Convective mixers consist of a vertical or horizontal static shell or container (cylindrical, conical, U- or W- shaped trough) in which powders is circulated around by a rotating blade, paddle or screw. The choice of mixer within the convective group is much wider. Particles are reoriented in relation to one another as a result of mechanical movement, also known as paddle mixing or plow mixing. Mixing consists of convection, diffusive and shear mechanism.




The transport of chunks of powder around the mixing element which is brought about by the rotation of the mixing element results in convective transport. The shear and diffusive transport takes place as the chunks of the power shear and inter disperse with each other causing the individual powder particles to randomly move from one point to another.

The particle size obtained after blending is determined by

• Impeller rotation rate (agitation speed)
• The number of blades
• Blade angle

Convective mixer subclass

Convective mixer sub classes are primarily distinguished by the vessel shape and impeller geometry. They include:

• Ribbon blenders
• Orbiting screw blenders
• Planetary blenders
• Forberg blenders
• Horizontal high intensity mixers
• Horizontal Double arm blenders
• Vertical high intensity mixer
• Diffusion (Tumbler) mixer with intensifier bar/agitator.

Of all the subclasses, the ribbon blender is perhaps the most widely used.

RIBBON BLENDER

Ribbon blender also referred to as a double helical ribbon agitator consists of inner and outer ribbons. The outer ribbon moves materials from the ends towards the centre of the trough, and the inner ribbon moves the material towards the outside of the trough (that is from the centre to the ends) during blending operations. Axial movement results from the difference in the peripheral speed of the outer and inner ribbons along the horizontal axis of the blender. Radial movement on the other hand is achieved as a result of the rational motion of the ribbons. These movements (radial and counter current axial movements) facilitate the formation of a homogenous mix of the material within 15-20 minutes of start up with 90-95 % or better homogeneity.

Mixing efficiency of a ribbon blender is affected by

• Solid particle size.
• Bulk density of the material.

The blended material is discharged from the discharge valve (which can be more than one in number depending on the size of the blender) located at the bottom of the trough. Practically, it is difficult to achieve 100% discharge in a ribbon blended material remaining after discharge depends on;

• The properties/nature of the material to be mixed.
• The clearance between the outer edge of the ribbon edge if the ribbon flight and the inside wall of the blender container.

A clearance of 3 to 6 is usually maintained depending on the application.

Pharmaceutical uses of Ribbon Blender

The ribbon blender is a popular blender because of its versatility for blending solids combined with its ability to perform heating, cooling, coating and other processes.

Advantages of Ribbon Blender

• Ribbon blender can be designed to operate in both batch and continuous modes
• It is best suited for free flowing and cohesive products.

Disadvantages of Ribbon Blenders

• It is not used for fragile materials
• It is not suitable for sticky materials
• Grinding of material may occur. 

V-BLENDER WITH INTENSIFIER BAR (V-BLENDER AGITATOR)

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

This modifications 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 short comings. 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.      

ORBITING SCREW BLENDERS

Orbiting screw blender also called nautamixer or vertical cone screw blender consists of a conical vessel with a screw that rotates on its axis which lifts powders from the base of the conical vessel to the powder surface. The screw rotates at low speed, typically 35-100m per minute. The drive system generally consist of two motors; one for rotation of the main drive and the other for rotation of the screw. For better and faster mixing, two orbiting screw may be used in a single vessel in which one screw is shorter and larger in diameter in some cases. The vertical design and the conical shape of the vessel makes orbiting screw blender have the flexibility to operate efficiently with batch size ranging from low volumes of about 10% up to 90% of the total working volume in capacity of the blender. The blended powder is discharged through a discharge value located at the bottom of the blender. Orbiting screw blender unlike horizontal ribbon blender achieves almost 100% material discharge.

Pharmaceutical uses of orbiting screw blender

• Orbital screw blender is used in mixing shear sensitive and heat sensitive powders.
• It is also used where the process parameters are critical.

