Static mixer

A static mixer or static mixer is a device for mixing fluids in which only the fluid movement causes the mixing and that does not have moving elements. It consists of the flow -influencing element in a tube. This share alternately on the material flow and then lead him back together, meaning that mixing is achieved. Static mixers are suitable for combinations of liquid / liquid, gas / gas and liquid / gaseous, under certain circumstances, for bulk materials. A further application is the homogenization of only one substance in terms of composition and temperature.

Design and operation

The static mixer consists of rows of elements, screws, fins or even are usually a lattice. The fluids to be mixed are pressed together and the desired mixing ratio in the mixer. The elements share the material flow, twist the currents and lead them back together. Divides the first element into the pumped two-layer fabric in mixture, arising after the merge after the element at least four layers. After the next there are eight layers, then 32, etc. With each level so better mixing is achieved. The following applies for the number of layers

Is the number of layers in the beginning, the number of channels, typically two, the number of mixing elements.

In addition to this effect, which occurs merely by laminar flow, there is also a radial mixing of the individual layers to one another, especially when turbulence occur.

There are several dozen different types of mixers, some examples are:

• Kenics mixer: it consists of 180 ° twisted sheets. Each coil is offset by 90 ° to the previous, and has the opposite direction of rotation to.
• Sulzer SMV mixer: Serrated blades direct the streams so that they cross.
• " Sulzer SMX mixers This mixer has a plurality of frame-like, arranged crosswise ridges.
• Fluitec CSE-X/6-12 mixer :: This mixer has six waisted, arranged crosswise ribs and has openings in the edge zones. This leads to a high mixing power at a significantly reduced pressure loss.
• Fluitec CSE XR mixer heat exchanger: The static mixer in addition to the mixing elements has an integrated tube bundle for the heat exchange.
• Ross ISG Mixer: The mixer is divided into segments by modules. The material can pass only through the holes in the modules from one segment to the other. The beginning and end of the bores are arranged such that a mixing takes place. The number of channels (bores) is four.

The mixers are either from the media suitable metal or plastic, then, usually manufactured as disposable products.

Interpretation

The selection of the suitable mixer type and the optimum geometry is determined primarily by:

• The properties of the fluids to be mixed, in particular the viscosity and the density
• The desired mixing quality
• The available installation length and the pipe diameter
• The desired operating pressures
• Flow rate
• The residence time in the mixer in reactive mixtures.

As a measure of the quality of mixing of the coefficient of variation is generally used as a basis, the average concentration of a component and its standard deviation. An ideal mix would thus be present at. The coefficient of variation decreases with increasing related mixer length, the ratio between total mixer length and diameter of the mixer, exponentially. That is, the longer the mixer, and the smaller the diameter, the better is the mixing effect. If the flow velocity so great that the Reynolds number reaches the critical value for this mixer 100-200, occur turbulent flows. These improve the mixing quality on.

For Kenics and Sulzer SMV mixer following measurements are available:

An improvement in the quality of mixing by reducing the diameter finds its limits in the consequent increase in pressure drop in the mixer. With the loss of pressure rise and the necessary funding and operating pressures. In the laminar regime applies

Wherein the average flow rate, the dynamic viscosity of the fluid and the pressure loss coefficient is. Halving the diameter thus means a fourfold increase in the pressure loss, or even a Versechzehnfachung, when the flow by increasing the flow rate should remain the same.

The product is in the laminar range constant and depends on the types of mixers:

From the context it follows that one can build an X - mixer and Sulzer SMV as a very compact mixer for example. The Sulzer SMV mixer shows a comparison but a high pressure loss. For highly viscous media rather X and Kenics mixers are with the lower pressure losses.

Pros and Cons

Compared to dynamic mixers such as agitators, static mixers have some advantages. They are inexpensive, do not take a lot of space, are designed to be maintenance free, require no energy supply and can be integrated directly into conveyor lines. With them, it is especially possible to mix continuous and short, minor variations in the residence time of components over a wide viscosity range. Upon stirring in separate mixing vessels, however, only batch processes are possible: the tank must only be filled on the actual mixing process takes place and only then is the finished mixture available. In particular in the processing of the curable material with a limited pot life, such as casting resin, static mixer is advantageous. The material will be filled directly from the mixer into the mold or applied, ie the components come until immediately before processing into contact, an attaching attributable to stock.

The disadvantage is that normally no influence on the mixing intensity is possible while at stirrers, the speed can be changed. When processing curing materials to static mixers can enforce during breaks in production. They should thus be rinsed or executed as disposable products.

Genesis and applications

In 1924, a spiral was used in a pipe for mixing purposes. The first patent application for a static mixer after Kenics principle in 1965 by Arthur D. Little. The purpose was the mixing of resins in small quantities, for example, for repair work. The Kenics Corporation stopped then this mixer under license from stainless steel. In the seventies they could be manufactured as disposable plastic products.

In addition to the processing of resins, adhesives, sealants, etc. the range of applications of static mixers today includes all procedural areas with mixing processes, for example in the chemical industry, food industry, water and wastewater treatment (eg enriching water with oxygen) oil and gas industry. In the plastics industry find static mixer insert in Vergussanlagen and mixing of polymer melts.