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
Dec., 1919
HEAT OF REACTJON- OF AMMONIA OXlDATION' By GUY B. TAYLOR Received July 9 , 1919
When a mixture of ammonia gas and air is brought in contact with a suitable catalytic surface maintained above a certain temperature, the predominating reaction which occurs is the formation of nitric oxide and water. The temperature most favorable to this reaction lies above 800' C. I t is, then, of considerable interest t o calculate the heat of the reaction in order t o determine how much external energy must be added t o maintain t h e catalyst a t t h e required temperature. A general formula has been developed for calculating the theoretical heat of reaction for any mixture of ammonia and air at any conversion efficiency, taking into account also the degree of humidity of the mixture. This calculation is based on the following equations: 4NH, 5 0 2 = 4NO 6Hz0 2 1 4 2 0 0 cal. (1) 4NH3 3 0 2 = 2N2 6 H z 0 300600 cal. (2) The heats of reaction were obtained from heats of forma.tion as follows: Ammonia, NHa.. ....................... f 12000 calories -21600 calories Nitric oxide, NO. .......................
+ +
+ +
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mixture. Multiplying each of the above fractional moles by its specific heat and adding the results, the value of C in the equation t
=
@
C
is obtained.
This
equation reduces t o the form: 7.08 -t 3 . 4 I X - 0 . 2 j y
+ 8.34 B -
V The third term in the denominator is small and may be neglected; the fourth term becomes zero when the mixture is dry.
+ +
I
Water, HzO.. ...........................
+58100 calories
The temperature rise is expressed by the formula
where t is degrees Centigrade, Q the heat of reaction i n calories, and C the specific heat of t h e products of reaction. The specific heats at constant pressure of t h e products involved, taken from Landolt and Bbrnstein's tables, in calories per mole are Nitrogen.. . . . . . . . . . . . . . . . 7 .OO (Holborn and Henning) Oxygen.. . . . . . . . . . . . . . . . . 7.36 (Holborn and Austin) Nitric oxide.. . . . . . . . . . . . . 6 . 9 3 (Regnault) Water vapor..
............
8.03
+ 0.00078t (Holborn and Austin)
Over t h e range 0'-800' C., water vapor is the only gas of the four having a n appreciable temperature coefficient. For the calculation a mean value of 8.34 n a y be used without appreciable error. I n an ammonia-air-water vapor mixture containing oxygen in excess of the stoichiometric proportion, let x = fraction in moles of 3" in ammonia-air mixture y = fraction in moles of NO produced by Reaction 1
z2r?= fraction in moles of =
N2
produced by Reaction 2
yield
Then in every mole of gas after t h e reaction there will be y moles nitric oxide 0 . 7 9 (I
X) $-
- moles nitrogen 2
0.21 ( I
3
+
2'
-- x )
V B -7-V
- -5 y 4
3-
(
Y) moles oxygen
moles water vapor
where I/ is partial pressure of water vapor and B the total pressure of the ammonia-air-water vapor 1
Yublisbed by permission of the Director of the U. S.Bureau of Mines.
8
/O II I2 J3 Yoff~J In air mixture li5eoretica/ rise in temperature
9
I4
FIG.I
I n Fig. I are plotted t h e temperature rises for ammonia concentration in air from 8 t o 14.4 per cent (dry basis) for IOO and 80 per cent conversion efficiencies for both wet and dry gas mixtures. The
'
value of the term ___ is assumed t o be 0.03 which B- V corresponds t o the humidity when ammonia liquor is used as the source of ammonia gas. From the plot it will be noted t h a t the four lines are very nearly parallel, diverging slightly as they rise. There is an increase of about 50' a t 80 per cent yield over t h a t of complete conversion.1 All the experimental evidence obtained in the Bureau of Mines laboratories, which has been corroborated by plant experience,2 tends t o show t h a t for 1 It may be considered doubtful whether in actual practice the oxidation of ammonia to elementary nitrogen and water takes place on the catalyst surface, or at least wholly on this surface, and thus contributes t o maintaining the gauze temperature. There are some grounds for believing that the nitrogen is formed after the gases pass the catalyst by interaction of ammonia and nitric oxide. See Baxter and Hickey, A m . Chem. J . , 33 (1905), 300. * C. L. Parsons, THISJOURNAL, 11 (1919), 541.
