Modeling of Sulfided Zinc Titanate Regeneration in a Fluidized-Bed

in a Fluidized-Bed Reactor. 1. Determination of the Solid Conversion Rate Model Parameters ... Publication Date (Web): December 1, 1997. Copyright...
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Ind. Eng. Chem. Res. 1997, 36, 5432-5438

Modeling of Sulfided Zinc Titanate Regeneration in a Fluidized-Bed Reactor. 1. Determination of the Solid Conversion Rate Model Parameters J. T. Konttinen,*,† C. A. P. Zevenhoven,‡ K. P. Yrjas,‡ and M. M. Hupa‡ Carbona Corporation, P.O. Box 610, FIN-33101 Tampere, Finland, and Department of Chemical Engineering, A° bo Akademi University, Lemminka¨ isenkatu 14-18 B, FIN-20520 Turku, Finland

Regenerable mixed metal oxide sorbents are prime candidates for the removal of hydrogen sulfide (a major pollutant) from the hot coal gas in the simplified integrated gasification combined cycle processes. As part of this sulfur removal process development, reactor models are needed for scale-up. It is essential for this work to apply a reliable and simple modeling correlation for the conversion rate of zinc sulfide or oxygen in the regeneration reaction. The fit of such a model, assuming uniform conversion of the ZnS, was obtained from ambient pressure thermogravimetric analyzer test data with sulfided zinc titanate samples. An activation energy of about 140 kJ/ mol was obtained for the regeneration reaction rate constant. Introduction The simplified IGCC (integrated gasification combined cycle) process incorporates pressurized fluidizedbed gasification of solid fuels and a hot gas clean-up train including a sulfur removal process with a regenerable sorbent (Salo and Hokaja¨rvi, 1994; Salo et al., 1995). The IGCC process has the advantages of improved power generation efficiency, high power-to-heat ratio for cogeneration, excellent environmental performance, and simple plant configuration and modularity. In the IGCC process, sulfuric gases (mainly H2S) are produced from the fuel-bound sulfur in the gasifier, which will convert to SO2 when the fuel gas is combusted in the gas turbine. Regenerable solid sorbents capable of operating at high temperature and high pressure (HTHP) can be used for sulfur removal in order to minimize the amount of solid wastes produced (which is the case when employing once-through calcium-based sorbents) and to recover fuel-bound sulfur for further use. Zinc titanate appears to be the leading sorbent for sulfur removal in HTHP fluidized-bed reactors (Harrison, 1995). Recent tests with pilot-scale fluidized-bed reactors proved that 95-99% sulfur removal efficiency as well as continuous production of gaseous SO2 from sorbent regeneration can be obtained with zinc titanates (Salo et al., 1995; Konttinen et al., 1996). The kinetics of sulfur removal with regenerable zinc titanate sorbents has been studied by many researchers (Woods et al., 1989, 1990; Lew, 1990; Gupta and Gangwal, 1992). Most of that work was made for the purpose of selecting the best sorbent candidate rather than for reactor-sizing purposes. A method to build a sulfidation reactor model based on kinetic data on the sulfidation reaction was recently presented (Konttinen et al., 1997a,b). Few experimental studies have been made on the rate of regeneration of the sulfided sorbents. On the basis of the reported experimental results (Bagajewicz, 1988; Woods et al., 1989, 1990; Siriwardane and Woodruff, 1995; Yrjas et al., 1996), the rate of the exothermal regeneration reaction is considered to * Author to whom correspondence should be addressed. Telephone: +358-3-358 0314. Fax: +358-3-358 0325. E-mail: [email protected]. † Carbona Corporation. ‡ A ° bo Akademi University. S0888-5885(97)00035-3 CCC: $14.00

be kinetically controlled; i.e., for (fluidized-bed) reactorsizing purposes, a reliable model to describe the reaction kinetics in the regeneration reactor should be found. A zinc sulfide conversion rate model, combined with fluidized-bed mass and energy balances, can be used to predict the performance of a large-scale regeneration reactor. Due to the narrow temperature operating window (600-750 °C) requirement in regeneration, it is essential to have a steady-state kinetic model for reactor sizing as well as a dynamic model for process control design. This paper concentrates on the experimental results of some laboratory-scale tests in order to determine kinetic parameters for a zinc titanate regeneration rate model. The recently reported experimental results of Yrjas et al. (1996) will be analyzed in connection with the other experimental data used in this text. The modeling work will continue in a separate paper (Konttinen et al., 1997 c), where the application of the solid conversion rate model into HTHP pilot-scale fluidized-bed regeneration test data will be reported and discussed. Zinc Sulfide RegenerationsReaction Chemistry Zinc titanate is a regenerable sulfur removal sorbent which is prepared by using a mixture of zinc and titanium oxides to form zinc titanate particles (Woods et al., 1989; Gupta and Gangwal, 1992). A general reaction of zinc titanate in sulfur (H2S) capture at HTHP conditions is (Lew, 1990) 1

