Application of Ultrasonic Technique in Multiphase ... - ACS Publications

Ind. Eng. Chem. Res. 2004, 43, 5681-5691

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Application of Ultrasonic Technique in Multiphase Flows Maytinee Vatanakul, Ying Zheng,* and Michel Couturier Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada

Ultrasonic technique was used as a tool for phase holdup measurement in multiphase systems. A new statistical approach using fluctuations of the ultrasound signals was applied to simultaneously detect the dispersed phase holdups in gas-liquid-solid flows. These phase holdups, measured by two methodssthe ultrasonic technique and the applications of pressure transducers and conductivity probessagreed well. In the system of a high liquid velocity (above 14 cm/s), the ultrasonic technique still provided reliable results. The local phase holdups and their radial distribution were measured in gas-liquid-solid three-phase circulating fluidized beds at two different elevations. A nonuniform radial distribution and a uniform axial distribution of gas and solid phases were observed. The nonuniform radial distribution of solid holdup did not depend on the solid circulating rate. The radial distributions of both phase holdups became uniform with increasing liquid velocity. Introduction Gas-liquid-solid fluidization is a complex system that consists of dispersion of gas bubbles and solid particles in a continuous liquid phase. These systems are widely used for chemical, physical, petrochemical, and biochemical processes.1 Gas-liquid-solid threephase circulating fluidized beds (GLSCFBs) are a special type of three-phase fluidized bed with a continuous outer particle circulation. For multiphase industrial applications, GLSCFBs provide many efficient properties, such as good phase contact, minimum dead zone, excellent heat and mass transfer characteristics, and high operational flexibility. GLSCFBs are suitable for the reactions involving light and/or small solid particles. Compared to the expanded three-phase fluidized bed, these reactors are commonly operated at high gas and liquid velocities, resulting in higher capacities. Thus, it is important to develop and achieve an efficient design of the fluidization systems. The hydrodynamic behaviors and phase holdup distributions need to be studied and well-understood. Several measurement techniques have been applied for phase holdups in multiphase systems, such as static pressure, direct sampling, shutter, optical-probe, and electrical-probe techniques. However, these methods involve some difficulties in obtaining reliable data. For example, the static pressure method is ineffective in the system where solids and liquids have similar densities. It is difficult to obtain reliable results with the direct optical-probe and the electrical-probe techniques. The direct sampling method is not accurate in measuring solid particles in the slurry.2 Ultrasonic measurement offers many advantages, such as high accuracy and rapidity. It is also suitable for optically opaque systems and potentially applicable to high temperature and pressure conditions. Furthermore, the ultrasonic technique is safer and simpler to use in comparison with existing techniques such as γ-ray and X-ray. There are several researchers developing this method to study the hydrodynamic character* To whom correspondence should be addressed. E-mail: [email protected].

istics of multiphase fluidization systems. However, the disadvantage of this technique is temperature sensitivity.3-5 For this ultrasonic system, the sound beams are transmitted through the studied volume. The acoustic properties of the transmitted ultrasound, such as velocity and amplitude, vary as the beam comes across different media along its path. In multiphase flows, a fluid contains dispersed inhomogeneities, which are the solid particles and/or gas bubbles. When an acoustic wave strikes the boundary between two different media and the acoustic impedances of the two media differ, some acoustic energy is reflected and some is transmitted. The reflected wave travels back through the incident medium at the original sound velocity. The transmitted acoustic wave continues to move through the new medium at the sound velocity of the new medium. In addition, while the ultrasound travels through different media, it is partially scattered and absorbed. This accordingly leads to the decrease of the amplitude of the sound wave, which is called attenuation.2,6-7 An accurate measurement of the variations in sound speed and amplitude can therefore be used to detect different media or discontinuities in a system. The ultrasonic technique has been widely applied in two-phase systems. In a liquid-liquid two-phase system, Smith8 applied the difference of the transmitted acoustic velocities in two liquids to simultaneously measure the volume fraction and the average drop size of one liquid dispersed in another one. Havlicek and Sovava9 applied the ultrasonic technique to monitor instantly the local phase fraction by measuring the velocity of sound in the liquid dispersion. Their approach, however, required mounting the probes through holes in the vessel walls. Later, Tavlarides and Bonnet10 demonstrated the nonintrusive use of this technique, which was used for monitoring a local phase fraction in a multistaged pilot extractor and for extractor control.11,12 Afterward, Yi and Tavlarides13 obtained the dispersed-phase holdups by using a simple volume average of sound velocity through the dispersion as compared to that through the liquid only. In liquidsolid systems, the acoustic properties can be used to

