Ind. Eng. Chem. Prod. Res. Dev. lQ82, 21, 617-620
617
Melt Granulation of Urea by the Falling-Curtain Process A. Ray Shlrley, Jr.,' Luther M. Nunnelly,' and Fred T. Carney, Jr. Tennessee Valley Authorlw, Natlonal Fertlllzer Development Center, Muscle Shoals, Alabama 35660
In this process, molten urea is sprayed onto cascading granules in a rotary drum to build seed granules to product size. Heat released by solidifying melt is removed primarily by evaporation of a fine mist of water sprayed into air passed through the granulation drum. Granules are discharged to a fluid-bed unit for further cooling before they are elevated to a screen for separation of oversize and undersize from product. Recycle rates vary from 0.25 to 1. Oversize can be crushed and metered to the process as seed granules. Size of product can be varied from small to very large granules. Formaldehyde is not needed to enhance product hardness. Dust generation in t h e process is less than 2 % of the urea granulation rate and is removed from the airstream in a low-pressure-drop, irrigated mist eliminator.
Introduction TVA is continuing efforts to develop new fertilizers and new and better manufacturing processes for existing fertilizers, with special emphasis placed on reducing energy requirements, eliminating pollution, and improving quality of the product. The newest TVA granulation process, the falling-curtain process for granulation of urea, looks especially promising in all three of these categories. Indications from pilot-plant work are that superior quality granules can be produced, that the process uses less energy than conventional granulation processes (about 4.07 X lo6 J/kg of product less), and that pollution, which is often a major problem in fertilizer manufacturing processes, is greatly reduced. Also, plant investment should be reduced since less equipment is required. Melt granulation by the falling-curtain method evolved from technology TVA developed for sulfur-coating urea granules to produce slow-release nitrogen fertilizer. In that work, a thin coating of sulfur was sprayed hydraulically onto granular urea substrate in a rotary drum (Blouin et al., 1967, 1974; Engelstad et al., 1972; Shirley et al., 1975; Tennessee Valley Authority, 1978; Myers et al., 1978). A significant discovery was that the coating process was much more uniform and efficient when the substrate was directed into the form of a thin, falling curtain and the sulfur sprays were directed onto this curtain. Special drum internals were designed to form the curtain and direct the sprays, and several patents were obtained (Blouin, 1975, 1976). The present falling-curtain process for production of granular urea (Tennessee Valley Authority, 1976,1980; Shirley et al., 1979, 1980) employs this same principle except that the spray is of molten urea onto urea substrate. The substrate is a combination of uncrushed, recycled undersize plus a controlled amount of seed material. Granule formation is by layering. A second salient and novel feature of the new process is the employment of evaporative cooling with water atomization within the granulation drum as a means of removing heat and thus promoting rapid solidification of the granules. This feature, which provides significant savings of equipment and energy, likewise is covered by patents (Shirley, 1979,1980) assigned to TVA. First tests of the falling curtain-evaporative cooling method of granulating urea melt were conducted by TVA in December 1978 in a modified sulfur-coated urea pilot plant. Granular urea with very good physical properties was produced in this modified pilot plant at rates up to Applied Chemical Technology, Inc., Muscle Shoals, AL 35660. This article not subject to
U.S.Copyright.
