ARTICLE pubs.acs.org/IECR
Microbubble Generation through Porous Membrane under Aqueous or Organic Liquid Shear Flow Sarah Khirani,†,‡,§ Papitchaya Kunwapanitchakul,†,‡,§ Frederic Augier,|| Christelle Guigui,†,‡,§ Pascal Guiraud,†,‡,§ and Gilles Hebrard*,†,‡,§ †
Universite de Toulouse, INSA, UPS, INP; LISBP, 135, avenue de Rangueil, F-31077 Toulouse, France INRA, UMR 792 Ingenierie des Systemes Biologiques et des Procedes, F-31400 Toulouse, France § CNRS UMR 5504, F-31400 Toulouse, France IFP Energies Nouvelles, Rond-point de l’echangeur de Solaize, BP 3, F-69360 Solaize, France
)
‡
ABSTRACT: Gasliquid reactors involving the production of bubbles are very useful in various industrial fields, such as chemistry, oil refining, or water treatment. Generating microbubbles at the inlet of gasliquid reactors could improve both hydrodynamics and mass transfer. In this paper, the generation of microbubbles using commercial porous filtration membranes is studied, in aqueous or organic liquid under a liquid cross-flow. Different combinations of dispersed phase (air or nitrogen) and continuous phase (water or heptane) are studied. Since the size of the microbubbles may be correlated with physicochemical properties of both phases and the membrane surface, several tubular ceramic membranes were tested (made of alumina or zirconium oxides) with mean pore diameters dp varying from 20 to 800 nm. It was observed that these membranes were able to generate bubbles which became microbubbles under the application of a liquid cross-flow. Microbubbles with bimodal distribution (microbubble diameters db in the range of 100200 and 500550 μm) were obtained in heptane when using zirconium oxide membranes (dp varying from 20 to 100 nm) under a shear stress τ varying from 27 to 39 Pa with a gas fraction of nearly 3%. The present study shows that microbubbles can be generated in heptane using commercial porous filtration membranes under a liquid cross-flow. This is a promising way to provide microbubbles that could be used to improve the usual gas/liquid mass transfer operation.
1. INTRODUCTION Many gasliquid reactors are based on the dispersion of gas bubbles in a continuous liquid phase. Microbubble generation could find numerous applications in the petrochemical industry, for example, in catalytic reactors used to perform the selective hydrogenation of diene to alkene. These fixed bed reactors are operated under upward cocurrent flows of gas and liquid. In these gas/liquid contactors, low gaseous hydrogen contents must be injected in the reactors to avoid the complete hydrogenation of diene to alkane. In this context, monitoring very low gas flow rates with very small bubble size populations is a new challenge that our study will deal with. Microbubbles have other potential applications in areas, such as pharmaceuticals, food, chemical and process engineering, water treatment, cosmetics, and medicine. The two most important advantages provided by the generation of a small-bubble population are the increase in mass transfer efficiency and the improvement in gas fraction homogeneity. In a bubble column, the specific exchange surface area developed by a monodispersed distribution of spherical bubble sizes can be expressed by eq 1 a¼
6εG db ð1 εG Þ
ð1Þ
where a (m1) is the exchange surface area developed in a liquid volume, εG the gas fraction, and db the bubble diameter (m). r 2011 American Chemical Society
