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Measurement of gas holdup profiles in a gas liquid cocurrent bubble column using electrical resistance tomography
Flow Measurement and Instrumentation Flow Measurement and Instrumentation 18 (2007) 191–196 www.elsevier.com/locate/flowmeasinst
Measurement of gas holdup profiles in a gas liquid cocurrent bubble column using electrical resistance tomography Haibo Jin a,∗ , Suohe Yang a , Mi Wang b , R.A. Williams b a Department of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China b Institute of Particle Science & Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, UK
Received 9 January 2007; received in revised form 18 July 2007; accepted 18 July 2007
Abstract Radial variation of the gas holdup and mean holdups were investigated in a 0.160 m i.d. bubble column using electrical resistance tomography with two axial locations (Plane 1 and Plane 2). In all the experiments tap water was used the liquid phase and air was the gas phase. Superficial gas velocity was varied from 0.02 to 0.25 m/s, and superficial liquid velocity varied from 0 to 0.011 m/s. The effect of liquid velocity on the mean holdups and radial gas holdup distribution was discussed. The experimental results showed the liquid velocity slightly influence the mean holdup and radial hold-ups distribution in the operating condition, and the liquid flow can improve the transition gas velocity for the homogeneous regime to heterogeneous regime. Meanwhile the mean gas holdups as a function of gas velocity were derived from using differential pressure method and electrical resistance tomography method. The agreement between results obtained by these two methods is generally very good in the homogeneous regime. But in the transition regime and heterogeneous regime, results with ERT are slightly larger than one with the differential pressure method. According to the experimental results, a correlation for the centreline holdup is obtained. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Gas liquid cocurrent bubble column; ERT; Radial gas holdup profile
1. Introduction Bubble columns are widely used as gas–liquid–solid contacting devices due to the ease of their operation and ability to enable good mixing and high heat transfer rates in a controlled manner [1]. During the past few years, special attention has been paid to local and global bubble flow characteristics as a key parameter that determines the overall reactor performance owing to the complexity of fluid mechanics in these systems [2,3]. It is essential to develop qualities as well as quantitative understanding of the axial and radial holdup distribution in bubble columns [4]. In recent years, applications of process tomography as a robust noninvasive tool for direct analysis of the characteristics of multiphase flows have increased in number [5]. Some tomographic techniques have been applied to measure
∗ Corresponding author. Tel.: +86 10 81292074; fax: +86 10 81292125.
E-mail addresses: [email protected] (H. Jin), [email protected] (M. Wang). c 2007 Elsevier Ltd. All rights reserved. 0955-5986/$ - see front matter doi:10.1016/j.flowmeasinst.2007.07.005
the axial and radial gas holdup distributions in bubble columns, e.g. gamma-ray computed tomography for measuring cross-sectional gas holdup distributions in bubble columns, gamma-densitometry tomography combined with electrical impedance tomography for measuring gas and solid holdup distributions in it should be gas–liquid–solid three-phase bubble columns, and multimodal computed tomography for measuring simultaneously cross-sectional gas and solid holdups in threephase bubble columns [6]. However, these data were captured in time-averaged mode, and all the tomographic techniques presented in these reports were too slow to capture the real-time image of highly fluctuating multiphase flows and eventually, some important information could be lost. Owing to its nonintrusive and fast data acquisition features, electrical resistance tomography might provide very useful tools for measuring and monitoring bubble column operation online. The speed of the technique to capture real-time data of highly fluctuating flow systems may be one of the most important concerns for the purpose of imaging multiphase flows in the process industries. In this regard, electrical resistance tomography is considered to
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Nomenclature AC D g H ∆H 1P ug ul
The cross-sectional area of the column, m2 Diameter of the column, m Gravitational constant, m/s2 Height of measurement location above the sparger, m The vertical distance between two pressure sensor points, m The differential pressure between two pressure sensor points, Pa, Gas superficial velocity, m/s Liquid superficial velocity, m/s
Greek Letters εg εg,centre εg σ1 σ2 σmc σ mc ρL
Gas holdup The centreline gas holdup Mean gas holdup Conductivity of the first phase Conductivity of the second phase Local mixture conductivity Mean mixture conductivity Liquid density, kg/m3
be the most powerful tool among other available tomographic techniques due to its high-speed capability, low construction cost, high safety and suitability for small or large vessels. Electrical resistance tomography (ERT) has been used in various industrial investigations for visualization of the concentration profiles and characteristics of the fluid dynamics in gas–liquid two-phase systems [7–9]. The aim of this study is to determine the gas holdup profiled in a gas–liquid cocurrent bubble column by the electrical resistance tomography. Based on the results of the overall gas holdup in the cocurrent gas liquid operation, the effect of liquid velocity on the gas liquid flow regime is discussed in this paper, and a correlation for the centreline holdup is obtained.