/xZnxTiyO(x+2y)(s) + H2S(g) f ZnS(s) + y/xTiO2(s) + H2O(g) (1)

After sulfidation of the sorbent to a certain predetermined level, the sorbent is regenerated. The desirable reaction in zinc titanate regeneration is

ZnS(s) + 1.5O2(g) f ZnO(s) + SO2(g)

(2)

The individual oxides of the sorbent are then assumed (Woods et al., 1989) to produce the original zinc titanium oxide starting material as follows:

xZnO(s) + yTiO2(s) f ZnxTiyO(x+2y)(s)

(3)

At regeneration conditions, undesired zinc sulfate can © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5433

Figure 1. Zn-O-S phase stability diagram (Barin et al., 1977; JANAF, 1985).

however be formed via the following side reactions (Woods et al., 1989; Siriwardane and Woodruff, 1995):

ZnS(s) + 2O2(g) f ZnSO4(s)

(4)

ZnO(s) + SO2(g) + 0.5O2(g) f ZnSO4(s)

(5)

Figure 1 shows the phase diagram of the Zn-S-O system as a function of O2 and SO2 partial pressures. At the relevant temperature range of the regeneration reactor (500-750 °C), the phase diagram suggests that the regeneration of sulfided zinc titanate according to reaction 2 is possible by using a combination of low O2 and SO2 partial pressures. Laboratory-scale results reported earlier (Bagajewicz, 1988; Gupta and Gangwal, 1992; Siriwardane and Woodruff, 1995; Yrjas et al., 1996), however, indicate that the kinetic rate of the reactions (rather than thermodynamics) plays a significant role in sulfided zinc titanate regeneration. Bagajewicz (1988) studied the regeneration of sulfided high surface area ZnO particles (dp = 600 µm), using pure ZnS (99.9 wt %) in an atmospheric thermogravimetric apparatus, at temperatures 650-700 °C. It was concluded that the oxidation of ZnS to ZnO follows the direct route according to reaction 2 with no intermediate compounds. Kinetic parameters were determined for a Langmuir-Hinschelwood type expression to describe the rate of reaction 2, in which the reaction of adsorbed oxygen with the solid is the rate-limiting step with the activation energy of 270 kJ/mol. The order of the partial pressure of O2 in the rate expression was found to be close to 1. Bagajewicz suggested that the ZnS regeneration rate (as a function of solid conversion) passes through a maximum, as a result of an increasing available reactive surface area of the solid. Zinc sulfate or oxysulfate formation was found to be caused by secondary reactions with freshly formed ZnO. The experiments yielded very low reaction rates for these secondary reactions, but some differences were found between fresh and regenerated ZnO, which were attributed to changing intraparticle morphology of the ZnO particles. Woods et al. (1989, 1990) studied the regeneration of sulfided zinc titanate pellets (dp = 4.8 mm) at temperatures 680-760 °C, mainly at ambient pressure. The rate of sulfided zinc titanate regeneration was found to increase with increased temperature, and the test data showed that regeneration kinetics are more sensitive to temperature than sulfidation kinetics are, which is in agreement with the findings of Bagejevicz (1988). An

