Ind. Eng. Chem. Res. 2009, 48, 11225–11229
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Hydrodynamic Characteristics of a Sparged Gas-Liquid Contactor for Fine Bubble Generation D. Chakraborty,† G. Siva Rama Krishna,‡ S. Chakraborty,‡ and B. C. Meikap*,‡,§ R&D DiVision, Tata Steel, Jamshedpur, 831003, India, Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharaghpur, PO: Kharagpur Technology, West Bengal, 721302, India, and School of Chemical Engineering, Faculty of Engineering, Howard College, UniVersity of Kwazulu-Natal, King George V. AVenue, Durban 4041, South Africa
In this study, hydrodynamics of a gas-liquid bubble column with a following sparger made of ceramic, a perforated metal plate, brick material, and polypor porous material has been presented. Experiments were conducted for water levels of 100, 125, and 150 cm with and without the addition of frother. An attempt has been made to investigate the hydrodynamics of the downward flow of a high-pressure water jet at different jet heights and different water velocities. The column was operated with air at superficial gas velocities in the range of 0.122-3.7 cm/s. Effects of superficial gas velocity, frother doses, and jet height on gas holdup have been presented. Depending on experimental operating conditions for gas holdup, flow regimes were identified for different spargers. Experimental results shows that a ceramic type sparger has better gas holdup compared to other spargers. It was also found that gas holdup is greater with the addition of frother (70%) compared to that without addition of frother (20%). 1. Introduction Gas-liquid contacting is an important unit operation in several chemical process industries and bubble column reactors are widely used for this purpose. The industrial importance of bubble column as a gas-liquid contactor is mainly due to their simple construction, low operating cost, high-energy efficiency, and good mass and heat transfer. The presence of a gas phase dispersed in a continuous liquid is the reason why such reactors can provide high interfacial area for mass and heat exchange, good mixing, and high thermal stability. In addition to the mass transfer qualities, it is very important to inject and distribute the gas efficiently with low-pressure drop and hence energy saving. The dispersion of the gas into the column is a critical aspect determining the performance of gas-liquid systems. In all these processes, gas holdup and bubble size distribution are important design parameters, since they define the gas-liquid interfacial area available for mass transfer. Small bubbles and a uniform distribution over the cross section of the equipment are desired to maximize the interfacial area and improve transport phenomena. In turn, these parameters depend strongly on the operating conditions, the physicochemical properties of the two phases, the gas sparger type, and the column geometry.1 In bubble column reactors depending on the gas flow rate, two main flow regimes are observed, i.e., the homogeneous bubbly flow regime and the heterogeneous (churn-turbulent flow) regime. The homogeneous regime is encountered at relatively low gas velocities and is characterized by a homogeneous distribution of small and almost identical bubbles and a radically uniform gas holdup.2 As the gas velocity increases, the heterogeneous regime is observed, where small bubbles coalesce and large bubbles are formed. In this regime, the gas holdup increase rate is lower than that in the homogeneous regime, since the larger bubbles * To whom correspondence should be addressed. Tel.: +27-312603802. Fax: +27-31-260-1118. E-mail address:
[email protected]. † Tata Steel. ‡ Indian Institute of Technology (IIT). § University of Kwazulu-Natal.
that are formed ascend with higher velocity. In this case (heterogeneous regime), most of the gas is transported through the reactor in the form of large fast-ascending bubbles, with few small bubbles, remaining (trapped) in the circulating liquid. The conversion of the gas phase reactant, achieved in the heterogeneous operating range is usually lower than obtained from the homogeneous regime due to the lower gas phase mean residence time and relatively lower gas-liquid interfacial area due to the bubble coalescence. There are considerable differences between the two regimes concerning the hydrodynamics and the transport characteristics; namely, the homogeneous regime, which offers a larger contact area and provides a low shear rate environment, is most desirable for practical applications, especially those involving sensitive materials (e.g., bioreactors, blood oxygenators). Gas holdup is one of the most important parameters characterizing bubble column hydrodynamics because it not only gives the volume fraction of the gas phase, it is also needed to estimate the interfacial area and thus the mass transfer rate between the gas and liquid phases. The gas distributor, superficial liquid velocity, and superficial gas velocity can also influence gas holdup in a bubble column. In the last few decades, several researchers have investigated important hydrodynamic and design oriented aspects of the bubble column reactors through extensive experimentation as well as theoretical analysis as reported by Schumpe and Grund.3 One of the main reasons behind focusing on the hydrodynamics is its strong influence on the design and hence on the performance of a bubble column. Some of the important design parameters include the average fractional gas holdup, mass transfer coefficient, heat transfer coefficient, effective interfacial area, bubble size distribution, etc. The external parameters that influence the performance include the superficial gas velocity, gas-liquid properties, and the sparger design. Meikap et al.2 studied bubble formation at gas distributors as well as the effects of distributor design and superficial liquid velocity on gas holdup and bubble size. A literature survey revealed that the combined effects of gas distributors and coalescence-inhibiting surfactants signifi-
10.1021/ie901322e 2009 American Chemical Society Published on Web 11/10/2009
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Figure 1. (a) Schematic of setup. (b) Detailed view of the sparger.
