Manufacture of Nitric Acid by the Oxidation of Ammonia’ Guy B. Taylor, Thomas H. Chilton, and Stanley L. Handforth ExPB)LIMBN~AI.
STATION, E. I ,
DU
Poar nld N B M v V R S AND COS~UANU. W I L X ~ N C T O NDEL. ,
YNTHETIC ammonia is rapidly becoming the primary source of nearly all fixed-nitrogen compounds. It is now the prime raw material from which nitric acid is made all over the world. The du I’ont Company has been a large producer of nitric acid from Chilean nitrate for a great many years, and has closely followed all work in the nitrogenfixation field because of its obviously important bearing on the manufacture of nitric acid. Since 1920 i t has appeared certain that nitric acid froni ammonia would supplant that from Chilean nitrate in America, so attention was directed to developing a process for ammonia oxidation.
S
As the products are cooled, the nitric oxide begins to react with exceSs oxygen to form the peroxide. 2NO
+ OS = 2NOs
(2)
Part of the peroxides polymerizes, 2NOz
Ns06
(3)
I n atmospBeric-pressure operation the gases then enter a series of packed towers through which acid is circulated so rapidly that tlre strength is substantially constant from top to bottom in each tower but becomes weaker in each succeeding tower. The reaction (4) taking place t.o form acid in all the towers is 3N03
+ HA1 =t 2HNOa + NO
(4)
l‘lie nitric oxide prodiiced by &action 4 repeats Reaction 2 with tile excess oxygen preseiit in the packed tower. Reaction 2 is irreversible at temperatures encouirtered in absorption practice, hit is a termoleeular reaction with a negative temperature coefficient (a, 3). Kcactions 3 and 4 involve equilibria ( I , 4, 7 , 8 ) favored by low temperature. Calculations of plant pcrforniances are possible from fomrrlae involving the rn,te constant of Renct.ion 2 and the equilibrium
Fieure I-General
View Low-Ressure Pilot Plant
From tlieoretical calculations the advantages of even a moderate increase in operating pressure were early established. The eornrnercial availability of a ductile metal not attacked by nitric acid made pressure operation feasible. Following lahoratory and semi-works investigations, a pilot plant working under 100 pounds pressure was erected which produced 5 tons of nitric acid per day at 60 per cent strength with ai1 absorption system one-twentieth the volume required for an atmospheric-pressure plant for tire same capacity at only 50 per cent strength. Since that time commercial plants producing in excess of 100 tons per day have heen erected and operated satisfactorily. Tile advantages of pressure operatioil lie largely in the absorption step, where nitrogen oxides are converted to acid by reaction with water. Before describing the development of the process and the plank, a brief outline of the chemistry of the reactions (8) involved will be given in order to make clear why a moderate increase in pressure so greatly reduces plant size. Reactions Involved
The primary product of annnonia oxidation is nitric oxide; obtained by passing a mixture of air and ammonia through a red-hot catalytic wiregauze. 4N& 501 = 4NO 6Hz0 (1)
+
+
~
I Received June 8, 1931. Contribution No. 64 from t h e Experimeirtri Station and from the Eastern Laboratoiy of E. 1. du Pont de Nemoure and CO.
