Mixing Primer

An Introduction to High Performance Fluid Mixing

There is vast range of technologies, machines and devices to perform fluid mixing tasks, although none can be used for all mixing duties.  Not surprisingly, this makes the selection of mixers both complex and confusing for most mixer users and is one of the main reasons why fluid mixing is still the subject of intense academic research even after thousands of years of development!

If you are new to the subject of fluid mixing and are looking for some help on terminology and an answer to the question, “why are there so many kinds of fluid mixer?”, then we hope that you will find this page helpful.  There are some suggestions on further reading at the bottom of the page.

What is Fluid Mixing?

The “mixing” of one or more components or materials in a “fluid system” can be described in terms of two separate but interlinked physical processes:

  • Blending (“distribution”) of different components of the mixture to create uniformity throughout the mix, and
  • Droplet or particle size reduction (“dispersion”) of one or more components of the mixture to give increased homogeneity of the system or to alter the nature of the system by increasing the contact surface area between the components, i.e. reducing the particle or droplet sizes increases their contact surface area to volume ratio.

A fluid system in this context means a combination of materials which combine to form a fluid, where a fluid is defined as matter which cannot sustain a shear force while at rest.  In particular, we are considering liquid-liquid and solid-liquid mixing systems here, as distinct from dry powder or gas-liquid mixing systems.

Most fluid mixing problems can be considered in terms of the miscibility (the ease of mixing) of the system components.  Miscibility can in turn be thought of as the ease of distribution and the ease of particle size reduction – this affects the mixing approach to be adopted.  For instance, where the rate of reaction between miscible components is to be improved, mixing efforts are focused on maximising distribution, while for mixing immiscible fluids, efforts are normally focused on reducing droplet or particle size to maximise the area of contact between the phases.

A further consideration is the type of production process involved, of which the fluid mixing is normally only a part.  The most important distinction that affects the mixing operation is whether the process is batch or continuous in nature.  In a batch process, a discrete volume of material is mixed, usually within a vessel; in a continuous process, a stream of material is mixed, usually piped to and from the mixer.

In an ideal world, it would be possible to choose the appropriate mixing action to suit the requirements of the fluid and then select either a batch or continuous form.  In practice, many mixing technologies are offered in either batch or continuous form but not both and in situations where both are offered, there are normally some performance trade-offs.  Many fluid production processes are actually defined by the kind of mixer that is used, often for reasons of expediency or “standard practice”.  This makes it difficult for those working to innovate new fluid products to make use of new or optimal production processes and methods.

Mixers offered by Maelstrom are mostly available in both batch and continuous forms and care has been taken to minimise performance trade-offs to make selection easier and more reliable.

Fluid Mixing Mechanisms

In terms of mechanical mixing mechanisms, a number of actions are employed by different types of mixers to create different effects for particular process results.

For distributive action, swirl created by rotating parts causes laminar thinning of the material interfaces, thereby increasing volumetric combination of the materials. A repeated cutting and folding action of the mixture also increases the distribution of different material components. The effectiveness and efficiency of a mixer in distributive mixing is therefore a function of how the machine interacts with the fluid in a geometric sense.

Geometric mixing

Conversely, the effectiveness and efficiency of a mixer in dispersive mixing is a function of how the machine interacts with the fluid in a stressing sense.  For most materials, the higher the stress, the smaller the resulting particles or droplets in the mixture.  However, another very important consideration is the uniformity of the stress field.  Without a reasonable uniformity, it is impossible to guarantee that the same stress is applied to all parts of the fluid.  This would result in a wide range of final droplet or particle sizes rather than a narrow range obtained with uniform stressing.

One or more of the three primary stressing mechanisms are used in most fluid mixers.  These mechanisms are:

Of these mechanisms, the most effective is extensional stressing. This is why nozzle valve homogenisers are used to create many of the ultra-fine dispersions demanded by process industries, despite their many practical disadvantages, and is why the common “high-shear” mixers are relatively ineffective and inefficient for dispersive mixing.

Mixer Types

Although there are as many types of mixers as there are terms for describing them, for fluid mixing they essentially break down into the following groups:

Impellers – normally comprising specially shaped blades on a rotating shaft, driven by some form of motor or geared drive – almost always for batch use, but more than 55% of the mixing equipment is for these types of devices, which come in a bewildering array of sizes and shapes.

Special agitators/blenders – this covers a range of special purpose machines which are normally for batch use only and are designed for a particular duty.  Although there are often many disadvantages in using these devices, such as cleanability, inefficiency and so on, their use is sometimes vital in creating certain mixing effects.  This group includes ribbon mixers, pin mixers, anchors, z-blades and dozens more.

