Solids Mixers (batch, convective)
There are different mixer designs that can be used for dry mixing solids and make the mix homogeneous. Those different designs are at 1st based on the mixing principle. The mixing can indeed be the result of diffusion, convection and shear.
Convective mixers are actually very often used industrially since they allow usually short mixing time compared to simpler but less efficient diffusive mixer. The object of this page will therefore focus on convective mixers.
Only the case of batch mixers is discussed in this page.
Convective mixers are equipped with a mixing tool which is basically agitating the powder and forcing its movement.
Such solids mixers can work at different regimes which are characterised by an adimensional number, the Froude number, which is comparing the forces given by the impeller vs the gravity force. When Froude is less than 1, the bed of particle will be gently agitated, while at more than 1 the bed of particles will be fluidized, this fluidization will increase with the Froude number.
Table 1 : Different types of powder convective mixers
|Ribbon blenders||Fr < 1|
|Screw mixers (type Nauta)||Fr < 1|
|Vertical mixers (type Amixon)||Fr < 1|
|Double shaft paddle mixers||Fr = 1 to 1.1|
|Pneumatically generated fluid beds||Fr > 1|
|Ploughshare mixers||Fr = 3 to 9|
The mixing tool design is particularly important in convective mixer since it will give the driving force to the movement of the particles which will ultimately lead to the homogeneity.
The way the agitator is designed and, linked to this, the speed at which it is operated will have influence on the mixing time, final homogeneity and possible damage to the product during the process of mixing.
Usual design, influence on product, but also design considerations are given in the following table.
Table 2 : Design of mixing tools
|Mixer type||Mixing tool design||Product impact||Design considerations|
|Ribbon blender||Ribbon (top view representation below)
||The ribbon is pushing the product, thus long mixing time can have significant impact on the particles.
The influence is more important if the mixer has been overfilled : product cannot move freely and the ribbon creates a severe attrition over an extended mixing time
|Screw mixer (type Nauta)||Screw (side view)
|Vertical mixer (type Amixon)||Agitator||This type of mixer appears to better perform on poor flowing solids compared to free fall mixers, or other thrust mixers like ribbon blenders and central screw blenders|
|Double shaft paddle mixer||2 shafts with paddles, paddles 1 shaft forming a 45 degrees angles with the corresponding paddle of the other shaft
||Very gentle if the mixer is operated in the proper range of filling and mixing speed.
Paddles actually send the powder in a fluidized zone at the center of the mixer where the mixing occurs.
|The distance paddle to mixer housing should always be higher than 4-5 mm in order to cope with slight misalignement of the mixing tool due to mechanical effort without risking to have the paddle touching the wall.
Classic designs have 2 bearings / shafts while other, more modern, use cantilevered shaft. This design allows to have, if required by the process needs (cleaning), extractable shafts.
|Pneumatically generated fluid bends||Such mixer presents the advantage to have no moving part, thus is very simple to manufacture, cheaper on CAPEX and offers better cleaning possibilities (no complex agitator to clean)||Gentle mixing||If the CAPEX is lower, compressed air is consumed during the mixing process, thus financial calculations should be made to ensure the operation of the mixer is profitable on long term.
Attention must be given to risks of segregation due to fines "floating" at the top of the mixer
|Ploughshare mixersr||Single shafts with profiled paddles (ploughshare) + additional choppers if required
||Very strong attrition allowing a deaglomerration of the powder||It is possible to add additional small agitators (choppers) turning at very high speed in order to break lumps that may form during the mixing process (especially in case of addition of liquid or when fats are involved in the mixing process)|
The powder is introduced on top of the mixer. There can be one inlet if the ingredients are preweighed in a same hopper, or manually tipped in the mixer, or several if the process upstream allows to dose individually the ingredients to the mixer.
Some considerations must be done during the design to the following requirements : the inlet must be large enough to allow a fast loading of the mixer (gain of cycle time), each inlet must be closed by a valve to avoid any uncontrolled entry of material in the mixer, the minor ingredient inlet should be positionned in the center of the mixer (or in the fluidizing area of the mixer, if any).
Defining firmly the position of the inlet at very beginning of a project is often mandatory since many mixers have top welded covers which require correct opening inputs for design and manufacture. It may be wise to keep 1 or 2 inlet spare for future evolution of the process.
On top of the mixer should also be located a filter, necessary to release air when loading the mixer, or admit air when discharging the mixer.
Once the mix has been completed, it must be discharged from the mixer. The discharge must generally be quick, in order not to impact too much the cycle time of the mixing process
Different designs can be found. One common point in between those design is that the valve closing the mixing must not allow dead zone in the mixing chamber where the product could accumulate and not be mixed properly, which could cause quality issues once the product is further used or sold.
Common design include bomb doors, square flap and round hygienic valves.
Bomb doors are called this way since they are usually covering the whole bottom of the mixer. Once opened, the mix will fall immediately since no surface is anymore keeping it in the mixer. Bomb doors allow therefore a very quick and complete discharge, only few hundred grams of product will stay in the mixer. On the other hand, they require large mechanical and pneumatic system to operate and make them tight during the mix. A hopper needs to be connected right below the mixer.
Square flaps are very common. They can be easily designed, implemented and manufactured. Compared to bomb doors, the discharge area is much reduced, thus the discharge time will take longer. Such design may not be very hygienic, depending on the care the supplier took in manufacture and designing the shape of the flap.
Round hygienic design are proposed by some manufacture only. They offer the best level of hygiene, when this characteristic is important for the plant operator. On the other hand, these are usually small valve requiring a long discharge time. In order to speed up the discharge it may be necessary to install 2 valves. Few kg of product may stay in the mixer after the discharge.
To be noted that the mixer should be on at low speed during discharge, it is necessary to push out the maximum of product and ensure high discharge rates.
The mixer should be the bottleneck of the installation of mixing, which means that it should not be slowed down by the process section upstream or downstream. The capacity of the installation should be a given and a batch size should be chosen in consequence, considering as well an estimated number of batches / h
Batch size (kg) = Capacity (kg/h) / Number batches per hour (/h)
The mixing process being actually volumetric, it is necessary to know the untapped (loose) density of the mixture to size properly the mixer.
Batch size (l) = Batch size (kg) / Loose density mix (kg/l)
On top of this, it is critical to consider that the system should never be filled at 100% of its capacity, in order to allow space for particles movement.
Total mixer size (l) = Batch size (l) / 0.7
Mixers have maximum filling coefficient in between 0.65 to 0.8 usually.
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