Jan., 1920
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
5
ORIGINAL PAPERS THE COMMERCIAL OXIDATION OF AMMONIA‘ By George Arthur Perley
apparent attempts have been made t o publish the complete d a t a upon t h e exact composition of the gas mixtures which were oxidized. The ammonia NEWHAMPSHIRE COLLEGE, DURHAM, NEWHAMPSHIRB Received September 4, 1919 oxidation reaction does not proceed t o nitric oxide A vast number of articles have appeared from time a t any appreciable rate in t h e absence of a catalyst t o time during recent years covering various phases a t even 800’ C. As t h e history of contact or catalyst of t h e work involved in t h e production of nitric acid poisons is well known, it seems rather sad t h a t previous b y means of ammonia oxidation. Most articles refer t o publications and experimental research have not est h e historic observations by Kuhlman in 1839 and then tablished their “zero setting” by t h e use of t h e purest trace t h e slow developments which include t h e patent type of ;ammonia available (or a t least have not used of TessiC du Motay in 1871 and t h e work of Ostwald anhydrous for scientific research). The least t h a t in I 900. This interesting historical development could be expected of a publication should be an exhas been so well covered in previous papers t h a t no act statement of t h e gas composition which was emeffort will be made t o cover t h e bibliography of the ployed. As evidence of this state of affairs a case subject. Excellent bibliographies may be found in may be cited with which t h e writer is familiar. An various publications.2 Several recent articles have unfortunate misunderstanding came about through t h e failure t o establish t h e exact quality of gas which appeared on this s ~ b j e c t . ~ was being oxidized. The situation arose over t h e TE RMI N 0 L 0 G Y possible presence of certain phosphorus compounds Many apparently conflicting statements have ap- within the ammonia-gas mixture. Even to-day a peared in some of t h e previous publications, yet some difference of opinion exists in reference t o t h e various of these can be reconciled b y a more careful state- statements which have been made relative t o t h e action ment of conditions and facts. Up t o t h e present time of phosphorus compounds as catalytic poisons in t h e there have been no attempts a t standardization of 9 ammonia. oxidation reaction. This is essentially due methods or nomenclature, and this alone has been t o the fact t h a t no analytical methods were perfected, accountable for a certain number of errors. The or adopted, t o distinguish between phosphorus which catalyst chamber is known as a converter, a tower, was present as phosphine and t h a t which was present a burner, a furnace, an oxidizer, etc., while t h e catalyst as a more stable form. Unfortunately, this is not itself has been variously termed a screen, a net, or a t h e only case where failure t o standardize t h e anagauze. A serious misunderstanding has resulted from lytical procedures has caused unfortunate misundera failure t o express t h e capacity factors in understand- standings. A similar state of affairs has existed in able terms. Much of t h e ammonia oxidation work reference t o t h e standardization of t h e methods of has been carried out with multiple layers of a platinum, sampling, t h e analytical procedures and the catalyst or platinum alloy gauze, and t h e yields or gas flows temperature measurements. have been expressed in terms of cubic feet of the amThe above matter has perhaps been treated in too monia-air mixture per square foot of gauze surface. great detail, b u t it is t o be desired t h a t greater care I n one instance a n author has referred his yields t o in standardization be secured in t h e future and t h u s t h e surface area of t h e first layer of exposed gauze, prevent a repetition of some of t h e past misunderwhile another has considered t h e surfaces of all of standings and inaccuracies. t h e separate layers. It is all folly t o attempt adSTABILITY 0 F INTERMEDIATE C 0 M P 0 U N D s-T he t h e ovancement along these lines of activities unless every one retical foundations involved in the production of will consider capacities in respect t o t h e actual pounds nitric acid by means of t h e oxidation of ammonia are (unit weight) of ammonia catalyzed t o nitric oxide well understood. However, it may not be amiss per unit weight, or ounces of platinum gauze or other briefly t o review t h e fundamental features in this catalyst. If i t is not convenient t o express the velocity, young and important industry. I t is true t h a t Ostor space-time d a t a , for such work, t h e future publica- wald‘ had developed his ideas as t o t h e law of successive tions should at least be quite explicit as t o t h e