Mechanistic Study of the Carbothermal Reduction of Sulfur Dioxide

Chongqing University, Chongqing, P.R. China 400044, and Syncrude Research Center, Syncrude Canada,. 9421-17th Avenue, Edmonton, Alberta, Canada ...
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Ind. Eng. Chem. Res. 2003, 42, 3731-3739

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Mechanistic Study of the Carbothermal Reduction of Sulfur Dioxide with Oil Sand Fluid Coke Cesar Bejarano,† Charles Q. Jia,*,†,‡ and Keng H. Chung§ Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5, Center for Eco-chemistry and Environmental Technologies, Chongqing University, Chongqing, P.R. China 400044, and Syncrude Research Center, Syncrude Canada, 9421-17th Avenue, Edmonton, Alberta, Canada T6N 1H4

Carbothermal reduction is a reducing reaction involving carbonaceous materials at high temperatures. Oil sand fluid coke is a high-carbon byproduct of the thermal cracking of oil sand bitumen via a process called fluid coking. To lay a foundation for the development of a process that removes and converts sulfur dioxide into elemental sulfur, the kinetics of the carbothermal reduction of SO2 by coke at 700-950 °C was investigated using a packed-bed reactor. Analysis using the shrinking core model revealed that the overall process is controlled jointly by surface chemical reaction and diffusion in a product ash layer. The existence of the layer was confirmed by SEM examination of a cross section of spent coke particles. The activation energy of the overall reaction was found to be 154 kJ/mol, which is in a good agreement with literature values. The sulfur balance was analyzed with data obtained using a total sulfur analyzer and a gas chromatograph. SEM-EDS analysis indicated that the ash layer was low in sulfur. At the ashcoke interface, however, an accumulation of sulfur was found that was attributed to C-S complexes. The chemical states of sulfur in the spent coke were determined using an X-ray photoelectron spectrometer. The sulfur in the raw coke was likely dominated by its thiophenic forms, whereas the sulfur in the ash layer was likely sulfite. Introduction The upgrading of heavy petroleum fractions is a process of raising the H/C ratio of the hydrocarbons. Higher H/C ratios can be achieved by removing carbon from or adding hydrogen to the heavy hydrocarbons. The coke produced by the petroleum industry is, basically, the carbon removed from heavy hydrocarbons. Two major types of petroleum coke are produced by upgrading processes, namely, delayed coke and fluid coke. Because of the difference in the technologies by which they are obtained, fluid coke often has a lower content of volatiles, a higher C/H ratio, a higher bulk density, and higher sulfur content than delayed coke. Consequently, it is less reactive and combustible. On a global basis, the amount of delayed coke produced is several times greater than the amount of fluid coke produced.1 There are about eight fluid coking units in the world, with a total capacity of 370 million barrels per day. Currently, about 6000 tons of fluid coke are produced daily in Alberta, Canada. Although fluid coke contains over 85 wt % of carbon and high heating value, it is not a desirable solid fuel because of its high sulfur content (6-8 wt %) and low combustibility. In a previous work,2 it was established that fluid coke could be used as a reductant for SO2. The goal was to develop a sulfurproducing flue gas desulfurization (SP-FGD) process that removes SO2 from flue gases and converts it into elemental sulfur. It is believed that such a process can be economically viable for several reasons: (1) Fluid * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (416)946-3097. Fax: (416)978-8605. † University of Toronto. ‡ Chongqing University. § Syncrude Canada.

coke is a byproduct that is essentially free. (2) Pretreatment of the coke might not be necessary, given its desirable physical properties (e.g., particle size). (3) Because this process is performed at high temperature, cooling of the flue gases can be avoided. The low volatiles contents in fluid coke could also be advantageous in terms of sulfur yield. Previous studies3-6 on coal suggest that the conversion of SO2 into elemental sulfur increases with the removal of volatiles from coal. Volatile substances react with SO2 and form H2S, lowering the yield of elemental sulfur. To develop an SP-FGD process, a better understanding of the reaction kinetics is essential. Ratcliffe and Pap5 studied the reduction of SO2 with lignite and low-rank coals, fitted kinetic data from a thermogravimetric balance using a model similar to shrinking core model (SCM), and established that the overall reaction rate was controlled by chemical reaction as well as diffusion in the ash layer. A kinetic study by Abramowitz et al.7 on the reaction of SO2 with carbon black powder revealed that the mechanism involves an oxygen-transfer step analogous to that occurring in the reaction between CO2 and carbon. The same study also indicated that, below 850 °C, the reaction is least affected by mass transfer and pore diffusion, whereas above 960 °C, it is most influenced by these processes. In this work, the reaction kinetics of SO2 with oil sand fluid coke was studied experimentally at temperatures between 700 and 950 °C. Data were examined using the shrinking core model. Reaction mechanisms were analyzed with a focus on the fate of the sulfur. Experimental Section Fluid coke was supplied by Syncrude Canada. Furimsky8 analyzed compositions of Syncrude fluid cokes