increase in O2 from 1 to 8 vol % concentration increased the rate of regeneration, with no sign of ZnSO4 formation. It was stated that part of the concentration effect may have been due to an increase in temperature due to the exothermic reaction in a large pellet. Although the regeneration reaction rate was shown to be dependent on gas temperature and O2 concentration, no kinetic parameters nor the order in O2 were determined. An increase in total pressure from 1.013 bar to 20 bar (using constant O2 concentration) decreased the regeneration rate, which was attributed to the decrease of external gas mass-transfer coefficients with pressure. No evidence of ZnSO4 formation was found in any of these tests. The change in regeneration reactivity of the ZnScontaining sorbent was found to be neglible after four consequent sulfidation/regeneration cycles. Siriwardane and Woodruff (1995) studied the interaction of oxygen with zinc sulfide samples by FTIR. They suggest that at 550-650 °C, the reactions in regeneration follow paths (2) or (5), which is in agreement with the findings of Bagajewicz (1988). At temperatures 550-650 °C they found that gaseous SO2 according to reaction 2 was the initial product which was then adsorbed by ZnO to form sulfate in the presence of oxygen (reaction 5) at oxygen partial pressures over 0.05 mbar. Several forms of adsorbed SO2 on the solid surface were detected at temperatures 600 and 650 °C. Zinc sulfate started to form after longer exposures, which, in agreement with Bagajewicz (1988), indicates that the overall rate of reaction 5 is much lower than that of reaction 2. After evacuation of the oxygen from the sample, the sulfate started to decompose. The effect of elevated total pressure was not studied. Experimental Methods During the development of the fluidized-bed sulfur removal process with regenerable sorbents, laboratoryscale data was produced rather for the needs of selecting the optimum sorbent for the process than for producing input data for fundamental modeling purposes. However, some information on kinetics of regeneration based on the data available can be obtained. The data in this paper were produced in thermogravimetric analyzers (TGA) at ambient and 20 bar pressures. Examples of the use of this experimental method for sulfur removal and regeneration kinetic modeling purposes can be found in the literature (Bagajewicz, 1988; Ayala and Kim, 1989; Lew, 1990; Krishnan and Sotirchos, 1994; Zevenhoven et al., 1996; Konttinen et al., 1997a). Atmospheric Pressure TGA. The kinetics of regeneration of sulfided zinc titanate sorbent A (UCI-3) was evaluated in a thermogravimetric analyzer DuPont 951, designed to handle corrosive gases (Gupta and Gangwal, 1992; Mojtahedi et al., 1996; Konttinen et al., 1997a). The inlet gas used in the atmospheric pressure thermogravimetric analyzer (ATGA) experiments consisted of O2, SO2, and N2 at compositions relevant for a large-scale regeneration reactor (Table 1). Approximately 50 mg of the sorbent in the 100-300 µm size range was used in each experiment. Each run included preparation of a standard sulfided sample by using a procedure of heating a 50 mg sample to 550 °C in helium, sample reduction in H2S-free gasifier gas for 10 min, and sulfidation in gasifier gas containing 1.2 vol % H2S for 60 min. The sulfided sample was then cooled to the desired regeneration temperature before contact with regeneration gases. Regeneration was

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Figure 3. PTGA sulfidation and regeneration test with sorbent D at 20 bar (Yrjas et al., 1996). Wend is the sample weight in the case of 100% H2S capture. Figure 2. TGA regeneration test results with sorbent A at 1.013 bar (Konttinen and Mojtahedi, 1993; Mojtahedi et al., 1996). Table 1. Ambient Pressure TGA Regeneration Test Conditions (Mojtahedi et al., 1996) O2 SO2 test no. (vol %) (vol %) 1 2 3 4 5

1.64 0.44 2.92 0.2 1.65

0.86 2.0 0.18 2.58 1.35

temperatures (°C)

hold time (min)

475, 525, 575, 625, 675, 725 same as above same as above and 775 same as above same as above

30 30 30 30 30

started at a low temperature (400-475 °C) and the temperature was increased stepwise up to 775 °C. The test temperature sequences are shown in Table 1. The partial pressures of O2 and SO2 in the inlet gas in each experiment at all regeneration temperatures were at a level where zinc sulfate was the thermodynamically stable solid phase, and, consequently, ZnO formation would not be possible. However, as the analysis of the results will show, this is not the case. Figure 2 shows two resulting time vs sample dimensionless weight curves of the ATGA regeneration tests (Konttinen and Mojtahedi, 1993; Mojtahedi et al., 1996). In the beginning of the tests, the reduction of the sample is shown by a slight decrease in the sample weight. During sulfidation (with H2S) the sample weight increases significantly due to ZnS formation. At the lowest temperatures of regeneration (475-525 °C) the sample weight increases slightly with certain gas (O2, SO2) compositions, which indicates zinc sulfate formation. The regeneration of sulfided zinc titanate (releasing gaseous SO2) is shown by a significant sample weight decrease at temperatures 575-725 °C, at each temperature with a different rate. The results will be analyzed later in this text. Pressurized TGA. The zinc titanate sorbent D (UCI-5) was previously tested in pilot-scale fluidized bed sulfur removal reactors, and the immediate results of these tests are reported elsewhere (Salo et al., 1995; Konttinen et al., 1996). The properties of sorbents studied in this text are shown in Table 2. Yrjas et al. (1996) tested the sulfidation and regeneration kinetics of sorbent D in a pressurized thermogravimetric analyzer (PTGA). A 100-150 mg sample of sorbent D was placed in a sample holder which was lowered into the pressurized (20 bar) reactor in a nitrogen atmosphere. The reactor was then heated to the desired sulfidation temperature (550 or 650 °C). The sorbent was sulfided with a gas mixture of H2S, H2, and N2 for 2.5 h. The gas composition was changed to a

