488
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 488-495
Apparently the hydrogen bonds will only form when the methyl groups are at specific locations on the benzene ring and pseudocumene and o-xylene possess the required structure (Welling, 1982). A future publication will present and discuss the results of the X-ray study. Literature Cited Angla, B. Ann. Chim. (Paris) 1948, 4(12), 639. Fetterly, L. C. I n “Non-Stoichlometric Compounds”; L. Mandiecorn, Ed.; Academic Press: New York, 1964; Chapter 10. Fuller. E. J. US. Patent 3684701, 1972. Gorton, P. J. M.S. Thesis in Chemical Engineering, Montana State University, Bozeman, MT, 1980. Love, R. M.; Pfenning, R. F. A&. Chem. Ser. 1951, No. 5 .
McCandless, F. P.; Cline, R. E.; Cioninger. M. 0. Ind. Eng. Chem. Prod. Res. D ~ V .i m , 13, 214. McCandless, F. P. I d . Eng. Chem. Rod. Res. D e v . 1980. 19. 612. Rony, P. R. Sep. Scl. 1868. 3, 239. Weast, R. C., Ed. “Handbook of Chemistry and Physics”, 57th ed.; CRC Press: Cleveland, Ohio, 1976. Welling, R. M.S. Thesis in Chemical Engineering, Montana State University, Bozeman, MT, 1982.
Received for review October 22, 1981 Accepted April 5, 1982
This material is based upon work supported by the National Science Foundation under Grant No. ENG 78-10035.
Production of Ammonium Polyphosphate Suspension Fertilizer. Phase I Horace C. Mann,” Kenneth E. McGlll, and Thomas M. Jones Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Alabama 35660
TVA is developing means for producing ammonium polyphosphate (APP) base suspensions by ammoniation of merchant-grade wet-process orthophosphoric acid. I n the process, acid derived from Florida rock is preheated and liquid ammonia is vaporized by use of process heat. The acid is then ammoniated in a pipe reactor to produce APP melt which is dissolved in water to give liquid with a pH of 6. The liquid is cooled in an evaporative-type cooler, and 2% clay is incorporated to yield a base suspension of nominal 9-32-0grade that contains about 25% of the P,05 as poiyphosphate. The product is free of crystals at 27 O C , has good physical properties, and is pourable at temperatures down to -26 OC. The process can be relrofitted with a minimum of equipment changes into existing pipe-reactor liquid fertilizer facilities that currently use superphosphoric acid.
There continues to be substantial growth in the use of fertilizers in fluid form. The latest available statistics show that in 1980,35% of all U.S. fertilizer was applied as fluids, including anhydrous ammonia and nitrogen solutions. Of these fluids, a substantial 26% consisted of two- or three-component mixtures in either solution or suspension form, while the remaining 74% were nitrogen liquids, including anhydrous ammonia. Production of solution mixes usually involves production of 10-34-0 base solution in a TVA-type pipe reactor, using low-conversion superphosphoric acid as the source of P205. Use of superphosphoric acid is required to achieve the polyphosphate level (60-75% of P206)considered necessary for high solubility and good storage properties of the product mixtures. Suspension fertilizers are made either by the same route or by use of solid diammonium phosphate (DAP), monoammonium phosphate (MAP),or ammonium polyphosphate (APP) as the source of Pz05. In attempt to lower the cost of phosphate in fluid fertilizers, TVA has continually explored methods for using relatively inexpensive merchant-grade orthophosphoric acid as the source of P205in these fluids. Considerable success has been achieved since 1976 when TVA began demonstration-scaleproduction and distribution of 13-38-0 grade orthophosphate base suspension made directly from merchant-grade acid. This material, which TVA still is producing, has good handling and shipping properties, with one exception-the solidification temperature (-9 to -7 “C) is somewhat too high to allow winter handling in northern locations. Another limitation of that product has been that the production process is not readily adaptable to use in
typical, small fluid-fertilizer plants, but rather is intended for use in larger regional plants. Now, however, TVA has developed and is ready to demonstrate a new process that utilizes merchant-grade acid without these drawbacks. In this process, a polyphosphate base suspension, instead of an orthophosphate suspension, is produced directly from low-cost merchant-grade acid. Because of the presence of the polyphosphate, which will be in the range of 25 to 35% of the PzOs, the suspension will have good low-temperature storage properties and should be usable in all sections of the United States. Additionally, the process is highly energy efficient and requires no external heat; this would be important to the small manufacturer who normally does not have a source of steam. Further, the process should be readily applicable to use in typical existing pipe-reactor fluid fertilizer plants, of which there are now an estimated 130 to 150 in the United States. Retrofitting the process into an existing plant in which 10-34-0now is made from superphosphoric acid would require some additional equipment, but it would provide a liquid manufacturer the flexibility of making in the same plant both a premium 10-34-0 liquid from superphosphoric acid and base suspension from a less expensive, readily available acid.