cantly affect gas holdup. Li et al.4 reported the hydrodynamics of a three-phase slurry bubble column for better hydrodynamic characteristics. Kazakis5 reported the hydrodynamics of bubble columns with fine pore spargers operating in the pseudohomogeneous regime. Literature survey indicates that the gas distributor plate could have a significant effect on gas holdup at low gas velocities. Gas sparger type is an important parameter that can alter bubble characteristics, which in turn affects gas holdup values and thus many other parameters characterizing bubble columns. The sparger used definitely determines the bubble sizes observed in the column. Small orifice diameter plates enable the formation of smaller sized bubbles. Some common gas sparger types that are used in literature studies are perforated plates, porous plates, membranes, ring type distributors, and arm spargers. The influence of the gas sparger on the hydrodynamics behavior of bubble columns including gas holdup depends strongly on the type of sparger as discussed by many researchers.6-9 These two flow patterns are separated by a transition regime that corresponds to the development of local liquid circulation pattern in the column, which establishes in the heterogeneous regime. Parthsarthy and Ahmed10 used a porous sparger to study the influence of the pore diameter on bubble size distribution by fine pores. The literature also suggests that the surface tension on the gas holdup distribution and the transition point changes from the homogeneous to the heterogeneous regime occurred at a lower gas flow rate when the pore diameter was smaller. Bubbles are uniformly distributed in the liquid when the gas flow rate is low. The bubble size distribution is relatively well-defined and is controlled by the sparger type and is uniform through the column. A good amount of literature is available on the effect of superficial gas velocity and the system properties on the flow pattern, while a detailed analysis of the intrinsic flow parameters subjective to the sparger design is still not available. In the present work, an attempt has been made to generate fine bubbles by using various spargers and to find the hydrodynamic characteristics, viz. gas holdup at different operating conditions in an air-water bubble column.
2. Experimental Setup and Technique Figure 1 shows the details of the sparger used and the experimental setup. The experimental setup consists of a vertical cylindrical Perspex column of diameter 18.6 cm and height 1.95 m. The column was operated batchwise with respect to the liquid phase and continuously with respect to gas phase. The setup consists of two sections: a top section and a bottom section. The bottom section of the column consists of a sparger where bubbles are generated by passing the gas through sparger. Air enters the bubble column from the bottom via sparger, which is installed at the center of the bottom plate. Experiments were carried out by using a ceramic sparger, perforated sparger, sparger made of brick material, and polypor porous sparger for gas dispersion. The airflow rate is adjusted with a needle valve and measured with rotameter. From the top of the column, water is drawn from water tank using a high-pressure reciprocating pump. A bypass valve is used at the bottom of the column to remove water. A high-pressure water jet of pipe diameter 2.54 cm, nozzle tip diameter of 0.6, and 106 cm height is used for downward flow of water for generating uniform bubbles and jet height is measured from the top of the water surface. All the experiments were conducted at ambient temperature and ambient temperature conditions. A digital camera is fixed on a stand very close to the cylindrical column and an appropriate lighting system is placed at top of the column to evenly distribute the light. Each experimental run starts by first filling the column with the appropriate water up to the 100, 125, and 150 cm level. Four different types of experiments were carried out to study the gas holdup. In experiment I, gas holdup experiments were done by using different spargers. Initially, the column was filled with water up to 100 cm. Air is passed at the bottom of column through rotameter via sparger and bubbles are generated. Holdup was measured from differences in height of liquid level. The air flow rate was increased by 2 L/min by using a needle valve until reaches 60 L/min. The same procedure was repeated for the 125 and 150 cm level of water and for different spargers. In experiment II, gas holdup experiments were carried out with and without the addition of frother. Initially, the column was filled with water up to 50 cm and frother of 9 ppm concentration