Figure 4. PTGA sulfidation and regeneration test with sorbent D at 20 bar. The sample was diluted with 66% inert silica sand (Yrjas et al., 1996). Table 2. Chemical and Physical Properties of the Fresh Sorbents Used in the Tests (Mojtahedi et al., 1996; Salo et al., 1995) property

sorbent A

sorbent D

zinc (wt %) bulk density (g/cm3) particle density (g/cm3) mercury pore volume (cm3/g) porosity surface area (m2/g) median pore diameter (Å) average particle size (µm)

47.3 1.24 1.97 0.28 0.55 2.5 4000 200

46.8 1.47 2.26 0.21 0.475 1.4 7000 309

mixture of O2 and N2 and the regeneration was started, to continue for 2 h. A detailed description of test procedures and analysis of results is reported by Yrjas et al. (1996). Figure 3 shows a typical PTGA test result. In the beginning of the test, the weight increases by about 7 mg, most probably due to a buoyancy effect during the heating of the sample. After H2S is introduced, the weight of the sample increases. During regeneration, O2 (partial pressure ) 0.6 bar) is introduced and the weight of the sample decreases rapidly, obviously as a result of the release of gaseous SO2. The rate of regeneration changes significantly after a short period. In Figure 4, it is interesting to note that the change in the regeneration rate does not take place when a lower 0.04 bar is used as oxygen partial pressure. Figure 5 shows the results of the test of introducing SO2 and O2 to fresh zinc titanate. The weight of the sample increases due to sulfate formation and starts to decrease when the SO2 supply is halted. The reasons for these phenomena will be discussed later in this text.

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Figure 5. PTGA test with fresh sorbent D showing zinc sulfate formation and decomposition at 650 and 750 °C (Yrjas et al., 1996). The maximum sample weights (in the case of 100% ZnSO4 formation) are shown above the curves.

The Determination of Rate-Controlling Steps Mass Transfer. In order to identify the ratedetermining mechanism, the unreacted shrinking core (USC) modeling approach can be used as a first approximation (Levenspiel, 1989; Zevenhoven et al., 1996). The rate-determining mechanism can be distinguished from a set of time-conversion data by plotting a function f(X) vs t/τ, which gives a straight line for the ratedetermining step. For more than one rate determining mechanism the principle of “additive reaction times” applies (Sohn, 1978). In order to evaluate the effect of sample bed diffusion with a USC model on the thermogravimetric analyzer test results, the equations reported in the previous paper in connection with sulfidation TGA tests (Konttinen et al., 1997a) will be applied. In the ATGA tests, the shape of the sulfided zinc titanate containing sample is approximated by a hemisphere. In the regeneration tests with PTGA the sulfided zinc titanate containing solid is located as a thin layer inside a cylindrical sample holder. The equations for the calculation of the relative effect of sample bed diffusion for this PTGA by Zevenhoven et al. (1996) will be applied in this text. If the time taken for sample bed diffusion is significant with respect to the total time to reach a certain sorbent conversion level, further analysis of the ATGA or PTGA data is omitted. Temperature Increase of Solid Reactant Due to Exothermal Reaction. Woods et al. (1989) tested the possibility of a temperature increase during exothermal regeneration by measuring the temperature inside a FeS- and ZnS-containing zinc ferrite pellet (4.8 mm). The increase was highest immediately after introducing oxygen and then leveled off. Temperatures 35-55 °C higher than the bulk gas were measured with 4-6.3 vol % oxygen, which showed that the temperature increase was directly proportional to the oxygen content. Bagajewicz (1988) reported certain maximum peaks in ZnS regeneration rates, which could be partly explained from increasing temperature inside the particle. A simple estimate of the maximum possible temperature difference across a gas film at the solid sample surface in the TGA tests is given by Lew (1990):

Ts - Tb ) r0

(-∆H)Pr2/3 jHCPG

where jH ) 0.91Re-0.51 (6)

The possibility of thermal gradients existing inside a

Figure 6. The comparison of time vs sulfided zinc titanate conversion TGA data with sorbent A at 1.013 bar and with sorbent D at 20 bar.

solid reactant can be estimated with the correlation for maximum temperature difference under steady-state conditions (Froment and Bishoff, 1989), assuming complete conversion of the gas inside the particle:

(∆Ts)max (-∆H)DeffCA ) Ts,S λeff,sTs,S

(7)

