July, 1927
IlVDCSTRIAL A S D ElVGIh’EERI,VG CHEMISTRY
789
Nitric Acid from Ammonia’ By Charles L. Parsons XIILLS BEILDIXG,WASHINGTON, D. C.
I
1- MASY industrial chemical applicatioris h e r i c a n
struction, pipes, valves, storage tanks, etc , which are almost a s
chemists must admit that Ive have fol1oTTf.d European 1mPervlOus to nltric acid as they are to water. ( c ) Experience in operation and construction so t h a t effipractice rather than being ourselm respollsible for ciencles of 96 per cent In oxidation, 99 per cent (3 per cent in their initiation and success. Sometimes we hare improT-ed nitrite tower) for absorption, and 97 per cent in concentration methods and cheapened production. To an extent this are obtained in practice, and even guarantees of operation within true \\-ith sulfuric acid. with most exnlosives. n-ith 2 per cent of these efficiencies are obtainable from engineering firms. alkalies, with dyes and pharmaceuticals. It is also true contracting ( d ) Operation a t higher temperatures, 1023’ C. (900” C. Of synthetic amnlonia and its oxidation to nitric acid. Today, observed), nearly doubling the capacity of a 1918 installation and in Europe, most of the nitric acid consumed is made by the rendering the burning of B-grade, or sulfur-free, ammonia as oxidation of arnmonia. I n America, although four plants easy and as efficientas A-grade aqua or synthetic anhydrous, with cost differential of a t least one cent a pound of “8. have been built and hare functioned successfully, only one a favorable (e) The designing of preheaters, either for air alone or mixed is now operating. Two war plants at Muscle Shoals lie air and ammonia, which makes this higher temperaturepossible idle, while the third small installation, profitable even on and when the mixture is preheated renders a separate ammonia still and gas-holder unneces12-cent ammonia, was desary. stroyed when the T. N. T. (f) The easy transportaplant a t Split Rock, N.Y., Nitric acid produced by the oxidation of ammonia tion of any strength of nitric w e n t u p i n smoke. The acid in drums or tank cars. can now be made in any concentration, including fixed success of the one modern (g) The c h e a p e n i n g of charges on plant, for at least $30 per ton less than it nitric acid concentration and 20-ton plant now operating can be made from Chilean nitrate even figured on the the easy and almost auto.leads the writer to prophesy basis of 2 cents a pound. Nitrogen in the form of matic recovery and reconcenthat within five years no ammonia is now obtainable for less than half the price tration of the sulfuric acid plant in America can proused in the process. of nitrogen in Chilean nitrate. This, together with duce nitric acid from Chilean ( h ) The development of other important developments in the last ten years, automatic controls and laborn i t r a t e and live. Today points to early obsolescence of plants producing nitric saving devices which have ammonia is available a t oneacid by the old processes, in America as well as in made the process almost foolhalf the price of ten years Europe. proof and reduced labor cost ago. Under the continual t o a minimum. cheapening of production These and other factors and the severe competition existing, i t is as likely to go lower to large consumers have vastly changed conditions in the last few years and as is Chilean nitrate, even considering the low prices warrant the prediction of virtual obsolescence for the older which both of these commodities now carry to this class of process the world over. trade. Plant Important improvements have been made in procedure, An ammonia oxidation plant for the production of conand economic conditions also have vastly changed to favor (1) ammonia storage and centrated nitric acid consists of: the production of nitric acid from ammonia, since the writer first published his conclusions on the subject in 1919.2 The supply; (2) oxidation unit; (3) absorption system, with details of processes, the rarious forms of converters used, or without nitrite tower; (4) nitric acid concentration with and a historical development of procedure are quite fully sulfuric acid recovery; and (5) nitric acid storage. d m i o m i STORAGE AKD SuPPLY-Steel storage tanks of treated in the first article. The circular type converter there described has been adopted, after careful study of sufficient capacity for a t least one and a half tank cars of other forms, for the one American plant operating for nitric ammonia, firmly placed on concrete foundations and prefacid, and contracts have been placed for it in three European erably of sufficient height for the ammonia to flow by nitric acid plants. It is already functioning with high gravity to the pump or vaporizing apparatus, are necessary. efficiency in several small plants for furnishing nitric oxide As the tanks for anhydrous ammonia have to be made of to sulfuric acid chamber plants and is being built into some very heavy metal, with special care of construction to resist fifteen similar installations now being erected. The changes the pressure involved, their cost is greater than for aqua which have taken place since 1919, when the cost of concen- storage. I n either case vaporizing apparatus and heat for trated nitric acid from ammonia was about equal to that same have to be furnished. The amount of heat is small from Chilean nitrate, have been so important that they will and is readily obtained from the waste heat of the oxidation soon force the scrapping of all old plants and the building