1160
Znd. Eng. Chem. Res. 1990,29, 1160-1167
acrylate. J. Macromol. Sci. Chem. 1977, A l l , 967. Marten, F. L.; Hamielec, A. E. Polymerization Reactors and Processes; American Chemical Society: Washington, DC, 1979; Vol. 140, p 43. Matheson, M. S.; Auer, E. E.; Bevilacqua, E. B.; Hart, E. J. Rate Constants in Free Radical Polymerization. J . Am. Chem. SOC. 1949, 71, 497. Mita, I.; Horie, K. Diffusion-Controlled Reactions in Polymer Science. J . Macromolec. Sci.-Reu. Macromol. Chem. Phys. 1987, C27, 91. O'Driscoll, K. F.; Dionisio, J. M.; Mahabadi, H. K. Polymerization Reactors and Processes. In The Temperature Dependence of the Gel Effect in Free-Radical Vinyl Polymerization; American Chemical Society: Washington, DC, 1979; Vol. 140, p 361. Prochazka, 0.; Kratochvil, P. An Analysis of the Accuracy of Determining Molar-Mass Averages of Polymers by GPC with an On-line Light Scattering Detector. J. Appl. Polym. Sci. 1987,34, 2325. Raval, D. K.; Patel, R. G.; Patel, V. S. Grafting of Methyl Methacrylate Onto Guar Gum by Hydrogen Peroxide Initiation. J. Appl. Polym. Sci. 1988, 35, 2201. Ray, W. H. On the Mathematical Modeling of Polymerization Reactors. J . Macromol. Sci.-Reo. Macromol. Chem. 1972, C8, 1. Schulz, G. V.; Harborth, G. Uben eine Dilatometrische Methode Zur Verfolgung von Polymerisationsvorgangen. Agnew. Chem. 1947, 59, 90. Skeirik, R. D.: Grulke, E. A. A Calculation Scheme for Rigorous
Treatment of Free Radical Polymerizations. Chem. Eng. Sci. 1985, 40, 535. Smoluchowski, M. 2. Phys. Chem. 1918,92,129. Soh, S. K.; Sundberg, D. C. Diffusion-Controlled Vinyl Polyrnerization. VI. Comparison of Theory and Experiment. J.Polym. Sci.: Polym. Chem. Ed. 1982,20, 1345. Tulig, T. J.; Tirrell, M. Toward a Molecular Theory of the Trommsdorf Effect. Macromolecules 1981, 14, 1501. Tirrell, M. V.; Pearson, G. H.; Weiss, R. A.; Laurence, R. L. An Analysis of Caprolactom Polymerization. Polym. Eng. Sci. 1975, 15, 386. Vansantha, R.; Rao, K. P.; Joseph, K. T. Synthesis and Characterization of Vegetable Tannin-Vinyl Graft Copolymers. Part I., Cutch-Poly(methy1 acrylate) Graft Copolymers. J . Appl. Polym. Sci. 1987, 33, 2271. Zeman, R. J.; Amundson, N. R. Continuous Models for Polymerization. AIChE J . 1963, 9, 297. Zeman, R. J.; Amundson, N. R. Continuous Polymerization Models-I. Polymerization in Continuous Stirred Tank Reactors. Chem. Eng. Sci. 1965a, 20, 331-361. Zeman, R. J.; Amundson, N. R. Continuous Polymerization Models-11. Batch Reactor Polymerization. Chem. Eng. Sci. 1965b, 20, 637-664.
