- Email: [email protected]
Effect of liquid properties on the performance of bubble column reactors with fine pore spargers
Chemical Engineering Science 60 (2005) 1465 – 1475 www.elsevier.com/locate/ces
Effect of liquid properties on the performance of bubble column reactors with fine pore spargers A.A. Mouza, G.K. Dalakoglou, S.V. Paras∗ Department of Chemical Engineering, Aristotle University of Thessaloniki Univ.Box 455, GR 54124 Thessaloniki, Greece Received 4 November 2003; received in revised form 20 September 2004; accepted 7 October 2004
Abstract This work is a study of the effect of liquid properties on the performance of bubble column reactors with fine pore spargers. Various liquids covering a range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video technique is used for visual observations and, combined with image processing, is used for gas holdup and bubble size measurements. New data on average gas holdup values, bubble size distributions and Sauter diameters are presented and are consistent with existing physical models on coalescence/breakage. A correlation based on dimensionless groups for the prediction of gas holdup in the homogeneous regime is proposed and found to be in good agreement with available data. 䉷 2004 Elsevier Ltd. All rights reserved. Keywords: Bubble column; Porous sparger; Gas holdup; Bubble size; Coalescence; Liquid properties; Flow regimes
1. Introduction Bubble columns are widely used in industrial gas–liquid operations (e.g. gas/liquid reactions, agitation by gas injection, fermentations, etc.) in chemical and biochemical process industries, due to their simple construction, low operating cost and high-energy efficiency. In all these processes gas holdup and bubble size are important design parameters, since they define the gas–liquid interfacial area available for mass transfer. In turn, bubble size distribution and gas holdup in gas–liquid dispersions depend largely on column geometry, operating conditions, physico-chemical properties of the two phases and type of gas sparger (Camarasa et al., 1999). The design of bubble columns has primarily been carried out by means of empirical or semi-empirical correlations based mainly on experimental data. Since the multiphase flow is in general complex in structure, the design and scale up of such type of equipment is still a difficult task and subject to errors (Deckwer and Schumpe, 1993). ∗ Corresponding author. Tel.: +30 23 10 996174; fax: +30 23 10 996209.
E-mail address: [email protected] (S.V. Paras). 0009-2509/$ - see front matter 䉷 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.10.013
Despite the extensive and long lasting study of bubble column performance many basic questions about the effect of important operational parameters remain unanswered. For example, although bubble column characteristics have been studied extensively in the past few decades, there is still considerable uncertainty concerning the prevailing mechanisms of bubble formation, as well as the most appropriate correlations for practical applications. Break-up and coalescence of fluid objects play a crucial role in a broad spectrum of multiphase flow processes, such as the evolution of the bubble size distribution in stirred tanks and bubble columns (Delhaye and McLaughlin, 2003). Shah et al. (1982) and Parasu Veera and Joshi (1999) have reviewed and summarized the work done in this area. It is generally accepted that, depending on the gas flow rate, two main flow regimes can be readily observed in bubble columns, i.e., the homogeneous bubbly flow regime encountered at low gas velocities and characterized by a narrow bubble size distribution and radially uniform gas holdup; and the heterogeneous (churnturbulent flow) regime observed at higher gas velocities and characterized by the appearance of large bubbles, formed by coalescence of the small bubbles and bearing a higher rise velocity hence leading to relatively lower gas holdup values
1466
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
(Joshi et al., 2002; Camarasa et al., 1999; Zahradnik et al., 1997). The two regimes differ from one another in their hydrodynamic and transport characteristics. Depending on the type of the gas distributor and the properties of the liquid phase, both regimes can be obtained in the same equipment by varying the gas input flow rate. Various types of gas spargers, the most common of which are perforated plate, membrane and fine porous plate, are in use. Among the above sparger types, the perforated plate requires a minimum gas velocity in order to produce a uniform bubble distribution over the whole sparger area. On the other hand, bubble columns equipped with porous spargers offer a greater gas/liquid contact area for efficient mass and heat transfer, because the bubbles created by this type of gas distributor are numerous and far smaller. Finally, the membrane, even though it is able to maintain a homogeneous flow up to greater flow rates, generates a greater pressure drop (Hebrard et al., 1996). As a result, the use of a fine porous plate as a gas sparger seems to be advantageous over the other types of multiple injection point distributors. To the authors’ best knowledge, limited information is available in the literature regarding the behavior of fine porous plate spargers. Hebrard et al. (1996), who studied the influence of gas sparger characteristics on the hydrodynamic behavior of bubble columns, concluded that bubble size and holdup values depend primarily on the physico-chemical properties of the liquid phase and the type of gas distributor. On the other hand, Parthasarathy and Ahmed (1996), who conducted their experiments with non-coalescing liquids, stated that there is confusion regarding the average size value and size distribution of bubbles generated by fine pore spargers. Zahradnik et al. (1997) investigated the effect of various parameters (i.e., column geometry, distributor type, liquid properties) on the gas–liquid flow regime stability and gas holdup in bubble column reactors equipped, among others, with porous spargers. Camarasa et al. (1999), who examined the influence of the liquid properties and of the gas sparging method on hydrodynamics and bubble characteristics in a bubble column, used water-alcohol solutions to simulate the behavior of non-coalescing organic liquids and compared their data with those concerning standard air–water systems. Kaji et al. (2001) studied experimentally the behavior of bubble formation using various porous spargers with pore diameter 5–400 m and investigated the effect of surface tension on the gas holdup distribution. The purpose of this work is to study the effect of liquid properties on bubble size distribution in a bubble column equipped with two different fine porous spargers. Various liquids covering a range of surface tension and viscosity values are employed, while atmospheric air is used as the gas phase for all experiments. Experimental data on average gas holdup, bubble size distribution and mean Sauter diameter, obtained from image analysis of fast video recordings, are
reported. A correlation is also proposed for the prediction of average gas holdup.
2. Experimental set-up and procedures The experimental set-up (Fig. 1) consists of a vertical rectangular Plexiglas䉸 column 1.5 m height, having a square cross-section (side length 10 cm). The column is equipped with appropriate rotameters for gas phase flow measurement and control. The rectangular geometry was preferred over the cylindrical one, because it facilitates both the direct flow visualization and the use of optical measuring methods by minimizing optical distortion. For the injection and uniform distribution of the gas phase, a gas sparger, i.e., a round metal porous disk, 2.0 cm in diameter, is installed at the center of the bottom plate. In the present experiments, two 316L SS porous disks (made by Mott Corp.) with nominal pore size of 20 and 40 m were alternatively used as gas spargers. However, in order to check the effect of the usage time on sparger performance various porous disks were employed during the period of the experiments. Several liquids, whose physical properties are presented in Table 1, were employed as liquid phase, whereas the gas phase was atmospheric air for all runs. All the experiments were conducted at ambient pressure and temperature conditions. Each experimental run started by first filling the column with the appropriate liquid phase up to 80 cm above the sparger. All the experiments were performed with no liquid throughput, while the gas phase was injected and distributed into the liquid phase by passing through the porous disk sparger. A high-speed digital video camera (Redlake MotionScope PCI 䉸 1000S) is used both for direct flow visualization and for bubble size and gas holdup measurements. The camera is fixed on a stand very close to the area of observation in such a way that the test section is located between the camera and an appropriate lighting system placed behind a diffuser to evenly distribute the light. Although the imaging system used was capable of recording up to 1000 full frames per second, a speed of 500 fps is considered a suitable recording rate for the present experiments. The shutter rate employed was of 1/10 000. It must be pointed out that the optical system used offers a very narrow depth of field (few mm). The recorded images were also used to extract quantitative information on bubble size distribution and gas holdup values. Using proper lighting, the gas–liquid interface around the bubble circumference can be clearly outlined on the pictures. The calibration of the measuring system, needed to ensure the accurate measurement of the bubbles, is accomplished by measuring a microscale placed at the focusing plane. Subsequent image processing (e.g. noise reduction, brightness improvement, contrast enhancement, shadow and double images removal) results to a sharp bubble–liquid interface. A detailed description of the technique can be also
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
1467
Fig. 1. Experimental set-up.
Table 1 Liquid phase properties at 20 ◦ C Index Liquid phase
w b1 b2 g1 g2 g3
Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
3. Results
Viscosity, L (mPa s)
Density, L (kg/m3 )
Surface tension, L (mN/m)
1.0 0.9 0.9 3.5 8.2 22.5
998 994 991 1081 1126 1173
72 60 48 70 68 67
found in a recent paper by Colella et al. (1999). Image analysis, using appropriate software (SigmaScan Pro䉸 ), allows bubble size prediction. The bubbles were approximated by ellipses whose major and minor axes were computed by the software. The equivalent diameter of a sphere with the same volume as the ellipsoid was computed (Colella et al., 1999; Polli et al., 2002). Approximately 800 bubbles were measured in each experimental run, a number considered to be adequate for statistical calculations (Tse et al., 2003; Hebrard et al., 1996). The advantage of the method is that it permits both in situ and non-intrusive measurements. The uncertainty of the measurements has been estimated to be less than 10%. The average gas holdup is estimated by the bed expansion. The liquid level is measured on two different pictures taken prior to gas inflow and after gas is injected and steady state is established. The difference in liquid level, measured by superposition of the two pictures, gives a measure of the average gas holdup. The uncertainty of the measurements is estimated to be less than 15%.