Determination of the Rate-Determining Step and Parameters from the TGA Test Data Zinc Sulfate Formation in TGA Tests. The regeneration of the ZnS-containing sample in Figure 2, at 1.013 bar with different mixtures of O2 and SO2, is started at temperatures 475-525 °C. At 525 °C, the weight of the sample slightly increases, which might be attributed to some sulfate formation. At 575 °C the weight starts to decrease probably due to SO2 release according to reaction 2. Significant decrease of sample weight takes place at temperatures 625-775 °C. The rate of weight decrease seems to be directly proportional to the oxygen content of the gases, despite different SO2 contents in the inlet gases. Thermodynamic equilibrium indicates that the regeneration reaction 2 is irreversible at temperatures 625-775 °C, so SO2 partial pressure should not play any role in regeneration. Near complete regeneration (using higher O2 contents), the weight of the sample slightly increases, which can, again, be attributed to sulfate formation. Most of the data at temperatures 625-725 °C will be used for determining the kinetic parameters of regeneration reaction 2, since the rate of possible sulfate formation should be relatively low at these temperatures (Bagajewicz, 1988; Siriwardane and Woodruff, 1995). The shape of the weight decrease curve in Figure 2 with 2.92 vol % O2 at 675 °C shows some sulfate formation; therefore the data at this condition and corresponding other data will be omitted from further analysis. External Mass- and Heat-Transfer Effects in TGA Tests. The conversion of sulfided zinc titanate to ZnO was calculated from the TGA curves (Figures 2-4) by using the following equation:

XZnS )

W0 - W(t) W0 - Wend

(8)

Examples of the resulting time vs ZnS conversion curves are shown in Figure 6. Two curves represent ambient pressure data and two represent PTGA data obtained at 20 bar.

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Figure 7. The relative importance of the time required by sample bed diffusion on the overall reaction time as a function of ZnS conversion to ZnO.

Gas film mass-transfer control can be suspected with the ATGA test data at 575 °C with 2.92 vol % O2 (Figure 2), due to a relatively straight line of solid conversion vs time (Levenspiel, 1989). The data at 625 °C with 0.2 vol % O2 in Figure 2 also shows a straight line, but this could result from the low rate of reaction due to the low oxygen concentration. The relative effect of sample bed diffusion for some ATGA and PTGA data is shown in Figure 7, using the method described earlier in this text. In the PTGA tests (Yrjas et al., 1996), despite a 66 wt % dilution of inert silica sand, the effect of sample bed diffusion is significant, i.e., up to 40%. Thus, further analysis of the SO2 release from PTGA tests is omitted. In the atmospheric tests, the relative effect of sample bed diffusion is less than 10%. Figure 7 shows the two highest contributions (from 0 to 0.15-0.25) with low O2 partial pressures at highest test temperatures, which indicates that the data of 0.2 vol % O2 at 775 °C and 0.44 vol % O2 at 725 °C must be omitted from further analysis. The rejected and selected ATGA data, including the reason for possible rejection are listed in Table 3. However, the PTGA data of zinc sulfate formation (reaction 5) in Figure 5 can be used since it does not suffer from masstransfer effects. Due to the moderate gas flow rate (200 cm3(std)/min) used in the TGA experiments in Figure 2 and relatively high regeneration rate of the sample, the concentration of the reactive oxygen gas was slightly reduced at the TGA exit. However, at temperatures of 625-725 °C, 90-98% of the initial oxygen gas flow remained at the exit. Thus, the effect of this on the assumption of constant gas concentration (used in modeling) is within the experimental error. The maximum possible temperature difference between the bulk gas and the solid surface was evaluated with eq 6 by using the ATGA test data. The results indicate that with the reaction rates of (1 × 10-9)-(7 × 10-8) mol/s the temperature increase is less than 4 °C. The highest values were obtained with some of the results at temperatures above 700 °C, which were already discarded due to sample bed diffusion limitation effects. The maximum temperature difference inside a solid reactant (eq 7) was calculated using an order of magnitude input value of 0.14 J/(s cm K) for solid effective thermal conductivity. The resulting value of (∆Ts)max equals about 0.02 °C in the temperature range 500-800 °C. It can be concluded that thermal gradients between bulk gas and solid sample during regeneration tests are not significant.

Figure 8. The initial rate of sulfided zinc titanate regeneration as function of O2 volumetric content obtained from TGA results at 1.013 bar. Table 3. The Selection of the Most Reliable Ambient Pressure TGA Data for Further Usea data at temperatureb test no.

O2 (vol %)

575 °C

625 °C

675 °C

725 °C

775 °C

4 2 1 5 3

0.2 0.44 1.64 1.65 2.92

1, 2 1, 2 1, 2 1, 2 1

X X 1 X X

X X X X 2

X 2, 3 2, 3 2 2, 3

3 2 2

a Symbols: X ) useful; 1 ) possible gas film diffusion; 2 ) possible simultaneous SO2 release and ZnSO4 formation; 3 ) sample bed diffusion. b All data at 425 °C and 475 °C rejected for further use.