rela- reactions a t about t h e period when he was working tionship of t h e gas flows in unit time t o t h e total sur- on t h e oxidation of ammonia. We realize t h e imface area of t h e catalyst employed. portance of t h e fact t h a t when there is t h e possible The grade of ammonia which has been utilized in formation of a consecutive series of intermediate comprevious work has varied between wide limits, and no pounds, t h a t compound which involves t h e smallest 1 An abstract of this paper was read before the Division of Physical loss of free energy will be formed first, then t h e one and Inorganic Chemistry, 58th Meeting of the American Chemical Society, involving t h e next smallest loss will come second. Philadelphia, Pa., September 2 to 6 . 1919. * “Literature of the Nitrogen Industries 1912-1916,” Helen R . Hosmer, The proof of t h e existence of t h e intermediate stages THISJOURNAL, 9 (1917). 424; John C . Boyce, Chem. b Met. Eng., 17 (1917). depends upon the stability of those compounds under 228; “ H o w the Nitrogen Problem Has Been Solved,” H. J. M. Creighton, t h e given conditions. In t h e early work of Ostwald, J . Frank. Inst., 187 (1919), 705, 733. * “The Oxidation of Ammonia, ’ W. S. Landis, Chem. & Met. Eng.,20 t h e formation of t h e possible intermediate nitrogen (1919), 470; “Commercial Oxidation of Ammonia to Nitric Acid,” C. I,. oxides was recognized merely as a step t o t h e final Parsons, THISJOURNAL,11 (1919), 541; “Analytical Method for Denitrogen. He realized t h a t t h e function of t h e catalyst termining Efficiency of Ammonia Oxidation.” n. P. Gaillard, Ibzd., 11 (1919), 745.
12. fihysik.
Chem., a2 (1897), 306.
T H E J O C R N A L OF I N D C S T R I A L A X D E N G I N E E R I N G C H E M I S T R Y
6
was towards t h e selective increase in velocity of one reaction. His views as t o t h e desirability of removing the oxygen compounds rapidly from the zone of reaction, through a very short ,contact time, were stated, b u t very badly qualified. The recent work of Lewis and Adamsl on "The Free Energy of Nitrogen Compounds," as well as their related publications on oxygen and water, enables us t o establish t h e quantitative relationships which can thermodypunically exist. They give us the values
+
-
-
NHa 3Fo = --lo700 5.5 T log T 0.001175 TZ 11.0 T 0 2 A F ' = 34600 2.75 T log T - 0.0028 T 2 0.00000031 T3 - 22.4 T NO 4F0 = -21 600 2.50 T NO, 4F0 = 10200 -I- 2.75 T log T 0.0028 T2 0.00000031 T s - 2.2 T Hz0 A F O = -57410 0.94 T log T 0.00165 'I?$ 0.00000037 T3- 3.66 T
+
-
+
-
+
+ +
-
The analogy t o t h e burning of carbon is most interesting. It has been considered t h a t even a t white heat coal burns completely t o carbon dioxide when the air is a t a high velocity. Yet t h e equilibrium indicates t h a t a t all temperatures above 800' C. t h e carbon monoxide should be t h e chief constituent of the gas mixture. There are, however, some differences of opinion relative t o this viewpoint. Oxygen seldom reacts b y a primary decomposition of its molecule into two atoms. The first product which usually forms when a substance burns contains t h e whole oxygen molecule.2 A discussion of this particular phase will be considered separately in connection with some studies upon the reaction velocity-catalyst surf ace effects. As far as Ostwald's work was concerned, it may be said t h a t he was successful in isolating his intermediate compound. Thus were established t h e facts t h a t platinum sponge would favor the velocity of certain of t h e intermediate reactions occurring in t h e ultimate oxidation of ammonia, and t h a t t h e ratios of gas space t o contact surface t o time were very important features. The close relationship of these factors t o the temperature-oxygen variables was not quite clearly stated. The equilibrium conditions existing between n ' 2 O2 p L z N 0 have been carefully investigated by 0 2 N e r n ~ twhile , ~ the dissociation of 2x02 I_ z N O had been well discussed prior t o Ostwald's work by Richardson.' It m-as shown by this last mentioned work t h a t a t 7 6 0 mm. there was complete decomposition of X 0 2 into K O and O2 a t 6 1 9 . 5' C., and t h a t this is a reversible change. The above decomposition of X 0 2 begins a t 140' C. Jellinekj has studied the
+
+
J . A m . Clzern. Soc., 37 (1915), 2308. 2. Eleklvochem., 7 (1901), 446, 447. 3 Z. anot'g. Chem., 49 (1906), 213. 4 J. Chem. SOL.,5 1 (1887), 397. The d a t a OF page 2 i 2 has a bearing 6 2. anorg. Chem.. 49 (1906), 229. on t h e ammonia oxidation work. At atmospheric pressure Time for half of decomposition 1" c. 1' N O ----f '/zNz '/z 0 2 T O 627 123 hrs 900 7.3.5.103 827 10 hrs 1100 5.80.102 1027 44 min 1300 4.43.10' 3 min. 1227 1500 3.30 1427 1700 2.47.10-1 15 sec. 1627 1 ,74,10-2 1900 1 sec. 2100 1827 0 . 0 7 sec. 1.21.10-3 1
2
+
1'01.
