US3050374A - Phosphorus burner assembly - Google Patents

Description

Aug. 21, 1962 R. B. BURT ETAL PHQSPI-IORUS BURNER ASSEMBLY Filed March 30, 1959 mamofwoza Q INVENTOR fiamm S g United States Patent 3,050,374 PHOSPHURUS BURNER ASSELY Reynold B. Burt, Sheifield, and James C. Barber, l lorence, Ala., assignors to Tennessee Valley Authority, a corporation of the United States Filed Mar. 30, 1959, Ser. No. 803,039 1 Claim. (Cl. 23-277) (Granted under Title 35, US. Code (1952), sec. 266) The invention herein described may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty therefor.

This invention is an improved phosphorus burner assembly. It is particularly useful in the production of phosphorus pentoxide for the manufacture of phosphoric acid, calcium or potassium metaphosphate, and other products formed by the reaction of phosphorus pentoxide and alkali or metal om'des or salts.

In the manufacture of phosphoric acid, metaphosphates, and similar materials, phosphorus is burned to produce phosphorus pentoxide. ignites spontaneously upon exposure to air, it is a very difiicult material to burn completely to form phosphorus pentoxide containing no appreciable proportions of lower oxides of phosphorus. Burners of very special types are required. 7

These burners usually are of extremely complicated design and require the use of air under high pressure of approximately 100 pounds per square inch in order to atomize phosphorus. It is customary to introduce molten phosphorus through a central chamber and through a nozzle at the outlet of such burners. Compressed air flows through a concentric outer chamber and passes through ports so that jets of compressed air strike the phosphorus stream at an angle of about 45 degrees and thereby atomize the molten phosphorus. A steam jacket is provided around the phosphorus chamber to heat the phosphorus and maintain its molten state. It has been necessary to have a water jacket to keep the burner assembly cool and prevent damage from the high temperatures inside a phosphorus combustion chamber.

Only a small proportion of the air required for com plete combustion of phosphorus has been introduced through the burner. It has been customary to add secondary air through ports in a combustion chamber and to mix atomized phosphorus in the combustion chamber by natural turbulence and the combustion that occurs. The efficiency of phosphorus combustion with such conventional burners depends on the degree of atomization imparted to the phosphorus by the small quantity of atomizing air and the relative position of the burner assembly with respect to secondary air ports and the combustion Although phosphorus chamber proper, which govern the subsequent mixing of air and atomized phosphorus by turbulence. These conditions and the capacity limitations inherent in the design of conventional burners contribute to inefficient combustion with the formation of objectionable lower oxides of phosphorus in the phosphoric acid produced.

Some conventional burners, such as that illustrated and described in US. Patent 2,355,080, assigned to the assignee of the present invention, depend on back pressure imposed by atomizing air on the phosphorus feed to regulate the burning rate. With phosphorus of variable qualities, properties such as density, tackiness, viscosity,

water and dirt content, often change rapidly; and this type of flow control permits variations in phosphorus rates which result in ineflicient combustion or loss of heat control.

It is an object of this invention to provide a phosphorus burner assembly which is simple and cheap in construccient combustion of phosphorus.

Another object is to provide such phosphorus burner assembly which permits efficient burning of phosphorus of variable qualities over a wide range of combustion rates under close control.

Another object is to provide such phosphorus burner assembly which eliminates the presence of lower oxides of phosphorus in the phosphorus pentoxide produced.

Another object isto provide such phosphorus burner assembly wherein phosphorus is burned more rapidly and more completely with a shorter flame.

Another object is to providesuch phosphorus burner assembly which will give improved reaction between phosphorus pentoxide and phosphate dust or other solid reactants.

Another object is to provide such phosphorusburner assembly which eliminates the need for careful cleaning of phosphorus before burning. 1

Another object is to provide such phosphorus burner assembly which gives improved control of phosphorus burning rate.

Still another object is to provide such phosphorus burner assembly in which the requirement for high pressure air is eliminated.

Other objects and advantages of our invention will become apparent as this disclosure proceeds.

,We have found that these objects'are attained in a.

phosphorus burner assembly comprising in combination an air duct having an inlet and an outlet, a source of low-pressure air having sufficient capacity to maintain a flow of air through the duct at a velocity in excess of 250 linear feet per second connected to the inlet of the air duct, said air duct being large enough to carry a major I proportion ofall air required for combustion of phosor assembly is maintained above the critical velocity of about 250 feet per second the phosphorus is broken up, or atomized, into tine droplets, which are burned immediately since'no further mixing of phosphorus and air is necessary. Also, we have found that finely divided solid material such as phosphate rock, metals, or alkali salts can be suspended in the air supplied to the burner, and there Will react with the P 0 formed more efiiciently than when they are fed to the reaction chamber through separate devices.

In the attached drawing, FIGURE 1 illustrates diagrammatically a burner assembly according to the principles of our invention disposed for the production of phosphorus pentoxide such as is used frequently in the production of phosphoric acid. FIGURE 2 illustrates a phosphorus burner assembly of similar type disposed for simultaneous production of phosphorus pentoxide and reaction of P 0 produced with phosphate dust or other solid reactants.

