November, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
1247
Action of Refining Agents on Pure Sulfur Compounds in Naphtha Solutions’ By Merrill A. Youtz a n d Philip P. Perkins STAKDAXD OIL COMPANY (INDIANA), WHITING, IND.
LTHOUGH sulfuric acid and other refining agents have available as an intermediate in the preparation of the trimethyllong been used to remove sulfur compounds from pe- e t h ~ ~ ~ ~ ~ ~ ~ m e t h y l esulfide t h y was l e na erepresentative of a except mercap- class of sulfur compounds not previously studied and only troleum, the tans as a class, have rarely been identified and only empirical recently synthesized. 6-Oxygen and nitrogen were known to be absent, or present knowledge has existed. It has seemed desirable, in view of the importance now being ascribed to sulfur removal, to only in very small amounts, in most light petroleum distillates containing sulfur, and hence sulfur compounds containing these learn about the action of these agents on sulfur elements did not need to be considered. pounds under conditions akin to those existing in practice. 7-other classes might have been included, such as the The work of Wood and others2 in this connection has been “thiophanes” or saturated cyclic sulfides, tertiary sulfides, and disulfides, thiophenes with one noted. Waterman and Peror two but longer side chains, quina have treated a few dithio ethers, etc. It is hoped sulfur compounds in naphto extend this work to include T h e desulfurizing action of 95 per c e n t sulfuric acid, some of these. tha solution with silica-gel. a l u m i n u m chloride, silica-gel, a n d a few o t h e r a g e n t s The present work deon naphtha solutions of several pure s u l f u r compounds Preparation of S u l f u r scribes the effect-in most Compounds has been studied. Each c o m p o u n d was dissolved in a cases only from the standstraight-run cleaners’ naphtha so that the solution The following sulfur COMpoint of the sulfur removed, contained 0.4 t o 0.6 per cent of sulfur. The r e s u l t s pounds were used as purand not the chemical reacshow that the a c t i o n of sulfuric acid is chiefly a solvent chased without further puri95 per tions involved-of action. A l u m i n u m chloride affects all t h e c o m p o u n d s fication: phenyl sulfide, isocent sulfuric acid, alumium except e t h y l a n d N-heptyl sulfides. Disulfides are amyl sulfide, benzyl sulfide, chloride, silica-gel, sodium largely decomposed only as f a r as m e r c a p t a n s u n d e r ethyl sulfide, allyl sulfide, sulfide, and hot fuller’s earth t h e s e conditions. Silica-gel removes sulfur f r o m all ethyl disulfide, and isoamyl on naphtha solutions of the the solutions, b u t the thiophenes are least affected, disulfide. following pure sulfur comt h e n the disulfides a n d t h e tetramethylethylene sulfide, The preparation o€ tripounds: ethyl sulfide, isoFuller’s earth a t 400’ C. (750’ F.) removes a l l b u t the methyl and trimethylethylamyl sulfide, N-pri-heptyl thiophenes fairly well. The u s e of m e t h y l iodide a n d t h i o p h e n e and of tetrasulfide, N4-sec-heptyl sulm e r c u r i c iodide in c o n j u n c t i o n is s h o w n t o be a very methylethylene sulfide will fide, phenyl sulfide, benzyl effective means of extracting aliphatic sulfides f r o m sulfide, allyl sulfide, ethyl be described elsewhere. petroleum fractions of high o r low molecular weight. disulfide, isoamyl disulfide, Normal primary heptyl s& t r i m e t h y l t hi o p hene, trifide-N-heDtvl bromide wm methvlethvlthioDhene. converted t o h e sulfide with and tGrarneihylethilene sulfide. These compounds do not alcoholic potassium sulfide. A solution of 50 grams potasby any means completely cover the various classes of sulfur sium hydroxide in 95 per cent alcohol (200 cc.) was divided compounds, but were nevertheless chosen with the follow- into two parts, one part saturated with hydrogen sulfide and the other part added. Ninety-nine grams of bromide were reing considerations in view: fluxed with this solution for 1.5 hours, the mixture cooled, 1-Mercaptans were not included, as they are known to be more or less completely oxidized by sulfuric acid to disulfides, diluted, and the upper layer separated, washed, dried, and thus changing the nature of the treating agent (presence of distilled in vacuo. Yield: 59 grams (93 per cent), boiling sulfur dioxide) and giving compounds (disulfides) which were point 140’ C. (cor.) at 5 mnm. to be studied separately, Furthermore, the practical treatment Normal secondary heptyl bromide-This compound was of mercaptans usually occasions little difficulty. 2-The sulfides were represented by ordinary aliphatic sul- prepared by a series of reactions:
A
fides of several molecular weights (ethyl sulfide, isoamyl sulfide, N-pri-heptyl sulfide), a secondary sulfide (N-sec-heptyl sulfide), an unsaturated sulfide (allyl sulfide), an aromatic sulfide (phenyl sulfide), and an aliphatic sulfide with a negative group (benzyl sulfide). 3-The disulfides were less well represented, although the effect of molecular weight in the saturated members was studied (ethyl and isoamyl disulfides). 4-In the thiophene series thiophene itself has been studied by Wood2 as t o the behavior with sulfuric acid, whichlreadily removes it, and aluminum chloride was known t o react vigorously with it. It was thought that a tetra-alkyl thiophene might be insoluble in and inert toward sulfuric acid, and hence correspond to the refractory sulfur compounds found in some oils. The trimethylthiophene was included only because it happened to be 1 Presented before the Division of Petroleum Chemistry at the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16,
1927.