0.5 t/h. Data from these preliminary tests were very promising, so TVA built a new pilot plant of 1.8 to 2.7 t / h capacity to further develop and refine the process. This plant, which was completed in September 1980, has operated well from startup. Operation was carried out at the 13th Demonstration of New Developments in Fertilizer Technology which was held at the National Fertilizer Development Center in Muscle Shoals, AL, Oct 7 and 8, 1980 (Shirley, 1979). Subsequently, the plant has been operated chiefly to obtain design data for a 12.7 t / h demonstration-scale plant planned for completion by TVA in late 1982. Process Description A basic flow diagram for melt granulation of urea by the falling-curtain process is shown in Figure 1. Granulation occurs in a rotary drum with specially designed internal equipment that forms a falling curtain of seed and recycled undersize onto which the molten urea is sprayed. The rotary drum is the heart of the process design; the drum and its operation are illustrated in Figure 2. As the drum rotates, small seed particles and recycled undersize particles are elevated from the bed by lifting flights and then discharged onto inclined collecting pans. Material flowing from the collecting pans forms a dense curtain of granules. Sprays of molten urea are directed onto this falling curtain of urea granules. As the melt strikes the surface of the granules, it quickly solidifies forming a coating. Thus, product granules of the desired size range are produced by successive layering of melt onto the seed and recycle granules while they pass through the drum. Cooling is provided by air flowing through the drum, and this air is cooled in the drum by the evaporation of fine water mist sprayed into the drum as shown in Figure 2. Granules discharge from the granulator drum into a fluid-bed cooler, which provides further necessary cooling. After cooling, the granules are elevated and screened as shown in Figure 1. Undersize material is conveyed to a recycle surge hopper from which it is fed back to the granulator at a metered rate. Small seed particles also are metered into the granulator drum to replace the product taken out of the system on a granule-to-granule basis; thus, a constant number of granules is maintained in the process streams at all times. Crushed oversize normally is used for seed. Dust from crushing of the oversize is separated from the seed-size material in a fluid-bed separator before the seed is metered into the granulator drum. This dust is collected in a cyclone and then is remelted and regranulated. In some of the TVA work, urea microprills were used a seed; this allowed all the oversize material to be
Published 1982 by the American Chemical Society
618
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
absorbs moisture as it passes through the granulator, the direction of airflow through the drum is made cocurrent with the urea to reduce chances of moisture being absorbed by the cool recycle entering the feed end of the granulator. The pilot plant was designed for air leaving the granulator to contain no more than 0.03 kg of water/kg of dry air, which is equivalent to 40% relative humidity at the discharge temperature of 49 " C . The air leaving the granulator is washed with recycle urea solution in a scrubber followed by an irrigated mist eliminator. Particles emitted from the fluidized-bed cooler are collected in a low-pressure-drop horizontal cyclone and are fed back to the recycle stream. Dust collected from miscellaneous transfer points in the system also is washed out of the air by a system similar to that used to remove particulates from the exhaust air from the granulator.
J
Figure 1. Falling-curtain granulation using evaporative cooling, flow diagram.
GRANULATOR STATISTICS DIAMETERLENGTHFLIGHTSR PM-
2 13 METERS 3 WMEFEERS VbRlABLE
MELT N O Z Z L E S 1 7 HPX
WATER
/
BED OF GRANULES
40
NOZZLES-5 _ .
SECT'ON P - A
MAX
Figure 2. Schematic diagram of granulation drum.
remelted and regranulated, and thus eliminated the crushing step in the process. If sufficient seed is not obtained from crushing oversize, some product-size material can be crushed. Feed requirements for seed depend on the size granules being produced. For example, when producing granules with an average diameter of 2.4 mm (8 Tyler mesh), the weight ratio of seed granules to product is approximately 150, based on use of seed material predominantly in the size range of 20 to 35 Tyler mesh (0.084-0.42 nim diameter). In production of larger granules, a lower proportion of seed would be used. Product size can be varied from that of typical prills to that of very large granules by changing screen sizes and making the necessary adjustments to the seed feed rate. Since urea is hygroscopic, the water spray into the drum is directed into areas of the drum that are free from falling urea to prevent direct contact of the granules with the water mist (see Figure 2). Also, the humidity in the air, and consequently the proportion of water spray, must be controlled to avoid exceeding the critical humidity of urea. For example, granules discharging from the drum at 97 "C would absorb moisture from contacting air that contained more than 0.27 kg of water/kg of dry air (relative humidity of 34% a t 97 "C). Recycled granules entering the drum at 60 "C would absorb moisture from contacting air that contained more than 0.08 kg of water/kg of dry air (58% relative humidity at 60 ' 0 . Since the air progressively
