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Classify Your Mixing Process or System:
Blending of Miscible Liquids Gas-Liquid Dispersion
Dissolving Slurry Mixing
Heat Transfer Extraction or Leaching
Solid Suspension High Viscosity Blending
Chemical Reaction Crystallization
Blending of Immiscible Liquids Dispersion & Homogenation

The above list is a general list of mixing classifications.  There are many more.  The intent is to present a generalized discussion to help you better describe your specific process conditions to achieve your overall objectives.  The classifications range from very basic to applications that are much more involved.  Some classifications overlap, such as the suspension of solids combined with heat transfer.  You mixer supplier will help to determine the controlling factors.  The objective is to assist you to generally describe and understand your mixing application.    

Blending of Miscible Liquids:
Miscible generically means that two or more fluids that will mix together readily and will not separate once mixed.  Oil and water are immiscible, where they will separate in and unmixed state.  Almost all of these applications are flow controlled applications.  Fluids can be miscible and can still separate, such as adding a small amount water to a large  batch that has a large viscosity difference, say 5,000 centipoise for example. 

Dissolving:
Almost all dissolving operations are flow controlled applications.  The solubility of a solid is the maximum percent by weight solids that will dissolve into a solution.  If more solid is added, the solution becomes supersaturated and is then classified a slurry.  The solubility in water is known for most substances.  The solubility is dependent upon the temperature, solvent, and the interaction with other chemicals.   

Heat Transfer:
Although heat transfer applications can be quite complex, most of these mixing operations are flow controlled applications understanding some very basic principles.

  1. The energy imparted by the mixers motor to the batch must be accounted for as a factor of heat added to the system.

  2. Heat transfer is highly dependent upon the heat transfer area and the driving force temperature differential, understanding that the heat transfer area is the primary controlling factor.  

  3. In order to double the heat transfer coefficient, the mixers horsepower would have to increase by a factor of 11 times or more, understanding that it is impractical to design a mixer to achieve a specific heat transfer coefficient.

  4. Flow must be directed across the heat transfer surface area.

Solid Suspension:
Free settling solids that are either insoluble or partially soluble are generally classified under Solid Suspension.  Most of these applications are generally classified as flow controlled. Parameters such as the density of the solvent, solid size distribution, percent by weight solids, and the specific gravity of the solids (not bulk density), and the degree of suspension (on-bottom, off-bottom, mid-depth, or uniform suspension) are required design parameter in the design of a mixer.  High percent by weight solid applications are classified as hindered settling and act closer to blending and slurry applications.  

Chemical Reaction:
Chemical Reactors can be quite complex, especially as viewed from the micro-molecular scale.  Generally speaking most are considered flow controlled applications as they most often include general chemicals blending.  A polymerizer, for example, could be considered as a chemical reactor but it is typically considered under a subcategory of high viscosity applications.  An very definite exothermic chemical reaction results from the manufacture of Magnesium Hydroxide from Magnesium Oxide, but again this application could be considered under the sub-categories of slurries or minerals processing rather than chemical reactions.

Blending of Immiscible Liquids:
A good example of two immiscible liquids would be oil & water such as seen in a common salad dressing.  Many of these types of immiscible liquid applications are flow controlled as the general objective is to contact two or more fluids to extract or strip a beneficial component from one of fluids, followed by a simple phase separation.  Where the process becomes complex, for example, is in consideration of a design bubble size distribution for diffusion in counter-current mixing columns.

Gas-Liquid Dispersion:
The study and requirements of a particular Gas-Liquid applications can be quite complex, involving a control vessel (temperature and pressure).  At relatively low gas-liquid concentrations, the process is considered flow controlled.  Most gas-liquid dispersions are consider flow controlled dispersions.  Pharmaceutical applications such as Fermentation, for example, require relatively high (gas-liquid concentrations) superficial gas velocities, where the mixer design combine both flow and fluid shear.  The fluid shear is necessary to create the required bubble size distribution for diffusion where the flow ensure that the bubble entrains throughout the vessel.  The smaller the bubble size, the greater the overall surface area to enhance diffusion.  If the sheared bubbles coalesce (to grow together) uncontrolled and do not entrain in an overall beneficial flow pattern, the efficiencies of the system will suffer.  If an adequate amount of torque (horsepower divided by speed) is not adequate, the result will be either not enough fluid shear or not enough flow to entrain the bubble flow distribution, which in either case efficiencies will suffer.  

