Page 208 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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& Mixer discharge may be rapid, or it may take sub-
6.2.7 Solids Mixing
stantial time, particularly if the mixer is used as a
. Give examples of solids mixing processes in pharma-
surge vessel to feed a downstream process.
ceutical, food, and chemical process industries.
& Ideally, a mixer should not be used for storage
& In the pharmaceutical industry, small amounts of a
capacity, because this can create a process bottle-
powdered active drug are carefully blended with
neck, as the mixer cannot perform operations of
materials such as sugar, starch, cellulose, lactose, or
storage and mixing concurrently.
lubricants.
& Batch mixers are often used where
& In the food industry, many powdered consumer pro-
➢ Quality control requires strict batch control.
ducts result from custom mixed batches. Examples
include cake mix, ice tea, or Indian curry, a blend of ➢ Ingredient properties change over time and com-
many fine spices. pensation must be on a batch-by-batch basis.
& Thousands of processes in the chemical process ➢ The mixer cannot be dedicated to a specific prod-
industries involve mixing or blending of specialty uct line.
chemicals, explosives, fertilizers, dry powdered ➢ Production quantities are small.
detergents, glass or ceramics, and rubber compounds. ➢ Many formulations are produced on the same
. What is the importance of uniformity in solids mixing production line.
on industrial scale? & Major advantages of batch over continuous mixing
& The costs to business are increased substantially with include the following:
poor mixing process implementation, when the re- ➢ Lower installed and operating costs for small to
quirements are to address larger batch sizes, faster medium capacities.
mixing times, energy conservation, and minimiza- ➢ Lower cleaning costs when product changes are
tion of segregation.
frequent.
& The goals of producing an acceptable mix, maintain-
➢ Production flexibility.
ing that mix through additional handling steps, and
➢ Premixing of minor ingredients is easily
verifying that both the mix and the finished product
accomplished.
are sufficiently homogeneous are often difficult to
➢ Control of mixing time.
achieve on the first attempt.
& In a continuous mixing process, the weighing, load-
& In many cases, the costs attributed to troubleshooting
ing, mixing, and discharge steps occur continuously
a poorly performing mixing system can far outweigh
and simultaneously. Product motion is generally
the initial investment costs. For example, an inade-
directed from the feed point toward the outlet.
quate mix or segregation of a pharmaceutical drug
can cause the batch to fail, wasting millions of dollars & Unlike batch mixers where product retention time is
involved, even though the equipment used to mix and carefully controlled, with continuous mixers, mate-
transfer the powder can be a small percentage of this rial retention time is not uniform and can be directly
cost. affected by mixer speed, feed rate, mixer geometry,
. Compare batch and continuous mixing processes for and design of internals.
solids mixing. & Continuous mixing is typically used when
& A batch mixing process typically consists of three ➢ A continuous, high production rate is required.
sequential steps, namely, weighing and loading the ➢ Strict batch integrity is not essential.
components, mixing, and discharge of the mixed ➢ Combining several process streams.
product.
➢ Smoothing out product variations.
& In a batch mixer, solids motion is confinedonly by the
& Some of the advantages of a continuous mixing
vessel, and directional changes are frequent and
system are the following:
critical with most units.
➢ Ease of equipment integration into continuous
& The retention time in a batch mixer is normally
processes.
controlled, while for a continuous mixer, this is not
➢ Less opportunity for batch-to-batch variation
the case.
caused by loading errors.
& Mixing cycles can last from a few seconds with high-
➢ Automation can improve quality and reduce labor
intensity units to 30 min or more where additional
costs.
processing such as heating or cooling may be
involved. ➢ Higher throughputs are often possible.