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Bubble characteristics in shallow bubble column reactors
chemical engineering research and design 8 8 ( 2 0 1 0 ) 197–203
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Bubble characteristics in shallow bubble column reactors Raymond Lau ∗ , Rujuan Mo, Wei Shan Beverly Sim School of Chemical and Biomedical Engineering (SCBE), Nanyang Technological University (NTU), 62 Nanyang Drive, Singapore, Singapore
a b s t r a c t The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Bubble column; Bubble size distribution; Shallow bed; Gas holdup
1.
Introduction
Bubble column reactor has a wide range of industrial applications due to its simple design and excellent heat and mass transfer properties. Traditionally, the design and scale-up of bubble column reactors are focused on high height to diameter ratio (H/D) to allow system hydrodynamics to reach fully developed state (Wilkinson et al., 1992; Kumar et al., 1997; Thorat et al., 1998; Parasu Veera and Joshi, 1999). However, the high H/D minimized the effect of gas distributors where small bubbles are generated. It would be logical to consider bubble column reactor operations with low H/D. A low H/D operation not only utilizes the small bubbles generated from the gas distributor, but also provides lower bed pressure drops. Potential application of shallow bubble column includes absorption, catalytic slurry reaction, bioreactions, wastewater treatment, etc. (Voigt and Schugerl, 1979; Gopal and Sharma, 1983; Haque et al., 1986; Bahadori and Rahimi, 2007). In fact, success application of shallows bed is well established in gas–solid operations (Harrison and Grace, 1971; Chakraborty and Howard, 1980; Broughton, 1983; Yan et al., 1984; Virr and Williams, 1985; Yang et al., 1987). The study on shallow bubble column reactors is quite limited. Experimental and simulation results are mostly focused on the effect of gas distributor design on the performance of shallow bubble column (Haque et al., 1986; Thorat et al.,
∗
1998; Ranade and Tayalia, 2001). The optimum gas distributor design is found to be dependent on the system properties but no design procedure is derived due to the complex flow pattern in the distributor region (Haque et al., 1986). On the other hand, there are mixed reports on the effect of static liquid height on the gas holdup of the system. Kumar et al. (1997) found no effect on the static liquid height on the overall gas holdup while the local gas holdup increases with the axial distance from the distributor. A decrease in gas holdup is also observed with an increase in static liquid height by other researchers and it is justified by the three-region concept (Wilkinson et al., 1992; Yamashita, 1998; Ruizika et al., 2001). Therefore, it would be beneficial to study the hydrodynamics in shallow bubble column reactor systematically for possible implementation in different industrial processes. In this study, the bubble characteristics such as bubble size distribution, bubble rise velocity distribution and gas holdup will be investigated in a bubble/slurry bubble columns with low height to diameter ratios. Effects of static liquid height, superficial gas velocity, solids concentration, and column diameter on the bubble characteristics are examined.
2.
Experimental
A schematic diagram of the experimental setup is shown in Fig. 1. The experiments are conducted in a cylindrical acrylic
Corresponding author. Tel.: +65 63168830; fax: +65 67947553. E-mail address: [email protected] (R. Lau). Received 22 October 2008; Received in revised form 20 May 2009; Accepted 7 July 2009 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.07.008
198
chemical engineering research and design 8 8 ( 2 0 1 0 ) 197–203
Fig. 1 – Schematic diagram of the experimental setup. column. The column has a height of 150 cm and an internal diameter of 14 cm. Pressure taps are installed every 10 cm starting from 10 cm above the distributor plate. A perforated plate with orifice diameter of 3 mm and triangular pitch distance of 0.9 cm is used as the gas distributor. The open area ratio of the perforated plate is 9.7%. Screen with an opening size of 100 m are covered on top of the perforated plate to prevent solid particles from dropping into the plenum region. Tap water and compressed air are used as the liquid and gas phases in the experiments, respectively. The particles used are soda-lime glass spheres having 425–600 m mean diameter and density of 2480 kg/m3 . The superficial gas velocity is varied from 3.25 to 10.8 cm/s. A solid concentration of 5% by weight is also used to study the effect of solids on shallow bed operation. A high speed camera is used to take bubble images at various height of the column at a frame rate of 600 Hz. The curvature of the column wall is found to have negligible image distortion when tested with square grids. A sample bubble image can also be seen in Fig. 1. The air bubbles rising in water generally has an ellipsoidal shape. As a result, the maximum dimensions of the ellipsoidal bubbles are used in the determination of the average bubble size. Two markers 10 mm apart from each other are placed on the surface of the bubble column. The size of bubbles observed on the digital images will be calibrated against the marker distance. The bubble travel time is determined by multiplying the number of frames needed for the bubble to rise from one marker to another with the frame rate. The rise velocity of the bubble is then determined by dividing the marker distance with the bubble travel time. Since there are bubble overlapping at high gas velocities, the bubble size and velocity determination are processed manually with i-SPEED software (Olympus). The focus of the camera is set at 5 cm behind the column wall. Only in-focus bubbles are considered in the bubble size and rise velocity determination to prevent the inclusion of bubbles at the wall. A minimum population size of 200 is considered for the determination of both size and rise velocity distributions. For bubble/slurry bubble column operation, the size distribution is found to follow lognormal distribution which can be represented by (Matsuura
and Fan, 1984; Yang et al., 2007):
(ln (b) − ˛) 1 Pdf (b) = √ exp − 2ˇ2 2bˇ
2
(1)
where ˛ and ˇ are the parameters to represent the mean and variance in the log-normal distribution. The parameters, ˛ and ˇ can be determined by applying least square method to compare the experimental results and theoretical prediction by the log-normal distribution. The mean, and variance, 2 of the log-normal distribution can be calculated by:
˛+
= exp
ˇ2 2
2 = exp(2˛ + ˇ2 ) exp(˛2 − 1)
(2)
(3)
A differential pressure transducer is installed at 10–20, 20–30, 30–40, 40–50, and 50–60 cm above the gas distributor to measure the axial gas holdup variation. A sampling frequency of 100 Hz is used for the collection of the pressure transducer signals. The pressure transducers measure the dynamic pressure gradient between two measurement locations. According to Epstein (1981), the relationship between the dynamic pressure drop and the individual phase holdups for a gas–liquid system is:
dP
−
dz
d
= (εg g + εl l − l )g
(4)
Since εg + εl = 1
(5)
the gas holdup can then be calculated by solving Eqs. (4) and (5): εg =
(dP/dz)d g(l − g )
(6)
chemical engineering research and design 8 8 ( 2 0 1 0 ) 197–203
199
Fig. 3 – Sectional gas holdup above the gas distributor. Fig. 2 – Comparison of bubble size distribution measured at different H/D ratios at Ug = 7.6 cm/s. The overall bed holdup is determined by using the bed expansion method.
