INDUSTRIAL A N D EXGIXEERING CHEMISTRY
1024
Operating Costs
I n considering the cost of the treatment system it should be recognized that there are certain costs which are constant, regardless of the amount of water treated, and other costs which vary almost in proportion to the amount treated. The attendance labor, for example, is in the former class, while the salt and acid costs are in the latter. This point is particularly important in the present case because of the wide seasonal variation in load. It should be taken into account in making comparisons between the cost of this and other systems. O p e r a t i n g Costs of Feed-Water T r e a t m e n t Systems (Data taken from March 15. 1928. to and including March 15. 1929) A-Costs which are fixed, regardless of amount of water treated: Labora.. ..................................... $2021.45 Miscellaneous 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120.00 T a b l e 111-Annual
Total fixed costs.. ............................. $2141.45 B-Costs which vary with amount of water treated: Salt, 297.8 tons a t $6.92 pet ton.. . . . . . . . . . . . . . . . $2062.10 Acid, sulfuric 356/8 tons a t $40.00 per ton.. ...... 1425.00 Acid, phosphoiic, 3l/s tons a t 8278.45per ton.. 870.18 Wash water, 95S5 M gal. a t $0.053per M gal.. . . . 508.00
...
Total variable costs..
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S4865.28
Total cost.. .................................... 57006.73 Amount of water treated, gallons.. ....................... 151,661,000 Cost of item A per M gallons.. .......................... $0.0141 Cost of item B per M gallonn.. . . . . . . . . . . . . . . . . . . . . . . . . . . 0,0321 Total cost per M gallons.. .............................. 0.0462 a Does not include time of supervising. chemist nor of plant operators who devote only a small part of their time to operation of the treating -iystem. b Includes laboratory supplies and replacements on acid pumps.
II1 shorvs the Operatirlg costs divided into the two classes noted above. The total cost of one full year of opera-
Vol. 21, No. 11
tion, during which 151,661,000 gallons of water were treated, was 37006.73, or a cost per 1000 gallons of 4.62 cents. For a plant in continuous operation, this cost might be lowered considerably. Conclusions
Experience with the system extending over three seasons of operation indicates that the zeolite method of treatment followed by acid feed is successful and satisfactory for the existing conditions. Although careful and intelligent supervision is required because of the acid feed, this would likewise be the case for any other system designed to meet the rigid requirements of the Beacon Street plant. The operating costs are, no doubt, relatively high; but there is probably no other system of treatment which would guarantee safe operation a t the high steaming rates made possible by this system. The increased steam output obtained from a plant of a given size as a result of this method of feed-water treatment renders the treating cost relatively unimportant. There is evidently room for improvement in the design of zeolite systems, particularly as regards flow control, salt utilization, and corrosion prevention. Literature Cited (1) Am. SOC.Mech. Eng., Rules for Care of Power Boilers.
(2) Parr and Straub, University of Illinois Eng. Expt. Sta., Brrll. 94, 165, and 177, refer to the use of sulfuric acid for the neutralization of the natural sodium bicarbonate feedwater of the power p l m t of the University of Illinois, and in the latter references, to the applica tion of this method of treatment t o power plants in the Chicago district. (3) White, Walker, Partridge, and Collins, J . A m . Works Assocn ,
18, 219 (1927).
A Converter for the Oxidation of Ammonia with Pure Oxygen‘ J. Y. Yee FBRTILIZER A N D FIXED
INVBSTIGXTIOSS, Brr~a.41: of CHEMISTRY A N D SOILS,WASHINGTON, D. C.
~ I I T R O G E N
HE process and converters for the oxidation of ammonia with air to nitric acid hare already been worked out successfully. However, in the production of over 90 per cent acid, required by most large-scale users, a large proportion of the cost is still chargeable to the absorption and concentrations steps, This has been partially overcome by the pressure system of oxidation. According to the information in the patent literature and some preliminary work done in this laboratory, acid of oyer 90 per cent strength can be obtained by the reaction of liquid nitrogen oxides with water under oxygen pressure. To produce such liquid oxides, thc ordinary oxidation process, using ammonia-air mixtures, is not an economical one, kince the concentration of the oxides in the final product is so low. This, however, can easily be overcome by oxidizing the ammonia with pure oxygen. Comparatively little work has been done on the oxidation of ammonia with oxygen, and no commercial plants, SO far as the writer knows, are using such mixtures in their oxidation units. This is undoubtedly due to the high cost of oxygen and the highly explosive property of the gas mixtures, which make commercial operation too costly and hazardous. A few years ago the Bureau of Mines appointed a special committee to make a thorough survey of the existing processes for the manufacture of 99 per cent oxygen. I n their
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Received August 7, 1929.