Fig. 1. Schematic diagram of bubble column.
2. Experimental
steel with a contact area of 8 mm (w) by 16 mm (h). The bottom ERT sensor ring was located 0.30 m above the air distributor. The details of the column configuration are shown in Fig. 1. The column was equipped with a perforated plate distributor of 0.003 m in thickness consisting of 55 holes in 1 mm diameter, which was arranged with an equilateral triangular pitch of 14 mm. A commercial electrical resistance tomography system was used for data collection. The data collection rate was 2 frames per second with an excitation signal frequency of 9.6 kHz. No conductivity–temperature compensation was applied since the water temperature was monitored and maintained.
2.1. Experimental setup
2.2. Experimental systems and operating conditions
The experimental setup was a transparent Perspex acrylic column with 2.5 m high and an inner diameter of 0.160 m operated in a gas–liquid cocurrent mode as shown in Fig. 1. A thermometer was used to provide a continuous monitoring of the water temperature. Two differential pressure sensors for measuring the differential pressure drop were placed along the column at 0.16 and 0.42 m above the distributor. The differential pressure sampling frequency was 100 Hz. Two neighboring pressure-sampling ports were connected to the high end and low end of each different-pressure transducer, respectively. Two rings of ERT sensors, each composed of 16 rectangular electrodes, were mounted in the inner wall of the column in a non-invasive fashion. The electrodes were made of stainless
All experiments were carried out at a temperature of approximately 22 ◦ C. Tap water was used as the liquid phase, and air, which was measured by two flowmeters, was introduced into the bubble column. The superficial gas velocity varied from 0.02 m/s to 0.25 m/s and the liquid velocity varied from 0 to 0.011 m/s. The water temperature was maintained under ambient conditions, at about 22 ◦ C. 2.3. Measurements and estimation of holdup profile The adjacent electrode pair strategy was adopted using 10 mA injection current at 9.6 kHz for parameter measurement. Data collection rates for plane 1 and plane 2 were 50 ms, with time for image reconstruction and other overheads, yielding
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
193
Fig. 2. The average gas holdups as a function of gas velocity at different liquid velocity.
