Chemical Engineering January 2017 - 26

Facts At Your Fingertips
Solids-blending processes
Department Editor: Scott Jenkins
ixing and blending of bulk solids
is common in pharmaceutical
and food manufacturing,
specialty chemicals, explosives, fertilizers,
glass and ceramics, detergents
and resin industries. This column provides
information on batch versus continuous
blending mechanisms.
M
Batch versus continuous
Batch blending processes typically
consist of three sequential steps:
weighing and loading blend components;
mixing; and discharge of the
blended product. In a batch blender,
solids motion is confined only by the
vessel, and directional changes are
frequent. The retention time in a batch
blender is carefully controlled, while
for a continuous blender, this is generally
not the case. Blending cycles
can take from a few seconds with
high-intensity units to 30 min or more
where additional processing, such as
heating or cooling, may be involved.
Blender discharge may be rapid or
take substantial time, particularly if the
blender is used as a surge vessel to
feed a downstream process.
In a continuous blending process,
weighing, loading, blending and discharge
steps occur continuously and
simultaneously. Blending occurs during
transport of the material from the
in-feed point toward the mixer outlet.
Unlike batch blenders, where product
retention time is carefully controlled,
material retention time with continuous
blenders is not uniform and can be directly
affected by blender speed, feedrate,
blender geometry and design of
internals (see Table 1).
Blending mechanisms
There
are
three
primary
blending.
blending
mechanisms: convection, diffusion
and shear.
Convective
Convective
blending involves gross movement
of particles through the mixer, either
by a force action from a paddle or by
gentle cascading or tumbling under
rotational motion.
Diffusion blending. Diffusion is a
slow blending mechanism and will
pace a blending process in certain
26
TABLE 1. BATCH VERSUS CONTINUOUS BLENDING
Batch processes are used when:
Quality control requires strict batch control
Ingredient properties change over time
The blender cannot be dedicated to a specific
product line
Production quantities are small
Continuous processes are used when:
A continuous, high-production-rate process is
required
Strict batch integrity is not essential
Combining several process streams
There is a need to smooth out product variations
Many formulations are produced on the same
production line
Advantages of batch processes over continuous Advantages of continuous processes over batch
Lower installed and operating costs for small to
medium capacities
Ease of equipment integration into continuous
processes
Lower cleaning costs when product changes are
frequent
Production flexibility
Pre-blending of minor ingredients is easily
accomplished
Control of blending time
tumbling mixers if proper equipment
fill order and method are not utilized.
Shear blending. The shear mechanism
of blending involves thorough incorporation
of material passing along
high-intensity forced slip planes in a
mixer. Often these mixers will involve
dispersion of a liquid or powdered
binder into the blend components to
achieve granulation.
Random versus ordered blend
There are two types of blend structures:
random and ordered. A random
blend occurs when the blend
components do not adhere or
bind with each other during motion
through the blend vessel. In this
case, dissimilar particles can readily
separate from each other and collect
in zones of similar particles when
forces such as gravity, airflow or vibration
act on the blend.
Ordered or structured blends are
more common in industrial processes.
This occurs when the blend components
interact with one another
by physical, chemical or molecular
means and some form of agglomeration
or coating takes place. The
process of granulation involves this
approach, whereby larger particles
are created from smaller buildingblock
ingredient particles, and each
" super " particle has ideally the correct
blend uniformity. A blend of perfect
superparticles of identical size
will not segregate after discharge
from the blender, which is clearly an
advantage over a random blend.
Segregation and sampling
If particles are not mono-sized, then
segregation by size may occur and
induce problems with bulk density,
reactivity or solubility in post-blend
processing. There are cases where
some ingredients have a tendency to
adhere only to themselves, without
adhering to dissimilar ingredients.
This often happens with fine materials,
such as fumed silica, titanium
dioxide and carbon black. At times, a
blend can reach " saturation, " where
minor fine components will no longer
coat larger particles, and concentrations
of the fine component will build
(and segregate from the blend).
With achieving a uniform blend as
the goal, defining uniformity strongly
depends upon the scale. For instance,
loading two components
into a tumble blender does not guarantee
blend uniformity across the
range of sample sizes. If the entire
quantity in the blender was analyzed,
then uniformity may be present.
However, taking smaller samples
from either side of the blender
will result in substantial differences,
which clearly does not meet uniformity
requirements.
n
Editor's note: This column was adapted from the following
article: Maynard, E., " Fundamentals of Bulk Solids Mixing
and Blending, " Chem. Eng., September 2013, pp. 66-71.
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JANUARY 2017
Less opportunity for batch-to-batch variation caused
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Higher throughputs are often possible

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Chemical Engineering January 2017

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