Kinetics and Yields for Sulfur Dioxide Reduction by Carbon Monoxide Charles W. Quinlan,' Vuranel C. Okay, and J. R. Kittrell Department of Chemical Engineering, Cniversity of Massachusetts, Amherst, Mass., 01002
Studies are reported on the kinetics of the reduction of sulfur dioxide b y carbon monoxide from a stream also containing nitrogen and carbon dioxide, using a copper-alumina catalyst in a plug-flow, integral reactor. A simple kinetic model i s reported relating sulfur dioxide conversion to process variables. It i s descriptive of system performance between 720" and 825" F, 3500 to 6500 ppm of carbon monoxide, 2000 ppm of sulfur dioxide, and contact times between 0.1 6 and 0.5 sec. A relationship between carbonyl sulfide production and sulfur dioxide conversion i s also presented, com pletely defining process selectivity as a function of the process variables.
T h e condition of the environment in which we live has become one of the most important technical, political, and social issues of our day. One key target of this concern is the decreasing quality of the air we breathe, particularly in and around metropolitan areas. X primary contributor to this air degradation is sulfur dioxide. In 1970, a n estimated 36.6 million tons of SOz (Chilton, 1971) were exhausted to the atmosphere by stationary sources alone. Nearly 50% of that figure is attributable to fossil fuel power plants. Projections indicate that this figure will rise in years to come, quadrupling by the year 2000 if controls are not implemented. Sulfur dioxide emission abat'ement from combustion sources is obtainable through modification of fuels or combustion (Chilton, 1 9 i l ) or tail gas treatment by a variety of wet' or dry processes (Naurin and Jonakin, 1950). The subject of this paper is work investigating catalytic reduction of SOz by carbon monoxide. This process represents one of the attractive dry stack gas treatment methods, for the products of this reduction are carbon dioxide and easily storable elemental sulfur. Work was done as early as 1918 on the direct reduction of sulfur dioxide by carbon monoxide (Ferguson, 1918). All early work, however, involved high reactant coiicentratioiis (25-357, SO? by volume) and large contact times. For adaptation t o flue gas treatment, concentrations on the order of 2000 ppm SO2 must be studied a t contact times on the order of a few tenths of a second (Slack, 1967). Some work a t low reactant concentrations was reported by Ryason (1967). Several catalyst's were evaluated; metals supported on alumina were found to be most effective, particularly copper, silver, and palladium. .in iron-alumina bifunctional catalyst was successfully tested by Khalafalla e t al. (1971) in SO2 reduction by carbon monoxide, Querido conducted a feasibility study on the SO2 reduct'ion process (Querido, 1 9 i l ) ; it was reported that a copper-oiialumina catalyst (Harshaw Cu-0803) containing 8y0 copper could achieve complete SO2 conversion, a t temperatures greater than 800°F, with carbon monoxide in excess of stoichiometric requirements, and a t contact times on the order of tenths of a second. However, a deleterious side reacTo whom correspondence should be addressed.
