CC-1, CC-2 = Cohen-Coon sett,iiig technique used with fit 1 and fit 2, respectively
ratio of dead time to major time constant controller transfer function minimum integral of the absolute error controller-setting technique = minimum integral of the error-squared controller-setting technique = minimum integral of the time absolute error controller-setting technique = controller gain = process gain = controller output, Laplace domain = controller output, time domain = unit step function applied a t t = 0 = controller-setting technique developed specifically for record order with dead time processes = major time coiistaiit = dead time = normalized time = load disturbance, Laplace domain = load disturbance, time domain = state variables = Ziegler-Nichols controller-setting technique based on ultimate period-gain rules = controller-setting technique based on ‘/4 decay ratio and minimum integral of the error = = =
GR1:ICK LETTERS 6(t)
=
deta function applied at t
=
0
Literature Cited
Ball, S. J., Adains, lt. K., Uak Ridge National Laboratory, Ilept. OliNI,-TAI-l933, 1967. Buckley, P. S., “Techniques of Process Control,” pp 69, 76, 90, 149, 238, Wiley, New York, N.Y., 1964. Coheii, G . B., Coori, G. A., Trans. ASJIE, 75, 827 (1953). Coughanowr, I>. ll., Koppel, I,. B., “Procesb Syitemr Analysis arid Control,” .-_- pp 241-4, 312-13, llcGraw-Hill, New York, .T
A\.
. r
Y
.)
I‘JU.).
Gallier, 1’. h-,, Otto, I{, E., Instrum. Techno/., 15, 6,; (1968). Graham, I)., Lathrop, It. C., A I E E , 72, 273 (1953). Haxebraek,. P.,, vaii der Waerden, B., Trans. ASJIE, 72, 309
(1930). Kegerreis, J. E., 1lS thesis, Purdue University, Lafayette, Ind., 1 n?o
Id,”.
Latour, P. It., Koppel, L. B., Coughanowr, L). It., I n d . Eng. Chem. Process Des. Debelop., 6 , 4.52 (1967). Lopex, A. AI., Miller, J. A4,, Smith, C. L., LIurrill, P. W., Instrum. Technol., 14, .i7 (1967). LoDez. A . 11..Smlth. C. L.. LIurrill, P. W., Brit. Chem. Enq., i 4 , i533 (1969). AIcAvoy, T. J., Johnson, E. F., Ind. Eng. Chem. Process Des. Develop., 6, 440 (1967). Miller, J. A , , Lopez, A. hl., Smith, C. L., 11urril1, P. W., Contr. Eng 1 4 , 7 2 (1967). XurriIl, P. W.,Smith, C. L., Hydrocarbon Process, 45, 105 (1966). Itovira A . A , . ~Iurrill.P. W.. Smith, C. L., Instrum Contr. S~pt.:42, 67 i1969). ’ Smith, C. L., LIurrill, P. W., I S A J . , 13, 30 (1966). Wills, D., Confr. Eng., 9 , 104 (1962). Ziegler, J., Nichols, N., Trans. A S d l E , 64, 739 (1942). RECEIVED for review February 9, 1971 ACCEPTEDOctober 12, 1971 James E. Kegerreis was supported by a fellowship provided by Union Carbide. Computer time was supplied by Purdue University. ~
Pilot-Plant Studies of Anhydrous Melt Granulation Process for Ammonium Phosphate-Based Fertilizers Robert G. lee, Robert S. Meline, and Ronald D. Young’ Tennessee Valley Authority, Muscle Shoals, Ala. 56660
An anhydrous melt process for producing ammonium phosphate-based fertilizers b y use of merchant-grade, wet-process acid has been developed by TVA on a pilot plant scale. Heat of reaction of anhydrous ammonia with the acid is used to form the melt. The melt i s produced in a tee-type reactor and granulated in a pug mill which provides the working action needed to induce crystallization of the polyphosphates. To facilitate granulation of the ammonium phosphate melt ( 1 2-57-0), the polyphosphate content is limited to a maximum of about 3070 b y controlling the temperature of the reaction. A wide range of grades, such as 28-28-0, 21 -42-0,24-24-0,19-19-19, and 17-1 7-1 7, can be produced b y using urea, ammonium nitrate, or ammonium sulfate to provide additional nitrogen and b y using potassium chloride or other sources to provide potassium. Since only small amounts of water are present in the materials fed to the granulator, no dryer is required in the process. The physical characteristics and storage properties of the products (1 5-30% of phosphates as polyphosphate) containing urea are much better than those of similar products made b y conventional processes.