12,
S O .I
influence of temperature upon t h e velocity of t h e decomposition of KO,and this data shows t h e quantitative relationships which exist as far as the temperature stability of t h e intermediate products of the ammonia oxidation reaction are concerned. It was shown t h a t N O was formed exceedingly slowly a t temperatures below 1 8 2 7 ' C., yet t h e rate of the decomposition of N O , in the absence of a catalyst, is a t least slow below 1 2 2 7 ' C. It is very apparent t h a t if we disregard any possible catalytic influence of t h e platinum we can commercially work a t ranges up t o 1 2 2 7 ' C. without fear of loss of N O from this reaction. However, this assumption, as applied t o t h e oxidation reaction, is probably not well founded, since the work of Jellinek in platinum tubes between 689' C. and 1387' C. showed a powerful catalytic action of platinum a t this temperature range. The necessity of a study of the influence of catalysts upon this decomposition reaction as applied t o the ammonia oxidation reaction is quite c1ear.l I t was recognized t h a t t h e theoretically possible reactions which might exist when ammonia was oxidized could include: 4NH8 802 = 4HT\TOs 4Hz0 (1) 4NH8 7 0 2 = 4N02 6Hz0 (2) 4"s 5 0 2 = 4NO 6H20 4NH3 3 0 2 = zN2 6Hz0 The possible side reactions include:
+ + + +
+ + + +
i:; i
+
2NO2 ZNO 0 2 (5) zNO N2 0 2 (6) 4NH3 6 N 0 = jNa 6Hz0 (7) J The original passage of a mixture of ammonia and air over platinum sponge had experimentally indicated a strongly exothermic reaction. As the catalyst temperature had a t least been above 6 j o " C., it has seemed wise t o consider Reactions 3, 4 and 7 as those worthy of closest study for t h e present. H E A T CONSERVATION-lve have seen t h a t t h e work of Ostwald resulted in the development of commercial apparatus in which the time of contact of t h e gases with the catalyst was small and thereby prevented excessive formation of nitrogen, and resultantly poor oxidation efficiencies. The ready decomposition of ammonia had also been considered prior t o Ostwald's work by W. Ramsay and S. Young,2 yet this work did not include air mixtures. The use of a heat interchanging equipment was adopted by Ostwald for the ammonia-air mixture, and t h e transfer of a certain quantity of heat was secured from the products of the oxidation reaction. The available illustrations of the type of interchanger which was utilized indicate t h a t the velocity of the ammonia-air mixture was sufficiently low t o give a contact time on the interchange surface which promoted a certain decomposition of t h e ammonia. There is abundance of evidence which shows t h a t there is a serious decomposition of ammonia by almost all bodies except silica a t the temperature of the gases resulting from t h e oxidation reaction; and silica is
Jr
+
+
+
IIelvetica Chcmzca A c t a , 1 (1918), 33 "The Decomposition of Ammonia b y Heat," (18841, 88. 1 2
J
Chcm
S o c , 46
Jan.,
1920
T H E J O U R N A L O F I N D U S T R I A L A N D ELVGINEERIXG C H E M I S T R Y
a very poor heat transfer medium. Kaiser's work' clearly shows t h a t it is easy t o obtain very poor oxidation efficiencies when preheating t h e ammonia-air mixture without serious regard as t o materials of construction or rates of gas flows. At t h e time when t h e Bureau of Mines began a careful study of this whole question in 1916, it was evident t h a t t h e commercial development had essentially followed t h e lines so clearly demonstrated by t h e work of Ostwald. Dr. W. S Landis,2 of t h e American Cyanamid Company, had studied t h e more recent developments in Germany. It is interesting t o note t h a t he states: "The writer is certain t h a t on January I , 1 9 1 5 , with t h e exception of t h e old Ostwald plant, there had been nothing new erected in Germany for t h e oxidation of ammonia." I n 1916 Dr. Landis continued t h e development of t h e electrically heated single gauze type of catalyst, which had previously given fair results in Germany. G. Schuphous3 has published a description of t h e electrically heated type as used in Germany. The catalyst which was utilized by t h e American Cyanamid Company bears a striking