10.1021/ie0206711 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/15/2003

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Table 1. Properties and Compositions of Syncrude Fluid Coke from Stockpiles8 coke sample (years) 1 (1979-1980)

2 (1980-1982)

3 (1982-1983)

4 (1983-1985)

5 (1985-5)

0.69 7.52 6.10 85.69

0.25 4.83 4.99 89.95

moisture ash volatiles fixed carbon

0.44 5.40 4.85 89.31

property proximate 0.60 0.50 7.21 5.18 5.11 6.23 87.08 88.09

carbon hydrogen H/C nitrogen sulfur oxygen

82.73 1.72 0.25 1.75 6.78 1.18

80.73 1.63 0.24 1.70 6.63 1.50

81.80 1.66 0.24 1.98 6.84 2.04

80.94 1.56 0.23 1.73 6.15 1.41

83.74 1.77 0.25 2.03 6.52 0.88

SiO2 Al2O3 Fe2O3 TiO2 CaO SO3 V2O5 other total

38.80 24.35 9.72 3.64 4.26 3.59 4.46 8.76 98.66

ash composition (wt %) 50.06 41.60 20.94 24.22 8.18 9.26 2.86 3.25 2.58 4.20 2.73 2.65 3.20 4.86 7.22 7.38 98.58 98.57

41.26 24.94 12.14 4.84 1.63 1.87 3.21 7.91 98.63

37.64 24.33 11.42 4.63 2.94 2.88 4.94 8.43 98.35

ultimate

produced from 1979 to 1995. As shown in Table 1, the change in composition is small through the years, suggesting the resistance of the coke to weathering. The coke used in this study was produced in 1998 and should have a similar composition. Fluid coke particles are spherical and have an onion-like internal structure resulting from the cyclic nature of the coking process. The coke particles were sieved prior to use; the 208295-µm size fraction was used without further treatment. According to Fairbridge et al.,9 coke particles with different particle sizes have different oxidation reactivities and surface structures. High-purity SO2 (99.8%, Matheson Gases) and He (UHP, BOC Gases) gases were supplied from compressed gas cylinders. The experimental setup was described in a previous report.2 The reaction was carried out in a packed-bed reactor typically charged with 5-20 g of coke. The amount of coke determines the height of the coke bed as well as the residence time of gas in the bed (in seconds). The reactor consisted of a 14-mm-i.d. and 600mm-long quartz tube with a porous quartz disk placed 45 cm from the inlet (top) to support the coke bed. The tube was mounted inside a tubular furnace with an automatic temperature controller. A thermocouple was fitted along the axis of the reactor, reaching the middle of the packing, to measure the temperature of the coke bed. Helium was used to purge the system before and after each run. During each experiment, a gaseous mixture of SO2 and He was introduced at a controlled flow rate (typically 100-150 mL/min). The exit gas was quenched in a sulfur condenser immersed in an ice bath. The gas, free of elemental sulfur, was manually sampled and injected into a gas chromatograph for determination of the composition. The mass of coke that reacted at any given time was calculated by integrating the molar flow rate of carbon species with respect to time. With a pressure drop of about 18 mmHg through the assembly, experiments were conducted under nearly atmospheric pressure. A Varian 3800 gas chromatograph (GC) was used to determine the concentrations of various gaseous compounds with He as the carrier gas at 30 mL/min. The GC was equipped with a 6.4-mm-o.d., 4.87-m-long Porapack QS column and a thermal conductivity detector (TCD). The oven temperature was 110 °C, and the