3.
Results and discussion
3.1.
Effect of axial distance from the distributor
There is no clear definition on the H/D ratio for a shallow liquid height. It is commonly accepted that when the H/D ratio is <4, it can be considered as shallow bed. In order to examine the performance of a shallow bubble column, it would be beneficial to look at the effect of axial distance near the distributor on the hydrodynamic properties. Fig. 2 shows the bubble size distribution measured at various axial distance (represented by H/D ratios) with a constant static liquid height of 100 cm (Hs/D = 7.2) and a superficial gas velocity of 7.6 cm/s. At H/D ratio of 6, the average bubble size is found to be around 9 mm. It is comparable to the bubble size results reported in the literature (Jiang et al., 1992; Kulkarni, 2007; Xue et al., 2008). As shown in the figure, as the H/D ratio is decreased from 6 to 2, there is negligible change in the bubble size and bubble size distribution. However, as the H/D ratio is reduced further from 2 to 0.5, a reduction in the average bubble size is observed. It is also found that the average bubble size is larger and the distribution is wider at H/D = 0.5 than that at H/D = 1.2. The perforated plate distributor used in this study generates relatively large bubbles. As the bubbles detached from the gas distributor, the bubbles start to accelerate. During this acceleration stage, the bubbles are subjected to relatively low shear. However, as height increases, the bubbles eventually reach their terminal rise velocities. The high rise velocities of large bubbles create vigorous gas–liquid interactions and causes bubble breakage, which results in a smaller average bubble size at H/D = 1.2. As the bubbles continue to rise in the column, the large amount of bubbles start to interact with each other and causes simultaneous bubble coalescence and breakup. At H/D higher than 2, the rising bubbles eventually reach an equilibrium state of bubble coalescence and breakup, and thus a relatively constant bubble size distribution. Based on the experimental results in this study, the entrance effect of bubbles is effective up to about an H/D of 2. It is in agreement with the Kulkarni (2007) that low H/D yields smaller bubble sizes and as H/D is >3, there is practically no change in the bubble size distribution with H/D.
The average sectional gas holdup is also determined every 10 cm from the distributor at Hs/H of 7.2 and superficial gas velocity of 7.6 cm/s. As shown in Fig. 3, it can be seen that the gas holdup increases from H/D of 1 to H/D of 2. Beyond an H/D of 2, the gas holdup decreases with an increase in H/D. The gas holdup result is consistent with the bubble size measurement results and shows a good representation that the gas bubble emerging from the gas distributor is relatively large and subjected to a larger shear for bubble breakage near the gas distributor. It is to note that this is likely to be caused by the design of the distributor. If porous distributor is being used, the initial bubble size generated is relative small, and this initial bubble breakage probably will not be present.
3.2.
Effect of static liquid height
Other than the entrance effect at the gas distributor, it is important to look at the exit effect at the bed surface for the operation of a shallow bubble column reactor. Since the static liquid height is <1 m in all experimental conditions, the hydrostatic head would only contribute to a pressure of <0.1 atm. Therefore, the effect of hydrostatic head on the bubble size and rise velocity can be considered negligible. As shown in Fig. 4, a comparison of the bubble size distribution measured at Hs/D = 2 for a static liquid height of Hs/D = 2, 4 and 7.2 indicates that the small bubble size is a combined effect of both the entrance and exit effect. The bubbles with Hs/D of 2 show smaller average bubble size compared with Hs/D of 4 and 7.2. It indicates that the presence of liquid surface enhances bubble breakup. When bubbles rise to the bed surface, the high disturbance generated by the bubble breakage at the bed surface
Fig. 4 – Compare bubble size distribution measured at H/D = 2 and Ug = 7.6 cm/s with various static liquid height.
