Gamble of the Scripps Institution of Oceanography who provided the box-core materials. Literature Cited
Chow, T. J., Bruland, K. W., Bertine, K . K., Soutar, A., Koide, M. Goldberg, E. D., “Lead Pollution: Records in Southern California Coastal Sediments,” Science, 181,551-2 (1973). Emery, K . O., “The Sea off Southern California.” 366 pp, Wiley, 1960. Goldberg, E. D., “Atmospheric dust, the sedimentary cycle and man. Comments on earth sciences,” Geophys. 1,117-32 (1971). John, W., Kaifer, R., Rahn, R., Wesolowski, J . J., “Trace element concentrations in aerosols from the San Francisco Bay Area,” Atmos. Enuiron., 7, 107-18 (1973). Koide, M., Bruland, K. W.,Goldberg, E. D., “Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments,” Geochim Cosmochim. Acta, 37,1171-87 (1973). Lazrus, A. L., Lorange, E., Lodge, J. P., Jr., “Lead and other
metal ions in United States Dreciuitation.” Enuiron. Sci. Technol., 4 , 5 5 4 (1970). Peirson. D. H.. Cawse. P. A,. Salmon, L., Cambray, R. S., “Trace elements in the atmosphere environment,” Nature, 241, 252-6 (1973). Presley, B. J., Kolodny. Y., Nissenbaum, A , , Kaplan, I. R., “Early diagenesis in a reducing fjord, Saanich Inlet, British Columbia-1. Trace element distribution in interstitial water and sediment,” Geochim. Cosmochim. Acta, 36, 1073-90 (1972). SCCWRP, “The Ecology of the Southern California Bight: Implications for Water Quality Management,” Southern California Coastal Water Research Project, 1973. Winchester. J . W.. Nifone. Gordon D.. “Water Dollution in Lake Michigan by trace elekents from pollution ‘aerosol fallout,” Water, Air, SoilPollut , 1,50-64 (1971). 1
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Received August 27, 1973. Accepted November 18, 1973. Work supported by a grant from the U.S Atomic Energy Commission, DiGision of BiologS)and Medicine, AT104-3)-34,Project 84.
Reactivity of SO, with Supported Metal Oxide-Alumina Sorbents Roger F. Vogel, Bruce R . Mitchell, and Franklin E. Massoth Gulf Research & Development Co., P.O. Drawer 2038, Pittsburgh, Pa. 15230
A number of impregnated metal oxides supported on alumina was evaluated for removal of SO2 a t 343°C from a mixture containing 02, N2, H20, and Son. A fixed-bed, flow reactor was used with the effluent monitored for SO2 removal by infrared analysis. A kinetic treatment of the system was developed and rates of SO2 sorption were calculated. A 5% SO2 breakthrough point was chosen for practical rating of the various sorbents. The relative capacities of the sorbents were in general agreement with those of bulk oxides in that the alkali and alkaline earth metals were superior in reactivity. Compared to the Bureau of Mines coprecipitated, alkalized alumina, the Naimpregnated alumina sorbent had a much higher rate for SO2 sorption. Supported Cu and Sr proved to be especially reactive. The presence of water vapor affected the capacity of several sorbents.