F l u r e 2-Inferlor
View Low-Reaaure Pilot Plant
coiistants of Reactions 3 and 4. The details of the method cannot he given here, but calcolations have been checked, with satisfactory results, against h o r n performance of many absorption systems in this country and abroad, for both arc and ammonia oxidation processes, with widely
August, ICJ31
INDUSTRIAL AhTUENGINEEEING CYiEhIISlXY
861
processes, the former will he called the low-pressure and the latter the pressure process. I n both processes air is preheated by interchange, mixed with ammonia gas, and the mixture is sent through the converter. The couverter products are cooled in a bank of water-cooled tubes, where weak acid is condensed, and are then scrubbed with water to absorb the residual nitrogen oxides. Low-Pressure Plant
'L'lic original low-pressure pilot plant at the Repauno Work8 a t Gibbstown, N. J., corisisted of one converter deliveriirg ioto an absorption system constructed of chemicalwarc and brick towers. Figure 1 slioivs a general view of this plant after a elirome-iron absorption tower had been added to it. Tliis tower was the first successful riveted tank o( tliis material made in America. Fib"lre 2 is a view of the experimental converter. This pilot plant served mainly to demonstrate tlie practicability of chrome iron for handling nitric acid. Its superiority over the old brick and chemicalware was so marked that when a full-scale plant was built, the entire acid section of the plant was constructed of this metal. Figure 3-Genernl View Commercial Low-Pressure Views of the low-pressure plant with a designed capacity Piant of 25 tons of nitric acid per day are shown in Fixures 3 varying gas compositions. A few examples of the applica- aud 4. It was provided with five converters of substantially tion of the method to ammonia oxidation may be quoted. the design (6) worked out at Muscle Shoals plant No. 1. In a case where the time of travel throny~lione tower was 40 Tire catalysts were in the form of cylinders 12 inches in diameter and 14 inclies high. seconds, eight towers in pracFour l a y e r s of p l a t i n u m tice gave 48 per cent strength gauze were used. There is acid with gas 90 per cent oxino particular advant.age in dized and a recovery of 90 per using gauze in cylinder form; cent when operated at 40" C. flat gauze gives equally good The c a l c u l a t e d ririmher of results. towers for these conditions The converters deliver into was seven. The calculated one absorpdion system, passnumber to raise the recoveq ing first to a tubular cooler to 95 percent and the strength and then tlirongh eight small to 50 per cent was eleven, hut tanks in series to allow oxidathe seven were calculated to tion of nitric oxide to the perdo t h e s a m e t h i n g i f run oxide. Intermediate coolers at half capacity. If operated between the tanks remove the at 25' C. nine towers would heat of this reaction. The be needed instead of eleven; gases are then scrubbed if o p e r a t e d a t 0" C., five Figure M o n v e r t e r Houee of Low-Pressure Plant countercurrent in ten large towers instead of eleven, and absorption towers, 10 by 50 a t this temnerature 60 ner cent strength could he made with seven. Ariotlier co~iclu- feet; in the last twosodium oitriteismade. Figure5isaview sion of interest is that 60 per cent strength acid cannot be inside the pump house, showing the motor-driven centrifugal made a t 40" C. with any number of towers and 70 per cent pumps used for circulating acid and allmli through tlie towers. cannot be made at 0" C. A t several atmospheres pressure, packed towers could he used, hut by theoretical considerations it can be shown that under pressure a large number of graded acid strengths are more important to securing high-strength acid than free gas volume-i. e., Reaction 4 is controlling instead of Reaction 2. F'ractically, the operation of circulating pumps for acid under pressure is a decided disadvantage. For these reasons it wm decided to use a bubbler-cap column for an ahsorption system. Pressure operation is advantageous because the time required for the constantly recurring oxidation of nitric oxide varies inversely as the syuare of the pressure and partial pressures of nitrogen peroxide are increased favorahle to producing higher strength acid. ~
Plants and Processes At present two types of commercial processes are operated, the conventional process at atmospheric pressure and a process at IO0 pounds gage. I n sketching the development of these
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Pressure Process