Slurry Mixing:
A slurry is created when solids partially dissolves in a particular solvent.  The most common solvent is water.  The extent to which a solid dissolves in a particular solvent is generally expressed in terms of its solubility.  Some solids, such as lime in water for example, is  only slightly or partially soluble, where the bulk of the solid does not dissolve in water.  What makes slurry mixing interesting, is that as the solid dissolves in a particular solvent, or combinations of solvents, the characteristics such as the viscosity of the slurry can change either slightly or significantly.  For some slurries, significant  changes to the viscosity may occur even at very low percent by weight solids concentrations.  For other applications, it is possible to have a relatively low viscosity at weight percent by solids approaching 80% or more.  Others slurries are time dependent, such as thixotropic slurries, where you could stand on a slurry left unmixed for weeks, where that same slurry, once sheared could be a thin as water.  The good news is that many liquid-solid applications generally are known, where it's not necessary to reinvent the wheel from a mixer viewpoint.  On the other hand, pilot plant studies may be required for unknown combinations of solids and solvents.     

Crystallization:
Crystallization involves two key steps, the formation of solid particles from liquid solution (normally referred to as nucleation), and growth due to the deposition of additional substances on existing particles.  The driving force behind both steps is the difference in chemical potential between the solution (liquid phase) and the crystal (solid phase).  Although the target impurity content is directly attributable to both the phase equilibrium and crystallization kinetics, crystallization is considered more of an art than a science.  The prime interrelated phenomena at play is mixing fluid shear.

Dispersion & Homogenation:
The intent of dispersion or homogenization is to obtain a particle size reduction generally into a range of 5-25 microns.  The process result of a dispersing or homogenation type mixer design is directly dependent u
pon how the energy or the horsepower is split and transformed either into flow, fluid shear and/or mechanical shear.  Homogenizers transform the bulk of the available energy primarily into fluid & mechanical shear with some flow.  Dispersers transform the available energy primarily into fluid shear with somewhat greater flow capability. Both dispersion and homogenization are tip speed dependent.  Tip speed = (p*RPM*Impeller Diameter), which is usually expressed in feet per second.  Both of these types of mixers are very poor flow designs, where the shear intensity is focused into a zonal region around the mixing head or impeller.  This can result in a hot spot within the tank where the available horsepower is transformed into heat energy, which can degrade the contents of a batch.

There are several methods used to impart both a shear and a flow requirement.  It is for this reason that some disperser and Homogenizer type mixers are equipped with hydraulic lift to provide vertical lift capability to enhance the tank flow patterns.  Dependent upon the application, trying to accomplish both flow and shear, using a stand alone homogenizer mixer or a disperser type mixer may be possible however, there maybe a more economical solution.  Homogenizer and disperser type mixers are significantly more expensive that common flow controlled open impeller designs, so economies of scale must be considered.  

If the intent is to circulate the fluid or slurry into a high intensity shear zone, a lower cost flow controlled mixer, in combination with a separate shearing mill, may offer the most economical solution.  For example, to manufacture magnesium hydroxide, reacting the raw material magnesium oxide, a common open flow controlled impeller can be used for this application.  Upon its transfer from the reaction tank to the storage tank, the process stream is milled to it final partical size distribution or consistency.  It is also quite common to see a separate lower cost homogenizer assembly, drawing materials from the lower region of a tank, in combination with a lower cost open flow controller impeller mixer assembly.  In short, although a homogenizer can achieve a similar process result on its own, applied to a commercial sized batch operation, its cost would be prohibitive.  

Extraction or Leeching:
The most common Leeching operation that we are all familiar with is in the making of coffee.  The solid (coffee) is contacted with a solvent water, to remove a desirable component from the coffee solid.  The same process is applied to other raw materials like mining ore (gold, copper, platinum, etc.), where the solvent removes the precious metal from the ore, and is later processed to precipitate and recover the precious metal from the solvent.  Extraction is somewhat similar in that two immiscible phases are contacted or mixed with each other, where a desirable material that is in one phase diffuses to another phase upon contact.  Bulb size distribution for efficient mass transfer is a key requirement.  

High Viscosity Blending:
Generally speaking fluids with a resulting viscosity above 10,000 centipoise would be considered beyond just a general blending application.  Since viscosity is a function of temperature, shear and time, applying a mixer design to a high viscosity application can be quite involved.  There are numerous designs used for high viscosity applications that include props, high solidity hydrofoils, pitch blade turbines, anchors, helical, double helical impeller and other designs.  The most common problem identified with high viscosity applications is due to the formation of an ellipsoid around an impeller, where the generated flow or mixing within the ellipsoid is vigorous.  Heat generation within this region can also be problematic, as there is no motion to transfer the heat to the tank wall or coils.  Outside the ellipsoid, the blend time becomes infinite, or in other words, no mixing occurs in that region.  In short, homogenity can never be reached due to a misapplied mixer to the application.  Obviously, there are various other issues to contend with dependent upon the applications requirements. 

04.12.11

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