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Hs/D ratio is low. At low bubble concentrations, each bubble can be considered to be rising in an isolated manner. The rise velocity would depend on the bubble size only. However, at high bubble concentrations, usually observed at high superficial gas velocities, the bubble–bubble interaction would cause the overall bubble rise velocity to be higher than the same bubble size rising in an isolated manner. Quite often, smaller bubbles are observed to rise at a velocity much higher than other bubbles that has the same size. This is maybe due to the entrainment of small bubbles by the wakes generated by the rise of adjacent large bubbles. Therefore, the bubble rise velocity normally has a wider distribution than the bubble size under the same conditions.
3.3.
Effect of superficial gas velocities
Fig. 5 – Overall gas holdup for different H/D. causes the preceding bubbles to break, resulting in smaller bubble size. In general, the bed height for shallow bubble column operation should be low enough that the distributor region and the bed surface region should cover a majority entire bed in order to maximize the relative effect of the end regions. However, for extremely low Hs/D of 0.5, a slight foaming problem is observed near the liquid surface which would deteriorate the gas–liquid contact. For the column diameter of 14 cm used in this study, an Hs/D ratio of 2–4 is favorable. The gas holdup of the bed is also determined for various static Hs/D ratios and the result is shown in Fig. 5. It can be seen that there is a significant decrease in gas holdup as the Hs/D is increased from 0.5 to 2 at a constant superficial gas velocity of 7.6 cm/s. The effect of end regions is diminishing as Hs/D is increased beyond 2. Comparison of the bubble size results with different Hs/D shows that even though both the entrance and exit effects contribute to the small bubble size generated, the increase in gas holdup is mostly contributed at the entrance. This is possibly due to the fact that the exit has a smaller effective region than the entrance. Due to the bubble breakage at the bed surface, the void generated has a very high inertia which attracts the preceding bubbles and causing a much faster bubble rise velocity compared to the bubble rise velocity that is purely a result of bubble–bubble and bubble–liquid interactions. The bubble rise velocity distribution at various Hs/D ratios is shown in Fig. 6. A similar trend as the bubble size distribution can also be observed. As the static Hs/D ratio is reduced, the average bubble rise velocity is also reduced. However, the bubble rise velocity distribution remains relatively wide compared to the bubble size distribution even when the
Fig. 6 – Bubble rise velocity distribution at different H/D ratios.
It has been confirmed that the use of shallow bed can provide a lower bubble size distribution and higher gas holdup which enhance the reactor performance. It would be beneficial to look at the performance of a shallow bubble column at different superficial gas velocities. Fig. 7 shows the bubble size distributions for Hs/D = 2, 4, and Hs/D = 6 at three different operating gas velocities. It is to note that because of the large orifice size used in the distributor, regime transition can happen at lower gas velocities compared to distributors having small orifice sizes. Nonetheless, the three velocities would cover the dispersed bubble, transition and coalescedbubble regime. For an Hs/D = 2, there is an increase in both the average bubble size and variance as Ug increases. Similar increase in both average bubble size and variance with an increase in Ug is also observed for Hs/D = 6. However, the effect of Ug on the variance of the bubble size distribution is much more significant for Hs/D = 6 than Hs/D = 2, especially at high Ug . As the result in the previous section indicates, for Hs/D as low as 2, the whole bed would be covered by the end regions. Bubble coalescence and breakup are suppressed due to the short axial distance for further bubble–bubble interaction. The regime transition is being delayed due to the reduced bed Hs/D ratio. There is insufficient distance for the bubbles to interact with other bubbles for coalescence. A higher gas velocity is thus needed to increase the bubble concentration to promote the bubble coalescence at a lower axial position. The bubble rise velocity at various velocities and Hs/D ratios are shown in Fig. 8. At low superficial gas velocity of 3.2 cm/s, the Hs/D ratio essentially has no effect on the bubble rise velocity. However, as Ug increases, the rise velocity distribution becomes much wider at higher Hs/D. For example, at Ug of 10.8 cm/s and H/D of 2, the variance of the rise velocity is 31.6 cm2 /s2 . The variance of the rise velocity becomes 66.6 cm2 /s2 for the same Ug of 10.8 cm/s but Hs/D of 6. This is because at a higher gas velocity, the bubble size is limited to the hydrodynamics. In order to compensate the high gas flow, the bubble concentration needs to be increased accordingly. The increased bubble concentration promotes bubble–bubble interactions, especially the entrainment of small bubbles by the adjacent large bubbles and results in a higher average rise velocity and larger variance. The effect of superficial gas velocity on the bubble rise velocity becomes more substantial at Hs/D = 6. It is also to note that at high gas velocities and large Hs/D ratios, the log-normal distribution cannot give a very good description of the rise velocity distribution since bubble size is no longer the sole factor for the rise velocity.
chemical engineering research and design 8 8 ( 2 0 1 0 ) 197–203
Fig. 7 – Bubble size distribution at different superficial gas velocities (a) H/D = 2, (b) H/D = 4 and (c) H/D = 6.
3.4.