Received for review J u n e 19, 1989 Revised manuscript received March 1, 1990 Accepted March 14, 1990
Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbents. 1. Single-Pellet Kinetic Studies M. C. Woods and S. K. Gangwal Research Triangle Institute, Research Triangle Park, North Carolina 27709
K. Jothimurugesan and D. P. Harrison* Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803- 7303
The reduction, sulfidation, and regeneration characteristics of high-temperature desulfurization sorbents composed of zinc and titanium oxides in varying molar ratios have been studied in an electrobalance reactor. The addition of titanium oxide reduces the tendency for zinc oxide reduction and subsequent volatization of metallic zinc, thereby increasing the maximum sorbent operating temperature. The reaction parameters studied include temperature, pressure, pellet diameter, and reactive gas concentration. The high-pressure test results are particularly significant since previous single-pellet desulfurization sorbent testing has been limited to atmospheric pressure. At a constant mole fraction of reactive gas, the global rate of both the sulfidation and regeneration reactions increased with pressure. However, at constant reactive gas concentration, increasing the pressure reduced the global reaction rate. In the temperature range of primary interest (650-760 "C),the sulfidation reaction rate was independent of temperature, while the regeneration rate was a relatively weak temperature function. Little deterioration in sorbent reactivity was observed after five reduction and sulfidation cycles with four intervening regeneration cycles. Advanced electric power generation processes such as the integrated gasification combined cycle (IGCC) and molten carbonate fuel cells (MCFC) are receiving attention because of their potential for increasing the efficiency with which coal can be used to generate electricity while at the same time reducing the environmental impact normally associated with coal combustion. Sulfur concentration limitations of approximately 1 ppmv for MCFC applications and 150 ppmv for IGCC systems have been established so that both require essentially complete sulfur removal from the typical gasifier product containing about 5000 ppmv of H2S. In order to satisfy the stringent sulfur removal requirements while at the same time conserving the sensible energy contained in the coal-derived gas, a high-temperature sulfur removal process involving the reaction of H,S with an appropriate 0888-5885/90/2629-1160$02.50/0
metal oxide sorbent is being developed. Zinc-based sorbents are potentially attractive for high-temperature applications because of their favorable thermochemical properties (Westmoreland and Harrison, 1976). Recently, the mixed metal oxide zinc ferrite (ZnFe204)has emerged as a candidate sorbent. Initial research by Grindley and Steinfeld (1981)at US.Department of Energy laboratories has progressed to the point that zinc ferrite has been tested in a fixed-bed process development unit operated by KRW Energy Systems (Schmidt et al., 1988). The results have been generally favorable. ZnFe204has a high sulfur capacity and will react rapidly and completely with H2S (Focht et al., 1988). H2S effluent concentrations of approximately 5 ppmv have been achieved prior to breakthrough from fixed-bed test reactors (Grindley and Steinfeld, 1981; Gangwal et al., 1988). The sulfided sorbent 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1161 has been regenerated with air and steam and then resulfided without major reactivity losses (Grindley, 1987; Focht et al., 1989; Gangwal et al., 1988). However, the operating temperature of zinc ferrite is limited to approximately 650 "C by the instability of both iron and zinc oxides in reducing atmospheres. A t normal operating conditions, Fe203will be reduced to Fe304while higher temperatures and/or more strongly reducing gas compositions will cause further reduction to FeO or even metallic Fe. ZnO is also subject to reduction to metallic zinc, which is somewhat volatile at the conditions of interest (Gibson and Harrison, 1980; Flytzani-Stephanopoulos et al., 1986). Flytzani-Stephanopoulos et al. (1986) and Lew et al. (1989) reported that the addition of Ti02 stabilized the ZnO, thereby permitting an increase in operating temperature. The stabilizing effect was attributed to the formation of mixed metal oxide compounds including ZnTi03, Zn2Ti04,and Zn2Ti308. However, the increased stability is offset by reduced sulfur capacity since Ti02 does not react with H,S at the conditions of interest. The theoretical capacity (based on stoichiometry) of a sorbent containing equimolar quantities of ZnO and Ti02is 198 g of sulfur per kg of sorbent compared to 398 g of sulfur per kg of sorbent for ZnFe204. In this study, pure ZnO and four sorbents containing ZnO to Ti02 molar ratios from 0.8:l.O to 2.O:l.O were subjected to reduction and sulfidation tests over a temperature range of 565-785 "C. The chemical reactions, based upon pure ZnO, are reduction ZnO + H2 (or CO) Zn + H 2 0 (or C02) sulfidation ZnO + H2S ZnS + H 2 0 Although reduction is generally quite slow at the temperatures of primary interest, the elemental zinc is volatile and serious zinc losses can occur after extended periods. The primary objective of these tests was to determine the quantity of Ti02needed to stabilize ZnO against reduction and to confirm that the ZnO remained available for sulfidation. On the basis of the results from these tests, the sorbent containing 1.5 Zn0:l.O Ti02was selected for more detailed kinetic testing. Single-pellet reduction, sulfidation, and regeneration tests were carried out in which temperature, gas concentration, pellet diameter, and total pressure were varied. In a steam/02 atmosphere, two regeneration reactions occur (Focht, et al. 1989): regeneration 1
-
ZnS regeneration 2
+ Y2O2
-
ZnO
+ SO2
-
ZnS + H 2 0 ZnO + H2S The first reaction is much faster than the second. Finally, a series of multicycle runs consisting of five reduction/ sulfidation cycles and four intervening regeneration cycles was conducted to gather preliminary information on long-term sorbent stability. The high-pressure tests are of particular significance since all previous single-pellet kinetic testing of desulfurization sorbents has been conducted at 1 atm.
