In many operations it is necessary to mix together miscible liquids, e.g., the blending of petroleum products. This is sometimes regarded as a simple mixing duty since it involves neither chemical reaction nor interphase mass transfer. It is necessary only to reduce the non-uniformities, i.e., variation in concentration to some acceptable level. However, such blending operations can be difficult to achieve when the liquids have widely different viscosities or densities. Also, problems can be encountered if one of the liquids to be mixed forms only a small volume fraction of the final mix.

When two immiscible liquids, often of low viscosity, are agitated a system is created having dispersed liquid droplets in a continuous liquid phase. Such a situation is often created in solvent extraction units where a high interfacial area between the two immiscible liquid phases is necessary to achieve interphase mass transfer. Thus agitation is used to create conditions favorable for mass transfer and if stirring is stopped the two phases will separate, leading to a greatly reduced interfacial area.

Another very common industrial mixing process involving immiscible liquids is emulsification. This is frequently encountered in the food and pharmaceutical industries when very small liquid droplets are created in a second liquid phase. In these cases the resulting mixture is often stable and will separate only after long period of time. furthermore, the stable emulsion will usually be relatively viscous and will often exhibit non-Newtonian rheological characteristics.

Process variables that can be controlled for blending and motion include: blend time (or time to uniformity), heat transfer rate, and reaction rate.

In low viscosity fluids (typically less than 5,000 cps) High efficiency axial flow impellers, such as the HF218, are the best choice. In intermediate viscosity fluids (5,000 cps to 100,000 cps) axial flow impellers such as the AFT454 work best. In fluids with a viscosity over 100,000 cps an anchor or helix style impeller is the best choice.

Maintaining solids in suspension within a liquid is another very common application for fluid agitation equipment. Industrial examples are diverse as suspending ore in cyanide leach systems or diatomaceous earth in a filter aide slurry. The nature of the solids, the concentration of solids, and the degree of suspension desired all contribute to agitator design. In order for solids to be suspended the impeller must create enough localized flow to over come the settling velocity of the material to be suspended.

High efficiency axial flow impellers (HF218) and the flow pattern they produce are best suited for suspending solids.

The degree of suspension is typically broken down into three different categories in order of increasing difficulty.

On-Bottom Motion—all particles moving on the bottom, with the lighter ones suspended off the bottom.

Off-Bottom Suspension—all particles suspended off the bottom, but not uniformly.

Complete Uniformity—all particles suspended uniformly throughout the tank.

The process variables that can be controlled in this category include dissolution rate for soluble solids, yields (crystallizers) and mass transfer.

Several major industrial operations, e.g., hydrogenation, and biological fermentations, involve the contacting of gases and liquids. It is the objective of such processes to agitate the gas-liquid mixture thus generating a dispersion of gas bubbles in a continuous liquid phase. Mass transfer then takes place across the gas-liquid interface. In some instance chemical reactions may also accompany the mass transfer in the liquid phase.

The main process variable that can be controlled with liquid/gas systems is the mass transfer rate.

Gas Dispersion has been done primarily with multiblade radial style impellers such as the RDT 710 (Rushton Impeller). More recently wide blade high efficiency impellers such as the HYF 518 have gained favor with lower energy requirements for equivalent mass transfer.