Statiflo International Ltd

Principles of Operation

Statiflo Static Mixers can be applied to a wide range of process operations including blending, dosing, dispersion and emulsion formation, laminar flow heat exchange, mass transfer and as an inline ‘plug flow’ reactor. These processes can be broadly grouped into the following mixing classifications. The principles of operation of the Motionless Mixer vary according to each mixing classification described below.

Liquid/Liquid laminar flow of miscible fluids

Flow Division

The main mechanism in laminar flow in static mixers (Reynold’s number < 2000) is flow division. Statiflo element styles STT, STS and STL are either helical or pseudo-helical and are arranged in a series of alternating left and right hand 180° twists. The leading edge of an element, which is on a diameter, is at 90° to the trailing edge of the upstream element.

In flow division, the leading edge of the first element splits the fluids entering the mixer into 2 streams, which are then rotated through 180°. The second element splits the flow again, this time into 4 streams, followed by a further rotation, in the opposite direction, through 180°. The third element repeats the process by splitting into 8 streams, and so on. As the number of streams or layers increases, the layer thickness decreases. Typically, 12 to 24 elements are required to provide a complete mix.

Mixture quality is a function only of mixer diameter and number of elements and, in laminar flow, is independent of flowrate and viscosity.

Liquid/Liquid turbulent flow of miscible fluids

At higher Reynold’s numbers, much greater than 2000, a second mixing mechanism, acting simultaneously with flow division, becomes important to the overall mixing process:

Radial Mixing

In general terms, the fluid viscosity in turbulent flow is lower than in laminar flow. The element shape is now able to impart a rotational spin to the fluids, which changes direction with each succeeding element. Fluids are constantly moved from the pipe centre to the pipe wall and back again, with the interface between elements a particularly active zone. This mechanism is called radial mixing, which dominates the flow division mechanism in turbulent flow. It very rapidly eliminates radial differences in, for example, composition, colour, pH, temperature and velocity.

The number of mixing elements required to achieve a fully homogeneous mix in turbulent flow applications is much less than in laminar flow and is typically 1.5 to 4 elements.