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V blenders are widespread in the process industries as they provide an easy and often sufficient way of mixing dry bulks solids (powder, granules, pellets...) in batch. They are simple, reliable and somehow easy to clean, thus constitute a good choice for small and medium size companies working in fields of food processing or pharmaceuticals. They can also be used for pre-mixing in any kind of industries, big or small. This page is giving many pieces of information interesting to plant designers, factory owners or factory operators for chosing a V mixer or optimizing an existing unit : applications, working principle, mixing volume, mixing time.
V blenders can be used to mix dry bulk solids. They are particularly used in the following applications :
- Food processing : premixing, cereals, coffee mixes, dairy powders, vitamin complements, soup, spices...
- Pharmaceuticals : mixing prior to granulation
- Plastics : mixing of masterbatches, mixing of pellets
- Chemicals : metallic powder mixtures, minerals
- Construction : steel preblends
V mixers are rotary mixers. The whole shell, which has a shape of a V and is made of 2 cylinders welded at 75° to 90°, is rotated allowing to mix the components thanks to a diffusive mixing.
There is no agitators inside the V blender, all the movement of the powder is due to the rotation of the shell which creates an avalanche of product. After a certain number of rotation, an optimum mixing is obtained. Due to the absence of agitator, it must be noted that this kind of mixer will perform satisfactorily with free flowing solids, but less with cohesive ones, cohesive solids cannot be sufficiently dispersed with the rotation movement.
These mixers are usually supplied with 2 opening on top of the "V". The V shape helps to discharge easily the blender at the end of the mixing.
Due to the (slow) rotation and the absence of agitator, the mixing is usually quite gentle for the components, thus one should expect minimal damage of the ingredients. However, the obligation to run free flowing materials can also be detrimental in terms of demixing, a point that must be controlled when validating a V blender.
Figure 1 : V blender drawing
For V mixers, the mixing time is typically 10-15 min. Common problems leading to a long mixing time or not achieving the right Cv Homogeneity is an overfilling of the blender or using materials that are not free flowing enough, or free flowing but with large particles / density differences. The way to load the mixer has also a very high influence on the mixing time.
The mixer performance, i.e. time to reach a desired homogeneity, is a
function of the following operating parameters :
- Mixing batch size : 40% to max 70% of mixer total volume. It is very important to leave free space in a rotary, free falling mixer. The solids need indeed to have space to roll over and the level of bulk solids should not be too high to allow a good material diffusion in between the 2 "legs" of the V blender. Filling at 70% may already be too high for some mixture, thus trials must be organized to optimize the filling rate of the blender. Some sources are reporting high increase in required mixing time due to an overfill of the mixer : 3 times longer at 60% vs 40% and 2 times longer at 70% vs 50%.
- Mixing speed : one should follow the supplier's recommendation, typical mixing speed for medium size blenders (500-2000 l) is around 10-25 rpm, depending on the mixer size, the bigger the slower, with a Froude number < 1. V blenders have a critical speed, where the centrifugal force is matching the gravity force : the optimum mixing speed is reported in the range of 50% to 80% of the critical rotation speed.
- It is very important to load such a mixer symetrically. For example, a very long mixing time will be needed if all ingredients 1 are in the 1st "leg" while all of ingredients 2 are on the other side. It is better to proceed by layers and, in case of small ingredients, to try to tip them in the middle of the major ones. If the level is higher than the junction of the 2 cylinders, one should tip equal amount of powder in both of them to optimize the cycle time.
Figure 2 : V mixer recommended and max mixing volume
- V blenders are very well efficient if the small ingredients represent min 1% of the blend size. Lower amount may lead to higher mixing time.The power input required for a V blender is quite low, in the range of 1-3 kW/m3.
V mixers are actually quite standardized among manufacturers, here is a typical range of blenders :
As the whole shell is rotating, the mixer must be disconnected from the rest of the process during mixing. It therefore involves manual operation to load and discharge the mixer. Loading and discharge can be directly done by the operator although this can generate dust emissions. To avoid this, the blender can be located to tipping station and discharge hopper thanks to flexibles, although still requiring some operator's intervention.
The discharge valve of V blenders is typically a manually operated butterfly valve
As the mixer is rotating, it is not equipped by many instruments, which don't appear necessary anyway. The control is usually limited to mixing time and rotation speed. To be noted that some suppliers are proposing for NIR sensors to follow the mixing and help optimizing the mixing time. Those sensors are actually in the axis of the mixer.
If the blender is processing powders that can trigger explosions, it must be the object of an ATEX risk analysis. In order to consider the risks associated in the mixer where a risky dust cloud can form but also around it during loading, discharge or in case of powder spill during rotation.
The rotation of the blender can constitute a safety risk if an operator is approaching too close. It is necessary to implement a safety cage around the mixer, high enough to prevent people entering the mixing area. The door must be locked when the mixer is rotating, and the mixer should not be started in case the door is opened.
flexibles are used to load or discharge the mixer, it could be also
necessary to have proximity switches that are detecting the
connection in order to avoid starting the rotation with the
flexibles still on.
It is possible to consider 2 cylindrical shells of different length making the blender assymetrical. Such a design can actually help the mixing from one shell to another and thus decrease the mixing time. This may also be achieved by baffles positionned in the mixer (one should however be careful of the drawbacks of such execution as it can reduce the accessibility and cleanbility of the blender).
Some manufacturers are proposing to install an agitator along the axis of the mixer and rotating at high speed. This intensifier is bringing shear mixing thus can improve the mixing performance with cohesive powders or mix having the tendency to agglomerate. It must however be noted that having such an intensifier bar will most likely create product breakage, this must be taken into consideration by the operator of the mixer.
When sourcing a new V mixer for your factory, the following questions need to be asked in order to buy the right specifications :
- What is the expected throughput of the line ? What is the product
density to be mixed ? What is the expected mixing
time and cycle time ? This will give the size of the V blender
to buy. Don't forget that the V blender should not be filled at more
than 40-60% of its total volume
and max 70%
- Is the mixture free flowing ? Or is it necessary to have an
intensifier bar to cope with some more cohesive blends ? Was there any
trial done to indicate if the use of an asymetrical geometry would be
beneficial to the mixing time ?
- Is it possible to load
the mixer by layer and avoid to have a vertical symetrical loading ?
- Is it in ATEX area ? If yes, the blender must be certified
Many used V blenders can be found on the market. When looking for a 2nd hand mixer, you should go through the following checks :
- Was the V blender used for a similar application to your needs ?
- Look for damages, scratches on the inside of the shell, hammer marks on the body
- Run the mixer, listen to the bearings, if possible measure vibrations
- If necessary, can the mixer be cleaned ?
- Can the opening doors and outlet valve be modified to accomodate your needs for the fittings
- Test the controls that may be coming with the mixer : mixing timer and mixing speed controller
- Is the mixer ATEX compliant for the area you defined, if not can it be retrofitted
Experience of the author
Scale Up factor determination of V Blender: An overview, V.S.C. Chopra et al, Der Pharmacia Lettre, 2010, 2(2): 408-433
Scale Up of Powder-Blending Operations, Muzzio and Alexander, Pharmaceutical Technology, 2005
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