LANTS Oxidation of Phosphorus with Steam Concentrated phosphoric acid and relatively pure h y drogen were produced continuously in a small pilot plant by the catalytic oxidation of phosphorus with steam at about 1400' F. Conversion of phosphorus to phosphorus oxides was maintained essentially complete by reactipation of a copper-zirconium pyrophosphate catalyst with metallic copper. The steam requirement was about 12% greater than the stoichiometric requirement for the production of 100% orthophosphoric acid. Graphite blocks joined with a sodium silicate-coke breeze cement as a material of construction satisfactorily resisted attack of condensing phosphorus acids. The process was shown to be technically feasible and offers promise of an inexpensive source of hydrogen to producers of phosphoric acid from elemental phosphorus.
S
*
TUDIES based on the laboratory work ( 7 ) indicated that application of the process for oxidation of phosphorus with steam to plant scale would be economically advantageous because the high energy level of elemental phosphorus would be utilized partially to produce by-product hydrogen. Calculations showed that 0.06 ton of hydrogen, equivalent to 0.35 ton of ammonia, could be made per ton of PzO6 produced as acid. Because of the indicated favorable economics, it was decided to study the process further in a small pilot plant. Although numerous attempts t o carry out the phosphorussteam reaction in laboratory scale equipment had been reported in the literature, detailed information on engineering scale studies was not found. One reference (8) was to a small pilot plant in which an impure arid was produced containing only about 35% phosphorus acids by weight by the reaction of phosphorus and water in the liquid phase at 575" F. and a pressure of 200 ntmospheres. Other references (3-5) stated that the Liljenroth vapor phase process had been studied on a semicommercial scale at Niagara Falls, K, Y.,and had been used for R while in Germany ( 2 ) . The pilot-plant work was directed primarily to a study of the production of pure, concentrated orthophosphoric acid and to the identification of satisfactory materials of construction since these factors appeared to be the ones most critical to the ultimate projection to large scale operation. Also, the catalysts that had given the best results in the laboratory were tested on a pilot plant scale. PILOT PLANT DESIGN
I t was considered that the process would be relatively simple and would consist in passing a mixture of vaporized phosphorus and steam at about 1300' F. through a bed of catalyst and then condensing the product acids. The proposed process differed chiefly from that described by Liljenroth (6) in the condensation step. Where Liljenroth passed hot gases from the catalyst directly into a cooler, the proposed process provided for retention of the hot gases in a vessel (secondary reactor) between the catalyst chamber and cooler; this permitted further oxidation of trivalent phosphorus to the pentavalent state. It was recog-
L. B. HEIN, G. H. MEGAR, AND M. W. STRIPLIN, JK. Tennessee Valley Authority, Wilson D a m , Ala.
nized that this step would result in the exposure of materials of construction to t'he severe corrosive act,ion of condensing phosphorus acids. 'However, this problem was solved by the use of graphite blocks which were joined with a mixture of acidproof cement and coke breeze as the lining for the reactor, The pilot plant reactor was designed for the use of 1 cubic foot of catalyst. Based on small scale studies, it' was expected that this would permit operation in the range of 10 to 20 pounds of phosphorus per hour. Preliminary operation of the pilot plant indicated that with a steam-phosphorus mole ratio of 18:1 which was close to the Btoichiometric requirement ( 16: I ) for orthophosphoric acid (considered desirable because of process economics), the maximum phosphorus f e d rate that would give complete conversion with fresh catalyst was about 10 pounds per hour. Consequently, a feed rat'e of 10 pounds of phosphorus per hour and a steam-phosphorus mole ratio of 18 : 1 was used in most of the subsequent work so that any decrease in the acativit,y of the catalyst could be detected. A carrier gas was not used as in the laboratory work. Calculat,ions showed that undcr these conditions and with inlet phosphorus and steam at about 1000" F. tJheheat evolved from the reaction should maint'ain the cat,alyst bed a t the desired operating temperature of about 1300' F. Catalysts tested were copper deposited on aluminum metaphosphate and copper deposited on zirconium pyrophosphate support materials. Copper was chosen instead of platinum or palladium because of its lover cost and because the laborat,ory work had shown it to be almost as active as the other materials. Although in the laboratory work copper had been removed from the supports during use, whereas the other two materials were not removed, it was believed that the copper could be renewed on the catalyst. in place, and therefore its use would be economically advantageous. DESCRIPTION OF PILOT PLAilT
Figure 1 is a quantitative flow sheet of the pilot plant as finally operated. Figure 2 is a general view of the plant before final change was madc in the secondary reactor and before addition of another entrainment separator, which also served as a cooler. In its final form, the plant consisted of phosphorus displacement tanks; a phosp'iorus vaporizer; a superheater, in which phosphorus vapor and steam were heated to the desired operating temperature; a reaction chamber that contained the catalyst and in Tvhich the reaction between phosphorus and steam took plare; a secondary reactor in which phosphorous acid and phosphine were oxidized to pentavalent phosphorus; and entrainment separators. Feed and Heating System. Phosphorus was fed continuously
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li 0
0
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d
w
02 'X
a
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 2.