10.1021/ie034184c CCC: $27.50 © 2004 American Chemical Society Published on Web 05/27/2004

5682 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004

determine particle size, concentration, and the mechanical properties of the constituents such as elastic modulus and rheology. Urick14 and Hampton15 investigated the systems of fine powders. Atkinson and Kytomaa16 studied the system of bigger particles. Moreover, the application of the ultrasonic technique was also found in gas-liquid systems to determine the properties of the gas bubbles such as the void fraction of the system,17 specific interfacial area,18 and bubble diameters.19 Attempts have been made to apply the ultrasonic measurement techniques to study the phase holdups in gas-liquid-solid three-phase fluidized beds as well as to correlate the transmission time and the attenuation of ultrasound with gas bubble and particle concentrations.2,20-22 Uchida et al.2,20 proposed an indirect method to measure the solid holdup by analyzing the shape and the phase lag or lead of the ultrasonic wave transmitted through the three-phase system. An accurate measurement of the variations in sound speed and amplitude could therefore be used to detect the different media or the presence of discontinuities in a system. The same method was applied by Maezawa et al.23 to measure the longitudinal distribution of gas holdup. Warsito et al.24 used the transmission time difference, which is based on the difference between the velocity of an ultrasound signal in the dispersion and that in the pure liquid, to investigate the gas and solid holdups. This variation of sound speed as well as the difference of the attenuation was applied to correlate the solid volume fraction in the slurry reactor using nitrogen as a gas phase.4-6 Later, Warsito et al.25 applied the ultrasonic computed tomography to obtain the cross-sectional distributions of gas and solid holdups. All the existing ultrasonic measurement methods have been developed on the basis of the phase difference of acoustic waves, transit time variation, and change in amplitude of ultrasounds with certain simplifications and assumptions. The assumption that gas bubbles have no effect on sound velocity was commonly used.4,5,26 Lately, some researchers have shown that the speed of ultrasound was independent of gas holdups, due to the great distortion of ultrasound around bubbles.22,29 This discrepancy challenged the approach of determining phase holdups in a multiphase system. Therefore, there is a need for a new approach to investigate the phase holdup in three-phase systems. A new analysis approach was proposed in recent publications.28,29,31 This analysis technique was used to determine the phase holdups for a system using 500µm glass beads as the solid phase. The method was based on the fact that the variations of signals are caused by the reflection and refraction when a sound beam encountered a boundary between two media with different characteristic impedances. Their observation showed that the fluctuations of sound waves, in terms of the standard deviations of transmission time and amplitude ratio, were highly correlated with gas and solid holdups. A new mathematical analysis of these fluctuations in two-phase flows can be employed to differentiate simultaneously the contributions of gas bubbles and solid particles in three-phase fluidized beds.30 However, this approach has been applied only to a three-phase flow with 500-mm glass beads as the solid phase. In addition, most of these studies were performed with small solid particles (below 1 mm). Only Macchi

Figure 1. Model gas-liquid-solid circulating fluidized bed column: (a) auxiliary liquid line, (b) primary liquid line, (c) air line, (d) gas distributor, (e) conductivity probes, (f) butterfly valve, (g) ultrasonic probes, (h) ultrasonic controller, (i) heat controller, (j) riser, (k) downer, (l) centrifugal filter, (m) heater, (n) flow meters, (o) cooling coil, (p) L-valve, (q) pressure transducers, (r) water reservoir

et al.22 presented results with 1.3-mm glass beads. They found that the ultrasonic system did not provide a good measurable signal and that this technique was better suited for smaller sizes of particles. The goal of this work is to assess the validity of the application of the ultrasound device and the new statistical approach for large particle sizes. The local dispersed phase holdups, the distribution of gas bubbles and solid particles, and the influences of operating conditions on the phase holdups were also investigated in the GLSCFB. Experimental Setup The experiments were performed in a fluidization loop, as shown in Figure 1. This loop consists of two main Plexiglas columns, called riser and downer, respectively. The riser is 7.6 cm in diameter and 2.0 m in height. The downer is 10.2 cm in diameter. The liquid pumped from the reservoir was divided into two streams and then fed into the bed. The primary liquid stream entered at the base of the riser as a continuous fluid to carry gas bubbles and solid particles up along the riser column. The second liquid stream was the auxiliary liquid stream entering at the side of the downer column to push and to control the amount of solid particles to the riser via the L-valve. After the three phases moved concurrently upward to the top of the riser and entered the downer, gas bubbles left the top exit of the downer while solid particles were separated by a centrifugal filter. This centrifugal filter is a cylindrical tube 5 cm