Pilot-Plant Equipment Granulation Drum. The granulation drum in the pilot plant (Figure 2) is 2.13 m in diameter and 3.05 m long. The retaining ring at the feed end is 38.1 cm high and the retaining ring at the discharge end is 12.7 cm high. Forty lifting flights are installed in the drum at 9 O intervals. The flights are straight with flat surfaces 7.62 cm wide and 2.74 m long. They are installed parallel to the axis of the drum and are canted 15' forward from the radii of the drum. Two collecting pans are installed parallel to the axis of the drum. The top pan is 66 cm wide and 2.84 m long. The lower pan is 53.3 cm wide and 2.9 m long. Both pans are sloped counter to the direction of rotation of the drum at an angle so that the granules will cascade down them. Each pan catches some of the material discharged from the lifting flights. All granules discharging from the top pan fall to the bottom pan, and the granules discharging from the bottom pan provide a curtain of falling granules onto which the molten urea is sprayed. The collecting pans catch the granules after only a short fall and break their momentum before they can develop enough force to shatter on impact and create dust. In addition, the pans provide a large open area inside the granulator in which heat transfer can occur by air-to-granule contact, but without allowing the granules to pass through the water sprays located adjacent to the pans. The double-pan configuration is designed to increase airflow between the area where most air-eo-granule contact occurs and the water evaporation area of the granulator. Melt Preparation and Handling. Molten urea is distributed in the granulator at gage pressures up to 3.45 X lo6 Pa through a steam-jacketed header which contains 27 spray nozzles. The urea melt is obtained from a single-pass steam-heated melter and is filtered before being pumped to the spray header to remove any particles larger than 10 wm, which would plug the spray nozzles. The purpose of the melter in the pilot plant is to provide urea melt that simulates the melt that would be obtained directly from a urea-plant evaporator in a large plant. The molten urea supplied by the melter is collected in a small tank located beneath the melter. This tank and the melt high-pressure piping are designed to minimize biuret formation by retaining urea in the molten state less than 30 s. Molten urea is pumped to the header by a doubleacting piston pump. For flexibility, the pump is driven by a piston-type air motor. The flow rate of urea is indicated by a turbine flowmeter and is manually controlled by changing the pneumatic pressure to the air motor. The pump and all high-pressure valves are submerged in a constant-temperature oil bath. All molten urea piping is steam-jacketed, and all equipment in contact with molten urea is made of T y p e 3 16L stainless steel, except the pump
Ind. Eng. Chem. Prod.Res. Dev., Vol. 21, No. 4, 1982 6 l g
Table I. Typical Operating Conditions for Urea Granulation by the TVA Falling Curtain-Evaporative Cooling Process urea feed rates, kg/h molten urea t o granulator undersize (recycle) to granulator seed feed t o granulator granulator discharge rates, kg/h product undersize (recycle) oversize dust and ammonia air and water flow rates water sprayed into granulator, kg/h airflow through granulator, actual m3/min airflow through cooler, actual m3/min process temperatures, "C urea as sprayed recycle entering granulator seed granules entering granulator urea leaving granulator air leaving granulator air entering cooler urea leaving cooler process pressures, Pa urea melt at spray nozzles air to atomize water urea drum granulator rotational rate, r/min relative humidity of air leaving: % feed-end retaining ring height, cm discharge-end retaining ring height, cm urea spray nozzles type pattern equivalent orifice diameter, mm angle of spray, deg number in operation spraying distance, cm water spray nozzles type number of nozzles used screened product characteristics crushing strength,b kg sphericity, % moisture content, wt % Tyler screen analysis, % t 6 mesh (3.36 mm diameter) -6 t 7 mesh (3.36-2.83 mm diameter) -7 + 8 mesh (2.83-2.38 mm diameter) -8 + 9 mesh (2.38-2.0 mm diameter) -9 mesh (2.0 mm diameter)
95-113 99 at 25 "C 85 at 28 "C 146-149 60-66 24-27 82-99 66-77 24-29 49-66 2.76-3.45 X l o 6 0.28 X l o 6 11.5 20-40 38.1 12.7 flat oval 0.53-0.66 65 27 21.6 pneumatic atomizing wide-angle round spray 5 3.2-3.4 75-90 0.1
2 20 64 13 1 aDependent on atmospheric conditions as well as water spray rate. bMeasured on -7 + 8 mesh granules. Procedure in TVA Bulletin Y-147.
