Mixers are often classified thanks to the Froude number . This adimensional number will define the regime of mixing depending on its value.
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.
The present page is focusing on Dry-Mixing of solids. You may wish to know more about other types of mixing with the following links : Solid / Liquid mixing (slurry) or Liquid / Liquid mixing
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 constitute 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.
Froude number is defined in equation 1 [Perry]:
Equation 1 : Froude number
With
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...)
With
u = tip speed mixing element
D = diameter of mixing element
Froude number is comparing 2 forces : Fr = (forces other than gravity - mainly centrifugal) / gravity
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 mixersAnother 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 timeThe 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
Process variable Continuous Batch 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 largeContinuous 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 developed 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
Ribbon Blender Paddle mixer Tumbler mixer
(drawing from US Machinery under license Creative Commons)
Figure 2 : Ribbon blender
Table 3 : ribbon blender process characteristics
Mixing tool Ribbon Froude regime < 1 Typical mixing speeds ~50-70 rpm for small mixers, ~10-20 rpm for large volumes 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 Note : some designs of Ribbon blenders are actually done with an extractible shaft. This promotes cleanability but complexifies the design. The ribbon is anyway difficult to clean.Note : some designs of Ribbon blenders are actually done with an extractible shaft. This promotes cleanability but complexifies the design. The ribbon is anyway difficult to clean.
Table 4 : paddle mixer process characteristics
Mixing tool Paddles Froude regime > 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 < 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 Top cover Clamped 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 cageContainers 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
Sources
[Perry] : Perry's 8th Edition, Solids Mixing, 21-39
[Gericke] : adapted from Different Methodes of Batch and Continuous Mixing of Solids, Bulk Solids Handling, H.R. Gericke,1993
This raises two questions:
The size reduction of lumps and agglomerates is of crucial importance. Only when all particles are separated/deagglomerated can they be optimally distributed. This is why fast-rotating choppers support the mixing process.
Such choppers have different names. For example, whirlers, choppers, cutter heads, cutting rotors, high-shear blades, homogenisers or mixing accelerators. Choppers can also be designed as rotor-stator machines. They then work in a similar way to a mill.
It is interesting to note, however, that deagglomeration in the conical mixer is effective even at low filling levels. Good efficiency is always achieved when the agitator is covered by the powder. Conical mixers can fulfil this requirement particularly well.