much lower metal concentrations in the bottom of the core. T h e sewage sludge showed a two-stage release. The first stage occurred a t day 1,followed by a slow constant release until day 50 when another release occurred. A slow but steady release would more likely result in a higher maximum uptake, whereas a two-stage release due to the estimated 6-h residence time for the water in the tanks and 8.5 L min-l flow rate produce a lower maximum net uptake. A definite net uptake of trace metals was measured in members of a Thalassia testudinumlRhizophorae mangle community in the two replicated experiments over the period from March 1975 to April 1976. The net uptake was especially significant in: Thalassia testudinurn leaves and rootslrhizomes, internal organs of the urchin L y t e c h i n u s uariegatus, “fouling organisms”, internal organs of Holothurea sp., and for some metals in Rhizophorae mangle roots. In general, the net uptake pathways followed the trophic levels in the food webs. This pathway was especially noticeable from the sludge to the fouling organisms and from the sludge to the Thalassia testudinurn leaves and thence t o the urchin herbivore L y t e chinus uariegatus. The net uptake in the Holothurea sp. was closely related to the net uptake in the fouling organisms and the trace metal loss rate of the sludge. The food web in this artificial system was dominated by the fouling organisms as would be expected. The uptake of trace metals, leached from the sludge, was always greatest in this complex group. T h e results of long-term exposure, in a simulated tropical ecosystem, to toxic trace metals more closely approximate the situation in nature than static laboratory experiments. Our results show that chronic exposure to toxic metals can lead to uptake and concentration of these metals in marine organisms. We were able to duplicate our results using this complex simulation of a marine ecosystem. The authors would not recommend, based on the results of this research, the dumping of sewage sludge in shallow, tropical marine environments. The potential exists for rapid concentration of toxic trace metals by members of the food web high on the trophic scale.
Acknousledgments The authors express their appreciation to the following organizations: the Puerto Rico Department of Agriculture for wet laboratory facilities and Harbor Branch Foundation Inc. and the Smithsonian Institution for use of facilities for completion of this research. The technical support of the following
people was invaluable for the completion of this research: Ms. Gina deCastro, Ms. Luz Cruz, Ms. Dee Zimmerman, Mr. John Thurston, Mr. Pedro Acosta, Mr. Jon Cole, Mr. Jose Echeverria, Mr. Thomas Prinslow, Mr. Jose Ramirez, Ms. Arlene Ramirez, Mr. Bob Quinn, and Dr. Kenneth Waters. We extend our sincere thanks to Dr. Frank Lowman for the initial conception of this project.
Literature Cited (1) Carmody, D. J., Pearce, J. B., Yasso, W. E., Pollut. Bull., 4(9), 132-4 (1973). ( 2 ) -Jacobs, S. A , , in “The Use of Flowing Biological Systems in Aauaculture Seware Treatment. Pollution Assav. and Food Chain Stbdies”, Woods Hole Oceanographic Institut e-Technical Report WHOI-73-2, 1973. ( 3 ) Kerfoot. W. B., Jacobs. S. A,. in “The Use of Flowing Biological Systems in Aquaculture,’Sewage Treatment, Pollution-Assay;and Food Chain Studies”, Woods Hole Oceanographic Institute Technical Report WHOI-74-2. 1973. ( 4 ) Kerfoot, W. B., Jacobs, S. A,, in ”The Use of’Flowing Biological Systems in Aquaculture. Sewage Treatment, Pollution Assay, and Food Chain Studies”, Woods Hole Oceanographic Institute Technical Report WHOI-73-2. 1973. ( 5 ) Kerfoot. W.B., in “The Use of Flowing Biological Systems i n Aquaculture, Sewage Treatment, Pollution Assay, and Food Chain Studies”, LVoods Hole Oceanorrauhic . . Institute Technical Reuort WHOI-73-2, 1973. (6) Shuster, C. M., Jr., Pringle, B. H., Proc. Natl Shellfish. Assoc., 59,91-103 (1969). ( 7 ) Valiela, I., Banus, M. D., Teal, J. M., Enuiron Pollut., 7,149-57 (1974) ( 8 ) Schroeder, P. R., Ph.D. Dissertation, University of Miami, Coral Gables, Fla., 1975. (9) Menzel. D. W.. Buii. M a r . Sci.. 27(1). 142-5 (1977). (10) Montgomery, .J. R., Price, M.. Thurston, J., deCastro, G., Cruz, L., Zimmerman, D., “The Release of Cadmium, Chromium, Copper, Nickel, Zinc by Sewage Sludge and Subsequent Uptake by Members of a Turtle Grass (Thaia.rsiu tehtudinum) Ecosystem”, Center for Energy and Environmental Research, Mayaguez, Puerto Rico, Report CEHR-2, 1977, 142 pp. (11) Parker, P. I,., P u b / . I n s t . M a r . Sci. I‘niLs. T e x . , 11, 102-7 (19661.