Advantages of orbiting screw blenders

• Flexibility of batch size
• Gentle blending action due to rotation of the screw on its axis.
• Nearly 100% discharge is obtainable
• Easy to clean after use.

Disadvantages of orbiting screw blenders

• Requires large freed room for installation.
• It is expensive.

PLANETARY BLENDERS

Planetary mixer also known as vertical mixer and commonly called beater consists of an anchor type of paddle/agitator revolving in a cylindrical pot with a hemispherical base. The paddle rotates on its axis and the axis equally rotates around. Planetary mixers are so named because they feature an agitator (which consists of several vertical blades or fingers) that rotates on an offset shaft, while the mixing bowl remains stationary.

The planetary motion of the beater is very effective in mixing the content of the bowl. The planetary mixer works on the principle of shearing and convective actions. Planetary mixer can be a single planetary mixer or a double planetary mixer.

Single Planetary Blender

A single planetary mixer consists of a bow with a blade that revolves in a planetary motion similar to the way the planet moves around the sun such that it visits all parts of the vessel. The vessel/bowl consists of an upper cylindrical section and a lower hemisphere section which is secured to a semicircular frame (fork) at the time of mixing. The beater which is shaped to match the lower curved surface of the bowl undergoes two types of movement.

• It revolves on its own vertical axis at high speed.
• The vertical axis on the other hand rotates around the centre of the bowl at a relatively lower speed.

The bowl is lowered and detached from the mixer assemble to discharge the mixed material which can either be done by hand scoping  when the material is pasty and does not flow or through a bottom discharge value when the material is flowable.

 Pharmaceutical uses of Single Planetary Blender

• It is used to mix dry and wet powders, light pastes, gel and dough.

Advantages of Single Planetary Mixer

• It is simple to construct, easy to operate and easy to clean after use.
• It is relatively cheap
• There is virtually no deed space in the mixing bowl.

Disadvantages of Single Planetary Mixer

• It requires high power
• It has limited size and is only useful for batch work only.

Double Planetary Mixer

Unlike the single planetary mixer, double planetary mixer includes two agitators that rotate on their own axis while being revolved around a central axis in the vessel. This motion allows the blades to pass through every point within the batch, not just along the periphery; thus removing materials from the vessel wall and transporting it to the interior. The drive of a double planetary mixer consists of a motor and a gear box that drives the head. Each planetary blade is generally driven by gears that rotate due to the motion of the planetary head. The material to be mixed is loaded into a cylindrical vessel with a nearly flat bottom. The mixed material is discharged through a bottom value or by manual scooping of the material from the bowl. The power consumption of a double planetary machine depends on

• The properties of the material to be mixed
• The blade type (rectangular, finger or helical blade).

Pharmaceutical uses of Double Planetary Mixer

• The double planetary mixer is efficient at mixing heavy materials like, dense powders, and filled slurries.

Advantages of Double Planetary Mixer

• It is less sensitive to liquid addition or shift of viscosity.
• It is easy to clean after use.
• The mixed materials are easily discharged.
• The equipment requires considerable less floor spaces.
• It is suitable for wet granulation, and drying operations.

Disadvantages of Double Planetary Mixer

• It requires high power
• It limited size restricts its used to batch work only

Other convective blenders used in the pharmaceutical industries include Forsberg blenders, horizontal double arm blender, horizontal high intensity mixers, and vertical intensity mixers.

REFERENCES

• CCPS (Center for Chemical Process Safety) (2010). Guidelines for Safe Handling of Powders and Bulk Solids. American Institute of Chemical Engineers, New York.
• http://www.brighthubengineering.com/manufacturing-technology/57262-double-planetary-mixer/
• http://www.webstaurantstore.com/guide/523/commercial-mixers.html

This article titled”Convective Mixers” discussed the operating principles of a Convective Mixers, the various sub classes of a  Convective Mixers, the uses of Convective Mixers, the advantages of convective mixers as well as disadvantages of Convective Mixers.