As can be seen in eq 1, a decrease in the bubble size leads to an increase of the specific exchange surface area and thus to an increase of the volumetric mass transfer coefficient kLa. The positive effect of the increase of the surface exchange area (by decreasing the bubble size) has to be very large as it could be counterbalanced by a decrease in the values of the liquid side mass transfer coefficient kL. The modeling of the liquid side mass transfer coefficient kL by the Frossling1 (1938) equation (for a rigid interface) shows that, for bubble sizes less than 1 mm, there is a diminution of the kL value by a factor of 4 compared to those measured with bubble diameters greater than 3 mm. At the same time, this change in diameter leads to an interfacial area 3 times greater, assuming a constant gas fraction in eq 1, and bubble size 3 times smaller. This is why it is important to obtain very small bubble sizes to provide a very high specific exchange area and thus to counterbalance the decrease of kL. Another advantage of generating very small bubbles is that a gas phase is injected without disturbing or modifying the main liquid stream developed in gas/liquid contactors. This can be explained by the low terminal velocities Special Issue: Nigam Issue Received: March 25, 2011 Accepted: July 21, 2011 Revised: July 13, 2011 Published: July 21, 2011 1997
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Industrial & Engineering Chemistry Research of the small bubbles and their low coalescence probability. In consequence, the generation of microbubbles could be a way to simultaneously improve the mass transfer and facilitate the gas distribution on the whole section of reactors. As millimetric bubbles are generally present in gas/liquid contactors, the generation of microbubbles is a very ambitious challenge for the industry. Many investigations have been reported in the literature concerning bubble populations. There are papers dealing with bubble formation, bubble coalescence or breakage, bubble trajectories, bubble slip velocity, bubble shape, bubbles in non-Newtonian liquid, bubble mass transfer, and bubble flotation. Many gas spargers have been investigated and proposed to create homogeneous bubble populations with large interfacial areas, to be robust over time, for low power consumption and high mass transfer efficiency. In industry, most of them generate bubbles between 3 and 5 mm. Generally, the smaller the bubbles are, the greater the power consumption is. Although they could improve the global efficiency of their process by decreasing bubble size, most engineers prefer to limit the power required by the process without taking the corresponding process efficiency into account. That is why fewer papers concern research to promote gas spargers providing very small bubble sizes even though they could increase the efficiency of many processes. For instance, various means are used to generate microbubbles, such as capillary tubes, pressurization (in Dissolved Air Flotation), ultrasonification or electrolysis. A few studies deal with the generation of microbubbles through membranes but they are limited to aqueous solutions. Compared to other systems, for example, dissolved air flotation, the main advantage of providing microbubbles through membranes is to reduce the power consumption and to use commercially available devices. Kukizaki et al.25 (2006, 2007, 2008, and 2009) studied the generation of monodispersed microbubbles or nanobubbles inside aqueous solutions, with surfactant addition, using Shirasu porous glass (SPG) membranes. These cylindrical or plane membranes were made in the laboratory and had cylindrical tortuous pores. Several factors were investigated during the studies, such as the physicochemical properties of the membrane (membrane pore size, membrane hydrophobicity, and the asymmetry or symmetry of the membranes) and the physicochemical properties of the fluids (surface tension, viscosity, and addition of several types of surfactants). Their main results are summarized in the following part (scope and literature review). The objective of our paper is to investigate the case of microbubble generation through a membrane in a nonaqueous liquid (heptane). The first part of the paper presents a review of the main results obtained on microbubble generation by different authors. The second part describes the materials and equipment used in this study, and the third part presents the results concerning the generation of bubbles formed in water and heptane.