The Process Overall development of the process has been divided into two phases. Phase I, which is the subject of the present paper, involves, as a fiist step, the pipe-reador production of an APP solution with grade limited to about 9-32-0. This limitation in grade ensures that the freshly made solution, even when cooled, will not contain crystals;
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 489 VENT
M E R C H A N T - GRADE WET-PROCESS ACI APP SUSPENSION 32-0.2% CLAY % POLY L E V E L 1
Figure 1. Pilot-plant apparatus for production of ammonium polyphosphate suspensions.
cooling, therefore, can be carried out in solution-type coolers, which are the type predominantly used in existing fertilizer plants that produce liquids from superphosphoric acid. The suspension product then is made by adding clay (2% by wt) to the solution and dispersing the clay by circulation of the mixture through a centrifugal pump. The purpose of clay addition is to suspend ammonium phosphate crystals or impurities that may precipitate during low-temperature or long-term storage and also to provide extra clay for cold blending with other ingredients to make grades such as 18-18-0 and 5-15-25. Demonstration-scale plant production of 9-32-0 suspension at TVA is scheduled for the fall of 1981. Construction of an 18 t / h (18-metric-ton-per-hour) demonstration-scale plant now is essentially complete. Phase I1 in the overall development of the process involves study of a modification of the process for production of the highest grade of satisfactory suspension possible. In that modification, crystals of suitably small size purposely will be produced in the cooling step. Therefore, special coolers of a type that can handle fluids containing crystals will be required. Pilot-plant studies of this second phase now are in progress and demonstration-scale production of such a suspension is planned for the future. A flowsheet of the TVA pilot plant for producing 9-32-0 grade Phase I suspension containing 25 to 35% of its Pz05 as polyphosphate is shown in Figure 1. The reactants are merchant-grade wet-process acid and anhydrous ammonia. The acid is pumped from a storage vessel to a heat exchanger in which it is heated to 77 to 82 "C with hot 93 "C to 99 "C APP liquid from the melt dissolution tank. Liquid anhydrous ammonia from another storage vessel flows to another heat exchanger in which it is vaporized and heated to about 82 "C with the APP liquid from the first heat exchanger. The hot acid and vaporized ammonia then flow into a mixing tee that is connected to the pipe reactor. The pipe reactor is an inverted U-shaped unit in which the acid is ammoniated and the polyphosphate is formed. The temperature at the discharge end of the vertical section ranges from 232 to 260 "C. Molten APP from the pipe reactor discharges under the surface of the liquid in a melt dissolution tank. In this tank, sufficient water is added to dissolve the melt and to obtain liquid of the desired density. The temperature in the melt dissolution tank generally ranges from 93 to 99 OC and is controlled by varying the amount of cool liquid returned from the evaporative cooler. The hot liquid from the melt dissolution tank is pumped to the top of a packed-tower evaporative cooler and flows down through the packing. Cooling of the liquid to be-
Table I. Chemical and Physical Properties of Nominal 9-32-0 Grade Ammonium Polyphosphate Suspension nominal grade 9-32-0 chemical analysis, % by wt total N 9.1 -9.8 total P,O, 31.I-32.6 available P,O, (% of total P@,) 100 polyphosphate (% of total P,Os) 25-35 0.28-0.31 N:P,O, wt ratio attapulgite clay, % by wt 2 6.2-6.6 PH 1401 density, kg/m3 viscosity, Pa.s at 27 "C 0.05-0.25 at 0 "C 0.10-0.40 at-18°C 0.56-0.65 solidification temperature, "C -20.5 t o -26.1 salt out temperature, "C 0 saturation temperature, "C 10
tween 38 and 43 "C occurs primarily by evaporation of some of the water in the liquid; the water is evaporated by air flowing countercurrent to the flow of liquid fertilizer. A demister section located above the liquid distributor separates entrained liquid from the exhaust air. The cooled liquid in the reservoir at the base of the tower then is pumped to a clay-mix tank. Sufficient attapulgite clay is added in the clay-mix tank to supply 2% by weight of clay in the final product. Fluid is withdrawn from the bottom of the tank by a centrifugal pump and is recycled to the top of the liquid surface to provide mixing and to gel the clay. Product suspension then is pumped from the clay-mix tank to storage.
