Radial Distributions of Phase Holdups and Phase Propagation

Ind. Eng. Chem. Res. 2009, 48, 281–289

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Radial Distributions of Phase Holdups and Phase Propagation Velocities in a Three-Phase Gas-Liquid-Solid Fluidized Bed (GLSCFB) Riser S. A. Razzak, J.-X. Zhu,* and S. Barghi Department of Chemical and Biochemical Engineering, UniVersity of Western Ontario, London, ON, Canada N6A 5B9

Electrical resistance tomography (ERT) and fiber optic were applied to investigate phase holdups and phase propagation velocities in a gas-liquid-solid circulating fluidized bed (GLSCFB). Since ERT is applicable only to conductive phase(s), e.g. the liquid phase in this study, a fiber optic probe was employed simultaneously to quantify all three phases. Saline water was used as the conductive and continuous phase. Glass beads and lava rocks constitute the solid phase and air as the gas phase. Glass beads were transparent and spherical in shape; however, lava rock particles were irregular in shape and opaque, which affected the signals obtained from the optical fiber probe. An empirical model was developed to measure the gas holdup using optical fiber probe data. Gas holdup was higher in the central region and decreased radially, while opposite trend was observed with solid holdup due to the drag forces imposed on solid particles by the gas and liquid flow in the riser. By applying cross-correlation between the data obtained at two different levels in the riser, nonconductive phase propagation velocity was obtained. The propagation velocity was higher in the central region compared to the wall region and increased with increasing liquid superficial velocity. 1. Introduction Good mixing and efficient heat and mass transfer have made fluidized bed reactors a unique choice in many processes in chemical, petrochemical, and biochemical industries. Recently, the liquid-solid circulating fluidized bed (LSCFB) and gas-liquid-solid circulating fluidized bed (GLSCFB) reactors have received growing interest on wastewater treatment, desulphurization of petroleum products, and in biochemical reactions.1 Most of the studies on gas-liquid-solid fluidization systems have mainly focused on conventional expanded bed regime in the past decades.2 Conventional fluidized beds also suffer from limitations such as liquid and gas velocities, solid particles size and density, etc. In GLSCFB, solid particles are circulated between the riser and the downer at higher velocities compared to conventional fluidized beds, which leads to formation of smaller bubbles and a better contact between phases and reduced backmixing. GLSCFB also offers great flexibility in terms of solid particles or catalyst regeneration in the downer. In spite of substantial work, the hydrodynamics of GLSCFB is not completely understood yet. The radial nonuniformity of phase holdup in liquid-solid circulating fluidized bed (LSCFB), using conductivity probe, was reported by Liang et al.4 Zheng et al.3 confirmed the radial nonuniformity using optical fiber probe. The solids holdup increased radially from the center to the wall. It was claimed that radial flow structure is affected significantly by operating conditions and particle properties. Zheng et al.3 showed radial distribution of the solids holdup under a wide range of operating conditions and tested the effect of particle density on the flow structure. Radial distribution of local liquid velocity was measured using a dual conductivity probe, with two probes, 20 mm apart, placed in the riser and a pulse injection of saturated NaCl electrolyte solution below the probes.5 Different methods have been employed in the study of hydrodynamics such as direct sampling, optical fiber, electric * Corresponding author. Tel.: 1-519-661-3807. Fax: 1-519-661-3498. E-mail: [email protected].

