CAMS 6 and 12 sites for January 1977 together with resultant wind speed and direction. (Resultant wind speed and direction, obtained from the Texas Air Control Board, are the hourly vectors of 1-s readings taken every 5 min). When these data were plotted for each month of 1977,similar figures were obtained for 9 of the 1 2 months, Le., peaks a t 08:OO and 20:OO hours for both stations, a peak of CAMS 6 for 16:OO hours and with CAMS 12 always having lower concentrations of CO than CAMS 6. The three atypical months, May, September, and October, have the peaks of the other nine months, but CAMS 12 has higher CO concentrations in the late evening than CAMS 6 (Figure 3). During January, a typical month, from 17:OO to 23:OO hours, winds at CAMS 6 were from 23' to 157" (NE to SE) 30% of the time and from 203' to 292' (SW to NW) 62% of the time; a t CAMS 12 they were from 23' to 157' 33% of the time and from 203' to 292' 40% of the time. In September, an atypical month, winds at CAMS 6 changed little-35% from 23' to 157' and 54%from 203' to 292'. However, a t CAMS 12,28%of the winds were from 23' to 157' and 49% from 203' to 292'. In January, the ratio of west: east winds during the 17:OO and 23:OO hours was 2.0:1.2; in September, the ratio was 1.51.7. These ratios and carbon monoxide concentrations are typical of all of the months which we have studied since 1975. CAMS 6 is located directly north of downtown Cd. Juarez and -1.8 km from the border. CAMS 12 is located directly east of downtown Cd. Juarez and -0.5 km from the border. During most of the year, CO from downtown Cd. Juarez is measured
a t CAMS 6; when the winds shift so as to blow more from the west, the CO from downtown Cd. Juarez is measured at CAMS 12. Conclusion The cities of El Paso and Cd. Juarez share a common airshed. Previous studies have shown that heavy metals from a local smelter crisscross the border. Parts of El Paso are farther south than the northern parts of Cd. Juarez. This means that whatever is put into the air of either city may end up in the air of the other city. International cooperation is needed if both cities are to meet the air standards of their respective countries. Acknowledgment
I thank Sabino Gomez, Regional Director, Texas Air Control Board, and Raul Munoz, Director, El Paso Environmental Unit, for furnishing data and fruitful discussions. Literature Cited (I) Barajas Weber, L. H. In "Air Pollution Along the United States-Mexico Border"; Applegate and Bath, Eds.; Texas Western Press; El Paso, TX, 1974;pp 50-7. (2) Dominguez C., C. A. In "Air Pollution Along the United StatesMexico Border; Applegate and Bath, Eds.; Texas Western Press; El Paso, TX, 1974; pp 140-54. (3) Environmental Protection Agency "Mobile Source Emission Factors"; Final Document, EPA-400/9-78-005. Received f o r review September 15, 1980. Accepted March 23, 1981.
Rate of Reaction of Hydrogen Sulfide-Carbonyl Sulfide Mixtures with Fully Calcined Dolomite Vivek S. Kamath and Thomas W. Petrie' Department
of
Thermal and Environmental Engineering, Southern Illinois University at Carbondale, Carbondale, Illinois 62901
Kinetic data are obtained by a gravimetric technique for rates of reaction of calcium oxide in fully calcined dolomite with hydrogen sulfide and hydrogen sulfide-carbonyl sulfide mixtures. The data are presented as values for a factor K defined by d[CaO] = -K[CaO] dt. At 600,700, and 800 'C with [H2S] from 0.5% to 5.0% by volume and [H2S]/[COS] = 20 for mixtures, expressions for K show apparent rate constants and the dependence on sulfurous gas concentration. For example, at 700 'C, K = 1.43 X 10-4[H2S]1.06 s-l and K = 1.70 X 10-4[H2S COS]l.oos-l. Since the data show first-order dependence on calcium oxide, K's for H2S alone as the sulfurous gas and H2S-COS mixtures can be obtained for the same sample, free from scatter due to variations from sample to sample. Addition of values for K from runs with H2S as the only sulfurous gas and runs with COS as the only sulfurous gas are compared to measurements with actual mixtures. K's for the mixtures are -30% higher than the sum of the appropriate separate values.
