selecting a right blender for your material

VORTEX INDUSTRIAL TECHNOLOGY CO.,LTD
E-mail: info@incmachine.com www.incmachine.com
Add: No.128 Huayuan Road, Hongkou District, Shanghai, China
Selecting the Right Blender for Your Material
Mixing is a critical unit operation in pharmaceutical, food, ceramic, cosmetic, chemical and
other industries. There are dozens of blenders and many vendors that all claim they can
mix powder materials. However, which of all the blenders is the best choice for your
material? Blending powders requires blender action and geometry induced velocity profiles
that cause all the particles to be active in the blending process (no stagnant zones). Each
blender style cause a unique set of blending velocity profiles, some of which rely on
induced velocities from mechanical interactions such as paddle, ribbon, or screw
movement. Other velocities are the natural result of bulk material motion in containers:
flow down piles, flow in hoppers, or particles carried by air currents. In either case, blending
velocity profiles in mixers must generate difference in residence time path at various
location in the blender or residence times measured at the blender exit as material passes
through. Mixing cannot occur if uniform plug flow exists in the blender; blending requires
non-uniform flow. Each blender can be characterized observing the type of motion during
blending.
Sometimes blending velocity profiles cause segregation in a blender. Proper
blender selection depends on the segregation mechanism occurring operation.
There are many reasons why material mixtures separate(de-mix) during
processing and handling. Here we consider the three most common: angle of
repose, sifting, and air entrainment. Each mechanism is enhanced or mitigated
by different types of blender-induced velocity profiles. Blending mixes the bulk

solid; since blending and segregation are opposite effects, the ideal blender
would attain blending velocities which do not also cause segregation. Optimal
blender choice depends on which action wins: blending or segregation.
To select the optimal blender we must understand the types of segregation
inherent with the material, rank the material relative to these active segregation
mechanisms, determine what velocity profiles exist in common blenders and
finally rank the blenders relative to their potential to segregate due to these
velocity profiles. Consider the segregation mechanisms that may be acting with
your material.
Angle of repose
Consider the case where two or more components have different inter-particle
friction characteristics caused by differences in particle shape (spheres and
angular materials) or by surface roughness effects. In either case, particles
sliding down a pile travel at different velocities, causing one types or particle to
separate from the other particle as piles are formed and reformed. Generally,
angle of repose segregation produces radial patterns as materials piles in
process equipment. Material with steeper repose angle accumulates near the pile
charge point while the flattest repose angle material concentrate toward the pile
edge.
Some blenders mix by continually forming and reforming piles. If a blender
forms piles during operation with a material sensitive to angle of repose
segregation, it separates the particles during blending. Thus, a blender that
operates the through repeated pile formation is a bad choice with this type of
material.
Sifting
In multiple component mixtures, fines may sift through a matrix of coarse
particles during handling. Sifting segregation requires that void space between
adjacent particles must be large enough to permit fine particles to
pass through (a particle size different of about 3:1). Inter-
particle motion exposes empty void spaces to fine particles, and fines must be
sufficiently free flowing to prevent arching
Between adjacent particles. Generally, sifting segregation also produces a radial
pattern as materials piles in process equipment. Fines accumulate near the pile

charge point, decreasing in concentration toward the pile edge. Noted that sifting
is separation is induced in the direction of gravity. A blender that wished to undo
this segregation effect must induce velocity profiles against the direction of
gravity.
Air Entrainment
In many systems, fine particles are carried by air currents and deposited in bins
wherever air currents reduce sufficiently for fine particles to drop out of the flow
stream. When the falling stream impacts the material level, entrained air is
pushed out of interstitial pores, carrying the fine particles in the resulting dust
cloud. This segregation typically caused a radial pattern during pile formation,
but fines are at the bottom of the pile and not the top. Mitigating this behavior
requires a blending action against the direction of the pile formation or operation
in such a way as to limit air currents. Since air currents separate the particles, a
blender that operates through repeated piles formation without free-fall may be
a good choice with this type of material.
Example
Let’s consider three blender types and rank each in terms of its ability to blend
via inter-particle motion counter to the direction of gravity, formation of piles
during operation, and formation of air currents during operation. We will consider
a rotary shell blender (v-blender), a ribbon blender, and a double paddle
blender. V-blender work by placing material into one or both legs of the V-
blender until the blender is about half full and rotating the blender. Material in
the blender undergoes ridged body rotation without inter-particle motion until
the material reaches the top of the pile formed during rotation. Material cascades
down the slope, inducing inter-particle motion in a narrow band on top of the
pile. All mixing occurs along the pile top. Piles are formed and reformed during
operation.
Ribbon blender work by placing components in a U-trough with left-handed and
right-handed dual ribbon screw flight connected to a common shaft. Blending
occurs because the ribbons move material in the axial direction. However, during
this process screw flights also lift material up on one side of the U-trough,
forming a pile. In ribbon blenders, mixing due to screw motion is nearly equal to
blending that occurs as material cascades down the pile.
Dual paddle blenders work by attaching paddles to counter-rotating shafts.
Paddles are angled relative to the shaft so that one paddle lifts the material up
and to the right while the next paddle lifts the material up and to the left. These
paddles rotate quickly making pile formation minimal. It should be pointed out

