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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol.18 no.1 São Paulo Mar. 2001 



G.S.E.Ayub, S.C.S.Rocha and A.L.I.Perrucci
Departamento de Termofluidodinâmica, Faculdade de Engenharia Química,
UNICAMP, Cx. P. 6066, 13083-970, Campinas - SP, Brazil


(Received: January 5, 2000 ; Accepted: September 19, 2000)



Abstract - In this work the quality of sulphur-coated urea was determined by the urea dissolution rate and analysed by electron microscopy. Particles of urea were coated in a two-dimensional spouted bed, having a 60º slanted base angle and an atomization nozzle installed at the base. Elementary sulphur was liquefied and atomized on the particles. The experiments were planned with the objective of verifying the influences of flowrates of sulphur and atomized air and the temperature of the air used in the spouted bed on the quality of the coated particle surface. The temperature of the spout air and the flowrate of the atomized air showed a significant influence on the quality of the coating.
Keywords: spouted beds, coating of particles in spouted beds, sulphur coating of urea.




Gishler and Mathur (1957) developed the spouted bed technique as a method for drying wheat, and the first industrial spouted beds were installed in Canada in 1962 to dry peas, lentils and linen. Research on this technique for gas-solids contact showed that the spouted bed could be applied to other operations, such as mixing of solids, cooling, coating, granulation, etc. Most results presented in the literature were obtained for conical and cone-cylindrical spouted beds and refer to problems of scaling-up processes, due to the difficulty of maintaining the bed fluid-dynamics in large beds. To overcome the scale-up problem, Mujumdar (1984) presented the retangular bed with an inclined base, named two-dimensional spouted bed. Passos (1991) extensively analysed this bed shape and concluded that there were some advantages to its use.

One of the possible applications of the spouted bed technique is the coating of particles in the fertilizer industries. Of the three main essential nutrients applied to the plants, N, P and K, nitrogen is the one that is rapidly lost by leaching. Urea has been used as an important nitrogen source, due to its high nitrogen content of around 45%. Also, urea is water soluble, making its feasible application on the soil and/or on the leaves. The main disadvantages encountered when using urea are its high hygroscopicity, the loss of nitrogen by NH3 volatilization when applied to the soil and the lost of urea by leaching before and during hydrolisis. A possible way to overcome these disadvantages is to coat the urea particles with a water-impervious material. The coated material can also permit control of the availability of nitrogen to the plant. Suplhur is an interesting coating material as it is water impervious, is used as a micronutrient with an important role in the development of many plants, is cheap and an is abundant industrial residue.

The objective of this work is to develop a process of coating urea with sulphur in a two-dimensional spouted bed and analyse the quality of the coated particles related to the process variables: temperature of the spout air and the mass flowrates of sulphur and atomized air. The product quality was evaluated based on the dissolution rate of urea, called D25%, which is defined as the percentage of urea that dissolves when 50g of the product, of which 25% is sulphur, is put in 250ml of water at 38º C for seven days. The surface of the coated particle was also analysed using an electronic microscope.

Coating of Urea with Sulphur

Rindt et al. (1968), at the Tennessee Valley Authority (TVA) produced the first work to consider the agronomic importance of coated fertilizers. They studied several cost-fertilizer combinations, but only the urea coated with sulphur showed the possibility of large-cale marketing.

The first process for coating urea with sulphur was developed by TVA. This process consisted of five stages: the pre heating of urea, the coating of the urea particles in a rotary drum, where liquid sulphur was atomized on the particles, a second coating stage with wax to cover imperfections on the sulphur coating, which presented some holes through which the urea could be leached, the cooling of the coated product and the adequate conditioning of the product to avoid particle agglomeration. The TVA process is mechanically complex and the investment and maintenance costs are high.

Meisen and Mathur (1978) verified the possibility of using the spouted bed for coating urea with sulphur. A batch-operated pilot plant was developed and the particles were coated in a cone-cylindrical spouted bed. The authors concluded that product quality is a function of air temperature and that the product obtained was comparable to that of the TVA process for some fixed process conditions.

