Jan., 1919
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 CHEiWISTRY
27
ORIGINAL PAPERS EFFECT OF PHOSPHINE AND HYDROGEN SULFIDE ON THE OXIDATION OF AMMONIA TO NITRIC ACID’ By GUY B. TAYLOR AND
JULIAN
H. CAWS
Received July 31, 1918
I n a recently published paper2 it has been shown t h a t acetylene gas derived from commercial carbide has a marked deleterious effect on t h e oxidation of ammonia t o nitric acid by catalytic platinum. Further experiments have shown t h a t this effect is t o be ascribed t o impurities in t h e acetylene rather t h a n t o acetylene gas itself. Lunge and Keane3 state t h a t t h e common impurities present in acetylene are hydrogen sulfide, phosphine, ammonia, carbon monoxide, hydrogen, methane, nitrogen, and oxygen. Silicon hydride may also be present in minute quantity. Captain G. A. Perley and Mr. J. D. Davis, of this Bureau, from experiment and observation on commercial ammonia converters oxidizing cyanamide ammonia, strongly suspected phosphine as the active poisoning agent. As a result, the present investigation was undertaken t o secure data on t h e effects of phosphine and of hydrogen sulfide. The apparatus and experimental methods have been previously d e ~ c r i b e d . ~Thoroughly activated, pure platinum gauzes were used, 0.0026 in. diameter wire, 80 wires t o linear inch. PURIFIED ACETYLENE
T o free acetylene from impurities t h e gas was passed through a train of wash bottles containing solutions as follows: Copper sulfate, acid cuprous chloride, 50 per cent nitric acid, alkaline sodium hypochlorite, and sodium hydroxide. The purified gas was collected in a large bottle and allowed t o stand over night in contact with sodium hypochlorite solution. Thiosulfate was added t o take care of any free chlorine t h a t might have been evolved, and copper sulfate t o precipitate hydrogen sulfide derived therefrom. This purified acetylene was used in a short run of about 2 hrs., during which the concentration in the ammoniaair mixture was gradually raised t o about 0.4 per cent. There was no noticeable effect on t h e gauze excepz a rise in the temperature, and t h e yields were quite satisfactory. Examination of t h e gauze after the run showed it had suffered no change and had the characteristic gray appearance of a n active gauze. PHOSPHINE
Phosphine was prepared by heating yellow phosphorus with sodium hydroxide solution. The gas was collected in bottles in which it was treated with hydrochloric acid t o free it from the self-inflammable variety. The acid was then neutralized with strong alkali. I n the preliminary experiments a gas was used containing 2 3 per cent P H z as determined by absorption 1
Published by permission of Director, U. S. Bureau of Mines. 10 (1918), 547. “Technical Methods of Chemical Analysis,” [I],2, 58?. Taylor and Capps, LOC.c i l .
* TFZIS JOURNAL, 8
4
in a solution of copper sulfate. The rest was presumably hydrogen. This was fed into the ammoniaair mixture at a rate of about I O cc. a minute for z min., corresponding t o about 2 j parts PHs per million. The electrically heated gauze blackened immediately. After shutting off t h e phosphine t h e gauze cleared in j min. The rate was then varied and i t was found t h a t I O parts per million was sufficient t o cause black areas t o appear on t h e gauze. A total of 590 cc. of the phosphine-hydrogen mixture was fed into the system, reaching a final concentration of 0.07 per cent in the gases passing t o the oxidizer. The gauze was black nearly all over most of the time. Examination of the gauze after t h e run showed in a pronounced way t h e same appearance as the gauze t h a t had been run with impure acetylene. It was brittle and pulled apart even though carefully handled. Quantitative experiments were then undertaken, using another gauze from t h e same sheet, in order t o determine t h e limit concentration of phosphine allowable. Phosphine, PHs, prepared as above, was diluted with air in a small metal gas holder which was water-sealed. This gas mixture was led into t h e ammonia-air feed line through 6 f t . of brass tubing, which formed t h e delivery pipe of t h e gas holder, at rates which should have added phosphine in amounts between 0.4 and 4 parts per million of t h e gas entering t h e oxidizer. The efficiency of oxidation was not modified b y this t o t h e extent t h a t t h e preliminary tests had indicated. On examination, however, it