Flow Regimes and Radial Gas Holdup Distribution in Three-Phase

Ind. Eng. Chem. Res. 2002, 41, 1877-1884

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Flow Regimes and Radial Gas Holdup Distribution in Three-Phase Magnetic Fluidized Beds Chia-Min Chen and Lii-Ping Leu* Department of Chemical Engineering, National Taiwan University, Taipei 106-17, Taiwan

This study examined the fluidization behavior of gas-liquid-solid magnetic fluidized beds to provide prerequisite knowledge for reactor design. Air, water, and iron shots (0.19, 0.39, and 0.93 mm in diameter) were used as gas, liquid, and solid phases, respectively. The radial gas holdup distribution and overall gas holdup in three-phase magnetic fluidized beds were measured. Flow regimes of three-phase magnetic fluidized beds with different particle sizes were also discussed. The radial gas holdup distribution was measured by using an optical fiber probe; the change of radial gas holdup distribution under different flow regimes was also studied. 1. Introduction Gas-liquid-solid three-phase fluidized beds are of major importance in chemical, petrochemical, and biochemical industry processes such as hydrogenation and hydrodesulfurization of residual oil, wastewater treatment, fermentation, coal liquefaction, and liquid-liquid extractors. The magnetic fluidized beds (MFBs) mean fluidized beds containing ferromagnetic particles under the influence of an external magnetic field. The fluidization behavior of magnetic fluidized beds was first described in the early 1960s by Filippov;1 early interest in magnetic fluidization was mainly centered around the gas-solid reactors. To date, the performance of gasliquid-solid magnetic fluidized beds has not been discussed in detail yet. Hu and Wu2 examined liquid-solid and gas-liquidsolid MFBs and determined the effect of magnetic field intensity on radial and overall gas holdup. Ouyang et al.3 and Kwauk et al.4 examined bubble size in threephase MFBs under the varying liquid velocity, gas velocity, particle size, and surface tension. Thompson and Worden5 presented experimental measurements of gas holdup, liquid axial dispersion, and gas-to-liquid mass-transfer coefficients in three-phase MFBs. Hristov and Hadzisavvas6 presented hydrodynamic study of three-phase fluidized beds in the presence of an external transverse magnetic field and drew a classification structure of magnetic fluidized beds. The influence of magnetic field on local and overall gas holdup has been studied in a three-phase magnetic fluidized bed by using the electroresistivity probe technique and quick closing valve technique.2,5,7 Sonolikar et al.7 reported that the application of magnetic field gives higher values of local gas holdup than those in the absence of magnetic field. The gas voidage variation in the two-phase as well as the three-phase fluidized beds is virtually important for estimating the real velocities and residence time distribution of each phase. Several methods have been proposed for the phase holdup measurements of threephase magnetic fluidized beds; these methods can be classified as 1. Physical methods: pressure profile technique, quick closing valve technique, bed height measurement (overall phase holdup measurement2,5,7). * To whom correspondence should be addressed.