Effect of solids concentration
Many bubble column operations involve the use of catalysts which transform the bubble column operation into a slurry bubble column operation. The presence of solids would also change the bubble behaviour in the slurry bubble column. Soda-lime glass beads of average diameter of 425–600 m are added to the bubble column as the solid phase. The bubble size distribution with H/D of 4 and solid concentration of 0% and 5% are compared in Fig. 9. The presence of solid particles clearly increases the bubble size and widens the bubble size distribution for all gas velocities. Similar effect of solid concentration is also observed by Koide et al. (1984), Saxena et al. (1989), De Swart and Krishna (1995), Li and Prakash (1997), Krishna et al. (1999), Luo et al. (1999) and Chilekar et al. (2005), etc. in bubble and slurry bubble columns. In general, small solid particles would form a stabilizing layer on the bubble surface, promotes bubble coalescences and inhibits the bubble breakup. More-
201
Fig. 8 – Bubble rise velocity distribution at different superficial gas velocities (a) H/D = 2, (b) H/D = 4 and (c) H/D = 6.
over, the solids in the liquid–solid suspension will result in a higher apparent viscosity than the actual viscosity of the liquid alone and thus larger bubbles are formed (De Swart and Krishna, 1995; Krishna et al., 1999). The addition of solids gives larger increase in the bubble size at low superficial gas velocity. For 5% solids concentration, the average bubble size at low gas velocities is larger than that at higher superficial gas velocities. This is possibly due to the settling of the solid particles at low gas velocities, which results in a higher solids concentration near the distributor and causes the formation of larger bubbles. As the gas velocity is increased, the gas velocity at the orifice also increases and the solids become more uniform in the column. The combined effects of high shear and lower solids concentration promote bubble breakup and result in the smaller bubble size are larger variance compared with low gas velocities.
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g l
variance gas density liquid density
Acknowledgement Support by AcRF Tier1 grant RG41/06 is gratefully acknowledged.
References
Fig. 9 – Effect of solids on bubble size distribution in a shallow bubble column reactor (H/D = 4) (a) 0% solids concentration and (b) 5% solids concentration.
4.
Concluding remarks
The bubble characteristics of bubble/slurry bubble columns have been investigated experimentally at various conditions. For a fixed static liquid height, the bubble size is higher in the distributor region than that in the central region, in which the bubble grows steadily within a H/D of 6. A reduction of Hs/D ratio is found to favor the formation of smaller bubbles when the end regions cover the majority of the column. The gas holdup is found to increase with a decrease in H/D ratio. A low Hs/D ratio also narrows the bubble size distribution with a more prominent effect at high superficial gas velocities and high solids concentrations. It is desirable to have an Hs/D ratio of 2–4 for the operation of shallow bed. The addition of solids causes an increase in average bubble size and the effect is less significant at higher superficial gas velocities.
Notations
d D g H Hs (dP/dz)d ˛ ˇ
a log-normally distributed variable column diameter gravitational constant liquid height at which measurements are performed static liquid height dynamic pressure gradient parameter used in log-normal distribution to represent the mean parameter used in log-normal distribution to represent the variance mean
Bahadori, F. and Rahimi, R., 2007, Simulations of gas distributors in the design of shallow bubble column reactors. Chemical Engineering Technology, 30(4): 443–447. Broughton, J., 1983, in Howard, J.R. (ed) (Applied Science Publishers, New York), pp. 305–320. Chakraborty, R.K. and Howard, J.R., 1980, in Grace, J.R. and Matsen, J.M., Matsen, J.M. (eds) (Plenum Press, New York), pp. 167–173. Chilekar, V.P., Warnier, M.J.F., van der Schaaf, J., Kuster, B.F.M., Schouten, J.C. and van Ommen, J.R., 2005, Bubble size estimation in slurry bubble columns from pressure fluctuations. AIChE Journal, 51: 1924–1937. De Swart, J.W.A. and Krishna, R., 1995, Influence of particle concentration on the hydrodynamics of bubble column slurry reactors. Transaction of Institution of Chemical Engineering, 73: 308. Epstein, N., 1981, Three-phase fluidization: some knowledge gaps. Canadian Journal of Chemical Engineering, 59: 649. Gopal, J.S. and Sharma, M.M., 1983, Mass transfer characteristics of low H/D bubble columns. Canadian Journal of Chemical Engineering, 61: 517–526. Haque, M.W., Nigam, K.D.P. and Joshi, J.B., 1986, Optimum gas sparger design for bubble columns with a low height to diameter ratio. Chemical Engineering Journal, 33: 63–69. Harrison, D. and Grace, J.R., 1971, in Davidson, J.F. and Harrison, K., Harrison, K. (eds) (Academic Press, New York), pp. 599–626. Jiang, P.J., Arters, D. and Fan, L.-S., 1992, Pressure effects on the hydrodynamic behaviour of gas-liquid-solid fluidized beds. Industrial & Engineering Chemical Research, 31: 2322–2327. Koide, K., Takazawa, A. and Komura, M., 1984, Gas holdup and volumetric liquid-phase mass transfer coefficient in solid–suspended bubble columns. Journal of Chemical Engineering of Japan, 17: 459. Krishna, R., Urseanu, M.I., Van Baten, J.M. and Ellenberger, J., 1999, Rise velocity of a swarm of large gas bubbles in liquids. Chemical Engineering Science, 54: 171–183. Kulkarni, A., 2007, Mass transfer in bubble column reactors: effect of bubble size distribution. Industrial & Engineering Chemical Research, 46: 2205–2211. Kumar, S.B., Moslemian, D. and Duducovic, M.P., 1997, Gas-holdup measurements in bubble columns using computed tomography. AIChE Journal, 43: 1414–1425. Li, H. and Prakash, A., 1997, Heat transfer and hydrodynamics in a three-phase slurry bubble column. Industrial & Engineering Chemical Research, 36: 4688. Luo, X., Lee, D.J., Lau, R., Yang, G.Q. and Fan, L.