Vol. 42, No. 8
Pilot Plant for Production of Phosphoric Acid and Hydrogen from Phosphorus and Steam
from metering tanks by displacement with water; the quantity fed was measured by the changes in level of floats in the tanks. Electrical heaters were used to vaporize the phosphorus and to heat phosphorus vapor and steam because of their simplicity of construction and convenience of control. It was recognized that for large scale operation a large amount of the heat required could be obtained by recovering heat of reaction; additional heat could be supplied through use of an energy source other than electricity. The vaporizer consisted of a section of 6-inch cast-iron pipe 2 feet long, flanged at each end, and heated with electrical strip heaters that were fastened to its outer surface. Liquid phosphorus entered a t one end of the pipe, which was in a horizontal position, and left the other end as a vapor at 750' F. During normal operation at a phosphorus feed rate of 10 pounds per hour, the vaporizer shell was maintained a t a temperature of 1000' F. The heater used for superheating steam and phosphorus vapor is shown in Figure 3. The steam-heating section consisted of a U-shaped section of 0.75-inch A.I.S.I. Type 316 stainless steel pipe that was 19 feet long and was heated by its resistance to flow of current. The temperature of the steam was controlled by regulating the current flow through the pipe wall by means of a rheostat; 13.7 volts were required to give a current flow of 480 amperes. Steam was heated from 270' to 1400' F. in this pipe a t a rate of 26 pounds per hour. The heat transfer coefficient between the pipe wall and steam was calculated to be 15 B.t.u. per hour per square foot per ' F. The pipe that carried phosphorus from the vaporizer entered the bottom of the superheater and passed upward midway between the two straight sections of the U. This 0.75-inch pipe was 8.5 feet long and was made of A.I.S.I. Type 316 stainless steel. Radiation .From the steam pipe heated the phosphorus vapor in the center pipe from about '750' to 1050' F. Reaction Chamber. From the superheater the phosphorus vapor and steam were passed into the reaction chamber where the oxidation of phosphorus took place in the presence of a catalyst. At the usual feed rate of 10 pounds of phosphorus per hour and a steam-phosphorus ratio of 18: 1, the space velocity
was 540 volumes of phosphorus and steam (standard temperature and pressure) per volume of catalyst per hour. The metal shell of the reaction chamber (Figure 4) was 5 feet 5 inches high by 8 inches in diameter and was constructed of two sections of S/l&ch A.I.S.I. Type 316 stainless steel. The 1 cubic foot of catalyst was supported on an A.I.S.I. Type 316 stainless steel plate, which was inch thick and contained 132 holes of 6/le-inch diameter. The catalyst bed was 59 inches deep and there was a 6-inch space above it. Each section of the chamber contained a 1-inch monolithic lining prepared from a mixture of 6.2 parts by weight of alundum RA-563 (the Norton Company) and 1 part 787, electric-furnace orthophosphoric acid. I n lining the shell the alundum-acid mixture was tamped into place and then fired a t 1800' F. for 1 hour. The shell was made of two short sections which could be fired in an available Globar furnace. When assembled, asbestos gaskets were used in the flanges. The alundum-phosphoric acid mixture was used for lining the reaction chamber because it had been found to be the most suitable from the standpoint of strength, resistance to cracking, and ease of installation. The lining remained in service for the entire period of operation (1438 hours) and was in excellent condition when the plant was dismantled. The catalyst support plate and thermocouple wells gave satisfactory service. However, they had been attacked and were pitted. Secondary Reactor. From the reaction chamber the hot gases were passed through a