I n d . E n g . Chem. Res. 1989, 28, 630-632
630
Satterfield, C. N.; Colton, C. K.; Pitcher, W. H. Restricted diffusion in liquids within fine pore. AIChE J. 1973, 19, 628. Schneider, P.; Smith, J. M. Adsorption rate constants from chromatography. AIChE J . 1968, 14, 762. Villadsen, J.; Michelsen, M. Solution of Differential Equation by Polynomial Approximation; Prentice-Hall: Englewood Cliffs, NJ, 1978; Chapters 2-4. Villadsen, J. V.; Stewart, W. E. Solution of boundary value problems by orthogonal collocation. Chem. Eng. Sci. 1967, 22, 1483. Weber, A. I.; Liang, S. Dual Particle Diffusion Model for Porous Adsorbent in Fixed Beds. Environ. Prog. 1983,2,3, 167.
Wen, C. Y.; Fan, L. T. Models for Flow Systems and Chemical Reactors; Mancel Dekker: New York, 1975. Wilson, E. J.; Geankoplis, C. J. Liquid Mass Transfer a t Very Low Reynold Number in Packed Bed. Ind. Eng. Chem. Fundam. 1966, 1 , 9.
Wu, P. D.; Debebe, A.; Ma, Y. H. Adsorption and diffusion of C6 and C, hydrocarbons in silicalite. Zeolites 1983, 3, 117. Received for review July 12, 1988 Revised manuscript received January 25, 1989 Accepted February 7, 1989
Polyethylene-Coated Urea. 1. Improved Storage and Handling Properties Omar A. Salman Products D e p a r t m e n t , Petroleum Petrochemicals and Materials Division, Kuwait I n s t i t u t e for Scientific Research, P.O. Box 24885, 13109 S a f a t , Kuwait
The effects of encapsulating urea prills with a low-density polyethylene (LDPE) film on improving the handling and storage characteristics of urea were investigated. Six tests were performed on uncoated and LDPE-coated urea samples: crushing strength, abrasion resistance, impact resistance, absorption-penetration test, caking tendency, and chemical compatibility with superphosphate. It was found that LDPE-coated urea is much superior to uncoated urea. In addition to being a controlled-release or slow-release fertilizer, its storage and handling properties are excellent. LDPE-coated urea granules are resistant to caking, abrasion, and crushing and are highly compatible when blended with superphosphate. Of the three primary plant nutrients-nitrogen, phosphorus, and potassium-nitrogen is lost most easily from the soil. Since the total loss is estimated to be between 30% and 5070,repeated application of nitrogen fertilizer becomes essential. This adds extra cost for materials and labor and causes inconvenience and a high solute concentration in the soil. One method of reducing nutrient losses is to use slowor controlled-release fertilizers. There are three types of these fertilizers: slightly soluble materials such as ureaformaldehyde; materials for deep placement such as urea super granules; and fertilizers coated with semipermeable or impermeable membranes. This paper deals with the last type, particularly polymer-coated urea. Coated fertilizers are physically prepared from granules of conventional fertilizers coated with materials that reduce their dissolution rate. Commercially available coated fertilizers can be divided into two categories: sulfur-coated urea (SCU) and polymer-coated urea. SCU has been under development by the Tennessee Valley Authority (TVA) since 1961 (Young, 1974). Sulfur was selected as a coating material because of its low cost. TVA started with coating small batches of urea in small rotating drums to continuous coating in a pilot plant with a capacity of 1 ton/h (Shirley and Meline, 1975). The first commercial polymer-coated fertilizers were developed by the Arthur Daniels Midland Co. (ADM). The main component of the coating is a copolymer of dicyclopentadiene with a glycol ester (Powell, 1968). Nutrients are released through osmotic exchange with moisture from the soil. The Sierra Chemical Co. currently produces this coated fertilizer under the trade name Osmocote. Two other polymer-coated fertilizers are produced commercially: Sierrablen and Agriform. Most of these products are based on ammonium nitrate mixed fertilizers and on single nutrients, according to the customer's requests. 0888-5885/89/2628-0630$01.50/0
In a previous paper (Salman,1988), the effect of coating urea prills with low-density polyethylene (LDPE) on reducing the release rate of urea was studied. In this paper, the physical characteristics of LDPE-coated urea compared to uncoated urea are investigated.