T h e Tennessee Valley A\ut’Iiority has developed o n a pilot plant scale a simple melt-type granulat,ion process for the production of ammonium phosphate-based fertilizers from merchant-grade, wet-process phosphoric acid. The process is a n outgrowth of TVA’s work on t,he direct, process for production To whom correspondence should be addressed.
90
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
of ammonium polyphosphate (Meline et al., 19iO). I n practice, this process consists of operating the direct-process reaction system in the temperature range required to produce an anhydrous ammonium phosphate melt of about 12- 57- 0 grade wit,h about 20- 30% of the Projin polyphosphate form. With t,he polyphosphate cont,ent in this range, the melt can be granulated withoiit difficulty in a pug mill. The polyphosphate con-
Figure 1. Flow diagram for direct production of low-polyphosphate granular products
tent is high enough to provide a fluid melt with good flow characteristics, but. is low enough to crystallize readily and granulate with moderate rates of recycle. Urea and potassium chloride can be added t o make products of such grades as 21-42-0, 28-28-0, and 19-l(t-19 or ammonium nitrate for grades such as 2 4 2 4 0 and 17-17-17. Grades such as 15-35-0 can be made by using by-product ammonium sulfate. The chief advantage of the process is that only anhydrous materials are fed to the pug mill, thus eliminating the need for a dryer. Also, the storage properties of the products containing urea are distinctly better than similar products made by conventional processes. I n the direct process, heat from the reaction of ammonia with merchant-grade acid is utilized to evaporate all of the free water and a portion of the combined water. Evaporation of the combined water results in the formation of polyphosphates. By this means, melts containing 50% or more of the PaOsin the polyphosphate form can be produced. Such melts are good materials for use in preparation of fluid fertihers hecause of the high water solubility of the phosphates in the ammonium phosphate-ammonium polyphosphate system. Attempts t o granulate the 50% polyphosphate melt were disappointing; granulation of this material required about 16 lh of recycle per pound of product and excessive power t o control plasticity in the pug mill because the melt did not crystallize readily. Subsequent pilot plant studies have shown that grannlation characteristics of the melt improve markedly when polyphosphate content is limited to 2C+30y0 by lowering the reactor temperature from 470' to about 400-430'F. The low-polyphosphate 'melt crystallizes readily and can he granulated easily at recycle ratios of 4:1 or less with or without supplemental materials. Description of Pilot Plant
The pilot plant consisted of a reaction system for preparation of the ammonium phosphate melt; a granulation section including a pug mill and cooling, screening, and crushing equipment; and facilities for adding urea and potash. A flow diagram of the entire process is shown in Figure 1. The pilot plant was operated at rates of 500-1500 Ib/hr of granular product. I n the countercurrent reaction system, the wet-process acid was fed to an X-in.-diam spray scrubber to remove unreacted ammonia from the reactor offgas. Partially neutralized acid was pumped a t a controlled rate from the scrubber into a ll/B-in. pipe tee reactor where all of the anhydrous ammonia was fed. The partially neutralized acid and ammonia reacted t o form an ammonium polyphosphate melt. Because of the short retention in the tee reactor, there was essentially no reversion of phosphate to a citrate-insoluble form. The melt from the tee reactor discharged in an extremely foamy state
Figure 2. Vapor disengager
Figure 3. Equipment arrangement for production of granular products