Composition and Physical Properties of 9-32-0 Grade Suspension The pertinent chemical and physical properties of the Phase I base suspension currently being made in the pilot plant and planned for the demonstration-scale unit are shown in Table I. The nominal grade of the suspension is 9-32-0. The suspension should have a pH (undiluted) of 6.2 to 6.6 and a density of about 1.4 kg/L. The polyphosphate content should range from 25 to 35% of Pz06, depending on the quality of the merchant-grade acid used. The suspension will contain about 2% by weight of attapulgite clay which will provide a substantial amount or all of the clay required when other grades of suspension are made by cold blending procedures. The suspension should store for short times at temperatures as low as -21 to -26 "C without solidification, and its viscosity is such that it should flow freely at all temperatures higher than this. As is the case with all suspensions, thorough agitation of the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
Table 11. Mixed Suspension Fertilizers Prepared from 9-32-0Grade Ammonium Polyphosphate Base Suspension,' Urea-Ammonium Nitrate Suspension,b Potassium Chloride,c and Water max init % clay viscosity, satisfactory N:P,O,:K,O suspension in mixed Pa.s at ratio graded suspension 27 "C 18-1 8-0 e 1.7 0.150 1:l:O 13-13-13e 1.2 0.200 1:1:1 11-11-22e 1.0 0.250 1:1:2 11-22-11 1.5 0.500 1:2:1 9-18-18e 1.3 0.300 1:2:2 8-16-24e 1.1 0.600 1:2:3 6-12-24 0.8 0.100 1:2:4 7-21-14e 1.3 0.150 1:3:2 7-21-21 1.3 0.650 1:3:3 6-18-24e 1.1 0.400 1:3:4 5-15-25e 1.0 0.250 1:3:5 4-14-28 0.9 0.150 1:3.5:7 22-11-oe 1.6 0.250 2:l:O 18-9-ge 1.3 0.400 2:l:l 16-8-16s 1.1 0.400 2:1:2 21-7-7e 1.3 0.500 3:l:l 18-6-12f 1.2 0.250 3:1:2 3:1:3 15-5-15f 1.0 0.200 3:2:1 18-12-6= 1.4 0.150 ~~
~
a Ammonium polyphosphate base suspension, 9-32-0, made in pilot-plant equipment; contained 2% clay. White solution grade. Contained 31.0%N, 1.5% clay. Suspensions with higher grade were unsatisfactory because of excessive viscosity and nonpourability. e Maximum grade limited by the grade of the constituents. f Contained t 20-mesh (850 Mm) urea-ammonium chloride crystals at 0 "C. The crystals dissolved at about 4.4 "C.
suspension at least once a week is recommended to prevent crystals from growing to excessive size and gels from becoming too strong. Tests at the TVA facilities in Alabama and tests in Iowa indicate that the suspension has excellent shipping and storage characteristics and that it should be an excellent product for regional markets. Various grades of mixtures can be prepared from this base suspension without addition of extra clay by cold blending the suspension with urea-ammonium nitrate suspension containing 1.5% clay and, in some cases,with potash. Typical grades possible are listed in Table I1 and discussed in a later section.