-S., 1999, Maximum stable bubble size and gas holdup in high pressure slurry bubble columns. AIChE Journal, 45: 665–680. Matsuura, A. and Fan, L.-S., 1984, Distribution of bubble properties in a gas–liquid–solid fluidized bed. AIChE Journal, 30: 894–903. Parasu Veera, U. and Joshi, J.B., 1999, Measurement of gas holdup profile by gamma ray tomography: effect of sparger design and height of dispersion in bubble column. Transaction of Institution of Chemical Engineering, 77: 303–317. Ranade, V.V. and Tayalia, Y., 2001, Modelling of fluid dynamics and mixing in shallow column reactors: influence of sparger design. Chemical Engineering Science, 56: 1667–1675. Ruizika, M.C., Drahos, J., Fialova, M. and Thomas, N.H., 2001, Effect of bubble column dimensions on flow regime
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transition. Chemical Engineering Science, 21–22: 6117–6124. Saxena, S.C., Vadivel, R. and Saxena, A.C., 1989, Gas holdup and heat transfer from immersed surfaces in two- and three-phase systems in bubble columns. Chemical Engineering Communications, 85: 63. Thorat, B.N., Shevade, A.V., Bhilegaonkar, K.N., Aglawe, R.H., Parasu Veera, U., Thakre, S.S., Pandit, A.B., Sawant, S.B. and Joshi, J.B., 1998, Effect of sparger design and height to diameter ratio on fractional gas hold-up in bubble columns. Transaction of Institution of Chemical Engineering, 76: 823–834. Virr, M.J. and Williams, H.W., 1985, Heat recovery by shallow fluidized beds. Chemical Engineering Progress, 81: 50. Voigt, J. and Schugerl, K., 1979, Absorption of oxygen in countercurrent multistage bubble columns—I. Aqueous solutions with low viscosity. Chemical Engineering Science, 34(10): 1221–1229. Wilkinson, P.M., Spek, A.P. and van Dierendonck, L.L., 1992, Design parameters estimation for scale-up of high-pressure bubble columns. AIChE Journal, 38: 544–554.
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Xue, J., Al-Dahhan, M., Dudukovic, M.P. and Mudde, R.F., 2008, Bubble velocity, size, and interfacial area measurements in a bubble column by four-point optical probe. AIChE Journal, 54: 350–363. Yamashita, F., 1998, Effect of clear liquid height and gas inlet height on gas holdup in a bubble column. Journal of Chemical Engineering of Japan, 31: 285–288. Yan, Z., Yao, J.Z., Wang, W.L., Liu, S.J. and Kwauk, M., 1984, in Kunii, D. and Toei, R., Toei, R. (eds) (Engineering Foundation, New York), pp. 607–614. Yang, G.Q., Du, B. and Fan, L.-S., 2007, Bubble formation and dynamics in gas–liquid–solid fluidization—a review. Chemical Engineering Science, 62: 2–27. Yang, J.-Y., Liu, Y.A. and Squires, A.M., 1987, Pressure drop across shallow fluidized beds: theory and experiment. Powder Technology, 53: 79–89.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Bubble characteristics in shallow bubble column reactors Raymond Lau ∗ , Rujuan Mo, Wei Shan Beverly Sim School of Chemical and Biomedical Engineering (SCBE), Nanyang Technological University (NTU), 62 Nanyang Drive, Singapore, Singapore
a b s t r a c t The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Bubble column; Bubble size distribution; Shallow bed; Gas holdup
1.
Introduction
Bubble column reactor has a wide range of industrial applications due to its simple design and excellent heat and mass transfer properties. Traditionally, the design and scale-up of bubble column reactors are focused on high height to diameter ratio (H/D) to allow system hydrodynamics to reach fully developed state (Wilkinson et al., 1992; Kumar et al., 1997; Thorat et al., 1998; Parasu Veera and Joshi, 1999). However, the high H/D minimized the effect of gas distributors where small bubbles are generated. It would be logical to consider bubble column reactor operations with low H/D. A low H/D operation not only utilizes the small bubbles generated from the gas distributor, but also provides lower bed pressure drops. Potential application of shallow bubble column includes absorption, catalytic slurry reaction, bioreactions, wastewater treatment, etc. (Voigt and Schugerl, 1979; Gopal and Sharma, 1983; Haque et al., 1986; Bahadori and Rahimi, 2007). In fact, success application of shallows bed is well established in gas–solid operations (Harrison and Grace, 1971; Chakraborty and Howard, 1980; Broughton, 1983; Yan et al., 1984; Virr and Williams, 1985; Yang et al., 1987). The study on shallow bubble column reactors is quite limited. Experimental and simulation results are mostly focused on the effect of gas distributor design on the performance of shallow bubble column (Haque et al., 1986; Thorat et al.,
∗
1998; Ranade and Tayalia, 2001). The optimum gas distributor design is found to be dependent on the system properties but no design procedure is derived due to the complex flow pattern in the distributor region (Haque et al., 1986). On the other hand, there are mixed reports on the effect of static liquid height on the gas holdup of the system. Kumar et al. (1997) found no effect on the static liquid height on the overall gas holdup while the local gas holdup increases with the axial distance from the distributor. A decrease in gas holdup is also observed with an increase in static liquid height by other researchers and it is justified by the three-region concept (Wilkinson et al., 1992; Yamashita, 1998; Ruizika et al., 2001). Therefore, it would be beneficial to study the hydrodynamics in shallow bubble column reactor systematically for possible implementation in different industrial processes. In this study, the bubble characteristics such as bubble size distribution, bubble rise velocity distribution and gas holdup will be investigated in a bubble/slurry bubble columns with low height to diameter ratios. Effects of static liquid height, superficial gas velocity, solids concentration, and column diameter on the bubble characteristics are examined.
