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 one- third 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 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 disad- vantages. 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, k L a, of triple- impeller configurations. 2. Experimental Section The experiments were carried out in a flat-bottomed cylindri- cal 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 configu- rations 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 m 3 ). The power consumption was determined by measuring the torque on the * To whom correspondence should be addressed. Tel.: (+86) 10 6255 4558. Fax: (+86) 10 6255 4558. E-mail: chaoyang@home.ipe.ac.cn. † Chinese Academy of Sciences. ‡ Jiangsu Marine Resources Development Research Institute. Ind. Eng. Chem. Res. 2009, 48, 8752–8756 8752 10.1021/ie900623m CCC: $40.75  2009 American Chemical Society Published on Web 08/04/2009 Downloaded by INSTITUTE OF PROC ENGINEERING on October 3, 2009 | http://pubs.acs.org Publication Date (Web): August 4, 2009 | doi: 10.1021/ie900623m