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EXPERIMEXTAL PLAwr-Laboratory experiments on the oxidation of ammonia nnder pressure were carried through to the semi-works scale a t the Experimental Station. Figure 6 is a diagrammatic sketch of the semi-works plant, not drawn to scale, which will also serve as a flow sheet of the pressure process in general. Liquid ammonia drawn from a cylinder was admitted to a steam-jacketed pipe, where it wa? gasified and superheated. The ammonia gas was mixed with air which had been preheated by interchange and filtered and passed to the converter. The conversion products from the heat-interchanger passed through a coil submerged in a tank of running water where acid, 35 to 55 per cent "03, was condensed and trapped into an appropriate plate of the absorption column. The gas then passed through empty cast chrome-iron vessels serving as oxidation space, thence to the column. Pipes and fittings handling the hot ammonia-air mixture were made of nickel. All the acid section of the plant was constructed of chrome iron and gave not the slightest trouble from corrosion. The column, oxidation chamber, and pipe fittings were all castings. Seamless drawn tubing was used for the pipe lines. Valves were made in the company's own shops by machining bodily from forgings. When this plant was built the art of working chrome iron in this country was very little advanced, S o one had succeeded in making a riveted tank from plate. Cast screwed elbows and the like were unsatisfactory and these finally had to be machined from solid bars. At the present time anything may be had in this material that can be made of ordinary steel. The catalyst in this plant was a cylinder of platinum gauze 3 inches in diameter, made of four layers in close contact. The absorption column was 15 inches in diameter, 14 to 16 plates, one 6-inch bubbler cap per plate. Both catalytic conversion a n d t h e a b s o r p t i o n s t e p were thoroughly studied over a range in capacity of 6 to 12 pounds ammonia per hour and a t pressures of 50, 75, and 100 pounds gage. PILOT PLAm-On the basis of experience gained with this experimental plant, a pilot plant with fifteen times its nominal capacity was built a t Repauno. The essential units of both plants were the same. In the pilot plant both the ammonia gas and heated air were filtered. The coolercondenser was the return-bend type with water trickling over a bank of tubes. The column of twenty plates was provided with two cooling sections, one between the fifth and sixth and another between the tenth and eleventh plates. It is necessary in large columns to cool both gas and liquid. T h e c o n d e n s a t e was fed to
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I'VD USTRIAL A N D ENGINEERING CHEMISTRY
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t,lie ninth plate. The oxidascending acid in the columns tion space was built into the w a e cooled by external tububottom of the column a n d lar coolers and pipe coils at one plate was placed below t h r e e points. Each of We it through which auxiliary air e i g h t c o l u m n s shown was was a d m i t t e d t e n d i n g to originally s u p p l i e d by two bleach blie acid. The air converters, as shown in figure 10. T h e c o n v e r t e r s and ammonia flows were controlled autoniatically. shown here were e q u i p p e d Figure 'Ishaws theair filter, with 4-layer gauze cylinders of platinum-rhodium alloy. converter, and water pimps as originally installed. The Recently, experiments were large Banged joint on the bot conducted looking toward increasing the conversion &converter was hard to keep tight and was subsequently ciency under pressure. By a Figure 7-Prcaame Pilot-Plant Converter new design of converter not e l i m i n a t e d by making the c o n v e r t e r a chrome-iron only was the conversion effic a s t i n g . Figure 8 shows a side view of the plant. The cienay incres?ed, but also the capacity per unit weight of catacolumn was made of riveted plate, its dimensions 3% by 35 lyst. Figure 11 shows a view of the new converter (right). feet. Under the shed may be seen the air receiver and heat Itssizemay becoinparedwith tlieolderstyleoii theleft. This exclianger. converter is capable of converting ammonia equivalent to Synthetic anhydrous ammonia was used exclusively in 25 tons of nitric acid per day, a t 96 per cent efficiency, which the pilot plant, pressure being maintained in scale tanks by is five times the capacity of the other converter shown in the pictiire and at, a better chemical efticiencv. In i930 all prmsure plants were equipped wiih this type of converter. Its flexibility is remarkable since i t gives the same high yield at low and liigli rates, say 3 to 25 toils of nitric acid per day. It is clear froni the theory presented earlier in this paper, t.liat low temperature as well as iiicrcased pressure favors both acid strength and capacity. Cost studies have indicated that refrigeration would not pay, but it was always realized that, if the whole of a pressure colunin could be cooled as riearly as possible to the teinperature of t.lic available cooling water, s u b stantial benefits might he e x p e c t e d . A new met.llod of cooling has been developed which removes the heat of reaction on individual plates of tiic ciilurriri inst,ead of a t a few particular ~roints. Several structural advantages were also gaiiicd. The tubular coolers, besides introducFigure 8-Outaide View Pressure Pilot I'lani ine a resistance to m s flow which limited the water jackets. The amnionia was vaporized by steam capacity of the column, introduced many joints throough which jackets on a large pipe just before mixing with heated air, leaks could occur int.0 the cooling watersithout being detected. 