ThOz Na HBr Butyric acid + dipropyl ketone + dipropyl carbinol ---f KzS bromide --f sulfide
Technical butyric acid was converted to the methyl ester by passing dry hydrogen chloride into a mixture of equal mols of acid and 100 per cent methanol and allowing to stand overnight. The ester was then washed, dried, and fractionated within very narrow limits, True boiling point 102.75’ C.4 a t 760 mm. It was then hydrolyzed by refluxing with lime and water, acidifying with 6 N hydrochloric acid, extracted with petroleum ether, and this solution fractionated. This purified butyric acid was passed over a thorium oxide catalyst held a t 380-420’ C.s and the product fractionated. The crude yield was 96 per cent, with a 75 per cent yield for the fraction boiling a t 140-147’ C. a t 760 mm.
* THISJOURNAL, 16, 1116 (1924);11,798 (1925); 18,169,823 (1926).
4
J . Chem. SOC.(London),63,1193 (1893).
a Brcnnsloff-Chem.,6, 255 (1925).
6
Sahatier and Mailhe, Bull.
SOC.
chim., [41 6, 483. 817 (1909).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1248
This ketone was reduced by the method of Wislicenuse by dissolving in ether, floating this on an equal volume of water, and adding sodium with cooling and using a reflux condenser. One hundred fourteen grams (I mol) of ketone were dissolved in 270 cc. of ether, 250 cc. of water added, and 55 grams mols) sodium in small pieces with shaking and cooling. The ether layer was dried with potassium carbonate, distilled and fractionated. Yield 72 grams, boiling point 153-157" C.; a t 750 mm. or 63 per cent yield. Another preparation gave a 61 per cent yield of N-4-sec-heptyl alcohol. The alcohol was refluxed with 4 mols of 48 per cent hydrobromic acid' for an hour. The upper layer was separated, washed with cold concentrated sulfuric acid, then with water, and dried. Upon distillation there was obtained a poor yield of material boiling a t 164-167" C. a t 748 mm., most coming over a t 166' C. A second preparation was made in which no sulfuric acid was used as a wash since the alcohol and bromide boiled far enough apart so that unchanged alcohol could be separated by distillation. A 72 per cent yield was obtained and also a small quantity of low-boiling material probably heptene formed from the alcohol and hydrobromic acid, This bromide was converted to the sulfide in the same manner as the previous bromide. Yields, 53 and 63 per cent, boiling point 114' C.(cor.), a t 5 mm. Preparation of Solutions The solutions were prepared by dissolving such amounts of the pure sulfur compounds in cleaners' naphtha that the resulting solutions had approximately 0.5 per cent sulfur. This naphtha was a refined straight-run distillate which was practically saturated. It gave only a slight color to 95 per cent sulfuric acid. Constants of "aphtha Distillation: Initial 88' C. (190' F . ) 50 per cent point 115' C. (240' F.) 165' C. (330' F.) Maximum 0.744 (58.1O A. P. I.) Specific gravity (15' C . ) 0 . 0 2 to 0.03 per cents Sulfur 2 per cent0 Sulfuric acid absorption
Treatment with Sulfuric Acid
In each case 100 cc. of solution were shaken violently for 3 minutes with 3.3 cc. of 95 per cent sulfuric acid (25 pounds of acid per barrel of naphtha solution) in a separatory funnel. After settling, the sludge was drawn off as completely as possible, 49 cc. of naphtha were poured off into another funnel, and 3.3 cc. of fresh acid (now 50 pounds per barrel) added to the remainder. The mixture was again shaken for 3 minutes, settled, the acid drawn off, and the naphtha transferred to a clean funnel. Each portion was washed thoroughly 6 A n n . , 219, 309 (1883); Houben-Weyl, "Die Methoden der organischen Chemie," 1'01. 11, p. 207, George Thieme, Leipzig, 1922. 7 A m . Chem. J . , 38, 633, 640 (1907); Morris, e t a l . , J. A m . Chem. Soc., 38, 1071 (1916). 8 Burton lamp method, A. S. T. M. D90-26T; Bur. Mines, Tech. Paper 8838 (1924),Method 620.1.p. 81. 0 Bur. Mines, Tech. Paper 323 (1923),Method 550.1,p. 86