Experimental Section All single-pellet reactions were studied by using an electrobalance reactor. With few exceptions, Louisiana State University (LSU) performed the atmospheric pressure tests, while the high-pressure tests were conducted
at Research Triangle Institute (RTI). The LSU reactor consisted of a Cahn System 113 electrobalance equipped with a temperature programmer/controller (MicRicon), a Bascom-Turner 113-DC data center, and a gas flow control center. All gases (except steam) were obtained from high-purity cylinders, and flows were regulated by needle valves using calibrated rotameters. The upper portion of the reactor system which contained the electrobalance mechanism was blanketed with inert gas. Additional inert plus reactive gases were premixed and entered the reactor through the side arm of the hangdown tube. The combined gases flowed downward over the sorbent pellet and were vented through a laboratory hood. Water was added to the reactive gas stream by using a Harvard Apparatus Model 944 precision syringe pump. At the point where water mixed with the other reactive gases, the line was heated to induce vaporization, and the reactive gas feed line was heat traced to prevent condensation. The reaction temperature was monitored by using a Chromel-Alumel thermocouple positioned approximately 1/4 in. below the sorbent pellet. The temperature was controlled by using the MicRicon temperature programmer/controller. Both the thermocouple signal and the electrobalance signal were transmitted to the BascomTurner data system for storage on a diskette and/or plotting on a x-y plotter. The RTI reactor was similar except for the high-pressure capability. A Cahn Model 1000 electrobalance was contained in a stainless steel housing to which an Alon-processed stainless steel reactor tube was attached. Gas flow rates were controlled with high-pressure mass flow controllers, and H 2 0 was added by using an Altex 100 highpressure positive displacement HPLC pump. The pressure control system consisted of a 3-way vent valve for atmospheric pressure studies and a back-pressure regulator for high-pressure work. Atmospheric pressure reaction tests at identical conditions were carried out in each reactor. On the basis of the similarity of the results, it was concluded that the large effect of pressure would overwhelm any differences between the two reactors. A stainless steel screen located in the isothermal zone downstream of the suspended pellet was incorporated in both reactors. This screen supported additional sorbent pellets which were subsequently used for sorbent structural characterization tests; all kinetic data were obtained from the single pellet suspended from the electrobalance. Controlled tests showed that the supplemental pellets were exposed to the same reaction conditions and reacted to the same extent as the suspended pellet. All test sorbents were obtained from United Catalysts Inc. (UCI) in the form of 3/16-in.-diameterextrudates. The composition and structural properties of each are summarized in Table I. The C7-1-02 zinc oxide is a commercial UCI product, while the zinc-titanium extrudes were prepared for this study. All zinc-titanium sorbents were calcined at 870 "C for 2 h, and the resultant properties are similar. Surface areas varied from 3.0 to 3.7 m2/g, pore volumes from 0.33 to 0.39 cm3/g, and porosities from 0.56 to 0.66. Skeletal density increased with increasing ZnO content from 4.64 to 5.15 cm3/g. In general, those properties measured by LSU and UCI are in good agreement. X-ray diffraction data supplied by UCI identified Zn2Ti308and free Ti02as the dominant phases present in the sorbent containing the lowest zinc concentration (L-3014). As the zinc to titanium ratio increased, free TiO, decreased and Zn2Ti04was identified along with Zn2Ti308. A t the highest zinc concentration (L-3025),Zn2Ti04was the only phase identified. No significant amounts of free ZnO were
1162 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table I. ComDosition and Structure of Zinc-Based Sorbents L-3014 L-3139 designation 0.8 ZnO/TiOz 1.0 ZnO/TiO, composition 3.0 3.0 bentonite, % cyl. extrude cyl. extrude shape nominal diam, in. 3~16 31 2.5 2.5 nominal LID BET surface area, mz/g 3.2a 3.7a 0.39," 0.37b 0.39,' 0.36b Hg pore vol, cm3/g 5000,' 4110b 5830,' 3540b av Dore diam, A porosity, dimensionless 0.56 0.58 crush strength, lb/mm 4.4" 4.7" He skeletal density, g/cm3 4.64* 4.74b calcination conditions temp, "C 870 870 time, h 2 2 a Measured
by United Catalysts, Inc.
L-3140 1.5 ZnO/TiOz 3.0 cyl. extrude
L-3025 2.0 ZnO/TiO, 3.0 cyl. extrude
C7-1-02 95% ZnO
3
3
2.5 3.6" 0.38,"0.37b 6200,' 4060b 0.66 4.5a 4.93b
2.5 3.0" 0.33,' 0.34b 5830,' 4630b 0.60 6.0" 5.15b
2.5 3.9b NA, 0.13b NA, 130Ob NA NA NA
870 2
870 2
NA NA
116
3/16
0
cyl. extrude I16
Measured by LSU.
Table 11. Test Gas Compositions (Volume Percent) reduction sulfidation regeneration HZ 10 10 0 co 15 15 0 5 5 0 COP 15 15 30 HzO 0 0.25-2.5 0 HZS 0 0 0.4-8.0 0 2 N2 balance balance balance
reported in any of the zinc-titanium sorbents. Gas compositions for the reduction, sulfidation, and regeneration phases are summarized in Table 11. Reduction and sulfidation gas compositions were identical except that no HzS was used during reduction. These gas compositions are representative of the product from a partially quenched, air-blown, fluidized-bed gasifier. Oxygen concentrations in the regeneration gas are typical, but the need to prevent corrosive or condensable gases from reaching the electrobalance mechanism required the use of lower H20 and, consequently, higher N2concentrations than would be expected commercially.