connecting pipe (Figure 4) into the secondary reactor where retention time was imposed at elevated temperatures for the oxidation of the phosphorus of the phosphorous acid and the phosphine to the pentavalent state. After preliminary tests of various materials under conditions somewhat similar to those expected in the secondary reactor and testing of some of the more suitable materials as linings in the reactor, a satisfactory lining was found. This reactor, as finally developed (Figure 5 ) , consisted of a stainless steel shell lined with graphite blocks which were joined with a 50:50 mixture of a silicate base acidproof cement and coke breeze to give a 2-inch lining on the sides and top. The
August 1950
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1619
lining in the bottom was somewhat thicker; a large circular THERMOCOUPLE section of it was in one piece and extended into the acid leg to STEAM INLET eliminate joints in this area, The graphite blocks were not STEAM OUTLET ELECTRICAL CONNECTORS, attacked noticeably by process materials during 1012 hours of ELECTRICAL CONNECTORS, STAINLESS S T E E L service; the joint compound showed some attack but was conWELDED TO PIPE sidered satisfactory since inspection after use indicated that a 10inch thickness would give satisfactory service for at least 1 year. ROCK WOOL INSULATION Operation of the secondary reactor as an empty unit a t a gas I”THlCK COVERED WITH /ASBESTOS CLOTH. inlet temperature of about 700’ F. and a gas outlet temperature of 550” F. resulted in a total product acid (reactor and en’ (100 X trainment separator) containing 12.3 mole % moles HaPO3/moles Hap08 plus moles HaP04). About 50% 0 I of the total acid produced was collected from the secondary b reactor. As the proportion of acid collected from the secondary reactor increased, the Hap03 content of the total acid decreased. In an attempt to increase further the proportion of acid collected from the secondary reactor, carbon Raschig rings were placed in PIPE,A.I.S.I., TYPE 3 1 6 the unit; 4-inch Raschig rings increased the proportion of acid STAINLESS STEEL collected in the secondary reactor to 85%, and the HaP03content TRICAL JUMPER, of the total acid decreased t o 4.9 mole 70. Replacing the 4inch NLESSSTEEL Raschig rings with 1-inch rings further increased the proportion DED TO PIPE of acid collected in the secondary reactor to 97%] and the HaPOa LATINC BRICK content of the total acid decreased to 1.5 mole %. During these tests the PHs content of the hydrogen varied between 1 and 2% by volume and had no apparent relation to the Hap03 content of the acid. Data for operation of the secondary reactor under the various conditions are given in Table I. The effects of temperature of operation of the secondary reactor Figure 3. Superheater for Steam and Phosphorus on impurities were not investigated in detail because the acid product was sufficiently low in HaP03 to be used in fertilizer manufacture (7), and even though the PHs content of the hydrometal. The concentration of phosphorus in the product acid gen might have been decreased, i t was believed that the gas still was equivalent to 41.1% HsP04 by weight, and 19.4 mole % ’ of would require purification before it would be suitable for use in this phosphorus was present as HaP03. the manufacture of ammonia. ’Investigation of the effects of Entrainment Separators. The gases from the secondary reachigher temperature in the secondary reactor also might have tor were passed successively through two cylindrical stainless greatly shortened the life of the reactor by increasing corrosion, steel vessels that were packed with 0.5-inch carbon Raschig rings; which would have brought about a premature end to the pilot the first of these was cooled with an external water spray. Enplant studies. trained acid that was separated from the gas in these units was However, in a short test near the end of the work the