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Basic Mixing Principles
When considering any mixing application it is important to realise that, there is no one solution to a given mixing problem. The optimum solution will depend on a variety of different factors. Mixing Solutions Limited have both the knowledge, experience and product range to specify precisely the right system configuration to best serve the total needs of a particular mixing operation.
Mixing Mechanisms
Mixing is achieved by a number of different mechanisms, as summarised
in the following table. The most important mechanism will vary for any
given application, and a given process may rely on any or all of these
mechanisms.
In order to arrive at an optimum mixer design a detailed understanding of the various mechanisms involved and their importance in achieving the process result is required.
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Nomenclature Where:
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Basic Mixing Applications
By far the most common of mixing applications are those which rely upon
flow to achieve the required process result. These applications are often
referred to as, 'Flow Controlled Applications', and include such applications
as Blending, Solids Suspension and Heat Transfer. Whilst it is not possible
to cover all possible applications these can be assigned to Application
Classes. Some of the more common application classes are considered here.
Blending / Homogenisation
of Miscible Liquids
Blending involves the mixing of two or more miscible liquids to achieve
a uniform mixture throughout the entire volume of the tank, usually within
a specified period of time. It is important to note that the blending
of liquids having widely varying density and/or viscosity requires special
attention and may require longer blend times to ensure the liquids are
mixed.
Typical Blending Applications
| • Chemical Reactions • Polymerization • Simple blending of miscible fluids • Make-Up Tanks • Storage, Feed, or Holding Tanks |
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Solids Suspension
The suspension of solid particles in a liquid is achieved by providing
a flow field with sufficient velocity to lift the solids to the required
level within the tank. Careful consideration should be given to the degree
to which the solids are suspended as this has a significant effect on
the required mixer power.
Principally there are five degrees
of suspension as follows:
| • Solids Just Suspended • Off Bottom Suspension • Moderate Uniformity • Nearly Uniform Suspension • Uniform Suspension |
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Heat Transfer
There are many applications that either exothermic, endothermic or simply
require heating or cooling. This requires heat to be transported to or
from the process fluid via a heat transfer surface usually in the form
of coils or a vessel jacket. Mixers are used to improve heat transfer
by effecting the mixer side film coefficient. It should however be recognised
that this represents only part of the overall heat transfer equation and
that variables not controlled by the mixer may have a much greater influence
on the overall heat transfer.
Typical Heat Transfer
| Applications • Chemical Reactions • Fermentations • Polymerizations • Esterifications • Hydrogenations |
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Optimum Mixer Selection
Criteria
A full and accurate specification of the mixing vessel, the process parameters,
and the required mixer performance is the first crucial step to arriving
at an optimum mixing operation. Some of the many other variables that
can affect mixer performance and so need to be considered in arriving
at the optimum mixer design are noted on these pages.
IMPELLERS
Impeller Type
The function of any mixing impeller is to convert the rotational energy
of the mixer shaft into the correct combination of flow, shear and turbulence
to achieve the required process result.
As no one-impeller design is capable of providing optimum performance under every process condition, optimum process performance is dependent upon selecting an impeller design that has the specific characteristics required by a given process.
At Mixing Solutions we have a range of impellers that is one of the most extensive in the industry and enables our engineers to select the most appropriate impeller for any given situation.
Number of Impellers
The use of a single impeller is the usual preferred option on the basis
of cost. However, changes in the ratio of liquid level (Z) to vessel diameter
(T) can have an adverse effect on the flow patterns generated within the
vessel. This can result in the need to consider the use of multiple impellers
in order to achieve an economic solution.
Z/T ratio alone is not the only consideration when determining the number of impellers required. Multiple impellers may also need to be considered for other reasons including, when high viscosity fluids are involved, for mixing at low level during filling and emptying or where draw down from the liquid surface is a requirement.
Impeller Positioning
Whether utilising a single or a multiple impeller configuration the positioning
of the impellers within the process fluid can have a significant effect
on the overall process performance. Incorrect positioning can lead to
staged flow patterns, poor dispersion of additives and impellers being
out of the liquid at crucial stages of the process.
D/T Ratio
The ratio of mixing impeller diameter (D) to vessel diameter (T) has a
very significant effect on the performance of most fluid mixers and the
optimum D/T is a function of both process conditions and process requirements.
Normally the optimum D/T will be in the range 0.2 < D/T < 0.5. Some special applications however, sometimes operate outside this range.
Bottom Clearance
The impeller bottom clearance (C/T ratio) can also have a very significant
effect on the overall performance of a mixer, effecting both power draw
and pumping efficiency. The optimum C/T ratio is essentially dependent
upon impeller type but can also be effected by process conditions.
Normally, the optimum C/T will be in the range: 0.1 < C/T < 0.3. Hydrofoils however can operate at much higher levels, up to C/T = 0.5 or more.
VESSEL DESIGN
Vessel Geometry
When designing a vessel for mixer duty it is important to understand the
role that tank geometry plays in determining the final mixer design. Poor
aspect ratios and or inappropriate bottom shapes can both result in increase
mixer cost and in certain circumstances make it impossible to optimise
the mixer design.
Aspect Ratio
It is generally accepted that the ideal aspect ratio for most mixing tanks
is one where the liquid depth (Z) is equal to the tank diameter (T) as
this allows for the optimum number of impellers, optimum power input and
optimum power distribution.
In practise, the optimum Z/T will be in the range 0.9 < Z/T ^ 1.2 as this does not significantly effect mixer design.
Bottom Shape
Tank bottom shape can have a significant effect on the flow patterns generated
within the mixing vessel and hence the mixers ability to achieve optimum
process performance. Normally a dish-bottom tank is the preferred bottom
shape. However, flat-bottoms and shallow cones (less than 15°) can
be used for many processes without any particular problem. In the case
of flat bottomed tanks mixer performance can often be significantly improved
by the introduction of corner fillets.
In general deep cones should be avoided especially where the requirement is solids suspension.
Baffles
| The importance of proper tank baffling in obtaining optimum mixer performance should not be underestimated. In a correctly baffled tank the mixer develops the fluid regime required to achieve the optimum process result.
An incorrectly baffled
tank on the other hand can lead to poor mixer performance and
may even result in the mixer not being able achieve the process
result for which it was designed. |
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Mixing Intensity
Within the process industries in general it has become convenient to characterise
the level of mixing required for a given application in terms of agitation
intensity. This practise has led to the introduction of terms like mild
or vigorous agitation. Whilst such general terms are convenient they can
mean different things to different people and so need quantifying if they
are to be of any practical use.
The following table gives a general overview of the various degrees of agitation in common use throughout the industry.
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Blending
Intensity
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Moderate
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Vigorous Critical mixing operations • Reactors • Most Heat Transfer • pH control • Rapid Blend Time Surface in rolling
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Violent |
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Mixer Related Dimensionless Numbers