approximately two images per second. Generally, 200–300 images per experiment were acquired. With conductivity data from ERT, the local gas volume fraction (εg ) can be determined by applying the Maxwell equation [10]: εg =
2σ1 + σ2 − 2σmc − σmc σ2 /σ1 σmc − σ2 /σ1 + 2(σ1 − σ2 )
(1)
where σ1 is the conductivity of the first phase, σ2 is the conductivity of the second phase, and σmc is the local value of mixture conductivity distribution. If the second phase is assumed to be nonconductive material, such as air in this study, the above equation can be simplified as follows: εg =
2σ1 − 2σmc 2σ1 + σmc
(2)
where AC is the cross-sectional area of the column. The average gas holdup (ε g ) can be obtained from Eq. (4), 2σ1 − 2σ mc . 2σ1 + σ mc
(4)
The axial holdup can be measured from the two differential pressure signals. On the assumption that values of liquid acceleration term and the wall friction term were normally small and could be neglected, the mean gas holdup (ε g ) was calculated by equation [11]. 1P εg = ρl g∆H
3.1. Effect of liquid velocity on the overall holdups Fig. 2 shows the relationship between average gas holdups and gas velocity using the electrical resistance tomography method with water/air in gas liquid cocurrent operation. The agreement between results obtained in different operating condition is generally very good. But there is a little error in the heterogeneous regime, namely the overall holdups slightly decrease with an increase of liquid velocity, which is similar to information reported in the literature [1]. The reason can be interpreted that the rising velocity of gas bubble increases with increasing the liquid velocity in cocurrent operation, and results in reducing the resistance time of gas bubble in the column and the overall holdup. 3.2. Effect of liquid velocity on flow regime
the local mixture conductivity (σmc ) is determined from the pixel conductivity of ERT image. Therefore, the average value of mixture conductivity over the cross-sectional area of the column (AC ) is defined by R A σmc dAC σ mc = C (3) AC
εg =
3. Results and discussion
(5)
where 1P is the differential pressure between two pressure sensor points, ∆H the vertical distance between two pressure sensor points, and ρl liquid density.
The homogeneous regime is characterized by the linearity of the gas holdup versus gas velocity at lower gas velocity, and the fully developed heterogeneous regime is observed at higher gas velocity from Fig. 2. But the transition regime cannot be obtained directly from the relationship between the gas holdup and gas velocity, such as Fig. 2. Numerous studies and methods have given the identification of the flow regime transition [12]. The classical method like the drift-flux model is a good technique to analyse to the flow regime. In this paper, the regimes and transition regimes were determined using the classical methods with data of electrical resistance tomography. The drift flux can be expressed as follows [13]: ug ul j = ε g (1 − ε g ) − . (6) εg εl The relationship between drift flux ( j) and holdups (ε g ) is shown in Fig. 3. From Fig. 3, we can see that the liquid velocity slightly influences the gas–liquid flow regime, such as the transition gas velocity point. The transition gas velocity at a certain liquid velocity condition is larger than one at batch operation. This can be interpreted as the liquid velocity affecting the gas liquid flow stability. Furthermore, when liquid
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Fig. 3. The effect of liquid velocity on flow regime.
Fig. 4. Comparison of average gas holdups using two measurement methods.
velocity is larger than 0.10 m s−1 in Reference [1], there is a significant effect on gas holdup in bubble columns with directional liquid flow. 3.3. Comparison of average gas holdups using two measurement methods The average column gas holdups as a function of gas velocity obtained from differential pressure method and electrical resistance tomography method with water/air in the gas liquid cocurrent operation are shown in Fig. 4. The agreement between results obtained by two methods is generally very good in homogeneous regime. But in transition regime and heterogeneous regime, results with ERT are slightly larger than the ones using differential pressure method. Moreover, the errors increase with an increase of gas velocity. The reason for this discrepancy is due to the effect of gas liquid churn-turbulent flow on the conductivity of mixture, and furthermore affects on the values of local holdup. However, the local holdup obtained by the differential pressure method is unaffected by gas liquid flow. The electrical resistance tomography has verified the validity and effectiveness of the measurement technique for bubbly flow regime. 3.4. The radial holdup profiles Radial holdup profiles for different liquid velocity at two axial locations as mentioned above are shown from Fig. 5. It
is seen from the Fig. 5 that the radial gas holdup increases with an increase of gas velocity. Meanwhile, the radial gas holdup profiles are steeper at the central region of the column with an increase of gas velocity, namely the higher gas holdups at the centre of the column and lower gas holdups at the wall of the column. Meanwhile the gas liquid belongs to the homogeneous regime at u g = 0.015 and u g = 0.041 m/s. In this range, there is a narrow bubble-size distribution, which results in a radially uniform gas holdups. With increasing gas velocity, bubble coalescence and breakup phenomena cannot be neglected, which a wide bubble-size distribution and notable radial gas holdup profiles appears. The bubble coalescence and breakup phenomena cannot be neglected. But the liquid velocity at such lower range influences slightly the gas holdup distribution from Fig. 5. According to these results, it is notified that the lower liquid velocity cannot affect the radial gas holdup distribution. From Fig. 5, a correlation is developed for the centreline holdup with respect to gas velocity (u g ) and height to diameter ratio (H/D) as shown Eq. (7) and Fig. 6 shows comparison calculated εg,centre values with experimental εg,centre values. The centreline holdup increases with an increase of gas velocity, however, it slightly decreases with an increase in distance from the sparger. εg,centre =
1.042u 0.523 g
H D
−0.096
.