(12) Huggett. R. -J., Bender, hl. E., Slone, H. D., Water Res., 1,451-60 (1973). (13) Camp, D. K., Cobt), S.P., VanBreedveld, J . F., R i o s c i e n c ~23(1), , ?7-8 (19731.
Rereiced for recieu M a ) 3 , 1978. Accepted December 5 , 1978. T h e research was funded under a joint E P A I E K D A grunt ( I A G - D d 0543/30-468-743. W e thank Harbor Branch Foundation Inc. and the Smithsoniun Institution for finanria/ support. T h i s is Harbor Branch Foundation, I n c . Contribution N o . 115.
Kinetics of Desulfurization of Hot Fuel Gas with Calcium Oxide. Reaction between Carbonyl Sulfide and Calcium Oxide Ralph T. Yang’’ and James M. Chen2 Brookhaven National Laboratory, Upton, N.Y. 11973
In developing coal gasification processes, removal of the sulfur compounds has been of great concern because of their detrimental effects on air quality, corrosion of equipment, and poisoning of the catalysts used in the processes. The sulfur compounds existing in the fuel gases are mainly hydrogen sulfide (H2S) and carbonyl sulfide (COS) ( I ) . Over the past few years, work on the use of metal oxides as sorbents for H2S removal has been reported in the literature, e.g., in ref 2-5. In Present address, Deparl ment of Chemical Engineering, State IJniversity of New York a t Buffalo, Amherst, N.Y. 14260. Present address, Engelhard Industries Division, Menlo ParkEdison, N.Y. 08817.
particular, studies have been made on the kinetics of the reaction between CaO and H2S (4-7). It is known that H2S exists in a greater concentration than does COS in the fuel gases from the coal gasifiers. The thermodynamic equilibrium ratio of H2S over COS is normally high under the gasification conditions. For example, the ratio is about 50 for the Lurgi process. Nevertheless, the equilibrium is not rapidly reached and, therefore, significant amounts of COS do exist in the raw gases from gasifiers. In addition, COS also exists in the effluents of other industrial processes. For the commercially available coal gasifiers, the typical concentration of COS is 0.01% ( I ) .
0013-936X/79/0913-0549$01 .OO/O @ 1979 American Chemical Society
Volume 13, Number 5, May 1979 549
Rates of sorption of COS by CaO were measured in the temperature range of 500 to 900 "C a t a total pressure of 1atm. The CaO sample was a reagent-grade fine powder with particle size of 1-5 pm. T h e overall rates in the temperature range of 600 to 900 "C were correlated satisfactorily with the grain model (with a shape factor of 1.5),where chemical reaction was the rate-limiting step. The correlation with the grain model was also supported by the SEM observation of the physical
structure of the CaO particles. Using the initial rates as the surface chemical rates, the temperature and concentration dependencies were determined and a rate expression was derived. The rate of sorption of COS by CaO was slightly higher than that of H2S, using the literature data on the H2S sorption. Significant sorption of COS takes place parallel to the sorption of HzS in the coal gasification processes in which lime is used.