Experimental Procedure Materials. The urea from the Petrochemical Industries Co. had a nitrogen content of 46.0% and a particle size range of 0.5-2 mm. Low-density polyethylene (LDPE) was purchased from CDP Chemie, and laboratory-grade toluene was purchased from BDH Chemical Ltd. Apparatus and Method. A schematic diagram of the fluidized-bed equipment used for coating is shown in Figure 1. In a typical experiment, the product container is filled with about 1 kg of urea prills and is inserted into the apparatus and sealed pneumatically. The turbine is run for 5-10 min to preheat and fluidize the urea particles. Polyethylene pellets are dissolved in toluene by heating and stirring to give a polymer solution concentration of 10% by weight. The spraying process begins by starting the dosing pump and the atomizing air. Solvent vapors are collected by a tubular condensor. After the filmcoating solution is sprayed, about 200 mL of pure solvent is sprayed to clean the tubes and nozzle assembly. The final step is drying. The air temperature is raised to 80 " C , and the atomizing air is shut off. Drying time (usually 5-10 min) depends on the required moisture content. The physical properties of uncoated and LDPE-coated urea which are related to storage and handling characteristics were evaluated. Six tests were conducted: crushing strength, abrasion resistance, impact resistance, absorption-penetration test, caking tendency, and chemical compatibility with superphosphate. All tests were performed according to standard procedures (International Fertilizer Development Center, 1986). Two LDPE-coated urea products were evaluated: PCU-3 refers to 3% total C 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 631 Table I. Physical Properties of LDPE-Coated U r e a a n d U r e a Granules materials test procedure urea PCU-3 PCU-6 crushing strength, kg/granule 1.23 1.53 1.55 0 27.8 0.3 abrasion resistance, % degradation 0.1 impact resistance, '70 degradation 10 0.2 absorption-penetration test 94 moisture absorption, mg/cm2 383 163 3.0 moisture penetration, cm 20 4.0 31.3 moisture-holding capacity, mg/cm3 19.3 40.8 caking tendency (small-bag method) none 1-mo storage; 9'0 +12.5-mm lumps 1 none 3-mo storage; % +12.5-mm lumps 3 none none none 6-mo storage; % +12.5-mm lumps 5 none
1
...
I
F i g u r e 1. Schematic diagram of the fluidized bed.
2o
A
Z.#XLDPE
Y
4.9YaLDPE
0
5.7XLDPE
t I/
I TIME IDAYE1
F i g u r e 2. Effect of coating percentage (low-density polyethylene) on urea dissolution in water a t 22 "C.
coating and PCU-6 refers to 6% total coating.