2. Electrical capacitance and conductance method: electric conductivity probe technique (local phase holdup measurement2,7). 3. Optical method: optical fiber probe (local phase holdup measurement, this work). The small particles (dp < 1 mm) have been widely used in biotechnology processes such as aerobic and anaerobic wastewater treatment and cultivation of immobilized and pellet-forming microorganisms.8 Bubble properties such as radial gas holdup distribution, bubble frequency, and overall gas holdup strongly affect the mass transfer rate and flow patterns of each phase. The hydrodynamic behaviors of three-phase magnetic fluidized beds have not been discussed in detail yet, so this study presents the experimental measurements of radial gas holdup distribution by using an in situ optical fiber probe, flow regimes, and overall gas holdup in threephase magnetic fluidized beds. The changes of radial gas holdup distribution and overall gas holdup with different particle sizes in different flow regimes were also studied. 2. Experimental Section Figure 1 shows a schematic diagram of the experimental apparatus. Tap water is used as the major fluidizing medium, which flows through a 0.08-m i.d., 0.5-m long Plexiglas column loaded with 2 kg of iron shots. The properties of iron shots are listed in Table 1. The air is dispersed in the liquid fluidized bed. A perforated plate is used as a distributor for the bed with a fractional free area of 2.0%, designed to prevent liquid channeling. Sampling port is placed at 0.07 m above the distributor, the initial bed height is 0.1 m, and the maximum bed height after expansion is about 0.17 m. A Helmholtz electromagnet comprised of two coils having an inner diameter of 0.16 m and separated by a gap of 0.08 m produces a 95% uniform and timeinvariant magnetic field with an intensity up to a maximum value of 23 880 A/m. Power for the solenoid is provided by a dc power supply rated at 4.8 A and 27 V. Oil-free compressed air is injected through 36 evenly spaced 1-mm-diameter holes on a stainless steel ring sparger which is put under the distributor. An in situ optical fiber probe is used to measure the local gas holdup directly. The probe (see Figure 2) is made of plastic optical fiber 1 mm in diameter. One end

10.1021/ie000800i CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002

Figure 3. Typical response signals of bubble measurements. Table 2. Comparison of Local Gas Holdup Measurement Technique in Three-phase Magnetic Fluidized Beds

Figure 1. Experimental setup.

probe

material and size

remarks

Sonolikar et al.7

electric conducting probe resistivity probe

this work

optical fiber probe

0.4-mm Pt needle 0.2-mm Pt wire 1-mm plastic optical probe

reflected type

Table 1. Physical Properties of Iron Shots dp (m)

Fs (kg/m3)

Ulmf (m/s) (from experiment)

applied superficial liquid velocity (m/s)

0.00019 0.00039 0.00093

7700 7850 7970

0.0016 0.007 0.033

0.015 0.026 0.043

Hu and Wu2

of time occupied by the gas bubbles, N

gr )

ti ∑ i)1 T

(2)

where ti is the fraction of signal occupied by the gas bubbles and T is the total sampling time. A comparison of gas holdup measurement techniques is shown in Table 2; the optical fiber probe is first used here to measure the local gas holdup in three-phase magnetic fluidized beds. The local gas holdup is measured directly by the optical probe at various radial positions; the crosssectional average gas holdup is calculated by the following equation:

jg )

Figure 2. Operation principle of optical probes.

of the single-fiber probe is made in a conical shape as described by Abuaf et al.9 With Snell’s law10 applied at the probe-fluid interface, one obtains

n sin i ) n0 sin i0

(1)

For a plastic probe tip with an index of refraction of 1.52, the light rays will be reflected back if n < 1.08 and will be refracted if n > 1.08, where n is the index of refraction of the medium surrounding the probe tip. Figure 3 is a historical diagram of the probe response during the measurements. In the case shown here, the local gas holdup can be assessed by counting the fraction

1 πR2

∫0Rgr2πr dr

(3)

The relationship between the sampling points and convergence of local gas holdup under different magnetic field intensities is shown in Figure 4. In this situation the optical fiber probe is located at the center of the radial position in the column and the sampling frequency is set at 100 Hz. In conventional three-phase fluidized beds the local gas holdup data converge at 6000 points, while in three-phase magnetic fluidized beds the local gas holdup data converge at 30 000 points. The valve technique11 is used here to measure the overall gas holdup over the entire bed. This is accomplished by simultaneously shutting off the gas and liquid flows and measuring the volume of gas trapped

Ind. Eng. Chem. Res., Vol. 41, No. 7, 2002 1879

Figure 4. Relationship between local gas holdup and measuring points.

Figure 6. Flow regime map of three-phase magnetic fluidizied beds.

Figure 5. Flow regime map of three-phase magnetic fluidized beds.