detector temperature was 170 °C, with a filament temperature of 190 °C. The retention times of all the species studied were less than 3 min. The Star Chromatography Workstation v.4.51 software package was used to collect and treat the data. Coke particles sampled before and after the reaction were examined using a Hitachi S2500 scanning electron microscope (SEM), coupled to a Siemens 5000 energy-dispersive X-ray spectroscope (EDS). Similar examinations were also performed on a cross section of coke samples prepared separately. To determine total sulfur contents in the coke samples, a sulfur analyzer (LECO CS 244) was used. The LECO unit was calibrated with a zinc sulfide standard (32.44 wt % S) and had a precision of 0.05 wt %. The surfaces of the raw and treated coke were examined by X-ray photoelectron spectroscopy (XPS) to determine the bonding properties of the sulfur contained in them. A Leybold-Heraeus Max 200 system (LH, Cologne, Germany) was utilized to obtain XPS spectra. The machine was equipped with an unmonochromatized Al KR source operated at 15 kV and 20 mA. To maximize the signal-to-noise ratio, large-area spot analysis (4 × 7 mm2) was conducted. Calibration of the energy lines was carried out against Cu 2p3/2 and Cu 3p lines (932.7 and 75.1 eV, respectively). With no differential charging observed, the energy scale was corrected by keeping the C 1s value for the main C-C component at 284.6 eV. Spectra were fitted using ESCATOOLS (Surface/Interface Inc., Mountain View, CA). Sensitivity factors used were supplied by the manufacturer of the machine (S 2p ) 0.76, O 1s ) 0.75, and C 1s ) 0.32). Results and Discussion Time Dependence of the SO2-Coke Reaction under Unsteady-State Conditions. To determine the long-term behavior of the coke as the reaction progressed, experiments were carried out with a large initial amount (about 20 g) of coke. Figure 1 shows the composition of the gaseous products as a function of reaction time at 900 °C, which provides an overall picture as to the influence of the amount of coke (or the residence time of the gas) on the reduction of SO2. As time progressed, the reaction with SO2 consumed coke

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Figure 1. Product composition as a function of reaction time at 900 °C, 150 mL/min of 15% SO2 gas, initial amount of coke ) 18.2 g.

and lowered the height of the coke bed as well as the residence time of the gas in the coke bed. As shown in Figure 1, there is a “critical point” at which the concentrations of elemental sulfur and CO2 reach a maximum. This point represents the amount of coke (or the gas residence time) required to just achieve the complete conversion of SO2 into elemental sulfur. Prior to this point, the large amount of coke will result in a prolonged residence time of the gas in the coke bed. In other words, the residence time of the gas in the coke bed is longer than the time required to completely reduce SO2. As a result, all of the SO2 is reduced, but with a significant production of reduced sulfur species, such as CS2 and CO,S along with CO. Beyond this point, the conversion of SO2 is incomplete because of the insufficient residence time of gas in the coke bed. Under these conditions, the only species in the gaseous product are CO2, unreacted SO2, and elemental sulfur. The concentration of sulfur decreases with time because of the decrease in the amount of SO2 reduced. The importance of this observation is twofold. First, it confirms the previous finding2 that, when carbon is the limiting reagent, the predominant reaction is C + SO2 f CO2 + S. Second, it provides the conditions under which experiments should be conducted so that the production CS2 and COS is minimized. Evaluation of Reduction Kinetics. Subsequent experiments were performed using carbon as the limiting reactant to ensure that CO2 was the only gaseous carbon species. The change in CO2 in the exit gas with time was followed. From the measured time dependence of the amount of CO2 and the flow rate, the total amount of CO2 produced was determined. Because all of the carbon in the CO2 came from the coke, from the amount of CO2 produced for a given time, the fraction of carbon converted into CO2 (i.e., the fractional conversion, X) at that time was calculated. To determine the control regime of the overall process, the data were analyzed using the SCM. According to the SCM theory, the time dependence of the fractional conversion, X, will take different mathematical forms, depending on the control regime (or the rate-limiting step) in the overall process. The theoretical development of these models is extensively covered in refs 10 and 11. For example, in the mixed-control regime, the dependence can be expressed as

[1 - (1 - XB)1/3] + σ2[1 - 3(1 - XB)2/3 + 2(1 - XB)] ) βt (1)

Figure 2. Correlations between observed fractional conversion of carbon at 900 °C and chemical-reaction-, ash-layer-diffusion-, and mixed-control equations.

