Simulation Of Solid-liquid Suspension In Stirred Vessels With Side-entering Agitators

Nowadays, the environment pollution of the world gets worse and worse. Many flue gas desulfurization(FGD) towers are installed in power plants. There is a typical side-entering agitation at the bottom of the FGD tower. Side-entering agitation is a special kind of stirring. It is widely used in the field of petrochemistry, paper-making, food and environment industry. Its characteristic is as following:the diameter of the stirrer is comparatively little, the installing location is especial, the power of the agitation is comparatively small. Because of the particularity of its flow pattern, four axial flow impellers are installed symmetrically at the bottom of the side wall of quite large stirred vessel with flat bottom. There is not only a horizontal deviating angle but also a vertical deviating angle of the impeller shaft. The vertical angle is always 10°. It makes the axial flows from the impellers go to the bottom of the tank and blow the solid particles up in order to reduce the deposit. The horizontal angle is usually between 4°and 12°. It produces circumfluence at the bottom and increases the area four axial flows pass by. So the liquid flow gets stronger and the solid deposit decreases. Four axial flows hit each other in the center of the tank bottom. A rising flow is made, and it goes to the surface and spreads around, and goes down along the side wall. So the flow goes everywhere in the vessel and the solid particles suspend.The effect of solid-liquid suspension at the bottom of the FGD towers plays an important role in the desulfurization effect of the whole system, and the horizontal deviating angle of the impeller shaft is the key factor on liquid flow field and solid suspension. But until now, the research on the simulation of solid-liquid suspension in stirred vessel with side-entering agitators is very few, so the following work with Fluent software was done:The numeric model of the vessel with side-entering agitators was built. The two-phase system consisted of gypsum crystal and mixture liquid at the bottom of FGD tower. Solid particle volume fraction was 15%, and the particle diameter was 50μm. Standard k-εturbulent model was used to simulate the steady liquid phase flow field, and the unsteady solid-liquid suspension process was simulated with the Eulerian multiphase model. The influences made by horizontal angle and rotate speed on solid-liquid suspension performance in stirred vessel with side-entering agitators was extensively investigated in the way of liquid velocity field, solid particle concentration distribution and power consumption. Optimum horizontal angle and rotate speed were confirmed.By analyzing the results of simulation, the following conclusions were obtained:the rising liquid flow in the center zone of the tank influnces the solid suspension most. The rising speed of liquid flow after adding solid is less than before adding solid, and the flow pattern also makes a certain change. With the horizontal angle a increasing, the fluid cycle at the bottom gets stronger, and the area of deposit reduces, but the rising flow in the center zone gets weaker so that the solid suspension gets weaker.αmakes different influnces on flow pattern and liquid velocity in different locations in side-entering agitation. Whenαis between 8°and 10°, flow pattern and liquid velocity is best for solid suspension. Currently, the deposit area at the bottom is relatively small and the average solid particle concentration is low. Considering power saving, the power consumption is the least whenα=10°. So optimum horizontal angle is 10°. The rotate speed increasing makes liquid velocity bigger everywhere in the vessel. But when rotate speed is beyond 2400rpm, the liquid rising speed increases slower. Increasing the rotate speed is good for solid suspension. When the rotate speed is equal to or bigger than 2400rpm, solid particles can suspend all over the tank. As rotate speed increases, the unit volume power increases nonlinearly. Considering all factors, the optimum rotate speed is 2400rpm. At this time, the power consumption is medium, the suspension satisfies the demand. All conclusions above offer some references for operations in the industry.