A HIGH TEMPERATURE
VI!'
18
INt4D~USTRlAL
AND ENGINEERING CHEMISTRY
SUPERPHOSPHORIC ACID PLANT H. Y. F. E. J. A. J. P.
ALLGOOD LANCASTER, JR. McCOLLUM SIMPSON
The effects of the main operating variables in various pieces of equipment have been analyzed while the plant was being operated under routine and special tests conditions. The plant burns elemental phosphorus to produce ortho- and superphosphoric acids at concentrations equivalent to
85 to 115% H3P04. Operating conditions can be easily changed to produce acid of the desired concentration. or several years TVA has operated thermal phosacid units of carbon and graphite construction to produce 75 to 90% phosphoric acid from elemental phosphorus for experimental fertilizer production (7). Pilot-plant studies showed that more highly concentrated phosphoric acids could be easily produced (70). T o test these findings on a plant scale, one of the old units was modified in 1956 so that dilute acid could be added in the hydrator and more heat could be removed from the product acid by use of an external heat exchanger. With these changes, the acid produced contained about 76% PZOEor the equivalent of 105% (8). Studies of the properties and potential uses of this “superphosphoric” acid in the fertilizer field (5, 6, 9) showed that the acid could be ammoniated to produce liquid fertilizer base solutions, such as 10-34-0 and 1137-0, as well as a 12-40-0 suspension. The polyphosphates in this acid or its base solutions sequester solids and thereby make possible the production of clear mixed fertilizer solutions, such as 10-5-5, 8-16-8, and 5-15-10, and the production of high-analysis fertilizer suspensions, such as 15-15-15, 10-30-10, and 9-18-27. Sequestration also permits the incorporation of more secondary nutrients and micronutrients, such as salts of iron, zinc, manganese, and molybdenum. With this acid, more concentrated solid fertilizers such as high-analysis superphosphate (0-54-0) and ammonium polyphosphate (15-61-0) can be produced (3, 4). Tests showed that the solids in wet-process phosphoric acid can be sequestered by the addition of 1 5 to 50% of superphosphoric acid or its ammoniated products ( 7 7). These developments were well received in the fertilizer industry (7). Therefore, TVA decided to produce enough superphosphoric acid to demonstrate the feasibility of the process and to provide acid and ammoniated
Fphoric
fluid fertilizers for evaluation and testing. Accordingly, in 1961 about 52,000 tons of this acid was produced in the old modified plants, but the higher temperatures required for making superphosphoric acid caused more rapid deterioration of the cemented joints between the graphite blocks, and this resulted in loss of acid and excessive maintenance. T o avoid such maintenance and to decrease the losses of acid, TVA had a stainless steel unit built for the production of superphosphoric acid. This unit, which went into operation in 1962, was designed to burn 6000 lb. of elemental phosphorus per hour to produce acid containing 76y0 PzO6 (equivalent to 105% H3P04). This plant has demonstrated the ability to produce acid with concentrations as high as 80 to 8370 P z O (equiva~ lent to 110 to 115y0 HsP04). Major objectives of the new plant were to lower maintenance expense, to increase efficiency of acid recovery, to produce a large part of the superphosphoric acid needed in TVA’s experimental fertilizer program, and to provide design and operational information about this type of plant for the industry. From a brief outline of essential features of the plant and the process, this paper moves to a more detailed discussion of operating experience-the problems, effects of operating variables, and operating indexes. More comprehensive information on the plant, particularly on the considerations underlying the design, will be presented in a bulletin to be published by TVA.
