Vertical impeller mixers

Vertical impeller mixers employ a screw-type impeller, which constantly overturns the batch. 

Figure 5-20 illustrates a vertical propeller mixer assembly, with vertical mounting with gear box and motor. Figure 5-21 is a typical right angle, vertical impeller shaft with horizontal gear and motor drive. 

Axial-Flow Fluidfoil Impellers For vessel volumes of 4 to 200 m (1000 to 50,000 gal), a turbine mixer mounted coaxiaUy within the vessel with four or more baffles should be the initial choice. Here also the vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5. Four vertical baffles should be fastened perpendicularly to the vessel wall with a gap between baffle and wall equal to Df/24 and a radial baffle width equal to Df/12. 

When the flow pattern in a mixed tank is primarily tangential, the fluid discharge from the impeller to the surroundings and its entrainment into the impeller are small. Also, fluid transfer in the vertical direction is at a minimum. The mixing effect is lowest when the rotational velocity of the liquid approaches that of the mixer. 

Physical depth or height of turbine mixer, ft or in., consistent with other dimensions. Figure 5-34 = Impeller blade wndtii, ft = width of baffles in vertical tank. Figure 5-34. 

Convection mixers use a different principle for blending. These mixers have an impeller. This class includes ribbon blenders, orbiting screw blenders, vertical and horizontal high-intensity mixers, as well as diffusion blenders with an intensifier bar. Scale-up considerations are similar to those for the tumble blenders. 

A vertical helical ribbon blender can be combined with an axial screw of smaller diameter (Fig. 18-25). Such mixers are used in polymerization reactions in which uniform blending is required but in which high-shear dispersion is not a factor. Addition of the inner flight contributes little more turnover in mixing newtonian fluids but significantly shortens the mixing time in nonnewtonian systems and adds negligibly to the impeller power [Coyle etal.. Am. Inst Chem. Eng.J., 15, 903 (1970)]. 

---