Fluid Dynamics and Transport Phenomena Application of KHX Impeller in a Low-shear Stirred Bioreactor ☆ Shifang Yang, Xiangyang Li ⁎, Gang Deng, Chao Yang ⁎, Zaisha Mao Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China abstract article info Article history: Received 18 January 2013 Received in revised form 20 June 2013 Accepted 5 July 2013 Available online 6 September 2014 Keywords: Bioleaching reactor Draft tube Mass transfer coefficient Ore suspension Shear stress In our previous work, a low-shear stirred bioreactor was explored. With a pitched blade turbine impeller downflow (PBTD) used, the shear stress generated is high compared with that in some low shear axial flow im- pellers. KHX impeller is an efficient axial flow impeller, which provides large onflow diffusivity and low shear force. In this work, the KHX impeller was applied in a lower-shear bioreactor and the performance of this reactor was evaluated and compared with that of the PBTD impeller. The experimental results show that the KHX impel- ler can disperse gas at lower power consumption and gives greater gas–liquid volumetric mass transfer coeffi- cients than PBTD at the same power consumption. An empirical correlation for evaluating the mass transfer coefficient of the KHX impeller in the bioreactor is presented to provide reference for its industrial application. © 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved. 1. Introduction In our earlier work [1], a low shear stirred tank bioreactor was de- signed based on the specific features characterizing bioleaching reactors and conventional stirred tank reactors. Experimental studies on suspen- sion of solid particles in gas–liquid–solid systems were conducted to simulate a real bioleaching system to examine the performance of this novel reactor. The results show that the flow field in the reactor will be more uniform with a draft tube installed, reducing the maximal shear rate and the deviation from the average shear rate, which is beneficial to a bioreactor. However, with the pitched blade turbine downflow (PBTD) impeller used, the average shear stress is high com- pared to that in some low shear axial impellers. Based on the mechanism of solid suspension and gas dispersion, the solid suspension relies mainly on the turbulence generated by the im- peller toward the bottom. When the downward discharge of the impel- ler goes to the vessel bottom directly and turns toward the side walls, the liquid and solid particles are directed upwards to make particles suspend [2–5]. For gas dispersion, radial impellers can offer a high rate of shear to liquid flow, which helps in dispersing the gas sparged into the reactor to smaller gas bubbles, increasing interfacial area between liquid and gas phases [6–8]. In a gas–liquid–solid system, both solid sus- pension and gas dispersion are important. In order to compromise the demand for low shear and large discharge, the ratios of power numbers and flow numbers, N p /N qd , of some different types of impellers are com- pared in Table 1. A high ratio means low discharge capacity and a low ratio represents low shear performance. The KHX impeller (devised by Zhejiang Great Wall Reducer Co., Wenzhou, China) is an efficient axial flow impeller with power number of 1 and flow number of 0.75, which can provide large onflow diffusivity and low shear force. In this work, a KHX impeller is installed in the previously tested lower-shear bioreactor [1] and the performance such as critical impeller speed, power consumption and gas–liquid vol- umetric mass transfer coefficient k L a of this reactor configuration is evaluated and compared with that of using the PBTD impeller in the ear- lier work. 2. Experimental 2.1. Experimental setup The experimental setup used in this study is shown in Fig. 1. The bio- reactor configuration is the same as that in [1] and has been depicted Chinese Journal of Chemical Engineering 22 (2014) 1072–1077 ☆ Supported by the National Basic Research Program of China (2010CB630904), the National Natural Science Foundation of China (21276004, 20990224), the National Natural Science Fund for Distinguished Young Scholars (21025627) and the National High Technology Research and Development Program of China (2012AA061503). ⁎ Corresponding authors. E-mail addresses: xyli@home.ipe.ac.cn (X. Li), chaoyang@home.ipe.ac.cn (C. Yang). Table 1 Ratio of power numbers and flow numbers of different impellers [9,10] N p N qd N p /N qd Rushton 6.00 0.75 8.00 Lightnin A310 0.30 0.56 0.54 Lightnin A315 0.75 0.73 1.03 PBTD 2.60 1.13 2.30 KHX 1.00 0.75 1.33 http://dx.doi.org/10.1016/j.cjche.2014.09.001 1004-9541/© 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved. Contents lists available at ScienceDirect Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJCHE