Optimization of process parameters for fluidized bed granulation of urea-based compound fertilizers

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1 Overview At present, the application of fluidized bed granulation in the fertilizer industry is mainly for the production of large-grained urea. Since our company built a fluidized-bed large-grain urea production facility at the beginning of this century, due to factors such as existing fertilization practices, farmers have been reluctant to accept large-grain urea; as a result, sales in the market have been poor and the utilization rate of this production facility has remained low. To make full use of the facility, our company’s engineering and technical staff, after evaluating various options and taking into account the company’s plan to focus on the production of compound fertilizers, ultimately chose the approach of using a large-grain urea production plant to manufacture urea-based compound fertilizers. 2 Test Conditions 2.1 Test Process Equipment The process flow used in the test is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/0710251650381255.jpg The purpose of this experiment is to, through intermediate tests, provide optimized process operation parameters for the commissioning and operation of industrial plants; at the same time, it aims to identify and resolve any problems that may arise during industrial commissioning and production. Therefore, the experimental setup is designed to simulate an industrial plant as closely as possible in terms of its process flow. Parameters such as flow rate, temperature, and pressure are continuously measured and regulated using instruments. This experiment is operated intermittently, with seed particles being added in one go through a hopper. The fluidizing air is supplied by fan 1, measured by turbine flow meter 2, and heated to the desired temperature by electric heater 3 before entering the fluidizing air distribution chamber at the bottom of the fluidized bed granulator 4. From there, it enters the bed through the air distribution plates, thereby keeping the seed crystals in a fluidized state. After being melted, urea is added to the molten urea solution tank 6 and then pumped to the urine two-fluid nozzle 5 under compressed nitrogen pressure. The air for urine atomization is supplied by the air compressor 10, measured by the rotameter 9, heated by the heater 11, and then led to the nozzle for atomizing the urine. The granulation exhaust gas is dried and dust-removed via a cyclone separator 13, then further wet-washed in a scrubber tower 12, and finally exhausted by an exhaust fan 14. The exhaust gas washing is carried out through continuous cyclic washing by the circulation pump 15. 2.2 Test Procedure and Analysis Methods 2.2.1 Test Procedure This test employs batch operation; ammonium phosphate seed particles with a particle size of 1.1–3.2 mm and an average particle size of 2.5 mm are added all at once. While these particles are in a fluidized state, urine with a concentration of over 95% and a temperature above 138°C is introduced into the bed to carry out continuous granulation. At the same time, parameters such as air volume, temperature, urine flow rate, and changes in bed pressure during the granulation process are continuously recorded. After the urine is added, the granulated particles are cooled, after which the resulting product particles are taken out for testing of their content, particle size, strength, and dust level. 2.2.2 Analysis and detection methods and instruments http://www.nmtech.com.cn/jishuwang/upload1/0710251651131059.jpg 3 Results and discussion The fluidized bed granulation process involves many factors, including bed temperature, flow rate of the fluidizing air, static bed height, and the size of the urine droplets after atomization. The experiment primarily examined the effects of the aforementioned factors on the stability of the granulation process and product quality (including moisture, strength, particle size, and inter-particle adhesion), as well as the dust generation during the granulation process. 3.1 Influence of temperature on product quality and the granulation process The temperature of the fluidized bed is the main control parameter in the fluidized granulation process; it has a significant impact on the granulation process and directly determines the product quality. It is generally believed that when selecting the temperature in the fluidized granulation process, two factors must be taken into account: one is the effect on the moisture content of the product. An increase in temperature facilitates the evaporation of moisture during the granulation process, thereby reducing the moisture content of the product; conversely, if moisture cannot be evaporated as much as possible during granulation, the moisture content of the product may exceed the acceptable levels. The second is the condition of the atomized droplets upon reaching the surface of the granulation seeds. If the temperature is too low, a large number of atomized droplets cool and condense before reaching the surface of the granulation seeds, preventing them from adhering to these seeds and resulting in dust formation; this leads to an increase in dust levels during the granulation process ； If the temperature is too high, the droplets of mist on the surface of the seed crystals do not have enough time to cool and crystallize, leaving the surface of the seed crystals \"wet.