DESCRIPTION OF PROCESS FLOWS AND EQUIPMENT Production of phosphoric acid by the thermal process involves the primary steps of oxidizing (burning) elemental phosphorus to produce PzOh, hydrating the PZOSwith dilute acid or water to produce phosphoric acid, and recovering the acid from the vapor stream. In the stainless steel unit, cooling of equipment in contact with hot process gases and acid streams is very essential to good service life. Figure 1 is a perspective sketch of the plant. Mainly the plant is comprised of two parallel combustion chambers, a hydrator, a venturi scrubber, and a separator tower. Two combustion chambers were provided to permit continued plant operation at reduced capacity if one should fail for any reason. Operating experience has shown that a single, larger combustion chamber should be feasible, and this would be recommended for any future plant. Auxiliary equipment consists of systems for storing and feeding phosphorus, filtering and compressing combustion air, controlling atomizing air, VOL. 5 9
NO. 6 J U N E 1 9 6 7
19
cooling and recycling product acid, collecting and distributing dilute acid, treating cooling water, and storing and pumping product acid. Supply or service connections available from outside the plant are liquid phosphorus, compressed air for atomizing phosphorus and the operating pneumatic instruments, filtered water for hydrating P206, process water for cooling the equipment, steam for heating the phosphorus and the water in phosphorus pipe jackets, aqua ammonia for neutraliziiig water over the phosphorus supply, and electricity. A small amount of sulfuric acid is used for adjusting the p H of the incoming process cooling water to minimize scaling. All of the equipment exposed to PzO5 vapor and phosphoric acid at elevated temperatures is constructed of Type 316L stainless steel. Piping and jackets that conduct cooling water are made of Type 304 stainless steel. The combustion chambers, the hydrator, the venturi, the bottom third of the separator tower, the ducts connecting these vessels, and the acid piping from the hpdrator to the product acid cooler are fully water jacketed. This stainless steel equipment is exposed to the phosphorus flame and P 2 0 5at temperatures as high as 5000" F. or to heated acid at temperatures up to 365" F. The key to protection of the metal is rapid and continuous removal of heat from the metal to the cooling water. In contrast with an old carbon-graphite plant, the new plant has the following features: construction of stainless steel cooled by treated water in jackets, operation under pressure of 3 to 4.5 lb. p.s.i.g. by forced airflow, operational control by more instrumentation, and capacity to burn 6000 lb. instead of about 3000 lb. phosphorus per hour in process equipment of equal or smaller size.
blower, a flow element, and thence through a 14-in. pipe to the combustion air nozzle at the combustion chamber. The atomized phosphorus discharges into an excess of combustion air and burns at a temperature of 3000" to 5000" F. in the bottom of a vertical combustion chamber that is 6.3 ft. in diameter and 29.3 ft. high. Then the PzO6 vapor, nitrogen, and excess air flow upward, lose heat through the water-cooled shell, and leave at 1200° to 1650" F. through a 3.5-ft.-diameter duct in the conical top. Water vapor present in combustion air reacts with P205 to form polyphosphoric acids, some of which deposit as a protective coating 0.1 to 0.3 in. thick on the walls of a chamber. Thus, the combustion chambers serve for oxidation of phosphorus to PzOb and for removal of heat from the products of combustion. Hydration of PBOS
Gas from the combustion chambers enters the conical top of the hydrator tower that is 9 ft. wide and 29.8 ft. high and flows downward with three streams of acid spray. One spray is dilute acid added at a controlled rate to hydrate the PzOj in the gas stream and yield product phosphoric acid of a desired concentration. The other two acid sprays are cooled product acid for cooling the gas stream. Sprays of cooled product acid also enter below the conical top to form a falling film that cools the vertical walls and the dished bottom of the tower. Acid collects in the bottom of the tower and flows by gravity to a cooler. Gas and acid mist leave through a duct connected to the side near the bottom of the tower. Thus, the hydrator serves for hydration of Pz06, for recovery of about one half of the product acid, and for cooling the gas stream.
Oxidation of Phosphorus
Phosphorus is transferred by a centrifugal pump from a feed tank through hot water-jacketed piping, a control valve, and a rotameter to the atomizing-type burner in the combustion chamber. Rates are between 1.5 and 4.5 g.p.m. Atomizing air from the general plant supply flows at 80 to 120 c.f.p.m. through a condensate trap, a flow control valve, and thence at 9 to 16 p.s.i.g. into the burner shown in Figure 2. This burner comprises a central pipe for phosphorus surrounded by successively larger annular channels for steam, atomizing air, and cooling water. Air passes through a gap between the phosphorus jet and burner tip and atomizes the stream of phosphorus. Combustion air at 3000 to 6000 c.f.p.m. flows through a dust filter, a damper flow controller, a centrifugal
AUTHORS H. Y. Allgood, F. E. Lancaster, Jr., J . A . McCollum, and J . P. Simpson are engineers for the Tennessee Valley Authority, Muscle Shoals, Ala. T h e authors wish to acknowledge that M . M . Striplin, Jr., D.McKnight, B. P. Dana, and S. A . Hardin were primarily responsible for planning and designing the plant. 20
