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RESEARCH NOTES Experimental Study on Surface Aerators Stirred by Triple Impellers Xiangyang Li,† Gengzhi Yu,† Chao Yang,*,†,‡ and Zai-Sha Mao† National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, and Jiangsu Marine Resources DeVelopment Research Institute, Lianyungang 222005, China
In a stirred vessel with the aspect ratio of 2.4, the gas-liquid mass transfer characteristics of triple-impeller configurations with surface aeration have been studied. The way by adding one or more impellers below the aerating impeller could not overcome the problem of poor gas dispersion in the region close to the bottom of the gas-liquid vessel stirred only by a surface aerator. With the combination of surface aeration and sparger aeration, gas dispersion is greatly improved. Triple-impeller configurations and volumetric gas flow rates are optimized based on the volumetric mass transfer coefficient per power consumption. It is found that the sparging gas has only a little effect on the surface aeration, because the vessel is stirred by the triple-impeller configuration and with a larger aspect ratio. 1. Introduction There are many industrial reactions, such as hydrogenation, alkylation, and oxidation, where the utilization of gas is quite low per pass. For these cases, compressing and recycling the unreacted gas from the headspace of a reactor always lead to considerable extra energy consumption, generally about onethird of the power consumed by the impellers for gas dispersion,1 and a more complicated flow sheet. In the case of surface aeration, the gas above the liquid surface can be entrained directly into the liquid by the surface aerating impeller, which eliminates the need for a recycle gas compressor. This not only means operating cost savings, but also the operation of the system becomes safer.2 So, surface aerator is of great potential in industrial applications. But, surface aerator has a severe flaw. Since the surface aerating impeller is often located very near the static liquid surface for surface aeration, there exists a region at the bottom of the vessel where bubbles hardly reach and gas holdup is very low.3 Many industrial vessels have aspect ratios significantly greater than unity, even up to two. When commercial scale equipments are to be designed, this problem becomes even more severe. This creates a barrier for the use of surface aerator in some industrial stirred vessels. In order to overcome this problem, some studies were conducted using dual-impeller configurations in surface aerators.3-8 The upper impeller located close to the liquid surface can be called the aerating impeller, playing the role of gas entrainer. The lower impeller located well below the aerating impeller is used for performing other duties such as gas dispersion, solid suspension, heat transfer, mixing, etc. However, the above reports are not sufficient to remove the barrier for using a surface aerator in the industrial stirred vessels. The reports on dual-impeller configurations in surface aerators were all conducted in the stirred vessels with the aspect ratio no more than 1.4. For the stirred vessels with a larger aspect ratio, the bubbles could not have been carried down to the * To whom correspondence should be addressed. Tel.: (+86) 10 6255 4558. Fax: (+86) 10 6255 4558. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Jiangsu Marine Resources Development Research Institute.
bottom zone only by adding one or more axial impellers to the above dual-impeller configuration at the bottom stage because of the buoyancy of the bubbles and the deficient pumping capability of the impellers. In gas-liquid stirred vessels, the aeration of gas is normally through sparger aeration or surface aeration. Multi-impeller configurations have been tested extensively in sparger aerators.9-17 Felicitous combination of sparger aeration and surface aeration in a stirred vessel with a large aspect ratio is perhaps able to exploit their respective advantages and overcome their disadvantages. The previous work on the combination of sparger aeration and surface aeration in a gas-liquid stirred vessel was aimed at studying the effect of sparger aeration on surface aeration.6,18,19 In this work, an experimental study was carried out in a surface aerating stirred vessel with a larger aspect ratio of about 2.4. Literature data indicate that the gas-liquid mass transfer is generally the rate-limiting step in many industrial processes,20 and hence, the focus of this paper is on the assessment of the volumetric gas-liquid mass transfer coefficients, kLa, of tripleimpeller configurations. 2. Experimental Section The experiments were carried out in a flat-bottomed cylindrical vessel of 0.380 m in diameter, equipped with triple impellers and a ring distributor with the diameter of 102 mm and 25 downward facing orifices of φ 1 mm below the bottom impeller as shown in Figure 1. The impellers used were Rushton disk turbine (RDT), half elliptical blade disk turbine (HEDT), and Techmix 335 hydrofoil impeller upflow (TXU) in Figure 1b-d. The shorthand notation used for defining the agitation configurations is straightforward: RDT + TXU + TXU means upper Rushton impeller for surface aeration with two upward-pumping Techmix 335 hydrofoil impellers (TXU) in lower positions. RDT + TXU + RDT and RDT + TXU + HEDT represent similar implications. In Figure 1, other dimensions are also labeled. In all experiments, tap water was used as the continuous liquid phase, and air was the dispersed gas phase. The liquid depth was 0.90 m (working liquid volume of 0.102 m3). The power consumption was determined by measuring the torque on the
10.1021/ie900623m CCC: $40.75 2009 American Chemical Society Published on Web 08/04/2009
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Figure 1. Schematic diagram of the reactor configuration: (a) reactor; (b) Rushton disk turbine (RDT); (c) half elliptical blade disk turbine (HEDT); (d) Techmix335 hydrofoil impeller upflow (TXU); (e) ring distributor.