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in diameter, which is located on the top of the downer. Solids were removed from the liquid stream by a radial centrifugal force exerted on the particles and were pushed downward in the downer. The butterfly valve was used for measuring the solid circulating rate, which is the mass flow rate of solids circulating within the fluidization loop. The solid circulating rate was determined by measuring the height of solid particles for a given time period. This solid circulating rate and overall liquid flow rate could be controlled independently by regulating the flow ratio between the two liquid streams. In addition, the superficial liquid velocity mentioned in this work was the sum of the primary and auxiliary liquid flow rates. Tap water was the continuous liquid phase. Glass beads with average diameters of 433, 700, and 1300 µm and a density of 2500 kg/m3 were used as the dispersed solid phases. Oil-free air bubbles generated by a gas distributor were the dispersed gas phase. The gas distributor was a perforated stainless steel plate with small pores of 35 µm diameter, located close to the base of the riser. A two-channel ultrasonic pulser-receiver system (model FUI2100, Fallon Ultrasonics Inc.) was used to measure the transmitted sound signals through the column in terms of the transmission time and the amplitude of the selected echoes within the gates. The system had a 0.130 MHz amplifier bandwidth and high-power HP modules. The ultrasonic probes (13 mm in diameter), a 3 MHz transmitter, and a 3 MHz receiver were mounted along the riser column and operated in a pulse mode. The gain of sound wave was continuously adjusted in order to maintain the signal above the instrument threshold limit. The instantaneous ultrasonic signals were analyzed to obtain the standard deviation of sound speed and attenuation in order to determine the gas and solid holdups. The horizontal space between the two ultrasonic probes was set at 7 or 2 cm apart. The local phase holdups were measured at two different axial positions of 0.5 and 1.2 m from the gas distributor. Besides, the transmitter and receiver were moved simultaneously at seven radial positions with the constant probe spacing of 2 cm. The local holdups are the mean of the phase holdups within the measuring gap. The water temperature is precisely maintained at 25 °C by a digital temperature controller during the experiments in order to achieve an accurate measurement of the ultrasonic system. Moreover, the phase holdups in the liquid-solid and gas-liquid systems were also determined by differential pressure transducers. To validate the ultrasonic results in the three-phase flow system, the pressure transducers and wall conductivity probes were used concurrently to determine the volume fractions of gas bubbles and solid particles. Since both gas and solid phases are nonconductive, the average value of conductivity data can be directly related to the liquid holdups.27 Conductivity probes were supplied with a 1 kHz ac current. Data from pressure transducers and the conductivity probes were taken at a data acquisition frequency of 150 Hz for 30 s at the same time. During the actual test, at least two repeated sets of data were obtained for each operating condition to ensure accuracy. Analysis of Ultrasonic Signals. The transmitted ultrasonic signals, the transmission time, and amplitude are recorded from a multiphase flow system. The

amplitude ratio or the attenuation is determined from the variation of system sound amplitude compared with the amplitude in pure liquid system. Vatanakul et al.28,29 studied the characteristics of the acoustic signals in the air/water and water/500-µm glass bead systems. From their work, it can be seen that the presence of dispersed gas and solid phases clearly had different effects on the fluctuation of the waveforms. Gas bubbles were apparently the significant factor in generating high fluctuations of sound signals, especially the transmission time within the multiphase flows. Besides, the standard deviations of transmission time and amplitude ratio were well-functioned with the concentrations of gas bubbles and solid particles. In addition, the determination of signals in three-phase flow showed that the fluctuations of the sound signal resulted from three factors, gases, solids, and the interaction between gases and solids.30 In this investigation, the statistical approach for the three-phase flow developed by Zheng and Zhang31 was applied. The gas and solid holdups of three-phase systems can be statistically estimated through the relationship between phase holdups and the sequence of the signal’s fluctuation obtained in gas-liquid and liquid-solid systems. This statistical approach is briefly presented. Only the signals of the transmission time determining the gas holdup are exhibited here in an example. A typical sequence of transmission time recorded in a three-phase flow is denoted as Tgls. To differentiate a subset representing the individual contributions of phase holdups from the standard deviation of Tgls, a separation coefficient, δ, is defined

Tgls,max/T h gls>δg1

(1)