Weiss and Meisen (1983) modified the basic equipment used by Meisen and Mathur (1978) and related product quality to the operational conditions. The best quality product was obtained when the bed was maintained at 80º C. Evaluating the dissolution rate, D25%, they concluded that quality increased at higher sulphur mass flowrates and at lower atomizing air flowrates.



Elementary sulphur was used as the coating material. Some of its physical properties are shown in Table 1 (Sander, 1984).



When the temperature of the melted sulphur is increased, its viscosity first decreases, as occurs with most of the liquids, from 17 cp at 120º C to 6.4 cp at 158º C. In this temperature range the melted sulphur is yellow. Above this temperature viscosity rapidly increases to a very high value at 160º C, and at this point the melted sulphur is dark red. This behavior of the viscosity of sulphur in relation to temperature is explained by the modification of its molecular structure.

The urea particles used in this work were produced by Ultrafertil S/A. The Sauter mean diameter was obtained by granulometric analysis in a standard sieve series and is 1.98 mm. A particle density of 1335.0 kg/m3 was determined by the manufacturer. With these values and the density of the air, we could verify in Geldart's diagram that urea particles are classified as a group D powder, which justifies the choice of the spouted bed as the proper gas-solids contact.

Experimental System

The two-dimensional spouted bed was constructed of Plexi glass to allow the observation of particle movement. Bed dimensions followed the relations proposed by Kalwar et al. (1988) to maintain spouted bed stability and adequate solids circulation. According to the authors the depth of the air inlet orifice must be equal to the depth of the bed and the width of the air inlet orifice must be within 1/6 and 1/20 of that of the bed. Figure 1 shows the two-dimensional spouted bed used in this work.



The double-fluid atomizer was set at the bed base. The complete experimental assembly developed and constructed is shown in Figure 2.



With the objective of studying the effects of the temperature of the spouted air, Tas, the sulphur mass flowrate, Ws, and the atomizing air flowrate, Wat, on the coating process, a two-level experimental factorial design was used. The results permit the evaluation of the statistical significance of the influence of the operational variables, specified above, on product quality in terms of D25%,.

Several preliminary experiments were conducted to establish the methodology and the ranges for the variables. Table 2 presents the operational conditions and the process times set for the experiments.



The mass of urea in the bed was fixed at 1300.0g for all experiments and the process time was fixed in 20 minutes. The spout air flowrate was set at 85% above the minimum spout flowrate, as the mass of sulphur added to the bed causes a mass increase in the bed of about 50%.

Methodology for Determination of Product Quality, D25%

The samples collected during the process of coating and after the end of the experiments were analysed to determine the sulphur content and the dissolution rate. The standard TVA test was used to obtain the dissolution rate after seven days: 10g of coated particles are put in a test tube containing 50ml of water and maintained at 38º C for seven days. After this period, the tube is shaken slightly and a sample of the solution is removed. The urea content is obtained by refractometry. The seven-day dissolution rate is thus given by

The dissolution rate is a function of the sulphur content of the coated material. Thus, to compare the quality of the products obtained for different operational conditions, the dissolution is evaluated for a fixed sulphur content. The sulphur content usually especified is 25%. The value of D25% is found fitting the curve of D versus sulphur content and by interpolation. The TVA dissolution test described and applied here is a measurement of the average dissolution of the sample and is not adequate to determine the dissolution of individual particles.



Analysis of the urea dissolution rate, D25%

The values obtained for the urea dissolution rate, D25%, can be used to evaluate product quality, as it represents the diffusion of urea through the sulphur layer. The values of D25% were determined every 24 hours until the 7th day for all coating process conditions, according to the experimental design established. Table 3 shows the D25%. results obtained in this work.



The statistical analysis of the results was based on Boxet al. (1978), and the variables Tas, Wat, as well as the interaction TasxWat, showed significant effects on the response, D25%, at a significance level of 5%. The significance of the effects can be seen in Figures 3 and 4.