Figure 2. Schematic of flow pattern with typical triple-impeller configuration based on visualization of bubble motions (QG ) 3 m3 · h-1, N ) 364 rpm).
stirring shaft and the stirring speed. The total power consumption is calculated by P ) PST + PSP
(1)
where PST is the agitation power calculated by PST ) 2πNTq and PSP is the expansion work of the sparged gas calculated by PSP ) Flg(H - h)QG. The gas-liquid volumetric mass transfer coefficient was measured by the steady-state sulfite feeding method (SFM).21 3. Results and Discussion 3.1. Observation on Flow Pattern and Mixing in Liquid Phase. In a gas-liquid stirred vessel, the bubbles could be taken as the tracer for the flow. So, the flow pattern was visually observed in the case of surface aeration combined with sparger aeration stirred by a triple-impeller configuration of RDT + TXU + RDT. As shown in Figure 2, four fluid recycling loops come into being. Once the sparging gas entered the stirred vessel below the bottom impeller, the stirring impeller sheared large bubbles and carried them along the liquid circulation 1 or 2. Thus, the bubbles were dispersed in the bottom zone. As a result, the dead zone at the bottom was reduced and even disappeared as the impeller speed or the volumetric gas flow rate increased. At the same time, the entrained bubbles were carried along the
liquid circulation 3 or 4. The bubble distribution with both surface and sparger aeration tended to be more uniform in the whole vessel than in the case of surface aeration only. 3.2. Mass Transfer Coefficient. The gas-liquid mass transfer of three triple-impeller configurations in the case of surface aeration and sparger aeration have been investigated at five volumetric gas flow rates as seen in Figure 3. For all gas flow rates, the kLa values of RDT + TXU + RDT and RDT + TXU + HEDT are always superior to that of RDT + TXU + TXU. When QG is very low at 1 m3 · h-1, just like that of no gas sparged, the kLa values of RDT + TXU + RDT and RDT + TXU + HEDT differ a little from each other. With the increase of QG of the sparged gas, the two lines separate gradually and RDT + TXU + HEDT shows better transport performance than RDT + TXU + RDT. Nothing is absolute. Multiple-impeller design should base itself on specific design objectives. When the dual-impeller configuration of gas-liquid surface aeration is used, the upflow impeller at the bottom helps the gas dispersion and liquid mixing by creating a favorable flow pattern. While the triple-impeller configuration in the case of surface aeration combined with sparger aeration is used, the major job of the bottom is to shear as vigorously as possible the bubbles sparged from the ring sparger and disperse them in the bottom region. The Rushton disk turbine (RDT) has high local shear performance and is suitable for dispersion purpose. Its central disk can retain the bubble bypassing the blades so that it is effective to prevent flooding. So up to now, RDT is one of the most commonly used mixer for gas-liquid systems in chemical industry, particularly in the cases of low or intermediate gassing rates.3,22 But the theory about the formation of gas cavities shows that the structure of RDT tends to form gas cavities in the impeller region and leads to decreased efficiency. HEDT has similar performance on gas dispersion as RDT, but its arched blades can prevent the formation of gas cavities in the impeller region. At the same time, its power number is lower than that of RDT so that energy consumption is reduced. Therefore, RDT + TXU + HEDT is the best tripleimpeller configuration for gas-liquid transport in our experiments. 3.3. Optimization of Flow Rate of Sparging Gas. Surface aeration combined with sparger aeration could remove the dead zone at the bottom of the stirred vessel, improve gas-liquid whole mixing, and also enhance the mass transfer coefficient.