Uniform Conversion Model Parameters. As first approximation for the porous sorbents of this study, gaseous reactant (O2) is assumed to be present evenly throughout the ZnS-containing solid particle and reacts with a solid reactant everywhere. Thus, the rate of regeneration is assumed to follow the expression of the “uniform conversion model” (Levenspiel, 1989; Kunii and Levenspiel, 1991):

dX N ) k(1 - X)CO 2 dt

(9)

where X is the fractional conversion of zinc sulfide to zinc oxide, t is time (s), CO2 is the concentration of oxygen (mol/cm3), N is the order of O2 in the reaction, and k is the reaction rate constant ((cm3N/(molN s)). This form of expression was specially encouraged by the results obtained with zinc titanates by Woods et al. (1989, 1990). The order of oxygen in eq 9 studied with the TGA data from 625 and 675 °C (Figure 8) show that the order of O2 in regeneration can reasonably be assumed to be 1. When extrapolating the slopes of the lines in Figure 8 to higher O2 partial pressures, the maximum regeneration rates reported by Bagajewicz (1988) at 675 °C with a ZnO-based sorbent seem to be at the same level or slightly lower than those indicated by sorbent A. It should be noted, however, that an exact comparison is not possible, due to a different test arrangement and different sorbent chemical composition. The Arrhenius plot of regeneration reaction rate constants with the selected atmospheric TGA test periods (listed in Table 3) is shown in Figure 9. The activation energy of the rate constant is about 140 kJ/ mol, which indicates that the regeneration reactivity is quite sensitive to temperature. The frequency factor of

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Figure 9. Arrhenius plot of the sulfided zinc titanate regeneration rate constant as defined by uniform conversion model with TGA data of sorbent A at 1.013 bar.

Figure 9 is 1.44 × 1011 (cm3(g)/(mol s)). The variation in reaction rate constant values at different temperatures might be partly due to simultaneous sulfate formation. Reliable data on sulfate formation rates at ATGA conditions was not available, but on the basis of the regeneration and sulfate formation rate data by Bagajewicz (1988) and Siriwardane and Woodruff (1995) the rate of sulfate formation was assumed to be neglible. The relative contribution of sulfate formation at high pressure (20 bar) can be estimated with the following: the rate data of sulfate formation in Figure 5 (in PTGA, 20 bar) can be used for actual rate comparisons (as discussed above). In Figure 5, the reaction rates according to reaction 5 are 2.9 × 10-5 at 650 °C (1/s) and 3.1 × 10-5 (1/s) at 750 °C. The rate of sulfided zinc titanate regeneration (reaction 2) obtained from ATGA results is (1.8 × 10-4)-(3 × 10-4) (1/s) at 675 °C. Within the ambient pressure conditions we have shown that the regeneration reaction rate is of first-order with respect to O2 concentration. Assuming that the first order dependence of O2 is valid at higher pressures (as indicated by high-pressure pilot-scale results in Konttinen et al. (1997c)), the extrapolated sulfided zinc titanate regeneration rates at 20 bar would be in the order of 5 × 10-3 (1/s) (675 °C), which is about two orders of magnitude higher than sulfate formation rates at corresponding conditions (Figure 5). This comparison indicates that the relative contribution of sulfate formation is insignificant at relevant regeneration temperatures and pressures. The results obtained here as part of a regeneration reactor model will be reported later in Konttinen et al. (1997c), by using the method reported previously by Konttinen et al. (1997b). Discussion All experimental results show that the rate of SO2 release in sulfided zinc titanate regeneration is strongly dependent on process temperature. Some evidence of ZnSO4 formation in laboratory-scale TGA results could be seen but based on literature (Bagajewicz, 1988; Siriwardane and Woodruff, 1995) and a comparison of ZnSO4 formation and sulfided zinc titanate regeneration rates at 20 bar pressure (of commercial interest) the sulfate formation was assumed not to be significant. Unfortunately, the rate of SO2 release at high pressure could not be determined directly with PTGA tests due to diffusion limitations inside the sample bed. This problem of external mass transfer was also observed by Woods et al. (1989, 1990). An alternative way to perform the high-pressure kinetic tests would be to use a small ZnS-containing sample in a fluidized bed with continuous O2 and SO2 measurement together with