Pilot-Plant Equipment and Operating Procedure TVA has one pilot plant capable of producing 227 kg/h of base suspension by this process, and another capable of producing 907 kg/h. A flow diagram of the 227 kg/h unit was shown in Figure 1. The 907 kg/h unit is similar, but it is not as energy efficient and has a shell-and-tube heat exchanger for a product cooler instead of an evaporative cooler. Most of the data for scaling up to the 18 t / h demonstration-scale unit were obtained from the 227 kg/h pilot plant. The equipment in the 227 kg/h pilot plant is described in the following paragraphs. Run conditions and results of a typical test made in the pilot plant (227 kg/h, nominal 2.5 in. diameter pipe reactor) are shown in Table 111. Acid Heater. Cold acid from storage is fed with a variable-volumeball-check-type metering pump to the acid heater. The acid heater consists of a coil of nominal 3/s in. diameter, Type 316 stainless steel tubing (0.69 m2 of heat-transfer area) inside a 25 cm diameter by 91.4 cm long cylindrical vessel. Acid is inside the tubing and hot liquid is outside the coil. The overall heat-transfer coefficient calculated for this unit is 278 W/m2.K when the acid is heated to 82 "C with liquid at 93 "C. Ammonia Vaporizer. The liquid ammonia from a trailer is metered with a rotometer before it enters the vaporizer. The ammonia vaporizer consists of ten 1.8 m long pipe heaters connected in series; each heater consists of a nominal 3/4 in. diameter Schedule 40 outer pipe made of mild steel and an inner nominal ' / z in. diameter tube made of Type 316 stainless steel. The ammonia passes through the inner tubing (with 0.6 m2of total heat-transfer area), and hot liquid passes through the annular space between the inner tubing and outer pipe. The liquid ammonia is vaporized and heated to about 82 "C in this heat exchanger. The overall heat-transfer coefficient calculated for this unit starting with liquid anhydrous ammonia at 0.6 "C is 681 W/m2-K. Pipe Reactor. The preheated acid and vaporized ammonia flow to a nominal 3/4 in. diameter pipe tee connected to the bottom of the insulated pipe reactor. The pipe reactors usually used are made from Schedule 40 stainless steel pipe. Pipes from 1to 6 in. (nominal) in diameter and from 0.9 to 2.7 m in length have been tested. The reactor most often used is nominally 2.5 in. in diameter and 1.8 m long. The pipe reactor is mounted vertically (Figure 2).
Table 111. Run Conditions and Results of Qpical Test Production of Phase I APP Suspension in 227 kg/h Pilot Plant nominal production rate, kg/h 227 packed-tower cooler airflow rate, m3/min 2.83 acid a source Florida air temperature (inlet/outlet), "C 32/72 151 pool temperature, "C 43 rate, kg/h temperature to pipe reactor, "C 76 retention time (in packed section), min 22b
16
5b
1.25 1
2*
1
1.83
2.5
2.5
0.91 1.83
2.5
2.14
Operation with a Nominal 2.5 in. Diameter Pipe Reactor 25 >8b >24b 0.01 22-25 >8b 0.01 26
Operation in 907 kg/h pilot plant.
Operation in
2 2 1 kg/h
1.83 1.83 1.83 1.83
pilot plant
section to separate entrained liquid from the exhaust air. Scaleup factors used to size the cooler are discussed in a later section of this paper. The cooled liquid in the reservoir at the base of the tower is pumped to a clay-mix tank. Clay-Mix Tank. The clay-mix tank (Figure 5) is 51 cm in diameter and 64 cm in height. Retention time in this vessel is about 30 min. Sufficient attapulgite clay is added from an auger-type volumetric feeder to supply 2% by weight of clay in the final product. Fluid is withdrawn from the bottom of the tank by a centrifugal pump with a tip speed of 11.6 m/s and recycled to the top liquid surface to provide the mixing and gel the clay. The material is circulated through the pump an average of 40 passes. About 227 kg of the suspension per hour is removed from the recirculation line as product and stored in a 1136-L tank. Design Parameters Results of pilot-plant operation were used to establish design parameters for the 18 t/h demonstration plant, as discussed below. Pipe Reactor. Several diameters and lengths of pipe