|2. Mixing principle|
|3. Mixing operating parameters|
|4. Detailed specifications|
|5. Mixer sizing|
|6. Common problems with ribbon
|7. Buying guide - Selection of
Ribbon mixers are very widespread in process industries for bulk solids dry Mixing. Although other Mixers designs exist which are over-performing ribbon blenders in some areas (mixing speed, hygienic design...), ribbon blenders are still a very simple and robust solution to mix dry materials offering decent mixing performances which are sufficient for many applications. It is not rare to have 40-50 years old industrial ribbon blenders in factories, still perfectly operational. Many manufacturers propose industrial ribbon blenders, from few hundreds liters to several cubic meters.
What is a ribbon blender used for ? Ribbon blenders are actually used in many industries and applications : pharmaceuticals, food and bakery, cosmetics, plastics, spices, mixes for drinks, cements grouts and mortars, coffee and tea, tobacco...
This webpage is focusing in the detail design of ribbon mixers in order to offer selection and buying guidelines to operators of mixers.
Ribbon blenders are convective mixers. The mixture movement is forced by the rotation of the ribbon which is circulating the product in 2 directions : the ribbon is actually made in 2 parts, 1 external ribbon circulates the product in 1 direction while another ribbon located inside the 1st one moves the product in the other direction (it can be possible to have even more complex profiles to attempt optimizing the mixing efficiency and thus the mixing time). By mixing enough time, those 2 axial movement, coupled with some radial movement (blades "cutting" the material when rotating), will allow to reach the required degree of homogeneity. The twist of the helix must be studied by the manufacturer so that the direction in which the outer ribbons is pushing is towards the outlet valve of the mixer, if it is not the case, good discharging rate of the mixer cannot be achieved.
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Contrary to double shaft paddle mixers the powder is not fluidized in a ribbon mixer. The optimal mixing speed is advised by the mixer manufacturer, some trials can however be done at higher or lower speed (provided the drive is able to sustain different speed) in order to verify the influence on the mixing quality and the product properties (in general, lower speed will require a longer mixing time but the product may be less damaged).
Figure 1 : Ribbon blender drawing
For ribbon mixers, the mixing time is typically 3-5 min. Ribbon blenders have the reputation not to be supposed have a short mixing time, which brings some operators to mix 10-15 min. If the mix has no particularity (injection of liquid...) such long mixing time should aler the producer that the mixer operation is not optimal.
The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :
Figure 2 : Ribbon mixer recommended and max mixing volume
The ribbon movement has quite some impact on the product being mixed. The thrust imposed to the product combined to mixing times that are not very short, usually cause some breakage on the solids processed. Breakage means that the Particle Size Distribution (PSD) will be changed, with creations of some smaller particles due to the breakage of bigger ones. The impact can be minimized by validating the exact time required for mixing (Homogeneity) and thus not mix longer without need, and by optimizing the mixing speed to mix as low as possible. However, breakage is expected to be higher than in twin shaft paddle mixers for example.
Ribbon blenders are operating well with free flowing powders, due to their mode of operation (thrust by a rotating agitator), ribbon blenders may be less effective with cohesive powders and may even block if the the mixture is very cohesive.
The power input required for a ribbon blender is quite low, in the range of 3-5 kW/m3.
Some manufacturers have adapted their ribbon blender design by changing the agitator through the use of paddles or plows. Those designs should not be confused with double shaft paddle mixers or shear mixers / ploughshare mixers. Going to a single shaft paddle design may be advantageous for very poorly flowing materials, fragile materials or if the batch size may be as low as 15-20% of the nominal batch size. When using plows, it is possible to have a better action close to the wall of the mixer, high centrifugal forces should however not be used and reserved to shear mixers / ploughshare which have a cylindrical shell, versus a U shaped trough design for ribbon blenders.
The access to the inside of a ribbon mixer for cleaning or maintenance is not very easy, considering that the ribbon is taking a lot of space in the mixer, and has a complex shape.
The most common access features for ribbon blenders is to have hatches on top of the mixer, with sometimes the possibility to open fully the cover of the mixer (tilting or sliding cover). Some manufacturers propose to have an extraction system for the ribbon which allows to take out the whole ribbon, it is possible but adds to cost and mechanical complexity.
The opening of the mixer must be detected so that the mixer cannot be started, for the safety of operators accessing the inside of the mixer. Accesses must be guarded by locks blocking the opening when the mixer runs and detecting when it has been opened.
Some care must be taken for hygienic applications, indeed the ribbon must be fully welded to avoid any loose part that could get in the product. The difficulty to access for cleaning means that risks of cross contamination must be well weighed and managed by the plant operator.
3 types of discharge valves can be found on the market, depending on the suppliers and the need of the customer.
Depending on the technology chosen, the mixer will discharge is a hopper that will be entirely connected to the mixer bomb doors, flap valves or that be connected through a short pipe to the mixer (round valve)
The following instrumentation can be found on ribbon mixers :
In order to process powders, ribbon blenders, which are mixing with a mixing tool tip speed > 1 m/s, must present the following characteristics in order to prevent dust explosion risks:
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.
Ribbon blenders are usually reliable equipment, however a certain number of issues may require some corrections :
Table 1 : common problems with ribbon blenders
|Issue||Root cause and action|
|Ribbon is blocked||Too high density of the powder - reduce density / batch size
Start mixer at low speed during filling and do not stop mixer until discharge
|Too long mixing time||Mixer is overfilled - reduce batch size
Mixing speed is too low - increase mixing speed
Filling sequence is incorrect - make sure the small ingredients are loaded in between majors
|Product damages, breakage||Too long mixing time
Too much speed
When sourcing a new ribbon mixer for your factory, the following questions need to be asked in order to buy the right specifications :
Many used ribbon blenders can be found on the market. When looking for a 2nd hand mixer, you should go through the following checks :
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