sample temperature measurement. The fluidizing gas at constant temperature would provide uniform temperature profile in all the particles in the sample. This way higher regeneration temperatures could be studied. Other possibilities to eliminate the problems discussed here have been reported: In order to avoid pore diffusion limitation in H2S capture tests with zinc titanates in ATGA, Lew et al. (1992) used very small samples (1-3 mg) and high gas flow rates. Yrjas et al. (1996) diluted the zinc titanate samples with inert silica sand. However, these actions can lead into problems such as vibration and inaccuracy in sorbent mass balance due to moderate maximum weight gain of the sample. For example, Yrjas et al. (1996) reported conversions of more than 100 mol % for sulfided zinc titanate regeneration with sorbent samples containing 66 wt % of SiO2. As conclusion, in order to obtain reliable kinetic data on gas-solid reactions in TGA, the possibilities of massor heat-transfer limitations in the sample should be eliminated, preferably with careful selection of test conditions as discussed by Yrjas et al. (1996) or with calculational methods, such as those presented in this paper. Conclusions This paper deals with experimental results and modeling of the regeneration of sulfided zinc titanate sorbents. The solid conversion rate data produced by ambient pressure thermogravimetric analyzer were used for model-parameter fitting. The ATGA data at different temperatures could be modeled by assuming uniform conversion of ZnS in the solid particles. The results indicate that the rate of sulfided zinc titanate regeneration reaction is strongly temperature dependent with an activation energy of about 140 kJ/ mol for the rate constant. The PTGA data on regeneration could not be directly used for model-parameter fitting due to sample bed diffusion limitations, but these results indicate that the rate of ZnSO4 formation at high temperature and pressure is neglible, in comparison with the rate of SO2 release. The results obtained will be used further as part of a regeneration reactor model. Acknowledgment The work on the development and testing of regenerable sulfur removal sorbents and processes has been carried out as part of the Combustion & Gasification Research Program “LIEKKI 2” and Energy and Environmental Technology Program “SIHTI 2”, which are partly financed by the Technology Development Centre of Finland. The pilot-scale testing of sorbent D was a part of the Cooperative Research and Development Agreement between Enviropower Inc. (the predecessor of Carbona Corporation) and the U.S. Department of Energy. Mr. Thomas Dorchak of the Federal Energy Technology Center, Morgantown, WV, is gratefully acknowledged for his valuable contribution. The development of the sulfur removal reaction models was funded partly by the Imatran Voima Foundation. The authors acknowledge the valuable contribution of Dr. Santosh Gangwal of the Research Triangle Institute, Research Triangle Park, NC, in producing the ambient pressure TGA data used in this article. Dr. Javad Abbasian of the Institute of Gas Technology, Des Plaines, IL, is acknowledged for his contribution in arranging the chemical and physical analysis of sorbent D studied in this article.

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Notation CA ) concentration of gaseous reactant (mol/cm3) Cp ) heat capacity (J/(g K)) D ) diffusion coefficient G ) gas flow rate (g/s) jH ) heat-transfer parameter k ) reaction rate constant (cm3/(mol s)) N ) reaction order parameter p ) pressure (bar) Pr ) Prandtl number r ) reaction rate (mol/s) Re ) Reynolds number t ) time (s) T ) temperature (K) W ) weight (g or mg) X ) overall particle fractional conversion x, y ) stoichiometric coefficients in eqs 1 and 3 ∆H ) heat of reaction (J/mol) λ ) thermal conductivity (J/(s cm K)) Fmol ) molar density (mol/m3) Subscripts b ) bulk eff ) effective end ) 100% conversion of the solid reactant g ) gas max ) maximum mol ) molecular p ) particle properties s ) solid S ) at surface sbd ) sample bed diffusion tot ) total 0 ) initial

Literature Cited Ayala, R. E.; Kim B. M. Modelling and Analysis of Moving-Bed Hot-Gas Desulfurization Processes. Environ. Prog. 1989, 8 (1), 19-25. Bagajewicz, M. J. Studies on Hot Gas Hydrogen Sulfide Removal Sorbents in Fixed-Bed Reactors at High Temperatures. Ph.D. Dissertation, California Institute of Technology, Pasadena, CA, 1988. Barin, I.; Knacke, O.; Kubaschewski, O. Thermodynamical Properties of Inorganic SubtancessSupplement; Springer-Verlag: Berlin, New York, 1977. Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; John Wiley & Sons: New York, 1990. Gupta R. P.; Gangwal S. K. Enhanced Durability of Desulfurization Sorbents for Fluidized Bed Applications. Topical Report, 96U-4274, Contract DE-AC21-88MC25006; Research Triangle Institute: Research Triangle Park, NC, November 1992. Harrison, D. Control of Gaseous Contaminants in IGCC Processes, An Overview. In Twelfth Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 11-15, 1995; Chiang, S.-H., Ed.; Pittsburgh Coal Conference: Pittsburgh, PA, 1995; pp 1047-1052. JANAF Thermochemical Tables, 3rd ed.; Chase et al., Eds.; Journal of Physical and Chemical Reference Data; National Bureau of Standards: New York, 1985; Vol. 14. Konttinen, J. T.; Mojtahedi, W. Gasifier Gas Desulfurization at High Temperature and Pressure. Kem.-Kemi (Finnish Chemistry) 1993, 20 (9-10), 847-851. In English. Konttinen, J. T.; Mojtahedi, W.; Abbasian, J. Coal Gas Desulfurization at the 15 MWth Pressurized Gasification Pilot Plant. In High Temperature Gas Cleaning; Schmidt, E., Ga¨ng, P.,

