An independent study (11) showed that converting a single-absorption acid plant to a feed-modulation plant would cost $1.95 million, whereas the addition of tail-gas scrubbing to the former using the Bureau of Mines citrate process would cost $0.88 million, while using the Wellman-Lord process would cost $0.86 million. Translating these figures to differences in operating costs, it was found that additional labor requirements, utilities, and waste disposal make scrubbing considerably more expensive than feed modulation: the Wellman-Lord process costs $960/day more while the Bureau of Mines process costs $1270/day more than feed modulation (11).Thus, the payout time for the extra capital cost of feed modulation over scrubbing is only -3 yr. In summary, feed modulation is less expensive than double absorption and more expensive than tail-gas scrubbing from a capital standpoint; however, in terms of total operating cost, feed modulation appears to be substantially less costly than any of the other SO2 emission control alternatives. Discussion Bench-scale simulation of a sulfur-burning acid plant shows that conversions very close to that required by U.S. federal regulations can be achieved by going to parallel final beds and cyclically blowing these beds with air. Experimental results using a simulated smelter stack gas feed, however, suggest that feed modulation will need further development before emission limits can be met. Provided regulations can be met, it appears that feed modulation is the most economical means of achieving the SO2 discharge standards for sulfur-burning plants. A number of important developmental problems remain to be solved for feed modulation. Figure 3 suggests that temperature changes of -30 “C over a 10-min period would be encountered in the final parallel beds for each cycle. The seriousness of this problem was discussed by correspondence with catalyst manufacturers. About half of the respondents pointed out that vanadia catalysts are exposed to severe thermal shock in start-up and shutdown, and they did not foresee any pronounced loss of catalyst strength or disintegration with continuous 30 “C fluctuations. Other respondents thought the problem needed study. Our long cycling runs used powdered catalysts, so the problem was not studied. A further problem is the design of the 6-ft butterfly valves required at 1000 STPD for redirecting flows. It is estimated that these valves would have to operate a t temperatures of -400 “C and remain trouble-free for periods up to 1yr. The feed-modulation plant shown in Figure 4 involves splitting the sulfur burner. Operators cite sulfur burner malfunctions most frequently as the source of plant shutdowns, so the splitting of this furnace could conceivably double shutdown frequency.
Acknowledgment Experimental data were obtained by Dr. J. P. Briggs and Dr. D. M. Kane. Catalyst was provided by Cyanamid of Canada, Ltd. The collaboration of Dr. John McIrvine of Canadian Industries Ltd. and of Dr. William Peterson, National Research Council, was greatly appreciated. Notation SO2/02 = molar ratio of SO2 to 0 2 in feed gas to the preconverter ( S O Z ) =~ SO2 concentration in the SOs-free final-stage effluent during continuous cycling (ppm) (SO& = SO2 concentration in the SOs-free final-stage effluent of bed A or B at steady state without dilution by the air blow from bed B or A, respectively (ppm) ts = time after switch at which a sample of effluent is injected into the GC column (min) tl/2 = half the period of a complete cycle (min) T F = temperature (“C) of the final stage (“C) Literature Cited (1) Rai, C., Siegal, R. D., Eds. AIChE Symp. Ser. 1975, No. 148,
71.
(2) Duprey, R. L. Chem. Eng. Prog. 1972,68,70. (3) Cameron, G. M.; Nolan, P. D.; Shaw, K. R. Chem. Eng. Bog. 1978, 74.47. (4) Briggs, J. P.; Hudgins, R. R.; Silveston, P. L. “Feed Modulation to Reduce SO2 Emissions from Sulfuric Acid Plants”, submission
to the Canadian Patent Office, Feb 1977. (5) Unni, M. P.; Hudgins, R. R.; Silveston, P. L. Can. J. Chem. Eng. 1973,51,623. ( 6 ) Silveston, P. L.; Hudgins, R. R. U S . Patent 3856927, Dec 1974. (7) Briggs, J. P.; Hudgins, R. R.; Silveston, P. L. Chem. Eng. Sci. 1977, 32,1087. ( 8 ) Briggs, J. P.; Hudgins, R. R.; Silveston, P. L. J . Chromatogr. Sci. 1976,14,335. (9) Stull, D. R., Prophet, H. Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.) 1971,37. (10) Fulford, B.; Katz, H.; Philip, D.; Thompson, D. “Design Study for a Sulfuric Acid Plant Based on the Air Cycling Principle”; written for the Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, May 1977;report available through P. L. Silveston. (11) Hamilton, D. W.; Johns, S. E.; Thomerson, J. F. R.; Urquhart, R. M. “Economic Analysis of Sulfur Dioxide Removal from Sulfuric Acid Plant Tail Gases by Citrate, Wellman-Lord and Modulated Bed Designs”, Design Project Report, Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, March 1977;report available through P. L. Silveston. Received for review February 26,1979. Accepted August 11,1980, We gratefully acknowledge financial assistance of the National Research Council of Canada in the form of a PRAI grant.
Correction 1980, Volume 14
Katherine Alben: Coal Tar Coatings of Storage Tanks. A Source of Contamination of the Potable Water Supply. Page 468. The statement “in New York State, coal tar is commonly applied to steel storage tanks and to cement-lined ductile iron pipes used in water distribution (2)” should be amended to “in New York State, coal tar is commonly applied to steel storage tanks used in water distribution ( 2 ) ” .The Ductile Iron Pipe Research Association has informed me that coal tar has not been commonly applied to ductile iron pipes used in water distribution.
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