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Recovery of Acid Mist
Gas and acid mist from the hydrator flow into the horizontal venturi scrubber unit that is 15 ft. long. A 9-in.-high dam near the inlet of the unit prevents backflow of dilute acid into the hydrator. The gas passes through a spray of dilute acid added to dissolve any solid acid from the walls and then through a spray of about 250 g.p.m. of dilute acid in the 3- by 3- by 42-in. venturi throat. I t is important to maintain a pressure differential of 35 to 60 in. of water at the venturi to obtain essentially complete recovery of PzOb. Consequently, the venturi hydrates the remaining PzOj vapor, coalesces acid mist, and cools the gas. The gas and acid then flow into the centrifugal section of the separator tow-er where the acid separates. The gas passes upward through an internal duct and enters a spray section. The spray consists of a controlled flow of make-up water and recycled dilute acid. The small amount of moisture in the combustion air and this makeup water provide all the water needed for hydration of P z 0 5 .The gas then flows through a mist eliminator pad made of Type 31 6L stainless steel mesh and into a stack. Dilute acid collected in the bottom of the tower flows by gravity to a dilute acid receiver. In summary, the
separator recovers liquid and mist from the gas, produces dilute acid, and releases heat in the stack gas. Acid and Wahr Systems
Product acid at 450 to 600 g.p.m. and 200' to 350" F. flows by gravity from the hydrator into the main cooler tank that is 9 ft. in diameter and 11.5 ft. high. The cooler tank contains 36 Platecoils with a total area of 1044 sq. ft. Water flowing through these coils and that spmyed-on the outside of the cooler tank cool the hot acid. An agitator unit with two turbine-type impellers is provided in the cooler tank. An improvised auxiliary cooler is used when higher production rates are required. The product is collected in a receiver tank. Recycled cool acid passes through a strainer that removes solid plyphosphoric acid and is pumped to spray nozzles in the hydrator. Dilute acid from the separator flows by gravity to a 1800-gal. receiver tank from which it is pumped to the sprays in the hydrator, the separator tower, and the venturi scrubber. River water is used for cooling. In its untreated state, it contains about 150 p.p.m. of dissolved solids and ,
has a pH of 7 to 8. It is passed through a strainer to remove large solids and then is treated with sulfuric acid to adjust the pH to 5.6 to 6.0. This treatment has prevented the formation of significant scale on the heat transfer surfaces when the temperature of the effluent water has been limited to 120" F.
OPERATION TO PRODUCE ACID OF DIFFERENT CONCENTRATIONS Start-up of the plant is a straightforward operation that involves starting flows of cooling water, acids, make-up water, and air, and then the phosphorus feed. After combustion of phosphorus is started, operators adjust process flows to attain operating conditions within set control limits. Shutdown of the plant involves stopping the flow of phosphorus and then the flow of dilute acid to the hydrator and the make-up water to the separator. Circulation of other streams continues until the combustion chamber and hydrator are sufficiently cooled. Conckntration of acid in the plant, the duration of the shutdown period, and the atmospheric temperature determine whether acid must be drained from equipment to avoid crystallization. When changing from the production of an acid of one concentration to another in the range 60 to 83% Pros content, operators change the dilute acid flow rate to the hydrator to cause the product acid concentration to change about 1 to 2% PzOoper hour until the desired concentration is being made. They also adjust operating conditions to control the temperatures within prescribed limits for a particular concentration of acid. Start-up, shutdown, and change in acid concentration in the new plant can be handled easily and cause no damaging expansion or contraction as was characteristic in the old plants.
OPERATING AND EQUIPMENT PROBLEMS Studies, frequent inspections, and observations have been made to improve the plant and to increase its production capability. Opeding Problems and Methods for Alleviating Them
m
L u*
I-
I" 0.0.
Several times during initial start-up with one combustion chamber in service, malfunction of the atomizing air controller caused a sudden decrease in airflow and a large flow which resulted in the - increase in .phosphorus e .. < I, blowing of the water safety seal at the combustion ;,g$~~,.,: , chamber. Without this seal, the excessive pressure .5 'i.; . 'could have caused severe damage to the thin-walled . , process vessels. Figure 3 shows that a decrease in atomizing airflow has a marked effect on the flow of phosphorus in the atomizing-type phosphorus burner. As the phosphorus burning rate was being gradually increased during plant start-up to produce 76% PPOS acid, the temperature of the acid leaving the hydrator . and cooler soon reached the maximum limits prescribed to avert corrosion in uncooled piping and pumps. So it was necessary to l i i i t the phosphorus burning rate to about 4500 lb. per hour until modifications demibed .L
~
V O L 5 9 NO. 6 J U N E 1 9 6 7
21
r
0
4u
w
w
IW
IZJ
IKI
Atoniziq lir M a 10hmr, C. F.
IW
1w 203
M.