Figure 7. Flow regime map of three-phase magnetic fluidizied beds.

in the bed. The overall gas holdup is calculated by the gas volume trapped divided by the total bed volume.

large particles was larger than that with small particles. Particle size had a major influence on bubble properties in three-phase fluidized beds, the enhanced bubble growth which occurred in beds of fine particles (dp < 3 mm), and the increased bubble splitting which took place in the beds of coarse particles (dp > 3 mm).12 Figure 8 shows the effect of particle diameter on the critical magnetic field intensity. The critical magnetic field intensity decreased with increasing particle diameter. The magnetic field had minor effects on the bubble behavior of three-phase fluidized beds with large particles compared to that with small particles. On the basis of the experimental data obtained from this work, the dependency of particle diameter on the critical magnetic field intensity Hc could be written as follows (all units in SI):

Results and Discussion 3.1. Flow Regimes. The regime map based on visual observations during the experiments of the type of bubble flow11 is shown in Figures 5-7 with different size particles. There are three flow regimes of threephase magnetic fluidized beds: dispersed bubble, coalesced bubble, and slug flow regimes. With the addition of magnetic field, the regime of dispersed bubble was expanded due to the enhanced bubble splitting. The magnetic field intensity that significantly affects the boundary between the dispersed bubble and the coalesced bubble regimes was defined as the critical magnetic field intensity here. The dispersed bubble regime in three-phase magnetic fluidized beds with

Hc ) 193dp-0.487

(4)

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Figure 8. Effect of particle diameter on the critical magnetic field intensity.

Figure 9. Radial gas holdup distribution in the coalesced bubble regime.

3.2. Radial Gas Holdup Distribution. Figures 9-11 show the radial gas holdup distribution of threephase magnetic fluidized beds in the coalesced bubble regime with differently sized particles. It is clear that the profile of gas holdup is far from flat; the gas holdup has a maximum value at the center of the column and a minimum one near the wall. The maximum value of gas holdup at the center of the column is higher for large particles than for small particles. With the introduction of magnetic field, the gas holdup at the center of the column increased with increasing magnetic field intensity, while the gas holdup near the wall decreased with increasing magnetic field intensity. The profile of the gas holdup became sharper under the effect of magnetic field in the coalesced bubble regime. It means the gas bubbles tended to flow upward from the center of the column under the effect of magnetic field. Figures 12-14 show the radial gas holdup distribution of three-phase magnetic fluidized beds in the



dispersed bubble regime with differently sized particles. With the introduction of magnetic field, the gas holdup at the center of the column decreased with increasing magnetic field intensity, while the gas holdup near the wall increased with increasing magnetic field intensity. The profile of gas holdup became more flat with the introduction of magnetic field in the dispersed bubble regime. The flow pattern of gas bubbles tended to be plug flow in the dispersed bubble regime. This trend was consistent with the finding of Hu and Wu2 that the local gas holdup is higher at the center of the column than near the wall for all magnetic field intensities. With weak magnetic fields when the bed is operated in the coalesced bubble regime, the radial distribution of gas holdup became sharper than those without applied magnetic field. When the fields were strong enough and the bed was in the dispersed bubble regime, the profile of gas holdup became more uniform.


Figure 12. Radial gas holdup distribution in the dispersed bubble regime.

Figure 14. Radial gas holdup distribution in the dispersed bubble regime. Table 3. Summary of Gas Holdup Distribution Curves

Hu and Wu2

low H (coalesced bubble regime)

high H (dispersed bubble regime)

gr became sharper

gr became more flat gr became more flat gr became more flat

Sonolikar et al.7 this work

gr became sharper

remarks no regime map no regime map

Figure 13. Radial gas holdup distribution in the dispersed bubble regime.

Hu and Wu2 reported that, with zero or weak magnetic fields, the radial distribution of gas holdup can be expressed by a parabolic equation. When the fields are quite strong, the profiles become more uniform. A comparison of the radial gas holdup distribution curve is shown in Table 3. With the interpretation of the change of flow regimes, the gas holdup distribution curve at the same flow regime was similar in trend to the increase of the magnetic field intensity. Figures15 and 16 show the comparison of radial gas holdup distribution with literature data. The gas holdup data taken from Hu and Wu2 show that when the bed is operated at the dispersed bubble regime, the radial gas holdup distribution becomes more uniform. A similar result was obtained in this work when the bed was operated at the dispersed bubble regime. The gas holdup data taken from Sonolikar et al.7 show a different trend compared to the results of Hu and Wu2 and this work.

