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Ind. Eng. Chem. Fundam. 1985, 2 4 , 443-449
the upset is in the other direction, the gain of the loop will decrease and the control response will be sluggish. This sluggish response can be seen in Figure 3 for tower 3 when xD is controlled by reflux flow. As shown, the controlled response for a positive change in XF is much more sluggish than for a negative change. When the controlled composition response is sluggish, this causes the uncontrolled composition to have a large peak in its transient response. This peak makes it necessary to have the steady-state concentration much purer than the product specification. When dual composition control is used, the nonlinear gains in a tower do not affect the control system as much. When either product composition is upset in the direction of decreasing purity, the gain of that loop increases, helping the controller to prevent the composition from violating the specification. Comparing the set point concentrations of single and dual control in Figures 3 and 4 shows how much more the uncontrolled product, in this case xB, has to be overpurified. For single composition control the XB set point is 0.0045, and for dual control it is 0.0063 compared to the product specification of 0.01. Gain scheduling could be used for both dual and single composition control to help offset the problem of nonlinear gains and thereby reduce energy use. A more detailed explanation of nonlinear gains and gain scheduling in distillation towers is given by Tsogas and McAvoy (1985). Conclusions
Dynamic simulations of distillation towers have shown that for some towers the actual energy savings achieved with dual composition control can be much larger than those predicted by a steady-state analysis. The amount of energy savings varies depending on the tower. Predicting energy savings is difficult without actually simulating the tower dynamically, but some general guidelines can be stated. The energy savings from using dual composition control can be expected to be favorable for highpurity towers and for components with a low relative volatility. For towers with large throughputs dual composition control may be beneficial. Acknowledgment
time for this project was partially funded by the Computer Science Center for the University of Maryland. Nomenclature
B = bottoms flow D = distillate flow F = feed flow K = controller gain h> = molal enthalpy of liquid on tray n h," = molal enthalpy of vapor on tray n H, = liquid holdup on tray n L = reflux flow L, = liquid flow from tray n t = time TR = reset time V = vapor boilup V,, = vapor flow from tray n X B = bottoms composition (mole fraction) xD = distillate composition (mole fraction) xF = feed composition (mole fraction) x, = liquid composition on tray n (mole fraction) y , = vapor composition in equilibrium with x, (mole fraction) L i t e r a t u r e Cited "Energy Conservation in Distillation"; Department of Energy: Springfield, VA, 1980; DOElCS14431-72. Cheung, A.; Marlin, T. E., paper presented at Distillation Control Short Course, Lehigh University, Bethelehem, PA, May 1982. Luyben, W. Ind. Eng. Chem. Fundam. 1975, 1 4 , 321. Luyben, W., paper presented at Distillation Control Short Course, Lehigh University, Bethlehem, PA, May 1982. McAvoy, T. J.; Weischedel, K.,paper presented at the Proceedings of the 8th International Federation of Automatic Control Congress, Kyoto, Japan, 1981. Perry, R. H. "Chemical Engineers Handbook", 5th ed.; McGraw-Hill: New York, 1976. Rljnsdorp, J. Automatika 1965, 1 , 29. Ryskamp, C. Hydrocarbon Process. 1980, 59, 51. Shlnskey, F. G. Oil Gas J . 1969, 67, 76. Shinskey, F. G. "Distillation Control for Productivity and Energy Conservation"; McGraw-Hill: New York, 1977; Chapters 5-6. Smith, J.; Van Ness, H. "Introduction to Chemical Engineering Thermodynamics", 3rd ed.;McGraw-Hill: New York, 1975. Stanley, G. T. M.S. Thesis, University of Maryland, College Park, MD, 1985. Stanley, G. T.; McAvoy, T. J.; Marino-Gallarraga, M. A. Ind. Eng. Chem. Process Des. D e v . . In press. Tsogas, A.; McAvoy, T., paper presented at the Proceedings of the 2nd World Congress of Chemical Engineering, Montreal, Canada, Oct 1981; Chem . Eng . Commun ., in press. Weischedel, K.; McAvoy, T. J. Ind. Eng. Chem. Fundam. 1980, 19, 379. Wood, R. K.; Berry, M. W. Chem. Eng. Sci. 1973, 28, 1707.
