Powder Dry Mixing
Shear mixers are developped in some process industries for some application of powder Mixing. They find applications when industrials need to mix particularly coehive, even lumpy, powders. They are also a mixer of choice when having to add some liquids in the dry blend.
This webpage is focusing in the detail design of shear mixers.
Shear mixers are using convection and shearing to achieve the mixing of material and its proper de-agglomeration if needed.
The mixing speed has to be set quite high in order to have enough shearing effect and even centrifugate the powder to the walls of the mixer. The design of the paddles is designed for this purpose, shearing and breaking agglomerates of cohesive powder..
It is difficult to give a typical mixing time for shear mixers, as the dependency on the product mixed is very high. A mixing time in the order of 3 min is common.
The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :
- Mixing batch size : 70-80% of mixer total volume. Some head space to the top cover must always be available. Volumes available range from around 100 liters to 30000 liters
- Mixing speed : the actual mixing speed must be adjusted to get a Froude number > > 1. Froude number range is from 2 to up to 9 in some cases, below 6-7 the solids bed is fluidized, above 6-7 centrifugal forces are preponderent and the solids form a layer all around the housing. . For Fr = 3, corresponding mixing speed is generally in the min 80-100 rpm range (small size mixers) depending on model and size.
- The mixing time will be greatly dependent on the nature of the solids mixed (highly cohesive or not) and if some liquid is added to the mixture
The power input required for a shear blenders is high, in the range of > 10 kW/m3.
When mixing highly cohesive powders, ploughshare mixers can be fitted with additional mixing tools. Those mixing tools are called deagglomerators, lump breakers or choppers and turn at very high speed. They are generally mounted on the side of the mixer, rather close to the bottom. It can of injection of liquid, it is preferable to direct the flux of liquid towards the deagglomerator to break lumps as early as possible.
Figure 1 : Ploughshare mixer
Side doors are generally used to access this type of mixer, giving a correct access to the inside of the mixer, although the presence of deagglomerators may make the access a bit less convenient than on other mixers type. The plough of the agitator are also generally quite large which makes the cleaning sometimes uneasy, although recent models are fitted with larger side doors allowing to overcome this drawback. Deagglomerators may also be difficult to clean (specific precautions are needed to avoid hurting hands when cleaning the blades).
Some design allow to have an extractable shaft for easier cleaning.
The following instrumentation can be found on ribbon mixers :
- Speed sensor : allows to confirm rotation and speed of the shafts
- Temperature sensors : positionned on the bearings, allow to detect abnormal heating due to broken bearings
- Flowmeter : positionned on the compressed air supply to bearing seal flush. Bearing seal flush constitute an important function to avoid ingress of product to the bearings, which would damage them or make the powder burn, which would constitute an ignition source causing dust explosion.
- Vale position sensor : allows to detect that the discharge valve is closed
- Locks : placed on each access door, ensure the safety of the machine by preventing operator to access the mixer while it runs, or start the mixer if an access point is opened.
In order to process powders, shear mixers, which are mixing with a mixing tool tip speed > 1 m/s, must present the following characteristics :
- Clearance in between the tip of paddles and the mixer body must be large enough to avoid any contact ribbon / housing
- The bearing seals must be pressurized
- During loading and discharge, the mixer speed must be such that the tip speed of the paddles is < 1 m/s
The ribbon tip speed can be calculated thanks to the following formula :
- R is the radius of the mixing tool (center of shaft to tip of paddle) in m
- n is the mixing speed in rpm
Equation 1 : Tip speed
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.
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
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