conductive probe, process tomography, static-pressure, ultrasound, and iso-kinetic separation. Phase holdup as a main parameter was of major concern. Lee et al.,6 de Lasa et al.,7 and Yong et al.8 used fiber optic probe to measure the gas holdup directly in a three-phase system. A single-core silica optical fiber of 400 µm U-shape probe was employed to detect gas bubbles. Wang et al.9 studied bubble behavior in a fluidized bed using 62.5 µm diameter optical fiber probe. Uchida et al.10 developed a new technique for solids holdup measurement in a three phase fluidized bed using ultrasonic sound waves. Later Vatankul et al.5 used similar concept for flow detection. This technique is based on the change in speed and amplitude of ultrasonic wave incident on a surface. Similar to light beams, when ultrasonic waves strike at the interface between two media, they may be partially/totally reflected, scattered or transmitted. Liang et al.11,12 used a horizontal probe for the measurement of solid holdup in the bubble wake and the emulsion phase. They also measured the solids holdup from the same signals using the conductivity of the pure liquid as the baseline. Process tomography is an area which has experienced a significant growth over the last ten years in the study of multiphase flow due to its nonintrusive technique.13 However, there are no imaging techniques available for the study of three phase systems in real time.14 George et al.15 developed a combined system of electrical impedance tomography (EIT) and gamma-densitometry tomography (GDT) to measure distribution of phases in a vertical three-phase flow system simultaneously. Razzak et al.16 measured phase holdups and velocities in a GLSCFB system by combining ERT and pressure transducers (PT). In this study, electrical resistance tomography (ERT), a newly developed method for the phase holdups measurement, is presented. However, ERT cannot measure phase holdups for all the phases, therefore an optical fiber probe and pressure transducers are used simultaneously to measure phase holdups for all three phases. In the experiments, water was used as the liquid (continuous and conductive) phase, air as the gas phase, and glass beads and lava rocks with 500 µm range as the solids phase. Combination of these measurement techniques provided

10.1021/ie800299w CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2008

282 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009

Figure 1. Schematic diagram of the experimental setup of the GLSCFB system.

valuable information about the hydrodynamics of GLSCFB riser. Average phase holdups and local distribution of phases were obtained. The phase propagation velocity was estimated by applying cross-correlation technique to the two sets of data obtained at upstream and downstream planes in ERT experiments. All measurements in this work were carried out at seven radial positions (distributed radially, centered at r/R ) 0, 0.2034, 0.492, 0.6396, 0.7615, 0.8641, 0.9518). 2. Experimental Setup All experiments were conducted with two different types of particles which were similar in size (average diameter 500 µm) but different in density and shape. Particle density of glass beads was 2500 kg/m3. Unlike glass beads which were spherical in shape with no internal porosity, lava rock particles were porous and irregular in shape with a particle density of 2210 kg/m3. Schematic diagram of the experimental setup of GLSCFB is shown in the Figure 1. The GLSCFB consists of two main sections, riser and downer, both made of Plexiglas. The riser is 5.97 m tall and 0.0762 m in diameter and the downer is 5.05 m tall and 0.2 m in diameter. A gas-liquid-solid separator is located at the top of the riser to separate out the solids from the gas and liquid flow and a solids circulation rate measurement device is located near the top of the downer to measure the solids circulation rate. The main liquid distributor made of seven stainless tubes occupying 19.5% of the total riser cross section and extending 0.2 m into the riser, and the auxiliary liquid distributor, a porous plate with 4.8% opening area at the base of the riser. The gas distributor is a tube of 19 mm in diameter and bent in a ring shape of approximately 0.0413 m in diameter, located at 0.34 m above the bottom of the riser. There are 460 small holes of 0.5 mm in diameter on the ring, giving a total