was found t h a t t h e phosphine in t h e gas holder decreased rapidly on standing over water. Furthermore a t t h e conclusion of t h e tests, t h e brass delivery t u b e of t h e gas holder was corroded and it seemed certain t h a t a further portion of t h e phosphine was destroyed in passing through it. On account of these known sources of error, t h e rather inconsistent results of these tests were discarded. The phosphine mixtures always contained hydrogen. While it did not appear probable t h a t hydrogen could have any deleterious effect, preliminary of t h e above tests, electrolytic gas from electrolysis t o sodium hydroxide solution was introduced and found t o be without effect up t o one-tenth per cent hydrogen in the ammonia-air feed. At about this time actual trials with added phosphine made b y Lieut. Brush a t Captain Perley’s direction, showed t h a t even two or three parts of phosphine per hundred million of mixed gases were serious in plant operation. Therefore, a second series of tests was made using special precautions t o insure t h e phosphine added actually reaching t h e gauze. The metal gas holder was varnished inside, a glass delivery t u b e arranged, and t h e phosphine-air mixture confined with an oil seal. The gas came in contact with no metal or water and did not vary in phosphine concentration from day t o day. About 2 0 0 cc. of 2 0 per cent PH3 was diluted in the gas holder t o about
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
28
.. . ... 459 458
466
...
461
... 46i ... 463 464 465
...
466
.. ,
467 468
464
... ... 471
470
Date July 16 July 16 July 16 July 16 July 16 July 16 July 16 Tulv 16 j u i y 17 July 17 July 17 July 17 July 17 July 17 Tulv 17 j u f y 18 July 18 July 18 July 18 July 18 July 18 July 19 July 19 July 19 July 19 July 19 July 19
Time 10:55 11:25 11:30 11:45 11:47 12:45 1:47 1:05 1250 1:oo 1130 .. . 1:35 2:lO 3 :oo 3:35 2:OO 2:08 2:35 2:40 3:15 3:35 9:45 10:EQ 1025 11:08 11:lQ 11:45 ~
air mixture per Cc. per PHs hr. min. per cent 170 0 170 0 170 55 0:OS 180 60 0.05 . 180 ' 0 180 0 180 25 0:OS 25 0.05 170 170 0 0:024 170 25 25 0.024 170 0.024 12 170 12 0.024 180 12 0.024 185 12 0.024 165 185 185 25 0:624 185 2.5 0.024 180 12 0.024 200 12 0.024 165 12 0.024 165 0 165 0 165 25 0:013 165 29 0.013 165 50 0,013 50 0.013 170
.. ..
..
..
NHs-air mixture NHs P. p. m. per cent PHa 0 9.74 0 0.3 9.80 0.3 ... 0 9.22 0 ... 0.14 9.22 0.14
... ...
...
HYDROGEN S U L F I D E
si15 9:s
9i:4
Si:,
10.5
0:07
7i:o
0.03 0.03 0.03 0.03
8;:s
...
10:;s
...
9.85 9.95
...
10.3
... ... 9.30 9.80
0:06 0.06 0.03 0.03 0.03
..
0:035 0.04 0.07 0.07
85.8 85.2
..
7415
8615 86.2
84:s
The gas in the holder was analyzed every day in duplicate by oxidation of 2 liter samples with sodium hypochlorite. The hypochlorite solution was strongly acidified with nitric acid and boiled down t o a small volume t o eliminate chlorine. The phosphate was then precipitated with ammonium molybdate and determined volumetrically with N / z o NaOH, taking all the usual precautions in this well-known procedure. The alkali was standardized against a known phosphate solution. We feel confident t h a t the calculated phosphine concentrations given in parts per million in Table I were actually present in the ammonia-air mixture t o =+=IO per cent. The rate of flow of t h e ammonia-air mixture was about go liters a minute. The phosphine-air mixture, less t h a n 0.1 per cent PH3, was fed in a t rates from o t o 60 cc. a minute. During these tests t h e electric current of 1 1 5 t o 140 amperes was never cut off, even momentarily, while phosphine was being admitted. I n connection with these tests, an attempt was made t o determine the phosphine concentration in the ammonia-air mixture directly by application of the nephelometric reagent of Kober and Egerer,' molybdate strychnine solution, for phosphates. By oxidation of a 2-liter sample with hypochlorite we were-able t o detect I part in a million where silver nitrate test paper showed nothing. Quantitative results could hardly be expected. Fair results were obtained nephelometrically2 in t h e analysis of acetylene and gas containing above 0 . 0 2 per cent PHI. The results presented in Table I show t h a t z or 3 parts phosphine in a hundred million undoubtedly reduce the yield. The results are consistent througho u t . The gauze, which had been used in previous 1
I
86:3
Yield HNOs per cent
10: is 9.73 9.80
No.