2.
Experimental
A schematic diagram of the experimental setup is shown in Fig. 1. The experiments are conducted in a cylindrical acrylic
Corresponding author. Tel.: +65 63168830; fax: +65 67947553. E-mail address: [email protected] (R. Lau). Received 22 October 2008; Received in revised form 20 May 2009; Accepted 7 July 2009 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.07.008
198
chemical engineering research and design 8 8 ( 2 0 1 0 ) 197–203
Fig. 1 – Schematic diagram of the experimental setup. column. The column has a height of 150 cm and an internal diameter of 14 cm. Pressure taps are installed every 10 cm starting from 10 cm above the distributor plate. A perforated plate with orifice diameter of 3 mm and triangular pitch distance of 0.9 cm is used as the gas distributor. The open area ratio of the perforated plate is 9.7%. Screen with an opening size of 100 m are covered on top of the perforated plate to prevent solid particles from dropping into the plenum region. Tap water and compressed air are used as the liquid and gas phases in the experiments, respectively. The particles used are soda-lime glass spheres having 425–600 m mean diameter and density of 2480 kg/m3 . The superficial gas velocity is varied from 3.25 to 10.8 cm/s. A solid concentration of 5% by weight is also used to study the effect of solids on shallow bed operation. A high speed camera is used to take bubble images at various height of the column at a frame rate of 600 Hz. The curvature of the column wall is found to have negligible image distortion when tested with square grids. A sample bubble image can also be seen in Fig. 1. The air bubbles rising in water generally has an ellipsoidal shape. As a result, the maximum dimensions of the ellipsoidal bubbles are used in the determination of the average bubble size. Two markers 10 mm apart from each other are placed on the surface of the bubble column. The size of bubbles observed on the digital images will be calibrated against the marker distance. The bubble travel time is determined by multiplying the number of frames needed for the bubble to rise from one marker to another with the frame rate. The rise velocity of the bubble is then determined by dividing the marker distance with the bubble travel time. Since there are bubble overlapping at high gas velocities, the bubble size and velocity determination are processed manually with i-SPEED software (Olympus). The focus of the camera is set at 5 cm behind the column wall. Only in-focus bubbles are considered in the bubble size and rise velocity determination to prevent the inclusion of bubbles at the wall. A minimum population size of 200 is considered for the determination of both size and rise velocity distributions. For bubble/slurry bubble column operation, the size distribution is found to follow lognormal distribution which can be represented by (Matsuura
and Fan, 1984; Yang et al., 2007):
(ln (b) − ˛) 1 Pdf (b) = √ exp − 2ˇ2 2bˇ
2
(1)
where ˛ and ˇ are the parameters to represent the mean and variance in the log-normal distribution. The parameters, ˛ and ˇ can be determined by applying least square method to compare the experimental results and theoretical prediction by the log-normal distribution. The mean, and variance, 2 of the log-normal distribution can be calculated by:
˛+
= exp
ˇ2 2
2 = exp(2˛ + ˇ2 ) exp(˛2 − 1)
(2)
(3)
A differential pressure transducer is installed at 10–20, 20–30, 30–40, 40–50, and 50–60 cm above the gas distributor to measure the axial gas holdup variation. A sampling frequency of 100 Hz is used for the collection of the pressure transducer signals. The pressure transducers measure the dynamic pressure gradient between two measurement locations. According to Epstein (1981), the relationship between the dynamic pressure drop and the individual phase holdups for a gas–liquid system is:
dP
−
dz
d
= (εg g + εl l − l )g
(4)
Since εg + εl = 1
(5)
the gas holdup can then be calculated by solving Eqs. (4) and (5): εg =
(dP/dz)d g(l − g )
(6)
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Fig. 3 – Sectional gas holdup above the gas distributor. Fig. 2 – Comparison of bubble size distribution measured at different H/D ratios at Ug = 7.6 cm/s. The overall bed holdup is determined by using the bed expansion method.
3.
Results and discussion
3.1.