tion producing carbonyl sulfide (COS) was observed, leading to a reaction scheme of: 2CO
+ SO2 co +
+ '/2
'/2
s2
s 2 +
+ 2c02
COS
(1) (2)
(3) Hence, from a n initial stream containing SOZ, CO, and Nz, the effluent would include those components plus elemental sulfur, COS and C02. I l s o , due to the presence of COS, process criteria must be established in terms of total sulfur compounds removed. Hence, the aim should be removal of, say, 90% of the original sulfur entering the reactor, by considering effluent levels of both SO2 and COS. The elemental sulfur could be removed from the exhaust gases by electrostat'ic precipitation or condensation. The result,s of the present work define t'he effects of temperature arid carbon monoxide concentration on SO2 conversion arid COS yield in a plug-flow reactor containing a n 8y0 copper-on-alumina catalyst (Harshan- Cu 0803). The intent of this work was to characterize this catalyst more completely with a semiempirical kinetic model to provide a basis for the process design and optimization efforts needed for ai1 evaluation of carbon monoxide reduction as a process scheme for removing sulfur dioxide from stack gases. Of course, since the present study does not include the effects of water, oxygen, and nitric oxide on the catalyst, the model must be extended for a complete description of stack gases. Experimental
I schematic flow diagram for the experimental apparatus is shown in Figure 1. Compressed gases consist'ing of X2 and mixtures of CO, CO2, and SO2 in K2 were blended to yield process streams consisting of approximately 2000 ppm SO2 and l4Y0 CO?. Carbon monoxide level was a variable under study and was adjusted from run to run, betiveen about 3500 and 6500 ppm. The balance of the stream was nitrogen. Once blended, the gas stream was directed to flow over a fixed catalyst, bed contained in a tubular flow reactor. The reactor ivas in. I.P.S. titanium pipe, selected because i t Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 1 , 1973
107
1GAS CYLINDER TI
Np
a
T2 z N O / N Z TO ANALYTICAL
TRAiU
FURYACE-r
13
i
CO/NZ
T4
i
COp/NZ
T 5 :S O p / N z
W
ROTAMETER X-VALVE
40
I
IO
I I 1
I 12
I I3
I 14
I
I
I 5
16
CO RATIO
Figure 1. Schematic flow diagram of equipment
Figure 2. Dependence of SO2 conversion on carbon monoxide concentration
Table 1. Catalyst Property Characterization.
a t steady-state reactor conditions. During longer runs, process variables of CO level and temperature mere often varied to take data a t a number of operating points. X o catalyst deactivation was observed during any of the runs. A fresh catalyst batch was started u p by subjecting i t to the reaction stream a t 825OF and a contact time of 0.23 see for about 6 hr, during which the SO2 conversion increased from about 20% to 100%. h lower temperature point was then routinely checked to ensure consistent activity. Startup considerations are discussed in more detail elsewhere (Quinlan, 1972). For the data discussed herein, a constant value of contact time was employed a t 0.230 see (15,000 hr-1 space velocity). Contact time variations between 0.16 and 0.5 see have also been made, and although they are not reported here, they conform to the model discussed below. Temperature was varied from 720-825°F while CO level was varied from 0.9 of the stoichiometric requirement to 1.6 times the requirement. For convenience, the CO level is reported in terms of a CO ratio defined as
Catalyst, cominercial name: Harshaw Cu-0803, T--1/* Copper level, wt % : 8.0 (10% as CuO) Surface area, m2/g: 140 Cumulative pore volume, cc/g: 0.33 a Data provided by the Harshaw Chemical Co.