or

gFp(X) + σ2[pFp(X)] ) βt

(2)

where

σ2 ) β)

k′′R 6De

bk′′CAg F BR

X is the fractional conversion of the solid; k′′ is the chemical reaction constant; De is the effective diffusivity of the gas; CAg is the concentration of the gas; FB is the density of the solid; R is the gas constant; b is the stoichiometric coefficient of the solid;and gFp(X) and pFp(X) are the chemical reaction and diffusion functions, respectively. Two similar sets of equations can be developed for the diffusion-controlled and the chemical-reaction-controlled regimes. Figure 2 shows the correlations between the values of X, calculated from the measured CO2 production (data points), and the three equations representing the three control regimes (lines). The highest correlation coefficient (R2 ) 0.999 98) was found for the case of mixed control, suggesting that both the diffusion in the ash layer and the chemical reaction play significant roles in controlling the overall process at 900 °C. Studying the reduction of SO2 with lignite and low-rank coals, Ratcliffe and Pap5 established that the overall reaction rate was controlled by chemical reaction as well as diffusion in ash layer. The shrinking core reaction modulus, σ2, represents the degree of control by diffusion. for σ2 e 0.1, chemical reaction controls, and for σ2 g 10, diffusion through the product layer and external mass transfer determine the overall rate. Values of σ2 between 0.1 and 10 indicate mixed control. In this study, all experiments were performed under conditions such that the effects of external mass transfer were negligible.19 Equation 2 can be rearranged so that σ2 is obtained from the slope of a plot of [gFp(X)]/t vs [pFp(X)]/t by linear regression. As reported in in Table 2, the σ2 values obtained at six different temperatures had values from 0.6 to 7.5, confirming the mixed-control mechanism. Temperature had a significant effect on the rate of coke consumption in the range of temperatures studied.

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Figure 3. Effect of temperature on β. Table 2. Effect of Temperature on Kinetic Parameters temperature (°C)

k′′ (m min-1)

Dc (m2 s-1)

σ2

700 750 850 900 950

5.09 × 10-4 1.362 × 10-3 6.168 × 10-3 1.095 × 10-2 3.010 × 10-2

5.5 × 10-10 11.1 × 10-7 6.1 × 10-7 3.5 × 10-7 2.1 × 10-7

2.9 0.6 0.6 1.3 7.5

Consumption of coke after 120 min increased from 2% to almost 40% when temperature was raised from 700 to 950 °C. The effect of temperature on the slope of the mixed-control model plot (β) is shown in Figure 3. The linearity shown suggests that the mixed-control model applies to all temperatures studied. A close examination of Figure 2 reveals that the curves corresponding to chemical reaction control and mixed control superimpose initially and then deviate from each other as time progresses, suggesting that, after a certain time, the effect of the product ash layer becomes significant. The initial slope of the chemical control curve corresponds to the slope of the mixedcontrol line (β). This observation implies that, in the initial stage of the C + SO2 reaction, no product ash layer is present, so that the only rate-controlling mechanism is the chemical reaction. Therefore, the slope of the initial portion of the chemical-reaction-control curve (β) was used to determine the value of the chemical reaction rate constant, k′′. Values of k′′ at various temperatures were calculated from β and are given in Table 2.

A plot of ln(k′′) vs 1/T for all six temperatures gives a straight line (R2 ) 0.991). According to the Arrhenius equation, the slope represents the parameter Ea/R, and the value of Ea obtained is 154 ( 16 kJ/mol (37 ( 4 kcal/ mol), which is indicative of chemical reaction control. The preexponential constant, k0, was found to be 9.73 × 104 m s-1. The Ea value is in a good agreement with values reported by Abramowitz et al.7 (48 ( 2 kcal/mol) and Ratcliffe and Pap 5 (150.7 ( 11 kJ/mol) for C-SO2 systems. The magnitude of Ea indicates that the reactivity of the coke is low and that a large amount of energy is required to achieve a high reaction rate, which corresponds to the behavior expected on the basis of the characteristics of the coke.1,8 The effective diffusivity of the reactant, De, can be obtained from the values of σ2 (defined in eq 2). Values of De at various temperatures are included in Table 2. The order of magnitude of the calculated values of De is lower than that of the diffusion coefficient for a binary SO2-air system (1.22 × 10-5 m2 s-1 at 0 °C).12 This difference can be attributed to the fact that the diffusion flux per unit cross section of a porous medium is equal to a constant, θ, multiplied by the flux under similar conditions with no solid present.13 The constant, θ, is the free cross section of the porous material and has a value less than 1.0. Furthermore, channels through which diffusion occurs are likely to be of irregular shape and varying cross section. Constrictions could offer resistance that is not offset by enlargements, so that the flux tends to be less than that of a porous material with uniform pore size and structure.13 De is expected to increase with T3/2; however, the calculated values are rather erratic, and no clear trend in their behavior could be observed. For an accurate estimation of De, it is necessary to know the evolution of the surface area with the reaction. A model taking into account the pore geometry of the ash layer would be necessary. In addition, at high temperatures, sintering might occur, resulting in significant changes in the structure of the ash layer. It is expected that the micro- and macropore characteristics of the coke will change during reaction. Pore structure can play an important role in determining the reactivity of coke.14

