Monitoring particle fluidization in a fluidized bed granulator with an acoustic emission sensor

Powder Technology 113 Ž2000. 88–96 www.elsevier.comrlocaterpowtec

Monitoring particle fluidization in a fluidized bed granulator with an acoustic emission sensor Hiroyuki Tsujimoto a,) , Toyokazu Yokoyama a , C.C. Huang b, Isao Sekiguchi c a

Research and DeÕelopment DiÕision, Hosokawa Micron Corporation, 1-9 Shoudai, Tajika, Hirakata-shi, Osaka, Japan b Research and DeÕelopment, Hosokawa Micron Powder Systems, Summit, New Jersey, USA c Department of Applied Chemistry, Faculty of Science and Engineering, Chuo UniÕersity, Tokyo, Japan Received 25 March 1999; received in revised form 30 August 1999; accepted 9 December 1999

Abstract A high-frequency Ž140 kHz. acoustic emission ŽAE. sensor with narrow-band receptors was developed and applied in monitoring the particle fluidization in a fluidized bed granulator. In particle fluidization processes, the impact and the friction of the fluidized particles on the wall of fluidized beds produce AE waves. By calibrating an AE sensor at various fluidization conditions with several uniform, spherical granules, the measurement of mean AE amplitudes can be used to monitor fluidization phenomena. It was found that there are direct correlations between the mean AE amplitude, dimensionless excess gas velocity, and dimensionless bed height. The AE sensor can be applied to detect the onset of unstable fluidization due to the increase of moisture content in the fluidized bed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Fluidized bed granulator; AE sensor; Acoustic emission; Monitoring; Fluidization

1. Introduction Sprayed fluidized bed granulators are widely used to produce granular products for solubility control, tablet production, and other special materials in various industries. In fluidized bed granulation, granules are formed by dispersing the liquid binder in powder feeds. Binder dispersion depends on the ratio of spray coverage to powder mass in the fluidized bed and the turnover of the powder mass, i.e., the fluidization conditions. Ideal dispersion can be achieved with a large spray coverage area and good fluidization, which leads to the production of desired granule types with high product yield. Therefore, the fluidization conditions need to be monitored and controlled during the granulation operation to prevent the occurrence of defluidization such as channeling and blocking. However, controlling fluidization conditions for conventional fluidized bed granulators is difficult because the operator cannot visually observe the granule growth process in the granulator. Fluidized bed granulation is more of an AartB ) Corresponding author. Tel.: q81-720-55-2231; fax: q81-720-552294. E-mail address: [email protected] ŽH. Tsujimoto..

than a Ascience,B and often depends on the supervision of experienced operators. For this reason, there is a strong demand for the automation of fluidized bed granulators to achieve stable operating conditions that will produce the desired granules and save manpower. As means of solving these problems, many sophisticated in-line sensors for controlling fluidization conditions and granule growth in fluidized bed granulators have been reported. Watano et al. w1x developed an infrared moisture sensor for measuring and controlling particle moisture content, which predominantly affects granule size. Watano et al. w2x additionally conducted stable fluidized bed granulation by controlling expanded bed height, which was measured with an ultrasonic displacement sensor. Watano and Miyanami w3x also achieved in-line monitoring of the granule growth process using a CCD camera for both controlling granule size and detecting the granulation end point. Measurements using in-line sensors have become a key technology for achieving stable automatic operation of fluidized bed granulators. In this study, a new measuring technique was tested by applying an acoustic emission ŽAE. sensor to monitor the fluidization conditions in a fluidized bed granulator. Powder processing machines generally release various AEs,

0032-5910r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 0 5 - 9

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because the solid particles vigorously move around in a limited space, colliding with each other and creating intense friction in the equipment during operation. The sounds generated during this process are manifestations of particle motion and are believed to be related to particle behavior in the operating system. Thus, particle behavior in a fluidized bed granulator can be monitored and characterized by assessing the sounds, once the correlation between particular sounds and particle motion is established. In fluidized bed granulators the three basic sources of AEs are Ž1. particle–particle or particle–chamber collisions Žimpact sound., Ž2. particle–particle or particle– chamber friction Žfriction sound., and Ž3. air turbulence in particle beds Žaerodynamic sound.. The sounds generated by the friction, collisions, and fluid turbulence in powder processing processes are not only the audible sounds detectable via air by a microphone, but also include highfrequency sounds in the non-audible range. Some of the high-frequency sounds invariably show up in the form of AEs called elastic waves, which can be measured by AE sensors that incorporate piezoelectric transducers. Hereinafter in this context, AE measurement means elastic wave measurement. A well-known characteristic of elastic waves is their ability to propagate easily through solid