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was added. To validate the experimental results after the air flow rate reaches a maximum value of 60 L/min, the reverse experiments were conducted by decreasing the flow rate of air for every 2 L/m and same procedure was repeated for 19 to 252 ppm frother concentration. In experiment III, gas holdup experiments were conducted for the downflow bubble column with a ceramic sparger at the bottom of the column for different flow rates of water and air at a constant jet height. The column was filled with water up to 100 cm initially, and the height of the water jet is placed at 70 cm from surface of the water level. Air was passed at the bottom of column through a rotameter and bubbles were generated by means of ceramic sparger. Differences in height were noted at various air flow rates by adjusting the needle valve. After adjusting the bypass valve of the high pressure reciprocating pump and the high pressure liquid jet flows downward into column for 30 s, the suspension height was recorded. Once the all water and air flow rates are simultaneously closed, the final height was recorded to calculate gas holdup from the differences in height. In experiment IV, gas holdup experiments were done for the downflow bubble column with a ceramic sparger at the bottom of the column for different flow rates of water and different jet heights at a constant air flow rate with the same procedure as discussed in the previous section. The gas-holdup is the difference between two-phase height of the mixing (hm) and the liquid height (h1) inside the column to the two-phase height of mixing, that is, Rg )
hm - h1 × 100 hm
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Figure 2. Effect of superficial gas velocity on gas holdup for a ceramic sparger at different heights.
(1)
3. Results and Discussion Experiments were conducted to find out the gas holdup of gas-liquid contactors (bubble column). Gas holdup experiments were conducted for different types of spargers at different water levels. Detailed investigations have been carried out to study the gas holdup with various superficial gas velocities of 0.122-3.7 cm/s by using different types of spargers (ceramic sparger, sparger made of brick material, polypor porous sparger, perforated sparger, etc). It has been observed that the effect of the type of spargers significantly changes the gas holdup. It has also been observed that with the change in the level of the liquid in the column, there is a considerable effect on holdup fraction. Effect of Superficial Gas Velocity on Gas Holdup for Various Spargers. Figure 2 shows typical gas holdup values versus superficial gas velocity for ceramic sparger at three different water levels. It has been observed that gas holdup increases with superficial gas velocity being approximately linear at low gas velocities up to 1 cm/s since the number of pores activated are less and uniformly small gas bubbles are generated. As gas velocity increases, there is little deviation from linear behavior. This is due to the fact that the number of the pores of the sparger activated is greater resulting in a larger number of small bubbles which ascend with relatively low velocity with no interactions among them, leading to a further increase of gas holdup. This is called the bubbly flow regime. In addition, since the sparger does not cover the whole bottom cross section, it is evident that there is not uniformity in the radial bubble distribution near the sparger region. However, as bubbles travel upward, they spread and are distributed more evenly and uniformly inside the column. This has been supported by other researchers conducting experiments in a column equipped with a porous sparger where the homogeneous discrete bubbles are generated and are uniformly dispersed without coalescence. In
Figure 3. Effect of superficial gas velocity on gas holdup for various types of spargers.
addition, many investigators describe the homogeneous as the regime at which gas holdup increases almost linearly with increasing gas velocity. Gas holdup is 19.5% for the 100 cm water level height, and gas holdup is 19.1% and 18.5% for 125 and 150 cm water level heights.11-16 In the case of the perforated sparger, it has been observed that the gas holdup found to be 10.5%, 9.68%, and 9.46% for 100, 125, and 150 cm water level, respectively. It is interesting to note that, for the sparger made of brick materials, the gas holdup is 16.1%, 15.3%, and 15.16% for 100, 125, and 150 cm water level, respectively. In the case of a polypor porous sparger, the gas holdup is slightly lower than the brick sparger. The gas holdup is 15.1%, 13.4%, and 13.29% for 100, 125, and 150 cm water level, respectively. Effect of Superficial Gas Velocity on Gas Holdup for Different Types of Spargers. Figure 3 shows a typical plot of the variation of overall gas holdup with superficial gas velocity for different spargers at the 100 cm water level. It has been observed that gas holdup increases with an increase in superficial gas velocity. It has also been observed that gas holdup is 19.5% for the ceramic sparger and gas holdup for a sparger made of brick material, a polypor porous sparger, and a perforated sparger are 16%, 13.4%, and 9.5%. Gas holdup is more for the ceramic sparger compared to other three spargers because the
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Figure 4. Effect of superficial gas velocity on holdup with frother doses.