Figure 4. SEM image and EDS analysis of coke particles before reaction.

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Figure 5. SEM image and EDS analysis of coke particles after 2 h of reaction.

Figure 6. SEM image and EDS analysis of the fallen ash after 13.5 h of reaction.

SEM-EDS Analysis of Coke before and after Reaction. SEM-EDS was used to estimate qualitatively the chemical composition of the coke surface before and after reaction. Figures 4 and 5 show SEM images of raw coke particles and spent coke after 2 h of reaction, respectively, indicating an increase in sulfur content as well as in the contents of other non-carbon elements (Al, Si, Ca, Fe) on the surface of coke after reaction, which is consistent with the increase of sulfur revealed later by total sulfur analysis. EDS analysis was conducted on the disrupted ash fallen from coke particles after a long period of reaction (often >10 h). In contrast to the coke surface, the ash itself had very little sulfur, its composition being mainly Si, Al, and Fe (Figure 6). This result agrees well with reported ash compositions (Table 1), suggesting that most of the sulfur must have been bound to carbon in the raw coke. Because the ash layer is low in sulfur, the accumulated sulfur on the coke surface should have come

from the surface layer of the coke particle underneath the ash layer, i.e., the ash-coke interface, where sulfur dioxide gas and the coke meet during carbothermal reduction. Cross-section analyses were also performed on reacted coke particles. Figure 7a confirms the existence of the ash layer. EDS analyses were performed on the reacted coke particle at three different spots: within the ash layer, at the ash-coke interface, and in the particle. As expected, the ash layer (Figure 7d) had a composition similar to that of the disrupted ash (Figure 6). On the other hand, the content of sulfur at the interface (Figure 7c) seems higher than that in the particle (Figure 7b), which is consistent with the interfacial sulfur buildup suggested earlier. Sulfur in Raw and Spent Cokes. Ultimately, the sulfur in the system came from two sources: the coke and the sulfur dioxide gas. A mass balance analysis was conducted for sulfur. The initial mass of sulfur in the system came from the original sulfur in the coke. SO2

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Figure 7. (a) SEM image and SEM-EDS analyses of cross sections of a coke particle after 13 h of reaction: (b) center, (c) interface, and (d) ash layer.

in the inlet gas was the only source of additional sulfur. During carbothermal reduction, sulfur left the system as elemental sulfur vapor, along with sulfur-containing gases (SO2, COS, and CS2), in the outlet gas. The spent coke also contained sulfur. Judging from the measured sulfur contents in the raw and spent coke and the flow rates and compositions of the inlet and outlet gases, the amount of elemental sulfur produced, Sprod (Calc), was calculated and compared with the amount measured, Sprod (Meas). As shown in Table 3, the differences between these values were within 15%. In 12 of 17 runs, the amount of sulfur collected was greater than that calculated. Table 3 also gives sulfur contents in the coke before and after reaction. In all cases, except run 20, the concentration of sulfur in the coke increased after the reaction with SO2. Although SO2 can be physically adsorbed by carbon, the temperature for effective physical adsorption to occur is normally low. According to Stacy et al.,15 physical adsorption decreases sharply with increasing temperature between 50 and 150 °C and is negligible above