This work was supported by the National Science Foundation under Grant CPE 8025301. The computer
Received for review April 5, 1984 Accepted February 28, 1985
Kinetic Study of Sulfate Reduction with Kraft Black Liquor Char John H. Cameron' and Thomas M. Grace The Institute of Paper Chemistiy, Appleton, Wisconsin 549 72
One of the principal reactions occurring in the Kraft recovery furnace is the reduction of sulfate with carbon. This paper presents the results of a kinetic study of sulfate reduction with Kraft black liquor char in a sodium carbonate melt. The reduction was found to be first order in the carbon content of the char, to be zero order in sulfate until only low levels of sulfate remain in the melt, and to have an activation energy of 122 kJ/mol. The following mechanism is proposed to account for these experimental results: Sulfate adsorbs on an active carbon site; it is reduced, forming either sulfide or an unstable reduced sulfur intermediate and carbon dioxide, and the reduced sulfur species then desorbs from the carbon surface.
Introduction
One of the principal steps in the Kraft pulping process is the recovery of the pulping chemicals. In the recovery furnace, the organic constituents of the spent pulping li0196-43 1318511024-0443$01 SO10
quor (black liquor) are burned and the sulfur compounds present are converted to sodium sulfide. One of the key reactions occurring in the furnace is the reduction of Na2S04to Na,S. This occurs in the bed of 0 1985 American Chemical Society
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the furnace in a melt consisting principally of NaZCO3, Na2S04,and carbon. Although this is an industrially important reaction, little information is available concerning its fundamental nature. This is due to the high temperatures and corrosive nature of the reaction environment and the difficulty of studying a reaction involving gas, liquid, and solid phases. In a previous paper (Cameron and Grace, 1983), sulfate reduction with graphite rods was described. The objective of this work was to define sulfate reduction with a wellcharacterized form of carbon. In the present paper, the study of sulfate reduction rates is extended to reduction with Kraft black liquor char, the form of carbon present in the Kraft recovery furnace. The effects of the reaction variables are described, a rate expression is developed, and a mechanism is proposed for NaZSO4reduction with Kraft black liquor char.
Previous Research The earlier paper by Cameron and Grace (1983) described the kinetics of sulfate reduction with graphite rods in alkali carbonate melts. Sulfate reduction with the rods was found to be autocatalytic. This was due to an increase in active surface sites on the graphite rods as reduction proceeded. Once this increase had been accounted for, reduction was shown to be first order in graphite surface area. The effect of sulfate varied from first order a t low sulfate concentrations to zero order a t high sulfate concentrations. Sulfate reduction was found to be catalyzed by the sodium carbonate melt. It was much slower in either sodium chloride-sodium sulfate mixtures or in pure sodium sulfate melts. Due to the initiating effect of C02 on the reduction rate, it was proposed that sulfate reduction was initially catalyzed by COz through the generation of active sites on the graphite surface. The active site was proposed to be an adsorbed oxygen. Sulfate adsorbs on this site and is reduced, and the reduced sulfur product desorbs from the carbon. The limiting step in this mechanism is the desorption of the reduced sulfur species from the graphite surface. To support their molten salt gasification process, Trilling (1974), Atomics International, has studied carbon oxidation in sodium sulfate-sodium carbonate melts. Two papers by Dunks et al. (1981, 1982) describe the oxidation of graphite particles with sulfate (sulfate reduction) in sodium carbonate. They reported a surface area dependence of 0.72, a variable effect of sulfate concentration dependent on carbon loading, and an activation energy ranging from 239 to 268 kJ/mol. The catalytic effect of alkali carbonates in reactions involving coal char is well-known. Among the reactions which alkali carbonates are known to catalyze are the gasification of coal char with steam, oxygen, and carbon dioxide (Veraa and Bell, 1978; McKee, 1981). It is likely that the mechanism by which alkali carbonates catalyze these reactions is similar to that by which alkali carbonates catalyze sulfate oxidation of carbon (sulfate reduction). Progress has been made recently in determining the mechanism by which alkali carbonates catalyze the carbon gasification reactions. Mims and Pabst (1983) have shown that potassium carbonate reacts with coal char or carbon to form surface salt complexes such as phenoxide groups. By surface methylation and solid NMR, the level of such surface complexes was correlated with the rate of gasification reactions of the carbon. This is consistent with these sites being active sites for the gasification reactions. Therefore, an important characteristic of the catalytic nature of alkali carbonate is the formation and stabilization
Figure 1. Experimental reactor.