7615
This was diluted twice during the tests ,with an equal volume of air.
TEST No.
11,
tests, was undoubtedly not in t h e best of condition, a s shown b y efficiencies taken when no phosphine was present. This points t o cumulative poisoning of the platinum. Several nephelometric analyses of the acetylene gas generated from the same lot of carbide used in our experiments with this gas, showed 0 . 0 2 - 0 . 0 3 per cent PH3. These analyses were confirmed by passing 2 3 liters of acetylene through sodium hypochlorite solution in a Friedrich spiral wash bottle in 16 hrs. The phosphate was determined gravimetrically and calculated 0 . 0 2 2 6 per cent PHs. ASsuming a value of 0 . 0 2 per cent PH3 in the acetylene used in the earlier experiments where 0 . 0 2 per cent acetylene in the ammonia-air mixture caused a drop of 2 or 3 per cent in the yield and 0.I per cent a drop of 2 5 or 30 per cent, corresponding t o 4 and 20 parts PH3 per hundred million, respectively, the results with acetylene are accounted for on the basis of phosphine alone. .
60 liters.
TABLEI-EFFECT OF PHOSPHINE Cq. ft. PHa-air Composition of
Vol.
J . A m . Chem. SOC.,37 (1915), 2373. nephelometer was on the Richards principle adapted t o a Du-
* The
boscq colorimeter. This instrument was kindly placed a t our disposal by Dr. W. M. Clark of the Department of Agriculture.
Hydrogen sulfide generated in a Kipp from ferrous sulfide and dilute sulfuric acid was measured into the feed line through a water wash bottle a n d the flowmeter. The results are given in Table I1 and show t h a t H2S has no deleterious effect in concentrations approaching 0 . I per cent. Tests were made on the same gauze used with phosphine. TABGE 11-EFFECT
HYDROGEN SULFIDEON AMMONIA OXIDATION c u . ft. air Intake Gas Yield TEST per AmNHs HzS HNOs No. Time Date hr. peres per cent per cent per cent June 20 130 1 :30 165 0 130 165 1 :46 io: i 2 0 June 20 447 130 160 1:48 June 20 0.02 160 130 June 20 10: 07 448 2 :oo 0.02 130 160 ... 0.04 ... 2:02 June 20 130 155 2:33 June 20 0.04 449 9.66 130 160 June 20 . . . 0.07 ... 2:35 130 160 2:47 Tune 20 9.62 450 0.07 ( a ) N o t corrected for Has04 formed. OF
...
SUMMARY
I-Pure acetylene has no effect on the catalytic action of platinum in oxidizing ammonia t o nitric acid. 11-Hydrogen sulfide in small concentrations has no immediate toxic effect. 111-Phosphine t o the extent of 2 or 3 parts per hundred million in the ammonia-air mixture affects the yield several per cent; 2 0 t o 30 parts are ruinous. IV-It has been shown t h a t phosphine is a sufficiently active poison t o account for previous results obtained with crude acetylene. I t seems certain t h a t the difficulty of oxidizing cyanamide ammonia with high efficiency is due t o its phosphine content. V-It seems probable t h a t phosphine has t o some extent a cumulative action, since the platinum appears t o become less efficient with use. ACKNOWLEDGMENT
The work described in this report was done under the general direction of the chief chemist of t h e Bureau of Mines, Dr. Chas. L. Parsons. BUREAUOF MINES WASHINGTON, D. C.