secondary removed through acid legs. From the second vessel, the by-prodreactor was insulated t o effect an increase in the temperature of uct hydrogen passed through a red brass exhaust pipe to the atoperation of about 300” F. This resulted in a further decrease of mosphere. H3P03in the total acid to about 0.5 mole % and a decrease in the PREPARATION OF CATALYSTS PHa content of the by-product hydrogen to about 0.2%. The concentration of the acid from the secondary reactor was Three catalysts were tested: copper deposited on supports of equivalent to 115% H ~ P O as I compared to 108% for the acid ob(1) zirconium pyrophosphate, (2) aluminum metaphosphate, tained from this unit before it was insulated. This test showed and (3) a mixture of alundum and phosphoric acid. The raw that increasing the operating temperature of the reactor increased materials used in preparing the supports were 78% TVA elecfurther the purity of the products and concentration of the acid. tric-furnace phosphoric acid, Insuloxide ( ZrOz) obtained from The duration of the test was not of sufficient length to give inthe Titanium Alloy Manufacturing Company, alumina prepared formation as to the effect of the higher temperature on the matefrom clay by the Walthall process (IO),and alundum RA-563 rials of construction. obtained from the Norton Company. Because of s e r i o u s corrosion to the secondary reactor during the TABLE I. EFFECTOF PACKING AND INSULATION OF SECONDARY REACTOR ON REACTOR ACID earlier phases of this CONCENTRATION AND IMPURITIES IN PRODUCTS study the pilot plant f ~ Reactor ~ y Graphite-Lined, Packed with Secondary $ ~ ~ ~Secondary operation had t o be Reactor Graphite1-Inch carbon stopped frequently. To I-Inch carbon Raschig rings, as Quench Lined, No 4-Inch carbon Cooler Packing Raschig rings Rasohig rings shell insulated expedite study of the Temperature a t top, F. 830 712 920 1150 catalyst, a simple, unTemperature a t bottom, F. 569 530 2i2 555 930 Reactor acid concn. lined, s t a i n l e s s s t e e l Hap04 by weight (equivalent)a, % 107.6 107.1 115.0 41.1 105.1 reactor was constructed H.PO.. mole qnb 1.4 3.5 19.4 10.0 0.5 T o ; [ LGd concn: which permitted water c HaPOa by weight (equivalent), % 85.1 40.6 94.5 69.3 c HaPOa mole % 1.5 4.9 12.3 19.7 quenching of the process PHa in h$drogen, % by volume 1.1 1.77 1.16 c.33 0,.2 gases. The shell of this Acid recovered from reactor, yo of total 84.5 51.5 97.5 .. reactor was sprayed with Based on total phosphorus present. moles HsPO water on the outside to loo’ % = moles HaPOs + moles HsPOd cool it and thereby deSecondary reaotor was not a t steady-state conditions long enough t o obtain reliable data. crease corrosion of the
u
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 8
TABLE 11. COMPOSITIONS ASD SUMMARY EVALUATIONS OF CATALYSTS TESTED Catalyst Cu-aluminum metaphosphate Before use After use (108 hours) Cu-alundum-phosphoric acid Before use After use (72 hours) Cu-zirconium pyrophosphate Before use After use (1213 hours)
cu
PzOa
AlzOa
0.8 0.5
74.4 77.0
24.4 21.7
1.0 0.8
11.6 26.9
78.6 66.5
0.8 7.2a
49.6 59.4
5.3 4.7
Composition, ZrOz
70 SiOz
Fez08
Total
.. .. ..
0.2 0.1
0.2 0.4
100.0 99.7
7.8 6.6
1.2 0.9
100.2 101.7
42.1 28.2
0.9 0.6
0.3 0.3
99.0 100.4
..
Evaluation High initial activity; poor physical stability (test of duration of activity could not be made) Low initial activity; good physical stability High initial activity; successfully maintained by addition of metallic copper to top of bed while operating; good physical stability
Copper added t o catalyst a t frequent intervals during use.
M E T A L JACKETED THERMOCOUPLE
1NSULATED WITH BLOCK MAGNESIA AND ROCK WOOL BATTING LINING I" THICK OF ALUNDUM CEMENT MIXED WITH PHOSPHORIC ACID
1 I: &
SUPPORT PLATE LINED TO 2"DIA. WITH ALUNDUM-ACID MIXTURE
4"PIPE LINED WITH GRAPHITE TO 2 " DIA.
.r,GRAPHITE TUBE (CORED) TAPERED TO FIT INTO TOP OF SECONDARY REACTOR
Figure 4.
