Influence of the nature and flow regime of the fluidizing medium on the patterns of entrainment of pharmaceutical preparations from a fluidized bed
Abstract Generated abstract
This study examines entrainment of granulated pharmaceutical preparations from a fluidized bed under steady and sinusoidally pulsating air flows, motivated by the design of a drying installation and its dust collection system. Experiments in a conical cylindrical batch apparatus measured entrainment over time for several materials, compared calculated and observed particle sizes and entrainment fluxes, and assessed changes in bed granulometry. The results indicate that fine and light components are entrained under all fluidizing regimes, with time dependent behavior attributed to initial segregation, ongoing removal of fines, and granule destruction. Pulsating flow could increase entrainment at equal average velocity, but at equal bed expansion it reduced entrainment markedly by allowing lower gas velocities, while existing calculation methods did not agree satisfactorily with the experimental data.
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EFFECT OF THE NATURE AND FLOW REGIME OF THE FLUIDIZING MEDIUM ON THE PATTERNS OF ENTRAINMENT OF PHARMACEUTICAL PREPARATIONS FROM A FLUIDIZED BED
A. D. ALEKSEEV, Yu. A. YANOVICH
(Presented at the scientific final conference of the Chemical-Technological Faculty)
The need to clarify the patterns of entrainment arose in connection with the development of a design for an installation for drying preparations manufactured by the Tomsk Chemical-Pharmaceutical Plant. The investigations were carried out in a conical-cylindrical batch apparatus with a cylindrical part 60 mm in diameter and 420 mm high. The ratio of the areas of the upper and lower bases of the cone was equal to two and corresponded to their ratio in the designed dryer. Granulated air-dry preparations were used as materials. The features of pseudo-fluidization of the preparations under the action on the materials of steady and sinusoidally pulsating air streams were determined beforehand. On the basis of the experimental results, pseudo-fluidization curves were plotted and the conditions for investigating entrainment were selected.
The purpose of the investigations was:
a) to clarify the general picture associated with the entrainment of pharmaceutical preparations from a fluidized bed under various process conditions;
b) to determine the possibility of extending existing calculation methods for the quantitative assessment of entrainment;
c) to obtain the data necessary for calculating the dust-collection system of a pulsed fluidized-bed dryer.
The basic information on the conditions under which the experiments were conducted, as well as the results obtained, are given in Table 1.
The maximum calculated values of the diameters of the entrained particles were determined from formulas (1), without taking into account their shape, as well as the nature of the motion of the fluidizing medium.
Microscopic analysis of the composition of the entrainment was carried out on the basis of the results of one experiment for each material.
The data presented show that the experimental values of the sizes of the entrained particles proved to be 20–30% greater than the calculated ones, and the relative difference practically did not depend on the nature of the motion of the medium. The initial weight of the material in all experiments was taken as 75 g. The duration of one experiment was 60 minutes and was adopted with a considerable margin relative to the possible duration of drying in a fluidized-bed apparatus. The change in the weight of the bed as a result of entrainment was determined by weighing the collected product. The duration of the interval between two weighings was 3 minutes.
Figure 1 shows the entrainment curves for vicalin obtained under different process conditions. For the other materials, the regular—
Table 1
Initial data and results of an experimental study of entrainment of pharmaceutical preparations
| Experiment No. | Material | Characteristic of the fluidizing medium, \(f\) | Velocity in the free cross section, m/sec | Bed expansion | Diameter of entrained particles, \(x \cdot 10^{-6}\) m, calculated | Diameter of entrained particles, \(x \cdot 10^{-6}\) m, experimental | Potential | Entrainment flux \(\times 10^{-4}\), kg/m², experimental | Entrainment flux \(\times 10^{-4}\), kg/m², calculation by method [2] | Entrainment flux \(\times 10^{-4}\), kg/m², calculation by method [3] | Entrainment flux \(\times 10^{-4}\), kg/m², calculation by method [4] |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Nitrofen | \(f = 0\) | 1.28 | 1.2 | 315 | — | 0.057 | 2.31 | 1.45 | 472 | 5140 |
| 2 | Nitrofen | \(f = 3.35\) | 1.0 | 1.2 | 225 | 290 | 0.06 | 4.44 | 2.45 | 397 | 665 |
| 3 | Nitrofen | \(f = 10\) | 1.166 | 1.2 | 287 | — | 0.048 | 1.96 | 1.51 | 333 | 0.40 |
| 4 | PASK | \(f = 0\) | 0.43 | 1.61 | 87.5 | 124 | 0.094 | 9.68 | 19.0 | 164.5 | 922 |
| 5 | PASK | \(f = 3.35\) | 0.34 | 1.59 | 78.6 | — | 0.055 | 8.63 | 27.5 | 48.6 | 87 |
| 6 | PASK | \(f = 10\) | 0.51 | 1.68 | 96.5 | — | 0.03 | 17.2 | 12.0 | 47.2 | 71 |
| 7 | PASK | \(f = 3.35\) | 0.24 | 1.5 | 66 | — | 0.032 | 5.86 | 40.2 | 11.25 | 26 |
| 8 | PASK | \(f = 10\) | 0.25 | 1.56 | 67.5 | — | 0.082 | 8.35 | 37.9 | 1.915 | 14.5 |
| 9 | Piperazine | \(f = 0\) | 0.152 | 1.13 | 44 | — | 0.18 | 51.0 | 18.6 | 34.2 | 0.543 |
| 10 | Piperazine | \(f = 3.35\) | 0.02 | 1.13 | 15.5 | — | 0.045 | 118.6 | 4.4 | 0.0847 | 0.0246 |
| 11 | Piperazine | \(f = 10\) | 0.032 | 1.13 | 19.6 | — | 0.085 | 103.4 | 3.75 | 0.516 | 0.0106 |
| 12 | Piperazine | \(f = 3.35\) | 0.152 | 1.4 | 44 | 62 | 0.2125 | 73.0 | 18.6 | 43.2 | 0.574 |
| 13 | Piperazine | \(f = 10\) | 0.152 | 1.38 | 44 | — | 0.2075 | 62.8 | 18.6 | 41.7 | 0.566 |
the entrainment patterns are qualitatively similar. The investigations carried out have established that entrainment of fine and light components occurs at all velocities and under all regimes of motion of the fluidizing medium that cause the material to pass into a pseudo-fluidized state. Entrainment continues throughout the entire period of bed blowing,
Fig. 1. Dependence of vicalin entrainment on the duration of blowing and on the process conditions. The numbers denote the experiment numbers in accordance with Table 1.