opening area of 361 mm2, pointing downward for gas flow. There is also a ring-type liquid distributor in the conical area near the bottom of the downer, which is a tube of 25.4 mm in diameter and bent in a ring shape of approximately 0.114 m in diameter, with 96 small holes of 1 mm in diameter on the ring, giving a total opening area of 301 mm2, pointing downward for gas flow. Solid particles are carried up in the riser mainly by the liquid flow, but also assisted by the gas flow. The auxiliary liquid flow is employed to facilitate the flow of solid particles from the downer to the riser, with the main purpose of controlling the solids circulation rate and acting as a nonmechanical valve. The combined effects of both primary and auxiliary liquid flow produce the total liquid flow, which carries the solid particles up in the riser. Air introduced from the gas distributor forms dispersed bubble flow in the riser. Entrained particles in the riser, collected in the gas-liquid-solid separator at the top of the riser, are returned back to the downer after passing through the solids circulation rate measuring device located near the top of the downer. All the measurements were carried out in the riser. 2.1. ERT Setup and Working Principle. A schematic diagram of the typical ERT setup is shown in Figure 2. The ERT consists of a sensor section, an electronic circuit and a PC-based data acquisition system. The inner diameter of the sensor section is built equal to the inner diameter of the riser so that the sensors can be lined up with the riser. The liner of the sensor section supports three planes of electrodes. Sixteen electrodes equally spaced on the first plane provide the voltage signals for reconstructing fine phase distributions, primarily for the distribution of solids holdup. Each of the two other planes contains eight electrodes, used to provide voltage signals for reconstructing coarse phase distributions. Cross-correlating between the latter two planes yields estimations of local or zoneaveraged phase propagation velocities. For the current study, the sensor section is installed between the heights of 1.93 and 2.24 m, to measure the flow structure at a height of H ) 2.02 m. For each driving current, the ERT measures the electrical potential distribution through the electrodes flush mounted on the pipe wall. With input values of the electrical potentials and currents, the local conductivity (or resistivity) of the mixture can be reconstructed through a state-of-the-art optimization algorithm. The conductivity distribution is then further converted into a local phase concentration distribution based on Maxwell’s relation. By cross-correlating two distributions from an upstream and a downstream plane, the phase propagation velocity can be obtained. The ERT system obtains data at 250 (500 optional) frames per second. Data can be collected for a certain period of time at steady-state condition. 2.2. Optical Probe Setup and Measurement Method. The optical fiber probes used in the present study were model PV5, produced by the Institute of Process Engineering, Chinese Academy of Sciences, are capable of measuring solids concentration in three phase fluidized beds. Details of the optical fiber probes system are shown schematically in Figures 3. They consist of both light emitting and receiving quartz fibers, arranged in an alternating array, corresponding to emitting and receiving layers of fibers. The diameter of the probe is approximately 4 mm and contains approximately 8000 emitting and receiving quartz fibers with a diameter of 15 µm each. These fibers are arranged in an alternating array, corresponding to emitting and receiving layers of fibers, within a 1.5 mm2 area at the center of the probe tip. Their small has negligible effect on the overall flow structure. The results were not significantly

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 283

Figure 2. Schematic diagram of the measurement prinicple of ERT.

Figure 3. Optical fiber probe system for gas holdup measurement in GLSCFB riser.

influenced by temperature, humidity, electrostatics, and electromagnetic fields. In order to prevent particles from occupying the blind zone, a Plexiglas cover of 0.2 mm was placed over the probe tip. The measuring section of the riser covered with black plastic sheet 0.4 m above and below to prevent external light may interference with the measurement. Figure 4 shows the typical voltage output signal produced by optical fiber probe for (a) liquid-solid and (b) gas-liquid-solid systems in the GLSCFB riser for glass beads with a segment of the data collected in 2 s. This situation may change from location to location not only under the same conditions but also for different operating parameters. The average voltage of the signal produced for this particular case was 1.334 V with a standard deviation of 0.241 V. The output voltage in the presence of bubbles was larger due to the reflection of light beams over the bubbles which increased the intensity of the light incident on the receptors of the fiber optic prob. The analysis of the data under different conditions showed that the critical voltage range, out of which the of bubbles could be detected, was the average voltage plus or minus 3 times the standard deviation, Vc ) V¯ ( 3σLS

(1)

j is the average voltage signal of the signals and σLS is where V the standard deviation for the liquid-solid system. Therefore, any voltage out of this range is the indication of a gas bubble. Gas holdup measurement in gas-liquid-solid systems is challenging when the gas forms a dispersed phase with small bubbles. In the range of operating parameters in these experi-

ments, average bubble size lies between five to twenty times of the solid particles size. Bubble sizes were reduced when the liquid velocity was sufficiently high and increased at lower liquid velocities. The signals obtained in the GLSCFB were similar to those of liquid-solid system. However, the detailed review of these signals was useful in developing an empirical model to detect the bubbles which should be at least three times larger than the average diameter of solid particles. The proposed model used to measure the gas holdup is Vc ) V¯ls +

(

)