(7)
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
Fig. 5. The radial gas holdup profiles at different liquid velocity.
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H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
method. So the measurement technique of electrical resistance tomography has verified the validity and effectiveness for bubbly flow regime in this operating condition. References
Fig. 6. Comparison calculated εg,centre values with experimental εg,centre values.
4. Conclusions Electrical resistance tomography measurements are used to study the overall and radial holdup in a gas liquid cocurrent bubble column. The effect of liquid velocity on the mean holdups and radial gas holdups distribution was discussed. The experimental results showed that the liquid velocity slightly influence the mean holdup and radial holdups distribution in the operating condition, and the liquid flow improves the transition gas velocity for the homogeneous regime to the heterogeneous regime. Meanwhile, the average column gas holdups as a function of gas velocity obtained from differential pressure method and electrical resistance tomography method. The agreement between results derived by these two methods is generally very good in the homogeneous regime. But in the transition regime and heterogeneous regime, results with ERT are slightly larger than the ones with the differential pressure
[1] Deckwer WD. Bubble column reactors. New York: John Wiley and Sons; 1991. [2] Dukovic MP. Opaque multiphase reactors: experimentation, modeling and troublesshooting. Oil Gas Sci Technol 2000;55:135–58. [3] Fan LS. Gas–liquid–solid fluidization engineering. Boston: Buttorworth; 1989. [4] Jin H, Wang M, Williams RA. The effect of sparger geometry on gas bubble flow behaviors using electrical resistance tomography. Chinese J Chem Eng 2006;14:127–31. [5] Williams RA, Beck MS. Process tomography priciples, techniques and applications. Oxford, UK: Butterworth-Heinemann; 1995. [6] Warsito W, Fan LS. Measurement of real-time flow structures in gas–liquid and gas–liquid–solid flow system using electrical capacitance tomography (ECT). Chem Eng Sci 2001;56:6455–62. [7] Wang M, Mann R, Dickin FJ. Electrical resistance tomography sensing system for industrial applications. Chem Eng Comm 1999;175:49–70. [8] Fransolet E, Crine M, Marchot P, Toye D. Analysis of gas holdup in bubble columns with non-Newtonian fluid using electrical resistance tomography and dynamic gas disengagement technique. Chem Eng Sci 2005;60:6118–23. [9] Fransolet E, Crine M, L’Homme G, Toye D. Analysis of electrical resistance tomography measurements obtained on a bubble column. Meas Sci Technol 2001;21:1055–60. [10] Maxwell JC. A treatise on electricity and magnetism. Oxford: Clarendon Press; 1873. [11] Jin H, Yang S, Zhang T, Tong Z. Bubble behaviour of a large-scale bubble column with elevated pressure. Chem Eng Technol 2004;27:1007–13. [12] Vial C, Pncin S, Wild G, Midoux N. A simple method for regime identification and flow characterisation in bubble column and airlift reactors. Chem Eng Process 2001;40:135–51. [13] Darton RC, Harrison D. Gas and liquid holdup in three-phase fluidization. Chem Eng Sci 1975;30:581–6.
