Chemical Engineering April 2012 - 64

Engineering Practice
tion is safer because, in its liquid state,
it can be metered more effectively. Gas
would be evaporated and mixed in the
reaction mass.
Solubility
Solubility is not an exotic property but
is an important and valuable property.
Mutual solubility or the lack thereof,
can be very useful in reactive chemical
processing and formulations. If the reactants
and reaction product dissolve
in the same solvent, the reaction rate
can improve through improved mass
transfer. Solubility of materials in the
solvents used in the formulated products
improves mixing and results in a
uniform product. Solubility characteristics
of chemicals are also critical for
product crystallization, purification
and separation of products.
Generally, higher solubility is considered
to be more valuable, but low
solubility can also be useful, especially
for separating chemicals. In reactive
processes, low solubility can be effectively
used to separate phases and
reaction products to improve process
yield. Creative use of solubility in reaction
systems to improve conversion
will be discussed more later.
Density
Density is defined as mass per unit
volume, and shows mass relative
to other chemicals. In a thoroughly
mixed system, density might not have
value, but it is one of the unique properties
that can be effectively used for
separation of two immiscible liquids.
I think of density as mother nature's
gift - imagine the challenge involved
with separating petroleum and water
if their densities were same. Density
differences can be effectively used for
product separation, and also for creating
a mix that is beneficial in the reaction
and formulation process.
Viscosity
The importance of viscosity is different
in chemical reactions versus formulated
products. By itself, viscosity
may not be considered a very important
property, but engineers must
be aware of it, especially when the
chemicals in the process mixtures are
in liquid form. Viscosity is important
while feeding, mixing and pumping
liquids. It is best to reduce the viscosity
to its lowest levels to facilitate addition,
pumping and mixing. This can
be accomplished either by dissolving
in suitable solvents, or by heating the
liquids, although the stability of the
liquid has to be carefully considered.
Preferred solvents should be the ones
that are being used in the process. In
certain formulated products, viscosity
control is necessary for product performance
and their applications.
Specific heat
Specific heat of a chemical is an important
property and is of value in chemical
reactions. For chemical reactions,
it is important to control the heat of
reaction, and specific heat determines
how much heat needs to be removed.
Specific heat values impact capital investment
because it affects the size of
the heat-transfer equipment required.
Heat of formation
Process reactions are either endothermic
or exothermic. In general, most reactive
processes are exothermic. How
we control the heat of reaction can significantly
impact the rate of reaction as
well as the size of the equipment necessary.
In addition, it is critical to control
the heat of reaction, as a runaway
reaction can rapidly raise the process
temperature and result in explosions
and other hazardous situations. Effective
control of the heat of formation
can reduce the reaction residence time,
which in turn, reduces the size of the
equipment (that is, investment). Heat
of formation is also influenced and
controlled by the method and sequence
used for raw material addition.
Azeotropic behavior
To many chemists or chemical engineers,
the azeotropic behavior of
chemicals might appear to be of limited
value - it does not have much
value in formulations. However, the
azeotropic behavior of chemicals can
be used very creatively in manipulating
reaction processes. Imagine a reaction
process where the liquid mix has
an azeotrope and the chemicals are
immiscible. Combination of these two
properties can be very effectively used
not only to control a reaction exotherm
but also used to improve yield.
64 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2012
EXAMPLES
Physical properties are valuable tools
for the process creator and manipulator,
and he or she can take advantage
of them when creating and simplifying
a process. Exploiting several different
physical properties in a process
is an exhilarating challenge with moments
of success and failure. However,
failures should be viewed as learning
experiences that will help in future
applications.
When developing processes, engineers
should look to incorporate anything
and everything they can imagine.
Imagination and creativity can lead
to unconventional ways of exploiting
unique chemical properties and their
interactions to arrive at excellent processes.
In the end, how the simplified
process is executed in a commercial
scenario matters a great deal. This is
very similar to creation of an excellent
musical composition. Some of the examples
of how physical properties can
be manipulated for process simplification
are included here:
Use of molten raw materials
Traditional approach: A primary
raw material " A " is solid at room temperature.
Its melting point is about
65°C. It reacts with chemical " B " , a
liquid, in presence of a solid catalyst.
The reaction is carried out at about
75°C. The resulting product " C " is
a liquid at room temperature. The
product-solvent mix is reacted further.
Traditionally, " A " would be dissolved
in a solvent and added to the reactor.
Catalyst would be added using appropriate
methods. Concentration of " A "
in the solvent is about 25% to have a
soluble solution.
Alternate approach: Since the melting
point of the raw material " A " is low
and to achieve a reasonable reaction
rate requires the reaction mass to be
heated to a temperature higher than
the melting point of " A, " it may be efficient
and productive to feed the raw
material " A " as a melt. Melt addition
raises the temperature of the reaction
mass faster than heating the reaction
mass from room temperature. This
will reduce the cycle time of the batch
process. Reaction temperature can be
controlled using conventional processcontrol
strategies.
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Chemical Engineering April 2012

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