1890
Ind. Eng. Chem. Res. 1992,31, 1890-1899
Sulfidation of Zinc Titanate and Zinc Oxide Solids Susan Lew,?Adel F. Sarofim, and Maria Flytzani-Stephanopoulos* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 The sulfidation of bulk mixed oxides of zinc and titanium of various compositions and Zn-Ti-0 crystalline phases in H+3-H2-H20-N2 gas mixtures was investigated in a thermogravimetric apparatus over the temperature range of 400-800 "C. Comparative sulfidation experiments with ZnO were also performed. In comparison to ZnO, the use of Zn-Ti-0 solids allows raising the operating temperature for desulfurization of hot coal-derived fuel gas, e.g., by as much as 94 "C for solids with (Zn/Ti)aumic= 2/3. The initial sulfidation rate of Zn-Ti-0 solids with (Zn/Ti),bmic I3 was approximately 1.5-2 times slower than for ZnO. Different zinc titanate phases (i.e., Zn2Ti04,ZnTi03, and Zn2Ti308)had the same initial sulfidation rate. Similar activation energies (9-10 kcal/mol) were measured for ZnO and Zn-Ti-0 sulfidation. No effect of H2 or H20 was observed on the initial sflidation rate. However, a t high conversions, rates were lower with increasing H2 concentrations. Fine particles and cracks were observed in sulfided ZnO as a result of ZnO reduction and subsequent vapor-phase reaction between Zn and H2Sto form ZnS. These structural effects were largely absent in Zn-Ti-0 solids.
Introduction The removal of hydrogen sulfide (H2S)to sufficiently low levels from coal-derived fuel gases at elevated temperatures is crucial for the efficient and economic coal utilization in emerging advanced power generation systems such as the integrated gasification-combined cycle (IGCC) and the gasification-molten carbonate fuel cell (MCFC). For these technologies, highly efficient sulfur removal from several thousand parts per million (pprn) down to -1 ppm for the MCFC or less than 100 ppm for the IGCC is needed. Commercial desulfurization processes are based on liquid scrubbing at or below ambient temperatures, resulting in considerable thermal efficiency loss as well as costly wastewater treatment. Previous studies (Jalan and Wu, 1980; Grindley and Steinfeld, 1981; Flytzani-Stephanopoulw et al., 1985) have investigated the potential use of zinc oxide as a high-temperature regenerable sorbent. Kinetic studies using single pellets of zinc oxide were also performed (Gibson and Harrison, 1980; Ranade and Harrison, 1981). The thermodynamic equilibrium for sulfidation of ZnO is quite favorable,yielding desulfurization down to a few parts per million (ppm) H2S. Zinc sulfide can be regenerated if sufficiently high temperatures or low oxygen concentrations are used to avoid zinc sulfate formation. A major drawback of zinc oxide is that in the highly reducing atmosphere of coal-derived fuel gases, it is partially reduced to elemental zinc, which at high temperatures is volatile. Consequently, sorbent loss is observed at temperatures above 600 "C. More recently mixed metal oxides have been studied in an effort to improve the properties of single oxide sulfur sorbents (Grindley and Steinfeld, 1983; Flytzani-Stephanopoulos et al., 1985). The mixed oxide sorbent zinc ferrite, ZnFe204,combining ZnO with Fe203 has been developed as an alternative to single zinc oxide sorbent (Grindley and Steinfeld, 1983) because of its high sulfur capacity, rapid reaction with H2S,and high H2Sremoval efficiency. Zinc femte decomposes into (ZnO + Fe304)in the reducing coal gas atmosphere. Hence, it is similarly limited (as ZnO) to an operating temperature of approximately 600 "C. Previous studies performed in this labo-
* To whom correspondence should be addressed.