\" This leads to adhesion and bridging between the seed crystal particles; in mild cases, this affects the uniformity of the product particles as well as the smoothness of their surfaces, while in severe cases it can result in a deterioration of the fluidization condition, or even a situation where the flow stops altogether. For this experiment, an increase in temperature during the granulation process will, on the one hand, accelerate the thermal decomposition of urea and ammonium phosphate. At around 115°C, urea and ammonium phosphate form a eutectic mixture; the material in the bed becomes in a slurry-like state, with a large amount of it sticking to the walls, which makes it impossible to carry out the granulation process at all. Therefore, temperature control is particularly important. Figure 2 shows the effect of bed temperature on the average crushing strength of the product particles. http://www.nmtech.com.cn/jishuwang/upload1/0710251651589708.jpg Using ammonium phosphate as a seed crystal, granulation can proceed normally at temperatures below 95°C; however, as the temperature decreases, the strength of the product significantly declines. When the bed temperature is above 110°C, the crushing strength of the product decreases ； Increasing the temperature further, at 115°C the granulation process becomes almost impossible, and almost all of the urea enters the cyclone separator in the form of dust. The reason for this phenomenon may be that as the temperature rises, the decomposition of urea under the action of ammonium phosphate intensifies, resulting in the production of large amounts of free ammonia. When the bed temperature exceeds 120°C, ammonium phosphate reacts with urea to form an eutectic (with a melting point of 115°C), causing the material to stick to the walls of the bed and hindering the granulation process. Therefore, taking the comprehensive test conditions into account, maintaining the temperature of the fluidizing air at 95–105°C during the granulation process ensures the proper progress of the granulation process, resulting in products with good strength and appearance. 3.2 Influence of fluidization gas velocity on particle fluidization and granulation process The fluidization gas velocity is an important factor affecting particle fluidization and the granulation process. According to classical fluidization theory, the fluidization gas velocity should be selected between the critical fluidization velocity (Umf) and the particle entrainment velocity (Ut). If the flow rate is below UMF, the particles will not be properly fluidized ； When the flow velocity is higher than ut, a large number of particles will be entrained into the cyclone separator. The fluidization gas velocity is influenced by various factors such as the properties of the particles themselves (such as average particle size, particle size distribution, particle shape, density, etc.), the properties of the fluid (such as density and viscosity), and the height-to-diameter ratio of the fluidized bed. In addition to obtaining preliminary velocity parameters through calculations, it is also necessary to determine these values experimentally. In the experiment, ammonium phosphate with an average particle size of around 2.5 mm was used as a seed crystal. Tests showed that in actual operation, by controlling the fluidization gas velocity at uf between 2.5 and 3.5 mm/s, the particle fluidization quality is good enough to meet the requirements of the granulation process. According to the Geldart classification, this particle belongs to Category D particles, characterized by a high critical fluidization velocity and poor fluidization properties. According to the traditional bubble theory, for Class D particles, once the gas velocity exceeds the critical fluidization velocity and the gas velocity is increased further, the excess gas passes through the bed in the form of bubbles, while the bed pressure drop remains constant. The actual measurement results also confirm this: as the fluidization number N (i.e., the ratio of the fluidization gas velocity uf to the critical fluidization gas velocity umf) increases, the pressure drop in the fluidized bed layer remains relatively constant (Figure 3), whereas the total pressure drop of the bed layer increases with rising fluidization numbers. The main reason for this is that an excessive pressure drop occurs across the air distribution plate as the fluidization number increases. Therefore, the selection of the operating fluidization number should take into comprehensive consideration factors such as the pressure drop loss of the air distribution plate, the critical fluidization velocity, and the stability of the fluidization operation. http://www.nmtech.com.cn/jishuwang/upload1/0710251652435002.jpg Tests showed that as the fluidization gas velocity increased, the height at which the particles were fluidized rose significantly. However, at high gas velocities, a large amount of gas passed through the bed in the form of large bubbles that formed and burst, resulting in a phenomenon of \"surgeing\"; the fluidization state of the particles became unstable, and bed vibration intensified. During the granulation process, collisions and fragmentation between particles as well as between particles and the inner walls of the bed increased, leading to more dust generation. 