Experimental Results Sorbent Screening Tests. Each of the test sorbents was exposed to the H2S-free reducing gas over the temperature range of 565-785 "C. The temperature was incremented in 55 "C step functions with isothermal periods ranging from 30 to 90 min at each temperature level. The exact temperature-time function along with the experimental results in the form of dimensionless pellet weight, W / Wo,versus time is shown in Figure 1. Initial weight loss occurred at 675 "C, and each of the sorbents lost approximately 0.1% of their original weight by the end of the 675 "C isothermal period. The rate of weight loss increased at 730 "C for all sorbents, and significant differences became apparent. The two sorbents containing high zinc concentrations (C7-1-02) and L-3025) lost appreciably more weight than the three sorbents containing lower proportions of zinc. This tendency continued at 785 "C, the highest temperature studied. Total percent weight loss at the conclusion of the overall test ranged from 1.5% for L-3014 (0.8Zn0:l.O TiOz) to almost 5.0% for C7-1-02 ZnO. By assuming that the entire weight loss is due to ZnO, the pellet weight losses correspond to approximately 3% ZnO loss for sorbents L-3139, L-3140, and L-3014 (low zinc levels), 4 % ZnO loss for sorbent L-3025 (2.0 ZnO: TiOz), and 5% for C7-1-02 (ZnO). The four zinc-titanium sorbents were then subjected to isothermal reduction and sflidation cycles at 675 and 785 "C. Each reduction phase lasted 30 min, while sulfidation was carried out for 170 min. The gas compositions were as reported in Table I1 with the sulfidation gas containing
1 .ooo
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880 f ......
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0.980
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L-3014
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60
160
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200
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300
360
Time. mln
Figure 1. Sorbent weight loss in reducing atmosphere.
c
I
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- L-3014 ..... L-3139
--
io i o i o
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1.1 282
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Throrsllcal wt 'We
i o i ~ ; oi i o i i o r i o i i o
240
Tlme, mln
Figure 2. Reduction/sulfidation results for four zinc oxidetitanium oxide sorbents (T= 675 "C;P = 1 atm).
2.5% by volume H2S. The experimental results in the form of W / Woversus time for the 675 "C runs are shown in Figure 2. Sulfidation behavior for each sorbent was similar, particularly during the early stages of the reaction. The global rate of suhidation of L-3140 sorbent was slightly higher with the global rate for the others bunched closely together. At longer reaction times, the four curves separated as each sorbent approached its separate final weight corresponding to complete sulfidation. The theoretical final values of Wf/ Wobased upon the stoichiometric reaction and no ZnO lost during reduction are shown on the figure. Although the reaction was still occurring at the end of the 200-min test, particularly for L-3025 sorbent, each sorbent had achieved greater than 98% of the theoretical final weight. The results at the higher temperature of 785 "C were similar except that a slight weight loss occurred during the reduction phase.
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1163 ______-----1.100-
1.I 00 \
1.060-
< 3
1.060-
a
1.040-
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260 C
tu
Temperature
m
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-
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160
IkO 200 Tlme, mln
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300
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Figure 3. Effect of temperature on L-3140sulfidation kinetics = 260-870 "C; P = 1 atm).
(T
On the basis of these screening tests, it was decided that L-3140 sorbent (1.5 Zn0:l.O Ti02) provided the best compromise between sulfur capacity and zinc loss due to reduction and vaporization. Hence, additional testing was limited to sorbent L-3140. Sulfidation Results. Tests were conducted using L-3140 sorbent to study the effects of temperature, H2S concentration, pressure, and pellet diameter on sulfidation kinetics. Each sulfidation was preceded by a 30-min exposure to the sulfur-free reducing gas at the desired temperature to provide additional data on the reduction reaction and to simulate the exposure of the sorbent in a fixed-bed reactor. Each test required that a new pellet be used, and care was exercised to select pellets whose diameter, length, and mass were as constant as possible. This was particularly true of the single pellet suspended from the electrobalance. The effect of temperature from 260 to 870 "C was investigated at 1atm using 2.5% H2Sin the sulfidation gas. The results at selected temperatures are shown in Figure 3. In the low-temperature range (1400"C), sulfidation was very slow. For example, the reaction was only about 12% complete after 350 min for the 260 "C run shown in Figure 3. In the intermediate temperature range (425-540 "C), the reaction was considerably faster, and there was a reasonably strong effect of temperature. The 540 "C run shown in Figure 3 was essentially complete after 350 min. It is the moderately high temperature range of 650-760 "C where zinc-titanium sorbents will most likely be used. Behavior in this range is represented by the 705 "C test in Figure 3. Sulfidation was rapid and approached completion after 150 min. However, sulfidation kinetics exhibited almost no temperature dependence in this range as shown by the results of Figure 4. The slightly lower value of W /Woat 760 "C in this figure may be attributed to the small weight loss during the reduction step. Since all tests used the same H2S mole fraction (0.025), the decrease in rate associated with lower concentration at higher temperature was almost exactly offset by increased rate constant and transport parameters to produce the same global rate. The 815 and 870 "C sulfidation tests shown in Figure 3 represent temperatures even higher than those studied in the reduction screening tests. The resulta are consistent, however, with weight loss during the reduction phase becoming even more severe. At 870 "C, approximately 3.5% weight loss occurred during the 30-min reduction period. However, the sulfidation reaction still occurred more or less normally with the lower final values of W / Woattributable to the reduction weight loss. These results are
I
0.060
1
100 Tlme. mln
50
760C
I
I
150
200
Figure 4. Effect of temperature on L-3140sulfidation kinetics = 650-760 "C; P = 1 atm).