2. SCOPE AND LITERATURE REVIEW Few studies deal with the generation of microbubbles through membranes and those that exist are limited to aqueous solutions plus surfactants. Five papers proposed by Kukizaki’s team relate to the generation of microbubbles through membranes. The
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following part focuses specially on the major parameter that controls bubble size. 2.1. Membrane Pore Diameter Effect. Kukizaki and Goto2 (2006) found a linear correlation between bubble sizes (from 360 to 720 nm) and membrane pore diameters (43, 55, 64, and 85 nm). The bubbles were generated in sodium dodecyl sulfate (SDS) solution (0.3%) using a tubular SPG membrane (surface area 1.57.103 m2) under a liquid cross-flow (mean liquid phase flow velocity UL = 0.7 m/s) and a ratio of transmembrane pressure (TMP)/bubble point (BP) equal to 1.1. Kukizaki and Baba6 (2008) observed the same trend for flat SPG membrane (surface area 12.16 cm2, membrane pore size varying from 1 to 10 μm). These bubbles were generated in different surfactant solutions (anionic, n-dodecylbenzenesulfonate; nonionic, polyxythylene (20) sorbitan monolaurate) at a concentration of 2 mol/m3. A liquid cross-flow and a TMP/BP ratio of 1.1 were also applied. 2.2. Membrane Hydrophobicity Effect. The bubble size is also influenced by the membrane hydrophobicity, which can be determined through the contact angle of a water droplet dropped on the membrane surface. For contact angles less than 90°, the water spontaneously enters the membrane pores. On the contrary, for contact angles higher than 90°, pressure is required to introduce the water into the pores and the membrane is thus classified as hydrophobic. The influence of membrane hydrophobicity was investigated in a study by Kukizaki and Wada6 (2008). The surface of an SPG membrane (membrane pore size of 0.97 μm) was modified chemically with organosilane (CnH2n+1(CH3)2SiCl; n = 1, 8, 18) to reduce the wettability of the membrane. Bubbles were generated in SDS solutions (0.3%) under a liquid crossflow (UL = 1.5 m/s) and under a TMP/BP ratio of 1.3. A narrow distribution of bubble sizes was observed when the contact angle was less than 45°, with a correlation between bubble size and membrane pore size. A wider distribution and greater bubble size were observed when the contact angle was higher than 45°, with an increase of the dispersal coefficient of the bubble size distribution. This was explained by the fact that the force expanding the bubbles is greater than the force contracting the contact zone between the bubble and the membrane. A higher contact angle will facilitate the bubble expansion. Many pores could thus participate in the generation of the bubble and the bubble neck could be regulated by pore diameter. To generate small bubbles in heptane, the membrane surface and pores must not be wettable with heptane i.e. must be very hydrophobic. To obtain such conditions, the membrane surface can be treated to reduce the value of the contact angle with water. 2.3. Effect of TMP/BP Ratio. Kukizaki5 (2009) found that when a TMP/BP ratio of less than 1.5 was applied, uniform microbubbles with a bubble diameter/membrane pore diameter ratio of 9 were generated in sucrose laurate solution (1%) using asymmetric SPG membrane (membrane pore diameter =1.58 μm, skin thickness =12 ( 2 μm) and with applied gaseous fluxes from 2.124.6 m3 m2 h1. Considering a TMP/BP ratio of 1.5, they observed that the gaseous fluxes were 40 times higher than those observed when using a symmetric membrane. This can be explained by the hydrodynamic resistance, which is lower for an asymmetric membrane operating with only one filtration layer located at the internal tube surface. The proportion of active pores 1998
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Table 1. Tubular Ceramic Membrane Characteristics mean pore diameter dp reference
a
material
supplier (μm)
water Permeabilitya (L/(hm2Bar))
experimental (μm)
BP (supplier) in water (mbar)
A
alumina
0.8
0.7
70009000
>230
B
alumina
0.1
0.7
13001600
>1700
C
zirconium oxide
0.1
1
D
zirconium oxide
0.02
0.6
E
titanium oxide
0.15
0.46
Fb
titanium oxide
0.15
7
Gb
zirconium oxide
0.1
2.17
250350
>1700
18002200
>1700
At 20 °C, given by the supplier. b Hydrophobic surface.
