The Art of Blending and Blender
Selection - A Few Basics
Guest article by Clark A. Beebe, Technical Services Manager, Gemco
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"blending" or ;mixing; (the terms are often used
interchangeably) is still an art. It has not yet arrived at the
stage of science in that it is a totally predictable process. There
are so many factors involved that the study of them individually and
then in combination is still on-going.
Among the properties that affect the
outcome are particle size, shape and density along with other
attributes such as the tendency to agglomerate, absorb moisture or
develop a static charge. Because of all this complexity, the common
approach to blending powders is to take a macro rather than a
(Photo above is a laboratory
blender. Any blender chosen should be able to work easily to meet
all the various competing processes, production needs and solve more
problems than it creates.)
What is a Good Blend?
The first question to ask is,
"What is a good blend?" The user must define what is an
acceptable precision in the final blend. Once an acceptable blend is
defined, then the process must be sampled and analyzed to ensure
that the goal is met. If you are making boxes of cake mix that are
sold in one pound boxes and the user further mixes during
preparation, the blend is not that critical.
Even if the flour or sugar is off by
five or ten percent from what is optimal, it won't really matter. In
this case the sample size for analysis would be the one pound unit
of sale. If on the other hand, the process involves stamping small
gears from powdered metal, the mixture at the base of each tooth
must have the correct properties or they will break in operation. In
this case the sample size would be much smaller than the part to
represent the smallest critical section of the part itself.
What Type of Blender is Best?
The next question is, "What type
of blender is best?" The answer is, of course, the one that
achieves the desired blend for the lowest overall or unit cost.
Which one that will be depends upon many things: the production
throughput required; the required precision of the blend; the
material handling requirements; cleaning and cross-contamination
issues; documentation requirements [and costs]; any liquid addition
requirements and their precision; etc. Each application will have
unique issues that drive the selection.
There are three basic types of powder
blenders commonly used today: diffusion, convection and pneumatic.
Pneumatic blenders typically use air
flow or pulses injected at the bottoms or sides of a hopper or bin
to carry materials from lower in the batch further up with the
rising air. Pneumatic blenders or blend adapters can be used on all
sizes of batches form laboratory to entire silos.
They are best used for non-critical
blends where particle sizes and densities are similar among all the
components of the mix so that fines are not carried to the top by
the air. If the fines contain most of one ingredient, or if there
are significant differences in size or density of the particles,
then pneumatic blenders are not the correct choice.
Convection blenders use mechanical
means to reorient particles relative to one another in order to
achieve a blend. Typical designs include ribbon, plow paddle and
conical orbiting screw mixers. There are many variations on this
theme with dual screws or ribbons, side auxiliary choppers, etc. if
liquid must be added to the point where the mixture starts to become
a paste or mass rather than still free flowing, this can be an
Advantages typically include cost,
fixed load and discharge points and limited space requirements. They
typically provide final blend variations of about plus or minus ten
percent and often have issues of some thin layers near the shell
where the powders are not touched and do not mix.
They are generally used in light
density mixes where there is not a very small amount of any
individual component. Due to the nature of the motion of a device
being pushed through the product to achieve the blend, high density
and abrasive products are typically to be avoided due to power
consumption and wear/maintenance issues. As thorough cleaning can be
quite involved, this aspect should be considered when cross
contamination is a major concern. Accurate scale-up predictions are
also difficult in this type of machine and should be considered
before laboratory work is started.
Diffusion or "Tumble"
blenders blend by placing particles in random motion allowing them
to reorient with respect to each other when inter-particular
friction is reduced as the result of bed expansion to achieve
uniformity. Typically this occurs in a rotating vessel commonly
called tumble blending. The "bed expansion" referred to is
actually the same mechanism as in a fluid bed unit or pneumatic
blender without the air flow. The advantage is that different
particle sizes and densities within certain bounds can be
accommodated without the separation of smaller particles due to
aeration or air flow through the product.
(Photo above is a double cone lab
blender with interchangeable vessels)
Tumble blenders typically offer the
best blend available of all the types mentioned. They are typically
able to achieve one to two percent variations and at times even
significantly better than that. When a precise blend is required or
some of the individual ingredients in the mix constitute less than
five to ten percent of the total, this is the technology that is
Tumble units are batch type blenders
suitable for small to medium sized production requirements. Typical
batch sizes run from a liter or less to 8,500 liters (300 cubic
feet). Larger units, typically used in the plastics industry, run to
30,000 liters (1,000 cubic feet) or more.
Types of Tumble Blenders
are three commonly recognized shapes of tumble blenders, the
V-shape, double cone and slant cone (Gemco trademark). There is also
a significant distinction between symmetrical and asymmetrical
designs within the three shapes.
The double cone and V-shape are
symmetrical blenders. They present a vertical line of symmetry
perpendicular to their axis of rotation. Studies on V-blenders run
at Rutgers University have demonstrated that there is no mechanism
to move powders of similar size and shape across this line of
symmetry therefore care must then be taken to load each side of the
blender equally to ensure the desired result. 