250 °C. Heating a variety of chars, activated carbons, or carbon blacks with sulfurous gases and vapors can form carbon-sulfur complexes of high stability and nonstoichiometric composition. The reactions involved are essentially heterogeneous processes in which both the nature and the magnitude of the surface area are of primary importance.16 Sykes and White17 studied the interaction of charcoal with sulfur and CS2 in the temperature range of 400-700 °C and concluded that sulfur and carbon disulfide were being adsorbed at the same set of sites, resulting in different interconvertible structures containing sulfur. Puri and Hazra16 found that, although the amount of sulfur fixed by carbon decreased with temperature, the C-S complex was very stable, and some complexes remained even at 1200 °C. Their results indicated that, in the case of SO2-treated carbon, readily oxidizable sulfoxide and sulfone groups, as well as sulfide groups, were formed. The formation of sulfoxide and sulfone groups was supported by the fact that an appreciable amount of oxygen was also chemisorbed on the reacting

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3737 Table 3. Sulfur Contents of the Coke before and after Reaction and Sulfur Balance sulfur balance (g) run 8 14 15 18 20 22 27 29 42 43 47 48 49 50 51 52 54

sulfur content (%) initial final difference 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30 8.30

9.86 9.39 9.58 9.55 7.57 9.35 9.12 9.29 10.26 10.08 9.10 8.80 8.88 8.60 11.01 9.60 11.20

1.56 1.09 1.28 1.25 -0.73 1.05 0.82 0.99 1.96 1.78 0.80 0.50 0.58 0.30 2.80 1.30 2.90

Sprod (Calc)

Sprod (Meas)

deviation (%)

0.414 1.540 1.366 1.316 1.585 0.980 1.431 2.662 0.971 1.239 1.257 1.198 1.055 0.478 2.233 2.324 9.126

0.379 1.565 1.422 1.452 1.647 0.963 1.508 2.704 0.919 1.367 1.288 1.316 1.135 0.552 2.092 2.558 8.779

-9.23 1.60 3.94 9.37 3.76 -1.77 5.11 1.55 -5.66 9.36 2.41 8.97 7.05 13.41 -6.74 9.15 -3.95

charcoals with the sulfur dioxide. The sulfur that could not be recovered at high temperature appeared to be present in the aromatic rings of the carbon layers. The fixation of sulfur was shown to take place partly by substitution of the quinonic and phenolic groups and partly by addition at the unsaturated sites. According to Puri,18 in the case of charcoal treated with SO2 at 1000-1200 °C, almost the entire amount of sulfur was fixed by addition at the unsaturated sites. Mateos and Fierro19 suggested the possibility that the sulfur from SO2 substitutes the carbon in graphene. Ratcliffe and Pap5 suggested that the initial chemisorption of SO2 on a coal char surface is followed by rapid chemical reduction of the SO2 to form carbonoxygen species (primarily CO2) and a thermally stable C-S complex. Subsequent interaction of this C-S complex with SO2 was the rate-limiting step in SO2 reduction with coal. The presence of mineral matter, especially alkali metal salts, provided the catalytic sites for the SO2 interaction with the C-S complex. It is quite possible that SO2 gas reacts with carbon in the coke to form C-S complexes that are stable at high temperatures. The fact that elemental sulfur was the major sulfur product under the conditions studied,20 however, suggests that the C-S complex was not the most stable form of sulfur, thermodynamically. A kinetically hindered reaction between the C-S complex and SO2 could, however, lead to the accumulation of sulfur at the interface. In a carbon matrix, S can be aromatic (bound to sp2-hybridized carbon) or aliphatic (bound to sp3hybridized carbon). Both seem possible in this case, although aromatic sulfur is thermodynamically more stable than aliphatic sulfur. To determine the relative atomic compositions of the surface, XPS analyses were performed on the raw coke and the spent coke reacted for 2, 4, and 13 h, respectively. As shown in Figure 8a, the coke sample treated for 2 h showed the highest surface S content, about 3 times that of the raw coke. It should be pointed out that the XPS signal could come from the surface of both the ash particles and the coke underneath. Although the ash layer could be as thick as 10 µm (Figure 7), it would not completely cover the coke surface. Because ash particles are low in sulfur, any increase in sulfur should be a result of the increase in sulfur at the coke surface (up to a few nanometers in depth), which explains the initial increase in S content (Figure 8a). The subsequent

Figure 8. Changes in (a) S and (b) O and C compositions of coke surface with reaction time.