3.1. Visual observations As already mentioned, depending on the gas flow rate, the two flow regimes observed in bubble columns are the homogeneous bubbly flow regime encountered at low gas velocities and the heterogeneous (churn-turbulent flow) regime observed at higher gas velocities. The photos in Fig. 2 give a visual image of the flow patterns observed in a bubble column according to the gas flow rate when the liquid phase is either water or 50% glycerin solution. At the beginning the bubbles are gathered at the core of the flow, but after the first 20 cm they are spread uniformly covering the whole column area. It is also worth noticing that after the first 40 cm of the column height the gas phase distribution does not seem to change significantly. For the lower gas velocities applied the homogeneous flow regime is encountered, where relatively small gas bubbles are formed and almost uniformly distributed throughout the whole column area (Figs. 2a and b). The bubbles have a symmetric ellipsoid shape and rise almost vertically with the same speed and without coalescence drifting an amount of liquid to the top of the column. As it is also described by Ruzicka et al. (2001), the amount of liquid carried up by the bubbles hinders the uprising bubbles on its way down, resulting in an increase of gas holdup. By increasing the gas flow rate the bubbles begin to grow in size and large bubbles appear to coexist with the smaller ones. The uprising bubbles begin to exhibit also a reciprocative movement which retards their upward movement enhancing coalescence. The above observations correspond to an intermediate transition regime. By further increasing the
1468
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
Fig. 2. Flow patterns in homogeneous (QG = 0.8 × 10−5 m3 /s) and heterogeneous (QG = 7.7 × 10−5 m3 /s) regimes for water–air (a, c) and glycerin 50.0%—air (b, d) systems (dp = 40 m).
gas flow rates (Figs. 2c and d) the heterogeneous flow regime is encountered where big gas masses, presumably formed by coalescence, begin to rise resulting in a kind of churn flow pattern. In this regime velocities display pronounced radial profiles resulting to strong circulations and enhancing bubble rise speed. This results to a further decrease of gas holdup value. A detailed and comprehensive description of the flow regimes in bubble columns is also given by Ruzicka et al. (2001). The flow patterns observed for some of the liquids used (i.e., water, 50% glycerin and 1.5% butanol) are presented in Fig. 3. It is worth noticing that the bubble shape and concentration obtained by the glycerin solutions is practically the same with that of water (Figs. 3a and b). The bubbles observed in the low surface tension non-coalescing butanol solutions (Fig. 3c) differ significantly from those encountered in the coalescence promoting media (i.e., water and glycerin solutions). The former are spherical, considerably smaller in size and hence, for a given gas flow rate, far more numerous than those of water (Papatzika, 2002). These bubbles form a kind of plume that quickly covers the whole column area providing an interfacial area much greater than that obtained with water. A radial circulation is also observed, which becomes more pronounced by increasing gas flow rate, resulting in a decrease of holdup value (transition regime). 3.2. Gas holdup In this section the measured gas holdup values are given. The flow regimes can be distinguished by plotting the average gas holdup (G ) versus the gas flow rate (QG ). Fig. 4
shows the dependence of gas holdup on corresponding gas superficial velocity for the two spargers used. A typical flow regime map (Ruzicka et al., 2001) is also included for comparison. The gas superficial velocity is defined as UGS =
QG , A
(1)
where QG is the gas flow rate and A the column cross section area. A first observation is that for the two spargers tested the gas holdup is not significantly affected by the sparger pore size, a remark that is in agreement with previous observations (e.g. Kaji et al., 2001). The first part of the curve corresponds to the homogeneous regime, where the gas holdup increases with the gas velocity. A transition regime follows where a slight decrease in gas holdup is observed. Finally, at the heterogeneous regime the gas holdup continues to increase but with a lower slope than the homogeneous regime. As it is also pointed out by Ruzicka et al. (2003), if the bubbles could travel unaffected at their terminal velocity, the gas holdup would increase linearly with the gas flow rate. In the homogeneous regime, as the gas holdup increases the hindrance progressively reduces the bubble velocity leading to a further increase of the gas holdup. The opposite holds true for the heterogeneous regime, where the bubble velocity increases in the central core of the column resulting in a decrease of the gas holdup value with gas flow rate. In Fig. 5 the data are plotted in terms of gas holdup versus gas phase superficial velocity for air–water, air–butanol and air–glycerin systems. As it is expected, gas holdup increases with gas flow rate. A slight increase in gas holdup
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
1469
Fig. 3. Characteristic photos of the homogeneous regime for various liquids (a) water, (b) glycerin 50.0% and (c) butanol 1.5% (QG = 1.5 × 10−5 m3 /s, dp = 40 m).
4.0
5 w b1
dp, µm 4
20
3.0
b2 g1
40
g2 g3
εG, %
εG, %
3 2.0
2 1.0 1 dp=40µm 0.0 0.0
0.2
0.4 0.6 UGS, cm/s
0.8
1.0
Fig. 4. Effect of pore diameter on gas holdup for the water–air system.
values is also observed when the lower surface tension butanol solutions are used as liquid phase, whereas air–water and air–glycerin solutions have almost identical gas holdup values. It is evident that the effect of viscosity is negligible. The present experimental data are compared with correlations included in a comprehensive review paper by Shah et al. (1982) and in general they are found to deviate considerably from the proposed empirical models. For instance, the prediction by the correlations of Akita and Yoshida (1973) and Hikita et al. (1980) exhibit for air–water systems a 20% deviation from the present data and those of
0 0.0
0.2
0.4
0.6
0.8
1.0
UGS, cm/s Fig. 5. Effect of type of liquid on gas holdup (dp = 40 m).
Camarasa et al. (1999) for porous spargers, whereas for glycerin (Fig. 6) and butanol solutions the corresponding deviations exceed 40%. This can be attributed to the fact that the aforementioned correlations are based on data obtained with single and multi-nozzle spargers (Shah et al., 1982). 3.3. Bubble size distributions The number frequency of the bubble size was calculated at five distances above the sparger surface (i.e., 3, 10, 20, 30
1470
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
50
3.0 µL=3.5 mPa.s
exp data
dp, µm
present work Eq.8 2.0 ε G, %
number frequency, %
Hikita et al. (1980)
1.0
0.0
h=40 cm 30
20
10
0
0.1
0.2
0.3
0.4
UGS, cm/s
(a)
0
3.0
0
exp data
µL=22.5 mPa.s
present work Eq.8 2.0
2
3 d, mm
4
5
6
εG , %
Hikita et al. (1980)
1.0
0.0
1
Fig. 7. Effect of pore diameter on bubble size distribution (water–air system, h ≈ 40 cm, QG = 0.8 × 10−5 m3 /s).
Akita & Yoshida (1973)
(b)
20 40
40
Akita & Yoshida (1973)
0
0.1
0.2 UGS, cm/s
0.3
0.4
Fig. 6. Comparison of the present experimental data with values calculated by proposed Eq. (8) and the models of Hikita et al. (1980) and Akita and Yoshida (1973) for two glycerin solutions: (a) 33.3% and (b) 66.7%. Error bars correspond to ±10 deviation.
