Modelling of multiphase flow containing ionic liquids in a stirred tank reactor

Abstract

Stirred tanks are widely used in the chemical reactions and the mixing operations for process industries to enable high product quality and process efficiency. Despite there being a large body of studies on the hydrodynamics of water in the stirred tanks, the understandings of the hydrodynamics of the ionic liquids in the stirred tanks are still very limited. In this study, Computational Fluid Dynamics (CFD) modelling is used to investigate the detailed flow characteristics of the single and multiphase ionic liquid flows in the stirred tanks which are experimentally validated using Particle Image Velocimetry (PIV). The ANSYS FLUENT was employed in this investigation to carry out the CFD simulation. Initially, the hydrodynamics of single phase flows were numerically studied where the single phase turbulent water flow and single phase transitional ionic liquid flow were modelled using a RANS and LES approach respectively in the three stirred tanks equipped with different bottom shapes and length of baffles. The simulation results indicated that the bottom shape and baffles’ length have significant effect on the flow field in a stirred tank when the water was operated in the turbulent state, where a large dead zone region was identified below the impeller. However, the magnitude of the dead zone region reduced a lot when the ionic liquid was operated in the transitional state. Before carrying out the gas-ionic liquid multiphase flow simulation in a stirred tank, the bubble size needs to be identified as it is crucial information for the accurate gas-ionic liquid multiphase flow modelling. In order to obtain the bubble size data, a high speed camera and a microscope were employed to experimentally measure the bubble size in the ionic liquid solutions. The correlations between the bubble size in the ionic liquid solutions and the impeller agitation speed were established. It showed that both the bubble breakage and coalescence has significant effect on determining bubble size in the ionic liquid. In addition, it was suggested that the surface tension of the ionic liquid is more important than the liquid viscosity on affecting the bubble size in the stirred tank. Afterward, the gas-ionic liquid multiphase flow modelling was carried out in the stirred tank at various impeller speeds and gassing rates. The simulation results indicated that the presence of gas phase did not have significant effect on changing the flow of liquid phase under the selected operation conditions due to the small bubble size, low gas flow rate and high viscosity of ionic liquid. The gas phase followed well with the liquid phase and circulated in the majority region of the stirred tank, which implied better gas holdup and mass transfer of the multiphase flow system. A correlation was proposed to predict the impeller power consumption of the gas-ionic liquid transitional flow in a stirred tank agitated by a Rushton turbine impeller. Finally, in order to validate the above single phase and multiphase flow CFD models adopted in this study, an experimental rig was established and the advanced visualization technique Particle Image Velocimetry (PIV) was used to measure the single phase water and ionic liquid flows and gas-ionic liquid multiphase flow in a stirred tank. The PIV data showed agreement with the CFD results in terms of the flow pattern and velocity components, which indicates good accuracy of the computational models and approaches presented in this investigation.

Date of Award	10 Nov 2017
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Binjie Hu (Supervisor), Xiaogang Yang (Supervisor) & Nick Miles (Supervisor)