through ahout 3 ft of l ' / h . pipe into a vapor disengager. The disengager (Figure 2) was a 24-in.-long horizontal tube 63/8 in. in diam containing a rotor with two helical blades. The rotor spread the melt against the wall of the vessel and thereby provided a large surface area for separation of the steam and unreacted ammonia from the melt. The pitch and rotation of the blade were such that the melt was conveyed to the discharge end of the disengager. Significantly, the shearing action of the blades transformed the foamy melt into a free-flowing fluid. A blower moved steam and unreacted ammonia from the vapor disengager through the spray scrubber. Normally, avacuumof about 1 in. of water wasmaintained a t the gas outlet of t h e scrubber. The disengager was placed directly above the pug mill so that the melt could fall freely into this granulator. A photograph of the disengager and pug mill is shown in Figure 3. The spray scrubber, disengager, and piping were made of Type 316L stainless steel. The pug mill had 19 ft' of effective working volume in a 7-ft-long bed which was 22 in. wide. Its 4-in.-diam shafts were equipped with 58/4-in.-long mixing paddles and wepe driven a t 57 rpm by a 25-hp motor. The bed sloped slightly toward the discharge end, and an adjustable gate was used to control the bed depth. Material from the pug mill was cooled with 600-1500 cfm of ambient air in a 30-in-diam rotary cooler 21 ft long. The cooler turned a t 10 rpm. The cooled material was screened a t -6 +9 mesh for the product. A chain mill crushed the oversize material in a closed circuit with an undersize screen so that no crushed material was in the product. Ind. Eng. Chem. Prmerr Der. Develop., Vol. 1 1 , No. 1, 1972
91
The urea and ammonium nitrate solutions were prepared from commercial unconditioned prills in a two-stage, steamheated melter. The melter was operated a t 280°F to make 99% urea solution and a t 340°F to make 9770 ammonium nitrate solution. I n tests with solid urea rather than solution, prills were fed to the pug mill with the recycle. I n other tests in which standard-size potassium chloride (60% K20) or byproduct crystalline ammonium sulfate was used, the solid was also fed with the recycle. Melt Production
The low-polyphosphate melt was produced from typical Florida merchant-grade, wet -process acid made from uncalcined rock. Analyses of the acids used in most of the work are : Chemicol composition,
-
% by wt
PzO:,
FezOs
A1203
SO3
F
Mg
Waterinsoluble solids
51.1 52.2 52.9
1.5 1.3 2.4
1.2 1.2 1.4
3.2 2.7 3.3
0.8 0.8 0.8
0.3 0.2 0.5
5.1 1.6 2.2
Typical operating conditions for the react'ion system and analyses of ammonium phosphate melts for tests a t a production rate of 500 lb/hr are shown in Table I. When the acid contained only 50-51% P205, preheating it to 140-70"F, as well as heating the ammonia to 260-320°F, was necessary to maintain reaction temperatures high enough to convert the desired proportion of I'2Oj to polyphosphate form. No preheat should be required for acid a t a concentration of about 5470. The flow of ammonia to the tee react,or was closely controlled so that enough unreacted ammonia passed to the scrubber to maintain the p H of the acid there a t about 1.8 (10% aqueous solution). At this p H the acid contained about 2 lb of ammonia per unit of PaOs, which is in a range of high solubility of the ammonium phosphate. Conversion of orthophosphate to polyphosphate in the tee reactor and vapor disengager usually ranged from 18-30%. For 19-19- 19 and 15-35-0 grades, increasing the polyphos-
Table I. Operation of Reaction System Teat no. 201 207 209 272 Product (melt) rate, Ib/hr 500 500 500 500 Feed acid Temp, "F 270 140 270 150 50 1 50 7 51 0 62 9 1'20,, % Scrubber Temp, "F 281 263 290 p H (10% .oh) 1 8 1 9 1 8 Temp, "F -1mmonia 90 300 260 320 Tee reactor 440 410 432 436 lmmoniuni phosphate melt p H ( I 0% 301n) 4 5 5 2 4 7 Rt% s 12 5 13 4 13 0 11 5 57 2 55 4 66 6 56 9 P205 yo of total P 2 0 j Polyphosphate 27 18 33 29 T a t e r soluble 99 98 98 91 Available 99 6 100 99 8 99 7 Degree of ammonification, lb SHp/ulilt of P20, 5 3 5 9 5 6 4 9
92
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
265 500
2 53 52.0
90 465
... 12.2 58,O 42 100 99.5 5.1