but its intensity changes with time. Entrainment is stable in character during the initial and final periods of blowing. The absolute magnitude of entrainment depends on many factors, including:
a) the initial particle-size distribution of the bed and the concentration in it of particles that can be carried away at the given velocity;
b) the strength of the bed components, since this determines the intensity of their destruction and the rate at which the loss is replenished;
c) the nature and velocity regime of motion of the fluidizing medium, since they determine the intensity of the dynamic effects on the bed.
At identical averaged values of air velocity and with different patterns of its motion, flow unsteadiness leads to an increase in entrainment by 13–50%. One reason for the increase in entrainment may be a more intensive process of granule destruction, which is associated with the features of pseudo-fluidization by unsteady flows. When equally expanded beds are blown with a sinusoidally pulsating flow, the entrainment of material proves to be much smaller than under the action of an undisturbed flow. Thus, for vicalin, the use of unsteady effects leads to a 3.3–4-fold reduction in entrainment. The decrease in entrainment becomes possible owing to an overall reduction in the velocities of motion of the fluidizing medium, which ensure the same boiling regime of the bed as in the steady regime.
Analysis of the curves permits the following mechanism of entrainment from a bed of granulated materials to be proposed. In the initial period of pseudo-fluidization, intensive separation of particles by size and density occurs.
...ness, as a result of which the fine and light components of the bed accumulate near the free surface, where they are captured by the ascending flow and carried out of the apparatus. At the same time, compression of the granules and their destruction take place. In the first period the process of granule destruction proceeds slowly, since it is hindered by the presence of fines, which impede direct contact of the granules with one another and with the walls of the apparatus. The entrainment intensity in this period is the highest, and more fine components are carried out of the bed than are formed as a result of destruction. The entrainment in this case increases with time according to a law close to linear.
In the second period, entrainment of fines continues and, at the same time, the losses are more fully made up owing to destruction. The entrainment intensity decreases somewhat, and the linear character of the dependence is disturbed.
By the beginning of the third period, entrainment of the fines introduced into the apparatus with the material is completely completed. This period is characterized by a decrease in the entrainment intensity, and the loss of material is fully made up by destruction. The entrainment curve practically becomes a straight line.
The duration of each period depends on the characteristics of the material, the composition of the bed, the velocity regime, the nature of the motion of the suspending medium, etc.
Calculations of the dust content of the flow were carried out by the methods of (2, 3, 4). The results are given in Table 1.
From a comparison of the experimental and calculated data it follows that none of the widely used methods for quantitative estimation of entrainment agrees with the experimental data and can be recommended for these materials. It should be borne in mind that none of the formulas used takes into account the residence time of the material in the chamber, which may be one of the reasons for the considerable discrepancies in the results. A second reason may be the continuous process of granule destruction, the intensity of which changes substantially with time; this also is not taken into account by the calculation formulas.
Change in granulometric composition
In chaotic motion of particles, their collisions with one another and with the internal surfaces of the apparatus are inevitable. As a result, some of the granules of all sizes are destroyed. In parallel with the destruction process, entrainment is observed of those particles whose terminal velocity is less than the velocity of motion of the suspending medium. The larger particles of the material acquire smaller sizes, taking the place of the fines, while the smallest are carried out of the apparatus.
It has been established that the nature of the motion of the suspending medium has a substantial effect on the change in the granulometric composition of the bed, but this effect is so complex that it was not possible to establish any regularities.
LITERATURE
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I. M. Razumov, L. I. Larionova. On calculation of the amount of fine-grained material carried out by a gas flow from a fluidized bed. Khimicheskoe mashinostroenie, No. 1, 14, 1961.
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E. V. Donat. Investigation of the process of entrainment of pulverulent and fine-grained material from apparatuses with a fluidized bed. Khimicheskaya promyshlennost’, No. 2, 134, 1962.
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I. M. Razumov. Fluidization and pneumatic transport of bulk materials. Moscow, Khimiya, 52, 1964.
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M. I. Fridland, A. I. Skoblo. Modeling of the process of entrainment of particles from a boiling bed. Khimicheskoe mashinostroenie, No. 5, 19, 1960.