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materials but attenuate rapidly in air in the high-frequency range above 100 kHz. AE measurement for detecting elastic waves takes advantage of this principle to differentiate elastic waves emitted by the fluidized particles from various background noises propagated through the air. In other words, AE measurement is not affected by the background noise generated by mechanical vibrations such as from the blowers or compressors used in fluidized bed granulators. By contrast, it is difficult to use audible sound measurement to separate AEs generated by fluidized particles from the disturbation signals generated by background noise, unless spectral analysis is employed. Moreover, the fact that elastic waves propagate in all directions through solid material, as well as the technique’s non-invasive nature, make the installation of AE sensors easy, and allow considerable freedom in positioning them. In view of these advantages, an AE sensor is used to measure the elastic wave portion of the sounds generated by fluidized particles in a fluidized bed granulator. AE sensing techniques have been applied to the monitoring of powder compaction processes, which produce two basic AE sources w4–11x from the non-stationary dislocation motion and the dislocation annihilation process of solid materials. Many studies have found a correlation

Fig. 1. Schematic diagram of the experimental apparatus.

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between the AE detected through die walls using piezoelectric transducers and the simultaneously measured force-displacement profile. In other applications of AE measurement in powder processing processes, Terashima w12x used AE measurement to identify incipient fluidization phenomena in a standard cylindrical fluidized bed device. Hirajima et al. w13x used an AE sensor to assess the production of pellet products from suspended particles in wet granulation using a tumbling drum granulator with an agitator. Although it was reported that AE measurement can be an effective technique for obtaining several types of information related to particle movement andror growth in various powder processing processes, there has been no detailed discussion on the application of AE measurement to actual fluidized bed granulators. This study is first attempt to develop an AE monitoring system for fluidized bed granulators. This paper describes the effects of fluidization conditions on AE wave properties and evaluates the feasibility of AE measurement for such development.

2. Experimental apparatus 2.1. Fluidized bed granulator Fig. 1 is a schematic diagram of the experimental apparatus used in the study. The main body of the fluidized bed granulator ŽAGM-2A-PJ, Hosokawa Micron. consists of two parts made of acrylic resin: a lower cylindrical chamber with an inside diameter of 156 mm and an upper conical–cylindrical chamber with a maximum inside diameter of 305 mm. A distributor plate made of sintered porous stainless steel is at the bottom of the lower chamber. As shown in Fig. 2Žb., when the granulator is used as an agitation fluidized bed granulator, the distributor is equipped with a rotating disk having five radial air slits and an agitator blade located over the distributor. This rotating disk turns around a central axis so as to tumble and agitate powder feeds. In the experiment, fluidizing gas was generated by a suction blower in the downstream. This fluidizing gas was heated by an electric heater and then entered the granulator through the air slits and the gap between the periphery of the disk and the inner surface of the lower chamber wall. In standard non-agitating use, the fluidized bed is formed by the fluidizing gas passing through the distributor, as shown in Fig. 2Ža.. Fine particles entrained by fluidizing gas were trapped by the bag filters at the top of the granulator and shaken off by pulsed air jets. The flow rate and the temperature of the fluidizing gas were measured and controlled with an orifice meter and a thermo-controller, respectively. Bed height was measured with an ultrasonic displacement sensor located in the upper part of the fluidized bed. The pressure drop in the fluidized bed was measured with a

Fig. 2. Two setups in the fluidized bed granulator.

digital manometer. A binary nozzle located 300 mm above the distributor was used to atomize a liquid binder and spray it toward the center of the fluidized bed. The moisture content of bed particles was measured with an in-line moisture sensor ŽWETRON, Hosokawa Micron.. A glass window on the side wall of the lower chamber located 50 mm above the distributor was used for the transmission and reflection of the infrared spectrum from the moisture sensor. 2.2. Particle samples Table 1 shows the physical properties of model particles, which were granulated spherical particles made of crystalline cellulose ŽCELPHER, Asahi Chemical Industrial.. They are widely used as core particles in solid formulation. These three kinds of model particles differ mainly in particle size. Their particle size range is roughly

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Table 1 Physical properties of feed samples Material

Symbol

Particle density rs = 10y3 Žkg my3 .