(0.27 to 0.43%), calculated from the measured gaseous fluxes, increases with the reduction of the thickness of the membrane skin layer. Considering the results of Kukizaki5 (2009), it is thus preferable to use low TMP/BP ratios and asymmetric membranes in order to increase the gas fraction and provide uniform microbubbles. 2.4. Surface Tension Effect. The effect of surfactant that induces a lower surface tension (σ) was investigated by Kukizaki and Goto2 (2006) using an SPG membrane (pore size 55 nm) under a liquid cross-flow (UL =0.7 m/s). Bubbles were generated in SDS solutions (from 0.01 to 0.5%), the surface tension (σ) varying from de 67.4 to 58.0 mN 3 m1. They observed that the decrease of surface tension led to a decrease of microbubble diameters because the capillary force decreased and kept the bubble attached to the pore for a shorter time, causing the formation of smaller bubbles. 2.5. Liquid Viscosity Effect. Kukizaki and Goto3 (2007) studied the effect of the air viscosity/liquid viscosity ratio on bubble size in liquids containing 0.3% of SDS and different concentrations of polyethylene glycol (PEG). The viscosity varied from 0.9 to 10.4 mPas. They used a flat SPG membrane with a membrane pore size of 3.7 μm. The bubbles were generated at a TMP/capillary pressure ratio of 1.1. Considering these operating conditions, they observed that bubble diameters increased with the increase of the viscosity of the liquid. The authors supposed that the increase of viscosity delayed the detachment of the microbubbles. 2.6. Shear Stress Effect. Kukizaki and Goto2 (2006) studied the influence of the shear stress applied at the membrane surface on the bubble size. They used tubular SPG membranes (membrane pore diameters of 55 nm, 550 nm, and 2.23 μm), and the bubbles were generated in SDS solution (0.3%) at a TMP/BP ratio of 1.1. When membranes with pore diameters of 550 nm and 2.3 μm were used, the bubble diameter decreased as the applied stress increased, to a limit value of bubble diameter beyond which the diameter was independent of the applied stress. Moreover, changes in the diameters of bubbles occurred at low stress values. This tendency was also observed in the work of Loubiere et al.7 (2004), who studied the influence of liquid shear stress on the size of the bubbles generated by a flexible membrane sparger. They observed that smaller bubbles were produced under liquid cross-flow conditions. For a UL = 0.20, 0.28, 0.40, and 0.53 m/s, the bubble diameters were reduced to 1015, 2040, 3050, and 50150%, respectively, in comparison with bubble formation under quiescent liquid conditions where bubble diameter and orifice gas velocity were respectively equal
to 2 103 m and 2 m/s. Colin et al.8 (2000) studied the growth of bubbles generated by ebullition under a liquid crossflow in a Couette reactor. The same trend of bubble size evolution was observed, the bubble size decreasing with the increase of the lateral drag force. They also observed that increasing liquid velocity tended to noticeably decrease the bubble formation time. Under the influence of the liquid velocity, the bubble frequencies were multiplied by a factor varying between 1.4 and 4.7 compared to bubble formation under quiescent liquid conditions. 2.7. Porous Structure Effect. In the main results obtained by Kukizaki et al.25 (2006, 2007, 2008, and 2009), the bubble diameter is always correlated to the membrane pore diameter. However, it is reasonable to think that the porous structure of membranes composed of adjacent, often interconnected, pores also plays an important role. Painmanakul et al.9 (2004) worked on the generation of bubbles using a flexible membrane sparger having four neighboring orifices and pointed out bubble coalescence when the distance between two orifices was less than a critical distance. In this case, coalescence was favored when the bubble generation time and so the interaction time between bubbles was longer than the liquid film drainage time. In addition, it can be noted that no information is available concerning the influence of the pore size distribution on the bubble size and size distribution. This review of the main results obtained in aqueous liquid by Kukizaki’s team provided clear guidelines for our investigation in heptane as an example of organic liquid: • The wettability of the membrane by the liquid phase is an important parameter as bubble sizes are smaller when the contact angle with this liquid phase is low. • Asymmetric membranes can have a higher gas flow than symmetric membranes. This encouraged the use of asymmetric membranes in our application to provide higher gas fraction. • The shear stress applied by the liquid flow is a key parameter of the bubble size; the experimental device was designed to investigate this effect. • Physicochemical properties of the liquid (viscosity, surface tension, presence of surfactants) also play a role that will not be examined here. The first work of this study was to characterize the membranes in terms of gas permeation, bubble point pressure, and membrane contact angle to determine the membrane properties. The objective of the study was to generate microbubbles in heptane (without surfactants) using commercially available membranes that would have a controlled distribution of pore size or porosity. 1999
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Figure 1. Simplified sketch for the generation of microbubbles under dynamic conditions of water flow.