(T
1.1 20
..
,
_------------
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o'oe%
' . 2.5
sb
3h
I60 lk0 260 2;O Tlmr, mln
360 4 0 0 4kO 5
Figure 5. Effect of Ha concentration on L-3140 sflidation kinetics (7' = 705 "C; P = 1 atm).
significant in that, while reactor temperature excursions are to be avoided, their existence appears to have a less significant effect on sflidation performance than observed by Focht et al. (1988) for zinc ferrite. Figure 5 shows the effect of H2S mole fraction over the range 0.0025-0.025 at 1atm and a constant temperature of 705 "C. The results are as expected, with the global rate increasing with H2S concentration. A t the lowest concentration, the reaction was only about 65% complete when the run was terminated after more than 8 h. However, the reaction was progressing normally, and it appears that complete sflidation would be achieved if the reaction time had been extended. Additional tests of the effect of the H2Sconcentration at 650 and 760 "C produced similar results. The L-3140 pellets as received from UCI were nominally 3/16-in.diameter. The diameters of selected pellets were carefully reduced to a nominal in. by using a fine grit sandpaper. The length was also reduced to maintain the pellet L I D = 2.5. The effect of pellet diameter on the sulfidation rate at 1 atm and 705 " C is shown in Figure 6. The smaller pellets reacted appreciably faster, requiring approximately 65 min to reach W / Wo = 1.10 compared to 120 min for the larger pellets. Once again, similar resulta were obtained at other temperatures. All previously described sulfidation tests were conducted at 1 atm. The effect of pressure (to 20 atm) was studied over the temperature range of 650-760 "C. H2S mole fraction and pressure combinations were selected to permit the effect of pressure to be studied both at constant H2S mole fraction and constant H2S concentration. The effect
1164 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 1.120 7
1
1 .I 20r---
1 .I 00,
1.I 00
T
705 C
1.020 Pellet Dlamcter
1.000
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f
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I
I
60
100 Tlme, mln
150
10 atm
0.26
I
I
I
I
I
200
0
Figure 6. Effect of pellet diameter on L-3140 sulfidation kinetics ( T = 705 "C;P = 1 atm).
Figure 8. Effect of pressure on L-3140 sulfdation kinetics (constant H2S concentration). I .I 20 1 .I00-
'3"5 1.080a i
%l.080-
3 a
8 1.040z
s,
3 1.020-
ij
I,000
-
-4
- 876
sb
I00
li0
2 0 0 2$0 Time, min
300
3iO
400
4 0
Figure 9. Effect of temperature on L-3140 regeneration kinetics.
variations could be attributed solely to regeneration conditions. The effect of regeneration temperature at 1atm in 2 vol 7'0 O2 is shown in Figure 9. Since reduction and sulfidation conditions were identical, the results during these phases indicate the level of reproducibility which can be achieved. The values of W l W, at the end of each sulfidation phase for the four runs ranged from 1.1064 to 1.1105 with an average of 1.1085. The two regeneration test results at 760 "C indicate that regeneration kinetics are also quite reproducible. Essentially complete regeneration was achieved at 760 " C in approximately 170 min (400-min elapsed time) and in approximately 200 min at 720 "C. A t 675 "C, regeneration was terminated after 200 min with the reaction only about 80% complete. There was no evidence of ZnS04 formation, and it appeared that complete regeneration could have been achieved if the test had been allowed to continue. The 675 "C regeneration temperature exceeded the ZnSO, decomposition temperature at these conditions reported by Focht et al. (1989). Regeneration kinetics are more sensitive to temperature than sulfidation kinetics (compare Figure 9 to Figure 4). Previous results on the regeneration of zinc ferrite (Focht et al., 1989) found that the intrinsic rate of reaction with O2was sufficiently fast that the global rate was transport controlled. The intrinsic reaction rate with steam was much slower, however, and the increased temperature dependence shown in Figure 9 could be due to the increased importance of the steam reaction at high temperature. Increasing oxygen concentration in the regeneration gas (see Figure 10) had the same qualitative effect as increasing
Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 1165
mi,,
c
1.1 201
I
,'.,
1.100
... . Cycle 1
T
5m P
ti
5 l.020t 1 .ooo--I
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= 875 C
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-- 2 . _ -, A -8
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1.1 00 .'