Present address: ARC0 Chemical Company, Newtown Square, PA 19073.
ratory (Lew, 1987; Flytzani-Stephanopoulos et al., 1987; Lew et al., 1989) have found that zinc oxide in association with titanium dioxide is reduced more slowly to volatile zinc than pure zinc oxide. In these earlier studies, cyclic sflidation-regeneration (six to eight cycles) of various Zn-Ti oxides was performed in a packed bed reactor. A simulated coal gas mixture with a molar composition of 1% H2S-13% H2-19% H20-67% N2 was typically used. H2S removal efficiency comparable to that of ZnO was measured for Zn-Ti4 sorbents. The same was true when 15% CO-10% COzwas substituted for N2 in the sulfidation gas mixture. Reduction experiments were performed with Hz, a stronger reducing agent than CO. A reduction rate lower by as much as 5 times was measured in these experiments for Zn-Ti-0 solids compared to ZnO reduction by hydrogen at 650 "C. More detailed reduction kinetic experiments were performed recently (Lew, 1990; Lew et al., 1992a). The overall reaction scheme in sulfidation-regeneration of Zn-Ti-0 materials is sulfidation + xH2S(g) ZnxTiyOx+Py(s) xZnS(s) + yTi02(s) + xH20(g) (1)
-
In addition to sflidation, some reduction can occur by the reaction reduction (>600 "C)
-
Zn,TiyO,+zy(s) + xH&) or xCO(g) xZn(g) + yTi02(s) + xH20(g)or xCOz(g) (2) regeneration xZnS(s) + yTi02(s)+ (3x/2)02(g) Zn,Ti,0,+2y(s) + xSOz(g) (3) The packed bed reactor experiments provide information on sulfur removal efficiency and utilization of sorbents. However, these are not suitable for kinetic measurements. In this work, the sflidation kinetics of Zn-Ti4 materials were examined in a thermogravimetric apparatus as a function of the operating conditions and sorbent composition. To determine the sflidation kinetics, a gas mixture of H2Sand N2 was used. A small amount of H2 (1%) was added to the gas mixture to prevent the decomposition of hydrogen sulfide. The effects of H2and H20on the initial sulfidation rates were examined separately. In practice, the actual coal gas is a mixture of H2S-H2-H20-CO-
0888-5885/92/2631-l890$03.00/00 1992 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1891 Table I. Chemical Properties of Sorbents Used in Sulfidation Experiments" crystalline phasesb (wt %) (Zn/ Ti) sorbent (atomic ratio) ZnO ZnsTiOl ZnTiOs ZnzTi3O8 0 0 69 0 Z3T7' 317 0 0 65 16 Z2T3-a 213 0 0 83 0 Z2T3-b 213 0 20 45 35 ZT 111 0 68 18 14 Z3T2 312 0 100 0 0 Z2T-a 211 0 100 0 0 Z2T-bd 211 28 72 0 0 Z3T 311 ZnO 100 0 0 0
Ti02(rutile) 31 19 17 0 0 0 0 0 0
"Prepared from zinc acetate and titanium(1V) isopropoxide with 1:l mole ratio of metal ions to citric acid (unless otherwise noted). All solids calcined at 720 "C for 12 h except for ZnO, which was calcined for 4 h. *Identified by X-ray diffraction. Prepared with titanium tetrachloride. Prepared with 1:2 mole ratio of metal ions to citric acid.
C02-N2. The complexity of this gas mixture makes it Micult to quantify the influence of different components. In this paper, we will report on the kinetics of sflidation and the structural changes associated with reductive sflidation. Several experiments in regeneration with OpN2were performed to determine the regenerability of the solids. These results (sulfidation and regeneration) were compared with those of ZnO to determine the level of improvement attained by the use of Zn-Ti oxides.