Two methods were considered for the bubble diameter measurement: the laser diffraction method and image recording by high speed camera. The images were analyzed using the VISILOG software to estimate the bubble diameters. For these purposes, different tubular membranes with different mean pore sizes (20800 nm) were tested using heptane under different gas flows and liquid cross-flows. Two pilots were set up to generate microbubbles through tubular membranes, one for water and the other for heptane.
Table 2. Tubular Ceramic Membrane Characteristics
3. MATERIALS AND METHODS 3.1. Membrane Characterization. Different ceramic tubular membranes (PALL, France) were used. Table 1 summarizes their characteristics as given by the supplier. These tubular membranes have an internal diameter of 7 103m, an external diameter of 10 103 m and a length of 0.25 m. To reduce the critical surface tension of membranes C and E, bifunctionalized molecules were grafted onto the ceramic membrane surface, giving membranes G and F (cf., Table 1). 3.1.1. Characterization Methods. The membranes were characterized in terms of gas permeation, bubble point and contact angles, using compressed air or tap water. Air Permeability. Dead-end permeation was performed to characterize the gas permeation. The tubular ceramic membrane was placed in a Membralox carter as shown in figure 1a. The compressed air, controlled by a valve, flowed from the external side of the tubular membrane to the internal side. The pressure was measured using a pressure gauge. The gas flow rate across the membrane was measured using a volumetric gas flow meter. The experiments were performed at ambient temperature (25 °C). The experimental setup is shown in figure 1. Bubble Point. The bubble point pressure was determined with the experimental setup shown in Figure 1. The internal
a
gas permeation N
contact angle θ
BP (mbar)
reference
L/h bar m2 ((100)
(°) ((1)
((5)
A
2
55 10
62
200
B
44 103
64
198
C
12 103
63
138
D
22 102
60
225
E
18 102
65
304
Fa
21 102
67
20
Ga
22 102
66
65
Hydrophobic surface.
side of the membrane was filled with water and then the compressed air was injected at a small pressure at the external side of the membrane. The pressure was increased gradually until the capillary pressure of the largest pores was reached. The bubble point pressure was obtained when the first bubble appeared. Contact Angle θ. The contact angle was measured by goniometry with a GBX-Digidrop system to characterize the surface state of the membrane. It should be noted that this method is very suitable for analyzing plane surfaces and nonporous surfaces. It was applied here simply to qualify the effect of the surface treatment applied to the membrane surfaces by the supplier, even in the case of the tubular membranes, for which a tiny droplet was deposited on the internal side of the tubular membrane. 3.1.2. Membrane Characteristics. Table 2 shows the measured gas permeation, the bubble point pressure and the contact angles of seven membranes used in this study. As can be seen from Table 2, membrane B had the highest gas permeability flux, while membrane E had the lowest. According 2000
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Figure 2. Characterization of membrane B by scanning electronic microscopy.
Table 3. Continuous Phase Characteristics at 20 °C continuous
specific
surface tension
viscosity
phase
density
σ (N m1)
(Pa s)
water heptane
1 0.681
3
73. 10 20. 103
1.103 4.104
to these results, for one operating pressure, membrane B was more suitable than the other membranes to provide a large gas fraction, because of its high gas permeability. The measured contact angles obtained for the different membranes with water droplets ranged between 60 and 67°. These membranes could thus be qualified as partially hydrophobic. Concerning the membranes F and G, the surface modification made by the supplier did not have an important effect as the contact angle value increased only slightly (from 63 to 67° with membrane F and from 65 to 67° with membrane G). Whatever the membranes tested, the heptane droplet placed on the membrane surface was adsorbed immediately into the membrane pores, which means that heptane wetted the membrane surface perfectly, certainly because the surface tension of heptane (20 103 N/m) is low. Assuming that the measured contact angles reported in the table 2 were equal to the contact angles in the membrane pores, the bubble point pressure was used to deduce a maximum pore diameter dp in the membrane according to the LaplaceYoung eq 211 PBP ¼
4σ cos θ dp
ð2Þ
The membrane pore diameters calculated are reported in Table 1 as “experimental” values. Large values of pore size were obtained, which were greater than those given by the supplier (except with membrane A). Concerning membranes C and G, greater pore sizes were obtained when surface modifications had been made by the supplier as reported in Table 1. The surface of alumina based membrane (B) was characterized by scanning electron microscopy (SEM JSM 840) to visualize the state and porosity of the membrane. Figure 2a presents the interface between filtration layer and support layer, and Figure 2b presents the state of the filtration layer. As can be seen from Figure 2b, the filtration layer had homogeneous hole sizes.