In FIGURE 1 the reference numeral .5 designates an air duct preferably ending in a tapered nozzle 6. Nozzle 6 maybe tapered or not, as desired. Duct 5 is disposed through port 7 in any suitable furnace in such manner as to direct a blast of air into combustion chamber 8. The diameters of duct 5 and nozzle 6 are such that air pres sure of approximately 3 pounds per squareinch, or in the ably in the range from 250 to 500 linear feet per second There is no operable upper limit on the velocity of such air other than that imposed by its cost and by undesirable dilution of phosphorus'pentoxide formed.

'Air duct has an inlet connected by any suitable means to a source of low-pressure air 9 having. sufficient capacity to maintain a flow of air through duct 5 at a velocity in excess of 250 linear feet per second. A heated tube 11" is disposed to inject a stream of molten phosphorus into a stream of air flowing in duct 5 at a point within the duct and adjacent to the outlet of nozzle 6. The location of thetip of tube 11 is immaterial within certain limits. it rnust not be too far back in ductS to permit a fiameof burning phosphorus to contact. nozzle 6, which would become overheated and burn off. The tip of tube 11 should not be farther out than the end of nozzle 6, where great turbulence and decreased'velocity occur, but should be within duct 5 where there is little turbulence, and atomization of phosphorus injected is caused by acceleration by air at high velocity and not i by turbulence; I v

We have found that in a burner of this type water the high velocity flow of air within duct 5 performs the cooling function.

Tube 11 is heated preferably means of steam jacket 7 12. The rate of flow of phosphorus may be controlled jackets or other means for cooling are unnecessary, since I preferably .in the range from about 250 to 500 feet per phosphorus. 'We prefer to introduce all the'air required for complete combustion through air duct '5. Air flowing at high velocity in duct 5 atomizes the phosphoruscompletely as it emerges from tube 11. The phosphorus burns completely to phosphorus pentdxide with aavel y shortgflame, thus reducing the;size of furnace required Y #01, conversely, greatly increasing the capacity of a furnace of givensizeh Substantially no lower oxides of phosphorus are formed.

The burner may be operated so that alloronly'part fof'the air required forrcombustion is used for atomiz'mg -the phosphorus, For example, aburner with a normal burning rate'of 2500 pounds per hour with all the'air used as atomizing air may be operated at a rate of 4000 pounds per hour by increasing the rate of flow of phosphorus to the desired extent and supplying additional secondary air for combustion. In this case the air flowing through duct 5 is insufficient for complete combustion of via line 26 and picks up and fluidizes the solid reactant. This-airborne mixture passes through a venturi 27 .and is discharged into an air duct 28 ata point upstream of.

.phorus-air ratios.

burner assembly permits wide latitude in the details of design within the limits of practical air velocity and phos- The phosphorus may be injected into the high velocity. air stream by one or more tubes equivalent to tube Il, which may be. centrally located as illustrated in duct 5, or may enter duct 5 from the outside at any. angle up to 90 degree. Thevelocity at Which the phosphorus isinjected is'not critical except when not centrally located, when it must be sufficient to' penetrate into the air stream but not great enough to pass through the air stream as a single'stream or jet. We have found that 25 to 30 linear feet per secondveloc'ity of phosphorus emerging from a line disposed at an angle to the flow of.

air in duct 5 is suflicient when the air flows at a rate'of approximately 300 linear feet per second.

The simplicity of the principle of the high-velocityburner permits its ready adaptation to processes for reacting phosphorus pentoxide with other materials by injecte ing the desiredreactant into the system by means of the high velocity air'stream. The burner operation imposes no back pressure on the phosphorus feed line. The phosphorus rate may be controlled independently of air volume, thereby permitting close control of the combustionrates. This also reduces the pressure requirements for the feed systemused'. Operation of a given,hi'gh-.

velocity burner is practical over a very wide range of phosphorus-burning rates Withoutsignificant sacrifices in combustion efficiency. FIGURE 2 illustrates an application of velocity burner assembly to the production of calcium metaphosphateor other products from simultaneous combustion of phosphorus and reaction of. phosphorus pentoxide formed with other reactants. ,Therein the refer.-

ence numeral 21 indicates a phosphorus tube heatedby a steam jacket 22. Flow'of phosphorus through this tube is controlled by a valve 23. Phosphaterock dust or other reactant desired is fed from a hopper, not shown, through 7 line 24. Compressed air, from a source not shown, enters the entrance of phosphorus through line 21. g

Low-pressure air, from a source not shown, enters via linef29 into an annular 'chamber31 and passes from p'ressure air andports 33 are ofsuch capacity, that air flows through duct 28 at a velocity in excess of 250 linear phosphorus, but this may be supplemented by secondary admitted through a line 15- and secondary ports 14.