decrease with treatment time is attributed to the increase in the thickness of the low-sulfur ash layer. Ratcliffe and Pap5 used XPS and analyzed carbon samples after reaction with SO2. They observed a uniform high concentration of the S signal over the surface and suggested that it confirmed the formation of C-S complexes. As shown in Figure 8b, although the C content decreases, O shows an increasing trend, supporting the argument of an ash effect. Other elements in ash exhibited trends similar to that of oxygen.20 As the ash consists of mainly salt- and oxideforming elements (V, Ni, and Si), the increase in surface “O” with time is expected. It should be noted that, although the ash contains no carbon, in principle, the ash could contribute to the carbon spectra if carboncontaining species were adsorbed on to the surface of ash particles. The decreasing trend of C, however, suggests that the C spectra mainly arose from the surface of coke underneath the ash. The XPS profiles of oxygen, nitrogen, and carbon species correspond to the 1s orbital (O 1s, N 1s, and C 1s, respectively), whereas those of silicon, aluminum, and sulfur species correspond to the 2p orbital (Si 2p, Al 2p, and S 2p, respectively). As shown in Figure 9, the raw coke showed a S 2p doublet (S1) at a binding energy (BE) of 163.9 eV, which accounts for about 85% of the entire sulfur in the surface. The other doublet (S2), at BE ) 167.7 eV, accounts for the remaining 15%. According to Mateos and Fierro19 and Kelemen et al.,21 S1 can be assigned approximately to sulfides (∼163.7 eV) or sulfur in a thiophenic environment (∼164.2 eV), which is a more stable form of sulfur at high temperatures. Elemental

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Conclusions To lay a foundation for the development of a process that uses fluid coke and converts SO2 into elemental sulfur, a mechanistic study was conducted. Kinetic data were analyzed using the shrinking core model, which revealed that the overall process was controlled jointly by surface chemical reaction and internal diffusion in a product ash layer. The existence of the ash layer was confirmed by SEM examination of a cross section of spent coke particles. The activation energy of the overall chemical reaction was found to be 154 kJ/mol, which is in a good agreement with literature values. An accumulation of sulfur at the surface of the SO2-treated coke particles was discovered, although the SEM-EDS analysis indicated a low sulfur content in the ash layer. The accumulation of sulfur was attributed to the C-S complexes formed at the coke surface. The chemical states of the sulfur in the spent coke were determined using an X-ray photoelectron spectrometer. Thiophenic sulfur is likely the dominating species of sulfur within the coke, whereas sulfite is the major form of sulfur in the ash layer. Acknowledgment Figure 9. S 2p spectra of the raw coke.

sulfur also has a binding energy of 163.7 eV.21 Given the high temperature (>500 °C) at which the coke was produced, however, the existence of elemental sulfur in the raw coke seems unlikely. Considering the rather labile nature of reduced sulfur groups (sulfide, mercaptan, thioether, etc.), thiophenic sulfur is likely the dominating form of sulfur giving the S1 doublet in the raw coke. On the other hand, the S2 doublet (167.7 eV) can be attributed to a range of sulfur compounds with higher oxidation states, including sulfoxides (∼166 eV), sulfones (∼168 eV), and even sulfates (∼169 eV). Because sulfoxides and sulfones are not very stable at high temperatures, if they did exist, they might have been formed as a result of air oxidation during storage. Analyzing heavy petroleum residua, Kelemen et al.21 found that thiophenic and sulfidic sulfurs dominate. It is expected that the forms of sulfur in fluid coke would be similar, perhaps with a larger proportion being thiophenic sulfur because of its thermal stability. After 2 h of reaction, the proportion of sulfur in S1 dropped to 80% and that in S2 increased to 20%. A longer reaction time, 4 h, results in a further decrease in S1 (to 50%); a similar S2 (20%); and a new doublet (S3, BE ) 170.6 eV), which accounts for the remaining 30% 20. According to Perkin-Elmer,22 the S3 doublet can be assigned to sulfites and/or sulfate. It is believed that the S3 doublet was not from the sulfur at the coke surface, but instead from the ash particles. The most logical form of sulfur in the ash is sulfite, given the conditions under which the coke was treated. The lower percentage of S1 species might not indicate a decrease in thiophenic sulfur at the coke surface, considering the “filtering” effect of the ash layer on the core electrons emitted from the sulfur at the coke surface. It should be noted that techniques are available for high-resolution sulfur speciation, including X-ray absorption near edge structure (XANES) spectroscopy.23,24