Figure 3. Effectof air rata on thcjow of p h p h in thc atmizingtype p h o s p h burnn
later were made. At the reduced phosphorus burning rate, the pressure differential at the venturi was less than 20 in. of water, and the stack gas carried some acid mist that fell into the plant area. Therefore, a damper was installed upstream from the venturi throat to permit control of the pressure differential in a range of 35 to 60 in. of water to cause the mist to coalesce and to be recovered in the separator tower. Several minor modifications of the plant as originally built were made to increase the phosphorus burning rate from 4500 to 6000 lb. per hour when producing acid of 76% PIOS content. These changes resulted in the plant previously described. They included the installation of a water jacket around the acid pipe between the hydrator and cooler, an external water-cooled gas seal at the cooler, an auxiliary product acid cooler, and a second product acid spray in the top of the hydrator. These changes also permitted the phosphorus burning rate to be increased to 7800 lb. per hour when acid of 80% PIOScontent was being produced. The development of 11-37-0 liquid fertilizer required acid with 75 to 85% of its P,Os in plyphosphate form. Most of the acid produced in the plant has been of 78 to 80% PZOScontent to satisfy this requirement. Because acid of this concentration crystallizes at room temperature, it is kept at 150' to 170' F. in plant storage until used in a fertilizer process. During the first few months, operating experience had shown that several minor changes were needed to overcome operating problems and to produce acid with 83% PZOScontent. It was necessary to install the following equipment in the plant: a strainer in the product acid cooler to retain and to permit dissolution of solid acids, a dilute acid spray to dissolve solids that deposited on the walls upstream from the venturi, and reinforced framework to secure the Platecoils in the turbulent viscous acid in the cooler. Also, the temperature of the acid leaving the hydrator was allowed to increase to about 350" F. and that leaving the cooler to about 250° F. These changes resulted in lower acid viscosity which permitted increased cooling of this acid. Tests had shown that acid containing 78 to 83% P106 22
INDUSTRIAL A N D EN GIN E E R I N G .C H EM1S T R Y
at 250" F. did not excessively corrode uncooled Type 31 6L stainless steel. Water-cooled stainless steel resisted attack by the acid at 350" F. Sludge that has about 30% phosphorus, 8% solids insoluble in benzene, 20% oils and tars, and 42% water gradually accumulates on the surface of the phosphorus in the feed tanks. It must be pumped from the tanks to a phosphorus recovery unit at intervals of 1 to 3 months. Impurities in the phosphorus, atomizing air, and filtered combustion air deposit residue in combustion chambers. This accumulates in the pool of polyphosphoric aeid in the bottoms of the chambers and is removed about once a year. The product acid has less than 0.01% solid impurities. A multipoint conductivity recorder is utilized to indicate any acid leaks into cooling water streams from the jackets and ducts. Leaks can be detected immediately and repairs made. Value of this instrumentation was proved when leakage of acid at a flexible pipe connection was stopped within 5 minutes after it started. Without this conductivity meter the leakage might have continued undetected and resulted in serious acid loss, corrosion, and stream pollution. Equipment Problems
During early operation while acid of 80% Pa05 content was being made, an internal weir for distributing acid for cooling the shell of the hydrator corroded through. This resulted from a stoppage caused by crystallized acid in the weir trough or from a low flow of acid to the trough. To correct this problem, the weir was removed, a water jacket was installed in this area, and four nozzles were inserted to spray a falling film down the inside walls of the hydrator. Failure of the shaft and bearings of one of the combustion air blowers was attributed to impeller imbalance caused by gradual accumulation of dust that passed through the air filter. Further failures were averted by periodically cleaning the blowers, and by doubling the area of the filter unit. While repairs were being made to the blower, the plant was operated at a reduced production rate with the other blower and one combustion chamber. Some equipment did not perform well and was eliminated. Gear-type metering pumps for phosphorus and the graphite rupture disk required with them were replaced with valves to control flow from existing centrifugal pumps in the feed tanks. Diaphragm pumps for metering sulfuric acid were eliminated, and compressed air was used to feed the acid into the cooling water. Carbon Intalox saddle packing was not needed so it was removed from the separator tower. Flexible hose sections for phosphorus and acid were replaced with rigid piping. Screwed fittings in acid piping were replaced with flanges. A rubber-lined pipe in which the sulfuric acid and process cooling water were mixed was replaced with a section of stainless steel pipe. Efforts to repair a crack in a horizontal weld in a combustion chamber resulted in progressive cracking.
Thedore, a 1-ft. band, 6 in. above and below the bad weld, was sawed out of the chamber and replaced by a I-ft. band properly welded in place. Corrosion
Corrosion of equipment has not presented major problem$ in the plant. Audigage measurements taken periodically have shown no apparent change in the thickness of metal in the combustion chamber, hydrators, and separator, or ducts that connect them. However, nome problems have been encountered at other locations. Leakage at welds required the relining of the bottom of the dilute acid receiver. Type 316L stainless steel unwetted by acid but exposed to vapor above the acid has required repairs because of corrosion. This corrosion is attributed partly to trace amounts of fluoride from impurities in phosphorus but mainly to the chloride (about 20 p.p.m.) present in the make-up water. A depmit found above the mist eliminator pad in the separator tower contained 0.01% fluoride and 1.0% chloride. Failure of acid piping and spray nozzles exposed to Pro, vapor at 1200' to 1600" F. inside the hydrator has occurred, but could be prevented by water jacketing. After service periods of 6 to 9 months, the phosphorus burners originally installed had to be repaired because of corrosion at the end of the burner. Mcdiied burners provided with improved cooling have lasted 15 to 18 months. Corrosion in the vapor space of the mild steel phosphorus feed tanks is minimized by keeping the tanks filled with water so that floating particles of phosphorus cannot burn to produce acid.