Measurement of gas holdup profiles in a gas liquid cocurrent bubble column using electrical resistance tomography Haibo Jin a,∗ , Suohe Yang a , Mi Wang b , R.A. Williams b a Department of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China b Institute of Particle Science & Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, UK
Received 9 January 2007; received in revised form 18 July 2007; accepted 18 July 2007
Abstract Radial variation of the gas holdup and mean holdups were investigated in a 0.160 m i.d. bubble column using electrical resistance tomography with two axial locations (Plane 1 and Plane 2). In all the experiments tap water was used the liquid phase and air was the gas phase. Superficial gas velocity was varied from 0.02 to 0.25 m/s, and superficial liquid velocity varied from 0 to 0.011 m/s. The effect of liquid velocity on the mean holdups and radial gas holdup distribution was discussed. The experimental results showed the liquid velocity slightly influence the mean holdup and radial hold-ups distribution in the operating condition, and the liquid flow can improve the transition gas velocity for the homogeneous regime to heterogeneous regime. Meanwhile the mean gas holdups as a function of gas velocity were derived from using differential pressure method and electrical resistance tomography method. The agreement between results obtained by these two methods is generally very good in the homogeneous regime. But in the transition regime and heterogeneous regime, results with ERT are slightly larger than one with the differential pressure method. According to the experimental results, a correlation for the centreline holdup is obtained. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Gas liquid cocurrent bubble column; ERT; Radial gas holdup profile
1. Introduction Bubble columns are widely used as gas–liquid–solid contacting devices due to the ease of their operation and ability to enable good mixing and high heat transfer rates in a controlled manner [1]. During the past few years, special attention has been paid to local and global bubble flow characteristics as a key parameter that determines the overall reactor performance owing to the complexity of fluid mechanics in these systems [2,3]. It is essential to develop qualities as well as quantitative understanding of the axial and radial holdup distribution in bubble columns [4]. In recent years, applications of process tomography as a robust noninvasive tool for direct analysis of the characteristics of multiphase flows have increased in number [5]. Some tomographic techniques have been applied to measure
∗ Corresponding author. Tel.: +86 10 81292074; fax: +86 10 81292125.
E-mail addresses: [email protected] (H. Jin), [email protected] (M. Wang). c 2007 Elsevier Ltd. All rights reserved. 0955-5986/$ - see front matter doi:10.1016/j.flowmeasinst.2007.07.005
the axial and radial gas holdup distributions in bubble columns, e.g. gamma-ray computed tomography for measuring cross-sectional gas holdup distributions in bubble columns, gamma-densitometry tomography combined with electrical impedance tomography for measuring gas and solid holdup distributions in it should be gas–liquid–solid three-phase bubble columns, and multimodal computed tomography for measuring simultaneously cross-sectional gas and solid holdups in threephase bubble columns [6]. However, these data were captured in time-averaged mode, and all the tomographic techniques presented in these reports were too slow to capture the real-time image of highly fluctuating multiphase flows and eventually, some important information could be lost. Owing to its nonintrusive and fast data acquisition features, electrical resistance tomography might provide very useful tools for measuring and monitoring bubble column operation online. The speed of the technique to capture real-time data of highly fluctuating flow systems may be one of the most important concerns for the purpose of imaging multiphase flows in the process industries. In this regard, electrical resistance tomography is considered to
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H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
Nomenclature AC D g H ∆H 1P ug ul
The cross-sectional area of the column, m2 Diameter of the column, m Gravitational constant, m/s2 Height of measurement location above the sparger, m The vertical distance between two pressure sensor points, m The differential pressure between two pressure sensor points, Pa, Gas superficial velocity, m/s Liquid superficial velocity, m/s
Greek Letters εg εg,centre εg σ1 σ2 σmc σ mc ρL
Gas holdup The centreline gas holdup Mean gas holdup Conductivity of the first phase Conductivity of the second phase Local mixture conductivity Mean mixture conductivity Liquid density, kg/m3
be the most powerful tool among other available tomographic techniques due to its high-speed capability, low construction cost, high safety and suitability for small or large vessels. Electrical resistance tomography (ERT) has been used in various industrial investigations for visualization of the concentration profiles and characteristics of the fluid dynamics in gas–liquid two-phase systems [7–9]. The aim of this study is to determine the gas holdup profiled in a gas–liquid cocurrent bubble column by the electrical resistance tomography. Based on the results of the overall gas holdup in the cocurrent gas liquid operation, the effect of liquid velocity on the gas liquid flow regime is discussed in this paper, and a correlation for the centreline holdup is obtained.