COMPOUND Ethyl sulfide Isoamyl sulfide N-heptyl sulfide N-sec-heptyl sulfide Allyl sulfide Phenyl sulfide Benzyl sulfide Ethyl disulfide
with water, dried by filtration through a dry filter, and analyzed for sulfur. The results are shown in Table I. of Sulfuric Acid Treatment on Sulfur Content AFTERFIRST AFTER SECOND ORIGINAL ACID ACID COMPOU D SOLUTION TREATMENT TREATMENT Per cenl Per cenl Per cent Ethyl sulfide 0.477 0.041 0.045 Isoamyl sulfide 0.185 0.100 0.586 A'-heptyl sylfide 0.418 0.131 0.032 A'-sec-heptyl sulfide 0.438 0.089 0.010 Allyl sulfide 0.051 0.824 0.038 Phenyl sulfide 0.416 0,507 0,464 Benzyl sulfide 0.210 0.400 0.295 Ethyl disulfide 0.439 0.573 0.278 Isoamyl disulfide 0.446 0.482 0.464 Trimethylthiophene 0.155 0.537 0.047 Trimethylethylthiophene 0.070 0.410 0.103 Tetramethylethylene sulfide 0.110 0.100 0.430
Table I-Effect
In several instances the sulfur has increased after the second treatment. It is believed that either the analyses are in error or possibly some sulfonation of the solvent or sulfur compound occurred. However, it is possible to distinguish certain results. Alkyl sulfides are removed rather readily, the more so if they are of low molecular weight or are secondary sulfides. The unsaturated allyl sulfide is also easily removed. But benzyl sulfide is not so completely removed and phenyl sulfide is little affected. Disulfides are not easily removed, although the lower one, ethyl disulfide, is removed to some extent. Isoamyl disulfide is practically unaffected. The two thiophenes are removed fairly well, although it is a little surprising that the tetra-substituted thiophene should be so well extracted. An inertness like that of hexamethylbenzene was expected, as sulfonation cannot readily occur unless an alkyl group is displaced. Apparently in this case and in that of most of the sulfides, the action of the acid must be purely a solvent action. I n support of this, the pure alkyl sulfides and the two thiophenes are found to be very easily soluble in concentrated sulfuric acid (soluble in 1 to 2 volumes of acid or completely miscible) while the disulfides and phenyl sulfide are much less so. The tetramethylethylene sulfide is fairly well extracted. With this compound probably a solvent action would also occur, but the material is polymerized by the strong acid and the acid layer is rendered very cloudy, then part of the polymer is redissolved by the naphtha. Treatment with Aluminum Chloride One hundred cubic centimeters of each solution were refluxed with 1 gram of aluminum chloride for 3 to 4 hours. The refluxing temperature of the liquid was 99-100" C. (210-212° F.). At the end of t h t time 50 cc. of the solution was poured off, washed, dried, and analyzed for sulfur. The remainder of the solution was distilled from the same flask through a very short column, practically to dryness. The distillate was then washed, dried, and analyzed. The results are given in Table 11. From Table I1 it is seen that sec-heptyl sulfide, allyl sulfide, and benzyl sulfide react very completely with the aluminum chloride on simple refluxing. The other com-
Table II- -Effect of A l u m i n u m Chloride Treatment on Sulfur Content AFTER ORIGINAL AFTER REMARKS DISTILLATION REFLUXING SOLUTION Per cent Per cenl Per cenl 0.448 Sweet to doctor; trace HzS during boiling 0.373 Slightly sour 0.477 Sour; yellow ppt. 0.170 0.380 0.511
0.445 0.446 0.624 0.507 0.400 0,573
0.324 0.019 0.056 0.377 0.012 0.422
0.330 0.294O 0.365 0.231
REMARKS
0.312 0.032
Sweet t o doctor Very sour: heavy yellow ppt. CaHaSH and HzS during boiling
Isoamyl disulfide Sweet; no HzS during boiling Trimethylthiophene 0.53; Trimethylethylthiophene 0 410 Tetr2methylethylene sulfide . . . a A solution in a heavier naDhtha, boiling 143-211' C. (290-430'' F.).