The percentages of heat removed will vary from those shown when heat removal in the combustion chambers is changed by operating steps described later. Heat flow through a square foot of metal shell of the combustion chambers was 20,000 B.t.u. per hour or about three times the flow through the shell of the hydrator and cooler. Because of the exceptionally high heat removal possible in a combustion chamber, it would seem that the metal surface in a plant could be saved by increasing the area exposed to the hot combustion gas. However, the size of the area must be limited because excessive cooling of the gases condenses polyphosphoric acids and then PIOsto form a thick lining that restricts heat flow through the metal to cooling water. Coatings of polyphosphoric acid are approximately 0.25 in. thick on the inside surfaces of the combustion chambers and as much as 0.75 in. thick in the ducts at the inlet of the hydrator. Thickness of the coating may increase with an increase in water vapor content in atomizing and combustion air, with a decrease in the temperature of the combustion products, and with a decrease in the temperature of the cooling water in the jackets. Water vapor in the air has ranged from 2 to 22 lb. per 100 lb. of phosphorus. Oxidation of Phosphorus
Figure 4 shows that increasing the phosphorus burning rate in a combustion chamber increases the temperature of the effluent gas, the heat removal per square foot, the total heat removal, and the overall heat transfer coefficient. The percentage of the beat input removed in the combustion chamber decreases with an increase
EFFECTS O F OPERATING VARIABLES Ha)R m o w l by Unih of Cho Plant
When the plant is being operated with any particular phosphorus feed rate and percentage of excess combustion air, the same percentage of the total heat input that is removed in any unit of the plant remains about the same regardless of the concentration of acid being made. However, when the concentration of the acid being made is decreased, the gas leaving the hydrator and separator removes a slightly higher percentage of the heat input because it carries out more heat in the water vapor. The following tabulation shows the typical heat removal in main components of the plant while phosphorus was being burned at rates between 3500 and 4300 lb. per
hnu:
Heat h U a i in
Coonbustionchamh Hydrator Roduet acid cmler Venturi scrubber Separatortowerbottom Gar leaving separator
by. Wow, Totd Sq. Ft. B.t.u./Hr. 1311 26,500,000 903 6,700,000 1300 9,100,000 157 400,000 396 900,000 2,400,000
. ..
cnruya of
B.t.u.1
Wr.1
InpuI
(Sq. Ft.)
57.4 14.7 19.7 0.8
20,OOO
2.0 5.4
7,400 7,000 2,300 2,400
. ..
fhsphwur )uln!q h,u. PJ HK.
Figure 4. Efftct of phosphorus burning rata ana' exctss air on h a t removal from n combustion chamber V O L 5 9 NO. 6 J U N E 1 9 6 7
23
in burning rate. With a lower percentage of excess air, the heat removal and gas temperature also increase, but the overall heat transfer coefficient remains about the same. At a constant phosphorus burning rate, the heat removal from the combustion chamber may be increased by lowering the percentage of excess combustion air to increase the temperature of the combustion products. An increase in the temperature of effluent cooling water to the maximum allowable limit decreases the thickness of the acid film in the chamber and also increases heat removal. If these steps do not sufficiently decrease heat in the effluent gas, as an operational control the phosphorus burning rate must be decreased. Metal walls near the bottom of the combustion chamber apparently have not been attacked by the phosphorus flame and gas at temperatures as high as 5180' F. In tests of about 2-hour duration, phosphorus was burned at a rate of about 5000 lb. per hour in one chamber and the temperature of gas leaving was 1800° to 1840' F. No apparent attack occurred on the walls of the chamber. These tests indicate that burning rates in a combustion chamber might be safely raised, but steam might have to be added at a controlled rate to provide a protective coating of polyphosphoric acid to avert corrosive attack by the gas. Hydration of P,Os
Cooled product acid sprayed into the top of the hydrator cools the hot gas stream and controls the temperature of the gas leaving. The dilute acid sprayed into the top hydrates the PeOa and controls the concentration of product acid. The falling film of acid on the inner wall is cooled by jacket water, and an increase in its flow rate lowen the overall temperature of acid leaving the hydrator: These flows are the main ones used to control the operation of the hydrator. The heat removed by the cooling water and by the product acid generally increases with the temperature, heat content, and mass flow of the inlet gas. Acid concentrations or acid flow rate within ranges used seem to have no apparent significant effect on PzOl removal. The hydrator recovers from 40 to 70% (average of about 55%) of the PnOs fmm the process gas stream. The remainder of the PzOa is recovered effectively in the venturi as described in the following section. Evidently the gas and acid 'spray do not intimately mix and come to equilibrium as they flow through this large (9-ft. diameter) tower. As shown in Figure 5, the dilute acid in the scrubberseparator system increases in P,06 content as the PzOs content of the product acid is increased. An increase in acid production rate or in acid concentration causes a rise in the temperature of the dilute acid.