Tgls)[Tgls(t1),Tgls(t2),Tgls(t3),...,Tgls(tn)]′

(2)

and

where Tgls(tn) is the transmission time in the threephase system recorded at time tn. Tgls,max is the maxih gls is the average Tgls value. mum element of Tgls. T The sequence Tgls is then arbitrarily partitioned by the defined separation coefficient, δ, into two subsets. The first subsets of the series are called the upper portion, defined as TU(δ). This subset consists of the data higher than or equal to T h glsδ. Under similar circumstances, the lower portion of Tgls, presented as h gls/δ. TL(δ), involves the elements less than or equal to T An increase in the value of δ consequently leads to a decrease in the standard deviation of the two subsets. When δ reached its optimum value, presented as δ*, TU(δ*) of a gas-liquid-solid three-phase flow was to statistically represent the contribution of gas bubbles to the standard deviation of Tgls. Moreover, this subset of Tgls was also highly correlated with the sequence of ultrasonic signal recorded in a two-phase flow system that had the same gas holdup. To achieve δ* the correlation coefficient based on canonical analysis, F, is used. This coefficient can be mathematically described as follows:

F)

Cov[L′TU(δ),M′Tgl] [Var(L′TU(δ))Var(M′Tgl)]

1/2

)

Cov(W,V) [Var(W)Var(V)]1/2 (3)


Figure 2. Transmission time difference signals recorded in the pure water, gas-liquid, liquid-solid, and gas-liquid-solid flows of 1.3-mm glass beads.

It is noted that |F| e 1. L and M are the coefficient vectors. The absolute value of F is maximized to achieve the highest correlation between W and V. Following the statistical procedure mentioned above, the canonical correlation coefficient between two subsets, F, is consequently a function of δ.

F)f(δ)

(4)

Therefore, if the separation coefficient is adjusted to its optimal value, δ*, the maximum canonical correlation coefficient, F*, is also reached. From a statistical point of view, the T(δ*) is an appropriate substitute for Tgl, since both of the two sequences represent the flow having the same gas holdup and reflect the effect of dispersed gas phase on the ultrasonic signal. Finally, the gas holdup of a three-phase system can be estimated by inserting the standard deviation of T(δ*) in to the relationship of the standard deviation and gas holdup that is experimentally obtained in a gas-liquid flow. The fluctuation of the amplitude ratio of an ultrasonic signal can be analogously analyzed by this approach. An appropriate portion is extracted from the amplitude ratio sequence obtained in a three-phase system and, then, replaced by the amplitude ratio of liquid-solid flow having the same solid holdup. Finally, the solid holdup of a three-phase system can be estimated using the relationship between the standard deviation of the amplitude ratio and solid holdup observed in liquidsolid flows. The statistical approach presented earlier provided a reliable analysis of gas and solid holdups in the 500µm glass bead system.31 The goal of this work is to apply the aforementioned method to determine the phase holdups in three-phase systems for three different sizes of glass beads (433 µm, 700 µm, and 1.3 mm). Results and Discussion Characteristics of Sound Signals in Different Flow Systems. Experiments were carried out in four different systems: a pure liquid flow, a gas-liquid flow, a liquid-solid flow, and a gas-liquid-solid flow. To study the influences of dispersed phases on the characteristics of the ultrasonic signals, all the experiments

took place at 0.5 m above the gas distributor. In a twophase fluidized bed, the solids or gas bubbles were uniformly distributed. Glass beads were in a narrow size distribution. Air bubbles were uniform in size. The average bubble size, observed through a video camera, increased from 2 to 6 mm as the gas velocity increased. Figures 2 and 3 show the ultrasound signals, in terms of transmission time difference (∆T) and amplitude ratio measured in systems with gas-liquid, solid-liquid, gas-liquid-solid flows and liquid one-phase flows, respectively. The solid phase was 1.3-mm glass beads. ∆T represents the difference between the transmission time of the ultrasound in a multiphase flow and that in a pure water system. The characteristics of acoustic signals will be discussed separately for each flow system: gas-liquid, liquid-solid, and gas-liquid-solid flows. In the gas-liquid system, the waveform of the transmission time difference caused by gas bubbles demonstrated a greatly fluctuating distribution (Figure 2). It is because the impedance of air and water is very different. When the sound beam strikes at the interface of these two phases, the sound wave is reflected/ reradiated. Thus, the traveling time of the ultrasound is longer, which gives a positive ∆T. The same characteristic was also reported in the time signal of a single bubble.28,29 From Figure 3, it is noted that the system of air-water also introduced the highest fluctuation of amplitude ratio compared to the other water-glass bead and air-water-glass bead fluidized systems. The attenuation of ultrasound is exponentially proportional to the interfacial area of suspended inhomogenities as follows18,32