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Figure 3. kLa values of three triple-impeller configurations with surface aeration and sparger aeration combined: (A) RDT + TXU + TXU; (B) RDT + TXU + HEDT; (C) RDT + TXU + RDT. (a) QG ) 1 m3 · h-1. (b) QG ) 2 m3 · h-1. (c) QG ) 3 m3 · h-1. (d) QG ) 4 m3 · h-1. (e) QG ) 5 m3 · h-1.
Figure 4 depicts the gas-liquid volumetric mass transfer coefficients as a function of volumetric gas flow rates at a fixed impeller speed with RDT + TXU + HEDT. The value of kLa at a zero gas flow rate is sorely the contribution of surface aeration, which constitutes the most important part in the overall gas-liquid mass transfer. It shows that at all rotational speeds the gas-liquid volumetric mass transfer coefficient increases quickly when the volumetric gas flow rate changes from 0 to 3 m3 · h-1, but from 3 to 5 m3 · h-1, the increase is not significant. Besides, a greater volumetric gas flow rate means more power consumption for compressing air and greater critical impeller speed for complete dispersion of the gas. So in this experiment, the volumetric gas flow rate has an optimal value of 3 m3/h, at which the advantages of sparger aeration are exploited suf-
ficiently and the disadvantages of sparger aeration are avoided at their full ability. 3.4. Effect of Gas Sparging on Surface Aeration. Another issue that has to be discussed is the effect of gas sparging on surface aeration. As reported by Nienow et al.,18 Matsumura et al.,19 and Veljkovic and Skala,6 the rate of surface aeration decreases with an increase in the superficial gas velocity or the sparging rate because of the effective density in the aerating impeller zone, and as a result, the effectiveness of the impeller is reduced. It is very difficult to determine the rate of surface aeration directly up to now. Second, it is also difficult to measure the power consumption of the surface aerating impeller only. In this experiment, the diameter of the aerating impeller is one-
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bubble holdup and the uniformity of bubble distribution throughout the vessel. 4. Conclusions
Figure 4. Gas-liquid volumetric mass transfer coefficient versus volumetric gas flow rate stirred by RDT + TXU + HEDT.
In a stirred vessel with the aspect ratio of 2.4, the gas-liquid mass transfer characteristics of triple-impeller configurations with the combination of surface aeration and sparger aeration have been studied. The following conclusions can be drawn: (1) The dead zones in the bottom zone of the vessel would diminish gradually and disappear at last with the increase of impeller speed or volumetric gas flow rate. Gas dispersion is improved greatly at the same time. (2) Triple-impeller configurations are optimized by changing the bottom impeller. It is found that RDT + TXU + HEDT is superior to RDT + TXU + RDT and RDT + TXU + TXU based on the kLa values vs power consumption. (3) At a fixed impeller speed, the kLa value increases rapidly with the increase of volumetric gas flow rate in its lower range, and once the volumetric gas flow rate is greater than 3 m3 · h-1, the increase of kLa tends to vanish. The value 3 m3 · h-1 is an optimal volumetric gas flow rate, at which the advantages of sparger aeration are exploited sufficiently and the disadvantages of sparger aeration are avoided at their full ability. (4) Unlike other researchers’ conclusions, the sparging gas has less effect on the surface aeration, because the vessel is stirred by the triple-impeller configuration and is of larger aspect ratio. Acknowledgment
Figure 5. Power consumption versus impeller speed at different gas flow rates stirred by RDT + TXU + HEDT.