of these oxygen groups. On the basis of these results, Mims and Pabst proposed that the mechanism for the carbon gasification reactions involved these surface oxygen sites serving as active sites, gasification occurring through a rapid reversible oxidation a t these sites followed by a slower desorption step. Wood et al. (1983) have studied the mechanism through which alkali carbonates catalyze coal char gasification reactions. They have also concluded that the catalytic effect of alkali carbonates is due to the formation of a phenolate-type intermediate on the carbon surface.
Experimental System Figure 1 illustrates the experimental reactor used to study sulfate reduction with the various forms of carbon. The reaction vessel consists of an alumina crucible 6.34 cm in diameter and 10.16 cm high, which is contained in a stainless steel retort. The steel retort is heated with an induction heating coil energized by a 20-kW Lepel highfrequency power supply. The alumina crucible is heated by radiation from the steel retort. To prepare for a sulfate reduction experiment, anhydrous, reagent-grade alkali carbonates and sodium sulfate were mixed, dried at 150 "C under vacuum for 2 h, and then added to the alumina crucible. A typical charge consisted of 0.51 mol of sodium carbonate, 0.26 mol of potassium carbonate, 0.01 mol of sodium sulfate, and 0.7 g of Kraft char. This mole ratio of sodium sulfate to carbon produced a reduction reaction that could be monitored. The mole fraction of sulfate in the reaction mixture approximates that found in a Kraft recovery furnace as reduction nears completion. Two methods of introducing carbon or carbonaceous char into the melt were used with this reactor. In the first method, the carbon was premixed with the carbonate and sulfate salts and placed in the reactor. This mixture was then rapidly heated to the desired reaction temperature. The sulfate-carbonate mixture normally required 5-10 min to melt and an additional 5 min to reach a steady temperature. The off-gases from the reactor were continuously monitored during this heating period. Although the temperature of the reactants increased continually, the effects of the sulfate and carbonate concentrations could be identified and the activation energy for the reduction reaction could be determined. In the second method of adding carbon, the alkali carbonate and sulfate salts were mixed, added to the alumina crucible, and brought to a temperature slightly above the desired reaction temperature; reduction was then initiated by introducing the carbon through the purge tube using the valves shown in Figure I . After the carbon was added,
Ind. Eng. Chem. Fundam., Vol. 24, No. 4, 1985
Figure 2. Experimental system.