reaction. With anhydrous ammonia, after vaporization of new. The early plants will have been amortized from a special reducing valve is inserted in the line, and the pure savings before the results of the new competition are severe. ammonia is fed to the air line either before or after preheating The changes rrhich have led to this condition n a y be sum- as desired. Its flow and the desired composition of the ammonia-air mixture are automatically adjusted. With marized: aqua ammonia two procedures are used, depending upon ( a ) The building of several synthetic ammonia plants both in America and Europe, with others in prospect, and the re- the decision as to preheating the air alone or preheating the mixed gases before entering the converter. I n the first duction of ammonia as a raw material from 12 to 6 cents a pound of NHI. case the ammonia must be vaporized by steam independ( b ) The development of chrome-steel alloys for tower con- ently in a small ammonia still and the gas so produced carried to a gas holder, from which it flows to the air line 1 Received June 6, 1927. 2 THISJOURNAL, 11, 541 (1919). after leaving the preheater. The only disadvantage is the
790
INDUSTRIAL A X D EXGINEERIXG CHEMISTRY
larger cost of the still and gas holder compared TTith that of a stripping column. I n the second case the aqua ammonia is elevated by a small pump to a constant-level tank, from which it flows into the stripping column as shown in Figure 5. Its flow is regulated and evenly controlled through a specially designed flowmeter. The aqua ammonia flows down the stripping column over the tower packing and comes in contact with the ascending air from the blower. A steam coil at the base of the tower keeps the waste water slightly below the boiling point, and under these conditions the air strips the ammonia gas from the liquid and the mixture passes to the filter to remove any particles of iron rust or dust which may by chance be found in the gas stream. The water slowly and automatically drips from a trap a t the bottom of the column, the loss of ammonia again being negligible. A stripping column has a distinct advantage in removing any dust which may enter from the factory as well as any gases like air, Figure 1-Converter sulfur dioxide or hydrogen - sulfide which are, to an extent, present around some manufacturing plants. OXIDATION Umr-The oxidation unit consists of a converter (Figure 1) and a heat exchanger, shown diagrammatically but not in detail in Figure 5 . The circular converter (Figure 1) has an advantage over flat gauze types in that it is especially adapted to large units. The heat of oxidation, too, soon brings the fire-brick walls to a red heat, BThich in turn radiate heat to the converter and keep the platinum gauze, even without preheating, at a temperature which is impossible with flat gauze as the heat is not immediately carried away. The circular form of burner also requires that only two selvage edges of platinum remain inactive for holding the gauze itself and the lower silica plate in position; while with flat gauzes all four edges have an inactive selvage about 1 cm. in width t o hold them in the frame. The largest circular converters in operation have a capacity for 5 tons of 100 per cent nitric acid per day. This is a convenient unit for large-size plant operation, and although it can readily be increased in capacity by additional layers .of platinum or increased size, it is the writer's opinion that nothing is to be gained thereby. Smaller converters are easily constructed and function equally well. Many small converters of this type are in use in sulfuric acid chamber plants and, as with the large-size units, an efficiency as high as 97 per cent is often obtained in daily runs. The circular converter should operate at 900" C., as observed through the Pyres-glass peephole with an optical pyrometer. The correction is thought to be approximately 125" f,so that the actual temperature of the gas a t best operation is 1025' C. The composition of the ammonia-air stream when it enters the converter should approximate 9.5 per cent KH3 by volume and the temperature of the mixed gases before entering the converter is usually around 300" C. At this higher temperature, as shown by long operation, the platinum has much greater capacity for oxidizing ammonia than a t lower temperatures, is much less affected by catalytic poisons, and greater efficiency is consequently obtained. In America gauze of 80-mesh and 0.003-inch wire (0.076mm. wire and 1020 meshes to the square centimeter) is quite generally used. This is a more rugged type than is used in Europe, where either 0.06-mm. wire, with 1020 meshes t o the sauare centimeter. or a 0.04-mm. wire, with 3600
T'ol. 19, No. 7
