted with cylindrical zinc titanate extrudates and spherical zinc titanate pellets of Merent formulations. This test series in comparison with previous tests (Everitt et al., 1990;Ayala et al., 1993) indicates that this formulation of molybdenum-containing zinc titanate spherical pellets has a higher attrition resistance than the cylindrical zinc titanate extrudates and spherical zinc titanate pellets not containing molybdenum utilized in previous testing (Everitt et al., 1990;Ayala et al., 1993). The degree of spalling is less severe with molybdenum-containing zinc titanate spherical pellets than with cylindrical zinc titanate extrudates and spherical zinc titanate pellets not containing molybdenum. Although the degree of spalling of the molybdenum-containing zinc titanate spherical pellets is less severe than that observed in previous tests, the spalling, which results in large pressure drops aceross the reactor causing lost emciency and increased operat-
6
Top of Bed
A BonomofBed
2
Figure 1. EDS analysis of unreacted sorbents: (a) Znfl'i ratio; (b) Mom ratio.
7
0
1
2
3 4 5 6 Pellet Numbers (b)
7
6
Figure 2. EDS analysis of sorbents reacted without H2S present (a) Znfl'i ratio; (b)Mom ratio.
2.2 2*4
t 0
I
I
I
I
0
I
Unreacted
A TopofBed
v Bonomof Bed
2
4 6 8 Pellet Numbers
1012
Figure 3. Plot of Zn/Ti ratio of pellets obtained from test OC6-
s1.
ing and maintenance costs, was such that it precluded the use of the sorbent for practical applications. Sorbent spalling was observed in all tests except where the sorbent sulfur loading was less than 3%. Observation of sample morphology was also carried out using secondary electron imaging (Figures 6 and 7). Average grain size is on the order of 0.5 pm. As can be seen, the grains in both the sulfided and regenerated samples were more aggregated than those in the unreacted sample. This aggregation appears to be a result of grain-grain fusion. This fusion is believed to be a result of one or more molten molybdenum compounds at the surface. The fusion of the grains is more prevalent in the samples that have undergone multicycle testing than in those that have undergone only single cycle testing. This observation appears to be
2814 Ind. Eng. Cbem. Res., Vol. 33, No. 11, 1994
.
2.4
Unreacted
. A
Top of Bed Bottom of Bed
2.2
5
N
2.0
1.8
2
4 6 8 Pellet Numbers
1 0 1 2
(8)
I
I
I
A
2.2
I
Unreacted
2.4
Top of Bed Bottom of Bed
.2.0
1.8
1.6
F i g u r e 4. Plot o f Z f l i ratio of pellets obtained from test series:
(a) MC2-S3; (bl MC2-R3.
consistent with the observation of molybdenum migration to the surface of the grains as observed with XPS, along with the lower melting point of molybdenum pentasulfide and lower sublimation temperature of molybdenum disulfide as compared t o ZnO and TiOz. The decomposition properties of molybdenum pentoxide, molybdenum trisulfide, and molybdenum tetrasulfide, which are not fully understood, may possibly also contribute to this observed effect (Handbook of Chemistry and Physics, 1983). Testing carried out under the condition of low sorbent sulfur loading (
