Powder Mixing Introduction
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A mixing process is at the heart of many bulk solids processes. Powder or solids mixing allows to get an homogeneous mixture of different component and constitue one of the process steps that adds the more value. However, to mix homogeneously major ingredients with minor ingredients and reach a good homogeneity (measured in practice by a coefficient of variation relative to one of the components used as tracer) is not easy. The page below will guide you through the main industrial blending equipment that exist, their pros and cons and will also introduce the notion of continuous mixing and batch mixing.
Powder mixing is based on the movement of the particles part of the recipe to be mixed. The movement can be of different type and different designs of mixers will correspond to different mixing principles.
Mixers are often classified thanks to the Froude number. This adimensional number will define the regime of mixing depending on its value.
Froude number is defined in equation 1 [Perry]:
Equation 1 : Froude number
R = mixer radius or mixer agitator radius
ω = angular velocity
It can be expressed in a more convenient form for powder mixers having a mixing element in equation 2:
Equation 2 : Froude number calculation for blender equipped with a mixing tool (ribbon, paddles...)
u = tip speed mixing element
D = diameter of mixing element
Froude number is comparing 2 forces : Fr = (forces other than gravity - mainly centrifugal) / gravity
If Fr < 1 it means that the gravity forces will be stronger than
the centrifugal forces, the powder will remain settled in the
mixer, moved, but not in a cloud
If Fr > 1 it means that the centrifugal forces will tend to be stronger than the gravity force : the powder will have a tendency to be suspended in air in the mixer.
Among the common mixers used industrially for powder mixing, the table below is proposing a classification according to Froude number
Table 1 : Mixer classified according to their Froude number and mixing principle
|Fr||Mixing class||Mixer type||Pros||Cons|
|< 1||Diffusion||Type free fall mixers
Double Cone blenders
Low energy required
No mixing elements in the equipment
Access for cleaning
Cannot achieve good mixing for powders of very different particle sizes
Segregation effects can be experienced
|< 1||Convection||Type thrust mixer
|Achieve generally better mixing results than diffusion
Low energy inputs
Generally less expensive than paddle or plough share mixers
Access for cleaning
Can damage product at long mixing time
|> 1||Convection||Paddle Mixers
|Short mixing time
Low energy input
Good access for cleaning (some design can be with extractible shafts
For paddle mixers, exist in continuous mixing execution
For padlle mixers, a liquid injection can be foreseen
|Cost compared to diffusion tumblers / ribbon blenders
If liquid injection, prone to agglomeration - then needs some additional mixing elements at higher shear
For pneumatically generated fluid bed, attention must be given to risks of segregation due to fines "floating" at the top of the mixer
|Plough Share Mixers
High shear mixing elements
|Short mixing time
Reduce risks of powder agglomeration
Exist in continuous mixing execution
|Higher powder breakage
High energy input
Another type of classification could be proposed depending on the type of process where mixers are integrated : Batch or Continuous. If batch mixer probably represent the majority of the industrial applications, some types of mixers (paddle mixers) can be used in a continuous mode, which can be useful for some kind of processes.
The following elements will influence the mixing time. As a general rule, mixers operators look for a mixing time as short as possible in order to increase the productivity of their line.
Table 2 : parameters influencing the mixing time
|Operating parameter||Influence on mixing time|
|Mixing volume||A higher mixing volume will lead to higher mixing time
But doubling the volume, does not mean doubling the mixing time
For mixers at Fr>1, after a certain volume, mixing time will be almost constant
Note : a mixer should not be over-filled otherwise the mixing may even become impossible
|Mixing speed||A higher mixing speed usually gives a shorter mixing time
Influence on the powder should however be considered (powder breakage
|Froude number||Higher Fr number should give shorter mixing time
Powder breakage will be a concern when Fr >>1
|Solids type||The more the particle sizes of the mix consituents are different, the more difficult it will be to mix|
The influence of the Froude number and the mixer volume are represented in the graph below :
Figure 1 : Mixing time = f(Fr,Volume) [Gericke]
Mixing time is also dependent on the sequence and place of filling the ingredients. In particular, the position of addition of micro ingredients is of prime importance. In case the micro ingredients are added on the side of a mixer, the mixing time can be greatly impacted. As a good practice, processes must be designed to have the small, minor and micro ingredients introduced in the middle of the mixer, in any case in the "active" area of the blender (the fluidization zone for instance).
Designing a solids mixing process batch or continuous is a question that will be asked at the very beginning of the project since the processes will require very different process equipments.