3.3 Influence of the static bed height on fluidization conditions and the granulation process Under certain fluidization gas velocities, the static bed height of the seed particles has a significant impact on the fluidization conditions, which in turn affects the granulation process. Tests revealed that at a certain fluidization gas velocity, if the amount of granulation seed particles is low, the particles in the bed are sparse, resulting in high dust generation during the granulation process; moreover, the seed particles tend to be carried away to the cyclone separator, preventing the granulation process from continuing ； If the amount of granulation seeds is too large, a continuous fluidization cannot be achieved; surging occurs, and the shaking of the bed intensifies. At a constant air volume, the effect of different static bed heights on the dust amount during the granulation process is shown in Figure 4. As can be seen from the graph, in the initial stage, during the granulation process the dust level shows a clear tendency to decrease as the amount of seed material increases. However, as the static bed height increases further, bubbling occurs and the dust level tends to rise again during granulation. Therefore, maintaining an appropriate static bed height is very important for ensuring the stable progress of the granulation process and reducing the dust ratio (the ratio of dust amount to urine amount) during this process. http://www.nmtech.com.cn/jishuwang/upload1/0710251653208568.jpg 3.4 The influence of urine atomization conditions on the granulation process and product quality: The impact of urea melt atomization conditions on the granulation process lies mainly in whether the granulation process can proceed smoothly and in the strength of the resulting products; it is also related to the temperature during the granulation process. For urea melt, when the particle size of the atomized droplets is between 60 and 120 μm, the wetting of the granulation seeds by these droplets is optimal. At this stage, the growth of the granulation seeds occurs primarily in a layered manner, with almost no agglomeration between the particles. The urea crystals on the surface of the product are small, and the particles are smooth. The atomized droplets are too large, resulting in agglomeration between particles; the urea crystals on the surface of the product are large, and the particle surfaces are rough ； At the same time, due to the large size of the atomized droplets, the cooling and crystallization rate of urine is slow; as a result, the stress between the seed crystals and urea does not have enough time to be released, leading to poor encapsulation and reduced particle strength. If the atomized droplets are too small, a large number of such tiny droplets during the granulation process can easily form dust, resulting in an increase in dust levels during granulation. The most important indicators for characterizing the atomization state of urine are the size and distribution of the atomized droplets. The atomization performance of nozzles of different models varies significantly. Figures 5 and 6 show, under a microscope, the size of the atomized droplets under different types of nozzles and their impact on the internal structure of the product. http://www.nmtech.com.cn/jishuwang/upload1/0710251653512520.jpg As can be seen from Figure 5, when the urine atomization droplets are too large, there is a clear boundary between the granulation seed and the outer coating layer, resulting in poor coverage; this is reflected in the product quality, as the particles have lower compressive strength and are more fragile ； As can be seen from Figure 6, when the atomized droplets are small, the bonding between the granulation seed and the outer coating layer is strong, with almost no distinct boundary; at this point, the compressive strength of the particles also increases significantly. http://www.nmtech.com.cn/jishuwang/upload1/0710251654205171.jpg Additionally, if the liquid spraying intensity is too high, the granulation process tends to proceed through agglomeration, resulting in irregularly large particles with multiple nuclei. At the same time, the fluidization state of the bed deteriorates rapidly, leading to what is referred to in the literature as \"wet quench\" – a condition in which the bed becomes inactive. Therefore, it is necessary to control the liquid spraying intensity appropriately during operation. Figures 7 and 8 are photographs of the appearance and cross-section of the multi-core particles. http://www.nmtech.com.cn/jishuwang/upload1/0710251654535711.jpg 4 Conclusion Numerous experiments have shown that by using ammonium phosphate with an average particle size of 2.5 mm as a seed, at a bed temperature of 95–105°C and an air flow rate of 3 m/s, with the static bed height maintained between 120–140 mm, and by employing GG-type urine nozzles that provide good atomization effects, it is possible to ensure a stable granulation process. The amount of dust generated during granulation is low; the average nitrogen content of the product is 32%, the phosphorus content is 18%, the average particle size reaches 3.0 mm, and the strength of the particles is greater than 25 N.