V¯gls - V¯ls σls + 4σls V¯ls

(2)

j ls is the average voltage of the signals produced from where V j gls is average voltage of the signals in the liquid solid system, V the GLSCFB, and σls is standard deviation for liquid solid j gls - V j ls)/V j ls used in the system. The multiplying factor (V equation is to find out the exact critical voltage above which all the data points represent the gas bubbles. The average voltage of the signals in the GLSCFB riser is influenced by the bubbles size and numbers. The dimensionless multiplying factor was then adjusted to measure the particular critical voltage. The above empirical model for the gas holdup measurement was suitable for the three phase system using glass beads as solids phase. However, some modifications required as the amplitude profile for lava rock particles was different compared to glass beads. The amplitude of fluctuations was higher for lava rock particles in liquid solid system as shown in Figure


Figure 4. Typical voltage output signal produce by optical fiber probe for (a) liquid-solid and (b) gas-liquid-solid systems in a GLSCFB riser for glass beads.

5a. Since lava rock particles are not transparent as glass beads the major portion of the light beams incident on the particles will be reflected back to the receptors on the probe. The presence of gas bubbles will amplify this phenomenon and consequently the average voltage would increase. As shown in Figure 6, the study of signals showed that the range of voltage signals fluctuations associated with lava rock particles was the average voltage plus or minus 3 times the standard deviation. Considering correlation 2, another correlation was developed for the measurement of gas holdup in the three phase system in the presence of lava rock particles;

(

)

V¯gls - V¯ls σls ( 2σls Vc ) V¯ls + V¯ls

(3)

Any signal out of the range of voltage estimated by eq 3 shows the presence of a gas bubble. The location of the gas bubble relative to the probe tip correlated with the numerical value of the signal which may be above or below the critical voltage range. The time averaged local gas holdup then can be obtained by counting the number of data points occupied by the gas bubbles. It is to mention that fine bubbles cannot be detected however the fraction of fine bubbles in this study was small and did not affect the results appreciably. Since higher superficial liquid velocities result in formation of small bubbles, the model should be cautiously applied in such experiments. 2.3. Solids Circulation Rate and Superficial Solids Velocity. The solids circulation rate measuring device is a special section of the downer located near the top of the downer and just below the solid returning pipe connecting to the riser. This section is divided in two sections by a vertical plate, where

Figure 5. Radial distribution of (a) solids, (b) gas, and (c) liquid phase holdups under different gas superficial velocity at Ul ) 5.6 cm/s and Us ) 0.62 cm/s using glass beads.

two half-butterfly valves installed at each end of this section. By properly flipping the two half-butterfly plates from one side to the other, solids circulated through the system can be accumulated on one side of the measuring section for a given time period,

Gs )

hFsεs

Ad 2

tAr

(4)

where Gs is the solid circulation rate, h is the height of the accumulated particle (m), t is the accumulation time (s), Fs (kg/ m3) is the solids density, εs is solids holdup, Ad is cross-sectional area of the downer, and Ar is cross-sectional area of the riser of the accumulated solid particles. Superficial solids velocity was estimated by dividing Gs with the density of the particles; Us )

Gs hFs(Ad ⁄ 2)εs h(Ad ⁄ 2)εs ) ) Fs tFsAr tAr

(5)

3. Results and Discussion 3.1. Solids Circulation Rate and Solids Superficial Velocity. Solids circulation rate is the mass flow rate of solids circulating between the riser and the downer at steady-state operation and is mainly controlled by the auxiliary liquid flow rate. At a constant auxiliary liquid flow, the solids circulation rate initially increased with increasing superficial liquid velocity and then reached to a constant value. It is practically important


Figure 7. (a) Solids circulation rate. (b) Solids superficial velocity for glass beads and lava rocks.

