Ribbon blender - Ribbon mixer - Design, calculation, and selection

Section summary
1. Introduction
2. Mixing principle
3. Mixing operating parameters
4. Detailed specifications
5. Mixer sizing
6. Common problems with ribbon blenders
7. Buying guide - Selection of ribbon blenders

1. What is a ribbon blender : definition

Ribbon blenders are made of shell having a U shape in which is rotating an agitator. The agitator is designed as a double helical ribbon which allows to create convective movement of the materials in 2 directions achieving mixing of powders and bulk solids.

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.

2. Ribbon blender working principle

What is the mixing mechanism of a ribbon blender ?

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).

Ribbon mixer

Figure 1 : Ribbon blender drawing

3. Mixing operating parameters

How to reach good homogeneity with a 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 alert the producer that the mixer operation may not be 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 (note that Froude number < 1 for ribbon blenders)
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

Level of solids in Ribbon mixer

Figure 2 : Ribbon mixer recommended and max mixing volume

Recent Discrete Element Modelling (DEM) [Basinskas] of ribbon blenders have confirmed that the key operating parameters influencing on the degree of mixing and thus the mixing time are :

The mixing speed : higher mixing speed means shorter mixing times
The filling level : a ribbon blender should not be under filled, typically if the level of powder is below the shaft the impact on the mixing quality will be very high
The filling sequence : according to the simulation the axial mixing of a ribbon blender is not so good (which is surprising as the ribbon are supposed to bring some axial thrust), which means that it is better to fill the mixer by layers rather than having one side of the blender with some components and the other side of the blender with the rest of the components.

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.

4. Ribbon blender design - Detailed specifications

Paddle and plow designs

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.

Mixer access

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.

Discharge valves

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, minimizes 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 bomb doors, flap valves or that be connected through a short pipe to the mixer (round valve)

Instrumentation

The following instrumentation can be found on ribbon mixers :

Speed sensor : allows to confirm rotation and speed of the shafts
Temperature sensors : positioned on the bearings, allow to detect abnormal heating due to broken bearings
Flowmeter : positioned 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.
Valve position sensor : allows to detect that the discharge valve is closed
Safety 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.

ATEX - dust explosion risks in ribbon blenders

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:

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 :

Tip speed

With :
- 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

Cleaning of ribbon blender (hygienic application such as food or pharma)

Ensuring the cleanliness and hygiene of ribbon blenders is important in industries such as pharmaceuticals, food processing, and cosmetics, where product purity is critical. Proper cleaning and sanitization procedures not only adhere to regulatory standards but also contribute to the longevity of equipment and the quality of the final product.

Accessible Design for Efficient Cleaning

The design of ribbon blenders significantly influences the ease and effectiveness of the cleaning process. Optimal cleaning requires accessible components, minimal crevices, and surfaces resistant to product adherence. Selecting ribbon blenders with well-designed access points, such as hatches on top, facilitates thorough cleaning and reduces the risk of cross-contamination between batches.

Evaluate Design Features : Prioritize ribbon blenders with top hatches, tilting or sliding covers, allowing convenient access to internal components.
Assess Material Properties : Opt for ribbon blenders constructed from materials resistant to product adherence, ensuring easier cleaning and minimizing the risk of residual material.

Safety Measures during Cleaning

Safety should be a paramount consideration during cleaning operations. Implementing safety features, such as locks that prevent the opening of access doors while the mixer is in operation, safeguards operators from potential hazards. Detection systems that signal when access points are opened contribute to a secure working environment.

Install Safety Locks : Equip ribbon blenders with safety locks on each access door to prevent unauthorized opening during operation. This can be done through electromechanical locks (Schmersal, Steute...) or a trapped key system (Fortress, Alen-Bradley...)

Hygienic Considerations

In applications where hygiene is critical, such as food and pharmaceutical industries, ensuring hygienic design is essential. Fully welded ribbons reduce the risk of loose parts contaminating the product. However, the complexity of accessing internal components should be weighed against the potential risks of cross-contamination.

Select Fully Welded Ribbons : Opt for ribbon blenders where ribbons are fully welded to eliminate loose parts that could compromise product integrity.
Evaluate Cleaning Challenges : Balance the hygienic benefits of full welding with the challenges associated with accessing internal components for cleaning. Employ additional cleaning validation processes when necessary.

Cleaning Validation and Documentation

Establishing a robust cleaning validation process ensures that ribbon blenders meet the required cleanliness standards. Documentation of cleaning procedures, including the materials used and the frequency of cleaning, contributes to quality control and compliance with industry regulations.

Develop Cleaning Validation Protocols : Create comprehensive protocols for cleaning validation, specifying the methods, materials, and criteria for successful cleaning.
Maintain Detailed Documentation : Keep meticulous records of cleaning activities, including dates, cleaning agents used, and any deviations from standard procedures. This documentation serves as evidence of compliance during regulatory inspections.

5. Solids mixer sizing

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.

6. Common problems with ribbon blenders

General troubleshooting guide for ribbon mixers

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, make sure the ingredients are loaded making layers all along the blender and are not concentrated on left or right of the blender before the mixing starts
Product damages, breakage	Too long mixing time Too much speed

7. Ribbon blender buying guide - How to select a ribbon blender

7.1 Buying a new ribbon blender

When sourcing a new ribbon 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 ribbon blender to buy. Don't forget that the ribbon blender should not be filled at more than 80% of its total volume
Is it an application requiring cleaning ? if yes, consider access doors on top of the blender to clean, consider safety locks on the access
Is the application handling an abrasive product ? if yes, discuss with the ribbon blender supplier the alloy in which the blender can be supplied
How fast is the blender to be discharged ? How ? This will give you the inputs necessary for the discharge valve
Is it in ATEX area ? If yes, the blender must be certified, the clearance paddle / housing guaranteed, the seals of bearing must be pressurized and possibly their temperature monitored

7.2 Second Hand ribbon blender

Many used ribbon blenders can be found on the market. When looking for a 2nd hand mixer, you should go through the following checks :

Was the ribbon blender used for a similar application to your needs ?
Look for damages on the ribbon, scratches on the inside of the shell, measure the gap in between the ribbon and the housing, there should not be metal metal contact
Run the mixer, listen to the bearings, if possible measure vibrations
If necessary, can the mixer be cleaned ?
Can the cover be modified to accomodate your needs for the fittings
Not all the ribbon are equivalent, if possible run a test with product to validate homogeneity and product degradation during mixing
Is the discharge valve functional
Test all instrumentation that may be coming with the mixer
Is the mixer ATEX compliant for the area you defined, if not can it be retrofitted

Sources

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
[Basinskas] Numerical study of the mixing efficiency of a ribbon mixer using the discrete element method, Basinskas et al, Powder Technology, 287 (2016) 380–394

Powder Dry Mixing - Ribbon blender