phate content to 35-40% resulted in better incorporation of the solids. The grade of the ammonium phosphate melt was 13-55-0 when the tee reactor temperature was 410°F; less nitrogen was fixed a t higher temperatures, resulting in a 1257-0 grade a t 440°F. The PZOsin the melt was usually 9899% water soluble and 99-100% citrate soluble. Most products made by this method contained a significantly higher portion of PnOr in a water-soluble form than those made by conventional orthophosphate processes. The ammonia loss and rate of fluorine evolution from the process were evaluated. The amount of ammonia lost from the spray scrubber was too small to be detected in gas-sampling tests. Gas samples analyzed for fluorine indicated that 2.4% of the fluorine in the feed acid was evolved. I n another test, the F:Pp05 ratio in the product was compared with that of the feed acid; this comparison indicated that about 7% of the fluorine in the acid had been evolved. Tests with corrosion specimens mounted in the pilot plant equipment showed that Type 316 stainless steel corroded at high rates, about 100 mils per year in the scrubber and as high as 500 mils per year when exposed to the hot melt. These tests indicated that Hastelloy G would he satisfactory for the scrubber, and Hastelloy C could be used for the disengager and tee reactor. Since the components in the reaction system are comparatively small, use of corrosion-resistant alloys should not be unduly expensive. Granulation
The principal objective of the granulation studies was to determine the best conditions for operation of the pug mill. The effects of recycle ratio and the polyphosphate content of the melt were investigated, and the pug mill slope and bed depth were varied to obtain the best operation. For each grade of product, a range of polyphosphate conversion in the melt was established which provided sufficient plasticity for good granulation but did not require excessive amounts of recycle. The pug mill gave the best mixing of melt and recycle when nearly level, so that the bed was full to about the top of the paddles. In a few tests, the pug mill was discharged to a rotary-drum granulator to improve granulation efficiency. Use of the drum did not affect t,he particle-size distribution, but the rolling action improved granule appearance. The test data in Table IT are typical of operation at a recycle ratio high enough to avoid excessive plasticity and stickiness in the pug mill and yet low enough to obtain adequate granulation for providing the necessary proportion of product. The mechanical working of the melt in the pug mill is beneficial in promoting crystallization and resultant granulation. Also, a pug mill is particularly suited for specific placement of the melt and urea solution to avoid contact a t high temperatures. However, rotary-drum or other types of granulators probably would be adaptable to this process. The 12-57-0 grade was the basis for all the other grades and was also the simplest to granulate, merely involving mixing melt with recycle. I n the pilot plant, 500 lb/hr of melt was granulated at a recycle ratio of 3.8:1, giving a temperature of 210°F in the pug mill. The proportion of oversize material was high (34%), but, it was easily crushed and handled. The optimum polyphosphate level for granulation of this product 25-30% of the total P20a. The 21-42-0 grade product was produced a t a rate of 1000 lb/hr using 750 lb of ammonium phosphate melt and 250 lb of concentrated (99%) urea solution per hour. When using urea in the solution form, we found it necessary to spray the solution onto the recycle a t the feed end of the pug mill followed
Table II. Pilot-Plant Granulation of Ammonium Phosphate Melt Product grade ~
12-57-0
2 1-42-0
20-20-0 ___ _ --
Test no. Test length, hr Product rate, lb/hr Feed acid
201-2 4.7 500
208 2.7 1000
207 5.0 1000
%
50.1 270
50.7 115
500 ...
p205,
Temp, O F Rate, lb/hr ,Melt Urea Potassium chloride Ammonium nitrate (97% soln) Ammonium sulfate (crystalline) Recycle Ratio Rate, lb/hr Temp, O F Screen analysis, % by w t +6 mesh -6 +10 mesh - 10 mesh Pug mill slope, in./ft Pug mill product Temp, O F Screen analysis, w t % f 6 mesh -6 $10 mesh - 10 mesh Analysis of screened product (- 6 +9 mesh), wt %
N
P206
Kz0 HzO
% of total PZOS
a
Polyphosphate Water soluble Available Urea solution, 99$& 280°F.
b
________~
19-1 9-1 9
24-24-0
15-35-0
122 4.8 1350
209-2 4.5 1480
272 3.0 1175
265 2.8 820
50.7 140
51.8 100
51.0 270
52.9 150
52.0 2 53
750 250.