Particle bulk density r b = 10y3 Žkg my3 .

Mean particle size d 50 = 10 4 Žm.

Range of particle size distribution d 50 = 10 4 Žm.

Standard deviation particle size distribution sg Žy.

Minimum fluidization velocity Umf Žmsy1 .

Crystalline cellulose particle

S

1.51

0.87

2.32

1.50–3.00

1.28

0.022

M L

1.52 1.46

0.97 0.97

4.32 5.63

3.00–5.00 5.00–7.10

1.26 1.13

0.061 0.128

equal to the size range of granules produced by coating and granulation operations in actual fluidized bed granulators. Accordingly, they were thought appropriate as model particles used to create a typical fluidized state and examine the characteristics of AE measurement. 2.3. Measurement of AEs An AE sensor ŽAE-901S, NF Kairo Setukei Block. designed for high frequency and equipped with narrowband receptors having a resonance point of 140 kHz was used in this study. The AE sensor was mounted on the outer wall of the lower chamber at 50 mm above the distributor as shown in Fig. 1. This sensor position was determined in the preliminary experiment to obtain maximum signal level in view of particle friction and collisions with the fluidized bed chamber walls. AE waves generated by friction and collisions of the fluidized particles with the fluidized bed chamber walls and other particles first propagate in all directions, and are then transmitted to the AE sensor through the chamber walls. The piezoelectric transducer in the AE sensor detects minute surface displacements associated with an AE wave propagating through the chambers and converts those displacements into electri-

cal signals by means of the piezoelectric effect. This is amplified at a 40 dB gain by a preamplifier ŽAE-912, NF Kairo Setukei Block. and processed by an AE analyzer ŽMUSIC, NF Kairo Setukei Block., thereby providing information about AE parameters based on the ring down counting method w14x. The AE output signals from the preamplifier are also frequency-analyzed using a Fast Fourier Transform ŽFFT. analyzer. In the frequency analysis, a second AE sensor ŽAE-900WB, NF Kairo Setukei Block. having a flat wide-band frequency response was used to avoid the influence of the sensor’s own resonance characteristics. The measured AE frequencies were varied from 100 kHz to 1 MHz by adjusting the band pass filter of the AE analyzer to filter out the background noises emitted from the fluidized bed granulator. Consequently, it was confirmed that AE measurements within this frequency region were immune to both audible noise and the low vibration frequencies associated with mechanical vibrations from the blower and the compressor, because such unwanted background noises were not noticed during idling of the fluidized bed granulator as described in Section 3. As an example, Fig. 3 shows the AE frequency spectrum generated in a fluidized bed granulator with sample M at the fluidizing gas velocity of 0.6 msy1 . As shown, the highest frequency spectrum peak obtained under all experimental conditions in this study appeared to be within the range of 125–375 kHz. For this reason, an AE sensor ŽAE901-S. having narrow-band receptors with a resonance point of 140 kHz was chosen to perform AE measurements to improve sensitivity.

3. Results and discussion 3.1. Mean AE amplitude in a standard fluidized bed without particle agglomeration

Fig. 3. Frequency spectrum, obtained by FFT analysis of the AE by fluidizing particles of sample M at Us s 0.6 msy1 .

