Study on solid-liquid suspension characteristics in a slurry polymerization reactor with different impeller combinations
Slurry process is the main production method for high-density polyethylene.For slurry of high-density polyethylene stirred polymerizer,three-blade-backswept impeller HQ,parabolic disc turbine impeller BTD,three blade hydrofoil impeller KHX,pitched blade disc turbine impellers ZY with blade placement angle of δ=45° andδ=75°,and straight blade disc turbine impellers PY160 were used to form four types of impeller combinations.Solid-liquid suspension experiments were conducted in a cylindrical perspex stirred tank with a diameter of T=480 mm.The CFD software Ansys Fluent 2020R2 combined multiple reference frame method and Euler-Euler multiphase flow model were used to study the solid-liquid suspension of each impeller combination at 30.71％solid holdup.The results showed that when the rotating speedN≤250 r/min,a clear liquid layer formed at the top of the mixing tank with impeller combination 2 and 3.Due to the inhomogeneous of solid holdup,the power of impeller combination 3 and 4 showed an inflection point at the rotating speed N=350 r/min.When the rotating speed N≤350 r/min,the combined power of each paddle type from low to high was:2,4,3,1.When the rotating speed N＞350 r/min,the mixing power from low to high was:2,3,4,1.Impeller combination 1 and 4 can achieve uniform mixing at lower speeds and power,and the power consumption of impeller combination 4 was about 30％lower than that of impeller combination 1,so impeller combination 4 was high efficiency and energy conservation.The simulated results of solid holdup were in good agreement with the experimental values which was measured by the sampling method.The simulated flow field indicated that the flow patterns of impeller combination 4 and 1 were similar,which can effectively avoid the clear liquid layer appearing in impeller combination 2 and 3 at low rotating speeds.
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