Experimental Section Preparation and Characterization of Solids. Bulk mixed oxide solids of zinc and titanium were prepared by a known method for synthesizing highly dispersed mixed oxides from amorphous citrate precursors (Marcilly et al., 1970; Courty et al., 1973). The preparation of &-Ti oxides consists of mixing a 2:l volume ratio solution of glacial acetic acid (Mallinckrodt, AR grade) and titanium(1V) isopropoxide (Strem Chemical, AR grade) with an aqueous solution of zinc acetate (Mallinckrodt,AR grade) and citric acid monohydrate (Mallinckrodt, AR grade). Typically, an equal mole ratio of citric acid to metal ions (zinc and titanium) is used in preparation of the solution. The fiial solution is first dehydrated rapidly (15-30 min) in a rotary evaporator at 65-75 "C under vacuum to form a viscous liquid and then dehydrated slowly (4-6 h) in a vacuum oven at 70-80 "C to form a porous solid foam. The solid foam was calcined in air in a muffle furnace at 720 OC for 12 h producing a porous, homogeneous mixed metal oxide. The solids were characterized by several bulk and surface analysis techniques. The elemental composition (zinc and titanium) of the solids was verified by atomic absorption spectroscopy (Perkin Elmer 360 spectrophotometer) of the solids dissolved in a hot HF-HC1-H20 solution (-90 OC). X-ray diffraction (XRD) for identification of crystalline phases in the mixed oxides was performed with a Rigaku RU300 instrument using Cu (Ka) radiation. Scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDS) using a Cambridge Stereoscan 250 MK3 instrument were used to observe the surface morphology, crystallite size, and compositional variation of the solids. Surface areas were measured by a Micromeritics Flow Sorb I11 2300 BET apparatus using N2gas, while pore volumes and pore size distribution were measured by a Micromeritics Autopore 9200 mercury porosimeter. Physicochemical Properties of Bulk Zn-Ti-0 Sorbents. Three distinct zinc titanate phases, namely Zn2Ti04,Zn2Ti308,and ZnTi03, can be formed through solids preparation by the citric acid complexation method using zinc acetate and titanium(IV) isopropoxide precursors followed by pyrolysis in air at different time-temperature conditions. The type of phases present depends
40
20
60
80
100
mol% ZnO (based on ZnO-Ti02)
Figure 1. Crystalline phases formed as a function of Zn-Ti-0 composition of solids calcined at 720 "C for 12 h.
on the &/Ti atomic ratio and the calcination temperature. The observed phase transformation with increasing temperature is Zn2Ti308 ZnTiO, Zn2TiOl. At high calcination temperature (11000 "C), Zn2Ti04is the only stable mixed oxide phase for all Zn-Ti4 solids, coexisting with either ZnO or TiOz phases depending on the solid stoichiometry (Zn/Ti ratio). Also, Zn2Ti04is the stable phase for solids with Zn/Ti 1 2 calcined a t temperatures 1700 "C for long periods of time (112 h). At temperatures below 800 "C, all three zinc titanate phases may be present (Lew, 1990). Table I shows the stoichiometry and XRD analysis of the various Zn-Ti4 materials used in sulfidation experiments in this work. All sorbents were calcined at 720 "C for 12 h. The type of phases present depended on the Zn/Ti ratio as shown graphically in Figure 1. Decreasing the Zn/Ti ratio of the solids produced phases in the order ZnO Zn2Ti04 ZnTi03 and Zn2Ti308 Ti02. There were some variations in the relative amounts of ZnTi03 and Zn2Ti308as exemplified by the solids Z2T3-a and Z2T3-b in Table I, prepared, respectively, in flowing and static air calcination. Effecta of varying the %/Ti atomic ratio on the physical properties of the &-Ti4 solids, i.e., surface area and pore volume, are shown in Figure 2. Solids with up to 50 mol % Ti02are characterized by higher surface area and pore volume than ZnO neat. Addition of small amounts of Ti02 into ZnO has the largest effect, with a maximum in surface = 9/1. These area and pore volume shown for (Zn/Ti)ab~c data indicate that Ti02 disperses ZnO, effectively preventing ZnO particle growth (sintering). High levels of Ti02, however, and compound formation (e.g., ZnTi03, Zn2Ti308)reduce the overall surface area.
- -
- -
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1892 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
0' 40
60
BO
I
100
mol% ZnO (based on ZnO-Ti02)
-4 0
60
80
100
mol% ZnO (based on Zn0-1102)
Figure 2. Effect of Zn-Ti4 composition on physical properties of the d i d particles (90-125 am): (a) surface area and (b) pore volume.