Reaction Chamber and Connecting Pipe
All metal of A.I.S.I. Type 316 stainless steel
In preparing the zirconium pyrophosphate and aluminum metaphosphate supports, the oxides were mixed with 110% of the quantity of 78% orthophosphoric acid required to convert the AlzOaand the ZrOn to aluminum metaphosphate and zirconium pyrophosphate, respectively. To prevent contamination by materials other than aluminum phosphate, which was not objectionable, the mixtures were heated in an aluminum vessel until reaction had occurred. The hard mass was fired to 1800' F. in a Globar furnace for 1 hour, cooled, broken in a jaw crusher, and screened to -1 to +0.5 inch size. The .alundum-phosphoric acid support was prepared by mixing alundum with 3.6% of the quantity of phosphoric acid required to convert all the A1203 to aluminum metaphosphate and extruding the mixture in 1-inch lengths, 3/8 inch in diameter. The extruded material then was fired to 1800' F. for 1.5 hours. This material and procedure were used in an attempt to obtain an aluminum-base catalyst support of greater physical stability than the support made with Walthallprocess alumina, which deteriorated rapidly in use; an alundumphosphoric acid composition had been shown to be stable as lining in the reaction chamber. Copper was added to the fired materials by absorption of copper nitrate solution. The quantity and concentration of the solution were such that drying and subsequent reduction of the copper nitrate resulted in the addition of about 1% copper, based on the total weight of the catalyst.
In evaluating the catalysts, phosphorus conversion was determined by weighing the elemental phosphorus collected from the secondary reactor and from the entrainment separators and calculating from the equation: - elemental Pd output x 100 % Conversion = total inPa puttotal Pq input Accumulation of elemental phosphorus in the system and losses from the stack were determined to be negligible. The copper-aluminum metaphosphate catalysts were tested in a number of short runs and found to be unsatisfactory. The catalyst made with Walthall-process alumina gave relatively high conversion of phosphorus but deteriorated physically to the extent that it was considered impractical for commercial use. The catalyst made with alundum RA-563 was of good physical stability but gave low conversion (below 80%) after only 40 hours of use. The copper-zirconium pyrophosphate catalyst showed good physical stability and gave high phosphorus conversion, which decreased to only 92% after 80 hours of service, so this catalyst was studied in greater detail. Methods of reactivation of the catalyst in place to maintain high conversion efficiency are described later. Chemical analyses of the catalysts before and after use are given in Table 11. The lower P20scontent of the original copper-alundum-phosphoric acid catalyst resulted from the method of preparation. The increase in PzO6 with use was attrib~ alumina. The deuted to the combination of process P z O with crease in percentage of ZrOz in the copper-zirconium pyrophosphate catalysts with use was consistent with the increase in weight of catalyst that occurred during use. The weight increase was shown to have been due to reaction of process PzO6 with the zirconium oxides and retention of about 26% of the metallic copper that had been added for reactivation. The ratio of PZO6 to ZrOz in the original catalyst was approximately equivalent to that of zirconium pyrophosphate, and the ratio in the used catalyst was approximately equivalent to that of zirconium metaphosphate. OPERATING PROCEDURE AND CONTROLS
Start-up and Shutdown. When the pilot plant was to be put into operation, the catalyst in the primary reactor was first heated to a minimum temperature of 1250' F. over a period of 12 to 15 hours by passing the gases from the combustion of carbon monoxide through the bed of catalyst. The carbon monoxide combustion chamber was connected to the reactor through a thermocouple opening in the top of the reaction chamber. After heating, the burner connection was removed and the thermocouple replaced. A temperature of a t least 1250° F. was necessary since a t lower temperatures metaphosphoric acid would condense on the catalyst and cause it to deteriorate rapidly. During the early part of the preheating period, superheated steam a t 1400' F. and atmospheric pressure was used to assist in heating the catalyst. A reducing atmosphere was maintained near the end of the preheating period to make sure that the copper was in the reduced state. The maximum temperature of the gases used in heating was set arbitrarily at 1800' F. to preclude heat damage to the catalyst.
August 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY 2'-2"DIA.
ci I
3" PIPE &FLANGE BATE PIPE SCREWED INTO GRAPHITE LINING BOTTOM LINING TURNED
ACID RECEIVER SHELL: &"THICK
Figure 5.