Removal of COS from hot gases with metal oxides has yet to be investigated. In this paper, we report the results of a kinetic study of the sorption of COS with CaO. Since lime is used in several promising second generation coal gasification processes, e.g., the CO2-acceptor and the Westinghouse fluidized-bed processes, the results of this study will be useful in understanding the sulfur removal mechanism in these processes. In studying the reaction between CaO and COS, we first consider the thermodynamic equilibria of the following reactions:
In all the rate measurements, about 50 mg of reagent grade calcium carbonate powder (Mallinckrodt) was used. The size of the powder was 1-5 pm as measured with scanning electron micrographs. The calcium carbonate sample was first calcined a t 900 "C in a N2 flow. Subsequently, the system was adjusted to the desired temperature and the rate measurement commenced upon admitting the reactant gas. The purpose of using the fine powder calcined CaO was to eliminate the possible pore diffusion resistance and to obtain the chemical rate. The gases used were directly obtained from gas cylinders with the compositions: nitrogen (99.99%), carbon monoxide (99.95%), and a custom-made nitrogen-carbonyl sulfide mixture (1.87%COS in Nz). Except where noted, the total gas flow rate was 2000 SCCM.
2cos = 2 c o + s2 CaO + COS = CaS + COz
(1) (2)
Equilibrium constants of the two reactions are shown in Figures 1 and 2. Now we shall examine the feasibility of removing COS from the hot fuel gases with CaO. Using the Lurgi process as an example, the temperature range is 1000-1400 O F and the COz composition is 15%.From Figure 2, the equior 0.7 ppm. Therelibrium composition for COS is 7 X fore, calcium oxide is thermodynamically a good sorbent for COS removal from hot fuel gases. The equilibrium curve in Figure 1 will be used in the experimental work as a guide to determine the maximum decomposition of COS.
Experimental T h e reaction rates were measured gravimetrically. The detailed experimental and calculation procedures have been described elsewhere (8)except that a Sartorius microbalance was used in this investigation. Also different in this work was the fact t h a t the sample holder was a porous alumina pan suspended with a quartz rod instead of a platinum wire because it was found early in the work that platinum was attacked by COS. Because COS decomposes a t high temperatures, a sufficient amount of CO was introduced into the system to prevent the decomposition. This was confirmed from the mass spectrometric analyses of the COS compositions in the inlet and outlet gases. T o test if the measured weight gain was indeed due to Reaction 2, the solid product samples were analyzed by X-ray diffraction.
Results and Discussion X-ray diffraction analyses usin standard powder technique gave the following d spacings (in ): 2.78,2.42,1.71,1.45,1.38, and 1.20 for the reactant and 2.87, 2.04, 1.72, 1.66, 1.43, and 1.28 for the product. They correspond to CaO and Cas, respectively. Preliminary experiments were also conducted which showed that the maximum weight gains of the solid reactants corresponded to full conversions of CaO to Cas. I t was concluded, therefore, that Reaction 2 was the only reaction attributable to the weight change. T o suppress the thermal decomposition of COS in the reaction zone, CO was used in the reactant gas mixtures. In the preliminary experiments, gas samples from the inlet and outlet of the reaction zone were taken and analyzed with a quadruple mass spectrometer for the COS concentrations. The sensitivity of the mass spectrometer was 30 ppm for COS a t a total pressure of 1atm. I t was found that, a t the highest temperature of this work, Le., 900 "C, 10%CO in the reactant gas was sufficient to suppress the decomposition of COS a t a concentration of 0.1%. The maximum thermodynamic limit of decomposition under the above conditions was, according to Figure 1,20% of the COS in the feed gas. However, the mass spectrometric analyses showed that the decomposition was below 3%. Consequently, we used 10% CO in all the reactant
1
I
TEMPERATURE I C )
IO00300 800
7O :
600
!
i: I -
N
7% $ Lo 5
O-
- 1
-
r
5
-2-
-3
-
-4
-
6'
08
Figure 1. Equilibrium
550
1.0
12 IIT IKI
Environmental Science & Technology
Flgure 2. Equilibrium
+
1
09
1
I O
,
I I
,
1 2
IIT I K I
1.4 x 10-3
constant for the reaction COS
,
08
CO
'/&
con
constant for the reaction CaO
I
13~10-3
+ COS
+
Cas
05.