Press. atm.
vol. X
20
2.0
'5 1.080
O2
r '
pa 1.060
-5
i aaaC
1
Tmperature T = 720 C
"---I
1
I
0.4
I
I
-
H2S concentration in the sulfidation gas (see Figure 5). Each test shown in Figure 10 was terminated at W / Wo 1.03 (corresponding to approximately 75% regeneration) to provide partially regenerated pellets for sorbent structural characterization. There was no evidence of ZnSOl formation, and complete regeneration would presumably have been possible if each test had been allowed to continue. The 8-fold change in O2 concentration produced a dramatic effect upon the global kinetics. Only 25 min was required to achieve W / Wo= 1.03 using 8 vol % O2compared to 200 min using 1vol ?& 02.Although the global rate is much faster at higher oxygen concentrations, pellet temperature excursions caused by the exothermic regeneration reaction also increase, which may accelerate the rate of sorent deterioration through multiple sulfidation/regeneration cycles. The effects of pressure at both constant O2concentration and constant O2 mole fraction are shown in Figure 11. Increasing the pressure from 1to 20 atm using 0.02 mole fraction of oxygen produced a significant increase in global rate. However, the increase was less than expected strictly on the basis of the increased O2concentration. An increase in pressure from 1to 5 atm, with a corresponding reduction in composition from 0.02 to 0.004 mole fraction of O2 (to produce a constant O2concentration), resulted in a marked decrease in the global regeneration rate. Both results are consistent with the previously discussed effect of pressure on the sulfidation reaction. Multicycle Test Results. Previous studies (Focht et al., 1989) have shown that zinc ferrite sorbent reactivity generally decreased during multicycle operation, particularly if regeneration temperatures were excessive. Mul-
ticycle tests consisting of five reductions and sulfidations with four intervening regenerations were carried out to provide initial information concerning the durability of L-3140 sorbent. All multicycle tests were at 1 atm, and standard reduction and sulfidation conditions were used. Regeneration temperature and O2concentration served as the reaction parameters. The most severe regeneration conditions studied were 760 "C using 4 vol70 OP The results in the form of W / Wo versus time for the complete test are shown in Figure 12. There was a small but noticeable change in sorbent performance between cycles 1and 2, but otherwise there was very little deterioration. The spread in the overall results is comparable to the expected spread between multiple runs at identical conditions, with each run using a different sorbent pellet. Similar multicycle tests under milder conditions of 720 "C regeneration temperature and 0.02 mole fraction of O2 were conducted. The lower temperature and oxygen concentration decreased the global reaction rate and extended the time of the regeneration cycle. Reproducibility between cycles, however, remained quite good. While more extensive multicycle testing is needed, these preliminary results are encouraging. L-3140 sorbent appears to possess better durability at higher temperatures than do presently available zinc ferrite sorbents. S t r u c t u r a l Property Changes. Pellet structural properties such as pore volume, pore size distribution, and surface area play an important role in determining the kinetics of both the sulfidation and regeneration reactions. Pore volume and pore size distribution are particularly important when the global kinetics are controlled by transport resistances. The stability of the sorbent through numerous cycles is determined by the stability of the structural properties as the pellet is alternately sulfided and oxidized. Because the molar volume of product sulfide is greater than that of the reactant oxide, structural property changes which accompany sulfidation are inevitable. However, such changes should be at least partially reversed during the regeneration step. In addition to the structural changes associated with reaction, the porous sorbents may experience sintering, which generally causes irreversible structural changes. The temperature must be controlled so that sintering is avoided, or at least minimized, if the sorbent is to maintain high capacity and high reactivity over the numerous cycles required for commercial operation. The dimensions of the single pellet suspended from the elctrobalance were carefully measured both prior to and immediately after a reaction test. The L-3140 pellet
1166 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table 111. Structural ProDertv Chanaes durina Reduction/Sulfidation/Reaenerationof L-3140 Sorbent pellet skeletal surface pore vol, av diam, density, density, area, no. of tests cm3/cr A dcm3 dcm3 mZ/g fresh sorbent 0.37 1.73 4.84 3.6 4060 reduced sorbent 650 "C 1 0.36 4150 1.69 5.16 3.5 705 "C 1 0.36 4130 1.71 5.05 3.5 760 "C 1 1.71 5.15 3.6 0.36 4230 partially sulfided sorbent (-70% sulfidation) 650 O C 1 0.26 1880 1.88 3.68 5.6 0.26 705 "C 1 1.85 4.03 5.4 1930 760 "C 1 0.26 2430 1.90 4.12 4.3 fully sulfided sorbent 650 "C 3 0.22 1480 1.97 3.82 6.1 705 "C 3 0.23 1700 1.94 3.87 4.9 3 760 "C 0.24 1.95 3.98 4.7 2075 1 815 "C 0.23 2560 1.96 4.07 3.6 870 O C 1 0.23 5120 2.02 3.96 1.8 partially regenerated sorbent (-70% regeneration) (sulfidation at 675 "C) 720 "C 4 fully regenerated sorbent (sulfidation at 675 O C ) 675 "C 1 1 720 "C 760 "C 1