and 40 cm) for the homogeneous regime. In Fig. 7 the bubble size distribution curve is presented for the air–water system and for both sparger types used. The measurements were performed at a height of 40 cm above the sparger surface where the flow was found to be developed. It seems that for the spargers tested the sparger pore size has practically no effect on the distribution curve and this holds true for all the liquids tested. The above observation agrees with the results of Parthasarathy and Ahmed (1996), who reported that below a sparger pore diameter of 50 m there is a negligible change in bubble size. Fig. 8 shows bubble size distributions at two locations i.e., just above the sparger surface and 40 cm away from it, for water and for the glycerin solutions. In all cases it is obvious that the form of bubble size distribution curve does not
practically change with height, an observation that is also reported by Colella et al. (1999) for air–water systems. The water data, both in the vicinity of (h ≈ 3 cm) and away from (h ≈ 40 cm) the sparger area, are well fitted by a lognormal distribution curve (Fig. 8a). Similarly, the two glycerin solutions (i.e., g1 and g2) data obtained close to the sparger follow a log-normal distribution curve (Figs. 8a and b). However, the g1 and g2 solutions data away from the sparger (h ≈ 40 cm) cannot be fitted by such type of curve because a second peak makes its appearance (presumably due to coalescence), which becomes more distinct for the higher viscosity glycerin solution (g3). In these cases the data are best fitted by the summation of two normal distribution functions (Figs. 8a and b). It must be noted that the bubble size distributions of the butanol solutions were not calculated due to the limitations of the measuring technique applied. In this case the problems arise from bubble overlapping on the focusing plane due to high bubble concentration. It is believed that the limited available data on bubble size distribution for low surface tension liquids is due to the difficulty in performing such measurements. The visual observations reveal a behavior for the butanol solutions similar to that of the low viscosity glycerin solutions. Moreover, Camarasa et al. (1999) report that for non-coalescing alcohol solutions a narrow distribution of small bubbles is observed and that the bubble size within the column is nearly the same as that of bubbles formed at the porous distributor. For the first two glycerin solutions (Figs. 8a and b) the unimodal bubble size distribution curve suggests that the majority of the bubbles have the same size. This implies that the bubbles, after detaching from the sparger surface,
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
Table 2 Mean Sauter diameter (d32 ) and standard deviation () for the 40 m pore sparger at h ≈ 40 cm
50 µL, mPa.s
h, cm 3 40 3 40
number frequency, %
40
1.0
Liquid phase
3.5 Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
30
20
0 0
1
2
(a)
3 d, mm
4
5
6
50 µL, mPa.s
h, cm 3 40 3 40
40 number frequency, %
QG = 0.8 × 10−5 m3 /s QG = 1.5 × 10−5 m3 /s d32 (mm) (mm)
d32 (mm) (mm)
3.1 1.5 1.1 2.2 2.4 3.0
3.8 1.7 1.3 2.8 3.5 4.8
0.5 a a
0.6 0.8 1.0
0.9 a a
0.8 1.0 1.5
a Not available.
10
8.2 22.5
30
20
10
0 (b)
1471
0
1
2
3 d, mm
4
5
6
Fig. 8. Bubble size distributions in the homogeneous regime (QG = 1.5 × 10−5 m3 /s) for: (a) water, glycerin 33.3% and (b) glycerin 50.0%, glycerin 66.7%.
retain their initial size and neither a coalescence nor a breakage process occurs during their course to the column exit. The bimodal distribution observed at the higher viscosity value (Fig. 8b) can be attributed to coalescence phenomena that seem to commence onto the sparger surface. The more pronounced bimodal distribution observed at h ≈ 40 cm implies that the bubbles continue to coalesce as they rise towards the liquid surface. This is in agreement with the observations of Zahradnik et al. (1997), who noticed an unfavorable effect of liquid viscosity that they ascribed to the existence of drag forces promoting bubble coalescence in
the distributor region. Finally, the broad distribution curve of the water, which retains its initial shape through the column, may be attributed to the fact that coalescence occurs onto the sparger surface but does not continue during the bubble course through the bulk of the liquid. The mean Sauter bubble diameter (d32 ), defined as N 3 i=1 di d32 = N (2) 2 i=1 di is a popular representation of the mean bubble size. Its values and the standard deviation of the drop size distribution () for all liquids as well as for two gas flow rates are given in Table 2. Values for the low surface tension solutions are only a rough estimation, because they are calculated from a series of small samples (e.g. 30 bubbles) taken from areas with low bubble concentration and for this reason values for the butanol solutions are not included in Table 2. As it is expected the mean bubble size increases with increasing gas flow rate but it is worth noticing that the increase is more pronounced for the high-viscosity glycerin solutions (30–60%) than it is for the low viscosity butanol solutions (∼ 15%). It is evident that d32 also increases with viscosity. The only exception is water which, despite its relatively low viscosity value, exhibits a d32 comparable to that of the high-viscosity glycerin solutions. It must be noted that the calculated d32 value of the glycerin solutions is much greater than the more frequent bubble diameter (< 1 mm), as it can be seen in Fig. 8, a fact that is also reported by Tse et al. (2003), who found that during coalescence the formation of large bubbles is accompanied by a great number of small daughter bubbles (100–200 m). 3.4. Homogeneous–heterogeneous regime transition A common procedure to locate the transition point between the homogeneous and heterogeneous regime is to apply the drift flux analysis, which is based on mass conservation equations and relates velocities and concentrations of the phases (Wallis, 1969). The model looks at the relative motion of the two phases and is suggested for flows with flat radial profiles. The basic quantity is the drift flux, j, which
1472
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
tion point to slightly higher velocities. The only exception is water whose transition velocity is lower than that of butanol solutions despite its slightly higher viscosity. This behavior can be attributed to the simultaneous effects of both relatively low viscosity and high surface tension. The difference between the calculated transition velocities and the ones reported in the literature (e.g. Zahradnik et al., 1997; Camarasa et al., 1999; Ruzicka et al., 2001, 2003) is not unexpected since it has been reported (e.g. Hebrard et al., 1996; Zahradnik et al., 1997) that differences in size and type of the distributor shifts the limit of the homogeneous regime.
0.010
drift flux, m/s
0.008
0.006 w b1
0.004
b2 g1 0.002
g3
trans 0.000
4. Result interpretation
g2
0
1
2
3
5
4
εG, %
a=
Fig. 9. Regime transition using the drift flux model.
Table 3 Transition between homogeneous and heterogeneous regime (dp =40 m) Liquid phase
trans (%)
Utrans (cm/s)
js (cm/s)
Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
1.2 2.2 2.2 1.1 1.1 1.2
0.20 0.23 0.23 0.29 0.28 0.28
6.4 7.3 7.3 9.2 8.9 8.9
represents the gas flux through a surface moving at the average velocity of the mixture and is given by j = UGS (1 − G ),
The optimum operating conditions of a bubble column would be the ones that enhance mass transfer and this is accomplished by maximizing the gas/liquid interfacial area, a measure of which is given by
(3)
where UGS is the superficial gas velocity and G the average gas holdup. If the drift flux is plotted versus the gas holdup, the change in the slope of the curve indicates the transition from the homogeneous to the heterogeneous regime (Shah et al., 1982). Fig. 9 presents the calculated j values (Eq. (3)) versus the corresponding gas holdup experimental data for all the liquids tested. The points where the change of the slope occurs are located and the corresponding superficial velocities (UGS ) are calculated and presented in Table 3. A general comment is that for all cases the transition occurs at a liquid superficial velocity between 0.2 and 0.3 cm/s. These superficial velocity values correspond to gas fluxes through the sparger, js , 6.0 to 9.0 cm/s (Table 3), which are in the same order of magnitude with those reported in the literature (e.g. Camarasa et al., 1999; Zahradnik et al., 1997). It has been determined that (for the two spargers employed) the transition point does not depend on the pore size. It is also evident that an increase in liquid phase viscosity shifts the transi-
6G , d32
(4)
where G is the gas holdup and d32 the mean Sauter diameter. Consequently, the homogeneous bubbly flow regime encountered at the lower gas flow rates is most desirable for mass transfer operations, since, by exhibiting a large gas holdup value accompanied by relatively small bubble size, provides a greater interfacial area. As it is already mentioned the mean bubble size depends on the liquid properties which may either promote or inhibit coalescence of the primary bubbles formed on the sparger surface. It is generally admitted that coalescence occurs in three steps, i.e., collision, liquid film drainage and rupture. When two bubbles collide, the liquid film formed by the small amount of liquid trapped between them begins to drain until it becomes sufficiently thin to be ruptured due to an instability mechanism. The above described sequence is leading to a coalesced bubble. It is also believed that the liquid film drainage is the rate controlling step while the rupturing step is almost instantaneous (Chaudhari and Hofmann, 1994). Bubble coalescence is also a function of the contact time between two bubbles that depends on the bubble rising velocity, which in turn is a function of the bubble size and the turbulence intensity. It is evident that, when a porous sparger is used, the proximity of the pores promotes coalescence on the sparger surface as soon as the bubbles enter the column. The viscosity seems to play a dual role. At relatively low viscosity values, an increase in viscosity hinders film drainage during the thinning process and thus inhibits coalescence. This remark is also supported by the shape of the distribution curves of the two lower viscosity glycerin solutions tested (Fig. 10) and the photos in Figs. 11a and b from which is evident that the bubbles away from the sparger area retain their relative monodispersity and coalescence mechanism plays a minor role. However, a further increase of
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
coalescence on the sparger surface as the bubbles enter the column. An attempt was made to formulate a correlation that would permit the prediction of gas holdup, a variable that greatly affects the bubble column operation. From the visual observations and the careful inspection of the experimental results (from various investigators) it can be concluded that the gas holdup value is the result of the interaction of several parameters, the most important of which are:
50 µL, mPa.s 1.0 3.5 8.2 22.5
number frequency, %
40
30
1473
h=40 cm
• the gas phase superficial velocity, • the physical properties of the liquid phase (i.e., surface tension, viscosity), • the column cross section and • the sparger cross section.