A/A0)exp(-RZ)

(5)

and the absorption coefficient, R, is generalized to

R)

( )( ) kd h s νZ 2 8θ

(6)

A and A0 are the amplitudes of the acoustic signals with and without the presence of dispersed phases, gas bubbles, and/or solid particles. ν is the volumetric interfacial area, Z is the traveled distance of the incident sound wave, R is the scattering coefficient, k is the


Figure 3. Amplitude ratio signals recorded in the gas-liquid, liquid-solid, and gas-liquid-solid flows of 1.3-mm glass beads.

wavenumber of ultrasonic wave, and d h s is the Sauter mean diameter of suspended objects. In the gas bubblewater system, the average size of bubbles is big (2-6 mm). This results in a small value of the volumetric interfacial area, which in turn gives a small absorption coefficient (eq 5). This leads to a high amplitude ratio in the flow system (eq 6). In the liquid-solid system, it was evident that transmission time slightly oscillated compared to that in gas-liquid flow. The partial penetration and the scattering of the acoustic beam through solid particles generated smaller fluctuations in the signals. For the waveform of amplitude ratio, the small variation generated by solid particles was observed as well. It can be attributed to the small difference in impedance at the liquid-solid boundary. Further observation was made in the three-phase system where both gases and solids were present. The variation of transmission time difference in three-phase flow was higher than those in two-phase flows (Figure 2). It can be explained that these variations of acoustic signals were created by discontinuous phases (gases and solids) and interaction between phases. Gas bubbles were clearly a dominant factor for the fluctuation of ultrasonic speed, which was further promoted by the interaction between gas bubbles and suspended solid particle in a three-phase system. The observations of the waveform characteristics for 1.3-mm glass bead system are in agreement with the study of the system using 500-µm glass beads reported by Vatanakul et al.28,29 In addition, in this work, the systems of glass beads with an average size of 433 and 700 µm also provided similar results. The contributions of gas bubbles and solid particles to the fluctuation of ultrasonic signals are measured in two-phase fluidizations. It can be observed that the volume fractions of gas bubbles and solid particles are highly correlated with the standard deviations of the transmission time and amplitude ratio of the ultrasound (Figure 4a,b). Figure 4a shows that the fluctuations of transmission time increase when concentrations of air bubbles or

glass particles increase. The transmission time’s fluctuations in the gas-liquid system are much higher than that in the solid-liquid system. This finding demonstrates that gas bubbles are certainly the major factor that generates the fluctuations of the transmission time. In the liquid-solid system, the glass beads with average sizes of 433 and 1300 µm were used as the solid phase. It can be seen that the particle size affected the standard deviation of transmission time. The system with bigger particles generated a higher standard deviation of transmission time. For a given solid holdup, the system with smaller beads has a higher number of particles. This means that the sound wave travels through a more consistent medium and therefore the transmission time presents less fluctuation than in the case of bigger beads. Figure 4b shows the standard deviation of the amplitude ratio of the gas-liquid system and the liquidsolid systems using three different sizes of glass beads. The standard deviation of the amplitude ratio in waterair flow increased gradually as gas holdup increased. The system of water and glass beads was operated in two different regimes, which were the expanded bed and solid circulating bed regimes. The results from these two regimes can be fitted with the same curve. Apparently, liquid velocity has no influence on the ultrasonic signals. Therefore, only one calibration line was needed to describe the solid holdup in terms of the standard deviation of amplitude ratio, independent of the system regime. The standard deviation of the amplitude ratio exponentially decreased with increasing solid holdup. The relationship between the fluctuation of amplitude ratio and the solid concentration for three particle systems demonstrated a similar profile. However, for a given solid holdup, the standard deviation of amplitude ratio increased with increasing particle size. This can be explained by eqs 5 and 6. The system with bigger beads has a smaller volumetric interfacial area, which leads to a smaller scattering coefficient and then to a higher amplitude ratio. This characteristic results in the higher fluctuation observed in a bigger particle system.


Figure 4. Standard deviation of (a) transmission time and (b) amplitude ratio as a function of gas and solid holdups in two-phase flows.