500 500a
67 5 675b
...
... ... ...
... ... ...
500 ... ... 075
500 ...
... ...
500 500a 480 ... ...
...
:320
3.8 1900 92
3.2 3200 102
3.9 3900 104
2.3 3108
3.0 3480 128
1.5 1340
...
3.0 4400 122
0 6 94 1.0
1 25 74 0.2
2 27 71 0.2
... ... ... 0
2 24 74 0.2
0 24 76 0
6 29 65 0
... ...
...
-
... ...
.
I
.
210
192
186
175
176
192
194
34 23 43
28 28 44
29 31 40
24 27 49
26 30 44
7 53 40
31 24 45
12.3 56.8 ... 0.8
20.8 42.3
28.1 30.6
27.4 30.4 ... 0.8
20.1 19 4 19.1 0.5
24.6 25.0
15.2 35.4 ... 0.8
25 99 99.4 Urea prills fed with recycle.
by the addition of the ammonium phosphate melt a t a point in the mill after the solution had crystallized. This procedure ensured incorporation of the urea into the granules by overcoating with melt and prevented hydrolysis of urea by the hot ammonium polyphosphate melt. Hydrolysis is undesirable because it results in loss of ammonia and foaming and stickiness in the pug mill. Also, this procedure of overcoating the urea with ammonium phosphate melt minimized dusting cf urea from the product. The sequential placement of recycle, followed by urea solution and then by the ammonium phosphate melt in the pug mill, was important. The best pug mill operation was obtained a t a recycle ratio of about 3 and a temperature of 192°F. 'The product grade was 20.8-42.3-0 and analyses showed that no biuret was formed during granulation. Production of the 28-28-0 grade a t a rate of 0.5 ton/hr required 500 lb each of melt and urea. Operation with urea solution was similar to that for 21-42-0 grade except that a higher recycle ratio, about 4:1, was required. The temperature in the pug mill was 186°F. Operation with solid (prilled) urea was satisfactory and easily controlled by use of melt with 27y0 of its P205as polyphosphate, but some uncoated urea prills could be seen in the product. This condition probably could be preyerited by using prills of smaller size such as micro-
I
.
...
.
0.7 17 98 100
0.6 18 98 100
27 ... 100
34 98 100
... 1.1 26 91 99.6
35 94 99,6
prills. Only about 2 lb of recycle was required per pound of product, and the temperature in the pug mill was 175°F when prilled urea was used. The 19-19-19 grade was granulated a t a rate of about 1500 lb/hr. Melt and urea solution were fed to the pug mill as in 28-28-0 production, and standard-size potassium chIoride was fed along with the recycle. Incorporation of the potash was somewhat difficult with the limited liquid phase provided by the anhydrous melt. Increasing the polyphosphate content to a range of 33-38y0 gave additional plasticity for incorporating the potash. A recycle ratio of 3:l and pug mill bemperature of 176°F gave the best operation. The ammonium phosphate melt and 97% ammonium nitrate solution used to granulate 24-24-0 grade were fed together in a trough and entered the pug mill as a single stream. Although this method of mixing cannot be used with melt and urea for bhe reasons mentioned above, it is iidvantageous with materials, such as nitrate solution, that do riot react with t8he melt because it produces a honiogeneous product and avoids the finely divided nitrogen material encountered when separate streams are used. At a recycle ratio of 3, the pug mill temperature was 192"F, and t,he pug mill product (26y0 polyphosphate) contained only 7% oversize. water solubility was only 91%; the insoluble The product P205 Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1, 1972
93
phosphate was present as amorphous iron orthophosphate. The acid used in 24-24-0 production contained substantially more iron (2.4y0 Fe203)than that used in most of the other tests. Only a few preliminary tests of 15-35-0 granulation with ammonium sulfate were made. Product containing 3501, polyphosphate was granulated at 194’F in the pug mill with a recycle ratio of 1.5. The ammonium sulfate used was steelmill by-product containing 19,870nitrogen. These preliminary tests indicated the melt granulation process would be quite suitable for a product of this type. The extent to which impurities in wet-process acid affect crystallization of the ammonium phosphate melt is not known. With acids of differing impurity distribution and content, the polyphosphate ranges required for granulation likely will differ. In the pilot plant pug mill, the degree of plasticity needed for good granulation also favored the formation of amounts of oversize material that usually were about equal to the amount of product size. I n a large plant, the proportion of oversize should be substantially less, however, because larger pug mills give better mixing due to the greater tip speed of the paddles. I n the TVA demonstration-scale plant for production of 15-62-0 ammonium polyphosphate from electric-furnace superphosphoric acid, the proportion of oversize is about one third of the product-size material (Kelso et al., 1968). Storage Properties