20
10
0 0
1
2
3 d, mm
4
5
6
Fig. 10. Effect of liquid viscosity on the form of bubble size distribution curve (QG = 0.8 × 10−5 m3 /s, h ≈ 40 cm).
liquid viscosity leads to a decrease of turbulence in the liquid phase favoring large bubble formation by coalescence (Figs. 10 and 11c) which leads to an increase of the larger bubble number in expense of the smaller ones. This explanation is also supported by other authors (e.g. Ruzicka et al., 2003). Thus at higher viscosity values the wide shape of the bubble size distribution curve implies that the smaller bubbles coalesce, an assumption also supported by the bimodal bubble size distribution observed. It is noted that Parthasarathy and Ahmed (1996) believe that in a coalescing medium bubbles continue to coalesce as they rise. The water is an exception to the above trends and the bubbles formed have larger sizes than that expected by considering only the physical properties, i.e., the mean bubble size has a higher value than that of the 50% glycerin solution whose viscosity value is 8.2 mPa s (Fig. 10). This peculiar behavior can be attributed to the simultaneous effects of both low viscosity and relatively high surface tension that favor
The experimental results of this study show that the sparger pore size used does not practically affect the holdup values, a remark that is also supported by other investigators (e.g. Parthasarathy and Ahmed, 1996; Kaji et al., 2001). It must be also noted that the use of several porous disks with the same nominal porosity during the period of the experiments proved that the data are quite reproducible and that the porous disk usage time does not significantly affect its performance. In order to formulate a generalized correlation that would incorporate the relative effect of all the above factors, dimensional analysis was performed. The results show that the effect of gas velocity and column dimensions can be taken into account by defining a Froude number: Fr =
2 UGS , dC g
(5)
where UGS is the gas superficial velocity and dC the column diameter or, in case of non-circular cross-sections, the hydraulic diameter. Similarly, the effect of the liquid phase properties can be included in the appropriate Archimedes (Ar) and Eötvos (Eo) numbers defined as follows: Ar =
dC3 2L g
2L
,
(6)
Fig. 11. Images illustrating the effect of viscosity on bubble size distribution for: (a) glycerin 33.3%, (b) glycerin 50.0% and (c) glycerin 66.7%. (QG = 0.8 × 10−5 m3 /s, dp = 40 m).
1474
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
25
20
present work Kaji et al.(2001) Camarasa etal. (1999) correlation
εG, %
15
10
5
0 103
104
105 Fr0.5Ar0.1Eo2.2
106
107
(dS/dC)
Fig. 12. Gas holdup correlation for the homogeneous regime.
Eo =
dC2 L g , L
(7)
where L , L and L are the liquid density, viscosity and surface tension respectively. Finally, the ratio (dS /dC ) of sparger to column diameter was also included to account for the different geometrical configurations of the gas entrance. The attempt to formulate a generalized relation that would be valid for both homogeneous and heterogeneous regimes was unsuccessful. Finally, a correlation is formulated that is valid only for the homogeneous regime: 2/3 dS G = 0.001 F r 0.5 Ar 0.1 Eo2.2 (8) dC and it is plotted in Fig. 12, where the data of Camarasa et al. (1999) and Kaji et al. (2001) are also included for comparison. It seems that the proposed correlation is in fairly good agreement (±10%) with all the available data for the homogeneous regime.
5. Concluding remarks In bubble column reactor design the homogeneous flow regime is usually the most desirable, because it enhances the efficiency of the equipment by providing a greater gas–liquid interfacial area. For this regime new data concerning average gas holdup values, bubble size distributions and Sauter diameters are given for a number of liquids covering a range of surface tension and viscosity values. It was found that bubble size depends on the gas flow rate and is affected by the liquid properties and that an increase in gas flow rate increases bubble collision probability resulting in greater
bubble sizes. An increase in liquid viscosity favors larger bubble formation by decreasing turbulence, a fact that both promotes bubble coalescence and hinders breakage. On the other hand, an increase in liquid surface tension favors small bubble formation by promoting breakage and demoting coalescence. The bubble size distribution data are generally unimodal. Only for the relatively high-viscosity liquids a second peak arises as a result of bubble coalescence, and therefore the data are best fitted by the summation of two normal distribution functions. It is found that for the geometry studied the superficial velocity that marks the homogeneous regime limit does not exceed 0.3 cm/s. Finally, a new correlation based on dimensionless groups for the prediction of gas holdup in the homogeneous regime is proposed and found to be in good agreement with available data. It is generally accepted that porous plates hold advantages over the other types of gas distributors used. It has been also proved that bubble size distribution in bubble columns equipped with fine porous spargers depends mostly on coalescence/breakage phenomena, which take place either directly onto or in the vicinity of the sparger surface. Delhaye and McLaughlin (2003) stated that both experimental and theoretical analyses are needed in order to establish rigorous criteria for the coalescence and breakage of fluid objects at the microscopic level. Consequently, future experimental work must be focused on the phenomena occurring onto the sparger surface with the intention to gain an insight on the bubble formation mechanisms. Moreover, the knowledge of the range of parameters over which a particular regime is encountered and the conditions under which a transition occurs would facilitate the design of a bubble column reactor. References Akita, K., Yoshida, F., 1973. Gas Holdup and Volumetric Mass Transfer Coefficient in Bubble Columns. Ind. Eng. Chem. Proc. Des. Dev. 12, 76–80. Camarasa, E., Vial, C., Poncin, S., Wild, G., Midoux, N., Bouillard, J., 1999. Influence of coalescence behaviour of the liquid and of gas sparging on hydrodynamics and bubble characteristics in a bubble column. Chemical Engineering and Processing 38, 329–344. Chaudhari, R.V., Hofmann, H., 1994. Coalescence of gas bubbles in liquids. Reviews in Chemical Engineering 10 (2), Colella, D., Vinci, D., Bagatin, R., Masi, M., Bakr, E.A., 1999. A study on coalescence and breakage mechanisms in three different bubble columns. Chemical Engineering Science 54, 4767–4777. Deckwer, W.-D., Schumpe, A., 1993. Improved tools for bubble column reactors and scale up. Chemical Engineering Science 48, 889–911. Delhaye, J.-M., McLaughlin, J.B., 2003. Appendix 4: report of study group on microphysics. International Journal of Multiphase Flow 29, 1101–1116. Hebrard, G., Bastoul, D., Roustan, M., 1996. Influence of gas sparger on the hydrodynamic behaviour of bubble columns. Transactions of the Institution of Chemical Engineers 74, 406–414. Hikita, H., Asai, S., Tanigawa, K., Segawa, K., Kitao, M., 1980. Gas Holdup in Bubble Column. Chemical Engineering Journal 20, 59–67. Joshi, J.B., Vitankar, V.S., Kulkarni, A.A., Dhotre, M.T., Ekambara, K., 2002. Coherent flow structures in bubble column reactors. Chemical Engineering Science 57, 3157–3183.
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475 Kaji, M., Sawai, T., Mori, K., Iguchi, M., 2001. Behaviors of bubble formation from a bottom porous nozzle bath. Proceedings of the 5th ExHFT, Thessaloniki, Greece. Parasu Veera, U., Joshi, J.B., 1999. Measurement of gas hold-up profiles by gamma ray tomography: effect of sparger design and height of dispersion in bubble columns. Transactions of the Institution of Chemical Engineers 77, 303–317. Papatzika, E., 2002. Experimental study of a bubble column. Diploma Thesis, Department of Chemical Engineering, AUTh (in Greek). Parthasarathy, R., Ahmed, N., 1996. Size distribution of bubbles generated by fine pore spargers. Journal of Chemical Engineering of Japan 29, 1030–1034. Polli, M., Di Stanislao, M., Bagatin, R., AbuBakr, E., Masi, M., 2002. Bubble size distribution in the sparger region of bubble columns. Chemical Engineering Science 57, 197–205. Ruzicka, M.C., Zahradnik, J., Drahos, J., Thomas, N.H., 2001. Homogeneous–heterogeneous regime transition in bubble columns. Chemical Engineering Science 56, 4609–4626.
1475
Ruzicka, M.C., Drahos, J., Mena, P.C., Teixeira, J.A., 2003. Effect of viscosity on homogeneous–heterogeneous regime transition in bubble columns. Chemical Engineering Journal 96, 15–22. Shah, Y.T., Kelkar, B.G., Godbole, S.P., Deckwer, W.-D., 1982. Design parameters estimations for bubble column reactors. A.I.Ch.E. Journal 28, 353–379. Tse, K.L., Martin, T., McFarlane, C.M., Nienow, A.W., 2003. Small bubble formation via a coalescence dependent break-up mechanism. Chemical Engineering Science 58, 275–286. Wallis, G.B., 1969. One-Dimensional Two-Phase Flow, McGraw-Hill, New York. Zahradnik, J., Fialova, M., Ruzicka, M., Drahos, J., Kastanek, F., Thomas, N.H., 1997. Duality of the gas–liquid flow regimes in bubble column reactors. Chemical Engineering Science 52, 3811–3826.