which is made of Type 303 and Type 304 stainless steels. The water for evaporative cooling is metered to wide-angle pneumatic atomizing nozzles with round spray patterns that spray in the drum countercurrent to the air being pulled through the drum. Fluid-Bed Cooler. The fluid-bed cooler has a bed area of 0.93 m2 and a stainless steel fluidizing screen that contains a large number of holes with equivalent diameters of 0.036 cm for a screen open area of 8.2%. Air for fluidization and cooling in the fluid-bed cooler is provided by two centrifugal fans (a blower and an exhauster). The damper arrangement in the ducts allows close control of airflow through the cooler. A horizontal cyclone removes any seed-size particles in the air leaving the cooler, and the particles collected are returned through an airlock to the elevator. The elevator is a continuous-discharge-type unit that is operated at a slow speed to prevent breakage of the material being handled. Screening and Recycle. The screen is a double-deck, gyrating-type unit. Screen sizes are changed as necessary to obtain the size of product desired. After screening, the
product is collected in a product hopper and is later transported to storage. Undersize leaving the screen is moved by a belt conveyor to the recycle surge hopper which has a capacity of about 2.9 t of urea. Seed Preparation and Feeding. The process calls for all oversize material, and some product if needed, to be fed on an interim basis to a thin-bladed hammer mill. The speed of the mill is variable and can be adjusted to optimize the efficiency of seed formation. The crushed material is fed to the dust separator which separates the dust from the seed size particles using the principle of air classification. The dust separator is a small fluid-bed unit with a screen area of 0.19 m2. It is designed to operate at an airflow of about 13.6 m3/min. At the resultant air velocity, the dust is entrained and the seed is left in the fluid bed. Usually, about 50% of the feed to the hammer mill is retained as seed granules. The dust separated from the seed is collected in a cyclone which has a diameter of about 30.5 cm and operates at a pressure drop of about 995 Pa. The crusher feeder and seed feeder, like the recycle feeder, have variable-speed belt control and are
620
Ind. Eng.
Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
mounted on scales for precise rate determination. The low-pressure-drop, wire-mesh mist eliminators are mounted in tanks 0.91 and 1.07 m in diameter. The tanks, mist eliminators, centrifugal exhaust fans, and inline centrifugal pump used for pumping scrubber solution are all constructed of stainless steel. General. All rotary equipment is driven by totally enclosed fan-cooled motors. Instruments and access points are provided throughout the pilot plant to facilitate the taking of data and samples. Operating Results Typical operating data from pilot-plant production of granular urea of -6 +9 mesh (3.36-2.0 mm diameter) particle size at a rate of 1.8 t / h are given in Table I. Heat Balance. The urea melt is usually sprayed onto the granules in the granulator drum a t temperatures between 146 and 149 "C. The granules discharge from the granulator drum a t temperatures between 82 and 99 "C, and they normally leave the fluid-bed cooler a t temperatures between 49 and 66 "C. When melt is sprayed at 149 "C and granules leave the cooler at 60 "C, about 4.09 X lo5 J/ kg of urea granulated is released. This includes heat of crystallization, about 2.42 X lo5 J/kg, the sensible heat released in cooling of melt to the crystallization temperature, and the sensible heat released in subsequent cooling of granules. In the pilot plant, about 68% (5.4 X lo8 J / h ) of this heat released in the granulator drum is removed by the combination of water evaporation, increase in temperature of the airstream passing through the granulator, and losses through the shell of the drum. The remaining 32% of the heat (2.5 X lo8 J/h) is removed in the fluid-bed cooler. This large transfer of heat in the granulator is possible with a relatively low airflow rate, 99 m3/min a t a production rate of 1.8 t/h, because of the rapid evaporation of finely atomized