meshes to the square centimeter, is ordinary practice. At the higher temperatures used in America the more rugged wire appears to stand up better and gire better service, although more platinum is required. The type of apparatus used has no bearing upon the amount of ammonia that a certain area of platinum gauze mill oxidize. ,4t a definite temperature the same surface of platinum will oxidize the same amount of ammonia in any fixed time. With the higher temperatures used in the circular converter, by which higher capacity and higher efficiency are obtained, the more rugged gauze appears to be best. This means that for the same surface approximately 25 per cent more platinum is desirable. As there is but little loss of platinum, however, which makes the platinum simply a capital charge, and as the total cost of platinum, including interest and repairs, for a 5-ton plant seldom exceeds $250 to $350 a year, the cost of this extra w e i g h t of platinum is negligible, as the difference is made up in length of life and replacement costs. The p l a t i n u m g a u z e s h o u l d be pure platinum, free from iron and all other impurities, except that it may contain without h a r m f u l effect not over 1 per cent of iridium. While with the circular converter temperat u r e s a r o u n d 800" C. may be obtained without preheating, the preheater is necessary to obtain the high temperature desirable for high efficiency and capacity. The preheater may be the ordinary h e a t -exc h a n g er form with non-corrosive special steel pipes where air alone is to be heated, the ammonia being mixed with the air after it passes the preheater. As a l l metals except alumiiium a t elevated temperatures cause d e s t r u c t i o n o f ammonia in the presence of air, aluminum piping is Figure 2-Absorption Tower used in preheaters where the mixture is heated. These preheaters are easily constructed and function perfectly. They are built so that the incoming mixture can be supplied at any temperature desired without the slightest pre-decomposition of the ammonia or danger of melting the aluminum pipe, The hot oxidized gases leaving the converter and the preheater may be passed to ordinary steel boilers, if desired, for the production of steam, and unless the gases leave the boiler below 175-200" C., no danger of corrosion exists. A noteworthy amount of steam can be so produced for evaporation purposes, but the temperature of the gases is not sufficiently high to give steam of much value for power purposes around the plant. From the oxidation unit the gases pass to the absorption system. ABSORPTIONSYsTm-The hot gases are first cooled i n special chrome-steel tubes in a gas cooler, shown in Figure 5. From the gas cooler the gases pass to a system of oxidation
INDUSTRIAL A N D ENGINEERING CHE-14ISl'R Y
July, 1927
and absorption towers, one of the absorption towers being shown in Figure 2 . The number and size of the towers will vary with local conditions and the capacity of the plant, and there is no part of the operation that requires more skilful design. The towers are usually operated under atmospheric pressure. Experimental work has been conducted both in America and in Europe on high-pressure operation which would decrease the size of abqorption
79 1
of the nitrogen from the nitrogen peroxide again to nitric oxide, the first reaction, which, although slow, gives off considerable heat, is repeated throughout the tower system, the amount of nitrous oxides gradually growing less until, with a properly constructed and cooled series of towers, the gases escaping from the final nitrite tolyer should not contain more than 2 to 2.5 mg. of HN03 per liter. This indicates a high absorption efficiency. Both of the reactions given above are assisted by cooling, so that the cooling of both gases and liquids in the absorption system is of the utmost importance. The first is a thirddegree reaction, and although it apparently takes place on the walls of the containing ~ e s s e lbeing , ~ speeded up or catalyzed by a number of substances such as charcoal and silicagel, it also has a negative coefficient of velocity, the rate increasing with falling t e m p e r a t ~ r e . ~It follows that the cooler all parts of the system can be kept, the better, and that reaction (l), as well as reaction (2), is assisted by the tower packing. The end tower of the system is an ordinary steel tower, through the packing of which a solution of sodium carbonate flows, taking up some 2 to 3 per cent of the total gases which have escaped absorption as nitric acid in the water and dilute nitric acid flowing through the packing of the previous towers. The sodium nitrite-nitrate mixture so obtained may be crystallized to obtain commercial sodium nitrite, or it may be treated with nitric acid to form pure sodium nitrate the nitric oxide gases from the sodium nitrite being returned to the tower system. The absorption towers are filled with 3-inch (7.5-em.) spiral tower packing rings, the Patrick and Latshaw, Trans A m Inst Chem Eng , 15, 221 (1923). Foerster and Blich, 2 Elektrochem , 22, 2017 (19101, Bodenstein, I h i d , 34, 183 (1918), Toniolo, Gzorn. chim and. appircata, 9, 9 (19271, Ckzmte--zndustrze, 17, 547 (1927), Chem &.Met Eng (in part), 34, 92 (1927) 4 5
Figure 3-Nitric
Acid Concentrator
tower-, h i t v hich ha. the distinct disadvantage of decidedly decrea.ing the efficiency of ammonia oxidat on and introducing difficulties and possibly dangers in the absorption unit itself. Important deyelopnients in absorption 1owers have recently been made in that chrome steel (17 per cent Cr) 1s used in their coristruction. This special steel has been shown by laboratory experiments and by experience to be as impervious to nitric acid as it is to water. It is also unaffected by atmospheric conditions, so that the towers are best left open to the weather. This not only saves building construction, but is of decided importance in the absorption process both in summer and winter owing to ihe 'act that the heat within the towers is greater than the heat without. I s the cooling of the gases and the absorbing liquid is the most important condition for the reaction, the larger heat losses to the outside air render them far more efficient than the granite, brick, stoneware, or brick-lined towers heretofore used.