Recent legislation against air pollution has forced many industries critically to review the quality of their stack gas emissions. Major sources of sulfur dioxide emissions are from power plants and sulfuric acid plants. The petroleum industry, although a relatively minor contributor, is not without its own sources of SO2 pollution, of which the most prevalent are Claus sulfur units and fluid cracking regenerators. One of the methods for SO2 removal from stack gases receiving much attention is sorption or reaction with solid sorbents, notably oxides. A successful sorbent must have a high capacity and rapid removal rate for SOz in low concentrations, as well as being regenerable. Theoretical examination of potential candidate materials for reaction with SO2 has centered mainly on oxides capable of forming stable sulfites or sulfates. On the basis of SO2 reactivity alone, the oxides of the alkali metals are best, followed by Ag, Hg, and then the transition metals (Welty, 1971). A comprehensive theoretical study of sulfite-sulfate formation and decomposition has suggested 432
Environmental Science & Technology
that the most promising oxides are: Al, Bi, Ce, Co, Cr, Cu, Fe, Ni, Sn, Ti, V, U, Zn, and Zr (Lowell et al., 1971). These studies have not considered mixed oxides, or supported oxides, both of which could increase the choice of potential candidate materials. Although these evaluations can eliminate some candidates from consideration, reaction rates of promising ones cannot be predicted. The Bureau of Mines was the first to demonstrate the potential of solid oxide for SO2 removal. Extensive work was done on a n alkalized alumina sorbent (Bienstock e t al., 1964). Other studies included a wide range of unsupported solid oxides (Bienstock e t al., 1961). Besides the alkalized alumina, bulk oxides of Mn, Co, and Cu were found to be active. Dispersing active metals and metal oxides on high area supports to considerably increase their available surface area over the bulk forms has been long recognized in the application of catalysts in the oil industry. A properly supported active sorbent for SO2 removal by either simple adsorption or chemical reaction should exhibit a n enhanced reactivity over the bulk material, although total capacity per volume of sorbent plus support may or may not improve depending upon active solid utilization. Recently, processes employing CuO supported on alumina have been reported (McCrea e t al., 1970) (Oil and Gas Journal, 1971). In the present paper we report on a screening program for SOz removal from a simulated stack gas employing a wide range of metal oxides supported on alumina. Experimental
Potential sorbents were prepared by impregnating Filtrol Grade 86 alumina with metal nitrates. An alkalized alumina (BM), supplied by courtesy of the U.S. Bureau of Mines, was chosen as the standard for a comparison of sorbent capacities. The BM standard contained 31% Na and 2% K by chemical analysis. All experimental sorbents contained about 30% of the metal equivalents found in the standard. This enabled high atomic weight elements to be evaluated on the same basis of theoretical capacity since
reasonable metal levels could be prepared. All sorbents were calcined in air a t 538°C for 10 hr prior to use and sieved to 10-20 mesh. Bulk densities ranged from 0.60 g/cc for the alumina to 0.93 g/cc for the 41% P b sorbent. Most were about 0.7 g/cc. The bulk voidage, E, was required for kinetic measurements, and a value of 0.38 cc/cc sorbent was assumed (Haughey and Beveridge, 1966). Two gas blends were examined: 4.7% S 0 2 , 6% 02, and 89.3% N2; and 4.7% S02, 6% 02, 2% H20, and 87.3% N2. This level of SO2 was chosen t o provide sufficient accuracy for the kinetic data analyses. Although the SO2 concentration is higher than normally encountered in flue gas applications, Russell et al. (1970) concluded that reaction rates are first order in S02. The effects of C 0 2 and NOx, components in normal flue gases, were not determined because of interference with the ir analysis. Four identical Pyrex glass reactors, 35 cm long by 3.2 cm i.d., were employed to facilitate screening of sorbents. Uniform heating was achieved by tube furnaces containing multiple heating elements manually controlled by variacs. Thermowells provided means of measuring sorbent bed temperatures. Gas flow, measured with standard flow rotameters, was downflow through the reactors. Reactors were charged with 25 grams of sorbent, the excess volume being filled with quartz. Sorbents were pretreated with hydrogen overnight a t 482°C to remove any residual nitrates remaining from the impregnation step. This was necessary because nitrogen oxides were liberated upon treatment with S02, interfering with the ir analysis. Reactors were maintained a t 343°C during sorption. Flow rates were 600 cc/min. A Perkin-Elmer model 137B infracord spectrophotometer, equipped with attached strip chart recorder, was used to monitor SO2 concentrations. The total effluent gas flow from the reactor was passed through a 4-cm long Pyrex ir cell containing sodium chloride windows. Water vapor was removed from the effluent gas by a tube containing Drierite placed immediately before the gas cell. The instrument was maintained a t the wavelength of maximum absorption for SO2 which was 7.3 p . The lower limit of detection was about 0.05% S02. Sorbents were tested until a t least 50% breakthrough of SO2 was observed in the exit gas. The time required to reach the 5% and 50% breakthrough points was used to calculate efficiency and capacity of sorbents.
Treatment of Data Two important parameters of sorbents for removing an active component from a gas stream are the loading capacity and the sorption (reaction) rate. For the alkalized alumina sorbent, Russell et al. (1970) found t h a t the sorption rate of SO2 could be fairly well described by the concentration of the reacting SO2 and the fraction of sorbent remaining unreacted, viz. d m = k,CE (1
-):
k,C,Et
where Co = reactant concentration a t reactor inlet, mol/cc Ce = reactant concentration a t reaction outlet, mol/cc L = sorbent bed length, cm u = interstitial gas velocity through bed, cm/min Under practical experimental conditions, exp ( k r L / u ) >> 1, and C,E/m,