reactors were tested to determine which factors were important in producing liquid with the highest polyphosphate content and which would allow the pipe reactor to operate longest before becoming clogged with scale. Results from these tests made at production rates of 227 and 907 kg/h of suspension are shown in Table IV. An attempt was made to correlate polyphosphate level of the product with production rate expressed as either kg of P,0,/h.cm2 em2 of pipe cross-sectional area or kg of P,0,/h.cm3 of internal pipe volume. Results show that there was a relatively large range of throughput in which about the same amount of polyphosphate was formed; operation in the range of 1.4 to about 14 kg of P20,/h.cm2 gave products with about 25 to 35% of their P,O, as polyphosphate, but further increasing the throughput to 53 kg of P206/h.cm2caused a significant reduction in polyphosphate content to 16% of P,Ok A series of tests a t constant throughput per unit area (2.5 kg of P,06/h.cm2) but with different lengths (0.9, 1.8, and 2.7 m) of nominal 2.5 in. diameter pipe showed that within the range tested, pipe length did not affect the polyphosphate content, which ranged from 22 to 26% of the P,Ok The P20, throughput per unit of pipe reactor volume in this series of tests ranged from 0.028 to 0.008 kg of P205/h.cm’. At TVA’s 18 t/h demonstration-scale plant, the pipe reactor will be 2.7 m long and will be constructed of nominal 14 in. diameter Schedule 40, Type 316 stainless steel pipe; throughput will be 6.68 kg of P,0,/han2 of pipe cross
’-
C L A Y ~ XPUMP ICLOSEO I M P I L L T S . #I, 6 L / U N T i P SPEED 8 8 6 U,I
Figure 5. Ammonium polyphasphate suspension pilot-plant elaymix unit. section and 0.025 kg of P,06/h.cm3 of reactor volume. All of the pipe reactors tested eventually became clogged if they were operated for long enough periods of time with feed of black Florida merchant-grade acid (see Table I11 for typical analysis). At TVA, acids from several manufacturers are received in tank cars and unloaded into a 1900-m3storage tank. The acid used in these studies was obtained from that tank, and usually the composition of the acid used was close to that shown in Table 111. At low throughputs of about 1.4 to 7.7 kg of P20,/h.cm2, the reactors could be operated continuously for at least 15 h before severe clogging occurred. However, a t higher throughputs, 13.4 and 53.4 kg of P,0s/h.cm2, the reactors became clogged in 5 h or less. In all cases, the scale that clogged the reactor was predominately an iron and aluminum ammonium orthophosphate fluoride salt, (Fe,Al)NH,HPO,F,, which is citrate soluble but water insoluble; occasionally some diammonium phosphate, (NH,),HPO,, also was present. Both of these salts are soluble in warm dilute aqua ammonia or acid. It was found that the scale could he removed from the pipe reactor by simply recirculating either an aqua ammonia or acid solution through the pipe. In the TVA 18 t/h demonstration plant,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 493
Table V. Heat Input to Pipe
-I
4 3 SUSPENSIONS
~~
% of
J/kg of P,O, heat obtained from hot APP liquid heat of vaporization of NH, sensible heat of NH, sensible heat of acid heat of reaction total
total 44 -
TESTED O=UNSATISFACTORY O V E R A L L EVALUATION O=SATlSF4CTORY O V E R A L L EVALUATION DATA RANGES 0205 AS P O L Y P H O S P ~ A T E ,*/e' 2 5 - 5 0 TOTAL PLANT F 0 3 0 , % 4 0 4 - 4 4 3
1; ,"
9.95 X 1.07 X 5.51 X 36.31 X 52.84 X
10' 10' 10'
19 2 10
lo5 2 10' 100