Effect of liquid properties on the performance of bubble column reactors with fine pore spargers A.A. Mouza, G.K. Dalakoglou, S.V. Paras∗ Department of Chemical Engineering, Aristotle University of Thessaloniki Univ.Box 455, GR 54124 Thessaloniki, Greece Received 4 November 2003; received in revised form 20 September 2004; accepted 7 October 2004
Abstract This work is a study of the effect of liquid properties on the performance of bubble column reactors with fine pore spargers. Various liquids covering a range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video technique is used for visual observations and, combined with image processing, is used for gas holdup and bubble size measurements. New data on average gas holdup values, bubble size distributions and Sauter diameters are presented and are consistent with existing physical models on coalescence/breakage. A correlation based on dimensionless groups for the prediction of gas holdup in the homogeneous regime is proposed and found to be in good agreement with available data. 䉷 2004 Elsevier Ltd. All rights reserved. Keywords: Bubble column; Porous sparger; Gas holdup; Bubble size; Coalescence; Liquid properties; Flow regimes
1. Introduction Bubble columns are widely used in industrial gas–liquid operations (e.g. gas/liquid reactions, agitation by gas injection, fermentations, etc.) in chemical and biochemical process industries, due to their simple construction, low operating cost and high-energy efficiency. In all these processes gas holdup and bubble size are important design parameters, since they define the gas–liquid interfacial area available for mass transfer. In turn, bubble size distribution and gas holdup in gas–liquid dispersions depend largely on column geometry, operating conditions, physico-chemical properties of the two phases and type of gas sparger (Camarasa et al., 1999). The design of bubble columns has primarily been carried out by means of empirical or semi-empirical correlations based mainly on experimental data. Since the multiphase flow is in general complex in structure, the design and scale up of such type of equipment is still a difficult task and subject to errors (Deckwer and Schumpe, 1993). ∗ Corresponding author. Tel.: +30 23 10 996174; fax: +30 23 10 996209.
E-mail address: [email protected] (S.V. Paras). 0009-2509/$ - see front matter 䉷 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.10.013
Despite the extensive and long lasting study of bubble column performance many basic questions about the effect of important operational parameters remain unanswered. For example, although bubble column characteristics have been studied extensively in the past few decades, there is still considerable uncertainty concerning the prevailing mechanisms of bubble formation, as well as the most appropriate correlations for practical applications. Break-up and coalescence of fluid objects play a crucial role in a broad spectrum of multiphase flow processes, such as the evolution of the bubble size distribution in stirred tanks and bubble columns (Delhaye and McLaughlin, 2003). Shah et al. (1982) and Parasu Veera and Joshi (1999) have reviewed and summarized the work done in this area. It is generally accepted that, depending on the gas flow rate, two main flow regimes can be readily observed in bubble columns, i.e., the homogeneous bubbly flow regime encountered at low gas velocities and characterized by a narrow bubble size distribution and radially uniform gas holdup; and the heterogeneous (churnturbulent flow) regime observed at higher gas velocities and characterized by the appearance of large bubbles, formed by coalescence of the small bubbles and bearing a higher rise velocity hence leading to relatively lower gas holdup values
1466
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
(Joshi et al., 2002; Camarasa et al., 1999; Zahradnik et al., 1997). The two regimes differ from one another in their hydrodynamic and transport characteristics. Depending on the type of the gas distributor and the properties of the liquid phase, both regimes can be obtained in the same equipment by varying the gas input flow rate. Various types of gas spargers, the most common of which are perforated plate, membrane and fine porous plate, are in use. Among the above sparger types, the perforated plate requires a minimum gas velocity in order to produce a uniform bubble distribution over the whole sparger area. On the other hand, bubble columns equipped with porous spargers offer a greater gas/liquid contact area for efficient mass and heat transfer, because the bubbles created by this type of gas distributor are numerous and far smaller. Finally, the membrane, even though it is able to maintain a homogeneous flow up to greater flow rates, generates a greater pressure drop (Hebrard et al., 1996). As a result, the use of a fine porous plate as a gas sparger seems to be advantageous over the other types of multiple injection point distributors. To the authors’ best knowledge, limited information is available in the literature regarding the behavior of fine porous plate spargers. Hebrard et al. (1996), who studied the influence of gas sparger characteristics on the hydrodynamic behavior of bubble columns, concluded that bubble size and holdup values depend primarily on the physico-chemical properties of the liquid phase and the type of gas distributor. On the other hand, Parthasarathy and Ahmed (1996), who conducted their experiments with non-coalescing liquids, stated that there is confusion regarding the average size value and size distribution of bubbles generated by fine pore spargers. Zahradnik et al. (1997) investigated the effect of various parameters (i.e., column geometry, distributor type, liquid properties) on the gas–liquid flow regime stability and gas holdup in bubble column reactors equipped, among others, with porous spargers. Camarasa et al. (1999), who examined the influence of the liquid properties and of the gas sparging method on hydrodynamics and bubble characteristics in a bubble column, used water-alcohol solutions to simulate the behavior of non-coalescing organic liquids and compared their data with those concerning standard air–water systems. Kaji et al. (2001) studied experimentally the behavior of bubble formation using various porous spargers with pore diameter 5–400 m and investigated the effect of surface tension on the gas holdup distribution. The purpose of this work is to study the effect of liquid properties on bubble size distribution in a bubble column equipped with two different fine porous spargers. Various liquids covering a range of surface tension and viscosity values are employed, while atmospheric air is used as the gas phase for all experiments. Experimental data on average gas holdup, bubble size distribution and mean Sauter diameter, obtained from image analysis of fast video recordings, are
reported. A correlation is also proposed for the prediction of average gas holdup.
2. Experimental set-up and procedures The experimental set-up (Fig. 1) consists of a vertical rectangular Plexiglas䉸 column 1.5 m height, having a square cross-section (side length 10 cm). The column is equipped with appropriate rotameters for gas phase flow measurement and control. The rectangular geometry was preferred over the cylindrical one, because it facilitates both the direct flow visualization and the use of optical measuring methods by minimizing optical distortion. For the injection and uniform distribution of the gas phase, a gas sparger, i.e., a round metal porous disk, 2.0 cm in diameter, is installed at the center of the bottom plate. In the present experiments, two 316L SS porous disks (made by Mott Corp.) with nominal pore size of 20 and 40 m were alternatively used as gas spargers. However, in order to check the effect of the usage time on sparger performance various porous disks were employed during the period of the experiments. Several liquids, whose physical properties are presented in Table 1, were employed as liquid phase, whereas the gas phase was atmospheric air for all runs. All the experiments were conducted at ambient pressure and temperature conditions. Each experimental run started by first filling the column with the appropriate liquid phase up to 80 cm above the sparger. All the experiments were performed with no liquid throughput, while the gas phase was injected and distributed into the liquid phase by passing through the porous disk sparger. A high-speed digital video camera (Redlake MotionScope PCI 䉸 1000S) is used both for direct flow visualization and for bubble size and gas holdup measurements. The camera is fixed on a stand very close to the area of observation in such a way that the test section is located between the camera and an appropriate lighting system placed behind a diffuser to evenly distribute the light. Although the imaging system used was capable of recording up to 1000 full frames per second, a speed of 500 fps is considered a suitable recording rate for the present experiments. The shutter rate employed was of 1/10 000. It must be pointed out that the optical system used offers a very narrow depth of field (few mm). The recorded images were also used to extract quantitative information on bubble size distribution and gas holdup values. Using proper lighting, the gas–liquid interface around the bubble circumference can be clearly outlined on the pictures. The calibration of the measuring system, needed to ensure the accurate measurement of the bubbles, is accomplished by measuring a microscale placed at the focusing plane. Subsequent image processing (e.g. noise reduction, brightness improvement, contrast enhancement, shadow and double images removal) results to a sharp bubble–liquid interface. A detailed description of the technique can be also
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
1467
Fig. 1. Experimental set-up.
Table 1 Liquid phase properties at 20 ◦ C Index Liquid phase
w b1 b2 g1 g2 g3
Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
3. Results
Viscosity, L (mPa s)
Density, L (kg/m3 )
Surface tension, L (mN/m)
1.0 0.9 0.9 3.5 8.2 22.5
998 994 991 1081 1126 1173
72 60 48 70 68 67
found in a recent paper by Colella et al. (1999). Image analysis, using appropriate software (SigmaScan Pro䉸 ), allows bubble size prediction. The bubbles were approximated by ellipses whose major and minor axes were computed by the software. The equivalent diameter of a sphere with the same volume as the ellipsoid was computed (Colella et al., 1999; Polli et al., 2002). Approximately 800 bubbles were measured in each experimental run, a number considered to be adequate for statistical calculations (Tse et al., 2003; Hebrard et al., 1996). The advantage of the method is that it permits both in situ and non-intrusive measurements. The uncertainty of the measurements has been estimated to be less than 10%. The average gas holdup is estimated by the bed expansion. The liquid level is measured on two different pictures taken prior to gas inflow and after gas is injected and steady state is established. The difference in liquid level, measured by superposition of the two pictures, gives a measure of the average gas holdup. The uncertainty of the measurements is estimated to be less than 15%.