Figure 15. Comparison of radial gas holdup distribution in the coalesced bubble regime.

The combination of flow regime and radial gas holdup distribution was first described in this work and the interpretation was quite reasonable. The gas holdup data taken from Lee and de Lasa13 and Yu and Kim14 without applied magnetic field show good agreement with those obtained from this work in Figures 12-14. 3.3. Overall Gas Holdup. There are two ways to obtain the overall gas holdup: (i) the valve technique

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Figure 16. Comparison of radial gas holdup distribution in the dispersed bubble regime.

Figure 17. Effect of magnetic field intensity on overall gas holdup (V: valve technique; O: optical fiber probe method).

and (ii) the optical fiber probe method. The overall gas holdup of a three-phase magnetic fluidized bed is shown in Figures 17-19. Those are the average of at least four replicates with an average standard deviation of 11%. The increase in average gas holdup was consistent with the common observation that bubbles break up with increasing magnetic field intensity. A similar result was reported by Hu and Wu;2 magnetic field resulted in smaller bubbles which rose more slowly than the larger ones, thus increasing the overall gas holdup. The overall gas holdup was relatively independent of magnetic field intensity at low magnetic field intensity where the bed is operated at the coalesced bubble regime. The breakup of bubbles in a coalesced bubble regime at low magnetic field intensity was not significant, so the overall gas holdup was not sensitive to magnetic field intensity.



Figures 17-19 also indicate that there was reasonable agreement between the gas holdup measurements using the optical fiber probe and the valve technique. The overall gas holdup obtained from the valve technique was about 22% higher than those obtained from the optical fiber probe method. The overall gas holdup data without applied magnetic field are compared to the correlation published by Begovich and Watson15 in Figure 20. Despite the differences in particle density the correlation by Begovich and Watson15 provide reasonable agreement with our data. The overall gas holdup data in three-phase magnetic fluidized beds are compared to the correlation published by Hu and Wu2 in Figure 21. The effect of magnetic field intensity on overall gas holdup was more pronounced for the smaller particles used in this study as compared to the larger particles (3.6 mm) used by


beds were measured. The effect of magnetic field intensity and particle diameter on the transfer of flow regime, radial gas holdup distribution, and overall gas holdup were also discussed. The critical magnetic field intensity decreased with increasing particle diameter. The radial gas holdup distribution became sharper with the introduction of magnetic field in the coalesced bubble regime, and it became more flat in the dispersed bubble regime. The overall gas holdup increased significantly when the critical magnetic field intensity was applied. The radial gas holdup distribution and overall gas holdup changed significantly with the change of flow regime. Acknowledgment The authors are grateful to the National Science Council, R.O.C., for financial support. Notation Figure 20. Comparison of overall gas holdup as a function of superficial gas velocity.

a ) parameter defined in eq 5 b ) parameter defined in eq 5 c ) parameter defined in eq 5 dp ) particle diameter (m) H ) magnetic field intensity (A/m) Hc ) critical magnetic field intensity (A/m) i ) angle between the probe tip and refraction light i0 ) angle between the probe tip and light ray n ) index of refraction of the surrounding medium n0 ) index of refraction of the fiber probe ng ) index of refraction of the gas phase nl ) index of refraction of the liquid phase N ) total sampling points r ) radial position (m) R ) radius of the column (m) ti ) fraction of signal occupied by the gas bubbles (s) T ) total sampling time (s) Ug ) superficial gas velocity (m/s) Ulmf ) minimum fluidization liquid velocity in a liquidsolid system (m/s) Greek Letters Fs ) true density of iron shots (kg/m3) g ) overall gas holdup gr ) local gas holdup jg ) cross-sectional average gas holdup

Figure 21. Comparison of effect of magnetic field intensity on overall gas holdup.