report (3) they stated that “large oxygen manufacturing plants can be built to serve metallurgical purposes directly, which will be capable of delivering oxygen for the processes a t a cost not to exceed $3.00 per gross ton.” If oxygen could be obtained a t this price, there is no reason why it cannot be used in the oxidation of ammonia economically. Wenger and Urfer ( 6 ) , using platinized asbestos, and -1ndrussov ( I ) , using a small piece of platinum gauze as catalyst, hare shown that high conversion can be obtained in the oxidation of ammonia with pure oxygen. Converters for use with such gas mixtures must be of different design from the ordinary types of ammonia-air mixtures, for they are handling a very explosive gas mixture and they must be able to prevent overheating of the catalyst caused by the highly exothermic reactions. The latter is not only unnecessary in the case of ammonia-air mixtures, but preheating of the inlet gas mixture is required to keep the platinum gauze a t the proper temperature, since so much heat is being carried away by the inert nitrogen. The patent literature contains several methods for overcoming these difficulties. For example, to prevent explosion and overheating of the gauze, steam (5) and a portion of the reaction product (4) are suggested as diluents for the ammonia-oxygen mixtures. Cederberg ( 2 ) accomplishes thc same by placing the platinum gauze between closely spaced cooling surfaces.
ISDUSTRIAL AAVDESGI.YEERIAt'!: CHEMISTRY
Novcniber, 1929
1025
Ammonia arid oxygen from flowmeters enter through inlets 14 and 15, respectively, and are well mixed as they pass through the series of baffles, 16. before entering thc gas chamber, 17. From here the gas mixture goes to the oxidation chamber through a series of holes, 18, which are made so small that the velocity of the gas mixture going through them is greater than that of the explosive waves of the ammonia-oxygen mixture. thus preventing an explosion a t the platinum gauze from traveling back into the gas chamber, 17. To insure further safety, the gas chamber is provided with a safety tube. 19. fitting lightly into the tube holder, 20. Through the center of this safety tube there is a sampling outlet for the ammonia-oxygen mixture. The reaction is started by removing the top of the converter and heating the gauze to red heat with a hydrogen flsme. It is advisable to keep the flow of cooling water so regulated that it reaches and maintains a temperature of about 80" C. The condensation of dilute acid in the apparatus and the attendant corrosion are thereby prevented. After leaving the gauze catalyst, the product of reaction passes through the water-cooled bottom to the outlet, 21. For the purpose of testing the converter, it is connected to a Pyrex condenser, 22 (part of which only is shown in Figure l), where the water of reaction is quickly condensed and removed from the gas stream. The joint is made gas-tight with packing, 23. After passing through a n oxidizing chamber, the gas goes to a condensing coil, where liquid oxide is condensed and stored in a reservoir. The reaction product was analyzed by Perley's method. Samples were taken in an evacuated bulb of known weight attached to the sampling tube, 24; the bulb was then reweighed and the nitrogen oxide titrated with standard dkali.
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Figure 1-Cross-Sectional
View of Converter
Process and Apparatus
The apparatus described in the present paper embodies the experience gained from a number of attempts in this laboratory to oxidize ammonia with pure oxygen. The converter is made of aluminum. This metal is chosen because it is a good conductor of heat, acid-resistant, and a poor ammonia cracker. The converter, a cross-sectional view of which is shown in Figure 1, consists of two main parts-a removable top, 1, which is cooled by a water jacket, 2, and a bottom member, 3, which is alqo cooled by a series of water tubes (Figure 4). The catalyqt gauze, 4, is held in place on the gauze holder, 5, by a retaicing ring, 6. The lower part of the removable top, shown in Figure 3, has a series of annular concentric taper grooves, 7, the sides of which meet in the sharp edges or apices, 8. An enlarged view of a portion of these is shoTm in Figure 2. The gauze holder also has on its upper surface a corresponding series of annular concentric grooves, 9. and apices, 10. The latter come in contact with the platinum gauze for the purpose of conducting the heat of reaction away from it rapidly, thus preventing overheating of the catalyst surface. A bottom view of the gauze holder is shown in Figure 5. There is a free space, 11, around the lower part of the removable top and the upper part of the gauze holder to prevent the loss of heat through conduction to the surface of the converter, which might be sufficiently rapid to result in a lower temperature at the edges of the gauze than a t the center. The top and bottom parts of the converter are held together by bolts, 12, and made gas-tight by means of an asbestos gasket, 13.
Experimental Results
Experiments were carried out with this converter, using single gauze catalysts (80-mesh, 0.003-inch or 0.076-mm. wire) 2.25 inches (5.72 em.) in diameter of platinum and platinum-
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Flg 4 Figure Figure Figure Figure
2-Enlarged View of Grooves 3-Lower Part of Removable T o p 4-Water-cooling Systems 5-Bottom View of Gauze Holder
rhodium alloy (10 per cent Rh). Various space velocities were employed. Oxygen-ammonia volume ratios of 5:4, 6:4, and 7.4 were used. These are the theoretical combining ratios for the formation of nitric oxide, liquid nitrogen trioxide, and liquid tetroxide, respectively. Electrolytic oxygen and synthetic anhydrous ammonia were used in these experiments. I n most cases the runs lasted 5 or 6 hours.
