Numerical Simulation Of D-T Model Horizontal Biaxial Agitated Tank

The separation and purification of the mixture with high viscosity, high freezing point and strongly heat-sensitivity is quite a hard problem in distillation process. There are lots of problems in separation equipment with the mixture: poor fluidity, long residence time and carbonization and expanded for local overheated. A new type of reboiler with horizontal biaxial stirring shafts could solve these problems. The purpose of this paper is to study on this reboiler through the computation of fluid dynamics and find the flow characteristics, power performances and residence time in the equipment.A proper three-dimensional model of the reboiler equipped with horizontal biaxial stirring shaft was constructed. Meshing with the unstructured grid technology; the flow characteristics and residence time (RT) in various viscosity in the tank, and power performances as well, are investigated. The influences of liquid velocity and rotation speed on the characteristics mentioned above are studied by use of dynamic mesh technique, multiphase flow and unsteady model.The results show that the axial flow of liquid was mainly concentrated near the clean-up shaft side, the velocity of liquid around the impellers and blades is slightly varied; The influence on the axial flow of the main trend by stirring became more obviously with the smaller liquid inlet/outlet velocity , larger rotation speed, and lower viscosity. Under the same conditions, axial flow changed into radial flow more clearly. Meanwhile there will be more vortices and dead zone in the liquid flow. At larger liquid inlet/out velocity, the space of main axial flow will be larger, and the velocity of the main axial flow is larger than the liquid inlet velocity; the space of slow velocity of liquid around the impellers and blades will get smaller at larger rotation speed; the larger the viscosity of the liquid , the more deviated from the level of the interface between gas and liquid, the more violently movements of the gas. And the vortices are easily formed.The power of clean-up shaft and agitating shaft change regularly in a operation cycle and the power of clean-up shaft is more large than the other; when liquid holdup is half the vessel and the viscosity is small, the power of clean-up shaft and agitating shaft increases linearly with rotation speed on a log-log plot, which slope of the curve is 2.94 and 2.96 respectively , and the power is almost proportional to the cube of rotation speed, that is Ntotal∝ω³; under the condition of half the liquid holdup and the viscosity is high , the power of clean-up shaft and agitating shaft increases linearly with rotation speed on a log-log plot, which slope of the curve is 2.20 and 2.23; the power number(Np) decreases linearly with the Reynolds Number (Re) on a log-log plot in the laminar regime, Np₁=3.28Re₁^-0.77,Np₂=17.26Re₂^-0.76, but the slope of the curve is not -1. For the higher viscosity flow, there will be longer RT and more seriously back mixing.