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
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high yields the temperature of platinum catalyzers should not fall below 800’ C. and the ammonia concentration should not exceed I O per cent. This is almost certainly equally true of any catalyzer for ammonia oxidation. A glance a t the flame temperature of Fig. I shows clearly why the reaction will not sustain itself unless external energy is supplied or converters are properly designed t o recover a portion of the sensible heat carried away with the reaction products. All types of commercial converters utilizing platinum catalyzers resort t o various expedients for maintaining proper temperatures. Converters of the Ostwald type preheat the ammonia-air feed by means of a heat tranqfer from the hot reaction products (plants a t Vilvorde, Angouleme and Dagenham abroad) ; others supply the extra energy required by electrical means (Frank Car0 a n d Landis types.)’ The Bureau of Mines type utilizes the hot reaction products t o heat a shell surrounding a cylindrical gauze. The shell by radiating heat back t o the gauze keeps its temperature up. A third method of maintaining temperatures is t o add oxygen t o the mixture and increase the ammonia concentration. This method was successfully applied t o non-platinum catalyzers il? laboratory experiments by the writer in 1916. EXPERIMENTAL
It is the purpose of the present paper t o describe experiments carried out in the Bureau of Mines laboratories with a platinum gauze converter, comparing results obtained by electrical heating, preheating, and the addition of oxygen. APPARATUS AND METHODS-The apparatus utilizing a 3 x 6 in. platinum gauze with provision for electrical heating has been described .elsewhere.2 Temperature of the gauze was observed through a mica window with a Holborn-Kurlbaum optical pyrometer, using the maker’s black-body calibrations, and making the necessary corrections3 for deviation from blackbody conditions by making additions according to t h e folhwing table: Actual temp. Deg. C. 600 700 800 900
Add. Deg. C. *
35 45 55
85
The temperature of t h e gauze varied from 10’ t o 30° in different places when electrical heating was used. The richer the mixture in ammonia, the more uniformly was the gauze heated. Efficiencies were determined by the vacuum-bottle method of Taylor and Davis.4 I n preheating experiments, a royal Berlin porcelain tube, 3 f t . long and in. outside diameter with walls in. thick, was fastened by means of a metal collar t o the cover plate of the top section of the aluminum oxidizer body. A 2-it. section of this tube about 3 in. above the cover plate was heated by means of an elec1
Parsons, LOG.cit.
THISJOURNAL, 10 (1918), 457; 11 (1919), 27 and 544. a Waidner and Burgess, “Optical Pyrometry,” Bureau of Standards, Bulletin 1, 247. 4 THISJOURNAL, 9 (1917), 1106. 2
Vol.
11,
No.
12
tric resistance furnace. The ammonia-air mixture on its way t o the platinum gauze passed through the heated porcelain tube, coming in contact only with porcelain and aluminum surfaces. The top section of the oxidizer was lagged with asbestos board. I n experiments using oxygen enrichment, this gas was separately metered and passed into the ammoniaair stream coming from the ammonia vaporizing drums. Preheating was not employed in these tests and electric current was used only t o ignite the gauze. The platinum gauze used in all experiments reported in this paper was I O O mesh t o linear inch, o.oo20 in. diameter wire. It contained some iridium as impurity. The ammonia gas used was derived from “A” grade liquor of coke-oven origin furnished by the SemetSolvay Company. D I S C U S S I O N AND R E S U L T S VWSUS E L E C T R I C HEATING-The importance of the nature of the surface and time of contact on the destruction of ammonia a t high temperatures has been shown by the investigations of Ramsey and Young,l Perman and Atkinson,2 and White and Melville.8 These experimenters showed t h a t ammonia may be easily decomposed under certain conditions a t moderately elevated temperatures. I n spite of the fact t h a t Ostwald’s4 original patent provided for preheating and t h a t the conditions t o be met in preheating are quite different from those obtaining in the experiments of t h e above-named investigators, there seemed t o be a pretty general impression a t t h e time the Bureau of Mines began its experiments that preheating was likely t o result in decomposition of ammonia and therefore should be avoided. Two United States patents lent support t o this idea, t h a t of Landis6 for precooling the ammonia-air mixture, and another issued t o Kaiser6 for preheating the air separately and previously t o its admixture with ammonia gas. Kaiser states in his patent t h a t by preheating the mixture in quartz t o 320° C. he obtained a yield of 66.8 per cent but when he preheated the air separately he obtained 99.8 per cent. These figures are nothing short of ridiculous. IS this another example of German camouflage patents? Among the very first experiments performed in t h e laboratory was one in which a IO per cent mixture of ammonia and air was passed through a column of bits of firebrick about 30 cm. long and 2 cm. in diameter contained in a glass combustion tube heated in a n electric furnace t o 700’ C. A t a rate of about I 1. per min. there was no destruction of ammonia whatever in three separate trials. Tests on other materialshave been given in Dr. Parsons’ paper. I n Table I are shown some efficiencies obtained with and without the use of electric current to aid in heating the platinum gauze. They show clearly t h a t the heat of reaction alone did not raise the gauze t o the re-
PREHEATING
1 2
3 4
6
J . Chem. SOC.,46 (1884), 88. Proc. Roy. Soc., 1 4 (1904). 110. J . A m . Chem. S o c . , 27 (1907), 373. U. S. Patent 853,904. U. S. Patent 1,193,796. U . S. Patent 987,375.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Dec., r g ~ g
1123
is no intrinsic merit in electric heating. Any suitable method of maintaining gauze temperatures will result in satisfactory conversion efficiencies. I n order t o determine whether ammonia suffered any decomposition by passage through the hot porcelain tube, a number of analyses of t h e air mixture were made before and after traversing the tube, with the results shown in Table 111. TABLEI11
24
19
215
/55
L7
14
/3 O2 NH, ratio
FIG. 2
quired temperature. I n Table I1 are shown some efficiencies obtained by preheating the ammonia-air mixture. The fourth*column of t h e table shows the temperature of the outside wall of the porcelain preheating tube and the fifth column the temperature of t h e ammonia-air mixture just after entering t h e oxidizer. The efficiencies of these tests compare very favorably with those of Table I a t t h e same concentrations and gauze temperatures. TABLEI-RESVLTS
OF
TESTSWITH
AND WITHOUT
E~ECTRICAL HEATINQ.