...
Vol. 19, No. 11
0.027 0.052
Sour, odor of CeHb
0.000 0.226
0.055per cent sulfur after SagPbOt treatm ent
0.122 0.180= 0.236 0.100
Sweet
...
November, 1927
IAYDUSTRIAL AND ENGINEERI,VG CHEMISTRY
1249
I n solvents inert to hydrogen sulfide they can only be partially decomposed, if a t all; in acetone the decomposition goes further, though not easily to completion, but the acetone also reacts with the hydrogen sulfide. They can be decomposed, apparently with regeneration of the original sulfur compounds, by boiling with zinc and 80 per cent acetic acid. Sulfuric acid can also be used. All these sulfur compounds except the thiophenes react, some even with evolution of considerable heat, In 0.5 per cent sulfur solution in naphtha, ethyl diwlfide, isoamyl disulfide, the tetrnmethylethylene sulfide, and phenyl sulfide reacted very slightly. This order is that of decreasing reactivity. The lower sulfides in pure form (not in solution) react with a large heat evolution. This method is in the writers’ opinion much superior to the methyl iodidesulfide or mercuric chloride-sulfide addition compound method of separation. The reaction occurs much more swiftly, is more nearly complete, and is available with almost equal sureness and speed for petroleum sulfides of molecular weight a t least as high as 360. In applying the method to a petroleum product it is desirable first to concentrate the sulfur compounds as far as possible by other means (distillation, selective solvents, silica-gel, etc.). h k r captans and disulfides should be absent. Some disulfides react slightly and give unstable compounds which decompose in a few hours, while the true sulfide addition products are stable almost indefinitely. The method can be used to purify the sulfides. A suitable procedure is to treat the oil (containing 0.5 per cent or more One hundred cubic centimeters of each solution were of sulfur) with solid red mercuric iodide and an excess (one-fifth shaken with 20 grams of gel for 2 hours; the solution was to equal volume) of methyl iodide and shake for an hour or then analyzed. The results are summarized in Table 111. less. The upper layer is poured off and 80 per cent acetic acid and granulated zinc added to the residue. A light T a b l e 111-Effect of Silica-Gel on S u l f u r C o n t e n t petroleum ether is also added and the mixture refluxed until BEFORE AFTER NET no further change occurs (1 to 3 hours). The petroleum TREATMENT TREATMEKT DECREASE SUBSTANCE Per cent Per cent Per cent ether solution then contains the liberated sulfides, having E t h y l sulfide 0.477 0,198 0.279 protected them from possible reduction by the zinc. For Isoamyl sulfide 0.586 0.350 0.236 N-heptyl sulfide 0,445 0.104 0.341 the lighter sulfides of petroleum distillates the petroleum ether N-sec-heptyl sulfide 0.560 0.369 0.191 could either be omitted entirely or perhaps replaced by Allyl sulfide 0.624 0.335 0.289 Phenyl sulfide 0.507 0.263 0.244 ether. A better plan might be to use a very closely fractionBenzyl sulfide 0.400 0.141 0.259 Ethyl disulfide 0.573 0.365 0.208 ated petroleum ether which could be fractionated from the Isoamyl disulfide 0.460 0.250 0.210 sulfur compounds. Trimethylthiophene 0.537 0.413 0.124
pounds react to varying degrees. Ethyl sulfide is very little affected. On distillation most of the compounds react further with the aluminum chloride, probably owing to the higher temperatures involved. This set of results (after distillation) is complicated by two other factors: (1) Some of the sulfur compounds have much higher boiling points than the naphtha and might not all be volatilized on distillation; (2) if the aluminum chloride formed a complex with a compound and largely or partly removed it from solution, the complex might be again decomposed by the higher temperature toward the end of the distillation.. Ethyl disulfide was very thoroughly decomposed, giving both hydrogen sulfide and mercaptan, but comparatively little of the former and very large amounts of the latter. After distillation nearly all the remaining sulfur was removed by dort’or, giving a yellow precipitate only-i. e., no hydrogen sulfide. The two thiophenes are affected to about the same degree, the same amount of sulfur being removed. There is a very striking difference in the behavior of the primary and secondary heptyl sulfides. On account of its great volatility, the tetramethylethylene sulfide was not treated with aluminum chloride. It should be noted that the amount of aluminum chloride used was relatively limited, in order that the comparative effect on the different compounds could be observed. Larger amounts decornpose nearly all the above compounds very completely. Treatment with Silica-Gel