half that of the vessel but the lower impellers are both onethird. At the same time, RDT is used as the aerating impeller and has a relatively large power number. So for the tripleimpeller configurations in this experiment, the aerating impeller accounts for the vast majority of the power consumption. In all the studies on the mechanisms of surface aeration, surface turbulence was thought of as the most dominant factor causing surface aeration. Therefore, the aerating impeller takes the majority of the energy of the whole configuration. So, the energy consumption can be used to index the effect of surface aeration indirectly. The power consumptions of the triple-impeller configuration RDT + TXU + HEDT are plotted in Figure 5. In this experiment, the maximum value of the decrease of power consumption is about 35% when the impeller speed is about 270 rev · min-1. As it is known for a stirred vessel with sparger aeration, this value can be approximated to 80%.26 Different from other reports, the present stirred vessel has a sufficiently large aspect ratio and the gas was always sparged at medium low volumetric gas flow rates in this experiment. Besides these, triple-impeller configurations are adopted. The gas sparged into the vessel was dispersed by the bottom impeller first, and then some ascending gas was dispersed by the middle impeller again. So, only a small part of the gas input arrived at the surface aerator directly. Thus, a conclusion can be drawn that the decrease of power consumption is mainly due to the bottom impeller and the middle impeller, and the effect of sparging gas on surface aeration can be ignored comparing its effect on increasing
The authors acknowledge financial support from the 973 Program (2007CB714305), the National Natural Science Foundation of China (Nos. 20676134, 20236050), 863 Program (2007AA060904), the National Project of Scientific and Technical Supporting Program (2008BAF33B03), and the Project of Scientific and Technical Supporting Program in Jiangsu Province (BE2008086). Notation g ) gravitational constant, m · s-2 H ) clear liquid height in a stirred vessel, m h ) clear height of ring distributor, m N ) agitation speed, rev · min-1 P ) total power consumption, kW · m-3 PSP ) expansion work of the sparged gas, kW · m-3 PST ) agitation power, kW · m-3 QG ) volumetric gas flow rate, m3 · h-1 Tq ) torque, N · m Greek Letters Fl ) liquid density, kg · m-3
Literature Cited (1) Oldshue, J. Y. Mixing and blending. In Unit Operation Handbook; McKetta, J. J., Ed.; Marcel Dekker: New York, 1993; Vol 2, pp 428-463. (2) Patwardhan, A. W.; Joshi, J. B. Design of stirred vessels with gas entrained from free liquid surface. Can. J. Chem. Eng. 1998, 76 (3), 339– 364. (3) Wu, H. An issue on application of a disk turbine for gas-liquid mass transfer. Chem. Eng. Sci. 1995, 50 (17), 2801–2811. (4) Matsumura, M.; Sakuma, H.; Yamagata, T.; Kobayashi, J. Gas entrainment in a new gas entraining fermentor. J. Ferment. Technol. 1982a, 60 (1), 457–467. (5) Matsumura, M.; Sakuma, H.; Yamagata, T.; Kobayashi, J. Performance of oxygen transfer in a new gas entraining fermentor. J. Ferment. Technol. 1982b, 60 (1), 551–563.