Effect of axial distance from the distributor
There is no clear definition on the H/D ratio for a shallow liquid height. It is commonly accepted that when the H/D ratio is <4, it can be considered as shallow bed. In order to examine the performance of a shallow bubble column, it would be beneficial to look at the effect of axial distance near the distributor on the hydrodynamic properties. Fig. 2 shows the bubble size distribution measured at various axial distance (represented by H/D ratios) with a constant static liquid height of 100 cm (Hs/D = 7.2) and a superficial gas velocity of 7.6 cm/s. At H/D ratio of 6, the average bubble size is found to be around 9 mm. It is comparable to the bubble size results reported in the literature (Jiang et al., 1992; Kulkarni, 2007; Xue et al., 2008). As shown in the figure, as the H/D ratio is decreased from 6 to 2, there is negligible change in the bubble size and bubble size distribution. However, as the H/D ratio is reduced further from 2 to 0.5, a reduction in the average bubble size is observed. It is also found that the average bubble size is larger and the distribution is wider at H/D = 0.5 than that at H/D = 1.2. The perforated plate distributor used in this study generates relatively large bubbles. As the bubbles detached from the gas distributor, the bubbles start to accelerate. During this acceleration stage, the bubbles are subjected to relatively low shear. However, as height increases, the bubbles eventually reach their terminal rise velocities. The high rise velocities of large bubbles create vigorous gas–liquid interactions and causes bubble breakage, which results in a smaller average bubble size at H/D = 1.2. As the bubbles continue to rise in the column, the large amount of bubbles start to interact with each other and causes simultaneous bubble coalescence and breakup. At H/D higher than 2, the rising bubbles eventually reach an equilibrium state of bubble coalescence and breakup, and thus a relatively constant bubble size distribution. Based on the experimental results in this study, the entrance effect of bubbles is effective up to about an H/D of 2. It is in agreement with the Kulkarni (2007) that low H/D yields smaller bubble sizes and as H/D is >3, there is practically no change in the bubble size distribution with H/D.
The average sectional gas holdup is also determined every 10 cm from the distributor at Hs/H of 7.2 and superficial gas velocity of 7.6 cm/s. As shown in Fig. 3, it can be seen that the gas holdup increases from H/D of 1 to H/D of 2. Beyond an H/D of 2, the gas holdup decreases with an increase in H/D. The gas holdup result is consistent with the bubble size measurement results and shows a good representation that the gas bubble emerging from the gas distributor is relatively large and subjected to a larger shear for bubble breakage near the gas distributor. It is to note that this is likely to be caused by the design of the distributor. If porous distributor is being used, the initial bubble size generated is relative small, and this initial bubble breakage probably will not be present.
3.2.
Effect of static liquid height
Other than the entrance effect at the gas distributor, it is important to look at the exit effect at the bed surface for the operation of a shallow bubble column reactor. Since the static liquid height is <1 m in all experimental conditions, the hydrostatic head would only contribute to a pressure of <0.1 atm. Therefore, the effect of hydrostatic head on the bubble size and rise velocity can be considered negligible. As shown in Fig. 4, a comparison of the bubble size distribution measured at Hs/D = 2 for a static liquid height of Hs/D = 2, 4 and 7.2 indicates that the small bubble size is a combined effect of both the entrance and exit effect. The bubbles with Hs/D of 2 show smaller average bubble size compared with Hs/D of 4 and 7.2. It indicates that the presence of liquid surface enhances bubble breakup. When bubbles rise to the bed surface, the high disturbance generated by the bubble breakage at the bed surface
Fig. 4 – Compare bubble size distribution measured at H/D = 2 and Ug = 7.6 cm/s with various static liquid height.
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Hs/D ratio is low. At low bubble concentrations, each bubble can be considered to be rising in an isolated manner. The rise velocity would depend on the bubble size only. However, at high bubble concentrations, usually observed at high superficial gas velocities, the bubble–bubble interaction would cause the overall bubble rise velocity to be higher than the same bubble size rising in an isolated manner. Quite often, smaller bubbles are observed to rise at a velocity much higher than other bubbles that has the same size. This is maybe due to the entrainment of small bubbles by the wakes generated by the rise of adjacent large bubbles. Therefore, the bubble rise velocity normally has a wider distribution than the bubble size under the same conditions.
3.3.
Effect of superficial gas velocities
Fig. 5 – Overall gas holdup for different H/D. causes the preceding bubbles to break, resulting in smaller bubble size. In general, the bed height for shallow bubble column operation should be low enough that the distributor region and the bed surface region should cover a majority entire bed in order to maximize the relative effect of the end regions. However, for extremely low Hs/D of 0.5, a slight foaming problem is observed near the liquid surface which would deteriorate the gas–liquid contact. For the column diameter of 14 cm used in this study, an Hs/D ratio of 2–4 is favorable. The gas holdup of the bed is also determined for various static Hs/D ratios and the result is shown in Fig. 5. It can be seen that there is a significant decrease in gas holdup as the Hs/D is increased from 0.5 to 2 at a constant superficial gas velocity of 7.6 cm/s. The effect of end regions is diminishing as Hs/D is increased beyond 2. Comparison of the bubble size results with different Hs/D shows that even though both the entrance and exit effects contribute to the small bubble size generated, the increase in gas holdup is mostly contributed at the entrance. This is possibly due to the fact that the exit has a smaller effective region than the entrance. Due to the bubble breakage at the bed surface, the void generated has a very high inertia which attracts the preceding bubbles and causing a much faster bubble rise velocity compared to the bubble rise velocity that is purely a result of bubble–bubble and bubble–liquid interactions. The bubble rise velocity distribution at various Hs/D ratios is shown in Fig. 6. A similar trend as the bubble size distribution can also be observed. As the static Hs/D ratio is reduced, the average bubble rise velocity is also reduced. However, the bubble rise velocity distribution remains relatively wide compared to the bubble size distribution even when the
Fig. 6 – Bubble rise velocity distribution at different H/D ratios.