two identical pipes will be provided, with provision for switching to allow cleanout without interruption of production. All of the water of formulation plus about 10% of the ammonia will be fed to the partially clogged reactor to clean it, while ammoniation of the acid to form polyphosphate is being carried out in the other pipe reactor. For comparison puposes, most existing liquid fertilizer plants that now ammoniate superphosphoric acid to make 10-34-0liquid fertilizer have single pipe reactors that are either 4 or 6 in. (nominal) in diameter. They normally produce 18 to 27 t / h of 10-34-0 containing about 70% of the P205as polyphosphate. With merchant-grade acid fed to these plants at a conservative rate of 7 kg of P205/h.cm2 of pipe cross section (based on pilot-plant results) about 1.8to 4.1 t / h of a 9-32-0 suspension containing about 25% P205as polyphosphate could be produced. A pipe with a larger diameter could be installed to allow increase in the production rate. Pipe-Reactor Heat Balance. In calculation of heat input to the pipe reactor, it was assumed that starting materials were acid and liquid ammonia, both at 21 "C, and that total heat input was the sum of the following heat inputs: heat of vaporization of ammonia; sensible heats (to feed temperature) of the acid and ammonia; and heats of ammoniation to form concentrated solution of monoammonium phosphate (6.3 X lo6 J/kg) and diammonium phosphate (5.6 X lo6 J/kg). It was assumed that 2.27 kg of ammonia per unit of P205would be fixed in the pipe, 2.2 kg as monoammonium phosphate and 91 g as diammonium phosphate. Polyphosphate level achieved in the suspension showed resonable correlation with heat input, expressed as J/kg of P205charged. Over the range of about 2.04 to 2.27 X loe J/kg P205heat input, corresponding polyphosphate levels were 15 to 27% of P20b In a typical pilot-plant run with heat input only from the product suspension (through ammonia vaporizer and acid preheater) and heat of reaction, the heat input to the pipe was as shown in Table V. Under these conditions, polyphosphate level of about 27% of the P205is achieved in the product. The tabulation shows that 31% of the total heat input to the pipe was reclaimed heat obtained from the hot APP liquid. It is obvious therefore, that this reclaimed heat is essential in achieving satisfactory polyphosphate level. Also, the energy-saving aspects of the process as compared with the production of superphosphoric acid and even as compared with use of steam for heating the feed acid are apparent. Packed-Tower Evaporative Cooler. The cooling tower in the pilot plant was purposely oversized to enable batch operation at a higher throughput to check design parameters that were assumed for the demonstration-scale plant. In operation to check these parameters, 9-32-0 grade liquid was accumulated from several pilot-plant runs, preheated to 93 "C, and then fed at a rate of 2177 kg/h to the 41 cm diameter by 4.6 m high cooler. The liquid was cooled 38 "C as it flowed downward through the 2.4-m bed of 4 cm diameter pall rings. The airflow into the tower was 7.5 m3/min and the pressure drop across the packing
p
~
43
J
a
3
41L
9-32-0
~o
I 40 0250
I
0260
0270
I
b
1
I
0283 N P20s
0290
0303
0310
O W
WEIGHT RATIO
Figure 6. Effect of total plant food content and NP2O5weight ratio on overall acceptability of 9-32-0(2% clay) APP suspensions.
was estimated to be about 245 Palm of packing. The inlet and outlet air temperatures and relative humidities were 25 and 64 "C and 71 and loo%, respectively. The viscosity and density of 9-32-0 suspension at 38,66, and 93 "C were 0.038, 0.022, and 0.018 P a s and 1.398, 1.387, and 1.375 g/mL, respectively. (Viscosity was measured with a Brookfield Model LVT viscometer.) The estimated specific heats of 9-32-0 at 38 and 93 "C were 0.67 and 0.69, respectively. Using the above data, it was calculated that at a flow rate of 2177 kg/h of 9-32-0 through the tower, 71 520 W/h was removed and the volumetric heat-transfer coefficient Uu was calculated to be 5494 W/m3.K.AT, where m3 refers to the volume of the packed portion of the tower and AT is the log-mean difference in temperature (Kelvin) between liquid and air in the tower. For scaleup, Uu was required because of the use of larger size packing rings. This adjustment was made and the cooler was scaled up to process 18 t/h of 9-32-0 grade liquid. The overall size of the cooler in the demonstration-scale unit is 2.29 m in diameter by 8.2 m in height. It has a 3.4 m deep bed of 9 cm diameter polypropylene pall rings. Air will be drawn in below the packing and exhausted through the top of the unit with an induced-draft fan rated at 467 m3/min.