N O PREHEATING O F AMMONIA-AIR MIXTURE
Test No.
Air
Cu. Ft.
per Hr.
NHa in Temp. of Air Mixture Gauze Amperes Per cent Deg. C. 86 82 0 60 58 0 58 0 0 49 0 0 0 0 115 0 90
0
0
NH? Escaping Oxidation Per cent 0.8
1.0 1.6 1.2 1.3 1.2 1.2 1.6 1.3 1.3 1.3 1.7 0.9 2.3 1.3 2.6 1.8 2.8 1.9 1.8
Conversion Efficiency 94.8 93.6 82.6 89.7 90.3 83.6 90.0 84.9 85.8 85.8 82.0 82.2 79.5 67.0 91.8 84.0 88.6 83.2 82.9 81.4
TABLE11-R .&SULTS O F TESTS PREHEATING T E E AMMONIA -AIR MIXTURE Preheating NH? ES-ConverNHa in Temperature caping sion Air Air Porcelain Oxi- Temp. of OxiEffiTest Cu. Ft. Mixture Tube dizer Gauze dation ciency Per cent Deg. C. Deg, C. Deg. C. Per cent Per cent No. per Hr 191 160 9.36 855 770 0.5 90.9 189 150 780 0.7 91.2 9.54 850 190, 140 9.76 780 0.4 91.2 905 185 150 10.00 680 770 0.8 88.2 188 150 10.43 830 90.8 815 0.6 186 150 10.47 795 0.6 680 88.9 187 150 11.20 820 0.4 780 87.8 183 150 11.55 665 0.9 805 88.3 184 160 820 12.07 540 0.7 86.3
The curve of Fig. 2 represents t h e most probable values of conversion efficiencies of the gauze we were using at various ammonia concentrations, air velocity 1 5 0 cti. f t . per hr. and gauze temperature Booo t o 900° C. All the points corresponding t o t h e above velocity from Tables I and I1 are plotted in the figure. All t h e points except those representing ammonia concentrations below 1 I . j per cent, where no energy was supplied except t h a t furnished by the reaction itself, Lie close t o t h e curve. Fig. 2 shows clearly t h a t there
Temp. Outside Wall of Tube Deg. C. 660 665 680 830 850 905
NHs Before Per cent 9.53 11.63 10.50 10.43 9.54 9.76
NHa After Per cent 9.44 11.58 10.45 10.33 9.54 9.70
While the gas probably passed too rapidly t o acquire anywhere near the temperatures of the tube, i t is clear t h a t ammonia may be easily preheated in a mixture with air in contact with a suitable surface without appreciable decomposition. Destruction of ammonia in t h e above table is seen t o be less than I per cent, not much more t h a n the error of analysis. O X Y G E N ENRICHMENT-In Table Iv are presented some results obtained in June 1 9 1 7 by using oxygen and air. Oxygen permitted t h e use of high enough ammonia concentrations t o make t h e reaction selfsustaining. The results of Table IV plotted in Fig. 2 on the basis of oxygen-ammonia ratio by volume lie practically on t h e curve. TABLEIV-RESVLTS O F TESTSUSING Air f Composition NH3 Oxygen Mixture by Volume Test Cu. f t . Oxygen Ammonia No. per Hr. Per cent Per cent 153 153 23.65 12.10 154 158 24.95 12.80 31.15 14.40 155 lS9 22.00 151 15.30 183 160 289 23.70 11.35 158 280 26.00 12.60
OXYGEN ENRICHMENT NHa Es- ConverTemp. caping sion EffiGauze Oxidation ciency Deg. C. Per cent Per cent 770 1.0 92.2 790 0.8 90.2 830 0.7 94.7 900 1.9 85.7 775 3.2 85.9 825 2.2 88.0
SUMMARY
I-A formula for calculating t h e heat of reaction of ammonia oxidation at any concentration and conversion efficiency has been developed. 11-The optimum temperature for conversion of ammonia t o nitric acid by the catalytic oxidation method has been shown t o lie above 800' C. 111-Both from calculation and experiment i t has been shown t h a t the heat of reaction developed at the optimum ammonia concentration is insufficient of itself t o maintain catalysts a t the optimum temperature. IV-It is shown t h a t electric heating, preheating the ammonia-air mixture, or utilizing oxygen, produce equally satisfactory results. ACKNOWLEDGMENT
The work described in this paper was carried out under the immediate supervision of Dr. C. L. Parsons, t o whom t h e writer is indebted for many helpful suggestions. A. S. Coolidge assisted in the heat of reaction calculations. Messrs. J. H. Capps, L. R. Lenhart, and R. C. Dabney assisted a t various times in t h e laboratory. BUREAUO F MINES WASHINGTON, D. C.
