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Chemical and Process Engineering Resources

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Category: Mass Transfer
Question: What are some good tank mixing rules of thumb?
Keywords: tank,mixing,rules,thumb,baffles,mix,vessel
Answer: Rules of thumb?....well, here are some basics of mixing:For fluid with viscosities under 10,000 Cp, baffles are highly recommended. There should be four baffles, 90 degrees apart. The baffles should be 1/12th the tank diameter in width and should be spaced off the wall by 1/5th the baffle width. The off wall spacing helps to eliminate dead zones. If baffles are used, the mixer should be mounted in the vertical position in the center of the tank.If baffles are not used, the mixer should be mounted on an angle, ~15 degrees to the right and positioned off center. This breaks up the symetry of the tank and simulates baffles although not nearly as good as baffles.The pupose of baffles is to prevent solid body rotation, all points in the tank are moving at the same angular velocity and no top to bottom turnover. The formation of a large central vortex is a characteristic of solid body rotation. However small vortices which travel around the fluid surface, collapse and reform are more a function of the level of agitation. Violent and vigorous agitation will have these vortices present. In fact, they are desired for processes which require solids addition from the liquid surface.The impellers are located at different positions depending on the design. Axial flow impellers, those pumping parallel to the shaft should be positioned between 0.5 and 1.5 impeller diameters off the bottom of the tank. Radial flow impellers, those pumping perpendicular to the shaft can be positioned just inches off the bottom. If multiple impellers are used, the spacing will depend on the liquid height to tank diameter ratio. Care must be taken to prevent impeller spacings on 1 impeller diameter. This can lead to a cancellation of flow.Torque is one of the most important factors. A large diameter slow spinning impeller is much better for blending than a small diameter fast spinning impeller at equal power levels. Torque = Work.Impeller diameters for relatively low viscosities should be between 0.25 and 0.45 times the tank diameter with some exceptions.