3.1.1. Effects of fluidizing gas Õelocity on mean AE amplitude In general, the activity of bed particles is proportional to the fluidizing gas velocity Us beyond minimum fluidizing gas velocity Umf . Fig. 4 shows the changes of both mean AE amplitude LAE and the expanded bed height Hf

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Fig. 4. Changes in mean AE amplitude LAE and fluidization activity expressed by bed height Hf above distributor with increasing fluidizing gas velocity Us ŽSample: L, Hi s 0.06 m, W s 0.9 kg..

obtained from a fluidized bed with 0.9 kg of sample L as Us was varied from 0 to 1.2 msy1 . Here the mean AE amplitude LAE was the mean amplitude value of the AE waves for 5 s calculated by using the AE amplitude distribution plotted in Fig. 5. As seen in Fig. 4, the bed height in the fixed bed was measured at 60 mm. There was no practical emission of AE waves because there was no movement of bed particles. Therefore, the mean AE amplitude measured in this region was only background noise from the equipment Ž LAE s 0.024 V.. Observed next was the beginning of bed expansion, i.e., the Atransition regionB from the fixed bed to the fluidized bed at the fluidizing gas velocity from Us s 0.110–0.128 msy1 . The mean AE amplitude in this region increased with the change in bed particle packing structure. Subsequently, both the mean AE amplitude and the bed height proportionally increased with fluidizing gas velocity. The cause of the increase in mean AE amplitude was assumed to be the increase in the frequency and strength of particle–particle andror particle–chamber wall friction and collisions due to the movement of bed particles beyond the minimum fluidization velocity ŽUmf s 0.128 msy1 .. The dotted lines in Fig. 4 show the bed height as measured with a ruler,

Fig. 5. Example of AE amplitude distribution obtained in fluidized bed of sample L at Us s 0.9 msy1 and W s 0.9 kg.

while the square symbols indicate the bed height as measured with the ultrasonic displacement sensor. The experimental data from both measuring methods tended to fluctuate with the fluidizing gas velocity. This phenomenon is caused by the sparse phase of the fluidized particles formed in the upper region of the dense fluidized bed. On the other hand, the mean AE amplitude LAE provided stable signals corresponding to the activity of the fluidized particles. This tendency was also observed with samples S and M. Fig. 6 shows the relationship between the mean AE amplitude LAE and the fluidizing gas velocity Us for various bed hold-ups Žthe mass of sample material in the fluidized bed. with samples L, M, and S. As seen in the figure, the mean AE amplitude LAE for all the tests tended to increase with an increase in Us and in particle size d 50 . 3.1.2. Effects of particle size on mean AE amplitude Hidaka et al. w15x studied the particle impact sound generated by flowing particles in a cylindrical hopper to simultaneously measure the flow rate and particle size. The impact sound as an audible noise was detected via air using a microphone. Some typical results of the correlation between the sound pressure of flow sound Pa , the particle flow rate Q, and the particle size d 50 were obtained using glass beads between 4 and 16.5 mm in size, and the relations are shown by the following equations. Pa A Q n 1.7 Pa A d 50

n s 0.45–0.50

Ž 1. Ž 2.

As the AE sensor used in this study only measures the elastic waves in the nonaudible region and assumes that mean AE amplitude corresponds to the above sound pressure Pa , the above relations expressed in Eqs. Ž1. and Ž2. are applicable in AE measurements. The relations in Fig. 6 were re-plotted as shown in Fig. 7, whose vertical axis indicates the mean AE amplitude based on the particle size effect of the sample S, and whose horizontal axis shows dimensionless excess gas velocity. This figure shows a favorable correlation between the AE amplitude and dimensionless excess gas velocity under all the experimental

Fig. 6. Effects of fluidizing air velocity Us on mean AE amplitude LAE obtained under various operational conditions.

H. Tsujimoto et al.r Powder Technology 113 (2000) 88–96

Fig. 7. Relationship between mean AE amplitude reflecting particle size effects, and dimensionless fluidizing air velocity ŽUs yUmf .r Umf .

conditions as shown below, which is nearly the same as the expressions in Eqs. Ž1. and Ž2.. LAE Ž 2.32 = 10y4rd 50 .

1 .70

s 1.40 = 10y2 Ž Us y Umf . rUmf

0.40

Ž 3.