'rliis plant made 4 tons of nitric acid per day at 60 per ceiit Figure 12 shows on the right a column remodeled for the strength in hot summer weather and 5 tons at 63 per cent improved cooling arrangement. The improved cooling o r better during the winter mnont\%s. The yield from am- easily doubled its capacity with no increase in size. omnia to nitric acid actually delivered approached 90 per rent. Tiie loss out of the column as determined analytically -...-.. seldom ran as high as 2 per cent, and 1.5 per cent was normal. Alkali scnihbing to recover this m a l l loss would pay only iiiider special circumstances. After this pilot plant served its original purbose, it was rmoved to one of thc company's other plants and is still in operation, with some later improvements, producing 6 tons or more per day. CO.VMRI~CIAL P u N ~ T l i elower investment cost and st,ronger acid resulting froni pressure operation more thaii counterbalanced the increase in power for compressing air and the lower conversion cfficieiicy obtained a t that time. Tliese factors estnl~lislied by the pressure pilot plant led Minitcdy in 19'28 to the adoption of this process for future eqmision. The largest of the pressure plants now in operation is locat,ed a t Hepaiino. It consists of eight units wit.), a nominal total cariacitv of 80 tons of nitric acid uer dav. Fimire 0 . I
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INDUSl'RIAL A N D ENGINBERIND CIIEMISTRY
The result of the development to date is the adoption of an operating unit consisting of a single converter about 2 feet in diameter delivering into a single column 5'/< feet in diameter and 40 feet high capable of producing 25 tons of nitric acid in 24 hours, a capacity equal t o the whole lowpressure plant shown in Figures 3 and 4. This standard unit bas given an over-all soaleweight yield from ammonia to nitric acid of 93 per cent of theory a t an acid strength of 61 per cent, with not more than 2 pcr cent loss in the spill gases. Discussion of Alternative Processes
The original spread in conversion efficiency of the pilot plants was 7 per cent in favor of low pressure when using pure platinum as the catalyst. With platinum-I0 per cent rhodium (5) this spread under optimum conditions was reduced to 4 per cent. The new design converter, besides allowing an increase some fivefold in the pounds of ammonia b u r n e d per ounce of metal per day, gives promise of subs t a n t i a l l y increasing yields nnder pressure. S y n t h e t i c a m m o n i a contains traces of oil and so does compressed air. For good conversions the most S C N ~ U ~ O U S care must be exercised to prevent oil from reaching the catalyst. At any pressure coiiversion increases with rise of catalyst temperature, but under pressure precombustion may occur ahead of the rontact if the mixture is preheated much above350" C. The yield falls off slowly with increase in capacity above the optimum at all pressures. The effect of running a 25 per cent over rating is hardly detectable on a commercial plant. P l a t i n u m s t e a d i l y loses weight while in service. The loss amounts to 5 per cent per month under conditions giving 95 per cent conversion a t atmospheric pressure. With tho
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temperature raised 100' C . where the conversion is 98 per cent this loss may he more than doubled. Yield, therefore, does not tell the whole story in costs. Catalyst replacement must be figured against ammonia costs for the most economical yield. With platinum-rhodium losses are approximately halved. The loss per pound of ammonia burned is not affected by pressure. It seems to be a function of the weight of oxygen passing and the temperature. The adsantages of pressure operation over operation at atmospheric pressure are (I) about half the initial investment in plant, and (2) large increase in acid strength, reducing both operating and investment costs of nitric acid concentration by sulfuric acid. In operation a t atmospheric pressure, over-all plant yields better than 90 per cent of theory from ammonia to nitric acid can be ohtained. Operation a t 100 pounds gage, 7.8 atmospheres, will produce the same or better over-all yield at 8 to 10 per cent liigherktrengtli. The increase in strenCth fmni 50 per cent to 00 &r cent means cutting by nearly one-half t h e c o s t of s u b s e q u e n t concentration to 90 per cent or higher. The only operating item of the pressure process significantly higher than that of the atmospheric process is the cost of power. If in any p a r t i c u l a r l o c a t i o n power rates are high, part of the power of compressing ajr may be economically recovered by use of an expansion engine on the gas exhausted from the columns. At one time in the develop ment when there was a significant difference in conversion between atmospheric and piessnre operation, the alternative of compressing after the cata1,ytic step vas considered. This involved the construction of a chromoiron compressor. Some experiments along this line were conducted bot cost studies indicatod that the expense of developing such a maebine was hardly warranted.