The reactions taking pIace within the absorption towers have been intmsively studied. According to one of the best printed re~earches,~ the reactions of importance are limited to two: 2N0 3x02
+
+ (32 = 2N02 = 2HN03 + NO
H20
(1) (2)
Unfortunately, the first reaction progresses rery slowly, and although the second, that of absorption, is almost instantaneous, it nevertheless comes to equilibrium short of completion. As the second reaction reduces one-third a
Burdick and Freed, J. A m Chem SOL.,43, 518 (1921).
Figure 4-Sulfuric
Acid Concentrator
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I N D U S T R I A L A S D EA-GINEERIXG CHEMISTRY
gas passing in countercurrent with the supplied water which is added a t the end of the system so that the strongest acid will meet the most concentrated gases and the weakest gases be scrubbed with nearly pure water. The acid obtained from the first tower is from 50 to 57 per cent strength and by special construction may be obtained even 3 or 4 per cent stronger, It is a C. P. acid except for the impurities which the water itself contains. The tower efficiency, exclusive of the nitrite tower, should be a t least 96 per cent in best operation, and guarantees of 95 per cent are obtainable. The towers themselves, the piping, the pumps, the valves, etc., are all made from chrome (17 per cent Cr) steel. The weak nitric acid from the absorption system is pumped to the nitric acid concentrator. NITRIC ACID CONCENTRATION-The nitric acid COnCentrator is shown in Figure 3. The tower is supplied with strong sulfuric acid and weak nitric acid, which is mixed in the top of the tower as shown. This mixture of nitric and sulfuric acid, as i t passes down the tower, is treated with steam, which enters a t the bottom and goes in countercurrent with the acid mixture. From the top of the tower the concentrated distilled C. P. nitric acid is drawn off and condensed in suitable condensing equipment, consisting of bleacher, condenser, and cooler. The bleacher produces a water-white acid, any nitrous fumes being returned to the absorption system as shown. From the bottom of the concentrating tower the sulfuric acid, now 66 to 69 per cent HzS04,is drawn off and reconcentrated in the sulfuric acid concentrator (Figure 4). As it is difficult to get complete denitration and quantitative recovery of the nitric acid from the concentrating tower, the diluted sulfuric acid is treated in a special denitrator for complete recovery of the nitric acid. The cooled nitric acid goes to the nitric acid storage, consisting of chrome-steel tanks if nitric acid itself is to be stored, or to steel tanks if mixed sulfuric and nitric acid is desired for use. The plant throughout is equipped with automatic devices, pyrometers, flowmeters, etc., so that very low labor costs enter into production.
Chilean Nitrate
us.
1701.
19, NO. 7
Ammonia as a Raw Material
In the writer’s opinion Chilean nitrate cannot long find place in the world’s markets, even should the Chilean Government abolish entirely the export tax of $10.40 a ton, and they are reported to have served definite notice that there will be no reduction prior to July, 1928. This is true not only as a source of nitric acid but for its much better understood use as a fertilizer. The rapid closing of plants-seventy operating in March, 1926, and twenty-eight in March, 19276and the falling off in production-228,490 metric tons in hfarch, 1926, to 90,479 metric tons in hfarch, 1927‘jpoint toward the end. It may be true that the much advertised new Guggenheim process of caliche extraction “will reduce production costs 50 per cent,” but this does not mean that delivered costs or prices will be reduced likewise, as too many have inferred from a cursory study of the problem. According to a careful survey of conditions,? the cost of production a t the mines is $12.96 per ton of finished nitrate. If 50 per cent of this is saved by change in process, it means a saving of $6.48 only in the cost of laying down nitrate a t American ports. Authorities familiar with the situation and able to judge the future deem a price of $40, f. 0 . b. consuming point, as about the lowest that can yield any profit even from the richest deposits and best managed ojicinas. Under financial stress stocks may temporarily go a few dollars lower, but such stress would also undoubtedly influence the ammonia market. Even today the price of nitrogen in NHB is less than half its price in KaNO,, and at least three more synthetic ammonia plants are in prospect in the United States alone. It is true that Chilean nitrate is a preferred form of fertilizer and that no nitrate is better. This preference is not so great, however, as t o overcome any notable differential in price. Calcium nitrate is becoming a real competitor, is growing in favor, carries lime also to the soil, and is being produced in granular form, by new processes of manufacture, quite acceptable to the consumer. It can be made from 6
7
Department of Commerce Reports. Dept. Commerce, Trade Information Bull. 226, P t . 11,p. 48.