3.2. Fluids. Air bubbles were generated in water and, for safety reasons, nitrogen bubbles were generated in heptane at ambient conditions (T = 20 °C, P = 1 atm). The characteristics of the continuous phases (specific density, surface tension and viscosity) are summarized in Table 3. 3.3. Experimental Setup. Two pilots were used to generate bubbles in the two continuous phases (water and heptane); their characteristics are presented in the following part. 3.3.1. Pilot 1 for the Generation of Microbubbles in Water. For the preliminary tests, a simplified pilot shown in Figure 1 was set up to generate the microbubbles under a water cross-flow. These tests investigated the influence of the shear stress of the continuous phase (tap water) on bubble size. Two alumina based membranes (A and B) were used. These membranes were fixed in a Membralox carter as presented in Figure 3a. Air was injected in the external side of the membrane and passed through the porous media at a pressure higher than the bubble point of the membrane (Table 1). The air that passed through the porous medium generated bubbles in the internal side of the tubular membrane where the continuous phase (tap water) was flowing, creating a shear stress. Figure 3a and b shows how the fluids circulated and how the bubbles were generated. The generated bubbles could be seen in the visualization cell connected to the internal flow of the Membralox carter and the exit bubble sizes were measured in situ by a laser diffraction method or by using a high-speed camera. 3.3.2. Pilot for the Generation of Microbubbles in Heptane. The second pilot (pilot 2, Figures 4 and 5) was then designed for the generation of microbubbles in heptane taking safety conditions into account. This pilot included two reservoirs (55 L) for the storage of heptane. An ATEX servo pump was used for the circulation of heptane from the feed tank (reservoir 1) to the upper tank (reservoir 2) passing through the internal side of the tubular membrane. The ceramic membrane was fixed in a Membralox carter. The heptane circulated at different flow rates and thus different shear stresses were applied at the internal surface of the membrane. Nitrogen bubbles were generated using nitrogen supplied at different pressures from a bottle. A gas pressure regulator with a sensor for nitrogen and a mass gas flow meter were used to regulate the gas flow rate. The nitrogen passed through the porous membrane to form bubbles in the internal side of the membrane where heptane was circulating (as explained in Figure3b). Two liquid pressure sensors were placed one on 2001
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Figure 3. Fluids circulation in Membralox module.
Figure 4. Simplified sketch for the generation of microbubbles under dynamic conditions of heptane flow.