As a result of the preparation method, the sorbents are produced in a highly macroporous form. Table I1 shows the pore size distribution (based on Hg porosimetry) of Zn-Ti4 particles (90-125-pm size) after calcination in air at 720 OC for 12 h. Macropores (>l-pm diameter) typically comprise more than 70% of the pores. Such pore structures allow for kinetic studies in the absence of pore diffusion limitationseven with relatively large particles ( 100 rm). Apparatus and Procedure. Kinetic sulfidation experiments with solids containing various Zn/Ti atomic ratios were performed in a Cahn System 113-X thermogravimetric analyzer (TGA) equipped with a Cahn 2000 electrobalance, a Micricon temperature controller, and a Bascorn Turner data acquisition system. The TGA reactor system is shown in Figure 3. The TGA measured the weight gain as a function of the time required for Zn-Ti oxide sulfidation to ZnS and TiOz. The solid was pretreated in a vacuum oven at 90 "C for 1 h to remove any absorbed HzO before it was reacted in the TGA. Gas flow rates were set by passing 6.3% HzS-in-Nz,Hz, and N2 gases through Brooks Model 58503 mass flow controllers. A gas flow rate of 350 cm3(STP)/minwas used in the experiments. Approximately half the gas flow (containing HzS, Hz, and N,) entered the reactor (TGA) through a side arm. The other portion of the gas containing only Nzentered the balance section of the TGA serving both to protect the balance from the corrosive HzS and as a diluent to the reactant gas. Water vapor was added to the gas by bubbling nitrogen and hydrogen through a water saturator maintained at 25 O C in a
Table 11. Pore Size Distribution of Sorbents Used in Sulfidation Experimentsa pore size distributionb sorbent (%), diameter (pm) Z3T7' 16.0, > 25 69.5, 25-1 13.3, 1-0.1 1.2, 0.1-0.003 Z2T3-a 58.5, >25 21.8, 25-1 10.8, 1-0.1 8.9, 0.1-0.003 ZT 15.7, >25 67.2, 25-1 7.9, 1-0.1 9.2, 0.1-0.003 Z3T2 44.2, >25 38.4, 25-1 10.5, 1-0.1 6.9, 0.1-0.003 Z2T-a 56.2, >25 28.1, 25-1 5.8, 1-0.1 9.9, 0.1-0.003 Z2T-bd 10.0, >25 53.4, 25-1 15.3, 1-0.1 21.3, 0.1-0.003 Z3T 19.6, >25 54.6, 25-1 10.0, 1 4 . 1 15.8, 0.1-0.003 ZnO 15.0, >25 62.2, 25-1 21.1, 1-0.1 1.7, 0.1-0.003
~~
'Prepared from zinc acetate and titanium (IV) isopropoxide with 1:l mole ratio of metal ions to citric acid (unless otherwise noted). All solids calcined at 720 "C for 12 h except for ZnO, which wae calcined for 4 h. bPore size distribution for 90-125-pm particles by mercury porosimetry. Prepared with titanium btrachloride. Prepared with 1:2 mole ratio of metal ions to citric acid. th e shield
r
-
m electrobalance a l q n
]
N
Ix]
mass flowmeter Figure 3. Schematic of TGA reactor system.
threeneck flask assembly. The saturated gas stream was then mixed with the HzS-in-Nzgas and entered the apparatus side arm. A thin layer of sample (typically 1-3 mg of 90-125-pm-size particles) was placed in a hemispherical-shaped quartz pan suspended by a quartz hangdown wire. Isothermal sulfidation experiments were performed at temperatures between 400 and 800 OC. It was experimentally verified by varying the gas flow rate,
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1893
1
H2S-l%H2-bal.N2
4t
ZnO: 700°C
I
Z2T-a: 600°C
I 20
40
60
80
100
" -
0
2
1
mol% ZnO (based on ZnO-Ti02)
4
3
5
mol% Hydrogen Sulfide
Figure 4. Initial sflidation rate of several Zn-Ti-O sorbents in 2% HzS-l% Hz-97% N2 at 600 and 700 "C.
Figure 5. Determination of reaction order for ZnO and Z2T-a reaction with H2S.
quantity of sample, and particle size that these experiments were performed in the absence of both mass transfer and pore diffusional limitations. Thus, the measured rate was due only to the intrinsic sulfidation kinetics and product layer diffusion. Typically, each sflidation experiment was repeated at least once to verify reproducibility. In regeneration, the sulfided solids were reacted in 21% 02-79% N2at 650 "C. T w o cycles of sflidationregeneration were performed.