Figure 6. Typical voltage output signal produced by an optical fiber probe for (a) liquid-solid and (b) gas-liquid-solid systems in the GLSCFB riser for lava rocks.

to know the range of change in superficial liquid velocity over which the solid circulation rate is no longer changing significantly. The results for two different types of particles glass beads and lava rocks are shown in Figure 7a. Glass beads are spherical in shape and not porous materials whereas lava rocks are irregular and porous by nature. At similar operating conditions the circulation rate of lava rock particles was lower than that of glass beads. Similar trend had been reported by Zheng et al.17 for the LSCFB. A similar trend was observed with the superficial velocity of solid particles as shown in Figure 7b. The lower values of lava rock particles superficial velocities may be attributed to the differences in apparent densities of solid particles and their shapes. Irregular shape of lava rock particles increases the drag force imposed by the flow of the liquid. 3.2. Phase Holdup. The cross-sectional area of the riser is divided equally into six sections (distributed radially, centered at r/R ) 0.2034, 0.492, 0.6396, 0.7615, 0.8641, 0.9518) to measure the zone based average solids and gas holdups. These locations were considered for dividing the cross-sectional area of the GLSCFB riser in six equal zones to measure zone and time based average solid and gas holdups. ERT system can provide phase distributions of a multiphase flow by differentiating between the conductive and nonconductive phases. Therefore, when the nonconductive phase consists of two phases, e.g. gas and solid phases as in this study, it would be impossible to determine the holdups of the two phases separately at particular location. However if the holdup of one of the phases can be determined by another method, the complete distribution of phases can be estimated. Therefore fiber optic probes were employed to measure the gas holdup. The combination of ERT and fiber optic data successfully revealed the phase holdups of all the phases at different locations.

3.2.1. Effect of Superficial Gas Velocity. Radial distribution of solids, gas and liquid phase holdups was determined at different operating conditions using glass beads as solids particles as shown in Figure 8. Nonuniformity of the gas and solids holdups distribution was found at different superficial gas velocities. As shown in Figure 8a, solids holdup is relatively constant at the central region and increased radially toward the wall region. This phenomenon can be explained by the momentum balance on the particles in the riser. Liquid velocity was higher at the central region and lower at the wall region due to wall effect. Therefore solids particle moved faster at the central region which resulted in lower solid concentration in that region. It was also noticed that solids holdup increased with the increase of superficial gas velocity in all radial positions. Similar profile was reported by Vatankul et al.5 and Razzak et al.16 Backmixing which is more plausible at higher solids circulation rate might have increased the solids holdup in the wall region. Unlike the solids holdup, gas holdup was higher in the central region and decreased radially toward the wall as shown in Figure 8b. Gas bubbles have a tendency to move toward the central region due to the higher liquid velocity and lower shear rate at this thing region. Local velocity gradient or shear rate increases toward the wall creating pressure differences. This pressure difference allowed more number of bubbles move to the central region. Local gas holdup increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity. As shown in Figure 5, in the liquid-solid system, liquid holdup was higher and almost flat nears the central region and decreases toward the wall. However in Figure 8c when the gas was introduced, liquid holdup was lower at the central region and increased toward the wall region. It was also noticed that in any local position liquid holdup decreased with the increases of superficial gas velocity. Radial distribution of solids and gas holdup at different operating conditions in the presence of lava rock particles and glass beads was depicted in Figure 9. Solids holdup radially increased toward the wall region. It also increased with the increases of superficial gas velocity. The difference between


Figure 8. Zone based average radial distribution of solids and gas holdup comparison between glass beads and lava rocks under different superficial gas velocities at Ul ) 22.4 cm/s and Ul ) 11.2 cm/s.

Figure 9. Zone based average radial distribution of solids holdup comparison between glass beads and lava rocks under different superficial liquid velocities at Ug ) 4.88 cm/s.

the holdups of glass beads and laves rock particles was small at higher superficial liquid velocities and increased with decreasing liquid velocities. On the other hand, gas holdup for glass beads and lava rocks was higher at higher superficial liquid velocity and decreased at lower liquid velocities. At any local point for both cases, gas holdup increased with the increase of superficial gas velocity.