does not catalyze either SO2 reduction or reduction of nitric oside (Quinlan, 1972). The catalyst bed was about 4 in. in length and consisted of 2 0 / 2 0 mesh particles of Harsliaw Cu-0803 (Table I). The catalyst was supported by 40-mesh stainless steel screening. Reactor pressure was essentially atmospheric with a system pressure drop OKI the order of 40 m m Hg. The tubular reactor was suspended in Lindberg 3-ZOile Heavi-Dut,y furnace mounted vertically. Temperature control was provided by a Lindberg Heavi-Dut'y controller, and temperature was measured with Omega Chromel-*klumel thermocouples niounted 011 the outer wall of the titanium reactor a t 1-in. intervals over the catalyst bed. Output of thermocouples was monitored on a Leeds and Sorthrup potentiometer. The catalyst bed was operated isothermally within 1 2 ° F . Dowistream of the catalyst bed, the stream passed through a n electrostatic precipitator and cold trap designed to remove elemental sulfur, thereby preventing line plugging. Heating tapes were initially employed bet'weeii the furnace exit a i d precipitator to prevent premature solidification of sulfur but were subsequently abandoned when the line heating was found to influence COS levels produced. Blank tests were conducted, demonstrating that no compoiients other than sulfur were removed before the analytical train. A h a l p i swas performed for upstream and don-nstream compositions by a Perkin-Elmer gas chromatograph (Xodel 900). For resolution and analysis of COS, SOe, and CO1, a '1s in. X 6 ft. stainless steel column of Porapak Q-S (80/100 mesh) was employed. On this column, Ns and CO passed as a single peak. These two components were resolved by a in. X 6 ft. lIolecular Sieve 5A column (80/'100 mesh). Helium was the carrier gas a t 30 cc/min arid 50 ccimin, respectively. The columns were operated isothermally a t about 65" C. Runs were of various durations, some lasting only a few hours while the longest continued 45 hr. 1 1 1 data were taken 108 Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 1, 1973
CO ratio
=
coiicii of CO (ppm) 2 [concn of SOz (ppm) 1
(4)
The range of CO ratios investigated was thus 0.9 to 1.6. This number is strictly meaningful as a variable only for relatively const'ant SOs coiicentrations and that condition was maintained in this experimentation. More details on experimental apparatus and procedures are coiitained in the work of Querido (1971) and Quinlan (1972). Results and Discussions
The work of Querido and Short' (1972) on this system has defined feasibility of operation t o attain greater than 90% conversion of SOUto elemental sulfur. However, to properly characterize system behavior, data must be obtained to generate a model capable of predicting SO2 conversion and COS yield as a function of temperature and CO level (and contact time, but that variable is not discussed herein). A truly mechanistic model would most probably be hyperbolic in nature, containing kinetic and adsorption terms for the principal reactants .(SO2 and CO) and perhaps for products as well. Definition of the form of the model and estimation of its parameters would require extensive experiment'al and statistical effort (Kittrell, 1970) not felt justifiable a t this
Table II. Summary
Run no.
Temp,
co
O F
ratio
of Conversion Data5
Contact time, sec.
%
%
SO2
COS prod.
red.
%
so2
+
COSin effluent
1.25 64.5 17.5 53.1 732 0.230 D-8 73.2 23.3 50.1 1.44 732 0.230 D-9 1.60 730 0.229 83.0 26.8 43.8 D-10 44.9 20.4 7 5 . 5 0.91 732 0.228 D-11 1.375 0.225 80.4 25.5 45.1 758 D-15 1.395 0.221 8 9 . 8 26.6 36.8 776 D-16 1.40 795 D-17 93.7 2 6 . 3 3 2 . 6 0.218 1.40 0.213 100.0 29.2 2 9 . 2 824 D-18 1.405 0.234 100.0 3 3 . 1 3 3 . 1 920 D-19 722 6 8 . 6 20.1 5 1 . 5 1.33 0.279 D-20 35.4 31.5 96.1 1.33 722 0.151 D-21 1.59 815 0.231 100.0 74.0 7 4 . 0 D-25 1.44 725 67.4 3 5 . 8 78.4 0.230 D-26 1.42 915 D-30 0.220 100.0 2 5 . 8 2 5 . 8 1.57 0.222 100.0 5 7 . 5 5 7 . 5 820 D-31 1.42 73.7 3 8 . 5 6 4 . 8 737 0.229 D-32 1.42 773 9 1 . 4 27.7 3 6 . 3 0.232 D-33 1.39 733 0,235 D-34 71.0 10.2 39.2 1.38 D-35 67.6 59.5 91.9 733 0.241 1.085 0.245 D-36 71.5 2 3 . 4 5 1 . 9 7 59 1.085 0.248 795 D-37 8 0 . 9 1 0 . 6 29.7 1.38 733 0.241 D-35.4 6 0 . 5 1 4 . 3 53.8 D-36A 9 . 3 42.9 66.4 1.085 0.245 759 1.50 812 0.237 100.0 4 7 . 8 4 7 . 8 D-39 1.50 812 0.237 100.0 4 7 . 8 4 7 . 8 D-39.4 1.50 795 0.232 100.0 27.7 27.7 D-40 92.4 11.6 19.2 1.31 795 0.232 D-41 1.14 795 D-42 8 . 6 25.2 83.4 0.232 1.21 795 0.226 D-45 9 . 3 23.3 86.0 1.34 795 D-46 90.6 11.8 21.2 0.226 754 8 2 . 9 11.0 2 8 . 1 1.46 0.228 DN-9 a Run D-34 and Runs after D-37 without heating tapes.