Table 111. Arrhenius Constants for the Sulfidation Reactions of ZnO and Z2T-a Sorbents Arrhenius const
Results and Discussion Initial Sulfidation Rate of Bulk Zn-Ti-O Sorbents. Initial rate experiments in the TGA were performed under isothermal conditions to determine the sflidation reactivity of the solid listed in Table I. The results of these experiments at 600 and 700 "C in 2% H2S-1% H2-97% N2are shown in Figure 4. The initial rate was expressed in mmol of ZnS formed/ (cm2-s).The rate was normalized with the initial surface area of the sorbent. No bulk titanium sulfide was formed. The initial rate of sulfidation for ZnO at 600 and 700 "C was approximately 1.5-2 times higher than for all Zn-Ti-0 sorbents containing 125 mol % Ti (based on Zn-Ti stoichiometry). The initial sulfidation rate was similar for different zinc titanate phases despite the fact that sorbents with different %-Ti compositions formed different zinc titanate phases as shown in Table I. For example, a sorbent with 30 mol % ZnO-70 mol % Ti02contained a mixture of ZnTiO, and Ti02,while a sorbent with 75 mol % ZnO-25 mol % Ti02 was composed of Zn2Ti04and ZnO. However, both of these sorbents had approximately the same initial sulfidation rates (Figure 4). Since all Zn-Ti-0 sorbents containing 125 mol 9% Ti (based on Zn-Ti stoichiometry) had approximately the same initial sflidation rate, further kinetic experiments were performed with just two different types of Zn-Ti-0 sorbents. Experiments were performed with Z2T-a, a sorbent prepared with 2:l atomic ratio of Zn:Ti, and Z2T3-a, which was prepared with 2:3 atomic ratio of Zn:Ti. Zn2TiOl was the only crystalline phase identified by XRD in Z2T-a, while Z2T3-a contained a mixture of ZnTiO,, Zn2Ti308,and Ti02. For the purpose of comparison and in order to establish a base-line sulfidation performance level to meet, similar kinetic experiments were also performed with ZnO. On the basis of SEM micrographs of the unreacted solids, the sorbents are composed of nonporous particles (or grains). In the absence of both diffusional and mass-
- + + -
ZnO + HzS ZnS 1/7Zn7TiOA H,S
HzO ZnS +
.
.
,
400-800
1.31
400-700
0.40
I
.
,
I
10.3 9.3
transfer resistances, the irreversible surface chemical reaction can be described as (4)
where Ro is the initialmolar rate of ZnS formation per unit surface area of the solid reactant [mmol/(cm2-s)],k is the intrinsic rate constant, CHp is the molar concentration of hydrogen sulfide [mmol/cm3], and n is the reaction order. The reaction orders (n)for ZnO and Z2T-a sulfidations were both determined to be 1. Figure 5 illustrates the results obtained for ZnO and Z2T-a sflidation. The linearity of the variation in initial sulfidation rate with changing hydrogen sulfide concentration indicated a reaction order of 1. This agrees with the kinetic data reported by Westmoreland et al. (1977) for zinc oxide. huation 4 was used to describe the initial sulfidation rate. As discussed in the previous section, the sulfidation experiments were peformed in the absence of pore diffusional and mass-transfer resistances by using small particles and high gas flow rates. An Arrhenius relationship can be used to express the intrinsic rate constant as
k = ko exp[-E/RT]
(5) In eq 5, ko is the Arrhenius frequency factor [cm/s], E is the activation energy [kcal/mol], R is the gas constant [1.987 X kcal/(mol*K)],and Tis the temperature [K]. The Arrhenius dependence for ZnO, Z2T-a, and Z2T3-a sflidation reactions was determined by measuring the initial sulfidation rate as a function of temperature. The experiments were performed in 2% H2S-1% H2+7 ?% N2. Figure 6 shows the resultant Arrhenius plots for ZnO, Z2T-a, and Z2T3-a sulfdations. The intrinsic sulfidation rate for ZnO was greater than Z2T-a and Z2T3-a at all temperatures between 400 and 800 "C,while the Arrhenius plots for Z2T-a and Z2T3-a virtually overlapped. The kinetic constants obtained for ZnO and Z2T-a sflidation are listed in Table 111. The activation energies for both ZnO and Z2T-a sulfidation were approximately the same (10.3 kcal/mol and 9.3 kcal/mol, respectively). The major
1894 Ind. Eng. Chem. Res., Vol. 31, No. 8,1992
'"
I
2XH25-1 WH2-97XN2 0
zno
T=KSO"C 2XH25-1 XH2-97KN2
I 0.0
1.0
1.1
1.2
1.3
1.4
0.0
1.5
I 20
40
KO
80
100
lime (mln)
Iff (K) * I000
Figure 6. Comparative Arrheniw plots for ZnO, m-8and , ZTl3-a sulfidation reactions.