The Maelstrom Fluid Division Mixer (FDM) falls into this category when operating at low speed as it is capable of very high distributive performance through its dynamic use of structured cutting and folding whilst imparting almost no shear stress into the product. This is very beneficial where shear-sensitive products need to be blended in either continuous or batch mode.

Static mixers – a relatively recent development (in the 1960s), these are devices for continuous use only, comprising a set of non-moving obstructions in a pipeline.  The obstructions are shaped and positioned in such as way as to create cutting and folding effects and/or turbulence for mixing of piped fluid streams.  Although cleanability is an issue, static mixers are a reliable and low cost alternative in a wide range of inline blending applications.  It should be noted however, that any high pressure drop across the mixer must be compensated for by larger and more expensive pumps.

Mills – available in various forms for both batch and continuous use, mills generally use compressive and/or shear stresses to create dispersions by crushing or grinding the fluid material between moving surfaces.  A two-roll mill, as the name suggests, comprises two rotating cylinders which rotate to crush and grind material between them.  The other common type, the bead mill, uses hardened metal, ceramic or glass beads inside a tumbling cylinder, through which the fluid is passed, to give a random crushing of the fluid.  Due to the way they work, mills are particularly suited to particle size reduction of solids which are suspended in fluids, although throughputs rates are generally quite low.

Rotor-stator dispersers – usually called “high-shear mixers”, are the most common form of dispersing mixer.  By placing a form of closely-fitting shroud around a high speed impeller, it is possible to create a shearing action between the blades and stator shroud.  As material is centrifugally pumped through the mixing head, some of it will see this high shear zone and experience shear stressing that results in dispersive mixing.  Where small or uniform dispersions are required, material must be cycled through the head many times to ensure statistically that all of the material has passed through the high shear zone at least once.  The viscosity range of fluids to be sheared is restricted, especially in batch mode, as the mixer uses a centrifugal pumping action which is not effective at higher viscosities.  Although their dispersion performance is limited, rotor-stator machines are fairly flexible in terms of their duties and capabilities and are also available in both batch and inline forms.

The Maelstrom Fluid Division Mixer (FDM) falls into this category when operating at high speed in turbulent mode as it combines an intense hydraulic shear with its excellent blending capability.  The uniformity of the high shear field in the mixing head means that some of the problems associated with stress uniformity in normal high shear mixers are avoided.  FDM machines typically put 5 times more energy into fluid than equivalent high shear mixers and achieve up to 25% improvements in dispersion performance.

Special purpose dispersers – a range of complex (usually inline) machines and systems which deliver very good uniform dispersions, normally in particular fluid applications, including those at the nano scale.  For example, high pressure valve homogenisers are used in the processing of milk to ensure that the milk fats droplets are reduced in size and evenly dispersed throughout the bulk.  This stops the cream separating from the milk.  The valve homogeniser comprises a very high pressure pump and a controlled valve nozzle through which the fluid is forced at very high velocity to rupture the fat droplets through extensional stressing.  The jet impinging mixer is another type of disperser which uses high velocity fluid streams, except that in this case, the fluid impinges on a plate or contra-jet to rupture the droplets or particles using impact stressing.  Ultrasonic mixers and membrane mixers provide extremely small droplet sizes, although their cost, complexity and fragility mean that few are used in medium to large volume production applications.

Maelstrom’s inline ConCor Milling technology competes in this class of machines with its ability to generate extremely high shear and extensional stressing forces with multiple mixing stages and very high rotational speeds.  Essentially a cross between a rotor-stator mixer and a two-roll mill, ConCor offers unique benefits for nano-scale emulsification and de-agglomeration with close control of fluid temperature.  ConCor milling is especially targeted at nano-scale deagglomeration where it can deliver dispersions down to tens of nanometres.  Its scalability to production volumes provides significant performance and cost advantages over state-of-the-art media mills.

Integral Pump Mixers (IPM) from Maelstrom – can really be treated as a separate class of mixing device due to the way that they combine a number of different stressing and distribution mechanisms to achieve both high dispersion and high distribution performance.  Available in both batch and inline forms, IPMs use internally-generated positive displacement pumping to force fluid through small nozzles at very high energies whilst extending and shearing it.  The fluid flowing through the nozzles at high velocity then impinges on an internal wall of the mixer.  A dynamic cutting and folding action added to vigorous turbulent flow provides distributive mixing.  IPMs are suited to a wide variety of applications due to their ability to handle a wide range of materials and viscosities, their high performance and their economic benefits.

——————————————————————————–

Further Reading

1. “Handbook of Industrial Mixing: Science and Practice”, Edward L. Paul (Editor) et al, 1st ed. 2003

2. “Mixing in the Process Industries”, Harnby, Edwards, Nienow, 2nd ed. 1992

3. “Fluid Flow for Chemical Engineers”, Holland, Bragg, 2nd ed. 1995