Comparison of Experimental Phase Holdups and Model Predictions. The accuracies of the proposed analysis method were evaluated in a gas-liquidsolid expanded fluidization system. The solid and gas holdups, determined by pressure transducer and a pair of conductivity probes, were considered as the experimental results, whereas the predicted phase holdups were the results obtained from the ultrasonic and the statistical approach. The distributions of solid particles and gas bubbles were assumed to be uniform under a wide range of operating conditions. The predicted phase holdups were compared with the experimental ones, as shown in parts a, b, and c of Figure 5 for the systems of air, water, and glass beads with three different bead sizes of 433, 700, and 1300 µm, respectively. It can be seen that the results of gas and solid holdups estimated from these two techniques are in very good agreement for every particle size. Thus, it can be concluded that the statistical analysis of the fluctuation of an ultrasound wave is very practical and accurate to predict values of gas and solid holdups in a three-phase fluidized bed. Moreover, the size of the solid particles has no influence on the accuracy of the new proposed procedure. This pair of 3 MHz ultrasonic transducers is proved to be a reliable application for the three-phase systems of solid particles with diameters in the range of 433-1300 µm, which is close to or larger than the wavelength of the sound wave (477 µm) emitted from the transmitter. If the particle size is smaller than the acoustic wavelength, the sound wave may pass around and have no change to be reflected at the solid particle boundary.33 Therefore, in a system of a smaller particle size, ultrasonic probes with a lower frequency are suggested. Application of the Ultrasonic Technique in GLSCFB. The ultrasonic technique was set up in the riser of a GLSCFB to measure cross-sectional averaged solid and gas holdups simultaneously. The location of transmitter and receiver probes was at 0.5 and 1.2 m from the gas distributor. The distance between the two probes was set at 7 cm apart. For the phase holdup’s measurement, the pressure transducers and conductivity probes were also employed and located at about the same axial positions. Experiments with the fluidized bed, consisting of air, water, and 433-µm glass beads, were performed under circulating operating conditions,

in which the liquid velocity is higher than the particle terminal velocity (6.3 cm/s for 433-µm glass beads). The predicted phase holdups, measured by the ultrasonic technique and the statistical approach, and the experimental phase holdups, determined by the pressure gradients and conductivity voltages, are reported in Table 1. At a low liquid velocity of 9.7 cm/s with constant gas and solid circulating rate, the experimental and the predicted volume fraction of 433-µm glass beads and air bubbles were very similar. However, from Table 1, at a higher liquid velocity of 15.2 cm/s, the experimental cross-sectional averaged axial solid holdups were smaller than the predicted ones for various gas velocities and solid circulating rates, since the experimental gas holdup was calculated from the fact that the summation of overall cross-sectional averaged phase holdup is equal to unity. Hence, the experimental value of gas holdup was expectably higher compared to the gas holdup obtained from the ultrasonic technique under the same operating conditions. Moreover, similar results were observed in the system with 1.3-mm glass beads as well. The difference between experimental and predicted phase holdups was obtained at the liquid velocity higher than 15 cm/s, where 1.3-mm glass beads started to be circulated in the fluidization loop. The high liquid velocity may affect the disagreement of the phase holdup’s results between two methods. The accuracy of both measurement techniques applied for the circulating fluidized bed, operated under high liquid velocities, needed to be investigated. The testing experiments took place in a liquid-solid two-phase circulating fluidized bed. The pressure transducer, conductivity probes, and ultrasonic analysis system were applied to simultaneously measure the cross-sectional averaged solid holdup at the same axial location in the riser. The twophase fluidized bed was operated at a liquid velocity higher than 12 cm/s with varying solid circulating rates, and 1.3-mm glass beads were used as a solid phase. Figure 6 shows a comparison of the results from these three different measurement methods. It can be seen that the solid holdups computed from every technique were fairly similar at liquid velocities less than approximately 14 cm/s. However, at the liquid velocity greater than 14 cm/s, the solid holdups measured by the ultrasonic technique and the pressure transducer were still in close agreement. In contrast, the values


Figure 5. Comparison of experimental and predicted phase holdups for the three-phase systems of (a) 434-µm, (b) 700-µm, and (c) 1.3-mm glass beads. Table 1. Comparison between the Predicted and Experimental Phase Holdups solid holdup l Ul Qg Gs (m) (cm/s) (mL/s) (kg/m2‚s) 0.5