Effect of Grade, N:P20SWeight Ratio, Polyphosphate Content, and Clay Content on Product Acceptability Numerous APP suspensions with various NP205weight ratios, grades, and polyphosphate contents were made and submitted to 3-month storage tests to determine the optimum grade and polyphosphate content for Phase I APP suspension. To be rated satisfactory, a first requirement was that the fluid, at the time of production, could contain no crystals at 27 "C. Subsequently, during 3 months' static storage, a satisfactory suspension could develop no settled and packed crystals on the bottom of the container; also, no crystals larger than +20 mesh (850 pm) could be present after the products had been stored for a minimum of 14 days at 0 "C or for 3 months at 27 and 38 "C and then evaluated at both 27 and 0 "C. Another requirement was that a satisfactory suspension would have a solidification temperature no higher than -21 "C even after 3 months in storage. Additionally, suspensions that were considered to be satisfactory had to be at least 98% pourable at 27 "C after mild agitation supplied by gently rotating a
494
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
,
- 0 4
EQUATION 1 9 4 TESTS, r 2 ~ 0 6 5 1
MODEL
'
,
SOLlDIFlCATlON t
TEMP 1*F1 : 5 5 3 3 7 - 3 5 9 0 6 I N P20s W T RATIO1
5 7 5 3 7 !N.P205 W T RATIO12-00089376 i%"20,
AS POLYPHOSPHATEIZ
-12.2DATA
"
~
15ck
TOTAL PL4NT FOOD, %
'
1
RANGES
P 2 0 5 AS POLYPHOSPHATE % 10 30 N PzO5 WT R4TlO 0 2 5 6 - 0 330
I J
37 7 4 4 7
-23.31 2 5 % POLYPHOSPUATE
\
I
I
I 28 9' 026
1 027
028
029 N P205
030
031
032
@33
WEIGHT RATIO
Figure 7. Effect of polyphosphate content on solidification temperature of 9-32-0 (2% clay) APP suspensions.
stirring rod twice around the inside of the 250-mL glass containers. Also, the viscosity (after agitation for 5 min) could not exceed 1Pa-s at 27 OC, 1.5 Pa-s at 0 O C , or 2 P a s at -17.8 "C. Clear liquid on top of the suspensions was considered undesirable, but such products were not ruled unsatisfactory, because either recirculation or air sparging would remix them. Overall results from evaluation of 167 suspensions showed that the initial polyphosphate content should be at least 25% of the P2O5 to prevent formation of large crystals during 3 months' storage and to maintain a solidification temperature of at least -21 "C after 90 days of storage. All but 43 of the 167 suspensions were eliminated because of failure to meet this polyphosphate level requirement. Storage characteristics of the remaining 43 suspensions (polyphosphate level 25-3070 of P2O5) then were correlated with N:PzO5 weight ratio and total plant 4
food content (N + P205)as shown in Figure 6. The suspensions,to be rated satisfactory, had to be satisfactory in all evaluations (initially and after static storage for 30, 60, and 90 days). The results in Figure 6 show that suspensions with total plant food concentration of 40% were satisfactory with N:P205weight ratios that ranged from about 0.281 to 0.314. A maximum total plant food concentration of 43.2% was possible, but only with control of N:P205ratio in the close range of about 0.295 to 0.300. From these results, the most satisfactory nominal grade for Phase I suspension appears to be about 9.5-32-0 (NP205wt ratio of 0.295-0.300 at about 41.5% total plant food). Suspensions of lower grade (40 and 41% plant food) with N:P205weight ratios and polyphosphate contents in the recommended ranges also were generally satisfactory, but transportation and storage of the lower grade product would be more expensive per unit of plant food. Several other nominal grades are indicated in Figure 6 for comparison. As shown, the recommended grade falls in the middle of the N:Pz05 weight ratio and total plant food concentration ranges of the satisfactory suspensions. The other grades shown for comparison were either on the limit of or outside one of these ranges, and it is anticipated that control of the plant by pH and density might not be accurate enough to ensure constantly that such a product would be satisfactory. For practical purposes, the nominal grades will be 9-32-0 but should be controlled near 9.5-32-0. The amount of clay required was found to range from 2 to 2.5% by weight. Reducing the amount of clay to 1.5% allowed syneresis to occur and crystals to settle and pack. Small ammonium phosphate crystals were present in many of the satisfactory suspensions after storage for 90 days, and in some cases strong gels were noted. The clay suspended the ammonium phosphate crystals or impurities and prevented them from precipitating during low-temperature or long-term storage. Sparging the suspensions will usually prevent gels from causing problems. Solidification Temperature Data from 94 of the suspensions were evaluated, with the aid of a computer, to determine the effect of totalplant food concentration, N:P205 weight ratio, and polyphosphate content on the solidification temperature. Solidification temperatures were determined by cooling the suspension over a period of 1to 2 h in an ethanol-dry ice bath with mild agitation supplied by a power stirrer operated at a tip speed of 2.1 m/s. The solidification