that, although there is significant inter-particle motion in this blender, that
motion is in the upward direction.
We must determine if the blender of interest can induce velocity profiles that
result in inter-particle motion, pile formation or gas velocities.
All blenders must induce inter-particle motion. However, some inter-particle
motion separates particles and some motions blend mixtures. The worst-case
scenario would be a blending velocity 90 degrees from the direction of gravity.
Material would sift across the flow direction, percolate through the void
structure, and remain in the stagnant layer below. This would cause particle
separation without any possibility of remixing the material. if the direction of flow
is aligned with gravity, then the velocity profiles can overcome sifting
segregation. We can compute a sifting segregation ranking if the angle of flow
relative to the horizontal (flow) is known. When a pile exists in the blender,
sifting segregation velocity relative to the direction of gravity is given by the pile
repose angle.
See Equation 1 (attached)
If the repose angle is 45 degrees, the rank computed from equation 1 is 3.6. In
a ribbon mixer, half the material mixes by cascading down the pile and half
mixes by transportation along the axial length of the screw. Transporting along
the screw results in inter-particle motion, but no slope, so the rank for sifting
due to a condition where inter-particle motion but little slope occurs, results in a
sifting rank of 1.0. The average rank for sifting in a ribbon blender is the average
of the 45 degree and zero slope piles, or 2.3. The rank for sifting in a paddle
mixer involves the deviation off the gravitational direction in which the paddle
throws material. In this case, the paddle action on the material goes against the
sifting direction. We estimated the average trajectory angle due to paddle action
is about 80 degrees, yielding a sifting ranking of 8.4.
The angle of repose ranking depends on how close the dynamic angle of the
slope in the mixing equipment (r) is to the actual static angle of repose (Sr).
We use the ratio in equation 2 as a means of scaling between expected angle of

repose effects. This equation relates acceleration between the dynamic and static
friction conditions caused by material sliding down piles. The derivation of this
equation will not be included here. There is always a difference between the
dynamic and static repose angle. For the purposes of this paper we assume that
difference is about 5 degrees. Thus, the expected ranking down the pile in both
the rotary shell blender and ribbon blender gives a rank of 3.4. However, in the
ribbon blender, half the material travels down the pile while the other half is
carried by screws with a repose angle of 0 degrees, resulting in a rank of 10 for
that part of the material that is conveyed. The overall rank for the ribbon blender
is the average of these values, or 6.7. We will assume the average repose angle
in the paddle mixer during operation is 6 degrees. This leads to a ranking of 9.9
for angle of repose segregation in a typical paddle mixer.
See Equation 2
(attached)
The ranking for air entrainment are somewhat subjective and depend on
operation speed. Rather than use simple correlations to approximate, these
ranking are based the author’s extensive experience with various blenders and
materials sensitive to air entrainment segregation. All rankings are summarized
in Table I. If a material is sensitive to a particular mechanism, this table
indicates how easily the blend will mix in the chosen blender.

See Table 1 (attached)
To achieve proper fine powder material blend, one must eliminate segregation
issues. Selection of the proper blender depends on the segregation
mechanism(s) occurring in the material during blender operation: one must
match blender choice to the material properties. Blender selection is often
considered an art rather than a science. However, knowledge of basic material
flow properties and segregation tendencies provides guidance in selecting the
right blender for the task.
Obviously we need a measure of segregation based on mechanism. Ideally we
would like to determine the fraction of each mechanism that might be active with
the particular material. Measuring segregation can be done by forming a pile,
measuring the concentration of key components along the pile and using this
data with particle size and angle of repose information of each component to
identify the active mechanisms Figure 1. This was done for a steak seasoning
mixture (Figure 2).
See Figure 1 (attached)

See Figure 2 (attached)
The data suggest that 65% of segregation occurring with this material is due to
angle of repose, 35% is due to sifting, and 0% is due to air entrainment
segregation. The question remains: which blender should be used to mix this
particular material? Overall rankings for each blender can be found by summing
the fraction of each mechanism (frmech) times the rank number (Rankmech) for
each mechanism for the blenders of interest (equation 3). For the steak
seasoning, blender ranks are found in Table II. Apparently, a paddle blender
would be the best choice for this material, followed by a ribbon blender.
See Equation 3 (attached)

See Table 2 (attached)
Conclusions
Blending bring distinct bulk material particles into intimate contact so as to
produce a mixture of consistent quality at a prescribed scale of scrutiny. Each
blender mixes by a particular set of actions (i. e. pile formation, paddle
movement0. Segregation undoes blending by inducing separation of distinct
particles. If material segregation due to a particular blending action, then any
blender causing that action is a poor choice for that material mixture. Thus, we
rank blending effectiveness based on segregation mechanism(s) which may
occur with the mixture. When we measure the segregation mechanisms and
determine the fraction of each active mechanism, we can deduce an overall
blender ranking and select the optimal blender based on science instead of trial
and error.

selecting a right blender for your material

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