resemblance t o this. It may be recalled t h a t Ostwald had suggested t h e use of platinum nets or gauzes. During t h e initial work upon the oxidation of ammonia in t h e United States, during 1916 and 1917, there was much discussion as t o t h e t y p e of catalyst t o adopt. T h e officials of t h e American Cyanamid Company h a d every confidence in t h e utilization of an electrically heated iridium-free platinum gauze as a catalyst. At least fairly satisfactory commercial results could be obtained from such a n installation. However, t o those who were just beginning a study of this problem it seemed most absurd when dealing with such a highly exothermic reaction as t h e oxidation of ammonia t o utilize external heat energy for the maintenance of a proper catalyst temperature. T h e employment of an electrically heated platinum gauze within a high heat conducting aluminum frame or converter casing seemed theoretically all wrong. The existence of the Ostwald4 patent as regards the utilization of a direct heat transfer from t h e hot exit gases t o t h e ammonia-air intake, as well as t h e knowledge of the ready decomposition o€ this intake gas mixture by t h e prevailing temperatures, made the commercial development a rather cautious one. After working with the electrically heated type of platinum gauze catalyst it became obvious t h a t t h e major function of this added heat energy involved the speeding up of the rate of reaction. An increase in temperature should make it possible t o oxidize unit quantity of ammonia in a much shorter catalyst contxct time. The less this contact time the better becomes t h e opportunity of preventing the catalytic decomposition of the desired intermediate compounds. The developments of t h e early work by t h e Bureau 8
I3-it. Patent 24.035 (1911): Chem.-Zlg., 40 (1916), 14. Chein. ? Mel..Eng., 20 (1919), 4 i O . MeLaZl und Em, 13 (1916), 21. T i . S. Patent 858,904 (1907).
7
of Mines has been briefly published.' An effort was made by the Bureau of Mines t o produce a catalyst and converter equipment whicx would promote t h e greatest conservation of heat within t h e catalyst itself. The r81e of temperature, contact surface, free space, reaction velocity relationships, oxygen concentration, ammonia decomposition, and poisons, y e r e all recognized as important. Unfortunately, only one of the above independent variables could be studied a t a time, and t h e one most promising of an early solution seemed t o be t h a t of t h e temperature. Could some sort of a catalyst element be devised which would eliminate the use of external heat energy without t h e uncertain decomposition of t h e ammonia of t h e intake mixture? The use of a heat interchanger on the incoming gas has caused all sorts of unsatisfactory results, according t o the previous published work. Only meager information was available as t o the use of multiple layers of gauzes up t o 1917. T H E O R E T I C A L T E M P E R A T U R E INFLUENCE-The theoretical influence of temperature upon t h e ammonia oxidation reaction has been well discussed. By t h e application of the law of ,,mass action t o t h e homogeneous system 4NH3 jOz 4x0 6H20 ( 31 \%-ehave
__
+
+
where (Pso), (PH~o), (PNH~) and (Po2) refer t o t h e partial pressures. This equilibrium coefficient K is independent of the relative proportions of t h e reacting bodies, but it is constant only for a given temperature. This equation by itself only enables one to determine t h e influence of changes in conceiitration a t a constant temperature. As applied t o this specific reaction, we may assume t h a t a n initial one gram molecule of ammonia is employed. x parts of this gram molecule are oxidized t o Y O by means of a n initial quantity of y gram molecules of oxygen. We would have a t equilibrium I - x gram mols. N H 3 x gram mols. N O y - j/4 x gram mols. O2 "4 x gram mols. H 2 0 z gram mols. Nz ( i f air was the source of the 0,) The total gram mols. present a t equilibrium are I--Z
or
+x + + I
+
1 . j ~
0.25X
Y-T
+y +
25%
+z
2
and m-e may call this G. Equation 8 can then be rewritten as
or (I.j x ) 6
(x')
~-
(I-%)~ 1
(y-
C. L. Parsons, LOG.cit.
P
x1.25%)~ G
=
.I