3.1. Visual observations As already mentioned, depending on the gas flow rate, the two flow regimes observed in bubble columns are the homogeneous bubbly flow regime encountered at low gas velocities and the heterogeneous (churn-turbulent flow) regime observed at higher gas velocities. The photos in Fig. 2 give a visual image of the flow patterns observed in a bubble column according to the gas flow rate when the liquid phase is either water or 50% glycerin solution. At the beginning the bubbles are gathered at the core of the flow, but after the first 20 cm they are spread uniformly covering the whole column area. It is also worth noticing that after the first 40 cm of the column height the gas phase distribution does not seem to change significantly. For the lower gas velocities applied the homogeneous flow regime is encountered, where relatively small gas bubbles are formed and almost uniformly distributed throughout the whole column area (Figs. 2a and b). The bubbles have a symmetric ellipsoid shape and rise almost vertically with the same speed and without coalescence drifting an amount of liquid to the top of the column. As it is also described by Ruzicka et al. (2001), the amount of liquid carried up by the bubbles hinders the uprising bubbles on its way down, resulting in an increase of gas holdup. By increasing the gas flow rate the bubbles begin to grow in size and large bubbles appear to coexist with the smaller ones. The uprising bubbles begin to exhibit also a reciprocative movement which retards their upward movement enhancing coalescence. The above observations correspond to an intermediate transition regime. By further increasing the
1468
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
Fig. 2. Flow patterns in homogeneous (QG = 0.8 × 10−5 m3 /s) and heterogeneous (QG = 7.7 × 10−5 m3 /s) regimes for water–air (a, c) and glycerin 50.0%—air (b, d) systems (dp = 40 m).
gas flow rates (Figs. 2c and d) the heterogeneous flow regime is encountered where big gas masses, presumably formed by coalescence, begin to rise resulting in a kind of churn flow pattern. In this regime velocities display pronounced radial profiles resulting to strong circulations and enhancing bubble rise speed. This results to a further decrease of gas holdup value. A detailed and comprehensive description of the flow regimes in bubble columns is also given by Ruzicka et al. (2001). The flow patterns observed for some of the liquids used (i.e., water, 50% glycerin and 1.5% butanol) are presented in Fig. 3. It is worth noticing that the bubble shape and concentration obtained by the glycerin solutions is practically the same with that of water (Figs. 3a and b). The bubbles observed in the low surface tension non-coalescing butanol solutions (Fig. 3c) differ significantly from those encountered in the coalescence promoting media (i.e., water and glycerin solutions). The former are spherical, considerably smaller in size and hence, for a given gas flow rate, far more numerous than those of water (Papatzika, 2002). These bubbles form a kind of plume that quickly covers the whole column area providing an interfacial area much greater than that obtained with water. A radial circulation is also observed, which becomes more pronounced by increasing gas flow rate, resulting in a decrease of holdup value (transition regime). 3.2. Gas holdup In this section the measured gas holdup values are given. The flow regimes can be distinguished by plotting the average gas holdup (G ) versus the gas flow rate (QG ). Fig. 4
shows the dependence of gas holdup on corresponding gas superficial velocity for the two spargers used. A typical flow regime map (Ruzicka et al., 2001) is also included for comparison. The gas superficial velocity is defined as UGS =
QG , A
(1)
where QG is the gas flow rate and A the column cross section area. A first observation is that for the two spargers tested the gas holdup is not significantly affected by the sparger pore size, a remark that is in agreement with previous observations (e.g. Kaji et al., 2001). The first part of the curve corresponds to the homogeneous regime, where the gas holdup increases with the gas velocity. A transition regime follows where a slight decrease in gas holdup is observed. Finally, at the heterogeneous regime the gas holdup continues to increase but with a lower slope than the homogeneous regime. As it is also pointed out by Ruzicka et al. (2003), if the bubbles could travel unaffected at their terminal velocity, the gas holdup would increase linearly with the gas flow rate. In the homogeneous regime, as the gas holdup increases the hindrance progressively reduces the bubble velocity leading to a further increase of the gas holdup. The opposite holds true for the heterogeneous regime, where the bubble velocity increases in the central core of the column resulting in a decrease of the gas holdup value with gas flow rate. In Fig. 5 the data are plotted in terms of gas holdup versus gas phase superficial velocity for air–water, air–butanol and air–glycerin systems. As it is expected, gas holdup increases with gas flow rate. A slight increase in gas holdup
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
1469
Fig. 3. Characteristic photos of the homogeneous regime for various liquids (a) water, (b) glycerin 50.0% and (c) butanol 1.5% (QG = 1.5 × 10−5 m3 /s, dp = 40 m).
4.0
5 w b1
dp, µm 4
20
3.0
b2 g1
40
g2 g3
εG, %
εG, %
3 2.0
2 1.0 1 dp=40µm 0.0 0.0
0.2
0.4 0.6 UGS, cm/s
0.8
1.0
Fig. 4. Effect of pore diameter on gas holdup for the water–air system.
values is also observed when the lower surface tension butanol solutions are used as liquid phase, whereas air–water and air–glycerin solutions have almost identical gas holdup values. It is evident that the effect of viscosity is negligible. The present experimental data are compared with correlations included in a comprehensive review paper by Shah et al. (1982) and in general they are found to deviate considerably from the proposed empirical models. For instance, the prediction by the correlations of Akita and Yoshida (1973) and Hikita et al. (1980) exhibit for air–water systems a 20% deviation from the present data and those of
0 0.0
0.2
0.4
0.6
0.8
1.0
UGS, cm/s Fig. 5. Effect of type of liquid on gas holdup (dp = 40 m).
Camarasa et al. (1999) for porous spargers, whereas for glycerin (Fig. 6) and butanol solutions the corresponding deviations exceed 40%. This can be attributed to the fact that the aforementioned correlations are based on data obtained with single and multi-nozzle spargers (Shah et al., 1982). 3.3. Bubble size distributions The number frequency of the bubble size was calculated at five distances above the sparger surface (i.e., 3, 10, 20, 30
1470
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
50
3.0 µL=3.5 mPa.s
exp data
dp, µm
present work Eq.8 2.0 ε G, %
number frequency, %
Hikita et al. (1980)
1.0
0.0
h=40 cm 30
20
10
0
0.1
0.2
0.3
0.4
UGS, cm/s
(a)
0
3.0
0
exp data
µL=22.5 mPa.s
present work Eq.8 2.0
2
3 d, mm
4
5
6
εG , %
Hikita et al. (1980)
1.0
0.0
1
Fig. 7. Effect of pore diameter on bubble size distribution (water–air system, h ≈ 40 cm, QG = 0.8 × 10−5 m3 /s).
Akita & Yoshida (1973)
(b)
20 40
40
Akita & Yoshida (1973)
0
0.1
0.2 UGS, cm/s
0.3
0.4
Fig. 6. Comparison of the present experimental data with values calculated by proposed Eq. (8) and the models of Hikita et al. (1980) and Akita and Yoshida (1973) for two glycerin solutions: (a) 33.3% and (b) 66.7%. Error bars correspond to ±10 deviation.
and 40 cm) for the homogeneous regime. In Fig. 7 the bubble size distribution curve is presented for the air–water system and for both sparger types used. The measurements were performed at a height of 40 cm above the sparger surface where the flow was found to be developed. It seems that for the spargers tested the sparger pore size has practically no effect on the distribution curve and this holds true for all the liquids tested. The above observation agrees with the results of Parthasarathy and Ahmed (1996), who reported that below a sparger pore diameter of 50 m there is a negligible change in bubble size. Fig. 8 shows bubble size distributions at two locations i.e., just above the sparger surface and 40 cm away from it, for water and for the glycerin solutions. In all cases it is obvious that the form of bubble size distribution curve does not
practically change with height, an observation that is also reported by Colella et al. (1999) for air–water systems. The water data, both in the vicinity of (h ≈ 3 cm) and away from (h ≈ 40 cm) the sparger area, are well fitted by a lognormal distribution curve (Fig. 8a). Similarly, the two glycerin solutions (i.e., g1 and g2) data obtained close to the sparger follow a log-normal distribution curve (Figs. 8a and b). However, the g1 and g2 solutions data away from the sparger (h ≈ 40 cm) cannot be fitted by such type of curve because a second peak makes its appearance (presumably due to coalescence), which becomes more distinct for the higher viscosity glycerin solution (g3). In these cases the data are best fitted by the summation of two normal distribution functions (Figs. 8a and b). It must be noted that the bubble size distributions of the butanol solutions were not calculated due to the limitations of the measuring technique applied. In this case the problems arise from bubble overlapping on the focusing plane due to high bubble concentration. It is believed that the limited available data on bubble size distribution for low surface tension liquids is due to the difficulty in performing such measurements. The visual observations reveal a behavior for the butanol solutions similar to that of the low viscosity glycerin solutions. Moreover, Camarasa et al. (1999) report that for non-coalescing alcohol solutions a narrow distribution of small bubbles is observed and that the bubble size within the column is nearly the same as that of bubbles formed at the porous distributor. For the first two glycerin solutions (Figs. 8a and b) the unimodal bubble size distribution curve suggests that the majority of the bubbles have the same size. This implies that the bubbles, after detaching from the sparger surface,
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
Table 2 Mean Sauter diameter (d32 ) and standard deviation () for the 40 m pore sparger at h ≈ 40 cm
50 µL, mPa.s
h, cm 3 40 3 40
number frequency, %
40
1.0
Liquid phase
3.5 Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
30
20
0 0
1
2
(a)
3 d, mm
4
5
6
50 µL, mPa.s
h, cm 3 40 3 40
40 number frequency, %
QG = 0.8 × 10−5 m3 /s QG = 1.5 × 10−5 m3 /s d32 (mm) (mm)
d32 (mm) (mm)
3.1 1.5 1.1 2.2 2.4 3.0
3.8 1.7 1.3 2.8 3.5 4.8
0.5 a a
0.6 0.8 1.0
0.9 a a
0.8 1.0 1.5
a Not available.