Results and Discussion When the urea prills are encapsulated with LDPE, the release can be controlled by changing the total amount of coating material. As shown in Figure 2, the dissolution of urea in water decreases with increasing coating percentage. This is due to the reduction in the number of pinholes which are the primary path for urea release from LDPEcoated urea. In addition to reducing urea release rates, LDPE coating film improves the physical properties of the fertilizer which are very important for storage and handling characteristics. Six tests were conducted to evaluate the physical properties of LDPE-coated urea in comparison with uncoated urea. The results for each test are discussed below. Crushing Strength. Crushing strength is a measure of the resistance of granules to deformation or fracture
under pressure. This is an important property for handling and bag storage. Low crushing strength results in the formation of urea dust which is a health hazard and not acceptable by farmers. The crushing strength was measured by applying pressure to individual prills of a size range 2.30-2.36 mm. For each sample, 25 prills were tested and the average value is reported. As presented in Table I, the crushing strength of both PCU-3 and PCU-6 is about 24% higher then uncoated urea. Generally, if the crushing strength is higher than 1.4 kg/granule, the product will have acceptable handling and storage properties. Thus, both PCU-3 and PCU-6, which have crushing strengths of 1.53 and 1.55 kg/granule, respectively, will exhibit no degradation during handling or bag storage. Abrasion Resistance. Abrasion resistance is the resistance to the formation of dust as a result of particleto-particle and particle-to-equipment contact. The test is conducted by placing a sample into a rotary drum with stainless steel balls and rotating the drum at 30 rpm for 5 min. The percentage of fines and dust formed is a measure of abrasion resistance. Abrasion resistance is very important in preventing fine and dust formation during transportation of urea in the plant. Fines must be removed and recycled to prilling tower, which increases the production cost of urea. In addition, fines formed during shipping and storage are rejected by farmers since the dissolution rates of fines are very high. The abrasion resistance value for uncoated urea is quite low. As presented in Table I, the percent degradation is about 28. Both types of LDPE-coated urea, however, exhibit very high abrasive resistance. The percent degradation for PCU-3 and PCU-6 is 0.3% and 0%. Thus, normal handling, shipping, and sorting will not cause any fine formation for LDPE-coated urea. Impact Resistance. Impact resistance is a measure of the mechanical strength of fertilizer particles. It is measured by calculating the percentage of degraded particles after subjecting a sample to a standardized impact. This impact is produced by pouring the sample through a 35-ft pipe onto a metal plate within the pipe cap. PCU-3 and PCU-6 had essentially no granule breakage during impact (Table I), indicating a high impact resistance. The impact resistance of uncoated urea is lower as indicated by the higher percentage of shattered granules (10%). Therefore, when the fertilizer material is discharged from an overhead conveyor into a bulk pile or when bags are dropped during handling, no degradation is expected for LDPE-coated urea.
Absorption Penetration Test Moisture absorption or hygroscopicity is a very critical property for fertilizers. Flowability of fertilizers during
632 Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989
field application and bulk storage are highly 'affected by the hygroscopic properties of the fertilizer. The test was carried out by exposing the fertilizer samples to 80% relative humidity at 30 "C for 72 h in open-top glass cylinders. Exposed samples were evaluated for (1)moisture absorption per unit of exposed surface, (2) depth of moisture penetration, and (3) moisture holding capacity. The results are summarized in Table I. The moisture absorption valve of uncoated urea was 383 mg/cm2. The moisture absorption valves for PCU-3 and PCU-6 were 163 and 94 mg/cm2, respectively, which are significantly lower than uncoated urea. Similarly, the moisture penetration valves for LDPE-coated urea were much lower than uncoated urea. The depths of moisture penetration for PCU-3 and PCU-6 were 4.0 and 3.0 cm. This compared to a valve of 20 cm for uncoated urea. The moisture holding capacity (mg/cm3) is the ratio of moisture absorption over moisture penetration. Thus, as expected, LDPE-coated urea had a higher capacity for holding moisture than uncoated urea.