C8-
404-
07-
4 O0
I
' 20
0
d
1
4c
60
1
1
1
80 100 TIME ( m i n i
1
I20
140
I60
180
Figure 3. Reaction rate of CaO with COS (0.1% ) a t500 O C ; in and in N2 with 10% CO ( V )
I O
r
-
N2 ( A )
p
0
0
2C
*C
6C 80 TIME l m i i l
I00
I20
I40
Effect of temperature on reaction rate with 0.1 % COS and 10% CO in NZ at 900 ( O ) ,800 (M),700 (A),600 (V),and 500 O C
Figure 5. (+)
0 89 I O
I -- ,
Oc-K
20
40
60
'
80 100 TIME ( m i n i
1
120
'
140
I60
'
180
Figure 4. Effect of flow rate on reaction rate at 900 O C with 0.1 % COS and 10% CO in N2; with flow rates of 1000 SCCM (0)and 2000 SCCM (0)
gas mixtures and assumed that the COS concentrations were the true values in all the subsequent experiments. T o ensure that the introduction of CO into the reactant gases did not affect the kinetics of Reaction 2, the rates of this reaction were measured a t 500 "C with and without 10% CO. The rates were then compared as shown in Figure 3. Since no difference in the rates was detected, CO may be regarded as a true inert in the reaction system. T o eliminate the resistance due to the gas film, or the external mass transfer effects, a certain minimum velocity of the reactant gas had to be exceeded. This was done by comparing t h e rates measured a t two gas flow rates. In Figure 4, we compared the rates a t 900 "C a t flow rates of 1000 and 2000 cm"min (STP), or SCCM. Inasmuch as the rates were the same in Figure 4, the flow rate of 2000 SCCM was used throughout this work, and the rates measured did not include the external mass transfer effect. Figure 5 shows the results of conversion of CaO to C a s , X , as a function of time a t various temperatures, with 0.1%COS in CO and Nz.Figure 6 gives the rates a t 800 "C with the concentration of COS varying from 0.02 t o 0.296. The initial rates, (dX/dt),=o, in 1his kind of rate curves approximate the intrinsic chemical rates. The initial rates obtained from Figure 6 were plotted against the concentration of COS and are shown in Figure 7. It is seen that, in the range of COS concentration below 0.2%,Reaction 2 is first order with respect to COS. The first-order rate dependence, in the low partial pressure range, was also observed for the reaction of CaO with SO2 ( 9 ) ,H2S ( 3 ) ,and NO (10). T o interpret the rate curves such as those shown in Figures
TIME ( m i n i
Figure 6. Effectof COS concentration on reaction rate at 800 O C with 10% CO and the marked COS compositions in N1
5 and 6, the interplay between the chemical rate and the diffusion rate would have t o be understood. Mathematical models exist for several simplified and idealized reaction systems ( I I ) . Before using the models to interpret the rate data, we examined the physical structure of the solid particles with a scanning electron microscope. The micrographs showed that most of the CaO particles had an overall size of 1-5 pm, and each particle consisted of randomly stacked grains, most of which were shaped as flakes. The pores or void in the particles were of the same dimensions as the grains. With the above physical observation, the grain models (11,121 appeared to be well suited for correlating the data. In the case where chemical reaction limits the overall rate, the following solutions to the grain model ( I 1 ) are used:
tlr
= 1- ( I
- X)1'F,
(3)
In the case where diffusion of COS through the reaction product layer is the limiting rate step, we have the following solutions t o the model (I 1):
t / r = x2
+ (1 - X ) In (1 - X ) = 1 - 3(1 - X)2'" + 2(1 - X ) =
X
F, = 1
(4)
F, = 2
15)
F, = 3
(6)
Volume 13, Number 5, May 1979
551
oo1st
!
0.016c
00141
2
4
000,I
4
0006F
20
0
60
40
80 100 T I M E , min
120
1
140
Figure 9. Correlation of rates with the grain model with the chemical reaction limiting case at 900 (0),800 (O), 700 ( A ) , 600 (m), and 500 OC (V) 014.