either side of the carter (at the entrance and outlet after the visualization cell) and the liquid flow meter was used to control the applied liquid flow rate and shear stress. For safety, different valves were used to isolate each component of the pilot and a condenser was used to condense heptane vapors to avoid their release into the environment. A sensor also served to measure the temperature of the heptane. The size of the bubbles generated was measured in the visualization cell using high-speed video camera. 3.4. Bubble Size Distribution Measurement. 3.4.1. Laser Diffraction Method. Bubble sizes were measured continuously using the “Spraytec” instrument (Malvern), which is generally used for the measurement of drop sizes in sprays. The Spraytech makes continuous measurements of the size distribution of bubbles using a HeNe (heliumneon) laser beam that passes through the bubble flow. Receiver optics detect the light diffraction pattern produced by the bubbles, converting the light detected into electrical signals. The signals are processed by analog and digital electronics boards. The light diffraction pattern is analyzed using an appropriate scattering model to calculate the bubble size distribution. The accepted theory,
which accurately predicts the light scattering behavior of all material under all conditions is known as the Mie theory. It was developed to predict the way light is scattered by spherical particles and deals with the way light passes through or is absorbed by the particle. This theory presumes that the particles being measured are perfect spheres. The data are recorded and sent to the acquisition system by the receiver. Finally, the Spraytec software calculates the data and presents the micro bubble size distribution directly. For each measurement, the bubble distribution can be plotted versus diameter. From each data item obtained, the software can also calculate the different average diameters (Di in μm), which are included in a database. This method can measure bubble sizes in the range of 0.1900 μm. 3.4.2. High Speed Video. A high speed camera (10 bit CMOS, PCO 1200 hs) was used for the image acquisition during the generation of bubbles. It had an acquisition capacity of 491 frames/s and was associated with an optical AF Micro Nikkor 105 mm with a 28/32 opening. Bubbles were assumed to be spherical. Their sizes were estimated from image analysis, the scaling being recorded for 2002
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Figure 5. Pilot for the generation of nitrogen bubbles in heptane.
Table 4. Shear Stress Influence on Bubble Size Generated Using Alumina Based Membranea
a
Membrane B, membrane pore diameter 0.1 μm; applied air pressure = 1.4 bar.
each frame sequence with a ruler on the wall of the visualization cell. More than 300 bubbles were analyzed for each operating condition, depending on the photo quality.
4. RESULTS 4.1. Bubble Generation in Water (Pilot 1). Air bubbles were
generated in water through an alumina based membrane having a mean pore diameter of 0.8 and 0.1 μm (A and B). Table 4 compares the images (obtained using a high speed camera) of the bubble in quiescent liquid or under liquid cross-flow for the membrane with a pore size of 0.1 μm (B). The air pressure imposed was 1.4 bar and the corresponding gas flow rate was low and not measurable. The liquid shear stress calculated according to eq 3 (under turbulent flow) varied from 3 to 40 Pa for corresponding liquid flow rates varying from 0 to 0.73 L
per second. τ¼
λFL UL 2 8
ð3Þ
where FL is liquid density; UL the liquid velocity, λ the Moody friction factor according to the Blasius equation λ = 0.3164Re0.25 and 2500 < Re < 100 000. As can be seen from Table 4, in quiescent liquid, the bubble size was close to 10 mm (not measurable by laser diffraction method); it was 10 times smaller when there was a liquid crossflow with a shear stress varying from 3 to 40 Pa. The bubble diameters obtained from the images taken by the high-speed camera were larger than those obtained with the laser diffraction method. However, these values were of the same order of magnitude. For an applied shear stress greater than 3.4 2003
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Figure 6. Influence of liquid shear stress on bubble size.
Table 5. Comparison of Calculated Tate’s Bubble Diameter and Measured Spraytech Bubble Diameter under a Shear Stress of 38 Pa (Air Bubble Generation in Tap Water) membrane
A
B
membrane pore diameter dp (μm) Tate bubble diameter db (μm)
0.8 327
0.1 164
Spraytech bubble diameter Dv(90) (μm)
310
800