approximately 2 min was required for complete mixing of the carbon and the melt. When this approach was used, a small amount of carbon was blown through the sample port and lost from the reactor. This loss of carbon caused some difficulty in the quantitative interpretation of the reduction data. Also, some of the carbon introduced by using this procedure was not wetted by the melt and tended to float. At high levels of carbon, this dry carbon reacted with carbon dioxide evolving from the melt to produce relatively high levels of carbon monoxide. Therefore, most of the data contained in this paper were obtained by premixing the carbon and salts. Figure 2 illustrates the configuration of the experimental system. To measure the flow rate of the nitrogen purge stream accurately, the nitrogen was metered from a pressurized gas cylinder through a thermal mass flowmeter. This meter provided an instantaneous reading of gas flow rate and a 0-5-V output signal. A mercury manometer monitored purge pressure and served as a pressure release valve. If the purge line from the reactor became plugged, the mercury in this manometer would be blown into a vial, releasing the purge pressure and preventing overpressurization of the reactor. The nitrogen purge stream plus any carbon dioxide or carbon monoxide generated by the reduction reaction was conveyed from the reactor in a 0.64-cm-i.d. steel tube. This gas stream then passed through a filter to remove any particles and to a carbon monoxide-carbon dioxide gas analyzer. This infrared analyzer could simultaneously measure both the carbon monoxide and carbon dioxide concentrations over a 0-30% range and provide a 0-100mV output signal. The temperature of the melt was monitored by a chromel-alumel thermocouple located near the bottom of the alumina crucible. The melt temperature was controlled by adjusting the surface temperature of the steel retort. Once the desired melt temperature was reached, the normal temperature variation was less than 1 "C. Since sulfate reduction with carbon produces both carbon dioxide and carbon monoxide as products, the reduction reaction was followed by monitoring the carbon dioxide and carbon monoxide content of the product gas. The production rates of these two gases and the sulfate reduction rate were determined from the nitrogen purge rate and the percentages of carbon dioxide and carbon monoxide in the sample gas. Kraft Black Liquor Char Carbonaceous black liquor char was produced by first drying Kraft black liquor under vacuum a t 150 "C and then pyrolyzing the dried solids in a covered ceramic crucible a t 950 "C for 7 min. Approximately 40% was volatilized during the pyrolysis. Different batches of char prepared in this manner provided very reproducible experimental data. The results of an elemental analysis of the Kraft black liquor char are shown in Table I. The elements listed in Table I accounted for 96% of the total mass. In addition to the elemental analysis, the char was analyzed for the
Table I. Elemental Analysis of Char element wt, 70 inorganic compd carbon 29.36 Na2C03 0.65 Na2S hydrogen 33.78 Na2S04 oxygen sulfur 2.12 chlorine 0.56 potassium 3.39 sodium 26.0 total 95.9
wt,
445
7 0
59.8 2.9 1.76 total 64.5
1500 Run 191 K,CO,=O
26mol
N.,CO,=O
Slmol
N**so.= o mmoi Char =07prams 0 CO, Gmeratlon 0 CO G-mtm T.m~rmur.*F
1400
1300
-
f
-: E
e
1200
,1100
o
i
i
3
i
5
6
i
6
b
to
ii
1000 12
13
TIME (Min)
Figure 3. Sulfate reduction with Kraft black liquor char.
major inorganic compounds. The results of this analysis are also shown in Table I. It was found that 77% of the char was water soluble. This may be compared to the major inorganic compounds identified in Table I, which constitute 65% of the char. Typical Experimental Results Typical sulfate reduction rates with char, illustrated by carbon dioxide and carbon monoxide generation rates in a K2C03-Na2C03melt, are shown in Figure 3. Here, the char was premixed with K2C03,Na2C03,and Na2S04salts and rapidly brought to the desired reaction temperature with the induction furnace. As the mixture was heated, the nitrogen purge stream continually removed any carbon dioxide and carbon monoxide generated in the reactor. Little carbon dioxide and carbon monoxide were generated below the melting point of the salts. The desired reaction temperature was reached a few minutes after a molten state had been achieved. The carbon dioxide and carbon monoxide generation rates increased as the melt temperature increased. Once the maximum temperature was reached, the carbon dioxide and carbon monoxide generation rates decreased due to the depletion of the carbon and sulfate in the reactor. The experimental data, used to define the reduction reactions, were obtained a t temperatures where a homogeneous molten system was believed to exist. The existence of this homogeneous molten phase was confirmed in
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Ind. Eng. Chem. Fundam., Vol. 24, No. 4, 1985
ZE-
Initial Levels
Na,CO,= 0 51 mol K F O , = 0 26 mol
= 0 01 mol Char = 0 7grams
Na$O,
Temperature = 1450'F
L-_----__
29-
- I E
3 0
-2
-
310;
m
K E
I?