Fig. 1. Schematic diagram of bubble column.
2. Experimental
steel with a contact area of 8 mm (w) by 16 mm (h). The bottom ERT sensor ring was located 0.30 m above the air distributor. The details of the column configuration are shown in Fig. 1. The column was equipped with a perforated plate distributor of 0.003 m in thickness consisting of 55 holes in 1 mm diameter, which was arranged with an equilateral triangular pitch of 14 mm. A commercial electrical resistance tomography system was used for data collection. The data collection rate was 2 frames per second with an excitation signal frequency of 9.6 kHz. No conductivity–temperature compensation was applied since the water temperature was monitored and maintained.
2.1. Experimental setup
2.2. Experimental systems and operating conditions
The experimental setup was a transparent Perspex acrylic column with 2.5 m high and an inner diameter of 0.160 m operated in a gas–liquid cocurrent mode as shown in Fig. 1. A thermometer was used to provide a continuous monitoring of the water temperature. Two differential pressure sensors for measuring the differential pressure drop were placed along the column at 0.16 and 0.42 m above the distributor. The differential pressure sampling frequency was 100 Hz. Two neighboring pressure-sampling ports were connected to the high end and low end of each different-pressure transducer, respectively. Two rings of ERT sensors, each composed of 16 rectangular electrodes, were mounted in the inner wall of the column in a non-invasive fashion. The electrodes were made of stainless
All experiments were carried out at a temperature of approximately 22 ◦ C. Tap water was used as the liquid phase, and air, which was measured by two flowmeters, was introduced into the bubble column. The superficial gas velocity varied from 0.02 m/s to 0.25 m/s and the liquid velocity varied from 0 to 0.011 m/s. The water temperature was maintained under ambient conditions, at about 22 ◦ C. 2.3. Measurements and estimation of holdup profile The adjacent electrode pair strategy was adopted using 10 mA injection current at 9.6 kHz for parameter measurement. Data collection rates for plane 1 and plane 2 were 50 ms, with time for image reconstruction and other overheads, yielding
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
193
Fig. 2. The average gas holdups as a function of gas velocity at different liquid velocity.