All the product grades had good storage and handling properties. The products that contained urea appeared to store much better in bulk than similar products in which the phosphate was entirely orthophosphate. Moisture absorption was limited to a light crust on the surface, and the piles were free from lumps or caking. The polyphosphate content apparently acts as a “built-in” conditioner for moisture protection. Bag-storage tests were made with 50-lb bags loaded under pressure equivalent to a stack 20 bags high and stored in an unheated, well-ventilated building. Unconditioned 12- 57- 0 grade (-6 $9 mesh) was not caked when examined after seven months’ storage in 4-ply paper bags with two asphaltlaminated plies. The grades containing urea were satisfactory in this type bag for one month without conditioner, but on further storage, the material absorbed excessive moisture and caked. However, these grades were in satisfactory condition after nine months when the 4-ply bag was sealed in a 4-mil polyethylene overbag. Bags with polyethylene inner liners should be satisfactory also. Addition of 2% conditioning
94
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1, 1972
dust, either diatomaceous earth or kaolin, prevented caking of the urea grades during nine months in the 4-ply paper bag. The 24-24-0 grade (containing ammonium nitrate) was in good condition after three months of storage without conditioner. These tests are being continued. No storage tests were made with 15- 35-0 grade containing ammonium sulfate. The critical relative humidity was 60-65% for the 12-57-0 and 15-35-0 grades and 5O-55y0 for all the grades containing urea or ammonium nitrate, compared with about 75% for commercial diammonium phosphate. For all the grades, however, resistance of the unconditioned bulk material to penetration by moisture from humid atmosphere was about the same as for commercial diammonium phosphate. No difficulties in handling the bulk materials are therefore expected. Bulk density of the products ranged from 46.4-49.3 lb/ft*. Conclusions
The process for the granulation of low-polyphosphate melt is technically proved, operationally simple, and should be adaptable commercially as a n alternative to present diammonium phosphate and triple superphosphate processes. It seems to be particularly well suited to preparation of ureaammonium phosphate-type fertilizers. The 12-57-0 grade product fixes less ammonia than diammonium phosphate, b u t is more concentrated in Pz06than either diammonium phosphate or triple superphosphate. It should be suitable for many uses as a n intermediate or finished fertilizer. The reaction system is simple, dependable, and easy t o control; corrosion-resistant materials of construction are required. However, eliminating the dryer from a granulation plant saves in both investment and operating costs as well as decreasing dust and fume problems. A process without a dryer is especially important in the production of mixed fertilizers containing urea or ammonium nitrate since drying is difficult and a long retention time and low-temperature air must be used (Meline et al., 1968). Only limited tests have been made with ammonium sulfate to provide supplemental nitrogen, but additional work is plamed. literature Cited
Kelso, T. M., Stumpe, J. J., Williamson, P. C., Commer. Fed., 116 (3), 10 (1968). Meline, R. S., Davis, C. H., Lee, R. G., Farm Chem., 133 (ll),26 11 a?n\ \‘Y’”,.
Meline, R. S., Hicks, G. C., Kelso, T. M., Norton, M. M., Znd. Eng. Chem. Process Des. Develop., 7, 124 (1968). RECEIVED for review February 16, 1971 ACCEPTED June 14, 1971