Dittler A., Eds.; G. Braun Printconsult GmbH: Karlsruhe, Germany, September 1996; pp 756-766. Konttinen, J. T.; Zevenhoven, C. A. P.; Hupa, M. M. Hot Gas Desulfurization with Zinc Titanate Sorbents in a Fluidized Bed. 1. Determination of Sorbent Particle Conversion Rate Model Parameters. Ind. Eng. Chem. Res. 1997a, 36 (6), 2332-2339. Konttinen, J. T.; Zevenhoven, C. A. P.; Hupa, M. M. Hot Gas Desulfurization with Zinc Titanate Sorbents in a Fluidized Bed. 2. Reactor Model. Ind. Eng. Chem. Res. 1997b, 36 (6), 23402345. Konttinen, J. T.; Zevenhoven, C. A. P.; Hupa, M. M. Modeling of Sulfided Zinc Titanate Regeneration in a Fluidized-Bed Reactor. 2. Scale-Up of the Solid Conversion Model. Ind. Eng. Chem. Res. 1997c, 36, 5439-5446. Krishnan S. V.; Sotirchos S. V. Experimental and Theoretical Investigation of Factors Affecting the Direct Limestone-H2S Reaction. Ind. Eng. Chem. Res. 1994, 33 (6), 1444-1453. Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Boston, 1991. Levenspiel, O. The Chemical Reactor Omnibook; OSU Book Stores: Columbus, OH, 1989. Lew, S. High-Temperature Sulfidation and Reduction of Zinc Titanate and Zinc Oxide Sorbents. Ph.D. Dissertation, Massachusetts Institute of Technology, October 1990. Mojtahedi, W.; Konttinen, J. T.; Gangwal, S. K. SulfidationRegeneration Kinetics of Zinc Titanate Sorbents. In High Temperature Gas Cleaning; Schmidt, E., Ga¨ng, P., Dittler A., Eds.; G. Braun Printconsult GmbH: Karlsruhe, Germany, September 1996; pp 557-568. Salo, K.; Hokaja¨rvi, R. Development of New Generation IGCC. In Eleventh Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 12-16, 1994; Pittsburgh Coal Conference: Pittsburgh, PA, 1994. Salo, K.; Konttinen, J. T.; Ghazanfari, R.; Feher, G.; Lehtovaara, A.; Mojtahedi, W. Pilot Scale Experience on IGCC Hot Gas Cleanup. In Proceedings of the Advanced Coal-Fired Power Systems ‘95 Review Meeting, Volume 1, June 1995. U.S. Department of Energy, Morgantown Energy Technology Center: Morgantown, WV, 1995. Siriwardane, R. V.; Woodruff, S. FTIR Characterization of the Interaction of Oxygen with Zinc Sulfide. Ind. Eng. Chem. Res. 1995, 34, 699-702. Sohn, H. Y. The Law of Additive Reaction Times in Fluid-Solid Reactions. Metall. Trans. 1978, 9B (3), 1978-1996. Woods, M. C.; Leese, K. E.; Gangwal, S. K.; Harrison, D. P.; Jothimurugesan, K. Reaction Kinetics and Simulation Models for Novel High-Temperature Desulfurization Sorbents. Final Report, Contract Number DE-AC21-87MC24160; Research Triangle Institute: Research Triangle Park, NC, February 1989. Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbents. 1. Single-Pellet Kinetic Studies. Ind. Eng. Chem. Res. 1990, 29, 1160-1167. Yrjas, K. P.; Hupa, M. M.; Konttinen, J. T. Regeneration of Sulfided Zn-Titanate by Oxidation under Pressure. In High Temperature Gas Cleaning; Schmidt, E., Ga¨ng, P., Dittler A., Eds.; G. Braun Printconsult GmbH: Karlsruhe, Germany, September 1996; pp 514-527. Zevenhoven, C. A. P.; Yrjas, K. P.; Hupa, M. M. Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 2. Sorbent Particle Conversion Modeling. Ind. Eng. Chem. Res. 1996, 35 (3), 943-949.

Received for review January 15, 1997 Revised manuscript received August 21, 1997 Accepted August 21, 1997X IE970035G X Abstract published in Advance ACS Abstracts, October 15, 1997.