Financial support in the form of a scholarship (to C.B.) from the Government of Ontario, Canada, and a research grant (to C.Q.J.) from the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. We thank Dr. Rana Sodhi for his assistance in XPS analysis. We also thank Syncrude Canada for supplying the coke. Literature Cited (1) Furimsky, E. Characterization of cokes from fluid/flexicoking of heavy feeds. Fuel Proces. Technol. 2000, 67, 205-230. (2) Bejarano, C.; Jia, C. Q.; Chung, K. H. A study of carbothermal reduction of sulfur dioxide to elemental sulfur using oilsands fluid coke. Environ. Sci. Technol. 2001, 35, 800-804. (3) Lepsoe, R. Chemistry of sulfur dioxide reduction. Ind. Eng. Chem. 1940, 32, 910-918. (4) Biswas, A. K.; Roy, N. C.; Rao, M. N. Conversion of waste sulphur dioxide into sulphur + carbon disulphide. Indian J. Technol. 1968, 157-158. (5) Ratcliffe, C. T.; Pap, G. Chemical reduction of sulfur dioxide to free sulfur with lignite and coal. 1. Steady-state reaction chemistry and interaction of volatile components. Fuel 1980, 59, 237-243. (6) Steiner, P. J.; Gutterman, C.; Dalton, S. M. Reduction of sulfur dioxide with recycled coal. U.S. Patent 4,328,201, 1980. (7) Abramowitz, H.; Insigna, R.; Rao, Y. K. Kinetics of reaction of sulfur dioxide with carbon.Carbon 1976, 14, 84-86. (8) Furimsky, E. Gasification of oil sand coke: Review. Fuel Process. Technol. 1998, 56, 263-290. (9) Fairbridge, C.; Palmer, A. D.; Ng, S. H.; Furimsky, E. Surface structure and oxidation reactivity of oil sand coke particles. Fuel 1987, 66, 688-691. (10) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley & Sons: New York, 1999. (11) Szekely, J. Gas Solid Reactions; Academic Press: New York, 1976. (12) Cussler, E. L. Diffusion and Mass Transfer in Fluid Systems; Cambridge University Press: New York, 1984. (13) Satterfield, C. N.; Sherwood, T. K. The Role of Diffusion in Catalysis; Addison-Wesley: London, 1963. (14) Zhao, G. H.; Lawson, F. Effect of pore diffusion on reactivity of lump coke. Trans. Inst. Min. Met. 1996, C105, 6371. (15) Stacy, W. O.; Vastola, F. J.; Walker, P. L., Jr. Interaction of sulfur dioxide with active carbon. Carbon 1968, 6, 917-923. (16) Puri, B. R.; Hazra, R. S. Carbon-sulphur surface complexes on charcoal. Carbon 1971, 9, 123-134.

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3739 (17) Sykes, K. W.; White P. The reactions of carbon with sulphur compounds. 4. Adsorption of gaseous sulphur and carbon disulphide by charcoal. Trans. Faraday Soc. 1956, 52, 660-671. (18) Puri, B. R. Chemistry and Physics of Carbon; Walker P. L., Ed.; Marcel Dekker Inc.: New York, 1971; Vol. 6. (19) Mateos, J. M. J.; Fierro, J. L. G. X-ray photoelectron spectroscopic study of petroleum fuel cokes. Surf. Interface Anal. 1996, 4, 223-236. (20) Bejarano, C. Carbothermal reduction of sulfur dioxide using oil-sands fluid coke. M.A.Sc. Thesis, University of Toronto, Toronto, Ontario, Canada, 2000. (21) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Direct determination and quantification of sulphur forms in heavy petroleum and coals, 1. The X-ray photoelectron spectroscopy (XPS) approach. Fuel 1990, 69, 939-944.

(22) Perkin-Elmer. Manual of XPS Analysis; Perkin-Elmer: Wellesley, MA, 1979. (23) Kasrai, M.; Brown, J. R.; Bancroft, G. M.; Tan, K. H.; Chen, J. M. Characterization of sulphur in coal from sulphur L-edge XANES spectra. Fuel 1992, 71, 649-653. (24) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Direct determination and quantification of sulphur forms in heavy petroleum and coals. 2. The sulphur K edge X-ray absorption spectroscopy approach. Fuel 1990, 69, 945-949.

Received for review August 27, 2002 Revised manuscript received May 16, 2003 Accepted May 23, 2003 IE0206711