Secondary Reactor
All metal of A.I.S.I. Type 304 stainless steel
When the desired temperature was reached, the system was purged with nitrogen. When analyses of the purge gas showed less than 0.5% oxygen, phosphorus feed was started. Simultaneously, steam was turned on at a rate to give the desired steamphosphorus ratio. The nitrogen was turned off when it was evident that hydrogen was being evolved. When it became necessary to stop operation, the phosphorus flow was turned off and nitrogen was bled into the vaporizer at a slow rate to prevent air from entering the system. If the interruption was of less than 3 hours, the catalyst bed was kept above the minimum allowable temperature by passing a small amount of superheated steam through the system along with the nitrogen. If operation was suspended for more than a few hours, the system was purged thoroughly with nitrogen and steam. The primary safety rule followed throughout operation of the plant was to maintain a positive pressure within the system a t all times so that no air could be drawn in. Sampling and Control. Gas samples were drawn periodically from the exhaust line through a sample train consisting of two glass U-tubes in series and surrounded by solid carbon dioxide, a small electrostatic precipitator, a glass tube filled with desiccant, a phosphine analyzer consisting of a glass U-tube filled with copper turnings and immersed in a small electric furnace, a wet-test meter, and a water-jet aspirator. Condensate and solids from the cold traps and precipitator were weighed and analyzed for PZOs, PiOa, and Pa. Phosphine concentration was calculated from the change in weight of the copper turnings in the phosphine analyzer and the volume of hydrogen as measured with the wettest meter. Condensate from gas samples (5 cubic feet) was collected in the cold traps and precipitator. Only 1 cubic foot of each gas sample was passed through the PHI analysis tube because of its limited capacity for PHI; the remainder was by-passed. The amount and specific gravity of acid from the reactor and entrainment separators were determined a t regular intervals, and samples of the acid were analyzed for H3PO4, HaPOa, and P4. The analytical procedures were similar to those described in the paper covering the laboratory studies (7). Temperatures were measured throughout the system by welltype thermocouples with stainless steel jackets. I n addition, thermocouples were tack-welded to the metal surface of the superheater piping and vaporizer shells at several points. Pressures in the system were determined with manometers. Phosphorus feed rates were controlled manually by regulating the flow of displacement water. Water flow rates were determined with a rotameter. Steam rates also were controlled manually from orifice meter readings. The temperature of the shell of the phosphorus vaporizer was
1621
controlled manually from recorder readings. The temperature of the phosphorus-steam mixture into the reaction chamber was controlled by use of a temperature indicator-controller which regulated the electric energy to the superheater. Strips of filter paper wetted with silver nitrate or mercuric chloride served as a warning system for the detection of PHa in the pilot plant area. These papers became colored brown and yellow, respectively, within 15 minutes, in dangerous concentrations of PHa (9). I n addition, a hand pump which pulled air over the wetted strips of paper had been calibrated and was used for making rapid tests for dangerous concentrations of PHS. Reactivation of the Catalyst. During continuous operation of the pilot plant with the copper-zirconium pyrophosphate catalyst for 5-day periods, conversion of phosphorus decreased during each period from essentially 100 to 95% in about 75 hours. Conversion could be restored to 100% by the standard purging procedure used for terminating a run and starting another run after the shutdown. It was not determined whether reactivation was caused by the sweeping action of gas over the catalyst or by a chemical action during some phase of the procedure. However, interruption of operation for a 30-minute steaming of the catalyst was not effective as a method of reactivation. It was recognized that loss of copper from the catalyst, as indicated by analysis of the product acid, might have been responsible for decrease in phosphorus conversion. Addition of metallic copper to the top of the catalyst bed was found to be a satisfactory means of reactivation, so this was studied in greater detail. The effect on conversion of the addition of small quantities of metallic copper to the top of the catalyst bed is given in Figure 6. The conditions of operation were a phosphorus feed rate of 10 pounds per hour, a steam-phosphorus mole ratio of 18:1 (space velocity, 540 hour-I), and a reaction chamber temperature of from 1550' F. a t the top to 1450' F. at the bottom. Copper additions (Figure 6) were made by placing Pounce copper bars on the upper surface of the catalyst bed. It is believed that the copper phosphide formed during operation flaked and was distributed over the catalyst. During operation the addition of copper improved the phosphorus conversion. In every instance, the standard shutdown and starting operations were effective in restoring phosphorus conversion to essentially 100%. IO0
95
.a
z z ; a
90
W
z
8 RUN 2 0 T
-11
RUN 21
7
I
F W R I-0Z.BARS OF COPPER WERE ADDED AFTER 16 70 30 MIN, OF STEAMING. b CONVERSIONS ARE AVERAGES OVER FROffl 3 T O 8 HR. PERIODS. I 480 520 500 000 040 680 720 CUMULATIVE LENGTH OF CATALYST SERVICE, HR.