32-
0
K
v,
TIME ( M i n )
Figure 4. Sulfate reduction with Kraft black liquor char injected into melt.
one experiment by inspecting the reactor immediately after the melting point of the system had been exceeded. Mixing effects were examined by using a different reactor configuration and were found not to affect the experimental results. One of the characteristics of sulfate reduction with graphite was the autocatalytic nature of this reaction (Birk et al., 1971; Cameron and Grace, 1983). It is difficult to determine whether an autocatalytic reaction was occurring in the experiments where the pulverized graphite and Kraft black liquor char were premixed with the alkali carbonate and sodium sulfate salts (Figure 31, due to the continually increasing temperature. To determine whether sulfate reduction with Kraft black liquor char is an autocatalytic reaction, an experiment was conducted in which char was injected into the molten salts. The carbon dioxide and carbon monoxide generation rates and the sulfate reduction rate from Kraft black liquor char injected into the molten salts are illustrated in Figure 4. Approximately 1 min was required for mixing of the carbon and the melt and for the generated product gases to reach the infrared carbon dioxide and carbon monoxide analyzer. As shown in Figure 4,the reduction rate continually decreased without any discernible autocatalytic effect. Therefore, sulfate reduction with char is not an autocatalytic reaction. With the char injected into the melt, there is a tendency for some of the char particles to remain dry and float on the melt. These floating particles then react with carbon dioxide evolving from the melt to produce carbon monoxide. Therefore, the carbon monoxide content of the product gas in Figure 4 where the char was injected into the melt is higher than that of Figure 3 where the char and salts were premixed and heated to the desired reaction temperature.
1
1
2 4
2 2
20
18
16
1 14
loglo(CarbonC o n t e n t , Moles)
Figure 5. Effect of Kraft black liquor char level on sulfate reduction.
v, -8
/ 38-
/ /
Effect of Char Level on Sulfate Reduction. The effect of the black liquor char level on sulfate reduction was determined by varying the initial level of char present and measuring the reduction rate with the same amount of sulfate reduced. The effect of the level of char on the sulfate reduction rate is shown in Figure 5 . In Figure 5 , the log of the sulfate reduction rate is plotted vs. the log of the carbon level present in the char. Since the slope of this curve is approximately 1.0, the sulfate reduction rate is proportional to the carbon present and reduction can be expressed as first order in carbon mass. Effect of Sulfate Level on Sulfate Reduction. The effect of sulfate on the reduction rate was determined by measuring the reduction rate at the same level of carbon
Ind. Eng. Chem. Fundam., Vol. 24, No. 4, 1985 447 io a
.6.0
9 0 Initial Concentrations Na,CO, = 0 51mol K,CO, = 0 2 6 mol Na,SO, = 0 01 mol
.
80
0 Carbonate Melt
70
E
\
0
c
x I
E
-. ;
6 4
5 60
pi
m
K
.-c5
.6.5
-
d
a -66-
Temperature = 1 5 O O * F lnltlal concentrat,onr NsC1=109moi Na2S0,= 0 0 3 m o l Cher=OZg
C
!4 0
=
U
I",
K
c
0 Sodium Chloride Mall
K
c
-4
50
m
W
a
' 0
Temperature =1500'F lnltlal Concantrat,onr ~a,co,=On m o i Na,SO,= 0 0 3 mol Char.0 Z g
J
-67-
-6.6
-
0
=5
3 0
In 20
.6.9
10
0
i
i
3
i
5
6
i
TIME (Min)
Figure 8. Effect of carbonate on sulfate reduction with Kraft black liquor char.
