Powder Dry Mixing
As rotating mixers, this webpage is considering all mixers for which the mixing driving force is achieved by rotating the whole shell of the mixer, with a mixing by free-fall within the rotating vessel and without agitators (with some exceptions). It gathers V mixers, Bi-cones, rotocubes, drum blenders or bin blenders. Free fall mixers are very widespread in process industries for bulk solids dry Mixing. Although other Mixers designs exist which are over-performing tumblers in some aspects, it remains a very simple solution for small operations or to perform a preblend for example. For bin blenders, the possibility to move the bin in which the mix is being done is also opening interesting possibilities to reduce handling, conveying and dust emission.
This webpage is focusing in the detail design of tumblers and more particularly bin blenders (IBC), a type of mixers which had an important development over the past years.
Most of free fall mixers are purely diffusive mixers, operating at Fr < 1. The movement of particles is not forced by an agitator as it is the case for Ribbon Blenders or Paddle Mixers. The movement of particles is created here by the rotation of the whole mixer which has as an effect to make the particles roll down (at the surface of the bed of solids in the mixer, the particles movement being similar to an "avalanche").
Since the mixing is based on the free movement of particles, those mixers will have limited performances with cohesive powders. Indeed, there is very little force dissipated in the mixer to separate particles of a same kind thus, if the mixer is filled with layers of different cohesive powders, they may stay together and not mix.
On the other hand, solids that are particularly free falling may also cause problems in diffusive mixers, leading to Segregation (demixing). A very common example is to fill a rotating mixer with balls of different densities, after appartently mixing they finally totally separate.
Despite these limitations, one should not discard at 1st sight this type of mixers. They indeed constitute a very simple solution, cheap, reliable and generally offer a good accessibility for cleaning in between 2 batches if needed. The Homogeneity achieved is very variable but can be more than satisfactory for some applications, especially when a preblend is produced that will be used in a main mixer. In addition, it must be noted that the product is not in contact with bearing, or bearing seal, removing a source of contamination (stagnant product released from time to time, lubricant or wearing of mechanical parts).
Figure 1 : Free Fall Mixer
For diffusive mixers, the mixing time is typically 5-15 min. This is quite long compared to other mixers and mainly due to the fact that no agitator is involved to speed up the movement of particles.
The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :
- Mixing batch size : 50-60% of mixer total volume. The filling rate is absolutely critical for free fall mixers. The bulk solids need indeed space to move during the rotation to promote the mixing. If the mixer is too full, bad CV and / or long mixing times are to be expected.
- Mixing speed : typical mixing speed are in the range of 15-25 rpm with a Froude number < 1 ; it is common to test the mixer at different speed and mixing time during the validation
- Mixing speed and time : for this kind of mixer, what usually matters is the number of rotation, passed a certain speed. If you reach the right homogeneity at 10 min and 15 rpm (150 rotations), you can try to validate the mixing at 7.5 min and 20 rpm (150 rotations). There may be some changes for big speed differences as the rotation speed may also influence the diffusion within the mixer.
- Presence of baffles : some mixers have baffles (fixed plates in the mixer) in order to promote mixing. Such baffles can improve the mixing in some cases but the effect can be neutral or even detrimental.
The power input required for a rotating blenders is quite low, in the range of 1 kW/m3.
Some manufacturers of bin blenders are proposing an option for adding an impeller within the mixer. The mixer is still rotating but additionally the agitator is providing a convective mixing effect. Such system can improve the performance of the mixer, especially with cohesive powders, however, it adds to the complexity of the system and the difficulty to clean, 2 points to consider before adding an agitator.
Many designs of rotating mixers are existing. Bi-cone, V blenders have a fixed shell and must be filled and emptied in place. However, the last years saw a growing trend in industrials and equipment suppliers shifting to drums and bin blenders. Such system present the flexibility to be able to bring the bin elsewhere than where stands the mixer. Thus, the bin can be brought to a dosing system for being filled, and can be moved from the mixer to a tipping point in order to be discharged.
Bins can be very small (20-50 liters ; more in the form of drums) to very large (2-5 m3).
Such system, based on drums and IBC bins is picking up pace in the industry. They offer a lot of flexibility and can open up the door to different manufacturing setups. A single mixing station is used but several containers can be used to increase the productivity. While a bin is being mixed, another one is being filled while another one is discharged. The occupancy of the system is therefore optimized. As well, the dosing can be performed in innovative ways like dosing trains or even the movement of the containers can be done via Automated Ground Vehicles (AGV). By using specific valves, filling and discharge can be done hygienically without spilling material.
Figure 2 : Typical IBC Bin Blender proecss
Mixer access is normally quite easy for rotating mixers. Indeed, as there is no agitator inside the mixer, inspection and cleaning between batches is relatively easy.
Drums are very easy to access while IBC bins may be more challenging for big containers, since it is necessary to reach the top of the bin to access the manhole. For this reasons, it is possible to find on the market IBC bin cleaning stations, either dry or wet.
For classical design like bi-cone or V blenders, valves are typically butterfly, sometimes even manual and the discharge can be done directly to a bag to another processing step.
For drums, there are most of the case no valves, the operator just opens the cover and tilt the drum to the next processing step. To be noted however that more and more suppliers are now proposing cone outlet for drums equipped with a valve, thus simplifying the manipulations.
These are for IBC containers that recent developments have been made with 2 different types of valves found in the market (and many different designs within each valve category, depending on manufacturers) : cone valves and split butterfly valves. Both types of valves attempt to answer to the following challenges : perform the docking of the IBC bin to the filling station or discharge station as automatically as possible, and have a tight system allowing to avoid dust emission during the process.
As the mixer rotates, no instrumentation is connected to the rotating part (level...), however the mixing station is equipped with different instruments :
- Speed sensor if required
- Safety cage equipped with locks : it is mandatory to keep anybody further away from the mixer when it rotates, and avoid that it starts when the cage is opened.
- The active parts of the discharge valves, if these are cone valves or split butterfly valves and also instrumented.
Rotating mixers are quite favorable fom an ATEX point of view, since there are no rotating equipment in the mixer. However, the possibility of leakage from the mixer during the rotating must be taken into account to define the ATEX rating of the area.
Mixing in the Process Industries, Harnby, Edwards, Wienow, Butterworth Heinemann, 1992
Food Mixing : Principles and Applications, Cullen, Wiley-Blackwell, 2009
Perry's Chemical Engineer's Handbook, McGraw Hill, 2008
|Powder properties||Unit operations||Equipment handbook|
Flow of solids
Mass and funnel flow silos
Particle Size Distribution (PSD)
Dosing and weighing
Solid Gas Separation
Grinding and Milling
Solid Liquid Suspension
Checking (sieve and magnets)
|Big Bag Tipping Stations
Airlock rotary Valve
Rotary Valves (pneumatic)
IBC Bin Blenders (rotating blender)