difference was in the frequency fadors. The frequency factor for ZnO sulfdation was approximately 3 times greater than that for Z2T-a. The lower frequency factor for the latter was probably caused by fewer reaction sites on the readant surface due to the presence of titanium on the surface. In contrast to the results obtained in this work, Woods et al. (1990) reported the sulfdation kinetics of Zn-Ti oxides to be independent of temperature in the range of 65Cb760 O C . This discrepancy is due to the fact that Woods et al. measured global sulfidation rates of Zn-Ti oxide pellets (3/16-in. diameter and LID = 2.5) under conditions not free from mass-transfer and pore diffusion limitations. Such measurements with pellets provide apparent reaction rates, are pellet-specific, and should not be used to obtain values for intrinsic kinetic parameters such as the activation energy. The initial sulfdation rate reported in the present study for ZnO powders was approximatelytwice as fast as that reported by Westmoreland et al. (1977). This difference in sulfidation rate is believed to be due to differences in crystallinity. Better agreement of the sulfidation rate with Westmoreland et al. (1977) was obtained using a commercially purchased ZnO (EM Science, AR grade). Comparison of SEM micrographs of both ZnO types shawed different crystal geometries present in each sample. The (EM Science) ZnO had a majority of rectangular crystals, while the ZnO prepared by the amorphous citrate technique consisted mainly of spherical crystals. It is known from the literature that exposure of different ZnO crystal faces can be obtained by varying the precursor (Krebs and Littbarski, 1981; Hindermann et al., 1988) during preparation. Variations in exposed crystal faces can cause differences in reactivity in structure-sensitive reaction systems. Conversion Profiles of Bulk Zn-Ti4 Sorbents. The sulfidation reactions of three sorbents, ZnO, Z2T-a, and Z2T3-a, were studied in detail. Figure 7 shows the conversion profiles for these sorbents at 650 'C. Conversion was defined as follows:
x = (Wi - W)/(Wi - w:,
'
0
I
(6)
where W is the weight, Wi is the initial weight, and W: is the fmal weight a t complete conversion assuming that the sorbent reacts completely to form ZnS and TiOz (for the Zn-Ti4 sorbents). For Z2T-a and Z2T3-a, the decrease of reaction rate at high conversion is attributed primarily to nonunifomity
Figure 7. Comparative aultidation conversion profiles for mrbents ZnO, Z2T-8, and ZZT3-a.
Figure 8. SEM micrographs showing (a, top) nonuniform grain sizee of sorbent Z2T-a and (h, bottom) uniform grains of sorbent Z2T-b.
in the grain size distribution of the solids. SEM micrographs of Z2T-a and Z2T3-a showed the presence of both small spherical grains and larger plate-like grains. In the early portion of the conversion profiles for Z2T-a and Z2T3-a (Figure 7). the small grains were mainly reacting while at higher conversion all the small grains had reacted and reaction was due only to the larger plate-like grains. The effect of nonuniform grain size was verified by sulfidation of the solid Z2T-b, which was prepared by using a 2 1 mole ratio of citric acid to metal ions instead of the
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1896 Table IV. XRD Analyses of Sulfided SorbentsD crystdine phases (wt %) sample
Z2T-a 48% sulfded 95% sulfided Z2T3-b 35% suhided ZOO 100% sulfided
ZnO
ZnlTiO,
Zn2Ti,08
ZnTiOa
u-ZnS
8-ZnS
Ti02(rutile)
0 0
23 0
35 6
0 0
16 17
26 53
negligible 24
0
0
0
48
15
0
37
0
0
0
0
33
67
0
'Sulfded at 650 OC in 2% H&l% H2-97% N,.