Fl
'T-7
W E T PROCESS ACID
0-54-0 ACID ,HEATER
AIR
4TTAPULGITE CLAY
PRODUCT STORAGE
I
DISSOLUTION
PUMP
Figure 8. Modification of a typical pipe-reactor plant to allow production of 9-32-0 low-polyphosphate APP suspension.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982
temperature was recorded when it no longer decreased with increase in cooling. The data evaluated were for suspensions that ranged in total plant food content (N P2O5) from 37.7 to 44.7, in N:P205 weight ratio from 0.256 to 0.330, and in polyphosphate content from 10 to 30% of Pz06. The accuracy of the solidification temperature measurements was h1 "C. Results from the study showed that, within the ranges studied, N:P205weight ratio and polyphosphate content had the greatest effect on solidification temperature. An equation relating these two variables with solidification temperature is shown in Figure 7 along with several curves that show the solidification temperature of APP suspensions of various N:P2O5 weight ratios and polyphosphate contents. Assuming that the N:P205 weight ratio of Phase I APP suspension will be restricted to between 0.29 and 0.31 and that the polyphosphate content will be at least 25% of the P205(both to ensure that the suspension is of good quality), the APP suspension should have an initial solidification temperature lower than -23 "C as is shown in Figure 7. Laboratory tests at TVA showed that at 38 and 27 "C, 5 and 3 polyphosphate percentage points, respectively, were lost over a 3-month storage period. Some loss of polyphosphate (hydrolysis) will occur in long-term storage, but data show that even with polyphosphate level reduced to 10% of P205,the solidification temperature is still lower than -21 "C. In comparison, 13-38-0ammonium orthophosphate suspension has a solidification temperature in the range of -12 to -7 "C. With such a low solidification temperature, the 9-32-0 grade base suspension should store without difficulty anywhere in the United States. Mixed Suspension Grades Possible with 9-32-0 Laboratory studies were made to determine the highest satisfactory grades of mixed suspension fertilizers that can be made by cold blending nominal 9-32-0 grade APP base suspension with TVA urea-ammonium nitrate suspension of 31-0-0 grade containing 1.5% clay, potassium chloride containing 62 % K20, and water. Highest satisfactory grades of some of the more common plant nutrient ratios that were tested included 18-18-0, 13-13-13, 9-18-18, 721-14,5-15-25,22-11-0,and 18-12-6; other ratios and grades are shown in Table 11. In all cases, no additional clay was required.
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Materials of Construction In the 18 t/h demonstration-scale plant, all of the metal that will be in contact with either wet-process phosphoric acid or hot (>52 "C) APP liquid will be AISI Type 316L stainless steel, except for the evaporative cooler where the metal will be AISI Type 304L stainless steel and the shell of the ammonia vaporizer where the metal will be mild steel. Other items of equipment to be constructed of stainless steel include the acid heat exchanger, pipe reactor, and melt dissolution tank. Seals in the acid heat exchanger (plate-and-frame type) will be Viton. The tubes in the ammonia vaporizer will be Type 316L stainless steel; ammonia will be inside these tubes. All equipment that will be in contact with suspension or liquid at temperatures less than 52 "C will be constructed of carbon steel and the piping will be either carbon steel or plastic (ABSor PVC). Items of equipment that will be constructed of carbon steel include the clay-mix tank, clay feeder, clay silo, and auger to transfer clay from the feeder to the clay-mix tank. Retrofit 9-32-0 Process into Existing 10-34-0 Plant There are 130 to 150 pipe-reactor plants within the United States in which 10-34-0 liquid is produced from superphosphoric acid. Although the design of most of the plants was based on the TVA process, the equipment used to make the 10-34-0usually varies somewhat in each plant. One possible approach for retrofitting the 9-32-0 process into an existing 10-34-0plant is shown in Figure 8. Items of equipment that would have to be installed are an acid heater and dry clay mixing system. Provisions would have to be made to have a source of merchant-grade acid to feed to the process. Also, if a separate melt-dissolution tank were not available, one would have to be installed. To obtain a production rate of 9-32-0 equal to that of 10-34-0, an enlarged pipe reactor also would have to be installed. Of course, each plant should be evaluated and the equipment required tailored for that particular unit. Received for review November 9, 1981 Accepted March 12, 1982
A patent application that covers the process and pipe reactor design is on file in the U.S.Patent Office. In accordance with usual TVA policy, nonexclusive licenses will be granted for use of the process. Paper presented a t 182nd National Meeting of the American Chemical Society, New York, NY, Aug 23-28,1981.