10
8.2 22.5
30
20
10
0 (b)
1471
0
1
2
3 d, mm
4
5
6
Fig. 8. Bubble size distributions in the homogeneous regime (QG = 1.5 × 10−5 m3 /s) for: (a) water, glycerin 33.3% and (b) glycerin 50.0%, glycerin 66.7%.
retain their initial size and neither a coalescence nor a breakage process occurs during their course to the column exit. The bimodal distribution observed at the higher viscosity value (Fig. 8b) can be attributed to coalescence phenomena that seem to commence onto the sparger surface. The more pronounced bimodal distribution observed at h ≈ 40 cm implies that the bubbles continue to coalesce as they rise towards the liquid surface. This is in agreement with the observations of Zahradnik et al. (1997), who noticed an unfavorable effect of liquid viscosity that they ascribed to the existence of drag forces promoting bubble coalescence in
the distributor region. Finally, the broad distribution curve of the water, which retains its initial shape through the column, may be attributed to the fact that coalescence occurs onto the sparger surface but does not continue during the bubble course through the bulk of the liquid. The mean Sauter bubble diameter (d32 ), defined as N 3 i=1 di d32 = N (2) 2 i=1 di is a popular representation of the mean bubble size. Its values and the standard deviation of the drop size distribution () for all liquids as well as for two gas flow rates are given in Table 2. Values for the low surface tension solutions are only a rough estimation, because they are calculated from a series of small samples (e.g. 30 bubbles) taken from areas with low bubble concentration and for this reason values for the butanol solutions are not included in Table 2. As it is expected the mean bubble size increases with increasing gas flow rate but it is worth noticing that the increase is more pronounced for the high-viscosity glycerin solutions (30–60%) than it is for the low viscosity butanol solutions (∼ 15%). It is evident that d32 also increases with viscosity. The only exception is water which, despite its relatively low viscosity value, exhibits a d32 comparable to that of the high-viscosity glycerin solutions. It must be noted that the calculated d32 value of the glycerin solutions is much greater than the more frequent bubble diameter (< 1 mm), as it can be seen in Fig. 8, a fact that is also reported by Tse et al. (2003), who found that during coalescence the formation of large bubbles is accompanied by a great number of small daughter bubbles (100–200 m). 3.4. Homogeneous–heterogeneous regime transition A common procedure to locate the transition point between the homogeneous and heterogeneous regime is to apply the drift flux analysis, which is based on mass conservation equations and relates velocities and concentrations of the phases (Wallis, 1969). The model looks at the relative motion of the two phases and is suggested for flows with flat radial profiles. The basic quantity is the drift flux, j, which
1472
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
tion point to slightly higher velocities. The only exception is water whose transition velocity is lower than that of butanol solutions despite its slightly higher viscosity. This behavior can be attributed to the simultaneous effects of both relatively low viscosity and high surface tension. The difference between the calculated transition velocities and the ones reported in the literature (e.g. Zahradnik et al., 1997; Camarasa et al., 1999; Ruzicka et al., 2001, 2003) is not unexpected since it has been reported (e.g. Hebrard et al., 1996; Zahradnik et al., 1997) that differences in size and type of the distributor shifts the limit of the homogeneous regime.
0.010
drift flux, m/s
0.008
0.006 w b1
0.004
b2 g1 0.002
g3
trans 0.000
4. Result interpretation
g2
0
1
2
3
5
4
εG, %
a=
Fig. 9. Regime transition using the drift flux model.
Table 3 Transition between homogeneous and heterogeneous regime (dp =40 m) Liquid phase
trans (%)
Utrans (cm/s)
js (cm/s)
Water n-butanol 0.6% w/w n-butanol 1.5% w/w Glycerin 33.3% w/w Glycerin 50.0% w/w Glycerin 66.7% w/w
1.2 2.2 2.2 1.1 1.1 1.2
0.20 0.23 0.23 0.29 0.28 0.28
6.4 7.3 7.3 9.2 8.9 8.9
represents the gas flux through a surface moving at the average velocity of the mixture and is given by j = UGS (1 − G ),
The optimum operating conditions of a bubble column would be the ones that enhance mass transfer and this is accomplished by maximizing the gas/liquid interfacial area, a measure of which is given by
(3)
where UGS is the superficial gas velocity and G the average gas holdup. If the drift flux is plotted versus the gas holdup, the change in the slope of the curve indicates the transition from the homogeneous to the heterogeneous regime (Shah et al., 1982). Fig. 9 presents the calculated j values (Eq. (3)) versus the corresponding gas holdup experimental data for all the liquids tested. The points where the change of the slope occurs are located and the corresponding superficial velocities (UGS ) are calculated and presented in Table 3. A general comment is that for all cases the transition occurs at a liquid superficial velocity between 0.2 and 0.3 cm/s. These superficial velocity values correspond to gas fluxes through the sparger, js , 6.0 to 9.0 cm/s (Table 3), which are in the same order of magnitude with those reported in the literature (e.g. Camarasa et al., 1999; Zahradnik et al., 1997). It has been determined that (for the two spargers employed) the transition point does not depend on the pore size. It is also evident that an increase in liquid phase viscosity shifts the transi-
6G , d32
(4)
where G is the gas holdup and d32 the mean Sauter diameter. Consequently, the homogeneous bubbly flow regime encountered at the lower gas flow rates is most desirable for mass transfer operations, since, by exhibiting a large gas holdup value accompanied by relatively small bubble size, provides a greater interfacial area. As it is already mentioned the mean bubble size depends on the liquid properties which may either promote or inhibit coalescence of the primary bubbles formed on the sparger surface. It is generally admitted that coalescence occurs in three steps, i.e., collision, liquid film drainage and rupture. When two bubbles collide, the liquid film formed by the small amount of liquid trapped between them begins to drain until it becomes sufficiently thin to be ruptured due to an instability mechanism. The above described sequence is leading to a coalesced bubble. It is also believed that the liquid film drainage is the rate controlling step while the rupturing step is almost instantaneous (Chaudhari and Hofmann, 1994). Bubble coalescence is also a function of the contact time between two bubbles that depends on the bubble rising velocity, which in turn is a function of the bubble size and the turbulence intensity. It is evident that, when a porous sparger is used, the proximity of the pores promotes coalescence on the sparger surface as soon as the bubbles enter the column. The viscosity seems to play a dual role. At relatively low viscosity values, an increase in viscosity hinders film drainage during the thinning process and thus inhibits coalescence. This remark is also supported by the shape of the distribution curves of the two lower viscosity glycerin solutions tested (Fig. 10) and the photos in Figs. 11a and b from which is evident that the bubbles away from the sparger area retain their relative monodispersity and coalescence mechanism plays a minor role. However, a further increase of
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
coalescence on the sparger surface as the bubbles enter the column. An attempt was made to formulate a correlation that would permit the prediction of gas holdup, a variable that greatly affects the bubble column operation. From the visual observations and the careful inspection of the experimental results (from various investigators) it can be concluded that the gas holdup value is the result of the interaction of several parameters, the most important of which are:
50 µL, mPa.s 1.0 3.5 8.2 22.5
number frequency, %
40
30
1473
h=40 cm
• the gas phase superficial velocity, • the physical properties of the liquid phase (i.e., surface tension, viscosity), • the column cross section and • the sparger cross section.
