Principles of Process Agitators

The Science behind Process Agitators

Technically speaking, for a liquid mixed in a tank with a rotating agitator, the shear rate is greatest in the immediate vicinity of the agitator. The shear rate decreases exponentially with distance from the agitator. Thus the shear stresses and strain rates vary greatly throughout an agitated liquid in a tank. As the viscosity of the fluids to be mixed increases, the physics fluid mechanics change from that of turbulent flow (like in liquid agitators) to that in which viscous drag forces dominate. Additionally, some fluids exhibit Non-Newtonian behavior – their viscosity cannot be designated by a single coefficient. Mixing of such fluids requires special heavy duty agitators. As the dynamic viscosity of a Newtonian liquid is independent of shear at a given temperature, its viscosity will be the same at all points in the tank. In contrast the apparent viscosity of a non-Newtonian liquid varies throughout the tank. This in turn significantly influences the mixing process. For shear thinning liquids, the apparent viscosity is at a minimum in the immediate vicinity of the agitator. The progressive increase in the apparent viscosity of a shear thinning liquid with distance away from the agitator tends to dampen eddy currents in the mixing tank. In contrast, for shear thickening liquids, the apparent viscosity is at a maximum in the immediate vicinity of the agitator. In general shear thinning and shear thickening liquids should be mixed using high and low speed agitators respectively.

Fluid Movement in Process Agitators

Wherever possible, the movement of the entire contents of the agitator vessel should be avoided as rapid movement tends to segregate the components due to centrifugal forces.

The mixing time is short if the components to be mixed undergo a large number of changes of location. This can take the form of movement of the agitator itself or of material flows generated by the agitator. They can be achieved by impact, flow around obstacles, crossing directions of flow and speed differentials at the interfaces of parallel flows.

Process Agitators – Design

The design and the operation of the agitator are crucial for efficient operation. However, there is no strict science that governs the best design. Rather, the designing of good process agitators is more a matter of experience. A core knowledge of fluid dynamics is must for good agitator design.

Other Types of Agitators