CFD-DPM simulation of gasliquid dispersion in a dual impeller mixing tank
LI Liang-Chao, XU Bin, XIANG Ke-Feng
(Key Laboratory of Testing Technology for Manufacturing Process of Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China)
Abstract：In order to study the gas dispersion and backmixing′s effect on its function in a gasliquid mixing reactor, on the basis of the computational fluid dynamics (CFD) software package Fluent 6.2, with a Euler model of liquid phase, and a discrete phase mode (DPM) of gas, discrete gas phase gasliquid flow in a dual sixbladediscturbine (6-DT) impeller mixing tank was numerically simulated. The macro flow field and gas residence distribution in the mixing tank was obtained. By counting bubbles escaped from the vessel at different times, gas residence time distribution in the tank was analyzed. The simulation results show that typical double circulation pattern is formed for every disc turbine and there are four circulation loops in the mixing tank. Gas easily gathers together at the center of the circulation loops, which leads to high local gas holdup there and relatively low gas holdup in the bottom region and regions between two impellers. The curve of gas residence time distribution displays as a singlepeak and a long tail, which is mainly affected by flow patterns, bubble sizes and operating circumstances. To increase impeller speed or decrease gas flow will be helpful for backmixing and gas dispersion in the mixing tank.
［1］Petitti M, Nasuti A, Marchisio D L, et al. Bubble size distribution modeling in stirred gasliquid reactors with QMOM augmented by a new correction algorithm［J］. AIChE Journal, 2010, 56(1): 36-53. ［2］Buffo A, Vanni M, Marchisio D L. Multidimensional population balance model for the simulation of turbulent gasliquid systems in stirred tank reactors ［J］. Chemical Engineering Science, 2012, 70: 31-44. ［3］Gao Zhengming, Smith J M, MullerSteinhagen H. Void fraction distribution in sparged and boiling reactors with modern impeller configuration［J］. Chemical Engineering and Processing, 2001, 40(6): 489-497.［4］Khopkar A R, Rammohan A R, Ranade V V, et al. Gasliquid flow generated by a Rushton turbine in stirred vessel: CARPT／CT measurements and CFD simulations［J］. Chemical Engineering Science, 2005, 60(8／9): 2215-2229.［5］Lane G L, Schwarz M P, Evans G M. Predicting gasliquid flow in a mechanically stirred tank［J］. Applied Mathematical Modeling, 2002, 26(2): 223-235.［6］Laakkonen M, Moilanen P, Alopaeus V, et al. Modelling local bubble size distributions in agitated vessels ［J］. Chemical Engineering Science, 2007,62(3):721-740.［7］李良超,王嘉骏,顾雪萍,等. 气液搅拌槽内气泡尺寸与局部气含率的CFD模拟［J］. 浙江大学学报:工学版,2010,44(12): 2396-2400,2415.Li Liangchao, Wang Jiajun, Gu Xueping, et al. Computational fluid dynamics simulation of bubble size and local gas holdup in stirred vessel ［J］. Journal of Zhejiang University: Engineering Science, 2010, 44(12): 2396-2400, 2415. (in Chinese)［8］Han Luchang, Liu Yuejin, Luo He′an. Numerical simulation of gas holdup distribution in a standard rushton stirred tank using discrete particle method ［J］. Chinese Journal of Chemical Engineering, 2007,15(6): 808-813.［9］Haider A, Levenspiel O. Drag coefficient and terminal velocity of spherical and nonspherical particles［J］. Powder Technology, 1989, 58(1): 63-70.［10］王凯,冯连芳. 混合设备设计［M］. 北京:机械工业出版社,2000.