porosity, dimensionless 0.66 0.61 0.62 0.62 0.49 0.48 0.49 0.43 0.44 0.47 0.45 0.46
0.31
3060
1.80
4.64
3.8
0.55
0.34 0.33 0.34
3490 3820 NA
1.77 1.77 1.77
4.84 5.42 5.10
3.9 3.5 NA
0.60 0.58 0.60
volume remained essentially constant throughout, including the sulfidation tests at 815 and 870 "C. The ratio of final to initial pellet volume (as calculated from measured diameter and length) was, with one exception, always in the range 0.98-1.0. This pellet volume stability contrasts significantly to the previous results of Sa et al. (1989) who reported approximately 20% volume reduction following the sulfidation of L-1442 zinc ferrite pellets at 700 "C. Since the L-1442 pellets contained no inorganic binder, it is not clear whether the improved volume stability of L-3140 pellets is an inherent property of the Zn0.Ti02 systems or is due to the 3% bentonite addition. The supplemental pellets positioned in the reactor just below the suspended pellet were used to characterize internal structural property changes. The key results are summarized in Table 111. Over the temperature range of primary interest, 650-760 "C, the internal structural properties were almost independent of temperature but varied significantly with reaction. Essentially no reduction of the sorbent occurred over the 650-760 "C temperature range, and the structural properties of the fresh and reduced sorbent were effectively equal. Sulfidation, however, resulted in significant reduction in pore volume and average pore diameter. These results, which occur as the larger sulfur atom replaces oxygen, are similar to previous results reported by Sa et al. (1989) for L-1442 zinc ferrite. In contrast to Sa et a1.k results, however, the surface area of the sulfided sorbent increased in the temperature range 650-760 "C, remained constant at 815 "C, and decreased upon sulfidation at 870 "C. Gangwal et al. (1989) have reported similar increases in the surface area of a 0.8 ZnO:Ti02 sorbent after two sulfidations with an interim regeneration. Flytzani-Stephanopoulos et al. (1986) have also reported surface area increases upon sulfidation of other sorbents, indicating that increases or decreases may be specific to individual systems. Since the average pore diameter is essentially inversely proportional to surface area, the average pore size decreases at conditions where the surface area increases, and vice versa. The increased pellet density which accompanies sulfidation simply reflects the increased mass
and constant volume of the pellets. Decreases in skeletal density accompany the conversion of ZnO (SG = 5.6) to ZnS (SG = 4.1) (SG = specific gravity). These data suggest that sulfidation produces a roughening of the interior surface which causes the increased surface area and decreased average pore size. While no specific sintering studies were conducted, the results also suggest that sintering becomes significant as the temperature approaches 800 "C. Sintering will have the effect of smoothing the roughened surface, thus reducing the surface area and increasing the average pore size. Regeneration appears to be largely successful in restoring the original sorbent structural properties. The pore volume increases from about 0.23 cm3/g for the fully sulfided sorbent to 0.34 cm3/g for the regenerated sorbent, which represents less than 10% reduction from the pore volume of the original sorbent. The surface area and pellet density are reduced during regeneration to values approximately equal to those of the original sorbent. The average pore diameter of the regenerated sorbent is more than twice that of the sulfided sorbent and is 5 1 5 % lower than the original material. Skeletal density increases during regeneration and, except for the unexpectedly high value following 720 "C regeneration, is approximately equal to that of the fresh sorbent. The X-ray diffraction patterns of the original and regenerated sorbent were essentially identical, confirming that crystalline phase changes associated with sulfidation were also reversible. The overall restoration of properties during regeneration is consistent with the experimental kinetics results from multicycle tests which showed good reproducibility during each of the 4 cycles. Acknowledgment This research was sponsored by the US. Department of Energy, Morgantown Energy Technology Center, under
Contract DE-AC21-87MC24160. We acknowledge Steve Bossart and Suresh Jain of METC for their contributions during the research and United Catalysts, Inc., for preparing the test sorbents. Registry NO. HpS, 7783-06-4; ZnO, 1314-13-2;TiOp,13463-67-7.