The principle of operation is radically different. For batch, there will be a discontinuous sequence of preparation, with the dosing of ingredients, then loading to the mixer, then mixing, then discharge of the mixer. This sequence will then be repeated each time a mix must be performed. For continuous process, all happens at the same time and with equipment that differs from the batch in the sense that they are able to dose in continuous the ingredients to the mixer. The mixer is as well able to move the ingredients and mix them at the same time which allows it to operate continuously.
The following grid is summarizing what imply each process type regarding key design parameters
Table 5 : comparison of batch and continuous mixing process
|Capacity||From 10 kg/h to very large||From very small to large|
|Mixer size (at similar output||Smaller||Larger|
|Segregation risks||Smaller||Higher due to steps following the mixing operation (sudden drop of the material|
|Space requirements (at similar output)||Smaller||Larger|
|Flexibility||Lower (continuous mixer designed for few recipe changes||Higher (mixing installation can start/stop on demand|
|Recipe complexity||Lower (limited number of ingredients||Higher (the process can accomodate more ingredients|
|Automation||Complex for the control of the Loss In Weight Feeders||Generally simple|
|Staff competency required||High because of the dosing systems||Lower|
|Space required||Comparatively low||Comparatively large|
Continuous mixing processes need to be supplied continuously in powder by special dosing systems. These dosing systems are made of Loss In Weight feeders. Feeders can use the following feeding units : screw conveyors, vibratory trays or weighing belts. Each feeder is on load cells and must be equipped with a very developped control system allowing to measure the loss of weight over time, filter perturbations, and adjust the feeder speed to keep a given set point in kg/h.
For a continuous mixer, the accuracy of the feeder influences greatly the homogeneity. A continuous blender must be able to provide radial but also axial mixing. In principle, better results will be achieved in plug flow, with minimal axial dispersion. However, if the continuous feeder is not stable, inhomogeneity will be witnessed at the outlet of the continuous mixer. In practice, an axial dispersion is required to cope with the feeder inaccuracies.
Low feeder accuracy will mean that : axial dispersion is required, thus higher mixing volume is required, thus higher mixing time is required.
The following common mixers will be presented below
(drawing from US Machinery under license Creative Commons)
Figure 2 : Ribbon blender
Table 3 : ribbon blender process characteristics
|Froude regime||less than 1|
|Typical mixing speeds||50-70 rpm, higher speed can be possible|
|Size||From few 100 l to more than 10000 l|
|Typical mixing time||More than 5 minutes|
|Impact on product||Can be severe if long mixing time at high speed|
|Number of bearings||2 - mixing tool not cantilevered|
|Top cover||Bolted, with gasket|
|Outlet||1 - generally round, some design hygienic|
|Cleanability||Limited, not a good access|
|Access||Generally doors on the top|
Table 4 : paddle mixer process characteristics
|Froude regime||More than 1|
|Typical mixing speeds||50 rpm for around 1000 l mixer|
|Size||From few 100 l to 5000 l|
|Typical mixing time||1 to 2 minutes|
|Impact on product||Mixing is quite gentle, low degradation of powder expected|
|Number of bearings||1 or 2 - mixing tool can be cantilevered|
|Top cover||Bolted, with gasket or welded|
|Outlet||1 or 2 - round hygienic design possible, bomb doors possible|
|Cleanability||Good, some design allow to take out the shafts|
|Access||Doors on side, some design with door on front (when cantilered)|
Table 5 : tumbler mixer process characteristics
|Mixing tool||Container - V shape, double cone, drum - generally no mixing tool|
|Froude regime||Less than 1|
|Typical mixing speeds||25 rpm|
|Size||From few l to 2000 l (when containers are mixed)|
|Typical mixing time||5 to 15 minutes|
|Impact on product||Mixing is quite gentle (if no agitator added), low degradation of powder expected|
|Number of bearings||The drum is agitated by a rotating arm|
|Cleanability||Good when the container is small, poor if bigger, for large container dedication of the container to one product is recommended|
|Access||Mixer must be protected by a safety cage|
Containers can be of very variable size. The smaller blenders will run with drums from few l to 200 liters. Those drums are often made of Stainless Steel, and can be equipped with baffles on the top cover. Those baffles promote the mixing and can result in a reduction in mixing time.
Bigger containers will generally have a conical bottom. Those containers can be 500 to 2000 l big. Some designs can actually be equipped with an agitator. The mixing will then combine the effect of the agitator with the classical avalanche effect of the container rotation
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