The behavior of the dissolution rate presented in Figure 3 shows that D25% is a function of spout air temperature and has a lower dependence on the atomizing air flowrate. To obtain lower values of D25%, the coating process may be conducted at the higher values of air temperature and atomizing air flowrate. The same behavior can be seen in Figure 4. The same tendency was verified by Weiss and Meisen (1983) for the influence of air temperature on D25% and the opposite behavior was obtained by the authors for the atomizing air flowrate. The main conclusion, however, was that air temperature is the principal variable in controlling the final value of D25%. We can attempt to explain this behavior as follows: when the coating process occurs at a temperature much lower than the melting point of sulphur, the droplets of sulphur atomized on the bed of particles are cooled and start to form a solid film as soon as they are ejected from the nozzle into the bed. When these sulphur droplets collide with the urea surface, the solid films break and the melted sulphur inside is released and bonds to the urea surface. For the coating process conducted at higher temperatures, close to the melting point of sulphur, the droplets are deposited on the urea surface and then solidify, forming a very fine film of sulphur. In this case, as the atomizing air flowrate increases, a better product is obtained, as the droplets atomized on the bed are smaller, leading to a fine and consequently uniform sulphur film over the particles.

When the process was carried out at 69.0ºC, the result was a dark yellow product, indicating the structure of rhombic sulphur, and a slight change in its color was verified after a short period of storage. Also, the product obtained at this temperature showed a complete urea dissolution in 7 days, as shown in Table 3, characterizing poor product quality. Yet, we can observe that, for the processes conducted at 69.0º C, no significant difference occurred in D25% for the two levels used for the sulphur and atomizing air flowrates, Ws and Wat.

When the process was carried out at the higher temperature level, 82.5º C, the color of the product obtained was a clear and bright yellow, indicating the formation of monoclinic sulphur on the urea surface. This color changed to dark yellow when stored for 24 hours at room temperature, characterizing a change in the molecular structure. The values of D25% lower than 100% on the 7th day indicate a better product quality obtained at this higher temperature.

By examining Table 3 we can verify that the process conditions which led to the product with the lowest D25% value or the best quality are the higher Ws (33.9g/min), the higher Wat (1.4m3/h) and the higher Tas (82.5º C), to obtain the value of D25% = 95.61% at the 7th day.



Dissolution of urea depends on the formation of sulphur film during the process and also on the initial characteristics of the particle used. The urea used in this work had some holes, sometimes deep, as can be seen in Figures 5 and 6. When the coating process was performed at 69.0º C, the sulphur did not completely penetrate the holes, probably due to the high superficial tension of the atomized drop, resulting in a non uniform coating, as can be seen in Figures 7, 8 and 9.







Figures 10 and 11 show the coated urea particles at the higher air temperature, 82.5º C. These particles still have an irregularly coated surface, but not as severe as the ones coated at 69.0ºC. Surface quality is better and the solidified sulphur is starting to cover the holes. These characteristics are in accordance with the lower dissolution rates obtained for Tas=82.5ºC. Figures 12 and 13 show a comparison of the coated particles at 69.0ºC and 82.5ºC, amplified 1000x. The irregularly coated surface and the imperfections are evidenced for the lower temperature. The microscopic analysis is in agreement with the statistical results obtained for the urea dissolution rate.







The experimental assembly constructed to coat urea with sulphur in a two-dimensional spouted bed permitted the study of product quality as a function of the operational variables Tas, WS and Wat.

The statistical analysis of the process showed that the urea dissolution rate is significant influenced by air temperature, Tas, the atomizing air flowrate, Wat, and the interaction between the two.

To obtain the lower urea dissolution rates through the sulphur layer, D25%, the process shall be carried out at higher Tas and Wat.

Operation of the coating process at 69.0º C resulted in a dark yellow product, indicating the formation of rhombic structure, and a slight change in color was observed after a short storage period. For the process conducted at 82.5º C, the product obtained was clear and bright, characteristic of the monoclinic structure. The color of the product changed to dark yellow after 24 hours of storage at room temperature.

The electronic microscopic analysis of the coated particles revealed an irregular surface and the presence of deep holes, which were already present in the urea particle, for the process operated at 69.0º C. For the process carried out at 82.5º C the holes were totally or partially covered after the coating process, leading to a coated particle with a satisfactory surface regularity and uniformity.

The better quality of the surface observed in the microscopic images for the process conducted at the higher temperature and atomizing air flowrate is in accordance with the lower value of the urea dissolution rate, D25%, obtained for this process condition. A uniform surface with fewer imperfections offers higher resistance to urea diffusion through the sulphur layer.



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