70 (96.71
7
65 (89.81
60 (82.91
-t-t
3 55 (76.01 s
I
35 (48.31
30 (41.41 25 134.5) (82.9)
60
(89.8) 65
(96.7) 70
(103.6) 75
(110.5)
80
?r&a Add Cornrntrotion, X Q, 0 ,
Figure 5. Effeci of cowenfration of acid being produced on cowenfra&on of dilutu acid in tha snubber-separator system
-4
I
30
$1
sa 5
5:
5
IO
d
5
R r o v w y of Acid Mist
In the venturi scrubber, the PzOsladen gas at 550" F. comes in intimate contact with sprays of relatively cool dilute acid and expands because of a decrease in pressure of 35 to 60 in. of water. Evaporation of 24
INDUSTRIAL A N D ENGINEERING CHEMISTRY
0 20
30
40
50
60
hnrurs Differordial krmr Venturi, In. H,O
Figure 6. Typical effect of gas pressure &ffmential at tha wntun on PtOa lossin gas to xtwk and on gar cooling
water from the acid and expansion of the gas quickly cool the gas stream to the temperature of the acid. The P206 vapor is hydrated, and the acid mist coalesces into droplets that separate as a liquid. The venturiseparator tower unit recovers more than 98% of the P2Os in the gas when the gas pressure differential across the venturi throat is less than 20 in. of water. However, a higher pressure differential, between 35 and 60 in. of water, must be imposed to recover 99.9yo of the Pzos from the gas. Figure 6 shows the effect of gas pressure differential on the P2O5 loss in stack gas and on cooling of the gas. At a differential of 35 in. of water or higher, the gas stream is actually cooled a few degrees below the temperature of the acid fed to the venturi. The rate of acid flow to the venturi throat also affects the differential as shown in Table I. The centrifugal separator, spray section, and mist eliminator in the separator tower effectively separate and recover acid from the gas leaving the venturi. Cooling of the Product Acid
In the product acid cooler, the agitator rapidly mixes the 450 to 600 g.p.m. of incoming acid at temperatures between 200" and 350" F. with the 3700 gal. of acid in the tank. The temperature of acid in the tank varies no more than =k4" F. throughout the tank. Generally, overall heat transfer coefficients between the acid and the water in the Platecoils decrease when the concentration and viscosity of acid increase. However, the coefficients sometimes vary for acids of the same concentration. Since an increase in temperature significantly decreases the viscosity of an acid, the acid being cooled is controlled at the maximum temperature permissible'without excessive corrosion. Ranges of the heat transfer rates measured for acids of different P 2 0 5content are tabulated below. When cooling the acids of high P205 content,
TABLE I.
Heat Transfer Overall Coefficient B.t.u./Hr. B.t.u./(Hr.) (Sq.Ft.)(O F.) (Sq. Ft.) I
% pzos
Acid O
65 76 80 83
F.
180 200 250 250
120-140 67-95 41-113 40-55
5000-10,000 7000-9000 5000-12,000 5000-7000
polyphosphoric acids may crystallize on the cool Platecoils and decrease the heat transfer rate. Reduced water rates and resultant high effluent water temperatures minimize crystallization of polyphosphoric acid on the surface of the Platecoils and cause a substantial increase in efficiency. At a temperature of 250' F. product acid containing 80 to 83% P205 will not significantly corrode the Platecoils even if the temperature of the outlet cooling water is allowed to rise above 120" F. and solids deposit to restrict heat transfer. At times the effluent water has been permitted to reach 190' F. Water leaving at a temperature of 189' F. permitted the high transfer rate of 12,000 B.t.u. per hour per sq. ft. for the acid containing 80% P20s. Permissible Phosphorus Burning Rates
Table I1 shows the maximum permissible acid temperatures and phosphorus burning rates for the plant when it is producing acids of different concentrations. Plant operating conditions for attainment of these rates are as follows: 20% excess air for phosphorus combustion; cooled product acid flow to two sprays in the top of the hydrator and a total flow of 550 to 600 g.p.m. of acid from the hydrator to the coolers; and the use of the main and auxiliary cooler with effluent cooling water at temperatures up to 150 F. Higher phosphorus burning rates than those shown
EFFECT O F D I L U T E A C I D FLOW O N GAS PRESSURE D I F F E R E N T I A L AND O U T L E T T E M P E R A T U R E A T T H E VENTURI"
Dilute Acid Temp.,
Flow, G.P.M.