It has been confirmed that the use of shallow bed can provide a lower bubble size distribution and higher gas holdup which enhance the reactor performance. It would be beneficial to look at the performance of a shallow bubble column at different superficial gas velocities. Fig. 7 shows the bubble size distributions for Hs/D = 2, 4, and Hs/D = 6 at three different operating gas velocities. It is to note that because of the large orifice size used in the distributor, regime transition can happen at lower gas velocities compared to distributors having small orifice sizes. Nonetheless, the three velocities would cover the dispersed bubble, transition and coalescedbubble regime. For an Hs/D = 2, there is an increase in both the average bubble size and variance as Ug increases. Similar increase in both average bubble size and variance with an increase in Ug is also observed for Hs/D = 6. However, the effect of Ug on the variance of the bubble size distribution is much more significant for Hs/D = 6 than Hs/D = 2, especially at high Ug . As the result in the previous section indicates, for Hs/D as low as 2, the whole bed would be covered by the end regions. Bubble coalescence and breakup are suppressed due to the short axial distance for further bubble–bubble interaction. The regime transition is being delayed due to the reduced bed Hs/D ratio. There is insufficient distance for the bubbles to interact with other bubbles for coalescence. A higher gas velocity is thus needed to increase the bubble concentration to promote the bubble coalescence at a lower axial position. The bubble rise velocity at various velocities and Hs/D ratios are shown in Fig. 8. At low superficial gas velocity of 3.2 cm/s, the Hs/D ratio essentially has no effect on the bubble rise velocity. However, as Ug increases, the rise velocity distribution becomes much wider at higher Hs/D. For example, at Ug of 10.8 cm/s and H/D of 2, the variance of the rise velocity is 31.6 cm2 /s2 . The variance of the rise velocity becomes 66.6 cm2 /s2 for the same Ug of 10.8 cm/s but Hs/D of 6. This is because at a higher gas velocity, the bubble size is limited to the hydrodynamics. In order to compensate the high gas flow, the bubble concentration needs to be increased accordingly. The increased bubble concentration promotes bubble–bubble interactions, especially the entrainment of small bubbles by the adjacent large bubbles and results in a higher average rise velocity and larger variance. The effect of superficial gas velocity on the bubble rise velocity becomes more substantial at Hs/D = 6. It is also to note that at high gas velocities and large Hs/D ratios, the log-normal distribution cannot give a very good description of the rise velocity distribution since bubble size is no longer the sole factor for the rise velocity.
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Fig. 7 – Bubble size distribution at different superficial gas velocities (a) H/D = 2, (b) H/D = 4 and (c) H/D = 6.
3.4.
Effect of solids concentration
Many bubble column operations involve the use of catalysts which transform the bubble column operation into a slurry bubble column operation. The presence of solids would also change the bubble behaviour in the slurry bubble column. Soda-lime glass beads of average diameter of 425–600 m are added to the bubble column as the solid phase. The bubble size distribution with H/D of 4 and solid concentration of 0% and 5% are compared in Fig. 9. The presence of solid particles clearly increases the bubble size and widens the bubble size distribution for all gas velocities. Similar effect of solid concentration is also observed by Koide et al. (1984), Saxena et al. (1989), De Swart and Krishna (1995), Li and Prakash (1997), Krishna et al. (1999), Luo et al. (1999) and Chilekar et al. (2005), etc. in bubble and slurry bubble columns. In general, small solid particles would form a stabilizing layer on the bubble surface, promotes bubble coalescences and inhibits the bubble breakup. More-
201
Fig. 8 – Bubble rise velocity distribution at different superficial gas velocities (a) H/D = 2, (b) H/D = 4 and (c) H/D = 6.
over, the solids in the liquid–solid suspension will result in a higher apparent viscosity than the actual viscosity of the liquid alone and thus larger bubbles are formed (De Swart and Krishna, 1995; Krishna et al., 1999). The addition of solids gives larger increase in the bubble size at low superficial gas velocity. For 5% solids concentration, the average bubble size at low gas velocities is larger than that at higher superficial gas velocities. This is possibly due to the settling of the solid particles at low gas velocities, which results in a higher solids concentration near the distributor and causes the formation of larger bubbles. As the gas velocity is increased, the gas velocity at the orifice also increases and the solids become more uniform in the column. The combined effects of high shear and lower solids concentration promote bubble breakup and result in the smaller bubble size are larger variance compared with low gas velocities.
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g l
variance gas density liquid density
Acknowledgement Support by AcRF Tier1 grant RG41/06 is gratefully acknowledged.
References
Fig. 9 – Effect of solids on bubble size distribution in a shallow bubble column reactor (H/D = 4) (a) 0% solids concentration and (b) 5% solids concentration.
4.
Concluding remarks
The bubble characteristics of bubble/slurry bubble columns have been investigated experimentally at various conditions. For a fixed static liquid height, the bubble size is higher in the distributor region than that in the central region, in which the bubble grows steadily within a H/D of 6. A reduction of Hs/D ratio is found to favor the formation of smaller bubbles when the end regions cover the majority of the column. The gas holdup is found to increase with a decrease in H/D ratio. A low Hs/D ratio also narrows the bubble size distribution with a more prominent effect at high superficial gas velocities and high solids concentrations. It is desirable to have an Hs/D ratio of 2–4 for the operation of shallow bed. The addition of solids causes an increase in average bubble size and the effect is less significant at higher superficial gas velocities.
Notations
d D g H Hs (dP/dz)d ˛ ˇ
a log-normally distributed variable column diameter gravitational constant liquid height at which measurements are performed static liquid height dynamic pressure gradient parameter used in log-normal distribution to represent the mean parameter used in log-normal distribution to represent the variance mean
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