stage of process development. Hence, a simple empirical predictor was sought. A rate of reaction first order in SOz concentration was postulated
5a
70
Y
-
795* F -732'F
h u.) O
t
9
/ I
IO
12 CO
realizing that kso, would be functionally related to temperature and CO ratio. It was then necessary to attempt t o fit experimental results to this form to achieve a n empirically sound prediction tool. Theoretically, this form of reaction rate is found wanting, but as will be shown, it does provide a first-approximation compatible with system performance over the range of variables investigated. The refined data taken in this work are contained in Table 11. Figures 2 and 3 illustrate t'he behavior of SOz conversion subject to changes in temperature and CO ratio. In Figure 2, data a t two temperatures indicate increasing conrersion of SO?to both COS and sulfur with increased CO rat'io a t const'aiit temperature and contact time. This would be expected since a n increase in one of the reactants would be expected to raise the rate of reaction from Equation 1. From Figure 3, the increase of conversion with increasing temperature at constant CO ratio and contact time is seen. Again, from Arrhenius dependence of intrinsic rabe constants on temperat'ure,this is as expected. For correlation purposes, the catalyst activity is presented in terms of kson. The expression for ksoz for anisothermal, plug-flow reactor beconies
13
14
15
I6
RATIO
Figure 4. Dependence of SO2 rate constant on CO ratio
Figure 4 illustrates the dependence of ksonon CO ratio for two constant temperatures. As can be seen, the points in both cases can be represented by a straight line. Hence, the value of In ksoz can be determined as ln (kso,)
=
A'
+ B ( C 0 ratio - 1.4)
(7)
Although the difference in slopes in Figure 4 is not significantly different, we expect B to be a function of temperature. -1s temperature increases, I n ksoZbecomes more sensitive to CO ratio and B increases. From the Arrhenius theory, In kso, would be expected to be a linear function of the reciprocal of the absolute temperature. I n Figure 5, this linearity is demonstrated by the data for a constant CO ratio. Here the expression in 1n ksonis In (kso,)
=
E l A -- RT
Although the simple model employed in this work is known to be theoretically weak, the correct .Irrhenius behavior of the data for the range of temperatures observed (a. well as the previously described contact time agreement) suggests that i t is an adequate representation of system behavior. The Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 1 , 1973
109
50
PUERIDO'S DATA II971 I
-
0
CURRENT WORK
ul
-
EXIT LINE
AT 75.F
c
2 40LL
ij 2
c 30
-
W
P
5
d
SYSTEM VARIABLES CATALYST HARSHAW Cu 0803 PARTICLE SIZES 20/30 MESH AND 1/8 in TEMPERATURE 7 0 0 9800 F CONTACT TIME 210 - 610 SECS CO RATIO 80 - I 6
-
Y
3t
2L 795
0 0
I
I
805
815
I
825
I
835
I
845 '0
RECIPROCAL TEMPERATURE x IO'.