energy, R is the ideal gas constant, T i s the absolute temperature, and f (composition) represents the compositiondependent terms. The activation energy was calculated by using a least-squares fit of the natural logarithm of the sulfate reduction rate (mol/min) vs. the reciprocal of the absolute temperature (K-') at 50% of the sulfate converted to sulfide (Figure 7). Since the initial concentrations of reactants were the same in each experiment, at 50% conversion the composition-dependent terms in eq 1 are constant and the reaction rates depend only on the temperature. Over the temperature range of this study, the activation energy was 122 f 4 kJ/mol. Effect of Carbonate on Sulfate Reduction. Sulfate reduction with graphite has been shown to be catalyzed by carbonate melts (Cameron and Grace, 1983). To determine whether sulfate reduction with Kraft char was also catalyzed by carbonate melts, sulfate reduction rates with Kraft char were also measured in NaC1-NazC03 melts. In Figure 8 the sulfate reduction rate with char in NaCl is compared with the reduction rate in Na2C03. In these experiments, the char was first washed to remove as much of the carbonate as possible and then injected into the melt at the desired reaction temperature. The reduction rate was significantly lower in the NaCl melt. The catalytic effect of carbonate on sulfate reduction with Kraft black liquor char is similar to that observed with pulverized graphite. Therefore, a common reaction mechanism is likely responsible for reduction with both forms of carbon. Discussion Although sulfate reduction with Kraft black liquor char is similar to reduction with graphite rods, there are some differences. Perhaps the most significant difference is that reduction with graphite rods is an autocatalytic reaction,
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Ind. Eng. Chem. Fundam., Vol. 24, No. 4, 1985
while reduction with Kraft char is not. The autocatalytic nature of sulfate reduction with graphite rods is due to an increase in active sites on the surface of the rod (Cameron and Grace 1983). With Kraft char, however, the carbon surface appears to be initially saturated with active sites and no increase in active sites occurs during reduction. Since the catalytic effect of molten alkali carbonate is similar for both graphite rods and Kraft char, both reactions probably occur through a common mechanism. Alkali carbonates are also known to catalyze the gasification reactions of carbon. The catalytic action of alkali carbonates in the gasification reactions of carbon is likely the same as that in sulfate reduction with carbon. Therefore, the progress made in determining the catalytic nature of alkali carbonates in carbon gasification reactions can be applied to defining the alkali carbonate catalyzed sulfate reduction mechanism. The catalytic nature of alkali carbonate in the carbon gasification reactions has been shown to be due to the formation and stabilization of oxygen on the carbon surface. This oxygen acts as an active site for the adsorption of the reactive gas (H20,C02, or H2). Due to an initial catalytic effect of COz, a similar mechanism was previously prepared for sulfate reduction with graphite rods (Cameron and Grace, 1983). Here, it was proposed that the oxygen on the carbon surface results from the reaction of C 0 2with the carbon. Since the catalytic effect of alkali carbonate on sulfate reduction with Kraft black liquor is similar to its effect on the carbon gasification reactions, it is likely that the catalytic action of carbonate on sulfate reduction with char is also due to the formation and stabilization of oxygen on the carbon surface. Proposed Mechanism. Based on the experimental results presented in this report, the following mechanism for sulfate reduction with Kraft black liquor char is proposed. Sulfate adsorbs on an active carbon surface site; reduction occurs on the carbon surface forming a reduced sulfur species, either sulfide or an unstable reduced sulfur intermediate and C02, which then desorbs from the carbon. The rate-limiting step in this mechanism is the reduction of sulfate on the carbon surface. This mechanism is described by the following equations. The adsorption of sulfate on an active carbon can be represented by with Here, [SO,] is the sulfate concentration in the melt, Cf represents the vacant active sites per unit surface area, and C(SO,),d, is the adsorbed sulfate per unit surface area. The total number of active sites available for sulfate adsorption is the sum of the vacant active sites and those already occupied by sulfate ("T
=
c, ic(SO4),ds
(3)
where CT is the total number of active sites per unit surface area. By combining eq 2 and 3 and eliminating the free active carbon site term, Cf, we can express the adsorbed sulfate in terms of sulfate concentration and total active carbon sites i4i