approximately two images per second. Generally, 200–300 images per experiment were acquired. With conductivity data from ERT, the local gas volume fraction (εg ) can be determined by applying the Maxwell equation [10]: εg =
2σ1 + σ2 − 2σmc − σmc σ2 /σ1 σmc − σ2 /σ1 + 2(σ1 − σ2 )
(1)
where σ1 is the conductivity of the first phase, σ2 is the conductivity of the second phase, and σmc is the local value of mixture conductivity distribution. If the second phase is assumed to be nonconductive material, such as air in this study, the above equation can be simplified as follows: εg =
2σ1 − 2σmc 2σ1 + σmc
(2)
where AC is the cross-sectional area of the column. The average gas holdup (ε g ) can be obtained from Eq. (4), 2σ1 − 2σ mc . 2σ1 + σ mc
(4)
The axial holdup can be measured from the two differential pressure signals. On the assumption that values of liquid acceleration term and the wall friction term were normally small and could be neglected, the mean gas holdup (ε g ) was calculated by equation [11]. 1P εg = ρl g∆H
3.1. Effect of liquid velocity on the overall holdups Fig. 2 shows the relationship between average gas holdups and gas velocity using the electrical resistance tomography method with water/air in gas liquid cocurrent operation. The agreement between results obtained in different operating condition is generally very good. But there is a little error in the heterogeneous regime, namely the overall holdups slightly decrease with an increase of liquid velocity, which is similar to information reported in the literature [1]. The reason can be interpreted that the rising velocity of gas bubble increases with increasing the liquid velocity in cocurrent operation, and results in reducing the resistance time of gas bubble in the column and the overall holdup. 3.2. Effect of liquid velocity on flow regime
the local mixture conductivity (σmc ) is determined from the pixel conductivity of ERT image. Therefore, the average value of mixture conductivity over the cross-sectional area of the column (AC ) is defined by R A σmc dAC σ mc = C (3) AC
εg =
3. Results and discussion
(5)
where 1P is the differential pressure between two pressure sensor points, ∆H the vertical distance between two pressure sensor points, and ρl liquid density.
The homogeneous regime is characterized by the linearity of the gas holdup versus gas velocity at lower gas velocity, and the fully developed heterogeneous regime is observed at higher gas velocity from Fig. 2. But the transition regime cannot be obtained directly from the relationship between the gas holdup and gas velocity, such as Fig. 2. Numerous studies and methods have given the identification of the flow regime transition [12]. The classical method like the drift-flux model is a good technique to analyse to the flow regime. In this paper, the regimes and transition regimes were determined using the classical methods with data of electrical resistance tomography. The drift flux can be expressed as follows [13]: ug ul j = ε g (1 − ε g ) − . (6) εg εl The relationship between drift flux ( j) and holdups (ε g ) is shown in Fig. 3. From Fig. 3, we can see that the liquid velocity slightly influences the gas–liquid flow regime, such as the transition gas velocity point. The transition gas velocity at a certain liquid velocity condition is larger than one at batch operation. This can be interpreted as the liquid velocity affecting the gas liquid flow stability. Furthermore, when liquid
194
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
Fig. 3. The effect of liquid velocity on flow regime.
Fig. 4. Comparison of average gas holdups using two measurement methods.
velocity is larger than 0.10 m s−1 in Reference [1], there is a significant effect on gas holdup in bubble columns with directional liquid flow. 3.3. Comparison of average gas holdups using two measurement methods The average column gas holdups as a function of gas velocity obtained from differential pressure method and electrical resistance tomography method with water/air in the gas liquid cocurrent operation are shown in Fig. 4. The agreement between results obtained by two methods is generally very good in homogeneous regime. But in transition regime and heterogeneous regime, results with ERT are slightly larger than the ones using differential pressure method. Moreover, the errors increase with an increase of gas velocity. The reason for this discrepancy is due to the effect of gas liquid churn-turbulent flow on the conductivity of mixture, and furthermore affects on the values of local holdup. However, the local holdup obtained by the differential pressure method is unaffected by gas liquid flow. The electrical resistance tomography has verified the validity and effectiveness of the measurement technique for bubbly flow regime. 3.4. The radial holdup profiles Radial holdup profiles for different liquid velocity at two axial locations as mentioned above are shown from Fig. 5. It
is seen from the Fig. 5 that the radial gas holdup increases with an increase of gas velocity. Meanwhile, the radial gas holdup profiles are steeper at the central region of the column with an increase of gas velocity, namely the higher gas holdups at the centre of the column and lower gas holdups at the wall of the column. Meanwhile the gas liquid belongs to the homogeneous regime at u g = 0.015 and u g = 0.041 m/s. In this range, there is a narrow bubble-size distribution, which results in a radially uniform gas holdups. With increasing gas velocity, bubble coalescence and breakup phenomena cannot be neglected, which a wide bubble-size distribution and notable radial gas holdup profiles appears. The bubble coalescence and breakup phenomena cannot be neglected. But the liquid velocity at such lower range influences slightly the gas holdup distribution from Fig. 5. According to these results, it is notified that the lower liquid velocity cannot affect the radial gas holdup distribution. From Fig. 5, a correlation is developed for the centreline holdup with respect to gas velocity (u g ) and height to diameter ratio (H/D) as shown Eq. (7) and Fig. 6 shows comparison calculated εg,centre values with experimental εg,centre values. The centreline holdup increases with an increase of gas velocity, however, it slightly decreases with an increase in distance from the sparger. εg,centre =
1.042u 0.523 g
H D
−0.096
.