3.2.2. Effect of Superficial Liquid Velocity. Radial distribution of zone based average solids holdup under different superficial liquid velocities at a superficial gas velocity of Ug ) 4.88 cm/s was obtained. Figure 9a shows the effect of solids holdup under different superficial liquid velocity for glass bead at a superficial solids velocity of Us ) 0.95 cm/s while similar graph for lava rocks particles at Us ) 0.48 cm/s is shown in Figure 9b. Solid holdup decreased with increasing superficial liquid velocity for both particles. Radials distributions of solid holdup for both particles showed similar trends under different superficial liquid velocities, almost flat at the central region and followed by an increase at wall region. Unlike lava rocks, solids holdup of glass beads particles changed appreciably with liquid superficial velocity. Figure 10 shows zone based average radial distribution of gas holdup comparing between glass beads and lava rocks under different superficial liquid velocity at Ug ) 4.88 cm/s. The solid particles superficial velocity was Us ) 0.95 cm/s for glass beads (Figure 10a) and Us ) 0.48 cm/s for lave rocks (Figure 10b). Similar trend was observed for gas hold up variation with both particles which was higher at the central area of riser and continuously decreased toward the wall. As shown in Figure 10a, gas holdup was not appreciably influenced by the liquid velocity at any specific radial position for glass beads; however in the case of lava rocks, the effect of liquid velocity was obvious at higher liquid velocity which can be attributed to the irregular shape of lave rock particles. The drag force on lave rock particles was higher and more solids were picked up by liquid flowing upward in the riser. 3.2.3. Effect of Superficial Solids Velocity. Comparison of zone based average radial distribution of solids and gas holdups between glass beads and lava rocks under different superficial solids velocity at Ul ) 11.2 cm/s and Ug ) 4.88 cm/s depicted in Figure 11. Due to the different shape and density of particles, under the same superficial liquid velocity, superficial solid


Figure 10. Zone based average radial distribution of gas holdup comparison between glass beads and lava rocks under different superficial liquid velocities at Ug ) 4.88 cm/s. Figure 12. Cross-sectional average solids and gas holdup comparison between glass beads and lava rocks under different superficial liquid, solids, and gas velocities.

solids velocity as shown in Figure 11b. Similar trend was observed for the radial gas holdup distribution under different solids superficial liquid velocities for both particles. 3.2.4. Cross-Sectional Average Holdups. Optical fiber probe data in a particular radial location was used to calculate the time averaged cross-sectional phase holdup, from which the area averaged cross-sectional phase holdup is calculated by

∫

2 R εr dr (6) R2 0 whereas in ERT, phase holdup was measured in six equal spaced zones. Therefore, cross-sectional average phase holdup was calculated from the time average data captured using ERT by ε)

6

∑εA

i i

ε)

Figure 11. Comparison of zone based average radial distribution of solids and gas holdup between glass beads and lava rocks under different superficial solid velocity at Ul ) 11.2 cm/s and Ug ) 4.88 cm/s.

velocities were different for both particles. Glass beads particles superficial velocities were Us ) 0.95, 0.98, and 1.15 cm/s while the corresponding values for lava rocks under the same operating conditions were Us ) 0.33, 0.54, and 0.68 cm/s, respectively. Consequently solids holdup was higher for glass beads in all three cases compared to lava rock particles. Solids holdup increased radially with the increase of superficial solid velocity. However gas holdup decreased with the increase of superficial

i)1

(7) A 3D cross-sectional average solids and gas holdups have been shown under different superficial liquid, solids and gas velocities in Figure 12. The size of gas bubbles decreases with increasing liquid superficial velocity, which impose limitations on fiber optic probe as the probe cannot detect fine gas bubbles. Since the superficial solids velocity for glass beads was higher than lava rocks, solid and gas holdups was higher glass beads compared to lave rocks. At any superficial gas velocity, crosssectional average solids and gas holdup decreases with the increase of superficial liquid and solids velocities. However, the cross-sectional average gas holdup increased with the increase of superficial gas velocity. 3.3. Propagation Velocity Measurement. The interfacial propagation velocities of the nonconductive phase/phases are obtained by cross-correlation technique.18 As shown in Figure 13, two imaging planes are placed in short distances in the ERT. The image reconstructed by the ERT was divided into a number


Figure 13. Cross-correlation technique used for obtaining propagation velocity profile in GLSCFB system.