Ind. Eng. Chem. Res. 1990,29,1167-1172
Literature Cited Flytzani-Stephanopoulos, M.; et al. Detailed Studies of Novel Regenerable Sorbents for High-Temperature Coal-Gas Desulfurization-I. Proceedings of the Sixth Annual Meeting on Contaminant Control in Coal-Derived Gas Streams, 1986; DOE/METC-86/6042. Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1988, 43, 3005. Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1989, 44, 2919. Gangwal, S. K.; et al. Bench-Scale Testing of Novel High-Temperature Desulfurization Sorbents. Final Report, Contract DEAC21-86MC23126, US.Department of Energy: Washington, DC, 1988. Gangwal, S. K.; et al. Environ. Prog. 1989, 8, 26. Gibson, J. B.; Harrison, D. P. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 231. Grindley, T. Sidestream Zinc Ferrite Regeneration Testing. Pro-
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ceedings of the Seventh Annual Gasification and Gas Stream Cleanup Systems Review Meeting, 1987; DOE/METC/6079. Grindley, T.; Steinfeld, G. Development and Testing of Regenerable Hot Coal-Gas Desulfurization Sorbent. DOE/METC/16545-1125, 1981. Lew, S.;Jothimurugesan, K.; Flytzani-Stephanopoulos,M. Znd. Eng. Chem. Res. 1989,28,535. Sa, L. N.; Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1989, 44, 215. Schmidt, D. K.; et al. Fluidized Bed Coal Gasification with Hot Gas Cleanup. Proceedings of the Eighth Annual Gasification and Gas Stream Cleanup Systems Review Meeting, 1988; DOE/METC88/6092. Westmoreland, P. R.; Harrison, D. P. Enuiron. Sci. Technol. 1976, 10, 659.
Received for review J u n e 7 , 1989 Revised manuscript received January 29, 1990 Accepted February 21, 1990
Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbents. 2. Single-Pellet Sulfidation Modeling K. Jothimurugesan and D. P. Harrison* Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803- 7303
In the temperature range of primary interest, 650-760 "C,the reaction between H2S and a desulfurization sorbent composed of 1.5 Zn0:l.O TiOz (designation L-3140) can be described by a special case of the unreacted core model in which the global reaction rate is controlled by mass transfer and product layer diffusion resistances. Effective diffusivities through the product layer predicted by the random pore model show reasonable agreement with best-fit effective diffusivities determined by numerical methods. Although the magnitudes of the predicted and best-fit values differ, on average, by about 33%, the observed effects of temperature and pressure are consistent with random pore model predictions. Mass-transfer coefficients predicted by a modified Froessling equation differ from best-fit values by an average of only 15%. However, neither the effect of pressure nor the effect of temperature is adequately described. This is attributed to the fact that chemical reaction resistance is not truly negligible near the beginning of the reaction so that best-fit mass-transfer coefficients actually reflect both mass-transfer and reaction effects. The processes that determine the global rate of a gassolid noncatalytic reaction within a single pellet are numerous and complex. Mass transfer of gaseous reactant from the bulk gas to the pellet exterior surface is followed by diffusion through the pores of the pellet and perhaps through a layer of solid product before the solid reactant is encountered and the surface reaction can occur. The reverse of this sequence must be followed for gaseous product to reach the bulk gas. The structural characteristics of the porous pellet are important in determining the rates of the various steps, and the process is further complicated by the fact that pellet structure may vary with the extent of the reaction. At sufficiently high temperatures, additional structural changes which are independent of reaction may be imposed by sintering. A general discussion of the importance of these phenomena is provided by Szekely et al. (1976). In spite of the overall complexity, it is often possible to describe the global rate in terms of relatively simple mathematical models that consider only the most important phenomena and neglect steps that contribute little to the global rate. Such an approach was adopted by Focht et al. (1988) in modeling the sulfidation reaction between H2S and single cylindrical pellets of zinc ferrite. A special case of the unreacted core model (Yagi and Kunii, 1955) in which the global rate was determined by the rates of mass transfer to the pellet surface and diffusion through
the porous product layer was used. A similar approach has proven to be successful in modeling the experimental sulfidation kinetics of L-3140 zinc oxide-titanium oxide sorbents reported in our previous paper (Woods et al., 1990).
Unreacted Core Model The unreacted core model requires that the reaction be confined to a surface separating the solid reactant core from an outer product layer. The initial reaction surface corresponds to the external surface of the pellet. The thickness of the product layer increases with time, producing a shrinking core of unreacted solid. Within the core of solid reactant, the concentration is unchanged from its initial value of CW,while within the product layer the solid reactant concentration is zero. The general sulfidation reaction can be represented by the following stoichiometry: A(g) + bB(s) products (1)
-
For the L-3140 sorbent of interest, the specific reaction is H2S(g) + 0.667(1.5Zn0.Ti02)
-
products
(2)
By considering the reaction to be isothermal and the pellet to be infinitely long, a simple algebraic relation exists
0888-5885/90/2629-1167$02.50/0 0 1990 American Chemical Society