I S D C S T R I d L A S D E.VGI-%ERILVG CHEMISTRY
July. 1927
793
$0 per cent nitric acid, produced by ammonia oxidation, includes overhead and an allowance for niter cake as deter~ estiat a cost far below any possible native nitrate competitor. mined by experience. According to T a y l ~ r ,these Ammonia, on the other hand, can be produced a t a profit mates would appear to be based on the actual experience a t even lower prices than now obtain. The fixation of ni- of America's largest nitric acid producer and show a distinct trogen is still an infant industry, but costs have already saving over like figures published in greater detail the year been cut in half in the last ten years. They are still de- before.l0 They are lower than published European expericreasing. S e w procedure applied to hydrogen production is ence and undoubtedly represent the best American practice. imminent which will have an important bearing not only They are adopted as the basis of the price line marked I1 a n ammonia costs but on other forms of high-pressure syn- in Figure 6. This line mill enable anyone a t once t o deterthesis using hydrogen and carbon monoxide. New forms mine the cost of nitric acid a t any given price for nitrate. of fertilizers made from ammonia are daily gaining favor, The allowance made for niter cake is apparently $3.60 a such as calcium nitrate, the double ammonium nitrate and ton. This may be somewhat low for those who use it as a sulfate, urea, and ammonium phosphate, so that the use of basis for the manufacture of salt cake in their own plants. ammonium sulfate with its high cost of production and its It should not be forgotten, however, that salt cake made useless acid carrier is already beginning to fade from the from sulfuric acid alone can utilize its hydrochloric acid picture, Few realize that even today the cost, of ammonia to neutralize crude ammonia liquor and thus substitute going into sulfate but little exceeds the cost of the sulfuric another carrier for the sulfuric acid that would have been acid necessary t o neutralize it. Therefore sulfuric acid consumed in making ammonium sulfate. Ammonium cannot long be used as a carrier. Still, by-product ammonia chloride can be used as a fertilizer about as well as the sulfate from gas works and coke plants must be sold or become a with all crops where the customary potassium chloride is nuisance. The outlet seems to be neutralization lvith unobjectionable. There can be little doubt that ammonium phosphoric acid or oxidation to nitric acid or nitrate. Both chloride will serve as an outlet for any surplus hydrochloric will be used. The important point from the standpoint acid produced in salt cake manufacture. The figures for of this communication is that it seems likely (a) that nitrogen operation include 15 per cent fixed charges with plant cost in the form of ammonia (SH,) can always be purchased estimated at $12 per ton yearly capacity. for less than 50 per cent of its cost in Chilean nitrate, and ( b ) that there will always be a market differential in favor of ammonia dissolved in water over ammonia in the anhydrous state. Aqua Ammonia
us.
Anhydrous Ammonia as a Raw Material
In the manufacture of nitric acid from ammonia there is little, if any, choice between the use of aqua ammonia and anhydrous ammonia, except the one item of cost both in raw material and in storage installation. Once in storage either can be used and with equal facility. Both require heat for their vaporization, but this may be easily supplied from the large amount of waste heat of oxidation. This waste heat is not yet utilized in practice for other purposes, chiefly because the temperature is sufficiently high to produce s n l y low-pressure steam. Aqua ammonia with the use of a stripping column instead of a still and gas holder has an advantage in cost of installation and the special advantage that the air used for oxidation passing up the column is washed free from dust. Dust in certain localities has proved distinctly troublesome. The cost of A-grade aqua is now on a parity with anhydrous in the markets. There is a differential of from one cent to .a cent and a half a pound of SHa in favor of B-grade (sulfurSree) aqua. This grade of aqua works perfectly for oxidation #purposes and the difference in cost should naturally continue. If it does not, crude liquor can be purchased and, with proper purification, used a t an even grclater saving. A saving of one cent a pound in the price of ammonia means a saving of $6.20 a ton of (100 per cent) nitric acid produced. .Single tank-car storage for ammonia will cost for installation about $5000 more for anhydrous than for aqua. This