+
Introduction Schemes for high-temperature desulfurization of raw coal gas seek removal of hydrogen sulfide and carbonyl sulfide. Many results are available for the kinetics of H2S removal, starting with the data for half-calcined and fully calcined dolomites obtained at the City University of New York ( I ) . 966
Environmental Science & Technology
-
Data for COS removal by calcined calcium carbonate were obtained by Yang and Chen ( 2 ) .Since hydrogen sulfide and carbonyl sulfide occur together in coal gas, data for mixtures are a valuable extension of the available results. We have experience with the gravimetric technique used in much research on high-temperature desulfurization. Our apparatus and technique are described in detail elsewhere ( 3 ) . To study H2S-COS mixtures, we added COS to H2S and other gases to maintain a molar ratio of [H2S]/[COS]equal to 20 and a total reaction gas flow rate of 500 SCCM. This total flow rate was selected to provide enough sulfurous gas atoms to the volume of solid removal agent so that the reaction is not dominated by gas film resistance. A few runs with 1000 SCCM of reaction gases through the apparatus showed no effect of flow rate. Besides the sulfurous gases, the reaction gas mixture contained 20% by volume of hydrogen and 30% by volume of carbon monoxide to prevent dissociation of H2S and COS, respectively. Dissociation effects were checked a t various volume percentages of hydrogen and carbon monoxide by comparative gas chromatography on inlet and exit gas streams in the absence of the solid-gas reaction. Nitrogen comprised the rest of the gas mixture. All gases were obtained from gas cylinders. The solid removal agent was dolomite supplied by the Pfizer Corp. from their quarry in Gibsonville, OH. Calcium-to-
0013-936X/81/0915-0966$01.25/0 @ 1981 American Chemical Society
magnesium ratio was determined to be -0.84 from the records of weight change during the calcination of individual samples of dolomite (magnesium carbonate and calcium carbonate crystallites with trace impurities). The uncalcined dolomite was ground and screened to -35 t 5 0 U.S. mesh (geometric mean particle diameter of 389 pm). This range was selected because it could be held in a basket made from platinum screen. We noted no corrosion of the platinum with the concentrations of COS which we used. The reaction gases passed over and through the dolomite in the basket at controlled conditions of temperature (600,700, and 800 "C), pressure (1 atm), and reaction gas composition ([H2S]/[COS] = 20, H2S from 0.5 to 5.0% by volume, 20% Ha, 30% CO, balance nitrogen).
Results The record of weight change of calcined dolomite during sulfidation is interpreted as the rate at which sulfur atoms from H2S and/or COS replace oxygen atoms in the calcium oxide available for reaction in freshly calcined dolomite at high temperatures. If the reaction is first order with respect to calcium oxide, then d[CaO]/dt = -K[CaO]
(1)
where [CaO] is the concentration of unreacted calcium oxide and K is a proportionality factor dependent upon temperature and gas composition ( I ) . The unreacted calcium oxide in the ground and calcined dolomite samples seemed to be accessible throughout the samples until fractions of CaO converted to Cas were greater than -0.9. At a particular temperature and H2S concentration, successful least-squares fits were obtained for the time rate of change in the natural logarithm of the fraction of CaO converted. The correlation coefficients were greater than 99%. Squires et al. ( I ) observed similar behavior with H2S and powdered dolomite for some of their runs. The value of the proportionality factor K for H2S-COS mixtures gives the rate at which calcium oxide is reacting with both sulfurous gases. When compared to data for H2S alone at comparable concentrations of H2S and CaO, the effect of COS is seen as the difference between K with the mixture and I
.
. , , I
that with H2S alone. It is fortuitous that a single value of K -an characterize reaction 1over a wide range in fraction of CaO converted for a single sample at a particular temperature and gas composition. It allows us to see the effect of COS without the severe scatter observed due to unavoidable variations in solid sample charging and calcination procedure from run to run with different samples. Values for K with H2S-COS mixtures a t a particular temperature were obtained after about half of the calcium oxide available for reaction in each sample had been converted to calcium sulfide by reaction with H2S alone. Adding COS to the existing flow was a simple procedure yielding a predictable approach to new values of K. Complete data are available in ref 4 . Figures 1-3 present the least-squares values for log K from individual samples vs. sulfurous gas concentration at each of the three temperatures studied. Equations of the lines through the data are also presented on the graphs. They give the apparent rate constants at the stated temperatures as well as the order of dependence on sulfurous gas concentration. The variation of results for two or more runs a t the same nominal conditions involving H2S-COS mixtures is shown by the bars. Scatter is similar in the data for H2S only, obtained in the first half of the runs with H2S-COS mixtures and in separate runs with H2S as the only sulfurous gas. The HZS-only runs verified that the effect of conversion on the factor K was not significant in our experiments. Values at 1%sulfurous gas concentration in the equations yield, when plotted vs. temperature, an activation energy of 3.6 kcal/mol for H2S only and 3.9 kcal/mol for H2S-COS mixtures. These are reasonable values, and no significance is attached to the slight difference. Attempts were made to fit the data for fractional conversion of calcium oxide by an ash diffusion statistic and a chemical reaction statistic. Reworking the raw data in this way could show whether the procedure used to get values for K with and without COS for the same sample was hiding interesting information about the controlling mechanism. The results were inconclusive in that correlation coefficients were greater than 95% for both statistics. The overall impression is that the data better fit the chemical reaction statistic. There appears to be
h
r
__ K,,
= .10 x lo'' (H2S)0'76
- - - K,,
= 3 7 x 10'' (H2S+
-K7,, = 1.43 x - - - K , ~ , = 1.70 x
0.5
1.0
5.0
H2S CONCENTRATION
(VOLUME %) Figure 1. Rate of change of In [CaO] at 600 "C.
1 0 . ~ (H~s+cos)'.''
t
10
0.1
l Q-' (H2S)"06
t
.
0.1
5 0.5
5
1.0
4 5.0
H p S CONCENTRATION (VOLUME %) Figure 2. Rate of change of In [CaO] at 700 "C.
Volume 15, Number 8, August 1981
967
Table 1. Synergistic Effect in HPS-COS Mixtures at 800 "C gas, vol %
1o
K (SEC")
0.5 HQS
I
0.5 H2S 1 .O H2S 1.0 H2S
.~
I
3.0H2S 3.0 HpS 5.0H2S 5.0 H2S
+ 0.025 COS
+ 0.05 COS + 0.15COS
+
0.25 COS 0.05 COS alone 0.25 COS alone
t t