A t high sulfate concentrations, according to eq 4, the adsorbed sulfate is independent of the sulfate concentration, but at low sulfate concentrations, the adsorbed sulfate level decreases as the sulfate concentration decreases and
Table 11. Experimental Parameters Describing Sulfate Reduction with Kraft Black Liquor Char parameter linear est of std dev K , L/(mol b ) 5.96 x 104 ~ . 8 x8 104 K", I,/mol 45.6 hl7.1 S, kJ/mcil 122 f4 ~
eventually becomes proportional to sulfate concentration. Although sulfate reduction with Kraft black liquor char never becomes distinctly first order in sulfate, the zeroorder sulfate dependence at high sulfate concentrations and the decrease in rate at low sulfate concentrations are described by eq 4. Therefore, the observed effects of the sulfate concentration on sulfate reduction with Kraft black liquor char can be described by the rate of sulfate reduction being dependent on the level of adsorbed sulfate. The rate-limiting step in the proposed sulfate reduction mechanism is the reduction of the adsorbed sulfate on the carbon surface, forming carbon dioxide and a reduced sulfur species. The reduction rate is proportional to the amount of adsorbed sulfate and can be described by the equation d[ SO,] - KqS04]CTe-s/KT __ _(5) dt 1 4- K[SO,] where K'is a constant, AE is the activation energy, R is the gas constant, and T is the absolute temperature. As described earlier, the reduction rate is first order in the moles of carbon present. Scanning electron microscope photographs of the char reveal these particles to be a few microns in diameter, extremely porous, and irregularly shaped. With such particles it is expected that diffusion into the particles would not be rate controlling. Therefore, the active sites able to participate in the reduction reaction are assumed to be proportional to the moles of carbon present and eq 5 can be rewritten as K"[SO4][ C] e-lE/R* d[SO,] _(6) dt 1 + K[SO,] _ I _
Here, [C] is the carbon concentration. To determine the rate constants K and K ", a nonlinear regression analysis program was used to fit eq 6 to a large number of experiments with different initial carbon and sulfate concentrations. The activation energy was determined by using independent experimental data as previously described. The experimental parameters in eq 6 that describe sulfate reduction with Kraft black liquor char are listed in Table 11. A typical reduction experiment used for this analysis is shown in Figure 9. The actual amount of sulfate reduced is compared with that predicted by the rate equation. Sulfate reduction with Kraft black liquor char is accurately described by eq 6, a rate equation. The slightly lower rate at the beginning of the reaction in Figure 9 is due to a lower temperature as the system is heated to the desired reaction temperature. Although Figure 9 represents only one reduction experiment, the reaction model with the parameters in Table I1 accurately fits the experimental data over the entire range of experimental conditions investigated during this study. The ability of the reduction model to describe sulfate reduction with Kraft black liquor char demonstrates the assumption that the active sites are proportional to the mass of the carbon present is valid. Carbon Monoxide/Carbon Dioxide Ratio. While carbon dioxide was the major product gas, some carbon monoxide was observed. The carbon monoxide generation rate was usually extremely low, but under certain exper-
Ind. Eng. Chem. Fundam., Vol. 24,
Run 255 Initial Concentrations
=
0 . 5 1 mol 0 . 2 6 mol Na,SO, = 0 01 mol Char = 1.0gram
Na,CO, K,C03
Temperature 1450'F
0 Experimental Data
- Model
0
0
1
I
I
I
I
1
1
2
4
6
8
10
12
14
TIME (Min)
Figure 9. Model f o r sulfate reduction w i t h Kraft black l i q u o r char.
imental conditions it reached approximately 10% of the carbon dioxide generation rate. The ratio of carbon monoxide to carbon dioxide increased with temperature and melt carbon content and decreased with an increase in sulfate concentration. As described in the Proposed Mechanism section, the active sites on the carbon surface are believed to be adsorbed oxygens. It is possible that these sites can either serve as sites for sulfate adsorption and consequential reduction or desorb from the carbon as carbon monoxide. If sulfate is reduced a t these sites, the product gas from reduction is carbon dioxide. The ratio of carbon monoxide to carbon dioxide in the product gas is then proportional to the fraction of sites that form carbon monoxide divided by the fraction that serve as sites for sulfate reduction. With the experimental conditions used in this study, the reduction reaction is considerably faster than carbon monoxide desorption and the major gas product is carbon dioxide.