I
Figure 6. Effect of Addition of Metallic Copper to Copper-Zirconium Pyrophosphate Catalyst on Phosphorus Conversion
The effectiveness of metallic copper introduced to the reaction chamber, as shown in Figure 6, indicated that more frequent copper additions would decrease fluctuations in phosphorus conversion and effect a higher average conversion. This was verified in the final test operation.
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Vol. 42, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
attributed chiefly to the cost of raw materials. If the byproduct hydrogen were used in ammonia plants originally designed for natural gas or coke, the ammonia cost would be about the same as that produced from natural gas and about 307, less than that produced from coke. V
STEADY OPERATING CONDITIONS, 2 0 DAYS
94.
i-02 METALLIC COPPER ADDED AT 6HR INTERVALS -
TABLE 111. MATERIALS BALAKCE FOR FINAL TESTOPERATION
POINTS REPRESENT AVERAGE CONVERSION FOR 8-HR PERIODS
(Basis, average for 1 hour of operation) Pounds Reactants Phosphorue Steam Total Products Reactor acid (106.1%HsPOa, 1.2% HaPOs) P4 equivalent Entrainment separator acid (13.3% HaPOa, 5.3% HsP03) P4 equivalent Unconverted phosphorus Off-gas (as HaPO4, HaPOa)
3”
25.54 (8.69) 4.26
(0.184) 0.10
0.02
1.45 0.28 (0.26) 3.63
0.24
35.52
Phosphorus Feed, % ’
...
01.3
...
2.3 1.o
0.2
...
... 2.7 ...
2.5 100.0
SUMMARY
The work reported here resulted in the successful development and operation of pilot plant equipment for production of phosphoric acid and hydrogen by the catalytic oxidation of phosphorus with steam. This process offers promise of an inexpensive source of hydrogen to producers of phosphoric acid from elemental phosphorus. Difficulties reported by earlier investigators were overcome. 1. A low cost catalyst of high activity, identified in the laboratory (7), was tested, and a method for its reactivation-by placing metallic copper on the top of the catalyst bed without interruption of operation-was developed. 2. Phosphoric acid equivalent of 95% HaP04was produced; about 99% of the phosphorus in the acid was present as H3POa and the remaining 1% was present as H3P03. The high purity and concentration resulted from the retention of the hot gases from the catalyst chamber in a packed secondary reactor, which allowed time for completion of the oxidation reactions. 3. Graphite bonded with an acidproof cement-coke breeze composition was found to be a material of construction that would satisfactorily resist the severe corrosiveness of condensing phosphorus acids. ACKNOWLEDGMENT
The authors wish to express appreciation for the advice and encouragement received from T. P. Hignett and for the assistance of J. Ill. Potts, D. W. Rindt, and J. A. Wilbanks in the experimental work. LITERATURE CITED (1) Bear, F. E., “Theory and Practice in Use of Fertilizers,’’ 2nd ed., p. 329, New York, John Wiley & Sons, Inc., 1938. (2) Bushmakin, I. N., Ruisakov, M. V., and Frost, A. V., J . A p p l i e d Chem. (U.S.S.R.),6, 577-87 (1933). (3) Chem. & M e t . Eng., 33, 378 (1926). (4) Cottrell, E’. G., “Proceedings of International Conference on Bituminous Coal,” pp. 584-90, Pittsburgh, Carnegie Institute of Technology, 1926. (5) Hackspill, L., Chimie & industrie, 25, 1058-63 (1931). (6) Liljenroth, F. G., E.S. Patent KO.1,594,372 (Aua. 3, 1926). (7) Shultn, J. F., et al., IND. EXG.CHEX.,42, 1608 (1950). (8) Volkov, V. L., and Ginstling, A. M., J . Chem. Ind. (U.S.S.R.), 13, 905-11, 987-91 (1936). (9) Von Oettingen, W. F., U . S. Pub. Health Service, Suppl. Pub. Health Repts., 203 (December 1947). (10) Waithall, J. H., Miller, P., and Striplin, M. hf., Jr., Trans. A m . Inst. Chem. Engrs., 41, 53-140 (1945). RECEIVED April 7, 1950.