One could assume that all samples used in this experiment were prepared in the manner described by Eq. Ž3. because sample properties such as particle density, sharpness of particle size distribution, and particle shape were nearly the same, with the exception of particle size. In this instance the presumed reason that mean AE amplitude increased, as fluidized particles grew in size while dimensionless gas velocity remained the same, is that, as individual particles grew in mass, the force of their collisions with the chamber wall and of their friction increased. The data obtained suggested that the mechanism which generates the elastic waves that are measured with the AE sensor is closely related to the mechanism that produces the flow sound measured with a microphone. When the fluidization condition is expressed as dimensionless bed height Ž Hf y Hi .rHi , there is a good correlation between the dimensionless bed height and mean AE amplitude, as shown in Fig. 8 and similarly to that in Fig. 7. The correlation can be expressed by the following equation. LAE Ž 2.32 = 10y4rd 50 .

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Fig. 8. Relationship between mean AE amplitude reflecting particle size effect, and dimensionless bed height.

cle–chamber wall friction and collisions that are invigorated as bed hold-up increases. In Fig. 6, all fluidized beds with Hi s 0.06 m ŽW s 0.9 . kg had higher mean AE amplitude values than those with Hi s 0.03 m ŽW s 0.6 kg.. But the mean AE amplitude obtained from fluidized beds with Hi s 0.09 m ŽW s 1.2 kg. had lower values than beds with Hi s 0.03 m and 0.06 m, even though they would be expected to generate the largest AE signals. Results were conceivably influenced by the bubble effect. That is to say, in fluidized beds with a bed height ranging from 0.03 to 0.09 m, the higher the bed height, the greater bed expansion is activated at the same dimensionless gas velocity. We also observed many bubbles rising around the inner wall of the lower chamber. Fig. 9 shows the relationship between dimensionless gas velocity and the void fraction of the fluidized beds as calculated by Eq. Ž5..

´ f s 1 y Wr Ž Hf A rs .

Ž 5.

The void fraction in Fig. 9, which expresses the volume ratio of the gas phase with bubbles to the fluidized bed, tended to increase with increasing bed hold-up at the same

1 .70

s 2.67 = 10y2 Ž Hf y Hi . rHi

0.540

Ž 4.

3.1.3. Effects of bed hold-up on mean AE amplitude Of particular interest is the influence of the hold-up of the fluidized bed on mean AE amplitude. The mean AE amplitude appeared to be larger when expanded bed height Hf became higher and when the bed hold-up increased at constant dimensionless gas velocity. That is to say, the AE generated in the fluidized bed should be proportional to the frequency and the strength of the particle–particle or parti-

Fig. 9. Relationship between void fraction of fluidized bed and dimensionless fluidizing air velocity ŽUs yUmf .r Umf obtained under various operational conditions.

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dimensionless gas velocity. Thus, the volume ratio of the passing bubbles, which should promote active fluidization leading to increasing AE generation, was also thought to increase as void fraction increased. Nevertheless, the mean AE amplitude measured in the fluidized bed with the largest height Ž Hi s 0.09 m. showed lower values. In view of the fact that AE waves are quickly and heavily attenuated in a gas phase, it was suggested that the tendency for mean AE amplitude to decrease as bed hold-up increases, is due to the attenuation of AE waves during propagation through the gas phase containing the bubbles. 3.1.4. Effects of particle moisture content on mean AE amplitude Fig. 10 shows the fluidization behavior with increasing moisture content over time in a fluidized bed using sample L and W s 0.9 kg at Us s 1.0 msy1 . The pressure drop across the fluidized bed D P, the moisture content of bed particles Mp , the expanded bed height Hf , and the mean AE amplitude LAE were shown as a function of operating time. The spray nozzle was used to atomize purified water into the fluidized bed at a constant feed rate of 50 g miny1 . Because the fluidized particles in this operation were not water soluble, the formation of granules by agglomeration did not occur at the beginning of moisture addition. At higher moisture content, however, the apparent viscosity of the fluidized bed increased and the particles stuck together, thereby tending to cause the channeling phenomenon. As seen in this figure, beyond approximately 600 s of operating time the fluidized bed exhibited unstable fluidization tending toward channeling, and at Q s 690 s defluidization suddenly occurred in conjunction with channeling.

