The mixing of highly viscous fluids is a common operation in the chemical, pharmaceutical,
biochemical and food industries. Nevertheless, efficient mixing remains a difficult task, as does the design and scale-up of the stirred tank itself. Close clearance impellers such as screws, helical ribbons and anchors are well adapted to the processing of highly viscous liquids; however, they are less efficient when a change in viscosity occurs throughout the process. For such applications, coaxial stirring systems combining a close clearance agitator with a turbine or axial flow impeller, and also large diameter ratio axial agitators, e.g. Intermig or double-flux impellers, may be more appropriate.