water in the airstream. At an air discharge temperature of 49 "C, the water absorbs about 2483 J/kg of water evaporated. Typically, between 52 and 63 L of water is evaporated in the granulator drum for each metric ton of urea produced. Product Quality. Pilot-plant work has shown that the uniformity of size of the final product can be controlled by the ratio of the recycle feed rate to the melt spray rate. The recycle consists only of undersize granules, and the rate at which recycle granules are fed to the granulator is controlled by a scale-mounted belt feeder. As the amount of recycle is increased for a given spray rate, the size of the product becomes more uniform. Data from the pilot plant indicate that in production of -6 +9 mesh (3.36-2.0 mm diameter) product, oversize generated is only about 1% of the melt spray rate a t a recycle-to-melt ratio of about 1:l. Higher recycle rates make the product extremely round and very uniform in size; conversely, lower recycle rates increase oversize production. Any oversize that is not used for seed generation must be remelted and regranulated. This requires additional energy consumption and increases biuret content of the final product; both are
undesirable. Ideally, the amount of oversize produced should be no more than that needed for seed generation. The pilot-plant tests of urea granulation have shown that the process consistently yields urea that is hard, dense, and spherical. It is believed that formaldehyde or other additives will not be required for development of satisfactory product hardness and storage properties, and tests to establish this are planned. However, in tests thus far, melt completely free of formaldehyde has not been available for testing and the lowest formaldehyde content of -6 +9 mesh (3.36-2.0 mm diameter) product has been 0.06%. This product showed good storage properties and good hardness, 2.3- to 3.2-kg crushing strength for minus -7 +8 mesh (2.83-2.38 mm diameter) granules. Dust Generation. Results from the pilot plant indicate that dust formation in the process is less than 2 % of the urea granulation rate. General. The process appears to be easy to control, and manpower requirements for a large production plant of this design should be less than those for granulation processes now in use. Overall energy consumption and initial capital expenditures for this process also should be lower than for conventional granulation processes.
Future Development Process development work is continuing, and results from pilot-plant tests have shown so much promise that TVA decided in April 1982 to build a prototype or demonstration-scale production unit to produce 300 t/day of granular urea as soon as possible. Operation of the demonstration plant should help to speed the transfer of this technology to the fertilizer industry which as shown considerable interest. Pilot-plant work presently is being directed toward obtaining the remainder of the scaleup data needed for design of the demonstration unit. In accordance with usual TVA policy, nonexclusive licenses will be granted for use of the process. Literature Cited Blouin, G. M.; Rindt, D. W. US. Patent 3295950, 1967. Blouin, G. M. US. Patent 3877415, 1975. Biouin, G. M. U.S. Patent 3991 225, 1976. Biouin, G. M. TVA Bull. 1974, No. Y-82. Engelstad, 0. P.; Getsinger, J. G.; Stangel, P. J. TVA Bull. 1972, No. Y-52. Myers, F.; Russel, D. A,; Young, R. D. TVA Circ. 1979, No. 2-102. Shirley, A. R.; Meline, R. S. A&. Chem. Ser. 1975, 140, 33-54. Shirley, A. R.; Nunnelly, L. M.: Carney, F. T. Proceedings, 30th Annual Meeting of the Fertilizer Industry Round Table, Atlanta, GA, 1980; pp 142-45. Shirley, A. R . , Jr. US. Defensive Publication T980 005, 1979. Shirley, A. R., Jr. US. Patent 4213924, 1980. Shirley, A. R.; Meline, R. S.; Nunneliy, L. M.:McCamy, I.W. TVA Circ. 1979, No. 2-100. Tennessee Valley Authority TVA Bull. 1978, No. Y-136. Tennessee Valley Authority TVA Bull. 1976, No. Y-107. Tennessee Valley Authority TVA Bull. 1980, No. Y-158.
Receiued f o r review November 9, 1981 Acoepted March 15, 1982
Presented at the 182nd National Meeting of the American Chemical Society, New York, NY,Aug 1981, Division of Fertilizer and Soil Chemistry, Paper No. 25
