Influence of the Nature and Flow Regime of the Fluidizing Medium on the Patterns of Fluidization
Abstract Generated abstract
This study examines how steady and sinusoidally pulsating air flows affect the fluidization of granular beds, with emphasis on pharmaceutical preparations and poppy seeds of differing particle size and composition. Experiments compared stable fluidization conditions under steady flow and low-frequency pulsating flow, recording optimal velocities, Reynolds criteria, bed expansion, resistance, and qualitative bed behavior. The results indicate that pulsating flow can stabilize pseudo-fluidization and achieve suspension at lower gas velocities and smaller bed expansion than steady flow, especially for fine or multicomponent materials prone to caking and channel formation. The effectiveness of the unsteady action depends mainly on material properties, particle-size composition, and pulsation frequency, while showing little dependence on the specific loading on the grate.
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INFLUENCE OF THE NATURE AND FLOW REGIME OF THE FLUIDIZING MEDIUM ON THE PATTERNS OF FLUIDIZATION
A. D. Alekseev
(Presented at the scientific final conference of the Faculty of Chemical Technology)
In work [1], a qualitative analysis was carried out of the influence of the nature of gas motion through porous media from the standpoint of the effectiveness of its action on the material, and the supposition was made that unsteady gas motions through a bed will cause its boiling at lower flow rates of the fluidizing medium than in the case of steady motions. To confirm the validity of this proposition, studies were carried out with steady, \(f = 0\), and sinusoidally pulsating modes of motion of the fluidizing medium. Air-dry granulated preparations produced by the Tomsk Chemical-Pharmaceutical Plant, and poppy seeds, were adopted as materials. The principal characteristics of the materials are given in Table 1. A description of the apparatus and of the experimental procedure is given in work [1].
The specific loads of material on the grate were \(346 \div 693\ \text{N}/\text{m}^2\). It was established that, when both a freely poured and a preliminarily compacted bed are blown through by a steady stream of air, there is a range of velocities in which stable fluidization is possible. The main group of materials investigated consists of multicomponent polydisperse mixtures, and their transition to the boiling state occurs gradually, as the flow rate of the fluidizing medium increases. At first the fine and light components, whose terminal velocity is less than the flow velocity, become suspended, while the main mass of the material remains motionless.
Fluidization of pharmaceutical preparations is accompanied by the blowing of light components into the upper parts of the bed and the formation of through channels that hinder the suspension of the heavy and coarse components. If the flow velocity reaches a value sufficient to destroy the channels, this causes intense boiling of the entire material. Operation at elevated velocities is accompanied by the formation of large gas bubbles, the ejection of particles beyond the limits of the bed, entrainment of fine and light components, and a change in the mineralogical composition.
The main part of the experiments with an unsteady flow was carried out at frequencies of 3.35 and 10 Hz. The external picture of the behavior of the bed in this case differs substantially from that observed when it is acted upon by an undisturbed flow. In the initial period, while the flow velocity is small, no visible changes in the state of the bed are observed. A subsequent increase in velocity and, consequently, in the intensity of the dynamic actions causes oscillations of the particles located…
Table 1
Characteristics of the materials studied and experimental results
| Parameter | Condition | Pirkofen 1 | Pirkofen 2 | Pirkofen 3 | PASK | Dormatol | Vikalin | Poppy seeds |
|---|---|---|---|---|---|---|---|---|
| Particle size, mm | 0.25–5.0 | 0.5–5.0 | 0–6.0 | 0–6.0 | 0–6.0 | 0–2.5 | 0.6–1.0 | |
| Effective diameter, mm | 0.578 | 1.235 | 1.8 | 0.253 | 0.739 | 0.18 | 0.75 | |
| Material density, kg/m³ | 1245 | 1245 | 1295 | 1807 | 1250 | 2670 | 1200 | |
| Bulk density, kg/m³ | 457–508 | 410–462 | 450–542 | 563–640 | 525–550 | 585–590 | 622–654 | |
| Archimedes criterion, \(Ar \times 10^2\) | 86 | 837 | 2700 | 10.6 | 181 | 5.5 | 181 | |
| Optimal value of velocity, m/sec | \(f = 0\) | 0.36 | 0.64 | 1.01 | 0.365 | 0.98 | 0.35 | 0.41 |
| Optimal value of velocity, m/sec | \(f = 3.35\) | 0.156 | 0.31 | 0.62 | 0.15 | 0.55 | 0.049 | 0.25 |