225 169 113 56 0 a Operating conditions: venturi throat.
F.
161 163 163 162
... phosphorus burning rate, 7100 lb./hr.;
Pressure at Spray Inlet, P.S.I.G.
Pressure Daperential across Venturi, In. Water
95 65 40 12 0
55 46 38 22 12
-.
Gas at Venturi, O F. Inlet
O U l:let
540 540 520 530
162 168 165 182 280+
...
excess air, 18%; PzOd i n product acid, 80.1 7 0 Hap04 in dilute acid, 74.8%;
and damper removed from
TABLE II. M A X I M U M PERMISSIBLE A C I D TEMPERATURES A N D PHOSPHORUS B U R N I N G RATES Temfi. of Acid Leaving, ' F. Phosphorus Burning Rate, Lb./Hr.b
a
Acid Being Produced, % P20P
Hydrator
Cooler
Dilute Acid Receiver
63 (87) 76 (105) 80 (111) 83 (115)
280 280 350 350
200 200 250 250
180 180 200 200
with Cooling Water at 45' F. 85' F.
7300 6000 7800 3800
7000 6000 7800 3800
Hap04 equivalent in parenthesis. In both combustion chambers.
VOL. 5 9
NO. 6 J U N E 1 9 6 7
25
Figure 7. Op&'ng data on predudion of phorphic add of diffmmt cowntralion
in Table I1 could be permitted if some bottlenedrs listed in the following tabulation were e l i i a t e d :
63
Exear qaeure from wata vapor m praas gas
76
Product add omla cooled produa Inadequate eombuation air Air blowas Exear tmpmamc of dilute Lack ofa dilute add coola acid
80 83
Venturi throat
EXMStmpaahlrc of
OPERATING DATA AND HEAT AND MATERIAL BALANCE
0-w Figure 7 is a flow diagram of the plant with operating data from NXHduring the production of acid of 66, 76, 80, and 83y0 P ~ Ocontent. I When these runs were not made, some modifications previously described been completed, and the plant was being operated near its maximum permissible phosphorus burning rates for the concentration of acid beiig made. Both combustion chambers were beiig operated under essentially the same conditions. The data show the effeets of important operating variables. With increased phosphorus feed rates, the temperaturu of the gas leaving the combustion chambers increased, and the cooling water requirements increased throughout the plant. At the hydrator, the flow of cooled product acid delivered by the pump to the top spray decreased when
had
the density and viscosity of the acids increased. When the plant was producing acid with 66% Pros content,the low temperature and vapor pressure of the recycle cooled acid permitted cooling of the effluent gas to 440' F. at the 7060-lb. phosphorus feed rate. Inadequate flow of cooled acid to the top spray caused the temperature of effluent gas to reach the maximum allowable temperature of 550" F. and limited production of acid containing 79.6% PiOr. I n the gas scrubber and separator section, the concentration of the dilute acid increased from 41 to 91% HaPO, with increasing concentration of product add being made. The acid cooler did not have capacity to cool acid of 76% PIOI content to 200" F. at the full phosphorus burning rate, and thii limited the production rate of this acid. Cooling limitations for dilute acid limited the production rate of 83% Pro, acid. Heal and Mokrial hlanco
Figure 8 gives heat and material balance data for production of acid of 79% Pros content at a phosphorus burning rate of 5300 lb. per hour. Combustion of phosphorus accounts for 90% of the heat input, hydration of Pro, for 8.5y0, and ddution of acid in the venturi and scrubber for 1.5%. Of the heat output, 54% is removed in the combustion chambers, 18% in the hydrator, 17%' in the cooler, 8% in the scrubberseparator, and 2% in product acid to storage. The percentages of heat removed by water in different jackets of a combustion chamber vary widely, but are
usually significantly higher in the bottom than in the top jacket. In this test, no dilute acid was sprayed into the separator tower. Tests had shown that this spray had no significant effect on P205 recovery in the scrubberseparator system. Percentages of material and heat accounted for were less than 100 mainly because of errors in the measurement of the flows of air and stack gas. OPERATING DELAYS AND INDEXES From May 1962 through May 1966 the acid plant was operated 88.5y0of calendar time.. Most of the operating delays have been caused by factors outside the plant, such as lack of phosphorus supply, failure of power or water supply, the necessity for removing phosphorus sludge from feed tanks, or acid production not required ; these factors accounted for 6% of the time. Scheduled inspections and modifications of the plant accounted for about 3%. The sum of delays for equipment repair was about 38 days or less than 3% of the time. During the past year, the plant was operated 92yC of the time, and the average burning rate per operating hour was 5810 lb. of phosphorus.