Figure 5. Dependence of
20
10
30 SO,
O R
SO,rate constant on temperature
40 50 60 70 80 90 REDUCED, PERCENT OF INLET SO,
100
Figure 6. Dependence of COS production on SO2 conversion
CO ratio = 1.4 8
o
PUERIDO'S DATA 11971) DATA FROM CURRENT WORK
*BO-
Table 111. Parameter Estimates for Equation 9 k S O I in See-1 Parameter
Estimate
A" B (at 732'F) B (at 795'F)
19.8 1.53 1.87 2 . 1 6 x 104 OR
EIR
activation energy calculated from the slope of Figure 5 is approximately 24 k caljg-mol. Haas and co-workers (1971) have reported a value of 18.3 k caljmol with a n iron-alumina catalyst in a similar temperature range. For CO oxidation by 02 over a copper-on-alumina catalyst, Mooi and Selwood (1952) reported a n activation energy of 26.5 i 0.6 k caljgmol. One ramification of this agreement of values is a n interpretation of mechanisms involved in SOz reduction. If the reaction is considered in light of the simultaneous oxidation of CO occurring, a number of known mechanisms might apply with SO2 replacing 0 2 as oxidant. The close agreement of activation energies mentioned above could be indicative that this interpretation of the reduct'ion of SO2 would be fruitful. Calculations were performed to determine the possible influence of mass t,ransfer on the observed rates of reaction. These calculatioiis suggest negligible external mass transfer resistance for SO, reduction aiid a negligible temperature gradient between the catalyst surface and bulk stream. This result is iii agreement with experiments conducted by Querido (1971) under conditions for which mass transfer should assume greater importance than in our studies. Depending upon the relative importance of Knudsen and bulk diffusion in the catalyst pores, effectiveness factors bettveeii 0.7 and 1.O are predicted from available correlations (Smith, 1971), assuming spherical catalyst particles and first-order kinetics. The combination of the two process variable effects generates a simple equation for prediction of kso2: 111 kS02 =
A" -
E l -RT
+ B ( C 0 ratio -1.4)
(9)
The parameters of Equation 9 representing our data are shown ill Table 111. Combining Equations 6 and 9 allows a satisfactory prediction of SOz conversion levels for all temperatures and carbon monoxide concentrations studied. 1 1 0 Ind.
Eng. Chem. Process Des. Develop., Vol. 1 2 , No. 1 ,
1973
"0 SO,
20 40 60 80 100 REDUCED, PERCENT OF INLET SOz
Figure 7. Dependence on total remaining effluent SO2 and COS upon SOzconversion
Since a first-order power model should not precisely describe the system kinetics, i t would be expected that the utility of Equation 9 mould be limited as a n extrapolating tool. In addition, SO?feed concentratioiis were maint'ained a t 2000 =t200 ppm for all data used to generate Equation 9. However, Haas et al. (1971) have showi that the dependence of the reaction rate on inlet SO, aiid CO? concentrations is slight. Having Equation 9 for a prediction of SO2 conversion, t'he yield of COS must now be related bo the process variables to define the total system behavior. Since both Equations 1 and 2 involve adsorbed carbon monoxide reacting to form products, i t \\-as reasoned that the process variables influelicing the reaction rate for sulfur dioxide removal might have a similar effect on the rate of formation of carbonyl sulfide. Such an effect would result i n a strong correlation between COS yield and SO?conversion without regard to the levels of t'eniperature and CO ratio. In Figure 6 the percent of original SO2 converted to COS is plotted against the percent of original SOz converted to either COS or elemental sulfur. This plot is an aggregate of data obtained by Querido (1971) aiid the work described herein, as noted in the legend of Figure 6. KO distinction was made in plotting these data