1.2

0.5

1.2

gas holdup

pred

exptl

pred

exptl

9.7 9.7 9.7 9.7 9.7 9.7 9.7 9.7

0 35.1 0 35.1 0 35.1 0 35.1

9.2 9.2 14.1 14.1 9.2 9.2 14.1 14.1

0.0731 0.0872 0.0772 0.0947 0.0744 0.0779 0.0683 0.0982

0.0727 0.0720 0.0745 0.0902 0.0708 0.0752 0.0729 0.0908

0 0.0265 0 0.0222 0 0.0215 0 0.0214

0 0.0222 0 0.0248 0 0.0212 0 0.0253

15.2 15.2 15.2 15.2 15.2 15.2 15.2 15.2

0 35.1 0 35.1 0 35.1 0 35.1

9.2 9.2 14.1 14.1 9.2 9.2 14.1 14.1

0.0349 0.0373 0.0509 0.0860 0.0312 0.0372 0.0514 0.0952

0.0308 0.0214 0.0400 0.0492 0.0285 0.0212 0.0458 0.0434

0 0.0114 0 0.0106 0 0.0109 0 0.0108

0 0.0206 0 0.0303 0 0.0219 0 0.0269

determined by the wall conductivity probes were lower than the data from the other two techniques. The disagreement between the results of the conductivity

Figure 6. Comparison of solid holdups measured by three different techniques in the liquid-solid circulating fluidized bed.


Figure 7. Symmetrical radial distribution of solid and gas holdups for the systems of (a) 433-µm and (b) 1.3-mm glass beads.

probes and those of the ultrasonic technique and the pressure transducer increases when the fluidization system applied a higher liquid velocity. Nevertheless, the results obtained with the three methods showed a similar profile. It can be concluded that the conductivity probes underestimated the solid holdups at relatively high liquid velocities. The variation of the conductivity data may be created by the high turbulence of the flow structure in the circulating system. The wall conductivity probes were not sensitive enough to detect the small changes of the cross-sectional phase holdups of the circulating fluidized system. The data shown in Figure 6 also revealed that the ultrasonic technique was a reliable measurement device. Consequently, the ultrasonic technique should give an accurate measurement of the phase holdups in three-phase circulating flow for the particle sizes and operating conditions studied. The explanation given in the previous paragraph could account for the difference between the phase holdups computed by the ultrasonic analysis technique and those measured by the pressure transducer and conductivity probes presented in Table 1, especially at liquid velocity higher than 14 cm/s. Local Phase Holdups and the Axial Variation of the Radial Phase Distribution. Figure 7 shows the local gas and solid holdups obtained at seven different radial positions in GLSCFB of 433-µm and 1.3-mm glass beads. The measurements were taken at a fixed liquid velocity and solid circulating rate with five gas flow rates. It was clear that nonuniform distributions of solid and gas holdups existed in both solid systems after the fluidized bed was completely operated under the threephase circulating fluidization, Ul ) 9.7 and 15.2 cm/s for 433-µm and 1.3-mm glass beads, respectively. Local solid concentration slightly increased from the center to the wall of the riser. The radial distribution of gas bubbles presented the opposite trend. The gas holdup was highest at the bed center. As a liquid flowed through a cylindrical pipe, the liquid velocity distributed nonuniformly across the pipe with higher liquid velocity in the center but lower at the wall. The difference in local liquid velocity generated an inward pressure,

which forced gas bubbles to move toward the center of the riser. Solid particles in the riser were accelerated by the flow of the fluid phase (liquid and gas). In the center of the bed, a higher fluid velocity carried more solids upward, which therefore was responsible for a higher solid concentration at the wall and the radial nonuniform profiles of the solid holdup. It was also seen that the radial distributions of solid and gas holdups were symmetric about the axis of the riser for both solid systems (Figure 7). The radial distribution of gas holdup was more uniform in the 1.3-mm glass bead system comparing to the 433-µm glass bead system. In the three-phase circulating system of 1.3-mm glass beads, the local gas and solid holdups were also investigated at a higher axial position, located at 1.2 m from the gas distributors. Figure 8 shows the comparison of the radial distributions of phase holdups at two different axial elevations. Similar nonuniform distributions of solid and gas holdups were observed at higher location as well. Local solid holdup was a maximum at the column wall and a minimum at the center of the riser. The opposite trend was obtained for the radial gas distribution. Moreover, under the same operating conditions, the concentrations of gases and solids measured at two locations were apparently similar. The nonuniformity of radial phase distributions was therefore developed longitudinally within the three-phase circulating fluidized bed. This characteristic indicated a uniform axial flow structure. Thus, in the GLSCFB, solid particles typically circulated in the unit system with liquid velocity higher than the particle terminal velocity. Beyond this point, a fully developed flow structure was formed within the GLSCFB riser. The influences of the operating conditions on the local phase holdups and their profiles were also determined. The effect of gas flow rate on solid holdup (Figures 7 and 9) was more significant in the 1.3-mm glass bead system. When the gas flow rate increased, the volume fraction of 1.3-mm glass beads increased; however, the radial profile of the solid phase was not changed. The gas holdups expectedly increased with increasing gas flow rate for all solid systems.