TABLE Ill. Production
OPERATING INDEXES
Product acid, % Pz05 HaPOdequivalent, yo Phosphorus rate, lb./hr. Acid production, tons/hr.
63.0 87 7000 12.7
76.0 105 6000 9.0
79.6 83.3 115 110 7800 3800 11 , 2 5.2
Indexes/ton of P z 0 6in acid Labor, man-hours Operating Operator Foreman Maintenance Materials for maintenance, $ Cooling water, million gai.5 I n at 45 O F., out at 120" F. In at 65 O F., out at 120 F. I n at 85 O F., out at 120" F. Make-up water, gal. Combustion air, million cu. ft. Atomizing air, million cu. ft Electricity, kw.-hr. Electricity for air blowers, kw.-hr. Steam, lb.
.
0.12 0.04 0.11
0.14 0.04 0.13
0.13 0.04 0.11
0.22 0.06 0.21
0.34
0.40
0.34
0.62
EVALUATION The stainless steel thermal phosphoric acid plant described in this paper operates smoothly to produce acid containing 60 to 83y0 P z O ~(equivalent to 83 to 115% H3P04). Only minor adjustments in the operating conditions are required to vary the concentration of the acid. About 99.9yc of the phosphorus burned is recovered as acid. Corrosion of the stainless steel equipment has been negligible and operating and maintenance costs have been reasonable. Operating experience and plant studies have shown ways to avert equipment failures, to achieve optimum operating conditions, and to increase the production capacity. The performance of this plant has demonstrated that acid containing 83ycP205 or the equivalent of 115% H3P04 can be easily produced. However, additional cooling of the dilute acid would be required for the production of more than 5 tons of such acid per hour in this plant. With careful operational control and preventive maintenance, this plant should continue to perform well for several years. Information gained from the operation of this plant has been helpful to other producers of phosphoric acid. Design and operating data from this plant have been used by the industry as a basis for the design of several plants of a similar type (2). Product acid is used in the manufacture of other products as well as in fertilizer production. TVA is constructing another stainless steel plant of similar design to replace the remaining older unit.
15
15
15
15
20
20
20
20
32 210
32 130
32 126
32
LITERATURE CITED
90
80
80
80
80
2 56
2 61
2 52
2 80
41 15
44 18
39 14
53 25
(1) Almond, L. H., Steinbiss, H. K., Chem. Eng. 5 5 , 105-9, (October 1948). (2) Com. Fertilizer 119 (3), 2 5 , 28 (1966). (3) Getsinger, J. G., Siegel, M. R., Mann, H . C., Jr., J . Agr. Food Chem. 10, 341-4 (1 962). (4) Phillips, A. B., Young, R. D., Heil, F. G., Norton, M. M., Ibid., 8, 310-15 (1960). (5) Scott, \V. C.; Wilbanks, J. A,, Burns, M. R., Fertilizer S o h . 9, 6-7, 10-11, 14-15 (November-December 1965). (6) Slack, A. V.,Potts, J. M., Shaffer, H . B., Jr., J . Agr. Food Chem. 13, 165-71 (March-April 1965). ( 7 ) Striplin, M. M., Jr., FerliiizerSoln. 4, 32-4 (1962). (8) Striplin, M. M. Jr., McKnight, David, Megar, G. H., J . Agr. Food Chem. 6 , 298-303 (195.3): (9) Striplin, M. M., Jr., Stinson, J. M., Wilbanks, J. A . , Ibid., 7, 623-8 (1959). (10) Walthall, J. H., Striplin, M. M., Jr., IND. ENG.CHEH.33, 995-1000 (1941). (11) Wilbanks, J. A., Nason, M . C . , Scott, W. C., J . Agr. Food Chem. 9, 174-8 (1961).
4 Approximateb 2.7 gal. of Q3'7 sulfuric acid is added per 700,000 gal. of river water to prevent scale formation. Water f o/cooizng o i l at blowers is approximately 10 gal./min.
28
Table I11 gives operating indexes per ton of PzOa produced when the plant is being operated at maximum permissible burning rates to produce acid of different concentrations. Man-hours of operating and maintenance labor, of course, decrease as the phosphorus burning rate is increased. Cooling water varies in temperature from about 42" in winter to about 89' F. in summer. Requirements range from 15,000 to 32,000 g.p.m. per ton of P z O in ~ acid. Owing to the method of flow control, the power for the blowers does not significantly change with the combustion air rate. In the past, two combustion chambers have been operated to burn 3800 lb. of phosphorus per hour when 83% P ~ o 5acid was being made. Cost of power for the blowers could be cut in half by operating only one combustion chamber.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