as to the values of the process variables involved (Le., temperature, CO ratio or contact time); t'he plot is valid for all levels studied. Comparison of the data of runs D-8 to D-33 (Table 11) to that of Figure 6 illustrates that heating the reactor exit lines to about 450'F contributes to the COS production level, probably owing to either a homogeneous reaction or t'o a wallcatalyzed reaction in the lines downstream from the reactor. Explicit data demonstrating the effect of heating the downstream lines are shown in Table 11; runs 3 5 1 and 36A were duplicates of runs 35 and 36, except the downstream lines \vere heated t'o 450'F for runs 35 and 36. These differences in COS yield were consistently greater than could be attributed to experimental error. Without heating the exit line, the final COS production for a given SOz conversion is suitably represented b y a single line despite a wide range of process variables. Hovever, ot'her data from our laboratories suggest that t'he line represented in Figure 6 is representative of t'he behavior of t'he copper catalyst, but will change with metals other than copper. I n any event, when Figure 6 is coupled with the ability to predict the SO2 conversion at' a set of process conditions (Equation 9), the overall system performance can be predicted. Also, the shape of the relationship in Figure 6 suggests that a minimum value of total sulfur compounds should be exhibited. Replotting the data on Figure 6 as show1 in Figure i confirms the existence of a minimum in total remaining sulfur compounds as a function of SO2 conversion. In the neighborhood of 90% SO2 conversion, t'he total remaining sulfur compounds (percent SO2remaining and percent COS formed) exhibit a minimum value of from 20-2570. SO2 conversion approaches lOOyo,the total sulfur compounds swing up owing t'o rising COS production, until a t lOOy0 SOn conversion, COS yields betm-een about 40 and lOOyoare obtainable depending upon t'he levels of temperature, CO ratio, and contact time. h s SO2 conversion approaches O%, the total sulfur compounds increase owing to the high residual SOz. It may be conduded, then, that sulfur dioxide reduction by carbon monoxide may be adequately correlated by a firstorder model for the purposes of predicting the dependence of SO?conversion on process variables. I n addit,ion, the carbonyl sulfide yield may be directly predicted from the sulfur dioxide conversion for a wide variety of temperatures, CO ratios, and contact times. Translating these results into a total sulfur compound basis, it appears that there are no
process conditions wherein a single bed of copper-on-alumina catalyst can remove more than about 7 5 4 0 % of the entering sulfur dioxide as elemental sulfur. Either catalyst compositional changes or reactor flow changes appear to be required to achieve a higher c.onversion to elemental sulfur. Nomenclature
A
= logarithm of pre-exponential factor for SO2 rate constant, Equation 8 A ' = logarithm of SOz rate constant, evaluated a t CO ratio of 1.4, Equation 7 A " = logarithm of pre-exponential factor for SOz rate constant, evaluated a t CO ratio of 1.4, Equation 9 B = empirical slope of plot of logarithm of SO2 rate constant vs. CO ratio Cso2 = gas phase concentration of SOz, lb mol/ft3 E = activation energy, cal/g mol k802 = first-order rate constant for SO2 conversion, sec-l R = universal gas constant, cal/g mol OR rgo2 = rate of disappearance of S02, Ib mol/ft3 sec T = absolute temperature, OR X802 = fractional conversion of inlet SO2 concentration
GREEKLETTER 0
= contact time, based on reactor inlet conditions and total catalyst volume, sec
literature Cited
Chilton, T. H., Chem. Eng. Progr., 67 ( 5 ) , 69 (1971). Ferguson, J. B., J . Amer. Chem. Soc., 40, 1626 (1918). Haas, L. A., McCormick, T. H., Khalafalla, S. E., Bur. 'Vines Rep. R17483 (1971). Khalafalla, S. E., Haas, L. A., Ind. Eng. Chem. Prod. Res. Develow.. 10. 133 11971). Kittrell,'J.'R.,'Advan. Chem. Eng., 8, 97 (1970). hlaurin, P. G., Jonakin, J., Chem. Eng., 77 (9), 173 (1970). Illooi, J., Selwood, P. W., J . Amer. Chem. Soc., 74,2461 (1952) Querido, R., PhD thesis, University of Massachusetts, Amherst Mass., 1971. Querido, R., Short, W. L., submitted to Ind. Eng. Chern. Process Des. Develow.. 119721. Quinlan, C. W:, ?*ISthesis, University of Massachusetts, Amherst, Mass., 1972. Ryason, P. R., Harkins, J., J . Air Pollut. Contr. Assoc., 17 (la), 796 (1967). Slack, A. V., Chem. Eng., 74 (25), 188 (1967). Smith, J. XI., "Chemical Engineering Kinetics," 2nd ed., hICGraw-Hill, Xew York, NY, 1971. RECEIVED for review May 26, 1972 ACCEPTEDSeptember 28, 1972 This investigation was supported by research grant AP01443-01, Air Pollution Control Office, Environmental Protection Agency.
Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 1, 1973
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