I
I
,
I
I
I
I
1
,
I
I
,
C O N C E N T R A T I O N OF COS i%l
Figure 7. Analysis of the reaction order with respect to COS at 800 "C
TIME m n
Figure 10. Correlation of the rates at 500 "C with the grain model where diffusion in the product layer is the limiting step. PF = f l (0),X + (1 - x ) In (1 - x ) ( A ) , and 1 - 3(1 - X)2'3 2(1 - X ) ( 0 )
+
20
40
60
80 I00 T I M E , min
120
140
L I60
Figure 8. Correlation of the rates at 700 "C with the grain model. Y = 1 - (1 - x ) l / F g , where Fg = 1 (0),1.5 (0), 2 ( A ) ,and 3 (0).Y = 9 (o),i-3(1-~)*/3+2(1-x)(m),and[1-(1-~)1/3]2(+)
Here t is time, 7 is the time for complete conversion, X is the fractional conversion, and F , and F , are the shape factors for grain and particle, respectively. F , has the values of 1 , 2 , and 3 for, respectively, slab, cylinder, and sphere. In addition t o the above equations, Jander's equation ( 1 1 ) was also used. Figure 8 shows the results of correlating the data a t 700 "C with these equations. As shown in the figure, all the diffusion-limited models (Equations 4-6) failed to correlate the data whereas Equation 3 with a shape factor of 1.5 fit the data satisfactorily. The same equation also correlated the data in the temperature range of 600 to 900 "C (Figure 9). The satisfactory correlation of the rate data with Equation 3 indicated that the reaction front progressed linearly with time within each "grain", as postulated in the original model. The shape factor of 1.5 lies between the shapes of slabs ( F g = 1) and cylinders ( F g = 2 ) . As mentioned, the shapes of the "grains" in the CaO sample were close to the slabs. In the model which results in Equation 3 with F , = 1,the grains were assumed to be slabs of infinite widths and of a uniform thickness. The deviation of the measured rates from the ideal model was attributed to the facts (1)that the slabs were not infinite and (2) that the grain size was nonuniform. The latter factor may be further explained as follows. During the reaction, the smaller "grains" reached completion first and the overall rate would slow down and hence the rate curve would convex, as shown in Figure 8. The rate at 500 "C exhibited somewhat different features 552
Environmental Science & Technology
than those a t higher temperatures in that the rate fell off rapidly. The same phenomenon has been observed for the reaction between CaO and H2S a t temperatures below 550 "C (4, 5'). The rate a t 500 "C could not be explained with the aforementioned grain models, as shown in Figures 9 and 10. Although the reason for the above rate behavior a t 500 "C is not known, it is possible that, a t such a low temperature, a n impervious layer of CaS formed on the CaO crystallite which prevented further reaction ( 5 ) . The activation energy of Reaction 2 was obtained from the temperature dependence of the initial rate. The value of 4.3 kcal was calculated from the data shown in Figure 11. The surface area of the CaO sample was 7.5 m2/g as measured with a Micromeritics dynamic surface area instrument. Taking this value as the area of the initial reaction, the chemicalarate of Reaction 2 assumed the following form: dX (7) dt Finally, because COS normally coexists with HzS, as in the fuel gases, it is interesting to compare the rates of the reactions of the two compounds with CaO. The initial rates a t 700 "C of the two reactions with 0.1%both of C 6 S and H2S and a total pressure of 1 atm are compared as the following: CaO COS: - = 78e-2160/TP~os(l - X)
+
(%)t-o CaO
= 1.3 X
s-l (reagent powder, this work)
+ H2S: = 1.0 X = 0.6 X
s-l (reagent powder, ref 3 ) s-l (dolomite powder, ref 4 )
Acknowledgment We are grateful for the assistance provided by 0. F. Kammerer (X-ray diffraction analyses), J. Forrest (mass spectrometric analyses), and I. W. Still, Jr. (scanning electron microscopic observations), all of the Brookhaven staff. Literature Cited
i
L
,
0001 110
, E O
90
I00 IiT
I10
120
,
1
130
140
iK1
Figure 11. Arrhenius plot of the initial rates
The comparison shows that, under the same conditions, the rates for COS and H2S are similar. As discussed, both rates are first order with respect to the concentration of the reactant gas. Therefore, with the ratio of compositions of COS and H2S known, the relative rates of reaction may also be known. Or, alternatively, knowing the rate of the reaction COS H2 H2S CO, and given the ratio H2/CO, the relative rates of reaction may be calculated. A conclusion may be drawn from this comparison that lime is an equally effective sorbent for COS as for H2S.