Thc latest typc converter gives such high yields that this course has been amply justified. There is certainly no advantage in compression after conversion.
local conditions determine whether in either case refngeration is justified. Handling Anhydrous Ammonia I n closing this article it is perhaps appropriate to say a few words about handling anhydrous ammonia. Ammonia is the most concentrated form in which nitrogen can be shipped. For several years now i t has been shipped in tank cars holding 50,000 pounds each. There is no difficulty in unloading it int.0 storage tanks without loss or accident. F i y r e 13 shows a row of storage tanks on an acid plant. Each tank is hammer-welded steel and of tJie same size as a tank car. Some of these tanks have been constantly under pressures up to 150 pounds for years. Acknowledgment
Figure 13-Anhydrous
Ammonla Sforaee Tanks
Cooling an absorption system by refrigeration to temperatures around 0” C., while it does materially decrease thc size of the system, is not nearly so effective &s a moderate increase in pressure. Refrigeration as a method of cheapening plant costs is not very effective, because it reduces only the size of the absorptiori system, leaving all other units in the plant the same as without it. Pressure operation reduces nearly all units in size. If refrigeration can he justified for an atmospireric pressure plant as a means of increasing the capacity per cuhic foot of absorption volume, leas refrigeration would do proportionally even more in this direction on a pressure plant. In the case of existing plants
So many pcople have contributed to the development described in this paper that proper acknowledgment to them individually cannot be made here. Without a metal unattacked by nitric acid, a pressure operation would hardly be practicable. This process owes much to the engineen of both the du Pout Company and the steel manufacturers in advancing the art of working chrome iron. Literature Cited (1) liodeustrin, L. Phyrih. Chom., 100, 68 (1922). ( 2 ) Uodlenririn,lhid.. 106, 105 (1922,. (31 14iinei. I’feiRer, ond Malet. I.i h i m . phys., 21. 25 (1924) (4) Burdick and Freed, J. Am. Chem. Sor., IS, 518-30 (1921): Mass. Insf. Technology, Thesis, 191X. ( 5 ) Ilavis, U. S. Patent 1,706,055 (Merch 19, 1929). 10) Perlcv. 1. INO. Euc. C N ~ M12.J-IB. .. 119-120 118201
Modern Methods in the Production of Porcelain Grinding Balls and Lining Blocks’ NE of the most interesting aspects of the engineering field is the almost overnight growth of fine grinding. Materials and products which were formerly acceptable a t 100 t o 130 mesh are now demanded 200-mesh fine; while products which were formerly thought very fine a t 200 mesh are now commonplace a t 300 and there is a steady d e mand for products ground as fine as 350 mesh. Great strides have been made in the development of apparatus for fine grinding in the chemical, ceramic, metallurgical, and mining fields. Pebble, ball, and tube mills occupy a prominent position in these fields because the amount of grinding surface presented by the linings and by the freely moving grinding mcdia is greater than in any other type of fine bender. This accounts for the extremely fine products which are now ground at very low cost with these machines. Mills of these types are usually lined with Silex blocks imported from Belgium or with porcelain blocks most generally made in this country, while French flint pebbles havc been cornnionly used as a grinding mcdium. The Silex linings are reasonahly satisfactory for the grindiiig of rough materials, but where higher priced products are being ground or where products are chdnged frequently and it becomes necessary to clean the mills, porcelain linings arc more de1
Received March 20, 1981.
sirable. Flint pebbles are imperfect, inany being fractured, cracked, chipped, and filled with holes, rendering them unfit for use with many products. The porcelain ball has made its appearance as a more nearly perfect, and consequently a more desirable, grinding medium and is here t o stay. Its use has heretofore bccn greatly FcJtrietod on account of its high cost, as balls have been made entirely by hand. Recently The Patterson Refractories Company, a snbaidiary of The Patterson Foundry & Machine Company, has developed an almost fully automatic method of manufacturing both porcelain lining blocks and porcelain grinding balls. This method and the plant in which i t is operated arc described in this article. A plant which makes a standardized product and turns it out a t a fairly uniform rate, month in and montli out, is susceptible to design of the straight-line production type and t o automatic fabrication and handling of its produets, not possible in plants where many items are produced. The plant under discussion has been developed along these lines. Preliminary Treatment of Materials The principal materials consist of English and American ball clays, china clays, pulverized silica, and pulverized feldspar. The materials are unloaded from railroad cars directly