Trimethylethylthiophene Tetramethylethylene sulfide
0.410 0.430
0.305 0.227
0.105 0.203
Other Experiments
A few experiments xere conducted in which the solutions To facilitate comparison of solutions with varying initial sulfur content, the net decrease in percentage of sulfur is were vaporized and passed over fuller’s earth a t 400” C. shown in the fourth column. From this it is seen that for The solutions were dropped into a pipe leading to a preall the sulfur compounds except the two thiophenes, the heater coil heated in a lead bath, so that all the material was decreases are considerable. This might perhaps have been vaporized. The fuller’s earth was contained in a I-inch (2.5-em.) pipe with electrical heating. Temperatures were predicted on the basis of the much greater “unsaturation” of the sulfides (as compared with the thiophenes) due to the measured by a thermocouple imbedded in the fuller’s earth. divalent sulfur. I n the case of n-heptyl sulfide the large T a b l e IV-Effect of Fuller’s E a r t h on S u l f u r C o n t e n t decrease is perhaps due to the very high boiling point (or lorn BEFORE AFTERT R E A T M E N T vapor pressure) of the substance combined with the presence COMPOUND TREATMENTSour Sweet Per cenl Per cent Per cent of the unsaturated sulfur, although the considerable differIsoamvl sulfide 0.511 0.380 0.141 ence between the normal and secondary sulfides suggests Benzyi sulfide 0.111 0.326 0.112 Isoamyl disulfide 0.460 0.237 0.300 that structure may have a good deal to do with the result’s. A-heptyl sulfide 0.530 Treatment with Methyl and Mercuric Iodides By mixing a sulfur compound or its hydrocarbon solution with a little methyl iodide and then adding mercuric iodide and shaking, all ordinary sulfides and some disulfides react with considerable readiness to form sulfur addition conipounds, usually liquid under these conditions (probably crystallization is merely hindered by the hydrocarbons present) and very insoluble in hydrocarbon solvents. They are relatively far more stable than the addition compounds with methyl iodide alone or with the mercuric halides alone.
A-sec-heptyl sulfide Allyl sulfide Trimethylethylthiophene Trimethylthiophene“ a At 370’ C.
0 . 171
0.446 0,624 0.466 0.870
o:i&
0.283
...
0.131 0,231 0.107 0.287 0.837
These experiments are somewhat limited a i d perhaps inconclusive, but apparently ordinary sulfides are well removed, disulfides and secondary sulfides less well, and the thiophenes still less readily. Sodium sulfide is reported to react readily with disulfides according to the equation 2Na2S R& = 2RSNa Na&
+
+
INDUSTRIAL AND ENGINEERING CHEMISTRY A naphtha solution of isoamyl disulfide (S = 0.460 per cent) was refluxed with alcoholic sodium sulfide solution (50 cc. C2HjOH, 10 cc. HzO, and 6 grams NazS.9HzO). After separating, washing, and drying, the oil had 0.459 per cent sulfur; but after treatment with sodium plumbite it had only 0.312 per cent sulfur. That is, the above reaction occurred to some extent, but the sodium mercaptide was at once hydrolyzed and the mercaptan retained in the oil layer until removed with sodium plumbite. Suggested Analytical Method for Mercaptans, Sulfides, and Disulfides
It seems very probable that an analytical method for mercaptans, sulfides, and disulfides in gasolines could be developed, using sodium plumbite for mercaptans, sodium sulfide for the disulfides, and methyl iodide plus mercuric
Vol, 19, No. 11
iodide for the sulfides. Since the method would depend on analysis of the oil after each treatment, it would probably only be satisfactory for oils containing large amounts of sulfur (over 0.20 per cent). A suitable procedure might be: I-Remove hydrogen sulfide with sodium carbonate solution and analyze for sulfur. 2-Wash with concentrated aqueous sodium sulfide to remove free sulfur. 3-Wash with 50 per cent alcoholic sodium plumbite containing some solid lead sulfide (to remove mercaptans) and again analyze. 4-Reflux with excess sodium sulfide in 70 to 80 per cent ethyl alcohol, treat with alcoholic sodium plumbite, and again analyze. 5-Finally, shake for several hours with excess methyl iodide and mercuric iodide, pour off the oil layer, distil off methyl iodide (to do this the naphtha being analyzed would have to be fractionated so as to have no portion boiling below about 80" C.), reflux the bottoms with zinc and acetic acid, and analyze for sulfur.