No. 4, 1985 449
Other Possible Mechanisms. Other sulfate reduction mechanisms examined include a carbon dioxide-carbon monoxide cycle. In this mechanism carbon dioxide reacts with the carbon to form carbon monoxide. This carbon monoxide desorbs from the carbon and reduces sulfate in the melt generating additional quantities of carbon dioxide. This mechanism predicts an autocatalytic reaction due to the increasing level of carbon dioxide in the melt and was originally proposed by Cameron and Grace (1982) to explain the autocatalytic nature of sulfate reduction with graphite rods. Although some reduction likely occurs through this mechanism, further research demonstrated that it is minor compared to the amoimt occurring through direct reduction of sulfate on an active carbon site. Sulfate reduction with Kraft char is a nonautocatalytic reaction, and the autocatalytic rate observed with graphite rods is due to an increase in active surface sites. Also, a study of carbon monoxide reduction of sulfate (Sjoberg and Cameron, 1984) demonstrated that carbon monoxide reduction of sulfate is a relatively slow reaction compared to carbon reduction of sulfate. Therefore, it is likely that only a minor amount of reduction occurs through this carbon monoxide-carbon dioxide cycle. Summary and Conclusions Sulfate reduction with Kraft black liquor is first order in the carbon content of the char, zero order in sulfate until low levels of sulfate remain in the melt, and has an activation energy of 122 f 4 kJ/mol. The experimental results obtained during this study can be explained by the following mechanism: Sulfate reduction begins by the adsorption of sulfate on an active carbon site, and it is reduced, forming either sulfide or an unstable reduced sulfur intermediate and carbon dioxide; the reduced sulfur species then desorbs from the carbon surface. The active carbon site is likely an adsorbed oxygen stabilized by the alkali carbonate melt. Literature Cited Birk, J. R.; Larsen, C. M.; Vaux, W. G.; Oldenkamp, R. D. Ind. Eng. Chem. Process Des. Dev. 1971, IO, 7-13. Cameron, J. H.; Grace, T. M. Tappi 1982, 65, 84-7. Cameron, J. H.; Grace, T. M. Ind. Eng. Chem. Fundam. 1983, 22, 486. Dunks, G. B.; Stelman, D.; Yosim, S.J. I n "Proceedings-International Conference on Coal Science", Duesseldorf, 1981. Dunks, G. B.; Stelman, D.; Yosim, S. J. Inorg. Chem. 1982, 21, 108-114. McKee, D. W. "Chemistry and Physics of Carbon"; Walker, P. L.. Thrower, P. A,, Eds.; Marcel Dekker: New York, 1981; Vol. 16. Mims, C. A.; Pabst, J. K. Fuel 1983, 62, 176. Sjoberg, M.; Cameron, J. AIChE Symp. Ser. 1984, 80 (No.239), 35-40. Trilling, C. A., paper presented in part at the Winter Annual Meeting of American Society of Mechanical Engineers, Houston, TX, Nov 1974. Veraa, M. J.; Bell, A . T. Fuel 1978, 57, 194. Wood, B. J.; Brittain, R. D.; Chan, B. L.; Fleming, R. H.; Lau, K. H.; McMillen, D. F.; Sancier, K. M.; Seridan, D. R . ; Wise, H. DOE Report DOEfMCI 14593-14 16 (DE83013078), 1983.
Received for review May 29, 1984 Accepted April 16, 1985