The reason that the pressure drop slowly increases up to point B where channeling occurred is thought to be the increase in the apparent viscosity of the fluidized bed, which occurs in conjunction with a moisture content increase in the bed. It was also observed that expanded bed height as measured with the ultrasonic displacement sensor declined somewhat when moisture was added, and then, as bed height variation decreased, stayed more or less constant to point B. This is presumed to result mainly from the gradually progressing inability to maintain the formation of the sparse phase of the fluidized particles formed in the upper region of the dense fluidized bed as the moisture value rises. However, neither the expanded bed height nor the bed pressure drop exhibited large changes until point B, whereupon they finally decreased rapidly just before defluidization occurred. It was therefore extremely difficult, as a means of avoiding defluidization, to control the fluidized bed by detecting changes in bed height or pressure drop. By contrast, the mean AE amplitude values began to decline approximately 250 s prior to the occurrence of defluidization, which could be avoided by controlling operational variables such as the feed rate of liquid binder, the fluidizing gas velocity, and its temperature from point A to point B. This tendency for mean AE amplitude values to undergo a pronounced drop in the vicinity of point A can perhaps be explained in the following manner. A characteristic of the model particles used in this experiment is their suitable degree of moisture absorbance. For that reason the agglomeration of particles was hardly observed in the low-moisture region near point A. And just as the particles that had adhered in places to the chamber wall were no longer seen there after moisture was added, the fluidity of particles at the chamber wall improved with the decline of

Fig. 10. Changes in mean AE amplitude, bed pressure drop and bed height with increased moisture content in fluidized bed Žsample: L, Us s 1.0 msy1 , W s 0.9 kg..

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electrostatic forces acting on the particles, and, as noted previously, particles collide with one another more frequently — thereby facilitating AE generation — owing to the reduction of the bed void fraction with decreasing bed height. But at the same time, moisture absorption by particles is thought to decrease the AE propagation capacity of particles themselves. Thus, it is presumed that no great variation is discerned in AE amplitude values measured in the low-moisture region because the increase in AE generation frequency and the decrease in propagation capacity cancel each other out. But when bed moisture grows over a certain extent, the amount of moisture on particle surfaces likely increases gradually. In fact, near point A and thereafter, the formation of weak particle agglomerations due to liquid bridges among particles came to be observed. Further, after channeling commenced, it was noted that particles and the inside chamber wall were wet, which likely means that the bed moisture value around point A, where the large drop in AE amplitude value begins, corresponds to the amount of moisture needed on particle surfaces to induce weak particle agglomeration. Then in the high-moisture region coming next, as the amount of moisture among particles increases, there is a pronounced reduction in the intensity of AE generated by particle–particle and particle–wall collisions and friction, and because of this the measured mean AE amplitude values decrease. As the foregoing illustrates, AE measurement results under this experiment’s conditions were likely affected by the particles’ characteristics vis-a-vis water, and by the bed ` moisture value, which makes this a good manifestation of the behavior of an unstable fluidized bed as bed moisture increases. 3.1.5. Mean AE amplitude in an agitator fluidized bed granulator The state-of-the-art fluidized bed granulator equipped with a rotating disk having an agitator blade at the bottom of a standard fluidized bed, i.e., the so-called agitation fluidized bed granulator w1–3,16x is widely used especially in the granulation of pharmaceuticals and ceramic materials. This is because the rotating disk applies optimal tumbling and compacting motion to powder feeds in the granulator so as to produce the spherical and well-compacted granules that cannot be produced by conventional fluidized bed granulation. The AE sensor was installed on the agitation fluidized bed granulator, which was the standard fluidized bed granulator used in Section 3 and equipped with a rotating disk. Fig. 11 shows the mean AE amplitude obtained from that agitation fluidized bed granulator using sample M at W s 0.9 kg, which was the optimum bed hold-up to obtain a good agitation fluidized bed in the granulator. The mean AE amplitude increased remarkably with the increased rotational speed of the disk at constant fluidizing gas velocity. This was because an agitated particle flow was

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Fig. 11. Effects of rotational disk speed on mean AE amplitude obtained under various fluidizing air velocities Žsample: M, W s 0.9 kg..