Oxidation of Ammonia'*' By Guy B. Taylor E. I. DU
P O N T Dl3
NEMOURS& COMPANY, WILMINGTON, DEL.
MMONIA oxidation in its special sense refers to that industrial process wherein ammonia gas is burned with air on a catalytic surface, producing oxides of nitrogen. Under certain conditions ammonia, cyanogen, or hydrocyanic acid gases can be burned with a free flame, in which case their nitrogen appears in the products of combustion as free nitrogen. If, however, the combustion is carried out on certain surfaces-e. g., platinum or some heavy metal o x i d e u n d e r suitable conditions, the nitrogen appears in the reaction products as nitric oxide. The oxidation of ammonia to nitric oxide was observed first toward the last of the eighteenth century. Platinum as a catalyst for this reaction was discovered by Kuhlman in 1839. No industrial use was made of the catalytic oxidation of ammonia until shortly before the World War, when Ostwald worked upon it in Germany. The catalytic oxidation of ammonia is an important technical process because it affords the means of transferring nitrogen combined with hydrogen (or carbon) into nitrogenoxygen compounds. The primary product of the catalysis is nitric oxide and by suitable series of steps this compound may be converted into practically any compound in which nitrogen and oxygen atoms are directly attached. Syntheses of such compounds are generally effected through the reactions of nitric acid, and up to the present by far the largest technical use of catalytic ammonia oxidation has been in the manufacture of nitric acid. A description of such a nitric acid process is probably the best way t o give a clear idea of the general subject of this talk. Manufacture of Nitric Acid-Theory
A mixture of ammonia gas and air is passed through a red-hot, fine-meshed platinum gauze, the reaction taking place being represented by the equation 4NH3
+ 502
4N0
+ GHzO
(1)
It is possible on an industrial scale to convert 95 per cent or more of the ammonia into nitric oxide. The other 5 per cent breaks down to free nitrogen and all the hydrogen 1 Presented before a General Conference a t the Institute of Chemistry of t h e American Chemical Society, State College, Pa., July 16, 1927. 2 Contribution No. 5 from t h e Isu Pout Experimental Station.
is converted to water vapor. This secondary reaction may be represented by the equation 4NH3
+ 30,
= 2Nz
+ 6Hz0
(2)
After passing the platinum gauze, the gas mixture is cooled and nitric oxide combines with excess oxygen spontaneously to form nitrogen peroxide,
+
2 N 0 0 2 = 2N02 2N02 $ NzOi
(3) (4)
The last step in the process consists in countercurrent absorption of nitrogen peroxide in water to form nitric acid, 3N02
+ HzO F! 2"03
+ NO
(5)
The nitric oxide formed in this reaction repeats reaction (3), which is followed by (5) until nearly all of it is converted to nitric acid. The mechanism of the course of these reactions is important to an understanding of the plant equipment and process details, so a discussion of their peculiarities will be presented before outlining the nature of the plant. There is no adequate theory to explain reaction (1). Little enough is known to account for any case of contact catalysis, and less about this one. The temperature, 1000" C., is higher, and time of contact, 0,0001 second, is shorter than for any other known case of contact catalysis. Practically speaking, the reaction is irreversible, thus differing fundamentally from contact sulfuric acid practice. I n spite of the thermodynamic instability of nitric oxide, no upper temperature limit has been found a t which the yield falls off, a t least up to 1300" C. I n practice the best temperature seems to be 900-1000' C. on account of wear and tear on platinum. At temperatures as low as 500' C. the ammonia is almost wholly converted to free nitrogen. The time factor is important. At excessive rates of gas flow free ammonia passes the catalyst and is destroyed by a secondary reaction between NHs and NO. At very low rates the yield falls off, but there is a wide range in rate in which the yield is little affected. The thickness of the contact bed is, in the case of platinum, a t most only a few hundredths of an inch. Inert catalyst supports such as are used in sulfuric acid contact plants have never proved SUC-