Figure 8. Symmetrical radial distribution of (a) solid and (b) gas holdups for the system of 1.3-mm glass beads at two axial locations.

Figure 9. Effect of liquid velocity on the radial distribution of solid and gas holdups for the system of 1.3-mm glass bead at axial location of 1.2 m.

Figure 10. Effect of solid circulating rate on the radial distribution of solid and gas holdups for the system of 1.3-mm glass beads at axial location of 1.2 m.

Figure 9 shows the effect of liquid velocity on phase holdup at higher location (1.2 m). A higher liquid velocity resulted in more uniform radial distributions of gas bubbles and solid particles and lower gas and

solid holdups. The increase of liquid velocity enhanced the turbulence in the column, resulting in a more uniform distribution of phase holdups. On the other hand, gases and solids stayed at a shorter location in


the riser, inducing lower concentrations of dispersed phases. The solid holdup increased with increasing solid circulating rate. However, the solid circulating rate had no observable influence on the uniformity of the solid profile (Figure 10). Moreover, the local gas holdup and its radial distribution appeared to be independent of the solid circulating rate. The results of the effects of operating conditions, presented in the last two paragraphs, were in agreement with the work of Vatanakul34 for the system of 434-µm and 1.3-mm glass beads at lower axial position. Conclusions The applications of ultrasonic technique and statistical approach were very practical and accurate to investigate the phase holdups in multiphase flows. This method was proved to be reliable to measure simultaneously the gas and solid holdups under high liquid velocity conditions within the two-phase and threephase circulating fluidized beds as well. In a GLSCFB, the radial distributions of gas and solid holdups were nonuniform, whereas the profiles were symmetric about the axis of the riser in both solid systems. The solid holdup was low in the center but high toward the column wall of the riser, whereas the gas holdup distribution presented an opposite trend. The local solid and gas holdups at two different elevations along the height of the riser column were found to be similar, as were the radial profiles of gas bubbles and solid particles. These observations confirmed the hydrodynamic behavior of gas bubbles and solid particles within the GLSCFB studied. There was a uniform distribution in the axial direction and a nonuniform distribution in the radial directions for gas and solid phases. The nonuniform radial distribution of the solid holdup did not vary significantly with the solid circulating rate within the operating range of this study. The radial distributions of both phase holdups became more uniform and the concentrations of gases and solids decreased when the liquid velocity increased. Larger solid particles generated a more uniform radial distribution of gas holdup. Acknowledgment The authors gratefully acknowledge financial assistance from NSERC. Help provided by Frank Collins, Jody Chessie, Keith Rollins, and Dr. Andy Patel is highly appreciated. Nomenclature A ) amplitude of ultrasonic signals, % A0 ) amplitude of ultrasonic signals in pure liquid phase, % Cov ) covariance e ) absorption coefficient k ) wavenumber of ultrasonic wave dp ) particle diameter, cm d h s ) Sauter mean diameter, cm Gs ) particle circulating rate, kg/m2‚s l ) axial position, m L ) canonical weight M ) canonical weight Q ) flow rate, mL/s r/R ) reduced radius ∆T ) transmission time difference U ) superficial velocity, (cm/s)

Var ) variance V ) weighted canonical variate, defined in eq 5 W ) weighted canonical variate, defined in eq 4 T ) Transmission time of the ultrasound T h ) average value of T Z ) traveled distance of the incident sound wave Greek Symbols ) phase holdup λ ) Lagrange multiplier ξ ) scattering coefficient F ) canonical correlation coefficient F* ) optimum canonical correlation coefficient R ) absorption coefficient δ ) separation coefficient δ* ) optimum separation coefficient ν ) volumetric interfacial area Subscripts s ) solid phase l ) liquid phase g ) gas phase gl ) gas-liquid two phase gls ) gas-liquid-solid three phase U ) upper portion L ) lower portion

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Received for review October 14, 2003 Revised manuscript received March 22, 2004 Accepted March 24, 2004 IE034184C

Application of Ultrasonic Technique in Multiphase ... - ACS Publications

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