formed, as shown in Fig. 2Žb., in the lower chamber where the particles were closely spaced. As a result, the friction and collisions of the particles against the chamber wall increased as rotational disk speed increased. At the same rotational speed, mean AE amplitude decreased as fluidizing gas velocity increased, which was because the agitation fluidized beds that are formed at higher fluidizing gas velocities resemble the fluidization conditions formed in standard fluidized bed granulators. However, when fluidizing gas velocity decreases, agitation fluidization conditions become stronger. This dependency of fluidizing gas velocity on disk rotational speed was also observed in agitation fluidized beds with samples S and L. The effect of particle size on mean AE amplitude as plotted in Fig. 12 was nearly the same as the results obtained from the standard fluidized bed granulator. LAE Ž 2.32 = 10y4rd 50 .

1.70

s 1.57 = 10y1 Ž Us y Umf . rUmf

y0 .170

at R s 700 rpm LAE Ž 2.32 = 10y4rd 50 .

Ž 6. 1.70

s 8.90 = 10y2 Ž Us y Umf . rUmf at R s 300 rpm

y0 .170

Ž 7.

Comparing the agitation fluidized bed granulator to the standard fluidized bed granulator using the same sample and bed hold-up revealed that the mean AE amplitude measured in the agitation fluidized bed granulator at a disk speed of 700 rpm is about four times of that of the standard fluidized bed granulator. This shows that the agitated particle flow existing typically in an agitation fluidized bed granulator generates higher mean AE amplitude values. Based on measurements of elastic waves generated by the friction and collisions of fluidized particles against chamber walls, the AE measuring technique proposed here gives crucial information about fluidization behavior. Test results suggested that the mean AE amplitude measured in

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as a monitor signal to detect the unstable fluidization caused by increased moisture content, which leads to defluidization. Our conclusion is that the AE measuring technique presented here is highly feasible as a practical method for monitoring particle fluidization in a gas–solid fluidized bed.

5. List of Symbols d 50

Fig. 12. Relationship between mean AE amplitude as a reflection of particle size effect, and dimensionless fluidizing air velocity ŽUs y Umf .r Umf obtained at the rotational disk speeds of 300 and 700 rpm with W s 0.9 kg.

both the standard fluidized bed granulator and the agitation fluidized bed granulator provides signals that correspond to the fluidization activity of the particles in the fluidized beds as actually observed and discerned by the operator. In addition, these signals can be used as an index to estimate the fluidization conditions in which granulation does not occur. Moreover, it appears that this finding could be helpful not only for monitoring particle fluidization in fluidized bed granulators but also for maintaining stable operation without defluidization in other types of gas–solid fluidized bed equipment such as reactors and dryers. However, it would be challenging to use AE sensors on larger fluidized bed granulators. In our study, using AE propagation to monitor all phenomena arising in fluidized beds tended to be progressively more difficult as the mass Žhold-up. of fluidized beds increased and as fluidized particle size and density decreased. Continued studies will investigate the feasibility of using multiple AE sensors to monitor a pilot-scale fluidized bed granulator, and to examine the application of AE measuring techniques to the wet granulation process with various powder samples. 4. Conclusion In this paper we have presented a method of using one AE sensor to monitor particle fluidization in a small fluidized bed granulator with spherical particles in the size range of 0.150–0.710 mm. AE wave measurement for fluidized particles in a fluidized bed was performed to examine the effects of operating variables such as fluidizing gas velocity, bed hold-up, particle size, moisture content, and rotational disk speed on mean AE amplitude. It was found that mean AE amplitude values correlated well with the dimensionless excess gas velocity and dimensionless expanded bed height, which corresponded to fluidization activity as discerned and evaluated by the operator. It was also confirmed that mean AE amplitude could be used

f Hi Hf LAE Mp DP Us Umf W

rb rs

volume median diameter of particles frequency height of fixed bed bed height at maximum expansion mean AE amplitude moisture content of particles or granules on a wet basis pressure drop of fluidized bed fluidizing gas velocity minimum fluidizing gas velocity bed hold-up, the mass of material in the fluidized bed bulk density of particles true density of particles

Žm. Ž–. Žm. Žm. ŽV. Ž%.

ŽPa. Žmsy1 . Žmsy1 . Žkg. Žkg my3 . Žkg my3 .

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