Hu and Wu.2 A correlation equation was found as follows (all units in SI).

g ) aUgb exp(cH)

(5)

The values of a, b, and c are shown here as

a ) 0.123dp-0.21

(6)

b ) 0.226dp-0.144

(7)

c ) (1.11 × 10-6)dp-0.23

(8)

The average relative error of the above correlation is 18.5%. 4. Conclusions The flow regimes, radial gas holdup distribution, and overall gas holdup in three-phase magnetic fluidized

Literature Cited (1) Filippov, M. V. The Effect of Magnetic Field on a Suspended Bed of Ferromagnetic Particles. Prik. Magnit. Lat. SSR. 1960, 12, 215. (2) Hu, T. T.; Wu, J. Y. Study on the Characteristics of a Biological Fluidized Bed in a Magnetic Field. Chem. Eng. Res. Des. 1987, 65, 238. (3) Ouyang, F.; Wu, Y.; Guo, C.; Kwauk, M. Fluidization under External Forces (I) Magnetized Fluidization. J. Chem. Ind. Eng. China (in Chinese), 1990, 5, 206. (4) Kwauk, M.; Ma, X.; Ouyang, F.; Wu, Y.; Weng, D.; Chang, L. Magneto-fluidized G/L/S Systems. Chem. Eng. Sci. 1992, 47, 3467. (5) Thompson, V. S.; Worden, R. M. Phase Holdup, Liquid Dispersion, and Gas-to-liquid Mass Transfer Measurements in a Three-phase Magnetofluidized Bed. Chem. Eng. Sci. 1997, 52, 279. (6) Hristov, J. Y.; Hadzisavvas, K. Gas-Liquid-Magnetic Solid Beds: A Classification of the Operating Modes and a Hydrodynamic Study in a Transverse Magnetic Field. In Proceedings of the 2nd European Conference on Fluidization; Olazar, M.; San Jose, M. J., Eds.; Bilbao, Spain, 1997; p 565. (7) Sonolikar, R. L.; Butley, G. V.; Saheb, S. P. Gas Holdup in a Three-Phase Magneto-Fluidized Bed. Chem. Eng. Sci. 1999, 54, 5273.

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(8) Schugerl, K. Three-phase-biofluidizationsApplication of Three-phase-fluidization in the BiotechnologysA Review. Chem. Eng. Sci. 1997, 52, 3661. (9) Abuaf, N.; Jones, O. C.; Zimmer, G. A. Optical Probe for Local Void Fraction and Interface Velocity Measurements. Rev. Sci. Instrum. 1978, 49, 1090. (10) Resnick, R.; Halliday, D. Fundamentals of Physics; John Wiley and Sons: New York, 1974; p 671. (11) Kim, S. D.; Baker, C. G.; Bergougnou, M. A. Holdup and Axial Mixing Characteristics of Two- and Three-Phase Fluidized Beds. Can. J. Chem. Eng. 1972, 50, 695. (12) Epstein, N. Three-Phase Fluidization: Some Knowledge Gaps. Can. J. Chem. Eng. 1981, 59, 649. (13) Lee, S. L. P.; de Lasa, H. I. Phase Holdups in Three-Phase Fluidized Beds. AIChE J. 1987, 33, 1359.

(14) Yu, Y. H.; Kim, S. D. Bubble Characteristics in the Radial Direction of Three-Phase Fluidized Bed. AIChE J. 1988, 34, 2069. (15) Beogovich, J. M.; Watson, J. S. Hydrodynamic Characteristics of Three-Phase Fluidized Beds. In Fluidization, Proceedings of the 2nd Engineering Foundation Conference; Davidson, J. F., Keairns, D. L., Eds.; Cambridge University Press: New York, 1978; p 190.

Received for review September 12, 2000 Revised manuscript received August 23, 2001 Accepted January 21, 2002 IE000800I

Flow Regimes and Radial Gas Holdup Distribution in Three-Phase

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