20
10
0 0
1
2
3 d, mm
4
5
6
Fig. 10. Effect of liquid viscosity on the form of bubble size distribution curve (QG = 0.8 × 10−5 m3 /s, h ≈ 40 cm).
liquid viscosity leads to a decrease of turbulence in the liquid phase favoring large bubble formation by coalescence (Figs. 10 and 11c) which leads to an increase of the larger bubble number in expense of the smaller ones. This explanation is also supported by other authors (e.g. Ruzicka et al., 2003). Thus at higher viscosity values the wide shape of the bubble size distribution curve implies that the smaller bubbles coalesce, an assumption also supported by the bimodal bubble size distribution observed. It is noted that Parthasarathy and Ahmed (1996) believe that in a coalescing medium bubbles continue to coalesce as they rise. The water is an exception to the above trends and the bubbles formed have larger sizes than that expected by considering only the physical properties, i.e., the mean bubble size has a higher value than that of the 50% glycerin solution whose viscosity value is 8.2 mPa s (Fig. 10). This peculiar behavior can be attributed to the simultaneous effects of both low viscosity and relatively high surface tension that favor
The experimental results of this study show that the sparger pore size used does not practically affect the holdup values, a remark that is also supported by other investigators (e.g. Parthasarathy and Ahmed, 1996; Kaji et al., 2001). It must be also noted that the use of several porous disks with the same nominal porosity during the period of the experiments proved that the data are quite reproducible and that the porous disk usage time does not significantly affect its performance. In order to formulate a generalized correlation that would incorporate the relative effect of all the above factors, dimensional analysis was performed. The results show that the effect of gas velocity and column dimensions can be taken into account by defining a Froude number: Fr =
2 UGS , dC g
(5)
where UGS is the gas superficial velocity and dC the column diameter or, in case of non-circular cross-sections, the hydraulic diameter. Similarly, the effect of the liquid phase properties can be included in the appropriate Archimedes (Ar) and Eötvos (Eo) numbers defined as follows: Ar =
dC3 2L g
2L
,
(6)
Fig. 11. Images illustrating the effect of viscosity on bubble size distribution for: (a) glycerin 33.3%, (b) glycerin 50.0% and (c) glycerin 66.7%. (QG = 0.8 × 10−5 m3 /s, dp = 40 m).
1474
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475
25
20
present work Kaji et al.(2001) Camarasa etal. (1999) correlation
εG, %
15
10
5
0 103
104
105 Fr0.5Ar0.1Eo2.2
106
107
(dS/dC)
Fig. 12. Gas holdup correlation for the homogeneous regime.
Eo =
dC2 L g , L
(7)
where L , L and L are the liquid density, viscosity and surface tension respectively. Finally, the ratio (dS /dC ) of sparger to column diameter was also included to account for the different geometrical configurations of the gas entrance. The attempt to formulate a generalized relation that would be valid for both homogeneous and heterogeneous regimes was unsuccessful. Finally, a correlation is formulated that is valid only for the homogeneous regime: 2/3 dS G = 0.001 F r 0.5 Ar 0.1 Eo2.2 (8) dC and it is plotted in Fig. 12, where the data of Camarasa et al. (1999) and Kaji et al. (2001) are also included for comparison. It seems that the proposed correlation is in fairly good agreement (±10%) with all the available data for the homogeneous regime.
5. Concluding remarks In bubble column reactor design the homogeneous flow regime is usually the most desirable, because it enhances the efficiency of the equipment by providing a greater gas–liquid interfacial area. For this regime new data concerning average gas holdup values, bubble size distributions and Sauter diameters are given for a number of liquids covering a range of surface tension and viscosity values. It was found that bubble size depends on the gas flow rate and is affected by the liquid properties and that an increase in gas flow rate increases bubble collision probability resulting in greater
bubble sizes. An increase in liquid viscosity favors larger bubble formation by decreasing turbulence, a fact that both promotes bubble coalescence and hinders breakage. On the other hand, an increase in liquid surface tension favors small bubble formation by promoting breakage and demoting coalescence. The bubble size distribution data are generally unimodal. Only for the relatively high-viscosity liquids a second peak arises as a result of bubble coalescence, and therefore the data are best fitted by the summation of two normal distribution functions. It is found that for the geometry studied the superficial velocity that marks the homogeneous regime limit does not exceed 0.3 cm/s. Finally, a new correlation based on dimensionless groups for the prediction of gas holdup in the homogeneous regime is proposed and found to be in good agreement with available data. It is generally accepted that porous plates hold advantages over the other types of gas distributors used. It has been also proved that bubble size distribution in bubble columns equipped with fine porous spargers depends mostly on coalescence/breakage phenomena, which take place either directly onto or in the vicinity of the sparger surface. Delhaye and McLaughlin (2003) stated that both experimental and theoretical analyses are needed in order to establish rigorous criteria for the coalescence and breakage of fluid objects at the microscopic level. Consequently, future experimental work must be focused on the phenomena occurring onto the sparger surface with the intention to gain an insight on the bubble formation mechanisms. Moreover, the knowledge of the range of parameters over which a particular regime is encountered and the conditions under which a transition occurs would facilitate the design of a bubble column reactor. References Akita, K., Yoshida, F., 1973. Gas Holdup and Volumetric Mass Transfer Coefficient in Bubble Columns. Ind. Eng. Chem. Proc. Des. Dev. 12, 76–80. Camarasa, E., Vial, C., Poncin, S., Wild, G., Midoux, N., Bouillard, J., 1999. Influence of coalescence behaviour of the liquid and of gas sparging on hydrodynamics and bubble characteristics in a bubble column. Chemical Engineering and Processing 38, 329–344. Chaudhari, R.V., Hofmann, H., 1994. Coalescence of gas bubbles in liquids. Reviews in Chemical Engineering 10 (2), Colella, D., Vinci, D., Bagatin, R., Masi, M., Bakr, E.A., 1999. A study on coalescence and breakage mechanisms in three different bubble columns. Chemical Engineering Science 54, 4767–4777. Deckwer, W.-D., Schumpe, A., 1993. Improved tools for bubble column reactors and scale up. Chemical Engineering Science 48, 889–911. Delhaye, J.-M., McLaughlin, J.B., 2003. Appendix 4: report of study group on microphysics. International Journal of Multiphase Flow 29, 1101–1116. Hebrard, G., Bastoul, D., Roustan, M., 1996. Influence of gas sparger on the hydrodynamic behaviour of bubble columns. Transactions of the Institution of Chemical Engineers 74, 406–414. Hikita, H., Asai, S., Tanigawa, K., Segawa, K., Kitao, M., 1980. Gas Holdup in Bubble Column. Chemical Engineering Journal 20, 59–67. Joshi, J.B., Vitankar, V.S., Kulkarni, A.A., Dhotre, M.T., Ekambara, K., 2002. Coherent flow structures in bubble column reactors. Chemical Engineering Science 57, 3157–3183.
A.A. Mouza et al. / Chemical Engineering Science 60 (2005) 1465 – 1475 Kaji, M., Sawai, T., Mori, K., Iguchi, M., 2001. Behaviors of bubble formation from a bottom porous nozzle bath. Proceedings of the 5th ExHFT, Thessaloniki, Greece. Parasu Veera, U., Joshi, J.B., 1999. Measurement of gas hold-up profiles by gamma ray tomography: effect of sparger design and height of dispersion in bubble columns. Transactions of the Institution of Chemical Engineers 77, 303–317. Papatzika, E., 2002. Experimental study of a bubble column. Diploma Thesis, Department of Chemical Engineering, AUTh (in Greek). Parthasarathy, R., Ahmed, N., 1996. Size distribution of bubbles generated by fine pore spargers. Journal of Chemical Engineering of Japan 29, 1030–1034. Polli, M., Di Stanislao, M., Bagatin, R., AbuBakr, E., Masi, M., 2002. Bubble size distribution in the sparger region of bubble columns. Chemical Engineering Science 57, 197–205. Ruzicka, M.C., Zahradnik, J., Drahos, J., Thomas, N.H., 2001. Homogeneous–heterogeneous regime transition in bubble columns. Chemical Engineering Science 56, 4609–4626.
1475
Ruzicka, M.C., Drahos, J., Mena, P.C., Teixeira, J.A., 2003. Effect of viscosity on homogeneous–heterogeneous regime transition in bubble columns. Chemical Engineering Journal 96, 15–22. Shah, Y.T., Kelkar, B.G., Godbole, S.P., Deckwer, W.-D., 1982. Design parameters estimations for bubble column reactors. A.I.Ch.E. Journal 28, 353–379. Tse, K.L., Martin, T., McFarlane, C.M., Nienow, A.W., 2003. Small bubble formation via a coalescence dependent break-up mechanism. Chemical Engineering Science 58, 275–286. Wallis, G.B., 1969. One-Dimensional Two-Phase Flow, McGraw-Hill, New York. Zahradnik, J., Fialova, M., Ruzicka, M., Drahos, J., Kastanek, F., Thomas, N.H., 1997. Duality of the gas–liquid flow regimes in bubble column reactors. Chemical Engineering Science 52, 3811–3826.