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(6) Veljkovic, V. B.; Skala, D. Effect of number of turbine impellers on surface aeration in laboratory fermentor. Biotechnol. Bioeng. 1989, 34 (2), 207–213. (7) Yu, G. Z.; Mao, Z.-S.; Wang, R. A novel surface aeration configuration for improving gas-liquid mass transfer. Chin. J. Chem. Eng. 2002, 10 (1), 39–44. (8) Li, X. Y.; Yu, G. Z.; Mao, Z.-S. Optimization of dual-impeller configuration of gas-liquid surface aerator. Chin. J. Process Eng. 2005, 5 (6), 601–604; in Chinese. (9) Dohi, N.; Matsuda, Y.; Itano, N.; Shimizu, K.; Minekawa, K.; Kawase, Y. Mixing characteristics in slurry stirred tank reactors with multiple impellers. Chem. Eng. Commun. 1999, 171 (1), 211–229. (10) Saravanan, K.; Patwardhan, A. W.; Joshi, J. B. Critical impeller speed for solid suspension in gas inducing type mechanically agitated contactors. Can. J. Chem. Eng. 1997, 75 (4), 664–676. (11) Pinelli, D.; Nocentini, M.; Magelli, F. Hold-up in low viscosity gas-liquid systems stirred with multiple impellers. Comparison of different agitator types and sets. Proceedings of the 8th European Conference on Mixing, Cambridge, U. K., September 21-23, 1994; pp 81-88. (12) Moucha, T.; Linek, V.; Prokopova, E. Gas hold-up, mixing time and gas-liquid volumetric mass transfer coefficient of various multipleimpeller configurations: Rushton turbine, pitched blade and techmix impeller and their combinations. Chem. Eng. Sci. 2003, 58 (9), 1839–1846. (13) Gogate, P. R.; Beenackers, A. A. C. M.; Pandit, A. B. Multipleimpeller systems with a special emphasis on bioreactors: a critical review. Biochem. Eng. J. 2000, 6 (2), 109–144. (14) Shewale, S. D.; Pandit, A. B. Studies in multiple impeller agitated gas-liquid contactors. Chem. Eng. Sci. 2006, 61 (2), 489–504. (15) Alliet-Gaubert, M.; Sardeing, R.; Xuereb, C.; Hobbes, P.; Letellier, B.; Swaels, P. CFD analysis of industrial multi-staged stirred vessels. Chem. Eng. Process. 2006, 45 (5), 415–427. (16) Bao, Y. Y.; Hao, Z. G.; Gao, Z. M.; Shi, L. T.; Smith, J. M. Suspension of buoyant particles in a three phase stirred tank. Chem. Eng. Sci. 2005, 60 (8-9), 2283–2292.
(17) Moucha, T.; Linek, V.; Erokhin, K.; Rejl, J. F.; Fujasova´, M. Improved power and mass transfer correlations for design and scale-up of multi-impeller gas-liquid contactors. Chem. Eng. Sci. 2009, 64 (3), 598– 604. (18) Nienow, A. W.; Chapman, C. M.; Middleton, J. C. Gas recirculation rate through impeller cavities and surface aeration in sparged agitated vessels. Chem. Eng. J. 1979, 17, 111–118. (19) Matsumura, M.; Masunaga, H.; Kobayashi, J. A correlation for flow rate of gas entrained from free liquid surface of aerated stirred tank. J. Ferment. Technol. 1977, 55, 388–400. (20) Mills, P. L.; Chaudhari, R. V. Reaction engineering of emerging oxidation processes. Catal. Today 1999, 48 (1-4), 17–29. (21) Imai, Y.; Takei, H.; Matsumura, M. A simple Na2SO3 feeding method for kLa measurement in fermentors. Biotechnol. Bioeng. 1987, 29 (8), 982–993. (22) Myers, K. J.; Fasano, J. B.; Bakker, A. Gas dispersion using mixed high-efficiency/disc impeller systems. Proceedings of the 8th European Conference on Mixing, Cambridge, U. K., September 21-23, 1994; pp 6472. (23) Veljkovic, V. B.; Bicok, K. M.; Simonovic, D. M. Mechanism onset and intensity of surface aeration in geometrically similar, sparged, agitated vessels. Can. J. Chem. Eng. 1991, 69, 916–926. (24) Topiwala, H. H. Surface aeration in a laboratory fermenter. J. Ferment, Technol. 1972, 50, 668–675. (25) Greaves, M.; Kobbacy, K. A. H. Surface aeration in agitated vessels. I. Chem. Symp. Ser. 1981, 64, H1. (26) Long, J. G.; Bao, Y. Y.; Gao, Z. M. Gas-liquid dispersion in a stirred tank with different impeller combinations. J. Beijing UniV. Chem. Technol. 2005, 32 (5), 1–5; in Chinese.
ReceiVed for reView April 20, 2009 ReVised manuscript receiVed July 14, 2009 Accepted July 27, 2009 IE900623M