+
+
-
(1) Slack, A. V., “Sulfur Dioxide Removal from Waste Gases”, Noyes Data Corp, Park Ridge, N.J., and London, 1971. (2) Bureau, A. C.. Olden. M. J. F.. Chem. Eng.. 49.55 (1967). (3) Westmoreland, P. R., Gibson, J. B., Harrison, D.’P.,Enuiron. Sci. Technol., 11,488 (1977). (4) Pell, M., Graff, R. A,, Squires, A. M., “Sulfur and SO2 Developments”, Chemical Engineering Progress Technical Manual, AIChE, New York, 1971, D 151. (5) Gibson, J. B., Dissertation, Louisiana State University, Baton Rouge, La., 1977. 16) Ruth. L. A.. Sauires. A. M.. Graff. R. A,. Enuiron. Sci. Technol.. 6,1009 (1972). ( 7 ) Habashi, F., Mikhail. S. A., Thermochim Acta, 18.319 (1977). (8) Yang, R. T., Cunningham, P . T., Wilson, I., Johnson, S. A,, Adu. Chem. Ser., No. 137,149 (1975). (9) Borewardt. R. H.. Enuiron. Sci. Technol.. 4.59 (1970). (10) James, N: J., Hughes, R., Enuiron. Sci. ‘Technol.,’ 11, 1191 (1977). (11) Szekely, J., Evgns, J. W., Sohn, H. Y., “Gas-Solid Reactions”, Academic Press, ew York, 1976. (12) Evans, J. W., yeon-Sucre, C., Song, S., Metal[. Trans., 7B, 55 (1976). Received for review July 31, 1978. Accepted December 6, 1978.
Concentration Dependence upon Particle Size of Volatilized Elements in Fly Ash Richard D. Smith’, James A. Campbell, and Kirk K. Nielson Physical Sciences Department, Pacific Northwest Laboratory, Richland, Wash. 99352
Fly ash, collected from a coal-fired steam plant, has been separated into 17 well-defined size fractions and analyzed for 29 elements using X-ray fluorescence techniques. The results show t h a t the concentrations of volatile trace elements increase as particle size decreases in the 1- to 10-pm size range. For submicron particles, the concentration becomes independent of particle size. A volatilization-condensation model, which is capable of explaining these results, is suggested. T h e particulate emissions from coal-fired steam plants, equipped with modern pollution control devices, consist primarily of submicron particles. A large body of recent work (1-7) has shown t h a t the smaller fly ash particles resulting from coal combustion may be considerably enriched in several toxic trace elements. The most widely accepted model ( I ) for trace element enrichment in fly ash formation involves the volatilization of these elements during combustion, followed by condensation or adsorption over the available matrix material (composed primarily of the nonvolatile oxides of Al, Mg, and Si). Fly ash surfaces have been found to be enriched in several of the same trace elements showing enrichment in the smaller particles, supporting this mechanism (8-10). The smaller particles, which show the highest concentration of several trace metals, are not efficiently collected by pollution control devices. These particles, enriched in potentially toxic trace metals, also have the highest atmospheric mobilities and are deposited preferentially in the pulmonary and bronchial regions of the respiratory system ( I ) . Two models have been proposed to rationalize the con0013-936X/79/0913-0553$01.00/0 @ 1979 American
Chemical Society
centration vs. particle size dependence in fly ash resulting from a volatilization-condensation mechanism. Several years ago, Natusch and co-workers ( I ) were able to rationalize their data by assuming that volatilized material condensed evenly over the surfaces of all particles (Le., a surface layer thickness independent of particle size). They were able to obtain reasonable fits to their data using the equation:
c = c , + -6C
PD where C, is the concentration in the matrix upon which the volatiles condense, C, is the surface concentration, p is the density, and D is the particle diameter. Flagan and Friedlander ( I I ) , however, have recently suggested that a direct dependence of C on D-’ should exist only in the free molecule regime where the Knudsen number (K,) is greater than 1.At lower values of K,, in the continuum regime, they suggest that the total concentration will be proportional to 0 - 2 . Physically, this model is essentially the same volatilization-condensation model proposed by Natusch and co-workers but is quantitatively quite different, predicting much greater concentration increases for volatilized elements in the smaller size fractions. While Flagan and Friedlander were able to fit Natusch’s data ( I ) equally well with their model, the analytical results and fits to these simple models are of limited value since the data were limited to size fractions greater than 1 pm. In this work, we report the results of concentration vs. particle size measurements for sized fly ash fractions ranging from 0.15 to 200 pm median diameter. These results allow a direct examination of fly ash for the large enrichments pre-
Volume 13, Number 5, May 1979 553
