Diffusion Mixers

Diffusion mixers, also known as tumbling mixers/blender is a powder/solid mixing equipment that consists of a closed metallic vessel (usually stainless steel) that rotates about an axis either manually or with the help of a motor at an optimum speed. Powder particles, unlike fluids, must first be set in motion by some external action to achieve a proper mix.

Diffusion is the main mechanism of mixing in tumbling mixer. The powdered materials to be blended are loaded into the blender container and the movement of the powdered particles occurs by tilting the material beyond the angle of repose using gravity to impel flow. Different shapes of the diffusion mixer result in the movement of the material in various planes, which is necessary for rapid mixing.

The degree of mixing/blending achieved by using tumbling mixer in carrying out a mixing operation is dependent on

• The fill-up volume (should not be more than 50-60% of the total blender volume)
• The residence time.
• The rotation speed (increasing the speed above the optimum speed causes adhesion of the powder on the walls of the mixer)
• The charging method used in charging the powder.
• Inclination angle of the mixer.

There are several sub-classes of tumbler but the popular and commonly used ones are the v-blenders and the double cone blenders. Others include Slant cone blenders, cube blenders, Bin Blender, Horizontal/Vertical/Drum Blenders, Static Continuous Blenders, and Dynamic Continuous Blenders. They are primarily distinguished by geometric shape and the posited of the axis of rotation.

i. V-Blenders

The V-blender, also known as twin-shell mixer consists of two hollow cylindrical vessels/containers that are joined at an angle of 75⁰ to 90⁰ which is mounted on trunnion to allow it tumble. The free fall of the material within the vessel, and the repetitive converging and diverging motion, coupled with increased frictional contact between the material and the vessel’s long, straight sides as the mixer tumbles, split the material and recombines them continuously results in a homogenized blend. Removal of the blended material from the V-blender is normally through the apex port which is fitted with a discharge tube.

A V-blender can be modified by providing it with a high-speed intensifier bar also known as lump breaker running through trunnion into the vessel along with spray pipes for liquid addition. This modified v-blender is called V-Blender with intensifier bar.

This modification makes V-blender broaden its application in the pharmaceutical industries and they include;

• Ability to accomplish wet blending in addition to the normal dry blending that the V-blender is known for.
• It makes it suitable for mixing fine powders as well as coarse powders.

Provision of intensifier bar to a V-blender is not devoid of some shortcomings. These disadvantaged includes;

• V- Blender with intensifier bar, unlike V-blender, has cleaning problems after use.
• It causes intensifier bar shaft problems.
• It causes undesired particle attrition.

ii. Double Cone Blender

The double cone blender is an efficient and multipurpose tumbler blender for mixing dry powders and granules homogeneously. It is made of two conical shaped stainless vessels (available in different capacity ranging from 5 – 200 kg or even more) that are separated by a cylindrical section. It is mounted at the center of the container between two trunnions that allow the blender to turn end over end.

Double cone blender has no dead spots mixing and it is easily cleanly after use. Double cone blender is not a suitable choice of blender for very fine particles and particles with greater particle size difference due to less shear.

iii. Cubical blender or Cube-Shaped Blender or Cubic Blender

A cubic mixer is a tumbling mixer suitable for gentle blending of dense powders and granules. As the name implies, it consists of a cube-shaped stainless container mounted in a titled manner at the center of the container or hanged from the corner between two trunnions that allows it to rotate about an axis. Mixing occurs by sliding of powders on its wall. Cubic blender gives mixing in three dimensions (3D) when hanged from the corner.

Despite the fact that corner blender gives mixing in three dimensions, the presence of different corners makes it difficult to clean after use sliding action causes abrasion of particles. The above can be said to be the advantages and disadvantages of using cube-shaped blenders as blending equipment.

iv. Drum blender/vertical blender/horizontal Blender/Tumbling Drum

Drum blender which can also be referred to as vertical blender, horizontal blender, or tumbling drum consists of a horizontal cylinder (or vertical cylinder) rotating about its axis. Its mixing action is essentially diffusive and gravity impels flow.

The powder to be mixed is placed inside the drum and as the drum rotates, the powder is lifted up until the angle of repose is exceeded. This leads to the falling back of the powder on the rest of the bulk.

Diffusive mix takes place as the cycle of lifting and falling keeps on repeating itself. The efficiency of the drum blender is increased if the blender is attached in an inclined manner. This allows light movement and so it makes it suitable for friable particles. This suitability of drum blender for friable particle is an advantage in its application as a blender. Drum blenders also have low shear as its main disadvantage.

Pharmaceutical uses of Diffusion Mixers

• Diffusion mixers are best suited for free-flowing, non-segregating powders
• They are widely used for blending dry powders and granules in the pharmaceutical industry because of close quality control on batch operation; an effective diffusive mechanism.
• They can also be used to accomplish wet blending as in the case of modified V-blender (V-blender with intensifier bar).
• Diffusion mixers find use in gentle mixing of friable solid material.
• They are also used for mixing powders of different densities.

Advantages of diffusion mixers    

• ­Ease of product charging operation and discharging of materials.
• The various shape of diffusion/tumble blender container ensures the complete discharge of product material.
• They are mild equipment since rotation on their axis is done slowly; thus diffusion mixers are suitable for friable materials.
• They are easy to clean after use.
• Diffusion mixers require minimal maintenance.
• Product contamination is minimal due to the absence of shaft projection and seal in the working are of the blender.
• Absence of moving blades or agitators minimizes the tendency of further size reduction of the milled material.
• Due to repeated reversal of direction of flow, they are preferable when powders of different particle sizes and densities are to be mixed.
• Most of the diffusion mixers are operated in batch modes.

Disadvantages of Diffusion mixers

• Diffusion mixers require headroom for installation
• Large differences in particle densities or wide particle size distribution of materials lead to segregation
• Highly cohesive
• Materials cannot be handled in diffusion mixers since they tend to form a bridge over the blender outlet

References

• CCPS (Center for Chemical Process Safety) (2010). Guidelines for Safe Handling of Powders and Bulk Solids. American Institute of Chemical Engineers, New York.
• Jack Zheng (2009). Formulation and Analytical Development for Low-Dose Oral Drug Products. John Wiley and Sons, Inc., Hoboken, New Jersey.
• Zeki Berk (2013). Food Process Engineering and Technology. Elsevier Inc., London.