(7)
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
Fig. 5. The radial gas holdup profiles at different liquid velocity.
195
196
H. Jin et al. / Flow Measurement and Instrumentation 18 (2007) 191–196
method. So the measurement technique of electrical resistance tomography has verified the validity and effectiveness for bubbly flow regime in this operating condition. References
Fig. 6. Comparison calculated εg,centre values with experimental εg,centre values.
4. Conclusions Electrical resistance tomography measurements are used to study the overall and radial holdup in a gas liquid cocurrent bubble column. The effect of liquid velocity on the mean holdups and radial gas holdups distribution was discussed. The experimental results showed that the liquid velocity slightly influence the mean holdup and radial holdups distribution in the operating condition, and the liquid flow improves the transition gas velocity for the homogeneous regime to the heterogeneous regime. Meanwhile, the average column gas holdups as a function of gas velocity obtained from differential pressure method and electrical resistance tomography method. The agreement between results derived by these two methods is generally very good in the homogeneous regime. But in the transition regime and heterogeneous regime, results with ERT are slightly larger than the ones with the differential pressure
[1] Deckwer WD. Bubble column reactors. New York: John Wiley and Sons; 1991. [2] Dukovic MP. Opaque multiphase reactors: experimentation, modeling and troublesshooting. Oil Gas Sci Technol 2000;55:135–58. [3] Fan LS. Gas–liquid–solid fluidization engineering. Boston: Buttorworth; 1989. [4] Jin H, Wang M, Williams RA. The effect of sparger geometry on gas bubble flow behaviors using electrical resistance tomography. Chinese J Chem Eng 2006;14:127–31. [5] Williams RA, Beck MS. Process tomography priciples, techniques and applications. Oxford, UK: Butterworth-Heinemann; 1995. [6] Warsito W, Fan LS. Measurement of real-time flow structures in gas–liquid and gas–liquid–solid flow system using electrical capacitance tomography (ECT). Chem Eng Sci 2001;56:6455–62. [7] Wang M, Mann R, Dickin FJ. Electrical resistance tomography sensing system for industrial applications. Chem Eng Comm 1999;175:49–70. [8] Fransolet E, Crine M, Marchot P, Toye D. Analysis of gas holdup in bubble columns with non-Newtonian fluid using electrical resistance tomography and dynamic gas disengagement technique. Chem Eng Sci 2005;60:6118–23. [9] Fransolet E, Crine M, L’Homme G, Toye D. Analysis of electrical resistance tomography measurements obtained on a bubble column. Meas Sci Technol 2001;21:1055–60. [10] Maxwell JC. A treatise on electricity and magnetism. Oxford: Clarendon Press; 1873. [11] Jin H, Yang S, Zhang T, Tong Z. Bubble behaviour of a large-scale bubble column with elevated pressure. Chem Eng Technol 2004;27:1007–13. [12] Vial C, Pncin S, Wild G, Midoux N. A simple method for regime identification and flow characterisation in bubble column and airlift reactors. Chem Eng Process 2001;40:135–51. [13] Darton RC, Harrison D. Gas and liquid holdup in three-phase fluidization. Chem Eng Sci 1975;30:581–6.