Caking Tendency Caking is defined as the tendency of a fertilizer to agglomerate or form lumps during bulk or bag storage. Caking of fertilizer is affected by the following parameters: moisture content, particle size and hardness, conditioners, storage temperature and pressure, and material composition. The caking tendencies were determined using a smallbag storage test. Samples of 1800 cm3 were charged into polyethylene-lined bags. The inner liner and outer polypropylene bags were then sealed and the filled test bags were placed in storage racks inside a controlled temperature chamber at 30 "C. A 10-15-mm-thick plywood board was placed on top of the test bags. A dead weight pressure of 0.28 kg/cm2 was applied by placing a weight on the top of a plywood board. Then, the test samples were inspected after 1, 3, and 6 months in terms of percentage of lumps larger than 12.5-mm size. No +12.5-mm lumps were formed for PCU-3 and PCU-6 even after 6 months of storage. Uncoated urea, however, had 1%,370, and 5% lumps after 1, 3, and 6 months of storage, respectively. The caking resistance of LDPEcoated urea increases its marketability. First, there will be no need for polyethylene-lined bags in shipping. Bulk shipment wihtout the addition of anticaking conditioners will be possible, thus reducing costs. Second, fertilizer material will be flowable and easy for the farmer to apply either manually or by machinery. Chemical Compatibility The chemical compatibility of bulk blends is the ability of two or more materials to remain dry and free flowing when blended together. When sources of plant nutrients (nitrogen, potassium, and phosphorus compounds) are blended, chemical compatibility is very important to assure the flowability, dryness, and integrity of fertilizer particles. Urea and superphosphate are known to have low compatibility. Thus, uncoated urea, PCU-3, and PCU-6 were blended separately with superphosphate in a 1:l ratio by volume. The blend was placed in a sealed glass bottle and kept in an oven at 30 "C for 60 days. Each blend was inspected daily, and any wetting, caking, disintegration, or gas evolution was recorded. Table I1 summarizes the results of chemical compatibility tests. The blend caking tendency test showed that
Table 11. Chemical Compatability of LDPE-Coated Urea a n d Urea Blended with Superphosphate materials test mocedure urea PCU-3 PCU-6 caking tendency (small-bag method) 1-mo storage, 70 +12.5-mm lumps 51 none none 3-mo storage, % +12.5-mm lumps 77 none none 6-mo storage, % +12.5-mm lumps 100 none none chemical compatability (small-bag method) I C C 1-day storage I C C 10-day storage I PC C 30-day storage I PI PC 60-day storage
uncoated urea had 51%, 77%, and 100% lumps (+12.5 mm) after 1, 3, and 6 months of storage, respectively. LDPE-coated urea (both PCU-3 and PCU-6) had no lumps during the whole storage period. Chemical compatability was determined visually according to the following guidelines: visual observation dry and free flowing damp but still free flowing damp and nonflowing wet and nonflowing
rating compatible (C) predominantly compatible (PC) predominantly incompatible (PT) incompatible (I)
Uncoated urea blended with suerphosphate became wet and nonflowing after only 1 day of storage. PCU-3 blended with superphosphate was completely compatible after 10 days. After 30 days, the blend was damp but still free flowing. After 60 days, the blend became nonflowing but no wetting was observed. PCU-6 blend showed full compatibility up to 30 days of storage. Even after 60 days, the blend was still free flowing with slight damping. In conclusion, LDPE-coated urea showed a huge improvement over uncoated urea in the storage and handling properties. This increases the marketability of LDPEcoated urea and offsets its increased cost compared to urea. The production cost of LDPE-coated urea depends mainly on the amount of LDPE added. For 3% LDPEcoated urea, there is a 20% increase in production cost over urea. This increase is quite acceptable since LDPE-coated urea has many advantages compared to uncoated urea: lower nitrogen losses, lower labor costs since LDPE-coated urea is applied in a single dose, and better physical properties.
Acknowledgment This work has been supported in part by the Kuwait Foundation for the Advancement of Sciences. R e g i s t r y No. LDPE, 9002-88-4.
Literature Cited International Fertilizer Development Center Manual for Determing Physical Properties of Fertilizers. Muscle Shoals, AL, 1986; procedure IFDC-R-6. Powell, R. Controlled Release Fertilizer; Noyes Development Corp.: Park Ridge, NJ, 1968. Salman, 0. Polymer Coating on Urea Prills to Reduce Dissolution Rate. J . Agric. Food Chem. 1988,36, 616-621. Shirley, A,; Meline, R. Sulfur Coated Urea from a One Ton-Per-Hour Pilot Plant. Adu. Chem. Ser. 1975, 146, 33-54. Young, D. TVA's Development of Sulfur Coated Urea. TVA Bulletin 4-79, 1974; Tennessee Valley Authority, Muscle Shoals, AL.
Received for review August 26, 1988 Revised manuscript received January 23, 1989 Accepted February 15, 1989
