Fluidization and Spouting of Fine Particles: A Comparison
The fluidization characteristics of fine particles have been studied in both the fluidized bed and spouted bed. The effect of different system parameters (viz. static bed height, particle size, particle density and superficial velocity of the fluidizing medium, rotational speed of stirrer, and spout diameter) on the fluidization characteristics such as bed expansion/fluctuation ratios, bed pressure drop, minimum fluidizing/spouting velocity, and fluidization index of fine particles (around 60 micron particle size) have been analyzed. A stirrer/rod promoter has been used in the bed to improve the bed fluidity for fluidization process and spout diameter has been varied for spouted bed. Mathematical expressions for these bed dynamics have been developed on the basis of dimensionless analysis. Finally calculated values of these bed dynamics are compared with the experimentally observed values thereby indicating the successful applications of these developed correlations over a wide range of parameters.
Fluidization quality is closely related to particle intrinsic properties. The difference between fluidized bed and spouted bed lies in the dynamic behaviours of the solid particles. In a fluidized bed, air is passed through a multiorifice distributor/plate to fluidize the particles. Fluidized bed is divided mainly in two regions; bubble phase and emulsion phase  as shown in Figure 1.
Spouted bed is gas-solid contactor in which the gas is introduced through a single orifice from the centre of a flat base, instead of a multiorifice system, resulting in a systematic cyclic pattern of solid movement inside the bed  as shown in Figure 1. A spouted bed has three different regions: the annulus, the spout, and the fountain [3, 4]. At stable spouting process, a spout appears at the centre of bed, a fountain appears above the bed surface and an annulus forms between the spout and wall, as shown in Figure 1. At stable spouting process, spout and fountain are similar to fluidized beds where particles are dynamically suspended. On the other hand the annulus region is more like a packed bed. At partial spouting case, there are two distinct regions, an internal spout which is similar to a fluidized bed and the surrounding packed particle region similar to a packed bed. It is observed that spouted bed perform better for defluidization or segregation of particles .
As the particle size decreases the cohesive force between the particles increases causing the fluidity of the bed to decrease  for which the fine powders exhibit channeling , rise up as a plug and form “rat holes” when aerated. As a result, the fluidization of fine particles becomes difficult in comparison with the larger sized particles [7, 8]. Therefore, there is a need for the development of reliable techniques to improve the fluidization of cohesive or fine powders which has been focused in the present study. Usually, there are two methods to improve the fluidization quality of fine particles. One method is of applying external force  such as vibration, magnetic force, acoustic force, electric force, and centrifugal force to the fluidized bed. The other method is alteration of the intrinsic properties of particles [10, 11] that is, by modifying surface characteristics by mixing with other particles having different size or shape because of higher gravity force.
This paper presents a comparative study on hydrodynamic behaviours such as minimum fluidization velocity () and/spouting velocity (), pressure drop (), bed expansion ratio (), bed fluctuation ratio (), and fluidization index (FI) for both the fluidized and spouted bed using fine particles. The experimentally observed results are analyzed and processed with the system parameters on the basis of dimensionless analysis.
2. Materials and Methods
The experiments are carried out to study the hydrodynamic behaviours of fluidized and spouted bed. Schematic diagrams of experimental set up for two processes are shown in Figure 2. A filter cloth is used as the distributor and is placed in between two faces of flange located at the bottom of the fluidizer and the conical bottom to prevent back flow of the bed materials for the fluidization process. The calming section, that is, conical bottom, is packed with 5 mm glass beads for uniform distribution of fluid to the bed to prevent channeling. A stirrer (a rod promoter) is used inside the column to provide constant stirring to the bed material. The stirrer is connected to a motor and speed of rotation is varied by a varriac. The column is also covered with filter cloth at the top to prevent the entrainment of fine particles.
In case of spouting process, the distributor made from card board with a single orifice at the centre is used. The distributor is then covered with a filter cloth to prevent back flow of materials. Orifice diameter is varied. The conical bottom for the spouting process is kept empty for allowing a jet or direct flow of fluid to pass through the orifice of the distributor.