Figure 5. Effect of jet pressure on gas holdup at different jet heights.
bubbles that are generated from ceramic sparger are small. Bouaifi et al.17 stated that, the smaller the bubbles, the greater the gas holdup values. Thus, they concluded that with small orifice gas distributors their gas holdup values were higher. Effect of Frother on Gas Holdup. Figure 4 shows a typical plot of variation of gas holdup with superficial gas velocity for a ceramic sparger with and without the addition of frother. It has been observed that gas holdup increases with superficial gas velocity. This increase is due to the decrease in surface tension, which decreases the bubble size. Gas holdup is nearly 70% when a frother is added to 31 ppm, and then, gas holdup does not increase with increasing frother dosage. Hence, this effect is due to the bubbles that reach the saturated state based on the fact that gas holdup has been changeless in the column. It has also been observed that gas holdup increases with an increase in superficial velocity for low frother concentrations and gas holdup is almost similar for high frother concentrations. Gas holdup is more with the addition of frother compared to that without addition of frother. Effect of Pressure on Gas Holdup for a High-Pressure Liquid Jet. Figure 5 shows a typical plot of the variation of gas holdup with pressure for a high-pressure liquid jet for different jet heights from the water level. It has been observed that gas holdup increases with an increase in pressure keeping the jet height constant. This is because of the impact of the jet is more with the increase in pressure. It has also been observed that gas holdup is almost same for four different jet heights
Figure 6. Effect of water superficial velocity on gas holdup at different jet heights and air velocities.
from the water level. This means that there is very little effect on gas holdup by changing jet heights. Effect of Superficial Water Velocity and Jet Height on Gas Holdup. Figure 6 shows a typical plot of the variation of gas holdup with water velocity for both a high-pressure liquid jet and ceramic sparger for different superficial gas velocities. It has been observed from the experimental data that gas holdup increases with an increase in the water flow rate. This is because of the increase in liquid jet velocity, which in turn increases the kinetic energy of the liquid jet and contacting perimeter between the jets and receiving liquid surface. It has also been observed that gas holdup increases with increase in air flow rates because of increased bubble population. In the case liquid jets for different jet heights, it has been observed that gas holdup increases with an increase in the superficial water velocity at a constant jet height. This is because of the bubbles that are generated in downflow spending more time in the column which thus increases the gas holdup. It has also been observed that gas holdup is 32.5%, 32%, and 30.9% for jet heights of 90, 70, and 50 cm, respectively. Conclusions In the present work, gas holdup was experimentally investigated in a cylindrical bubble column fitted with different spargers for an air-water system. It can be concluded that gas holdup is more for a ceramic sparger than for a sparger made of perforated plate, brick material or polypor. It is preferred to operate the column in the bubbly flow regime, where gas holdup varies approximately linearly with gas rate. Gas holdup experiments were also done for different concentrations of frothers and without addition of frother for the air-water system. It was found that gas holdup is nearly 70% when a 31 ppm concentration of frother is added, and then, gas holdup does not increase by increasing the frother dosage further. It was found that the overall gas holdup is strongly dependent on gas flow rate and frother dosage. In the case of a downflow bubble column, it was found that height of the high-speed water jet from the water surface has not much effect on gas holdup. However, gas holdup increases with increase in pressure for a downflow bubble column. Acknowledgment The authors thankfully acknowledge the financial grant received from M/S TATA STEEL, Jamshedpur (IIT Kharagpur sanction no. IIT/SRIC/MBG).
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Notation hm ) total height of gas liquid mixture (cm) hl ) height of the clear liquid (cm) UGS ) superficial gas velocity (cm/s) UWS ) superficial gas velocity (cm/s) Rg ) gas holdup (%)
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ReceiVed for reView August 24, 2009 ReVised manuscript receiVed October 9, 2009 Accepted November 2, 2009 IE901322E
