Experimental Investigation And Numerical Simulation On Gas-Liquid Mixing Process For Polyether Polyol

The thesis were investigated the characteristics of gas-liquid mixed for polymerization system in a stirred tank with triple impellers by experimental research and numerical simulation. Polyether polyol(PPG)was chosen in this study as a research medium. The effects of temperatures, rotary speeds, gas flow rates and different combination of triple impellers on the characteristic of gas-phase flow, stirring power and gas holdup were investigated in the air-PPG system. The results supplied a theoretical basis to the design, scale-up and optimization of polymerization mixing equipment and the numerical simulation for gas-liquid two-phase flow in polymerization system.Firstly, using two types of multiple impellers (Multiple impellersⅠis a 6-HEDT as bottom and two WHu above 6-HEDT. Multiple impellersⅡis the 6-XYK as bottom, the 6-XDT as middle and 6-HEDT as upper). Aim for air-PPG or air-water as gas-liquid systems, the power consumption and gas holdup characteristics of two types of multiple impellers were investigated in a stirred tank at 20℃～120℃. In the same conditions, the results showed that gas holdup increased, while Relative Power Demand (RPD) decreased along with the raising of gas flow rate and rotary speeds in the two systems. With the temperature rising, the gas holdup in the above two systems and the RPD in the air-polyether polyols system decreased. While, the RPD in the air-water system increased. Additionally, multiple impellersⅠhad better performance than multiple impellersⅡfor its higher gas holdup and stirring power under the aerated condition. So it was more suitable for the operation of gas-polyether polyols dispersion. In the current study, the empirical equations between the gas holdup and the RPD were attained by multiple linear regression, which could provide a theoretical basis for designing and magnifying different gas-liquid systems stirred tank at high temperature conditions.Secondly, the flow characteristics of the PPG stirred in the tank with two types of triple impellers was investigated by using simulation and Laser Doppler Velocimetry (LDV) Especially the flow field, velocity distribution and power consumption were analyzed. The result showed that multiple impellersⅠhad wide distribution of high-speed zone, small range of tank "dead zone", large axial and radial velocity, and small power consumption. Numerical simulation results corresponded well with LDV experimental values. Numerical simulations showed that the chosen method was completely feasible, reliable and more suitable for application in the polymerization system in stirred tank. Three velocity components in the tangential velocity of the larger, and thus how to reduce the tangential motion of fluid, increase the radial and axial mixing is the key to improve the mixing effect of polymer mixing tank.Finally, the Euler-Euler model, multiple frames of reference (MFR) method and Laminar model were used to simulate the multiple impeller in the flow characteristics of air-PPG in stirred tank with triple impellers. Through the analysis of simulation, macro-velocity field, the overall and local gas holdup distribution details, and ventilation mixing power were obtained under different rotary speeds and gas flow rates. Then the RPD and overall gas holdup simulation were compared with experimental values. The results showed that Euler-Euler model and the assuming a single bubble size were feasible in air-PPG numerical simulation, which can accurately reflect the actual fluid flow pattern. However, under the high gas fluxes and rotating speeds, simulation values were lower than the experimental.