A cylindrical column, 5 cm ID and 100 cm high made of perspex sheet material, is used for both the fluidization and spouting processes. The rotameter is used for measuring the flow rate of air. A U-tube manometer is used for measuring the pressure drop across the bed with the carbon tetra chloride (CCl4) and mercury (Hg) as the manometric fluid for fluidization and spouting process, respectively. The experiments are carried out by varying different system parameters as discussed in scope of the experiments (Tables 1 and 2). The expanded bed heights and manometer readings are noted down at each flow rate of air.
3. Results and Discussion
The hydrodynamic behaviours of the fluidized bed or spouted bed have been studied in terms of bed pressure drop, bed expansion/fluctuation ratios and fluidization index using fine particles. The pressure drop profiles for different conditions are obtained from which the minimum fluidization/spouting velocities for fine particles are determined. A sample plot of pressure drop profile for both the processes is shown in Figure 3. It is observed that the bed pressure drop increases gradually with increase in velocity up to certain limit after which it remains constant for fluidization process. In spouting process, initially the bed pressure drop increases gradually with increase in minimum gas velocity up to certain limit. After that it increases abruptly due to bubbling and slugging, the bed is observed to fluctuate vigorously which is seen from the plot of pressure drop. Then the bed pressure drop decreases up to certain point after which it remains constant indicating stable fluidization. The peak part of the pressure drop profile due to sudden rise and fall in pressure drop values indicates the spout formation and breaking of spout respectively. From the Figure 3, it is also observed that the bed pressure drop () of fine particle is about 6.5 kPa and 3.3 kPa for fluidization and spouting process, respectively. Minimum fluidization velocity () and spouting velocities () are found to be 0.04 m/s and 8 m/s for fine particles, respectively, which is being validated (2). The fine particles are observed to have higher bed pressure drop in fluidization process than spouting process. The reason may be due to multiple orifices of small apertures used in fluidization instead of single large orifice. Again calming section in fluidized bed restricts the flow of air. That is why fluidization process needs more fluid for fluidizing fine particles than spouting process.
The overall changes in values of bed expansion and fluctuation ratios of fine particles are observed to increase with the increase in superficial velocity () for both, the fluidized bed and spouted bed (Figures 4 and 5). This may be due to the fact that when superficial velocity exceeds minimum fluidization/spouting velocity and bubbles form and then break because of collision with the particles/with other bubbles thereby resulting in bed expansion and fluctuation, respectively. As bubble size increases with excess superficial gas velocity the bed expansion ratio increases. Again, with the increase in initial static bed height (), the bed expansion and fluctuation ratios are observed to decrease because of more materials in the bed requires more flow of fluid to fluidize. It is also observed that the bed expansion and fluctuation ratios decrease with the increase with particle size () for both the processes because of more void spaces among the bed materials which leads to decrease in the number of the bubble formation in the bed.
The bed expansion and fluctuation ratios for the fluidized bed are observed to decrease with the increased speed of the stirrer () as the rotation of the stirrer prevents the bubble formation. In case of spouted bed these ratios were observed to decrease with the increase in spout diameter () because of higher flow of fluid through the distributor orifice at a higher force breaks the bubble quickly. The bed expansion ratio is observed to increase with the increased density of the particles () for both types of beds. The bed fluctuation ratio is observed to decrease for the fluidized bed and increase for the spouted bed for increasing densities. Increase in such bed dynamics may be due to the bubble formation whereas decrease may be due to the breakage of bubbles followed by quick settling of denser particles to the bottom of the bed. From Figure 4, it is observed that bed expansion ratio is more with fluidization process than spouting ones. This may be due to the fact that most of the bed materials are in bubble phase during fluidization process when compared to the spouting process. From Figure 5, it is observed that bed fluctuation ratio is more in spouting process than fluidization ones. This may be due to the fact that the minimum expanded bed height () for spouting process is much less than that for the fluidizing process due to single number of orifice in distributor for the former case.
