che_february-2025 - 18

Facts At Your Fingertips
Blending and Segregation Mechanisms for Solids
Department Editor: Scott Jenkins
I
n solids-handling, blending (combining
two or more materials to
achieve a combined product) and
segregation (the separation of particles
into distinct zones according to
particle size, shape, density or other
physical attributes) are competing
processes. This one-page reference
outlines common mechanisms by
which these processes occur.
Random and ordered blends
A random blend occurs when the
blend components do not adhere or
bind with each other during motion
through the blend vessel [1]. Particles
that form random blends can be easy
to blend, because they move easily
relative to one another, but they also
can readily separate from each other,
collecting in zones of similar particles
when forces, such as gravity, airflow
or vibration, act on the blend.
In most dry-blending applications,
particles have some tendency to interact
with one another via chemical,
molecular, physical or other means
such that individual particles can agglomerate,
coat or bond to one another
[2]. When particle interaction
occurs, the blend is referred to as an
ordered or structured blend. In most
industrial processes, the reality is
somewhere between the two blend
types. Some particles of the blend
may have very little tendency to interact
while other blend components
may have significant interaction.
Blending mechanisms
Three primary blending mechanisms
are convection, diffusion and shear.
Convection. Convection is the transfer
of a collection of particles from
one location to another. This can occur
as a result of material cascading
in a tumble blender, material moving
against the blade of a ribbon or paddle
blender or as a result of gas-pressure
pulses in a pneumatic blender.
Diffusion. Diffusion is the random
redistribution of particles that occurs
as a result of increased particle
mobility. Increased mobility typically
occurs when the bulk density of the
material is decreased sufficiently to
18
allow individual particles to
move relative to one another.
Fluidization in fluidized-bed reactors
or granulators results in
diffusion. Mechanical blenders
move collections of particles
by convection, but when the
speed of the agitator is sufficient
to locally fluidize material,
diffusion occurs.
Shear. Shear occurs in a flowing
granular solid as a result
of a velocity gradient, and can
develop as either a discontinuous
shear (for instance, a shear or
slip plane) or as a continuous gradient
of velocity. In either case, there
can be some overlap in what could
be called shear and what might be
characterized as convection. The
important difference, as it
applies
to solids blending, is in the intensity.
Shear planes that develop in gravity
blending or in a tumble blender predominantly
result in mixing by convection.
Shear in a high-speed mixer
is more effective in breaking up agglomerates
of fine powders and distributing
small-particle-size material
with high surface activity.
Segregation mechanisms
Segregation can be driven by factors
including gravity, electrostatic forces,
fluid-drag forces and elastic forces.
Common segregation mechanisms
are sifting, fluidization and dusting.
Sifting. The most prevalent type of
segregation is sifting, which results
in separation by particle size. Sifting
segregation is the most common
means for particles to separate.
It occurs when small, fine particles
move through large, coarser particles
(Figure 1). Sifting segregation occurs
to some degree in most bulk-solids
operations. Sifting will occur in mixtures
of different-sized particles when
particles are sufficiently large that surface
forces are weak relative to gravity
(usually larger than 100 μm), when
particles have mobility relative to one
another and when there is some
mechanism
to
allow
particles
to
move relative to one another [2]. Collections
of different size particles will
Jenike & Johanson
FIGURE 1. Sifting segregation, where particles of varying
sizes separate due to gravitational forces, is particularly common
in processes that involve piling materials or filling bins
not spontaneously segregate when
at rest, but will often readily segregate
when allowed to move. For sifting to
occur, material must be free-flowing,
have a range of particle sizes, have
some fairly large (>100 mesh) particles,
and have some means of interparticle
motion [3].
Fluidization. This type of segregation
results when finer, lighter particles rise
to the top surface of a fluidized blend
of powder, while the larger, heavier
particles concentrate at the bottom of
the bed. The fluidizing air entrains the
fines and carries them to the top surface.
This mechanism generally only
occurs with powders with an average
particle size smaller than 100 µm [1].
Fluidization segregation is likely to occur
when fine materials are pneumatically
conveyed, when they are filled
or discharged at high rates, or if gas
counter-flow occurs. The more cohesive
the material, the less likely it will
segregate by this mechanism.
Dusting. Particle entrainment or
dusting segregation occurs when fine
particles in a blend are carried by air
currents (such as during transfer of a
blend into a container) and then settle
preferentially at the container walls.
This mechanism requires four conditions:
difference
in
particle
sizes;
relatively large particles (average size
greater than 100 µm); free-flowing
material; and inter-particle motion. n
References
1. Maynard, E., Blender Selection and Avoidance of Post-Blender
Segregation, Chem. Eng., May 2008.
2. Troxel, T.G., Blending, Sampling and Segregation, Chem. Eng.,
October 2012, pp. 41-46.
3. Marinelli, J., Will Mass Flow Solve All Your Segregation Problems?,
Chem. Eng., April 2006.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2025
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