(Photo above is a 20 cubic foot
production v-shape blender with drum loading system and portable
design including removable safety gates, drum tray and air skids.)
blenders are represented by the slant cone or long leg V-shape
design where one leg is longer than the other. Asymmetrical blenders
superimpose an axial flow of material on the normal material in the
direction of rotation. Material is forced across the vertical axis
of the unit each half revolution creating a better, faster blend.
The slant cone has the advantage that it is available with internal
agitator bars, liquid addition and other options not available in
other asymmetrical units.
(Photo to the right is a 3 cubic
foot slant-cone portable blender with self-contained safety gates.)
Tumble Blender Options
Tumble blenders offer a large array
of options and accessories to customize them to the specific process
and plant needs. Process enhancements include internal agitator bars
to extend minute ingredients, break up unwanted agglomerates and, in
certain instances, dissipate static charges. More exotic options
such as heating or cooling jackets, interchangeable or split vessel
designs, on-line blend end point monitoring, humidity and/or
atmosphere control, sampling assemblies and portable designs-even
for production sized units-are all available.
of the type or style of blender chosen, customizing it to smoothly
meet plant needs by addressing such issues as material handling and
the interface with upstream and downstream product flow, cleaning
and product changeover issues, containment and worker exposure,
automation, control and monitoring, and industry standards is
critical. The blender should be designed with the custom features
required to improve work flow, not obstruct it.
(Photo to the left is a 50 cubic
foot Porta Hopper blender with quick-change, interchangeable hopper
sections for high production and reduced handling operations.)
If drums are commonly used to handle
product, one-floor automated drum loading/unloading systems are
standard options on most units. Or drum loading systems can be
adapted to provide discharge into bins through the use of cover
valves, extended supports and special positioning/control systems.
the unit is to be bulk loaded from above, then an automatic one
button positioning system should position the blender for loading
and discharge precisely and repeatedly to prevent worker error and
speed the process. Perhaps a portable unit, either pilot or
production will free up processing areas and allow for multiple
purpose suites. If precise positioning is required to match up with
an alpha-beta split butterfly valve, specify a positioning accuracy
of 1 mm or whatever is required. If feedback of the process is
required to tell when a valve is open or prevent rotation if the
cover is not in place or if the loading chute is not retracted,
state so in the specification.
(Photo to the right is a 30 cubic
foot slant cone blender with special, high clearance extended drum
with a loading/unloading system and cover valve for discharge into
bulk bins and hoppers.)
Such items should be designed into
the system from the start and not added on later. They often draw
the line between a smooth and successful integration of a new unit
and one that people have to learn to work around. The blender
manufacturer should be able to help in this regard. Ask for
suggestions. Tell them more about the plant and the process so that
hey have the background to make appropriate suggestions.
Tumble Blender Selection Criteria
In a nutshell, blender selection
depends upon three major criteria.
- The first is the blend required
and the quantities/ time frames involved.
- Is the throughput rated in tons
- Do you have multiple ingredients
and a very small unit size that must have the precise
percentages of the various components in order to fulfill the
expectations of the end user?
First determine the blend requirement
and sample size needed for the analysis of achieving that goal.
(Photo above is a
large, 4-disc internal agitator bar used for delumping and breaking
up agglomerates of minor ingredients and extending minor
The second criterion involved looks
inside the blender at the processing of the powders themselves. Any
process enhancements in the same unit enhance productivity. If you
can delump in the blending step by adding an agitator bar, you may
eliminate the need for a milling step afterwards. If you can add
only one percent liquid vs. 20 percent, you can reduce the drying
required later. The process designer must look beyond what was
previously done to what can be done. Every process step eliminated
means reduced labor, maintenance, cycle time, worker exposure, floor
space requirements, etc.
The third criterion is outside the
blender and its interface with the rest of the plant's operation.
Determine the best way to load, unload, control and clean the unit.
A well designed system will smoothly integrate with the rest of the
operations. Worker dissatisfaction with a poorly designed
installation will last years after the machine is put into service
and affect attitudes as well as output.
Before buying a blender, make sure
that the installation will meet as many goals as it can. Talk with
maintenance and operations people about current processes and
problems. Go beyond the standard answer of "no problems"
to make sure that they haven't just learned to live with a poor
scenario for loading or discharging or cleaning or whatever. View a
new machine as an opportunity to improve the productivity or
maintenance or worker satisfaction. Be sure to get the most bang for
About our author:
Mr. Clark A. Beebe is the Technical
Services Manager for Gemco, a leading manufacturer of tumble
blenders and dryers. Clark is in charge of engineering, new product
design and customer applications.
For more information contact:
Clark A. Beebe
301 Smalley Ave.
Middlesex, NJ 08846
Telephone: 800-654-3626 or 732-752-7900
 AIChE Journal, February 1998,
Volume 44, No. 2; Particle Technology and Fluidization; Quantitative
Characterization of Mixing of Dry Powders in V-Blenders by Dean
Brone, Albert Alexander and F.J. Muzio, Dept. of Chemical and
Biochemical Engineering, Rutgers University, Piscataway, NJ 08855.
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