| Optimal value of velocity, m/sec | \(f = 10\) | 0.14 | 1.34 | 0.68 | 0.15 | 0.54 | 0.052 | 0.26 |
| Optimal value of the Reynolds criterion \(Re_{\mathrm{opt}}\) | \(f = 0\) | 13.9 | 52.4 | 120.6 | 6.13 | 48 | 4.18 | 20.4 |
| Optimal value of the Reynolds criterion \(Re_{\mathrm{opt}}\) | \(f = 3.35\) | 6.0 | 25.4 | 74.2 | 2.52 | 27 | 0.586 | 12.4 |
| Optimal value of the Reynolds criterion \(Re_{\mathrm{opt}}\) | \(f = 10\) | 5.4 | 27.9 | 81.3 | 2.52 | 26.5 | 0.62 | 12.95 |
| Expansion of the bed at \(Re_{\mathrm{opt}}\) | \(f = 0\) | 1.21 | 1.145 | 1.165 | 1.27 | 1.265 | 1.367 | 1.09 |
| Expansion of the bed at \(Re_{\mathrm{opt}}\) | \(f = 3.35\) | 1.056 | 1.031 | 1.092 | 1.145 | 1.137 | 1.26 | 1.02 |
| Expansion of the bed at \(Re_{\mathrm{opt}}\) | \(f = 10\) | 1.028 | 1.012 | 1.01 | 1.09 | 1.02 | 1.15 | 0.98 |
Fig. 1. a) dependence of the porosity of a poppy-seed bed \(\varepsilon_m\) on the velocity in the free cross-section of the apparatus \(w\) and on the pulsation frequency \(f\); b) change in the resistance of the bed \(\Delta P\) under the same conditions
Fig. 2. Graphs of the dependence of pseudo-fluidization efficiency on the Archimedes criterion \(Ar\) and on the pulsation frequency \(f\)
near the free surface. The frequency of oscillation corresponds to the frequency of the external disturbing actions. Simultaneously with the oscillation of the particles, their rearrangement begins, which leads to a decrease in the volume of the bed. Compaction is accompanied by a corresponding increase in resistance. A further increase in flow rate leads to more noticeable throwing-up of particles, and ever larger portions of the bed are gradually drawn into motion. This pattern is observed until the entire mass of material begins to be thrown up, resulting in its detachment from the distributor grate and periodic suspension. This process is accompanied by the breakthrough of small gas bubbles through the bed, which causes mixing of the particles and helps stabilize the fluidization regime. As the velocity increases, the resistance of the bed rises up to the moment when, during one pulse, the entire bed or a significant part of it remains suspended for most of the period. A subsequent increase in velocity does not cause substantial changes in the resistance of the bed.
Typical pseudo-fluidization curves are given in Fig. 1.
An increase in the specific loading of material on the grate causes a slight shift of the expansion curve toward higher gas velocities. A gradual decrease in the gas flow rate likewise shifts the expansion curve toward higher velocities. When the air supply is completely stopped, the bed proves to be strongly compacted.
The averaged values of quantities characteristic of a stable fluidization regime of the bed are given in the table.
From the data presented it is evident that the numerical values of the pseudo-fluidization velocities depend on the character of motion of the medium. In pseudo-fluidization by a steady flow, the greatest flow rates of the suspending medium are required. To suspend the same materials by a disturbed flow, smaller flow rates are required, and the degree of reduction depends on the properties and particle-size composition of the bed. The greatest reduction in velocities is observed for vicalin, characterized by a multicomponent chemical composition and comparatively small particle sizes, as is clearly seen from Fig. 2.
The deviation from the general pattern for two materials (dormatol, poppy) is explained by the substantial difference between their properties and the properties of the other materials studied.
The observed tendency toward an increase in the effectiveness of nonstationary actions as particle size decreases appears very important, since it makes it possible to carry out processes with finely dispersed materials, which have an increased tendency toward caking and channel formation.
Conclusions
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The use of low-frequency sinusoidal actions on the air flow stabilizes the pseudo-fluidization regime of granular materials and ensures stable fluidization at lower flow rates of the suspending medium and with smaller bed expansion than under steady-state conditions.
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The effectiveness of sinusoidal-pulsating actions depends on the particle-size composition of the bed, the properties of the material, and the pulsation frequency, and is practically independent of the specific loading of material on the distributor grate.
References
- A. D. Alekseev. Influence of the nonstationarity of the gas flow on the behavior of a bed of granular material. Collection: Izvestiya TPI, 133, 69–74, 1965.