The variations of fluidization index of fine particles with superficial velocity of fluid for both fluidized bed and spouted bed are shown in Figure 6. Fluidization index gives an idea of the degree of the uniformity in expansion of the bed. A high value of fluidization index is observed for sand and silicon carbide powders implying that the bed can hold more gas between the minimum fluidization and bubbling point. A low value of fluidization index is observed for the talcum powder, magnetite, and dolomite powder because of more cohesive forces among particles thereby indicating a brittle fluidization state where a small change could cause a break from the uniformly fluidized state to a bubbling regime or a packed bed. A sample plot of fluidization index versus superficial velocity of fluid for the Alumina powder is shown in Figure 6. It is observed from this figure that the fluidization index of Alumina powder is approximately one indicating proper or smooth fluidization process. Fluidization index in spouting process always deviates from the fluidization process. Fluidization index directly depends on pressure drop. From Figure 6, it is also observed that the value of fluidization index is more in fluidization process than spouting process. This may be due to the fact that bed pressure drop is more in fluidization process than those for the spouting process because of the presence rotational speed of stirrer and mini aperture size of spout.
Dimensionless analysis helps one to understand how the typical value of the dependent variable changes when any one of the independent variables is varied, while the other independent variables are held fixed. The calculated values of the results for dependent variables, namely, bed expansion ratio/bed fluctuation ratio and fluidization index, obtained from the developed correlations through (1) to (6), are shown for both the fluidized bed and spouted bed, respectively.
3.1. Correlations for Fluidization Process
Consider the following:(a)bed expansion ratio is (b)bed fluctuation ratio is (c)fluidization index is
3.2. Correlations Spouting Process
Consider the following:(a)bed expansion ratio is (b)bed fluctuation ratio is (c)fluidization index is The correlation plots for the bed expansion/fluctuation ratio and fluidization index against the system parameters for both types of beds (fluidized bed and spouted bed) are shown in Figures 7, 8 and 9, respectively. The high value of the correlation coefficients, for the bed expansion ratio for both beds indicates that the dimensionless analysis explains well the variation of dependent variable. Calculated values of the bed expansion and fluctuation ratios through these developed correlations are compared with their respective experimental values. The bed expansion/fluctuation ratio and fluidization index decreases with increase of spout diameter because of more amount of air passing in the central region only in case of spouted bed. The standard deviation and mean deviation for the bed expansion ratio (), bed fluctuation ratio (), and fluidization index (FI) for both types of beds are shown in Table 3. The standard deviations of calculated values are found to be within +15% to −15% and mean deviation within −1 to 5% for fine particles in both fluidization and spouting process.
The calculated values of the bed dynamics such as expansion/fluctuation ratios and fluidization index obtained through the dimensionless analysis method are found to be agreeing well with the respective experimentally observed values of bed dynamics. Low values of deviations indicate the applicability of these correlations over a wide range of parameters. These models can suitably be scaled up for pilot plant units or for industrial uses. The developed correlations can also be used as the basis of designs for the industrial processes.
The use of a stirrer in the fluidized bed provides agitation by which fine powders do not stick to the wall of the column. Thus the formation of agglomerates is prevented. Thus uniform fluidization is achieved. Therefore, the stirrer may also be used in fluidized bed reactor where catalysts are mainly smaller in size to provide large surface area for effective reactions to occur. The knowledge of bed dynamics also gives the fundamentals for optimum design of fluidized bed reactor, gasifiers and combustors, especially in the fixation of bed heights for such units. Apart from this, the fluidized bed and spouted bed can also be used in pharmaceutical industry for different purposes. This method can also be extended to nanoscale where different modes of mechanical disturbance such as vibration, rotation, and use of sound amplifier may be used to break the cohesive forces among the particles.
|:||Bed height (cm)|
|:||Particle diameter (microns)|
|:||Velocity of air (m/s)|
|:||Velocity of rod promoter (rpm)|
|:||Pressure drop (N/)|
|:||Weight of material (gm)|
|:||Area of cross-section ().|
This work was supported by funding from the National Institute of Technology, Rourkela. The authors thank Dr. (Mrs.) Abanti Sahoo of National Institute of Technology, Rourkela, for her support.
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