When secondary air and ports are used, the flame is not as short as that'obtained when all air required is admitted through duct- 5; but atomization of'entering phosphorus is complete when a major portion of-the air required for complete combustion of phosphorus flows through 'duct .5

at proper-velocity, The secondary air entering from ports 14 then can be mixed with the atomized phosphorus-air 7 'mixture by turbulence in the furnace combustion cham- The high velocity air stream flowing through duct 5- 'violently accelerates and'disrupts phosphorus into very small droplets. In this finelyatomized condition the phosphorus immediately begins to burn, '-'b'ecausethe' V atomizing force also supplies the oxygen necessary for combustion. The size of the atomized droplets of phosphorus varies inversely withairvelocityl Therefore, rapid combustion is promoted. by high air velocity. We have I found that a minimum air velocity in duct'S in'the range place with the dust'and a smallerproportion inthe tower chamber 31- vto .chamber 3,2 via ports 33.".Chamber'3 2 communicates With-air duct 28. The source of lowfeet per second into a furnace 34. In this arrangement, phosphorus is injected into the high-velocity mixture of phosphate dust and air. Rapid and complete combustion of phosphorus takes place. In addition, intimate mixing and reaction of the P 0 vapor and phosphatedust 'is promoted by the high velocity air stream and by contact vat the point ofmaximum temperature.

We have constructed a burner of. the type shown in FIG-- iURE-l and applied itto the manufacture of calcium "metaphosphate according o the process shown and described in US. Patent 2,589,272. We have foundrth'at l this burner givesbetter contact between phosphorus pentoxide and phosphate dust in the combustion chamber,

and the improvement in reaction gives a metaphosphate product of improved quality with lower citrate-insoluble P 0 than it was possible to produce in the same furnace with conventional separate burners anddust injector; The

improvement in reaction taking place in the combustion a... high- 7 chamber with the combination burner-dust injector i re-' 1 sulted in a larger proportion of the total reaction. taking 7 charge. Use of this type, burner assembly reduced the activity in the furnace tower and the effects of variations 'in operating conditions on trouble in the tower. We have found that the burners arranged as described Weremore suitable for burning phosphoruscontaining sludge, since troubles due to stopping of air orifices-were eliminated.

Such burners are easier to control, simple to construct, and require less maintenance than conventional burners and, as an additional advantage, do not require highpressure air for atomization of phosphorus.

We do not mean to limit ourselves to the use of a single burner in connection with one furnace. Multiple burners often may be desirable and more eficient than use of a single burner.

Many minor modifications of the burner shown will readily suggest themselves to the average engineer concerned with the production of phosphorus pentoxide. Our invention contemplates the use of many minor variations and is limited only by the following claim.

We claim as our invention:

A burner structure for efficiently eliminating the presence of lower oxides of phosphorus in the production of phosphorus pentoxide powders produced by mixing a stream of molten phosphorus and a stream of air which comprises a substantially cylindrical inner-body member having a central duct terminating in a first outlet for the passage of a stream of first reactant therethrough, a substantially cylindrical first outer-body member of larger diameter than said inner-body member and disposed about said inner-body member to define a first annular passageway therebetween and terminating in the form of a first inwardly flared passageway coaxial with and extending from the periphery of said first outer-body member inwardly to juxtaposed relationship with the peripheral terminating surface of said inner-body member, said first flared passageway being in the form of the peripheral surface of a frustum of a cone coinciding at its maximum diameter with said first outer-body member and coinciding at its minimum diameter with said inner-body member, whereby said first defined annular passageway contains a fluid heating medium which circulates therethrough and which medium is restrained from contact with materials to be mixed by such burner structure, a substantially cylindrical second outer-body member of larger diameter than said first outer-body member and disposed about said first outer-body member to define a second annular passageway therebetween and terminating in a second outlet adjacent to said first outlet for the passage of a stream of second reactant therethrough, said second outlet having the form of a second inwardly flared passageway coaxial with and extending inwardly from the periphery of said second outer-body member, said second flared passageway being in the form of the peripheral surface of a frustum of a cone, the largest diameter of which coincides with said second outer-body member, the smallest diameter of which is substantially equal to the outer diameter of said first outer-body member, said second flared passageway being coaxial with said first outer-body member, and having an apex angle substantially equal to the apex angle of said first inwardly flared passageway, said burner structure in combination with a furnace having a port in a wall thereof, said burner structure disposed through said port, said first and second outlets in said burner assembly disposed within said furnace, and first and second reactant-supply means disposed outside of said furnace, said first reactant-supply means communicating with said central duct, said second reactant-supply means communicating with said second annular passageway, auxiliary air-supply means in combination with said burner structure, said auxiliary air supply means disposed adjacent to and in communication with said port, means independent of said air-supply means for controlling the flow of first reactant through said central duct, and means in combination with said burner structure for introducing an air-borne solid reactant to said auxiliary air-supply means upstream of said first and second reactant outlets.

References (Iited in the file of this patent UNITED STATES PATENTS 1,524,314 Schaeffer et a1 Jan. 27, 1925 1,575,369 Juillard Mar. 2, 1926 2,725,933 Gaucher Dec. 6, 1955 2,767,785 Eastman Oct. 23, 1956 2,763,749 Greco Dec. 9, 1958

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