Pa, the bubble size measured by the camera was close to one millimeter whereas the volume averaged size provided by the Spraytech was smaller than 850 μm. We can thus consider these results as reliable since the Spraytech measurement is restricted to bubble sizes smaller than one millimeter. Figure 6 presents the evolution of the volume averaged bubble sizes Dv(10), Dv(50), and Dv(90) for both alumina based membranes B and A (Mean pore diameters of 0.1 and 0.8 μm) as a function of the shear stress τ. As can be seen in Figure 6, whatever the membrane used, a small imposed shear stress (3 and 7 Pa respectively for membrane B (dp = 0.8 μm) and membrane A (dp = 0.1 μm)) reduces the bubble size considerably, from 10 mm to 700 μm for membranes B (dp = 0.8 μm) and from 10 mm to 370 μm for membrane A (dp = 0.1 μm). In addition, it can be observed that the membrane with the larger pore diameter (membrane A), surprisingly, allows the generation of smaller bubbles. According to the data reported in table 2, the contact angle and the BP of membranes A and B are similar whereas their gas permeation values differ: 55 102 N L/h bar m2 for membrane A and 44 103 N L/h bar m2 for membrane B. Regarding these two membranes, a decrease of gas permeation with an increase of hole diameter should be related to a decrease of membrane B porosity. To explain the trends observed, Table 5 compares the measured diameters Dv(90) with diameters calculated from the Tate eq 4 applied for a static bubbling regime (a balance between buoyancy and surface tension and gravity forces): VbulleT ate ¼
πdp σ ðFL FG Þg
ð4Þ
Tate’s equation gives the bubble size generated by a single hole. As the porous medium used in this study has adjacent pores, the application of a shear stress will add a lateral drag force on bubble growth which can, first, isolate the bubbling holes and, second, reduce the bubble size and so the coalescence phenomena (Pereira and Dias10). In this situation, the measured bubble diameter obtained can be appropriately compared with Tate’s
diameter. In fact, the shear stress favors the formation of bubbles by a single hole. As can be seen from the Table 5, Tate’s diameter is of the same order of magnitude for the membrane having a pore size of 0.8 μm and the lower porosity. This suggests that, for this pore size and this low porosity, the bubbles are generated as though through a single hole; that is, there is no interaction between neighboring bubbles during bubble generation, and there is only one bubble neck per pore. For the membrane with a smaller pore diameter and greater porosity, Tate’s diameter is five times lower than the measured bubble diameter. This leads us to think that bubble formation is not controlled by a single hole, and several adjacent holes participate in the bubble formation, even at a high shear stress. As membrane porosities are really unknown, it is difficult to confirm this bubble generation mechanism. However, these tests are encouraging as they allow the generation of micrometric bubbles with a mean diameter of 150 μm in water, without addition of surfactants. 4.2. Bubble Generation in Heptane (Pilot 2). Bubbles were also generated in heptane. Some experiments were run using the alumina based membrane with the pore size of 0.8 μm, but the bubble sizes obtained were not less than 600 μm. For this reason, two zirconium oxide based membranes were used (C and D with mean pore diameters of 100 and 20 nm respectively) to test another membrane material. Different liquid flow rates were tested for a constant gas flow rate to study the influence of shear stress (varying from 0 to 40 Pa) on the bubble size. 4.2.1. Membrane with a Mean Pore Diameter of 20 nm (D). Table 6 summarizes the operating conditions tested when using the membrane with a mean pore diameter of 20 nm (D). The shear stress varied from 0 to 40 Pa, and the void fraction varied from 2.5 to 4.5%. Figure 7 presents images of the bubbles for the different cases summarized in Table 6. Bubble diameters were calculated from the images and the void fraction was approximated by the ratio of gas and liquid throughputs. • In case a, in quiescent liquid, there was a large spherical cap of some millimeters in diameter and two populations of smaller bubbles of 550 and 270 μm. • In case b, when a liquid cross-flow was induced, there was less spherical cap. Two populations of bubbles were still generated, one having a diameter ranging between 90 and 270 μm and the other between 370 and 1100 μm. • In case c, for the same void fraction as b but with increased liquid cross-flow velocity and gaseous fluxes, the proportion of small 2004
dx.doi.org/10.1021/ie200604g |Ind. Eng. Chem. Res. 2012, 51, 1997–2009
Industrial & Engineering Chemistry Research
ARTICLE
Table 6. Operating Conditions for Bubble Generation Using Zirconium Oxide Membrane with Pore Diameter of 20 nm (Membrane D) case
a
b
c
d
e
f
UL (m/s)
0
1.58
2.13
3.03
3.82
4.58
τ (Pa) Qg (NL/h)
0
5.9 9.6
9.92 13.2
18.41 13.2
27.66 13.8
37.95 16.2
4.4
4.5
3.1
2.6
2.5
550
90270
