Powder Dry Mixing - Ribbon blender
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 ribbon blenders in factories, still perfectly operational. Many companies propose ribbon blenders, from few hundreds liters to several cubic meters.
This webpage is focusing in the detail design of ribbon 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.
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
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 :
- Mixing batch size : 70-80% of mixer total volume. Visually, the top of the blades must be slightly above the level of product, and some head space to the top cover must always be available. It is a very common problem to find ribbon blenders overloaded which is strongly decreasing their performance. To be noted that it is not advised to under-fill a ribbon blender, since the level of powder must reach inner ribbon in order to be moved and thus mixed.
- Mixing speed : one should follow the supplier's recommendation, typical mixing speed for medium size blenders (500-2000 l) is around 50 rpm with a Froude number < 1
- Small and Minor ingredients to be introduced in the mixer after the main ingredients (or in sandwich), preferably in the central area of the mixer
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 time 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.
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.
The access to the inside of a ribbon blender 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 (tiltable or sliding cover). Some manufacturers propose to have an extraction system for the 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.
- Simple flap valves : discharge valves have a rectangular shape. It is the most common design.
- Hygienic round discharge valve : the valve has a round shape and, once closed, mimimizes the gaps where product can settle and avoid being mixed, contrary to the 1st type of valve that is presenting higher gaps. Those valves have a limited size that can increase the discharge time and thus reduce the mixer capacity.
- Bomb doors : the bottom of the of the mixer can entirely be opened by large flaps. The key advantage is that the mixing time is very short (less than 30 s, and that the quantity of product remaining in the mixer is very low. However, one must be careful to the tightness of the doors once closed, as well as to the access for cleaning below the doors.
Depending on the technology chosen, the mixer will discharge is a hopper that will be entirely connected to the mixer bomd 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 :
- 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, ribbon blenders, 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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