Mixing How To: Ribbon Blenders, Tank Baffles and Avoiding Hazards
For many years David S. Dickey, senior consultant and founder of MixTech Inc., has been answering questions from our readers. His experience is unique in the field of mixing and scale-up because he has had exposure to both the theoretical and practical aspects of real problems and has learned about both successes and failures. Before starting MixTech, he had over 25 years of experience with process equipment manufacturers. He has built pilot-plant reactors and systems and spent 16 years working directly with manufacturers of liquid mixing equipment. He has also engineered dry-solids mixing equipment, static mixers, heat exchangers, pumps, distillation and other process equipment.
Here are three of the questions he’s answered.
Is there a general formula to apply to a ribbon blender?
Q: I am currently designing a 10-ton horizontal ribbon mixer. The medium is dry cocoa powder; rotation: 20 rpm; housing size: diameter is 2,200mm x 2,300mm (height) x 4,000mm (length).
I am looking for ways or formulas to help me decide on the outer and inner ribbon pitch, the width of the pitch, the distance between the outer blade and inner blade required, the direction of the inner and outer ribbon with an outlet at the center of the housing, gap between the outer ribbon tip and housing required.
A: What your are asking is whether there is a general formula to apply to a ribbon blenders. The right answer is that no absolute formulas exist, but rather, the blenders follow some general relationships that work well for a range of powder properties.
You should start with your housing dimensions, which fit typical designs. First, the direction of the ribbon pitch for the outer ribbon is always toward the discharge. For a center discharge, the outer ribbons are divided at the middle of the mixer, with each half of the ribbons pushing toward the center discharge. The inner ribbons must push material in the opposite direction toward the ends of the blender. All of the ribbons should be double-flight ribbons. Double flight means that each outer ribbon starts on one side of the center shaft, and directly opposite that ribbon is another identical ribbon, both pushing in the same direction. The two outer ribbons balance the loads on the blender shaft. The inner ribbons also are double flight for mechanical balance. In effect, you have two outer and two inner ribbons on each end of the blender, with the ribbons on each end pushing in the opposite directions and each inner ribbon pushing in the opposite direction of the adjacent outer ribbon.
Dimensions then become as much practical as they are a formula. With the housing being 2,200 mm wide, the outer ribbons should be about 2,000 mm in diameter, providing about a 5% clearance on each side of the blender. With 2,000 mm diameter ribbons, their length will be half the length, or 2,000 mm, which makes 2,000 mm a logical pitch for the ribbons. A 1:1 pitch to diameter is common. As with most helical ribbons, whether for powders or liquids, a ribbon width of 200 mm (10% of the diameter) is practical. Although not equivalent in material movement, the inner ribbon should be 200 mm to 300 mm wide. The inner ribbon is typically positioned about halfway between the outer ribbon and the center shaft. The idea is to allow sufficient area between the ribbons for powder motion in opposite directions.
The best technical book on mixing is the Handbook of Industrial Mixing, Wiley, 2004. However, that source is more about the functional design of mixers than the mechanical dimensions and design of mixing equipment. Equipment manufacturers provide little specific information about the design of their equipment.
How do I calculate the forces on a baffle?
Q: I am trying to design a tank baffle. I do not know how to calculate the forces on it. The parameters are:
- Tank height: 7.4m
- Tank Diameter: 7.5m
- Baffle width: 0.625m
- Agitator power: 22kW
- Material in tank: Slurry of finely ground rock
- SG of slurry mixture: approximately 2.0
How do I determine the pressure on the baffle plate due to the rotation of the slurry?
A: Your question is good, but you have failed to provide essential information and some unnecessary information. The only information you need is the maximum torque of the mixer, which is power divided by speed and the tank diameter. You have provided the motor power, which is essential, but you failed to provide the mixer's rotational speed, which is necessary to calculate the mixer torque. The input torque from the mixer must be taken out by the baffles, so the torque load by the mixer becomes the force load on the baffles divided by the tank diameter.
The tank height, baffle width and slurry density will have no effect on the maximum load on the baffles. The only other piece of information of possible interest is the number of baffles. The logical assumption would be that the load on each baffle would be the same and a fraction of the total imposed torque.
Is blending wax with a solvent a bad idea?
Q: We would like to blend a solid wax with a solvent to make it flowable for our customer. Some information available:
1) The wax melting point is around 40 deg C to 60 deg C
2) The solvent used is aromatic solvent with a flash point less than 40 deg C
Questions are:
1) Is a jacket tank suitable for this type of blending? Any recommended blade design?
2) Once we melt the solid wax, the temperature is above the flash point of the solvent; which type of blending tank design will be suitable? Should we have a nitrogen purge?
3) What is the normal way to transfer the solid wax into the tank since the wax is received in a 55-gal drum?
A: This whole process sounds like an extremely bad idea and potentially a very hazardous situation.
Heating the wax to a temperature above the flash point of the solvent means that all oxygen must be purged from the vessel, and vapors must not be allowed to escape.
If the wax must be heated to remove it from a drum, directly transferring it into the solvent would create a seriously dangerous situation.
If the flammable solvent will actually dissolve the wax, then the best suggestion is to melt the wax in the drum and pour it into small blocks. Then, allow the blocks to cool before adding them to the solvent and mixing them until the wax dissolves. Even under those conditions, a closed tank with a nitrogen purge and pressure relief/fire suppression system may be needed. This question is much less about mixer design and much more about a hazards analysis.
The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user.