Figure 14. Radial distribution of combined gas-solid phase propagation velocity profile under different superficial liquid and solid velocities for glass beads.

of finite elements, each having a value which was indicative of the resistivity/conductivity of the region occupied. Propagation velocity was estimated by applying cross-correlation analysis to phase distributions at the two levels. The total number of finite elements used to measure propagation velocities was 256 per plane. If it takes a time lag, τ, for a void wave to propagate from upstream plane to downstream plane, the wave velocity is d (8) τ where d and τ are the distance between the two sets of electrodes and the time lag of the interfacial wave propagation, respectively. The distance is set by the ERT manufacturer and the time lag is obtained from cross-correlation analysis. Basically, a cross-correlation function can be defined as Ck )

∫

1 T⁄2 σA(t) · σB(t + τ) dt (9) Tf∞ T -T⁄2 for a certain period of time, T. The function describes the general dependence between the upstream, σA(t), and the downstream, σB(t), conductivities. Radial distribution of nonconductive phase (gas and solid combined) propagation velocity under different superficial liquid and solid velocities for glass beads is shown in Figure 14. the nonconductive phase propagation velocity was higher at the central region and decreased radially toward the wall region for all superficial liquid velocities applied. Gas bubbles have a tendency to move upward in the central region. The liquid velocity was higher at the central region and imposed higher SAB ) lim

Figure 15. Radial distribution of combined gas-solid phase propagation velocity profile under different superficial liquid velocity for glass beads at Ul ) 11.2 cm/s.

drag force on particles in that region. Liquid superficial velocity had also a profound effect on the phase propagation velocity. The rate of change in propagation velocity was much higher at higher liquid superficial velocities. Figure 15 shows the radial distribution of the nonconductive phase propagation velocity at superficial liquid velocity of 11.2 cm/s for different solid superficial velocities. 4. Conclusions Radial distribution of phase hold-ups and phase propagation velocities were determined successfully by applying electrical resistance tomography and fiber optic methods. ERT signals are influenced by conductive phase(s) only, therefore the application of fiber optic method was necessary to determine phase hold-ups for all three phases. An empirical method was developed for quantification of the fiber optic data. The fiber optic signals obtained for glass beads and lave rocks were substantially different in amplitude and average values, which required minor modifications in the model. Although the fiber optic was not sensitive to the small gas bubbles the model provided reasonably good results. Different results were obtained for glass beads and lava rock particles but both particles showed similar trends in radial distribution of phase hold-ups and propagation velocities. Gas holdup was always higher in the central region and decreased radially toward the wall while solid holdup showed opposite trend. Solid propagation velocity was also higher in the central region compared to the wall region due to higher velocities of the gas and liquid phases which imposed larger drag force on particles in that region.


Acknowledgment The authors would like to acknowledge the Natural Science and Engineering Research Council of Canada for financial support and the Canada Foundation of Innovation for the infrastructure fund that was used to purchase the ERT. Notation A ) cross-sectional area, m2 Ck) wave velocity, m/s Gs) solids circulation rate, kg/m2s h )height of the accumulated particles in the solids circulation rate measurement device, m P ) pressure, psia r) radial position, m R ) radius of the riser, m SAB ) cross-correlation function Ul ) superficial liquid velocity, m/s Ug ) superficial gas velocity, m/s Us ) superficial solid velocity, m/s Z ) height, m Greek Letters F ) density, kg/m3 σ ) conductivity, µSi/cm σm ) estimated local conductivity, µSi/cm σ1 ) local conductivity for single phase, µSi/cm σ0 ) local conductivity for mix phases, µSi/cm ε ) holdup τ ) time lag, s Subscripts g) gas phase l) liquid phase s) solid phase b) bulk d ) downer r ) riser bed) fluidized bed ls ) liquid-solid phase gls ) gas-liquid-solid phase

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ReceiVed for reView February 20, 2008 ReVised manuscript receiVed April 16, 2008 Accepted May 23, 2008 IE800299W

Radial Distributions of Phase Holdups and Phase Propagation

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