Chemical Engineering April 2012 - 63

Feature Report
Engineering Practice
Focus on Physical Properties
To Improve Processes
Girish Malhotra, PE
EPCOT International
M
usic is nature's gift to humans.
Irrespective of nationality,
we all like to hum
melodies, tap rhythms and
sway to pleasing tunes. Even children
just learning to stand enjoy wiggling
to good music. In all cultures, the notes
that comprise musical scales are used
in varying sequences and different
combinations to compose music. These
note can be thought of as the the physical
properties of music. Just as composers
use notes to create music, designers
of chemical processes use scientific and
engineering principles to assemble processes.
And just as creating the most
enjoyable music takes time, skill, hard
work and imagination, creating elegant,
cost-effective chemical processes
requires exploiting the physical properties
of chemicals (the musical " notes " of
chemicals) in imaginative ways to craft
excellent chemical manufacturing and
formulation processes.
Whether an architect envisioning
a building, an automotive engineer
building cars or a metallurgist producing
metal ingots, all use the principles
of science and engineering, along with
the physical characteristics of the materials
and their own creativity and
imagination to create a product via
a process that is sustainable, costcompetitive
and has the desired quality
for the price. Similar factors are at
play when simplifying and improving
existing chemical processes.
KEY PROPERTIES REVIEW
The following list represents several
of the critical physical properties that
should be examined in process design
and process optimization:
1. Physical state of a material at room
temperature, along with its melting,
boiling or freezing point
2. Solubility
3. Density
Exploiting the physical
properties of chemicals
can offer pathways to
more simplified and
elegant processes
4. Viscosity
5. Specific heat
6. Heat of formation
7. Azeotropic behavior
In addition to those listed above,
other physical properties may have
to be considered for specific processes
and applications, but those above are
the most commonly used in process
development, scaleup and process
design. Molecular weight is also important,
but is excluded from the list
because every developer has to know
the formula and molecular weight of
every chemical they deal with.
Although chemists and chemical engineers
are familiar with the properties
mentioned in the list, it is worth
discussing the value of each individually,
as well as how each interacts with
others and how they can be exploited
in process development, scaleup and
commercialization of products.
Physical state
A chemical's physical state at room
temperature, its melting, boiling or
freezing point, along with solubility,
tells process developers a great deal
about how the product can be handled
during process development and in a
commercial operation. It is advisable
to handle every chemical with respect,
even if the product literature suggests
low or no toxicity. Chemical quantities
used in the laboratory are small, so
handling them in a safe manner is generally
easier. However, the quantities
required for scale-up experiments and
commercial-scale operations are higher
and different methods are used to handle
and feed materials. Care needs to
be exercised for each chemical.
Solid chemicals, when used in a
scale-up or commercial operation,
will require proper equipment to feed
at the desired rate while controlling
dust emissions. Additional precautions
might be necessary when handling
toxic materials. Another way to handle
solid materials can be by dissolving or
slurrying them in appropriate solvent,
preferably the one that is being used in
the process. When slurries are used in a
process, it is important that the slurry
be uniform. If feed to the reactor or the
formulation vessel is not uniform, the
product quality would vary for a continuous
process and could result in potential
financial loss. Variable feed rate
can influence batch-process product
quality also. Each situation has to be
considered on an individual basis.
Since we do not handle molten materials
in the laboratory, we do not consider
melt addition on a larger scale.
This can be due to our lack of experience
in handling molten materials or lack of
availability of bulk molten material.
Melt feed addition might be economical
for high volume or selective products
as molten liquid metering systems
are commercially available. We should
consider melt use as it can reduce solvent
need, which in turn can improve
process productivity and sustainability.
Lower product cost and higher profits
are additional incentives.
When a gas is needed for a reaction,
we end up bubbling the gas using
a weight-loss system or, in the case
of ammonia, using its solution. This
works well for the laboratory, but on
a commercial scale, the large volume
of water required to dissolve ammonia
takes up reactor volume. This
can lower productivity significantly.
Weight-loss systems work well for
batch operations, but are not very efficient
for continuous processes. If the
volume is justified, liquefied gas addiCHEMICAL
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Chemical Engineering April 2012

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