lime (min)
Figure 9. Sulfdation eonversion profiles for Z ~ T . ~ Z Z T - ~ ,
usual 1:l. A solid with a more uniform, small grain size was, thu8,pduced. Figure 8 shows the SEM micrographs of the solids Z2T-a and Z2T-b, while Figure 9 shows the corresponding conversion profiles obtained a t 600 OC. A uniform reaction rate was measured for sorbent Z2T-b up to very high conversions (Figure 9). The formation of a product layer of ZnS and TiO, around the unreacted solid core also contributed to the slower reaction rate at higher conversion. A more detailed discussion of the effect of product layer diffusion on zinc titanate sulfidation is presented in Lew et al. (1992b). XRD analysea were performed on samples reactedin the TGA at 650 "C in 2 % H2S-1% H2-97% Nz. To provide sufficient amount of samples for analysis, 20-30 mg was used. This was a much larger quantity than what was used for the h e t i c experiments (1-3 mg).On the basis of XRD analyses of partially sulfided sorbents (Table IV), the different zinc titanate compounds are believed to react according to ZnTi03 + H2S ZnS + Ti02 (7)
- -
+
Zn2Ti04+ %H2S y3Zn2Ti308+ y3ZnS + %H20 (8) Zn,Ti308+ 2H2S 2ZnS + 3Ti0, + 2H20 (9) No si&icant amount of free Ti02was detected for the Z2T-a sample sulfided to 48%. This indicates that Zn,Ti308reacted much slower than Zn,TiO,. However, sorbent ZT which contained 20 w t % Zn2Ti0,, 35 wt % ZnzTi30s,and 45 wt % ZnTiO, (Table I) did not have an initial sulfidation rate significantly different than sorbent Z2T-b (100 wt % Zn,TiO,). The reason that no free TiO, was detected in the partially reacted Z2T-a sorbent must then be attributed to a rapid reaction of any TiO, formed with Zn2Ti04to produce Zn2Ti30,: Zn2Ti04+ 2Ti02 ZnzTi30s (10) Physical Changes of Sorbents during Sulfidation. Surface area changes along with morphological changes of
-
Figure IO. SEM micrograph
of unreacted sorbent Z2T3-a.
the Zn-Ti4 sorbents were examined. The samples were subided in the TGA at 650 "C with 2 % H2S-1% H2-97% N,. To obtain sufficient quantity of samples for accurate measurements of the surface area, a large amount (40-60 mg) of sample was used. The solids were sulfided to approximately 90% conversion, and then their surface areas were measured. Typically, all Zn-Ti4 materials showed an increase in surface area upon sulfdation. This in an agreement with our previous reports with this type of sorbent (e.g., Flytmi-Stephanopouloa et al., 1987). For Z2T3-a after 90% subidation,the surface area increased from 2.3 to 2.8 m2/g. By constrast, the surface area of sulfided ZnO was 1.7 m2/g, lower than the value for the fresh sorbent (2.4 m2/g). On the basis of SEM micrographs of the sulfded samples, it is apparent that while ZnS tends to sinter, TiOz' inhibits this sintering by acting as a physical barrier to prevent growth of ZnS particles. Thus, TiO, acta as a dispersant of ZnS. SEM photographs of unreacted and 9(t95% subided Z2T3-a are shown, respectively, in Figures 10 and 11. On the basis of EDS elemental analms of the surface, more sintered regions contained lower levels of titanium. The sulfided region in Figure l l a contains (Zn/Ti)abmic= 0.32 and ( S / Z n ) a h ~=c 1.0 and preserves small grain sizes. In contrast, the more sintered region shown in Figure l l b contains (Zn/Ti),,, = 1.8 and ( S / Z n ) . ~
